Theoretical Analysis on the Optoelectronic Properties of Single Crystals of Thiophene-furan-phenylene Co-Oligomers: Efficient Photoluminescence due to Molecular Bending

We theoretically analyze the optoelectronic properties of single crystals of 2,5-bis(4-biphenylyl) bithiophene (BP2T) and 2-(4-biphenyl)-5-[5-(4-biphenyl)-2-thienyl] furan (BPFT) molecules, aiming to provide a guiding principle for the material design of organic light-emitting transistors. The X-ray structure analysis and the density functional theory (DFT) calculations indicate that half of the BPFT molecules bend the π-conjugation plane in the crystal. The Marcus theory parametrized by the DFT calculations indicates anisotropic charge mobilities. The emission spectra of the BP2T and BPFT crystals are analyzed by the time-dependent DFT calculations in conjunction with the Frenkel exciton model and the vibronic coupling analysis. We revealed that the high photoluminescence efficiency of the BPFT crystal originates from the symmetry breaking of the H-aggregate, where the transition dipole of the dark state does not cancel out.