HOFs Built from Hexatopic Carboxylic Acids: Structure, Porosity, Stability, and Photophysics

Abstract

Hydrogen-bonded organic frameworks (HOFs) have attracted renewed attention as another type of promising candidates for functional porous materials. In most cases of HOF preparation, the applied molecular design principle is based on molecules with rigid π-conjugated skeleton together with more than three H-bonding groups to achieve 2D- or 3D-networked structures. However, the design principle does not always work, but results in formation of unexpected structures, where subtle structural factors of which we are not aware dictate the entire structure of HOFs. In this contribution, we assess recent advances in HOFs, focusing on those composed of hexatopic building block molecules, which can provide robust frameworks with a wide range of topologies and properties. The HOFs described in this work are classified into three types, depending on their H-bonded structural motifs. Here in, we focus on: (1) the chemical aspects that govern their unique fundamental chemistry and structures; and (2) their photophysics at the ensemble and single-crystal levels. The work addresses and discusses how these aspects affect and orient their photonic applicability. We trust that this contribution will provide a deep awareness and will help scientists to build up a systematic series of porous materials with the aim to control both their structural and photodynamical assets.

Recent progress in gas separation platforms based on hydrogen-bonded organic frameworks (HOFs)

“…18,63 However, some methods to improve the stability of this category of materials, particularly the porosity, have been proposed, such as (1) the creation of multiple hydrogen bonds, in this regard, promising options for creating stable HOFs would be multitopic organic ligands that can produce additional hydrogen bonds, (2) making use of π–π interactions using organic building blocks with a big π-conjugate system would encourage the development of a structure with numerous, potent π–π interactions (3) incorporation of appropriate interpenetrations within the HOFs’ structure, (4) following self-assembly, avoiding leftover hydrogen donors and acceptors, and (5) using stiff backbone compounds as a building block. Since many review articles on HOFs have been published to date, and much has been explained about the methods of improvement of the HOF stability, 64 this section focused on the importance of the stability and porosity of the HOF materials used for gas separation. 65 According to our application, which deals with HOF applications in the field of separation, permanent porosity and stability in the separation environment are of vital importance.…”

In situ generated dimethylamine constructs a robust hydrogen-bonded organic framework for selective fluorescence detection

“…Supramolecular chemistry has played a pivotal role in guiding the synthesis of HOFs at the molecular level using organic building blocks known as supramolecular synthons, particularly those containing carboxyl ligands. 36 However, organic ligands containing carboxyl groups should not be limited to the formation of HOFs through dimerization, they possess more possibilities. 37 Porphyrin, a type of porphin derivative with a large conjugated structure, is an excellent building block for HOF design and synthesis due to its rigid structure, tunable external substituents, and metallized ring center.…”

Recent advancements of photo- and electro-active hydrogen-bonded organic frameworks

“…Nevertheless, the relatively weak hydrogen-bonding interactions within HOFs can potentially limit their stability and further applications. Since the design rules regarding the formation of stable MOFs were not applicable [68], researchers had to develop several unprecedented design strategies to overcome these challenges related to HOF stability, including shape-fitted π-π stacking [24,[69][70][71][72][73], interpenetration [74,75], charge-assisted hydrogen bonds [35,76], and chemical cross-linking [32]. Among these strategies, the shape-fitted π-π stacking is the most effective one and has afforded a large number of robust HOFs with permanent porosity, as well as high chemical (pH from 1 to 12) and thermal stabilities (up to 300 °C).…”