Photophysical and DFT studies on cycloplatinated complexes: modification in luminescence properties by expanding of π-conjugated systems

Abstract

To investigate the factors influencing the luminescent properties of cycloplatinated complexes containing aryl and SMe₂ ligands, a detailed study of the photophysical and structural properties of the complexes [Pt(p-MeC₆H₄)(ĈN)(SMe₂)], (ĈN = benzo[h]quinolate (bzq), 1, or 2-phenylpyridinate (ppy), 2), was conducted in solution and solid state at different temperatures. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) investigations were carried out on both the singlet (S₀, ground) and triplet (T₁, excited) states of these complexes for explanation of the electronic structures, absorption and emission spectra. In the solid state, these complexes display an intense phosphorescence emission which was red-shifted from 2 to 1 due to expansion of the π conjugated system in the cyclometalated bzq ligand. These complexes are strongly emissive at 77 K, giving emission peaks similar to those observed at ambient temperature. The complex 1 in deoxygenated CH₂Cl₂ solution (at low concentration) gave a phosphorescence band which was accompanied by a higher-energy fluorescence band (ascribed to ¹LC transition, centered on bzq ligand), and these bands disappeared when a concentrated solution or glassy state (77 K) of the complex is used. On the basis of theoretical calculations, the HOMO level is a mixture of platinum center and ligand orbitals, while the LUMO level is predominantly ĈN ligand based. The lowest-lying excited state responsible for the phosphorescence emission (with a significantly long lifetime) originates from mixed ³ILCT/³MLCT with some ³L′LCT excited state and the emission is red shifted in comparison to the corresponding free cyclometalating ligand.