Observation of hydrogen-bonding effects on twisted intramolecular charge transfer of p-(N,N-Diethylamino)benzoic acid in aqueous cyclodextrin solutions

The effects of α- and β-cyclodextrins (CDs) on the twisted intramolecular charge transfer (TICT) emission of p-(N,N-diethylamino)benzoic acid (DEABA) in aqueous solution have been investigated by using steady-state and time-resolved fluorescence techniques. The ratio of the TICT emission to the normal emission increases as β-CD concentration increases, while it decreases significantly upon addition of α-CD. This behavior is in accordance with CD dependent decay times of the TICT and normal emissions, indicating the formation of different 1:1 DEABA/CD inclusion complexes. An increased pK_a value of DEABA in the α-CD inclusion complex is determined relative to that of the β-CD complex. The rise time for the TICT emission increases as the β-CD concentration increases, whereas no rise time is observed in the case of the α-CD complex. These results and the size of the diethylamino group clearly suggest that the orientation of the DEABA in the β-CD complex is different from that in the α-CD complex; the carboxyl group of DEABA is encapsulated in the α-CD cavity, whereas it is exposed to the water phase in the β-CD complex. The CD dependences of the TICT fluorescence properties demonstrate that the specific hydrogen bonding between the carboxyl group and water plays an important role in the excited-state TICT.