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 2 position were comprehensively investigated. Their emission quantum yield ranged between 0.2-0.7 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.