Tuning Second‐Order Nonlinear Optical Properties via Electronic Interaction Engineering: Distinct Roles of Nonconjugation, σ–π Conjugation, and Intramolecular Charge Transfer in Chalcogen‑Functionalized Closo ‐Carborane Hybrids

The rational design of carborane‐based second‐order nonlinear optical (NLO) materials requires fundamental understanding of how distinct electronic interactions govern the first hyperpolarizability. Herein, a systematic DFT/TD‐DFT investigation is performed on a series of chalcogen‐functionalized closo ‐carborane derivatives to establish explicit structure–property relationships and to achieve a comparative understanding of how distinct bonding motifs—nonconjugation, σ–π conjugation, and intramolecular charge transfer (ICT)—affect the second‐order NLO properties. Our results demonstrate that nonconjugated compound 1 exhibits a modest NLO response. In chalcogen‐bridged monomers 2–4 , σ–π conjugation significantly enhances hyperpolarizability through improved orbital delocalization and moderate ICT. Notably, dimer 5 achieves the largest first hyperpolarizability ( β tot = 786.4 a.u.), owing to its relatively extended CT distance and highly asymmetric spatial charge separation. In contrast, dimers 6 and 7 show markedly lower NLO activity despite stronger ICT, due to their symmetric, convergent charge redistribution that cancels the net dipole moment change. These results reveal that optimal second‐order NLO performance requires a synergy of extended conjugation, directional ICT, and asymmetric charge redistribution. The quantitative correlations between electronic descriptors (e.g., overlap integral S r , CT distance D , charge separation t index, and etc.) and first hyperpolarizability provide a predictive framework for engineering high‑efficiency NLO chromophores in hybrid inorganic–organic architectures.