Conformation-dependent photophysical properties of butadiyne-linked pi-extended BODIPY dimers

A hitherto unknown butadiyne-linked BODIPY dimer (4) has been synthesized under the conventional Glaser coupling reaction conditions using the ethynyl-substituted pyrrolyl-BODIPY (2) prepared by gold-catalyzed C-H alkynylation. Due to the pi-extended structure of the BODIPY chromophore in 4, a characteristic broad absorption band in the near-infrared (NIR) region has emerged. Upon photoexcitation, the environment-dependent fluorescence spectral features were observed. The fluorescent response varied with solvents, viscosity, and temperature, which was found to originate from the conformational changes between the coplanar and twisted conformers in the otherwise rigid butadiyne-bridged dimer 4. Therefore, the BODIPY dimer 4 would be a potential NIR fluorescence material for use in the probes responding to the distinct viscous environment in biological tissues.