Bimolecular Crystals Research Articles

Thermal decomposition behavior of CL-20 co-crystals

The thermal stability of bimolecular crystals 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazawurtzitane (CL-20) with 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), 1,4-dinitropipyrazine (DNPP), benzotrifuroxan (BTF), and 1-methyl-3,4,5-trinitro-1H-pyrazole (MTNP) has been studied using isothermal and non-isothermal kinetics methods. The studied co-crystals can be divided into two groups: in the case of co-crystallization of two nitramines 2CL-20/HMX and CL-20/DNPP, the molecular structure of the co-crystal does not undergo significant changes up to the decomposition temperature and the stability of the CL-20 does not change. In the case of co-crystallization of CL-20 with compounds of another class 2CL-20/MTNP and 2CL-20/BTF, the intermolecular interaction is weakened. Being more volatile and/or easily melted than CL-20, the second components (BTF and MTNP) of the co-crystals evaporate/melt when heated, leaving the CL-20 in an amorphous form, which leads to a decrease in its stability. Based on the reaction kinetic model, thermal hazard indicators such as adiabatic time to maximum rate (TMRad), and self-accelerating decomposition temperature (SADT) have been predicted to provide necessary safety information concerning the usage and storage of studied co-crystal. The combustion studies of bimolecular crystals showed that the burning CL-20/HMX and CL-20/DNPP co-crystals obey the mechanism with the leading reaction in the condensed phase. The burning rate of CL-20/BTF co-crystal is determined by reactions in the flame.

2D Inorganic Bimolecular Crystals with Strong In-Plane Anisotropy for Second-Order Nonlinear Optics.

2D inorganic bimolecular crystals, consisting of two different inorganic molecules, are expected to possess novel physical and chemical properties due to the synergistic effect of the individual components. However, 2D inorganic bimolecular crystals remain unexploited because of the difficulties in preparation arising from non-typical layered structures and intricate intermolecular interactions. Here, the synthesis of 2D inorganic bimolecular crystal SbI3 ·3S8 nanobelts via a facile vertical microspacing sublimation strategy is reported. The as-synthesized SbI3 ·3S8 nanobelts exhibit strong in-plane anisotropy of phonon vibrations and intramolecular vibrations as well as show anisotropic light absorption with a high dichroism ratio of 3.9. Furthermore, it is revealed that the second harmonic generation intensity of SbI3 ·3S8 nanobelts is highly dependent on the excitation wavelength and crystallographic orientation. This work can inspire the growth of more 2D inorganic bimolecular crystals and excite potential applications for bimolecular optoelectronic devices.

Real‐Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X‐ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl‐c61‐butyric acid methyl ester (PC60BM) and poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) named (pBTTT‐C14) from their 1,2‐orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive‐tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented.

Bimolecular crystal instability and morphology of bulk heterojunction blends in organic and perovskite solar cells

The in situ crystallization study reveals that size compatibility plays a key role in governing the formation of bimolecular crystals.

Combustion of CL-20 cocrystals

Combustion behavior, flame structure, and thermal decomposition of bimolecular crystals of hexanitrohexaazaisowurtzitane (CL-20) with glycerol triacetate (GTA), tris[1,2,5]oxadiazolo[3,4-b:3′,4′-d:3″,4″-f]azepine-7-amine (ATFAz), 4,4′′-dinitro-ter-furazan (BNTF), oxepino[2,3-c:4,5-c′:6,7-c′′]trisfurazan (OTF), oxepino[2,3-c:4,5-c′:6,7-c′′]trisfurazan-1-oxide (OTFO) have been studied by using a constant-pressure bomb, microthermocouple technique, and TG/DSC analysis. It has been found that the introduction of volatile and thermally stable compounds into the composition with CL-20 brought about unexpected results: first, the thermally stable component decreased the thermal stability of CL-20, and second, even twofold dilution of rapidly-burning CL-20 with slow-burning compound may practically does not change the burning rate of the former. It was suggested that a reason for the observed phenomena might be amorphous CL-20 which remains after evaporation of the volatile component in the combustion wave. The above assumption was confirmed by combustion modeling of bimolecular crystals in a wide pressure interval. It has been found that the combustion mechanism of the CL-20 cocrystals depends both on the burning rate of the second component and its volatility. Depending on these parameters, several combustion models of cocrystals are implemented: the burning rate is determined by (1) the CL-20 heat release kinetics at its boiling temperature, (2) the CL-20 heat release kinetics at the boiling point of the second component and (3) by the heat flow from the gas phase. It is abundantly clear that all these combustion mechanisms can be realized in real CL-20-based propellants.

Breaking the Bimolecular Crystal: The Effect of Side-Chain Length on Oligothiophene/Fullerene Intercalation

Polymer/fullerene bimolecular crystal formation has been investigated using a variety of conjugated polymers and fullerenes to understand the design rules that influence donor–acceptor interaction. Modifications of the polymer by varying the substitution side-chain position, density, and branching have demonstrated the importance of the “pocket” dimensions (free volume between side chains where the fullerene resides) for controlling intercalation. Yet the effect of pocket height has not been systematically explored because of the solubility limitations in polymers. In this report, we present an experimental investigation into the effect of the pocket height by synthesizing poly[2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene] dimers with varied side chain lengths and track the morphological changes of the dimer/fullerene blends using grazing-incidence X-ray scattering, thermal measurements, and photoluminescence quenching. We identify two regimes: (1) oligomers with side chains greater than or equal t...

Cubic‐Like Bimolecular Crystal Evolution and over 12% Efficiency in Halogen‐Free Ternary Solar Cells

AbstractIn this study, the solubility properties of a given ternary blend set, with two donors (poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl (PTB7‐Th) and benzo[1,2‐b;4,5‐b′]dithiophene‐based small molecule (DR3TSBDT)) and one acceptor ([6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM)), in a series of solvents are determined, and active material–solvent interactions are used as an aid for finding suitable nonchlorinated solvents to achieve effective ternary organic solar cells (OSCs) based on PTB7‐Th:DR3TSBDT:PC71BM. An exceptional power conversion efficiency (PCE) as high as 12.3% (certified 11.94%) is obtained using the developed nonhalogenated processing system. In‐depth investigations (morphology, charge mobility, recombination dynamics, and OSC characteristics) uncover the underlying structure–property relationships as a function of the chosen nonhalogenated systems. Another intriguing finding of this study is the formation of a cubic bimolecular crystal structure of PTB7‐Th:PC71BM in a nonhalogenated system, which is the first such demonstration in blend films. This sheds light upon the fact that the physical properties of a material applied from different solutions may surpass the variation in the properties between two material having totally different molecular structure. Therefore, this work not only offers important scientific insights into developing highly efficient and eco‐friendly OSCs but also improves our understanding of achievable bimolecular crystals with an intercalated structure.

Standard Enthalpy of Formation of the Bimolecular Crystal of CL-20 with Tris-Oxadiazolo-Azepine and Its Thermal Stability

The standard enthalpies of combustion and formation of 7H-tris([1,2,5]oxadiazolo) [3,4-b:3′,4′-d:3″,4″-f]azepine, its bimolecular crystal with the γ-polymorph of CL-20, and the γ-polymorph of CL-20 have been experimentally determined. The standard enthalpies of formation of the bimolecular crystal and an equimolecular mechanical mixture of γ-CL-20 with azepine differ by less than 12.8 kJ/mol. This small difference is validated by quantum chemical calculations. It has been experimentally found that the presence of azepine in the bimolecular crystal inhibits the thermal decomposition of γ-CL-20 and increases the thermal stability of γ-CL-20 in the bimolecular crystal as compared to original γ-CL-20.

Polymer:Fullerene Bimolecular Crystals for Near-Infrared Spectroscopic Photodetectors.

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and biointegrated electronics, is hampered by the use of bulky dispersive optics. Here, solution-processed organic donor-acceptor blends are employed in a resonant optical cavity device architecture for wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge-transfer states of the intercalating poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, near-infrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors, is fabricated by using a blade-coated PBTTT:PCBM thin film with a thickness gradient. As an application example, a measurement of the transmittance spectrum of water by this device is demonstrated.

Polymorphism of bimolecular crystals of CL-20 with tris[1,2,5]oxadiazolo[3,4-b:3´,4´-d:3″,4″-f]azepine-7-amine

New bimolecular crystals (BMCs) of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) with tris[1,2,5]oxadiazolo[3,4-b:3´,4´-d:3″,4″-f]azepine-7-amine (AZ2) were synthesized. Four different crystal structures — two polymorphic modifications with the CL-20: AZ2 ratio of 1: 1 (BMCs 1 and 2) and two polymorphic modifications with the ratio of 1: 2 (BMCs 3 and 4) — were obtained depending on the crystallization conditions. These crystals were studied by X-ray diffraction. The results of quantum chemical calculations indicate that the new η-conformation found in crystal 1 is not a stable conformer of CL-20 and its existence is attributed to the crystal packing effects in structure 1.

Structure of a bimolecular crystal of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane and methoxy-NNO-azoxymethane

The crystal structure of the bimolecular crystal of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and methoxy-NNO-azoxymethane (МАМ) (1:2) is studied. The CL-20 molecules adopt a ζ-conformation. The crystal structure is formed by layers of CL-20 and МАМ molecules, between which there are shortened NOδ–⋯Nδ+O contacts of the neighboring CL-20 and МАМ molecules.

Structure and properties of a bimolecular crystal (2CL-20 + MNO)

A novel bimolecular crystal (BMC) of CL-20 and N,N'-dimethyl-N,N'-dinitrooxamide (MNO) has been prepared. Molecular and crystal structures have been determined. Thermal stability has been studied with DSC technique.

Energetic potential of solid composite propellants based on CL-20-containing bimolecular crystals

The energetic potential of bimolecular crystals (BMCs) containing CL-20 as components of solid composite propellants (SCPs) was investigated. The experimental and calculated values of the standard enthalpies of formation are reported. The maximum heats of explosion, which correlate with the impact sensitivity, were calculated. The specific impulse values and the densities of SCPs based on an active binder, aluminum, and oxidizer (BMC + a small portion of ammonium dinitramide) were evaluated.

Real‐Time Investigation of Intercalation and Structure Evolution in Printed Polymer:Fullerene Bulk Heterojunction Thin Films

The complex intermixing morphology is critical for the performance of the nanostructured polymer:fullerene bulk heterojunction (BHJ) solar cells. Here, time resolved in situ grazing incidence X‐ray diffraction and grazing incidence small angle X‐ray scattering are used to track the structure formation of BHJ thin films formed from the donor polymer poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) with different fullerene derivative acceptors. The formation of stable bimolecular crystals through the intercalation of fullerene molecules between the side chains of polymer crystallites is investigated. Such systems exhibit more efficient exciton dissociation but lower photo‐conductance and faster decay of charges. On the basis of the experimental observations, intercalation obviously takes place before or with the formation of the crystalline polymer domains. It results in more stable structures whose volume remains constant upon further drying. Three distinct periods of drying are observed and the formation of unidimensional fullerene channels along the π‐stacking direction of polymer crystallites is confirmed.

Preparation, structure, and main properties of bimolecular crystals CL-20—DNP and CL-20—DNG

New bimolecular crystals of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) with 2,4-dinitro-2,4-diazapentane (DNP) and 2,4-dinitro-2,4-diazaheptane (DNG) were obtained in a ratio of 2 : 1 and 1 : 1, respectively. Their X-ray diffraction analyses were carried out. The thermal stability, detonation characteristics, and sensitivity to friction of bimolecular crystal of CL-20 with DNP (CLD) were studied. The detonation characteristics of bimolecular crystal CLD are comparable with the properties of the known bimolecular crystal CL-20—HMX (2 : 1).

Bimolecular crystals with an intercalated structure improve poly(p-phenylenevinylene)-based organic photovoltaic cells.

The exciton dissociation, recombination, and charge transport of bulk heterojunction organic photovoltaic cells (OPVs) is influenced strongly by the nanomorphology of the blend, such as the grain size and the molecular packing. Although it is well known that polymers based on amorphous poly(p-phenylenevinylene) (PPV) have a fundamental limit to their efficiency because of low carrier mobility, which leads to increased recombination and unbalanced charge extraction, herein, we demonstrate that the issue can be overcome by forming bimolecular crystals of an amorphous PPV-based polymer:phenyl-C61 -butyric acid methyl ester (PCBM) intercalated structure. We used amorphous poly(2,5-dioctyloxy-p-phenylene vinylene-alt-2',5'-thienylene vinylene) (PPVTV), which has a simple chemical structure. A reasonably high power conversion efficiency (∼3.5 %) was obtained, although the material has an intrinsically amorphous structure and a relatively large band gap (2.0 eV). We demonstrate a correlation between a well-ordered bimolecular crystal of PPVTV:PCBM and an improved hole mobility of a PPVTV:PCBM film compared to a pristine PPVTV film by using 2 D grazing incidence XRD and space-charge-limited current measurements. Furthermore, we show that the bimolecular crystal structure in high-performance OPVs is related to an optimum molecular packing, which is influenced by the PPVTV:PCBM blending ratio, side-chain length, and molecular weight of the PPVTV polymer. Improved charge transport in PPVTV:PCBM bimolecular crystals leads to a fast sweep out of charges and thus suppression of nongeminate recombination under the operating conditions.

Spatially Resolving Ordered and Disordered Conformers and Photocurrent Generation in Intercalated Conjugated Polymer/Fullerene Blend Solar Cells.

Resonance Raman spectroscopy was used to identify ordered and disordered conformers of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) blended with the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in bulk heterojunction (BHJ) solar cells where PCBM intercalates into PBTTT side groups. We show that the PBTTT thiophene ring symmetric C=C stretching mode consists of contributions from ordered (ℏωC=C = 1489 cm–1, fwhm ∼ 15 cm–1) and disordered (ℏωC=C = 1500 cm–1, fwhm ∼ 25 cm–1) components and their relative amounts are sensitive to PCBM loading, annealing and excitation energy. The 1500 cm–1 PBTTT component originates from twisted thiophene rings and disordered side groups due to PCBM intercalation in a mixed kinetic phase and thermal annealing promotes ordering of PBTTT chains from the formation of bimolecular PBTTT/PCBM crystals. Density functional theory (DFT) Raman simulations of PBTTT monomers support these assignments. Resonance Raman images of annealed PBTTT/PCBM model solar cells confirm that ordered PBTTT chains are most concentrated in PCBM-rich bimolecular crystals and corresponding intensity modulated photocurrent spectroscopy (IMPS) and imaging measurements show increased nongeminate charge recombination at the boundaries of ordered/disordered regions.

The Crucial Influence of Fullerene Phases on Photogeneration in Organic Bulk Heterojunction Solar Cells

The conjugated polymer, poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (pBTTT‐C16), allows a systematic tuning of the blend morphology by varying the acceptor type and fraction, making it a well‐suited structural model for studying the fundamental processes in organic bulk heterojunction solar cells. To analyze the role of intercalated and pure fullerene domains on charge carrier photogeneration, time delayed collection field (TDCF) measurements and Fourier‐transform photocurrent spectroscopy (FTPS) are performed on pBTTT‐C16:[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) solar cells with various stoichiometries. A weak influence of excess photon energy on photogeneration along with a photogeneration having a weaker field dependence at increasing fullerene loading is found. The findings are assigned to a dissociation via thermalized charge transfer (CT) states supported by an enhanced electron delocalization along spatially extended PC61BM nanophases that form in addition to a bimolecular crystal (BMC) for PC61BM rich blends. The highly efficient transfer of charge carriers from the BMC into the pure domains are studied further by TDCF measurements performed on non‐intercalated pBTTT‐C16:bisPC61BM blends. They reveal a field dependent charge generation similar to the 1:4 PC61BM blend, demonstrating that the presence of pure acceptor phases is the major driving force for an efficient, field independent CT dissociation.

Crystal structure of cocrystals 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo [5.5.0.05.9.03.11]dodecane with 7H-tris-1,2,5-oxadiazolo (3,4-b:3′,4′-d:3″,4″-f) azepine

The crystal structure of the bimolecular crystal of CL-20 with new promising highly energetic compound 7H-tris-1,2,5-oxadiazolo[3,4-b:3′,4′-d:3″,4″-f] azepine is obtained and studied.

Intrinsic charge transport in single crystals of organic molecular semiconductors: A theoretical perspective

Abstract