Circular polarization of fluorescence

M Voigt, M Chambers, and M Grell, On the circular polarization of fluorescence from dyes dissolved in chiral nematic liquid crystals, Chem. Phys. Lett., 347 173-177, 2001. 

Schessinger, J., Steinberg, I. L., Givol, D., Hochmann, J., and Pecht, I., Antigen-induced conformational changes in antibodies and their Fab fragments studied by circular polarization of fluorescence. Proc. Natl. Acad. Sci. U.S.A. 72, 2775-2779 (1975). 

Steinberg I (1975) Circular polarization of fluorescence. In Chen RF and EdeUioch H (eds) Biochemical Fluorescence Concepts. New York Marcel Dekker. 

ScHLESSiNGER, J., R. S. RocHE, and I. Z. Steinberg A Study of Subtilisin Type Novo and Carlsberg by Circular Polarization of Fluorescence. Biochemistry 14, 255-262 (1975). 

The optically active 1,2-dioxetane of 2,4-adamantanedione (89) was synthesized. Thermal activation of 89 yielded chemiluminescence (Xmax = 420 nm characteristic of ketone fluorescence), pointing to intermediate 90 which is chiral only in its excited state due to the out-of-plane geometry of one of the two carbonyl groups. However, circular polarization of chemiluminescence measurement of 90 has not detected optical activity at the moment of emission. The authors have concluded that fast, relative to the lifetime of ketone singlet excited state, intramolecular n, it energy transfer caused racemization of 90196. 

Circular dichroism provides an additional spectroscopic tool for characterization of excited states 35). Considerable interest has also been extended to esr-spectra of anion radicals of a-diketones 36). Circular polarization of the phosphorescence of camphorquinone has been determined, 51). Biacetyl has been the subject of a CIDNP study 152), of fluorescence quenching by a variety of substrates 153), and of steric effects in quenching of triplet states of alkylbenzenes 154). 

Circular polarization of luminiscence, stopped-flow fluorescence, fluorescence-monitored chemical relaxation, the evaluation of relative orientation by polarized excitation energy transfer, time-resolved fluorescent polarization ( nanosecond polarization ), and other new techniques have become valuable means for studying protein structures, their interactions and structural changes in relation to various treatments (e.g. denaturation). New fluorescent probes and quenchers have enabled the research field to expand from isolated proteins to more complicated systems such as membranes, muscle and nerve components and other subcellular structures (see also 7.3). 

Use of polarized light to excite fluorescence, and measurement of the state of polarization of the emitted light introduce another set of measurable parameters that can characterize structures and dynamics of molecules. The anisotropy of the polarization of fluoresence after excitation by linearly polarized light provides the rotational diffusion coefficient, or rotational correlation time, of the fluorophore. When there is fluorescence energy transfer, analysis of the anisotropy of both donor and acceptor can reveal the relative orientation, and the relative motion. Measurement of fluorescence after excitation by circularly polarized light provides the fluorescence-detected circular dichroism. This measurement characterizes the chiral environment of the ground state of the fluorophore. If the circular polarization of the fluorescence is measured, the circularly polarized luminescence is obtained. This measurement characterizes the chirality of the excited state. 

In a second experiment,ENDOR measurements were performed in the optically populated excited p5/2> Es/2 state of Tm " in Cap2, using the same apparatus. The ENDOR transitions were monitored via the circular polarization of the fluorescence. The authors obtained the ligand hyperfine structure constants A, = 4.83 (3) MHz and Ap = 3.59 (3) MHz of the first shell of fluorine neighbors, thus providing the first ENDOR results of an optically excited state of an impurity center. 

The overall experimental arrangement at Berkeley is shown in Figure 9. The polarization of the IP state is measured not by its fluorescence, which is far in the infrared, but by using another laser at 2.18 fim which is circularly polarized to sweep all the 7P atoms of the proper polarization up to the 85 state. One then observes the fluorescence at 323 nm from the 85 state as a function of the polarization of the 2.18-/itm beam, and thereby measures the polarization Jj of the IP state. Since one does not need to know the polarization of this fluorescence, one can detect a greater solid angle and improve the counting rate. One defines A similarly to Eq. (36), except and n refer to the numbers of fluorescent photons for the two states of circular polarization of the 2.18-/im laser beam. 

Figure 9.29 Two-photon fluorescence excitation spectrum of 1,4-difluorobenzene. The upper and lower traces are obtained with plane and circularly polarized radiation, respectively, but the differences are not considered here. (Reproduced, with permission, Ifom Robey, M. J. and Schlag, E. W., Chem. Phys., 30, 9, 1978)...

Schippers and Dekkers reported on the CD and circularly polarized fluorescence of 4,4-dideuterio-adamantan-2-one (85)193. The CD of 85 originates in transitions to a totally symmetric n -+ n excited state with double minimum potential in the C=0 out-of-plane bending mode. 

Maupin, C. L. Riehl, J. P. Circularly polarized luminescence and fluorescence detected circular dichroism. In Encyclopedia of Spectroscopy and Spectrometry, Lindon, J. C. Trantner, G. E. Holmes, J. L., Eds. Academic Press, 2000 pp 319-326. 

To visualize the depolarization fields we may consider the following idea assume we find a molecule with its absorption dipole moment in the plane of the sample. We now adjust the polarization of the excitation beam such that the fluorescence is maximized. We may assume that this happens if the electric field vector in the focus is parallel to the dipole moment. Now, if we turn the incoming polarization by 90° the dominant electric field component will not be able to excite the molecule and the presence of other field components should become visible as weak, but distinctly non-circular spots. Figure 6 shows the result of such an experiment. In Fig. 6(b) the polarization has been turned by 90° as compared to (a) as indicated by the white arrows. The bright spots become dim and their symmetry changes to a four-lobed structure. These weak structures can be made visible if the excitation intensity is increased by a factor of five [see Figs. 6(c) and (d)]. 

Analogous g-values may be defined for the degree of circular polarization in emission [or circularly polarized photoluminescence (CPPL)] and circularly polarized electroluminescence (CPEL), eg. gCppL = 2(JL - 1R)/(1L + 1r), where IL and IR denote the intensity of left- and right-handed circularly polarized emission, respectively. CPPL should not be confused with fluorescence-detected CD. 

Fig. 2.6. Geometry for the calculation of the degree of circularity of fluorescence C at excitation by lefthanded circularly polarized light.

A similar analysis of expected signals can be performed in the case of circularly polarized excitation. If we observe the circularly polarized fluorescence in the xy plane, the righthanded Ir and lefthanded // polarized fluorescence components can be written in accordance with (2.24), and in... 

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