Well-Structured Narrow Near-Infrared Absorption Based on Nonaggregated Hexarylene-Bisimide toward a Colorless Dye

Extended π-conjugated systems are often insoluble, and their aggregation manner greatly affects their absorption spectra. This study produced a planar, soluble, and nonaggregated hexarylene-bisimide (HB) with appropriate substituents. The single-crystal X-ray structure of HB confirmed the planar molecular structure with small twist angles and the dimerization behavior of HB in the solid state. The concentration-dependent ¹H NMR experiments in CDCl₃ indicated that the association constant K_dimer is 4.6 × 10³ M^-1 at 298 K and ΔG_dimer (298 K) = -20.8 kJ mol^-1. The longest absorption of HB at the monomeric state exhibits a sharp and intense peak at 921 nm (ε = 230,000 M^-1 cm^-1, full width at half-maximum = 718 cm^-1) in toluene. 75% of the absorption of HB above 400 nm appears in the near-infrared region, thus giving a practically colorless solution. Magnetic circular dichroism spectra of a series of oligorylene-bisimides reveal the predominant contribution of the linear polyene-like conjugation over the annulene-like conjugation for larger [n]oligorylene-bisimides.