Triplet impure

FIGURE 13 15 The 200 MHz H NMR spectrum of ethyl bromide (BrCH2CH3) showing the char acteristic triplet-quartet pattern of an ethyl group The small peak at 8 1 6 is an impurity... 

If the cross-coupling is strong enough this may include a transition to a lower electronic level, such as an excited triplet state, a lower energy indirect conduction band, or a localized impurity level. A common occurrence in insulators and semiconductors is the formation of a bound state between an electron and a hole (called... 

The impurities may capture this migrating exciton and lose its excess energy. The mutual annihilation of two or more triplet excitons occurs in the same polymer chain and delayed fluorescence is observed. 

Finally, one has to consider that an unknown impurity present in the benzene is responsible for quenching the triplet state/8,12,16 We then have four viable possibilities to explain this effect. 

There is experimental evidence that triplet states indeed play an important role in radical ion reactions. The formation of excimers has been suggested on the basis of chemiluminescence emission spectra, e.g. in the case of N-phenylcarbazole 15> and in some other experiments 18>. Other authors 19>20> have observed that the excimer fluorescence reported is probably produced by decomposition products of the radical ions or other impurities, as is very probably the so-called preannihilation chemiluminescence which occurs in electrogenerated chemiluminescence (see 21>). 

As mentioned above, phosphorescence is observed only under certain conditions because the triplet states are very efficiently deactivated by collisions with solvent molecules (or oxygen and impurities) because their lifetime is long. These effects can be reduced and may even disappear when the molecules are in a frozen solvent, or in a rigid matrix (e.g. polymer) at room temperature. The increase in phosphorescence quantum yield by cooling can reach a factor of 103, whereas this factor is generally no larger than 10 or so for fluorescence quantum yield. 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

Figure 5.2 GHSQC spectrum of a colored impurity formed during the synthesis of Tipranavir in 2 h 9 m in with a 160 Hz optimization for the one-bond coupling constant. The triplet resonating at 7.8 ppm gave a very weak response within the boxed region that was below the threshold used to plot this spectrum.

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

Molecular oxygen is an important participant in reactions of triplet carbene because of its triplet ground state and its ubiquity as an impurity in reaction... 

Excimers have been suggested in several other photochemical processes. The triplet lifetime of o-xylene decreases from 900 nsec at 0.01 M in methylcyclohexane to 13 nsec in 8M solution.50 The inefficiency of the triplet sensitized dimerization of indene has been attributed to formation of an indene excimer.51 The emission from acetone originally52 believed to arise from an excimer has now been shown53 to be the true monomer fluorescence while that attributed to the monomer appears to have been due to an impurity. 

We first note that an isolated atom with an odd number of electrons will necessarily have a magnetic moment. In this book we discuss mainly moments on impurity centres (donors) in semiconductors, which carry one electron, and also the d-shells of transitional-metal ions in compounds, which often carry several In the latter case coupling by Hund s rule will line up all the spins parallel to one another, unless prevented from doing so by crystal-field splitting. Hund s-rule coupling arises because, if a pair of electrons in different orbital states have an antisymmetrical orbital wave function, this wave function vanishes where r12=0 and so the positive contribution to the energy from the term e2/r12 is less than for the symmetrical state. The antisymmetrical orbital state implies a symmetrical spin state, and thus parallel spins and a spin triplet. The two-electron orbital functions of electrons in states with one-electron wave functions a(x) and b(x) are, to first order,... 

At — 20 °C. the lifetime in both solvents was approximately the same, as was the lifetime at 77°K. (which was assumed to be the natural lifetime ro). In view of the high viscosity of glycerol at — 20°C., it is reasonable to assume that impurity quenching is negligible, and the same must therefore have been true for ethanol at this temperature. The ratio of ro to r at — 20 °C. is equal to (kp + kh)/kp. The values of the rates of intersystem crossing from triplet to ground state (fa) at —20° were thus derived (see Table I). They are apparently independent of viscosity, a... 

For a given value of Iai, the lifetimes show an appreciable decrease with increasing pyrene concentration. This may be due to quenching of the triplet by the pyrene itself, or by traces of impurity present in the pyrene. The lifetimes are, of course, also critically dependent on the efficiency of deoxygenation of the solutions and slight variations from one cell filling to the next may thus account for part of the variation shown in Table XI. Such variations will not, however, affect the validity of the calculations. 

It has been suggested that one reason for the nonreproducibility is the high triplet energy of phenanthrene which makes its triplet susceptible to quenching by far more kinds of impurity molecule than, say, that of anthracene. However, this is not the whole explanation because naphthalene also has a high-lying triplet but seems to be less susceptible to quenching. 

Following the procedure outlined above, there is a 3-5% impurity (by H NMR) of an /5-indenyl product the triplet of the five-membered ring of the impurity is centered at <5 6.87. The initial crude product is suitable for further use. However, sublimation at 0.10 torr and 85°C lowers the amount of impurity to <1 with >85% compound recovery. 

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