Ab initio Hartree-Fock investigation of π-conjugated compounds presenting large βv/βe ratio:: merocyanines

CHF/6-311G* calculations of the first electronic and vibrational hyperpolarizabilities reveal that merocyanines present a substantial beta(v)/beta(e) ratio under their quinonoid nonpolar form. It originates from a large vibrational first hyperpolarizability whereas its electronic counterpart is small for this class of push-pull pi-conjugated molecules. The transition from the quinonoid to the aromatic configuration is accompanied by an increase of beta(e) and a decrease of the beta(v)/beta(e) ratio as well as by a approximate to 180 degrees rotation in the plane of the molecule of beta(e) and beta(v) with respect to the molecular frame. Our results support the recent experimental discovery that antiparallel aggregation of aromatic and quinonoid forms of merocyanine is energetically favoured and that their first hyperpolarizabilities, which combine constructively, present both electronic and non purely electronic origins.