Spectroscopic Determination of Proton Position in the Proton-Coupled Electron Transfer Pathways of Donor−Acceptor Supramolecule Assemblies

A homologous set of porphyrin derivatives possessing an isocyclic five-membered ring appended
with an amidinium functionality has been used to examine proton-coupled electron transfer (PCET) through
well-characterized amidine−carboxylic acid interfaces. Conjugation between the porphyrin chromophore
and the amidinium interface can be altered by selective reduction of the isocyclic ring of an amidinium−purpurin to produce an amidinium−chlorin. The highly conjugated amidinium−purpurin displays large spectral
shifts in the visible region upon alteration of the amidinium/amidine protonation state; no such change is
observed for the chlorin homologue. Analysis of the UV−vis absorption and emission profiles of the
amidinium−purpurin upon deprotonation allows for the measurement of the porphyrinic-amidinium acidity
constant for the ground state (pKa = 9.55 ± 0.1 in CH3CN) and excited state (pKa* = 10.40 ± 0.1 in
CH3CN). The absorption spectrum of the purpurin also provides a convenient handle for determining the
protonation state of assembled interfaces. In this way, the purpurin macrocycle provides a general tool for
PCET studies because it can be used to determine the location of a proton within PCET interfaces formed
from carboxylic acid electron acceptors including dinitrobenzenes (DNBs) and naphthalenediimide (NI),
which have been used extensively in previous PCET studies. An amidine−carboxylic acid interface is
observed for electron-rich acceptors, whereas the ionized amidinium−carboxylate interface is observed
for electron-poor acceptors. The PCET kinetics for purpurin/chlorin associated to NI are consistent with an
amidine−carboxylic acid interface, which is also verified spectrally.