Electron-withdrawing substituent position effect on the optoelectronic properties of new photochromic diarylethenes

Three new photochromic diarylethenes bearing cyano groups at the ortho-, meta-, or para-positions of both terminal phenyl rings, namely 1,2-bis(2-methyl-5-(2-cyanophenyl)-3-thienyl)perfluorocyclopentene (1a), 1,2-bis(2-methyl-5-(3-cyanophenyl)-3-thienyl)perfluorocyclopentene (2a), and 1,2-bis(2-methyl-5-(4-cyanophenyl)-3-thienyl)perfluorocyclopentene (3a), have been synthesized. The substituent position effects on their optoelectronic properties, including photochromism, fluorescence and electrochemical properties in solution, were investigated systematically. The molar absorption coefficient (ε = 7.48 x 104 L mol-1 cm-1), the absorption maxima and the quantum cyclization yield (Φ 0-c = 0.53) of 3a are the biggest while its cycloreversion quantum yield (Φ c-0 = 0.004) is the smallest. In hexane, the fluorescent emission peaks of diarylethenes 1a, 2a and 3a showed a bathochromic shift upon increasing concentration. The oxidations of diarylethenes 1a, 2a and 3a are initiated at 1.85, 1.58 and 1.58 V, respectively, which are higher than those of their closed-ring forms 1b, 2b and 3b (1.61, 1.43 and 1.49 V, respectively). The results indicate that the optoelectronic features of diarylethenes 1, 2 and 3 are distinctly different, which may be attributed to the different electron-withdrawing ability and steric effect of cyano groups with different substituted positions