Quantum yields intersystem crossing

Intersystem crossing quantum yields can be determined by the flash photolysis technique in several different ways. The accuracy of each method depends to a large extent upon how much is known about the particular molecule under study. 

A method for the determination of intersystem crossing quantum yields involving triplet-sensitized isomerization has been developed by Lamola and Hammond.<55) In this method the compound whose intersystem crossing... 

Intersystem crossing quantum yields determined by the methods discussed in this section and other methods<24) are presented in Table 5.6. 

When Hammond and co-workers(59) found that the intersystem crossing quantum yield for aromatic ketones was unity (see Chapter 3) it was a short but very important step to realize that these compounds should be ideal triplet sensitizers. Thus one can excite the triplet state of molecules that otherwise would be formed inefficiently, if at all, by intersystem crossing. This idea resulted in a number of papers in the early 1960 s from the Hammond group on this topic. It is not possible in this short section to survey this area, but a few of the early studies are indicated by the following reactions ... 

Triplet state (cont d) intersystem crossing quantum yields, table of, 239-240 lifetime, 12 lowest triplet energies of carbonyls, table of, 224-225 of hydrocarbons, table of, 226 of various organic molecules, table of, 227... 

Since the conditions can be easily adjusted such that Aret[02] kd, then 4>et = 1. The intersystem crossing quantum yield in the presence of oxygen is... 

Table 5.6. Intersystem Crossing Quantum Yields for Various Organic Molecules...

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

An alternate method can be used to obtain separate from the intersystem crossing quantum yield. In this case, we will also use benzophenone (BP) and naphthalene (N) as our example. Now one of our samples will contain both compounds, and we will choose the concentration of naphthalene so that it will quench at least 98% of the benzophenone triplets. This will require knowledge of the... 

Simple expressions for these three reaction parameters are given in Eqs. 6-8. Subscripts specify the multiplicity of the excited states. The expression for Du includes the singlet component 0 as well as the triplet one which is given by the product of the intersystem crossing quantum yield 013 and the fraction of triplets which eventually result in products 3. As mentioned previously, the singlet related terms in the three equations are considered negligible when alkyl aryl ketones are the reactants. 

Carbonyl compounds usually have intersystem crossing quantum yields, d 1Sc, of very close to unity. In the case of a-guaiacoxyacetoveratrcHie, we have shown that some of the chemistry is coming from the singlet state, which necessarily implies that die intersystem crossing quantum yield is no longer 1. 

Photoacoustic spectroscopy was used to determine intersystem crossing quantum yields, d>-lsc, by a method previously described elsewhere (20). These values of d sc show a solvent dependence and are 0.4 in ethanol and 0.6 in dioxane. This is consistent with singlet involvement in the chemistry of a-guaiacoxyacetoveratrone. 

TT,TT )min is ultimately reached. The initial steps of the photochemical mechanism after absorption of a photon involve the decay of its lowest singlet excited state S in times shorter than the experimental time resolution to an intermediate related to Tn. The population transfer occurs before the S -minimum is encountered, which is consistent to the fact that the fluorescence quantum yield of the flavins compounds is lower than the intersystem crossing quantum yield and triplet state formation [85], Polar solvents seem to increase ISC and decrease fluorescence quantum yields in... 

The cycloaddition of enones to olefins is a reaction of considerable synthetic interest 14°). Oxetane formation and cyclobutane formation are sometimes competitive 141>, but the latter reaction is the more common. The photodimerization of enones 142> is a special case of such cycloaddition. It has been shown that triplets are involved in these cycloadditions, since intersystem crossing quantum yields are unity 143> and cycloaddition is totally quenchable by triplet quenchers. Careful kinetic analysis indicates an intermediate which can partially revert to ground state reactants, since quantum yields are lower than unity even when extrapolated to infinite substrate olefin concentration. That a diradical is... 

Numerous photochemical transformations of pyridazines and their N-oxides have been reported by several groups. The lowest singlet-triplet transition observed for pyridazine in rigid-glass solution was later revised, and the intersystem crossing quantum yield was determined. Photolysis of pyridazine in the gas phase gives nitrogen and vinylacetylene as main products. ... 

In benzene.933,948 n and 3 t>M are the quantum yields of the type II elimination from the singlet and triplet state, respectively (Scheme 6.116) is the intersystem crossing quantum yield frH and 3frH are the rate constants of 1,5-hydrogen abstraction. 

A lot of information are currently available, for example, transient absorption spectra, lifetimes of excited states x, interaction rate constants k, intersystem crossing quantum yield <3>jSC, triplet-state energy levels Zsx, dissociation quantum yields of cleavable photoinitiators, bond dissociation energies (BDEs) of amines or thiols used... 

If ES2 has a different spin multiplicity from that of ESi, the efficiency of its formation is the intersystem crossing quantum yield... 

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