Anthracenes 2 molecules

In cases where there is more than one molecule per unit cell (e.g. anthracene 2 molecules, 6T 4 molecules) and the molecules are related by symmetry operations, the crystal wavefunctions are constructed out of the subsets of non-equivalent molecules which leads to representations of the crystal states which are the symmetric or the antisymmetric combinations of the subset wavefunctions [3]. These wavefunctions do then represent different irreducible representations of the crystal space group and in the following we wiU sketch the theoretical ideas for the example of 6T. 

Due to the steric requirement of these substituents the formation of a columnar structure with infinite M M interactions is inhibited, and only the association of pairs of molecular units is allowed. The Ni Ni distance is 3.21 A [164]. If the same compound is crystallized in the presence of benzimidazole, the [Ni(dmg-BF2)2]2 dimer units are sandwiched between sheets of benzimidazole molecules due to n-n interactions resulting in an increased Ni Ni separation of 3.358 A [165]. With anthracene the n-n interactions seem to be stronger, because in this case the parent dimer molecule is cleaved. Each monomer now has a conformation of type B (Fig. 32) and is sandwiched by anthracene molecules [166]. Compound 121 has the same configuration [163d]. 

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

Table 2.13. Some Physical Characteristics of the Anthracene Molecule...

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

Molecules of anthracene consist of three shared quasi-hexagonal rings of carbon (14C-atoms) plus ten H-atoms attached to ten of the carbons (Figure 12.1). In three dimensions the anthracene molecules have the shape of elongated tablets. They stack in crystals side-by-side in a staggered pattern to form sheets which pack together in herringbone arrays. 

Kojima (1981) discovered that a photo-plastic effect occurs in anthracene. It is largest for light of 430 nm. wavelength and is partially reversible. The effect probably results from a change in the polarizabilities of the anthracene molecules caused by photo-excitation. This is expected to increase the cohesion in the crystals slightly. The magnitude of the effect is up to about ten percent. 

Figure 4.4 Energy-level diagram showing how the electronic and vibrational energy levels in the ground-state (S0) and first excited-state (Si) anthracene molecule are related to the absorption and fluorescence emission spectra...

A well-known example of an exciplex is the excited-state complex of anthracene and N,N-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene molecule. In nonpolar solvents such as hexane, the quenching is accompanied by the appearance of a broad structureless emission band of the exciplex at higher wavelengths than anthracene (Figure 4.9). The kinetic scheme is somewhat similar to that of excimer formation. 

Thermal treatment of solid Cgg(anthracene) was successfully employed for the selective synthesis of the trans-l-C,5Q(anthracene)2 bisadduct by the disproportionation of two Cgo(anthracene) molecules into C5o(anthracene)2 and CgQ [10]. 

This quantum of energy is contained in a photon of wavelength 365 nm. An Avogadro number of photons is called an einstein. The amount of energy absorbed to promote one mole of anthracene molecules to the first excited electronic state will be... 

Figure 4. IS Probability of absorption and emission by differently oriented anthracene molecules. The dotted cutves represent vector direction of emitted radiation.

The analysed value predicts 2.5 atoms of chlorine per anthracene molecule which indicates the solid to be a dimer with an empirical formula of CscHl7Cl6. When a small quantity of the white residue is sublimed under the reduced pressure of a filter pump with the help of a small open flame, it sublimes giving vapours acidic to litmus. The absorption curve of the sublimate is identical with that of anthracene. The most plausible structure that could be assigned to this white solid seems to be... 

In this reaction the excited anthracene molecule is supposed to abstract a chlorine atom from CC14, a process facilitated by the resonance energy of. CCIS radical. As the energy of 1A is 322 kJ (77 kcal) mol-1 and the bond energy of C—Cl in CC14 is 293 kJ (70 kcal) mol-1 there is enough energy for the process. 

Some of the products and resin formation may be accounted for in this way. These heavily substituted benzene derivatives are likely to have absorption in the uv regions A < 300 nm. Since more HC1 is formed on keeping, the mode of polymerization seems to be as presented in (C) above and not by attack on another anthracene molecule by the AC1 radical. This is concluded from the low quantum yield of anthracene removal which does not predict chain reaction. 

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

This early work was followed by the study of many other aromatic systems and the molecules were generally found to be essentially planar. During the 1930 s the X-ray work was mainly two-dimensional and the accuracy was not high by modem standards. Limits of error in the region of 0-05 to 0-10 A can generally be assumed. Many of the important structures have now been refined by more accurate methods and the naphthalene and anthracene molecules are known to be planar to within 0-01 A. 

Cruickshank (1962b) has pointed out that all four sets of deviations show the same trend, so that it is likely that, in the crystal, the anthracene molecule is not strictly planar, although the matter is not... 

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