Analysis of spatial contribution to the second hyperpolarizabilities of π-conjugated systems involving sulfur atoms

We study the third-order optical nonlinearity for some interesting pi-conjugated systems involving sulfur (S) atoms. First, we investigate the static second hyperpolarizabilities (gamma) for 1,6,6a-tritkiapentalene (TTP) and its donor (D)- and acceptor (A)-disubstituted model species by using ab initio molecular orbital and density functional methods. By using the second hyperpolarizability density analysis, which can elucidate spatial contributions of electrons to gamma, these molecules are found to exhibit interesting differences in contributions of unusual binding structure, i.e., S-S-S bridged structure, to the longitudinal gamma. Secondly, we investigate the static gamma for another pi conjugated system involving sulfur atoms, i.e., cation radical state of tetrathiafulvalene (TTF). This molecule is found to provide unique third-order optical nonlinearity, i.e., large negative gamma. We finally discuss a possibility of multiproperty aggregate systems combining the unique third-order optical nonlinearity and high electrical conductivity.