Molecular crystals excitons

For dilute solutions of essentially independent donor and acceptor molecules the Forster or resonance interaction is quite important in molecular aggregrates and in molecular crystals exciton interactions are likely to be important. When the interaction is strong the excitation is not localized on the donor or acceptor but is spread over both. If larger aggregrates are involved, the excitation can be spread over many molecules/33-3 This can easily be seen for the case of a dimer where the donor and acceptor are... 

In order to demonstrate the NDCPA a model of a system of excitons strongly coupled to phonons in a crystal with one molecule per unit cell is chosen. This model is called here the molecular crystal model. The Hamiltonian of... 

The lowest excited states in molecular crystals are singlet and triplet excitons [3]. Since it costs coulombic energy to transfer an electron that has been excited optically from the HOMO (highest occupied molecular orbital) to the LUMC)... 

Craig, D. P., WalmsUy, S. H. Excitons in molecular crystals Theory and applications. London-New York Benjamin 1968. 

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... 

Another example of simple bimolecular reaction is mobile exciton annihilation which is well studied in molecular crystals A + A —> 0 (zero means that usually we are not interested what is happening with reaction products) [10]. In this case the kinetic equation is obvious... 

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

Because of periodic symmetry, the electronic excitation states in a molecular crystal are also of collective nature. These are the well-studied exciton states.81 82 Their energy is close to that of discrete electronic states of isolated molecules (4-8 eV), but the excitation envelops a large group of molecules, migrating efficiently up to 100 nm along the crystal.82 In the same manner, because of efficient migration, the excitation of a fragment of a polymer chain rapidly spreads over the whole molecule.37... 

Photoexcitation of the complex is accompanied by charge transfer from the donor TBPDA molecule to the acceptor C6o one and CT-exciton is formed. Depending on mutual orientation of spins of components (electrons and holes), CT-exciton can be either a singlet or a triplet one. Free charge carriers are formed in molecular crystals mainly due to thermal or impurity dissociation of triplet CT-excitons [6],... 

The increased yield of the reaction products of (1) results in lower concentration of triplet excitons and, consequently, a lower number of free charge carriers formed in molecular crystals that is justified by two resonance peaks of negative polarity in the RYDMR spectrum of... 

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 this section we analyze the surface investigation of molecular crystals by the technique of UV spectroscopy, in the linear-response limit of Section I, which allows a selective and sharp definition of the surface excited states as 2D excitons confined in the first monolayer of intrinsic surfaces (surface and subsurfaces) of a molecular crystal of layered structure. The (001) face of the anthracene crystal is the typical sample investigated in this chapter. 

In the case of electronic spectroscopy in molecular crystals, the first singlet state of the isotopic mixed crystal of anthracene has been investigated in reflectivity and shown to be of amalgamation type.129 Actually, while the gap of resonances A = va — vg) is about 70 cm-1, the excitonic bandwidth is estimated at 500 cm-1. Very few mixed crystals have been investigated in the whole range of concentrations,120 whereas examples of low concentrations (of impurities, for instance) are very numerous The anthracene crystal contains j8-methylanthracene naturally and shows an impurity level below the excitonic band.120 (This does not suffice, however, to predict that a mixed anthracene-j8-methylanthracene crystal will be of the persistence type.)... 

D. P. Craig and S. H. Walmsley, Excitons in Molecular Crystals, Benjamin, New York, 1968. 

The working substance of the ITEP-group /9 source was a doubly tritiated crystalline amino acid, DL-valine (Mallikarjunan and Rao, 1969), which is a molecular crystal. As a result of decay, during the time 10 18 sec the valine molecule transforms into the corresponding helium-containing ion (RHe)+. Since the translational symmetry of the crystal is disturbed, no exciton states are produced, and only molecular states of the complex (RHe)+ are excited. Since in molecular crystals the intermolecular interactions are considerably weaker than the intramolecular ones, one can neglect the influence of the valine crystalline surrounding on the /S-decay process and consider only an isolated valine molecule. 

Kalinowski, J. Excitonic Interactions in Organic Molecular Crystals Technical University of Gdansk Publ. Gdansk, 1977 (in Polish with English Abstract). 

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