Switching nonlinear optical properties by proton transfer in hydrogen-bonded merocyanine dyes: a theoretical investigation at the semi-empirical level

Abstract

The synthesis of [H(M²)₂]⁺I⁻·3/2H₂O [M² being a zwitterionic cyanine dye of formula HOCH₂CH₂(NC₅H₄)CHCH(C₆H₄)O] is reported. The crystal structure indicates that two M² entities share one proton in the solid state. INDO calculations reveal that the protonation strongly affects the molecular hyperpolarizability β of these dyes, while PM3 calculations indicate a potential barrier of 0.76 eV for the proton transfer between (HM²)⁺ and M². The possibility of an NLO switch for this type of compound is discussed.