Synthesis and the effect of substituent position upon unsymmetrical isomeric diarylethenes bearing a pyrrole unit

Four unsymmetrical diarylethenes bearing a pyrrole moiety were synthesized, and the structures of two isomeric compounds were determined by single-crystal X-ray diffraction analysis. The properties of these compounds, including photochromism, fluorescence and electrochemical properties of these diarylethenes were also investigated systematically. Each of the four diarylethene derivatives exhibited good photochromism and functioned as an effective fluorescent photoswitch both in solution and in PMMA films. The absorption maxima, the quantum yields of cyclization and cycloreversion, and the fluorescence quantum yields of the open-ring isomers increased whereas the emission peaks decreased evidently when the methoxy group was attached at any of the three positions on the terminal benzene ring. Compared with the analogous diarylethenes bearing a pyrazole moiety, the introduction of the pyrrole moiety could increase the absorption maxima and decrease the cycloreversion quantum yields significantly. Cyclic voltammograms testified that methoxy group and its substituted position could effectively modulate the electrochemical behaviours of these diarylethene derivatives.