Observation of Proton-Coupled Electron Transfer by Transient Absorption Spectroscopy in a Hydrogen-Bonded, Porphyrin Donor−Acceptor Assembly

Proton-coupled electron transfer (PCET) kinetics of a Zn(II) porphyrin donor noncovalently bound to a naphthalene-diimide acceptor through an amidinium-carboxylate interface have been investigated by time-resolved spectroscopy. The S₁ singlet excited-state of a Zn(II) 2-amidinium-5,10,15,20-tetramesitylporphyrin chloride (ZnP-β-AmH⁺) donor is sufficiently energetic (2.04 eV) to reduce a carboxylate-diimide acceptor (ΔG° = −460 mV, THF). Static quenching of the porphyrin fluorescence is observed and time-resolved measurements reveal more than a 3-fold reduction in the S₁ lifetime of the porphyrin upon amidinium-carboxylate formation (THF, 298 K). Picosecond transient absorption spectra of the free ZnP-β-AmH⁺ in THF reveal the existence of an excited-state isosbestic point between the S₁ and T₁ states at λ_probe = 650 nm, providing an effective ‘zero-kinetics' background on which to observe the formation of PCET photoproducts. Distinct rise and decay kinetics are attributed to the build-up and subsequent loss of intermediates resulting from a forward and reverse PCET reaction, respectively (k_PCET(fwd) = 9 × 10⁸ s^-1 and k_PCET(rev) = 14 × 10⁸ s^-1). The forward rate constant is nearly 2 orders of magnitude slower than that measured for covalently linked Zn(II) porphyrin−acceptor dyads of comparable driving force and D−A distance, establishing the importance of a proximal proton network in controlling charge transport.