Conformation-dependent molecular optical nonlinearities in bisnaphthol derivatives

The ‘vibrational technique’ has been applied to measure first-order and second-order molecular nonlinearities β and γ of bisnaphthol derivatives constrained in a non-coplanar conformation and containing electron donor and acceptor groups. The vibrational chemical shifts observed in the IR spectra of model molecules show that in solution a conformational equilibrium exists with only one conformer favouring electron injection by the oxygen lone pair into the aromatic ring, with the consequent increase of the weight of a quinoid structure. It follows that the hyperpolarizabilities are conformationally dependent and may show temperature dependence. The same spectroscopic approach to the bisnaphthol derivatives shows that the conformationally dependent role of the electron injecting property of the oxygen is more relevant than the inter-ring electron hopping between pz orbitals.