Excitons model

In a regime of strong interaction between the chains no optical coupling between the ground slate and the lowest excited state occurs. The absence of coupling, however, has a different origin. Indeed, below 7 A, the LCAO coefficients start to delocalize over the two chains and the wavefunclions become entirely symmetric below 5 A due to an efficient exchange of electrons between the chains. This delocalization of the wavcfunclion is not taken into account in the molecular exciton model, which therefore becomes unreliable at short chain separations. Analysis of the one-electron structure of the complexes indicates that the... 

The small and weakly time-dependent CPG that persisLs at longer delays can be explained by the slower diffusion of excitons approaching the localization edge [15]. An alternative and intriguing explanation is, however, field-induced on-chain dissociation, a process that does not depend on the local environment but on the nature of the intrachain state. The one-dimensional Wannier exciton model describes the excited state [44]. Dissociation occurs because the electric field reduces the Coulomb barrier, thus enhancing the escape probability. This picture is interesting, but so far we do not have any clear proof of its validity. 

Ferrell, R. A., and Quinn, J., Phys. Rev. 108, 570, Characteristic energy loss of electrons passing through metal foils Momentum exciton model of plasma oscillations/ ... 

Bittner ER (2007) Frenkel exciton model of ultrafast excited state dynamics in AT DNA double helices. J Photochem Photobiol A 190 328-334... 

Figure 6.5. Exciton model for the coupling of dimers. When the transition dipoles are aligned (ft, W) in a card pack fashion only the transition to n is allowed thus a blue shift in the spectrum is expected.

Kasha, M., H.R. Rawls, and M. Ashraf El-Bayoumi. 1965. The exciton model in molecular spectroscopy. Pure Appl. Chem. 11 371-392. 

Packard, B. Z., Toptygin, D. D., Komoriya, A. and Brand, L. (1996). Profluorescent protease substrates intramolecular dimers described by the exciton model. Proc. Natl. Acad. Sci. USA. 93, 11640-11645. 

On the other hand in the exciton description occasionnally adopted by some authors to interpret the main absorption peak in the polydiacetylenes one finds x " negative and its values two orders of magnitude lower than expression 6 since electron correlation (28) is essential in the exciton model, the calculation of even the simplest optical properties becomes prohibitively complicated and the physical insight is obscured. 

Kasha M (1963) Energy transfer mechanisms and the molecular exciton model for molecular aggregates. Radiat Res 20 55-71... 

The fluorescence and absorption spectra of [2.2]paracyclophane have been investigated and interpreted by a number of authors 23-26). Theoretical calculations of the energy levels with the aid inter alia of an exciton model are in agreement with spectroscopic findings. The methods of calculation are, of course, based on a n approximation and neglect the s skeleton. 

J > D. There is rapid excitation energy transfer between the two porphyrins (exciton model). The values of the zfs and kinetic parameters for the dimer are determined by the monomer values and relative orientation of the porphyrin planes (5-8,24). (Assuming that the porphyrin structure is not modified by dimerization). For instance, with parallel porphyrin planes, the value of D is not affected by dimerization, whereas the effect on E depends on the relative orientation of the in-plane axes of the two rings ( ). 

Dimerization causes shifts of optical absorption and emission bands of the order of several hundred cm (cf. Table I). This suggests that the triplet EPR data must be interpreted in terms of exciton and charge transfer effects. In the case of ZnTCP the effect of dim.erization on zfs values can be accounted for on the basis of rapid triplet excitation transfer between essentially unperturbed porphyrin moieties. If the exciton model applies the principal components of the zfs tensor in the dimer (X, , Z ) can be related... 

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 exciton model of polyene spectra assumes that each excited state of the polyene may be described by a linear combination of basis states, each having only one (singly) excited ethylenic unit. Only states with neighboring excited ethylenic units can interact. 

Fig. 3. Bond transition moment diagrams (Exciton model) for first three electronic transitions of 4a,4b-dihydrophenanthrene and of the iinear hexaene...

Xmax is shifted to 410 nm in 44 (c and g annelation) it is shifted to 422 nm. The explanation of this effect is quite straight forward. Benzo-annelation across a double bond substitutes a bond with half double bond character for a double bond. In terms of the exciton model this decreases both the length of the interacting system and the strength of the interaction. 

We mentioned the main models for generation, transfer, and recombination of the charge carriers in polymers. Very often, these models are interwoven. For example, the photogeneration can be considered in the frame of the exciton model and transport in the frame of the hopping one. The concrete nature of the impurity centers, deep and shallow traps, intermediate neutral and charged states are specific for different types of polymers. We will try to take into account these perculiarities for different classes of the macro-molecules materials in the next sections. 

D McBranch, MB Sinclair. Ultrafast photoinduced absorption in nondegenerate ground-state conjugated polymers Signatures of excited states. In NS Sariciftci, ed. Nature of the Photoexcitations in Conjugated Polymers Semiconductor Band vs. Exciton Model, New York World Scientific, 1997. 

Exciton model for optical resonances in carbon nanotubes... 

The paper is organized as follows. The next section quotes details of the Frenkel exciton model necessary for the later discussion. Comments on a full quantum dynamical description of all those quantities which are of interest in the mixed description are shortly introduced in Section 3. The used mixed quantum classical methodology is introduced in Section 4. Its application to EET processes is given in 5, to the computation of linear absorbance in Section 3.2, and to the determination of emission spectra in Section 7. The paper ends with some concluding remarks in Section 8. 

Abstract. We consider here the theoretical and quantum chemical description of the photoexcitated states in DNA duplexes. We discuss the motivation and limitations of an exciton model and use this as the starting point for more detailed excited state quantum chemical evaluations. In particular, we focus upon the role of interbase proton transfer between Watson/Crick pairs in localizing an excitation and then quenching it through intersystem crossing and charge transfer. 

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