Steric crowding of a series of pyridine based ligands influencing the photophysical properties of Zn(II) complexes

Abstract

Zn(II) complexes containing N, N,N and N,N,N pyridine (dPy) ligands tend to display improved fluorescence efficiencies in comparison with their starting ligands benefiting from the chelation enhanced effect (CHEF) and preventing photoinduced electron transfer (PET) mechanisms by the coordination of their lone pair electrons. Nevertheless, the size of Zn(II) makes steric crowding an important factor to be considered, since it can promote the elongation of the coordination bonds that weakens their strength and favors the reduction of fluorescence efficiencies through PET processes. For that reason, in this contribution we have performed a systematic study of Zn(II) compounds based on α-acetamidocinnamic acid (HACA) and a comprehensive range of dPy ligands with increasing bulkiness (pyridine (py) (1), 3-phenylpyridine (3-phpy) (2), 2,2′-bipyridine (2,2′-bipy) (3), 1,10-phenanthroline (1,10-phen) (4), 2,2′:6′,2′′-terpyridine (terpy) (5) and 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (mpterpy) (6)), which has allowed the equilibrium between CHEF and steric crowding effects to be studied. The elucidation of the six crystal structures revealed the formation of one coordination polymer (1) and five monomeric complexes (2-6). All of them have been characterized by analytical and spectroscopic techniques, and their molecular and supramolecular structures have been discussed. Furthermore, the UV-vis and photoluminescence properties have been recorded and their quantum yield (Φ) values have been calculated. Finally, it has been observed that even though the CHEF has a favorable impact on the fluorescence properties, the steric crowding of the ligands has been imposed, leading to fluorescence quenching.