Sensitization singlet mechanism

Me and co-workers studied the intramolecular photocycloaddition of naphthyl groups in poly(l-vinylnaphthalene) (335) in cyclohexane, benzene, and dichloromethane [338], In cyclohexane, the reaction proceeds via first-order kinetics to a high conversion of 70%, whereas in dichloromethane, the reaction levels off at a very low conversion of 20%. Quenching and sensitizing experiments proved that the Mplet mechanism is predominant in cyclohexane, whereas in dichloromethane, a singlet mechanism is predominant. The addition of trifluo-roacetic acid to the illuminated sample restores the initial absorbance. 

A similar study of the photooxidation of some spiropyrans and spironaphthox-azines indicates that the spiro and open forms of these dyes are singlet oxygen quenchers and that the colored form does not act as a sensitizer. A mechanism is proposed that involves the formation of a superoxide radical anion by photoinduced electron transfer to oxygen from a merocyanine form of the dye, followed by nucleophilic attack of the radical anion on the radical cation of the dye.174... 

Photostationary trans/cis ratios in the presence and absence of ferrocene or azulene either under direct or sensitized excitation conditions give different results. The simplest explanation is that 4 is quenched in the former and 3t in the latter cases. Further evidence for a mere singlet mechanism is the linearity of the Stern-Volmer plot (l/0, c vs. [Q]). This is not in agreement with a quenching of both 4 and 3t in the direct trans - cis photoisomerization. 

This photorearrangement was not sensitized by triplet sensitizers such as benzophenone and MK and was not quenched by molecular dioxygen and 2-methyl-1,3-butadiene, and thus a singlet mechanism is proposed. The homolysis of the C1-C2 bond of 7 from the excited singlet state of 7 generates the 1,3-singlet... 

The transfer of energy must proceed with net conservation of spin. In the usual case, the acceptor molecule is a ground-state singlet, and its reaction with the triplet state of the sensitizer will produce the triplet state of the acceptor. The mechanism for triplet photosensitization is outlined below ... 

This behavior has been verified with bithiazole derivative 94 (Scheme 36). Neither acetophenone nor benzophenone sensitized the reaction, in agreement with an ICI mechanism via the excited singlet state (86TL6385 87JA938). 

A/E) (Roth, 1972b). In contrast, no polarization is observed in the absence of a sensitizer, despite the fact that ethylbenzene is produced. This is consistent with a direct insertion of singlet methylene into the C—H bond, but it could also arise from an abstraction-recombination mechanism if the lifetime of the intermediate radical-pair were too short to permit a significant amount of Tq-S mixing. 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

Several mechanisms have been postulated in order to account for ketone-sensitized photodehydrochlorination. Benzophenone and acetophenone have been suggested to act as singlet sensitizers via a collisional deactivation process (13). An alternative mechanism proposed for benzophenone involves abstraction of a methylene hydrogen from PVC by the triplet ketone (Equation 2), followed by 3 scission of a... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

---