Absorption dissymmetry factor

Further studies reported similar CPL spectra for [Yb(28a)]3+, as compared to [Yb(28b)]3+ (Dickins et al., 1999), with dissymmetry factor gium for the peak corresponding to the maximum of luminescence (at 995 nm) of —0.18 (Maupin et al., 2000). A quantitative comparison with the CD spectrum for which the absorption dissymmetry factor gabs = —0.11 is difficult in view of the spectral overlap of various transitions from differently populated LF sub-levels, as demonstrated in a previously cited study (Di Bari et al., 2000a). However, such a comparison is useful to verily the sign of the CPL emission ... 

The extra factor of 1/2 in this equation results from the fact that the equivalent expression for the absorption dissymmetry factor (or ratio), gabs > employed in CD spectroscopy is defined in terms of the difference in extinction coefficients, Ae, divided by the average extinction coefficient, e. 

Circular dichroism employs standard dispersive or interferometric instrumentation, but uses a thermal source that is rapidly modulated between circular polarization states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polarized light, A A = AL — AR, is recorded as a function of wavenumber. Relative differential absorptions (dissymmetry factors) AA/A at absorption maxima are typically 0.1—0.01 for uv—vis electronic transitions and 10-4 —10-5 for vibrational modes in the ir. 

The ratio between As and e at a given wavelength yields the absorption dissymmetry factor ... 

A positive CD was found in the 500-nm quinonoid band which is formed on reaction of tryptophanase with L-alanine and oxindolyl-L-alanine (Fig. 9.10). The dissymmetry factor in this band is much smaller than in the absorption bands of the unliganded enzyme (Table 9.2). A negative 315-nm peak, which appears in the presence of L-alanine (Fig. 9.10), may be caused by interaction of an aromatic amino acid residue with the quinonoid coenzyme ring. 

Table 9.2 1 The dissymmetry factor and reduced linear dichroism values in the coenzyme absorption bands of tryptophanase and its complexes with inhibitory amino acidst2...

Enzyme form Ligand concentration mM Absorption maximum nm Dissymmetry factor X 104 Reduced linear dichroism X 103... 

The ratio of the circular dichroism signal and the corresponding absorption is called dissymmetry factor g ... 

Finally, consideration is given to the results appearing in column 7 of Table VII, which apply to the situation in which the two laser beams have circular polarizations of opposite handedness. The results here differ from those in either columns 5 or 6 in each value of j. This represents the fact that changing the helicity of one beam produces a dichroism associated with a discrimination of the handedness of the Aj—A2 pair dressed by the chirality of the other circularly polarized beam. This again is a known feature of two-photon absorption (Thirunamachandran 1979), and one which persists even when the pair is not held in a fixed mutual orientation. In this case the dissymmetry factor has both numerator and denominator given by the leading jo terms, and the explicit result is as follows ... 

CPL is the emissive pendent of CD and therefore probes the excited state chirality it also reflects the molecular motions taking place between absorption and emission. In this case, the parameter of interest is the luminescence dissymmetry factor ... 

In the context discussed above, characterizing a chromophore means an assignment of the absorption band to its corresponding quantum mechanical states. The electric transition moment direction of the absorption band belonging to the chromophore that can be experimentally determined from the degree of anisotropy R (eqn [26]) is an important information for an unequivocal assignment. Furthermore, the dissymmetry factor g (eqn [30]) rewritten by eqns [14] and [17] into... 

The fourth, the so-called inherent dissymmetric chromophore, does not possess local symmetry. Therefore, the transitions belonging to these chromophores are magnetically and electrically allowed. Inherent dissymmetric chromophores are often found with so-called form chiral molecules for which atropisomers like binaphthols are typical representatives. Further examples are chromophores that come into being by exciton coupHng. In both cases the dissymmetry factor g is in the order of 10 to 10 and the CD is easily measurable in spite of the fact that the absorption coefficients of these compounds are often very high (e between 10" and 10 ). 

K) [33], The CPL signal is noisy because it is very weak. The lower part of the figure shows the values of the absorption and emission dissymmetry factors measured at specific frequencies... 

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