Theoretical Study on the Design and Optical Properties of a Carbon Nanoring System

The rational design of supramolecular assemblies enables precise modulation of molecular optical properties through controlled intermolecular interactions. This study employs density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to investigate the structure–property relationships governing the second-order nonlinear optical (NLO) responses in host–guest complexes formed between C₆₀ and carbon nanoring derivatives (B-PLY-CPP and N-PLY-CPP). The optimized geometries of B-PLY-CPP@C₆₀ and N-PLY-CPP@C₆₀ confirm stable convex–concave π–π stacking, with interaction energies of approximately –35 kcal/mol. The static​ first hyperpolarizability (β_tot), calculated at the CAM-B3LYP/6-31G(d) level, reveals that the N-doped complex (N-PLY-CPP@C₆₀) achieves a β_tot value of 1.01 × 10⁴ au, which is significantly higher than that of its B-doped analogue and is competitive with classic push–pull NLO chromophores. This enhancement correlates directly with a more efficient intermolecular charge transfer from the nanoring host to the C₆₀ guest, as evidenced by a substantial red-shift in the calculated low-energy absorption band. Analysis using a two-level model confirms that the superior NLO response originates from a lower transition energy and a larger transition dipole moment associated with this charge-transfer excitation. The results provide comparative mechanistic insights​ into how heteroatom doping and supramolecular organization can be used to tailor charge-transfer excited-state characteristics and enhance second-order NLO activity in carbon-based materials, highlighting a viable supramolecular strategy for property modulation.