Polar molecules, fluorescence

The polarization properties of single-molecule fluorescence excitation spectra have been explored and utilized to detennine botli tlie molecular transition dipole moment orientation and tlie deptli of single pentacene molecules in a /7-teriDhenyl crystal, taking into account tlie rotation of tlie polarization of tlie excitation light by tlie birefringent... 

There are two important drawbacks of such an approach (1) a polarity scale based on a particular class of probes, in principle, does not account, for example, sizes of probes, which should strongly effect the interactions (2) betain dyes do not fluoresce, which restrict essentially the field of application of this approach, because in many cases, absorption spectrum could not be measured accurately (small volumes of samples, study of cells, and single molecules spectroscopy). Therefore, polarity-sensitive fluorescent dyes offer distinct advantage in many applications. 

FIGURE 10.7 Power consumption simulation for a 2.2-in. full-color OLED display using Universal Display s phosphorescent OLEDs, small-molecule fluorescent devices, and polymer OLEDs along with a comparison of the power consumed by an active-matrix liquid crystal display backlight. R G B= 3 6 1, 50% polarizer efficiency, and 30% of pixels lit. (From Mahon, J.K., Adv. Imaging, June, 28, 2003. With permission.)... 

This important parameter can provide information on the excited states. For instance, when the dipole moment of a fluorescent molecule is higher in the excited state than in the ground state, the Stokes shift increases with solvent polarity. The consequences of this in the estimation of polarity using fluorescent polarity probes is discussed in Chapter 7. 

Because there is no phase relation between the light emitted by different molecules, fluorescence can be considered as the result of three independent sources of light polarized along three perpendicular axis Ox, Oy, Oz without any phase relation between them. Ix, Iy, Iz are the intensities of these sources, and the total intensity is I = Ix + Iy + Iz. The values of the intensity components depend on the polarization of the incident light and on the depolarization processes. Application of the Curie symmetry principle (an effect cannot be more dissymmetric than the... 

The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section I.A assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. 

Compounds 268a and 269a showed large Stokes shifts in polar solvents. Such large Stokes shifts have been observed in many TICT (twisted intramolecular charge transfer) molecules. Fluorescence decay time measurement indicated that there were two kinds of excited states, fast and slower decaying components, and the latter was the emission from the more polar state. 

Jeyes, S.R., McCaffery, A.J. and Rowe, M.D. (1978). Energy and angular momentum transfer in homonuclear diatomic molecules from polarized laser fluorescence, Mol. Phys., 36, 845-867. 

Acrylamide is an uncharged polar molecule, so it can diffuse within a protein and quenches fluorescence emission of Trp residues. The quencher should be able to collide with tryptophan whether it is on the surface or in the interior of a protein. Nevertheless, Trp residues, mainly those buried within the core of the protein, are not all reached by acrylamide. For a fully exposed tryptophan residue or for Tryptophan free in solution, the highest value of kq found with acrylamide is 6.4 x 109 M-1 s-1. 

Most fluorescent substances have rigid structures associated with fused aromatic rings and their fluorescence intensity is practically independent of the viscosity of the environment (4). On the other hand, their rotational movement as a whole will, of course, depend upon the local environment and since such rotations sweep out a larger volume than would be the case for auramine O, for example (i.e., larger V in eq. 2), so that larger domains in the polymeric system can be studied. Any fluorescent system will exhibit a polarization of fluorescence by virtue of the fact that the fluorescent molecules are anisotropic in regard both to emission and to absorption. This anisotropy can be described by fixed axes within the molecule, namely the dichroic axis of the molecule and the emission... 

Fig 10. Degree of polarization as a function of polymer concentration for aqueous solutions of fluorescein conjugates of polyacrylamide represented by open circles. The weight average molecular weights are 3.5 104 and 23.0 10, for a and b, respectively. Filled circles represent the degree of polarization of fluorescence for free fluorescein molecules (sodium salt) dissolved in pure polyacrylamide solutions (15)... 

As an example of application of the method we have considered the case of the acrolein molecule in aqueous solution. We have shown how ASEP/MD permits a unified treatment of the absorption, fluorescence, phosphorescence, internal conversion and intersystem crossing processes. Although, in principle, electrostatic, polarization, dispersion and exchange components of the solute-solvent interaction energy are taken into account, only the firsts two terms are included into the molecular Hamiltonian and, hence, affect the solute wavefunction. Dispersion and exchange components are represented through a Lennard-Jones potential that depends only on the nuclear coordinates. The inclusion of the effect of these components on the solute wavefunction is important in order to understand the solvent effect on the red shift of the bands of absorption spectra of non-polar molecules or the disappearance of... 

Spectroscopy. It has been shown previoiasly L that the fluorescence spectrum of Ruthenim tris-bipyridine, RuII, is solvent dependent, showing a red shift with increasing solvent polarity. The fluorescence spectrvnn of RuII on silica particles is identicel to that of the excited molecule in water. It will be shown subsequently that the RuII is essentially all botmd to the silica particle, hence the data show that the environment of a probe molec ile such as RuII on silica particles is very polar and similar to water. 

The laboratory coordinate system chosen for TIR fluorescence anisotropy measurements is illustrated in Figure 12.2. SRIOI molecules located at a water/oil interface (in the x-y plane) are excited by an s-polarized laser beam along the x -axis. The TIR fluorescence is then detected along the z-axis and its polarization is selected by a polarizer. The fluorescence decay profile observed under such a configuration is analysed for two limiting cases, depending on the structure of a water/oil interface two-dimensional or three-dimensional. 

The degree of polarization of fluorescence after the absorption of polarized light can tell us whether the excitation has been transferred from one molecule to another. If the same chlorophyll molecules that absorbed polarized light later emit photons when they go back to the ground state, the... 

In contrast to the absorption maxima of ANS, the fluorescence maxima are sensitive to solvent changes, but to a different extent in solvents of varying polarity. The fluorescence of ANS arises from two different excited-state molecules, the apolar locally excited state Si p and the zwitterionic excited state Si t, which emerges from Si p by intramolecular electron or charge transfer. The first emission, predominant in nonpolar solvents, varies only modestly with solvent polarity, as expected for a transition Si,np So between two states of similar charge separation. The second emission, observed in more polar solvents, is quite sensitive to solvent polarity, as anticipated for a transition Si,ct So between two states of very different charge separation [119, 120, 340]. 

Here, we discuss the polarization of fluorescence emission. The spatial orientations of emitting fluorophores determine the polarization of photons emitted. This relationship is the basis of fluorescence depolarization experiments as illustrated in Fig. 4a. When a sample of randomly oriented molecules (e.g., proteins... 

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