Prototropic reactions for 1-, 2-, and 4-fluorenamides in the ground and excited singlet states

Abstract

Absorption and fluorescence spectra of 1-, 2- and 4-fluorenamides (1-FAm, 2-FAm, 4-FAm) were studied in different solvents and at various acid concentrations. This study indicates that the long wavelength absorption band is long-axis polarised in 2-FAm whereas in 1-FAm and 4-FAm, it is composed of long- and short-axis polarised transitions. The —CONH2 group in 1-FAm and 4-FAm is coplanar with the fluorene moiety, whereas the same group in 2-FAm is non-coplanar with the fluorene ring in non-polar solvents, both in S0 and S1 states, but attains coplanarity in the polar and hydrogen bonding solvents in the S1 state. Hammett's acidity scale is not valid for the protonation reaction of amides and a medium effect is observed on the absorption spectra of the monocations in strong sulphuric acid. The prototropic equilibrium for the protonation of ring carbon atom is not established in the S1 state, due to the slower rate of protonation reaction. Proton-induced fluorescence quenching of the monocations is followed before the monocations are being protonated.