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

[Image: see text] 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.