Photochemical Bleaching of an Elaborate Artificial Light-Harvesting Antenna
Résumé
The target artificial light-harvesting antenna, comprising 21 discrete chromophores arranged in alogical order, undergoes photochemical bleaching when dispersed in athin plastic film. The lowest-energy component, which has an absorption maximum at 660 nm, bleaches through first-order kinetics at arelatively fast rate. The other componentsbleach more slowly, in part, because their excited-state lifetimes are rendered relatively short by virtue of fast intramolecular electronic energy transfer to the terminal acceptor.Two of the dyes, these being close to the terminal acceptor but interconnected through a reversible energy-transfer step, bleach by way of an autocatalytic step. Loss of the terminal acceptor, thereby switching off the energy-transfer route, escalates the rate of bleaching of these ancillary dyes. The opposite terminal, formed by aseries of eight pyrene-based chromophores, does not bleach to any significant degree. Confirmation of the variousbleaching steps is obtained by examination of an antenna lacking the terminal acceptor, where the autocatalyticroute does not exist and bleaching is very slow.