Effect of Fluorination on Properties of Thiophene–Phenylene Co-Oligomers with Annulated Core: A DFT Study

Organic light-emitting transistors are a new type of optoelectronic devices that combine the functionality of organic light-emitting diodes and the control of a transistor. These devices require organic semiconductors as the active layer, which should possess both high charge mobility and high quantum yield of photoluminescence. One promising class of such materials is thiophene-phenylene co-oligomers, whose properties can be tuned by introducing various substituents. In this study, we investigate the effect of hydrogen-to-fluorine substitution on the properties of two model representatives of the thiophene-phenylene co-oligomer class with an annulated central fragment: P–TTA–P and P–BTBT–P. It is demonstrated that fluorination of both molecules lowers the energy levels of the frontier orbitals, which is expected to facilitate electronic transport in their crystals and films. At the same time, fluorination exerts qualitatively different effects on the delocalization of frontier orbitals, optical bandgap, oscillator strength, exciton binding energy, and the Raman spectrum, which can be attributed to the differences in the equilibrium geometry of fluorinated molecules. It is expected that the revealed correlations between the structure and properties of the investigated compounds will contribute to a targeted molecular design of organic semiconductors for efficient light-emitting devices.