Advances in pH-Sensitive Luminescent Transition Metal Complexes for Theranostic Applications

pH sensitive luminescent transition metal complexes are emerging as next generation molecular probes for sensing, bioimaging, and cancer theranostics. Small variations in protonation state can markedly influence their photophysical behavior by modulating charge-transfer processes, excited-state energetics, and noncovalent interactions, resulting in pronounced and often reversible changes in emission intensity or wavelength. In this review, we examine recent advances in the design of pH-responsive luminescent transition metal complexes and discuss the mechanistic origins of their pH sensitivity, including protonation-deprotonation equilibria, hydrogen-bonding effects, and aggregation-induced phenomena. Particular emphasis is placed on ruthenium(II) and related complexes, which offer long-lived excited states, structural robustness, and wide tunability through rational ligand design. We critically analyze representative systems across different metal platforms with respect to their pK_a ranges, emission responses, and suitability for biological environments. By highlighting both recent progress and existing limitations, we aim to clarify design strategies that are most likely to translate into reliable tools for real-time pH sensing, high-resolution imaging, and multifunctional theranostic applications.