Electric-Field-Induced Second Harmonic Generation Nonlinear Optic Response of A(4) beta-Pyrrolic-Substituted Zn-II Porphyrins: When Cubic Contributions Cannot Be Neglected

In this work, we have prepared a series of A(4) Zn-II porphyrins, carrying in the beta-pyrrolic-position one or two pi-delocalized ethynylphenyl moieties with a -NO(2 )acceptor or a -NMe2 donor pendant, and measured their second-order NLO response in CHCl3 solution at 1907 nm via the electric-field-induced second harmonic generation (EFISH) technique. For some of these compounds, we have recorded an unexpected sign and/or absolute value of mu beta(1907). Since their sterically hindered A(4) structure should ensure the lack of significant aggregation processes in solution, we explain such anomalous EFISH results by invoking a non-negligible contribution of the electronic cubic term gamma(-2 omega; omega, omega, 0) to gamma(EFISH), as supported by a qualitative evaluation of the third-order response through the measure of the cubic hyperpolarizability (gamma(THG)) and by computational evidence.