Fluorenone, photoreduction

Thus for the photoreduction of benzophenone in the presence of benzhydrol (RH = < 2CHOH, C—H x 78 kcal/mole) the overall reaction is exothermic by 26 kcal/mole. For the photoreduction of acetophenone the value calculated in this way is —31 kcal/mole, while for fluorenone the abstraction is still exothermic by 10 kcal/mole. Clearly, the energetics of the abstraction reaction is not the reason why benzophenone and acetophenone photoreduce but fluorenone does not, since the values for the overall reaction would indicate that all three compounds should react. However, this conclusion is not necessarily valid if large activation energies are involved. ... 

The above ideas fit in nicely with the work of Cohen and Guttenplan223 who reported the photoreduction of fluorenone by triethylamine. Dilution of neat triethylamine with cyclohexane increased the quantum yield of reduction by decreasing the amount of singlet quenching.221... 

As a function of N,N-dimethyl ethanolamine concentration, the pKslymerization rate passed throi h a maximum, as shown in F. 2. Such behaviour parallels that observed for photoreduction of fluorenone by other tertiary amines, e.g. the quantum yield for photoreduction by triethyiamine has been measured (5) as 0.09 in neat amine, 0.9 in... 

The photoreduction of aromatic ketones by tertiary amines is reported [38] to proceed at rates which are substantially faster than those observed for the corresponding photoinduced hydrogen abstraction from, e.g. alcohols. A limit case is given by fluorenone, the photoreduction of which does not occur in alcohol, ether or alkane solution, but readily takes place in the presence of amines, tertiary amines being the most effective [39,40]. Xanthone has also been reported to be easily photoreduced by iV,A-dimethylaniline [41], but not by 2-propanol [42]. However, the oxidation of tertiary amines photosensitized by fluorenone and xanthone is much less efficient than when sensitized by benzophenone, apparently because of lower rates of hydrogen abstraction [43]. Fluorenone/tertiary amine systems have been used successfully to photoinitiate the polymerization of MMA, St, MA and AN [30,38,44] and rather similar results have been obtained in the photoinitiated polymerization of MA by the benzophenone/EtsN system [45]. Thus, the great variety of substrates participating in exciplex formation has been readily extended to polymer-based systems. 

Solubility characteristics of the polymeric product allow us to exclude the presence of homopoly(styrene). Moreover, taking into account that semipinacol-type radicals, as those produced by photoreduction of fluorenone, are not significantly active for initiating MMA polymerization [8], the presence of homopoly(MMA) can also be ruled out. 

The photoreduction of the above dye, 5,7-diiodo-3-pentoxy-9-fluorenone, in the presence of (phenylthio)-acetic acid and its tetrabutylammonium salt occurs via a photoinduced electron transfer process. On the basis of the known photochemistry of sulfur-containing aromatic carboxylic acids, it is postulated that the existence of the carboxyl group in an ionic form allows a rapid decarboxylation, yielding a neutral very reactive a-alkylthio-type radical (R-S-CH2 ). 

A model reaction of hydrogen transfer has been proposed that describes well the results of kinetic studies of the photoreduction of o- and p-quinones and fluorenone in the presence of p-substituted A,A-dimethylanilines and polyalkylbenzenes/° Photosubstitution of the sulfo group for hydrogen is observed upon irradiation of sulfonated derivatives of hydroquinone. The reaction can be promoted by visible light, with eosin as sensitizer/°°... 

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