Multi-chromophore systems

In the strong coupling case, the transfer of excitation energy is faster than the nuclear vibrations and the vibrational relaxation ( 10 12 s). The excitation energy is not localized on one of the molecules but is truly delocalized over the two components (or more in multi-chromophoric systems). The transfer of excitation is a coherent process9 the excitation oscillates back and forth between D and A and is never more than instantaneously localized on either molecule. Such a delocalization is described in the frame of the exciton theory10 . 

It is fair to say from the above discussions that the distance dependence of ET dynamics has been extensively measured and explained in terms of the superexchange (TB) mechanism. These investigations, involving covalently linked multi-chromophoric systems, have also shed light on aspects of the mechanism of the... 

A complex multi-chromophoric system comprises the purple membrane patches from Halobacterium salinarium. These patches are composed of about 3000 bacter-iorhodopsin proteins. The hyperpolarizability of solubilized monomeric bacterio-rhodopsin was measured by HRS and found to be 2100 x 10 esu at 1064 nm. This high value is due to the presence of a chromophore in the protein, the proto-nated Schiff base of retinal. A purple membrane patch can be treated as a two-dimensional crystal of bacteriorhodopsin proteins, and its structure is known in considerable detail. The analysis of the purple membrane tensor was performed by adding the hyperpolarizabilities of the individual proteins in the purple membrane. From (depolarized) HRS measurements on purple membrane suspensions, the structure of the purple membrane patches, and an average membrane size measured by atomic force microscopy, a fi value of 2200 x 10 esu was calculated for bacteriorhodopsin [22]. The organization of the dipolar protonated Schiff base chro-mophores in the membranes was found to be predominantly octopolar. 

A tutorial review has been devoted to the means of controlling energy after absorption, by using bi- or multi-chromophoric systems, between which energy is shuttled in a way precisely determined kinetically and thermodynamically. 

Keywords Luminescence, Multi-Chromophore systems. Nanoparticles, ResOTiance energy transfer. Signal amplification... 

Exciton theory provides a theoretical model for investigating the collective excitation properties of multimeric or multi-chromophoric systems. It can be thought of as a subtype of the typical Cl method described in Sect. 3, where configurations of generalized Hartree products replace the usual configurations of symmetry-adapted Slater determinants. In the exciton method, the Hartree-product configurations... 

The discrete behavior of PPV polymers depends on the conformation of polymer chain. If the polymer sample was prepared from a good solvent, or if PPV was embedded in a matrix of low molecular weight PS, the fluorescence intensity transient revealed quite different characteristics from those previously mentioned. The spot showed a gradual decrease in intensity without discrete jiunps to the dark level The fluorescence spectra also gradually decreased in intensity while keeping the peak positions and the vibronic bands as shown in Fig. 5d. These results are usually expected for a multi-chromophoric system, in which the individual chromophores are in-... 

Zinc tetrasulfophthalocyanine is an attractive choice for the chromophore in multi-component systems for the photoreduction of water, but recent studies have shown that only low yields of photoproducts are obtained, and that porphyrin-based chromophores are more efficient.1181 1182... 

The rate of energy and electron transfer in any of these systems is an ensemble average of all sites within the layered assembly, and thus there is no single rate for any of the processes in the assemblies. Nonetheless, in addition to the increased quantum yield, assemblies constructed with the HTiNbOs spacer exhibit a long-lived charge-separated state component not observed in the a-ZrP spaced assemblies, of x = 900 ps. While there is still much to be learned from the study of multi-chromophore arrays, this synthetic approach appears to hold much promise for the creation of organic-inorganic hybrid solid state devices. 

Hofkens J, Schroeyers W, Loos D, et al. Triplet states as non-radiative traps in multi-chromophoric entities single molecule spectroscopy of an artificial and natural antenna system. Spectrochim Acta 2001 A57 2093-107. 

The triplet states of free-base, zinc-, and ruthenium-porphyrins can be conveniently monitored in nanosecond laser flash photolysis (Fig. 25). The triplet transient spectra exhibit significant differences between the free-base and the metalloporphyrins that can be used for diagnostic pmposes in multi-chromophore supramolecular systems. 

The covalent assembly of functional Jt-systems is a general synthetic principle and in some cases they can even be achieved in a multi-component fashion. One of the most impressive examples is the very elegant access to covalently linked donor-fullerene arrangements by 1,3-dipolar cycloadditions with in situ-generated azomethine ylids [59]. However, here only the multi-component de novo synthesis of the chromophore structures will be considered. The major developments have been achieved in condensation-based and cross-coupling strategies. 

FDCD measurements, and a basic theoretical formalism for this technique, were first reported by Turner, Tinoco and Maestre in 1974 [5]. In this experiment one uses the selectivity and sensitivity of luminescence measurements to probe the local chiral environment of fluorescent chromophores. The ultimate goal in many applications of FDCD is to relate the observed differential fluorescence signal to the conventional CD measurement. In certain multi-component absorbing systems this procedure may be difficult. This technique is sometimes applied to systems for which CD measurements are impossible or very difficult. FDCD, like CPL and other polarization sensitive techniques, is not immune to troublesome background and noise problems, and these will be discussed in Section 3. The only detailed discussion of the applicability of FDCD measurements, and other characteristics of the technique has been presented by Turner in 1978 [6]. In this chapter we will also list some of the more recent applications of FDCD. 

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