A set of four porphyrin trimers (1-4) consisting of an energy-accepting 5,15-diphenylethynyl-substituted Zn(II)-porphyrin core flanked by two energy-donating peripheral Zn(II)-porphyrins have been prepared as a new efficient energy-transfer functional unit. The peripheral porphyrin donor is either a TPP-type Zn(II)porphyrin for 1 and 2 or a OEP-type Zn(II)-porphyrin for 3 and 4 and the diphenylethynyl substitution axis of the core porphyrin is aligned either orthogonal in 1 and 3 or parallel in 2 and 4 with respect to the long axis of the trimeric arrays. Femtosecond transient absorption spectroscopy and femtosecond up-conversion fluorescence measurement have revealed the very efficient S 1 -S 1 energy-transfer reactions in these porphyrin trimers. The S 1 -S 1 energy transfer is faster in the parallel trimers 2 and 4 than in the orthogonal trimers 1 and 3, reflecting larger electronic coupling in the former pair. The peripheral porphyrin S 2 -state lifetime is considerably shortened in 1-4, which has been ascribed to S 2 -S 2 energy transfer. Probably the strong Sorettransitions of both the donor and acceptor lead to large Coulombic interactions, thereby rendering S 2 -S 2 energy transfer effective enough to compete with rapid internal conversion to S 1 -state. These results encourage a new strategy for construction of porphyrin-based supramolecular artificial photosynthetic antenna.
Metalloporphyrins bearing two crown ether moieties (benzo-15-crown-5) at the 5-and 15-postions have been prepared by condensation of 4Ј-formyl-benzo-15-crown-5 with either unsubstituted or meso-aryl-substituted dipyrromethanes, followed by oxidation and metallation. The meso-meso-coupled Zn II diporphyrin Zn4 was prepared by the oxidative coupling of Zn1 with AgPF 6 . UV/vis, fluorescence, and 1 H NMR spectroscopic studies revealed that addition of K + and Rb + to solutions of monomeric porphyrin derivatives M1 (M = 2 H, Zn, Co, Ni, Pd, Cu) in CHCl 3 :MeCN (2:1 v/v) induced face-toface dimerization with high stability constants (ca. 10 15 −10 19 M −3 ). 1 H NMR analysis showed that the two Zn1 molecules in the face-to-face dimer were packed more closely than in TPP-type Zn II porphyrin bearing four crown ethers (Krishnan et al, J. Am. Chem. Soc. 1982, 104, 3643), but such dimeriz-
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