(2-Pyrene-1-yl-vinyl)pyridine: a highly transparent chromophore with an unexpected large second-order nonlinear optical response

In the field of electrooptic materials, the rational design and synthesis of chromophores endowed with high transparency together with large second order nonlinear optical (NLO) response is a field of intense research activity. Many organic and metallorganic 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 -electron bridge to generate an intramolecular charge transfer transition [1]. On the other hand, far less attention has been devoted to low dipole moment chromophores, though in some instance they have been shown to possess large hyperpolarizability  [2]. Here we report on the second order NLO properties of (2-pyrene-1-yl-vinyl)pyridine, 1, a chromophore based on the pyrene moiety which is well studied for its outstanding fluorescence properties (long lifetime and high quantum yield) while its derivatives have never been investigated for their NLO response. Preliminary measurements of the second order NLO properties of 1 in solution by the EFISH technique revealed an unexpectedly high and strongly concentration dependent µβ value, comparable to that of benchmark NLO chromophores. At high concentrations the decrease of the second order NLO response seems to point to the formation of centrosymmetric aggregates, a feature that is supported also by the emissive behaviour of 1. Interestingly 1 is quite transparent, a relevant property for potential technological applications. These experimental results have been supported by theoretical investigation performed at DFT and Time Dependent DFT level, which has allowed an analysis of the electronic origin of the NLO response. [1] T. Verbiest, et al. J. Mater. Chem. 7, 1997, 2175. [2] J. Gradinaru, A. Forni, et al. Inorg. Chem. 46, 2007, 884.