Unique Reversible Crystal-to-Crystal Phase TransitionStructural and Functional Properties of Fused Ladder Thienoarenes

Donor–acceptor type molecules based on fused ladder thienoarenes, indacenodithiophene (IDT), and dithienocyclopenta–thienothiophene (DTCTT), coupled with benzothiadiazole, are prepared, and their solid-state structures are investigated. They display a rich variety of solid phases ranging from amorphous glass states to crystalline states, upon changes in the central aromatic core and side group structures. Most notably, the DTCTT-based derivatives showed reversible crystal-to-crystal phase transitions in heating and cooling cycles. Unlike what has been seen in π-conjugated molecules, variable temperature XRD revealed that structural change occurs continuously during the transition. A columnar self-assembled structure with slip-stacked π–π interaction is proposed to be involved in the solid state. This research provides the evidence of unique structural behavior of the DTCTT-based molecules through the detailed structural analysis. This unique structural transition paves the way for these materials to have self-healing of crystal defects, leading to improved optoelectronic properties.