Crystalline Packing Research Articles

Effect of molecular packing on modulation of electronic properties of organic donor–acceptor hybrid gels

Here we report gelation properties and molecular packing of phenyl alanine conjugated perylenebisimide (PPA, P) in dimethyl formamide (DMF) and tetrahydrofuran (THF). In both solvents, PPA self-assembles to crystalline nanofibers and exhibits solvent dependent packing polymorphism of PPA molecule. The gels reveal almost comparable optoelectronic behavior in solutions, but xerogels prepared from P3TAA-PPA hybrid(PP), (a donor-acceptor system) in THF exhibits four times higher conductivity than that in DMF and 12 times higher conductivity in former than the later on illumination with light of 1 sun. Spectroscopy, microscopy, X-ray scattering and electrical measurements suggest that the robust crystalline packing of PPA in THF compare to DMF results in better percolation of charges. Due to different packing arrangement, PP-THF hybrid xerogel exhibits negative differential resistance whereas PP-DMF xerogel acts as electronic memory.The impedance results indicate lower charge transfer resistance and higher capacitance in P-THF than in P-DMF xerogel for the difference of molecular packing and this is also greatly reflected in hybrid xerogels. This study proves that although different packing of PBI may have small effect in optical properties and conductivity of pristine assemblies, but such packing differences are critical in determining the efficiency of charge transport when incorporated into donor-acceptor composites.

Polaron spin dynamics in high-mobility polymeric semiconductors

Polymeric semiconductors exhibit exceptionally long spin lifetimes, and recently observed micrometre spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin–orbit and hyperfine interactions lie at the origin of their long spin lifetimes, but the coupling mechanism of a spin to its environment remains elusive. Here, we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges’ hopping motion at low temperatures, whereas an Elliott–Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime, charge, spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains. The long spin lifetimes observed in polymeric semiconductors hold promise for potential applications. A careful study untangles the main mechanism behind them.

Heavy atom substitution — A strategy for improving conductivity in conjugated polymers

Abstract Heavy heteroatom substitution is a strategy for tailoring crystalline packing, carrier mobilities, and optoelectronic properties of the poly(3-chalcogenophenes) (P3AEs). In this study, the structure-property relationship is explored between S, Se, and Te substituted P3AEs and how heteroatom substitution affects the integration of dopants. Two oxidants with were used to positively-dope the P3AEs: iron(III) chloride and iron(III) p-toluenesulfonate hexahydrate. We observe that the size of dopant anion affects the doping efficiency, topology, and morphology of the P3AEs depending on the heteroatom substituted. Outstandingly, poly(3-alkyltellurophene) dopes most efficiently greatly improving its conductivity at low doping concentrations. Moreover, polymers doped with iron(III) p-toluenesulfonate hexahydrate all had considerable improvements at lower doping concentrations. Lightly doped P3AEs could be favorably incorporated into transistors, photovoltaics, and thermoelectrics without compromising conductivity or causing unwanted side-reactions between excess dopant and other components of the device.

Carbon-Sulfur Bond Cleavage During Adsorption of Octadecane Thiol to Copper in Ethanol.

The effect of the solvent on the formation of thiol self-assembled monolayers (SAMs) on oxide-covered, reactive metals is more involved than in the well-studied gold-thiol system. In this work, copper covered with a native oxide was modified with 1-octadecanethiol (ODT) in either tetrahydrofuran or ethanol. Infrared spectroscopy indicated the formation of crystalline chain packing of alkyl chains from both solvents. Surface coverage was approximately equal in both systems, with differences in tilt angles of the chains. A detailed analysis by X-ray photoelectron spectroscopy showed the formation of Cu2S and copper-bound carbon when the adsorption was carried out in ethanol. This observation can be explained by the cleavage of the C-S bond in ODT during adsorption. Based on the analogy of preparations, we reason that the solvation of ODT in ethanol must be such that it weakens the C-S bond in ODT, thus enabling the cleavage of this bond. Based on the evidence presented here, it is not possible to distinguish between surface solvation and bulk solvation. Electrochemical linear sweep voltammetry shows that SAMs from both solvents have an enhancing protective effect compared to the native oxide layer. The results from this work show interesting possibilities for the preparation of adsorbed monolayers with chemical interaction to reactive metals, with some similarities to carbene-based SAMs.

Periphery Design of Macrocyclic Materials for Organic Light-Emitting Devices with a Blue Phosphorescent Emitter.

Cyclo- meta-phenylenes were modified with trifluoromethyl groups at their periphery to create host materials suitable for use in blue phosphorescent organic light-emitting devices. The periphery design resulted in molecules with high triplet-state energies, which were required to support the blue emission from Ir phosphors. As a result, an external quantum efficiency of 9.9% was achieved. The most successful host, a pentameric congener, preferred CF-π/CH-π interactions in its crystalline packings, which could be beneficial for the host performance.

Local Structure and Bonding of Carbon Nanothreads Probed by High-Resolution Transmission Electron Microscopy.

Carbon nanothreads are a new one-dimensional sp3-bonded nanomaterial of CH stoichiometry synthesized from benzene at high pressure and room temperature by slow solid-state polymerization. The resulting threads assume crystalline packing hundreds of micrometers across. We show high-resolution electron microscopy (HREM) images of hexagonal arrays of well-aligned thread columns that traverse the 80-100 nm thickness of the prepared sample. Diffuse scattering in electron diffraction reveals that nanothreads are packed with axial and/or azimuthal disregistry between them. Layer lines in diffraction from annealed nanothreads provide the first evidence of translational order along their length, indicating that this solid-state reaction proceeds with some regularity. HREM also reveals bends and defects in nanothread crystals that can contribute to the broadening of their diffraction spots, and electron energy-loss spectroscopy confirms them to be primarily sp3-hybridized, with less than 27% sp2 carbon, most likely associated with partially saturated "degree-4" threads.

Atomic packing and size effect on the Hume-Rothery rule

Atomic packing problems have been widely investigated by physicists, mathematicians, material scientists as a long-standing scientific issue. While it is widely known that the monodisperse particles are closely packed into the face-centered cubic structure, the increase of polydispersity will suppress the crystallization. It is still unsolved when and how the crystalline packing collapses as the particle-size difference increases over a critical value. On the other hand, the atomic-size factor of the solution has gone deeply into the concept of applied physicists and physical metallurgy scientists since the proposal of Hume-Rothery rules in 1934. Although great efforts have been devoted to understanding this simple but important empirical rule of the atomic size effect, it remains a question without a complete answer even in binary alloy systems. Here, the two outstanding issues were solved in a rigorous geometric packing instability model even in the multicomponent system. The physical scenario presented here are helpful for understanding the close packing instability and the atomic-size effect in the Hume-Rothery rules.

Crystallization Mechanisms Applied to Understand the Crystal Formation of Rotaxanes

The present study discusses the crystalline packing formation of several [2]rotaxanes with Leigh‐type tetralactam macrocycle bearing different threads. The presence of solvent molecules in some structures are also addressed to shed some light on this matter. Additionally, the degree of similarity between supramolecular structures of rotaxanes was discussed using similarity indices. For this, new descriptors and crystallization mechanisms, which were proposed in terms of contact area and stabilization energy, were carried out to evaluate the rotaxane molecules. It was possible to observe similar general stages of crystallization dominated by the formation of 1D‐blocks and, in fewer cases, by dimers in the first stage of nucleation. The preference for the formation of 1D nuclei resides in the large contact area and complementarity involved in the large set of interactions between the rotaxanes at the earliest stages of crystallization. In this context, it was possible to propose when solvent molecules are trapped between the rotaxanes during crystal formation. Therefore, a unique example of a rotaxane whose topology favored the entrapment of water molecules between rotaxanes during the first stage of the crystallization process is presented. Crystallization mechanisms showed to be a valuable asset in the supramolecular investigation of rotaxanes in the crystalline state.

Assembling-Induced Emission: An Efficient Approach for Amorphous Metal-Free Organic Emitting Materials with Room-Temperature Phosphorescence.

Pure organic emitting materials with room-temperature phosphorescence (RTP), showing large Stokes shifts with long emitting lifetime, low preparation cost, good processability, and wide applications in analysis, bioimaging, organic light emitting diode, and so forth, have been drawing great attentions recently. Related to the design strategy for metal-free RTP materials, the phosphors containing heavy atoms (Br, I, etc.) and other heteroatoms (O, S, etc.) to facilitate the singlet-to-triplet intersystem crossing (ISC) to populate the triplet are usually employed. Besides this factor, the pathways of nonradiative relaxation are inhibited as much as possible. Crystalline packing was the commonly used strategy to engender the rigid environment to suppress the nonradiative decay, and thus to enhance the RTP emission. However, crystal RTP materials might usually be provided with not good enough repeatability and processability, which would restrict their specific practical applications special for biosystem. Instead, amorphous metal-free RTP materials could overcome such deficiencies. Recently, great efforts have been devoted to develop challengeable amorphous metal-free materials and expand their potential applications. This Account mainly focuses on the recent progress on amorphous pure organic RTP system, focusing on the rigid effect to restrict the nonradiative decay to induce or enhance the RTP emission via supramolecular interactions such as host-guest interaction and hydrogen-bonding rigid matrix. Typical host-guest assembling and supramolecular polymer systems, hydrogen-binding copolymers, and small molecules for RTP emission, as well as the heavy-atom free assembling systems for RTP emission are well illustrated in this Account. In the summary, we also give some future perspectives and research direction of the area of pure organic RTP systems, such as enhancement of emission quantum yield, emission color tuning, possible device applications, and the remaining challenge. Moreover, based on these amorphous RTP material examples and beyond, we herein would like to conclude and propose a new concept as "Assembling-Induced Emission", the key thought of which systems is "control molecular motions, then control emission" via supramolecular dynamic assembling. This assembling-induced emission strategy is applicable in many emissive assembling systems besides such amorphous RTP materials introduced in this Account. We hope this concept will be a helpful guide for understanding the emissive mechanism and constructing strategy of various emissive materials.

High temperature molten flux growth, structural and optical characteristics of KTiOPO4:Ho and KTiOPO4:Er single crystals

KTiOPO4:Ho and KTiOPO4:Er single crystals are grown by a top-seeded solution growth method from potassium phosphate and MoO3 flux. The crystalline phase, packing structure and chemical composition of the crystals are analyzed by X-ray diffraction (XRD), energy dispersive spectrum (EDS). The doping of Ho and Er induced strain in the crystal is determined from Williamson–Hall relation. The bonding between titanium and oxygen of Ho and Er doping in the KTP crystal is changing the pseudo-symmetrical deviation in the lattice. The bonding of O and Ti decreases with doping Ho and Er and Ti3+ content in the crystal. The bonding, distortion of the TiO6-TiO6 chain and change in inter-atomic distances by erbium and holmium doping are assessed by Raman spectroscopy and low temperature ESR spectroscopy which evidences the low concentration of Ti3+ presence in the crystal. The KTiOPO4:Ho and KTiOPO4:Er single crystal shows second harmonic generation properties which shows that the efficiency ratio of ηKTiOPO4:Ho/η KTiOPO4 = 0.63 and ηKTiOPO4:Er/η KTiOPO4 = 0.80.

Impact of Eumelanin–PEDOT Blending: Increased PEDOT Crystalline Order and Packing–Conductivity Relationship in Ternary PEDOT:PSS:Eumelanin Thin Films

AbstractA recently developed protocol allows to prepare high quality thin films integrating eumelanin pigment with standard commercial poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The new blend combines noteworthy water stability and substrate adhesion with valuable electro‐optical properties, so it can be used as anodic material in indium tin oxide‐free organic light emitting diodes. Starting from these results, the present work systematically addresses the parameters affecting the blend conductivity. In particular, X‐ray scattering analysis discloses that low quantities of eumelanin modifies molecular packing of the ternary blend, mainly in intermolecular distances and possibly in the degree of crystalline order of PEDOT, having anyway very little effect on the decay of the blend conductivity. Because of the synergetic effects observed here between the eumelanin and the PEDOT:PSS, this study provides a promising ground to access multifunctional materials for applications in advanced electronic devices and bioelectronics.

Solving the enigma of weak fluorine contacts in the solid state: a periodic DFT study of fluorinated organic crystals.

The nature and strength of weak interactions with organic fluorine in the solid state are revealed by periodic density functional theory (periodic DFT) calculations coupled with experimental data on the structure and sublimation thermodynamics of crystalline organofluorine compounds. To minimize other intermolecular interactions, several sets of crystals of perfluorinated and partially fluorinated organic molecules are considered. This allows us to establish the theoretical levels providing an adequate description of the metric and electron-density parameters of the C–F⋯F–C interactions and the sublimation enthalpy of crystalline perfluorinated compounds. A detailed comparison of the C–F⋯F–C and C–H⋯F–C interactions is performed using the relaxed molecular geometry in the studied crystals. The change in the crystalline packing of aromatic compounds during their partial fluorination points to the structure-directing role of C–H⋯F–C interactions due to the dominant electrostatic contribution to these contacts. C–H⋯F–C and C–H⋯O interactions are found to be identical in nature and comparable in energy. The factors that determine the contribution of these interactions to the crystal packing are revealed. The reliability of the results is confirmed by considering the superposition of the electrostatic potential and electron density gradient fields in the area of the investigated intermolecular interactions.

Enforcing Extended Porphyrin J-Aggregate Stacking in Covalent Organic Frameworks.

The potential of covalent organic frameworks (COFs) for realizing porous, crystalline networks with tailored combinations of functional building blocks has attracted considerable scientific interest in the fields of gas storage, photocatalysis, and optoelectronics. Porphyrins are widely studied in biology and chemistry and constitute promising building blocks in the field of electroactive materials, but they reveal challenges regarding crystalline packing when introduced into COF structures due to their nonplanar configuration and strong electrostatic interactions between the heterocyclic porphyrin centers. A series of porphyrin-containing imine-linked COFs with linear bridges derived from terephthalaldehyde, 2,5-dimethoxybenzene-1,4-dicarboxaldehyde, 4,4′-biphenyldicarboxaldehyde and thieno[3,2-b]thiophene-2,5-dicarboxaldehyde, were synthesized, and their structural and optical properties were examined. By combining X-ray diffraction analysis with density-functional theory (DFT) calculations on multiple length scales, we were able to elucidate the crystal structure of the newly synthesized porphyrin-based COF containing thieno[3,2-b]thiophene-2,5-dicarboxaldehyde as linear bridge. Upon COF crystallization, the porphyrin nodes lose their 4-fold rotational symmetry, leading to the formation of extended slipped J-aggregate stacks. Steady-state and time-resolved optical spectroscopy techniques confirm the realization of the first porphyrin J-aggregates on a > 50 nm length scale with strongly red-shifted Q-bands and increased absorption strength. Using the COF as a structural template, we were thus able to force the porphyrins into a covalently embedded J-aggregate arrangement. This approach could be transferred to other chromophores; hence, these COFs are promising model systems for applications in photocatalysis and solar light harvesting, as well as for potential applications in medicine and biology.

A Potassium Metal-Organic Framework based on Perylene-3,4,9,10-tetracarboxylate as Sensing Layer for Humidity Actuators

We have synthesized a novel three-dimensional metal-organic-framework (MOF) based on the perylene-3,4,9,10-tetracarboxylate linker and potassium as metallic centre. We report the formation of this K-based MOF using conventional routes with water as solvent. This material displays intense green photoluminescence at room temperature, and displays an aggregation dependent quenching. Correlation of the optical properties with the crystalline packing was confirmed by DFT calculations. We also demonstrate its potential to build humidity actuators with a reversible and reproducible response, with a change of 5 orders of magnitudes in its impedance at about 40% relative humidity (RH). This 3D-MOF is based on an interesting perylene derivative octadentate ligand, a moiety with interesting fluorescent properties and known component in organic semiconductors. To the best of our knowledge, this is the first time to build such a printed and flexible actuator towards humidity with a reversible response, enabling precise humidity threshold monitoring.

Exploring photophysical processes in a ternary-blended polymer solar cell

Ternary bulk-heterojunction polymer solar cells have been fabricated using a polymer blend (PBDB-T: PBDTTPD) as a donor in combination with a small molecule ITIC as a non-fullerene acceptor. In addition to the similarity in their energy levels, both PBDB-T and PBDTTPD polymers demonstrated rather similar crystalline or lamellar packing structure to support an alloy-type blend model, which well-explained the progressively monotonic change in photovoltaic VOC against the compositions. In addition, photoinduced electron-transfer (PET) between the PBDB-T/PBDTTPD polymers forming an intermediate charge separation state between the donors. A synergistic improvement in exciton generation rate, enhancement in charge carrier transport with good balance, and charge separation rate due to PET all contribute to the achieve a photovoltaic device with high efficiency.

Thermal conductivity at the high-density limit and the levitating granular cluster.

The granular Leidenfrost state consists of a dense granular cluster levitating above a hot granular gas. The density of particles inside the cluster can be very high and even close to the density of crystalline packing. To describe this state theoretically, one needs to know the density dependence of constitutive relations (pressure, heat losses, thermal conductivity) at these very high densities. However, the accurate expression for the coefficient of thermal conductivity is lacking. In this work, the constitutive relations were measured at high densities in molecular dynamics simulations in three different settings: a uniform freely cooling dense granulate (to measure heat losses), a uniform ensemble of elastically colliding particles (to measure pressure), and a dense granular medium between two thermal walls under gravity (to measure thermal conductivity). Next, the hydrodynamic equations with the resulting expressions were solved to describe the levitating cluster state in various parameter regimes. Separate molecular dynamics simulations were performed to test the theoretical predictions and measure the density and temperature profiles of the granular Leidenfrost state, and a good agreement with theoretical results was observed.

Structural and Optical Properties Investigation on H-Bonded 1D Helical Self-Assembly of 1,1-Dibenzyl-3-(2-Bromobenzoyl)thiourea Molecules For Nonlinear Optical Application

A benzoylthiourea molecule namely 1,1-dibenzyl-3-(2-bromobenzoyl)thiourea (2BrBT) was synthesized and characterized by C, H, N and S elemental, mass spectrometry and spectroscopic analyses (infrared, ultraviolet-visible and nuclear magnetic resonance). The 2BrBT compound crystallized in a tetragonal system with the space group P43 and exhibits an acentric crystalline packing due to the presence of intermolecular H-bonding network that forms a self-assembly of 1D helical motif. The asymmetric delocalisation of electrons in the molecule retains its transparency throughout the visible and near-infrared region and hence, essentially propagates the macroscopic helical motif in the solid state. The highest-occupied and lowest-unoccupied molecular orbital (HOMO/LUMO) are mainly found on the thiourea moiety and the benzoylthiourea fragment, respectively and shows an optical bandgap of 3.50 eV. The influence of its geometrical characteristics to the optical properties of 2BrBT is established and discussed in view of nonlinear optical (NLO) application.

Reactivity of O2 versus H2O2 with polysaccharide monooxygenases

Enzymatic conversion of polysaccharides into lower-molecular-weight, soluble oligosaccharides is dependent on the action of hydrolytic and oxidative enzymes. Polysaccharide monooxygenases (PMOs) use an oxidative mechanism to break the glycosidic bond of polymeric carbohydrates, thereby disrupting the crystalline packing and creating new chain ends for hydrolases to depolymerize and degrade recalcitrant polysaccharides. PMOs contain a mononuclear Cu(II) center that is directly involved in C-H bond hydroxylation. Molecular oxygen was the accepted cosubstrate utilized by this family of enzymes until a recent report indicated reactivity was dependent on H2O2 Reported here is a detailed analysis of PMO reactivity with H2O2 and O2, in conjunction with high-resolution MS measurements. The cosubstrate utilized by the enzyme is dependent on the assay conditions. PMOs will directly reduce O2 in the coupled hydroxylation of substrate (monooxygenase activity) and will also utilize H2O2 (peroxygenase activity) produced from the uncoupled reduction of O2 Both cosubstrates require Cu reduction to Cu(I), but the reaction with H2O2 leads to nonspecific oxidation of the polysaccharide that is consistent with the generation of a hydroxyl radical-based mechanism in Fenton-like chemistry, while the O2 reaction leads to regioselective substrate oxidation using an enzyme-bound Cu/O2 reactive intermediate. Moreover, H2O2 does not influence the ability of secretome from Neurospora crassa to degrade Avicel, providing evidence that molecular oxygen is a physiologically relevant cosubstrate for PMOs.

Conjugated Polymer Nanoparticle–Graphene Oxide Charge‐Transfer Complexes

AbstractThe game‐changing role of graphene oxide (GO) in tuning the excitonic behavior of conjugated polymer nanoparticles is described for the first time. This is demonstrated by using poly(3‐hexylthiophene) (P3HT) as a benchmark conjugated polymer and employing an in situ reprecipitation approach resulting in P3HT nanoparticles (P3HTNPs) with sizes of 50–100 nm in intimate contact with GO. During the self‐assembly process, GO changes the crystalline packing of P3HT chains in the forming P3HTNPs from H to H/J aggregates exhibiting exciton coupling constants as low as 2 meV, indicating favorable charge separation along the P3HT chains. Concomitantly, π–π interface interactions between the P3HTNPs and GO sheets are established resulting in the creation of P3HTNPs–GO charge‐transfer complexes whose energy bandgaps are lowered by up to 0.5 eV. Moreover, their optoelectronic properties, preestablished in the liquid phase, are retained when processed into thin films from the stable aqueous dispersions, thus eliminating the critical dependency on external processing parameters. These results can be transferred to other types of conjugated polymers. Combined with the possibility of employing water based “green” processing technologies, charge‐transfer complexes of conjugated polymer nanoparticles and GO open new pathways for the fabrication of improved optoelectronic thin film devices.

Bistetracene Thin Film Polymorphic Control to Unravel the Effect of Molecular Packing on Charge Transport

AbstractPolymorphism, the ability for a given material to adopt multiple crystalline packing states, is a powerful approach for investigating how changes in molecular packing influence charge transport within organic semiconductors. In this study, a new “thin film” polymorph of the high‐performance, p‐type small molecule N‐octyldiisopropylsilyl acetylene bistetracene (BT) is isolated and characterized. Structural changes in the BT films are monitored using static and in situ grazing‐incidence X‐ray diffraction. The diffraction data, combined with simulation and crystallographic refinement calculations, show the molecular packing of the “thin film” polymorph transforms from a slipped 1D π‐stacking motif to a highly oriented and crystalline film upon solvent vapor annealing with a 2D brick‐layer π‐stacking arrangement, similar to the so‐called “bulk” structure observed in single crystals. Charge transport is characterized as a function of vapor annealing, grain orientation, and temperature. Demonstrating that mobility increases by three orders of magnitude upon solvent vapor annealing and displays a differing temperature‐dependent mobility behavior.