Synthesis, crystal structure and INDO calculation of the molecular hyperpolarizability of a highly polarizable merocyanine dye

Abstract

A zwitterionic cyanine dye of formula HOCH₂CH₂(NC₅H₄)CHCH(C₆H₄)O (M₂) is reported with its crystal structure. Crystal data: monoclinic (P2₁), a=15.038, b=5.385, c=17.301 Å, β=111.68, Z=2. The nonlinear optical properties are investigated through solvatochromism and found to be very similar to those of a related cyanine dye (M₁) having an extremely high molecular hyperpolarizability (β). INDO calculations performed on both crystal structures indicate small modifications in the electronic properties, but which result in a large modification of the hyperpolarizability in the solid: β(M₂)=2.5×β(M₁). In addition, the hydroxyethyl substituent can be employed for engineering the chromophore into acentric environments. Therefore, increasing NLO responses may be expected at both the molecular and macroscopic levels with such systems.