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WIREs Comput Mol Sci
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Cross‐conjugation

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Abstract Cross‐conjugation, even though very common in chemistry, has only sparsely been visited by experiment or by theory. Until very recently, this form of conjugation, in which two separate conjugated branches are linked to the same carbon atom which is part of a (shared) double bond, was considered to be much less able to promote electron delocalization than through‐conjugation. Therefore, exploiting cross‐conjugation to design new materials was considered difficult and not very promising. In this article, we show that this view, at least for neutral molecules in their electronic ground state, is essentially correct. For cross‐conjugated radical ions, the situation is different: There is appreciable charge delocalization across the bifurcation point, i.e., the point where the two conjugated branches meet. Simple Hückel molecular orbital considerations show that the connectivity pattern encountered in cross‐conjugation will lead to enhanced electron delocalization effects. This observation, confirmed by density functional calculations, also applies for electronically excited species. Therefore, cross‐conjugation may be exploited to build molecular switches or may be used to design devices such as molecular transistors. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 477‐486 DOI: 10.1002/wcms.16 This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Computational Materials Science

Through‐ and cross‐conjugated isomers of diethinylethene and its all‐trans (polytriacetylene) or all‐geminal (iso‐polytriacetylene) oligomers.

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The evolution of the scaling exponent bα for polarizabilities and bγ for second hyperpolarizabilities as a function of chain length n for different backbone types (panel a). When b is plotted against the inverse chain length the initial slope is a measure for the conjugation efficiency (panel b). For better readability, the oscillating graph of iso‐PA is not shown in (b).

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The single, double, and, if present, triple bond lengths (columns) for five different backbone types (rows) for increasingly large chains. Each graph represents a particular chain length, with the bond lengths scanned from the left to the right end of the respective chain. The graph of the longest chain (n = 40 for PA, comb and iso‐PA, or n = 20 for PDA and iso‐PDA) is marked in red. The data are from CAM‐B3LYP/6–31G* density functional calculations, which were shown to predict geometries close to experiment for π‐conjugated oligomer chains.38,39

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Different types of oligomer chains derived from PA and iso‐PA using spacers and tethers. The numbering scheme is such that an n‐mer corresponds to a chain containing exactly n double bonds.

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HMO occupations, spin densities as well as single and double bond orders for a polyene (black) and a dendralene (red) of length n = 20 in different electronic (ground state, first excited triplet state) and oxidation states (cation, anion). The symbols used are explained in the box on the right hand side.

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The π orbitals of geminal‐DEE ordered according to their energy. The upper row shows filled MOs with the HOMO on the right, the lower row shows the first virtual MOs with the LUMO on the left.

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The spectra of the Hückel Hamiltonian for the different types of connectivities a, b, c, d, and chain lengths n. The orbital energies are given in terms of the (negative) Hückel β‐parameter. For n = 5000 the orbital density is shown.

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Different connectivity schemes for molecules containing one to four double bonds.

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Hyperconjugation
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