Mechanistic insights into photochromic 3H-naphthopyran showing strong photocoloration

3,3-Diphenylbenzo[f]chromene (1) represents an important architectural platform for photochromic systems. Since the practical utility of such chromophores is largely dependent upon the kinetics of coloration and decoloration, elucidating the mechanistic details of these processes is of great value. Toward this end, we studied the photochromic reaction of (3-(2-methoxyphenyl)-3-phenyl-3H-benzo[f]chromene (2) by both time-resolved UV–vis and mid-IR spectroscopies. We found that irradiation of 2 at 365 nm generates long-lived colored transoid-cis isomers with lifetimes of 17.1 s and 17.5 min (at 21 °C) and even longer-lived transoid-trans isomers with a lifetime of 16 h. These experimental results were supplemented with ab initio ground-state and excited-state calculations, and the resulting theoretical interpretation may be useful for the design of new photochromic systems with optimized photofunctionality.

When the application requires a relatively fast decoloration rate, the presence of the long-lived TT form is unwanted. Efforts are focused then on designing derivatives of 1 with a minimized TT contribution in the photoreaction. Under typical experimental conditions with UV irradiation, the active channel for TT generation is TC → TT photoisomerization, which is a single-twist long amplitude motion 11,13 occurring in the excited state (S 1 ). Strong competition of the other TC excited state deactivation channels can minimize the TC → TT photoisomerization yield. For example, if the potential-energy landscape of the initially photoexcited TC S 1 state favors a bicycle-pedal motion, the isomerization path might be quickly aborted through S 1 → S 0 internal conversion populating the form with the geometry close to that of the starting TC form 12,13 . Such a deactivation path can reduce the TC → TT photoisomerization yield, as reported for derivative of 1 with a methoxy group inserted at the position 10 13,14 .
The thermal fading rate of colored TC and TT isomers is also a key parameter of photochromic materials based on derivatives of 1. Appropriate substituents in the skeleton of 1 can be used to tune the TC color fading rate which depends on the ground state potential energy landscape 15 . A key role in this process is played by the cisoid-cis intermediate form (INT) lying midway between the colored TC form (see Fig. 2), and the colorless CF form 15 (Fig. 2), which makes the apparent TC lifetime relatively long (9.3 s at 21 °C in cyclohexane).
The opposite situation (ΔE INT-TC > ΔE INT-CF ) has been reported for 3H-naphthopyrans with phenyl substitution at position 2 15 . The apparent TC lifetime is remarkably short (only 30 μs) in solution at room temperature 16 . On the other hand, one can expect that a slow decoloration would help to achieve a strong photocoloration  (2), Fig. 1, as a candidate for a material with the strongest UV activated coloration. The relatively simple structure of 2 facilitates ab initio quantum-chemical calculations for the excited state, while the stages of the photochromic reaction can be studied in detail with experimental techniques such as the time-resolved UV-vis and mid-IR spectroscopies.

Experimental
Materials. Compound 1 was purchased from TCI. Compound 2 was synthesized following the procedures described in the Supplementary Information. In the time-resolved spectroscopic investigation cyclohexane of spectroscopic grade from Sigma Aldrich was used for solution preparation.
Time-resolved UV-vis and mid-IR spectroscopies. Changes in UV-vis absorption spectra and kinetics were recorded using three configurations.
(1) Jasco V-550 spectrophotometer with a modified cell compartment. The solution in 1 cm × 1 cm fused silica cuvette was placed into a temperature-controlled cuvette holder (Flash 300, Quantum Northwest) with stirring switched on. UV LED (λ exc = 365 nm, Thorlabs M365LP1) was used to induce the photochromic reaction (as in the two other configurations below). (2) Similar arrangement with a temperature-controlled cuvette holder and white light generated by a xenon lamp (Applied Photophysics), equipped with a bundle fiber, as a probing beam. The probing beam was passed through an almost-closed iris to ensure low white light intensity. The UV-vis spectra were recorded by an Ocean Optics FLAME-T-VIS-NIR-ES USB spectrometer at the sampling rate of 10 spectra per second.  (1) converts the closed form CF to color isomers TC (short-lived) and TT (long-lived), with the intermediacy of cisoid-cis form INT (ultrashort-lived) 11 . Compound 2 with a methoxy group implemented at one of the phenyl rings at ortho position is selected for studies of a strong photocoloration effect. www.nature.com/scientificreports/ (3) Acquisition mid-IR spectra upon simultaneous measurements in UV-vis spectral range has been described recently 11 . A Bruker Tensor 27 FT-IR spectrometer was equipped with an MCT detector, with a spectral resolution of 4 cm −1 and a sampling rate of 1.15 s −1 . The UV-vis probing light was generated from the xenon lamp. An Ocean Optics spectrophotometer was used for recording of UV-vis spectra. A Harrick Scientific cell was used with 2 mm thick CaF 2 window and a 0.63 mm spacer.
Theoretical calculations. The equilibrium geometries of the CF conformers and their isomers in the closed-shell ground state (S 0 ) were obtained with the MP2 method 23 with no imposed symmetry constrains. The energy of the most stable form CF c (Table S1, Supplementary Information) was assumed as the reference energy for higher energy structures. The excited-state (S 1 ) equilibrium geometries were determined with the secondorder algebraic diagrammatic construction ADC(2) method [24][25][26] . The correlation-consistent valence double zeta basis set with polarization functions on all atoms (cc-pVDZ) 27 was used in these calculations as well as in the potential energy profiles and surfaces. The vertical excitation energies and response properties of the lowest singlet excited states were calculated using the CC2 methods 28,29 . The basis set augmented with the diffuse functions aug-cc-pVDZ was also used to compute vertical excitation energies of the molecular system. All calculations were performed using the TURBOMOLE program package 30 .

Results and discussion
The stationary UV-vis absorption spectrum of 2 in cyclohexane ( Figure S1, Supplementary Information) is similar to that of the reference compound 1 11 , reflecting structural similarities between the two derivatives. In the structure of 1, each phenyl ring has two ortho positions that can be substituted by a methoxy group, thus, four respective conformers of 2 are taken into account in the calculations. These conformers of the CF form are in thermal equilibrium in freshly prepared solution (indicated with subscripts: a, b, c, and d, see Table S1). The selection of wavelength at 365 nm induces the electronic transition S 0 → S 1 (π,π*) separately in each CF form, which opens up along the C 3 -O 4 photodissociation pathway, as in 1 11 . The photoinduced pyran ring-opening process can lead to the colored-isomers CTC and TTC . All the CF, TC and TT conformers potentially involved in the photoreaction, along with their calculated vertical excitation energies (ΔE VE ) simulating UV-vis absorption spectra, are collected in Table S1. The strong S 0 → S 2 (π, π*) transitions for colored TC forms are slightly redshifted vs. those of TT forms; a similar trend was observed for the parent compound 1 11 .
To study the photochromic reaction by changes in UV-vis absorption spectra, a UV LED light at 365 nm was used for sample excitation, while a xenon lamp was used for probing. Figure 3a shows the kinetics probed at 425 nm upon 60-s exposure a solution of 2 in cyclohexane to UV light.
Compared to 1, compound 2 produces a stronger absorption signal upon UV irradiation (Fig. 3a), and after cessation of UV irradiation its color fading occurs at a lower rate. The quantum yield of TC formation is similar for 2 and 1 (0.745 11 ), since the early signal rise in the − 55 to − 60 s time window shows the same slope and the TC molar extinction coefficients are comparable (see Supplementary Information). Figure 3b  www.nature.com/scientificreports/ TT isomers, since the C=O stretching absorption band of TC is located at a lower frequency than that of TT, as has been already observed for the reference compound 1 (TC at 1644 cm −1 and TT at 1655 cm −111 ). Photocycle reproducibility observed for 2 in cyclohexane is similar to that for 1, which is known to have high fatigue resistance (Fig. 4).

Why do TC forms differ in decoloration kinetics?
The TC form in 3H-naphthopyrans is photogenerated in a single-photon absorption process 11 . A single TC isomer is generated from compound 1, while four TC isomers from compound 2 should be taken into consideration. The geometries of these conformers are presented in the first column in Table S1. CTC a and CTC b are energetically more stable than TTC c and TTC d . Each of the four TC isomers is a colored species which, under thermal conditions, fades towards the respective closed CF form via a single cisoid-cis intermediate INT geometry located on the ground-state potential energy pathway. Each of the four S 0 -state pathways is characterized by a slightly different energy barriers separating the intermediate INT structure from the local TC and CF minima, which influence the apparent fading rate of each TC isomer. Energy barriers are shown in Table 1.
The first reaction step in the TC fading process is TC → INT isomerization, which must overcome a relatively high energy barrier ΔE TC-OUT (~ 0.38−0.48 eV), to reach a highly energetic cisoid-cis INT intermediate in an endothermic process. In the second step, this intermediate can follow (1) an exothermic reaction towards the final closed-pyran ring CF form or (2) an alternative reverse process towards the TC form ( Figure S4, Supplementary Information). As one can see in Table 1, the ΔE INT-CF barrier towards the final CF for each fading pathway is usually higher than the respective ΔE INT-TC barrier towards the initial TC form (except for the TTC d pathway, for which the TS2 d structure (E a = 1.145 eV) is slightly more destabilized due to O…O repulsion). To support the idea that the high value of the ΔE INT-CF barrier is a decisive parameter in longer fading time of CTC forms, we analyzed the geometries of the transition state TS1 structures. Indeed, the presence of a methoxy group is the cause of the O…O repulsion in TS1 a (E a = 1.185 eV, see Table 1) or 10H…OCH 3 steric hindrance in TS1 b (E a = 1.223 eV). The ortho-methoxy substituent increases the electron density on that aromatic ring, which has a stabilizing effect on the cisoid-cis INT geometry through increased π-stacking of the aryl and naphthalenone moieties. This is responsible for a higher ΔE INT www.nature.com/scientificreports/ be classified as a single-twist mechanism (rotation around the C 14 =C 1 bond) or as a bicycle-pedal motion (if the concerted rotation about both double bonds takes place). Both mechanisms are visualized in the excited-state potential energy surface spanned over the two double bonds (Fig. 5). The single-twist motion can be seen as a motion along the X axes (C 14 =C 1 rotation) from the green dot representing the initial TC geometry (upper left corner of the contour plot) towards the red dot (TT form, in the upper right corner). This motion meets the excited state S 1 TW minimum for which C 13 -C 14 =C 1 -C 2 dihedral angle is usually a little less than 90°, but it should be considered as the active channel in TC → TT photoisomerization. Alternatively, the bicycle-pedal motion is followed along the diagonal of the plot-from the TC form (green dot, upper left corner) towards the bottom right corner. This path encounters the S 1 BP minimum midway through, which may deactivate through S 1 → S 0 internal conversion to repopulate eventually the initial TC geometry in the S 0 state. Inspection of the contour plots in Fig. 5 and energetics in Table S2 shows that the relative energies of S 1 TW are below S 1 BP thus the photoisomerization TC → TT is expected to be efficient, as observed in the experiment.
According to theoretical calculations, the replacement of the methoxy group in 2 by a methyl substituent has a low impact since its potential energy surface is also tilted towards the S 1 TW minimum (see Table S3). Consequently, this system should also easily produce the TT form.

Conclusions
The synthesized compound 2 shows exceptional properties among the members of 3H-naphthopyran family. In the photochromic reaction its colored isomers are formed with longer lifetimes in comparison to that of the reference compound 1. Under UV irradiation a high accumulation of colored forms is observed in a photostationary Table 1. Energetics of the two-stepwise process of the TC form depopulation (TC ↔ INT → CF). Adiabatic S 0 -state energies, E a (in eV), and dipole moment, μ g (in Debye), for the relevant minima: TC, INT, CF, and transition states separating these minima: TS1 and TS2, calculated at the MP2/cc-pVDZ theory level. www.nature.com/scientificreports/ www.nature.com/scientificreports/ state. The mechanism of the colored TC form fading process can be analyzed using the energetic landscape of the thermal TC → INT → CF reaction. The determined ΔE INT-CF energy barrier seems to hamper CF repopulation and extends the apparent TC lifetime. Thus, the experimentally determined long lifetime of 17.5 min can be assigned to CTC forms, while the short lifetime of 17.1 s to TTC conformers. The theory suggests also that all TC isomers can undergo photoisomerization by the single-twist mechanism around the C 14 =C 1 angle. The photogenerated TT form is long-lived-the population decay occurs with a single time-constant of 16 h at 21 °C. The proposed photochromic reaction mechanism explains the strong photocoloration effect observed for 2.
Theoretical investigations should be considered as an efficient tool for designing new 3H-naphthopyrans derivatives with optimized properties. In other words, the implementation of a promising substituent in 3,3-diphenyl-3H-naphtho[2,1-b]pyran skeleton can be first theoretically tested for desired photochromic properties prior to the synthesis efforts.

Data availability
The datasets generated during the current study are available from the corresponding author on request.