A nanotubular metal-organic framework with a narrow band gap from extended conjugation.

We report a one-dimensional nanotubular metal-organic framework (MOF) [Ni(Cu-H6TPPA)]∙2(CH3)2NH2+ (H8TPPA = 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin) constructed using the arylphosphonic acid H8TPPA. The structure of this MOF, known as GTUB-4, was solved using single crystal X-ray diffraction and its geometric accessible surface area was calculated to be 1102 m2/g, 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 band gaps (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 band gap, and high thermal stability, GTUB-4 could find applications as electrodes in supercapacitors.