Second-order nonlinear optical properties of zwitterionic chromophores

The molecular linear and second-order nonlinear optical (NLO) properties of a series of donor (D)-π-acceptor (A) merocyanine molecules have been studied in three solvents, dimethylformamide (DMF), tetrahydrofuran (THF), and chloroform (CHCl 3 ). All the compounds have a cyanodicyanomethylidenedihydrofuran electron acceptor system with either a pyridinylidene or quinolinylidene donor group. In high polarity solvents the molecules with a quinolinylidene donor have larger first hyperpolarizabilities than those with a pyridinylidene donor, while the opposite is true in low polarity solvents. The molecules under investigation have an aromatizable donor unit, which leads to a high degree of charge separation in the ground-state; as a result they have a strong tendency to aggregate. To minimize these interactions arene-rich bulky groups have been introduced in a number of these compounds.