The influence of molecular structure on collision radius for optical sensing of molecular oxygen based on cyclometalated Ir(iii) complexes

Three triphenylamine (TPA) substituted cyclometalated Ir(iii) complexes IrA1, IrA2, and IrA3 based on Ir(ppy)(3) were synthesized and applied as phosphorescent probes for the monitoring of molecular oxygen. The phosphorescence intensity of all the Ir(iii) complexes in tetrahydrofuran (THF) was gradually quenched with an increase of oxygen concentration. The increase of TPA substituents on the meta-position of 2-phenylpyridine (IrA1-IrA3) gradually improved the oxygen sensitivity of cyclometalated Ir(iii) complexes. IrA3 showed the highest oxygen sensitivity in THF with a KappSV of 204.8 bar(-1) and a limit of detection (LOD) of 0.27 mbar. The relationship between molecular structure and the collision radiuses (sigma) of all the Ir(iii) complexes has been investigated on the basis of the Demas model and the fundamental expression of luminescence quenching systems by oxygen. The ratio of collision radiuses are sigma(IrA1)/sigma(Ir(ppy)3) = 1.27 +/- 0.05, sigma(IrA2)/sigma(Ir(ppy)3) = 1.72 +/- 0.10, and sigma(IrA3)/sigma(Ir(ppy)3) = 2.13 +/- 0.07, respectively. The introduction and increase of TPA substituents can obviously increase the collision radiuses of cyclometalated Ir(iii) complexes which leading to potential oxygen sensitivity. And the incremental effect of collision radiuses caused by the introduction of TPA substituents resulted in outstanding oxygen sensitivity of IrA3. The results demonstrate for the first time evidence between molecular structure and oxygen sensitivity of the emitters for optical sensing.