Molecular Design of Pyridinium Betaines for Second-Order Non-linear Optical Materials

Abstract

The present study describes a new approach to the molecular design for the second -order non-linear optical (NLO) material by the use of heterocyclic betaines. The new approach is based on a short-range charge transfer from the charged electron donor group to the charged electron acceptor group linked with directly, which will cause a large difference between the dipole moments in the excited and the ground states. Based on the enhanced charge separation between the pyridinium and benzimidazole rings, the betaines can be represented as a D^- (donor)-A⁺ (acceptor) system having a very large ground -state dipole moment in the direction from D^- to A⁺ The inversion of the dipole moment direction upon electronic excitation is the most remarkable feature of the heterocyclic pyridinium betaines. This nature of the betaines makes it possible to manipulate the ground-state dipole moment and molecular hyperpolar-izabilities (β). The ab initio and INDO/S molecular orbital calculations have shown that the reduction of the ground-state dipole moment, and the enlargement of the excited-state dipole moment and β would be achieved by a D^-A⁺-π-A system, in which a traditional A is introduced through a π-conjugation path attached to At On the other hand, the enlargement of the ground-state dipole moment and the reduction of β was achieved by a D^-A⁺-π-D system. It is shown from the measurement of β with the hyper-Rayleigh scattering technique that this concept for manipulation of the dipole moments and β can be used to construct more efficient second-order NLO materials based on the heterocyclic pyridinium betaine structures.