Structure–property correlation in static electronic second-order hyperpolarizabilities of centrosymmetric squaraines Original Research Article

A relationship between the static electronic second-order hyperpolarizabilities, γ, of centrosymmetric 2,4-diphenyl-squaraine derivatives and their ground-state geometric parameters, quinonoid–benzenoid characters (QBC), was established on semiempirical MO calculations. The QBC values, which are sensitive to the geometric variations when electron-donating or electron-withdrawing groups are attached to the benzene rings, relate closely to the signs and magnitudes of γ values. With an increasing QBC, γ decreases monotonously from positive to negative. It is found that a squaraine may exhibit negative or positive third-order optical nonlinearity depending on its molecular structure determined by the substituents at both terminals. The negative γ can be significantly enhanced by ortho-hydroxys at the benzene rings. Our results are supported by some experimental evidences and are rationalized using three-level model combining with valence-bond charge-transfer (VB-CT) model.