Electronic defects and conjugation length in mesoscopic π-systems

The intrinsic non-linearity of π-conjugated materials shows up clearly in the non-additivity of their physical properties. This offers unique opportunities for studying defects that limit the conjugation length in polymers whose linear absorption and emission is generally discussed in terms of finite segments. We apply a simple Hückel approach to the electronic and vibrational properties of polyenic chains and consider (a) a weak central bond due, for example, to chain torsion and (b) the more complex “defect” due to a central para-conjugated phenyl. The phenyl generates local and extended states that are shown to break conjugation much as a weak bond. Defects alter the length dependence of polyene properties and lead us to the concept of mesoscopic chain lengths. We find comparable vibrational and electronic contributions to the static second hyperpolarizability γ.