Synthesis and characterisation of photochromic dithienylcyclopentene liquid crystal with thermal irreversibility

ABSTRACT A series of photochromic liquid crystal dithienylcyclopentene derivatives were synthesised from the initial material 2-chloro-5-methylthiophene. They were characterised by 1H NMR, high resolution mass spectrometer, differential scanning calorimetry (DSC) and polarised optical microscopy (POM) with a hot stage. All of them exhibited excellent photochemically reversible isomerisation between the open-ring form and the closed-ring form in both organic solvents and LC media, resulting in the colour change between colourless and purple under different light irradiations. The typical textures from POM showed that their isomers both had nematic phase and smectic phase with different colours. These compounds with bent shape were more sensitive to light than those of diarylethenes reported. Graphical Abstract


Introduction
As a kind of important function material, photochromic compounds attract much attention since they were discovered in which the azo-dyes are one of the most applied. Azo-dyes with N = N double bond have been studied much for applications in photonic and display devices, [1] such as photo-alignment material. [2][3][4][5] The isomerisation from trans to cis form can be induced by UV light at about 365 nm, and the return to trans-isomers can be induced under visible light irradiation of 400-550 nm.
Unfortunately, the thermal relaxation [6] of cis-isomers to trans-isomers occurs spontaneously in the dark, which is unfavourable for widespread applications. Dithienylcyclopentenes with excellent thermal stability and fatigue resistance can change their absorption spectra upon UV light irradiation, corresponding to the phototransformation from a colourless open-ring form to a coloured closed-ring form. The reverse process is thermally stable and occurs only by visible light irradiation, [7][8][9][10][11][12] which is different from the azobenzene compounds with the property of spontaneous isomerisation in the dark to the thermodynamic stability of trans isomer. [13,14] Furthermore, these kinds of intelligent materials are more sensitive to light, resulting in their application in remote, spatial and temporal controllability. [15] In recent years, this sort of photochromic diarylethenes are attracting more and more attentions because of their potential applications as photo-memories, photo-switches, [16][17][18][19][20] photochromic polymer [21] and other photochromic devices. The design and synthesis of these materials with new structures or practical applications have been a very popular area. [22][23][24][25] Li et al. [15,[26][27][28][29] also have synthesised several series of chiral dithienylcyclopentene derivatives and carried out the application research on selforganised cholesteric LCs.
Here, we report a new class of photochromic dithienylcyclopentenes that contain typical structure similar to the bent-shaped molecules. They showed excellent photochromic property of transforming between the open-ring and closed-ring isomers upon light irradiation with thermal irreversibility. Interestingly, their open-ring forms and closed-ring forms both displayed the same liquid crystalline phase sequence at the photostationary states, whereas the trans-isomers with rod-like shape usually exhibit liquid crystalline property and the liquid crystalline phase disappears during the isomerisation process to cis forms of azo-dyes. The dithienylcyclopentenes can be doped into liquid crystal host, [30] the clear colour change under light irradiation promoted its practical applications.

Materials and methods
All chemicals and solvents were purchased from TCI (Tokyo, Japan), Sigma-Aldrich (Shanghai, China) and Energy Chemical Co., Ltd. (Shanghai, China) and used without further purification unless specified. Tetrahydrofuran (THF) was purified by distillation and B (OCH 3 ) 3 was dried by Na; CH 2 Cl 2 was distilled in CaH 2 . 1 H NMR (Bruker AVANCE, 400 MHz) spectra were recorded in CDCl 3 and DMSO-d 6 . Chemical shifts are in δ units (ppm) with tetramethylsilane as an internal standard. The coupling constant (J) is reported in hertz (Hz). NMR splitting signals are designated as s, single; d, double; t, triple; and m, multiple. The phase behaviour was observed by the polarised optical microscopy (POM) (ECLIPSE LV100POL, Nikon, Tokyo, Japan) with a hot stage (LN2-P6U, Instec, Boulder, CO, USA) and differential scanning calorimeter (Diamond DSC, Perkin Elmer, Waltham, MA, USA). The UV light irradiation was carried out by a xenon lamp (50 W) through a filter at 310 nm.
The visible light irradiation was carried out by a green laser at 532 nm. The liquid crystal mixture TEB300 (Chengzhi Yonghua Display Material Co., Ltd., Shijiazhuang, China) was used in the study.

Synthesis
The photochromic compounds were synthesised from starting material 2-chloro-5-methylthiophene, as shown in Scheme 1. Their structures were identified by 1 H NMR and high resolution mass spectrometer.

4-(Iodophenyl)tetrahydropyran-2-yl ether (a) [22]
To a solution of 4-iodophenol (2.20 g, 10 mmol) in dry CH 2 Cl 2 (60 mL) was added 3,4-dihydro-2H-pyran (1.37 mL, 15 mmol), followed by addition of pyridinium p-toluenesulfonate (PPTS) (251 mg, 1 mmol). Then the mixture was stirred for 3 h at room temperature. The resulting solution was washed twice with saturated brine, and the organic phase was dried by anhydrous Na 2 SO 4 , filtered. The solvent was removed under reduced pressure and the crude product was purified by flash silica gel column chromatography with petroleum ether elution to obtain colourless crystal a (2.  [22] A mixture of 2-chloro-5-methylthiophene (8.76 mL, 80 mmol) and glutaryl dichloride (5.12 mL, 40 mmol) in CS 2 (150 mL) solution was added into a 250 mL round-bottom flask, followed by addition of AlCl 3 (11.60 g, 87 mmol) at 0°C with vigorous stirring. After stirring the mixture for 2 h at room temperature, icecold water was added carefully and the water layer was extracted with acetic ether. The combined organic phase was washed with water, dried by Na 2 SO 4 , filtered, and the solvent was removed under reduced pressure. The crude product was purified by flash silica gel chromatography to afford a white solid b (6.77 g, 47%). 1 [31] To a suspension of zinc (2.24 g, 35 mmol) in THF (200 mL) under N 2 atmosphere was slowly added TiCl 4 (2.80 mL, 25 mmol) at 0°C by syringe and the mixture was refluxed for 2 h. Then b (1.09 g, 3 mmol) in THF was added very carefully into the reaction mixture at room temperature and stirred for 24 h. After that, the reaction was quenched with 40% K 2 CO 3 aqueous solution (20 mL) and stirred for 12 h in the dark. The solid was filtered and washed with petroleum ether (50 mL). The combined organic layer was evaporated under reduced pressure. The residue was purified by flash silica gel chromatography with petroleum ether as eluent to afford a colourless crystalline solid c (0.6 g, 61%

1,2-Bis[2-methyl-5-(4-hydroxyphenyl)-3thienyl]cyclopentene (d) [12]
A solution of n-BuLi (1.6 M, 3.1 mL) in hexane was added dropwise slowly to a stirred solution of c (0.66 g, 2 mmol) in dry THF (20 mL) at −78°C under argon atmosphere. Then, without taking temperature control system away till the temperature rose to ambient temperature, the solution was stirred at ambient temperature for 2.5 h. B(OCH 3 ) 3 (0.70 mL, 6 mmol) was added dropwise carefully to the reaction mixture at −78°C, and then stirred for 1.5 h at ambient temperature. A mixture of a (1.22 g, 4 mmol) and Pd(PPh 3 ) 4 (50 mg, 0.04 mmol) in THF (10 mL) was added to the reaction mixture, and the resulting solution was stirred for 30 min at ambient temperature. Then, 20% Na 2 CO 3 aqueous solution (15 mL) was added. The reaction mixture was refluxed for 10 h and cooled down to ambient temperature and extracted with ethyl acetate (50 mL), washed with water (50 mL), dried over Na 2 SO 4 and filtered. The solvent was removed under reduced pressure to give a crude product. The crude product and PPTS (10 wt%, 0.1 g) were dissolved in methanol (80 mL) and methylene chloride (80 mL). The mixture was stirred for 12 h at ambient temperature. The solvent was removed under reduced pressure. The residue was purified by flash silica gel column chromatography to obtain d as a purple solid d (0.32 g, 36%). 1

Target compound (e)
A mixture of d (0.7 g, 1.5 mmol), 4-pentyloxybenzoic acid (0.7 g, 3 mmol), EDCl (0.8 g, 4 mmol) and DMAP (60 mg, 0.5 mmol) in dry CH 2 Cl 2 was stirred for 48 h at ambient temperature. Then, the solution was washed with water twice (100 mL) and the combined organic phase was dried over Na 2 SO 4 . The solvent was removed under reduced pressure and the crude product was purified by silica gel column chromatography to get a white solid e (0.6 g, 48%). 1

Results and discussion
Compounds e-h were thermally and chemically stable, and exhibited the expected reversible photochromic property in both organic solvent and LC host under different light irradiation (Scheme 2). Their photoresponsive property in solution was characterised by 1 H NMR and UV-Vis spectroscopy. For instance, the CDCl 3 solution of white solid e in the dark was detected and the results showed that there was no remarkable 1 H NMR signal change for protons on the thiophene moiety (Figure 1     at 532 nm took 350 s (60 mW/cm 2 ) (Figure 4). The cycle was repeated many times without fatigue.
The phase behaviour of the open-ring form e-h was investigated using differential scanning calorimetry (DSC) and POM equipped with a hot stage. For this series compounds, two mesophases (N, S m ) were observed, as evidenced by the typical textures in Figure 5(a) and (b), with broad temperature range during cooling process shown in Table 1. The cell was heated up to a slightly above isotropic temperature of these open-ring materials and then put under light irradiation of 310 nm. Interestingly, for the closed-ring series, the same liquid crystalline phase sequence was found and the typical textures of nematic and smectic phases were observed from the POM during the cooling mode at the same rate, which are shown in Figure 5(c) and (d). The open-ring form e (2 wt%) was doped into the liquid crystal host TEB300 (98 wt%), and the mixture was filled into the cell (15 μm). The experiment results showed that the cell turned from colourless to purple upon UV irradiation at 310 nm for 5 min (Figure 6). If this cell was irradiated with visible light of 532 nm in the dark, the colour of the irradiation area was changed from purple to colourless, as shown in Figure 7.

Conclusion
A series of new photochromic dithienylcyclopentene compounds were synthesised. Their photochromic   properties were characterised in both organic solvents and LC media. Furthermore, their phase behaviours were studied by POM and DSC, showing that both isomers had nematic and smectic phase. The colour changed from colourless to purple at room temperature under light irradiation at 310 nm and the decolouration process did not occur in the dark unless there was visible light irradiation at 532 nm, which is important for potential practical applications, such as indicators and photochromic devices.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The financial supports from the National Science Foundation of China [61435008, 61575063] are greatly appreciated.