Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study

Isoindigo-based donor–acceptor small molecules are promising candidates for organic photovoltaic applications because of their strong electron-accepting character and tunable π-conjugated structures. In this work, seven isoindigo-based derivatives bearing phenyl, thiophene, thiophene, benzothiophene, fluorene, carbazole, phenoxazine, and julolidine terminal donor units were computationally designed and investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Following a benchmark study, B3PW91/6-31+G(d,p) was selected for the systematic evaluation of structural, electronic, optical, and OPV-related properties. The results indicate that donor modification mainly affects the highest occupied molecular orbital (HOMO) energy, while the lowest unoccupied molecular orbital (LUMO) remains largely localized on the isoindigo acceptor core. Stronger donor units narrowed the electronic gap and induced bathochromic shifts in absorption, but excessive HOMO destabilization decreased the estimated open-circuit voltage from 1.28 to 0.59 V. Among the investigated molecules, the benzothiophene derivative I3 showed the most balanced OPV-related descriptor profile, with λmax = 672.55 nm, f = 1.67, LHE = 0.98, Eb = 0.36 eV, ΔLUMO = 0.64 eV, and an estimated Voc of 1.27 V. In contrast, phenoxazine and julolidine derivatives exhibited more strongly red-shifted absorption maxima at 803 and 838 nm, respectively, but their less favorable voltage-related descriptors and higher exciton binding energies reduced their overall OPV-related descriptor balance. These findings indicate that moderate donor strength is more favorable than excessive electron donation for achieving a balance among absorption, exciton dissociation, and voltage retention in isoindigo-based small-molecule OPV candidates.