Hyperpolarizability density analysis of the enhancement of second hyperpolarizability of π‐conjugated oligomers by intermolecular interaction

Abstract

AbstractIn a previous paper we found that the cofacial intermolecular π–π orbital interaction in stacking dimers significantly changes the longitudinal second hyperpolarizability (γ) of the isolated monomer. On the basis of this result, we investigate the longitudinal γ values of π‐conjugated main chains (CnHn+2, 6 ≤ n ≤ 16) interacting in both‐end regions with two small‐size cationic perturbing π‐conjugated molecules, that is, allyl cations (C3H). These interacting model systems exhibit remarkable enhancement of γ values as compared with those of isolated main chains in the whole chain‐length region. The γ density analysis reveals that this enhancement is described by the virtual charge transfer between both‐end perturbing molecules via the main chain. The analysis of orbital correlation diagram between the perturbing molecules and main chain molecule also clarifies that such feature of γ density distribution originates in the “weak intermolecular antibondinglike coupling” between the (lowest unoccupied molecular orbital [LUMO], LUMO+1) of cationic perturbing molecules and (highest unoccupied molecular orbital [HOMO], HOMO−1) of main chain molecule. The current result suggests the possibility of novel nano‐size control of nonlinear optical (NLO) properties by adjusting the intermolecular orbital interactions between the main molecule and perturbing molecules. A possible control scheme of longitudinal γ for novel intermolecular interacting NLO systems using modified DNA wires is also proposed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005