A C ₆₆ Polycyclic Aromatic Hydrocarbon with Six Azulene Units and NIR-II Absorption: Toward Azulene-Based Carbon Allotropes

The development of small-molecule dyes with strong and tunable absorption in the near-infrared II (NIR-II, 1000-1700 nm) window remains a central challenge for optoelectronic and biomedical applications, motivating the exploration of unconventional π-conjugated frameworks. In this context, nonalternant polycyclic aromatic hydrocarbons (PAHs) have emerged as promising candidates, with azulene-embedded architectures offering access to small electronic gaps. However, the lack of efficient synthetic strategies has largely restricted the use of such systems, reducing systematic structure-property relationships and limiting their use as functional NIR-active materials. Here, we report the synthesis of 2-diMes, a C₆₆ PAH that represents the first nonalternant PAH incorporating six azulene subunits within a fully conjugated framework. This purely hydrocarbon, closed-shell analogue of hexabenzoperylene (HBP) exhibits an exceptionally small HOMO-LUMO gap, among the smallest reported for neutral closed-shell π-conjugated hydrocarbons. The nonalternant topology of 2-diMes gives rise to intense near-infrared absorption with an onset extending to 1450 nm (0.85 eV) and a small electrochemical gap of 1.13 V. Notably, 2-diMes functions as a highly photostable NIR-II photoacoustic dye, combining a strong photoacoustic response with excellent solubility in common organic solvents. Spectroelectrochemical and titration studies revealed further red-shifted absorption upon oxidation, with the radical cation [2-diMes]•+ absorbing beyond 3200 nm (0.39 eV) and the dication [2-diMes]2+ absorbing up to 2600 nm (0.48 eV). These results highlight nonalternant azulene-rich PAHs as a powerful and underexplored platform for the rational design of next-generation NIR-II dyes and functional organic materials.