HOFs under light: Relevance to photon-based science and applications

Hydrogen-bonded Organic Frameworks (HOFs) are an appealing, newly emerging classes of porous materials whose bright potential as multifunctional resources is reflected in important applications like gas storage and separation, molecular recognition, electric and optical materials, chemical sensing, catalysis, and biomedicine. HOFs are assembled from organic building blocks through H-bonding interactions. The resultant framework can be further reinforced via weak connections such as π-π, van der Waals, and/or C-H-π interactions. The highly flexible and reversible HOF structures are exceptionally suitable for the realization of smart HOF materials. To this end, it is crucial to unravel and control the photobehavior of these compounds at intimate levels by the use of advanced laser-based spectroscopy and microscopy techniques. The use of light to study the photophysical processes of HOF-based systems will help to trigger further research to expand their applicability in the related fields. This Review surveys the past-10-years contributions on the spectroscopy and photoinduced fast/ultrafast dynamics of HOFs, the interactions between their building units, the effect of light on their photostability, and most important photonic applications. The aim of this work is to give a rich up-to-date summary of photochemistry and related applications of HOFs and their composites. The reviewed HOFs have been divided into different families based on the nature of the linker, with the purpose of offering to the reader a concise understanding of the related photoinduced processes within each family. The relevant applications of HOFs are also briefly summarized to validate their potential use in modern science and technology.     

An Ultra‐Robust and Crystalline Redeemable Hydrogen‐Bonded Organic Framework for Synergistic Chemo‐Photodynamic Therapy

The low structural stability of hydrogen‐bonded organic frameworks (HOFs) is a thorny issue retarding the development of HOFs. A rational design approach is now proposed for construction of a stable HOF. The resultant HOF (PFC‐1) exhibits high surface area of 2122 m² g⁻¹ and excellent chemical stability (intact in concentrated HCl for at least 117 days). A new method of acid‐assisted crystalline redemption is used to readily cure the thermal damage to PFC‐1. With periodic integration of photoactive pyrene in the robust framework, PFC‐1 can efficiently encapsulate Doxorubicin (Doxo) for synergistic chemo‐photodynamic therapy, showing comparable therapeutic efficacy with the commercial Doxo yet considerably lower cytotoxicity. This work demonstrates the notorious stability issue of HOFs can be properly addressed through rational design, paving a way to develop robust HOFs and offering promising application perspectives.     

Docking Strategy To Construct Thermostable,Single‐Crystalline,Hydrogen‐Bonded Organic Framework with High Surface Area

Enhancing thermal and chemical durability and increasing surface area are two main directions for the construction and improvement of the performance of porous hydrogen‐bonded organic frameworks (HOFs). Herein, a hexaazatriphenylene (HAT) derivative that possesses six carboxyaryl groups serves as a suitable building block for the systematic construction of thermally and chemically durable HOFs with high surface area through shape‐fitted docking between the HAT cores and interpenetrated three‐dimensional network. A HAT derivative with carboxybiphenyl groups forms a stable single‐crystalline porous HOF that displays protic solvent durability, even in concentrated HCl, heat resistance up to 305 °C, and a high Brunauer–Emmett–Teller surface area [SA_(BET)] of 1288 m² g⁻¹. A single crystal of this HOF displays anisotropic fluorescence, which suggests that it would be applicable to polarized emitters based on robust functional porous materials.     

Docking Strategy To Construct Thermostable,Single‐Crystalline,Hydrogen‐Bonded Organic Framework with High Surface Area

Enhancing thermal and chemical durability and increasing surface area are two main directions for the construction and improvement of the performance of porous hydrogen‐bonded organic frameworks (HOFs). Herein, a hexaazatriphenylene (HAT) derivative that possesses six carboxyaryl groups serves as a suitable building block for the systematic construction of thermally and chemically durable HOFs with high surface area through shape‐fitted docking between the HAT cores and interpenetrated three‐dimensional network. A HAT derivative with carboxybiphenyl groups forms a stable single‐crystalline porous HOF that displays protic solvent durability, even in concentrated HCl, heat resistance up to 305 °C, and a high Brunauer–Emmett–Teller surface area [SA_(BET)] of 1288 m² g^?1. A single crystal of this HOF displays anisotropic fluorescence, which suggests that it would be applicable to polarized emitters based on robust functional porous materials.     

Co-encapsulating Cofactor and Enzymes in Hydrogen-Bonded Organic Frameworks for Multienzyme Cascade Reactions with Cofactor Recycling

Use of hydrogen-bonded organic frameworks (HOFs) for enzyme immobilization faces challenges in the improvement of enzyme activity recovery and the assembly of cofactor-dependent multienzyme systems. Herein, we report a polyelectrolyte-assisted encapsulation approach (PAEA) that enables two cascades with four oxidoreductases and two nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) cofactors co-encapsulated in BioHOF-1 with excellent cargo loading and over 100 % cascade activity. The key role of the polyelectrolyte is to coat enzymes and tether NAD(P)H, thus interacting with HOF monomers in place of enzymes, avoiding the destruction of enzymes by HOF monomers. The versatility and efficiency of PAEA are further illustrated by an HOF-101-based bio-nanoreactor. Moreover, the immobilization by PAEA makes enzymes and NAD(P)H display excellent stability and recyclability. This study has demonstrated a facile and versatile PAEA for fabricating cofactor-dependent multienzyme cascade nanoreactors with HOFs.     

Computational investigation of the heat of formation, detonation properties of furazan-based energetic materials

The heats of formation (HOFs) for a series of furazan-based energetic materials were calculated by density functional theory. The isodesmic reaction method was employed to estimate the HOFs. The result shows that the introductions of azo and azoxy groups can increase the HOF, but the introduction of azo group can increase the more HOF, when compared with azoxy group. The detonation velocities and detonation pressures of the furazan-based energetic materials are further evaluated at B3LYP/6-31G* level. Dioxoazotetrafurazan and azoxytetrafurazan may be regarded as the potential candidates of high-energy density materials because of good detonation performance. In addition, there are good linear correlations between OB and detonation velocities, and OB and detonation pressures. The energy gaps between the HOMO and LUMO of the studied compounds are also investigated. These results provide basic information for the molecular design of novel high-energy density materials.     

多硝基金刚烷生成热和稳定性的理论研究

在密度泛函理论（DFT）B3LYP/6-31G水平下，通过不破裂金刚烷分子骨架（即 选择金刚作为参考物）的等键反应设计，比较精确地计算了系列多硝基金金刚烷的 生成热。经验性基团加和法和半经验MO法（AM1，PM3，MNDO，MNDO/3）均不适用于 标题生成熟的估算。4种半经验MO方法中，以MP3计算结果略好些。探讨了生成热和 分子结构的关系，发现桥头C上硝基使生成热减小，而偕二硝基使生成热增大。运 用生成热、前沿轨道能级差和C-NO_2键级等计算结果，阐明了标题的相对稳定性， 为新一代高能量密度材料（HEDM）的分子设计提供了基础数据和规律性。     

Radiochromic Hydrogen-Bonded Organic Frameworks for X-ray Detection

Porous materials have been investigated as efficient photochromic platforms for detecting hazardous radiation, while the utilization of hydrogen bonded organic frameworks (HOFs) in this field has remained intact. Herein, two HOFs were synthesized through self-assembly of tetratopic viologen ligand and formic acid (PFC-25, PFC-26), as a new class of “all-organic” radiochromic smart material, opening a gate for HOFs in this field. PFC-26 is active upon both X-ray and UV irradiation, while PFC-25 is only active upon X-ray irradiation. The same building block yet different radiochromic behaviors of PFC-25 and PFC-26 allow us to gain a deep mechanistic understanding of the factors that control the detection specificity. Theoretical and experimental studies reveal that the degree of π-conjugation of viologen ligand is highly related to the threshold energy of triggering a charge transfer, therefore being a vital factor for the particularity of radiochromic materials. Thanks to its convenient processibility, nanoparticle size, and UV silence, PFC-25 can be further fabricated into a portable naked-eye sensor for X-ray detection, which shows obvious color change with the merits of high transmittance contrast, good sensitivity (reproducible dose threshold of 3.5 Gy), and excellent stability. The work exhibits the promising practical potentials of HOF materials in photochromic technology.     

A Microporous Hydrogen-Bonded Organic Framework Based on Hydrogen-Bonding Tetramers for Efficient Xe/Kr Separation

Hydrogen-bonded organic frameworks (HOFs) have been emerging as a new type of very promising microporous materials for gas separation and purification, but few HOFs structures constructed through hydrogen-bonding tetramers have been explored in this field. Herein, we report the first microporous HOF (termed as HOF-FJU-46) afforded by hydrogen-bonding tetramers with 4-fold interpenetrated diamond networks, which shows excellent chemical and thermal stability. What's more, activated HOF-FJU-46 exhibits the highest xenon (Xe) uptake of 2.51 mmol g⁻¹ and xenon/krypton (Kr) selectivity of 19.9 at the ambient condition among the reported HOFs up to date. Dynamic breakthrough tests confirmed the excellent Xe/Kr separation of HOF-FJU-46a, showing high Kr productivity (110 mL g⁻¹) and Xe uptake (1.29 mmol g⁻¹), as well as good recyclability. The single crystal X-ray diffraction and the molecular simulations revealed that the abundant accessible aromatic and pyrazole rings in the pore channels of HOF-FJU-46a can provide the multiple strong C−H⋅⋅⋅Xe interactions with Xe atoms.     

Chemically Gradient Hydrogen-Bonded Organic Framework Crystal Film

Hydrogen-bonded organic frameworks (HOFs) are ordered supramolecular solid structures, however, nothing much explored as centimetre-scale self-standing films. The fabrication of such crystals comprising self-supported films is challenging due to the limited flexibility and interaction of the crystals, and therefore studies on two-dimensional macrostructures of HOFs are limited to external supports. Herein, we introduce a novel chemical gradient strategy to fabricate a crystal-deposited HOF film on an in situ-formed covalent organic polymer film (Tam-Bdca-CGHOF). The fabricated film showed versatility in chemical bonding along its thickness from covalent to hydrogen-bonded network. The kinetic-controlled Tam-Bdca-CGHOF showed enhanced proton conductivity (8.3×10⁻⁵ S cm⁻¹) compared to its rapid kinetic analogue, Tam-Bdca-COP (2.1×10⁻⁵ S cm⁻¹), which signifies the advantage of bonding-engineering in the same system.     

Pore-Environment-Dependent Photoresponsive Oxidase-Like Activity in Hydrogen-Bonded Organic Frameworks

Mimicking the bioactivity of native enzymes through synthetic chemistry is an efficient means to advance the biocatalysts in a cell-free environment, however, remains long-standing challenges. Herein, we utilize structurally explicit hydrogen-bonded organic frameworks (HOFs) to mimic photo-responsive oxidase, and uncover the important role of pore environments on mediating oxidase-like activity by means of constructing isostructural HOFs. We discover that the HOF pore with suitable geometry can stabilize and spatially organize the catalytic substrate into a favorable catalytic route, as with the function of the native enzyme pocket. Based on the desirable photo-responsive oxidase-like activity, a visual and sensitive HOFs biosensor is established for the detection of phosphatase, an important biomarker of skeletal and hepatobiliary diseases. This work demonstrates that the pore environments significantly influence the nanozymes’ activity in addition to the active center.     

Coordination-bond-directed synthesis of hydrogen-bonded organic frameworks from metal–organic frameworks as templates

Controlled synthesis of hydrogen-bonded organic frameworks (HOFs) remains challenging, because the self-assembly of ligands is not only directed by weak hydrogen bonds, but also affected by other competing van der Waals forces. Herein, we demonstrate the coordination-bond-directed synthesis of HOFs using a preformed metal–organic framework (MOF) as the template. A MOF (Cu^I-TTFTB) based on two-coordinated Cu^I centers and tetrathiafulvalene-tetrabenzoate (TTFTB) ligands was initially synthesized. Cu^I-TTFTB was subsequently oxidized to the intermediate (Cu^II-TTFTB) and hydrated to the HOF product (TTFTB-HOF). Single-crystal-to-single-crystal (SC-SC) transformation was realized throughout the MOF-to-HOF transformation so that the evolution of structures was directly observed by single-crystal X-ray diffraction. The oxidation and hydration of the Cu^I center are critical to breaking the Cu–carboxylate bonds, while the synergic corbelled S⋯S and π⋯π interactions in the framework ensured stability of materials during post-synthetic modification. This work not only provided a strategy to guide the design and discovery of new HOFs, but also linked the research of MOFs and HOFs.

The MOF-to-HOF transformation was realized in a single-crystal-to-single-crystal manner by the oxidation and hydration of the Cu^I center in Cu^I-TTFTB. The corbelled S⋯S and π⋯π interactions ensured the framework stability during transformation.     

Substituent effects on heats of formation, group interactions, and detonation properties of polyazidocubanes

We have calculated the heats of formation (HOFs) for a series of polyazidocubanes by using the density functional theory (DFT), Hartree-Fock, and MP2 methods with 6-31G* basis set as well as semiempirical methods. The cubane skeleton was chosen for a reference compound, that is, the cubane skeleton was not broken in the process of designing isodesmic reactions. There exists group additivity for the HOF with respect to the azido group. The semiempirical AM1 method also produced reliable results for the HOFs of the title compounds, but the semiempirical MINDO3 did not. The relationship between HOFs and molecular structures was discussed. It was found that the HOF increases 330-360 kJ/mol for each additional number of the azido group being added to the cubane skeleton. The distance between azido groups slightly influences the values of HOFs. The interacting energies of neighbor azido groups in polyazidocubanes are in the range of 2.3 approximately 6.6 kJ/mol, which are so small and less related to the substituent numbers. The average interaction energy between nearest neighbor --N3 groups in the most stable conformer of octaazidocubane is 2.29 kJ/mol at the B3LYP/6-31G* level. The relative stability related to the number of azido groups of the title compounds was assessed based on the calculated HOFs, the energy gaps between the frontier orbitals, and the bond orders of the C--N3 and C--C bonds. The predicted detonation velocity of hepta- and octa-derivatives is over 9 km/s, and the detonation pressure of them is ca. 40 GPa or over.     

Theoretical studies on heats of formation for cubylnitrates using density functional theory B3LYP method and semiempirical MO methods

The heats of formation (HOF) have been calculated for all the 21 cubylnitrate compounds using the semiemprical molecular orbital (MO) methods (MINDO/3, MNDO, AM1, and PM3) and for 8 of 21 cubylnitrates containing 1–4 ? ONO₂ groups using the density functional theory (DFT) method at the B3LYP/6‐31G* level by means of designed isodesmic reactions. The cubane cage skeletons in cubylnitrate molecules have been kept in setting up isodesmic reactions to produce more accurate and reliable results. It is found that there are good linear relationships between the HOFs of the 8 cubylnitrates calculated using B3LYP/6‐31G* and two semiempirical MO (PM3 and AM1) methods, and the linear correlation coefficients of PM3 and AM1 methods are 0.9901 and 0.9826, respectively. Subsequently, the accurate HOFs at B3LYP/6‐31G* level of other 13 cubylnitrates containing 4–8 ? ONO₂ groups are obtained by systematically correcting their PM3‐calculated HOFs. Compared with noncaged nitrates, all the 21 cubylnitrates have high heats of formation implying that they may be very powerful energetic materials and have highly exploitable value. The relationship between the HOFs and the molecular structures of cubylnitrates has been discussed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

多氰基立方烷生成热的DFT-B3LYP和半经验MO研究

运用密度泛函理论(DFT)B3LYP方法和半经验MO(MINDO/3,MNDO,AM1和PM3)方法系统计算了全部21种多氰基立方烷的生成热,首先,在DFT-B3LYP/6-31G^*水平下通过不破裂立方烷笼状骨架(亦即选择立方烷为参考物)的等键反应设计,精确计算了9种多氰基立方烷的生成热;发现B3LYP/6-31G^*结果分别地均与上述四种半经验MO方法求得的生成热之间存在良好的线性关系(相关系数均在0.9994以上),且以AM1生成热与B3LYP/6-31G^*计算值最为接近,其次,其它12种多氰基立方烷的精确生成热借助上述线性关系通过校正对应的AM1结果而获得,多氰基立方烷的生成热很高,且随-CN基数目的增加而线性地增大,表明它们属于极具潜力的“新一低高能炸药”而具开发价值。     

Computational studies on tetrazole derivatives as potential high energy materials

The tetrazole is an important functionality of the most of energetic materials due to 80% nitrogen content, stability, and high enthalpy of formation. The present structure–property relationship study focuses on the optimized geometries of tetrazole derivatives obtained from density functional theory (DFT) calculations at B3LYP/6-31G* levels. The heat of formation (HOF) of tetrazole derivatives have been calculated by designing the appropriate isodesmic reactions. The increase in nitro groups on azole rings shows the remarkable increase in HOF. Density has been predicted by using CVFF force field. Increase in the nitro group increases the density. Detonation properties of the designed compounds were evaluated by using the Kamlet–Jacobs equation based on predicted densities and HOFs. Designed tetrazole derivatives show detonation velocity (D) over 8 km/s and detonation pressure (P) of about 32 GPa. Thermal stability was evaluated via bond dissociation energies (BDE) of the weakest C–NO₂ bond at B3LYP/6-31G* level. Charge on the nitro group has been used to assess the sensitivity correlation. Overall, the study implies that designed compounds of this series are found to be stable and expected to be the novel candidates of high energy materials (HEMs).     

Accurate heats of formation of polycyclic saturated hydrocarbons predicted by using the XYG3 type of doubly hybrid functionals

Polycyclic saturated hydrocarbons (PSHs) are attractive candidates as hydrocarbon propellants. To assess their potential values, one of the key factors is to determine their energy contents, such as to calculate their heats of formation (HOF). In this work, we have calculated HOFs for a set of 36 PSHs including exo-Tricyclo[5.2.1.0^(2,6)] decane, the principal component of the high-energy density hydrocarbon fuel commonly identified as JP-10. The results from B3LYP, B3LYP-D3BJ, M06-2X, B2PLYP, B2PLYP-D3BJ, and the XYG3 type of doubly hybrid (xDH) functionals are presented. It is demonstrated here that the xDH functionals yield accurate HOFs in good agreement with those from experiments or the G4 theory. In particular, XYGJ-OS, a low scaling xDH functional, is shown to hold the promise for accurate prediction of HOFs for PSHs of larger sizes. © 2018 Wiley Periodicals, Inc.     

Theoretical prediction on heats of formation for polyisocyanocubanes

The heats of formation (HOF) for all the 21 polyisocyanocubanes are calculated systematically with density functional theory (DFT) B3LYP and semiempirical MO(MINDO/3, MNDO, AM1 and PM3) methods. First, the accurate HOFs for the 8 title compounds are obtained by means of designed isodesmic reactions at DFT-B3LYP/6-31G* level, and the cubane cage skeleton has not been broken (i.e. choosing cubane as a reference compound) to produce more accurate and reliable results. It is found that there are good linear relationships between the HOFs calculated using the B3LYP/6-31G* and four semiempirical MO methods, respectively, and all of the linear correlation coefficients are more than 0.9971. The HOFs obtained from PM3 calculation are the best among the four semiempirical MO methods. Then, the accurate HOFs at B3LYP/6-31G* level of other 13 polyisocyanocubanes are obtained by systematically correcting their PM3-calculated HOFs. Polyisocyanocubanes have very high HOFs, and the HOFs increase linearly with the increasing of the number of isocyano groups in a molecule. The results show that polyisocyanocubanes are the new generation explosives with highly potential and exploitable value.     

Triaxially Woven Hydrogen-Bonded Chicken Wires of a Tetrakis(carboxybiphenyl)ethene

Interpenetration of low-dimensional networked structural motifs crucially affects porosity, stability, and properties of the whole reticular framework. However, varying and controlling the manner of interpenetration is still challenging. Herein, a porous hydrogen-bonded organic framework (HOF) with wvm -like weave constructed by triaxially woven chicken wires of X-shaped tetra-armed tetrakis(carboxybiphenyl)ethene CBPE , formally 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(1,1′-biphenyl-4-carboxylic acid), is reported. The structure is a contrast to a non-interpenetrated layered framework composed of tetrakis(4-carboxyphenyl)ethene CPE . This exotic framework of CBPE is due to the disproportionate conformation of the outer four phenylene rings in the peripheral biphenyl arms. The activated framework CBPE-1a exhibits thermal stability up to 220 °C and a BET surface area of 555 m² g⁻¹. Additionally, the HOF shows mechanochromic behavior in terms of fluorescence color and quantum efficiency. The achievement of the present HOFs can provide insight into constructing a new type of functional porous organic materials with interwoven network structures.     

Tetraphenylethylene-Based Hydrogen-Bonded Organic Frameworks (HOFs) with Brilliant Fluorescence

By synergistically employing four key strategies: (I) introducing tetraphenylethylene groups as the central core unit with aggregation-induced emission (AIE) properties, (II) optimizing the π-conjugated length by extending the building block branches, (III) incorporating flexible groups containing ethylenic bonds, and (IV) applying crystal engineering to attain dense stacking mode and highly twisty conformation, we successfully synthesized a series of hydrogen-bonded organic frameworks (HOFs) exhibiting exceptional one/two-photon excited fluorescence. Notably, when utilizing the fluorescently superior building block L2, HOF-LIFM-7 and HOF-LIFM-8 exhibiting high quantum yields (QY) of 82.1 % and 77.1 %, and ultrahigh two-photon absorption (TPA) cross-sections of 148959.5 GM and 123901.1 GM were achieved. These materials were successfully employed in one and two-photon excited lysosome-targeting cellular imaging. It is believed that this strategy, combining building block optimization and crystal engineering, holds significant potential for guiding the development of outstanding fluorescent HOF materials.