Tuning Photophysical Properties by p-Functional Groups in Zn(II) and Cd(II) Complexes with Piperonylic Acid

Francisco Sánchez-Férez,Joaquim Mª Rius-Bartra,Josefina Pons,Mercè Font-Bardia,Teresa Calvet,José A Ayllón

doi:10.3390/molecules27041365

Francisco Sánchez-Férez, Joaquim Mª Rius-Bartra + Show 4 more

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https://doi.org/10.3390/molecules27041365

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Abstract

Aggregation between discrete molecules is an essential factor to prevent aggregation-caused quenching (ACQ). Indeed, functional groups capable of generating strong hydrogen bonds are likely to assemble and cause ACQ and photoinduced electron transfer processes. Thus, it is possible to compare absorption and emission properties by incorporating two ligands with a different bias toward intra- and intermolecular interactions that can induce a specific structural arrangement. In parallel, the π electron-donor or electron-withdrawing character of the functional groups could modify the Highest Ocuppied Molecular Orbital (HOMO)–Lowest Unocuppied Molecular Orbital (LUMO) energy gap. Reactions of M(OAc)2·2H2O (M = Zn(II) and Cd(II); OAc = acetate) with 1,3-benzodioxole-5-carboxylic acid (Piperonylic acid, HPip) and 4-acetylpyridine (4-Acpy) or isonicotinamide (Isn) resulted in the formation of four complexes. The elucidation of their crystal structure showed the formation of one paddle-wheel [Zn(μ-Pip)2(4-Acpy)]2 (1); a mixture of one dimer and two monomers [Zn(µ-Pip)(Pip)(Isn)2]2·2[Zn(Pip)2(HPip)(Isn)]·2MeOH (2); and two dimers [Cd(μ-Pip)(Pip)(4-Acpy)2]2 (3) and [Cd(μ-Pip)(Pip)(Isn)2]2·MeOH (4). They exhibit bridged (1, µ2-η1:η1), bridged, chelated and monodentated (2, µ2-η1:η1, µ1-η1:η1 and µ1-η1), or simultaneously bridged and chelated (3 and 4, µ2-η2:η1) coordination modes. Zn(II) centers accommodate coordination numbers 5 and 6, whereas Cd(II) presents coordination number 7. We have related their photophysical properties and fluorescence quantum yields with their geometric variations and interactions supported by TD-DFT calculations.

Highlights

The correlation between composition, structure and properties has always been the foundation of materials design
They commonly present fluorescence quenching associated with intramolecular charge transfer (ICT) and intra-ligand charge transfer (ILCT) transitions, photoinduced electron transfer (PET) processes, or aggregation-caused quenching (ACQ) by exciplex formation that promote non-radiative decays
As a continuation of this work, in this paper, we have studied the reaction between M(OAc)2 (M = Zn(II) and Cd(II); OAc = acetate) with HPip and two N-donor ligands (dPy = 4-acetylpyridine (4-Acpy) and isonicotinamide (Isn)), considering that 4-Acpy has a good electron withdrawing group and Isn has a better electron donor group

Summary

Introduction

The correlation between composition, structure and properties has always been the foundation of materials design. Pyridine based fluorophores have been developed as a fitting family of ligands with which to design fluorescent complexes They excel at being sensitive to electronic perturbation and are capable of coordinating both soft and hard metal ions [6]. They commonly present fluorescence quenching associated with intramolecular charge transfer (ICT) and intra-ligand charge transfer (ILCT) transitions, photoinduced electron transfer (PET) processes, or aggregation-caused quenching (ACQ) by exciplex formation that promote non-radiative decays. Diamagnetic metal ions as Zn(II) and Cd(II) are able to coordinate with the fluorophore and present outstanding performance by minimizing relaxation via non-radiative decays [7,8] Their fully populated d orbitals do not partake in electronic transitions and, ligand centered π. Being coordinated with N- and O-donor ligands, they tend to construct highly blue emissive materials [10]

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