Photoexcitation sensitization

There are a variety of photochemical reactions that non-conjugated dienes can undergo. One of these that is currently of considerable interest is the reactivity brought about by electron-accepting sensitizers such as the cyanoarenes. The photoreactivity of these systems involves the photochemical excitation of the sensitizer to an excited state7. Thereafter, the reactivity is dependent on the ease of oxidation of the alkene or diene. With the transfer of an electron from the diene to the photoexcited sensitizer a radical cation is formed. It is this intermediate that brings about the various processes which occur within the diene systems under investigation. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Table 5, The free energy changes11 (AG) of the electron transfer reaction of photoexcited sensitizers with pyridinium ions...

Turro s group196 has investigated and conducted a statistical analysis of luminescence quenching of the photoexcited sensitizer, [Ru(phen)3]2+, using [Co(phen)3]3+ on the surface... 

Photoinduced decomposition of the cationic photoinitiator can also be achieved by electron transfer reaction between photoexcited sensitizer and onium salt [15,16] (Scheme 13.4). 

SCHEME 13.4 Electron transfer from photoexcited sensitizers to onium salts. 

More information on the initial attack of the photoexcited sensitizer on the alcohol and about the subsequent behavior of the sensitizer is given by Wells in a comprehensive series of kinetic investigations. It was shown that deactivation of the photo-excited sensitizer proceeds in two ways only, as shown in reactions (9) and (10). 

Photoinduced electron transfer among the donor (D) photoexcited sensitizer (P), and acceptor (A) in a polymer solid phase. 

Pyrrole, thiophene, etc. are polymerized by electrolytic oxidative polymerization. It is considered also that photopolymerization may proceed if the oxidation potential of a photoexcited sensitizer is high enough to polymerize them. Photopolymerization is commonly used in a photo-resist patterning process with a photomask. 

When exposed to both light and oxygen, thiophene photo-oxidation can occur [66, 67]. Singlet oxygen formed from photoexcitation, sensitized by the polymer absorption, can then undergo a Diels-Alder 1,4-addition to the thienyl double bonds of the thiophene backbone, breaking the n-electron system. Free radical-assisted chain scission has also been proposed. 

Another common loss process results from electron—hole recombination. In this process, the photoexcited electron in the LUMO falls back into the HOMO rather than transferring into the conduction band. This inefficiency can be mitigated by using supersensitizing molecules which donate an electron to the HOMO of the excited sensitizing dye, thereby precluding electron—hole recombination. In optimally sensitized commercial products, dyes... 

Excited states can also be quenched. Quenching is the same physical process as sensitization, but the word quenched is used when a photoexcited state of the reactant is deactivated by transferring its energy to another molecule in solution. This substance is called a quencher. 

In most studies, heterogeneous photocatalysis refers to semiconductor photocatalysis or semiconductor-sensitized photoreactions, especially if there is no evidence of a marked loss in semiconductor photoactivity with extended use. It is meant here that the initial photoexcitation takes place in the semiconductor catalyst substrate and the photoexcited catalyst then interacts with the ground state adsorbate molecule [209]. 

Many fluorophores are sensitive to changes in the hydropho-bicity of the immediate environment. Therefore, bringing these fluorophores into a different environment may also produce a change in FRET, when a second fluorophore is affected by the emission change of the first. Fluorophores like Nile Red with changes of up to 100 nm when transferred from water to an aprotic organic solvent are principally suitable for such an approach [71], Molecular rotors have the characteristic of having a quantum yield that depends on the viscosity. Such dyes are formed by an electron donor unit and an electron acceptor unit that can rotate relative to each other upon photoexcitation with a behavior that depends on the viscosity of the environment. These dyes have been included in FRET probes for viscosity studies [53],... 

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