Light-Harvesting and Photoisomerization in Benzophenone and Norbornadiene-Labeled Poly(aryl ether) Dendrimers via Intramolecular Triplet Energy Transfer

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Abstract
A series of benzophenone (BP) and norbornadiene (NBD)-labeled poly(aryl ether) dendrimers (Gn-NBD), generations 1−4, were synthesized, and their photophysical and photochemical properties were examined. The phosphorescence of the peripheral BP (donor) chromophore was efficiently quenched by the NBD (acceptor) group attached to the focal point. Time-resolved spectroscopic measurements indicated that the lifetime of the triplet state of the BP chromophore was shortened due to the proximity of the NBD group. Selective excitation of the BP chromophore resulted in isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that an intramolecular triplet energy transfer occurs in Gn-NBD molecules. The light-harvesting ability of these molecules increases with generation due to an increase in the number of peripheral chromophores. The energy transfer efficiencies are ca. 0.97, 0.54, 0.45, and 0.37 for generations 1−4, respectively, and the rate constant of the triplet−triplet energy transfer is ca. 106−107 s-1, which decreases inconspicuously with increasing generation. The intramolecular triplet energy transfer is proposed to proceed mainly via a through-space mechanism involving the closest donor (folding back conformation) and acceptor groups.

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