Electrochromic (EC) behaviors of 1,1'-diheptyl-4,4'-bipyridinium (heptyl viologen; HV) and N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) were studied spectroelectrochemically. Suitable darkening potentials for HV and TMPD were determined to be - 1.3 and - 0.3 V (vs. Ag/AgClO4), respectively, according to voltammetric and spectroelectrochemical measurements. A solution-phase electrochromic device (ECD) was fabricated by sandwiching both HV and TMPD in between two parallel indium tin oxide (ITO) electrodes with an intervening space. The influences of the applied voltage and the cell gap on the single-compartment ECD performance are discussed. The transmittance transients measured at 615 nm suggest that there is no need to darken the ECD beyond 1.0 V, while the ECD can be bleached at 0 V Self-bleaching was possible with an open circuit. At 615 nm, a typical transmittance change of over 75% was observed. The reversible electrochromism was achieved only when the cell gap was maintained at 0.28 mm or less. Once reversible operating conditions were maintained, a coloration efficiency of 169 cm(2)/C at 615 mu for the ECD was obtained. (C) 2003 Elsevier B.V. All rights reserved.