Luminescence polarization

A light quantum of appropriate energy can be absorbed by a molecule fixed in space only if the light electric field vector has a component parallel to the molecular transition moment. If the directions of the transition moment and of the electric field vector form an angle p, the absorption probability is proportional to cos q . (Cf. Section 1.3.5.) The light quanta of luminescence are also polarized, with the intensity again proportional to cos p. 

The polarization direction of an electronic transition may be determined by measurement of the absorption of polarized light by aligned molecules. (Cf. Michl and Thulstrup, 1986.) Orientation may be achieved in a number of ways. When single crystals are used or when the molecules of interest are incorporated into appropriate single crystals, a very high degree of orientation can be obtained if the crystal structure is favorable. Other methods accomplish orientation by embedding the molecules in stretched polymer films (polyethylene, PVA Thulstrup et al., 1970) or in liquid crystals further possibilities are orientation by an electric field (Liptay, 1963), or, in the case of polymers such as DNA, by a flow field (Erikson et al., 1985). 

Here /g and I, are the intensities of the components of the emitted light parallel and perpendicular to the electric vector of the exciting light, respectively. The curves P X) or P(v) are called the polarization spectrum. Depending on whether the measurement is carried out with constant excitation 

The relationship between the degree of polarization P and the angle a between the transition moments of absorption and emission is given by 

The observed phosphorescence polarization direction may be accounted for by the fact that singlet-triplet transitions acquire their intensity by spin-orbit coupling of the So state with triplet states and particularly, of the T, state with singlet states. (Cf. Section 1.3.2.) Under usual conditions the phosphorescence is an unresolved superposition of emissions from the three components of the triplet state, which in the absence of an external magnetic field are described 

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The versatility of luminescence goes beyond intensity-, wavelength- and kinetic-based measurements. Fluorescence polarization (or anisotropy) is an additional parameter still largely unexplored for optical sensing yet widely used in Biochemistry to study the interaction of proteins, the microfluidity of cell membranes and in fluorescence immunoassays. Although only a few optosensors based on luminescence polarization measurements can be found in the literature, elegant devices have recently been reported to measure chemical parameters such as pFI or O2 even with the bare eye41. 

Zircon belongs to the tetragonal system and is a positive uniaxial. The typical form shows the ill and the 110 planes. The two orientations selected for luminescence polarization study were the (110) plane, parallel to the basal section and the [100] row. In such cases the axis perpendicular to the (110) plane will be called X. The orientation notation is made according to the so-called Porto notation (Porto et al. 1956). The Xi(ZX2)Xi orientation means that the laser light entered parallel to the Xi axis of the crystal and is polarized in the Z direction, while the emission is collected along the Xi axis with X2 polarization. By polarization spectroscopy with a high spectral resolution (less then 0.1 nm) six lines are observed for the Dq- Fi transition of the Eu-II center instead of the maximum three allowed for an unique site (Fig. 5.12). In Z(XX)Z geometry which corresponds to observation of a-polarized luminescence we... 

In the intrinsic sensors, generally a tapered, fased-silica fiber is used, on which and after decladding of the core, specific biological receptors are immobilized via a well-establish chemical procedure. Changes in the absorbance, luminescence, polarization or refractive index are detected. But direct measmement is not usually possible and competitive configurations are employed using fluorescent labels (see Fig. 5.4b). 

Thus, BNZ-doped polystyrene appears to possess two structural features which influence the characteristics of the delayed luminescence. One of these involves clusters of BNZ molecules consisting of relatively densely populated regions along with others which are much less densely populated. The other consists of molecular pairs which are capable of trapping excitons by virtue of kinetic barriers as opposed to energetic ones. Although it might seem that features of this sort would be common to all molecularly doped polymers, a search for them in N-ethylcarbazole (NEC) doped polystyrene revealed no indication of their presence. In particular, there was found to be no time dependence of the delayed luminescence polarization on the millisecond time scale(8). With NEC, however, one does find a dependence of the delayed luminescence polarization on dopant concentration. That is, the polarization decreases monotonically as the concentration increases. 

Another photophysical technique used to probe the dynamics of these interactions is steady state luminescence polarization (18, 19). The ruthenium complex bound to the DNA is excited with polarized light. If, on... 

Enantiomer and DNA Relative Emission Enhancement Luminescence Polarization Relative Stern-Volmcr Quenching Rates ... 

This is the consequence of photoselective excitation of luminophores by polarized light, which selectively excites a population of molecules oriented with respect to the electrical vector of excitation. Emission also occurs with the light polarized along a fixed axis in the luminophore. The angle between these moments determines the maximal polarization of fluorescence. - Luminescence polarization is defined as follows ... 

Batch mode can be performed in membranes and in powdered solids. The simplest procedures use the solid phase as a medium to support the analytes and carry out the measurement. Among membranes, a variety of cellulosic supports have been used, and the nature of the analyte plays an important role in producing luminescence. Polar or ionic molecules give off a strong adsorption interaction due to hydrogen bonding and induce intense phosphorescent signals, while nonpolar compounds produce weak dispersion interactions and small shifts in fluorescence spectra and weak phosphorescence. External heavy-atom perturbation can be used to induce... 

A large amount of work has been published on Re complexes of the general type [(L)Re(CO)3(a-diimine)] +. These complexes exhibit Re —> (diimine) MLCT emission at room temperature in solution and the emission energy can be tuned by variation of the diimine, ancillary ligand, L, and solvent. Several reviews have appeared that discuss the luminescence behavior of these complexes. " Recently, detailed temperature-dependent luminescence measurements have been made on several methylated phenanthroline (w-phen) complexes of the type [ClRe(CO)3( i-phen)] the emission from the complexes was comprised of components from the MLCT and m-phen localized tt-tt states. Emission from this class of chromophores has been plied recently to immunoassays based on luminescence polarization of Re diimine complexes and the development of unique luminescent arylethynylene polymers. ... 

The steady-state polarization of (Re-L) -HSA was sensitive to the binding of anti-FI S A, resulting in a significant increase in luminescence polarization. Consequently (Re-L) -HSA can be used as a tracer in a competitive immunoassay with unlabeled HSA acting as an antigen (Fig. 8.18). 

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