Effect of pH on the photophysical properties of two new carboxylic-substituted iridium(iii) complexes

Two cyclometalated iridium(iii) complexes have been prepared based on 2-(4-diphenylamino-phenyl)-quinoline and incorporating carboxylic acid ethyl ester (-COOC₂H₅, (TPAQCE)₂Irpic and carboxylic acid (-COOH, (TPAQCOOH)₂Irpic) substituents at the 4-position of the quinoline ligand, respectively. The absorption, emission and ¹H NMR spectra of (TPAQCE)₂Irpic and (TPAQCOOH)₂Irpic under alkaline or acidic conditions demonstrate that they respond to the pH of the surrounding solvent environment. The deprotonation of the carboxylic acid group significantly blue-shifts the metal-to-ligand charge transfer absorption band of (TPAQCOOH)₂Irpic by 48 nm and enhances the emission quantum-yield in DMSO. In addition, ¹H-NMR titration reveals that (TPAQCOOH) ₂Irpic is deprotonated into negatively charged (TPAQCOO ^-)₂Irpic in free DMSO-d₆ solution, and the acid-induced N^O ancillary ligands cleavage or replacement in (TPAQCOOH) ₂Irpic could be ignored. A water-soluble near-neutral optical pH probe (TPAQCOOH)₂Irpic with pK_a of ∼7 is also reported. In aqueous buffer, (TPAQCOOH)₂Irpic possesses an obvious emission response with an excellent linearity in the pH range of 6.50-8.00, showing a promising application in bioprocessing.