Synthesis and fluorescence quenching studies of a series of carotenoporphyrins with carotenoids of various lengths

A series of molecular dyads, each consisting of a porphyrin covalently linked to a carotenoid, was synthesized. The carotenoid conjugated chain length was systematically varied from 5 to 11 double bonds (not including the carbonyl group) and each carotenoid was linked to 5-(4-aminophenyl)-10,15,20-tris(4-methylphenyl)porphyrin (TPPA) by an amide bond. The analogous zinc carotenoporphyrins were prepared by metallation of the free base species. Porphyrin fluorescence quenching by the carotenoids was studied as a function of the length of the carotenoid conjugated system. Carotenoids containing from 5 to 7 double bonds do not quench porphyrin fluorescence. Carotenoids with 10 or 11 double bonds quench the fluorescence of the attached porphyrin to ∼10% of the value of the parent porphyrin, and carotenoids with 8 or 9 double bonds exhibit an intermediate quenching effect that is solvent dependent. In the intermediate cases greater fluorescence quenching is observed for the zinc carotenoporphyrins.