Molecular design for squaraines with large positive or negative third-order optical nonlinearity

Abstract

The second hyperpolarizabilities γ have been investigated for a series of centrosymmetric squaraines by using semiempirical finite-field (FF) and time-dependent Hartree–Fock (TDHF) methods. The calculation shows that the squaraines dyes may exhibit significant negative or positive γ values, depending on their molecular structures. By choosing 2,4-diphenyl-squaraine as a prototype, the para and/or ortho substitution effect on the magnitude and sign of γ were investigated. The dye with powerful electron-donating groups at the para positions exhibits a large negative γ value, while the less powerful donors lead to a small γ value. When the donors are very weak or replaced by electron-withdrawing groups, then γ may become positive. The dye with powerful electron-withdrawing groups at the para positions may exhibit a large positive γ value. The introduction of ortho hydroxy groups helps to polarize the carbonyl groups, making the central acceptor more powerful in its electron-withdrawing ability and enhancing greatly the negative γ values.