Synthesis and characterization of tris(5,7-diphenyl-8-quinolinolato) aluminum(III), gallium(III), and indium(III) complexes: Effect of metal ions on the structural, photoluminescence, thermal and electrochemical properties

The 5,7-diphenyl-8-hydroxyquinoline (5,7-Phq) organic ligand was successfully synthesized via Suzuki cross-coupling reaction followed by synthesis of the novel organometallic complexes tris(5,7-diphenyl-8-quinolinolato) metal(III) [M(5,7-Phq)₃; M = Al, Ga, or In]. All synthesized M(5,7-Phq)₃ complexes were characterized using various analytical techniques. The molecular structure of In(5,7-Phq)₃ was confirmed using single-crystal X-ray crystallography, which indicates that the three 5,7-Phq ligands are arranged in a meridional configuration around the central metal In³⁺ ion, and the complex is distorted from ideal octahedral geometry. There are no significant differences in the UV–visible absorption spectra of the complexes in the tetrahydrofuran (THF) solution and they are all red-shifted compared to that of the parent Mq₃ (M = Al³⁺, Ga³⁺, In³⁺) compounds. The broad absorption band at around 417 nm is assignable to the π-π* transition. The synthesized M(5,7-Phq)₃ complexes in the solution and in the solid-state show red-shifted emission compared to that of the parent unsubstituted compounds. The decrease in the solution and the solid-state emission efficiency and intensity from Al to In complexes is attributed to the heavy atom effect. Despite the electrochemical band gap energies (E_g^ec) of the complexes determined from cyclic voltammetry being in good agreement with those reported for the unsubstituted complexes, the optical band gap energies (E_g^opt) determined from the Tauc plot method are smaller. The results of the fluorescence decay of the M(5,7-Phq)₃ complexes both in CHCl₃ solution and in solid state at 298 K show that the decays fit a bi-exponential model with two lifetimes (τ₁ and τ₂). The average values of fluorescence lifetimes of the complexes in CHCl₃ are significantly greater than those of the complexes in the solid state.