Molecular Packing Structure Research Articles

Solution processed uniaxially oriented thin film of tert-butyl-substituted phthalocyanine

Uniaxially orientated thin film of phthalocyanine with bulky tert-butyl substituents (t-BuPcH2) was fabricated by bar-coating method. The thin film exhibited high dichroic ratio of more than 50 in the polarized absorption. Molecular packing structure of t-BuPcH2 in the thin film was suggested by using the results of XRD measurements, in which the molecular plane of t-BuPcH2 was determined to be perpendicular to the substrate and bar-sweep direction. In addition, the X-ray rocking curve measurements demonstrated that t-BuPcH2 possessed highly molecular orientation in the thin film fabricated by bar-coating method.

π-π Stacking Distance and Phase Separation Controlled Efficiency in Stable All-Polymer Solar Cells.

The morphology of the active layer plays a crucial role in determining device performance and stability for organic solar cells. All-polymer solar cells (All-PSCs), showing robust and stable morphologies, have been proven to give better thermal stability than their fullerene counterparts. However, outstanding thermal stability is not always the case for polymer blends, and the limiting factors responsible for the poor thermal stability in some All-PSCs, and how to obtain higher efficiency without losing stability, still remain unclear. By studying the morphology of poly [2,3-bis (3-octyloxyphenyl) quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl](TQ1)/poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]] (PCE10)/PNDI-T10 blend systems, we found that the rearranged molecular packing structure and phase separation were mainly responsible for the poor thermal stability in devices containing PCE10. The TQ1/PNDI-T10 devices exhibited an improved PCE with a decreased π–π stacking distance after thermal annealing; PCE10/PNDI-T10 devices showed a better pristine PCE, however, thermal annealing induced the increased π–π stacking distance and thus inferior hole conductivity, leading to a decreased PCE. Thus, a maximum PCE could be achieved in a TQ1/PCE10/PNDI-T10 (1/1/1) ternary system after thermal annealing resulting from their favorable molecular interaction and the trade-off of molecular packing structure variations between TQ1 and PCE10. This indicates that a route to efficient and thermal stable All-PSCs can be achieved in a ternary blend by using material with excellent pristine efficiency, combined with another material showing improved efficiency under thermal annealing.

Polymer Pre‐Aggregation Enables Optimal Morphology and High Performance in All‐Polymer Solar Cells

Herein, all‐polymer solar cells (all‐PSCs) are studied based on PTzBI:N2200 system processed from two different solvents, chlorobenzene (CB) and 2‐methyltetrahydrofuran (Me‐THF). It is found that the preaggregation of the donor and acceptor polymers in Me‐THF is the key factor that enables a drastic enhancement in cell efficiency from ≈1% (processed by CB) to ≈11% (processed by Me‐THF). When using CB as the solvent, both donor and acceptor polymers are well dissolved and mostly disaggregated. In contrast, the donor and acceptor polymers both exhibit strong aggregation in Me‐THF. As a result, the donor and acceptor blend films processed from Me‐THF exhibit pure domains with appropriate molecular packing structure, which leads to high charge mobilities (10−3–10−4 cm2 V−1 s−1) and fill factors (FFs; 75%), whereas the blend films processed by CB suffer from highly miscible and impure domains, hence decreasing the charge mobilities by 1–2 orders of magnitude compared with those of the corresponding pure films. The current work reveals that the polymer preaggregation is the key reason enabling optimal morphology and high performance in N2200‐based all‐PSCs, and this strategy may be potentially applied in other systems to optimize the morphology and performance of all‐PSCs.

Acene-Modified Small-Molecule Donors for Organic Photovoltaics.

A series of acene-modified small molecules have been designed and synthesized, and their photovoltaic characteristics were studied by using the small molecules in organic photovoltaics (OPVs). Different cores were introduced to modulate the conjugation lengths of the small molecules and the bulk heterojunction (BHJ) morphologies. Three small-molecule donors were prepared, namely Ph-TTR, Na-TTR, and An-TTR, which have phenyl, naphthalene, and anthracene moieties, respectively, as conjugated cores. These donors were synthesized in a few steps and exhibited favorable BHJ morphologies, thereby giving promising power conversion efficiencies (PCEs). The donors showed excellent miscibility with the acceptor PC71 BM, and the use of the additive 1,8-diiodooctane (DIO) led to a remarkable increase in crystallinity, thereby increasing the PCEs of their OPVs. Of the three donors, Na-TTR showed the most efficient charge carrier generation and favorable molecular packing structures; hence, of the three types of devices tested, the Na-TTR:PC71 BM devices exhibited the highest PCE, specifically 6.27 %, without pre- or post-treatments. The promising PCEs achieved from these easily synthesized acene-modified small molecules suggested that acene-modified small molecules can be useful materials in OPVs.

Impact of Graphene Work Function on the Electronic Structures at the Interface between Graphene and Organic Molecules.

The impact of graphene work function (WF) on the electronic structure at the graphene/organic interface has been investigated. WF manipulation of graphene is realized using self-assembled monolayers (SAMs) with different end groups. With this method, the upper surface of the functionalized graphene remains intact, and thus precludes changes of molecular orientation and packing structures of subsequently deposited active materials. The WF of NH2-SAM functionalized graphene is ~3.90 eV. On the other hand, the WF of graphene increases to ~5.38 eV on F-SAM. By tuning the WF of graphene, an upward band bending is found at the ZnPc/graphene interface on F-SAM. At the interface between C60 and NH2-SAM modified graphene, a downward band bending is observed.

Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells.

Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives often-but not always-exhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.

Investigation of the free volume characteristics of PEO based solid state polymer electrolyte by means of positron annihilation spectroscopy

Ion conduction in poly (ethylene oxide), PEO, based electrolytes are explained considering the ion movement coupled with polymer segmental motions or free volume. Herein, we describe the application of positron annihilation spectroscopy to investigate the free volume structure of PEO-LiTFSI solid state electrolyte with varying electrolyte concentration. Addition of LiTFSI indicates plasticization effect through the increase in free volume size of polymer matrix. Depth dependent Doppler broadening measurements have shown a decrease in positron/positronium diffusion length and S-parameter value on addition of electrolyte. The observed decrease in S-parameter and positronium intensity could not be ascribed to positronium reaction (inhibition) with dissociated ions or salt aggregates as the variation does not follow the established inhibition expression. The SW correlation curves indicate that the chemical environment at positron/positronium annihilation sites in PEO-LiTFSI is similar to pristine PEO. It again confirms that reduction in positronium intensity and S-parameter cannot be attributed to positronium inhibition. The present study shows that resultant positron/positronium annihilation parameters can be directly attributed to the change in molecular packing or free volume structure of the solid state electrolytes.

Remarkable photo-induced crystal transition accompanying with room temperature phosphorescence change

Limited works are reported about photochromism type room temperature phosphorescence luminogens (RTPs) owing to notorious quenching effect and lacking of reliable guidance. Here we have demonstrated an organic crystal (4-iodophenyl terpyridine) exhibiting photochromism and slowly spontaneous heat recovery. The photochromic crystal can be formed within a few minutes, and it once formed is stable for hours. Fortunately, the single crystals before and after photochromism are all analyzable and clearly indicate molecular conformation and packing structure change. It is deduced that photoactivation has induced single bond internal rotations and intermolecular slipping to form a metastable crystal with more planar molecular conformation and looser stacking. The different molecular conformation and packing structure have changed intermolecular interaction nature, which should be responsible for their different RTP properties.

Nanostructured assemblies of liquid-crystalline supermolecules: from display to medicine

ABSTRACTLiquid crystal oligomers have a supermolecular structure in which two or more mesogenic units are interconnected through flexible spacers. They can form various molecular packing structures in their liquid-crystalline phases. We present flexible LC oligomers stabilising optically isotropic phases, i.e. blue phases (BPs) and chiral conglomerate phases. Hysteresis free switching, high contrast and wide viewing angle are obtainable in the amorphous BPIII without surface treatment. Spontaneous mirror symmetry breaking in layered phases of achiral flexible linear trimers produces periodic nanostructured surfaces. Then we demonstrate the effects of liquid-crystallinity on anticancer activities. Biological systems have links with liquid crystallinity. We found a molecular assembly of mesogenic molecules possessing an active site as a novel therapeutic approach against solid cancer cells.

Molecular Orientation of Polymer Acceptor Dominates Open-Circuit Voltage Losses in All-Polymer Solar Cells

Low open-circuit voltage (Voc) induced by energy loss in organic solar cells is considered to be one of the most influencing factors limiting device performance, in which morphology of the active layer plays a crucial role in determining energy loss. By employing a bilayer structure of the P3HT:N2200 all-polymer system, we have identified the isolated impact of a molecular packing structure on device Voc with analysis of energy loss processes. Thermal annealing and various solvents were used to control molecular orientation in P3HT:N2200 bilayer devices, in which different Voc spanning from 0.45 to 0.54 V could be obtained. It was found that energy of charge-transfer state (Ect) differed in these bilayer devices. Besides, increased charge recombination could be observed in bilayer devices when N2200 layers exhibited face-on orientation, which caused an additional energy loss and decreased Voc. Our results suggest that rational control of polymer molecular orientation is essential to reduce the energy loss...

Homo/hetero-epitaxial growth in tetrabenzotriazaporphyrin derivative thin film fabricated by contact freezing method with seed crystal

Molecularly oriented thin films of 1,4,8,11,15,18,22,25-octahexyltetrabenzotriazaporphyrin (C6TBTAPH2) have been fabricated by the contact freezing method with a seed crystal. By dropping a seed crystal of C6TBTAPH2 or the analog molecule, 1,4,8,11,15,18,22,25-octahexylphthalocyanine, onto the C6TBTAPH2 thin film at supercooled liquid crystal state, the homo- or hetero-epitaxial growth spontaneously occurred, respectively, and the C6TBTAPH2 thin films with the molecular packing structure obeying the seed crystal were selectively obtained. The different crystal structures in the thin films were confirmed by grazing-incidence wide-angle X-ray scattering technique.

A Polarity-adjustable Columnar Liquid Crystalline Compound by Intermittent Voltage Application

A novel bisurea compound showed columnar liquid crystal phases possessing a new functionality, polarity-adjustability. Its molecular packing structures were analyzed by X-ray diffraction (XRD), and...

Polymorphic self-assembly of pyrazine-based tectons at the solution-solid interface.

Exploring the surface self-assembly of small molecules that act as building blocks (tectons) for complex supramolecular structures is crucial for realizing surface-supported functional molecular devices. Here, we report on the synthesis and surface self-assembly of a new pyrazine-derived molecule with pyridine pendants. Ambient scanning tunneling microscopy investigation at the solution–solid interface reveals polymorphic self-assembly of these molecules on a HOPG substrate. Two different molecular packing structures with equal distribution are observed. Detailed analysis of the STM images emphasizes the crucial role of weak intermolecular hydrogen bonding, and molecule–substrate interactions in the formation of the observed polymorphs. Such weak hydrogen bonding interactions are highly desirable for the formation of modular supramolecular architectures since they can provide sufficiently robust molecular structures and also facilitate error correction.

Hydrophobicity and CH/π-interaction-driven self-assembly of amphiphilic aromatic hydrocarbons into nanosheets.

The hydrophobicity and CH/π-interaction-driven self-assembly of an amphiphile that contains a biphenylanthracene group furnishes two-dimensional aggregates in dilute aqueous solution. A windmill-shaped molecular packing structure that arises from multiple intermolecular CH/π interactions of the aromatic hydrocarbons is the key motif for the self-assembly into micrometer-scale nanosheets.

Investigation of hydrogen-bonding mediated molecular packing of diketopyrrolopyrrole based donor-acceptor oligomers in the solid state

Two diketopyrrolopyrrole (DPP)-based donor-acceptor oligomers with thermal labile 2-methylhexyl-2-oxylcarbonyl (mHoc) groups, mHocPBT and mHocTBT, were synthesized and characterized to study the hydrogen-bonding mediated molecular packing in solid state. The soluble oligomers can be converted to strong intermolecular hydrogen-bonded and pigment-like materials HPBT and HTBT by annealing the casted solid film at a temperature between 180 and 200 °C. The formation of intermolecular hydrogen bonding significantly altered the conformation of the molecules, molecular packing structure and related optical properties of the oligomers, with HPBT molecule exhibiting bathochromic shift and HTBT molecule exhibiting hypsochromic shift on UV/Vis spectra. Spin-casted mHocTBT and HTBT films also exhibited interesting morphology and charge transport properties. Even though the single mHocTBT oligomer molecule has a better coplanar molecular conformation as compared with that of HTBT, an enhanced π-π stacking structure and larger crystal grain size rendered HTBT film better charge transport performance.

Solvent Effects on the Structural Packing and Spin-Crossover Properties of a Mononuclear Iron(II) Complex

Solvent molecules are known to have a remarkable effect on the crystal structures and magnetic properties of spin-crossover (SCO) compounds. On the basis of our previous works on this topic, we have synthesized a series of SCO Fe(II) compounds, [Fe(tpa)(NCSe)2]·Solv (tpa = tris(2-pyridylmethyl)amine; Solv = 0.5CH3OH for 1·CH3OH, 0.5C2H5OH for 1·C2H5OH, 0.25H2O for 1·H2O, 0.5CH3CN for 1·CH3CN-A, CH3CN for 1·CH3CN-B, and CH2Cl2 for 1·CH2Cl2), by crystallization of the molecular complex [Fe(tpa)(NCSe)2] from the respective solvents. Single-crystal X-ray crystallographic studies show that the molecular packing structures and intermolecular interactions of these compounds are subtly changed by the lattice solvent molecules; thus, their SCO properties can be differentiated from each other. All of the solvated compounds undergo one-step SCO behavior with the order of critical temperatures being Tc(1·CH3CN-B) < Tc(1·CH2Cl2) < Tc(1·CH3CN-A) < Tc(1·C2H5OH) ≈ Tc(1·CH3OH) < Tc(1·H2O), of which thermal hysteresis loop...

Direct observation of van der Waals two-dimensional crystal and small clusters of methane on copper surfaces

Methane is the simplest hydrocarbon and its aggregation structures are determined merely by van der Waals (vdW) interaction. Here we present the direct observation of a vdW two-dimensional crystal and small clusters of methane on Cu surfaces using a low-temperature scanning tunneling microscope. Methane shows a hexagonal molecular-packing structure on the Cu(110) and Cu(111) surfaces. However, methane preferentially forms a dimer and a trimer on the $\mathrm{Cu}(110)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)\text{-O}$ added-row structure, indicating different aggregation behavior from the bare Cu surfaces. The adsorption structures are examined in detail by density functional theory calculations, providing atomistic insights into the adsorption geometry and aggregation mechanism of methane on the Cu surfaces, which are governed by a subtle balance between the intermolecular and molecule--surface interactions.

Collagen-the Ultrastructural Element of the Bone Matrix

There is an increased interest and more studies highlight the fact that bone strength depends not only on bone tissue quantity, but also on its quality, which is characterized by the geometry and shape of bones, trabecular bone microarchitecture, mineral content, organic matrix and bone turnover. Fibrillar type I collagen is the major organic component of bone matrix, providing form and a stable template for mineralization. The biomedical importance of collagen as a biomaterial for medical and cosmetic purposes and the improvement of the molecular, cellular biology and analytical technologies, led to increasing interest in establishing the structure of this protein and in setting of the relationships between sequence, structure, and function. Bone collagen crosslinking chemistry and its molecular packing structure are considered to be distinct features. This unique post-translational modifications provide to the fibrillar collagen matrices properties such as tensile strength and viscoelasticity. Understanding the complex structure of bone type I collagen as well as the dynamic nature of bone tissues will help to manage new therapeutic approaches to bone diseases.

Homeotropic alignment of non-peripheral octahexyl phthalocyanine in thin film and its photovoltaic properties

The homeotropic alignment of non-peripherally hexyl-substituted phthalocyanine (C6PcH2) molecules in thin films has been investigated. A spin-coated film C6PcH2 was covered with a poly(vinyl phenol) layer, and a thermal treatment through a liquid crystalline phase was carried out. The molecular packing structure and induced homeotropic alignment in the thin films were clarified by the grazing incidence wide-angle X-ray scattering technique. The bulk heterojunction solar cell with the homeotropically aligned C6PcH2 was fabricated, and the photovoltaic properties were discussed by taking the optical properties and carrier mobility in the active layer into account.

Kinetically Controlled Polymorphic Superstructures of Pyrene-Based Asymmetric Liquid Crystal Dendron: Relationship Between Hierarchical Superstructures and Photophysical Properties.

To understand the relationship between kinetically controlled hierarchical superstructures and photophysical properties, pyrene-based asymmetric liquid crystal (LC) dendrons (abbreviated as PD) were newly synthesized by covalently attaching a pyrene moiety (P) at a biphenyl-based LC dendritic group (D). The phase transition behavior of PD has been systematically studied with a combined technique of thermal analysis, microscopy, spectroscopy, and scattering analysis. PD formed two different crystalline structures depending on the cooling rate: a stable crystalline phase (Ks , slow cooling) and a metastable crystalline phase (Kms , quenching). The kinetically controlled molecular packing structures of PD depend on the competition and cooperation of intermolecular physical interactions with nanophase separation. Upon slow cooling, the PD dimer formed by intermoelcular H-bonding constructed a layered hierarchical structure with the help of nanophase separation. Owing to the strong π-π stacking (J-aggregation) with weak H-bondings, the PD dimer in the layer was slightly tilted to give a monoclinic layered structure with a periodic layer d-spacing of 6.6 nm. In contrast, the metastable Kms phase formed by the quenching process showed a significant tilt of the PD dimer in the layer (d-spacing=4.4 nm) due to the weak π-π stacking (H-aggregation) and the strong H-bondings.