PHOTOCHEMICAL ELECTRON-TRANSFER IN CHLOROPHYLL PORPHYRIN QUINONE TRIADS - THE ROLE OF THE PORPHYRIN-BRIDGING MOLECULE

The photochemistry of four chlorophyll-porphyrin-naphthoquinone molecu les possessing both fixed distances and orientations between the three components is described. These molecules consist of a methyl pyropheo phorbide a or pyrochlorophyllide a that is directly bonded at its 3-po sition to the 5-position of a ,12,18-tetraethyl-3,7,13,-17-tetramethyl porphyrin, which is in turn bonded at its 15-position to a 2-triptycen enaphthoquinone. In addition, porphyrin-quinone compounds in which the chlorins are replaced by a p-tolyl group were also prepared as refere nce 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 ZCZP NQ and ZCHPNQ, and HC in HCHPNQ dissolved in butyronitrile yields ZC+Z PNQ-, ZC+HPNQ-, and HC+HPNQ- with rate constants of 1.0 X 10(11), 9.0 X 10(9), and 8.2 x 10(9) s-1, respectively, while the corresponding io n-pair recombination rate constants are 1.4 x 10(10), 4.0 X 10(9), and 4.0 x 10(9) s-1, respectively. The fact that ZCZPNQ possesses faster rates of charge separation than do ZCHPNQ and HCHPNQ is inconsistent w ith an electron transfer mechanism involving superexchange with virtua l states possessing anionic character on the bridging porphyrin. The d ata 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 ove r to a charge transfer state in which the bridging porphyrin is oxidiz ed 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.