Chromophore benzophenone

Correct experimental conditions are vital. In particular, it is essential to irradiate only the benzophenone chromophore, which can be achieved by employing an appropriate u.v. laser, for direct excitation of the azo compound produces a singlet diradical that collapses to bicyclopentane. Oxygen pressure (150 psi) and reaction time (60-70 h) must be carefully regulated to obtain optimum yields (ca. 20%) of 9. 

The above results may be rationalized In terms of Scheme 2, which proposes that the benzophenone chromophore catalyzes... 

No exciton coupling was observed for the dialdehyde 165. The CD of (+)-166 also shows a simple pattern. The small amplitude of those Cotton effects can be attributed to the complicated polarization spectra of benzophenone chromophore and to the conformational flexibility of the 2-tolyl group. In contrast to (+)-165 and (+)-166, the quinone (+)-167 exhibits relatively strong Cotton effects ascribed to exciton interaction between favorably oriented transition moments in the 9,10-anthraquinone chromophore359. 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

It is noteworthy that the addition of lEE to poly(ABP) (Table 5) does not appreciably change the activity of poly(ABP), thus suggesting that the photoreduction of excited benzophenone chromophores is much more efficient when both ethereal groups and aromatic ketone moieties are attached to the same macromolecule. This behaviour can easily be explained in terms of a higher local concentration of ethereal functions around the benzophenone chromophores in poly(ABP-co-AEE) than in the corresponding poly(ABP)/IEE system. 

One of the earliest examples of photodecomposable, but thermally stable, polymeric systems contains pendant perester substituted benzophenone moieties, the photodissociation of the former group being promoted by the benzophenone chromophore which behaves as a triplet sensitizer. Indeed, the homopolymer of 4-vinylbenzoyl-4 -tert-butyl perbenzoate and its copolymers with styrene [poly(VBPE) and poly(VBPE-co-St), respectively] have been prepared and checked as photoinitiators in the polymerization of styrene and MMA by irradiation at 366 nm [79,80]. 

A FBrster-type transfer mechanism (48) was proposed to occur. Batochromic shifts and hyperchromicity were observed In the absorption spectra of systems [10b,1,3] and to the largest extent In [10b,2] (44,45). On specific excitation of the benzophenone chromophore, triplet transfer to the naphtalene moiety occurs with 100 percent efficiency. The rate of transfer is greater than lO s l g d not measurably dependent on chain length. Since the ratio of Intersystem crossing in benzophenone is of the order of (49), energy transfer will occur, after inter-... 

Poly(vinyl halides) - The photocatalysed oxidation of PVC has been undertaken in the presence of titanium dioxide and zinc oxide pigments and the extent of dehydrochlorination measured. Acetic and formic acids were major products along with carbon dioxide. Copper(II) dialkyldithiocarbamate complexes are also sensitisers. Photodegradable PVC has also been developed by grafting with benzophenone chromophores. Plasticised PVC also degrades and discolours on irradiation but this is due mainly to the plasticiser. ... 

Finally, diphenylsulfonium-bis-(methoxycarbonyl)methylide, IX [87], and salts of p,p -bis-[(triphenylphosphonio)methyl]-benzophenone, X [88], have been claimed as photoinitiators of free radical polymerizations [89]. They presumably photolyze homolytically in the case of X, intramolecular triplet energy transfer from the benzophenone chromophore to the phosphonium moieties may likely be involved. 

Bosca, F. and Miranda, M.A., 1998, Photosensitizing drugs containing the benzophenone chromophore, J. Photochem. Photobiol. 43, 1-26. 

THIOXANTHONES (CTX, DTX, ITX) Thioxanthones are a subclass of the benzophenone chromophore modified by the presence of a bridging sulfur atom, resulting in a greatly enhanced UV absorption spectra as is shown in Table 4. Like benzophenone, these aromatic ketones require an amine synergist for efficient curing. Fig. 6 shows 3 examples of this class of photoinitiators chlorothioxanthone (CTX),... 

For example, by attaching the benzophenone chromophore to position 3a of a steroid molecule (197), a series of selectively functionalized derivatives have been obtained under various experimental conditions (Fig. 5.19). 

In the case of poly[bis(4-benzoylphenoxy)phosphazene] (4.91), which contains the benzophenone chromophoric group, the photocrosslinking... 

Martinez, L. J. and Scaiano, J. C., Transient intermediates in the laser flash photolysis of ketoprofen in aqueous solution unusual photochemistry for the benzophenone chromophore, /. Am. Chem. Soc., 119, 11066, 1997. 

Bosca, E, Miranda, M. A., Carganico, G., and Mauleon, D., Photochemical and photobiological properties of ketoprofen associated with the benzophenone chromophore, Photochem. Photobiol., 60, 96,1994. 

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