Soluble Two-Dimensional Donor–Acceptor Aza-Fused Aromatic Frameworks and their Electrochromism between the Visible and Near-Infrared Regions

Two-dimensional (2D) covalent fused-ring aromatic frameworks (FAFs) can be regarded as holey graphene and hold considerable potential in many applications. However, the development of such materials has been seriously limited by their poor solubility. In this work, four soluble 2D fused-ring aromatic frameworks, FAF1–FAF4, soluble in common organic solvents, were successfully synthesized by creating large pore structures and attaching long side-chain substituents. They possess high molecular weights (M_w = 56,500–87,000 Da) and large hydrodynamic diameters (190–250 nm). Although FAFs are soluble in common organic solvents, they nevertheless show excellent ordering in the solid state with a layer spacing of ca. 0.36–0.37 nm, typical for π–π stacking. Due to the presence of electron-deficient pyrazine rings in the main skeleton and electron-rich alkoxyphenyl or alkylthienyl side-chain groups, these FAFs are electroactive in both n- and p-doping processes (E_red = −(0.80–0.93) V, E_ox = 0.84–0.94 V vs Fc/Fc⁺). They possess low-lying lowest unoccupied molecular orbital (LUMO) energy levels of −(3.87–4.00) eV and narrow band gaps (E_g^CV = 1.64–1.87 eV), suggesting that they are good candidates as n-type semiconductors. Taking advantage of their good solubility, spray-coated films of FAF1 and FAF2 were prepared, and their cathodic electrochromic behavior in the ultraviolet–visible–near-infrared (UV–vis–NIR) region was investigated for the first time. The 2D framework with 4-(2-ethylhexyloxy)­phenyl solubilizing groups, FAF1, demonstrated superior performance with short switching times between the neutral and n-doped states (few seconds), good contrast (ΔT up to 44%), high coloration efficiency (up to 171 cm² C^–1 at 515 nm), and good operational stability (>200 redox cycles). We envisage that such a type of 2D fused-ring framework possessing good solubility and strong π–π interactions between the layers should be a promising material for a wide range of electronic and optoelectronic applications.