Open Access

Many organic dyes are fluorescent in solution. In the solid state, however, quenching processes often dominate, hampering material science applications such as light filters, light-emitting devices, or coding tags. We show that the dimethylene-cyclopropanides caffold can be used to form two structurally different types of chromophores, which feature fluorescence quantum yields up to 0.66 in dimethyl sulfoxide and 0.53 in solids. The increased fluorescence in the solid state for compounds bearing malonate substituents instead of dicyanomethide ones is rationalized by the induced twist between the planes of the cyclopropanide core and a pyridine ligand.

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.