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 OLED and a transistor that controls it. The working layer of these devices requires organic semiconductors that combine high charge mobility with a high photoluminescence quantum yield. One of the promising classes of such materials are thiophene-phenylene co-oligomers, the properties of which can be tuned over a wide range by adding various substituents. In this work, we address the effect of fluorination on the properties of two model thiophene-phenylene co-oligomers with an annulated central fragment, P–TTA–P and P–BTBT–P. It is shown that fluorination of both molecules lowers their frontier orbitals energy levels, and this should enable electron transport in their crystals and films. At the same time, fluorination has a qualitatively different effect on the delocalization of frontier orbitals, the width of the optical gap, the oscillator strength, the exciton binding energy, and the Raman spectrum, which is explained by the difference in the equilibrium geometry of fluorinated molecules. It is expected that the revealed relationships between the structure and properties of the studied compounds will contribute to the rational design of organic semiconductors for efficient light emitting devices.