Zinc porphyrin energy donors

Fluorescence lifetime measurements on the aggregate have shown that the rate constant of the intermolecular energy transfer from the zinc porphyrin unit to the free-base porphyrin unit has been evaluated to be 3.0 x 109 s-1. This value is reasonable from a model in which dendritic donor 6b and acceptor 5a contact each other directly at their exterior surfaces (Scheme 2). Therefore, electrostatic assembly of positively and negatively charged dendrimers provides a promising supramolecular approach to construct photofunctional materials with nanometric precision. 

The design of dendritic multiporphyrin systems [18] permits energy transfer over longer distances. The outer shell of the dendrimer shown in Fig. 5.14 is made up of eight porphyrin-zinc complexes as energy donor units. Excitation of the units of the outer shell leads to fluorescence emission of the metal-free porphyrin core as a result of energy transfer from the periphery to the energy acceptor [19]. 

The self-assembled diad Zn P-PH2P consisting of a zinc porphyrin donor and a free base porphyrin acceptor (Scheme 7.4) was studied by time-resolved fluorescence [21]. The driving force of the assembly is the site selective binding of an imidazole connected to a free base porphyrin. Evidence for Forster back transfer was obtained from the analysis of the fluorescence decay (Fig. 7.8) and the relevant rate was quantitatively evaluated for the first time. The transfer efficiency [13] is 0.98, and the rate constants for direct and back transfer were found to be 24.4 x 10 s and 0.6 X 10 s respectively. These values are consistent with the Forster energy transfer mechanism. 

To mimic energy transfer processes in light-harvesting antenna complexes, Sessler and coworkers developed in 1991 the idea of assembling by noncovalent bonds a zinc porphyrin acting as an energy donor and a free-base... 

In a second paper, Sanders and coworkers presented a simple model mediated by pyridine ligation to zinc porphyrin. " In the donor-spacer-acceptor system, photoinduced ET occurs with a rate con.stant of 2.13 x I0 s for zinc mesoporphyrin II dimethyl ester 141 and 53.3 X lO s" for zinc tetraphenylporphyrin, while the recombination rate constants were 6.35 x 10 s and 3.81 X 10 s , respectively. Although the two porphyrins have similar redox potentials, the forward ET rates were unambiguously different. The authors speculated that the two photodonor porphyrins have different solvent reorganization energies. 

One problem that is still consistently recurrent in the study of porphyrin assemblies stems from very similar absorption and excitation properties of the species involved in the charge separation or the energy transfer processes. Very often, it is difficult to selectively generate the excited state of a zinc porphyrin donor without partial excitation of the free base porphyrin acceptor, even though no electronic coupling can be detected in ground state absorption spectra. This also stands for excitation of the electron acceptor or hole donor free base with a zinc electron donor or hole acceptor. It is thus useful to connect the multiporphyrin assembly to a nonporphyrinic electron/energy donor or acceptor, or both, that can be selectively excited in order to initiate the photoinduced process. This class of assemblies will now be examined. 

Bis(zinc porphyrin)-oxoporphyrinogen [(ZnTPP)2-OxP] is an example of a donor-acceptor-type triad. Excitation of the zinc porphyrin gronps leads to an efficient energy or electron transfer in the triad and this process depends on the polarity of the solvent used. Further elaboration of this system was accomplished by complexing this compound and other derivatives with fullerene-containing compounds capable of coordinating at the porphyrin zinc metal ions. The structures of the complexes are shown in Figure 14. 

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