Synthesis and Luminescent Properties of 1,4,5-Triphenylimidazole‒Phenothiazine Fluorophores

Two blue donor–acceptor fluorophores with 1,4,5-triphenylimidazole as the electron-transporting unit and phenothiazine as the hole-transporting unit were synthesized by grafting 1,4,5-triphenylimidazole moieties onto 3- and 3,7-position of the phenothiazine core and characterized by spectroscopic methods. Their thermal stability, photophysical, electrochemical and electroluminescence properties were systematically investigated. These compounds exhibit good thermal stability and show blue emission in dichloromethane solution and thin solid films. The solution-processed doped devices were fabricated by using these fluorophores as the emitting dopant in 1,3-bis(N-carbazolyl)benzene host, in which the device fabricated from the fluorophore containing two 1,4,5-triphenylimidazole moieties exhibited blue emission with a luminance of 648 cd/m2 and external quantum efficiency of 1.48%.


Introduction
Organic electroluminescence diodes (OLEDs) are becoming one of the leading technologies for full-color displays and environment-friendly light source due to the thinness, high brightness, wider viewing angle, lower power consumption, higher color gamut, faster response speed, better flexibility, wider operating temperature range and lower cost [1][2][3][4][5][6]. Over the last decades, great development of the light-emitting materials and device structures for high-performance OLEDs has been made in both scientific research and commercial applications [7][8][9][10][11][12][13][14][15]. Generally, OLED light-emitting materials can be divided into two main types according to their chemical and photophysical properties: small molecule light-emitting materials and polymeric light-emitting materials [16][17][18][19][20][21][22][23]. Compared with polymeric light-emitting materials, small molecule luminescent materials are easier to synthesize and purify, and their structure can be fine-tuned through appropriate molecular design. OLEDs based on small molecules can be fabricated either by vacuum deposition method or by solution spin-coating method [24][25][26]. Among the small-molecule luminescent materials, luminescent materials with high photoluminescence quantum (Φ PL ) and excellent charge transport characteristics can effectively improve the electroluminescence (EL) efficiency [17,27].
The twisted 1,4,5-triphenylimidazole as an acceptor could cause a wider band-gap and lead to deeper blue emission than the planar and rigid phenanthroimidazole acceptor, and the twisted 1,4,5-triphenylimidazole could also hamper the tight π-π intermolecular interactions. Therefore, it was often used to construct blue-emitting materials [36,37]. Based on the structural feature of 1,4,5-triphenylimidazole, in this work, we have synthesized two blue D-A fluorophores containing 1,4,5-triphenylimidazole as the electron-acceptor and phenothiazine as the electron-donor by grafting 1,4,5-triphenylimidazole moieties onto 3-and 3,7-position of the phenothiazine core to investigated the effect of the number of electron-acceptors on the optoelectronic properties of the materials.

Materials and Methods
Phenothiazine, 1-bromobutane, benzyl and 4-tert-butylaniline were obtained from Energy Chemical (China). All the other reactants and solvents were obtained from commercial sources. All organic solvents used in this study were dried with the suitable desiccants and freshly distilled prior to the relevant reactions.
The NMR spectrum was measured on Bruker 500. The mass spectrum was recorded using a Bruker Esquire 3000 mass spectrometer. Thermogravimetric analysis (TGA) was performed on Shimadzu thermal analyzer in Japan. The UV-vis absorption spectrum was recorded on a Shimadzu UV-2550 spectrometer. The cyclic voltammogram was performed on an electrochemical analyzer (CHI Instruments 760 B). At room temperature, using Edinburgh instrument FLS920 integrating sphere and Xe lamp, the quantum yield of photoluminescence was measured by the absolute method. At room temperature, the Edinburgh instrument FLS920 was used with a microsecond flash lamp as the excitation source (repetition frequency 90 Hz), and the photoluminescence decay lifetime was measured by a time-correlated single photon counting spectrometer.

Photophysical, electrochemical properties and thermal stabilities of the compounds
In their absorption spectra, the stronger absorption bands at 260 nm for PTZ-PIM and 272 nm for PIM-PTZ-PIM were attributed to the π-π* transitions of the phenothiazine units, while the weak absorption bands at 375 nm for PTZ-PIM and 392 nm for PIM-PTZ-PIM could be assigned to the intramolecular charge transfer (ICT) transitions from phenothiazine cores to imidazole units [35]. The moderately intense absorption bands located in the range of 280 -360 nm could be the π-π* transitions of imidazole and the substituted benzene rings [38]. Compared with PTZ-PIM, the absorption spectrum of PIM-PTZ-PIM was red-shifted due to more conjugation of A-D-A type structure for PIM-PTZ-PIM than that of A-D type structure for PTZ-PIM. The optical absorption edges (λ onset ) of PTZ-PIM and PIM-PTZ-PIM in dichloromethane solution were estimated to be at ca. 443 nm and 461 nm, the corresponding optical band gaps of PTZ-PIM and PIM-PTZ-PIM were about 2.80 eV and 2.69 eV, respectively, from the equation: E opt = 1241/λ onset .
As shown in Fig. 1, PTZ-PIM and PIM-PTZ-PIM exhibited strong blue emission with a maximum peak at 459 nm and 476 nm in dilute chloromethane solution, while their PL spectra in thin solid films were obviously blue-shifted, and the maximum emission peaks of PTZ-PIM and PIM-PTZ-PIM in thin solid films were observed at 443 nm and 469 nm, respectively. The presence of the butterfly conformational PTZ core and the twisted 1,4,5-triphenylimidazole units in PTZ-PIM and PIM-PTZ-PIM could efficiently limit π-conjugation and inhibit the intermolecular aggregations, which could cause the blueshifts of their PL spectra in thin solid films than that in dilute chloromethane solution. In addition, the fluorescence quantum yields (Φ f ) of PTZ-PIM and PIM-PTZ-PIM in dichloromethane solutions (1.0 × 10 -6 mol/L) were measured to be 18.49% and 28.92%, respectively, at room temperature under air atmosphere, and their decay lifetimes (τ) were measured to be 5.6 and 5.8 ns, which belong to typical fluorescent materials.
In order to investigate the influence of solvent polarity on the PL spectra of the compounds, the steady-state fluorescence spectra of PTZ-PIM and PIM-PTZ-PIM were measured in different polar solvents (1.0 × 10 -5 mol/L) (Fig. 2). Compared with the PL spectra of PTZ-PIM in different solvents, from low polarity toluene to high polarity DMF, the PL spectrum of PIM-PTZ-PIM in toluene, dichloromethane, tetrahydrofuran and dimethylformamide exhibits obvious solvatochromic effects, and the emission band of PIM-PTZ-PIM is obviously red-shifted from 452 nm in toluene to 477 nm in DMF, and the total redshift of 25 nm was observed. For PTZ-PIM, its emission peak is located at 452 nm in toluene and red-shifted to 458 nm in DMF. As the solvent polarity increases, the emission peak of PTZ-PIM was only red-shifted 8 nm. The results were indicated that PIM-PTZ-PIM has a more obvious charge transfer characteristic.  the compounds were very weak. With the increase of the concentrations, the relative emission intensity of the compounds gradually increased. When the concentrations were 1.0 × 10 -5 mol/L, the relative emission intensity of the compounds reached the maximum. At higher than 1.0 × 10 -5 mol/L of their concentrations, the relative emission intensity of the compounds decreases own to the intermolecular aggregation. Furthermore, at lower than 1.0 × 10 -3 mol/L, the emission spectra of the compounds did not significantly change within the entire range of the concentration changes. The thermal stabilities of PTZ-PIM and PIM-PTZ-PIM were measured by thermogravimetric and differential thermal analyses (TG-DTA) under nitrogen atmosphere (Fig. 4). As shown in Fig. 4, PTZ-PIM and PIM-PTZ-PIM show excellent thermal stability, and they exhibit higher thermal decomposition temperatures (corresponding to a 5% weight loss) of 365 °C for PTZ-PIM and 378 °C for PIM-PTZ-PIM.

Theoretical Calculation
The optimized molecular geometries and the HOMO and LUMO density maps of PTZ-PIM and PIM-PTZ-PIM were calculated by the density functional theory (DFT) calculation at B3LYP/6-31G(d) level (Fig. 6). From their optimized molecular geometries, it was found that these compounds exhibit a highly distorted configuration, which could effectively inhibit the intermolecular aggregations and weaken the π-π intermolecular interactions.  Table 1.

Electroluminescence Investigation
To   Table 2.
The doped devices based on PTZ-PIM and PIM-PTZ-PIM exhibited the turn-on voltage from 5.6 to 6.4 V. Among the compounds, the device based on PIM-PTZ-PIM exhibited the best performances with a maximum current efficiency (CE max ) of 2.45 cd/A, a maximum power efficiency (PE max ) of 1.48 lm/W, a maximum brightness (L max ) of 648 cd/m 2 and a maximum external quantum efficiency (EQE max ) of 1.48% at 15 wt% doping concentration. From Table 2, it was found that the devices based on PTZ-PIM showed poor EL performances in comparison to that of PIM-PTZ-PIM, which was consistent with the higher photoluminescence quantum yield of PIM-PTZ-PIM in solution than that of PTZ-PIM. The device based on PTZ-PIM exhibited the best EL performances with 1.37 cd/A of CE max , 0.68 lm/W of PE max , 638 cd/m 2 of L max and 1.13% of EQE max at 20 wt% doping concentration.

Conclusions
Two blue D-A fluorophores, PTZ-PIM and PIM-PTZ-PIM, were synthesized by grafting 1,4,5-triphenylimidazole moieties onto 3-and 3,7-position of the phenothiazine core and characterized by NMR, high-resolution mass spectrometry and elemental analysis. These compounds exhibit strong blue emission and good thermal stability. The doped devices based on PIM-PTZ-PIM had better EL performances than that of PTZ-PIM because of more conjugation of A-D-A type structure for PIM-PTZ-PIM than that of A-D type structure for PTZ-PIM, which was consistent with the higher photoluminescence quantum yield of PIM-PTZ-PIM in solution than that of PTZ-PIM. The doped device fabricated from PIM-PTZ-PIM exhibited a maximum brightness of 648 cd/m 2 , a maximum current efficiency of 2.45 cd/A and a maximum external quantum efficiency of 1.48% at 15 wt% doping concentration.