Complexation effect of γ-cyclodextrin on a hydroxyflavone derivative: Formation of excluded and included anions

In this work, we report on steady-state and time-resolved picosecond measurements of the effect of γ-cyclodextrin (CD) on the UV–visible absorption and fluorescence characteristics of 4′- dimethylaminoflavonol (DMAF) in N,N-dimethyl formamide (DMF). We first show that the electronically excited DMAF in DMF undergoes an equilibrated reaction between an intramolecular charge transfer (ICT) and a tautomer (T) formed after an intramolecular proton transfer in the former. A very small amount of anion of DMAF is also found at the ground state. Upon addition of CD, the results show larger formation of anions of DMAF through H-bonding interactions inside of the CD and at its gate giving birth to inclusion and capped complexes, respectively. Both anions emit at the same regions. However, the encapsulated one has a shorter fluorescence lifetime (∼1 ns) due to the lower polarity of the caging nano-cavity when compared to DMF. Rotational times of free (97 ps) and complexed (130 and 770 ps) DMAF agree with those expected from the hydrodynamics theory and the optimized structures using PM3 calculations. The results add new information on the interaction of CD with phenol-containing probes, and it may be relevant to the design of sensors using these nano-hosts and to drug delivery field using CD.