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In this research, two 4-styrylpyridines carrying an acceptor –NO₂ (<b>L1</b>) or a donor –NMe₂ group (<b>L2</b>) were axially coordinated to A₄ Zn^II porphyrins displaying in 5,10,15,20 <i>meso</i> position aryl moieties with remarkable electron withdrawing properties (pentafluorophenyl (<b>TFP</b>)), and with moderate to strong electron donor properties (phenyl (<b>TPP</b>) < 3,5-di-<i>tert</i>-butylphenyl (<b>TBP</b>) < bis(4-<i>tert</i>-butylphenyl)aniline) (<b>TNP</b>)). The second order nonlinear optical (NLO) properties of the resulting complexes were measured in CHCl₃ solution by the Electric-Field-Induced Second Harmonic generation technique, and the quadratic hyperpolarizabilities β_λ were compared to the Density Functional Theory (DFT)-calculated scalar quantities β_||. Our combined experimental and theoretical approach shows that different interactions are involved in the NLO response of <b>L1</b>- and <b>L2</b>-substituted A₄ Zn^II porphyrins, suggesting a role of backdonation-type mechanisms in the determination of the negative sign of Electric-Field-Induced Second Harmonic generation (EFISH) β_λ, and a not negligible third order contribution for <b>L1</b>-carrying complexes.