In situ surface plasmon study of the electropolymerization of Fe(vbpy)3 onto a gold surface

Sequential multilayer electropolymerization of Fe(vbpy)₃²⁺ (vbpy=4-vinyl-4′-methyl-2,2′-bipyridine) onto a thin gold electrode was followed in situ with surface plasmon spectroscopy (SPS) using a 1 mW HeNe laser at 6328 Å. The robustness of the gold film electrode necessary for electrochemical deposition in 0.10 M tetraethylammonium perchlorate+acetonitrile is imparted by use of a thin film of 3-mercaptopropyl-trimethoxysilane attached to a SF10 slide to which the metal is covalently bonded. As each polymer layer is deposited by cycling a potentiostat from 0.0 to −1.75 and back to 0.0 V, a plasmon spectrum (reflectivity versus prism angle) is obtained. SP analysis of the angular shift of the spectrum, which increases as the polymer layer thickens, yields an estimate of both the thickness and index of refraction of the polymer film. We found that the plasmon spectrum shifts to higher angles as the polymer layer thickens, along with a progressive decrease in the depth of the resonance minimum. Our modeling shows this unusual spectral behavior involving the resonance minimum is consistent with a Fe(vbpy)₃²⁺ chromophore absorption at 6328 Å, along with thickening of the polymer film. This work demonstrates that SPS is a viable in situ technique for obtaining thickness measurements of electrodeposited thin films.