Electric-Field-Induced Second Harmonic Generation Nonlinear Optic Response of A 4 β-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 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.