The presence of KIE hints at a possibility of excited-state proton transfer (ESPT) as the quenching mechanism for Ru-bpm. Similar ESPT processes were observed previously in Ru-bipyrazine complexes and have been assigned to the excited-state protonation of the bpm ligand. This behavior is consistent with the increased basicity of non-coordinated N atoms in Ru → bpm MLCT excited states and similar effect is expected to occur in Ru-bpm. The excited-state protonation of the bpm ligand at pH=7 is thermodynamically favorable: our Förster cycle analysis predicts that the pKa value of protonated bpm shifts from pKa=-0.7 in the ground state to pKa * =6.15 in the MLCT state (Eq. S1, Supp Info). If such protonation event occurred adiabatically, excited-state dynamics would be controlled by the excited-state lifetime of the protonated Ru(bpmH + ), which we evaluated by measuring transient absorption spectra at pH=-1, where the ground-state protonation of bpm ligands is expected to occur (Figure ). The transient absorption signal exhibits a bleach in the 550-800 nm range, consistent with the red shift of the MLCT absorption band upon protonation (Figure ). Additional excited-state absorption bands appear at 350 and 500 nm and the overall signal decays biexponentially with lifetimes of 1=1.5 and 2=7 ps. Given that the excited state of Ru-(bpmH + ) is indeed quite short-lived, the adiabatic excited-state protonation of Ru-bpm is expected to result in the observed short-lived excited states. To test the possibility that the ESPT mechanism is taking place, we collected transient absorption spectra of Ru-bpm at pH=9 (Figure ). This pH value was selected because it is more basic than the pKa*=6.15 value estimated for Ru-bpm. If ESPT were indeed taking place, the excited-state lifetime of Ru-bpm was expected to become significantly longer in the basic pH region, where the solvent is insufficiently acidic to protonate bpm ligands. Similar suppression of ESPT was observed in other Ru-bipyrimidine complexes in basic solutions, resulting in detectable emission from MLCT state of the unprotonated complex. To our surprise, we found that the spectral features and kinetic traces obtained for Ru-bpm at pH=9 are quite similar to those obtained at pH=7, clearly eliminating the possibility that the ESPT is involved as the quenching mechanism for the Ru-bpm excited state.