Electrochromic Polymers Covalently Anchored to Electrode Surfaces. Optical and Electrochemical Properties of a Viologen‐Based Polymer

Abstract

Smooth Pt and optically transparent SnO2 electrodes can be functionalized with redoxactive polymeric material derived from the hydrolytically unstable {N,N'-bis[-3-(trimethoxysilyl)propyl]-4,4'-bipyridinium}dibromide, I. The redox polymer on the surface [(pQz+),,]~f is reversibly reducible to [(PQ+),]surf. in either CH3CN/or H20/electrolyte solutions, and in CH3CN the reduction of [(PQ+)~]~f to [(PQ~ is reversible. The E~ =-0.55 • 0.05V vs. SCE, and-0.45 • 0.05V Vs, SCE in H20'and CH3CN, respectively. The E~176 =-0.85 • 0.05V vs. SCE in CH3CN. The optical properties of [(PQ~+),J~u~f (colorless), [(PQ+),]~rf. (purple, ;~m~ ~ 545 rim, e545 ,m ~-107 cm~mol-~), and [(PQ~ (yellow, ~ ~ 375 rim, e~T~ ~ 4 • 107 cm~mol-~) depend somewhat on solvent/electrolyte. The optical spectral features associated with the three redox states of the surface polymer are easily distinguishable with the naked eye for coverages corresponding to >10-8 mol/crn 2 of redox active centers. Electrodes bearing [(PQ2+~+),,]~rf are very rugged and do not deteriorate upon repeated cycling between [(PQ~+),]~u~f and [(PQ+),,]~f. Potential step/chronoamperometry experiments establish that the current for oxidation/reduction of the polymer is nearly proportional to t-'2, indicating that the rate of oxidation or reduction is controlled by a diffusion process with a diffusion constant, D, of 10-9 to 10-~~ cm2/sec depending on electrolyte and its concentration. In practical terms a potential step from 0.0 to-0.80V vs. SCE results in ~50% reduction of [(PQ~+),,]~f to [(PQ+),,]~f in a time as short as 5 • 10-~ sec at a coverage of 1 • 10-s mol/cm 2. The electrode surface derivatizing reagent ! (1-6) is related to N,N'-dialkyl-4,4'-bipyridinium reagents that are known to become intensely SdC h2) 3-+I~~N +~ (C H2)sSt (OMe),x] Br 2