Influence of pH on the emission properties, reversibility, and acid–base behavior of a water-soluble Ir(III) complex

This study describes the synthesis, purification, and characterization of the water-soluble iridium(III) complex [Ir(fppy)₂(bqdc)], where bqdc (2,2′-biquinoline-4,4′-dicarboxylic acid) enhances aqueous solubility. The complex was characterized by HRMS, ESI-TOF, FTIR, ¹H NMR, UV-Vis, and photoluminescence spectroscopy, confirming its structure and high purity. Photophysical studies showed intense red emission centered at 617 nm. The pH-dependent luminescence behavior was investigated in Britton–Robinson buffer (pH 2.0–12.0), revealing maximum emission intensity between pH 6.5 and 9.0 and progressive quenching under acidic conditions. A Boltzmann sigmoidal fit of the emission data provided a pKa value of 3.85 ± 0.03. The complex also exhibited reversible emission changes upon protonation and deprotonation, demonstrating its stability and reproducibility. These findings highlight the potential of this water-soluble Ir(III) complex as a luminescent sensor for acidic pH in biological environments.