Electronic defects and conjugation length in mesoscopic pi-systems

The intrinsic non-linearity of pi -conjugated materials shows up clearly in the non-additivity of their physical properties. This offers unique opport unities for studying defects that limit the conjugation length in polymers whose linear absorption and emission is generally discussed in terms of fin ite segments. We apply a simple Huckel approach to the electronic and vibra tional properties of polyenic chains and consider (a) a weak central bond d ue, for example, to chain torsion and (b) the more complex "defect" due to a central para-conjugated phenyl. The phenyl generates local and extended s tates that are shown to break conjugation much as a weak bond. Defects alte r the length dependence of polyene properties and lead us to the concept of mesoscopic chain lengths. We find comparable vibrational and electronic co ntributions to the static second hyperpolarizability gamma. (C) 2001 Elsevi er Science B.V. All rights reserved.