Transient conductivity OF 1,3-dimethyluracil, uridine and 3-methyluridine in aqueous solution following 20-ns laser excitation at 248 nm

Abstract

Uridine, 3-methyluridine and 1,3-dimethyluracil in aqueous solution were studied by timeresolved conductimetry after excitation at 248 nm by 20-ns laser pulses. The conductivity signal increases to the maximum value at the pulse end (Δκ m) and decreases then with time, depending on the saturating gas (Ar, N 2O, O 2 or McCI) and pH. The Δκ m signal is suggested to originate from hydrated electrons (e aq −) and protons, the latter resulting from radical cations after rapid reaction with water. Biphotonic photoionization occurs in the whole pH range 3–11 with a quantum yield of 0.016 or smaller for laser intensities of ⩽ 8 MW/cm 2. The reaction of eaq with uncharged bases in Ar-saturated solution at pH 5–8 generates radical anions which are subsequently protonated. The neutralization reaction kinetics of the uracil derivatives depend essentially on the transient proton concentration and lead to the disappearance of most of the conductivity (>90%) within a few microseconds or less in neutral or acidic solution, respectively. For 3-methyluridine and uridine after neutralization, the presence of a long-lived species with acidic properties was observed (in small yield) upon biphotonic (but not monophotonic) excitation. The time-resolved conductivity pattern in the alkaline pH range is different for each of the three pyrimidines, depending essentially on the generation or consumption of OH − in the radical termination reactions.