Theor Chim Acta (1993) 87:175-194 Theoretica Chimica Acta © Springer-Verlag 1993 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 Aiain Botrel 1, Bertrand Illien 1, P~ter Rajczy 1'*, Isabelle Ledoux 2, and Joseph Zyss 2 I Laboratoire de Physicochimie Analytique et Th~orique, Ecole Nationale Sup6rieure de Chimie de Rennes (E.N.S.C.R.), avenue du G6n~ral Leclerc, F-35700 Rennes, France 2 D~partement d'Electronique Quantique et Mol6culaire, FRANCE TELECOM, Centre National d'Etudes des T616communications, Paris B, 196, avenue Henri Ravera, BPI07, F-92225-Bagneux Cedex, France Received February 25, 1993/Accepted April 29, 1993 Summary. 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 l(nn*) 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 contribu- tion to the quadratic nonlinear susceptibility estimated and shown to be the main feature. Theoretical and experimental solvent effects on rc ~ re* absorption bands are discussed. MNDO-PM3 finite-field first-order hyperpolarizabilities are com- pared 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. Key words: 5-Phenyl-3H-1,2-dithiole-3-(one or thione) - Hyperpolarizability - Dipole moment - Charge transfer - Solvent effect 1 Introduction The large scale development of theoretical and experimental studies, at the micro- scopic and macroscopic scale, of the nonlinear optical (NLO) properties of organic materials has for the past twenty years been driven by some specific advantages of these materials with respect to inorganic ones: high NLO efficiency, processability, strong flexibility of the molecular structure, short time NLO response in paramet- ric and to some extent in resonant regimes [1-3]. Incorporation and orientation of Correspondence to: A. Botrel * Permanent address: Department of Atomic Physics, Roland Ertnrs University, Budapest, Hungary