Silver Colloidal Effects on Excited-State Structure and Intramolecular Charge Transfer of p-N,N-Dimethylaminobenzoic Acid in Aqueous Cyclodextrin Solutions

[摘要] The silver colloidal effects on the excited-state structure and intramolecular charge transfer (ICT) of p-N,N-dimethylaminobenzoic acid (DMABA) in aqueous cyclodextrin (CD) solutions have been investigated by UV-VIS absorption, steady-state and time-resolved fluorescence, and transient Raman spectroscopy. As the concentration of silver colloids increases, the ratio of the ICT emission to the normal emission (I_a /I_b) of DMABA in the aqueous �?-CD solutions are greatly decreased while the I_a /I_b values in the aqueous �?-CD solutions are significantly enhanced. It is also noteworthy that the ICT emission maxima are red-shifted by 15-40 nm upon addition of silver colloids, implying that DMABA encapsulated in �?-CD or �?-CD cavity is exposed to more polar environment. The transient resonance Raman spectra of DMABA in silver colloidal solutions demonstrate that DMABA in the excited-state is desorbed from silver colloidal surfaces as demonstrated by the disappearance of �?_s (CO₂^-)(1380 cm^-1 ) with appearance of �?(C-OH)(1280 cm ^-1) band, respectively. Thus, in the aqueous �?-CD solutions the carboxylic acid group of DMABA in the excited-state can be readily hydrogen-bonded with the secondary hydroxyl group of �?-CD while in aqueous and �?-CD solutions the carboxylic acid group of DMABA has the hydrogen-bonding interaction with water. Consequently, in the aqueous �?-CD solutions the enhancement of the I_a /I_a value arises from the intermolecular hydrogen-bonding interaction between DMABA and the secondary hydroxyl group of �?-CD as well as the lower polarity of the rim of the �?-CD cavity compared to bulk water. This is also supported by the increase of the association constant for DMABA/ �?-CD complex in the presence of silver colloids.

[学科分类] 化学（综合）

[关键词] p-N;N-Dimethylaminobenzoic acid;Intramolecular charge transfer;Cyclodextrin silver colloidal effects.