Triplet and Singlet Energy Transfer in Carotene-Porphyrin Dyads: Role of the Linkage Bonds

A series of carotenoporphyrin dyad molecules in which the carotenoid is covalently linked to a tetraarylporphyrin at the ortho, meta, or para position of a meso aromatic ring has been prepared, and the molecules have been studied using steady-state and transient fluorescence emission, transient absorption, and ¹H NMR methods. Triplet-triplet energy transfer from the porphyrin moiety to the carotenoid has been observed, as has singlet-singlet energy transfer from the carotenoid polyene to the porphyrin. In addition, the carotenoid quenches the fluorescence of the attached porphyrin by a mechanism which increases internal conversion. The rates of all three of these processes are slower for the meta isomer than for the corresponding ortho and para molecules. Analysis of the data suggests that the triplet-triplet energy transfer is mediated by a through-bond (superexchange) mechanism involving the π-electrons of the linkage bonds, rather than a direct, through-space coupling of the chromophores. The same appears to be true for the process leading to enhanced internal conversion. The results are consistent with a role for the through-bond mechanism in the singlet-singlet energy transfer as well. Simple Hückel molecular orbital calculations are in accord with the proposed through-bond process.