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.

Keywords: hexatopic carboxylic acids; hydrogen-bonded organic frameworks; photophysics; porosity; stability; time-resolved spectroscopy.

Figure 1

Examples of carboxylic acids for HOFs construction: (a) tri- and hexa-substituted benzene derivatives; (b) C₃-symmetric π-conjugated molecules (C₃PIs) providing layered HOFs; (c) C₃PIs providing 3D-networked rigid HOFs; (d) C₃PIs providing HOFs with unexpected H-bonded networks. The methyl ester derivatives corresponding to the carboxylic acids are denoted by the postfixed -COOMe (e.g., the ester derivative of T12-COOH is described as T12-COOMe).

Figure 2

Hierarchical representations of (a) a flexible, layered HOF based on H-bonded hexagonal network and (b) a rigid, interpenetrated pcu framework, composed of C₃-symmetric hexacarboxylic acids Tp and CPHAT, respectively.

Figure 3

Crystal structure of T12-apo: (a) H-bonded HexNet structure; (b) LA-H-HexNet.

Figure 4

Emission (a–d) spectra and (a′–d′) decays of small T12-apo crystals. The insets in (a–d) show the fluorescence lifetime imaging (FLIM) images of the samples, and those in (a′–d′) give the lifetime distribution histogram. Reproduced with permission from the Royal Society of Chemistry, 2018.

Scheme 1

Schematic representation (not to scale) of the photodynamics of solid (A) Ex-ester and (B) Ex-apo showing the S₀-S₁ electronic transitions, involved processes, and relative time constants of the emission decays. The green, orange, and red arrows correspond to the fluorescence from LE, ICT, and anionic (only for the HOF Ex-apo) species, respectively. Reproduced with permission from the American Chemical Society, 2020.

Figure 5

H-HexNet structures of: (a) Tp; (b) TpF; (c) TpMe. Crystallographically disordered parts in the frameworks are omitted for clarity.

Figure 6

Crystal structures of HAT-based HOFs: (a) CPHAT-1; (b) CBPHAT-1; (c) ThiaHAT-1.

Figure 7

(a) Framework of CPHATN-1a. (b) Ps-time-resolved emission spectra and (c) femtosecond (fs)-emission transients of CPHATN-1a crystals. The observation wavelengths and gating times are indicated in the insets. The dashed line in (c) is the instrumental response function (IRF).

Figure 8

(a) FLIM of CPHATN-1a crystals. (b) Emission spectra of the crystals (or sites) shown in (a). (c) Emission decays at selected spectral ranges as indicated in (b). (d) Histogram of the emission anisotropy of the crystals (inset). Reproduced with permission from the American Chemical Society, 2019.

Figure 9

(a) Pictures showing the color changes of crystalline bulks of CPHATN-1a (i) before and (ii) after exposure to 37%-HCl, and (iii) after heating at 150 °C for 30 min. (b) Absorption and (c) emission spectra of solid CPHATN-1a upon exposure to HCl atmosphere for 40 min and after equilibrating the sample with ambient air for 48 h. Adapted with permission from the American Chemical Society, 2019.

Figure 10

(a) Emission decays of solid CBPHAT-1a, recorded at the indicated wavelengths. The dashed signal is the IRF of the setup. (b) Normalized TRES of CBPHAT-1a in the solid-state at the indicated gating time. (c) Absorption and (d) emission spectra of CBPHAT-1a crystals before and after exposure to HCl atmosphere. The upper pictures in (c,d) show the color of the crystals before and after interaction with HCl atmosphere in ambient light and upon irradiation with a 360 nm lamp.

Figure 11

Fs-emission decays of ThiaHAT-1 in DMF suspension. The solid lines are from the best multiexponential global fit, and the IRF is the instrumental response function. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0, accessed on 14 January 2022).

Figure 12

Acid responsiveness of ThiaHAT-1. (a) Photographs of the crystalline powder before (left) and after (right) being exposed to HCl under daylight (top) and UV light (365 nm, bottom). (b) Absorption and (c) emission spectra of ThiaHAT-1 before and after being exposed to vapors of HCl.