Stacked molecules electrical properties

In crystals that are formed from small molecules, it is often found that large, flat molecules stack together like a pile of plates or saucers. This stacking imposes considerable order and can generate unusual optical or electrical properties. 

Unusual magnetic and electrical properties might also arise from quasi one-dimensional crystal structures of these compounds. The acceptor stacks /especially TCNQ/ may be either regular, i, e. with equally spaced molecules, or alternating when composed of diads, triads or tetrads. In the latter case some substances exhibit EPR spectrum characteristic of mobile, thermally activated triplet states /triplet excitons/, The spectrum may result from the excitation of two or more coupled TCNQ entities /6, 7/. The triplet character of the paramagnetic excitation is shown by the anisotropic two-lines EPR spectrum which results from a zero field splitting of the triplet levels being described by the spin Hamiltonian /8/j... 

A superlattice is a multilayer structure with coherent stacking of atomic planes (7). An idealized superlattice structure with square-wave modulation of composition and/or structure is shown in Figure 1. The thicknesses of the layers are not necessarily equal, as long as the structure is periodic. When the modulation wavelength is in the nanometer range, each layer is only a few unit cells thick. Quantum confinement of carriers in rianomodulated materials often leads to technologically important optical and electrical properties which are intermediate between those of discreet molecules and extended network solids. 

The DFT scheme is more suitable to compute the electronic properties of the extended DNA molecules that are proposed as candidates for electrical wires, and has been successfully applied to a number of different structures. Provided DFT reproduces the main structural features (e.g., bond lengths and angles, stacking distances) in agreement with the MP2//HF calculations, the electronic properties thus derived are fully reliable. DFT is the method of choice for studying reactive chemistry [113]. 

While complexes in which the metal is coordinatively unsaturated frequently oligomerize utilizing bridging jS-diketonate ligands, a different mode of aggregation is believed to occur in M(CO)2-(MeCOCHCOMe) (M = Rh, Ir), The near planarity of the acetylacetonate permits these flat molecules to stack so that short (3.20 A for Ir) metal-metal contacts are formed. This leads to highly anisotropic DC electrical transport properties and these compounds are semiconductors. 

The crystal structure and electrical conduction properties of (Et4N)0,5[Ni(dmit)23 (where dmit = isotriethione dithioiate) have been reported.120 In the crystal, face-to-face stacking and side-by-side arrangement of Ni(dmit)2 molecules form a two-dimensional S—S network. The compound is an anisotropic semiconductor with a maximum conductivity of 4.5 x 10-2 Q"1 cm 1 in the stack direction.120... 

---