WATER QUENCHING BEHAVIOR OF EXCITED 9-XANTHYLIUM CATIONS IN AQUEOUS SULFURIC-ACID-SOLUTIONS

A series of 9-alkyl- and 9-aryl-substituted xanthylium cations (H, Me, i-Pr, c-Pr, t-Bu; Ph, p-CF3, p-Me, m-Me, p-OMe, m-OMe) were generated from the corresponding 9-xanthenols in strongly acidic aqueous acetonitrile (2:1) solutions (39-78% H2SO4). Steady-state irradiation at 370 nm produced the excited cations, all of which were fluorescent in the 520-nm region. Fluorescence lifetimes varied considerably with acid concentration, from subnanosecond values at low acidity to 20-35 ns at high acidity. Stern-Volmer plots of PHI-F-degrees/PHI-F versus the ''free'' water concentration in these acid solutions gave linear plots in all cases, yielding k(q) values in the 10(6)-10(9) M-1 s-1 range, depending on substituents. For the 9-aryl systems, a plot of log k(q) versus a recently determined sigma-h-nu scale was reasonably linear (r = 0.974) with a rho-value of -1.5, which is opposite in sign to that recently determined for H2O quenching of the same cations in their ground states. Possible explanations for this surprising result are considered.