Photochromism of dithienylethenes with electron-donating substituents

Bis(2,4-dimethyl-5-phenylthiophene-3-yl) having hydrogen, methoxy, diethylamino, or cyano substituents at para-positions of the phenyl groups were synthesized to reveal the effect of the substitution on the absorption coefficient epsilon of the closed-ring forms and the photochemical reactivity. Electron-donating substituents, such as methoxy or diethylamino groups, were found to be effective to increase the absorption coefficient and to decrease the ring-opening quantum yield. The cyclization quantum yield was scarcely affected by the substitution. The conversion from the open- to the closed-ring forms of the diethylamino-substituted compound in the photostationary state under irradiation with 313 nm light was close to 100%.