The electronic transitions in Ru-bpm were studied using pH-dependent steady-state UV/Vis absorption spectroscopy. The absorption spectra of Ru-bpm are pH-independent at pH>1 and consist of one absorption bands in the visible range, centered at =520 nm (Figure ). As the pH is lowered below pH=1, the absorption band shifts to the red. The new intermediate is detected at pH=-0.73, with absorption at centered at  590 nm, while further acidification to pH=-0.88, leads to the formation of new product that absorbs at =680 nm. These spectral changes were assigned to the stepwise protonation of the uncoordinated N-centers on the bpm ligand to form Ru-bpmH + and Ru-bpmH2 2+ species. The associated pKa values were determined to be pKa1=-0.7 and pKa2=-0.8 (Figure , Supp Info) and the value are in the agreement with the previously published pKa values of similar Ru-bipyrimidine complexes. These experimental spectral changes are in excellent agreement with the calculated electronic transitions of Ru-bpm, Ru-bpmH + and Ru-bpmH2 (Figure , calculation details are shown in the Supplementary Material), which predict the visible absorption bands for Ru-bpm, Ru-bpmH + and Ru-bpmH2 2+ to appear at 470, 615 and 730 nm, respectively. These absorption bands were evaluated using difference density plots (Figure ) and show that, in each case, the electronic density shifts from Ru and terpy units toward the bpm moiety. Detailed assignment of the three lowest-energy electronic transitions (S1, S2 and S3) of Ru-bpm was achieved using charge-transfer matrix calculations. Calculated transition wavelengths and the orbitals involved in each transition are shown in Figure . The highest contribution to each transition is attributed to metal-to-ligand charge transfer (MLCT) transitions: S1 and S2 states are associated with the charge accumulation at the bpm ligand (Ru → bpm MLCT state), while the S3 state is associated with the charge accumulation at the terpy ligand (Ru → terpy MLCT state). The lower energy of the Ru → bpm MLCT states relative to Ru → terpy states is consistent with better electron-accepting ability of the N-rich bpm ligand relative to terpy ligands. Each of the three transitions also involves a small contribution due to ligand-associated transitions, such as ligandcentered (LC) bpm → bpm and terpy → terpy transitions and intra-ligand charge transfer (ILCT) terpy → bpm transitions. Finally, the S3 state also exhibits a small contribution from the Ru → Ru metal-centered (MC) transitions. The presence of these low-energy MC states is likely associated with the terpy ligands, which are known to have a small bite angle, resulting in inefficient ligand field induced splitting of metal d-orbitals. The oscillator strengths of S1 (f=0.0013), S2 (f=0.1144) and S3 (f=0.0097) transitions indicate that the excitation into the lowest-energy transition populated predominantly the S3 state. To better understand the pH-dependent photophysics of Ru-bpm, we employed femtosecond transient absorption (fsTA) spectroscopy (Figure ). At pH=7 (Figure ), transient spectra consist of the ground state bleach in the 500-650 nm range and excited-state absorption with maximum at 350 nm. These spectral features are indicative of the Ru → bpm 3 MLCT state with the band at 350 nm assigned to the absorption by the reduced bpm moiety. This absorption band is red-shifted relative to the bpm-centered absorption of the molecule in the ground state (which appears at 350 nm, Figure ), as expected for the increased electronic density on bpm upon the formation of the 3 MLCT states. Furthermore, the assignment of MLCT state is consistent with the DFT-predicted Ru → bpm MLCT transition at 485 nm (Figure ). The observed transients were found to decay biexponentially with lifetimes 1=4 ps, 2=65 ps. Such short excited-state decay in transition metal complexes is often associated with the involvement of MC transitions that are known to undergo fast excited state deactivation to the ground state or photochemical ligand loss. Since the low-lying metal-centered (MC) transition is predicted by DFT to appear at 477 nm (0.04 eV above the photogenerated MLCT state) it is quite possible that the observed fast kinetics are associated with the thermally activated MLCT → MC state transition. However, we rule out this scenario for several reasons. First, the observed lifetimes appear to be too short for MC state mediation. A very similar compound, [Ru(bpy)(terpy)(H2O)] 2+ , was found to exhibit an excited state lifetime of 270 ps, which was assigned to the mediation by MC states whose energy is lowered due to the small bite angle of the terpy ligand. Thus, if the same mediation took place in Ru-bpm, the observed decay would be an order of magnitude slower than the observed. Second, the excited-state decay of Ru-bpm exhibits a large kinetic isotope effect (KIE) of 1.75 for the 1 component and 1.61 for 2 component (Figure ). Such KIE is not expected for the MC-mediated deactivation. As a matter of fact, the inverse isotope effect was observed due to the stabilization effects caused by the aqua ligand. Based on these arguments, we conclude that the fast excited-state decay in Ru-bpm is associated with another process, likely involving the contribution from the solvent. Corresponding kinetic traces at selected wavelengths with data plotted as dots and fits, derived from target analysis using the A→B→GS (ground state) model, plotted as curves. Samples at pH =-1 were excited at 700 nm while samples at pH 7 and 9 were excited at 550 nm.