Photochromism of 1,2-bis(2-n-alkyl-5-formyl-3-thienyl)perfluorocyclopentene derivatives

Photochromic diarylethene derivatives 1o–6o bearing long alkyl chains (⩾seven carbon atoms) have been synthesized and their structures with the exception of compound 3o have been determined by single-crystal X-ray diffraction analysis. The effect of alkyl chain length on their properties, including photochromism both in solution and in PMMA film and their electrochemical properties were investigated in detail. All of these diarylethenes showed good photochromism both in solution and in PMMA film. The diarylethene derivatives with n-heptyl and n-octyl substituents at the 2-positions of the two thiophene rings exhibit photochromism even in the single-crystalline phase, but those with n-decyl, n-undecyl and n-dodecyl groups showed no photochromism in the single-crystalline phase. Compared to the photochromic features of 1,2-bis(2-methyl-5-formyl-3-thienyl)perfluorocyclopentene, introduction of the long alkyl groups was found to decrease the cycloreversion quantum yield and induce bathochromic shifts of the absorption spectra of the closed-ring isomers. The absorption coefficients of both open- and closed-ring isomers increased remarkably when replacing the methyl groups at 2-positions of the two thiophene rings with n-heptyl ones. However, further increase was not observed the alkyl chain becoming longer from n-heptyl to n-dodecyl groups. The cyclic voltammetry indicated that the HOMO and LUMO energy levels of these diarylethenes increased with the increase of the carbon numbers of alkyl chains from n-heptyl to n-dodecyl groups, but their band gaps were essentially not influenced by the alkyl chain length.