Through-Space Charge Transfer and Nonlinear Optical Properties of Substituted Paracyclophane

The introduction, within a π-conjugated donor−acceptor molecule, of an intermediate barrier to electron tunneling and its size scaling and influence on electronic polarization properties have remained so far elusive issues of great potential interest toward the fine-tuning of the linear and nonlinear optical properties of molecular materials. Paracyclophane (pCP) provides a most relevant cornerstone for more elaborate compounds where donor and acceptor substituents are made to interact through a sterically constrained π−π stack. A first attempt in this direction is reported here with the synthesis of a model dipolar 4-(4-dihexylaminostyryl)-16-(4-nitrostyryl)[2.2]paracyclophane and the subsequent experimental and theoretical study of its quadratic nonlinear optical properties. A major outcome of this investigation is the evidence of a significant “through-space” charge transfer as unambiguously designated by the strong departure of the β quadratic hyperpolarizability tensor of the full doubly substituted molecule (60 × 10-30 esu) from the additive β value (18 × 10-30 esu) expected for strictly noninteracting singly substituted pCP moieties. This desired increase of nonlinear efficiency upon substitution is not offset by the usual red-shift of the absorption spectrum which generally curtails application perspectives in more common uninterrupted conjugated chains. The collective nonlinear polarization behavior involving the full end-to-end molecular structure is confirmed by theoretical calculations using the Collective Electron Oscillator (CEO) approach which furthermore indicates a significantly enhanced role of electron−hole pair delocalization in the higher order nonlinear response, compared to the linear polarizability or the static dipole moment.