Estimation of average orientation of surface-confined chromophores on electrode surfaces using electroreflectance spectroscopy

A novel method was proposed to estimate the orientation of chromophores confined on a planar electrode surface using electroreflectance (ER) spectroscopy (potential-modulated UV-vis reflectance spectroscopy). The intensity ratio of the ER response to p-polarized and s-polarized incident light was formulated in an equation as a function of incident angle and the orientation of the chromophore by adopting approximations. The method was applied to estimate the orientation of viologen cation radical chromophore in a monolayer of N-butyl-N′-(4-mercaptobutyl)-4,4′-bipyridinium dihexafluorophosphate adsorbed on a gold electrode. The orientation of the long axis of the viologen cation radical was found to be approximately 70° with respect to the surface normal. Both two-phase and three-phase models were examined and the validity of these models was discussed.