Aromatic molecular crystals

The photoinduced absorption and the electrical characteristics of the conjugated LPPP show that the optoelectrical properties are strongly dependent on charge carrier traps in the bandgap. From aromatic molecular crystals it is known that impurities and structural imperfections form localized states [34]. LPPP forms homogeneous and dense films with a mean interchain distance of about 20 A and ncgligi-... 

In the strong-binding approximation, valid for an aromatic molecular crystal, the intramolecular degrees of freedom are practically uncoupled in... 

Over its nearly 40-year history, ONP has permitted a wide range of novel photochemical and photophysical experiments in various aromatic molecular crystals unfortunately, this approach has not yet seen significant application beyond such fundamental studies in these systems. However, tantalizing recent results - particularly (1) the observation of extremely high polarization values under modest experimental conditions (2) the transfer of high polarization to rare nuclei for enhanced, high-resolution NMR studies ... 

At the same time we must have in mind that for the majority of aromatic molecular crystals such as, for example, naphthalene, the lowest molecular electron excited state is a triplet state. In these circumstances triplet excitons exhibit a number of specific properties, which has been used for their study. 

Devos, A. and Lannoo, M., Electron-phonon coupling for aromatic molecular crystals Possible consequences for their superconductivity, Phys. Rev. B, 58, 8236, 1998. 

There are also aromatic molecular crystals with only one molecule in the imit cell, so that all the molecules in the crystal are equivalent under translations. The perhaps best-known example of this type of compound is hexamethylbenzene. 

Another very sensitive method is measurement of the lifetime of triplet exci-tons (Sect. 6.9), especially in aromatic molecular crystals. It can be very sharply decreased by specific impurities. Here, again, a detection limit of 10 " impurity molecules/cm can be attained [4]. An example is shown in Fig. 3.4. 

All together, there are 3 acoustic phonons. The velocities of sound in all aromatic molecular crystals lie in the range of a few 10 cm/s. 

Such CT excitation states are found, in addition to the Frenkel excitons, in the aromatic molecular crystals such as anthracene which we have already treated, that... 

MS H. C. Wole, The Electronic Spectra of Aromatic Molecular Crystals, in Adv. in Solid State Physics, edited by E. Seitz and D. Turnbull, vol. 9, Acad. Press, New York (1959)... 

The charge carrier mobilities p differ fundamentally in ultrapure aromatic molecular crystals from those in less-perfect organic crystals or disordered organic sohds or polymers. Fig. 8.1 shows the temperature dependence of /c in an ultrapure perylene crystal between 300 K and 30 K, the mobility increases with decreasing temperature. This holds generally for high-purity single crystals. In this particular case, p increases from about 1 cm /Vs at room temperature to around... 

Although the exact details are not known at present, evidence is available, then, that the triplet level in addition to the singlet level can yield charge carriers efficiently. In a recent detailed theoretical consideration of triplet excitons in aromatic molecular crystals, Jortner, Rice and Katz have again pointed out that the most favored triplet-triplet annihilation process should... 

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

As in molecular chemistry, an alternative path to compensate for electron deficiency is the formation of multiple bonds, through 7r-interactions, as in unsaturated and aromatic molecular systems. Our work in Houston focuses on probing the efficacy of the ZintI concept in rationaUzing stoichiometries, crystal structures and chemical bonding of complex electron-poof ZintI phases that exhibit novel i-systems. Their chemical bonding is reflected by their unusual crystal structures related to unsaturated hydrocarbons [53]. 

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... 

The picture is less clear for molecular crystals when the molecules deviate strongly from a globular form. NMR data and tracer diffusion data are then often in disagreement. Diffusion profiles (In c, vs. distance) are found to be curved, which is usually attributed to additional heterogeneous and fast diffusion pathways. For plastic crystals, this could indicate that many of them possess a highly defective structure. Even for the aromatic ring molecule benzene, which forms a non-plastic crystal, one finds a D (NMR)/D (tracer) ratio on the order of 103. This cannot be understood unless one invokes other than bulk lattice mechanisms of diffusion. 

Determination of the crystal structure of phase II by Lonsdale in 1929 unequivocally settled over 70 years of debate concerning the geometry and bonding of aromatic molecular systems. The measured bond lengths and crystal structure of hexamethylbenzene are shown in Fig. 9.6.1. The hexamethylbenzene molecules lie within planes approximately perpendicular to (111). Phase III is structurally very similar to phase II, but differs from it mainly by a shearing process between molecular layers that results in a pseudo-rhombohedral, more densely packed arrangement. 

A simple possibility for the synthesis of esters, the reaction of an acid chloride with an alcohol, was used by Schrage and Vogtle [58] for a two-step synthesis of the macrocycle 63 from the alcohol 61 and the acid chloride 62. Compound 63, an example from the field of host/guest chemistry, forms a cavity, as studied with CPK-models, which could include planar, aromatic guests. Crystals obtained from benzene/ -heptane point to a 1 2 stoichiometry of 63 and benzene according to NMR-spectroscopic data. However, whether this is a molecular inclusion complex or just a clathrate is not yet known. 

Dunitz, J. D. and Gavezzotti, A. (1999). Attractions and repulsions in molecular crystals what can be learned from the crystal structures of condensed ring aromatic hydrocarbons Acc. Chem. Res., 32, 611-84. [153,169]... 

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