The effect of the cyano group position upon unsymmetrical isomeric diarylethenes bearing a pyrrole unit

Four photochromic diarylethenes bearing a pyrrole moiety were synthesized, and the structures of two of the new isomeric compounds were determined by single-crystal X-ray diffraction analysis. Each of the diarylethene derivatives exhibited good photochromism both in solution and in poly(methyl methacrylate) films, which their photoconversion ratios from open-ring to closed-ring isomers were larger than 89% in hexane at photostationary state. They also functioned as effective fluorescent photoswitches both in hexane and in poly(methyl methacrylate) films. The absorption maxima, cyclization/cycloreversion quantum yields, and fluorescence quantum yields increased whereas the molar absorption coefficients and the emission peaks decreased notably when the cyano group was attached at any of the three positions on the terminal benzene ring. Cyclic voltammograms indicated that the introduction of cyano group decreased the band-gap of the open-ring isomer and increased that of the closed-ring isomer. The cyano group and its -substituted position can availably modulate the optical and electrochemical behaviours of these diarylethene derivatives.

Research highlights
► Four photochromic diarylethenes bearing a pyrrole moiety were synthesized.
► They exhibit photochromism and fluorescent switch in solution and in PMMA films.
► The cyano group position had a notable effect on their optoelectronic properties.