Intermolecular coupling enhancement of the molecular hyperpolarizability in multichromophoric dipolar dendrons

Nonlinear optical dendritic macromolecules, called azobenzene dendrons, were synthesized, and their conformational properties and molecular nonlinear optical properties were clarified by second-order nonlinear optical measurement. The synthesized molecules were modified by introducing 1-15 numbers of azobenzene branching units as the nonlinear optical chromophore and by placing aliphatic chains at the end the dendritic chains. In these topologically complex molecules, each chromophore contributed coherently to the macroscopic nonlinear optical activity. The first-order molecular hyperpolarizability of the azobenzene dendron having 15 chromophoric units was measured to be 3010 x 10^-30 esu using the hyper-Rayleigh scattering method. This level of molecular hyperpolarizability was much higher than that for an azobenzene monomer (150 x 10^-30 esu). The polarized nonlinear optical measurement provided structural information on the dendrons and indicated that each chromophore was oriented noncentrosymmetrically along the molecular axis to become a cone shape rather than a spreading or spherical shape. This structure gave rise to a large electronically dipolar macromolecule system, in which each chromophoric unit coherently contributed to the second harmonic generation.