Nonlinear optical processes of spiroconjugated dimers

Electronic and vibrational contributions to the second hyperpolarizability of spiroconjugated dimers have been calculated for different nonlinear optical processes. It is shown that the intermolecular vibrational modes that twist one monomer relative to the other significantly contribute to the gas-phase second hyperpolarizability, although not to an extent that can explain the experimentally observed order of magnitude increase of the hyperpolarizability in the dimer relative to the monomer for the degenerate four-wave mixing optical processes.