20-Electron Antiaromatic Benziphthalocyanines with Absorption Reaching the Near-Infrared-II Region

Although second near-infrared (NIR-II, 1000-1500 nm) light has attracted considerable attention, especially for life sciences applications, the development of organic dyes with NIR-II absorption remains a formidable challenge. Herein we report the design, synthesis, and electronic properties of 20 pi-electron antiaromatic benziphthalocyanines (BPcs) that exhibit intense absorption bands in the NIR region. The strong, low-energy absorption of the antiaromatic BPcs is attributed to electric-dipole-allowed HOMO-LUMO transitions with narrow band gaps, enabled by the reduced structural symmetry of BPc compared with regular porphyrins and phthalocyanines. The combination of peripheral substituents and a central metal decreases the HOMO-LUMO energy gaps, leading to the extension of the absorption bands into the NIR-II region (reaching 1100 nm) under reductive conditions. 20 pi-Electron antiaromatic benziphthalocyanines, low symmetry analogues of phthalocyanine, were synthesized by hydrogenation of 18 pi-electron benziphthalocyanines. They exhibit intense absorption in the near-IR (NIR) region despite their antiaromaticity, which is attributed to electric-dipole-allowed HOMO-LUMO transition as a result of symmetry-lowering. Further structural modification extends the absorption into the second NIR region (reaching 1100 nm).+ image