Mapping dynamic concentration profiles with micrometric resolution near an active microscopic surface by confocal resonance Raman microscopy. Application to diffusion near ultramicroelectrodes: first direct evidence for a conproportionation reaction

Confocal Raman microspectroscopy is a very efficient means for probing the molecular composition of micrometric-sized samples. Its coupling with Raman resonance spectroscopy allows the specific tracking of very dilute species by considerably enhancing its Raman bands. Thus, spatially resolved information on the chemical composition of diffusion layers, which build up spontaneously near an active surface placed in a solution, can be obtained with a micrometric resolution. In this work, the applicability of the method for imaging diffusional transport towards ultramicroelectrodes with a micrometric resolution is examined. The efficiency and versatility of confocal resonance Raman microspectroscopy have been tested by probing the composition of the two different diffusion layers which build up in the vicinity of an ultramicroelectrode during the reduction of tetracyanoquinodimethane (TCNQ) on its first or second electrochemical wave. Besides the establishment of the method, this work affords the first direct experimental evidence of the existence and role of conproportionation reactions, which take place on the second reduction wave of EE electrochemical systems. In both cases, the concentration profiles of the radical anion TCNQ− agree extremely well with the theoretical predictions.