Photoinitiated electron transfer processes

Each of the terms in Eq. (1) must be known in order to calculate the free energy change for a photoinitiated electron transfer process (Eq. (2)). 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

A limited number of reports of indoles arising from o-bromoaniline and enolates have appeared (equation 107) (80JOC1546,8lT(S9)393). The final cyclization step is of the condensation type already recognized in several other procedures. The inertness of unactivated halobenzenes, such as o-bromoaniline, requires an alternative to direct aromatic nucleophilic substitution and those cases where success has been reported depend upon photoinitiated substitution by an electron transfer process. The scope of this method remains to be explored but it appears that alkyl, alkoxy and carboxy groups can be tolerated on the aromatic ring. When the enolates are derived from an unsymmetrical ketone in which one group is methyl, there appears to be a preference for exclusive involvement of the less substituted enolate, leading to 2-alkylindoles. 

Redox substitution reactions can be photoinitiated. Taube first proposed that the photo-catalyzed substitution of PtCll- occurs by an electron-transfer process (equation 560) to give a kinetically labile platinum(III) intermediate.2040 Further work on this system has shown that the exchange occurs with quantum yields up to 1000,2041-2043 and the intermediate has beer assigned a lifetime in the fis range.2044 Recently the binuclear platinum(III) complexes Pt2(P2OsH2)4Xr (X = Cl, Br, I) have been found to show similar behavior and both photoreduction and complementary redox reactions are again proposed to explain the substitution behavior.1500... 

Photoinitiated and optical electron-transfer processes and their relationship to corresponding ground-state thermal processes provide important new tests of theory, especially when comparisons are made for a given DBA system, of charge separation (CS) and charge recombination (CR), or of thermal and optical electron transfer (e.g., [27]). Photoinitiated processes have also been valuable in providing access to the dynamics of electron transfer in the activationless and inverted kinetic regimes (e.g., [43, 44]). 

In the present ehapter we consider the inter- or intramolecular photoinduced electron transfer phenomenon. We mainly focus on photoinduced electron transfer processes that lead to the photoinitiation of polymerization, and on processes initiated by photoredueed or photooxidized excited states. We concentrate especially on a description of the kinetic schemes, a description of the reactions that follow the primary proeess of eleetron transfer, and the characteristics of intermediates formed after electron transfer. Understanding the complexity of the processes of photo-initiated polymerization requires a thorough analysis of the examples illustrating the meehanistie aspects of the formation of free radicals with the ability to start polymerization. 

Considering these principles and the interaction between the dye (chromophore) and an electron donor in the ground state and after an electron transfer process, dye photoinitiators can be classified in three different groups ... 

Since photoinitiation occurs in specific monomeric mixtures, as Marcus theory predicts, the properties of the to-be-photopolymerized mixture (polarity, viscosity, electron-donating or electron-accepting properties) may play an important role in the overall efficiency of the process. Considering this, the monomer can participate in the photoinduced electron transfer process, either as a light-absorbing chromophore, as a hydrogen atom source, or as an electron donor/electron acceptor. 

Photoinitiating Donor-Acceptor Pairs with Electrostatic Interaction in the Ground State (Ground-state Ion Pair) and Neutral after Photoinduced Electron Transfer Process... 

Kinetic analysis gives additional information related to the reactivity of the free radicals obtained during the processes following a photoinduced electron transfer process. A variation in radical reactivity could be caused, for example, by their stability or their reactivity with monomer, as in the case of photoinitiation by 4-carboxybenzophenone-sulfur-containing carboxylic acid of free-radical polymerization [184]. 

In spite of this success, dyads like 1 suffer from a major limitation as mimics of the natural electron transfer process. The very structural and electronic features which ensure rapid photoinitiated electron transfer, and consequently a high quantum yield, in these molecules also favor rapid charge recombination (step 3 in Figure 2). Thus, the P -Q " state lives at most a few hundred picoseconds in solution. The P-Q systems, and indeed other dyad-type artificial photosynthetic molecules, are unable to reproduce the long-lived charge separation characteristic of the reaction center. The stored energy is quickly lost as heat. 

Excitation of the porphyrin moiety of 2 in dichloromethane solution yields the first excited singlet state, which can decay according to the pathways detailed in Figure 4. As with P-Q dyad 1, photoinitiated electron transfer competes with other decay processes to yield a C-P -Q charge-separated state. Fluorescence decay studies yielded a fluorescence lifetime r of 0.10 ns for 2 [27]. The hydroquinone form of the triad, 3, in which such electron transfer is not possible, has a fluorescence lifetime of 3.4 ns in the same solvent (see Section 111.A.). Application of Eq. (1) yields an electron transfer rate constant fcj in Figure 4 of 9.7 x 10 s", and consequently a quantum yield for this step of essentially unity. Thus, the addition of the carotenoid moiety to the molecule has had little influence upon the initial photodriven electron transfer step. 

Structure on hydrogel properties of 2-hydroxyethyl acrylate determined. " Polymers bearing tertiary amino groups have been synthesised and their fluorescence spectra found to be significantly quenched while maleic anhydride " and cyclododecanones have been found to be effective initiators of the photopolymerisation of styrene. Poly(methylphenylsilane) is also an effective photoinitiator for styrenes and acrylates via a photolytic process to give silyl radicals. Iron oxalate is also an effective photo initiator for acrylate monomers while a theoretical description of the kinetics of free radical dye-initiated polymerisation via an electron transfer process has been proposed. Using the Marcus theory it has been shown that the rate of electron transfer can affect the rate of initiation. 

Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and inter-molecular photoinitiated electron transfer. 

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