An ab initio time-dependent Hartree-Fock study of solvent effects on the polarizability and second hyperpolarizability of polyacetylene chains within the polarizable continuum model

The solvent effects upon the longitudinal polarizability (α_L) and second hyperpolarizability (γ_L) of small all-trans polyacetylene (PA) chains ranging from C₂H₄ to C₁₀H₁₂ have been evaluated at the time-dependent Hartree-Fock (TDHF) level within the framework of the polarizable continuum model. The solvent effects, which correspond to the solvent-induced modifications of the solute properties, result in large increases of the linear and nonlinear responses even for solvents with low dielectric constants. When the dielectric constant is increased, the α_L values tend to saturate at values 30%-40% larger than in vacuo, whereas for γ_L it ranges from 100% to 400% depending upon the nonlinear optical process and the length of the PA chain. These solvent-induced α_L and γ_L enhancements can partially be accounted for by the corresponding decrease of the energy of the lowest optically-allowed electronic excitation. The geometrical parameters of the ground state of the PA chains are almost unaffected by the solvent. This shows that the solvent effects are mainly of electronic nature. In addition, the local field factors, which relate the macroscopic or Maxwell field to the field experienced by the solute, tend towards unity with increasing chain length for the longitudinal PA axis.