Enhancing the photothermal conversion of tetrathiafulvalene-based MOFs by redox doping and plasmon resonance

Porous Crystalline Materials Based on Tetrathiafulvalene and Its Analogues: Assembly, Charge Transfer, and Applications

ConspectusThe directed synthesis and functionalization of porous crystalline materials pose significant challenges for chemists. The synergistic integration of different functionalities within an ordered molecular material holds great significance for expanding its applications as functional materials. The presence of coordination bonds connected by inorganic and organic components in molecular materials can not only increase the structural diversity of materials but also modulate the electronic structure and band gap, which further regulates the physical and chemical properties of molecular materials. In fact, porous crystalline materials with coordination bonds, which inherit the merits of both organic and inorganic materials, already showcase their superior advantages in optical, electrical, and magnetic applications. In addition to the inorganic components that provide structural rigidity, organic ligands of various types serve as crucial connectors in the construction of functional porous crystalline materials. In addition, redox activity can endow organic linkers with electrochemical activity, thereby making them a perfect platform for the study of charge transfer with atom-resolved single-crystal structures, and they can additionally serve as stimuli-responsive sites in sensor devices and smart materials.In this Account, we introduce the synthesis, structural characteristics, and applications of porous crystalline materials based on the famous redox-active units, tetrathiafulvalene (TTF) and its analogues, by primarily focusing on metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). TTF, a sulfur-rich conjugated molecule with two reversible and easily accessible oxidation states (i.e., radical TTF•+ cation and TTF2+ dication), and its analogues boast special electrical characteristics that enable them to display switchable redox activity and stimuli-responsive properties. These inherent properties contribute to the enhancement of the optical, electrical, and magnetic characteristics of the resultant porous crystalline materials. Moreover, delving into the charge transfer phenomena, which is key for the electrochemical process within these materials, uncovers a myriad of potential functional applications. The Account is organized into five main sections that correspond to the different properties and applications of these materials: optical, electrical, and magnetic functionalities; energy storage and conversion; and catalysis. Each section provides detailed discussions of synthetic methods, structural characteristics, the physical and chemical properties, and the functional performances of highlighted examples. The Account also discusses future directions by emphasizing the exploration of novel organic units, the transformation between radical cation TTF•+ and dication TTF2+, and the integration of multifunctionalities within these frameworks to foster the development of smart materials for enhanced performance across diverse applications. Through this Account, we aim to highlight the massive potential of TTF and its analogues-based porous crystals in chemistry and material science.

Radical-Induced Photochromic Silver(I) Metal-Organic Frameworks: Alternative Topology, Dynamic Photoluminescence and Efficient Photothermal Conversion Modulated by Anionic Guests

Photogenerated radicals are an indispensable member of the state-of-the-art photochromic material family, as they can effectively modulate the photoluminescence and photothermal conversion performance of radical-induced photochromic complexes. Herein, two novel radical-induced photochromic metal-organic frameworks (MOFs), [Ag(TEPE)](AC) ⋅ 7/4H2O ⋅ 5/4EtOH (1) and [Ag(TEPE)](NC) ⋅ 3H2O ⋅ EtOH (2), are reported. Distinctly different topological networks can be obtained by judiciously introducing alternative π-conjugated anionic guests, including a new topological structure (named as sfm) first reported in this work, describing as 4,4,4,4-c net. EPR data and UV-Vis spectra prove the radical-induced photochromic mechanism. Dynamic photochromism exhibits tunability in a wide CIE color space, with a linear segment from yellow to red for 1, while a curved coordinate line for 2, resulting in colorful emission from blue to orange. Moreover, photogenerated TEPE* radicals effectively activate the near-infrared (NIR) photothermal conversion effect of MOFs. Under 1 W cm-2 808 nm laser irradiation, the surface temperatures of photoproducts 1* and 2* can reach ~160 °C and ~120 °C, respectively, with competitive NIR photothermal conversion efficiencies η=51.8 % (1*) and 36.2 % (2*). This work develops a feasible electrostatic compensation strategy to accurately introduce photoactive anionic guests into MOFs to construct multifunctional radical-induced photothermal conversion materials with tunable photoluminescence behavior.

The Influence of Light-Generated Radicals for Highly Efficient Solar-Thermal Conversion in an Ultra-Stable 2D Metal-Organic Assembly

Solar-thermal water evaporation is a promising strategy for clean water production, which needs the development of solar-thermal conversion materials with both high efficiency and high stability. Herein, we reported an ultra-stable cobalt(II)-organic assembly NKU-123 with light-generated radicals, exhibiting superior photothermal conversion efficiency and high stability. Under the irradiation of 808 nm light, the temperature of NKU-123 rapidly increases from 25.5 to 215.1 °C in 6 seconds. The solar water evaporator based on NKU-123 achieves a high solar-thermal water evaporation rate of 1.442 and 1.299 kg m-2 h-1 under 1-sun irradiation with a water evaporation efficiency of 97.8 and 87.9 % for pure water and seawater, respectively. A detailed mechanism study revealed that the formation of light-generated radicals leads to an increase of spin density of NKU-123 for enhancing the photothermal effect, which provides insights into the design of highly efficient photothermal materials.

Accessible Tetrathiafulvalene Moieties in a 3D Covalent Organic Framework for Enhanced Near-Infrared Photo-Thermal Conversion and Photo-Electrical Response

Redox-active tetrathiafulvalene (TTF)-based covalent organic frameworks (COFs) exhibit distinctive electrochemical and photoelectrical properties, but their prevalent two-dimensional (2D) structure with densely packed TTF moieties limits the accessibility of redox center and constrains their potential applications. To overcome this challenge, an 8-connected TTF linker (TTF-8CHO) is designed as a new building block for the construction of three-dimensional (3D) COFs. This approach led to the successful synthesis of a 3D COF with the bcu topology, designated as TTF-8CHO-COF. In comparison to its 2D counterpart employing a 4-connected TTF linker, the 3D COF design enhances access to redox sites, facilitating controlled oxidation by I2 or Au3+ to tune physical properties. When irradiated with a 0.7 W cm-2 808 nm laser, the oxidized 3D COF samples (I X - ${\mathrm{I}}_{\mathrm{X}}^{-}$ @TTF-8CHO-COF and Au NPs@TTF-8CHO-COF) demonstrated rapid temperature increases of 239.3 and 146.1 °C, respectively, which surpassed those of pristine 3D COF (65.6 °C) and the 2D COF counterpart (6.4 °C increment after I2 treatment). Furthermore, the oxidation of the 3D COF heightened its photoelectrical responsiveness under 808 nm laser irradiation. This augmentation in photothermal and photoelectrical response can be attributed to the higher concentration of TTF·+ radicals generated through the oxidation of well-exposed TTF moieties.

Supra-Photothermal CO2 Methanation over Greenhouse-Like Plasmonic Superstructures of Ultrasmall Cobalt Nanoparticles

Improving the solar-to-thermal energy conversion efficiency of photothermal nanomaterials at no expense of other physicochemical properties, e.g., the catalytic reactivity of metal nanoparticles, is highly desired for diverse applications but remains a big challenge. Herein, a synergistic strategy is developed for enhanced photothermal conversion by a greenhouse-like plasmonic superstructure of 4 nm cobalt nanoparticles while maintaining their intrinsic catalytic reactivity. The silica shell plays a key role in retaining the plasmonic superstructures for efficient use of the full solar spectrum, and reducing the heat loss of cobalt nanoparticles via the nano-greenhouse effect. The optimized plasmonic superstructure catalyst exhibits supra-photothermal CO2 methanation performance with a record-high rate of 2.3 mol gCo -1 h-1 , close to 100% CH4 selectivity, and desirable catalytic stability. This work reveals the great potential of nanoscale greenhouse effect in enhancing photothermal conversions through the combination with conventional promoting strategies, shedding light on the design of efficient photothermal nanomaterials for demanding applications.

Photothermal Conversion Perylene-Based Metal-Organic Framework with Panchromatic Absorption Bandwidth across the Visible to Near-Infrared

Recently, facilely designable metal-organic frameworks have gained attention in the construction of photothermal conversion materials. Nonetheless, most of the previously reported photothermal conversion metal-organic frameworks exhibit limited light absorption capabilities. In this work, a distinctive metal-organic framework with heterogeneous periodic alternate spatial arrangements of metal-oxygen clusters and perylene-based derivative molecules was prepared by in situ synthesis. The building blocks in this inimitable structure behave as both electron donors and electron acceptors, giving rise to the significant inherent charge transfer in this crystalline material, resulting in a narrow band gap with excellent panchromatic absorption, with the ground state being the charge transfer state. Moreover, it can retain excellent air-, photo-, and water-stability in the solid state. The excellent stability and broad light absorption characteristics enable the effective realization of near-infrared (NIR) photothermal conversion, including infrequent NIR-II photothermal conversion, in this perylene-based metal-organic framework.

Engineering of CdS-chain arrays assembled through S⋯S interactions in 1D semiconductive coordination polymers

One-dimensional (1D) Cd(II) coordination polymers [Cd(x-SPhOMe)2]n (x = ortho, meta, and para; HSPhOMe = methoxybenzenethiol) containing inorganic 1D (-Cd-S-)n chains were synthesized. Among these, the KGF-31 polymer bearing para-SPhOMe featured a three-strand chain structure assembled via interchain S⋯S interactions and exhibited high photoconductivity and longevity.

A 3D tetrathiafulvalene-based metal-organic framework with intramolecular charge transfer for efficient near-infrared photothermal conversion

The selection of metal centers can endow donor-metal-accepter (D-M-A) type MOFs with progressive framework dimensions. 3D Cd-based MOFs with intramolecular charge transfer caused by D-M-A exhibit a satisfactory photothermal conversion efficiency of 35.7%, with the temperature rapidly rising from 25 °C to 201 °C in 7 s under 808 nm laser irradiation.

A route to metalloligands consolidated silver nanoclusters by grafting thiacalix[4]arene onto polyoxovanadates

Metalloligands provide a potent strategy for manipulating the surface metal arrangements of metal nanoclusters, but their synthesis and subsequent installation onto metal nanoclusters remains a significant challenge. Herein, two atomically precise silver nanoclusters {Ag14[(TC4A)6(V9O16)](CyS)3} (Ag14) and {Ag43S[(TC4A)2(V4O9)]3(CyS)9(PhCOO)3Cl3(SO4)4(DMF)3·6DMF} (Ag43) are synthesized by controlling reaction temperature (H4TC4A = p-tert-butylthiacalix[4]arene). Interestingly, the 3D scaffold-like [(TC4A)6(V9O16)]11- metalloligand in Ag14 and 1D arcuate [(TC4A)2(V4O9)]6- metalloligand in Ag43 exhibit a dual role that is the internal polyoxovanadates as anion template and the surface TC4A4- as the passivating agent. Furthermore, the thermal-induced structure transformation between Ag14 and Ag43 is achieved based on the temperature-dependent assembly process. Ag14 shows superior photothermal conversion performance than Ag43 in solid state indicating its potential for remote laser ignition. Here, we show the potential of two thiacalix[4]arene modified polyoxovanadates metalloligands in the assembly of metal nanoclusters and provide a cornerstone for the remote laser ignition applications of silver nanoclusters.

The progress of research on the application of redox nanomaterials in disease therapy

Redox imbalance can trigger cell dysfunction and damage and plays a vital role in the origin and progression of many diseases. Maintaining the balance between oxidants and antioxidants in vivo is a complicated and arduous task, leading to ongoing research into the construction of redox nanomaterials. Nanodrug platforms with redox characteristics can not only reduce the adverse effects of oxidative stress on tissues by removing excess oxidants from the body but also have multienzyme-like activity, which can play a cytotoxic role in tumor tissues through the catalytic oxidation of their substrates to produce harmful reactive oxygen species such as hydroxyl radicals. In this review, various redox nanomaterials currently used in disease therapy are discussed, emphasizing the treatment methods and their applications in tumors and other human tissues. Finally, the limitations of the current clinical application of redox nanomaterials are considered.

An Iron-NDC Framework with a Cage Structure and an Optothermal Conversion in NIR Window

Pursuing novel materials with efficient photothermal conversion under irradiation at the near-infrared region windows (NIR, 750-850 nm; NIR-I and NIR-II, 1000-1320 nm)) is of great importance due to their irreplaceable applications, especially in the biomedical field. Herein, on the basis of a coordination chemistry strategy, an iron-based metal-organic framework (MOF) of [N(CH₃)₄]₂[Fe₃(NDC)₄]·DMF·3H₂O (Fe-NDC, 1,4-H₂NDC = 1,4-naphthalenedicarboxylic acid, N(CH₃)₄⁺ = tetramethyl-ammonium, and DMF = N,N-dimethylformamide) was prepared and characterized. Due to the d-d transition effect introduced by coordination with the transition-metal ion of iron and the highly conjugated naphthalenic moiety in 1,4-H₂NDC, guaranteeing an energy transfer between iron and the organic module, Fe-NDC shows a remarkable broad absorption, which could be extended into the NIR-II section. As a result, Fe-NDC could be irradiated by NIR laser (both 808 and 1064 nm) to achieve photothermal conversion. This work sets a good example to inspire the future designation of NIR light-irradiated photothermal materials based on the first-row transition metals.

A radical photochromic metal-organic framework for boosting NIR-II photothermal conversion and therapy

This work demonstrates for the first time that a photochromic metal organic framework (pMOF) can be employed as a promising class of NIR-II photothermal material based on the photoinduced donor-acceptor intermolecular charge transfer process. After further surface-modification, such UV-activated pMOF-a nanoparticles allow the strong inhibition of 4T1 cancer cells under 1064 nm laser irradiation.

Redox-Active Metal-Organic Frameworks with Three-Dimensional Lattice Containing the m-Tetrathiafulvalene-Tetrabenzoate

Metal-organic frameworks (MOFs) constructed by tetrathiafulvalene-tetrabenzoate (H₄TTFTB) have been widely studied in porous materials, while the studies of other TTFTB derivatives are rare. Herein, the meta derivative of the frequently used p-H₄TTFTB ligand, m-H₄TTFTB, and lanthanide (Ln) metal ions (Tb³⁺, Er³⁺, and Gd³⁺) were assembled into three novel MOFs. Compared with the reported porous Ln-TTFTB, the resulted three-dimensional frameworks, Ln-m-TTFTB ([Ln₂(m-TTFTB)(m-H₂TTFTB)_0.5(HCOO)(DMF)]·2DMF·3H₂O), possess a more dense stacking which leads to scarce porosity. The solid-state cyclic voltammetry studies revealed that these MOFs show similar redox activity with two reversible one-electron processes at 0.21 and 0.48 V (vs. Fc/Fc⁺). The results of magnetic properties suggested Dy-m-TTFTB and Er-m-TTFTB exhibit slow relaxation of the magnetization. Porosity was not found in these materials, which is probably due to the meta-configuration of the m-TTFTB ligand that seems to hinder the formation of pores. However, the m-TTFTB ligand has shown to be promising to construct redox-active or electrically conductive MOFs in future work.

Enhanced Photocatalytic Hydrogen Production Activity by Constructing a Robust Organic-Inorganic Hybrid Material Based Fulvalene and TiO2

A novel redox-active organic-inorganic hybrid material (denoted as H₄TTFTB-TiO₂) based on tetrathiafulvalene derivatives and titanium dioxide with a micro/mesoporous nanomaterial structure has been synthesized via a facile sol-gel method. In this study, tetrathiafulvalene-3,4,5,6-tetrakis(4-benzoic acid) (H₄TTFTB) is an ideal electron-rich organic material and has been introduced into TiO₂ for promoting photocatalytic H₂ production under visible light irradiation. Notably, the optimized composites demonstrate remarkably enhanced photocatalytic H₂ evolution performance with a maximum H₂ evolution rate of 1452 μmol g⁻¹ h⁻¹, which is much higher than the prototypical counterparts, the common dye-sensitized sample (denoted as H₄TTFTB-5.0/TiO₂) (390.8 μmol g⁻¹ h⁻¹) and pure TiO₂ (18.87 μmol g⁻¹ h⁻¹). Moreover, the composites perform with excellent stability even after being used for seven time cycles. A series of characterizations of the morphological structure, the photoelectric physics performance and the photocatalytic activity of the hybrid reveal that the donor-acceptor structural H₄TTFTB and TiO₂ have been combined robustly by covalent titanium ester during the synthesis process, which improves the stability of the hybrid nanomaterials, extends visible-light adsorption range and stimulates the separation of photogenerated charges. This work provides new insight for regulating precisely the structure of the fulvalene-based composite at the molecule level and enhances our in-depth fundamental understanding of the photocatalytic mechanism.