Correlation of fluorescence quenching in carotenoporphyrin dyads with the energy of intramolecular charge transfer states. Effect of the number of conjugated double bonds of the carotenoid moiety

The electrochemistry of a series of non-symmetric synthetic carotenoids, with different conjugated double bounds chain lengths (5 to 11) is reported. The values of the first oxidation potentials of the carotenoids were evaluated by digital simulation of the experimental cyclic voltammograms. There is a clear relationship between calculated (AM1) HOMO energies of neutral carotenoids with their conjugated chain length, indicating that the change of solvation energy of carotenoids is small throughout the series, and that the electron-donating ability of carotenoids increases with the length of the conjugated chain. Carotenoids had been previously used to design carotenoporphyrin (C-P) molecular dyads. Carotenoid oxidation potentials and the reduction potential of the porphyrin moiety were used in order to calculate the energy of intramolecular charge transfer state in C-P dyads. Correlation of porphyrin fluorescence quenching of these dyads with the energy of the charge transfer state is reported, showing that effective quenching is only possible for carotenoids with more than eight conjugated double bonds.