Benzophenones excited states

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). 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

These reactions usually occur via the triplet excited state T,. The intersystem crossing of the initially formed singlet excited state is so fast (fc 10 s ) that reactions of the S state are usually not observed. The reaction of benzophenone has been particularly closeh studied. Some of the facts that have been established in support of the general mechanisir. outlined above are as follows ... 

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

From these equations one can approximate kf for benzophenone to be 5 x 10s sec-1. This, however, is the expected rate constant for fluorescence, which should be in competition with radiationless deactivation of the excited state kd. In actuality no fluorescence is observed for benzophenone although the fluorescence techniques are sensitive enough to detect fluorescence occurring with a quantum yield as low as 0/ = 0.001. Therefore kd must be at least 1000 times greater than kf We have... 

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... 

In an attempt to sensitize the thiosulfate bond cleavage, benzophenone (10% by weight) was incorporated into the polymer film. Upon photolysis at 366 nm, the 639 cm 1 thiosulfate band was reduced (Figure 10) as in the case of direct photolysis at 254 nm and 280 nm. Since benzophenone is a known triplet sensitizer it is likely that the S-S bond cleavage in the thiosulfate group occurs from a triplet excited state in the sensitized reaction. Incidentally photolysis of a PATE film at 366 nm in the absence of benzophenone resulted in no loss of the 639 cm 1 IR peak. Unfortunately due to the film thickness, we were unable to obtain accurate quantum yields for either the direct or sensitized photolysis. Finally it should be noted that no chemical evidence has been presented to confirm disulfide formation. Results from the photolysis of a PATE-type model compound will be offered to substantiate the claim of disulfide formation as well as quantitate the primary photolysis step. But first, we consider photolysis of a PASE polymer film. 

Many aromatic aldehydes and ketones (e.g. benzophenone, anthrone, 1- and 2-naphthaldehyde) have a low-lying n-n excited state and thus exhibit low fluorescence quantum yields, as explained above. The dominant de-excitation pathway is intersystem crossing (whose efficiency has been found to be close to 1 for benzophenone). 

It is noteworthy that the absolute rate constants for the reaction of the benzophenone triplet with Et3SiH, n-C5HnSiH3, PhSiH3, and Cl3SiH have been measured by LFP,56 and comparison of the kinetic data with corresponding data for reactions of /-BuO radicals shows that these two transient species have a rather similar reactivity toward silanes. Furthermore, the xanthate and the p-methoxyacetophenone triplets were found to be more and less reactive, respectively, than the benzophenone triplet with Et3SiH.56 Similar behavior of excited states in reactions with tin hydrides is discussed in Section V. 

Lastly, photochemically unstable ligands should be avoided. Re(bpy)(CO)3Cl shows a moderately efficient MLCT emission at room temperature (R. M. Ballew, unpublished results from our laboratory). However, the apparently closely related Re(dpk)(CO)3Cl (dpk = 2,2 -dipyridyl ketone) shows a benzophenone like phosphorescence at 77K indicating that the n-n excited state of the ketone in complex is the lowest state of the complex. No luminescence is seen at room temperature, and even at 77K the dpk triplet state is such a powerful hydrogen atom extractor that it removes protons from alcohol glasses as seen by the formation of the intense blue color of the keto free radical. The absence of an MLCT emission is caused by the greater difficulty of reducing dpk relative to bpy, which pushes the MLCT states above the dpk ligand states. 

As mentioned, electronically excited states are both easily reduced (since an electron can be accepted in the half vacant HOMO) and easily oxidized (since an electron can be donated from the half-hlled LUMO) under much milder conditions than the corresponding ground states. As an example, reducing the ground state of benzophenone requires the use of a highly reactive reductant, such as sodium metal... 

Figure 3.5 Reduction of benzophenone by sodium and of the excited state by Et3N.

The photohydrolysis of 2-fluoro-4-nitroanisole to 2-methoxy-5-nitrophenole is sensitized by benzophenone and completely quenched by sodium sorbate The excited state multiplicity in photoaminations has also been studied. Photolysis of mNA in liquid ammonia yields m-nitroaniline. If the amination is carried out in a large excess of benzophenone, 2-methoxy-4-nitroaniline is formed instead and thus an excited singlet state as reacting species is envisaged in the unsensitized photoamination loo.ioi). it may well be that uptake of the nucleophile present in high concentration successfully competes with intersystem crossing. 

It is well-known that many organic excited states (e.g. the triplet state of benzophenone) can effectively abstract hydrogen atoms from organic compounds such as alkanes and alcohols. This behaviour is not commonly found for metal-containing compounds - a notable exception being the lowest excited state of uranyl ion which abstracts H atoms from alcohols, sugars etc., with the resultant formation of free radicals and U(V) compounds. Recent work has shown that it is very effective in inducing strand breaks in DNA (see Sect. 8). 

The excited states of carbonyl compounds are often considered to be similar to alkoxyl radicals because of the unpaired electron on the oxygen atom. In particular, the benzophenone n—tt triplet mostly reacts in the same manner as and at a similar rate to t-BuO radicals. 

An intriguing possibility, which appears close to realization, is that the excited state reagent may be generated by methods other than irradiation. For example, diphenylcarbene reacts with oxygen (in the dark) to yield ultimately the benzophenone triplet.26 The excited state thus prepared should be fully capable of participating in the cycloaddition reaction. 

The assignment of the excited state of benzophenone as a triplet which could act as a sensitizer was made by Hammond and Moore in 1959 (equation 49)/ and this led to a great surge in radical study using photochemical techniques. The role of photoexcited benzophenone as a diradical initiator for benzaldehyde oxidation was previously shown explicitly by Backstrom in 1934 (equation 34). ... 

These compounds are often used as photostabilizers in plastic materials. 2-methyIbenzophenone (A) appears to undergo intramolecular H-abstraction in 2-propanol in the excited state. The unstable enol reverts to the ketone in the ground state. The possibility of such a six-membered transition state does not exist for 2-t-butyl benzophenone, and it is as reactive towards H-abstraction as benzophenone itself. 

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