Theoretical study of solvent effects of first-order hyperpolarizabilities of nitro–azobenzene dendrimers

Abstract

The molecular structures of four generations of nitro–azobenzene dendrimers in the nonpolar solvent, carbon tetrachloride (dielectric constant, ϵ=2.2) and the polar solvents, chloroform ( ϵ=4.8) and acetonitrile ( ϵ=37.5), have been calculated using a molecular dynamics (MD) method. The first-order hyperpolarizabilities ( β) were calculated for the MD-calculated structures using the semi-empirical molecular orbital, CNDO/S-CI (complete neglect of differential overlap for spectroscopy–configuration interaction) level of theory. It was found that the dendrimers show larger β values in the polar solvents with larger dielectric constants, whose characteristics reproduce the experimental results. Although the experimentally observed enhancement of the second harmonic generation (SHG) activity that is greater than the number of chromophores is not sufficiently reproduced, the β values for the second, third, and fourth generations in acetonitrile solution are, respectively, 4.8, 8.0, and 21.7 times that of the first one (98×10 −30 esu) of the nitro–azobenzene dendrimers, whose tendency is in good qualitative agreement with the measurements compared to the azobenzene dendrimers without the nitro groups.