%0 Journal Article %A Damrauer, Niels H. %A Hodgkiss, Justin M. %A Rosenthal, Joel %A Nocera, Daniel G. %D 2004 %T Observation of Proton-Coupled Electron Transfer by Transient Absorption Spectroscopy in a Hydrogen-Bonded, Porphyrin Donor−Acceptor Assembly %U https://acs.figshare.com/articles/journal_contribution/Observation_of_Proton_Coupled_Electron_Transfer_by_Transient_Absorption_Spectroscopy_in_a_Hydrogen_Bonded_Porphyrin_Donor_Acceptor_Assembly/3338737 %R 10.1021/jp049296b.s001 %2 https://acs.figshare.com/ndownloader/files/5177953 %K T 1 states %K kinetic %K Transient Absorption Spectroscopy %K THF %K formation %K acceptor %K Zn %K ZnP %K porphyrin %K S 1 lifetime %K k PCET %K donor %X 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 λprobe = 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 (kPCET(fwd) = 9 × 108 s-1 and kPCET(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. %I ACS Publications