Intersystem crossing, definition

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

The competition between intramolecular vibrational relaxation and chemical reaction has been discussed in terms of the applicability of transition state theory to the kinetic analysis [6], If the environment functions mainly as a heat bath to ensure thermalization among the vibrational modes in the excited complex, then transition state theory is a good approximation. On the other hand, when the reaction is too fast for thermalization to occur the rate can depend upon the initial vibronic state. Prompt reaction and prompt intersystem crossing are, by definition, examples of the latter limit. 

At alkaline pH the fluorescence of Class B proteins is found to be that of tyrosinate isi-i ). A report of Vladimirov and Zimina )that the fluorescence of serum and egg albumins at pH 13 is entirely due to tryptophan is probably in error the observed luminescence is most likely that of tyrosinate. Tryptophan and t3n-osinate fluorescence spectra are quite similar and hfetime measurements are sometimes necessary for definite identification. The phosphorescence of Class B proteins at alkaline pH generally has a considerable tr5 tophan component along with a dominant tyrosinate contribution 26,164). Thus, trp- -t T < > transfer appears to be very efficient at the singlet level and enhanced intersystem crossing to the tryptophan triplet at high pH 26) also contributes to the tryptophan fluorescence quenching and to the production of tryptophan phosphorescence of Class B proteins at high pH. It is possible that tyr( ) - -trp triplet transfer also occurs to an extent in some proteins. 

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