Influence of solvent-solute H-bonds of p-nitroanilines on the hyper-Rayleigh scattering in solutions. Preferential environment for non-linear optically active molecules in binary solvent mixtures

In previous work it was shown that for the non-linear optically active (NLO) molecules para-nitroaniline and N,N-dimethyl para-nitroaniline in pure solvents, in the absence of specific solute-solvent interactions, good linear correlations for the hyperpolarizability beta in a given solvent were obtained of the form beta= beta(gas) + a root mu(S)/V-S where beta(gas) is the first hyperpolarizability in the gas phase, mu(S) is the average gaseous dipole moment of the solvent, Vs is the molar volume of the solvent and a is a constant. When solute-solvent H-bonds are formed, the value of beta is always larger than that given by the former correlation. In this case beta = beta(gas) + gamma degrees a root mu(S)/V-S + (1 - gamma degrees)b root mu(S) where gamma degrees is the fraction of the solute molecules which, at a given time, do not form H-bonds with the solvent. In the present work this study was extended to binary solvent mixtures. The study of the two-photon scattering coefficient of NLO molecules, related to their hyperpolarizabilities, reveals the existence of strong preferential environments around the NLO molecules and is also confirmed by measurements of the wavelength of maximal absorption, lambda(max), in these mixtures. Often the preferential environment results from the formation of hydrogen bonds between the NLO molecule and one of the solvents. In this case the change of both NLO coefficients and lambda(max) with the composition of the mixtures is quantitatively predicted using the stability constants of the I-I-bonds deduced from the solubilities. However, strong non-specific dipole-dipole interactions may compete with the H-bonds to establish the preference in the environment. (C) 1998 Published by Elsevier Science B.V. All rights reserved.