Noncovalently Linked Donor-Acceptor Pairings via Metal Coordination

Noncovalently Linked Donor-Acceptor Pairings via Metal Coordination 

Sanders and coworkers have constructed noncovalently linked donor-acceptor ET models by use of axial ligation of pyridine ligands to zinc porphyrin units.For example, bis(4-pyridyl)porphyrin 139 coordinated to a host porphyrin dimer 140 with an association constant of 2.6 x 10 M  

In this system, the fluorescence at 630 nm due to free-base 139 was at least 90% quenched, while fluorescence of 139 was not quenched in the presence of simple zinc porphyrin, suggesting that the photoinduced ET occurs from 139 to pyromellitic diimide groups in the porphyrin dimer 140. 

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. 

Additionally, Hunter and coworker presented another model for the noncovalently linked donor-acceptor complex which was made up of triad components, quinone-cyclophane-porphyrin 144. H NMR measurements indicated the formation of a ternary complex and the association constant of BQ via hydrogen bonds was determined to be 1.4 x 10 M, while the association constant of pyridine of the cyclophane and zinc porphyrin was 3.2 X lO M in dichloromethane. The fluorescence emission from the zinc porphyrin in the presence of the cyclophane was clearly quenched by the addition of BQ, compared with a control experiment, due to diffusional quenching. This evidence indicated that the intracomplex ET occurred within the triad system. 

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F. Noncovalently Linked Donor-Acceptor Pairings via Metal Coordination. 318... 

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