Effect of magnetic field on radical yields during the photoreduction of xanthene dyes in viscous media

Abstract

The effect of an external magnetic field (H⩽ 0.18 T) on the yield of semiquinone radicals during photoreduction of xanthene dyes (fluorescein, ethyleosine, erithrosine and rose bengal) in a viscous ternary mixture of glycerol, p-cresol and ethanol has been studied by the flash-photolysis technique. The effect of the magnetic field depends upon field strength and solvent viscosity. Magnetic enhancement (an increase of radical yield up to 2.5 times) was observed for dyes in solvents of moderate viscosity [3 ⩽(η/T)/cP K^–1⩽ 8]. The influence of internal and external heavy atoms on magnetic field effect in the above-mentioned reactions has been investigated. In the case of fluorescein and erithrosine the existence of an external heavy atom (KI, 0.1 mol dm^–3 leads to a decrease in magnetic field effect in contrast to the behaviour of rose bengal. The effect of external heavy atoms on the e.s.r. spectrum of fluorescein-semiquinone radical (HDH˙) has been also studied. Peak broadening and poorer resolution were found in the presence of CsCl for the spectrum of HDH˙. The influence of external heavy atom on the g-factor of free radicals as well as the role of spin–orbit coupling in a magnetic field are discussed.