Charged Frenkel excitons

An important issue in the nature of the excited states in stacks of DNA bases is whether or not the states extended over a number of the bases are neutral Frenkel excitons or if they carry some degree of charge transfer character (exciplex or excimer).3 [2-4] A purely excitonic model neglects configurations... 

Excitation of the polymer creates one electron and a hole on the chain. This effect is particularly important when the electron-hole interactions are strong. Coulomb attraction keeps them together and we consider the two opposite charges as a bound electron-hole pair. An exciton (Fig. 1.11) is named according to its delocalisation. If it is localised, it is called a Frenkel exciton and, if it is delocalised, i.e., it extends over many molecular units, it is a Mott-Wannier type of exciton. ... 

A Mott-Wannier exciton is a neutral quasi-particle, consisting of an excited bound-state electron and its associated "Coulomb hole" in a high-dielectric constant solid, that can also travel throughout the lattice without transporting net charge since the exciton radius is several lattice constants, its binding energy is as low as 0.01 eV it thus tends to be more "delocalized" than the Frenkel exciton. 

Electro-absorption (EA) spectroscopy, where optical absorption is observed under the application of an electric field to the sample, is another method that can distinguish between localised and inter-band excitations. The electric field produces a Stark shift of allowed optical absorptions and renders forbidden transitions allowed by mixing the wavefunctions of the excited states. Excitons show a quadratic Stark (Kerr) effect with a spectral profile that is the first derivative of the absorption spectrum for localised (Frenkel) excitons and the second derivative for charge transfer excitons, i.e. 

Electroabsorption of Azo-TPA (Scheme 5) is interpreted in terms of a charge transfer exciton rather than a Frenkel exciton as formed by Azo-FO [341]. From this and other studies it is suggested that carrier generation from charge transfer excitons is more efficient than from Frenkel excitons [34m]. 

Deviations from OGM were recognized early on spectroscopic properties of molecular crystals Davydov shifts and splittings of absorption bands in molecular crystals are clear deviations from OGM and were rationalized based on the excitonic model (EM) [10, 14, 15, 16, 17]. This same model proved extremely successful to describe the complex and technologically relevant spectroscopy of molecular aggregates, i.e. of clusters of molecules that spontaneously self-assemble in solution or in condensed phases [IS]. Much as it occurs in molecular crystals, due to intermolecular electrostatic interactions the local bound electron-hole pair created upon photoexcitation travels in the lattice and the corresponding wave function describes an extended delocalized object called an exciton. We explicitly remark that the Frenkel picture of the exciton, as a bound electron-hole pair, both residing on the same molecule, survives, or better is the basis for the excitonic picture. The delocalization of the exciton refers to the fact that the relevant wave function describes a Frenkel exciton (a bound e-h pair) that travels in the lattice, and this is of course possible even when electrons and/or holes are, separately, totally localized. In other terms, the EM describes localized charges, but delocalized excitations. 

In our book we present methods of computation of Frenkel exciton states in molecular crystals, which are not based on the molecular two-level model and Heitler-London approximation (Ch. 3). The methods allow us, in particular, to obtain the Frenkel exciton spectra for arbitrary strength of the intermolecular interaction, assuming that the interaction does not violate the charge neutrality. However, in this section we use the simplest form of the Heitler-London method to construct the wavefunctions and to obtain some qualitative results on the properties of the spectra which occur by the aggregation of molecules into a crystal. 

In the analysis of the lowest electronic excitations in quasi-one-dimensional crystals, it is natural to take into account not only Frenkel excitons, but also one-dimensional charge-transfer (CT) excitons. We will show below that the spectrum of excited states in the molecular chain is strongly sensistive to the mixing of Frenkel and CT states. 

Mixing of Frenkel and charge-transfer excitons in a finite molecular chain... 

Hoffmann, M. (2003). Mixing of Frenkel and Charge-Transfer Excitons and their Quantum Confinement in thin Films. In Agranovich, V. M. and Bassani, G. F. (Eds.) Electronic Excitations in Organic Based Nanostructures. Thin Films and Nanostructures 31. Elsevier Academic Press, Amsterdam, pp. 221-292. 

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