B/N-Doped <i>p</i>‑Arylenevinylene Chromophores: Synthesis, Properties, and Microcrystal Electron Crystallographic Study

Linearly conjugated systems have long served as an archetype of conjugated materials, but suffer from two intrinsic structural problems: potential instability due to intermolecular interactions and the flexibility of the C–C bonds connecting CC bonds. Efforts to solve these problems have included the insertion of aromatic units as a part of the conjugation and the introduction of carbon bridges to stop the bond rotation. We report here B/N-doped p-arylenevinylene chromophores synthesized through the incorporation of a cyclopenta­[c]­[1,2]­azaborole framework as a part of the conjugated system. The ring strain intrinsic to this new skeleton both flattens and rigidifies the conjugation, and the B––N+ dative bond is much easier to form than a C–C bond, which simplifies the synthetic design. The B–N dative bond also reduces the HOMO–LUMO gap, thereby causing a significant redshift of the absorption and emission compared with their all-carbon congeners while retaining high photostability and high fluorescence quantum yield in both solution and film states. A doubly B/N-doped compound showed emission peaks at 540 nm with a small Stokes shift of 20 nm and a fluorescence quantum yield of 98%. The molecules serve as excellent lipophilic fluorescent dyes for live-cell imaging, showing a higher photostability than that of commercially available BODIPY-based dyes.