BAr₂‐bridged Azafulvene Dimers with Tunable Energy Levels for Photostable Near‐infrared Dyes

Abstract

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)₂C₆H₃) 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.