Intermolecular Charge‐Transfer Luminescence by Self‐Assembly of Pyridinium Luminophores in Solutions

Designing a luminophore for application both in solution and in the solid state is a highly challenging task given the distinct nature of intermolecular interactions in these phases. In this context, we demonstrate that self‐assembly of non‐emissive charged pyridinium luminophores enables luminescence in solutions through a mechanism that is characteristic for the crystal state. Specifically, protonation of pyridine luminophore subunits in a solution promotes oligomer formation through intermolecular π(+)‐π interactions, leading to an intermolecular charge‐transfer type luminescence. The luminescence turn‐on by protonation is utilized for a highly efficient solution‐state luminescent sensing of hydrogen chloride and sulfonic acids (TfOH, TsOH and MsOH) with detection limits spanning the range from 0.06 to 0.33 ppm. The protonation followed by self‐assembly results in a bathochromic shift of the emission from 420 nm to 550 nm.