Organic Cocrystals Research Articles

Crystallization and phase transitions of C6H6:C6F6 complex under extreme conditions using laser-driven shock

The C6H6:C6F6 cocrystal is one of the simplest organic cocrystals with a molecule having a C–F bond and without any hydrogen bonding. It has a crystal structure very different from its constituents, C6H6 and C6F6, and its higher melting point indicates its increased stability relative to these two materials. So far, no studies are available on the phase transitions of this interesting adduct under dynamic compression. In this study, we present the findings of phase transitions of an equimolar mixture of C6H6:C6F6 observed under rapid shock compression at pressures of up to 4.15 GPa using time-resolved Raman spectroscopy. The compression is driven by a 2 J Nd:YAG laser with an 8 ns pulse length. Four prominent modes at 370 cm−1 (ν10F mode), 443 cm−1 (ν6F mode), 560 cm−1 (ν1F mode), and 991 cm−1 (ν1H mode) exhibit a blue shift with scaling factors of 2.41, 2.26, 2.39, and 2.67 cm−1/GPa, respectively. The liquid → solid-I phase transition is observed at around 0.49 GPa shock pressure. The second phase transition from solid-I → solid-VI is observed between 1.32 and 2.60 GPa, and no signature of the solid-V phase is observed unlike in the case of static compression[Wang et al., J. Phys. Chem. C 120, 29510 (2016)]. Another phase transition solid-VI → solid-VII is observed between 3.9 and 4.15 GPa. The shock velocities in the sample at two laser intensities, 1.47 GW/cm2 (300 mJ) and 2.46 GW/cm2 (500 mJ), are calculated by measuring the intensity ratio of Raman modes emerging from the shocked region to that of the whole sample and are 3.13 and 4.05 km/s, respectively. To compare with the experimental results, 1D radiation hydrodynamics simulations are also performed. The experimental and simulated shock velocities are in good agreement. The mode Grüneisen parameter for the ν1H, ν1F, ν6F, ν10F, and ν10' F modes are γi = 0.011(2), 0.022(2), 0.011(1), 0.024(3), and 0.379(14), respectively.

Two New Organic Co-Crystals Based on Acetamidophenol Molecules

Herein we present two new organic co-crystals obtained through a simple solution growth process based on an acetamidophenol molecule, either paracetamol or metacetamol, and on 7,7,8,8-tetracyanoquinodimethane (TCNQ). These co-crystals are part of a family of potential organic charge transfer complexes, where the acetamidophenol molecule behaves as an electron donor and TCNQ behaves as an electron acceptor. Due to the sub-micron size of the crystalline domains, 3D electron diffraction was employed for the structure characterization of both systems. Paracetamol-TCNQ structure was solved by standard direct methods, while the analysis of metacetamol-TCNQ was complicated by the low resolution of the available diffraction data and by the low symmetry of the system. The structure determination of metacetamol-TCNQ was eventually achieved after merging two data sets and combining direct methods with simulated annealing. Our study reveals that both paracetamol-TCNQ and metacetamol-TCNQ systems crystallize in a 1:1 stoichiometry, assembling in a mixed-stack configuration and adopting a non-centrosymmetric P1 symmetry. It appears that paracetamol and metacetamol do not form a strong structural scaffold based on hydrogen bonding, as previously observed for orthocetamol-TCNQ and orthocetamol-TCNB (1,2,4,5-tetracyanobenzene) co-crystals.

Evaluating Hydrogen Bonding in Organic Cocrystals Using Low-Frequency Raman Vibrational Spectroscopy and Quantum Mechanical Simulations

Cocrystallization can provide a potential route to usability for active pharmaceutical ingredients that are eliminated in the drug discovery process due to their low bioavailability. In this work, cocrystals of urea and thiourea with glutaric acid and tartaric acid were used as model systems to experimentally and computationally investigate the intermolecular energy factors within heterogeneous molecular crystals. The tools employed in this study were low-frequency Raman vibrational spectroscopy and solid-state density functional theory (ss-DFT). The sub-200 cm–1 Raman spectra give insights into vibrations that are characteristic of the crystal packing and the intermolecular forces within the samples. ss-DFT allows for the analysis of these vibrations and of the specific energies involved in the collective cocrystal. Moreover, ss-DFT permits the computational investigation of hypothetical cocrystals, utilized here to predict the properties of the unrealized thiourea:dl-tartaric acid cocrystal. These analyses demonstrated that it is both experimentally and computationally favorable for the urea and thiourea glutaric acid cocrystals to form, as well as the urea:dl-tartaric acid cocrystal, when compared to the crystallization of the pure component materials. However, changes in the hydrogen bonding network yield a thiourea:dl-tartaric acid cocrystal that corresponds to an energetic minimum on the potential energy surface but has a Gibbs free energy that prevents it from experimental formation under ambient conditions.

Segregated Array Tailoring Charge-Transfer Degree of Organic Cocrystal for the Efficient Near-Infrared Emission beyond 760nm.

Harvesting the narrow bandgap excitons of charge-transfer (CT) complexes for the achievement of near-infrared (NIR) emission has attracted intensive attention for its fundamental importance and practical application. Herein, the triphenylene (TP)-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 TCNQ) CT organic complex is designed and fabricated via the supramolecular self-assembly process, which demonstrates the NIR emission with a maximum peak of 770nm and a photoluminescence quantum yield (PLQY) of 5.4%. The segregated stacking mode of TP-F4 TCNQ CT complex based on the multiple types of intermolecular interaction has a low CT degree of 0.00103 and a small counter pitch angle of 40° between F4 TCNQ and TP molecules, which breaks the forbidden electronic transitions of CT state, resulting in the effective NIR emission. Acting as the promising candidates for the active optical waveguide in the NIR region beyond 760nm, the self-assembled TP-F4 TCNQ single-crystalline organic microwires display an ultralow optical-loss coefficient of 0.060dBµm-1 . This work holds considerable insights for the exploration of novel NIR-emissive organic materials via an universal "cocrystal engineering" strategy.

Organic Cocrystals of TCNQ and TCNB Based on an Orthocetamol Backbone Solved by Three-Dimensional Electron Diffraction

Organic Cocrystals of TCNQ and TCNB Based on an Orthocetamol Backbone Solved by Three-Dimensional Electron Diffraction

Charge transfer and hydrogen bonding motifs in organic cocrystals derived from aromatic diamines and TCNB

Herein, we have successfully fabricated two Charge transfer (CT) cocrystals 1 and 2 using 1,2,4,5-tetracyanobenzene (TCNB) as an acceptor with aromatic diamine (o-phenylenediamine and benzidine) as hydrogen bond (H-bonds) donor compounds. These binary compounds were characterized comprehensively by Single Crystal X-ray diffraction (SCXRD), powder diffraction, thermal, and spectroscopic analysis (Infrared and UV–vis spectroscopy) in the solid state. CT π···π interactions help to achieve the binary (1:1) mixed staking (DADA) supramolecular self-assembly while the dominant N−H··N and C−H··N H-bonds are exclusively involved in accomplishing the 2D structure as evidenced by the single-crystal analysis in these two cocrystals. Owing to the strong absorption in the near-infrared region (NIR) these materials appear black by virtue of charge transfer which is a fascinating phenomenon in TCNB based complexes. Not to mention, Hirshfeld surface analyses and density functional theory (DFT) were performed to assess the contribution of different intermolecular interactions and HOMO-LUMO energy levels. The present study will help to expand the understanding of non-covalent interactions in these hybrid materials which may further expand the library of functional materials that can find fascinating applications in the future.

The competitive role of C–H⋯X (X = F, O) and π–π interactions in contributing to the degree of charge transfer in organic cocrystals: a case study of heteroatom-free donors with p-fluoranil (FA)

Four binary organic charge transfer cocrystals were grown by the slow cooling method. The competitive role of C–H⋯X (X = F, O) and π–π interactions in contributing to the degree of charge transfer in the cocrystals was investigated.

The organic co-crystals formed using naphthalenediimide-based triangular macrocycles and coronene: intermolecular charge transfers and nonlinear optical properties.

Formation of organic co-crystals is an effective strategy to synthesize near infrared emission and nonlinear optical (NLO) materials, which often show "1 + 1 > 2" performance. Moreover, the crystallization process can be effectively regulated through supramolecular interactions; thus the properties of co-crystal materials can also be flexibly regulated. Here, in order to further understand the nature and formation mechanism of co-crystals from the perspective of theoretical research, we studied the structures, intermolecular interactions, absorption spectra, charge transfer (CT) characteristics and nonlinear optical (NLO) properties of the newly synthesized organic co-crystals formed between naphthalenediimide based triangles (NDI, acceptor) and coronene (COR, donor). According to the analysis of decomposition of intermolecular interaction energy, dispersion energy played a major role, so the co-crystal properties can be regulated by regulating the intermolecular dispersion energy. More importantly, the formation of co-crystals NDI-COR and NDI-2COR reduced the Egap values with respect to those of their components. And there was significant intermolecular CT from COR to NDI and the degree of CT in NDI-COR was larger than that in NDI-2COR, so that the αtot and γtot values of NDI-COR and NDI-2COR were significantly greater than those of their components. Thus, the NLO properties of organic co-crystals can be further improved by enhancing the electron-donating ability of the donor and the electron-withdrawing ability of the acceptor to enhance the degree of intermolecular interaction energy and CT.

Cooperative non-covalent interactions and synthetic feed as driving forces to structural diversity within organic co-crystals containing isosteric perhalobenzenes

Structural diversity is achieved in a series of halogen-bonded co-crystals based upon cooperative non-covalent interactions and the synthetic feed of the components.

Cocrystallization tailoring radiative decay pathways for thermally activated delayed fluorescence and room-temperature phosphorescence emission.

Modulation of excited-state processes in binary organic cocrystals has been rarely explored so far. Here, we develop two charge-transfer (CT) cocrystal microrods with a 1 : 1 stoichiometric ratio where halogenated dibenzothiophene (DBT) compounds act as π-electron donors and 1,2,4,5-tetracyanobenzene (TCNB) acts as an acceptor. Unexpectedly, the cocrystal containing one bromine (Br) atom at the 3-position of DBT (3-BrTC) presents thermally activated delayed fluorescence (TADF), while the other one comprising one Br atom at the 4-position of DBT (4-BrTC) exhibits both TADF and room-temperature phosphorescence (RTP). Experimental and theoretical calculation results reveal that CT interactions in 3- and 4-BrTC decrease the S1-T2 energy gap, whereas abundant lone-pair electrons from the Br atom in 4-BrTC facilitate the n → π* transition. As a consequence, single TADF and dual-emissive TADF/RTP were realized, respectively. The present work offers wonderful insight into the effect of molecular structures on the excited-state pathways of organic CT cocrystals.

Growth direction dependent separate-channel charge transport in the organic weak charge-transfer co-crystal of anthracene-DTTCNQ.

Co-crystallization is an efficient way of molecular crystal engineering to tune the electronic properties of organic semiconductors. In this work, we synthesized anthracene-4,8-bis(dicyanomethylene)4,8-dihydrobenzo[1,2-b:4,5-b']-dithiophene (DTTCNQ) single crystals as a template to study the crystal growth direction dependent charge transport properties and attempted to elucidate the mechanism by proposing a separate-channel charge transport model. Single-crystal anthracene-DTTCNQ field-effect transistors showed that ambipolar transport properties could be observed in all crystal growth directions. Furthermore, upon changing the measured crystal directions, the electronic properties experienced a weak change from n-type dominated ambipolar, balanced ambipolar, to p-type dominated ambipolar properties. The theoretical calculations at density functional theory (DFT) and higher theory levels suggested that the anthracene-DTTCNQ co-crystal motif was a weak charge-transfer complex, in line with the experiment. Furthermore, the detailed theoretical analysis also indicated that electron or hole transport properties originated from separated channels formed by DTTCNQ or anthracene molecules. We thus proposed a novel separate-channel transport mechanism to support additional theoretical analysis and calculations. The joint experimental and theoretical efforts in this work suggest that the engineering of co-crystallization of weak charge-transfer complexes can be a practical approach for achieving tuneable ambipolar charge transport properties by the rational choice of co-crystal formers.

Formation of organic ion cocrystals, phase transition and ion conduction

Organic ionic plastic crystals (OIPCs) have been increasingly drawing attention recently owing to their many practical applications.

Four strategies towards customized rainbow/white light emission from a series of organic charge-transfer cocrystals and their heterostructures

Four progressive strategies were proposed based on a series of structurally isomorphic charge-transfer cocrystals for a broad and fine tuning of luminescence towards rainbow/white light emission in crystals.

An organic cocrystal based on phthalocyanine with ideal packing mode towards high-performance ambipolar property

The identical macrocyclic pair promotes desired packing mode with tight and large-overlapped π–π stacking in phthalocyanine-based cocrystal, which ensures high-performance ambipolar property.

A controllable and defectless cutting postprocess method via cleavage of an elastic cocrystal based on pyrene and tetrachloroterephthalonitrile

A controllable and defectless cutting postprocess method of an organic cocrystal based on pyrene and tetrachloroterephthalonitrile was proposed. The nature of that is revealed as cleavage, which shed light on the design of such materials.

Tunable multiple light emissions of core-shell structures based on rare earth ions doped on the surfaces of organic cocrystals.

The charge transfer (CT) interactions play a vital role in tuning the luminescence of organic crystals. The enhanced energy transfer (ET) effect in rare earth (RE) ions is a significant method to achieve long-lifetime fluorescence. These studies are of great significance in the fields of photoelectric functional materials. However, the effect of CT interactions on the process of ET from the cocrystal ligand group to RE ions is unknown. In this work, we have doped Eu3+ ions, Tb3+ ions and Eu3+/Tb3+ mixed ions on the surfaces of Phen-TCNB (Phen = 1,10-phenanthroline, and TCNB = 1,2,4,5-tetracyanobenzene) to construct organic cocrystal-type core-shell structures by the epitaxial growth method. The core-shell structures exhibited multiple photoluminescence depending on the types and proportions of RE3+ ions that are doped on the surfaces of the cocrystals. Experimental and theoretical investigations prove that the ET enhancements from ligand groups to Eu3+ ions originate from appropriate energy differences between the lowest triplet states of Phen-TCNB and the lowest excited state of RE3+ ions. In contrast, the reduced Tb3+ 5D4 lifetime is caused by the energy back transfer process since the energy difference becomes small. These results reveal that the multiple luminescences of the cocrystal-type core-shell structures can be adjusted by the CT and ET, and this study provides a new strategy for developing novel optoelectronic materials.

Doped organic charge-transfer cocrystal with tunable fluorescence of wide band emission

Supramolecular crystals with tunable fluorescence emission have been demonstrated for many applications. Guided by quantum chemical calculations, a series of (TCNB)x(OFN)1-x(Cor) doped cocrystals were synthesized by charge transfer induced self-assembly with coronene (Cor) as electron donor and octafluoronaphthalene (OFN) or 1,2,4,5-tetracyanobenzene (TCNB) as electron acceptors. Structural similarity between OFN-Cor and TCNB-Cor are necessary for full proportion doping of (TCNB)x(OFN)1-x(Cor). Based on experimental and theoretical analysis, doped cocrystals can be tuned in photophysical properties (luminescent color, fluorescence lifetime, quantum yield). With the doping ratio increasing, the doped cocrystals emit fluorescent in different colors from blue to yellow and then to orange. Through molecular design to select and synthesize organic fluorescent materials is an efficient strategy to deeply understand their structure–activity relationship, which pave a potential path to develop novel functional supramolecular systems.

Organic Cocrystals: Recent Advances and Perspectives for Electronic and Magnetic Applications.

Cocrystal engineering is an advanced supramolecular strategy that has attracted a lot of research interest. Many studies on cocrystals in various application fields have been reported, with a particular focus on the optoelectronics field. However, few articles have combined and summarized the electronic and magnetic properties of cocrystals. In this review, we first introduce the growth methods that serve as the basis for realizing the different properties of cocrystals. Thereafter, we present an overview of cocrystal applications in electronic and magnetic fields. Some functional devices based on cocrystals are also introduced. We hope that this review will provide researchers with a more comprehensive understanding of the latest progress and prospects of cocrystals in electronic and magnetic fields.

Rücktitelbild: Hyperbranched Microwire Networks of Organic Cocrystals with Optical Waveguiding and Light‐Harvesting Abilities (Angew. Chem. 52/2021)

Wie Bäume in der Natur bilden sich hyperverzweigte Nanobäume durch sequenzielles Wachstum von Generationen ausgerichteter Äste an den Stämmen, was eine effiziente Lichtabsorption ermöglicht. In ihrem Forschungsartikel auf S. 27252 entwickeln Yilong Lei et al. eine Coassemblierungsstrategie, um geordnete binäre und ternäre hyperverzweigte Mikrodrähte aus Aren-Perfluoraren-Cokristallen zu erhalten, die optische Wellenleiter- und Lichtsammelfähigkeiten aufweisen.

Back Cover: Hyperbranched Microwire Networks of Organic Cocrystals with Optical Waveguiding and Light‐Harvesting Abilities (Angew. Chem. Int. Ed. 52/2021)

Like well-manicured trees in nature, hyperbranched nanotrees are generated by sequential growth of multiple generations of aligned branches on the trunks, giving efficient light absorption and trapping. In their Research Article on page 27046, Yilong Lei and co-workers develop a solution co-assembly strategy to access ordered binary and ternary hyperbranched microwires comprising arene-perfluoroarene cocrystals, which display optical waveguiding and light-harvesting abilities.