Intersystem crossing quantum yield for

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 ... 

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

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... 

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. 

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. ... 

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... 

It is a well documented fact that the main pathway for the Si state of polyme-thinic dyes at room temperature is the trans-cis isomerization (see [16, 88] and references quoted therein). For monomeric cyanines and in the absence of steric hindrance, both fluorescence quantum yields and intersystem crossing quantum yields are usually very low. For TCC and 9-MeTCC entrapped within the polymer chains of microcrystalline cellulose F becomes close to unity, evidence of the decrease of the nonradiative pathways of deactivation. 

The sum of fluorescence and intersystem crossing quantum yields from the Boltzmann vibrational levels of Si appears to be slightly less than unity In both CgHe and CeDs- Although the uncertainty is sufficient to mask a real difference from unity, it seems reasonable to inquire further for other relaxation channels, and to identify if possible the nature of the very inefficient collision-induced electronic relaxation that seems to be present. 

The singlet-triplet intersystem crossing quantum yields (isc) for the compounds with unknown values, sc tit known triplet molar absorption coefficient, 8 , can be obtained by comparing the AODl, in the triplet-triplet absorption maximum of the compounds, with the AODj in the triplet wavelength absorption maximum of a reference compound with known intersystem crossing quantum yield, (j> Q, and triplet molar absorption coefficient, using Eq. (15.14) [17]. 

The photoacoustic calorimetry technique together with the triplet-triplet energy transfer method (see below) has been used to characterise the non-emissive triplet excited state of indigo, that is, to evaluate the intersystem crossing quantum yield and triplet energy values for this compound [20]. 

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