Electronic interactions and photoinduced electron transfer in covalently linked porphyrin-C60(pyridine) diads and supramolecular triads formed by self-assembling the diads and zinc porphyrin

Supramolecular triads of the type (donor-1)-acceptor:(donor-2) composed of free-base porphyrin, fullerene, and zinc porphyrin, have been formed by a "covalent-coordinate" approach. Toward this, two diads, namely, 5-(3'-(2"-(3'" or 4"'-pyridyl)fulleropyrrolidinyl-N)ethoxyphenyl)-10,15,20-triphenylporphyrin, bewing C-60 as acceptor and free-base porphyrin, H2P, as donor were first synthesized. The diads and self-assembled supramolecular triads, which were formed by coordinating the pyridine group located on the diads to zinc tetraphenylporphyrin, ZnP, have been characterized by semiempirical PM3, electrochemistry, and steady-state and time-revolved spectroscopic techniques. Subpicosecond and nanosecond transient absorption spectral studies of diads revealed the occurrence of the electron transfer from the H2P Moiety to the C-60 entity via the excited singlet state of H2P. Clear evidence for the formation of triads in o-dichlorobenzene was obtained from the steady-state and time-resolved fluorescence measurements, which revealed quenching of the ZnP emission on addition of the pyridine bearing diads. Semiempirical PM3 energy optimized structures of the triads suggested substantial intramolecular interactions between the H2P and C-60 entities. Cyclic voltammetric studies on these triads exhibited a total of 12 one-electron redox processes involving the three redox active ZnP, H2P, and C60 entities. Nanosecond transient absorption studies revealed additional charge separation in the triads as compared to that observed for the diads, suggesting that the coordinated ZnP accelerates the charge-separation process. However, in a coordinating solvent such as benzonitrile, intermolecular electron transfer from the (ZnP)-Zn-3* to the diads is shown to take place.