Nanoscale View Article Online REVIEW View Journal | View Issue Pore surface engineering of covalent organic frameworks: structural diversity and applications Cite this: Nanoscale, 2019, 11, 21679 Harsh Vardhan,a Ayman Nafady,b Abdullah M. Al-Enizib and Shengqian Ma *a Connecting molecular building blocks by covalent bonds to form extended crystalline structures has caused a sharp upsurge in the field of porous materials, especially covalent organic frameworks (COFs), thereby translating the accuracy, precision, and versatility of covalent chemistry from discrete molecules to two-dimensional and three-dimensional crystalline structures. COFs are crystalline porous frameworks prepared by a bottom-up approach from predesigned symmetric units with well-defined structural pro- perties such as a high surface area, distinct pores, cavities, channels, thermal and chemical stability, struc- tural flexibility and functional design. Due to the tedious and sometimes impossible introduction of certain functionalities into COFs via de novo synthesis, pore surface engineering through judicious functionalization with a range of substituents under ambient or harsh conditions using the principle of coordination chemistry, chemical conversion, and building block exchange is of profound importance. In Received 31st August 2019, this review, we aim to summarize dynamic covalent chemistry and framework linkage in the context of Accepted 28th October 2019 design features, different methods and perspectives of pore surface engineering along with their versatile DOI: 10.1039/c9nr07525a roles in a plethora of applications such as biomedical, gas storage and separation, catalysis, sensing, rsc.li/nanoscale energy storage and environmental remediation. giant structure of Vitamin B12 and other solid networks exist- Introduction – ing in nature.20 22 In 2005, a revolution was made by success- The utilization of confined space for molecular purposes is fully connecting boronic acid and catechol to form one of the basic principles of living systems and is profoundly extended structures6 using the principle of dynamic covalent believed to be one of the vital steps in the origin of life. The chemistry.23,24 Since then, the sustained expansion and biological process proceeds in the spatial confinement, com- development of COFs have been the answer to both long- Published on 29 October 2019. Downloaded by University of South Florida 11/21/2019 2:40:47 PM. monly known as the enzyme pocket, to drastically influence lasting and newly recognized problems demanding new the reactivity and selectivity of biochemical reactions. To and evolving synthetic routes, characterization and applied mimic this, linking organic building blocks by strong covalent studies. bonds to form crystalline materials is vital; however, one The development of elementary skeletons in frameworks cannot neglect the possibility of thermodynamically favored primarily relies on the geometry of the building blocks. In 2D amorphous disordered structures.1,2 In comparison to non- COFs, the building blocks are covalently bonded with a well- porous materials, porous materials have various characteristics defined stacking of π-building units under the influence of π–π such as a high surface area, nanometer-scale pores with stacking interactions, whereas the lattice structure of 3D COFs peculiar properties and functions, tunable pore sizes, and is preserved by robust covalent bonds between precursor units – – chemical and thermal stability.3 11 Covalent organic frame- using the principle of reticular chemistry.25 29 The uniquely works (COFs) are porous, crystalline extended solids con- confined nanometer-sized pores have practical implemen- structed from organic building units entirely composed of tation as an outstanding platform for future challenges. With light elements (C, H, N, B, O) and connected by covalent this goal in mind, it is important to examine the peculiar pro- – bonds, which are robust and diverse in nature.12 19 The perties of COFs, starting with crystallinity. Due to the ordered strength of covalent bonds is unambiguously reflected in the structure, crystallinity enables characterization using powder X-ray diffraction patterns (PXRD). The porous nature is a result of the crystalline structure and the uniform and periodic a Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, assembly of monomer units exhibiting surface areas as high as Tampa, Florida-33620, USA. E-mail: [email protected]; Fax: +1-813-974 3203; − 4000 m2 g 1. Chemical/thermal stability (>400 °C) emerges Tel: +1-813-9745217 bDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451, from the robust and diverse nature of the covalent bonds, Saudi Arabia which resist severe conditions such as hydrolysis, oxidation, This journal is © The Royal Society of Chemistry 2019 Nanoscale,2019,11,21679–21708 | 21679 View Article Online Review Nanoscale reduction, and pH range variations. Also, the low density, by stability, crystallinity, diversity, etc. The chemical stability and virtue of uniform composition with lightweight elements allows crystallinity are two peculiar properties that are dependent and a high gravimetric performance. De novo synthesis, to a lesser come at the expense of others. In recent years, numerous the- – extent, suffers from various challenges such as active metal matic perspectives and reviews have discussed16,30 37 in great complexation, chemical conversion, tedious synthetic and puri- detail the role of dynamic covalent chemistry in constructing fication procedures, etc. However, in order to address these chal- frameworks of different linkages under the umbrella of reticu- lenges the tunable pore size of pristine frameworks must lar chemistry, and their implementation in different pore undergo pore surface engineering using established molecular sizes, pore volumes and dual pore frameworks. We are not organic chemistry to tune the structural and functional pro- trying to duplicate those aspects, however, we have briefly perties of COFs. In this review, we will briefly mention the extra- highlighted different linkages and pore geometries present in ordinary role of dynamic covalent chemistry in the COFs to create cohesion for a better understanding of the synthesizing COFs of specific pore size and broadly summarize central theme of this review. the principle and examples of pore surface engineering in – frameworks along with their role in a vast number of appli- B O linkages cations such as drug delivery, catalysis, environmental remedia- Yaghi and co-workers reported the first class of boroxine-based tion, gas adsorption and separation, sensing and energy frameworks6,38 (COF-1, COF-102) by the self-condensation of storage. boronic acid-based building units, and the co-condensation of the same building blocks with catechol generates the boronate ester linkage framework (COF-5, COF-105). Due to the vulner- Dynamic linkages ability of the B–O linkage, the frameworks hydrolyze in moisture. The Dichtel research group also highlighted the dissociation of Since the advent of the first COF in 2005, the last decade has HHTP-DPB COF in aqueous solution within minutes.39 To observed substantial development in COFs from the perspec- enhance the stability, Yaghi’s group introduced the B–O–Si tive of design, construction and application. There are ranges linkage in a borosilicate-linked framework (COF-202) prepared of linkages, as shown in Fig. 1, which have been employed to by the reaction of silanols with boronic acid, showing stability in construct extended frameworks of different pore structures moisture ranging from minutes to days.40 Additionally, the stabi- and morphologies. Unlike traditional interactions, dynamic lity of frameworks in water and basic medium was visualized in covalent bonds possess error-checking capabilities, and the spiroborate-linked ionic COFs by varying the hybridization of robustness of reversible bonds bestow COFs with thermal boron by incorporating alkali metal ions.41 Harsh Vardhan received his Ayman Nafady is currently a Master of Science (M.Sc.) degree Professor of Nanomaterials/ in Chemistry from the Indian Inorganic Electrochemistry at Published on 29 October 2019. Downloaded by University of South Florida 11/21/2019 2:40:47 PM. Institute of Technology- King Saud University, Riyadh, Kharagpur, India. In 2016, he Saudi Arabia & Sohag obtained his Ph.D. in Material University, Egypt. He is also an Physics and Chemistry under the adjunct/visiting professor at supervision of Prof. Francis RMIT University, Melbourne, Verpoort from the Laboratory of Australia and Editor in Chief of Organometallics, Catalysis, and the International Journal of Ordered Materials, Wuhan Nanomaterials and Chemistry. University of Technology, China. Prof. Nafady obtained his Ph.D. Harsh Vardhan At present, he is a graduate Ayman Nafady (2000–2004) under the supervi- student in the research group of sion of Prof. William Geiger at Prof. Shengqian Ma at the University of South Florida, USA. His the University of Vermont, USA and has been a Research Fellow at research interest is focused on developing novel covalent organic Monash University (2005–2011), Australia working with Prof. frameworks as organic scaffolds for heterogeneous catalysis, Alan Bond. He has made significant contributions to the fields of including C–H activation. inorganic/organometallic electrochemistry and nanomaterials science and has been a pioneer in the development of novel electrochemical and photochemical