Odd–Even Effect in Photoreactivity and Transistor Property of Benzo[<i>b</i>]-benzo[4,5]thieno[2,3-<i>d</i>]thiophene Tethered with a Diacetylenic Group through a Spacer Chain

A series of benzo[b]-benzo[4,5]thieno[2,3-d]thiophene (BTBT) derivatives carrying a diacetylenic unit through a hydrocarbon chain spacer of different lengths (BTBT-Cn-DA-C12, n = 4–7) were synthesized and characterized. A clear trend of odd–even effect can be observed in their photophysical property in that the ones with odd carbon chain spacers undergo topochemical polymerization upon UV-irradiation, whereas those with even carbon chain spacers do not. Thin-film transistors based on aligned films of these compounds after UV-irradiation also exhibit an odd–even effect in that the ones with odd carbon chain spacers gave much higher mobility than the ones with even carbon chain spacers, presumably due to the polymerization of diacetylene chains. The results can be attributed to the different relative disposition of the neighboring diacetylenes resulting from the trans zigzag chain extended from the crystalline lattice of BTBT cores. These findings underscore the importance of chain spacers as structural leverages for modulating photoreactivity and electronic properties in organic semiconductors.