Chromophore concentration

The stoichiometric relationship between chlorine dioxide added and color removed during bleaching is nonlinear, but it is independent of temperature, pH, and pulp concentration under conditions normally used. Models used to explain the kinetics and stoichiometry show a strong dependence on chromophore concentration that probably results from differences in the reaction rates of the various chromophores present in the pulps (80). 

In order to better quantify the absolute value of chromophore concentrations, time of flight (TOF) must be measured in addition to light attenuation. This may be achieved using time-resolved or frequency domain methods. Time-resolved spectroscopy (TRS) was first pioneered by Delpy et. al. [19], Patterson et. al. [85] and Chance et al. [12, 13]. 

Oxy- and deoxy-hemoglobins are mainly of interest because they are related to the regional cerebral blood flow (rCBF). The focal change in rCBF determines the activation state. The term activation usually refers to the focal increase in rCBF whereas a decrease is called deactivation [78]. With the dual wavelength approach, one can derive two simultaneous equations to be solved for each of the two chromophore concentration changes. To this end, Equation (4) is split into two parts, separating the contributions from HbO and Hb. Equation (4) is then, rewritten as... 

The photoinduced absorbance anisotropy in a TPD experiment relaxes according to the same correlation function as in Eq. (4.16).(29) Effects of spatial variations in the excitation and probe beams, and chromophore concentration, have been treated and shown not to alter the final result.(29) NMR dipolar relaxation rates are expressed in terms of Fourier transforms of the correlation functions, 4ji< T2m[fi(0)] T2m[i2(f)]>> where fl(f) denotes the orientation of a particular internuclear vector. In view of Eq. (4.7), these correlation functions are independent of the index m, hence formally the same as in Eq. (4.16). For the analysis of NMR relaxation data, it is necessary also to evaluate Fourier transforms of the correlation functions. Methods to accomplish this in the case of deformable DNAs have been developed and applied to analyze a variety of data.(81 83)... 

Doornbos RMP, Lang R, Aalders MC, Cross FW, Sterenborg HJCM. The determination of in vivo human tissue optical properties and absolute chromophore concentrations using spatially resolved steady-state diffuse reflectance spectroscopy. Physics in Medicine and Biology 1999, 44, 967-981. 

This relationship holds when the polymer chromophore (or any chromophore) is uniformly distributed in a solution or bulk. In polymers with a high chromophore concentration, l is small and the photochemical/photophysical phenomenon occurs largely in a thin surface area. 

The photoisomerization of several copolymers was studied, in order to determine the effects of the structure and switching of the chiral side chain on the helicity of the main chain. A delicate balance of parameters was found, including separation and nature of the stereocenters, solvent, and concentration of azobenzene moieties J77 Stereoselectivity was often greatly enhanced if the chiral moieties were closer to each other. Accordingly, it was found that the incorporation of the stereocenter into a short, two-carbon spacer resulted in much more pronounced helical preference, as well as CD effects at lower chiral chromophore concentrations. The greater helical twist and improved thermal stability of the cis form (half-life 40h at RT) are notable features. 771 It was also found that the relationship between the trans-cis... 

The order parameter, , will thus play a critical role in defining the maximum achievable electro-optic activity that can be realized, particularly if this quantity is dependent on chromophore concentration. 

To simplify our calculations, let us define a reference lattice by assuming that the shape of a chromophore can be approximated as an ellipsoid (see Fig. 10). Taking the long axis of the ellipse as 10, we can define a closely packed cubic lattice of spheres of diameter 10. The number density associated with this lattice is N0=(l0) 3. For chromophore concentrations (loading in a host matrix) below this reference number density (i.e., N=N0x lattice with lattice constant, a=b=(N0) 1/3(x) l/3. For N >N0, only the intermolecular distances in the plane perpendicular to the principle ellipsoid axis can decrease with increasing concentration, x. We have a simple tetrahedral lattice with lattice constants, a=(N0) 1/3(x) l/3 b=(N0) l/3. 

Figure 9.3 Optical density (OD) of fluorescein at 495 nm as a function of chromophore concentration.

Free and polymer-bound 2-benzyloxy-thioxanthone exhibit similar flash photolysis behaviour and the same photoreduction quantum yield in the presence of 2-(A, AT-diethylamino) ethanol. This clearly shows that the polymeric nature does not appear to affect photophysical properties of the thioxanthone moiety. The photoinitiated polymerization of MMA in benzene solution, using BOTX and poly(StX-co-St) in combination with 2-(MA -dieffiylamino) ethanol, indicates that the polymer-bound chromophore seems to operate in the same way and with similar efficiency as the free photoinitiator, at least in conditions of dilute chromophore concentration. 

Single concentration approach. An alternative, less rigorous, technique is to enploy kinetic data derived for a sin e chromophore concentration in combination with the unquenched lifetime Any attempt to derive equivalent rate coefficients in homopolymers must of necessity be confined to the less rigorous single concentration approach (2) This inevitably produces a pseudo first order koM terra which is... 

Examination of the data in Table 8 reveals that T3 (and hence X3) is a function of the intramolecular chromophore concentration. In previous studies of vinyln h-thalene copolymers X3 has been found to vary linearly with the mde fraction of aromatic, fg. Figure 34 demonstrates the reasonable fit attained in such a plot for the acenaphthylene/methyl methacrylate stem. However, unlike the vinylnaphthylene copolymer systems, the linear fit of X3 to fg as a concentration term is not unique. 

This deformation can be followed by UV-vis spectroscopy directly at the air/water interface. Upon compression of the monolayer, the absorbance changes not very much around the point where the first increase in surface pressure is recorded (see Figure 6.17), This is caused by the disappearance of the voids between the domains. Above the kink in the isotherm, howevei the K-n band (-323 nm) decreases while the band at 248 nm increases. This increase is due to the increase in chromophore concentration upon compression of the monolayer. The decrease of the 7t-ic band is caused by a preferential orientation of the chromophorra perpendicular to the surface. Chromophores oriented perpendicularly are not detected because the transition dipole of the n-n band is parallel to the probing light. The band at 248 nm, however is not sensitive to the orientation of the chromophore, because the transition dipole moment has a component perpendicular to the long axis of the molecule (see Figure 6.17). 

The first question that had to be answered was. What were the optimal experimental conditions to photoinduce and photoeliminate birefringence. A typical experimental setup is shown in Figure 13.3. Since the polymer film is absorbing the pump light, we could calculate a maximum film thickness that would not completely absorb the pump beam. This is approximately 4 divided by the initial film absorptivity thus, in the case of pDRlA and pDRlM homopolymers, the useful thickness cannot exceed 300 nm. Obviously, polymers with lower absorptivities at the laser wavelength or with lower chromophore concentration can afford thicker useful films. 

DBS) are employed. The rate of electron transfer and its dependence upon chromophore concentration suggest that the electron tunnels from a chromophore on the inner surface of the vesicle to one on the outer surface rather than being carried by flipping of the surfactant chromophore from one surface to the other. In the system involving amphiphilic zinc porphyrin... 

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