TRANSIENTS OF URACIL AND THYMINE DERIVATIVES AND THE QUANTUM YIELDS OF ELECTRON EJECTION AND INTERSYSTEM CROSSING UPON 20 NS PHOTOLYSIS AT 248 NM

Abstract— Transients of uracil and a series of 17 correlated pyrimidines, e.g. methylated bases, nucleosides, nucleotides, and polyuridylic acid [poly(U)] were studied after 248 nm excitation by 20 ns laser pulses. The transient absorption spectra in aqueous solution at room temperature reveal the triplet state and the hydrated electron (e−nq), while the corresponding radical cation could not be observed at pH6–7. Fast loss of the chromophore in the260–290 nm range within 0.1 μs was observed in aqueous solution in some cases [e.g. poly(U), 5'‐UMP, uridine, uracil] and in others (thymine, thymidine) virtually not. This photobleaching is assigned to formation of the photohydrate. The concentration of e−eq shows a quadratic dependence on the laser pulse intensity (I,) in the range (0.2‐2) × 107 W cm −2 and the quantum yield of electron ejection (Φs.‐) thus depends linearly on IL. This behaviour, suggesting that the photoionization involves a two‐step absorption process, was found for poly(U) and all pyrimidine monomers examined. At a constant I, value of 2 × 107 W cm 2, Φe. ranges from 3 × 10 −3 for 1,3‐dimethylthymine to 4 × 10˜2 for poly(U). The triplet state shows a much larger transient absorbance (DTA. typically in a broad range, e.g.290–500 nm) than that of the neutral radical resulting from the radical cation. The triplet state in organic solvents (acetonitrile and ethanol) shows generally a significantly larger DTA value than in aqueous solution. The estimated quantum yields of intersystem crossing at room temperature are compared with those of phosphorescence at ‐196°C. Copyright © 1990, Wiley Blackwell. All rights reserved