Luminescence dissymmetry

In circularly polarized luminescence spectroscopy, one normally reports the ratio of Al to the total intensity 7. The results are most often reported in terms of the luminescence dissymmetry ratio, glum, which is defined as the ratio of the differential emission intensity to the average total emission intensity.75... 

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 ... 

The Pfeiffer effect, the outer-sphere interaction of a chiral substrate with a rapidly interconverting racemic solution of a chiral lanthanide complex, can be investigated by measurement of the luminescence dissymmetry factor (the ratio of circularly polarized luminescence to total luminescence) for Eu or Tb " complexes. Thus the racemic D chiral complexes [M(dpa)3], where M = Eu or Tb, interact in an outer-sphere manner with the following optically active spiecies cationic chiral transition metal complexes, ascorbic acid, aminocarboxylates, tartrates, amines and phenols. Association constants can be obtained from limiting values of the dissymmetry factors. In some cases, inner-sphere complexation can be demonstrated, as judged by changes in the general nature of the circularly polarized luminescence spectrum and pH irreversibility of the complexation. 

The CPL results were placed on a quantitative basis by calculating the luminescence dissymmetry factor lum as defined by Richardson and Riehl [Ri 77] ... 

Allowing for the time-dependence of the excited state population, we may substitute eqs. (29) and (30) into eq. (27), and ejqiress the time-dependence of the luminescence dissymmetry as follows... 

Luminescence dissymmetry ratio values (glum) the spectral range of the D4 transition for 0.01 M... 

We thus define the luminescence dissymmetry ratio, as follows ... 

A = absorbance c = concentration d = path length / = frequency 1,5 = absorption dissymmetry ratio glujjj = luminescence dissymmetry ratio I = light intensity J = total angular quantum number t = time = extinction coefficient A = wavelength a = standard deviation. 

Because of the difficulty in measuring absolute emission intensities, it is common to report the degree of CPL in terms of the luminescence dissymmetry ratio (or factor)... 

Under the assumptions that the hneshapes for CPL and total luminescence are identical (this is appropriate for the usually sharp isolated pure electronic transitions that are often target of CPL measurements), and that the number of molecules in the emitting state is independent of their orientation, the luminescence dissymmetry ratio can then be related to the molecular... 

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 ... 

Because of the difficulty in measuring absolute emission intensities, in CPL spectroscopy one commonly reports the ratio of AI(X) to the total intensity I(X). This ratio, glum(X), is referred to as the luminescence (or emission) dissymmetry ratio, and is explicitly defined at wavelength, X, as... 

Crystalline Conjugated Polymers Showing Circularly Polarized Luminescence with High Dissymmetry Factors... 

Lyotropic Di-substituted Polyacetylenes that Exhibit High Dissymmetry Factors in Circularly Polarized Luminescence Through the Chiral Nematic Liquid Crystal Phase... 

B.A. San Jose, S. Matsushita, K. Akagi, Lyotropic chiral nematic liquid crystalline aliphatic conjugated polymers based on di-substituted polyacetylene derivatives that exhibit high dissymmetry factors in circularly polarized luminescence. J. Am. Chem. Soc. 134, 19795-19807 (2012)... 

Absorption (gabs) luminescence (gium) dissymmetry ratios for selected transitions of a chiral crystal of... 

For the simple model enantiopure systems described above, it was concluded that the time dependence of the CPL and total luminescence were identical, and, therefore, the dissymmetry ratio contained no dynamic molecular information. This, of course, would not be the case if intramolecular geometry changes, that would effect the chirality of the molecular transitions, were occurring on the same time scale as emission. However, no such examples of this type of study have yet appeared. Time-resolved CPL measurements have been useful in the study of racemic mixtures of lanthanide complexes in which racemi-zation or excited state quenching is occurring on the same time scale as emission. 

Measurement of CPL from racemic mixtures is not a technique that can be applied to all racemic solutions. For moderately luminescence systems values of approximately 10 are measurable. This means that the intrinsic absorption and emission dissymmetry ratios need to be on the order of 10 for this experiment to be successful. Although there have been a couple of examples using this technique in organic systems, by far the most widely studied systems are racemic lanthanide complexes, such as given in Figure 5, because of the large and g bs values that may exist for certain f o f transitions. It is also useful to perform these experiments in a time-resolved mode and as a function of temperature to determine racemization rate constants. 

The experimental determination of the emission intensity in absolute units is quite complex as it commonly happens in luminescence measurements, /(A) and A/(A) are often measured in arbitrary units, which are dependent on the equipment and the experimental conditions adopted. The dissymmetry factor gem(- ) is a significant quantity because it is a ratio of emission intensities and is therefore unaffected by the instrumental and experimental parameters. Its value gives an absolute quantification of the chirality of the emitting excited state. 

As noted for cireular diehroism (Seet. 6.1.3), the extent of circular polarization of a luminescence band is proportional to the rotatory strength of the corresponding electronie transition(s). The emission dissymmetry factor for a sample of molecules isotropieaUy distributed both in the ground and in the excited state has the following expression ... 

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