Microelectrode systems for the study of photochemical processes in solution

Three systems for photoelectrochemical measurements on the microscale have been implemented and compared: (i) an arrangement in which a fibre illuminates a disc-shaped ultramicroelectrode (UME) face-on; (ii) face-on illumination of a ring UME; and (iii) illumination through a fibre-coupled ring UME assembly. These set-ups have been used to study the classic system in which photoexcited tris(2,2′-bipyridine) ruthenium (II), Ru(bpy)32+, reduces methyl viologen dication, MV2+, to the radical cation, MV+. Detection of MV+ at the UME allows the kinetics to be determined and mass-transport at the various configurations to be assessed. It is shown that face-on illumination of either disc or ring-shaped UMEs can be treated in a straight forward way with simplifications such as a uniform light intensity distribution. On the other hand, while the fibre-coupled ring arrangement is potentially easier and simpler to deploy, it does not appear as amenable to such straightforward treatments. Deviations between experiment and theory in the latter case are attributed to complications such as natural convection and non-uniform light distribution, to which this format appears to be more sensitive.