Second Order Nonlinear Optical Properties of 4-Styrylpyridines Axially Coordinated to A4 ZnII Porphyrins: A Comparative Experimental and Theoretical Investigation

Stefania Righetto,Alessio Orbelli Biroli,Francesca Tessore,Gabriele Di Carlo,Alessandra Forni,Francesca Limosani

doi:10.3390/inorganics8080045

Abstract

In this research, two 4-styrylpyridines carrying an acceptor –NO2 (L1) or a donor –NMe2 group (L2) were axially coordinated to A4 ZnII porphyrins displaying in 5,10,15,20 meso position aryl moieties with remarkable electron withdrawing properties (pentafluorophenyl (TFP)), and with moderate to strong electron donor properties (phenyl (TPP) < 3,5-di-tert-butylphenyl (TBP) < bis(4-tert-butylphenyl)aniline) (TNP)). 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 βλ 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 L1- and L2-substituted A4 ZnII porphyrins, suggesting a role of backdonation-type mechanisms in the determination of the negative sign of Electric-Field-Induced Second Harmonic generation (EFISH) βλ, and a not negligible third order contribution for L1-carrying complexes.

Highlights

Since the beginning of the nineties, organometallic and coordination compounds have been extensively studied as chromophores for second order nonlinear optical (NLO) applications
Electron donor groups lead to positive values, since the second order NLO response is dominated by an intraligand
Results sheds and Discussion approach light on the different interactions involved in the second order response of L1 and L2 axially coordinated to A4 ZnII porphyrins, suggesting a role of backdonation-type interactions in the

Summary

Introduction

Since the beginning of the nineties, organometallic and coordination compounds have been extensively studied as chromophores for second order nonlinear optical (NLO) applications. Inorganics 2020, 8, 45 second order NLO properties of π-delocalized nitrogen-donor push–pull monodentate and chelating polydentate ligands (such as pyridines [4], bipyridines [5], phenanthrolines [6], and terpyridines [7,8]). When the metal center is ZnII , a remarkable enhancement of the second order NLO response is recorded due to its inductive acceptor strength and its Lewis acid properties [5,6,7,8]. The modulus of their quadratic hyperpolarizability β (which is the figure of merit of the second order NLO response) increases significantly, with a sign that depends on the nature of the substituent in para position. Electron donor groups lead to positive values, since the second order NLO response is dominated by an intraligand

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Conclusion