Excitons transfer

If these two photosystems are located close to each other, the higher excitation energy of PS II (p680 versus p700) would pass a large fraction of its absorbed photons to PS I, via exciton transfer. These two particles are placed 10 nm away, thus eliminating this difficulty [37],... 

Baldo et al. [ 164] used the platinum complex of 2,3,7,8,12,13,17,18-octaethyl-21 //,23//-porphine (PtOEP, 66) as efficient phosphorescent material. This complex absorbs at 530 nm and exhibits weak fluorescence at 580 nm but strong phosphorescence from the triplet state at 650 nm. Triplet transfer from a host like Alq3 was assumed to follow the Dexter mechanism. Dexter-type excitation transfer is a short-range process involving the exchange of electrons. In contrast to Forster transfer, triplet exciton transfer is allowed. 

FIGURE 19-43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text. 

Chlorophyll Funnels the Absorbed Energy to Reaction Centers by Exciton Transfer... 

FIGURE 19-45 Organization of photosystems in the thylakoid membrane. Photosystems are tightly packed in the thylakoid membrane, with several hundred antenna chlorophylls and accessory pigments surrounding a photoreaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (black arrow). Also embedded in the thylakoid membrane are the cytochrome bkf complex and ATP synthase (see Fig. 19-52). 

ATP synthase 704 FiATPase 708 rotational catalysis 711 P/O ratio 712 P/2e ratio 712 acceptor control 716 mass-action ratio 716 light-dependent reactions 723 light reactions 723 carbon-assimilation reactions 723 carbon-fixation reaction 723 thylakoid 724 stroma 724 exciton transfer 725 chlorophylls 725... 

Only the radically symmetric (s-wave or 1 = 0) solution is of interest since rotational relaxation has been assumed to be very much more rapid than the processes of molecular diffusion or exciton transfer. The solution of eqn. (78) subject to the boundary conditions (68) and (69), in the steady-state where dp/dt = 0, leads to the density [129,140]... 

Figure 23-27 Illustration of proposed exciton transfer of the energy of light absorbed by bacteriochlorophyll a of purple bacteria. Energy absorbed by the light harvesting complex LH2 is transferred in steps to another LH2, to LH1 and to the reaction center. The short lines within the circles represent the edges of the BChla chromophores. After Kiihlbrandt300 with permission.

In conclusion, we present herein a rather compelling model for the short-time dynamics of the excited states in DNA chains that incorporates both charge-transfer and excitonic transfer. It is certainly not a complete model and parametric refinements are warranted before quantitative predictions can be established. For certain, there are various potentially important contributions we have left out disorder in the system, the fluctuations and vibrations of the lattice, polarization of the media, dissipation, quantum decoherence. We hope that this work serves as a starting point for including these physical interactions into a more comprehensive description of this system. 

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