NIR‐II Absorption/Fluorescence of D–A π‐Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron‐Fused Azobenzene Complexes

Abstract

Luminescence in the second near‐infrared (NIR‐II, 1,000–1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron‐acceptors with low‐lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor–acceptor (D–A) π‐conjugated polymers (CPs) with the NIR‐II emission property, however, limited kinds of molecular skeletons are still available. Herein, D–A CPs involving fluorinated boron‐fused azobenzene complexes (BAz) with enhanced electron‐accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to −4.42 eV, which is much lower than those of conventional strong electron‐acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR‐II. Furthermore, owing to the inherent solid‐state emissive property of the BAz skeleton, obvious NIR‐II fluorescence from the film (up to λFL = 1213 nm) and the nanoparticle in water (λFL = 1036 nm, brightness = up to 29 cm–1 M–1) were observed, proposing that our materials are applicable for developing next‐generation of NIR‐II luminescent materials.