Phosphorescent cyclometalated Ir(iii) complexes comprising chelating thiolate ligands as pH-activatable sensors

A series of cyclometalated Ir(iii) complexes possessing the general formula of [Ir(pqe)(2)(S<^>N)], pqe = 2-phenyl-quinoline-4-carboxylic acid methyl ester, S<^>N = pyridine-2-thiolate (Spy, B1); pyrimidine-2-thiolate (SpyN, B2); 5-(trifluoromethyl)-pyridine-2-thiolate (SpyCF(3), B3); 2-thiazoline-2-thiolate (Stz, B4) and benzothiazole-2-thiolate (SBt, B5), were synthesized and characterized using several spectroscopies and analytical techniques. These compounds were photophysically investigated using UV-Vis absorption and photoluminescence spectroscopies. The complexes demonstrate strong red emissions in their CH2Cl2 solutions at 298 K but with different quantum efficiencies (B3 > B2 > B1 > B5 > B4). The emissions majorly originate from a (MLCT)-M-3 (metal to ligand charge transfer) character along with smaller contributions of (ILCT)-I-3 (intra ligand charge transfer) and L-3 ' LCT (ligand to ligand charge transfer; L = pqe, L ' = S<^>N). The esteric functional group (COOMe) on the pqe ligand can be easily converted to COO-Na+ in a hydrolysis process, resulting in the corresponding complexes [Na-2{Ir(pqc)(2)(S<^>N)}], S<^>N = Spy, C1; SpyN, C2; SpyCF(3), C3, being very water-soluble. Based on this hydrolysis, a large blue-shift is observed for the emissions of C1-C3 compared to B1-B5 which is supported by density functional theory (DFT) calculations. Also, the emission intensity of these complexes is sensitive to pH and steadily increases with the pH of solutions. The lifetime values as the other function of emission is changed by varying the pH values, wherein the graphs of lifetime/pH become quasi-linear. The slopes of the lines, as an index for the pH-sensitivity property, demonstrate the trend of C3 > C2 > C1 for the pH sensitivity of the complexes.