Nowadays there is a great interest in using individual molecules as nanometer-scale switches and logic devices, with the aim of reaching higher power and smaller size. Demonstrating that one molecular switch can be turned on and off in room temperature, simply by applying a current to a neighboring molecule has interesting implications. Herein we report the synthesis, characterization and behavior of three generations of polyphosphorhydrazone (PPH) dendrimers, fully functionalized with 6, 12

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... 24 redox active perchlorotriphenylmethyl (PTM) radicals in the periphery, capable of undergoing an electrochemical reversible switching by multi-electron reduction and oxidation. An electrical input was used to trigger the physical properties of these radical dendrimers in a reversible way, modifying their optical, magnetic and electronic properties. Our Gn(PTM • )x radical dendrimers are paramagnetic, exhibit an absorbance band at 386 nm and red fluorescence emission, if in radical state. When they are switched to their anion state, these dendrimers convert to diamagnetic species with a maximum absorbance band at ca. 520 nm and no fluorescence emission. Due to two different molecular states, the switch undergoes a reversible and important color change, from light brown for G0(PTM • )6 and bright yellow for G1(PTM • )12 and G2(PTM • )24 dendrimers, when in radical state, to either deep wine color for G0(PTM -)6 or purple colors for G1(PTM -)12 and G2(PTM -)24 dendrimers, when in the anion state. Furthermore, there exists a viable opportunity to control the exact number of electrons transferred during the switching process, that could lead not only to a two-state but also to a multi-state switch in the near future. This is the first molecular switch based on organic radical dendrimers, in our best knowledge. Moreover, these species can act as electron accumulative molecules able to accept and release up to 24 electrons per molecule at very accessible potentials and in a reversible way. On state λabs ~ 386 nm, red emission Off state λabs ~ 520 nm, no emission