Coumarin electron transfer processes

In the initial description of the cationic dye-borate system [24, 76], it was postulated that electron transfer was possible because, in nonpolar solvents, dye/borate salts exist predominantly as ion pairs. Since the lifetime of the cyanine singlet excited state is quite short [24, 25], this prerequisite is crucial for eflfective photo-induced electron transfer. Recently initiator systems in which neutral dyes are paired with triarylalkylborate anions have appeared in the literature [77]. In the latter case, the borate ion acts as the electron donor while neutral merocyanine, coumarin, xanthene, and thioxanthene dyes act as the electron acceptors. It is obvious that these initiating systems are not organized for effective electron transfer processes. The formation of an encounter complex (EC) between excited dye and electron donor is required. 

The coumarin dyes 98 is an ideal system to investigate the importance of the hydrogen bond in an electron transfer process, which involves the carbonyl group of 98. 

Aminocoumarins 101-103 present ultrafast fluorescence quenching by the fluorescence up conversion method when the electron donating partners (aniline or N,N-dimethylaniline) are present as solvents. An intermolecular electron-transfer process between the coumarins and the anilines is indicated to be active in depressing the quenching rate171. 

Pyrylium and Thiopyrylium Salts In the presence of additives such as a perester and in conjunction with ketocoumarins and coumarins, pyrylium, and thiopyrylium salts allow to decompose peresters. Addition of a diphenyl iodonium salt or a bromo compound such as CBr4 to a thiopyrylium salt leads to an electron transfer process resulting in the generation of radicals [211]. 

The reaction mechanisms have been explored in the coumarin, ketocoumarin, or titanocene derivative/bis-aryl imidazole Cl-HABPmercaptobenzoxazole system [240]. For example (10.67), when the sensitizer is the coumarin Cl, an electron transfer occurs and the formed Cl-HABI radical anion leads to a lophyl radical that undergoes a hydrogen transfer with the thiol SH (the lophyl radical anion reacts with the thiol radical cation). The Cl/SH interaction can also participate in the initiation process. The role of new thiols and disulfides has been recently studied [241]. 

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