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.