Molecular Materials Research Articles

Switching Spin‐States: Spin Crossover vs. Coordination‐Induced Spin‐State Switching

AbstractSwitchable molecular materials have continued to attract the interest of chemists and physicists for decades. A prominent example are spin crossover complexes, where numerous textbooks, review articles, and special issues are available. A highly interesting alternative is the coordination‐induced spin‐state switching. In this concept, we will compare the two spin‐state switching mechanisms and discuss common aspects and differences. Additionally, a short overview of coordination‐induced spin state switching will be given.

A C3‐symmetric like structurally twisted molecular architecture: mechano/fluorochromic behavior and selective multiphase detection of Pd (II)

Triaminoguanidinium (TG)‐based C3‐symmetric‐like molecular architectures with diverse donors and acceptors are well established as organic molecular materials for promising stimuli‐responsive and metal sensing‐based applications. However, the mechanofluorochromic (MFC) response was unremarkable upon applying external mechanical stress. Even though some show MFC features, there was no optical shift under ambient light. In this paper, we developed a novel C3‐symmetric molecule, TAC3, containing the TG core linked with conformationally twisted thiophene‐linked anthracenyl terminals. Three conformationally deformed hands in TAC3 respond against external mechanical stimuli with a redshifted absorption and emission, easily detectable through the naked eye. Although MFC features were reported before for such C3‐symmetric molecules, the change in absorbance was not identified. In addition, TAC3 could selectively detect Pd2+ through extreme emission quenching (green to black). The mechanochromism and sensing outcomes are elucidated by the experimental and theoretical support. Compared to the previous reports, the notable 2:3 (metal: ligand) binding due to geometric restriction creating a specific pocket size for Pd+2 is unusual for this system. The onsite application on Pd2+ detection and measurable MFC features are demonstrated to validate the potential of this C3‐symmetric molecule in advanced optoelectronic applications such as security writing and Pd2+ ion detection.

Room‐temperature magnetocapacitance spanning 97 K hysteresis in molecular material

Magnetic capacitor, as a new type of device, has broad application prospects in fields such as magnetic field sensing, magnetic storage, magnetic field control, power electronics and so on. Traditional magnetic capacitors are mostly assembled by magnetic and capacitive materials. Magnetic capacitor made of a single material with intrinsic properties is very rare. This intrinsic property is magnetocapacitance (MC). The studies on MC effect have mainly focused on metal oxides so far. No study was reported in molecular materials. Herein, two complexes: (CETAB)2[CuCl4] (1) and (CETAB)2[CuBr4] (2) (CETAB = (2‐chloroethyl)trimethylammonium) are reported. There exist strong H‐Br and Br‐Br interactions and other weak interactions in complex 2, so the phase transition energy barrier is high, resulting in the widest thermal hysteresis loop on a molecular level to date. Furthermore, complexes 1 and 2 show large MC parameters of 0.247 and 1.614, respectively, which is the first time to observe MC effect in molecular material.

Room-temperature magnetocapacitance spanning 97 K hysteresis in molecular material.

Magnetic capacitor, as a new type of device, has broad application prospects in fields such as magnetic field sensing, magnetic storage, magnetic field control, power electronics and so on. Traditional magnetic capacitors are mostly assembled by magnetic and capacitive materials. Magnetic capacitor made of a single material with intrinsic properties is very rare. This intrinsic property is magnetocapacitance (MC). The studies on MC effect have mainly focused on metal oxides so far. No study was reported in molecular materials. Herein, two complexes: (CETAB)2[CuCl4] (1) and (CETAB)2[CuBr4] (2) (CETAB = (2-chloroethyl)trimethylammonium) are reported. There exist strong H-Br and Br-Br interactions and other weak interactions in complex 2, so the phase transition energy barrier is high, resulting in the widest thermal hysteresis loop on a molecular level to date. Furthermore, complexes 1 and 2 show large MC parameters of 0.247 and 1.614, respectively, which is the first time to observe MC effect in molecular material.

Nickel Bis(dithiolene) Complexes and Tetrathiafulvalenes from Sterically Hindered Di‐ and Tetra‐substituted Dithiine‐dithiolo‐thione Precursors

Three variants of 5,6‐dihydro‐1,4‐dithiin‐2,3‐(1’,3’‐dithiole‐2’‐thione) (dddt) bearing up to four substituents on the ethylene bridge have been obtained by hetero‐Diels‐Alder reaction. The substituted dddt derivatives have the advantage of providing modulation of the steric hindrance and stereogenic centres, and represent valuable intermediates for both tetrathiafulvalene (TTF) and metal‐dithiolene complexes. Within this work, disubstituted and tetrasubstituted dddt derivatives have been used for the synthesis of Ni(II) monoanionic and neutral complexes, as well as for tetra‐ and octa‐methylated EDT‐TTF and BEDT‐TTF (EDT = ethylenedithio, BEDT = bis(ethylenedithio)) derivatives. While the influence of the bulkiness and number of the substituents is observed in the solid state packing of both types of materials (complexes and TTF), the absence of the counterion in the case of the neutral complexes translated into increased intermolecular interactions between the donors and the formation of 1D and 2D layers. The two examples of tetrasubstituted TTF radical cation salts reported in here show semiconducting behaviour due to the formation of dimers and lateral interactions through the S atoms. Our findings highlight the importance of substitution on these type of dddt precursors and their potential for electroactive molecular materials.

Exceptional Field Effect and Negative Differential Conductance in Spiro-Conjugated Single-Molecule Junctions.

The advancement of molecular electronics endeavors to build miniaturized electronic devices using molecules as the key building blocks by harnessing their internal structures and electronic orbitals. To date, linear planar conjugated or cross-conjugated molecules have been extensively employed in the fabrication of single-molecule devices, benefiting from their good conductivity and compatibility with electrode architectures. However, the development of multifunctional single-molecule devices, particularly those with unique charge transport properties, necessitates a more rigorous selection of molecular materials. Among different assortments of molecules suited for the construction of molecular circuits, Spiro-conjugated structures, specifically spirobifluorene derivatives, stand out as promising candidates due to their distinctive electronic properties. In this work, we focus on the charge transport characteristics of Spiro-conjugated molecules sandwiched between graphene nanogaps. Experiments reveal significant Coulomb blockade and distinct negative differential conductance effects. Beyond two-terminal device measurements, solid-state gate electrodes are utilized to create single-molecule transistors, successfully modulating the molecular energy levels to achieve an on/off ratio exceeding 1000. This endeavor not only offers valuable insights into the design and fabrication of future practical molecular devices, blessed with enhanced performance and functionality, but also presents a new paradigm for the investigation of fundamental physical phenomena.

Spot the difference in reactivity: a comprehensive review of site-selective multicomponent conjugation exploiting multi-azide compounds.

Going beyond the conventional approach of pairwise conjugation between two molecules, the integration of multiple components onto a central scaffold molecule is essential for the development of high-performance molecular materials with multifunctionality. This approach also facilitates the creation of functionalized molecular probes applicable in diverse fields ranging from pharmaceuticals to polymeric materials. Among the various click functional groups, the azido group stands out as a representative click functional group due to its steric compactness, high reactivity, handling stability, and easy accessibility in the context of multi-azide scaffolds. However, the azido groups in multi-azide scaffolds have not been well exploited for site-specific use in molecular conjugation. In fact, multi-azide compounds have been well used to conjugate to the same multiple fragments. To circumvent problems of promiscuous and random coupling of multiple different fragments to multiple azido positions, it is imperative to distinguish specific azido positions and use them orthogonally for molecular conjugation. This review outlines methods and strategies to exploit specific azide positions for molecular conjugation in the presence of multiple azido groups. Illustrative examples covering di-, tri- and tetraazide click scaffolds are included.

Heterojunction Organic Solar Cells with Efficient Charge Mobility and Separation Capabilities Studied by DFT.

The properties of the active layer materials play a decisive role in determining the power conversion efficiency of organic solar cells (OSCs). Chlorophyll and its derivatives are abundant and environmentally friendly functional organic molecular materials. Using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), we have calculated the absorption spectra and their excited state properties based on optimized ground state structures. It was found that bacteriochlorin exhibits superior structural properties, a smaller energy gap and hole reorganization energy, redshifted absorption spectra, and higher hole mobility compared to the donor D18. This suggests that bacteriochlorin exhibits superior donor properties. Comparative studies between o-AT-2Cl and m-AT-2Cl showed that o-AT-2Cl had superior acceptor properties, implying that differences in substitution positions can influence the physicochemical properties of non-fullerene acceptors (NFAs). Subsequently, six bulk heterojunctions (BHJs) were constructed by combining three donors with nonfused ring electron acceptors, o-AT-2Cl and m-AT-2Cl. The bacteriochlorin-based BHJs performed well among them, with BChl3/o-AT-2Cl and BChl4/o-AT-2Cl having the largest interfacial charge separation rate. The results suggested that BHJs composed of bacteriochlorin and NFAs can improve OSCs' photovoltaic performance, providing a feasible scheme for designing efficient OSCs.

Reductive Photocatalytic Proton‐Coupled Electron Transfer by a Zirconium‐based Molecular Platform

Reductive proton‐coupled electron transfer (PCET) has important energetic implications in numerous synthetic and natural redox processes. The development of catalytic systems that can mediate such transformations has become an attractive target, especially when light is used to generate the reactive species towards solar‐to‐chemicals conversion. However, such approach becomes challenged by kinetic competition with H2 evolution. Here we describe the excited state reactivity of a molecular Zr‐based platform under visible light irradiation for the efficient reduction of multiple bonds. Mechanistic investigations shine light on a charge separation process that colocalizes an excited electron and an acidic proton to promote selective PCET. We further leveraged this reactivity for the photocatalytic reduction of a variety of organic substrates. Our results demonstrate the promise of this molecular platform to design strong photocatalytic PCET mediators for reductive transformations. More broadly, we also show the potential relevance of PCET mechanisms in the (photo)redox chemistry of Zr‐based molecular materials.

Reductive Photocatalytic Proton-Coupled Electron Transfer by a Zirconium-based Molecular Platform.

Reductive proton-coupled electron transfer (PCET) has important energetic implications in numerous synthetic and natural redox processes. The development of catalytic systems that can mediate such transformations has become an attractive target, especially when light is used to generate the reactive species towards solar-to-chemicals conversion. However, such approach becomes challenged by kinetic competition with H2 evolution. Here we describe the excited state reactivity of a molecular Zr-based platform under visible light irradiation for the efficient reduction of multiple bonds. Mechanistic investigations shine light on a charge separation process that colocalizes an excited electron and an acidic proton to promote selective PCET. We further leveraged this reactivity for the photocatalytic reduction of a variety of organic substrates. Our results demonstrate the promise of this molecular platform to design strong photocatalytic PCET mediators for reductive transformations. More broadly, we also show the potential relevance of PCET mechanisms in the (photo)redox chemistry of Zr-based molecular materials.

Improving Friction Sensitivity of Molecular Perovskite Energetic Material DAP‐4 by Polymer Binders Coated

AbstractThe high detonation and thermostability of the molecular perovskite energetic material DAP‐4 attracted a lot of interest, but its high friction sensitivity hindered its applicability. It is critical to minimize the friction sensitivity of DAP‐4 for future applications. DAP‐4‐based composites with core‐shell structures coated by polymer binders were devised and manufactured in this study. DAP‐4 was discovered to be polymer binder compatible, and the thermal breakdown temperature remained constant at 380°C. The friction sensitivity of DAP‐4‐based core‐shell composites increased from 36 N (raw DAP‐4) to 60 N (DAP‐4@Estane‐5703, DAP‐4@F2602, and DAP‐4@F2311) when 10% polymer binders were added as shell‐structured layers. Furthermore, their theoretical detonation performance was calculated, and each sample demonstrated superior performance. Among them, the theoretical detonation velocity of DAP‐4@F2602 reached 8081 m/s, and the heat of detonation was 5.118 kJ/kg. This provided proof‐of‐concept work for increasing the mechanical safety of molecular perovskite energetic materials.

Porous flexible molecular-based piezoelectric composite achieves milliwatt output power density

Molecular ferroelectrics have made breakthrough progress in intrinsic piezoelectric response that can be on par with advanced inorganic piezoelectric ceramics. However, their successful applications in high-density energy harvesting and self-powered flexible devices have been great challenge, owing to the low elastic moduli, intrinsically brittle, and fracture proneness of such material systems under mechanical loading. Here, we have developed a flexible porous composite piezoelectric material by using soft thermoplastic polyurethane (TPU) and molecular ferroelectric materials. Benefiting from the porous structure of TPU, the flexible piezoelectric composites enable effectively large doping ratio (50%) of [Me3NCH2Cl]CdCl3 (TMCM-CdCl3) and highly efficient stress absorption, coupled with the excellent piezoelectric properties of TMCM-CdCl3, to realize a superior power density (636.9 µW cm−2 or 1273.9 µW cm−3). This output is 2000 times higher than that of flexible piezoelectric materials represented by poly(vinylidene fluoride) (PVDF). We believe that the outstanding performance of the porous composite piezoelectric material would pave a feasible way for real industrial applications of molecular ferroelectrics.

Tailored Permeation Through ≈1 nm Thick Carbon Nanomembranes by Subtle Changes in Their Molecular Design.

Due to their nanoscale thickness (≈1 nm) and exceptional selectivity for permeation of gases, nanomembranes made of 2D materials possess high potential for energy-efficient nanofiltration applications. In this respect, organic carbon nanomembranes (CNMs), synthesized via electron irradiation-induced crosslinking of aromatic self-assembled monolayers (SAMs), are particularly attractive, as their structure can be flexibly tuned by choice of molecular precursors. However, tailored permeation of CNMs, defined by their molecular design, has not been yet demonstrated. In this work, it is shown that the permeation of helium (He), deuterium (D2) and heavy water (D2O) for CNMs synthesized from biphenyl-based SAMs on silver (C6H5-C6H4-(CH2)n-COO/Ag, n = 2-6) can be tuned by orders of magnitude by changing the structure of the molecular precursors by just a single methylene unit. The selectivity in permeation of D2O/D2 with an unprecedented value of 200000 can be achieved in this way. The temperature-dependent study reveals a clear correlation between the molecular design and the permeation mechanisms facilitating therewith tailored synthesis of molecular 2D materials for separation technologies.

Constitutive modelling for understanding stress-stretch behaviour of Lennard-Jones non-crystalline molecular Solid

Abstract Non-crystalline molecular solid materials have many scientific and engineering applications. This study develops a constitutive equation for understanding stress-stretch behaviour of non-crystalline molecular solid using Lennard-Jones (LJ) intermolecular interaction. The strain energy derived from Lennard-Jones interactions between molecules. Based on the excluded volume (spherical volume occupied by the molecules maintaining centre to centre distance with a reference molecule) and density of the molecules, strain energy density is developed. In order to relate the molecular approach with continuum approximation, the excluded volume and density are expressed as a function of strain invariants of right Cauchy-Green deformation tensor. Finally, the constitutive equation in the form of Cauchy stress tensor is developed using the present strain energy density function. The present constitutive model is used to study finite deformations of the molecular solid like uniaxial extension. We compare our theoretical results with the experimental data of flexible polyurethane foams and obtain very good agreements. The current constitutive model can predict the deformation of micro/nano engineering system components.

Acylhydrazone Bond: A Pivotal Bridge for Modifying Julolidine Derivative with Pyrazole Group as MultiStimuli‐Responsive Fluorescent Materials

AbstractTo screen julolidine derivatives containing a pyrazole group with excellent fluorescence properties and promising applications in multistimuli‐responsive fluorescent materials, three novel compounds were designed and synthesized by connecting pyrazoles through typical conjugated bonds: imine (T1), dihydrazone (T2), and acylhydrazone (T3). The molecular structure of the title compounds was characterized by 1H NMR, 13C NMR, and HR‐MS. The representative compound T3 was verified by single crystal X‐ray diffraction. Although the three compounds have roughly similar conjugated structures, dissimilarity in their fluorescence properties was apparent. The results of fluorescence studies showed that the compound T3, which was connected to julolidine and pyrazole by an acylhydrazone bond, exhibited excellent fluorescence performance compared to the other compounds T1 and T2. Compound T3 can also respond to external stimuli, such as solvent viscosity, organic acids/bases, and mechanical forces. In addition, T3 could also specifically detect Cu2+ ions, and the fluorescence titration data indicated that the low limit of detection (LOD) of compound T3 for Cu2+ is 0.26 µM within the range from 0 to 50 µM. Therefore, this work provides a preliminary basis for the development of multistimuli‐responsive fluorescent molecular materials containing acylhydrazone fragment.

Shape-stable composite of synthetic phase change materials from balsa wood by epoxy/PEG through alkaline method

Phase change materials have excellent thermal energy storage capacity, making them an appealing option for energy management applications. Stabilizing the form of phase change materials enhances their practical effectiveness. This research focuses on sustaining the shape of low molecular weight PEG phase change materials in wood structures made from natural balsa wood. The alkalization process is used to modify the wood, after which the PEG/epoxy polymers are infused into the wood structure to form a synthetic phase change material. The process of treating wood with alkali involves partially removing lignin from the wood, creating specific spaces. Results from optical transmittance analysis demonstrate that wood treated with alkali and wood infused with phase change polymer exhibit high optical transmittance. SEM images also illustrate the presence of spaces in delignified wood treated with Epoxy/PEG 400 and Epoxy/PEG 600. The synthetic phase change material remains stable at temperatures below 310°C and possesses a ∆Hc of 170.1J/g to 162.8 J/g. The phase transition temperatures during heating are 56.4°C and 57.2°C, cooling is 25.7°C and 30.1°C.

Predicting Molecular Ordering in Deposited Molecular Films

AbstractThin films of molecular materials are commonly employed in organic light‐emitting diodes, field‐effect transistors, and solar cells. The morphology of these organic films is shown to depend heavily on the processing used during manufacturing, such as vapor co‐deposition. However, the prediction of processing‐dependent morphologies has until now posed a significant challenge, particularly in cases where self‐assembly and ordering are involved. In this work, a method is developed based on coarse‐graining that is capable of predicting molecular ordering in vapor‐deposited films of organic materials. The method is tested on an extensive database of novel and known organic semiconductors. A good agreement between the anisotropy of the refractive indices of the simulated and experimental vapor‐deposited films suggests that the method is quantitative and can predict the molecular orientations in organic films at an atomistic resolution. The methodology can be readily utilized for screening materials for organic light‐emitting diodes.

Inverse Design of Singlet Fission Materials with Uncertainty-Controlled Genetic Optimization.

Singlet fission has shown potential for boosting the power conversion efficiency of solar cells, but the scarcity of suitable molecular materials hinders its implementation. We introduce an uncertainty-controlled genetic algorithm (ucGA) based on ensemble machine learning predictions from different molecular representations that concurrently optimizes excited state energies, synthesizability, and singlet exciton size for the discovery of singlet fission materials. The ucGA allows us to efficiently explore the chemical space spanned by the reFORMED fragment database, which consists of 45,000 cores and 5,000 substituents derived from crystallographic structures assembled in the FORMED repository. Running the ucGA in an exploitative setup performs local optimization on variations of known singlet fission scaffolds, such as acenes. In an explorative mode, hitherto unknown candidates displaying excellent excited state properties for singlet fission are generated. We suggest a class of heteroatom-rich mesoionic compounds as acceptors for charge-transfer mediated singlet fission. When included in larger conjugated donor-acceptor systems, these units exhibit strong localization of the triplet state, favorable diradicaloid character and suitable triplet energies for exciton injection into semiconductor solar cells. As the proposed candidates are composed of fragments from synthesized molecules, they are likely synthetically accessible.

Inverse Design of Singlet Fission Materials with Uncertainty‐Controlled Genetic Optimization

Singlet fission has shown potential for boosting the power conversion efficiency of solar cells, but the scarcity of suitable molecular materials hinders its implementation. We introduce an uncertainty‐controlled genetic algorithm (ucGA) based on ensemble machine learning predictions from different molecular representations that concurrently optimizes excited state energies, synthesizability, and singlet exciton size for the discovery of singlet fission materials. The ucGA allows us to efficiently explore the chemical space spanned by the reFORMED fragment database, which consists of 45,000 cores and 5,000 substituents derived from crystallographic structures assembled in the FORMED repository. Running the ucGA in an exploitative setup performs local optimization on variations of known singlet fission scaffolds, such as acenes. In an explorative mode, hitherto unknown candidates displaying excellent excited state properties for singlet fission are generated. We suggest a class of heteroatom‐rich mesoionic compounds as acceptors for charge‐transfer mediated singlet fission. When included in larger conjugated donor‐acceptor systems, these units exhibit strong localization of the triplet state, favorable diradicaloid character and suitable triplet energies for exciton injection into semiconductor solar cells. As the proposed candidates are composed of fragments from synthesized molecules, they are likely synthetically accessible.

DNA-Based Complexes and Composites: A Review of Fabrication Methods, Properties, and Applications.

Deoxyribonucleic acid (DNA), a macromolecule that stores genetic information in organisms, has recently been gradually developed into a building block for new materials due to its stable chemical structure and excellent biocompatibility. The efficient preparation and functional integration of various molecular complexes and composite materials based on nucleic acid skeletons have been successfully achieved. These versatile materials possess excellent physical and chemical properties inherent to certain inorganic or organic molecules but are endowed with specific physiological functions by nucleic acids, demonstrating unique advantages and potential applications in materials science, nanotechnology, and biomedical engineering in recent years. However, issues such as the production cost, biological stability, and potential immunogenicity of DNA have presented some unprecedented challenges to the application of these materials in the field. This review summarizes the cutting-edge manufacturing techniques and unique properties of DNA-based complexes and composites and discusses the trends, challenges, and opportunities for the future development of nucleic acid-based materials.