Electron transfer reactions between 1,8-dihydroxyanthraquinone and pyrimidines: A laser flash photolysis study

• Intramolecular proton transfer was identified for the photoexcited DHAQ in transition absorption spectra.
• Electron transfer and hydrogen abstraction processes were clarified for photochemical reactions between triplet DHAQ and three pyrimidines, C, T and U.
• Electron transfer rates of these reactions were determined.
• Structural differences of photosensitizers were suggested to alter the trend of electron-transfer reactivity of nucleobases from their redox potential sequence.

Electron transfer (ET) and hydrogen abstraction (HA) reactions between a photosensitizer, 1,8-dihydroxyanthraquinone (DHAQ), and three pyrimidines, cytosine (C), thymine (T) and uracil (U), have been investigated with a method of nanosecond time-resolved laser flash photolysis. Under photo-irradiation at 355 nm, both the triplet DHAQ of normal structure and tautomer structure are identified via intersystem crossing (ISC) in pure acetonitrile and CH3CN/H2O solvent, and they have the very similar behavior in the reaction with nucleobases. With the aid of a complete spectral assignment, decay dynamics of various intermediates have been measured and discussed. A photo-induced ET process followed a HA reaction is confirmed for the reaction between DHAQ and C, while there is no distinct evidence for ET and HA between 3DHAQ* and T (or U). Interestingly, the quenching rate of triplet DHAQ by three pyrimidines is contrary to the redox potential (Eox) order of these DNA bases. By comparing structural difference of two quinones and the ET efficiency from these pyrimidines to DHAQ with the case of menadione (MQ), we can infinitely demonstrate an alternation in trend in reactivity of these bases caused by substituent group on pyrimidine ring.