The photochromism of unsymmetrical diarylethene isomers with an electron-withdrawing cyano substituent

Abstract

Three unsymmetrical isomeric diarylethenes bearing an electron-withdrawing cyano group were synthesized and their structures determined using single-crystal X-ray diffraction analysis. Each of the compounds displayed excellent photochromism in solution, in PMMA film, as well as in the crystalline phase. The isomeric compounds also functioned as a fluorescence switch in PMMA films. The cyclization quantum yield and the absorption maxima of both the ring-opened and ring-closed isomers increased in the order: ortho- < meta- < para-substitution of the cyano group. Compared with the unsubstituted parent compound, the cycloreversion quantum yield decreased whereas the molar absorption coefficient of the ring-opened and the ring-closed isomers increased significantly when the cyano group was attached at any of the three positions on the terminal benzene ring. Cyclic voltammetry studies indicated that the position of cyano substitution had a marked effect on the electrochemical behaviours of these isomeric diarylethenes.