Bright, Fluorescent Dyes Based on Imidazo[1,2-a]pyridines that are Capable of Two-Photon Absorption

A library of imidazo[1,2-a]pyridines was synthesized by using the Gevorgyan method and their linear and non-linear optical properties were studied. Derivatives that contained both electron-donating and electron-withdrawing groups at the 2position were comprehensively investigated. Their emission quantum yield ranged between 0.20.7 and it was shown to depend on the substitution pattern, most notably that on the phenyl ring. Electron-donating substituents improved the luminescence performance of these compounds, whereas electron-withdrawing substituents led to a more erratic behavior. Substitution on the six-membered ring had less effect on the fluorescence properties. Extension of the delocalization increased the luminescence quantum yield. A new quadrupolar system was designed that contained two imidazo[1,2-a]pyridine units on its periphery and a 1,4-dicyanobenzene unit at its center. This system exhibited a large Stokes-shifted luminescence that was affected by the polarity and rigidity of the solvent, which was ascribed to emission from an excited state with strong charge-transfer character. This quadrupolar feature also led to an acceptable two-photon absorption response in the NIR region.