Synthesis and photochromic mechanism of 3,4-bis[2,5-dimethylthiophene-3-yl]-2,5-dihydrothiophene

A novel photochromic compound, 3,4-bis[2,5-dimethylthiophene-3-yl]-2,5-dihydrothiophene (BT), has been synthesized. The photochromic reactions of BT in a polar solvent (acetonitrile) and a non-polar solvent (cyclohexane) were investigated by means of steady state absorption spectra and time-resolved transient absorption spectra. The maximum absorption wavelengths of BT in acetonitrile and cyclohexane are at 232 and 233 nm, respectively, and the maximum absorption wavelengths of the photocyclization product (CBT) in acetonitrile and cyclohexane are 445 and 453 nm, respectively. Upon irradiation with ultraviolet light, BT is first excited to the excited singlet state, and then to the excited triplet state via inter-system crossing. Only the excited singlet state generates CBT. The excited triplet state, which has a maximum absorption wavelength at 390 nm, decays rapidly to the starting compound (BT) alone.