Geometry-Driven Control of Linear and Nonlinear Optical Responses in <i>p</i> -Nitroaniline: Insights into Structure–Property Relationships and Thermal Robustness

The nonlinear optical (NLO) response of organic chromophores is intrinsically governed by their molecular geometry; yet, the microscopic mechanisms that link specific deformations to optical properties remain unclear. Here, we reveal how controlled torsional and bond-length distortions in <i>p</i>-nitroaniline (pNA) dictate its dipole moment, polarizability, and hyperpolarizabilities, establishing clear geometry-property relationships. Using density functional theory and coupled-cluster calculations combined with thermal ensemble modeling, we uncover threshold-like behaviors: NH₂ and NO₂ torsions dominate the optical response, while local vibrational displacements have negligible effects. Remarkably, ensemble-averaged properties remain stable under thermally accessible fluctuations, demonstrating the intrinsic robustness of pNA's NLO response. These results provide fundamental physical insight into geometry-driven modulation of optical properties, offering guiding principles for the rational design of molecular photonic materials.