Photosensitizers, aromatic ketones

From the foregoing discussion it might seem fruitless to utilize MNP to investigate photochemical reactions. However, the monomer is transparent between ca. 270 and 550 nm, and by irradiating reaction mixtures in this window excellent results have been obtained without complications from DTBN formation (Leaver et al., 1969 Leaver and Ramsay, 1969a,b Torssell, 1970). This expedient is unfortunately not infallible, there being good evidence that aromatic ketones can photosensitize MNP dissociation (Ikeda et al., 1978). 

Type 2 initiators are bimolecular systems and consist of a photosensitizer and a hydrogen donor. The most well known system is benzophenone/tertiary amine. Because of the relatively weak absorption of benzophenone at 360 nm the efficiency of this system is rather low. A more efficient aromatic ketone is thioxanthone (TX) and its derivatives simply because of the increased at the exposure wave length. 

Canonica, S., U. Jans, K. Stemmier, and J. Hoigne, Transformation kinetics of phenols in water Photosensitization by dissolved organic material and aromatic ketones , Environ. Sci. Technol., 29, 1822-1831 (1995). 

The homolysis of iodonaphthalene can be photosensitized in the presence of ketones.216 Homolysis of phenyl-iodine and phenyl-bromine bonds is a general photochemical reaction217,218 but even in the case of aromatic ketones,217 it is not as yet known whether the triplet state is involved in the direct photolysis. 

Hammond and Cole reported the first asymmetric photosensitized geometri-r cal isomerization with 1,2-diphenylcyclopropane (Scheme 2) [29]. The irradiation of racemic trans-1,2-diphenylcylcopropane 2 in the presence of the chiral sensitizer (R)-N-acetyl-1 -naphthylethylamine 4 led to the induction of optical activity in the irradiated solution, along with the simultaneous formation of the cis isomer 3. The enantiomeric excess of the trans-cyclopropane was about 1% in this reaction. Since then, several reports have appeared on this enantiodifferentiating photosensitization using several optically active aromatic ketones as shown in Scheme 2 [30-36]. The enantiomeric excesses obtained in all these reactions have been low. Another example of a photosensitized geometrical isomerization is the Z-E photoisomerization of cyclooctene 5, sensitized by optically active (poly)alkyl-benzene(poly)carboxylates (Scheme 3) [37-52]. Further examples and more detailed discussion are to be found in Chap. 4. 

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. 

Subsequently to the work on DIBF above, two other aromatic ketones, 2-chlorothioxanthone (CTX) and Michler s ketone (MK) were studied as photosensitizers for the decomposition of onium borates [55, 56]. Each absorbs light at 365 mn and their photochemistry and photophysics are well known. The time dependence of the photopolymerization of tetrahydrofurfuryl acrylate (THFA) in MeCN solution in the presence of CTX with selected onium borates (Figure 12) is shown in Figure 13. The rate of polymerization is dependent on the solvent with the fastest polymerizations observed when the onium borates are used in solvents where they form tight ion pairs. Thus higher rates are observed in the less polar CH2CI2. It is obvious that the rate of photopolymerization without sensitizer is much slower (two orders of magnitude) than that observed for sensitized photoinitiated polymerization. 

Other examples of such photosensitizers include carbonyl compounds such as ketones, both aliphatic and aromatic. Aromatic ketones are, however, more useful in commercial practice, since their absorptions occur at longer wavelength and their efficiency of initiation (quantum yield) is higher. 

Charge-transfer initiator systems are attracting increased interest, as is cationic photopolymerization. Photopolymers of acetylenes and polyacetylenes are of potential importance as conductors and semiconductors, for example in solar cell applications (Fouassier et al., Yee, Day, et al., inter alia). Fouassier et al. report that vinyl polymerization photosensitized by aromatic ketones can proceed faster in a micellar system through enhanced initiation rather than propagation. 

Poly( vinyl cinnamate) itself is only weakly absorbent above 320 nm. Its photoresponse is generally of the order of a tenth of that of dichromated colloids, but the rate can be accelerated by the use of photosensitizers, such as nitroamines (increase of the order of 100 times), quinones (increase of the order of 200 times for specific ones), and aromatic amino ketones (increase of the order of 300 times for specific ones). A commonly used aromatic ketone is 4,4 -bis(dimethylamino)-benzophenone, also known as Michler s ketone. 

Unfortunately, NBD does not absorb in the visible and UV-A (400-320 nm) range of the solar spectrum. This problem may be overcome by the use of a wide variety of photosensitizers, which includes transition metal complexes and organic sensitizers like acridones and aromatic ketones . In this section, however, new trends in the exothermic back-isomerization Q NBD are of considerable interest because in this two cyclopropane ring moieties are involved. This ring-opening reaction may be achieved by transition metal complexes " or by Lewis acid catalysis the latter, however, leads to side reactions which include cationic oligomerization ". ... 

Based on the above, an initiating composition for cationic photopolymerization, with visible and long-wavelength UV light was described by Crivello et al. The structure of the monomers plays a key role in these photosensitization processes. Useful aromatic ketones are camphoquinone, benzyl, 2-isopropylthioxanthone, or 2-ethylanthraquinone. The monomer-bound radicals reduce diaryliodonium salts or dialkyl phenacylsulfonium salts rapidly to form monomer-centered cations. These cations then initiate the polymerization of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potential, however, such as triarylsulfonium salts, do not undergo this reaction. 

A wide assortment of electron transfer photosensitizers for diaryiiodonium salts have been described in the journal and patent literature. Among these are polynuclear aromatic hydrocarbons with three or more rings such as anthracene, alkoxyanthracenes, pyrene, and perylene heterocyclic compounds of low basicity such as carbazoles " and phenothiazines aromatic ketones such as benzophenone, Michler s ketone, and thioxanthone and substituted thiox-anthones coumarins phenanthrene-9,10-quinone Mannich bases and (dimethylamino)benzylidyne compounds. In addition, the use of dyes such as eosine, acridine orange, acridine red, and benzoflavin has been employed to provide photosensitization in the visible region of the spectrum. 

The photolysis of dialkyl-4-hydroxyphenylsulfonium salts is not photosensitized by aromatic hydrocarbons. Instead, aromatic ketones are excellent photosensitizers for these photoinitiators Scheme 12 gives the mechanism which has been proposed for the photosensitization of dialkyl-4-hydroxyphenylsulfonium salts. 

Electron transfer is often observed for aromatic ketone-amine pairs and always with dye/coinitiator systems. The photosensitization by dyes is dealt with in detail in Section V. 

Electron transfer photosensitization of spiro-diaziridines with aromatic ketones is a method for the generation of carbenes initiated by CN bond fragmentation at the cation radical stage. 

Various organic molecules are used as photosensitizers in liquid-phase reactions, for example, anthraquinones, aryl ketones, polycyclic aromatic hydrocarbons, dyes, etc. The following mechanism, as the most probable, was suggested for the initiation by the organic photosensitizer Q with the aromatic ring [204-208] ... 

The reaction of formamide with aromatic compounds under ultraviolet irradiation is still unexplored and only preliminary results have so far been obtained. In the cases already studied it has been found that this reaction must be sensitized with a ketonic sensitizer, usually acetone, in order to take place. The mechanism of the photoamidation of aromatic compounds certainly differs from the one of simple olefins. The detailed mechanism still awaits further experimental evidence, and in some cases involves, most probably, radical combinations and not addition of radical to unsaturated systems. Interactions of the excited sensitizer with aromatic compounds, having in some cases triplet energies similar or just a bit higher than those of the sensitizers used, must be brought into consideration. Experimentally it has been shown that the photosensitized amidation of benzene leads to benzamide (11),... 

Substrates containing an electron-rich double bond, such as enol ethers and enol acetates, are easily oxidized by means of PET to electron-deficient aromatic compounds, such as dicyanoanthracene (DCA) or dicyanonaphthalene (DCN), which act as photosensitizers. Cyclization reactions of the initially formed silyloxy radical cation in cyclic silyl enol ethers tethered to an olefinic or an electron-rich aromatic ring, can produce bicyclic and tricyclic ketones with definite stereochemistry (Scheme 9.14) [20, 21]. 

In view of the biological activity of taxodione, some attention has been directed at the introduction of a C-6 carbonyl group. The nor-abietatetraene (56) was converted52 into a cis-glycol which on reaction with formic acid gave the C-6 ketone. As an alternative sequence, hydroboronation and oxidation also gave a C-6 ketone. The photosensitized oxidation of 6,7-dehydro-aromatic diterpenoids [e.g. (57)] has also been studied53 with this objective. However, the products were unsaturated ketones [e.g. (58)]. 

Typical photosensitizers for diaryliodonium salts are condensed ring aromatic hydrocarbons, diaryl ketones, and acridinium dyes. Condensed ring aromatic hydrocarbons are particularly effective photosensitizers for triarylsulfonium salts. The use of photosensitizers in onium salt photolysis permits the photoimaging processes induced by these compounds to be optimally fitted to the specific irradiation source used for their exposure. 

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