Dimethylene‐Cyclopropanide Units as Building Blocks for Fluorescence Dyes

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‐cyclopropanide scaffold 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. The dimethylene‐cyclopropanide moiety is used as key scaffold for the design of fluorescent dyes which emit light in solution and in solids. In particular, malonic ester groups induce a remarkable twist angle of about 20° between a pyridinium substituent and the cyclopropanide plane, resulting in enhanced light emission in solids.