Polarization of emission

Selected entries from Methods in Enzymology [vol, page(s)] Additive properties of polarization, 246, 286 angle-resolved, assessment of peroxidation effects on membranes, 233, 274-275, 281-283, 287-288 binding isotherm construction, 246, 287-288 effect of inner filter effects, 246, 288 incoherent systems, 246, 263-264 orientational averaging, 246, 265, 269-270 Perrin equation, 246, 284-285 polarization of emission, 246, 284 rotational diffusion, 246, 9, 260 time-resolved, assessment of peroxidation effects on membranes, 233, 274, 283-285, 285-287. 

Rowe, M.D. and McCaffery, A.J. (1978). The influence of hyperfine coherence and of elastic collisions on the circular polarization of emission from Li2, Chem. Phys., 34, 81-94. 

Presentation and discussion of the experimental results will proceed in the following manner. First the absorption spectra of the polysilylenes will be described and a conq)arative analysis of the spectra in room temperature fluid solvent media and rigid low temperature glasses at 77°K made. This will be followed by a description of the rather remarkable emission properties of these materials with emphasis on results obtained at 77°K. Included as part of the emission spectroscopic properties are the results of photoselection or polarization of emission measurements obtained in a rigid glass at 77°K. Based on these results a model is developed which describes individual chains of these silicon polymers in terms of a distribution of all-trans sequences with variable effective conjugation lengths. 

The polarization of emission bands may give information on the orientation of immobilized molecules or their mobility (fluorescence depolarization). 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

An alternative approach to obtaining microwave spectroscopy is Fourier transfonn microwave (FTMW) spectroscopy in a molecular beam [10], This may be considered as the microwave analogue of Fourier transfonn NMR spectroscopy. The molecular beam passes into a Fabry-Perot cavity, where it is subjected to a short microwave pulse (of a few milliseconds duration). This creates a macroscopic polarization of the molecules. After the microwave pulse, the time-domain signal due to coherent emission by the polarized molecules is detected and Fourier transfonned to obtain the microwave spectmm. 

Orlova et al. (2003) theoretically studied the mechanism of the firefly bioluminescence reaction on the basis of the hybrid density functional theory. According to their conclusion, changes in the color of light emission by rotating the two rings on the 2-2 axis is unlikely, whereas the participation of the enol-forms of oxyluciferin in bioluminescence is plausible but not essential to explain the multicolor emission. They predicted that the color of the bioluminescence depends on the polarization of the oxyluciferin molecule (at its OH and O-termini) in the microenvironment of the luciferase active site the... 

Clearly, if a situation were achieved such that exceeded Np, the excess energy could be absorbed by the rf field and this would appear as an emission signal in the n.m.r. spectrum. On the other hand, if Np could be made to exceed by more than the Boltzmann factor, then enhanced absorption would be observed. N.m.r. spectra showing such effects are referred to as polarized spectra because they arise from polarization of nuclear spins. The effects are transient because, once the perturbing influence which gives rise to the non-Boltzmann distribution (and which can be either physical or chemical) ceases, the thermal equilibrium distribution of nuclear spin states is re-established within a few seconds. 

The polarization of biphenyl, deserves special comment. If, as indicated in Scheme 2, its immediate precursor is a radical pair consisting of two phenyl radicals, then it should be formed without detectable net polarization since if Ag = 0. Analogous results have been reported in the decomposition of other peroxides for example, ethane formed from acetyl peroxide shows net emission. To account for this, it has been suggested (Kaptein, 1971b, 1972b Kaptein et al., 1972) that nuclear spm selection which occurs in the primary radical pair—in... 

Fig. 8. Product yields and polarization (100 MHz) in the thermal decomposition (126°C) of 4-chlorobenzoyl peroxide (0-1 m) in hexachloroacetone containing iodine and water. Curve A, 4-ohlorobenzoic acid ([HjO], ca. 0-03 M) Curve B, p-chloroiodobenzene ([H2O], CO. 0-03 m) Curve C 4-chIorobenzoic acid ([HaO], ca. 1-6 m) Curve D, p-chloro-iodobenzene ([H O], ca. 1-6 m) Curve E, maximum intensity of emission from p-chloro-iodobenzene (O, co. 0-03 m H2O , ca. 1-6 M H2O). Data of Blank and Fischer, 1971b.

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Jablonski (48-49) developed a theory in 1935 in which he presented the now standard Jablonski diagram" of singlet and triplet state energy levels that is used to explain excitation and emission processes in luminescence. He also related the fluorescence lifetimes of the perpendicular and parallel polarization components of emission to the fluorophore emission lifetime and rate of rotation. In the same year, Szymanowski (50) measured apparent lifetimes for the perpendicular and parallel polarization components of fluorescein in viscous solutions with a phase fluorometer. It was shown later by Spencer and Weber (51) that phase shift methods do not give correct values for polarized lifetimes because the theory does not include the dependence on modulation frequency. 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

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