Fluorescence properties of the derivatives of oxazolo[4,5-b]pyridyne

The fluorescence behaviour of a family of new dyes based on the oxazolo[4,5-b]pyridine ring has been investigated. It was found that introduction of electron withdrawing and electron donating groups into the oxazolo[4,5-b]pyridine molecule causes an increase of the ground and excited state dipole moments. The observed fluorescence has a strong charge transfer character and was investigated in terms of radiative back electron transfer theory. In water–acetonitrile solutions the compounds behave as bases forming fluorescing cations in the presence of inorganic acid. The protonation centres are different depending on the substituent. In the case of 2-(4-N,N-diethylaminophenyl)oxazolo[4,5-b]pyridine the protonation is complex and formation of a bication of this compound has been determined. These observations are consistent with quantum chemical calculations.