Photophysical Properties of Cinchona Organocatalysts in Organic Solvents

The photophysical properties of cinchona organocatalysts 1 (dehydroquinidine) and 2 (9-benzylcupreidine) in several organic solvents have been studied by means of absorption and fluorescence spectroscopy. In addition to a locally excited (LE) state emission near 360 nm for both compounds, broad emission bands were observed at longer wavelengths. For 1, the long-wavelength emission bands are pronounced in polar solvents, and their appearance is accompanied by a strong reduction of the LE emission. Electron transfer from the quinuclidine amino group to the locally excited quinoline ring is a likely mechanism that accounts for these observations. For 2, on the other hand, the long-wavelength emission is relatively strong even in nonpolar solvents. Protonation of the quinuclidine amino group by a strong acid in acetonitrile solution leads to an increase of the LE emission at the expense of the long wavelength bands, both for 1 and 2. Upon addition of water to 1 and 2 in acetonitrile, however, the emission spectra change in a very different way: the long wavelength emission of 1 is suppressed, but that of 2 is enhanced. This indicates that the long-wavelength emission of 2 is to be attributed to proton transfer rather than to electron transfer.