Fluorescence Spectroscopic Investigation of the Effect of α-Cyclodextrin on the Twisted Intramolecular Charge Transfer of p-Dimethylaminobenzoic Acid in Aqueous Media

Abstract

The effect of α-cyclodextrin (α-CD) on the twisted intramolecular charge transfer (TICT) of p-dimethylaminobenzoic acid (DMABOA) in aqueous media is investigated with the use of TICT-typical dual fluorescence. With the increase of α-CD concentration in aqueous DMABOA solution, the LE fluorescence is slightly quenched and is not shifted in position, while the TICT fluorescence is strongly enhanced and shifted to blue. The correlation of the intensity ratio of the TICT band to the normal band, Ia/ Ib, of DMABOA with medium polarity in aqueous α-CD media is opposite to that in pure organic solvent, which is explained in terms of the different TICT behavior in aqueous solution. The p Ka of DMABOA is not affected by the presence of α-CD, and the orientation of the DMABOA molecule in the DMABOA/α-CD inclusion complex is determined to be with the dimethylamino group in the cavity. The stoichiometry and association constant of the DMABOA/α-CD inclusion complex are investigated by monitoring the Ib/ Ia of DMABOA as a function of α-CD concentration. The Ia/ Ib value of DMABOA in α-CD media is generally much lower than that of DMABN in the same media, which is interpreted by the difference in the energy barrier from the locally excited (LE) to the TICT state and in the orientation of the guest molecules in the α-CD cavity. The effect of the cyclodextrin cavity is clearly demonstrated to be a micro-environmental effect.