Effects of substitution on the optoelectronic properties of photochromic diarylethenes bearing a pyrrole moiety

Five diarylethenes bearing a pyrrole moiety were synthesized in order to investigate the effects of the substituents on the properties of these compounds. The structures of three of the diarylethenes were determined by single-crystal X-ray diffraction analysis. All of the diarylethene derivatives exhibited notable photochromism and functioned as effective fluorescent photo-switches in solution and in poly(methyl methacrylate) films. The absorption maxima and the quantum yields of cyclization and cycloreversion increased whereas the emission peaks and the emission intensities decreased evidently when the electron-withdrawing/donating substituent was attached at the para-position on the terminal benzene ring. Cyclic voltammograms indicated that the electron-withdrawing/donating substituents decreased the bang gaps of the open-ring isomers but enhanced those of the closed-ring isomers, compared with the unsubstituted parent diarylethene. The properties of these diarylethene derivatives were significantly different from each other, which might be attributed to the different substituent effects.