One-electron reduction of acridine-1,8-dione in aqueous solution: a pulse radiolysis study

Abstract

In a neutral aqueous solution of acridine-1,8-dione (AD) the e_aq¯ was observed to react with a bimolecular rate constant of 1.5 × 10¹⁰ dm³ mol^-1 s^-1 and form transient optical absorption bands with λ _max = 305, 475 nm, a shoulder at 680 nm, and increasing absorption with λ > 800 nm. The entire spectrum from λ > 450 nm decayed by first-order kinetics with k = 1.5 × 10⁵ s^-1, whereas the absorption at 315 nm showed growth with k = 1.6 × 10⁵ s^-1. It decayed by second-order kinetics with 2k/el = 1 × 10⁴ s^-1. The transient optical absorption bands at 305, 475, 680, and >800 nm are assigned to the electron adduct, AD^•-, which on protonation forms H adduct (ADH^•) with λ _max = 315 nm and a pK value of 4.7. In basic solutions (pH = 13), AD^•¯ does not show slow transformation to ADH^• while in acidic solutions (pH = 3), only ADH^• was observed (λ _max = 315 nm). ADH^• in highly acidic solutions (pH < 3) protonates to form (ADH₂)^•+ (λ _max = 315, 560 nm) with a pK value of 2.6. AD/AD^•¯ established an equilibrium with MV²⁺/MV^•+ and the reduction potential for AD/AD^•- couple was determined to be -0.586 ± 0.03 V. α-Hydroxyalkly radicals (CH₃)₂C^•OH, CH₃C^•HOH, and ·CH₂OH) were able to reduce AD with bimolecular rate constant values of 3.2 × 10⁸, 2.3 × 10⁸, and 1.2 × 10⁸ dm³ mol^-1 s^-1, respectively. The radical anion AD^•- was able to transfer electron to p-nitroacetophenone with a bimolecular rate constant of 6 × 10⁹ dm³ mol^-1 s^-1.