- 6-Methoxyquinoline (6MQ) and acridine are known to become more basic when excited to their lowest singlet state. We utilized a direct time-resolved subnanosecond technique and steady-state fluorescence to determine the acid-base kinetics of these excited compounds ΦN* + H2O ⇄ ΦN*H+ + OH- (k1, k-1). The protonation rates (k1) were found to be 5.2 × 106 M-1 s-1 and 0.2 × 106 M-1 s-1 for 6MQ (pK* = 11.8) and acridine (pK* = 10.7), respectively. The neutralization rate (k-1) was found to be diffusion controlled (6MQ) k-1 = 3.1 × 1010 M-1 s-1. The deuterium isotope effects were found to be k1H/k1D = 3.5 and k-1H/k-1D = 2.2. The protonation rate increased in the presence of metal cations; for acridine in 2 M Mg(NO3)2 solution k1 = 73 × 106 M-1 s-1.