The absence of photochromic switching in semiconducting films of thioindigo

Thioindigo is reported to undergo a photochromic cis/trans conformation change in both liquids and in epoxy resins: This characteristic could be be useful for the writing and erasing of information with visible light. In this paper, solid films of thioindigo and indigo are examined and they are shown to be organic semiconductors with an observable photovoltage on the order of 50–100 mV. The spectral dependence of the photovoltage indicates, however, that it is originating from T–T transitions undergone by an excited triplet state and there is no evidence for photochromic switching between the trans and cis singlet electronic states.