Stabilizing the Hexacyanotrimethylenecyclopropane Electron Acceptor-Structural and Photophysical Characterization

Hexacyanotrimethylenecyclopropane (CN6CP) is an exceptionally strong organic electron acceptor in its neutral form, and widely applied for molecular doping to induce charge transfer processes and enable electrochemical systems. Yet, its fundamental molecular properties have remained largely unknown. Here, we show the first comprehensive structure-analytical characterization of CN6CP, enabled by an improved, low-temperature synthesis and the first solid-state structure of the neutral compound. The resulting procedure affords isolable, crystalline CN6CP that is stable for weeks at -30°C and can be recrystallised. Across all redox states, combined IR/Raman, UV-Vis and NMR measurements, together with NICS calculations, reveal an oxidation-state-dependent redistribution of electron density. These data show that CN6CP possesses a σ-aromatic cyclopropane core with tunable π-delocalisation, which is enhanced upon reduction while the additional charge is predominantly localised on the exocyclic acceptor framework. Cyclic voltammetry experiments unveil two reversible one-electron processes and an exceptionally low LUMO energy of -5.85 eV, which is the lowest reported for small organic molecules being significantly lower than those of benchmark acceptors such as F₄TCNQ or F₆TCNNQ. All together, these findings establish CN6CP as a structurally unique, extremely strong electron acceptor and provide the molecular basis underlying its performance in organic electronics and redox-active materials.