Narrow-Band Electrochromism of Ferrocene-Substituted Rhodamine B

Here, we report on a rhodamine dye containing a redox-active ferrocene group, in which we investigate the coupling of the light-absorbing properties of the rhodamine dye to the electroactive properties of the ferrocene unit. The closed and reduced form of the ferrocene-substituted rhodamine dye, 3′,6′-bis(diethylamino)-2-ferrocenyl-4a′,9a′-dihydrospiro[isoindoline-1,9′-xanthen]-3-one, was obtained following the reaction of aminoferrocene with the corresponding open rhodamine acyl chloride. The effect of the oxidation state of the ferrocene on the equilibrium between the closed and open forms of the dye was examined using chemical and spectroelectrochemical studies. One-electron oxidation of ferrocene unit to the ferrocenium species (at +0.45 V vs FeCp*2+/0; Cp* = –C5Me5) resulted in a small extent of opening of the rhodamine spirolactam moiety, leading to a moderate increase in the absorbance at 560 nm. However, a more oxidizing potential (ca. +1.0 V vs FeCp*2+/0) resulted in a dramatic increase in the absorbance at 560 nm, which we attribute to the formation of a ring-opened rhodamine cation linked to a FeCp(η6-6-azafulvene) cation. The dye could be repeatedly switched between its colorless neutral closed form, and this vividly colored doubly oxidized species.