Unlike most other electron donor–acceptor (EDA) molecules, aminochalcones exhibit
unusual solvent polarity-dependent fluorescence behavior. The photophysical
behavior of two aminochalcones, namely, 4-aminochalcone (AC) and
4-dimethylaminochalcone (DMAC), has been studied in a viscous room-temperature
ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate,
[bmim][PF6], by steady-state and time-resolved fluorescence
techniques. The observation of a single emission band in viscous IL, which is
similar to the one observed in less viscous polar conventional solvents,
suggests no twisting is necessary for the formation of the charge-transfer state
from which the emission of aminochalcones originates. The fluorescence decay
profiles, solvation dynamics, and excitation-wavelength-dependent emission
behavior of AC are found to be quite different from those of DMAC in the IL. The
observed difference is attributed to specific H-bonding interaction between AC
and [bmim][PF6].