Effects of cyclodextrins on the fluorescence of 2-methylnaphth[2,3-d]oxazole in aqueous solution

In organic solvents, 2-methylnaphth[2,3-d]oxazole (MNO) exhibits a single emission, whereas it exhibits dual emissions (the 350-nm band fluorescence and the longer-wavelength fluorescence) in water (pH 6.1). In acidic aqueous solution, the 350-nm band fluorescence alone is observed with a disappearance of the longer-wavelength fluorescence. In pH 6.1 buffers, alpha-CD has been found to form a 2:1 inclusion complex with MNO. Upon the addition of alpha-CD to MNO solution, the longer-wavelength fluorescence is significantly reduced in intensity, relative to the 350-nm band fluorescence. The incorporation of an MNO molecule into the alpha-CD cavities obstructs the generation of the excited state responsible for the longer-wavelength fluorescence. The longer-wavelength fluorescence is most likely due to the intramolecular charge transfer (ICT) state of MNO, which is induced by water molecules of high polarity. MNO forms a 1:1 inclusion complex with beta-CD. In the presence of beta-CD, the intensity ratio of the longer-wavelength fluorescence to the 350-nm band fluorescence is decreased. When 1-propanol is added to MNO solution containing beta-CD, a ternary inclusion complex is formed among beta-CD, MNO, and 1-propanol. In beta-CD solution containing 1-propanol, the intensity ratio of the longer-wavelength fluorescence to the 350-nm band fluorescence is slightly reduced. These findings support the conclusion that the longer-wavelength fluorescence of MNO is due to the ICT state, whose generation is promoted by the water environment.