Bis-cyclometallated Ir(iii) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands; influence of substituents and cyclometallating ligands on response to changes in pH

Abstract

Bis-cyclometallated Ir(III) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands have been synthesised. Their absorption is almost unchanged with changes in pH however the emission intensities vary by a factor of up to three and the complexes have emission pK_as in the range 8.0 to 10.0. Substituents on the pyrazole have only a minor effect on the emission pK_a. Surprisingly the complexes with phenylpyrazole cyclometallated ligands 3aL_1–3 showed an intensity decrease with increasing pH (switch off) whilst the corresponding phenylpyridine ones 3cL_1–3 showed an increase in emission intensity with increasing pH. Putting electron-withdrawing CF₃ substituents on the cyclometallating phenyls reduced the pK_a of the complexes to 6.8–7.8, thereby extending the useful pK_a range; however, in general it tended to reduce the magnitude of the change in emission intensity. Surprisingly the CF₃-substituted complexes also showed a complete reversal in the direction of the intensity change when compared to their respective unsubstituted congeners.