How much can catenated boronated benzene-like nanobelts enhance effective nonlinear optical responses: a molecular inverse study via DFT

Boron-rich belt-like nanoclusters constitute a compelling frontier in low-dimensional materials science, combining intrinsic electron deficiency with highly delocalized multi-center bonding to afford exceptional tunability of optoelectronic properties. Here we present a comprehensive density functional theory (DFT) and time-dependent DFT (TD-DFT) investigation of a benzene-like boron-catenated (BC) belt and five exocyclic super-alkali(SA)-doped derivatives, BC@FLi 2 , BC@LiMg, BC@NLi 4 , BC@OLi 3 , and BC@Li 3 , with emphasis on frontier orbital engineering, global chemical reactivity indices, and the prerequisites for enhanced nonlinear optical (NLO) response. The SA doping introduces strong donor–acceptor asymmetry, compressing the HOMO–LUMO gap from 1.431 eV in boron-carbon nanobelt (BC) to as low as ∼0.19 eV in BC@OLi 3 . Koopmans' theorem-based reactivity descriptors reveal dramatic increases in electrophilicity (ω, 1.80 → 180.79 eV) and global softness (σ), quantitatively linking frontier orbital compression to enhanced charge-transfer susceptibility. TD-DFT analysis of BC identifies three near-infrared singlet excited states (0.84–0.96 eV) with negligible oscillator strengths, while doping systematically reduces exciton binding energies (0.140 → 0.124 eV), favoring free-carrier generation. These findings collectively establish exocyclic SA functionalization as a powerful and chemically tunable strategy for designing high-performance boron-belt NLO chromophores, with implications for photonic switching, frequency conversion, and two-photon absorption applications.