Photochemical Reaction Between 1,2-Naphthoquinone and Adenine in Binary Water-Acetonitrile Solutions

The photochemical reaction between 1,2-naphthoquinone (NQ) and adenine was investigated using nanosecond time-resolved laser flash photolysis. With photolysis at 355 nm, the lowest triplet state T₁ of NQ was produced via intersystem crossing from its singlet excited state. The triplet-triplet absorption of the state contributes three bands of transient spectra at 374, 596 and 650 nm, respectively, in pure acetonitrile and binary water-acetonitrile solutions. In the presence of adenine, the observation of A·⁺ (at 363 nm) and NQ+H· radical (at 343 and 485 nm) indicates a multistep mechanism of electron transfer process followed by a proton transfer between ³ NQ* and adenine. By fitting with the Stern-Volmer relationship, the quenching rate constant k_q of ³ NQ* by adenine in binary water-acetonitrile solutions (4/1, volume ratio, v/v) is determined as 1.66 × 10⁹  m^-1  s^-1 . Additionally, no spectral evidence confirms the existence of electron transfer between ³ NQ* with thymine, cytosine and uracil.