Photobleaching products

When tetrabutylammonium triphenylalkylborate was used as the electron donor, the dye radical anion (DIBF ) was observed as the only transient formed by quenching the dye triplet. The electron transfer rate ealeulated for the electron transfer process is 7.6 x 10 s , i.e., three orders of magnitude below the diffusion-controlled limit. The photobleached products and the transient phenomena observed... 

Purines. - A study of anhydrous deoxyadenosine (compound iv) single crystals x-irradiated (70 kGy) and investigated at 10 K found four base radicals (21-24) and one deoxyribose radical (25) (see p. 251).20 Radical 21, a de-protonated electron loss product, was stable to 100 K and readily photobleached at 10 K. Its nitrogen hyperfine coupling tensors were estimated by using values that resulted in acceptable EPR spectral simulations these were A(N10) = (1.76, 0, 0) mT, A(N3) = (0.99, 0, 0) mT and A(N1) = (0.34, 0, 0) mT. 

The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-151], Photoreduction by amines such as tribenzylamine (R = CH2, R" = ) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152], In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [153]. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [154],... 

Relief Maps. A unique and unusual application of dye sensitized photopolymerization is the production of three-dimensional topographical maps from aerial photographic negatives (91). This application is specific to such dye sensitized processes by virtue of the photobleaching of the dye which accompanies many of the polymerizations. The technique faithfully reproduces relief structures, and can be used to produce parent molds for the fabrication of terrain map reproductions. 

Many photobleaching herbicides act by inhibiting the enzyme protoporphyrinogen oxidase (Protox), which catalyzes the last step in common between chlorophyll and heme biosynthesis. Usnic acid shares some structural features in common with these herbicides, such as the diphenyl ether scaffolding. The inhibitory activity of (-)-usnic acid on Protox was similar to that of the herbicide, acifluorfen, (I50 ca. 3 pM). However, these compounds did not displace acifluorfen from its binding site on Protox (data not shown), indicating that this natural product interacts with Protox differently than other photobleaching inhibitors. 

Up to now, most proposals for photobleaching image enhancement have relied on linear photochemistry, in which the transmittance is a function only of total dose, and not on the rate at which that dose is delivered. The kinetics of such linear photochemistry are well understood and have been described analytically (28). The exposure depends solely on a single parameter which is the product of extinction coefficient, quantum yield, intensity, and time. No increase in contrast can be obtained by changing extinction coefficient or quantum yield, since this merely scales the dose. Contrast can be increased only by increasing the initial optical density, which increases the dose requirement. Only with nonlinear (intensity dependent) photochemistry can one obtain steeper bleaching curves at a specified optical density. 

Dyes from phenoxazine, phenothiazine, and phenoselenazine undergo photoreduction in various conditions yielding radical products. The subject has a long history and, as intimated at the outset, treatment here is cursory. Parker reported an early flash-photolytic study of the photobleaching of Methylene Blue, which was followed by others in the 1960s. This technique permitted the recognition of transient species in the system. 

Fluorogenic substrate molecules that produce fluorescent product molecules by enzymatic reactions, as proposed for horseradish peroxidase [28]. Because fluorophores are continuously replenished and diffuse away from the excitation volume, this assay circumvents the photobleaching problem that hampers all the other assays, and provides extremely long time traces [29,30]... 

---