Origin of transition dipole-moment polarizability and hyperpolarizability in hydrazones

The origin of transition dipole-moment polarizability and hyperpolarizability in organic materials was investigated using electroabsorption (EA) spectroscopy. It is well established that the following three nonlinear optical properties can be derived from the analysis of EA spectrum; (1) the transition dipole-moment polarizability and hyperpolarizability $(D$ factor), (2) the change of polarizability upon photoexcitation (Δα), and (3) the change of static dipole moment upon photoexcitation (Δμ). We prepared hydrazones derived from a series of biological polyenes to investigate the physical meaning of the D factor. The hydrazones derived here have two features; (1) they are polar carotenoid analogs with a large third-order nonlinear polarizability, and (2) they have two transition dipoles nearly collinear with the molecular axis. The number of conjugated C=C bonds was systematically varied and then the relationship between their structures and nonlinear optical properties of the molecules was investigated. We found that the D factor was the main contributor to the EA spectra of the present set of hydrazones and it had a nearly constant value regardless of the lengths of polyene fragments investigated. Based on the sum rule of the transition dipole moments, the D factor was attributed to charge transfer between the neighboring transition dipoles in the excited states. We conclude that the presence of more than two transition dipole moments in one molecule and the intramolecular charge transfer between them, are necessary for the D factor to be predominant in the nonlinear optical process of organic materials.