Theoretical study on the influence of a polarizable environment on the second-order nonlinear optical properties of substituted naphthalene isomers

The design of molecular configurations for high-efficiency nonlinear optical (NLO) materials has attracted considerable interest in recent years. In this work, quantum chemical methods are employed to investigate the second-order NLO properties of a series of mono- and disubstituted naphthalene derivatives in both the gas phase and a solid polarizable environment. The results demonstrate that the substitution position strongly influences the charge-transfer architecture and, consequently, the total first hyperpolarizability (<italic>β</italic>_tot) of the studied systems. In the gas phase, the calculated <italic>β</italic>_tot values for the 1,5- and 2,6-naphthylene-substituted derivatives (N1,5-TPA-TRZ and N2,6-TPA-TRZ), which exhibit through-bond charge transfer (TBCT) character, are markedly higher than that of the 1,8-naphthylene-substituted analogue (N1,8-TPA-TRZ) with through-space charge transfer (TSCT) character. The extended π-conjugation in the TBCT systems leads to a pronounced red-shift in the absorption spectra and lower electronic transition energies, contributing to their enhanced second-order NLO responses. Furthermore, solid-state polarization effects were examined using a combined approach of range-separated hybrid (RSH) functionals and a polarizable continuum model (PCM). This analysis reveals a substantial increase in <italic>β</italic>_tot for N2,6-TPA-TRZ, from 5.73×10^‒⁴⁹ C³·m³·J^‒² in the gas phase to 1.59×10^‒⁴⁸ C³·m³·J^‒² under solid-like conditions. These findings highlight that introducing donor and acceptor groups at the 2,6-positions of naphthalene derivatives offers a promising strategy for developing high-performance NLO materials.