Influence of symmetry on the second-order NLO properties: insights from the few state approximations

A series of valence bond charge-transfer (VB- n CT) models ( n ∈ [1, 4]) are enacted to assess the influence of molecular symmetry on the second-order nonlinear optical (NLO) response, the first hyperpolarizability, β . This is accomplished for different amplitudes of the ground-state charge-transfer character, quantified by the VB-CT mixing parameter, m CT . The analysis focuses on quantities accessible from the second harmonic scattering (SHS) phenomenon and measurements, β SHS and its depolarization ratio. Among the symmetries considered, D 3 h and C 3 v (VB-3CT, with large θ , i.e. for almost planar tripod-shape molecules) yield the largest β SHS values provided m CT is large, followed by T d (VB-4CT). These cases correspond to dominant octupolar responses and depolarization ratios close to 3/2. Other symmetries result in comparatively weaker responses, and a variety of depolarization ratios. Maximizing the charge-transfer dipole moment μ CT and minimizing the ratio ξ = T / t , are also identified as critical factors for enhancing the second-order NLO responses.