Triplet zero-field splitting

At 77 K the position of the 0-0 band is generally blue shifted for exposed tryptophans and red shifted for buried tryptophans. Along with a shift in wavelength to the red, the phosphorescence lifetime decreases.(28) The single tryptophan of human serum albumin shows red-shifted phosphorescence and D - L triplet zero-field splitting, indicating that it is in a hydrophobic environment. 29 ... 

The zero Field splitting (ZFS) parameters D and of the triplet exciton are ... 

Figure 6.2 Zero-field splitting of a triplet state.

FIGURE 11.1 Schematic triplet spectra as a function of zero-field splitting. The X-band S = 1 spectra have been calculated for indicated /l-values and with E = D/10 to illustrate increasing intensity of the IAm5l = 2 transition at half field with increasing D-value. 

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... 

In 1987 we reported26 that the three possible C3H2 isomers S-2, T-36, and S-37 can be transformed into each other under photochemical conditions. For several reasons propargylene (36) attracted our attention On the one hand the first C3H2 parent species, identified by direct spectroscopic methods, was triplet propargylene (T-36). Its ESR spectrum was published in 1965,63 and, based on the zero-field-splitting parameters, a linear or nearly linear structure was derived. On the other hand, the structural elucidation of 36 by comparison of the calculated and experimental IR spectra turned out to be rather difficult.64... 

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.

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 4. Zero-field splitting parameters and derived bond angles for triplet carbenes...

The arylcarbenes are also bent, the angle being about 150—155° for phenyl-and diphenylcarbenes 63,64) jj g zero-field splitting parameters were shown to be appreciably dependent on the host matrix used >. However diphenylcarbene prepared from different precursors proved to be identical and is believed to be the triplet species of diphenylcarbene. 

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

Table II. Zero-field Splitting parameters (in 10 cm ) and average triplet lifetimes of porphyrin monomers and dimers ...

The transitions in the X-band ESR spectra of triplet species occur in two regions. The so-called Anis = 1 region represents transitions between energetically adjacent pairs of the three triplet sublevels. These are characterized by two so-called zero-field splitting parameters, D and E. The parameter D is inversely proportional to the cube of the average separation of the electron spins, and E is related to the molecular symmetry. The number of lines depends on the molecular symmetry. If all three magnetic axes of the molecular carrier of the spectrum are distinct, the spectrum in the Anis = 1 region will show six major resonances, plus any hyperfine lines that may be visible. If two of the principle axes are equivalent by symmetry, only four lines will be observed. In the latter case, the parameter E has the value of... 

EPR Spectroscopy. The structure of triplet carbenes is unequivocally characterized by EPR zero-field splitting (ZFS) parameters, which are analyzed by D and E values. In a simple model, the ZFS parameters D and E for a triplet depend on the distance between electrons with parallel spins as given by... 

TABLE 9.14. Zero-Field Splitting Parameters" and Kinetic Parameters for Persistent Triplet Diarylcarbenes (106)... 

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