Modulating the structural framework with electron-withdrawing groups and studying their effects on charge mobility in a new series of D-A-D based organic semiconductors

Abstract

A systematic computational study was conducted on a new series of donor-acceptor-donor (D-A-D) architecture molecules, featuring bithiazolimide as the acceptor (A) and thiophene as the donor (D), to evaluate the effect of electron-withdrawing substituents on their charge mobility. Density functional theory (DFT) and time-dependent DFT were used to analyze HOMO/LUMO stability, electron affinity (EA), ionization potential (IP), reorganization energy (λ), charge mobility (μ), absorption spectra, and density of states (DOS), revealing the impact of steric and electronic properties on charge transport in the D-A-D framework. The observed trend of variation of the ionization potential and electron affinity suggests increasing charge transfer behaviour with increasing substitution constant. The study reveals that electron mobility is directly proportional to the strength of the electron-withdrawing substituent. Molecules containing -F, -Cl, -CSCH_3, -COCH₃ and -CF₃ exhibited higher electron mobility pointing towards their dominating n-type character, whereas molecules containing -CN and -NO₂ showed balanced electron and hole mobilities, suggesting an ambipolar nature. In contrast molecule 4, with a -CSCH₃ substituent, demonstrates the highest electron mobility and the smallest HOMO-LUMO gap, making it particularly promising for n-type organic semiconductor applications. The high electron mobility of molecule 4 is further supported by non-covalent interaction (NCI) analysis, which shows S⋯S interactions within the thiophene ring and electron-withdrawing substituents present in the donor moiety. Further, electrostatic potential (ESP) and transition density matrix (TDM) analyses support effective charge separation within these molecules, suggesting an optimized D-A-D system with strong electron-withdrawing substituents and minimal steric hindrance. These findings highlight the potential of these systems in developing high-mobility n-type organic semiconductors, contributing valuable insights for advancing organic electronic devices.