A Low Temperature Fluorescence Study of a 4-Dimethylamino-2′-Hydroxy Chalcone: From Solvent Matrix to Crystalline State

4-Dimethylamino-2′-hydroxy chalcone (DHC) 1 is an important natural compound that is nearly non-fluorescent in solution but highly fluorescent in its crystalline state. At room temperature, the weak fluorescence from the DHC solution is exclusively from its keto tautomer, without notable contribution from its enol tautomer. By using low-temperature fluorescence, the study found that the enol emission could be detected upon cooling with liquid N2 in a protic solvent (e.g., EtOH). This led to observation of the fluorescence vibronic structure of enol tautomer, in addition to its enol emission λem ≈ 473 nm that is well separated from its keto tautomer emission (λem ≈ 600 nm). By freezing DHC in a solvent matrix, the study revealed the fluorescent characteristics of a single molecule in a rigid environment. Further comparison of DHC in a solvent matrix and crystalline state disclosed that the emission of crystalline DHC was primarily from the keto tautomer, along with some minor contribution from the enol tautomer, despite the tight packing environment in the crystalline state.