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 pKa 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.