Tetracene and pentacene

Crystals of tetracene and pentacene are triclinic, with two molecules per unit cell. The dimensions of the unit cell in the directions of the a and b axes are similar to those of naphthalene and anthracene the dimension in the direction of the c-axis increases proportionally to the number of benzene rings in the molecule. In all four crystal structures the long axes of both molecules in the unit cell are approximately parallel to the c crystal axis. However, in the triclinic structures of tetracene and pentacene, the only crystal symmetry operation is inversion. The results of calculation for tetracene crystals are shown in Table 3.6. Analogous results for pentacene can be found in (60), (61). The experimental data of the splitting cm-1 for the io-o transition in the tetracene crystal are in the interval 600-700 cm-1. For the /q o transition in a crystal of pentacene the 

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The several methods of energy dissipation, including fluorescence, are strongly dependent on the structure of the excited molecule. The existence of rigid planar aromatic structures is usually favourable to fluorescence. The size of the aromatic system also directly affects the fluorescence intensity and the excitation and emission wavelengths. For the series benzene, naphthalene, anthracene, tetracene and pentacene the emission and excitation wavelengths increase from SOS to S80 nm and from 278 to 640 nm, respectively [28]. 

Polyacenes are condensed aromatic molecules such as naphthalene, anthracene, tetracene, and pentacene. The most widely studied is anthracene The use of anthracene as a photoreceptor was first described in Carlson s original patent (1942). Because of its poor physical and mechanical properties and lack of absorption in the visible or near infrared, it is not of practical interest. The photogeneration processes, however, are perhaps the best understood of any... 

TABLE 7.1 Optimization of CUEES-EC for Benzene, Naphthalene, Anthracene, Tetracene, and Pentacene with MINDO/3 and AMI Parameters... 

Table 3.2 Summary of free molecule data for naphthalene, anthracene, tetracene, and pentacene, a) Vapor phase data b) No data available, the Franck-Condon factors of tetracene were used.

The origin of the small reorganization energy valnes in tetracene and pentacene can be traced to a combination of macrocyclic rigidity and full delocalization of the frontier molecular orbitals [24,26,27], Accordingly, other molecules that have been found to present small intramolecular values are fullerenes, as described by Devos and Lannoo [43], phthalocyanines [39,44], or discotic macrocycles [45],... 

The emission spectra of non-crystalline tetracene films have been interpreted in terms of emission from a molecular pair in a sandwich configuration.32 Substitutional and surface fluorescence in pentacene-anthracene crystals33 and the changes in luminescence of single crystals of pure anthracene, pure tetracene, and pentacene-doped tetracene as a result of electrode-induced changes in electrical charge carriers 34>35 have been examined. Hot bands in the fluorescence and phosphorescence of coronene have been analysed.38... 

Polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and pentacenes, as well as cyclopentadienes, cyclohexa-l,3-dienes, cyclo-hepta-l,3-dienes, and furans, have been found to be suitable diene systems to which the singlet oxygen adds as a dienophile in a 1,4-cycloaddition reaction. Thus, endoperoxides (transannular peroxides) and, in the case of furans, ozonides of the corresponding cyclobutadienes are the primarily produced, more or less stable addition products (2, 21, 22). 

Fig. 2.10 The ct7stal structure of naphthalene, anthracene, tetracene, and pentacene. These aromatic compounds crystallise in the herringbone pattern. Their crystal-structure data are given in the first part of Table 2.3, and the orientation of the individual molecules in these crystals are indicated in the lower part of the Table.

The aromatic compounds are of special interest. As an example, we show here the crystal structure of the very intensively investigated molecule anthracene (Fig. 2.10). The molecular structure, with the charge clouds of the n electrons perpendicular to the molecular plane (compare Fig. 1.2), shows in an understandable way that the polarisability is strongly anisotropic and has by far its largest value in the molecular plane. From the conditions of maximum dispersive interactions and optimum packing in space, the herringbone structure seen in Fig. 2.10 is most favourable in a monodinic crystal lattice with two molecules in the unit cell. Fig. 2.10 also shows the structiures and Table 2.3 the structural data for the other crystals of polyacene molecules, i.e. for naphthalene, tetracene and pentacene with 2, 4 or 5 aromatic rings. All of these substances crystallise like many others also in the same pattern. Table 2.3 also contains data about the orientations of the individual molecules in the unit cell for these crystals. 

Fig. 8.8 A comparison of the term diagrams of the ionised states of the polyacene crystal series benzene, naphthalene, anthracene, tetracene, and pentacene, with the levels of the isolated molecules. Notation as in Fig. 8.6. From [19].

Fig. 8.38 shows the energy bands F (k) for electrons and holes for five different directions of the k vector in the naphthalene crystal and also the densities of state (DOS) in the conduction band and the valence band. Here, a, b, and c are the crystal axes and di = (1/2,1/2, 0) and A2 = (-1/2,1/2, 0) are the directions along the near neighbours in the (a - b) plane, which are degenerate in the monocUnic crystals naphthalene and anthracene, but not in the triclinic crystals tetracene and pentacene. The dispersion along the c direction is smaller than within the (a - b) plane, which is due to the smaller intermolecular interactions between the (a - b) planes compared to those within the (a-b) planes. Similarly, the mobilities are also higher in the (a-b) plane than between the (a-b) planes (see below). 

The bandwidths of the all together sixteen subbands in the monocUnic crystals (or twenty in the triclinic crystals) - both for electrons and for holes, there are two bands each, F+ and F, for each of the five directions of k calculated, of which two are degenerate in the monocUnic crystals - vary between 4meV and 500 meV, depending on the direction and the crystal (cf. Fig. 8.38). The overaU bandwidths W vary, depending on the crystal and the LUMO or HOMO band (CB or VB) between 372 meV (naphthalene, LUMO) and 738 meV (pentacene, HOMO). Table 8.3 Usts the values of the overall bandwidths for the polyacene crystals naphthalene, anthracene, tetracene, and pentacene. 

As one may deduce from Table 1, the computational cost of full-direa MP2 calculations grows with the fifth power of the number of basis functions. The timings given in Table 1 are for runs constrained to 14 MW of the core memory. Decreasing the size of the core memory to 7 MW would result in a doubling of the CPU time for calculations on benzene, naphthalene, and anthracene. Calculations on tetracene and pentacene would not run with this amount of the core memory. 

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