Twisted intramolecular charge transfer of methyl p-dimethylaminobenzoate in aqueous β-cyclodextrin solution

Abstract

This paper reports the investigation of the twisted intramolecular charge transfer (TICT) of methyl p-dimethylaminobenzoate (MDMAB) in aqueous β-cyclodextrin (β-CD) solution by the TICT-typical dual fluorescence. In pure water, MDMAB emits only LE fluorescence, and the TICT fluorescence band is developed with the addition of β-CD. Both the LE and TICT fluorescence bands are continuously enhanced upon the increase of β-CD concentration. The intensity ratio of the TICT band to the LE band shows a hillshaped dependence on β-CD concentration, and a blue shift in both TICT and LE bands is observed with the increase of β-CD concentration, of which the blue shift in TICT band is more appreciable. Formation of a 1:1 MDMAB-β-CD inclusion complex, with an association constant of 580±80 l mol −1, is evaluated. The effect of β-CD on TICT of MDMAB is discussed, with consideration of the fact that aqueous β-CD solution is pseudoaquaorganic binary mixture and that TICT in aqueous solution acts differently than in organic solvents. A comparison is made between the TICT of MDMAB and of DMABN in aqueous β-CD solution.