Rose bengal radicals and their reactivity

Abstract

The one-electron oxidised (radical cation) and one-electron reduced (radical anion) forms of the photosensitiser rose bengal have absorption maxima at 470 nm (molar absorption coefficient 21 100 dm³ mol^–1 cm^–1) and 420 nm (molar absorption coefficient 37 600 dm³ mol^–1 cm^–1), respectively. The radical anion (RB^˙–) undergoes electron-transfer reactions both with oxygen (k= 1.5 × 10⁸ dm³ mol^–1 s^–1) and diethylenetriaminepentaacetic acid (DTPA)(k= 8.6 × 10⁸ dm³ mol^–1 s^–1). Rose bengal reacts both with the solvated electron and the semireduced nicotinamide adenine dinucleotide radical (NAD˙) to give RB^˙– with second-order rate constants of 3.1 × 10¹⁰ and 9.6 × 10⁸ dm³ mol^–1 s^–1. The one electron-reduction potential E₁ ⁷ for rose bengal, measured via pulse radiolysis, was found to be –511 mV.