Effect of cyclodextrin inclusion complex formation on the twisted intramolecular charge transfer (TICT) of the included compound: the p-dimethylaminobenzoic acid-β-cyclodextrin system

Abstract

The effect of cyclodextrin inclusion complex formation on the twisted intramolecular charge transfer (TICT) of the included compound was examined using the TICT-typical dual fluorescence for the p-dimethylaminobenzoic acid (DMABOA)-β-cyclodextrin (β-CD) system in buffered (pH 4.12 and 8.00) and unbuffered (pH 5.06) solutions. The host-to-guest ratio of the DMABOA-β-CD inclusion complex is evaluated to be 1 : 1 in the three solutions from an analysis of the fluorescence intensity data by two methods. Enhanced LE (locally excited state) and TICT emissions and I a/ I b ratio, a blue-shifted TICT fluorescence band and a stronger excitation wavelength dependence of the TICT fluorescence for DMABOA in the β-CD inclusion complex are observed. The correlation of the I a/ I b ratio with the medium polarity is opposite in the inclusion complex to that observed in organic solvents. The TICT fluorescence of the DMABOA-β-CD inclusion complex with DMABOA in the basic form shows a stronger excitation wavelength dependence than that with DMABOA in the acidic form. A decreased p K a of DMABOA in the β-CD inclusion complex is determined. This is taken as evidence that the Me 2N group of the DMABOA molecule in the β-CD cavity. TICT is shown to be viscosity dependent in the cyclodextrin inclusion complex. The dependence of the TICT fluorescence of DMABOA in the inclusion complex on the polarity, viscosity and excitation wavelength is discussed in detail with regard to the dynamic scheme of the TICT model and the structural characteristics of the ground and excited states.