Proton-induced fluorescence quenching study reveals that the quenching rate constants for fluorene and 9-fluorene methanol (9-FM) are very small (about 10 7 dm 3 mol −1 s −1), indicating that the contribution of the intramolecular charge transfer structure is not large, whereas the presence of methyl group leads to an increase in the charge density at position 2 ( k q ≈ 10 9 dm 3 mol −1 s −1). Prototropic equilibria between the monocation and the neutral species are not established in the first excited singlet state, indicating that the radiative lifetimes of the conjugate acid-base species are much shorter than the time required to establish the equilibria. Monoanions produced by the deprotonation of ▪ and —OH groups are non-fluorescent and become stronger acids in the S 1 state.