Electric-Field-Induced Second Harmonic Generation Nonlinear Optic Response of A4 β-Pyrrolic-Substituted ZnII Porphyrins: When Cubic Contributions Cannot Be Neglected

In this work, we have prepared a series of A 4 Zn II porphyrins, carrying in the β-pyrrolic-position one or two π-delocalized ethynylphenyl moieties with a -NO 2 acceptor or a -NMe 2 donor pendant, and measured their second-order NLO response in CHCl 3 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 μβ 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 γ(-2ω; ω, ω, 0) to γ EFISH , as supported by a qualitative evaluation of the third-order response through the measure of the cubic hyperpolarizability (γ THG ) and by computational evidence.