Bridge-Controlled Excited-State Interactions in Cofacial Phenylene-Vinylene Oligomers

Rigid cofacial π-conjugated systems provide a well-defined platform for investigating how molecular geometry governs excited-state interactions in organic optoelectronic materials. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed on unbridged p-phenylenevinylene-stilbene (1PV) and oligo(p-phenylenevinylene) (o-PV) dimers together with xanthene- and dibenzofuran-bridged cofacial analogs. Natural transition orbitals (NTOs) and fragment-resolved transition-density analyses were employed to characterize local excitation character, direct interstrand charge transfer, bridge participation, and the limitations of the simplified S1-S2 splitting model. Xanthene bridges promote compact cofacial geometries with shorter interchain distances and smaller bite angles, whereas dibenzofuran bridges generate more open structures. The fragment-resolved analysis reveals that the low-lying excited states remain predominantly local in character, with only minor direct interstrand charge-transfer contributions, while bridge participation is substantially greater in the 1PV systems than in the corresponding o-PV analogs. The effective S1-S2 half-splitting, J_eff, ranges from 27.5 to 110.0 meV and is interpreted as a comparative descriptor of the low-energy excited-state separation. These results show that bridge-dependent excited-state behavior reflects the combined effects of molecular geometry, excitation localization, bridge participation, and multipair NTO character rather than interchain distance alone.