One Molecule, Four Colors: Discovering the Polymorphs of a Thieno(bis)imide Oligomer

The crystallization of 2,2′-(5,5′-(ethyne-1,2-diyl)bis(thiophene-5,2-diyl))bis(5-hexyl-4H-thieno[2,3-c]pyrrole-4,6(5H)-dione), hereafter NTA, in different solvents and/or conditions yielded the identification of five different polymorphs which are characterized by impressive differences in their photophysical behavior. Phase I, phase II, phase III, and phase IV are characterized by a yellow, green, red, and orange emission under illumination with 365 nm light. The crystal structures of phase II and III were determined by single-crystal X-ray diffraction. Phase II is characterized by the elongated molecules that lie parallel to each other in layers and hence by the presence of a long axis in the cell parameters. The two thienoimide-thiophene systems do not lie in the same plane which is consistent with the higher energy emission. In phase III, the NTA molecules pile up and form columns with strong π-π interactions. The packing is consistent with the presence of red emission and the absence of a long axis in the cell parameters. The thermal characterization revealed that phase I, II, and III converts into phase IV, which, upon melting, recrystallizes in the elusive phase V. Quantum-chemical studies were carried out to investigate optical and charge transport properties. Promising charge transport properties with a dominant n-type character were predicted; however, the role of internal conformational disorder could influence the efficiency of charge transport.