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

Gabriele Di Carlo,Alessandra Forni,Maddalena Pizzotti,Stefania Righetto,Francesca Tessore

doi:10.1021/acs.inorgchem.0c00451

Abstract

In this work, we have prepared a series of A4 ZnII porphyrins, carrying in the β-pyrrolic-position one or two π-delocalized ethynylphenyl moieties with a −NO2 acceptor or a −NMe2 donor pendant, and measured their second-order NLO response in CHCl3 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 μβ1907. Since their sterically hindered A4 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 γEFISH, as supported by a qualitative evaluation of the third-order response through the measure of the cubic hyperpolarizability (γTHG) and by computational evidence.

Highlights

Coordination and organometallic compounds, due to the presence of a metal center, offer in comparison to organic chromophores some interesting additional electronic features acting on their second-order nonlinear optical (NLO)response, such as rather strong charge transfer (CT)transitions, which can be tuned by working on the oxidation state of the metal and on its coordination sphere
The synthetic strategy (Scheme 1) to obtain the chromophores investigated in this contribution required to adopt building-blocks as 2bromo (1a and 1b) and 2,12-dibromo (2a) functionalized ZnII porphyrins,[21,22,31] for the preparation of the monosubstituted porphyrins (BP1, BP2, and BAP1) and of the disubstituted ones (BP3, BP4, and BP5), respectively
According to Density functional theory (DFT) calculations, providing negative γ|| = γ(−2ω; ω, ω, 0) values[28] for all the porphyrin chromophores here investigated (Table 4), we suggest that the electronic third-order term may overwhelm the positive value of the dipolar orientational term μβ|| for symmetric BP4 and BP5 and for slightly asymmetric BP2 chromophores, providing a negative γEFISH value (Table 3)

Summary

■ INTRODUCTION

Coordination and organometallic compounds, due to the presence of a metal center, offer in comparison to organic chromophores some interesting additional electronic features acting on their second-order nonlinear optical (NLO). According to DFT calculations, providing negative γ|| = γ(−2ω; ω, ω, 0) values[28] for all the porphyrin chromophores here investigated (Table 4), we suggest that the electronic third-order term may overwhelm the positive value of the dipolar orientational term μβ|| for symmetric BP4 and BP5 and for slightly asymmetric BP2 chromophores, providing a negative γEFISH value (Table 3) Such a suggestion is further supported by comparing the negative value of γ|| calculated for BAP1 (Table 4) with the relevant negative value of its measured γTHG. In the case of asymmetric monosubstituted BP2 carrying an ethynylphenyl linker with a donor pendant, we have a negative value of γEFISH, which suggests a rather low second-order NLO response This evidence is totally opposite to what reported in our previous investigation for a structurally related monosubstituted ZnII porphyrin with simple phenyl groups in the 10,15,20-meso-positions, lacking the steric effects and electronic effects induced by more hindered and slightly donor 3,5-di-tert-butylphenyl groups.[4]. J intermolecular aggregation is well-known to produce an increase of the second-order NLO response.[17]

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES