Exciton Phenomena

Figure 4.11 An exciton phenomenon where the incident irradiation is absorbed by a single chromophore along the backbone (hVj), where the energy is reversibly transmitted to the neighboring chromophore which can re-emit the light (hv, hv, hv, ...) at any given moment...

In Figure 4.24, the distances between the two N and M atoms are about the same depending whether the N=C—axes are parallel or not. This means if the atomic contributions on N and C are important for Cu- and Ag-containing chromophores, but less or absent for Pd- and Pt-containing species, the exciton phenomenon will be as important for the U-conformation for both types of chromophores. On the other hand in the Z-conformation, the N- N separation is about 5.8 A. If the atomic conttibution on Ihe N atom is important as indicated for Cu- and Ag-containing chromophores (Table 4.10), then energy migration is still possible. On the other hand, if this contribution is weak or nil, then transfer is very inefficient. In the cases... 

Leading theoreticians were, however, attracted to the phenomenon and soon suggested models for F centers. In 1930 Frenkel suggested that an F center was an electron trapped in a distorted region of crystal structure, an idea that was incorrect in this instance but led directly to development of the concepts of excitons and... 

The effect of intermolecular interactions can be readily observed when comparing the absorption spectrum of a molecule in solution to that in the solid state. In solution, where the molecules can be considered as isolated, the spectra are characterized by sharp lines corresponding to absorption bands. However, in the solid, intermolecular interactions cause the formation of exciton bands and splitting of the levels. This phenomenon is often referred to as Davydov splitting. This splitting is thus a measure of the strength of the interactions and for MOMs it can amount to 0.2-0.3 eV. 

The physical phenomenon of current generation in simple D/A systems can be thought of in terms of the six chemical steps depicted in Fig. 4. (1) The absorption of a photon leads to a localized exciton with energy oo on either the donor or... 

Cadmium sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect (28-37) is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For the CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change (28,33,34). This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon occurs as the cristallite size is comparable or below the excitonic diameter of 50-60 A (34). In a first approximation, a simple electron hole in a box model can quantify this blue shift with the size variation (28,34,37). Thus the absorption threshold is directly related to the average size of the particles in solution. 

If two (or more adjacent) distinct chromophores exist in a chiral arrangement with respect to one another in a dendrimer molecule, the CD spectrum shows two intense Cotton effects of opposite signs which merge with each other [80]. This phenomenon, known as an exciton couplet, arises from the interaction of two electronic transition moments in a mutually chiral orientation, such as oc-... 

Compared to the dispersion equation c2K2 = ew2, (1.81) introduces the dependence of e(a>) on K This is the spatial dispersion phenomenon studied first by Pekar.34 For molecular crystals spatial dispersion may be ignored, since the slope of the excitonic branch is often negligible near K ai/c. This is not the case for several ionic crystals, for which the spatial dispersion is strong enough for a new wave, corresponding to the excitonic branch, to be excited. New boundary conditions must then be found for the study of the reflectivity of the exciton-polariton system.35... 

In contrast with the region cK > cu, the region cK < a> shows no analogy with the 3D case. Instead of a complete upper polariton branch, a new phenomenon appears, which may be represented, equivalently, either as a radiatively unstable exciton (3.19)—(3.20) or as scattering by the exciton-contaminated continuum states (3.21)—(3.22). 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

The plane waves of a perfect 2D lattice diagonalize the electromagnetic interactions, giving rise to the excitonic dispersion through the Brillouin zone, and to the surface-exciton-polariton phenomenon around the zone center.148,126 The corresponding hamiltonian may be written as... 

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