In situ spectroelectrochemical studies of the fluorescence of 5-substituted indole trimer films

A novel in situ spectroelectrochemical cell has been constructed for the simultaneous measurement of fluorescence and current-voltage characteristics during redox cycling. This involves the use of a detachable rotating disc electrode (RDE), which allows the characterisation of luminescent redox active films produced electrochemically under controlled hydrodynamic conditions. Using this cell, the fluorescence of 5-cyanoindole (CI) and indole-5-carboxylic acid (ICA) films has been measured as a function of redox composition during cyclic voltammetry at slow sweep rates. At faster sweep rates hysteresis has been observed in the recorded fluorescence-charge response, indicative of a structural change in the film. The rate of this structural change appears to be decreased by linking the trimer redox centres in the film and by switching from CI to ICA, consistent with the influence of intertrimer hydrogen bonding. Variation in the observed fluorescence emission as a function of wavelength has also been observed, which gives spatial information on the nature of the film redox reaction. This technique shows potential as a method for probing the kinetics and mechanisms of the reaction of electrochemiluminescent films.