Sensitization by electron transfer

Sensitization by electron transfer can occur in resist systems. A good example is the sensitization of azide photolysis by aromatic hydrocarbons, which proceeds by the coupled reactions,  

In this system, the hydrocarhon sensitizer is the electron acceptor, and the azide functions as the electron donor. The change in the free energy on charge transfer between two molecular species is given hy an expression derived hy Rehm and Weller  

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Electron Level Position. One essential condition of spectral sensitization by electron transfer is that the LUMO of the dye be positioned above the bottom of the conduction band, eg, > —3.23 eV in AgBr or > —4.25 eV in ZnO (108). To provide the desired frontier level position respectively to the valence and conduction bands of the semiconductor, it is necessary to use a polymethine with suitable electron-donor abiHty (Pq. Increasing the parameter (Pq leads to the frontier level shift up, and vice versa. Chain lengthening is known to be accompanied by a decrease of LUMO energy and hence by a decrease of sensitization properties. As a result, it is necessary to use dyes with high electron-donor abiHty for sensitization in the near-ir. The desired value of (Pq can be provided by end groups with the needed topological index Oq or suitable substituents (112). 

Steady-state irradiation of Eosin and PDO leads to the formation of ethyl benzoate, benzil, and benzoyloxycarboxylate. Energy transfer from Eosin to PDO is unfavorable and the decomposition of PDO is most likely sensitized by electron transfer. In addition, at the concentration of PDO employed the fluorescence of Eosin is not significantly quenched and it is concluded that the excited state involved is the triplet of Eosin. 

In type II oxygenation, oxygen is sensitized by electron transfer and the chemical reaction involves singlet oxygen. ... 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

The photochemical deprotection of sulphonamides to free amines is synthetically useful193. This process is caused by electron transfer from an electron-donating sensitizer such as 1,2-dimethoxybenzene or 1,4-dimethoxybenzene and the presence of reductants like ammonia or hydrazine is also required194 (equation 139). 

A number of papers have reported studies on pyrimidine radical cations. 1-Methylthymine radical cations generated via a triplet-sensitized electron transfer to anthraquinone-2,6-disulfonic acid were detected by Fourier transform electron paramagnetic resonance (FTEPR). The parent 1-methylthymine radical cation, and its transformation to the N(3)-deprotonated radical cation, were observed. Radical cations formed by addition of HO and POs" at C(6) were also detected depending on the pH. Similarly, pyrimidine radical cations deprotonated at N(l) and C(5)-OH were detected from the reaction of 804 with various methylated pyrimidines." These radicals are derived from the initial SO4 adducts of the pyrimidines. Radical cations of methylated uracils and thymines, generated by electron transfer to parent ions of... 

The DCA-sensitized irradiation of 107a for 13 hr affords, after column chromatography on silica gel, the rrans -cyclopropane derivative 108a (10%) as a 1 1 mixture of C=N bond fiZ-isomers. Similarly, irradiation of the oxime acetate 107b under these conditions for 2.5 hr affords, after chromatography, the rrans -cyclopropane derivative 108b (12%). These results show that the novel 1-ADPM rearrangement promoted by electron-transfer sensitization can be extended to other C—double-bond derivatives. 

The NU-Ge-1 exhibits optical absorption effects that are tunable by changing the wall thickness of the germanium framework. The pore structure of this material can be both active and sensitive toward electron-transfer adsorbates. We have observed that the electronic structure of the NU-Ge-1 can be altered in a controllable manner... 

It has been assumed so far that the sensitizer acts by an energy-transfer mechanism, but in some cases other modes of interaction may occur. It is possible that electron transfer takes place to give the radical anion or the radical cation of the alkene, which is the species that subsequently isomerizes. This is likely to be the case in the chlorophyll-sensitized isomerization of vitamin A acetate, which is used commercially to obtain the required all-trans isomer 12.8) from the mixture of Isomers resulting from the synthesis. Unlike triplet-sensitized reactions, electron-transfer isomerizations frequently lead to a predominance of the most thermodynamically stable isomer. 

The electron transfer sensitized photooxygenation reactions have been the subjects of numerous investigations and of several reviews [181-183]. In the presence of oxygen, CP is oxygenated during the DCA photosensitized reaction via the reaction of CP"+ and 0 2", which is formed by electron transfer from DCA to 02, to give the corresponding dioxolanes (Scheme 20) [173] ... 

Time resolved laser flash photolysis and electric spin resonance (ESR) spectroscopic investigations were used to get further insight to the reaction mechanism. Both methods demonstrate the formation of do using PET conditions [175,214,215], Upon addition of H donors the signal of do is quenched [214], The oxidation of do is followed by H abstraction from the H donor as shown in Scheme 9. Nucleophilic addition can be excluded because no alkoxyfullerenes were detected at all [173], After reduction of H-do, e.g., by electron transfer from the reduced sensitizer molecule H-do might recombine with R" to the final product. Decay experiments of do by the addition of alcohols support the proposed mechanism of H abstraction as a first step. The involved radical products reveal do as an electrophilic radical. 

Dynamic spectral sensitization (16). Based upon the kinetic scheme given above (Scheme 1), the dynamic sensitization pathway leading to the desired product, e.g. by electron transfer, ray be written as shown in Equation 2. 

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