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 RuII polypyridyl complex (RuIICOO–)
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 RuIICOO– (H4DPP2+(RuIICOO–)2 and H4DPP2+(RuIICOO–)­(Cl–)) and BV2+COO– (H4DPP2+(BV2+COO–)2) was confirmed in acetone by 1H NMR measurements,
CSI-TOF-MS, and X-ray crystallographic analysis. The photodynamics
of H4DPP2+(RuIICOO–)2 and H4DPP2+(BV2+COO–)2 was elucidated by femto-, pico-, and nanosecond
time-resolved transient absorption spectroscopy. In H4DPP2+(RuIICOO–)2, intrasupramolecular
photoinduced electron transfer (ET) occurred from the RuII center to the singlet excited state of H4DPP2+ to afford
an ET state involving one-electron-reduced H4DPP•+ and
the corresponding one-electron-oxidized RuIII 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•+, following intrasupramolecular thermal ET proceeded from H4DPP•+ to BV2+COO– to form H4DPP2+(BV•+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.