Dual-Function Fluorescent Covalent Organic Frameworks: HCl Sensing and Photocatalytic H2 Evolution from Water

Abstract

Two ultrastable luminescent covalent organic frameworks (COFs), PyTA-BC and PyTA-BC-Ph, are synthesized through polycondensations of 4,4′,4″,4′″-pyrene-1,3,6,8-tetrayl)tetraaniline (PyTA-4NH2) with two carbazole-based derivatives having different degrees of conjugation. The PyTA-BC and PyTA-BC-Ph COFs exhibit ultrahigh thermal stabilities (up to 421 °C), excellent crystallinity, and high Brunauer–Emmett–Teller surface areas (up to 1445 m2 g−1). These COFs display strong fluorescence emissions in various solvents, with their emission maxima gradually red-shifting upon increasing the polarity of the solvent (solvatochromism). Upon exposure to HCl, they respond very rapidly and sensitively in terms of changing their colors and fluorescence emission maxima. In the presence of a sacrificial electron donor, these COFs mediate the highly efficient photocatalytic evolution of H2 from water. In the absence of a noble metal cocatalyst, the COFs and ascorbic acid provide a photocatalytic H2 production of up to 1183 µmol g−1 h−1 (λ ≥ 420 nm); this value is the highest reported to date for a COF. Such COFs appear to be potentially useful as chemosensors for the naked-eye and sensitive spectroscopic detection of HCl and as cocatalysts for the sustainable photocatalytic production of H2 from water.