Amorphous diarylethenes

Fig. 11 shows the photoswitching of the injection current. Upon UV irradiation, the hole injection current increased, while decreasing to zero on irradiation with visible light. Very thin amorphous diarylethene film as thin as 0.2 pm could also control the hole injection to the organic hole transport layer (Fig. 9b). These results are potentially applicable to optical memory-type organic photoconductors.

TABLE 17.1 Glass Transition Temperatures of Amorphous Diarylethenes... 

Thin films of the amorphous diarylethenes can be formed easily by a conventional spin-coating method on appropriate substrates with various solvents such as toluene, hexane, dichloromethane, and so forth. Both the open-ring and the closed-ring form isomers and their mixtures form thin films. Figure 17.5 shows the reversible absorption spectral changes of a bulk amorphous film of 2. The spectral changes are almost the same as those in the solution. 

State was attained. After UV-light irradiation, about 80% of the absorbance at 580 nm was recovered. The conversion of 80% from 2a to 2b was almost the same as that in the solution phase. The conversion of the film prepared from 2a solution was 40%, which is half of the conversion of the film prepared from a solution of the closed-ring form isomer. This difference in the maximum conversion to 2b is caused by the conformation of the open-ring form isomers. The isomer has two conformations, anti-parallel and parallel conformations. The former is photoactive whereas the latter is inactive, and half of the open-ring form isomers are in the inactive parallel conformations in solution. Half of 2a molecules in the film prepared from 2a solution are in the inactive parallel conformation. The conformational change is difficult in the amorphous film below Tg. In contrast, 2a in the bleached film prepared from the solution of 2b is in an anti-parallel conformation, and the maximum conversion to 2b at the photostationary state is about two times larger than that in the film prepared from 2a solution. A similar increase in the conversions in the film prepared from the closed-form isomer has been observed in amorphous diarylethenes, 3-10. It should be noted that heat treatment of the bleached 7a film at a temperature above Tg resulted in a decrease in the maximum conversion, which indicates that conformational change takes place at temperatures above Tg. 

Kmi, M.-S., Kawai, T, and Irie, M. Synthesis of fluore.scent amorphous diarylethenes. Chem. Lett. 1188, 2000. 

---