INTRAMOLECULAR CHARGE-TRANSFER IN 5-PHENYL-3H-1,2-DITHIOLE-3-THIONE AND 5-PHENYL-3H-1,2-DITHIOLE-3-ONE DERIVATIVE MOLECULES FOR QUADRATIC NONLINEAR OPTICS

This paper deals with the theoretical and experimental studies of new sulfur-containing NLO chromophores: 5-phenyl-3H-1,2-dithiole-3-thione and 5-phenyl-3H-1,2-dithiole-3-one derivatives; ground-state and first (1)(pi pi*) excited states characteristics are determined within the framework of MNDO-PM3 and CNDO/S-CI methods. These results are confronted with the NLO response of these molecules. The intramolecular charge transfer is identified and its contribution to the quadratic nonlinear susceptibility estimated and shown to be the main feature. Theoretical and experimental solvent effects on pi --> pi* absorption bands are discussed. MNDO-PM3 finite-field first-order hyperpolarizabilities are compared to EFISH measurements. A preliminary CNDO/S parametrization of the sulfur atom in thio-carbonyl compounds is proposed and shown to be adequate for the description of electronic spectra.