Computational investigation of functionalized BTBT-based molecules for organic solar energy harvesting: DFT and TD-DFT study

In this study, we present the design and theoretical investigation of four novel π-conjugated acceptor molecules (M1-M4), derived from the recently synthesized core structure 2,7-bis(4-octylthiophene-2-yl)BTBT, by functionalizing it with different terminal electron acceptor groups. These materials were created essentially to improve the optoelectronic characteristics necessary for use in organic solar cells. Density functional theory (DFT) and time-dependent DFT (TD-DFT) simulations were employed to analyze the electronic structures and optical behaviors of these compounds. After benchmarking several functionals, B3PW91/DGDZVP was identified as the best choice for accurately reproducing experimental HOMO-LUMO gaps and absorption spectra. The designed molecules exhibit near-planar conformations and significantly reduced HOMO-LUMO gaps (1.95-2.10 eV), with M4 showing the narrowest gap at 1.95 eV. This reduction correlates with a pronounced red shift in absorption maxima (603.56-636.82 nm), compared to 397.12 nm for the reference, enabling enhanced visible light harvesting. Excited-state lifetime values show that all designed molecules possess longer excited-state lifetimes compared to the reference, indicating improved photophysical stability. Transition density matrix and electron–hole overlap analysis confirm efficient and directional intramolecular charge transfer, while RDG analysis highlights the stabilizing effect of terminal acceptors through non-covalent interactions. Calculated open-circuit voltages (1.05-1.15 V) further support the potential of these materials in organic photovoltaic devices. These findings emphasize the necessity of targeted molecular design for tuning optoelectronic properties in organic semiconductors.