BAr₂-Bridged Azafulvene Dimers with Tunable Energy Levels for Photostable Near-Infrared Dyes

Organic dyes with strong absorption in the near-infrared (NIR) region are potentially useful in medical applications, such as tumor imaging and photothermal therapy. In this work, new NIR dyes combining BAr2-bridged azafulvene dimer acceptors with diarylaminothienyl donors in a donor-acceptor-donor configuration were synthesized. Surprisingly, it was found that in these molecules the BAr2-bridged azafulvene acceptor adopts a 5-membered, rather than 6-membered ring structure. The influence of the aryl substituents on the HOMO and LUMO energy levels of the dye compounds was assessed from electrochemical and optical measurements. Strong electron-withdrawing fluorinated substituents (Ar=C6F5, 3,5-(CF3)(2)C6H3) lowered the HOMO energy while preserving the small HOMO-LUMO energy gap, resulting in promising NIR dye molecules that combine strong absorption bands centered around 900 nm with good photostability.