SCIENTIFIC HIGHLIGHTS RL4, Articles

Multifunctional molecular switches based on radical dendrimers

New radical dendrimers based on polyphosphorhydrazone (PPH) dendrimers fully functionalized with perchlorotriphenylmethyl (PTM) radicals as electrochemical molecular switches with optical (absorption and fluorescence) and magnetic responses.

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 at room temperature, simply by applying a current to a neighboring molecule has interesting implications.

In this work, we report the synthesis and characterization of three generations of polyphosphorhydrazone (PPH) dendrimers, fully functionalized with 6, 12 and 24 redox active perchlorotriphenylmethyl (PTM) radicals in the periphery, capable of undergoing an electrochemical reversible switching (on/off) by multi-electron reduction and oxidation, for many cycles. An electrical input is used to trigger the physical properties of these radical dendrimers in a reversible way, modifying their optical and magnetic properties. Gn(PTM)x radical dendrimers are paramagnetic, exhibit an absorbance band at 386 nm and red fluorescence emission, if in radical state (on). 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 (off). Furthermore, in this work we open the perspective of controlling the exact number of electrons transferred during the switching process, that could lead not only to a two-state (on/off) but also to a multi-state switch in the near future.

In addition, the high electron accepting capacity of these radical dendrimers able to accept and donate up to 24 electrons per molecule at the same time at very accessible potentials and in a reversible way makes them good electron-reservoir molecules.

Undoubtedly, the high control over the synthesis, stability and reversibility of systems like the ones reported in this work, further supports the perspective of using macromolecules as scaffolds in the electronic devices of the future.

Redox active PTM radical dendrimers as promising multifunctional molecular switches
Vega Lloveras, Flonja Liko, José L. Muñoz-Gómez, Jaume Veciana, José Vidal-Gancedo
Chem. Mater. 31, 22, 9400-9412, 2019
DOI: 10.1021/acs.chemmater.9b03015

Figure: Representation of the electrochemical switch (on/off) of PTM radical dendrimers (Gn(PTM·)x, n=0, 1, 2; x=6, 12, 24). The PTM molecule has two redox states, the radical (left) and anion (right) forms, which can be electrochemically interconverted. Radical dendrimers Gn(PTM·)x exhibit spin S=x.½ (EPR active), an absorbance band at 386 nm (yellow-brownish colour) and red fluorescence emission, and their corresponding anionic forms Gn(PTM-)x are diamagnetic species with S=0 (EPR silent), with a maximum absorbance band at ca. 520 nm (deep wine or purple colour) and are not fluorescent.

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