Controlled Storage of Ferrocene Derivatives as Redox-Active Molecules in Dendrimers

Dendritic polyphenylazomethines (DPA) could encapsulate ferroceniums by complexation of the electron-donating skeleton of the DPA imines. Upon addition of ferroceniums to a series of dendritic polyphenylazomethines (DPAGX, where Xis the generation number, X= 1-4), the UV-vis spectra showed changes in a manner similar to that observed for the complexation of metal ions with DPAGX. Stepwise shifts in the isosbestic point were consistently observed with the number of imine groups in the first and second layers of the generation-4 dendrimer (DPAG4). DPAG2 and DPAG3 were also found to trap 6 equiv of ferroceniums. To investigate the complexation, UV-vis spectroscopy, Fe-57 Mossbauer spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS), cyclic voltammetry (CV), and fluorescence spectroscopy were performed. We confirmed that neutral ferrocenes cannot complex with the imine group while ferroceniums can. Utilizing the redox property of ferrocenes, we were able to electrochemically control the encapsulation and release of ferrocenes into the DPA in a manner similar to redox-responsive proteins such as ferritin. In addition to ferrocenes, oligoferrocenes could also be trapped in the DPA. The biferrocene cation(1+) was particularly suitable for electrochemical switching due to its stable mixed valence condition. The terferrocene dication(2+) encapsulated into DPAG4 could be fabricated into a thin film, which exhibited the near-infrared absorption of an intervalence charge-transfer (IV-CT) band, pointing the way toward the use of such systems in material science.