Design Of Molecular Materials Research Articles

Investigating the function and design of molecular materials through terahertz vibrational spectroscopy.

Terahertz spectroscopy has proved to be an essential tool for the study of condensed phase materials. Terahertz spectroscopy probes the low-frequency vibrational dynamics of atoms and molecules, usually in the condensed phase. These nuclear dynamics, which typically involve displacements of entire molecules, have been linked to bulk phenomena ranging from phase transformations to semiconducting efficiency. The terahertz region of the electromagnetic spectrum has historically been referred to as the 'terahertz gap', but this is a misnomer, as there exist a multitude of methods for accessing terahertz frequencies, and now there are cost-effective instruments that have made terahertz studies much more user-friendly. This Review highlights some of the most exciting applications of terahertz vibrational spectroscopy so far, and provides an in-depth overview of the methods of this technique and its utility to the study of the chemical sciences.

A data-driven interpretation of the stability of organic molecular crystals.

Due to the subtle balance of intermolecular interactions that govern structure-property relations, predicting the stability of crystal structures formed from molecular building blocks is a highly non-trivial scientific problem. A particularly active and fruitful approach involves classifying the different combinations of interacting chemical moieties, as understanding the relative energetics of different interactions enables the design of molecular crystals and fine-tuning of their stabilities. While this is usually performed based on the empirical observation of the most commonly encountered motifs in known crystal structures, we propose to apply a combination of supervised and unsupervised machine-learning techniques to automate the construction of an extensive library of molecular building blocks. We introduce a structural descriptor tailored to the prediction of the binding (lattice) energy and apply it to a curated dataset of organic crystals, exploiting its atom-centered nature to obtain a data-driven assessment of the contribution of different chemical groups to the lattice energy of the crystal. We then interpret this library using a low-dimensional representation of the structure-energy landscape and discuss selected examples of the insights into crystal engineering that can be extracted from this analysis, providing a complete database to guide the design of molecular materials.

Rapid calculation of internal conversion and intersystem crossing rate for organic materials discovery.

The internal conversion (IC) process from S1 to S0 and the intersystem crossing (ISC) transition from T1 to S0 are two essential processes in functional molecular material design. Despite their importance, it is currently impossible to evaluate the rate of these processes for a large set of molecules and, therefore, perform high-throughput virtual screening in large-scale data to gain more physical insight. In this work, we explore possible approaches to accelerate the calculations of IC and ISC rates based on a systematic reduction of the number of modes included in the computation and the study of the importance of the different parameters and the influence of their accuracy on the final result. The results reproduce the experimental trends with systematic errors that are ultimately due to the approximations of the theory. We noted that plausible results for ISC in planar molecules are only obtained by including the effect of Hertzberg-Teller coupling. Our method establishes the feasibility and expected accuracy of the computation of nonradiative rates in the virtual screening of molecular materials.

Dual Emission, Aggregation, and Redox Properties of Boron Difluoride Hydrazones Functionalized with Triphenylamines

Abstractπ‐Conjugated molecular materials often exhibit interesting photoluminescence, redox, and optoelectronic properties that grant them utility as light‐harvesting materials in solar cells, as the emissive component of organic light‐emitting diodes, and as fluorescence sensors. Preparing dye conjugates by appending multiple dyes together to generate π‐conjugated molecules is a common strategy to enhance the properties of these materials as it often imparts traits that cannot be achieved by the building blocks independently. Herein, we report molecular materials that incorporate boron difluoride hydrazone (BODIHY) and triphenylamine (TPA) units that exhibit charge‐transfer character, dual‐emission, and aggregation‐induced emission. The title compounds have low‐energy absorption bands (λabs=455–493 nm) that are red‐shifted relative to BODIHYs with smaller π‐systems and exhibit dual‐emission in the solution, solid, and aggregate states. TPA–BODIHY (donor‐acceptor) conjugates show multiple reversible redox events and, in combination, the data presented herein indicate that there is electronic communication throughout the donor‐acceptor and acceptor‐donor‐acceptor π‐systems. These results are rationalized with the use of computational chemistry and solid‐state structural analysis. This study provides new opportunities for the design of molecular materials that are comprised of redox‐active photoluminescent dyes, including BODIHY, and their potential applications.

Refine the evaluation of photophysical properties of organometallic chromophores under confined molecular crystal conditions

Many impressive results have been achieved in the researches and developments of luminescent chromophore materials by combining experimental synthesis and characterization with the cooperative theoretical calculation. However, the existing theoretical studies are usually based on the intrinsic properties of isolated molecules and extend their properties to the whole molecular material directly, which will lead to the persistence of errors and affect the computational design of molecular materials with different morphology. Therefore, the study of multimolecular systems needs to further consider the environmental effects on molecules. This work is based on the calculation of a series of crystalline Ir(III) complexes under background charge conditions to reveal how the surrounding charge affects the photophysical properties of a series of transition metal Ir(III) complex materials. Through this method, the study of crystalline complexes is found to be more authentically reproduced the charge transfer state, energy level, and reorganization energy, etc., and shows the changes of luminescence characteristics and efficiency. The change of the electronic structure of the target molecule would be characterized more comprehensively, thus obtaining more accurate results for the excited states properties of molecular materials.

Tuning the Electronic Properties of Main-Group Species by N-Heterocyclic Vinyl (NHV) Scaffolds

ConspectusMolecules and materials with easily tunable electronic structures and properties are at the forefront of contemporary research. π-Conjugation is fundamental in organic chemistry and plays a key role in the design of molecular materials. In this Account, we showcase the applicability of N-heterocyclic vinyl (NHV) substituents based on classical N-heterocyclic carbenes (NHCs) for tuning the structure, properties, and stability of main-group species (E) via π-conjugation and/or π-donation.NHVs such as [(NHC)═CR] (R = H or aryl) are monoanionic ligands formally derived by the deprotonation of N-heterocyclic olefins (NHOs), (NHC)═CHR. Further deprotonation of [(NHC)═CR] (R = H) is viable, giving rise to N-heterocyclic vinylidene (NHVD) species such as (NHC)═C. NHVs and NHVDs feature a highly polarizable exocyclic CNHC═C bond because of the presence of adjacent π-donor nitrogen atoms. The nature of the NHC, in particular the π-acceptor property, has a direct consequence on the polarity of the CNHC═C bond and hence on the magnitude of π-conjugation in the derived molecules. Thus, the electronic structure, especially the energy and shape of frontier molecular orbitals, HOMO and LUMO, of derived species can be fine-tuned by a judicious choice of the carbene unit. For instance, the HOMO of classical diphosphenes (RP═PR) (R = alkyl or aryl) is invariably the phosphorus lone-pair orbital, while the P═P π-bond is HOMO - 1 or HOMO - 2. In strong contrast, the HOMO of divinyldiphosphenes (R = NHV) is mainly the P═P π-bond. This is owing to the π-conjugation, resulting in the lowering of the LUMO and raising of the HOMO energy. They have a remarkably small HOMO-LUMO energy gap (4.15-4.50 eV) and readily undergo 1e-oxidations, giving rise to stable radical cations and dications.By employing a similar approach, one can access divinyldiarsenes and the corresponding radical cations and dications as crystalline solids. The use of divinyldiphosphenes and divinyldiarsenes as promising ligands in the stabilization of metalloradicals has been shown. By a logical selection of singlet carbenes, stable 2-phosha-1,3-butadiene and 2-arsa-1,3-butadiene compounds, as well as related radical cations and dications, can be prepared as crystalline solids.The relevance of NHV ligands as potent π-donors has been demonstrated for the stabilization of elusive electrophilic phosphinidene and arsinidene complexes {(NHV)E}Fe(CO)4 (E = P or As). Moreover, stable singlet diradicaloid [(NHC)CP]2 and p-quinodimethane derivatives [(NHC)CP2]2 based on an NHVD framework are accessible as stable solids.In this Account, a special emphasis is given to the contributions from this laboratory. The author hopes that this Account will serve as a useful reference guide for researchers interested in studying and applying NHV and NHVD scaffolds in modern molecular chemistry and materials sciences.

Computational and data driven molecular material design assisted by low scaling quantum mechanics calculations and machine learning.

Electronic structure methods based on quantum mechanics (QM) are widely employed in the computational predictions of the molecular properties and optoelectronic properties of molecular materials. The computational costs of these QM methods, ranging from density functional theory (DFT) or time-dependent DFT (TDDFT) to wave-function theory (WFT), usually increase sharply with the system size, causing the curse of dimensionality and hindering the QM calculations for large sized systems such as long polymer oligomers and complex molecular aggregates. In such cases, in recent years low scaling QM methods and machine learning (ML) techniques have been adopted to reduce the computational costs and thus assist computational and data driven molecular material design. In this review, we illustrated low scaling ground-state and excited-state QM approaches and their applications to long oligomers, self-assembled supramolecular complexes, stimuli-responsive materials, mechanically interlocked molecules, and excited state processes in molecular aggregates. Variable electrostatic parameters were also introduced in the modified force fields with the polarization model. On the basis of QM computational or experimental datasets, several ML algorithms, including explainable models, deep learning, and on-line learning methods, have been employed to predict the molecular energies, forces, electronic structure properties, and optical or electrical properties of materials. It can be conceived that low scaling algorithms with periodic boundary conditions are expected to be further applicable to functional materials, perhaps in combination with machine learning to fast predict the lattice energy, crystal structures, and spectroscopic properties of periodic functional materials.

Efficient Multiexciton State Generation in Charge-Transfer-Coupled Perylene Bisimide Dimers via Structural Control.

The singlet fission (SF) process is generally defined as the conversion of one singlet exciton (S1) into two triplet excitons (2·T1), which has the potential to overcome thermalization losses in the field of photovoltaic devices. Among the applicable compounds for SF-based photovoltaic devices, perylene bisimide (PBI) is one of the best candidates because of its electronic tunability and photostability. However, the strategy for efficient SF in PBIs remains ambiguous because of numerous competing relaxation pathways in PBI-based molecular materials. In this regard, for the first time, we observed the SF mechanism in PBI dimers by controlling the intrinsic factor (exciton coupling) and the external environment (solvent polarity and viscosity). Time-resolved spectroscopic measurements and quantum chemical simulations reveal that efficient SF occurs through the charge-transfer-assisted mechanism, entailing a large structural fluctuation. Our findings not only highlight the SF mechanism in PBI dimers but also suggest the factors responsible for an efficient SF process, which are important considerations in the design of molecular materials for photovoltaic devices.

Prediction of Thermal Conductivity for Guiding Molecular Design of Liquids

There is a need for molecular-structure-based predictive models that guide the molecular design of materials with desired properties. Herein, we developed a general model based on group-contributio...

Applications of charge-density analysis to the rational design of molecular materials: A mini review on how to engineer optical or magnetic crystals

Low-temperature, high-resolution X-ray diffraction has become a rather established technique to unveil fine details of the electronic structure of molecules and even polymeric units in crystals. The most recent advances in methods, hardware and software have enabled the reconstruction of crystalline charge densities with unprecedented high accuracy. A natural step in the evolution of the field would be to apply these results to the rationalization and prediction of macroscopic response properties of crystals, such as thermal expansion and conductivity, electrical polarization, magnetization, mechanical elasticity, etc. Consequently, charge-density analysis is nowadays not limited to find ever more accurate refinement models, but it has also accepted the challenge of helping to engineer application-oriented molecules and materials. Therefore, in the last years, experimentally estimated charge densities have been of interest not only to the crystallographic community, but also to the broader fields of chemistry and materials science. In particular, detailed insight can be gained on the optics or magnetism of molecular materials when their crystalline macroscopic behavior is correlated with structural and electronic features (as obtained from X-ray diffraction or first-principle quantum-mechanical calculations) of their molecular and sub-molecular building blocks. In this mini review, such relationships are outlined. The emphasis is put on amino acids and coordination polymers as prototypical systems, but the conclusions are general in the sense that they may apply to different classes of molecular systems. This work can also be read as a tutorial review by the newcomers interested in the entanglement between charge-density analysis and crystal engineering.

Towards the design of molecular materials

Towards the design of molecular materials

Nano-objects - sculpting and shape in molecular material design (The Pierre Gilles de Gennes ILCS prize lecture)

ABSTRACTEarly liquid crystal compounds often had relatively simple molecular structures, but with the invention of electro-optical devices, the design of materials became increasingly important. Property-structure correlations have promoted computer-aided rational design to the point where it is becoming a necessity in the selection of compounds for preparation. For simple molecular architectures, allowable electrostatic interactions in the face of steric repulsion were used in a phenomenological way to generate new mesophases, such as the hexatic phase, and to engineer materials with desirable physical properties. Materials chemists, in the creation of substances of greater complexity, have built upon these forms of research. Interestingly, the larger the system often the more simply it becomes as the molecular bulk and shape begin to dominate over the surface interactions. Nano-objects of unique shapes, such as ‘Janus’ grains, created in the process of design can kindle the formation of new mesophases, such as the twist-bend and splay phases generated through the crowding of molecular particles, and minimisation of the free volume. This article explores the development of the shapes of molecular grains and the mesophases they spawn.

Microcrystalline array structures induced by heat treatment of friction-transferred organic semiconductor films

The correlation between molecular orientation and optoelectrical properties is most critical to the future design of molecular materials. We made highly-anisotropic microcrystalline array structures with an organic semiconductor, a methoxy-substituted thiophene/phenylene co-oligomer (TPCO), by depositing it on friction-transferred poly(tetrafluoroethylene) (PTFE) layers fabricated on substrates with several heat treatments. Polarising microscope observation, polarised emission and absorption spectra measurements indicated that the TPCO molecules aligned along the drawing direction of PTFE. Using these films, we fabricated two types of field-effect transistors (FETs) and compared them with those using non-heated TPCO films which provide aligned pleats structures. Ones had the channel length direction parallel to the drawing direction of PTFE and the others had the channel length direction perpendicular to that drawing direction. As for the microcrystalline array films, the mobility ratio of the former FET to that of the latter device was about 27 in the saturation region, while the emission polarisation ratio was 4.5. The heat treatment promoted the crystal growth to enhance the mobility while retaining the high anisotropy. The results demonstrate that the heat treatments of the TPCO films on the friction-transferred layers were useful for controlling crystallinity and orientation of the molecules.

Fluorene, pyrene, and thiophene-based donor-acceptor asymmetric small molecules for solution-processable memory performance

In this paper, two new asymmetric donor-bridge-acceptor molecules (PFTCH and PFTCE) containing the same electron donor (pyrene group) and bridge (fluorene group) but different terminal electron acceptors (cyanoacetic acid for PFTCH and ethyl cyanoacetate for PFTCE) have been synthesized and characterized by NMR, HR-MS etc. Their photophysical properties including Uv-vis absorption and emission spectra were studied. They were further used as the memory active materials and the ITO/PFTCH or PFTCE/Al sandwich memory devices were prepared, in which the organic films were obtained by solution process. The I-V measurements display satisfactory memory performance; they have a low threshold voltage, high ON/OFF ratio (106) and a low current value (10−4 A) at the ON state. Although the electron-withdrawing ability of the cyanoacetic acid group is stronger than that of the ethyl cyanoacetate group, the two memory devices (ITO/PFTCH/Al or Au and ITO/PFTCE/Al or Au) display different memory behaviors (volatile DRAM for ITO/PFTCH/Al or Au devices and nonvolatile WORM for ITO/PFTCE/Al or Au devices). The mechanism is speculated and the different memory behaviors can be tentatively attributed to the different migration processes of the charge carriers under the influence of an electric field. This can offer a new idea for the molecular design of materials with volatile memory.

Synchronized Collective Proton-Assisted Electron Transfer in Solid State by Hydrogen-Bonding Ru(II)/Ru(III) Mixed-Valence Molecular Crystals.

A proton-coupled electron transfer (PCET) reaction was widely studied with isolated organic molecules and metal complexes in solution in view of the biological catalytic reaction, while studying this reaction in the crystalline or solid-state phase, which has a novel example, would give insight into the rather internal environment of proteins without solvation and a creation of new molecular materials. We tried to crystallize a hydrogen-bonded (H-bonded) coordination polymer with one-dimensional nanoporous channels, formed from redox-active RuIII complexes, [RuIII(Hbim)3] (Hbim- = 2,2'-biimidazolate monoanion). As a result, a synchronized collective PCET phenomenon was observed for the molecular nanoporous crystal by novel solid-state cyclic voltammetry (CV), which could be measured by only setting some crystals on the electrode surface. The nanoporous crystals, {[RuIII(Hbim)3]}n (1), are simultaneously induced to a synchronized collective RuIIRuIII mixed-valence state, {RuIIRuIII}n, with alternating arrays of RuII and RuIII complexes by PCET in a way of the reductive state of {RuIIRuII}n. Further, a new crystal with {RuIIRuIII}n, {[RuII(H2bim)(Hbim)2][RuIII(bim) (Hbim)2][K(MeOBz)6]}n (2), was also prepared, and the solid-state CV revealed the same electrochemical behavior of {RuIIRuIII}n with 1. The single crystal with {RuIIRuIII}n of 2 was unusually a semiconductor with 5.12 × 10-6 S/cm conductivity at 298 K by an impedance method (8.01 × 10-6 S/cm by a direct-current method at 277 K). Thus, an unprecedented electron-hopping conductor driven by a low-barrier proton transfer through a PCET mechanism (Ea = 0.30 eV) was realized in the H-bonding molecular crystal with {RuIIRuIII}n. Such studies on a PCET reaction in the crystalline state is not only worthwhile as a model of essential biological reactions without solvation, but also proposed to a new design of molecular materials to occur an electron transfer by using an intermolecular H-bond.

Are metals made from molecules?

In the traditional view, covalently bound materials differ in a fundamental way from metallic substances. Though both are built from more basic units that are, in turn, constructed from a small number of atoms, for these two materials classes the nature of these units is thought to be quite different. For covalent solids and liquids, these units are considered to be molecular, meaning that they possess properties and bonding that are retained in the condensed phase and thus they continue to be identifiable within the larger system. For metallic materials, these basic units are considered to be mere constructs that are not observable against the delocalized bonding of metals or alloys. The perceived dissimilarity of metallic and covalently bound materials has fostered distinctly different approaches to their design and improvement. Here, the delocalized view of metallic bonding is examined. This examination suggests that much of the rationale used in the design of molecular materials my be applied to metals and alloys as well.

Versatile and Resilient Hydrogen-Bonded Host Frameworks.

Low-density molecular host frameworks, whether equipped with persistent molecular-scale pores or virtual pores that are sustainable only when occupied by guest molecules, have emerged as a promising class of materials owing to the ability to tailor the size, geometry, and chemical character of their free space through the versatility of organic synthesis. As such, molecular frameworks are promising candidates for storage, separations of commodity and fine chemicals, heterogeneous catalysis, and optical and electronic materials. Frameworks assembled through hydrogen bonds, though generally not stable toward collapse in the absence of guests, promise significant chemical and structural diversity, with pores that can be tailored for a wide range of guest molecules. The utility of these frameworks, however, depends on the resilience of n-dimensional hydrogen-bonded motifs that serve as reliable building blocks so that the molecular constituents can be manipulated without disruption of the anticipated global solid-state architecture. Though many hydrogen-bonded frameworks have been reported, few exist that are amenable to systematic modification, thus limiting the design of functional materials. This Account reviews discoveries in our laboratory during the past decade related to a series of host frameworks based on guanidinium cations and interchangeable organosulfonate anions, in which the 3-fold symmetry and hydrogen-bonding complementarity of these ions prompt the formation of a two-dimensional (2-D) quasi-hexagonal hydrogen-bonding network that has proven to be remarkably resilient toward the introduction of a wide range of organic pendant groups attached to the sulfonate. Since an earlier report in this journal that focused primarily on organodisulfonate host frameworks with lamellar architectures, this unusually persistent network has afforded an unparalleled range of framework architectures and hundreds of new crystalline materials with predictable solid-state architectures. These range from the surprising discovery of inclusion compounds in organomonosulfonates (GMS), as well as organodisulfonates (GDS), structural isomerism reminiscent of microstructures observed in soft matter, a retrosynthetic approach to the synthesis of polar crystals, separation of molecular isomers, storage of unstable molecules, formation of a zeolite-like hydrogen-bonded framework, and postsynthetic pathways to inclusion compounds through reversible guest swapping in flexible GS frameworks. Collectively, the examples described in this Account illustrate the potential for hydrogen-bonded frameworks in the design of molecular materials, the prediction of solid-state architecture from simple empirical parameters, and the importance of design principles based on robust high dimensional networks. These and other emerging hydrogen-bonded frameworks promise new advanced materials that capitalize fully on the ability of materials chemists to manipulate solid-state structure through molecular design.

Effects of alkyl chain length on film morphologies and photocurrent responses of tetrathiafulvalene-bipyridinium charge-transfer salts: a study in terms of structures

Viologen dication derivatives with a series of alkyl chains are used to assemble charge-transfer salts and films with dimethylthio-tetrathiafulvalene-bicarboxylate which aims to explore the relationship of the alkyl chain length to molecular structures and film morphologies which are important points for the design of molecular materials and manipulation of molecular devices. Four charge-transfer salts generally formulated as [(CnV)(HL)2] [n = 4(1), 8(2), 12(3) and 16(4)] are prepared and 1–3 are characterized by single-crystal X-ray structural analysis. In the short alkyl chain compound, TTF and C4V moieties are regularly arranged and effectively stacked with strong charge-transfer interactions. Lengthening the alkyl chain of the bipyridinium cation reduces the regular and close arrangement of the cations and anions in the crystals, which weakens the charge-transfer interaction while increasing the amphiphilic assembly of the films. Their film morphologies change from crystals to fused crystals, band-like structures and finally to smooth films with the increase of the alkyl chain length. The effects of alkyl chain length on photocurrent intensity are different between the crystal sample-modified electrodes (in the order of 1 > 2 > 3 = 4) and the film-modified electrodes (in the order of 2 > 1 = 3 > 4), because both inter-ion interactions and film-forming properties should be considered for the film electrodes. This is a systematic study of the effect of alkyl chain substitution on film morphologies in terms of crystal structures.

Self-catalyzed carbon dioxide adsorption by metal-organic chains on gold surfaces.

Efficient capture of CO2 by chemical means requires a microscopic understanding of the interactions of the molecule–substrate bonding and adsorption-induced collective phenomena. By molecule-resolved imaging with scanning tunneling microscopy (STM), we investigate self-catalyzed CO2 adsorption on one-dimensional (1D) substrates composed of self-assembled metal–organic chains (MOCs) supported on gold surfaces. CO2 adsorption turns on attractive interchain interactions, which induce pronounced surface structural changes; the initially uniformly dispersed chains gather into close packed bundles, which are held together by highly ordered, single molecule wide CO2 ranks. CO2 molecules create more favorable adsorption sites for further CO2 adsorption by mediating the interchain attraction, thereby self-catalyzing their capture. The release of CO2 molecules by thermal desorption returns the MOCs to their original structure, indicating that the CO2 capture and release are reversible processes. The real space microscopic characterization of the self-catalyzed CO2 adsorption on 1D substrates could be exploited as platform for design of molecular materials for CO2 capture and reduction.

International conference: Modeling & Design of Molecular Materials—MDMM 2012 (September 10–14, 2012, Wrocław, Poland)

International conference: Modeling & Design of Molecular Materials—MDMM 2012 (September 10–14, 2012, Wrocław, Poland)