Femtosecond spectroscopic study on photochromic diarylethenes with terthiophene

Photochromic ring-closure reactions of diarylethene derivatives with terthiophene (T3H, T3Me) have been examined by femtosecond transient absorption spectroscopy. The rate of the ring-closure reaction of T3H in n-hexane was estimated to be similar to 2.5 ps, which is very similar to that of T3Me in the same solvent. It was suggested that the ring-closure reaction proceeds through a concerted mechanism without any intermediates but not a two-step mechanism. The rate of ring-closure reaction was also dependent on the solvent polarity, suggesting that the charge transfer state in the excited singlet state plays an important role in the photochromic reaction.