A Diprotonated Porphyrin as an Electron Mediator in Photoinduced Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies

We have successfully constructed hydrogen-bonded supramolecular assemblies based on a diprotonated saddle-distorted porphyrin (H4DPP2+) and redox-active molecules bearing a carboxylate group, such as a Ru-II polypyridyl complex ((RuCOO-)-C-II) that can act as an electron donor and a benzyl viologen derivative (BV2+COO-) that can act as an electron acceptor. Formation of supramolecular assemblies of H4DPP2+ with (RuCOO-)-C-II(HH4DPP2+((RuCOO-)-C-II)(2) and H4DPP2+((RuCOO-)-C-II)(Cl-)) and BV2+COO- (H4DPP2+(BV2+COO-)(2)) was confirmed in acetone by H-1 NMR measurements, CSI-TOF-MS, and X-ray crystallographic analysis. The photodynamics of H4DPP2+((RuCOO-)-C-II)(2) and H4DPP2+(BV2+COO-)(2) was elucidated by femto-, pico-, and nanosecond time-resolved transient absorption spectroscopy. In H4DPP2+((RuCOO-)-C-II)(2), intrasupramolecular photoinduced electron transfer (ET) occurred from the Ru-II center to the singlet excited state of H4DPP2+ to afford an ET state involving one-electron-reduced H4DPP center dot+ and the corresponding one-electron-oxidized Ru-III complex with the lifetime of 150 ps. On the other hand, in the presence of an external electron donor, intermolecular photoinduced ET occurred from an electron donor to the triplet excited state of H4DPP2+(BV2+COO-)(2) to afford H4DPP center dot+, following intrasupramolecular thermal ET proceeded from H4DPP center dot+ to BV2+COO- to form H4DPP2+(BV center dot+COO-)(BV2+COO-) with the lifetime of 1.1 ms. Thus, H4DPP2+ can act as an electron mediator in hydrogen-bonded supramolecular assemblies. This is the first example of porphyrins as a photosensitizer and an electron mediator in hydrogen-bonded ET systems.