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

The second hyperpolarizabilities gamma 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 gamma 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 gamma were investigated. The dye with powerful electron-donating groups at the para positions exhibits a large negative gamma value, while the less powerful donors lead to a small gamma value. When the donors are very weak or replaced by electron-withdrawing groups, then gamma may become positive. The dye with powerful electron-withdrawing groups at the para positions may exhibit a large positive gamma 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 gamma values.