On the origins of large interaction-induced first hyperpolarizabilities in hydrogen-bonded π-electronic complexes

In this article we elucidate the origins of interaction-induced linear and nonlinear electro-optic properties of model hydrogen-bonded π-electronic complexes. In particular we report on contributions due to various interaction energy terms to excess dipole moments (Δμ), electric dipole polarizabilities (Δα), and first hyperpolarizabilities (Δβ), focusing on the latter. The analysis of intermolecular interaction-induced electric properties is performed for selected model systems including quasi-linear dimers of urea, diformamide, 4-pyridone, 4-nitroaniline, and the complex of hydrogen fluoride with nitroacetylene. The nature of intermolecular interactions as well as of the Δμ and Δα is very similar in all studied complexes. However, partitioning of Δβ into physically well-defined components reveals that the origins of this term, the magnitude of which is often comparable to the hyperpolarizabilities of isolated monomers, are different in each case. Our results indicate that, even though hydrogen bonding usually diminishes the nonlinear response of interacting species, the first hyperpolarizability of complexes with the nitro group acting as a proton acceptor is substantially increased, essentially due to field-induced changes of electrostatic interactions between subsystems. However, in the remaining complexes the origins of Δβ are much more involved. Even though at large intermolecular separations the origins of interaction-induced electric properties are essentially due to the field-induced electrostatic and induction interactions, in the vicinity of van der Waals minimum the overlap effects cannot be neglected since they may substantially alter the predicted excess properties or even determine their magnitude and sign. On the other hand the Δβ contribution due to dispersion interactions is usually negligible. Interestingly, the values of interaction-induced first hyperpolarizability in some cases depend strongly on the intermolecular separation in the vicinity of equilibrium geometry.