Synthesis, Photochromic Property and Application for Optical Recording of a New Asymmetrical Diarylethene

The reversible molecule switching of an assymmetrical photochromic diarylethene


Introduction
Photochromic materials have been extensively investigated for their potential applications in erasable optical memories, displays, and optical switches [1].The properties of photochromic materials, such as their absorption spectrum, refractive index, dielectric constant, oxidation and reduction potentials, photoluminescence, dipole moment, and ionization potential (Ip), can be changed reversibly during the photochromic process.This unique character can be applied to different types of photonic devices [2][3][4][5].Among the photochromic compounds, diarylethenes are regarded as the best candidates for such devices [1], because of their advantages such as a high efficiency of photoisomerizations, sufficient thermal stability of both the open and the close forms, a very high resistance to photofatigue, and the ease with which the reaction can be monitored by UV-vis spectroscopy [6][7][8][9].These differences have been utilized to control their functions such as electrical conductivity, alignment of liquid crystals, light-driven organogelators, and photocontrolled release and uptake.
Among diarylethene derivatives so far synthesized [10][11][12][13], most of the heteroaryl moieties have been thiophene or benzothiophene rings, and other heteroaryl moieties, such as furan, thiazole, benzofuran, pyrrole and indene [1], have also been reported partially.But diarylethene bearing thiophene and indole moieties haven't been reported until now.In this paper, we have designed and prepared a new asymmetrical diarylethene derivative with thiophene and indole moieties.i.e., 1-[2-methy1-5- We synthesized asymmetrical diarylethenes having an indole moiety, and the compound showed good thermal stability and strong fatigue resistance in the solid state.The results that asymmetrical diarylethene with an indole moieties have better thermal stability and stronger fatigue resistance than the symmetrical one are very interesting and important.Its photochromic reactivity, fluorescence property and application for optical recording were investigated.The photochromic reaction of diarylethene 1a is shown in Scheme 1.

Experimental
The synthetic route for diarylethene 1a is shown in Scheme 2. It was synthesized according to the similar procedure of Pu et al. [12].The structure of compound 1a was confirmed by 1 H NMR spectroscopy, IR and 13 C NMR spectroscopy. 1 H NMR (400 MHz, CDCl 3 ): δ1.80 (s, 3H, -CH 3 ), 2.04 (s, 3H, -CH 3 ), 3.65 (s, 3H, -CH 3 ),  The recording film was prepared as follows: diarylethene 1a (10 mg) was dissolved ultrasonically in PMMAchloroform solution (10%, w/w, 1 ml).The mixture solution was then spin coated on a glass substrate (20 mm × 20 mm × 1 mm) which was then dried in air at room temperature.The thickness of the film was about 12 μm.The sample was colored homogeneously by irradiation with 313 nm UV light to convert the open-ring isomer (1a) to the closed-ring isomer (1b) before being recorded.

Photochromism of Diarylethene
The diarylethene 1a exhibits good photochromism both in hexane solution (2 × 10 −5 mol•L −1 ) and in PMMA amorphous film (10% w/w).As shown in Figure 1, in hexane solution, the absorption maximum of compound 1a was observed at 220 nm (ε = 5.5 × 10 4 L•mol −1 •cm −1 ).Upon irradiation with 313 nm light, the colorless solution of 1a turned blue, in which the absorption maximum was observed at 557 nm (ε = 8.9 × 10 3 L•mol −1 •cm −1 ).Correspondingly, the blue colored solution returned to colorless upon irradiation with visible light (λ > 500 nm), indicating that 1b returned to the initial open-ring isomer 1a.Similarly, upon irradiation 313 nm light, the color of dithienylethene 1b/PMMA film (λmax = 275 nm) changed from colorless to blue with the appearance of a new broad absorption band at λmax = 575 nm, which was assigned to the formation of the closed form 1b. The red shift of the ring-closed form of diarylethene 1 in PMMA film in comparison with that in hexane solution can be ascribed to the stabilization of molecular arrangement in solid state.

Fluorescence of Diarylethene 1a
The fluorescence properties in hexane and in PMMA amorphous film of the compound 1a were measured using a Hitachi F-4500 spectrophotometer, and the breadths of excitation and emission slit were selected 10.0 nm and 20.0 nm, respectively.As shown in Figure 2 light in hexane and in PMMA film.When irradiated by light of 313 nm, the photocyclization reaction was carried out and the non-fluorescent closed-ring form of the compound was produced.The hexane solution and the PMMA film of 1a exhibited relatively strong fluorescence at 421 and 453 nm when excited at 355 nm.The back irradiation by appropriate wavelength visible light regenerated its open-ring isomer and recovered the original emission intensity.

Optical Storage
The evaluation of potential of photochromic material as an erasable storage medium by recording, reading and erasing the optical image in a real-time operation was investigated.For the sake of long-term stability and environmental durability of storage devices, photoresponsive materials must be processed into large-area, high quality solid films.Usually, photochromic materials are doped into a polymer matrix for investigation of applications, and PMMA was chosen in this paper to be used as polymer matrix for optical recording.Using dithieny-

Conclusion
A novel photochromic diarylethene 1a was synthesized and its photochromic, reaction kinetics and fluorescent properties were investigated.The results showed that this compound exhibited good reversible photochromism both in solution and in PMMA amorphous film.It also exhibited remarkable fluorescence switching.Using diarylethene 1b/PMMA as recording medium, the results demonstrated that the diarylethene compound had attracttive properties for high capacity optical storage.

Figure 2 . 3 .
Figure 2. Fluorescent intensity changes of diarylethene 1(a) in hexane and (b) in PMMA film.lethene1c/PMMA as the recording medium, optical storage was performed by a 650 nm He-Ne laser.The film was colored homo-geneously by irradiation with UV light before optical recording.With a He-Ne laser in the experimental setup, the exposure time for each spot was 0.08 s and the patterns were about 20 μm.The polarizations of the patterns recorded on dithienylethene 1b/ PMMA film is shown in Figure3.The result indicated that the new photochromic diarylethene can be applied in high capacity optical storage.As shown in Figure3, the quality of optical recorded in 1c/PMMA film is also fairly good.