Linear -conjugated molecular systems

Figure 12.7 A summary of the molecular orbitals and relative energies for linear conjugated pi systems of 1 to 6 p orbitals. The phase of the ends of a linear pi system molecular orbital alternates as the energy of the MO increases. The fundamentai (lowest MO) is always symmetric.

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

The 7i orbitals of longer linear conjugated systems are derived in essentially the same way. The energies and coefficients of the n molecular orbitals for all six systems from an isolated p orbital up to hexatriene are summarised in Fig. 1.35. The viewpoint in this drawing is directly above the p orbitals, which appear therefore to be circular. This is a common simplification, rarely likely to lead to confusion between a p orbital and an s orbital, and we shall use it through much of this book. 

In the examples of our work on organic molecular and polymeric solids that follow, first some contributions to the UPS line widths in condensed molecular solids are discussed for two prototype systems, anthracene and isopropyl benzene then the UPS of two.aromatic pendant group polymers, polystyrene and poly(2-vinyl pyridine), are discussed and compared with some spectra concerning the simplest linear conjugated polymer, polyacetylene. 

The conjugated polyene systems taking part in pericyclic reactions are linear, so all the tt molecular orbitals of the starting materials have a different energy. The distribution of energy levels is important because it affects the electronic configuration of the starting materials (see Example 6.19) and consequently the course of the reaction. 

The molecular orbital model as a linear combination of atomic orbitals introduced in Chapter 4 was extended in Chapter 6 to diatomic molecules and in Chapter 7 to small polyatomic molecules where advantage was taken of symmetry considerations. At the end of Chapter 7, a brief outline was presented of how to proceed quantitatively to apply the theory to any molecule, based on the variational principle and the solution of a secular determinant. In Chapter 9, this basic procedure was applied to molecules whose geometries allow their classification as conjugated tt systems. We now proceed to three additional types of systems, briefly developing firm qualitative or semiquantitative conclusions, once more strongly related to geometric considerations. They are the recently discovered spheroidal carbon cluster molecule, Cgo (ref. 137), the octahedral complexes of transition metals, and the broad class of metals and semi-metals. 

The energies of the molecular orbitals can also be deduced by the same device, used for linear conjugated systems, of inscribing the conjugated system inside a circle of radius 2/3. There is no need for dummy atoms, since the sine curves go right round the ring, and the picture is therefore that shown in Fig. 1.44. 

Prasad (1988) further demonstrated that 7 increases remarkably with the increase of the molecular length n. The relation of 7 and n of the para-acetylene (C H +2) is shown in Figure 6.35 in which 7 increases exponentially with n. Therefore, the long molecular shape and the linear conjugated system are still the essential requirements for a high x 3" -Apparently, liquid crystals are a good candidate. 

Linear-conjugated systems (mainly, poly- and oligothiophenes) deriva-tized with C6o-fullerene as molecular heterojunctions for organic photovoltaics 05CSR483. 

The linear and nonlinear optic properties of organic materials, including a -conjugated molecular polymers, make it possible to develop numerous technological and industrial applications of these systems. The applications in the field... 

An electrocyclic reaction is a molecular rearrangement that involves the formation of a cT-bond between the termini of a fully conjugated linear rr-electron system and a decrease by one in the number of 7U-bonds, or the reverse of that process. Thus if the open chain partner contains n 7u-electrons, the cyclic partner has (n — 2) 7c-electrons and two electrons in a new a-bond. For example, let us consider electrocyclization of butadiene and hexatriene systems as shown in Scheme 2.1. 

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