Theoretical studies on second hyperpolarizabilities for cation radical states of tetrathiafulvalene and tetrathiapentalene

The static longitudinal second hyperpolarizabilities (γ) for cation radical states of symmetric alternant and condensed-ring conjugated systems involving sulfur atoms, i.e., tetrathiafulvalene (TTF) and tetrathiapentalene (TTP), are calculated by using a density functional (B3LYP) method with extended basis sets. The spatial contributions of electrons to the γ are investigated by using the hyperpolarizability density analysis. These molecules are found to provide unique third-order optical non-linearity, i.e., negative γ. We also discuss a possibility of multi-property aggregate systems combining the unique third-order optical non-linearity and high electrical conductivity.