Distribution of chromophores

Continuing on a related theme, the practical importance of observations such as FRET is their ability to convey information about the spatial distributions of chromophores and consequently structural information about the condensed phase under study. It is important to keep in mind that diffusion of the donor and acceptor species, if taking place on the fluorescence timescale, will influence the observed kinetics. 

Although both theories correctly predicted the shape of the time-dependent anisotropy, the EF theory predicted values for theta-condition PMMA which were lower than expected (as compared to determinations by light scattering and neutron scattering) while the FAF theory resulted in values which were too high. The reason for the disparities in the excitation transport size determinations has been shown to be due to Inadequate models of the spatial distribution of chromophores on a polymer chain (18). 

If the samples to be compared do not have similar scattering coefficients or do not maintain a homogeneous distribution of chromophores through the thickness of the sample, the PC number will not be strictly proportional to chromophore concentration. In such cases, Schmidt and Heitner [3,94,95] propose explicit calculation of absorption and scattering coefficients. This requires measurement of the reflectance of transmitting samples over white and black backgrounds of known reflectance, and additional computation. Eurther details are available in recent reviews [3,94,95]. 

Thus the diffraction pattern contains spatial information that can be used to deduce information about the distribution of chromophore in the diffusion layer. Although the full potential of the spatial properties of the diffraction pattern remains to be realized, it is apparent that regions very close to the electrode surface can be monitored, or conversely, that the whole diffusion layer may be monitored by examining a large portion of the diffracted beam. 

In the absence of reactions between two excited states or environmental effects resulting in significant inhomogeneities in the distribution of chromophores, emissive decay from a single excited state (i.e., either Si or Ti of Figure 1) will follow first order kinetics and the observed rate... 

A second, more complex model which can approximate energy migration kinetics involves the relaxation of the condition that there must be an even distribution of chromophores. Such a relaxation can involve, say, a random distribution of chromophores in three dimensions interspersed by a random distribution of traps. The GAF [34] and LAF [35] models are of this type and, in addition,... 

Here, the function N( >o) describes a distribution of chromophores over their resonant ftequencies. We know from Fourier analysis that the broader function N( o) becomes the narrower function N(x — x ). The function N(tOo) describes... 

The influence of the intramolecular distribution of chromophores upon the spectral characteristics of polymer luminescence is evident in the works of Nishijima [42] and David et al [43]. The latter workers were the first to attempt to characterize the extent of excimer formation in polymers in terms of the microcomposition of the macromolecule. It was argued that in styrene-methyl methacrylate copolymers excimer formation required energy migration from the site of absorption to that suitable for excimer formation. It was proposed that excimer formation was dominated by interactions between adjacent chromophores upon the polymer chain and that the excimer site concentration was consequently proportional to the fraction of pairs of styrene residues in the co-pol3nner faa. ... 

The extent of fluorescence polarization at a time t following a brief excitation pulse is directly related to the time dependent, ensemble averaged probability that an excitation resides on the chrom-ophore where it was created at t = 0 [4,6,7]. This probability, which will be designated G (t), depends strongly on the density and distribution of chromophores, and can be obtained from a theoretical analysis of the many body EET problem in the polymeric system. A fit of transient fluorescence depolarization data to a theoretical expression for G (t) provides parameter values that are directly related to chromophore distribution. G (t) and G (t) are portions of the total Green function for excitation transport. 

Dale RE and Teale FWJ (1970) Number and distribution of chromophore types in native phycobiliproteins, Photochem. Photobiol. 12, 99-117. 

Jen a al. have developed dendronized polymeric NEO materials that have shown significantly improved poling efficiency by encapsulating chromophore with dendritic substituents that can electronically shield the core, ii-electrons, and form spherical molecular shapes." "" " Figure 6 illustrates different molecular architectures of dendronized side-chain NLO polymers with crosslinkers. The diverse selections of molecular architectures provide additional flexibility in the molecular engineering of high-performance polymeric NLO materials. Moreover, the unique nanoscale environment created by the shape and size, dielectric properties, and distribution of chromophores in crosslinkable polymers with dendrons and dendrimers can all play critical roles in maximizing the macroscopic EO properties of polymeric NEO materials. 

Del Vecchio, R. and Blough, N.V. (2004). Spatial and seasonal distribution of chromophoric dissolved organic matter and dissolved organic carbon in the Middle Atlantic Bight. Mar. Chem., 89(1 ), 169-187. 

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