Thermodynamics and Photodynamics of a Monoprotonated Porphyrin Directly Stabilized by Hydrogen Bonding with Polar Protic Solvents

Addition of 1 equiv of TFA to an acetone solution containing dodecaphenylporphyrin (H₂DPP) in the presence of 10 % MeOH (v/v) resulted in selective formation of a monoprotonated form (H₃DPP⁺), in sharp contrast to protonation of H₂DPP directly affording a diprotonated form (H₄DPP²⁺) in acetone in the absence of MeOH. The crucial role of MeOH for selective H₃DPP⁺ formation was interpreted as hydrogen-bonding stabilization of H₃DPP⁺, since a MeOH molecule was found to form hydrogen bonds with an NH proton of H₃DPP⁺ in the crystal. The selectivity of H₃DPP⁺ formation was evaluated by the formation yield of H₃DPP⁺, which increased when elevating the portion of MeOH (0–10 %) in acetone with saturation behavior, suggesting that H₃DPP⁺ is stabilized by hydrogen bonding with MeOH even in solution, together with the thermodynamic parameters determined from a van't Hoff plot based on the spectroscopic titration. Femto- and nanosecond laser flash photolysis allowed us to elucidate the photodynamics of H₃DPP⁺ in intermolecular photoinduced electron transfer (ET) from ferrocene derivatives as one-electron donors to the triplet excited state of H₃DPP⁺ as an electron acceptor. The second-order rate constants of the ET reactions were evaluated in light of the Marcus theory of ET. The reorganization energy of ET was determined to be 1.87 eV, which is slightly larger than that of H₄DPP²⁺ in acetonitrile (1.69 eV), due to larger structural change upon ET than that of H₄DPP²⁺.