Tuning the Linear and Nonlinear Optical Properties of Pyrene-Pyridine Chromophores by Protonation and Complexation to d10 Metal Centers †

Elena Lucenti,Daniele Marinotto,Andrea Previtali,Elena Cariati,Stefania Righetto,Alessandra Forni

doi:10.3390/inorganics7030038

Abstract

The linear and second-order nonlinear optical (NLO) properties of two pyrene-pyridine chromophores, namely, 4-(pyren-1-yl)pyridine (L1) and 4-(2-(pyren-1-yl)ethyl)pyridine (L2), were investigated and modulated by performing protonation/deprotonation cycles or by complexation to d10 metal centers such as Zn(II) and Cu(I) to form the monomeric [Zn(CH3CO2)2(L1)2] complex and the [CuI(L2)]n coordination polymer, respectively. The structures of L1, L2, [Zn(CH3CO2)2(L1)2] and [CuI(L2)]n were determined by means of single-crystal X-ray diffraction studies. The NLO response, measured by the electric-field-induced second harmonic generation (EFISH) technique, was positive for both chromophores and showed an inversion of the sign after exposure to HCl vapors. This process was completely reversible and the original values were restored by simple exposure to NH3 vapors. Coordination of L1 to Zn(II) also resulted in a negative NLO response, although smaller in magnitude compared to the protonated form, due to the weak Lewis acidity of the “Zn(CH3CO2)2” fragment. The results were also interpreted on the basis of DFT/TDDFT calculations.

Highlights

In recent years compounds with second-order nonlinear optical (NLO) properties have received increasing attention due to their potential use in optical communications, signal processing, data storage and electro-optical devices [1,2]
The investigation of Cr(0), W(0), Rh(I), Ir(I), and Os(II) complexes with para-substituted pyridines has revealed the possibility of tuning the second-order NLO response of the ligand depending on the electron properties and oxidation state of the metal center or on the ligand environment [6,7]
Our previous findings on the linear and NLO optical properties of pyrene-pyridine chromophores were here extended to L1, with the pyridine directly bound to the pyrene moiety and to L2, where the dimethylene bridge connecting the aromatic systems breaks the conjugation between them

Summary

Introduction

In recent years compounds with second-order nonlinear optical (NLO) properties have received increasing attention due to their potential use in optical communications, signal processing, data storage and electro-optical devices [1,2]. In this context, pyridine-based chromophores have been extensively studied as promising building blocks for the preparation of bulk organic materials [3] and coordination networks [4], or at the molecular level for the syntheses of organometallic complexes [5]. Inorganics 2019, 7, 38 electron donor through metal-to-ligand (MLCT) charge-transfer transition or as an electron acceptor [5]. The investigation of Cr(0), W(0), Rh(I), Ir(I), and Os(II) complexes with para-substituted pyridines has revealed the possibility of tuning the second-order NLO response of the ligand depending on the electron properties and oxidation state of the metal center or on the ligand environment [6,7].

Methods

Results

Conclusion