Back-crossing, transfer

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Scheme 11.5 Photophysics of sensitised lanthanide sensors sens-Ln such as 11.32 and 11.33. Key k = rate constant, em = emission, eT = electron transfer, q = quenching, isc = inter-system crossing to triplet state, (B)ET = (back) energy transfer (reproduced with permission from Reference 24).

Exciplexes are usually considered to be singlet, but triplet exciplexes have also been found. Many photoreactions proceed by way of the triplet state. In these cases, the triplet exciplex may be involved. A triplet state can be produced by three pathways 1) intersystem crossing of the singlet state of the exciplex or one of the partners 2) recombination of the radical-ion pairs or back electron-transfer 3) energy transfer by triplet sensitizers. 

The spin status of the ion pair is another crucial variable affecting the overall efficiency of the process. The forward electron transfer from (or to) a diamagnetic molecule is not affected by the spin status of the excited component. The back electron transfer, however, is forbidden within the triplet ion pairs (it would violate Pauli s exclusion principle). In situations like that the intersystem crossing will very often determine the efficiency of BET. In practice, the triplet state acceptors or donors lead to overall efficiencies that are higher than those observed with singlet state acceptors or donors [38,78,102,103,116]. An additional bonus is the fact that triplet states have longer lifetimes [2] and are efficiently ET-quenched with lower concentrations of the ground state component. Quinones and ketones are the most common triplet acceptors, while aromatic amines often serve as triplet donors. 

The reactions of SO2 with [Cr(NN)3] (NN = bpy, phen, and derivatives) when subjected to visible light (laser pulse) are threefold. " Quenching yields [Cr(NN)3] which undergoes back electron transfer. The predominant reaction is electron transfer between SO2 and [Cr(NN)3] yielding the transient SO2. The rate constants obtained may be utilized in a Marcus cross-correlation relation to calculate a self-exchange rate of between 1 x 10" M s and 18 x 10" s for... 

Figure 1.10 Modified JablonskI diagram Illustrating the antenna effect. Abs - absorption, FI -fluorescence, Ph -phosphorescence, L - luminescence, ISC-intersystem crossing, ET - energy transfer, BT - back energy transfer, NR - non-radiative deactivation, - first excited singlet state, T - lowest excited triplet state, GS - ground state, f - emissive f excited state...

Lay-up proceeds by laying down the veneer which is to be the back surface of the panel. Then a sufficient number of pieces of core veneer are passed through the glue spreader to form the next layer of cross-oriented veneer. The glue spreader commonly used in hardwood plywood manufacture is a roU coater in which a pair of opposing mbber roUs are coated with a thin layer of adhesive. As the veneer is passed between the roUs, the adhesive is transferred to the surfaces of the veneer. Adhesive is appHed only to the cross-pfles and in sufficient quantity to provide a continuous layer on both opposing faces of veneer. Thus, in the case of a three-ply panel, only the core layer is spread with adhesive and in that of a five-ply panel, the second and fourth layers both of which are cross-pfles, are spread with adhesive. Then the top surface veneer, which is normally the decorative surface, is placed on the assembly. 

Referring back to equation 47, the other quantity necessary in calculating the gas conductivity is the coUision cross section, Gases contain at least four types of particles electrons, ionized seed atoms, neutral seed atoms, and neutral atoms of the carrier gas. Combustion gases, of course, have many more species. Each species has a different momentum transfer cross section for coUisions with electrons. To account for this, the product nQ in equation 47 is replaced by the summation where k denotes the different species present. This generalization also aUows the conductivity calculation to... 

Carbonless Copy Paper. In carbonless copy paper, also referred to as pressure-sensitive record sheet, an acid-sensitive dye precursor, such as crystal violet lactone or /V-hen2oy11eucomethy1ene blue, is microencapsulated with a high boiling solvent or oil within a cross-linked gelatin (76,83,84) or in synthetic mononuclear microcapsules. Microcapsules that have a starch binder are coated onto the back of the top sheet. This is referred to as a coated-back (CB) sheet. The sheet intended to receive the image is treated on the front (coated-front (CF)) with an acid. When the top sheet is mechanically impacted, the dye capsules mpture and the dye solution is transferred to the receiving sheet where the acid developer activates the dye. 

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