Ammonium pentaborate crystals with adjustable and bright phosphorescence and long lifetime

As a small molecule boron-based compound, ammonium pentaborate was prepared by a solvothermal synthesis by controlling the crystal growth process. As-prepared ammonium pentaborate crystals consisted of two perpendicular six-membered rings with powerful intermolecular interaction. A resonance hybrid of Lewis acid formed by the combination of five monovalent boric acids and coordinated with an ammonium ion. The planar six-membered ring and alternating B–O provide π-bond orbitals and anti-π* bond orbitals. The crystals exhibited a novel phosphorescence phenomenon at room temperature with excited wavelength dependence and long lifetime. Broad phosphorescence spectra show bright blue- and green-emitting excited at 250 and 360 nm while their phosphorescence lifetime is up to 1.63 and 1.49 s, respectively. The change of phosphorescence with excited wavelength is ascribed to the electronic energy level. This result provides a new idea for synthesizing new-type phosphorescent materials. © 2020 Elsevier B.V.