pH-Responsive colorimetric, emission and redox switches based on Ru(ii)-terpyridine complexes.

We have undertaken a thorough investigation on pH-responsive optical and redox switching behaviors of our recently reported trans form of bis-tridentate Ru(ii) luminophores, [(Hpbbzim)Ru(tpy-pvp-X)] where X = H, Me, Cl, NO, and Ph. The complexes possess two benzimidazole protons in their second coordination sphere, which became acidic upon coordinating influence of Ru and could be successively deprotonated with the increase of pH. The effect of pH on photophysical and electrochemical behaviours of the complexes was thoroughly studied. Substantial quenching of emission together with the red-shift of both absorption (color change) and emission bands is noticed for all complexes upon dissociation of NH protons. Absorption vs. pH data were employed for determination of ground-state pK values, while excited-state pK (pK*) values were estimated by employing the Förster cycle based equation. The electronic nature of X induces a small but finite effect on the pK values and a linear correlation is found by plotting pKvs. Hammett σ parameters of X. Proton-coupled electrochemical behaviours were investigated within the pH range of 1-10. From the Evs. pH plot, acid dissociation constants in different protonation states of the complexes were estimated in both Ru and Ru states. Compared with their protonated forms which exhibit reversible oxidation within 0.91-0.95 V, the oxidation potential of the doubly deprotonated forms shifted remarkably to the cathodic region (0.61-0.66 V). In essence, the present complexes act as potential pH-responsive colorimetric, emission and redox switches.