Through-space charge transfer and nonlinear optical properties of substituted paracyclophane

The introduction, within a pi -conjugated donor-acceptor molecule, of an in termediate barrier to electron tunneling and ifs 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 nonlin ear optical properties of molecular materials. Paracyclophane (pCP) provide s a most relevant cornerstone for more elaborate compounds where donor and acceptor substituents are made to interact through a sterically constrained pi-pi stack. A first attempt in this direction is reported here with the s ynthesis of a model dipolar 4-(4-dihexylaminostyryl)-16-(4-nitrostyryl)[2.2 ]paracyclophane and the subsequent experimental and theoretical study of it s quadratic nonlinear optical properties. A major outcome of this investiga tion is the evidence of a significant "through-space" charge transfer as un ambiguously designated by the strong departure of the beta quadratic hyperp olarizability tensor of the full doubly substituted molecule (60 x 10(-30) esu) from the additive beta value (18 x 10(-30) esu) expected for strictly noninteracting singly substituted pCP moieties, This desired increase of no nlinear efficiency upon substitution is not offset by the usual red-shift o f the absorption spectrum which generally curtails application perspectives in more common uninterrupted conjugated chains. The collective nonlinear p olarization behavior involving the full end-to-end molecular structure is c onfirmed by theoretical calculations using the Collective Electron Oscillat or (CEO) approach which furthermore indicates a significantly enhanced role of electron-hole pair delocalization in the higher order nonlinear respons e, compared to the linear polarizability or the static dipole moment.