To find out why azobenzene dendrimers, which are branched three-dimensional macromolecules with a branch point at each of the donor-acceptor-functionalized azobenzene monomeric units, have large first hyperpolarizabilities, we used theoretical calculations for three types of dimers: (1) two azobenzenes linked covalently head-to-tail; (2) linear dimers where the two azobenzenes are nearly superimposable on the covalently bonded dimers but are not covalently linked; (3) parallel dimers where the two azobenzenes are roughly side by side. We found from ab initio calculations that the optimized geometry is roughly linear for the first type and is nearly parallel and slightly staggered for the third type. We also found that the first hyperpolarizabilities for the linear dimers are larger than those obtained from the sum of the hyperpolarizability tensors for the individual monomers and that those for the parallel dimers are smaller. These findings led us to conclude that the noncentrosymmetric arrangement of the monomeric units forming the azobenzene dendrimers, which causes the large first hyperpolarizabilities, can be partly ascribed to the interaction between the monomeric units and the first hyperpolarizability is increased by the interaction between monomeric units arranged in series. In addition, calculations based on electrical interaction models showed that the changes of the first hyperpolarizability that are due to the interaction between the monomeric units can be qualitatively explained in terms of classical electrostatic interaction, whereas quantum mechanical interactions and/or a more complete electrostatic model need to be considered in accounting for the extent of these changes quantitatively.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry