B/N-Doped <i>p</i>-Arylenevinylene Chromophores: Synthesis, Properties, and Microcrystal Electron Crystallographic Study
Hua Lü, Takayuki Nakamuro, Keitaro Yamashita, Haruaki Yanagisawa, Osamu Nureki, Masahide Kikkawa, Han Gao, Jiangwei Tian, Rui Shang, Eiichi Nakamura
Abstract
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.