N-Methylquinolinium derivatives for photonic applications: Enhancement of electron-withdrawing character beyond that of the widely-used N-methylpyridinium

A series of pi-conjugated styryl quinolinium push-pull chromophores have been designed and synthesized in order to examine the electron-withdrawing strength of various quinolinium electron acceptor groups, and their influence on the photophysical properties and in particular on the second-order nonlinear optical response. The static molecular first hyperpolarizabilities measured by long-wavelength hyper-Rayleigh scattering are found to follow the order of the electron withdrawing strength of their acceptor groups as determined by NMR analysis. The quinolinium chromophores based on the strongest electron acceptor groups (1,2- and 1,4-dimethylquinolinium) exhibit remarkably large first hyperpolarizability values of 233 and 256 x 10(-30) esu respectively, which is higher than that of the well-known and widely-used pyridinium analogue 4-(4-(dimethylamino)styryl)-1-methylpyridinium 4-methylbenzenesulfonate with first hyperpolarizability = 183 x 10(-30) esu. The dimethylquinolinium electron acceptor groups exhibit increased electron-withdrawing strength compared to the dimethylpyridinium group used in 4-(4-(dimethylamino) styryl)-1-methylpyridinium 4-methylbenzenesulfonate, and therefore have a high potential for photonic applications. (C) 2014 Elsevier Ltd. All rights reserved.