Theoretical predictions of first-order hyperpolarizabilities of azobenzene dendrimers

The geometric structures of four generations of novel azobenzene dendrimers in chloroform solution and a model of monolayer Langmuir-Blodgett (LB) film have been calculated by using a molecular dynamics (MD) method. On the basis of the MD-calculated structures of the dendrimers, the first-order hyperpolarizabilities (β) and UV-vis absorption spectra were calculated by using the semi-empirical molecular orbital, complete neglect of differential overlap for spectroscopy-configuration interaction level of theory. It is found that the rod-shaped dendrimers have significantly large β values in the solution. The β values (for incident light with λ = 1064 nm) for the second, third, and fourth generations in chloroform are respectively 2.0, 6.5, and 9.1 times that for the first one (75 × 10^-30 esu). This trend is in good agreement with experimental results, though the experimentally observed enhancement of the second harmonic generation (SHG) activity is not reproduced. It is assumed that the β values of the dendrimers in the LB films are smaller than those in the solution because the azobenzene chromophores in the films have staggered conformations.