A Nanotubular Metal-Organic Framework with a Narrow Bandgap from Extended Conjugation*

A one-dimensional nanotubular metal-organic framework (MOF) [Ni(Cu-H₄ TPPA)]⋅2 (CH₃ )₂ NH₂ ⁺ (H₈ TPPA=5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin) constructed by using the arylphosphonic acid H₈ TPPA is reported. The structure of this MOF, known as GTUB-4, was solved by using single-crystal X-ray diffraction and its geometric accessible surface area was calculated to be 1102 m²  g^-1 , making it the phosphonate MOF with the highest reported surface area. Due to the extended conjugation of its porphyrin core, GTUB-4 possesses narrow indirect and direct bandgaps (1.9 eV and 2.16 eV, respectively) in the semiconductor regime. Thermogravimetric analysis suggests that GTUB-4 is thermally stable up to 400 °C. Owing to its high surface area, low bandgap, and high thermal stability, GTUB-4 could find applications as electrodes in supercapacitors.