CIDEP Under Photodissociation of Initiators

Laser flash photolysis of Pis in the cavity of an ESR spectrometer is often accompanied by chemically induced dynamic electron polarization, CIDEP, that is, by the formation 

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

The reactions following photolysis of IRG651 and TPO lead to polarized free radicals of Pis  

Photolysis of Pis occurs via a triplet state and leads to polarized reactive free 

FIGURE 12.2 TR ESR spectra of TPO in ethyl acetate were taken at different observation times after a laser pulse. The two outmost components correspond to a large h3 perfine coupling (HFC) constant on the P atom. The signal in the center of the spectrum corresponds to 2,4,6-trimethylbenzoyl radical (an envelope of small HFC.)  

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