Hydroxy-substituted electron deficient Pd porphyrin cofactors illuminate ultrafast proton transfer reactions

Coordinated electron and proton movement drive bioenergetic functions. Relative to electron transfer reactions, tracking proton transport over fast-to-ultrafast time scales is challenging. Optical resolution of proton transfer dynamics can take advantage of chromophoric photoacids that not only trigger proton migration upon photoexcitation, but produce distinct spectroscopic changes associated with protonation/deprotonation. In this work, we report the design of a hydroxy-substituted electron deficient Pd porphyrin, PPd(C₆F₅)₃OH; upon photoexcitation, the acidity constant of this weak acid (pKa = 6.49) dramatically drops (pKa* = 0.97). Electronic excitation of PPd(C₆F₅)₃OH triggers an ultrafast proton transfer reaction (PPd(C₆F₅)₃OH +:B + hυ → ¹[PPd(C₆F₅)₃OH]* +:B → ¹[PPd(C₆F₅)₃O]^-* + HB⁺; τ_PT = 342 fs) to a H-bonded base (B) in solution. Both PPd(C₆F₅)₃OH and its conjugate anion PPd(C₆F₅)₃O^- exhibit distinct vis-NIR spectral features for their respective ground and excited states. Because the electroni-cally excited triplet lifetimes of these species exceed tens of microseconds, the PPd(C₆F₅)₃OH photoacid defines an ideal cofactor to probe light-triggered proton release and track long-range proton migration in protein environments.