AN AB-INITIO TIME-DEPENDENT HARTREE-FOCK STUDY OF SOLVENT EFFECTS ON THE POLARIZABILITY AND 2ND HYPERPOLARIZABILITY OF POLYACETYLENE CHAINSWITHIN THE POLARIZABLE CONTINUUM MODEL

The solvent effects upon the longitudinal polarizability (alpha(L)) an d second hyperpolarizability (gamma(L)) of Small all-trans polyacetyle ne (PA) chains ranging from C2H4 to C10H12 have been evaluated at the time-dependent Hartree-Fock (TDHF) level within the framework of the p olarizable continuum model. The solvent effects, which correspond to t he solvent-induced modifications of the solute properties, result in l arge increases of the linear and nonlinear responses even for solvents with low dielectric constants. When the dielectric constant is increa sed, the alpha(L) values tend to saturate at values 30%-40% larger tha n in vacuo, whereas for gamma(L) it ranges from 100% to 400% depending upon the nonlinear optical process and the length of the PA chain, Th ese solvent-induced alpha(L) and gamma(L) enhancements can partially b e accounted for by the corresponding decrease of the energy of the low est optically allowed electronic excitation. The geometrical parameter s of the ground state of the PA chains are almost unaffected by the so lvent. This shows that the solvent effects are mainly of electronic na ture. In addition, the local field factors, which relate the macroscop ic or Maxwell field to the field experienced by the solute, tend towar ds unity with increasing chain length for the longitudinal PA axis. (C ) 1998 Elsevier Science B.V. All rights reserved.