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

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