π-Conjugation

In the current overview, the concept of pi-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 Huckel 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. (C) 2011 John Wiley & Sons, Ltd.