Mixed crystals guest molecules

Mixed crystals are formed by replacement of some host molecules by others similar in dimensions and in their interactions with neighbours. Impurity or "guest molecules with lower transition energies can capture or trap excitation energy from the host crystal. 

Fig. 3. Schematic diagram of levels in a one-dimensional mixed crystal containing several guest molecules. One level from each manifold moves out of the band, finally coalescing at large trap depths.

In the inverted band a quite different pattern of intensity distribution is to be expected. In the pure crystal the topmost level alone is active it remains the strongest under all conditions. As the trap is deepened, some intensity moves from the topmost level downward through the band into the bottom level, which breaks out of the band and eventually becomes practically a localized state of the trapping molecule. Thus the presence of guest molecules awakens spectral activity in normally inactive levels, and should enable the extent and character of the pure crystal band structure to be studied experimentally. The point is illustrated in the diagrammatic spectra in Fig. 6, illustrating the transitions in one-dimensional mixed crystals for trap depths from zero (pure crystal) to d = 3.6. In each case the intensities are adjusted to make the lowest transition have unit intensity this... 

This MCPA calculation has been carried out for the triplet naphthalene mixed crystal, with a square cell of nine sites (3 x 3). The density-of-states distribution for cA = 30% is shown in Fig. 4.14. One first notices that the smoothed shape disagrees generally with the numerical-simulation spectrum. The finest structure, at the maximum of the spectrum, corresponds to a configuration with a guest molecule A completely surrounded by host... 

The effect that is dominant in pure crystals, the Davydov splitting, is missing in mixed crystals. There are no first order effects of the crystal field. Second order coupling terms like (3.3), where the superscripts refer to the lowest transitions in the host and guest molecules, cause intensity transfers. In the b direction the tetracene intensity is increased by a factor of 3 by transfer from the anthracene 250 nm and 380 nm systems in the a direction there is little change. 

In an inclusion crystal formed by mixing crystals of guest and host compounds, molecules of the former are ordered regularly, therefore, photoreaction of the guest in the inclusion crystal proceeds efficiently and stereoselectively to give one stereo-... 

Centrosymmetric crystals can be transformed into chiral or polar mixed crystals and this enables us to obtain novel means for absolute asymmetric synthesis. The principle is based on selective introduction of a guest molecule into a centrosymmetric host structure, thus reducing the symmetry of the mixed crystal. Crystallization of (E)-cinnamamide (space group P2i/c) in the presence of (E)-... 

We believe that direct dipole-dipole coupling among the guest molecules, which leads to an exciton -type state in the mixed crystal, is responsible for the effect." More work on these highly doped molecular crystals needs to be done. 

Scheme 8. Enantioselective Photoreactions in TADDOL Inclusion Compounds with a Cou-marin, a Methacryl Anilide, and an Oxocyclohexenyl-carboxamide. In the first case, the packing of the coumarin molecules in the mixed crystal is such that the double bonds are predisposed for the (2+2) cycloaddition. In the second example, a photochemical electrocychc reaction is followed by a sigmatropic H shift. The third reaction is an intramolecular (2+2) cycloaddition with dia- and enantioselective formation of three new stereogenic centers. There are several more reactions of this type, described in the literature [54], and the Toda group has determined the crystal structures of a number of inclusion compounds to show the correlation between the crystal packing and the configuration of the photoproducts. EMastereoselective solid-phase reactions of chiral guests in TADDOL-host lattices have also been described by the...

In the coming years, excitons in disordered systems and strongly-coupled excitons will be a focus of attention. In mixed crystals consisting of two components, the guest molecules can be included into the exciton band of the host, i.e. amalgamated. The influence of such an amalgamation on the electrical, optical, and mag-nehc properties of the mixed crystals is an interesting, old, but still current research topic in the physics of molecular crystals [43]. 

The first experiments with ESR on electronically excited molecular triplet states were published by Hutchison et al. [1]. A few years later, the first optical detection of ESR in a magnetic field followed [2], as well as the first optical detection of the zero-field resonance [3]. Eor these experiments, molecular crystals (host crystals doped with guest molecules) were investigated. The host crystal in these mixed-crystal systems serves mainly as an orienting matrix and the result of these ESR measurements are values for the fine structure and the dynamic properties of the Ti state of the guest molecules. 

Fig. 7.3 The ESR spectrum of a perdeuteronaphthalene-naphthalene (N-dg + 2% N-hg) mixed C7Stal at T = 4.2 K. A IA and 6 /6 are ESR transitions in isolated N-hg molecules with the two different orientations A and B (see Fig. 7.11) are transitions in N-hg A-B pairs (mini-excitons) the lines in the centre (g 2) are transitions in free radicals of the crystal mount. The relative concentrations of the T- states of the isolated guest molecules [A,B)...

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