Triplet state zero field splitting

B. Smaller, E. C. Avery, and J. R. Remko, Triplet-state zero-field-splitting correlations in substituted molecules, / Chem. Phys. 46, 3976-3983 (1967). 

Table 3. Triplet state zero-field splitting parameters ) of the aromatic amino acids and related compounds 6)...

Infra red spectra of triplet phenylnitrenes , zero-field splitting of the T, state in biradicals measured by magnetic field effects on fluorescence decay , and oiplet-triplet fluorescence and spin polarization of 1- and 2-naphthylphenylcarbenes ° are experimental studies reported on biradical species. 

Figure 6. Energy level scheme for a typical aromatic hydrocarbon. So denotes the electronic ground state, S the first excited singlet state and T the first excited triplet state. The triplet state is actually split into three sublevels by magnetic dipolar interaction of the triplet electrons (zero-field splitting). The dots and arrows denote the approximate populations and lifetimes of the sublevels for a typical, planar aromatic hydrocarbon. The lower panel shows schematically the time distribution of fluorescence photons (photoelectric pulses) for a single emitter undergoing singlet-tiiplet transitions leading to photon bunching.

Figure 6.2 Zero-field splitting of a triplet state.

The detection of zero-field splitting for dianions of [18] or [21] is very important it reveals not only the existence of a triplet state, but it also provides information on the mode of spin density distribution. Even more... 

Figure 1.2. Jablonski energy level diagram showing the singlet state and the triplet state with its zero-field splittings for a planar aromatic chromophore.

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. 

The g-tensor components are usually not resolved for a triplet state of a porphyrin or chlorin molecule at X and Q band frequencies and only the zero field splitting (ZFS) parameters are obtained. This situation changed with the advent of high field/high frequency EPR techniques. The first data of 3P700 and 3Chl a were published by Poluektov et a/. 194 the spectra obtained at D-band are shown in Fig. 5. 

In a few papers an attempt has been made to interpret quantum-mechanically the zero-field splitting parameters, which have been determined from the analysis of the electron spin resonance spectra on the triplet state of the pyrimidine bases. 

Figure 12 Zero field splitting of an orbitally non-degenerate triplet ground state. D and E are related to the zero field splitting tensor by D = 3/2Dzx and E - DXX — Dyy). The states are labelled according to the z components of the spin...

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