Minimal modification approach to red-shifted absorption and fluorescence in 1,8-naphthalimides

As a minimum modification approach toward longer wavelength chromophores, a series of (4-diethylamino-1,8-naphthaloyl)-aminopyridines were prepared with the pyridine nitrogen located at the ortho, meta, and para positions. Comparison between these isomeric neutral dyes and their corresponding pyridinium-1,3-propanesulfonate salts reveals a red-shift in both absorption (up to an emission of 1682 cm−1 in ethyl acetate) and emission. These observed shifts along with increased fluorescence quantum yield are attributed to polarization induced by the quaternary nitrogen of the pyridinium cation.