Selective and sensitive detection of CO₂ using phosphorescent iridium(<scp>iii</scp>) complexes containing 1,10-phenanthroline derivatives as neutral ligands

Based on the neutral ligands of 2-phenyl-1<i>H</i>-imidazo[4,5-<i>f</i>][1,10]phenanthroline (pip) and 2-(4-(trifluoromethylphenyl)-1<i>H</i>-imidazo[4,5-<i>f</i>][1,10]phenanthroline) (tfpip), iridium(III) complexes [(F₄ppy)₂Ir(pip)](PF₆) (1, F₄ppy = 2-(4-fluorophenyl)pyridine), [(F₄piq)₂Ir(pip)](PF₆) (2, F₄piq = 3-(4-fluorophenyl)isoquinoline), [(F₄ppy)₂Ir(tfpip)] (PF₆) (3), [(F₄piq)₂Ir(tfpip)](PF₆) (4), [(F_3,5ppy)₂Ir(pip)](PF₆) (5, F_3,5ppy = 2-(3,5-difluorophenyl)pyridine), [(F_3,5piq)₂Ir(pip)](PF₆) (6, F_3,5piq = 3-(3,5-difluorophenyl)isoquinoline), [(F_3,5ppy)₂Ir(tfpip)](PF₆) (7) and [(F_3,5piq)₂Ir(tfpip)](PF₆) (8) were synthesized and characterized. Complexes 1-8 exhibited yellow-green and red luminescence (<i>λ</i> = 528-588 nm) in acetonitrile solution. The photoluminescence quantum yields (PLQYs) of complexes 1-8 ranged from 0.44 to 0.80, and complex 7 with yellow-green emission had the best PLQY of 0.80. An electrochemical study and density functional theory (DFT) calculations showed that the luminescence characteristics of the complexes are mainly controlled through the modification of cyclometalated ligands. Upon addition of CH₃COO^- to the solution of complexes 1 and 5, the luminescence intensity of 1 and 5 was quenched by 80% and 91%. After bubbling CO₂ gas into the solution of iridium complexes containing CH₃COO^-, complexes 1 and 5 both showed turn-on phosphorescence responses with high sensitivity, excellent selectivity, good reversibility and a low detection limit as low as 22.8 μM.