Exciton theory

Numerous studies of carotenoid aggregates have focused on the molecular organization of the aggregates (Simonyi et al. 2003), but little is known about aggregation-induced effects on carotenoid excited states. Classical exciton theory can qualitatively explain the aggregation-induced shifts of absorption bands (Section 8.3.1), but a detailed understanding of the parameters governing the... 

Ohta, K., M. Yang, and G. R. Fleming. 2001. Ultrafast exciton dynamics of J-aggregates in room temperature solution studied by third-order nonlinear optical spectroscopy and numerical simulations based on exciton theory. J. Chem. Phys. 115 7609-7621. 

Figure 2. Molecular orientation model of bilayer membranes and schematic explanation of Kasha s molecular exciton theory (see equation (2)).

In this expression 1, m, n denote the direction cosines specifying the relative orientations of the principal axes in the monomer and dimer. (The expressions defining the values D and E are D = -3/2 Z and E = 1/2 (Y - X).) For the face-to-face structure proposed for [ZnTCP]2 (14) exciton theory predicts that dimerization should not affect the out-of-plane component (Z) of the tensor. The in-plane component, and therefore E, depends on the angle of rotation of one porphyrin plane relative to the other. According to the exciton model the observed reduction in E (cf. Table II) corresponds to an angle of rotation of about 23. This is reasonably close to the value predicted by molecular models (14). 

The simplest theoretical description of the electronic absorption spectra of 4a,4b-dihydrophenanthrenes seems to be provided by Simpson s exciton theory of the spectra of polymers. Compared with equally applicable but more complicated MO treatments (e.g. see Ref. and for tr-electron SCF MO analyses of 1, 44 and of 45), Simpson s model offers (at least for 1) some advantages such as numerical simplicity and sufficient transparency without losing too much of physical meaning. In the case of 1 Simpson s exciton model predicts the correct number of transitions and gives estimates of their energies and of their relative intensities. 

The chirality of [2.2]paracyclophane derivatives has been deduced as being (—)(R) on the basis of the exciton theory of coupled oscillators 67) and confirmed by experimental results (see 2.9.1 and 2.9.3). In these compounds a negative Cotton effect at 270 nm (corresponding to the p-band) seems to be specific for the (R)-chirality 54). 

Empirical conformational energy calculations are performed on helical poly(2,3-quinoxaline)s to predict stable conformations. Two energy minimum conformations are found by varying the dihedral angle, y, between two adjacent quinoxaline units from 5 to 180°. Circular dichroism spectra are calculated for the two stable conformations (v - 45 and 135°) on the basis of exciton theory. 

Consider, now, radiationless transitions in pure molecular crystals of aromatic molecules. At the very outset we must realize that crystal field effects may lead to the inversion of the order of the triplet and singlet exciton levels relative to the ordering of the corresponding molecular states.12 The Davydov tight binding formulation of exciton theory leads to the following representation for the manifold of optically accessible (k = 0) energy levels in a pure molecular crystal 138... 

Exciton theory may also be applied with success to nonidentical dipole-dipole coupling in mixed ligand complexes such as [M(bpy)2(phen)]"+ or [M(bpy) (phen)2]"+ (bpy = 2,2 -dipyridyl phen = 1,10-phenanthroline). These complexes exhibit the expected characteristic exciton CD patterns with three components, (+), (+), (—), from the lower frequency for the A-isomers. There are... 

The conjugated systems in Fig. 2 have inversion and e-h symmetry. The acenes are the prototypical systems of molecular exciton theory[23]. The photophysics of stilbene and polyparaphenylene vinylene (PPV) have been extensively studied [14], separately at first and together since the preparation [24] of light-emitting diodes based on PPV. Pyrene and perylene appear in many contexts, while the polydiacet-... 

We summarize here the triplet-exciton theory only for the properties involved in high-resolution (0.1 cm-1) optical absorption spectroscopy in particular, the triplet spin is left out. 

Several theories have been developed to explain how energy absorbed by one molecule is transferred to a second acceptor molecule of the same or a different species. At first sight exciton theory,20 66 which accounts for excitation transfer in molecular aggregates or crystals and the Davydov splitting effects connected with it, appears to bear little relationship to the treatment of long-range resonance transfer as developed, for example, by Forster.81-32 However, these theories can be shown to arise from the same general considerations treated at different well-defined mathematical limits.33-79... 

Excited States and Correlation Effects in Polymers Intermediate Exciton Theory of Excited States.—It is well known that the HF picture does not permit a reasonable calculation of excited states and optical... 

Abstract The analysis of ZnO exciton spectra is given. In the band and exciton theory,... 

Coupled oscillator models are extensions to the simple models developed for electronic circular dichroism. They are well known under the name exciton theory (see e.g. Harada and Nakanishi, 1972). These models, extended to vibrational transitions, describe the coupling of pairs of electric dipole transition moments. They predict equal amounts of positive and negative VCD intensity ... 

Exciton theory deals with the theory of electronic excitation processes in ordered arrays of absorbers (Kasha, 1959). Applications of exciton theory to protein problems have not been numerous, but some important results, especially on the conformation-dependence of the peptide absorption around 2000 A, will be discussed in the section on peptide-bond absorption. 

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