Linearly polarized luminescence

Since side-chain-type LC polyacetylene derivatives were first synthesized [10-18], various types of LC conjugated polymers have been synthesized and evaluated from the standpoint of their electrical and optical properties [19-46]. Among them, LC aromatic conjugated polymers, including LC polythiophene derivatives, have been of recent interest because they are available for linearly polarized luminescent materials, and also anisotropically conducting materials [28-30, 40-53]. 

The possibility of conformational changes in chains between chemical junctions for weakly crosslinked CP in ionization is confirmed also by the investigation of the kinetic mobility of elements of the reticular structure by polarized luminescence [32, 33]. Polarized luminescence is used for the study of relaxation properties of structural elements with covalently bonded luminescent labels [44,45]. For a microdisperse form of a macroreticular MA-EDMA (2.5 mol% EDMA) copolymer (Fig. 9 a, curves 1 and 2), as compared to linear PM A, the inner structure of chain parts is more stable and the conformational transition is more distinct. A similar kind of dependence is also observed for a weakly crosslinked AA-EDMA (2.5 mol%) copolymer (Fig. 9b, curves 4 and 5). 

Optical fluorescence microscopy is a powerful and sensitive method for obtaining information about the orientation of luminescent dye molecules in small crystals. In Figure 1.10, we show unpolarized and linearly polarized fluorescence of two perpendicularly lying zeolite L crystals loaded with DSC. 

Polarized luminescence "" is a radiation for which the amplitudes (or intensities) of vibrations of the li t vector in two directions normal to each othra and lying in a plane normal to the direction of the incident light are not identical. We will deal only with PL observed in the direction normal to the linearly polarized or natural incident light. 

In classical terms, radiation is represented by an electromagnetic wave. The polarization of plane-wave radiation is defined by the way the oscillating electric field evolves in space, in a plane perpendicular to the direction of propagation. The most general polarization state is called elliptical polarization [23], but for luminescence applications the subset of linear polarization states usually suffices. In these cases the electric field vector oscillates along a well defined direction in a plane perpendicular to the direction of propagation. This direction is the polarization direction, and radiation with this characteristic is said to be linearly polarized. 

As polarization arises from the mode of vibration of an atom or molecule, both bands and lines in luminescence spectra suffer polarization. Since polarization of fluorescent light is closely associated with the life time of the excited state, a lower limit for the life time of excited state of many organic compounds was obtained37 by consideration of the linear polarization of fluorescence in media of high and low viscosity by using Perrin s law of depolarization41, which connects polarization with the life time of the excited state and the relaxation time of molecular rotation. 

Luminescence of Probe Molecules. These studies permit evaluation of polymer properties. In particular, measurement of the relative Intensities of fluorescence of a probe molecule polarized parallel to and perpendicular to the plane of linearly polarized exciting radiation as a function of orientation of a solid sample yields Information concerning the ordering of polymer chains. In solution, similar polarization studies yield Information on the rotational relaxation of chains and the viscosity of the microenvironment of the probe molecule. More recently, the study of luminescence Intensity of probe molecules as a function of temperature has been used as a method of studying transition temperatures and freeing of subgroup motion in polymers. 

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. 

Dynamic Switching of Linearly and Circularly Polarized Luminescence of Liquid Crystalline Photoresponsive Conjugated Polymers... 

In addition to investigating ferroelectric liquid crystalline conjugated polymers with dynamic switching functionahties under an electric field, our group has also developed photoresponsive liquid crystalline conjugated polymers with dynamic switching of linearly and circularly polarized luminescence. This was accomplished through the use of a photoisomerizable moiety in the polymer side chain. 

The measurement of the time dependence of gium may be used to probe various chiral aspects of excited state energetics, molecular dynamics, and reaction kinetics. Although there are some time-dependent circular polarization effects due to molecular reorientations that parallel time-dependent linear polarization measurements, the most interesting studies are those that involve the time-dependence of intrinsic molecular chirality. For a sample containing one chiral luminescent lanthanide chromophore, it might be the case that there are processes that affect chirality occurring on the same time scale as emission that could be probed by time-dependent CPL. To date, however, there have been no reports of such studies, and all of the time-dependent CPL measurements have involved racemic mixtures. 

For highly luminescent samples, the error may be reduced by decreasing the time-window (the error is approximately 1% for a 20% window) (Schippers and Dekkers, 1981), or one might be justified in simply correcting the value to account for this known effect. Obviously, there are other sources of error in polarization measurements of this type. The PEM itself is not a perfect optical element, and other elements including linear polarizers, sample containers, filters, mirrors, etc. all lead to sources of error. One also needs to consider the statistical nature of these measurements as described below, and, as a result, lum values obtained with 50% time-windows are usually presented as obtained without correction. 

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