Targeted fluorescence enhancement of acridones by hydrogen bonding in solution and solid state

Abstract

The fluorescence of many aromatic carbonyl compounds is strongly enhanced by protic solvents. Yet, not all applications allow for protic surroundings. Employing steady state and time-resolved spectroscopy it is shown that N-methylacridone (NMA) forms a hydrogen-bonded conjugate with urea derivatives (Schreiner's catalyst, SCat). In toluene solution, the hydrogen bonding results in a 9-fold increase of the fluorescence quantum yield. As evidenced by X-ray crystallography, NMA and SCat form a co-crystal featuring a 3-fold increased fluorescence quantum yield. It further exhibits sharper fluorescence bands. By the aid of quantum-chemical computations, the increased fluorescence quantum yields are attributed to an energetic up-shift of nπ* states.