Photochemical Electron Transfer in Chlorophyll-Porphyrin-Quinone Triads: The Role of the Porphyrin-Bridging Molecule

The photochemistry of four chlorophyll-porphyrin-naphthoquinone molecules possessing both fixed distances and orientations between the three components is described. These molecules consist of a methyl pyropheophorbide a or pyrochlorophyllide a that is directly bonded at its 3-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, which is in turn bonded at its 15-position to a 2-triptycenenaphthoquinone. In addition, porphyrin-quinone compounds in which the chlorins are replaced by a p-tolyl group were also prepared as reference compounds. Selective metalation of the macrocycles with zinc gives the series ZCHPNQ, ZCZPNQ, HCZPNQ, HCHPNQ, HPNQ, and ZPNQ, where H, Z, C, P, and NQ denote free base, Zn derivative, chlorophyll, porphyrin, and naphthoquinone, respectively. Selective excitation of ZC in ZCZPNQ and ZCHPNQ, and HC in HCHPNQ dissolved in butyronitrile yields ZC⁺ZPNQ⁻, ZC⁺HPNQ⁻, and HC⁺HPNQ⁻ with rate constants of 1.0 × 10¹¹, 9.0 × 10⁹, and 8.2 × 10⁹ s⁻¹, respectively, while the corresponding ion-pair recombination rate constants are 1.4 × 10¹⁰, 4.0 × 10⁹, and 4.0 × 10⁹ s⁻¹, respectively. The fact that ZCZPNQ possesses faster rates of charge separation than do ZCHPNQ and HCHPNQ is inconsistent with an electron transfer mechanism involving superexchange with virtual states possessing anionic character on the bridging porphyrin. The data support an electron transfer mechanism in which the lowest excited singlet state of the bridging porphyrin weakly mixes with the lowest excited singlet state of the chlorophyll. This mixed state crosses over to a charge transfer state in which the bridging porphyrin is oxidized and the quinone is reduced. This charge transfer state then relaxes to yield the final state which possesses an oxidized chlorophyll and a reduced quinone.