Electronic charge transfer properties of COF-5 solutions and films with intercalated metal ions

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Research Letter

Electronic charge transfer properties of COF-5 solutions and ﬁlms with intercalated metal ions William S. Owen, Michael S. Bible, Emma F. Dohmeier, Lindsey R. Guthrie, Michael J. Parsons, Justin W. Hendrix, Joseph R. Hunt, and Michael S. Lowry, Naval Surface Warfare Center – Dahlgren Division, Dahlgren, VA 22448, USA Address all correspondence to Michael S. Lowry at [email protected] (Received 8 August 2019; accepted 7 January 2020)

Abstract To investigate the manipulation of electromagnetic properties of two-dimensional materials, this effort characterizes charge transfer behavior of colloidal COF-5 (covalent organic framework) in the presence of various metal ions. A series of metal chloride compounds was introduced to COF-5 in solution and solid ﬁlm phases and the interaction of the material with electromagnetic radiation was monitored across the visible region using electronic absorption spectroscopy. Notable changes were observed, quantiﬁed, and discussed for copper (II) chloride (CuCl2), chromium (III) chloride (CrCl3), and iron (III) chloride (FeCl3) with COF-5. Ligand-to-metal and metal-to-ligand charge transfer are explored as a possible mechanism for the observed electronic behaviors.

Introduction The ability to tailor the geometric arrangement and electronic structure of materials is appealing for a broad range of applications including energy storage,[1,2] micro-electronics[3,4], and electromagnetically active materials.[5,6] This paper describes an effort to develop and study a series of new materials to gain insight into methods for controlling the electromagnetic properties of two-dimensional covalent organic frameworks (2D-COFs). 2D-COFs are porous, semi-planar, stacked macromolecular structures,[7] and are explored here as a scaffold for the inclusion of additives to yield novel performance. Recently, there has been a surge in the development of 2D-COF materials using a variety of organic linker reactions.[7,8] 2D-COFs can be designed with speciﬁc and widely ranging pore sizes and varying chemical,[9] semi-conducting,[8,10,11] conducting,[12] solvatochromic,[13] and luminescent[14] behaviors. Accordingly, 2D-COFs offer a viable platform to generate materials with a high degree of control over the placement of additives and/or functional groups. Some work has been reported on the incorporation of functional additives into 2D-COFs[9]; however, the effects of additives on electromagnetic behavior are relatively unexplored. Complementary research has been performed using graphitic structures; graphite-intercalation compounds have been created in which ionic compounds were incorporated between atomically thin sheets (or clusters of sheets) of graphene.[15,16] These structures were generated using a vapor-phase sublimation process in a sealed-tube reaction vessel, and intercalants were reported to inﬂuence in-plane and inter-plane electrical conductivity under certain conditions.[15] Herein, the hope is

to extend the effects of intercalation compounds on electronic behavior onto

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