Optically detected magnetic resonance zero-field splitting

The rate constants for radiationless decay of the triplet state of mesoporphyrin IX dimethyl ester at 77 are 26 s-1 in EtOD and 57 s-1 in EtOH.352 The most probable cause is a decrease in the rate of tautomerism in the porphyrin due to deuteriation of the N—H hydrogen. However, for TPPH2 triplet state in n-octane matrices 363 such tautomerism does not appear to be an important mechanism for radiationless deactivation. Several recent reports deal with the low-temperature e.s.r. spectra of triplet states of porphyrins.364-368 The zero field splittings and depopulation rates of the various spin sub-levels of the triplet state of Zn-chlorophyll-a have been determined by an optically detected magnetic resonance method.359... 

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. 

UV spectra were obtained with a Varian spectrometer (Cary 15 and 17). Fluorescence, phosphorescence spectra, and the zero-field splitting parameters D and E of the triplet state were determined at 1.3K with an apparatus (31) for optical detection of magnetic resonance (ODMR) which was similar to the one described by Zuclich et al. (32). 

If the separation among the sublevels is in the range of microwave frequency, sublevel properties can be obtained by observing the effect of microwave resonance on the emission from this state. The zero-field splitting is of the order of microwave frequency for most of rr/r states. Thus, the sublevel properties can be obtained by analyzing the effect of microwave resonance on the phosphorescence intensity. The method is called phosphorescence-microwave double resonance (PMDR) or optical detection of magnetic resonance (ODMR). 

Spin selective information on the lowest triplet state decay was obtained by optical detection of magnetic resonance transitions between the spin components of the T state of FBP in n-octane. Because of the absence of phosphorescence at these conditions, the ODMR signals were detected via changes in the 5i->5o fluorescent intensity [6, 29], Our calculations reproduce the fluorescent frequency and radiative constant rather well. In order to complete the interpretation of the microwave-induced fluorescent ODMR measurements [6, 29] one has to calculate the zero-field splitting in the T state and hyperflne coupling between electron and nuclear spins. 

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