Benzophenone excitation transfer

Intramolecular transfer of excitation is of considerable importance in photochemistry. Leermakers, Byers, Lamola, and Hammond (332) have demonstrated by optical emission the occurrence of intramolecular electronic energy transfer in 4-(l-naphthylalkyl)benzophenone. de-Groot and van der Waals (333) have examined the temperature-dependent ESR spectra of the phosphorescent states of benzene, toluene, triptycene, and tribenzotriptycene. The latter two molecules consist of three benzene or naphthalene systems joined together, and their ESR spectra reveal the intramolecular excitation transfer between the benzene or naphthalene subsystems. At 20°K, the excitation is mainly localized in one of the subsystems, but at such... 

Intermolecular triplet-state energy transfer is well known in photochemistry. McDowell et al. (334) first demonstrated by ESR the triplet energy transfer from benzophenone to naphthalene in a rigid glass. The excitation transfer is found to be independent of orientation (335a), while the probability of excitation transfer per lifetime is dependent on concentration (335b). 

In a DISP 2 mechanism the second-order disproportionation step is rate limiting (see Section 2). An example of such a process involves the photoreduction of the dye fluorescein in basic aqueous solutions at mercury electrodes (Compton etal., 1988b). The photoreduction of benzophenone (86) and fluorobenzophenone in acetonitrile also proceeds via a DISP 2 type mechanism as verified by channel electrode voltammetry (Leslie et al., 1997). The rate-limiting step is electron transfer (86c) between photoexcited radical anion and the initial anionic species formed on electron transfer at the electrode surface. This process is further complicated by significant con-proportionation (86e) and quenching of the benzophenone excited state (86f). 

The conclusions of Hammond and of Backstrom were confirmed by a flash spectroscopic investigation which permitted direct measurement of the rate constants for triplet decay, k i, = 1X10 sec in benzene, hydrogen abstraction from benzhydrol, fcss = 2 X 10 I mole - sec h triplet excitation transfer from benzophenone to suitable acceptors (Bell and Linschitz, 1963). 

These authors observed phosphorescence from 4,4-diphenylcyclohexadie-none at 77°K (0-0 band at 68.8 kcal per mole) and pointed out that the similarity between the phosphorescence spectrum of the dienone and that of benzophenone indicates that the lowest triplet state of the former compound is n — IT. This evidence suggests that a triplet state of the dienone may be involved in the unsensitized rearrangement but is not sufficient to justify Zimmerman and Swenton s conclusion that such a state is demanded. Interestingly, naphthalene (triplet 0-0 band at 61 kcal per mole) sensitizes rather than quenches Reaction (110). Such sensitization has also been observed by Caspar and Hammond (unpublished results) in a similar system and can be best interpreted as involving singlet excitation transfer from naphthalene to the dienone. 

Upon exposure to uv light, ground-state benzophenone is excited to the ttiplet state (a diradical) which abstracts an alpha H atom from the alcohol, resulting in the formation of two separate initiating radicals. With amine H atom donors, an electron transfer may precede the H-transfer, as in ttiplet exciplex formation between benzophenone and amine (eq. 43) ... 

As for the energy transfer to the subsurface layers of zinc oxide from the singlet oxygen molecules, the transfer should lead to an intn ease in the electrical conductivity of semiconductor either due to ejection of electrons into the conduction band h-om shallow traps [67], or due to the injection of electrons into zinc oxide by excited particles [68]. Effects of this kind were observed in the interaction between a ZnO surface and excited pairs of benzophenone [70], and also in adsorption of singlet oxygen on the surface of ZnO monocrystal in electrolyte [69]. 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

The decomposition of methanesulphonyl azide in isopropyl alcohol could be effected by selective irradiation of 2-acetonaphthone instead of benzophenone 21>. Since 2-acetonaphthone triplets are incapable of hydrogen abstraction from isopropyl alcohol 22>, initiation must occur via transfer of excitation energy to the azide. A marked difference was observed from benzophenone sensitization in that the reaction was extremely slow, gave a nitrogen yield of only 68%, and produced a yellow solution 21>. 

There is some controversy regarding the decay mechanism of 2-hydroxy benzophenone. Scheme 1 summerizes all of the possible decay paths which involve proton transfer in the excited state... 

The electron transfer reaction of excited benzophenone and trialkylamines has been applied to design photochemical cells54. 

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