Electron transfer in the supramolecular donor-acceptor dyad of zinc porphycene

The electron transfer processes of Zn octaethylporphycene (ZnPcn), a structural isomer of Zn octaethylporphyrin, have been investigated mainly using transient absorption spectroscopy. To form a supramolecular donor-acceptor dyad, imide compounds bearing a pyridine group at the N position of the imides have been used as an acceptor. The N atom of the pyridine ring can coordinate to the central Zn ion of ZnPcn. Formation of a supramolecular donor-acceptor dyad, that is, pentacoordinated ZnPcn, was confirmed by steady-state absorption spectroscopy using toluene as a solvent. Charge separation upon excitation of ZnPcn was indicated by efficient fluorescence quenching, especially when pyromellitic diimide was used as the acceptor. Electron transfer processes were confirmed by subpicosecond transient absorption spectroscopy, in which generation of a radical anion of the acceptor and a radical cation of ZnPcn, which was identified by means of gamma-ray radiolysis, was confirmed. It became clear that the charge separation rate was smaller than that of the corresponding supramolecular dyads of Zn tetraphenylporphyrin and Zn octaethylporphyrin despite a similar driving force. This observation indicates a larger internal reorganization energy and a smaller coupling element of the ZnPcn dyad.