Triplet spin polarization

The utility of CIDEP in photochemistry was greatly enhanced when it was realized (131) that the radical-pair mechanism is not the exclusive spin polarization mechanism. Initial triplet spin polarization produced by the different intersystem-crossing rates to the excited triplet sublevels can be "transferred" to radicals formed by the photochemical reaction of the polarized triplet. 

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

Atkins P W and Evans G T 1974 Electron spin polarization in a rotating triplet Mol. Phys. 27 1633—44... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

Koga T, Ohara K, Kuwata K and Mural H 1997 Anomalous triplet mechanism spin polarization... 

Electron-electron repulsion integrals, 28 Electrons bonding, 14, 18-19 electron-electron repulsion, 8 inner-shell core, 4 ionization energy of, 10 localization of, 16 polarization of, 75 Schroedinger equation for, 2 triplet spin states, 15-16 valence, core-valence separation, 4 wave functions of, 4,15-16 Electrostatic fields, of proteins, 122 Electrostatic interactions, 13, 87 in enzymatic reactions, 209-211,225-228 in lysozyme, 158-161,167-169 in metalloenzymes, 200-207 in proteins ... 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

A number of attempts to transfer the spin polarization directly from the triplet electrons to the 13C spins that are present in the sample have been unsuccessful. Nevertheless, it is possible to polarize the 13C spins indirectly, by polarizing the 1H spins first, and then transporting the enhanced 1H polarization to the I3C spins by the conventional crosspolarization technique. Figure 18B shows the enhanced 13C spectra obtained in this way using the same crystal of pentacene-doped naphthalene. 

According to this sequence, formation of cis- and trani -stilbenes is preceded by formation of a magnetosensitive ion-radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and trani -stilbene, and the isomerization rate must depend on the intensity of the magnetic held. These predictions were conhrmed experimentally (Lyoshina et al. 1980). Hence, the ion-radical route for trans —f cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account single-electron transfer. The electron transfer takes place in reality and makes the... 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

Figures Spin-polarized pulse EPR spectra of the light-induced triplet slate JP7W in...

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