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WIREs Comput Mol Sci
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π‐Conjugation

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In the current overview, the concept of π‐conjugation in organic compounds is reexamined. Starting from valence bond and molecular orbital (MO) theory, the principal elements of conjugation are worked out starting from a simple Hückel picture; this includes the definition of the conjugated path, bond length alternation, as well as MO localization effects, and requires the distinction between formal and effective conjugation. The latter will be needed to understand the electronic and optical properties of polyconjugated molecules, highlighting the importance of a correct quantum chemical description of ground and excited state properties. Finally, implications of conjugation on interacting systems are shortly discussed. © 2011 John Wiley & Sons, Ltd.

Figure 1.

Examples of conjugated oligomers where n is the number of repetition units and N is the conjugation length. The conjugation path is shown in blue.

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Figure 2.

Energy and topology of the frontier MOs in ethylene and butadiene, and electronic configurations (nΦi) describing the ground (S0), first (S1), and second (S2) singlet excited states of butadiene in the framework of HMO theory. The symmetry of the orbitals and the states is given in lower case and capital letters, respectively. $\Gamma _{\hat \mu }$ gives the symmetry elements of the transition dipole moment operator.

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Figure 3.

Carbon–carbon bond lengths (d) in Å along the conjugated thiophene core (1 = terminal bond) of oligothiophenes 6T, 14T in the ground (solid red line), and first excited state (dashed blue) as calculated at the B3LYP/6‐311G* level of theory; the bond pattern for ideal polythiophene in the ground state is given as thin dotted black line. Orbital pictures depict the HOMO (down) and LUMO topologies (up) of the n‐ring oligomers.

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Figure 4.

Molecules with stiffed backbone, sterical hindrance, and throughomni‐ and cross‐conjugation; p, m, and o indicate para‐, meta‐ and ortho‐substitution.

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Figure 5.

Representative examples for donor–acceptor substituted conjugated molecules and their frontier MO topologies as calculated at the DFT(B3LYP/6‐311G*) level of theory.

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Figure 6.

Electronic excitation for benzodithiophene as calculated at the (TD‐)DFT(B3LYP/6‐311G*) level of theory.

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Figure 7.

Vertical transition energies in vacuo for oligothiophenes as function of 1/N; experiment (open symbols, Ref 43), calculations with different combinations of ground state optimizations with time‐dependent calculations; (TD‐)DFT calculations were done at the B3LYP level, the 6‐311G* basis set was used in all cases. The ECL is extracted graphically by the auxiliary (dashed) lines, see Ref 11.

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