Selenium-Substituted Nonfused Ring Acceptors: Theoretical Insights into Optoelectronic Properties via Noncovalent Interaction Engineering.

Selenium (Se) substitution critically modulates the acceptor electronic structures and optoelectronic properties. This study systematically modifies the high-performance nonfused ring electron acceptor (NFREA) 2BTh-2F through π-bridge S-to-Se substitutions. Using density functional theory (DFT) and time-dependent DFT (TD-DFT), we designed nine novel acceptors (Z1-Z9) and characterized their electronic/optoelectronic properties. Complementary atoms in molecules (AIM) topological analysis and reduced density gradient (RDG) analysis reveal enhanced Se···O noncovalent interactions (NCIs), though molecular planarity remains governed by steric constraints from three-dimensional (3D) side chains. Crucially, Se substitution optimizes the electrostatic potential (ESP) distribution, frontier molecular orbital (FMO) energy levels, and excited-state properties. Z5, a symmetrical Se-substituted derivative of the outer thiophene on the π-bridge, established itself as the most promising NFREA candidate due to its minimum band gap, maximum open-circuit voltage and fill factor, and minimum energy loss. These findings underscore rational substitution strategies for NFREA design and provide critical guidelines for the development of high-performance organic solar cells.