A face-to-face porphyrin-sapphyrin pseudo dimer

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Abstract
The synthesis and characterization of a new, β-to-meso-linked cofacial porphyrin-sapphyrin pseudo dimer ([por]-[sap], 3) is described. This system is the first example in which an expanded porphyrin has been conjugated covalently to a porphyrin and one of the few examples wherein an expanded porphyrin has been incorporated into any type of larger supramolecular array. The first excited singlet state of the free-base porphyrin in 3 was found to lie ca. 0.19 eV higher in energy than that of the metal-free sapphyrin subunit. Excitation of 3 at 408 nm, where only porphyrin subunit absorbs, gives rise to virtually no porphyrin-like fluorescence, but instead, gives fluorescence characteristic of the sapphyrin. This is attributed to very efficient intramolecular singlet energy transfer from the porphyrin subunit to the sapphyrin subunit. From time-resolved fluorescence studies the rate of energy transfer must exceed 2 × 1010 sec−1. Studies with the isolated compounds indicated that the rate of Förster energy transfer in 3 should be ca. 3 × 1013 sec−1. As such, 3 can be considered to be a good model for the final steps in natural light harvesting complexes.

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