pH-driven optical changes of platinum(ii) complexes having carboxy-appended salophen ligands

Platinum(ii) complexes with salophen ligands bearing carboxy substituents at different positions, [Pt{(COOH)(n)-salophen}] (n = 2 (1), 3 (2), 1 (3)), were synthesized and characterized by acquiring UV-vis and luminescence spectra. These complexes exhibited systematic variations in absorption spectra depending on the number of carboxy groups, and this effect was attributed to metal-ligand charge transfer with support from density functional theory calculations. The luminescence properties of these complexes were also correlated with structural differences. Complexes 1-3 showed systematic spectral changes by addition of organic acid and base, respectively. This is based on the protonation/deprotonation of the carboxy substituents. Furthermore, aggregation-induced spectra change was investigated in DMSO-H2O mixtures with various proportions of water. Peak shifts in the range of 95 to 105 nm occurred in the absorption spectra in conjunction with pH changes. These variations resulted from molecular aggregation and diffusion associated with protonation/deprotonation of the carboxy groups. Variations in luminescence emission intensity and peak shifts were also observed. This work provides new insights into the correlations between the optical properties of carboxy-appended molecular complexes and pH changes and will assist in the future design of pH sensing devices based on molecular metal complexes.

Automated summary

Platinum(ii) complexes with carboxy-substituted salophen ligands at different positions were synthesized and their optical properties were studied by UV-vis and luminescence spectroscopy. The complexes showed systematic variations in absorption spectra depending on the number of carboxy groups, which were attributed to metal-ligand charge transfer. The luminescence properties were also correlated with structural differences. The study also investigated aggregation-induced spectra changes in mixed solvents and observed variations in luminescence intensity and peak shifts. This research provides insights into the correlations between the optical properties of carboxy-appended molecular complexes and pH changes.