Natural photosynthesis uses elaborate superstructures to harvest incident sunlight and channel the photonic energy to a site where electron-transfer reactions take place. These light-harvesting antennae comprise discrete rings of tetrapyrrolic pigments that function as nano-scale cyclotrons able to direct the photon around the ring and between adjacent rings. The net result is that a substantial fraction of the solar spectrum can be collected and ultimately focussed at a catalytic site. Now research led by Prof Anthony Harriman has established an artificial analogue that closely resembles the chemistry taking place within the leaf.1 Thus, a C60 hexa-adduct has been decorated with ten yellow and two blue boron dipyrromethene (Bodipy) dyes in such a way that the dyes retain their individuality. Intra-particle electronic energy transfer (EET) from yellow to blue dyes occurs with high efficiency such that photons absorbed by the yellow dye are emitted by the blue dye. Films can also be prepared to contain C60 nanoparticles loaded with the yellow dye but lacking the blue dye and, under these circumstances, electronic energy migration occurs between yellow dyes appended to the same nano-particle and to dye molecules on nearby particles. Consequently, long-range energy migration occurs among yellow dyes attached to different particles before the photon is trapped at a blue dye.
The leaf is equipped with special machinery that allows the photon flow to be regulated and redirected as required. This latter feature, being a remarkable achievement of evolution, is controlled by a light-induced proton gradient. In a second publication,2 Harriman et al. have demonstrated how to switch the direction of electronic energy transfer in an artificial system. Here, a sequence of efficacious intramolecular EET steps follows from selective illumination of the fluorescent centre (DPP) present in a new class of molecular triads (see picture). In the resting state, EET occurs preferentially to the blue dye (B) but, because of the unique spectral properties inherent to this system, a second EET step occurs to redirect the photonic energy to the green (G) terminal. Protonation of the green dye completely reverses the EET flow. A novel aspect of this work is that protonation is achieved by one- or two-photon excitation of a photo-acid generator (PAG) present in the solid film.
published on: 15th April 2012