4-(2-Pyren-1-yl-ethyl)-pyridine: a highly transparent chromophore with unexpected large second-order nonlinear optical response

The rational design and synthesis of electrooptic materials endowed with high transparency together with large nonlinear optical (NLO) response is a field of intense research activity. Many second order NLO chromophores have been designed according to the ''push-pull'' strategy, where electron-donating (D) and electron-withdrawing (A) end groups interact through a delocalized pi-electron bridge to generate an intramolecular charge transfer transition. On the other hand, far less attention has been devoted to low dipole moment chromophores. Here we report on the measurements, by the solution-phase electric-field induced second-harmonic (EFISH) generation method, of the second-order NLO properties of 4-(2-pyren-1-yl-ethyl)-pyridine 1, a simple and highly transparent chromophore. In addition, the presence of a dimethylene bridge ensures a greater stability with respect to standard D-pi-A dyes. In spite of its relatively low dipole moment mu, 1 gives rise to concentration dependent and unexpectedly large mubeta values, comparable to that of benchmark NLO chromophores.