SYNTHESIS AND PHOTOEXCITED-STATE DYNAMICS OF AROMATIC GROUP-BRIDGED CAROTENOID PORPHYRIN DYADS AND CAROTENOID PORPHYRIN PYROMELLITIMIDE TRIADS

A series of conformationally restricted carotenoid-porphyrin dyads (C-H2P and C-ZnP) was prepared by trichloroacetic acid-catalyzed condensation of a carotenoid-substituted aromatic aldehyde and 4-(methoxycarbonyl)benzaldehyde with bis(3-hexyl-4-methyl-2-pyrrolyl)methane in a benzene-acetonitrile mixture. Spacers between the carotenoid and the porphyrin cover aromatic groups including benzene, naphthalene, biphenyl, diphenylmethane, spirobiindane, and benzanilide. Carotenoid-porphyrin-pyromellitimide triads (C-H2P-Im) were similarly prepared using N-hexyl-N'-(4-formylphenyl)methyl)pyromellitimide instead of 4-(methoxycarbonyl)benzaldehyde. Photoexcited-state dynamics of these models were studied by steady-state fluorescence spectra, fluorescence excitation spectra, picosecond time-resolved fluorescence lifetime measurements, and picosecond time-resolved transient absorption spectroscopy. Intramolecular singlet-singlet energy transfer in C-H2P has been revealed to be a reversible process, while only one-way singlet-singlet energy transfer from zinc porphyrin to carotenoid has been observed in C-ZnP. Rates of the intramolecular energy transfer 1(C)*-H2P --> C-1(H2P)* were determined from efficiencies of the energy transfer that were obtained by comparing their fluorescence excitation spectra with the absorption spectra, and rates of C-1(H2P)* --> 1(C)*-H2P and of C-1(ZnP)* --> 1(C)*-ZnP were determined on the basis of their fluorescence lifetimes. The rates of the intramolecular energy transfer depend on the spacer between C and P in a rather similar manner through these three different energy-transfer processes and are quite sensitive to the substitution position of the aromatic spacer where the porphyrin is linked to the carotenoid. These spacer dependencies observed indicate the through-bond electronic coupling to be the most important interactions in these intramolecular energy transfers. In C-H2P-Im triads, excitation at C with 532-nm light led to an accumulation of (C)+-H2P-(Im)- within several tens of picoseconds. This rapid accumulation had been interpreted in terms of long-distance electron transfer from 1(C)* to Im mediated by a superexchange interaction involving the pi-electronic orbital of the intervening H2P. Selective excitation of the triad to C-1(H2P)*-Im at 585 nm lod to much slower formation of (C)+-H2P-(Im)-, probably via a 1(C)*-H2P-Im State which may be formed by intramolecular singlet-singlet energy transfer.