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

Abstract

The combination of α-acetamidocinnamic acid (HACA) and different N, N,N and N,N,N pyridines (dPy) leads to crowded Zn(ii) metal centers. The increasing bulkiness competes with the chelation enhanced effect (CHEF) in the resulting quantum yields.

Influence of a series of pyridine ligands on the structure and photophysical properties of Cd(ii) complexes

“…70 Finally, compound 2 shows the lowest Φ s , probably because of the presence of two coordinated water molecules, generating unwanted quenching of its fluorescence (Table 2). 71 The comparison of these results with that from our previous work based on Zn(II) complexes 31 shows that the higher ionic radius of Cd(II) (0.95 Å) than that of Zn(II) (0.74 Å) 23 can avoid the negative effects generated by steric crowding around the metal centers and maximize the positive effects of CHEF, obtaining complexes with enhanced fluorescent efficiencies.…”

“…Geometry a S value ata value (°) Twist angles b (°) other Zn(II) CPs containing ACA and py or 4-phpy, observing a limit outer atom angle of 80.86°. 30,31 In 2, the increase in the outer atom angle up to 83.70°does not allow the coordination of the carbonyl oxygen atom of ACA. Instead, two water molecules are coordinated to the Cd(II) center, providing additional stabilization to the structure arising from the strong intramolecular interactions of the coordinated water molecules.…”

“…Indeed, the formation of a dimeric structure in 3 is probably influenced by the intra-and intermolecular N-H⋯O COO interactions, which force the acetamide moieties to point inside the dimer, differently from previous compounds described in the literature. 30,31 In addition, this change in position also influences its supramolecular scaffold given that the nonusual position of the acetamide moieties approach the methyl groups of ACA to the 2,2′-bipy ligands, avoiding the possibility to form interactions involving the aromatic rings such in 1, 2, 4 and 5 (Fig. 7).…”

“…Finally, the coordination of terpy in 5, being a bulkier ligand and presenting an outer atom angle of 217.36°, only allows the formation of the monomeric specie as in its Zn(II) analogue. 31 Photophysical properties UV-vis spectroscopy. All the samples were dissolved in MeOH and their UV-vis spectra recorded at 298 K. Additive measurements were performed for all the compounds and ligands in the concentration range of ∼1 × 10 −9 to 1 × 10 −4 M to select the optimal concentration at which aggregation does not occur, thus avoiding aggregation caused quenching (ACQ) processes (ESI: † Fig.…”

“…In addition, the photophysical properties of these compounds were analyzed, observing the dominant impact of steric crowding over CHEF as the size of the coordinated dPy increased, which was reflected in the Φ values. 31 Following this study and aiming to prove the suitability of Cd(II) complexes as efficient fluorescent compounds, herein, we studied the impact of adding a series of N-donors (pyridine, py; 3-phenylpyridine, and 3-phpy), N^N-donors (2,2′-bipyridine, 2,2′-bipy; 1,10-phenanthroline, and 1,10phen) and N^N^N-donor (2,2′:6′,2″-terpyridine, terpy) ligands on the structure and photophysical properties of Cd(II) complexes incorporating ACA in their structure. The increasing denticity of the selected dPy was expected to favor the formation of chelate coordination modes, maximizing CHEF.…”