Disentangling solvent effects on optical response via p-π and sp² lone-pair contributions in non-aromatic fused systems

In this study, we disentangle solvent effects on optical responses in non-aromatic fused systems via p-π and sp² lone pair contributions. Donor-π-acceptor (D-π-A) molecules based on 5,7,9,14,16,18-hexahydroheptacene (6 H-Hep) were designed across three chalcogen series: oxygen (R1, IM1-IM3), sulfur (R2, IM4-IM6), and selenium (R3, IM7-IM9). DFT and TD-DFT studies on D3-B3LYP/6–31 + G(d) level of theory were used to explore electronic structure, absorption, and nonlinear optical (NLO) properties. HOMO-LUMO gaps ranged from 4.23 to 1.95 eV, with IM3 showing the smallest gap and strongest intramolecular charge transfer (ICT) due to enhanced p-π conjugation and sp2 lone pair donation. Absorption maxima spanned 307–394 nm (π→π*), with IM6 red-shifted to 394 nm from solvent-polarity-amplified lone pair effects. First hyperpolarizability (βx) increased dramatically from negligible values (< 0.02 a.u.) in parent compounds to 46.75 × 10⁶ a.u. for IM3 in water, far exceeding benchmarks. Solvent-dependent studies (water, benzene, ethanol) confirmed βx dominates total hyperpolarizability, underscoring solvent polarity’s critical role in modulating p-π/lone pair-driven NLO responses. NICS(1)zz values and electrostatic potential mapping verified enhanced polarization in oxygen derivatives, while π-π stacking in IM3 stabilized charge delocalization. Radiative lifetimes (τr = 18–47 ns) support efficient optical activity. Collectively, IM3, IM6, and IM9 emerge as top candidates, with IM3 showing exceptional NLO performance. This work establishes heteroatom substitution, solvent-tuned electronic contributions, and donor-acceptor modulation as effective strategies for advanced electro-optical materials.