s2fieldsofstudy: Chemistry Physics

text:
Effect of 2-Methylthiazole Group on Photoinduced Birefringence of Thiazole-Azo Dye Host–Guest Systems at Different Wavelengths of Irradiation

The photoinduced birefringence behaviors of host–guest systems based on heterocyclic thiazole–azo dyes with different substituents, dispersed into PMMA matrix, were investigated under three excitation wavelengths, i.e., 405 nm, 445 nm or 532 nm. The wavelengths fell on the blue side, near the maximum or on the red side of the absorption bands of trans-azo dyes, respectively. We found that photoinduced birefringence was generated at a similar extent in all studied systems, except the system containing a 2-methyl-5-benzothiazolyl as thiazole–azo dye substituent. For this material, the achieved birefringence value was the highest among the whole series, regardless of the excitation wavelength. Moreover, we identified the optimal irradiation wavelength for efficient birefringence generation and showed that large absorption of excitation light by trans isomer does not account for achieving a significant degree of molecular alignment. The obtained results indicate that thiazole–azo dye with a 2-methyl-5-benzothiazolyl substituent shows promising photoinduced birefringence, and can be considered a dye potentially suitable for optical applications.

Introduction
In the last decades, light-responsive molecules have been of interest in many applications, such as optical data storage, optical switches, optical memory, etc., [1][2][3][4].
Azo dyes are the best known family of photoresponsive compounds, which have different properties and can also have a refractive index depending on the polarization and propagation direction of light, namely birefringence.
Azobenzenes attract the attention of the scientific community due to their photochromic nature, ease of processing and simplicity of design [5][6][7]. These features allow changing their physico-chemical properties for a particular application by appropriate modification of their chemical structure [8][9][10][11]. It is well known that the core of azo dyes is formed by the conjugated azo (-N = N-) chromophore group in combination with one or more aromatic or heterocyclic systems. The addition of electron withdrawing and/or electron donating substituents to the backbone of azo moiety can significantly influence the absorption spectra of azo dyes by affecting the reorganization of electronic density [12][13][14]. The D-π-A system of azo compounds can provide a prerequisite ground state charge asymmetry [15] as well as efficient intramolecular charge transfer (ICT) between donor and acceptor groups [16,17] because the π-conjugated bridge ensures a pathway for electronic charges movement [15]. The transition from the ground state to the excited state upon excitation causes almost instantaneous electronic polarization, which changes the dipole moment of the molecules and generates a dipolar push-pull system [16][17][18][19]. Therefore, substitution of one or more benzene rings with easily delocalizable electron-excessive and/or electron-deficient hetero-aromatic rings, acting as an auxiliary electron donors and/or acceptors, can result in enhanced intramolecular charge transfer [15][16][17].
The most important feature of azo compounds is a possibility of trans-cis photoisomerization, which can be induced and reversed depending on the wavelength of the incident light. It is well known that this photochromic behavior can differ due to the structure of studied compounds (i.e., location and the shape of π-π* and n-π* bands) [6,8,9,20,21].
Another interesting feature of azo compounds is the photoinduced orientation. Generation of optical anisotropy in azo dye-containing materials (e.g., azo dyes dispersed in polymer matrix or azopolymers) results from the orientation of azo chromophores induced by linearly polarized light due to processes of selective absorption and reactions of trans-cis isomerization [22,23]. After numerous trans-cis-trans isomerization processes, the long axes of azo molecules tend to align in directions perpendicular to the polarization of light. As a result, the material becomes birefringent and dichroic in the plane perpendicular to the direction of light propagation [6,24,25].
The efficiency and dynamics of the light-induced birefringence generation strongly depend on various factors, which are related to the chemical structure of azo dye (such as substituents of the azo group and its bulkiness), a chromophore content and a type of polymer matrix, but also with the experimental conditions (e.g., an excitation wavelength and intensity) [26][27][28]. While some general principles govern the efficiency of the lightinduced processes in azo compounds, the optical response of a given material may be substantially different than expected [29,30].
A polymeric material with unique properties, such as light weight, high flexibility and low-cost of production, can be used to improve the quality of a prepared thin layer [31]. One of the most popular and widely used polymeric materials is poly(methyl methacrylate) (PMMA). Its main advantages are excellent mechanical properties, high chemical resistance, simple synthesis, low cost, good tensile strength, low optical loss in visible spectral range, good insulation properties and thermal stability. PMMA-based matrices are well known not only for their good optical transparency, but also for high resistance to laser damage [32,33]. PMMA is an excellent and suitable host material in the host-guest systems due to its optical clarity and known chemical and physical properties. It should also be added that for the samples based on PMMA it is possible to conduct research of the structure of matrices and photophysical transitions connected with changes in the mobility of low molecular structural units.
Studying the correlation between structure and material properties is a fascinating field of research, which is very important for the development of novel materials for specific applications such as optical data storage. Typically, measurements of photoinduced birefringence generation were carried out at a single excitation wavelength located on the red side of the azo moiety absorption band. However, the measurements performed for various excitation wavelengths may provide valuable information on the optimal experimental conditions leading to the most efficient process of azo chromophore alignment.
The aim of this work was to characterize the photoinduced birefringence generation in thiazole-azo dyes host-guest systems under irradiation with linearly polarized violet, blue or green light. The motivation for this research is the possibility of using thiazole-azo dyes in photonic devices for recording optical information (optical data storage), which are becoming increasingly important in many fields. In this article, we focus on the effect of an additional 2-methylthiazole group on the efficiency photoinduced birefringence generated in thiazole-azo dyes dispersed into a poly(methyl methacrylate) (PMMA) matrix. Furthermore, we introduce the benzene ring into a thiazole fragment in the position of 4, which can improve solubility of the compounds. The key structural feature of the investigated materials is the presence of a heterocyclic thiazole fragment in the azo molecule, which leads to the change in the distribution of the electron density of the conjugation system in comparison with azobenzenes without a heterocyclic fragment. We show that upon irradiation with polarized violet, blue or green light, the studied azo dye systems can exhibit photoinduced birefringence. To the best of our knowledge, the photoinduced birefringence generation for these heterocyclic thiazole-azo dyes dispersed into a PMMA matrix at 405 nm, 445 nm and 532 nm are presented for the first time. Figure 1 shows the UV-Vis spectra of the studied thiazole-azo dyes dispersed in the PMMA matrix thin films (T-azo-OCH 3 , T-azo2-OCH 3 , T-azo-H). One can see that the π-π* and n-π* bands are completely overlapped in this region, and the absorption bands of T-azo-OCH 3 , T-azo2-OCH 3 samples are redshifted relative to the thin film of T-azo-H without any substitution in para-position [8]. We also found that the absorption band of T-azo2-OCH 3 thin film with 2-methyl-5-benzothiazolyl moiety is redshifted compared to T-azo-OCH 3 film with a phenyl ring.

UV-Vis Spectra
It should also be noted that the excitation wavelength of 445 nm used in the photoinduced birefringence measurement was the most strongly absorbed by the examined samples compared to the excitation wavelengths of 405 nm and 532 nm. The samples were transparent at the probing wavelengths (690 nm or 783 nm, respectively). (PMMA) matrix. Furthermore, we introduce the benzene ring into a thiazole fragment in the position of 4, which can improve solubility of the compounds. The key structural feature of the investigated materials is the presence of a heterocyclic thiazole fragment in the azo molecule, which leads to the change in the distribution of the electron density of the conjugation system in comparison with azobenzenes without a heterocyclic fragment. We show that upon irradiation with polarized violet, blue or green light, the studied azo dye systems can exhibit photoinduced birefringence. To the best of our knowledge, the photoinduced birefringence generation for these heterocyclic thiazole-azo dyes dispersed into a PMMA matrix at 405 nm, 445 nm and 532 nm are presented for the first time. Figure 1 shows the UV-Vis spectra of the studied thiazole-azo dyes dispersed in the PMMA matrix thin films (T-azo-OCH3, T-azo2-OCH3, T-azo-H). One can see that the π-π* and n-π* bands are completely overlapped in this region, and the absorption bands of T-azo-OCH3, T-azo2-OCH3 samples are redshifted relative to the thin film of T-azo-H without any substitution in para-position [8]. We also found that the absorption band of T-azo2-OCH3 thin film with 2-methyl-5-benzothiazolyl moiety is redshifted compared to T-azo-OCH3 film with a phenyl ring.

UV-Vis Spectra
It should also be noted that the excitation wavelength of 445 nm used in the photoinduced birefringence measurement was the most strongly absorbed by the examined samples compared to the excitation wavelengths of 405 nm and 532 nm. The samples were transparent at the probing wavelengths (690 nm or 783 nm, respectively).     Figure 3 present the birefringence growth and relaxation curves for the thiazole-azo dyes dispersed in PMMA matrix thin films, where λexc. = 405 nm and λprobe = 690 nm were used. We found that the irradiation time of a few hundred seconds was already sufficient to observe saturation of birefringence in the studied compounds (see Figure 3a). We also found that thiazole-azo-PMMA samples T-azo-OCH3 and T-azo2-OCH3 have a higher saturation level of birefringence compare to the T-azo-H one without substituent in paraposition. Moreover, T-azo2-OCH3 with a heterocyclic fragment (R1) has the highest final birefringence, which is almost twice as that for T-azo-OCH3 with a phenyl fragment (R1). At the same time, T-azo2-OCH3 exhibits the most stable birefringence after irradiation among the series (Figure 3b). One can also see that the T-azo-OCH3 sample with a phenyl ring (R1) and electron donating group (R2) has almost a similar birefringence value after relaxation with T-azo-H and it is almost two times smaller than for T-azo2-OCH3.  Figure 3 present the birefringence growth and relaxation curves for the thiazole-azo dyes dispersed in PMMA matrix thin films, where λ exc. = 405 nm and λ probe = 690 nm were used. We found that the irradiation time of a few hundred seconds was already sufficient to observe saturation of birefringence in the studied compounds (see Figure 3a). We also found that thiazole-azo-PMMA samples T-azo-OCH 3 and T-azo2-OCH 3 have a higher saturation level of birefringence compare to the T-azo-H one without substituent in paraposition. Moreover, T-azo2-OCH 3 with a heterocyclic fragment (R1) has the highest final birefringence, which is almost twice as that for T-azo-OCH 3 with a phenyl fragment (R1). At the same time, T-azo2-OCH 3 exhibits the most stable birefringence after irradiation among the series (Figure 3b). One can also see that the T-azo-OCH 3 sample with a phenyl ring (R 1 ) and electron donating group (R 2 ) has almost a similar birefringence value after relaxation with T-azo-H and it is almost two times smaller than for T-azo2-OCH 3 .  The curves of birefringence growth under 445 nm excitation and birefringence relaxation for the studied films are shown in Figure 4. It is interesting that despite a strong film  The curves of birefringence growth under 445 nm excitation and birefringence relaxation for the studied films are shown in Figure 4. It is interesting that despite a strong film absorbance at this wavelength, the values of final birefringence observed for all the samples were lower than the values obtained in the case of 405 nm excitation. The result may be explained on the basis of the recorded changes in the absorption spectra under irradiation. Both transand cis-isomers are involved in the process of optical birefringence generation, and thus, light absorption by the cis form is essential for obtaining a significant degree of molecular order. Cis-isomers more effectively absorb the 405 nm wavelength than 445 nm light, which compensates for the effect of a lower absorption of 405 nm light by trans-isomers.

Photoinduced Birefringence
As in the case of 405 nm excitation, we found that the highest birefringence, under 445 nm, was also induced in T-azo2-OCH 3 film (thiazole-azo dye with 2-methyl-5benzothiazolyl substituent-R 1 and methoxy group R 2 ), which again correlates with the slowest birefringence relaxation rate (see Figure 4b). Its final birefringence is almost twice as large as the final induced birefringence compared to T-azo-OCH 3 due to an additional 2-methylthiazole group. We also found that host-guest film of thiazole-azo compound T-azo-OCH 3 with a phenyl ring (R 1 ) and electron donating group (R 2 ) demonstrate higher birefringence saturation level compared to T-azo-H without a substituent in para-position (R 2 ). Figure 4b shows the normalized birefringence relaxation curves after turning off the beam at 445 nm. One can see that the type of substituent strongly affects the relaxation of birefringence. We found that T-azo2-OCH 3 exhibits the lowest relaxation, which may be associated with different geometry of chromophores T-azo2-OCH 3 vs. T-azo-OCH 3 and T-azo-H with more compact structure. It is difficult to relax the molecules to the isotropic state by thermal movement if the chromophores have a big volume. The 2-methyl-5-benzothiazolyl group in T-azo2-OCH 3 increases the steric effect and slows down the relaxation of birefringence of T-azo2-OCH 3 . However, a thiazole-azo compound with a phenyl ring (R 1 ) and electron donating group (R 2 ) (T-azo-OCH 3 ) has similar birefringence to T-azo-H. Therefore, the role of various substituents in thiazole-azo dyes in the photoinduced birefringence measurements is evident.   Figure 5 presents the birefringence growth and relaxation curves for the thiazole-azo dyes dispersed in PMMA matrix thin films, where λexc. = 532 nm and λprobe = 783 nm were used. Apart from T--azo2-OCH3, the irradiation time of about 300 s was sufficient to observe birefringence saturation for all samples. We also found that the highest birefringence saturation level was induced in T-azo2-OCH3 with the highest absorption value at 532 nm. This can be explained by a significantly red-shifted absorption band and arising strongest absorption of 532 nm light among the series. Nevertheless, the values of photoinduced birefringence generated under 532 nm irradiation were very low. The result can be attributed to low sample absorbance, i.e., the excitation wavelength falls on the tails  Figure 5 presents the birefringence growth and relaxation curves for the thiazole-azo dyes dispersed in PMMA matrix thin films, where λ exc. = 532 nm and λ probe = 783 nm were used. Apart from T-azo2-OCH 3 , the irradiation time of about 300 s was sufficient to observe birefringence saturation for all samples. We also found that the highest birefringence saturation level was induced in T-azo2-OCH 3 with the highest absorption value at 532 nm. This can be explained by a significantly red-shifted absorption band and arising strongest absorption of 532 nm light among the series. Nevertheless, the values of photoinduced birefringence generated under 532 nm irradiation were very low. The result can be attributed to low sample absorbance, i.e., the excitation wavelength falls on the tails of trans absorption bands for all the samples. Figure 5 presents the birefringence growth and relaxation curves for the thiazole-azo dyes dispersed in PMMA matrix thin films, where λexc. = 532 nm and λprobe = 783 nm were used. Apart from T--azo2-OCH3, the irradiation time of about 300 s was sufficient to observe birefringence saturation for all samples. We also found that the highest birefringence saturation level was induced in T-azo2-OCH3 with the highest absorption value at 532 nm. This can be explained by a significantly red-shifted absorption band and arising strongest absorption of 532 nm light among the series. Nevertheless, the values of photoinduced birefringence generated under 532 nm irradiation were very low. The result can be attributed to low sample absorbance, i.e., the excitation wavelength falls on the tails of trans absorption bands for all the samples.  From Figure 5a, it can be seen that the birefringence saturation level decreases as follows: T-azo2-OCH 3 > T-azo-OCH 3 > T-azo-H. Thus, the thiazole-azo compound with 2-methyl-5-benzothiazolyl substituent (R 1 ) (T-azo2-OCH 3 ) has a higher saturation level of birefringence compared to thiazole-azo dyes with a phenyl ring (R 1 ). Similar behavior was visible in absorbance. For 532 nm excitation (see Figure 5b), the relaxation of birefringence was similar for all studied compounds. Figure 6 presents the examples of birefringence growth and relaxation curves for the thiazole-azo dyes dispersed in PMMA matrix thin films for three excitation wavelengths, i.e., 405 nm, 445 nm and 532 nm. In all cases, we observed the rapid increase of birefringence at the beginning of pumping (see Figure 6), which was due to the molecular arrangement orientation of the thiazole-azo dye, which gradually tended to be perpendicular to the polarization direction of the pumping light; thus the detecting light intensity began to increase gradually. Then we can see a slow increase to the saturation level with different speeds depending on the type of substituent. When the pumping light was turned off, the curves decreased sharply due to the molecular relaxation. The anisotropic state reestablishes the originally mixed and disordered distribution. However, this type of recovery is not complete, because some azo molecules achieve equilibrium, and some still remain at an orientation distribution state.
The decay of birefringence after turning off the excitation light was caused by the thermal cis-trans isomerization of thiazole-azo chromophores and a thermal randomization of the molecular orientation.
We found that the final birefringence generated after irradiation with 405 nm light was the highest for all studied thiazole-azo dyes. The difference between the increase in the birefringence for the studied wavelengths strongly depends on the type of substituents in thiazole-azo compounds. state re-establishes the originally mixed and disordered distribution. However, this type of recovery is not complete, because some azo molecules achieve equilibrium, and some still remain at an orientation distribution state.
The decay of birefringence after turning off the excitation light was caused by the thermal cis-trans isomerization of thiazole-azo chromophores and a thermal randomization of the molecular orientation.  The birefringence growth and birefringence relaxation with time are often described by the following biexponential equations [25]: where τ 1 , τ 2 are time constants for writing processes, τ 3 , τ 4 are time constants for relaxation processes, A, B, C and D are amplitudes associated with different physical processes appearing upon illumination, and E is the residual birefringence. Using Equations (1) and (2), one can perform the curve fitting, which allows us to quantitatively compare the obtained birefringence signals. It should be noted that the biexponential growth and biexponential relaxation reproduced the results of the experiment well. The values of the fitted parameter for T-azo-H, T-azo-OCH 3 and T-azo2-OCH 3 are presented in Tables 1 and 2. The contributions of various processes to birefringence growth and relaxation were calculated using Equation (3): where X i = A, B and C, D, E for birefringence growth and relaxation, respectively. 3 and T-azo2-OCH 3 for 405 nm, 445 nm and 532 nm.  3 and T-azo2-OCH 3 for 405 nm, 445 nm and 532 nm. When excited with 405 nm light, for samples T-azo-H and T-azo-OCH 3, the fast and slow processes contributions to the birefringence growth are the same for both materials and are 0.65 and 0.35, respectively. The time factors for both components are of the same order. For the sample T-azo2-OCH 3 , the slow component has more impact on birefringence growth than for the other materials, and the fast and slow processes' contributions are 0.51 and 0.49, respectively. The time factors are noticeably longer than for the other two samples, especially for the slow component, whose time factor is one order of magnitude higher.

T-azo-H T-azo-OCH 3 T-azo2-OCH
Upon excitation with 445 nm light, the fast process contribution to birefringence growth is higher than the slow process contribution. For the samples T-azo-H and T-azo2-OCH 3 T-azo-H and T-azo2-OCH 3 , and it is the lowest for the Tazo-OCH 3 sample, while for the fast process, the time factors are similar for the T-azo-H and T-azo-OCH 3 samples, and it is slightly higher for the T-azo2-OCH 3 sample. Table 2 shows the fitted parameters for the birefringence relaxation. After excitation with 405 nm light, the fast process contribution to birefringence relaxation is slightly higher than the slow process contribution. The sample T-azo2-OCH 3 exhibits the highest residual birefringence, while for the sample T-azo-H, it is the lowest. Both time factors for the fast and slow processes are the lowest for the T-azo-H sample, and they are the highest for the T-azo2-OCH 3 sample.
The fast processes contribution to birefringence relaxation after 445 nm excitation is slightly higher than the slow process contribution. Again, the sample T-azo2-OCH 3 exhibits the highest residual birefringence, and the sample T-azo-H has the lowest. Time factors are the same order of magnitude, and both time factors are the lowest for the T-azo-OCH 3 sample. They are the highest for the T-azo2-OCH 3 sample.
The fast process contribution to the birefringence relaxation is considerably higher than the slow process contribution after excitation with 532 nm light, for all the samples. The residual birefringence is similar, and it is around 0.1 of the maximum birefringence value. Time factors, separately, are the same order of magnitude. Both time factors are the lowest for the T-azo-OCH 3 sample. The fast process time factor is the highest for the T-azo2-OCH 3 sample, while the slow process time factor is the highest for the T-azo-H sample. Table 3 summarizes the ratios between the maximum birefringence and absorbance for the given excitation wavelengths. As can be seen, there is no clear influence of the amount of the absorbed light on the maximum birefringence value. Even though the absorbance at 405 nm and 445 nm is the lowest for the T-azo2-OCH 3 sample, the birefringence values are the highest. For the 532 nm light, the absorbance of the T-zo2-OCH 3 sample is the highest amongst the three studied samples, and the birefringence value is the highest as well. However, the ratio between the two parameters is the lowest. Table 3. Maximum birefringence values, absorbance at the excitation wavelengths and their ratio for T-azo-H, T-azo-OCH 3 and T-azo2-OCH 3 samples.  Figure 7 shows the chemical structure of the studied thiazole-azo dyes. The synthesis procedure for T-azo-H, T-azo-OCH 3 and T-azo2-OCH 3 is described elsewhere [8,9,20,34]. 1

Preparation of Thin Films
The standard procedure was used to prepare thin films of studied thiazole-azo dyes dispersed in the PMMA (poly(methylmethacrylate)) matrix using a spin-coating method [8,9]. THF solutions including PMMA and the thiazole-azo dyes were prepared first. PMMA was purchased from Sigma-Aldrich and was used as it was. Films were formed on glass substrates using a spin-coating method with the spinning time of 60 s. After that, films were baked at 60 °C for 3 h in a vacuum chamber. The thickness of the samples was in the range of 900-1300 nm.

UV-Vis Absorption
The absorption spectra of all studied thin layers of heterocyclic thiazole-azo compounds dispersed in the PMMA matrix were measured with a spectrometer (Shimadzu UV-1800) in the range 350-600 nm.

Photoinduced Birefringence Measurements
Photoinduced birefringence measurements were performed for 405 nm, 445 nm and 532 nm excitation wavelengths. The experimental configuration used in the studies with violet and blue irradiation was presented elsewhere [35]. The intensity of each beam (from diode lasers) was 100 mW/cm 2 . The time-evolution of birefringence generation and birefringence decrease after switching on and off the excitation light was probed by 690 nm wave. The excitation and probe beams were linearly polarized in the directions forming an angle of 45°. The measurement technique is based on detecting the intensity of the probe beam after passing through the thin film situated between two crossed polarizers [35]. The details of the experimental configuration were described elsewhere [36][37][38], whereas Figure 8 shows the experimental configuration of photoinduced birefringence at excitation of a CW laser (λexc. = 532 nm, 0.365 mW, I~29 mW/cm 2 ). The details of this setup were described elsewhere [24].

Preparation of Thin Films
The standard procedure was used to prepare thin films of studied thiazole-azo dyes dispersed in the PMMA (poly(methylmethacrylate)) matrix using a spin-coating method [8,9]. THF solutions including PMMA and the thiazole-azo dyes were prepared first. PMMA was purchased from Sigma-Aldrich and was used as it was. Films were formed on glass substrates using a spin-coating method with the spinning time of 60 s. After that, films were baked at 60 • C for 3 h in a vacuum chamber. The thickness of the samples was in the range of 900-1300 nm.

UV-Vis Absorption
The absorption spectra of all studied thin layers of heterocyclic thiazole-azo compounds dispersed in the PMMA matrix were measured with a spectrometer (Shimadzu UV-1800) in the range 350-600 nm.

Photoinduced Birefringence Measurements
Photoinduced birefringence measurements were performed for 405 nm, 445 nm and 532 nm excitation wavelengths. The experimental configuration used in the studies with violet and blue irradiation was presented elsewhere [35]. The intensity of each beam (from diode lasers) was 100 mW/cm 2 . The time-evolution of birefringence generation and birefringence decrease after switching on and off the excitation light was probed by 690 nm wave. The excitation and probe beams were linearly polarized in the directions forming an angle of 45 • . The measurement technique is based on detecting the intensity of the probe beam after passing through the thin film situated between two crossed polarizers [35]. The details of the experimental configuration were described elsewhere [36][37][38], whereas Figure 8 shows the experimental configuration of photoinduced birefringence at excitation of a CW laser (λ exc. = 532 nm, 0.365 mW, I~29 mW/cm 2 ). The details of this setup were described elsewhere [24]. 532 nm excitation wavelengths. The experimental configuration used in the studies with violet and blue irradiation was presented elsewhere [35]. The intensity of each beam (from diode lasers) was 100 mW/cm 2 . The time-evolution of birefringence generation and birefringence decrease after switching on and off the excitation light was probed by 690 nm wave. The excitation and probe beams were linearly polarized in the directions forming an angle of 45°. The measurement technique is based on detecting the intensity of the probe beam after passing through the thin film situated between two crossed polarizers [35]. The details of the experimental configuration were described elsewhere [36][37][38], whereas Figure 8 shows the experimental configuration of photoinduced birefringence at excitation of a CW laser (λexc. = 532 nm, 0.365 mW, I~29 mW/cm 2 ). The details of this setup were described elsewhere [24].

Conclusions
The optical birefringence was induced in three heterocyclic thiazole-azo dyes with different substituents dispersed in a PMMA matrix, upon polarized violet, blue and green irradiation. We found that the role of the substituents in thiazole-azo dyes and irradiation wavelength is visible during the birefringence generation.
We noticed that the photoinduced birefringence response at 405 nm, 445 nm or 532 nm of most studied host-guest thin films of PMMA-thiazole-azo dyes with different substituents is similar, except for thiazole-azo dye with 2-methyl-5-benzothiazolyl substituent (i.e., T-azo2-OCH 3 ). It was found that this molecule had the highest saturation level of birefringence compared to other studied thiazole-azo dyes for all three induced irradiation wavelengths (i.e., 405 nm, 445 nm and 532 nm). This dye also exhibited the lowest relaxation after ceasing the irradiation 405 nm and 445 nm wavelengths. We suppose that the high ∆n value obtained for T-azo2-OCH 3 , despite its lower absorption at these wavelengths, can be attributed to the free space in the polymer, created by the bulky T-azo2-OCH 3 chromophores, giving them the opportunity to reorient.
The chemical structure is the main factor influencing the photoinduced behavior of the studied thiazole-azo dyes. The introduction of the thiazole-azobenzene unit into PMMA matrix, restricts the chromophore motions during the writing process. In the hostguest polymers, the chromophores are typically more mobile, which can induce a faster inscription of ∆n. Therefore, the appropriate design of thiazole-azo dyes can increase the properties of photoinduced birefringence, which contributes to their use in new photonic devices such as optical data storage.  Data Availability Statement: Data supporting the results of this study are available from the appropriate author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds T-azo-H, T-azo2-OCH 3 and T-azo-OCH 3 are available from the appropriate authors upon reasonable request.


Replica-Exchange Enveloping Distribution Sampling: Calculation of Relative Free Energies in GROMOS

: Free-energy calculations based on molecular dynamics (MD) simulations are playing an increasingly important role for computer-aided drug design and material discovery in recent years. Free-energy differences between pairs of end-states can be estimated using well-established methods such as thermodynamic integration (TI) or Bennett’s acceptance ratio (BAR). An attractive alternative is the recently developed replica-exchange enveloping distribution sampling (RE-EDS) method, which enables estimating relative free-energy differences between multiple molecules from a single simulation. Here, we provide an introduction to the principles underlying RE-EDS and give an overview of the RE-EDS pipeline. In addition, we provide a description of the two complementary tools RestraintMaker and amber2gromos. We briefly discuss the findings of three recent applications of RE-EDS to calculate relative binding or hydration free energies. In all three studies, good agreement was found between the results obtained using RE-EDS and experimental values as well as values obtained using other free-energy methods.

Salomé R. Rieder obtained her Bachelor's and Master's degree in Computational Science and Engineering with a specialization in fluid dynamics and computational chemistry from ETH Zurich. For her Master's thesis she was awarded the ETH Medal in 2019. She has been working as a PhD candidate in the CSMS and CCG research groups since 2018. Her research is focused on the CombiFF force-field parameterization project and on the development and application of the RE-EDS multistate free-energy method.

Introduction
Molecular dynamics (MD) simulation is a well-established tool to investigate the properties of molecular systems in silico. In classical MD simulations, Newton's equations of motion are integrated numerically to propagate the particles of a system in time. The accuracy of MD calculations relies critically on the quality of the underlying force field, i.e. the functional form and parameters that determine the covalent and non-bonded interactions between the particles. Free-energy calculations are an important and challenging task in the field of MD simulation, specifically for computer-aided drug design and material discovery. The free energy of a state determines the probability that the system will adopt this specific state. [1] For example, the (relative) binding free energy calculated in simulations is directly related to the experimental binding affinities of ligands to a protein. [2] Persistent methodological, software and hardware advances continue to make the calculation of binding free energies more efficient and accurate. Furthermore, the automation of free-energy pipelines renders the setup of such calculations less error prone. However, they can still be quite timeconsuming. Owing to much smaller system sizes, the calculation of solvation/hydration free energies of small organic molecules is much faster, making them a good test case to compare different free-energy methods and force fields.
Here, we provide an overview of the replica-exchange enveloping distribution sampling (RE-EDS) [3,4] pipeline, [5] as well as of the two complementary tools RestraintMaker [6] and amber2gromos. [7] We also briefly discuss the results of three recent studies

RE-EDS Pipeline
Recently, Ries et al. [5] published an improved RE-EDS pipeline consisting of three main phases: parameter exploration, parameter optimization, and production. During the parameter exploration, a favorable configuration is generated for each end-state to use as initial coordinates for a starting state mixing (SSM) approach. Further, a lower bound for the s-values is determined and initial energy offsets are estimated. The second phase involves optimizing the s-distribution (number and location of replicas) and rebalancing of the energy offsets such that all end-states are sampled roughly equally at s = 1. Finally, the relative free-energy differences of all end-state pairs in the system are calculated from the production run. The workflow is controlled by the Python3 reeds module, freely available at https://github.com/rinikerlab/reeds/.

RestraintMaker
In free-energy calculations, the end-states can be represented as 'single', 'hybrid', or 'dual topology'. [6] In (RE-)EDS, a dual topology representation is typically used, i.e. the coordinates of all ligands are present separately in the system, independent from each other. Consequently, the molecules might drift away from each other during a simulation. One strategy to prevent this drift is the application of (atomic) distance restraints to the common core of the molecules. As the number of end-states in the system and/or the size of the ligands grows, the choice investigating the performance of RE-EDS, RestraintMaker and am-ber2gromos to calculate relative binding or hydration free energies.

Calculation of Free-energy Differences
In the thermodynamic cycle, two equivalent pathways can be taken to obtain the relative hydration free energy of two end-states A and B, either via absolute hydration free energies or via pairwise free-energy differences in water and in vacuum (Eqn. (1)) Thermodynamic integration (TI) [8] and (multistate) Bennett's acceptance ratio (MBAR) [9,10] are two well-established pathwaydependent methods to estimate free-energy differences from MD simulations using a coupling parameter λ. They can be used, for example, to estimate the free-energy difference of a molecule upon a change in the environment, or the relative free-energy difference of a pair of molecules. Enveloping distribution sampling (EDS), [11,12] on the other hand, is a pathway-independent multistate free-energy method, i.e. the free-energy difference of any end-state pair in the system can be calculated from a single simulation. In EDS, a reference state V R combining N end-states is defined in Eqn. (2) (Fig. 1) where s > 0 is the smoothness parameter, E R is a set of energy offsets, and β = 1/(k B T) with k B being the Boltzmann constant and T the absolute temperature. The energy offsets govern the contribution of the end-states to the reference potential energy. At high s-values, the reference state is dominated by the end-state with the lowest value of . As the smoothness parameter decreases towards zero, it flattens the potential-energy landscape such that each end-state contributes to the reference state simultaneously (Fig. 1). The free-energy difference between any pair of end-states in the system can then be calculated as, To obtain accurate free-energy estimates with EDS, an optimal choice of the s-value and of the energy offsets is essential in practice. To mitigate the necessity for an optimal s-value, replicaexchange EDS (RE-EDS) was introduced, [3,4] combining EDS with Hamiltonian replica exchange. [13,14] In RE-EDS, the sampling of the end-states is enhanced by simulating multiple replicas with different s-values and attempting replica exchanges at fixed intervals (Fig. 2). Three 'sampling regions' can be distinguished: physical sampling (s ≈ 1) with typically one dominant end-state, an intermediate region where more end-states start to contribute to the sampling, and a so-called 'undersampling' region with all end-states contributing roughly equally. [5,15]  with FEP+ [23] and QligFEP [24] (Fig. 3, left). With an RMSE of 3.3 kJ/mol against the experimental values, the accuracy of the results obtained with RE-EDS SSM was considerably better than with RE-EDS 1SS (RMSE = 4.8 kJ/mol). The accuracy of RE-EDS SSM was found to be comparable to that of FEP+ and QligFEP. With a total simulation time of about 50 ns, QligFEP was the most efficient of the three approaches, followed by the RE-EDS pipeline with about 160 ns, requiring only about a quarter of the simulation time of FEP+. One of the main advantages of RE-EDS is the fact that all transformations are sampled explicitly, avoiding the error propagation that occurs when only selected pairs are calculated with pathway dependent methods.

Validation of RestraintMaker: Relative Hydration Free Energies of Small Molecules
In another study, [6] relative hydration free energies were calculated for two sets of small molecules in order to validate the quality of the distance restraints selected by RestraintMaker. The first set (labeled A) consisted of six molecules and the second set (labeled B) contained ten molecules. The selected molecules contained a small ring core and involved R-group changes and core-hopping transformations, e.g. benzene to cyclohexane. Dual topology TI and RE-EDS simulations were carried out in vacuum/water in GROMOS to estimate the relative hydration free energies. RestraintMaker was used to select atomic distance restraints. For the multistate RE-EDS calculations, each molecule was restrained to two other molecules, forming a cyclic chain of pairwise restrained molecules. The obtained relative hydration free energies were compared to experimental values as well as to the results obtained from TI calculations as reported on the AT B server [24] (Fig. 3, center). For a reasonable force constant (5000 kJ mol -1 nm -2 ), the molecules remained well-aligned during the simulations and the sampling behavior was not (or only negligibly) affected by the employed distance restraints. For both selected sets, the results obtained with the different free energy estimators agreed well with each other and with the experimental values. The simulation time required to obtain converged freeenergy estimates with RE-EDS was shorter than for the dual topology TI calculations by about a factor 5 for set A and a factor 3.5 for set B.
of appropriate distance restraints becomes increasingly nontrivial. To address this, the Python3 program RestraintMaker was developed by Ries et al., [6] which is freely available at https://github.com/rinikerlab/restraintmaker/. Given a set of pre-aligned molecules, RestraintMaker employs a greedy algorithm to select locally optimal distance restraints for dual topology free-energy calculations, e.g. with RE-EDS or TI.

amber2gromos
A popular small molecule force field is the generalized AMBER force field (GAFF). [16] One of the advantages of GAFF is the availability of antechamber, [17] a tool to automatically parameterize molecular systems. GAFF was originally developed for the AMBER MD engine. [18] There are several differences between AMBER and GROMOS force fields, such as the file formats, the units, and the functional form. [19] As RE-EDS is currently implemented in the GROMOS MD engine, [20] the C++ program amber2gromos [7] was developed to convert GAFF (or generally AMBER) topologies into a format compatible with the GROMOS MD engine, facilitating the preparation of new systems of interest for (RE-EDS) simulations in GROMOS. amber2gromos will be part of the next scheduled release of the GROMOS++ package, [21] freely available at http://gromos.net/.

RE-EDS Case Studies
The performance of RE-EDS has been assessed in several studies. [3][4][5][6][7] In this section, we would like to highlight three recent publications.

Validation of the RE-EDS Pipeline: Relative Binding Free Energies of CHK1 Inhibitors
In a study by Ries et al., [5] the improved RE-EDS pipeline was tested by estimating the relative binding free energies of five inhibitors of human checkpoint kinase 1 (CHK1). [22] The five selected ligands contained R-group modifications as well as different core-hopping transformations such as ring size change and ring opening/closing. Two RE-EDS pipelines were compared, one with a single set of starting coordinates (labeled 1SS) and one with SSM. The relative binding free energies obtained were then compared to experimental values [22] and to reported results obtained Fig. 3. Comparison of relative binding free energies (left) or hydration free energies (middle, right) obtained with different free-energy methods to experimental values. The underlying datasets and simulation setups are discussed in detail in refs. [5][6][7]. The gray line corresponds to perfect agreement with ±4.184 kJ/mol (±1 kcal/mol). The reported metrics (RMSE, MAE, r Spearman ) compare the RE-EDS results to the experimental values. Overall, there is a good agreement between RE-EDS and both experiment and other free-energy estimators. (Left): relative binding free energies of five CHK1 inhibitors obtained with FEP+ with/without cycle closure (CC), [23] QligFEP, [24] and RE-EDS (SSM and 1SS) [5] compared to experimental results. [22] (Middle): relative hydration free energies of two sets of small molecules with a ring core obtained with TI [25] (direct via absolute free-energy calculations), TI [6] (indirect via relative free-energy calculations), and RE-EDS [6] compared to experimental results. [25] (Right): relative hydration free energies for two sets of benzene derivatives obtained with MBAR [26,27] and RE-EDS [7] compared to experimental results. [26,27]

License and Terms
This is an Open Access article under the terms of the Creative Commons Attribution License CC BY 4.0. The material may not be used for commercial purposes. The license is subject to the CHIMIA terms and conditions: (https://chimia.ch/chimia/about).
The definitive version of this article is the electronic one that can be found at https://doi.org/10.2533/chimia.2022.327

Validation of amber2gromos: Relative Hydration Free Energies of Small Benzene Derivatives
A third study [7] focused on the validation of the GAFF topologies converted with amber2gromos for RE-EDS simulations in GROMOS. Two sets of benzene derivatives were selected from the FreeSolv [26,27] database. A small set (labeled A) containing six molecules was used as an initial sanity check. To assess the accuracy of the relative free-energy differences in vacuum/water, complementary single topology TI simulations were carried out with the GROMACS [28] MD engine for set A. In order to explore the performance of multistate RE-EDS, a larger set (labeled B) with 28 molecules was selected. To investigate whether the performance is adversely affected by the larger number of end-states, set B was additionally divided into two subsets (Ba and Bb, with one molecule in common). For all sets, amber2gromos was used to generate the topologies from the input GAFF topologies, RestraintMaker to generate appropriate distance restraints, and the RE-EDS pipeline to carry out the simulations. The results were compared to the experimental and calculated (using MBAR) values reported in FreeSolv (Fig. 3, right). With an overall RMSE of 2.6 kJ/mol against experimental data and 1.1 kJ/mol against the MBAR results, the agreement was excellent. The relative hydration free energies obtained with RE-EDS from the two combined subsets Ba and Bb were slightly more accurate than the results for the full set B (RMSE of 2.5 kJ/mol versus 2.6 kJ/mol). As for the previously described study, the required simulation time was considerably shorter for RE-EDS than for TI (at least a factor 6 for set A) and MBAR (about a factor 5.5 for set A, 4.5 for set B).

Conclusions
We provided an overview of the RE-EDS pipeline and the complementary tools RestraintMaker and amber2gromos. Combined, they provide a convenient, robust, and largely automated workflow for the setup and execution of free-energy calculations with RE-EDS in GROMOS.
The results from three applications of RE-EDS to calculate relative binding or hydration free energies were briefly discussed. In all three studies, the relative free energies obtained with RE-EDS accurately reproduced experimental values, as well as results obtained with different free energy methods, even for a large number (e.g. 28) of end-states. A major advantage of the RE-EDS pipeline is the comparably small amount of total simulation time required.


Enhanced Photocatalytic Hydrogen Production Activity by Constructing a Robust Organic-Inorganic Hybrid Material Based Fulvalene and TiO2

A novel redox-active organic-inorganic hybrid material (denoted as H4TTFTB-TiO2) based on tetrathiafulvalene derivatives and titanium dioxide with a micro/mesoporous nanomaterial structure has been synthesized via a facile sol-gel method. In this study, tetrathiafulvalene-3,4,5,6-tetrakis(4-benzoic acid) (H4TTFTB) is an ideal electron-rich organic material and has been introduced into TiO2 for promoting photocatalytic H2 production under visible light irradiation. Notably, the optimized composites demonstrate remarkably enhanced photocatalytic H2 evolution performance with a maximum H2 evolution rate of 1452 μmol g−1 h−1, which is much higher than the prototypical counterparts, the common dye-sensitized sample (denoted as H4TTFTB-5.0/TiO2) (390.8 μmol g−1 h−1) and pure TiO2 (18.87 μmol g−1 h−1). Moreover, the composites perform with excellent stability even after being used for seven time cycles. A series of characterizations of the morphological structure, the photoelectric physics performance and the photocatalytic activity of the hybrid reveal that the donor-acceptor structural H4TTFTB and TiO2 have been combined robustly by covalent titanium ester during the synthesis process, which improves the stability of the hybrid nanomaterials, extends visible-light adsorption range and stimulates the separation of photogenerated charges. This work provides new insight for regulating precisely the structure of the fulvalene-based composite at the molecule level and enhances our in-depth fundamental understanding of the photocatalytic mechanism.

Introduction
H 2 , as a high calorific value and zero pollution, has attracted widespread attention [1][2][3]. Photocatalytic water splitting is considered to be one of the significant strategies for converting solar energy into chemical energy without pollution to the environment [4][5][6][7][8][9]. Using semiconductor photocatalysis to split water is an effective clean hydrogen production technology. Among many photocatalytic semiconductor materials, n-type TiO 2 has become one of the most promising photocatalysts due to its excellent photoelectrochemical performance, low cost, non-toxic and strong redox activity [10][11][12][13]. However, TiO 2 has a wide band gap and low visible-light absorption ability, and it can only absorb ultraviolet light (about 4% of solar energy) [14]. Simultaneously, photogenerated carriers are easy to recombine, and quantum efficiency is low, which seriously affects the development of TiO 2 in the field of photocatalytic hydrogen production. Adjusting the morphological structure and the energy band position on TiO 2 , such as doping of metal and non-metal ions [15][16][17], dye sensitization [18][19][20] and semiconductor combination [14,[21][22][23], has been carried out to enhance the photocatalytic activity of TiO 2 . Especially, dye-sensitized composites have been applied to utilize visible-light sections for photocatalytic conversion. However, dyesensitized composites have poor stability and are easy to deactivate due to dyes detaching and degrading from the surface of the semiconductor under light irradiation [24,25]. To remedy the situation, we used the facile sol-gel method to design and synthesize a series of hybrid materials based on functional organic molecules (calixarene or pillararene derivative) and TiO 2 for photocatalytic H 2 production from splitting water under visible light irradiation [20,26,27]. Compared with the dye sensitization systems, these hybrid photocatalysts show outstanding photocatalytic stability due to high robust coordination linkage and efficient photogenerated charges separation and transfer from organic dyes to TiO 2 . However, these calixarene or pillararene derivatives still have some disadvantages in the hybrid system, such as weak optical responsiveness ability (an absorption band edge <500 nm), which hinder its development in photocatalysis. Thus, it is essential to exploit functional organic molecules with their extended visible-light absorption wavelength range to achieve efficient solar-energy conversion.
Tetrathiafulvalene (TTF) and its derivatives own unique electron-rich and redox-active properties [28], which commonly work as functional building blocks to construct twodimensional (2D) and three-dimensional (3D) framework materials (COF or MOF) and organic-inorganic hybrid materials, and have been intensively studied in the fields of photothermal conversion, photocatalysis [29][30][31][32], superconductors [33], sensors [34][35][36] and solar cell [37][38][39]. For example, Zhou et al. reported a composite (Ag@Dy-m-TTFTB) containing Ag nanoparticles and tetrathiafulvalene-based MOFs materials with a wide absorption wavelengths range (200-1000 nm) and efficient NIR photothermal conversion owing to the doping of the partially oxidized TTF ligands (TTF •+ ) and the plasmonic Ag nanoparticles [40]. Lan et al. designed and synthesized 2-dimensional COFs (TTCOF-Zn) based on porphyrin and tetrathiafulvalene, which worked as catalysts for the photocatalytic coupling reaction of CO 2 and H 2 O, achieving high photocatalytic CO production with nearly 100% selectivity and accompanying with H 2 O photooxidation to O 2 [41]. Dai et al. synthetized a self-condenses black organic-inorganic polymeric nanomaterials (B-1) assembled by a cyclic titanium-oxo cluster and organic TTF units, which owned multiporous nanostructure and generous electron density on TTF moiety for outstanding dye selection adsorption, and would even work as photocurrent-responsive electrode materials [42]. The studies above indicate that TTF derivatives have a huge potential application in the field of artificial photocatalysis and could greatly embellish the photophysics property of TiO 2 nanomaterials. However, the synthesis methods for TTF-based composites are still hard and complicated. Therefore, it is absolutely necessary to exploit TTF-based composites with a flexible, simple preparation processed to endow its wide optical spectrum responsiveness range to fully use solar energy and high optical quantum conversion efficiency for photocatalytic reaction.
Herein, we designed and synthesized binary organic-inorganic composite nanomaterials, which combine TTF derivatives with TiO 2 nanoparticles by a simple sol-gel method. Among them, TTF derivatives are tetrathiafulvalene-3,4,5,6-tetrakis(4-benzoic acid) (H 4 TTFTB) structure, as superior electron donors/acceptors, enabling the formation of intermolecular π-π stacking with relatively short S-S interactions, which could efficiently facilitate charges separation and transfer under illumination [40]. The organic-inorganic composite nanomaterials loaded with a co-catalyst are expected as an excellent photocatalyst owing to some advantages as below: (1) H 4 TTFTB possesses donors-conjugation-acceptors (D-π-A) electric structure, achieving excellent photoresponse ability and promoting effective photogenerated charge separation from excited H 4 TTFTB to TiO 2 nanoparticles; (2) high photostable and thermostable H 4 TTFTB molecule has multiple -COOH functional groups that can integrate with Ti(OC 4 H 9 ) 4 to come into being steady TiO 2 -O-TTFTB bonds, improving the stability of the hybrid; (3) the composite nanomaterials have a large surface area due to a multiporous nanostructure, which endues the photocatalysts with ample light-response active sites, leading to effective light harvesting and utilizing in the pores and channels. In this work, we confirm the advantage of the composite based on H 4 TTFTB and TiO 2 with Pt nanoparticles (Pt NPs) as a co-catalyst. We have optimized the activity of photocatalytic H 2 evolution from water for the hybrid materials with a mass percentage difference of H 4 TTFTB and TiO 2 . Remarkably, the hybrid materials with a 5.0% mass percentage of H 4 TTFTB, denoted as H 4 TTFTB-TiO 2 -5.0, exhibit the best photocatalytic H 2 evolution activity with a rate of 1452 µmol g −1 h −1 (TON = 1101). Moreover, the photocatalytic mechanisms of the H 4 TTFTB-TiO 2 composite have been explored. It testifies that the outstanding photocatalytic performance of H 4 TTFTB-TiO 2 composite is attributed to a large surface area, suitable energy band position and multifunctional synergistic catalysis between H 4 TTFTB and TiO 2 .

Synthesis of H 4 TTFTB-TiO 2 Materials
The structure of H 4 TTFTB and H 4 TTFTB-TiO 2 hybrid materials are shown in Figure S1a,b, respectively. To prepare H 4 TTFTB-TiO 2 -5.0, THF (2.0 mL), acetic acid (336 µL), distilled water (212 µL) and Tetrabutyl titanate (2 mL) were subsequently added to a 20 mL glass sample bottle, and then H 4 TTFTB (24.0 mg) in DMF (2 mL) was added. The color of the mixture changed from light yellow to dark red and, when stirred, became transparent gelatum. The gelatum was heated at 75 • C overnight to obtain a red solid. The residue was soxhleted with distilled water at 100 • C for 48 h and evaporated to dryness. The hybrid nanomaterials were calcined in a Muffle furnace at 250 • C for 2 h to become yellow-brown powder. Similarly, the hybrid material was handled as the above process with different calcination temperatures (150 • C, 250 • C and 350 • C) and then denoted as H 4 TTFTB-TiO 2 (150 • C), H 4 TTFTB-TiO 2 (250 • C) and H 4 TTFTB-TiO 2 (350 • C). In order to elaborate the formation process of the hybrid materials H 4 TTFTB-TiO 2 , the intermediate products at various reaction times were collected to characterize their crystalline states by checking their XRD patterns ( Figure S2).

Synthesis of Dye-Sensitized Composites
The a-TiO 2 powder was dispersed in 0.7 mM DMF solution of H 4 TTFTB at room temperature in the dark for 6 h and then centrifuged and washed with DMF and methanol until no color was observed from the eluent. The solid was dried under a vacuum oven at room temperature, denoted as H 4 TTFTB-5.0/TiO 2 .

Synthesis of Composites Materials Loaded with Pt NPs
The hybrid material above (20 mg) was dispersed in a photocatalytic reaction cell containing 4 mL H 2 O and 10 mL methanol. K 2 PtCl 4 aqueous solution (0.01 M) was added under stirring, and then the resulting mixture was stirred another 1 h under full-light irradiation by a 300 W xenon lamp. After illumination, the material was centrifugally washed with water and methanol and then dried in a vacuum. Pt NPs' actual loading amounts tested by inductively coupled plasma atomic emission spectrometry (ICP-AES) were 0.39 wt%, 0.88 wt% and 1.0 wt% (Table S1).

Photocatalytic Hydrogen Production
The photocatalytic hydrogen evolution by water splitting was carried out in a glass reaction cell with a quartz cover connected to a closed gas circulation, which was swept by high-purity Ar before illumination. A total of 20 mg photocatalysts were dispersed in 20 mL of 10 vol% TEOA aqueous solution. Then the suspension was exposed to a 300 W Xe lamp equipped with an optical filter (λ > 400 nm) to cut off the light in the ultraviolet region. The reaction solution was stirred continuously and cooled to maintain a suitable temperature by the circulation of cooling water. The amount of hydrogen evolved was determined at an interval of 1 h with online gas chromatography.

Photolectrochemical Methods
The Mott-Schottky curves were conducted on an Ivium-n-Stat electrochemical analyzer (Ivium Technologies B.V., Eindhoven, Netherlands) in a three-electrode cell. A platinum plate was used as a counter electrode, and an Ag/AgCl electrode (3 M KCl) was used as a reference electrode. The electrolyte was a 0.5 M Na 2 SO 4 solution. In order to prepare the working electrolyte, 5 mg samples were added to a mixed solution of ethanol and Nafion. The photocurrent response and electrochemical impedance measurements were performed using a 0.5 M Na 2 SO 4 solution electrolyte, using a 300 W xenon lamp with a 400 nm cut-off filter as the light source. Cyclic voltammograms (CVs) were recorded using an Ivium-n-Stat electrochemical analyzer with the platinum plate as the working electrode, Ag/AgCl as the reference electrode and Pt wire as the counter electrode. CVs were measured using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as a supporting electrolyte in DMF with a scan rate of 50 mV s −1 .  (Figure 1a). Calcination at 250 • C can remove impurities and facilitate the separation and transfer of photogenerated carriers. Simultaneously, the XRD analysis of H 4 TTFTB-TiO 2 hybrid materials calcined at 250 • C is shown in Figure 1b. As can be seen ( Figure S3), no obvious diffraction peaks of H 4 TTFTB can be found, and it shows little impact on the crystallinity due to low contents and high dispersion of H 4 TTFTB in H 4 TTFTB-TiO 2 hybrid nanomaterials.  Figure 1a,b display the XRD patterns of a-TiO2 and H4TTFTB-TiO2(1.7/3.4/5.0/6.5) hybrid materials. The obtained diffraction peaks at 25.54°, 37.85°, 47.90°, 54.22° and 62.70° were consistent with the standard card of anatase TiO2 (JCPDS 21-1272), which demonstrated the formation of anatase TiO2 (Figure 1a). Calcination at 250 °C can remove impurities and facilitate the separation and transfer of photogenerated carriers. Simultaneously, the XRD analysis of H4TTFTB-TiO2 hybrid materials calcined at 250 °C is shown in Figure  1b. As can be seen ( Figure S3), no obvious diffraction peaks of H4TTFTB can be found, and it shows little impact on the crystallinity due to low contents and high dispersion of H4TTFTB in H4TTFTB-TiO2 hybrid nanomaterials.  Figure 2 shows the N2 adsorption-desorption isotherms at 77K of a-TiO2 and H4TTFTB-TiO2−5.0 materials. The isotherms plots have a typical type IV pattern, which is evidently characteristic of mesoporous materials. The prepared a-TiO2 and H4TTFTB-TiO2−5.0 possess high BET specific surface area, 184.1 m 2 /g and 212.7 m 2 /g, respectively. These results show that the introduction of H4TTFTB could appropriately increase the specific surface area of the hybrid nanomaterials. Moreover, the pore size was simulated and calculated by the Barrett-Joyner-Halenda (BJH) method. From the pore size distribution diagram ( Figure S4), it could be observed that the pore size distribution (3.630 nm) of H4TTFTB-TiO2−5.0 nanomaterial is smaller than that (4.411 nm) of a-TiO2. The detailed data of the surface areas and the pore sizes are summarized in Table S2. In general, H2 production involves surface adsorption and interface reaction of reaction substrates in the heterogeneous photocatalyst system, which could be significantly affected by the morphology and structure of the photocatalyst. Thus, H4TTFTB-TiO2 composite nanomaterials prepared through a sol-gel process endow mesoporous hierarchical structure and multifunctional synergistic effect, which results in broad visible-light response and more efficient photocatalytic performance.  Figure 2 shows the N 2 adsorption-desorption isotherms at 77K of a-TiO 2 and H 4 TTFTB-TiO 2 -5.0 materials. The isotherms plots have a typical type IV pattern, which is evidently characteristic of mesoporous materials. The prepared a-TiO 2 and H 4 TTFTB-TiO 2 -5.0 possess high BET specific surface area, 184.1 m 2 /g and 212.7 m 2 /g, respectively. These results show that the introduction of H 4 TTFTB could appropriately increase the specific surface area of the hybrid nanomaterials. Moreover, the pore size was simulated and calculated by the Barrett-Joyner-Halenda (BJH) method. From the pore size distribution diagram ( Figure S4), it could be observed that the pore size distribution (3.630 nm) of H 4 TTFTB-TiO 2 -5.0 nanomaterial is smaller than that (4.411 nm) of a-TiO 2 . The detailed data of the surface areas and the pore sizes are summarized in Table S2. In general, H 2 production involves surface adsorption and interface reaction of reaction substrates in the heterogeneous photocatalyst system, which could be significantly affected by the morphology and structure of the photocatalyst. Thus, H 4 TTFTB-TiO 2 composite nanomaterials prepared through a sol-gel process endow mesoporous hierarchical structure and multifunctional synergistic effect, which results in broad visible-light response and more efficient photocatalytic performance. The morphology and structure of the hybrid were characterized by SEM, TEM, HRTEM and SAED, as well as the element mapping. The SEM image of H4TTFTB-TiO2−5.0 material is recorded in Figure S5, showing the size of ellipse nanoparticles as about 10-15 nm. To enhance the activity of photocatalytic H2 production, H4TTFTB-TiO2−5.0 was  Table S3). In addition, XPS measurements for a-TiO 2 , H 4 TTFTB-TiO 2 -5.0 and Pt@H 4 TTFTB-TiO 2-5.0 were carried out ( Figures S7-S9). Generally, pollution carbon C1s (284.8 eV) is used for charge calibration for all samples. In Figure S8, the survey spectrum displays that C, O, Ti and S exist in the H 4 TTFTB-TiO 2 -5.0. The binding energy of Ti 2p 3/2 and Ti 2p 1/2 is at 458.7 and 464.4 eV, respectively, and the binding energy of O 1s is at 529.9 eV, which could be attributed to the Ti-O bond of TiO 2 for a-TiO 2 and H 4 TTFTB-TiO 2 -5.0 ( Figures S7 and S8). Moreover, the compositions of Ti and O for both a-TiO 2 and H 4 TTFTB-TiO 2 -5.0 display the expected 1:2 ratio, indicating the existence of TiO 2 in the hybrid. Compared to a-TiO 2 , the binding energy at 164.2 eV and 165.2 eV, 168.6 eV and 169.6 eV belongs to S 2p 3/2 and S 2p 1/2 , respectively, demonstrating that H 4 TTFTB is successfully doped within TiO 2 . Furthermore, the two different types of peaks represent the existence of the neutral sulfur and oxidized sulfur of H 4 TTFTB in the composite ( Figure S8d). However, the partially oxidized sulfur of H 4 TTFTB in the composite indicates that the electron-rich tetrathiafulvalene (TTF) could be oxidized easily to its stable radical oxidation states (TTF •+ ) during the synthesis process of the hybrid. Furthermore, when the composite H 4 TTFTB-TiO 2 -5.0 was loaded with the Pt NPs by the photoreduction with the aid of methanol, the proportion of neutral sulfur of H 4 TTFTB in the hybrid would increase owing to the reduction of the oxidized sulfur by methanol. In addition, the banding energy at 71.1 eV and 74.4 eV belongs to Pt 4f 7/2 and Pt 4f 5/2 , respectively, which could be attributed to Pt (0), also the weak banding energy at 72.5 eV (Pt 4f 7/2 ) and 75.6 eV (Pt 4f 5/2 ), which could be attributed to a small amount of unreduced Pt 2+ ( Figure S9).

Results and Discussions
The FT-IR spectra of a-TiO 2 , H 4 TTFTB and H 4 TTFTB-TiO 2 -5.0 hybrid are displayed in Figure 4. Notably, 1621 cm −1 and 3417 cm −1 of absorption peaks are attributed to the bending and stretching vibrations of -OH groups from a-TiO 2 , which is pivotal for the combination with H 4 TTFTB molecules. The stretching vibrations of C=O groups and C-O groups from H 4 TTFTB are at 1696 cm −1 and 1103-1378 cm −1 , respectively. Moreover, the characteristic vibration bands of the benzene skeleton are around 650-900 cm −1 , which conforms to the structure feature of H 4 TTFTB. Compared with a-TiO 2 , the characteristic vibration bands of C=C from H 4 TTFTB and Ti-O from a-TiO 2 appear at 1500 cm −1 and 500-700 cm −1 , respectively, indicating the existence of H 4 TTFTB and TiO 2 in the hybrid materials. Furthermore, the C=O stretching vibration of H 4 TTFTB at 1696 cm −1 disappears, and two new bands appear at 1407 cm −1 and 1621 cm −1 in H 4 TTFTB-TiO 2 -5.0, which could be attributed to the symmetric and anti-symmetric stretch of -COO-. Thus, H 4 TTFTB should be chemically combined with TiO 2 through titanium ester bonds (O=C-O-Ti), which could also enhance the separation and transfer of photogenerated charges between H 4 TTFTB and Ti (3d) orbital manifold of a-TiO 2 [43,44]. Further, thermogravimetric analysis (TGA) of H 4 TTFTB and of H 4 TTFTB-TiO 2 -5.0 were carried out to evaluate their thermostability, revealing that the organic molecular H 4 TTFTB and the hybrid materials could be stable up to 300 • C ( Figure S10). The FT-IR spectra of a-TiO2, H4TTFTB and H4TTFTB-TiO2−5.0 hybrid are displayed in Figure 4. Notably, 1621 cm −1 and 3417 cm −1 of absorption peaks are attributed to the bending and stretching vibrations of -OH groups from a-TiO2, which is pivotal for the combination with H4TTFTB molecules. The stretching vibrations of C=O groups and C-O groups from H4TTFTB are at 1696 cm −1 and 1103-1378 cm −1 , respectively. Moreover, the characteristic vibration bands of the benzene skeleton are around 650-900 cm −1 , which conforms to the structure feature of H4TTFTB. Compared with a-TiO2, the characteristic vibration bands of C=C from H4TTFTB and Ti-O from a-TiO2 appear at 1500 cm −1 and 500-700 cm −1 , respectively, indicating the existence of H4TTFTB and TiO2 in the hybrid materials. Furthermore, the C=O stretching vibration of H4TTFTB at 1696 cm −1 disappears, and two new bands appear at 1407 cm −1 and 1621 cm −1 in H4TTFTB-TiO2−5.0, which could be attributed to the symmetric and anti-symmetric stretch of -COO-. Thus, H4TTFTB should be chemically combined with TiO2 through titanium ester bonds (O=C-O-Ti), which could also enhance the separation and transfer of photogenerated charges between H4TTFTB and Ti (3d) orbital manifold of a-TiO2 [43,44]. Further, thermogravimetric analysis (TGA) of H4TTFTB and of H4TTFTB-TiO2−5.0 were carried out to evaluate their thermostability, revealing that the organic molecular H4TTFTB and the hybrid materials could be stable up to 300 °C ( Figure S10).  The UV-vis diffraction spectra of H4TTFTB, H4TTFTB-TiO2−5.0 and a-TiO2 are displayed in Figure 5a. As we can see, H4TTFTB shows a strong absorption range of about 300-650 nm, ascribing to n-π* or π-π* transition of the phenyl-TTF moiety. The small shoulder at around 650-800 nm could be attributed to the partially oxidized TTF ligands The UV-vis diffraction spectra of H 4 TTFTB, H 4 TTFTB-TiO 2 -5.0 and a-TiO 2 are displayed in Figure 5a. As we can see, H 4 TTFTB shows a strong absorption range of about 300-650 nm, ascribing to n-π* or π-π* transition of the phenyl-TTF moiety. The small shoulder at around 650-800 nm could be attributed to the partially oxidized TTF ligands (TTF •+ ) [40]. The UV-vis absorption spectrum and the emission spectra of H 4 TTFTB are exhibited in Figure S11. The maximum emission wavelength of H 4 TTFTB is located at 650 nm; thus, its transition energies (E 0-0 ) are calculated to be 2.34 eV. However, a-TiO 2 only has a strong UV absorption peak with a 400 nm absorption edge, which matches the verified band gap (about 3.2 eV) (Figure 5b) [12]. H 4 TTFTB is reacted with tetrabutyl titanate in situ by sol-gel method to obtain composite H 4 TTFTB-TiO 2 . The composite has a broad absorption range at about 300-600 nm, along with a smaller shoulder adsorption peak at 650-800 nm, owing to the low contents of H 4 TTFTB doped. Compared with pure TiO 2 , the H 4 TTFTB-TiO 2 -5.0 composite has two band gaps: a wide band gap of 2.8 eV and a narrow band gap of 2.0 eV (Figure 5b), attributed to a-TiO 2 and H 4 TTFTB, respectively. The band gap of H 4 TTFTB in the composite is lower than the E 0-0 value of 2.34 eV in pure H 4 TTFTB, indicating that a robust combination exists between H 4 TTFTB and TiO 2 in the hybrid.  The UV-vis diffraction spectra of H4TTFTB, H4TTFTB-TiO2−5.0 and a-TiO2 are displayed in Figure 5a. As we can see, H4TTFTB shows a strong absorption range of about 300-650 nm, ascribing to n-π* or π-π* transition of the phenyl-TTF moiety. The small shoulder at around 650-800 nm could be attributed to the partially oxidized TTF ligands (TTF •+ ) [40]. The UV-vis absorption spectrum and the emission spectra of H4TTFTB are exhibited in Figure S11. The maximum emission wavelength of H4TTFTB is located at 650 nm; thus, its transition energies (E0-0) are calculated to be 2.34 eV. However, a-TiO2 only has a strong UV absorption peak with a 400 nm absorption edge, which matches the verified band gap (about 3.2 eV) (Figure 5b) [12]. H4TTFTB is reacted with tetrabutyl titanate in situ by sol-gel method to obtain composite H4TTFTB-TiO2. The composite has a broad absorption range at about 300-600 nm, along with a smaller shoulder adsorption peak at 650-800 nm, owing to the low contents of H4TTFTB doped. Compared with pure TiO2, the H4TTFTB-TiO2-5.0 composite has two band gaps: a wide band gap of 2.8 eV and a narrow band gap of 2.0 eV (Figure 5b), attributed to a-TiO2 and H4TTFTB, respectively. The band gap of H4TTFTB in the composite is lower than the E0-0 value of 2.34 eV in pure H4TTFTB, indicating that a robust combination exists between H4TTFTB and TiO2 in the hybrid. In order to elucidate the photocatalytic mechanism of H 4 TTFTB-TiO 2 under visible light irradiation, Mott-Schottky (M-S) measurements of a-TiO 2 were carried out at different frequencies of 500 Hz, 1000 Hz and 1500 Hz ( Figure S12). The positive slopes of the M-S plots match with an n-type semiconductor. The flat band positions (V fb ) of a-TiO 2 are calculated to be −0.7 V (vs. Ag/AgCl). As well known, the conduction band (CB) of n-type semiconductors is approximately the same as the value of V fb [25]. Consequently, the CB of a-TiO 2 is about −0.7 eV (vs. Ag/AgCl), being more negative than the redox potential of H + /H 2 , thus, favoring the photoreduction of the proton from water to produce H 2 . The valence band (VB) of a-TiO 2 is counted to be 2.5 eV (vs. Ag/AgCl). Moreover, the first oxidation potential of H 4 TTFTB was detected by CVs characterization to acquire the highest occupied molecular orbital (HOMO) value ( Figure S13). The redox potential of excited H 4 TTFTB* is about −1.64 eV (vs. Ag/AgCl); namely, the bottom of the lowest unoccupied molecular orbital (LUMO) is counted from the HOMO and the E 0-0 value of H 4 TTFTB, which is more negative than the CB of a-TiO 2 . Thus, it is thermodynamically favorable for the transfer to be directly from the H 4 TTFTB* to CB of a-TiO 2 .
The photocatalytic activities of H 4 TTFTB-TiO 2 -5.0 catalysts with different amounts of H4TTFTB-doping and with different contents (0.5/1.0/1.5 wt%) of Pt NPs loading have been optimized under visible light irradiation. A series of experiment data are presented in Figure 6a. The results demonstrate that H 4 TTFTB-TiO 2 -5.0 with 1.0 wt% Pt NPs has the optimized photocatalytic activity ( Figure S14). Moreover, the photoactivity increases with the addition of H 4 TTFTB-doped amount (1.7 wt%, 3.4 wt% and 5.0 wt%), reaching a maximum for H 4 TTFTB-TiO 2 -5.0. However, further increasing the percentage of H 4 TTFTB to 6.5 wt% with a decrease in photoactivity indicates that the photoactivity of H 4 TTFTB-TiO 2 nanomaterials could not correlate only to H 4 TTFTB amounts. Reasonable speculation is that excessive H 4 TTFTB in the composite could block porous channels and impede electron migrating and transporting. The photocatalytic activity of the composites nanomaterials is influenced by many factors, such as temperature, pH, solvents, additives, etc. Among them, the calcination temperature is pivotal in building up the structural morphology of composites and enhancing the photocatalytic activity. Thus, the calcination temperature of H 4 TTFTB-TiO 2 -5.0 was optimized. In addition, H 4 TTFTB-TiO 2 -5.0 with 250 • C calcination exhibits the best photocatalytic rates of H 2 production, which is greatly superior to higher or lower calcination temperature (150 • C or 350 • C) ( Figure S15). It is believed that appropriate calcination temperature not only could eliminate the impurity but also improve the crystallinity of the hybrid materials. However, a lower temperature (150 • C) could not meet the requirements for the synthesis of the hybrid, and a higher calcination temperature (350 • C) could destroy the organic component H 4 TTFTB. In contrast, the rate of photocatalytic H2 production of a-TiO2 and sensitization H4TTFTB-5.0/TiO2 with only 1.0 wt% Pt (0) loading are 18.87 and 390.8 μmol g −1 h −1 which are both less than the rates of H4TTFTB-TiO2−5.0. It demonstrates that the composites have excellent photocatalytic activity than the dye sensitization TiO2 system owing to the more efficient migration of photogenerated electrons from H4TTFTB* to TiO2 through the robust linkages between H4TTFTB and TiO2. Moreover, to assess the stability of H4TTFTB-TiO2 nanomaterial, we conducted photocatalytic H2 evolution experiments at intervals of 5 h. The H4TTFTB-TiO2 composite was reused by centrifugation, washed with water and methanol and then vacuum dried. As shown in Figure 6b, the photocatalytic activity of H4TTFTB-TiO2 shows no significant decrease, revealing high photocatalytic stability in photocatalytic reducing H2 with seven recycles. The TONH2 is calculated as the mole numbers of H2 over the mole numbers of Pt NPs, giving significant results of 1101 after 35 h of photoreactions. In general, compared with other reported photocatalysts (Table S4), the photocatalytic activity of the H4TTFTB-TiO2−5.0 system is moderate. However, the simple In contrast, the rate of photocatalytic H 2 production of a-TiO 2 and sensitization H 4 TTFTB-5.0/TiO 2 with only 1.0 wt% Pt (0) loading are 18.87 and 390.8 µmol g −1 h −1 which are both less than the rates of H 4 TTFTB-TiO 2 -5.0. It demonstrates that the composites have excellent photocatalytic activity than the dye sensitization TiO 2 system owing to the more efficient migration of photogenerated electrons from H 4 TTFTB* to TiO 2 through the robust linkages between H 4 TTFTB and TiO 2 . Moreover, to assess the stability of H 4 TTFTB-TiO 2 nanomaterial, we conducted photocatalytic H 2 evolution experiments at intervals of 5 h. The H 4 TTFTB-TiO 2 composite was reused by centrifugation, washed with water and methanol and then vacuum dried. As shown in Figure 6b, the photocatalytic activity of H 4 TTFTB-TiO 2 shows no significant decrease, revealing high photocatalytic stability in photocatalytic reducing H 2 with seven recycles. The TON H2 is calculated as the mole numbers of H 2 over the mole numbers of Pt NPs, giving significant results of 1101 after 35 h of photoreactions. In general, compared with other reported photocatalysts (Table S4), the photocatalytic activity of the H 4 TTFTB-TiO 2 -5.0 system is moderate. However, the simple and flexible synthesis approach has been proposed in this study to obtain the robust fulvalene-based composite, which is significant to shed light on the rational design of photocatalysts and understand the photocatalytic inherent nature. Thus, our next work will focus on regulating precisely the structure of the fulvalene-based composite at a molecule level, which would enhance our in-depth fundamental understanding of the photocatalytic mechanism.
The photocurrent-time (i-t) curves and the electrochemical impedance spectroscopy (EIS) of a-TiO 2 , H 4 TTFTB-5.0/TiO 2 and H 4 TTFTB-TiO 2 -5.0 have been monitored under visible light illumination to further verify the influence of enhanced photoresponse on the photocatalytic process (Figure 6c,d). It is clear that the photocurrent of H 4 TTFTB-TiO 2 -5.0 is higher than H 4 TTFTB-5.0/TiO 2 , indicating that effective interfacial electrons migration and transfer between H 4 TTFTB and TiO 2 in H 4 TTFTB-TiO 2 -5.0 (Figure 6c). Figure 6d displays the EIS Nyquist plots for samples. Compared with a-TiO 2 and H 4 TTFTB-5.0/TiO 2 , H 4 TTFTB-TiO 2 -5.0 exhibits a smaller semicircular diameter in Nyquist plots, suggesting that the smaller resistance of photogenerated charge migration and transfers on the electrode surface. Thus, the result demonstrates that the hybrid materials possess a smaller interfacial charge-transfer resistance, which could significantly enhance the photoactivity for H 2 generation, hinting that more effective interfacial photogenerated electrons migrate and transfer between H 4 TTFTB and TiO 2 in H 4 TTFTB-TiO 2 composites.
According to the results and discussions above, a possible photocatalytic mechanism for the H 4 TTFTB-TiO 2 catalytic system is proposed, as shown in Figure 7. H 4 TTFTB owns excellent light absorbance ability to act as a light harvester under visible light irradiation. The excited electrons jump from HOMO into LUMO of H 4 TTFTB and inject into the CB band of TiO 2 . The electrons separated successfully could reduce proton to H 2 production with the aid of Pt (0) NPs. Herein, Pt (0) NPs are loaded on the surface of H 4 TTFTB-TiO 2 by the in-situ photodeposition and commonly work as co-catalysts to provide reactive sites, owing to the formation of the Schottky barrier at the interface of Pt NPs and the hybrid, which could be beneficial to promote the separation and transfer of photogenerated charges and enhance the photocatalytic activity. The electrons on the CB of a-TiO 2 could be captured by the co-catalysts and then efficiently photoreduce H + to H 2 . The holes retained in the VB of H 4 TTFTB are neutralized by sacrificial reagent triethanolamine (TEOA). Furthermore, the oxidative H 4 TTFTB + could also be reduced by TEOA to obtain relative H 4 TTFTB.
Nanomaterials 2022, 12, x FOR PEER REVIEW 11 The photocurrent-time (i-t) curves and the electrochemical impedance spectrosc (EIS) of a-TiO2, H4TTFTB-5.0/TiO2 and H4TTFTB-TiO2−5.0 have been monitored under ible light illumination to further verify the influence of enhanced photoresponse on photocatalytic process (Figure 6c,d). It is clear that the photocurrent of H4TTFTB-TiO2 is higher than H4TTFTB-5.0/TiO2, indicating that effective interfacial electrons migra and transfer between H4TTFTB and TiO2 in H4TTFTB-TiO2−5.0 (Figure 6c). Figure 6d plays the EIS Nyquist plots for samples. Compared with a-TiO2 and H4TTFTB-5.0/T H4TTFTB-TiO2−5.0 exhibits a smaller semicircular diameter in Nyquist plots, sugges that the smaller resistance of photogenerated charge migration and transfers on the trode surface. Thus, the result demonstrates that the hybrid materials possess a sm interfacial charge-transfer resistance, which could significantly enhance the photoacti for H2 generation, hinting that more effective interfacial photogenerated electrons mig and transfer between H4TTFTB and TiO2 in H4TTFTB-TiO2 composites.
According to the results and discussions above, a possible photocatalytic mechan for the H4TTFTB-TiO2 catalytic system is proposed, as shown in Figure 7. H4TTFTB o excellent light absorbance ability to act as a light harvester under visible light irradia The excited electrons jump from HOMO into LUMO of H4TTFTB and inject into the band of TiO2. The electrons separated successfully could reduce proton to H2 produc with the aid of Pt (0) NPs. Herein, Pt(0) NPs are loaded on the surface of H4TTFTBby the in-situ photodeposition and commonly work as co-catalysts to provide reac sites, owing to the formation of the Schottky barrier at the interface of Pt NPs and hybrid, which could be beneficial to promote the separation and transfer of photoge ated charges and enhance the photocatalytic activity. The electrons on the CB of acould be captured by the co-catalysts and then efficiently photoreduce H + to H2. The h retained in the VB of H4TTFTB are neutralized by sacrificial reagent triethanolam (TEOA). Furthermore, the oxidative H4TTFTB + could also be reduced by TEOA to ob relative H4TTFTB.

Conclusions
In summary, we have in situ synthesized organic-inorganic H4TTFTB-TiO2 compo containing organic light-harvesting unit H4TTFTB and inorganic semiconductor TiO the sol-gel method. H4TTFTB-TiO2−5.0 material exhibits high photocatalytic efficiency

Conclusions
In summary, we have in situ synthesized organic-inorganic H 4 TTFTB-TiO 2 composite containing organic light-harvesting unit H 4 TTFTB and inorganic semiconductor TiO 2 by the sol-gel method. H 4 TTFTB-TiO 2 -5.0 material exhibits high photocatalytic efficiency under visible light irradiation. A series of characterizations on structural morphology and photoelectric physical and chemical properties suggest that H 4 TTFTB is successfully doped into H 4 TTFTB-TiO 2 , which is attributed to the formation of stable titanium ester bond linkage between H 4 TTFTB and TiO 2 . The absorption ability of H 4 TTFTB extended to visible light range could fully use the solar energy. The multifunctional synergistic effect of organic and inorganic components of H 4 TTFTB-TiO 2 composite could promote efficient electron migration and transfer in the interface. This work sheds light on the relationship of structure, photocatalytic activity and reaction pathway of TTF-based composite nanomaterials for splitting water to H 2 production under visible light irradiation.