Quencher triplet

An alternative approach envisages the stimulating idea to produce an all-carbon fullerene polymer in which adjacent fullerenes are linked by covalent bonds and align in well characterized one-, two- and tliree-dimensional arrays. Polymerization of [60]fullerene, with the selective fonnation of covalent bonds, occurs upon treatment under pressure and relatively high temperatures, or upon photopolymerization in the absence of a triplet quencher,... 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

The benzoin ethers (75, R-alkyl R H) and the ot-alkyl benzoin derivatives (75, R=H, alkyl R =alkyl) undergo a-scission with sufficient facility that it is not quenched by oxygen or conventional triplet quenchers.276 This means that the initiators might be used for UV-curing in air. Unfortunately, it does not mitigate the usual effects of air as an inhibitor (Section 5.3.2). The products of a-scission (Scheme 3.53) are a benzoyl radical (13) and an ( -substituted benzyl radical (76) both of which may, in principle, initiate polymerization, 76 2"... 

It is possible to obtain kjkr from this expression either by keeping [BH2] constant and varying [Q] or by measuring the slope of l/4> vs. l/[BHa] with and without quencher. The former is the preferred procedure. In this way one finds, using oxygen as triplet quencher,... 

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... 

This reaction was run in the presence of the triplet quencher 1,3-pentadiene to prevent epimerization due to a-cleavage (a triplet reaction). 

Since the first two processes are spin-forbidden, it can clearly be seen that in the absence of triplet quenchers (e.g., oxygen) the triplet will be long lived. Consequently the experimental determination of the lifetime of triplet states... 

The triplet lifetime (in the absence of triplet quenchers) is given by... 

In the absence of triplet quenchers two processes compete for triplet deactivation radiative (phosphorescence) and nonradiative decay of the triplet to the ground state ... 

In order to determine the multiplicity of the reactive species, the photodimerization was carried out in the presence of the triplet quenchers oxygen and ferrocene. The results of these experiments are shown in Table 10.4.<41) It is obvious that the presence of oxygen exerts a large quenching effect on the production of the tram dimer and a smaller but significant effect on the formation of the cis dimer (the formation of tram dimer is decreased by oxygen by a factor of 25, while the cis dimer is decreased by a factor of 1.2). As with oxygen, the production of the tram dimer was quenched in the... 

Product (41) was also obtained in the presence of xanthone as sensitizer. This product could be totally eliminated by the triplet quencher piperylene, indicating (41) to be derived from the diphenylethylene triplet state. 

The direct photolysis of (6) was found to involve no triplet intermediates, since the same product distribution was obtained in the presence of the triplet quencher piperylene as in its absence. 

Evidence that the reactive state is a triplet follows from the use of triplet quenchers (oxygen and paramagnetic salts). For this we must add a further step to the mechanism ... 

The preceding discussion applied to aromatic ketone triplet reactions. With aliphatic ketones the situation is quite dilferent. As stated previously, aliphatic ketones undergo type II cleavage from both the excited singlet and the triplet state. By studying the reaction with and without added quencher, one can determine the characteristics of the reaction for each state, that is, the singlet reaction can be studied in the presence of a strong triplet quencher while the triplet reaction characteristics can be obtained by the difference between the reaction without quencher and that when quencher is added. For example, for the reaction... 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

The measurement of Tg is straightforward when Trp<triplet lifetime can be shortened by addition of triplet quenchers, and the values of Tg for PPVK and CoPT(l) have been obtained using this approach (6). Figure 2 shows a typical trace corresponding to the decay of the biradical from PTVK, as monitored at l 15nm. The triplet state is in this case too short lived to be detectable the residual absorbance observed after decay of the biradical is due to the enol. 

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). 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

The photochemical reaction can also proceed via the triplet state and in this case no cyclization is observed. Especially when acetophenone is added as a triplet sensitizer, 41 is not formed. Remarkable is the observation that in the presence of anthracene or pyrene as triplet quencher, the yield of the cyclization product 41 was not enhanced and that nitrene insertion into CH bonds of anthracene or pyrene was observed. When the photochemical cyclization reaction was performed with the tosyl azide derivative 42a or the azido nitrile derivative 42b (Scheme 6), only low yields of the tricyclic amide 41 (32% from 42a, 9% from 42b, respectively) were obtained <2001JCS(PI)2476>. 

When a photochemical reaction is quenched by a compound that is known to be a triplet quencher, the chemically-active state is very likely... 

The inhibition of product formation by a triplet quencher readily allows a triplet mechanism to be identified where the absence of inhibition by a triplet quencher indicates a singlet mechanism. 

Suitable triplet quenchers should have the following properties ... 

Conjugated dienes, such as penta-1,3-diene H2C=CH-CH=CH3, are excellent triplet quenchers. 

If several different triplet quenchers of different triplet energy are used and the efficiency of the photochemical reaction is determined, a value for the energy of the reactive Ti state of the reactant can be obtained. By considering the relative efficiency of quenching for each quencher and knowing its triplet energy, it will be found that the efficiency of... 

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