The pH-dependent behavior of Ru-bpm is very similar to that observed previously for Ru and Os complexes with dipyridophenazine (dppz) ligands. For example, Ru-dppz complexes were found to exhibit short excited-state lifetimes in water, while a significant increase in the lifetimes was observed in acetonitrile and other non-aqueous media. This light switching behavior found application in biosensing of DNA and other biomolecules. Mechanistically, the short excited state lifetimes observed in aqueous media were found to be insensitive to the pH changes, even though significant kinetic isotope effect was observed. This behavior was hypothesized to occur due to the formation of MLCT states with strong hydrogen bonds between aza centers of the dppz radical anion and the water molecules. Optical transient absorption measurements showed that such hydrogen-bonded complex forms within several picoseconds, a timescale associated with water reorganization dynamics, while the time-resolved infrared measurements provided direct evidence for the hydrogen-bonded complex through their vibrational signatures. While strong experimental evidence exists that the hydrogen-bonded complex is involved in the fast excitedstate deactivation of Ru-complexes with dppz ligands, it is still not clear what is the mechanism of increased rates of nonradiative decay in these adducts. Based on the observed strong coupling of the vibrational modes of the dppz ligand and hydrogen-bonded water, it was hypothesized that fast vibrational energy transfer to the solvent causes the observed short excited-state lifetimes. It is highly likely that the hydrogen-bonded complex between Ru-bpm in its MLCT state and the solvent molecules causes the observed short excited-state decay in our system. To investigate this hypothesis further, we collected the transient absorption spectra of Ru-bpm in acetonitrile, where hydrogen-bonded complexation was not expected to occur. Results of this experiment are summarized in Figure and show that both lifetime components increased from 1=4 ps and 2=65 ps in water to 1=7 ps, 2=76.5 ps in acetonitrile. These results show that the nature of the solvent plays an important role in the excited-state lifetimes. However, the observed lifetime increase is quite modest relative to that observed for dppz ligands, where a 2560-fold increase was reported. We assign this difference in behavior to the higher basicity of the aza moiety of bpm radical anion relative to that of the dppz analog. Due to smaller size of the bpm ligand relative to dppz, we expect that the aza moiety of the bpm radical anion formed in the 3 MLCT state of Ru-bpm is a strong hydrogen-bond acceptor, capable of forming hydrogen bond adducts with acetonitrile. While acetonitrile is not as strong hydrogen bond donor as water, it has a reasonable hydrogen bond donating coefficients, making it likely that the observed fast dynamics are due to the hydrogenbonded adduct. Based on our experimental findings and the previous literature on similar complexes, we propose the Jablonski diagram shown in Scheme 2 to describe the excited state behavior of Ru-bpm. The excitation into the lowest energy absorption band is expected to populate S2 and S3 states of Rubpm. The S1 state is not populated directly due to the low oscillator strength calculated for this transition (Figure ). The nonradiative internal conversion and intersystem crossing from these states to the T1 state are expected to occur at timescales shorter than the 300 fs instrument response function of our setup. The T1 state is predominantly characterized by the Ru → bpm MLCT character, based on the transition orbitals calculated for the corresponding S1 state. The experimentally observed 1 lifetime is assigned to the solvent reorganization dynamics around the photogenerated molecular dipole, and this reorganization also includes the specific hydrogenbonding interactions between the solvent molecules and the aza-based hydrogen bond donors of the bpm radical anion moiety. The T1' state of the molecule/solvent adduct then decays back to the ground state with lifetime 2, and this fast nonradiative decay is likely associated with efficient vibrational energy transfer to the solvent, facilitated by the strong coupling of the vibrational modes of bpm moiety with those of the hydrogen-bonded solvent.