Cooperative effects in static polarizabilities and second hyperpolarizabilities of hydrogen-bonded 4-pyridones

We investigate the intermolecular interaction effects on the static polarizabilities (α) and second hyperpolarizabilities (γ) of hydrogen (H)-bonded 4-pyridones, which have been recently found to produce extraordinarily strong H-bonds. The longitudinal α and γ are calculated by the finite-field approach at the HF, MP2 and DFT levels of theory. The increase ratio of γ per monomer for the increase of the number of monomers is found to be significantly enhanced compared to that of α per monomer. This feature suggests the presence of strong cooperative effects on γ originating in the covalent interaction between polarizable π-systems through H-bonds in the virtual excitation processes.