Organic Ionic Crystals Research Articles

Ferroelastic ionic organic crystals that self-heal to 95%

The realm of self-healing materials integrates chemical and physical mechanisms that prevent wear and fracturing and extend the operational lifetime. Unlike the favorable rheology of amorphous soft materials that facilitates efficient contact between fragments, the efficiency of recovery of atomistically ordered materials is restricted by slower interfacial mass transport and the need for ideal physical alignment, which limits their real-world application. We report drastic enhancements in efficiency and recovery time in the self-healing of anilinium bromide, challenging these limitations. Crystals of this material recovered up to 49% within seconds and up to 95% after 100 min via ferroelastic detwinning. The spatial evolution of strain during cracking and healing was measured in real time using digital image correlation. Favorable alignment and strong ionic bonding across the interface of partially fractured crystals facilitate self-healing. This study elevates organic crystals close to the best-in-class self-healing polymers and sets an approach for durable crystal-based optoelectronics.

Recent Advances in Room-Temperature Phosphorescence Metal-Organic Hybrids: Structures, Properties, and Applications.

Metal-organic hybrid (MOH) materials with room-temperature phosphorescence (RTP) have drawn attention in recent years due to their superior RTP properties of high phosphorescence efficiency and ultralong emission lifetime. Great achievement has been realized in developing MOH materials with high-performance RTP, but a systematic study on MOH materials with RTP feature is lacking. This review highlights recent advances in metal-organic hybrid RTP materials. The molecular packing, the photophysical properties, and their applications of metal-organic hybrid RTP materials are discussed in detail. Metal-organic hybrid RTP materials can be divided into six parts: coordination polymers, metal-organic frameworks (MOFs), metal-halide hybrids, organic ionic crystals, organic ionic polymers, and organic-inorganic hybrid perovskites. These RTP materials have been successfully applied in time-resolved data encryption, fingerprint recognition, information logic gates, X-ray imaging, and photomemory. This review not only provides the basic principles of designing RTP metal-organic hybrids, but also propounds the future research prospects of RTP metal-organic hybrids. This review offers many effective strategies for developing metal-organic hybrids with excellent RTP properties, thus satisfying practical applications.

Innenrücktitelbild: Fast and Tunable Phosphorescence from Organic Ionic Crystals (Angew. Chem. 36/2023)

Innenrücktitelbild: Fast and Tunable Phosphorescence from Organic Ionic Crystals (Angew. Chem. 36/2023)

Inside Back Cover: Fast and Tunable Phosphorescence from Organic Ionic Crystals (Angew. Chem. Int. Ed. 36/2023)

Inside Back Cover: Fast and Tunable Phosphorescence from Organic Ionic Crystals (Angew. Chem. Int. Ed. 36/2023)

Fast and Tunable Phosphorescence from Organic Ionic Crystals

AbstractCrystalline diphosphonium iodides [MeR2P‐spacer‐R2Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λem values from 550 to 880 nm) and intensities (Φem reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion‐π interactions. Time‐resolved and variable‐temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46–92.23 μs at 297 K. Radiative rate constants kr as high as 2.8×105 s−1 deduced for salts 1–3 were assigned to strong spin‐orbit coupling enhanced by an external heavy atom effect arising from the anion‐π charge‐transfer character of the triplet excited state. These rates of anomalously fast metal‐free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.

Fast and Tunable Phosphorescence from Organic Ionic Crystals.

Crystalline diphosphonium iodides [MeR2P-spacer-R2Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λem values from 550 to 880 nm) and intensities (Φem reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-π interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46-92.23 μs at 297 K. Radiative rate constants kr as high as 2.8 × 105 s-1 deduced for salts 1-3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-π charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.

Design of the Ionic Organic Nonlinear Optical Material NH4 [LiC3 H(CH3 )O4 ] with Ultrawide Band Gap and Moderate Birefringence.

Ionic organic crystals containing organic planar π-conjugated units has become one of the hot spots as nonlinear optical (NLO) materials. However, although this type of ionic organic NLO crystals commonly have remarkable second harmonic generation (SHG) responses, they also suffer from overlarge birefringences and relatively small band gaps that be hardly beyond 6.2 eV. Herein, a flexible π-conjugated [C3 H(CH3 )O4 ]2- unit was theoretically revealed, showing great potential for designing NLO crystals with balanced optical properties. Accordingly, through the reasonable NLO-favourable layered design, a new ionic organic material, NH4 [LiC3 H(CH3 )O4 ], was successfully obtained. As expected, it achieves not only a large SHG effect (4×KDP), but also a suitable birefringence (0.06@546 nm) and an ultrawide band gap (>6.5 eV). This study provides a new flexible π-conjugated NLO-active unit, contributing to design more ionic organic NLO materials with excellent balanced optical properties.

Ultralong room temperature phosphorescence and reversible mechanochromic luminescence in ionic crystals with structural isomerism

Photofunctional ionic crystals have become more and more appealing in the applications of light-emitting devices, optical imaging, and anti-counterfeiting. However, it is still limited to achieve ultralong room temperature phosphorescence (RTP) and stimuli-responsive property in ionic crystals due to the lack of valid design principles. Herein, we developed a new class of organic ionic crystals (sodium o-, m-, p-carboxyphenylborate, o-, m-, p-Nacpb) by a structural isomerization strategy, which could exhibit ultralong RTP and mechanochromic emission. Among the three isomers, m-Nacpb crystal exhibits the longest RTP lifetime up to 732.8 ms, which is ca. 120 times longer than that (6.1 ms) of o-Nacpb crystal. Interestingly, o-Nacpb could display mechano-responsive luminescence with tunable fluorescence-phosphorescence emissions and longer phosphorescence lifetime (increased by about 20-fold). Both experimental and theoretical studies indicate that the distinct RTP and mechanochromic luminescence properties of the organic ionic crystals are attributed to the alternation of molecular conformations and spatial packing modes. Moreover, potential applications including data encryption and decoration are demonstrated, benefiting from the varied phosphorescence lifetimes and good processability. Therefore, this work not only provides a rational isomerization route to prepare various organic ionic crystals with ultralong RTP and stimuli-responsive emission, but also supplies a facile way to develop the advanced information encryption applications.

Exceptionally high work density of a ferroelectric dynamic organic crystal around room temperature

Dynamic organic crystals are rapidly gaining traction as a new class of smart materials for energy conversion, however, they are only capable of very small strokes (<12%) and most of them operate through energetically cost-prohibitive processes at high temperatures. We report on the exceptional performance of an organic actuating material with exceedingly large stroke that can reversibly convert energy into work around room temperature. When transitioning at 295–305 K on heating and at 265–275 K on cooling the ferroelectric crystals of guanidinium nitrate exert a linear stroke of 51%, the highest value observed with a reversible operation of an organic single crystal actuator. Their maximum force density is higher than electric cylinders, ceramic piezoactuators, and electrostatic actuators, and their work capacity is close to that of thermal actuators. This work demonstrates the hitherto untapped potential of ionic organic crystals for applications such as light-weight capacitors, dielectrics, ferroelectric tunnel junctions, and thermistors.

Organic Crystalline Solid Electrolytes with High Mg-Ion Conductivity Composed of Nonflammable Ionic Liquid Analogs and Mg(TFSA)2

The development of solid electrolytes with Mg-ion conductivity at room temperature is an important issue to achieve all-solid magnesium batteries. We focus on organic ionic crystals with Mg-ion conduction paths in addition to nonflammable and nonvolatile features as an innovative candidate of solid electrolytes with Mg-ion conductivity. Herein, we show the development of novel organic ionic crystals, [N(CH3)4-n(CH2CH3)n][Mg{N(SO2CF3)2}3] (n = 0 or 2), using analogs of ionic liquids, [N(CH3)4][N(SO2CF3)2] (N1111TFSA) and [N(CH3)2(CH2CH3)2][N(SO2CF3)2] (N1122TFSA), and magnesium salt, Mg{N(SO2CF3)2}2 (Mg(TFSA)2). We also report the crystal structures of the obtained crystals and the high Mg-ion conductivity of 10-4 S cm-1 under mild conditions of 80 °C in the solid state. These results indicate that organic ionic crystals with ion conduction paths have significant potential as safe solid electrolytes and provide insights into developing innovative Mg-ion conductors.

Growth, linear and nonlinear optical property and dielectric studies of an efficient organic ionic single crystal: 2-amino-5-methyl-pyridinium trifluoroacetate for optoelectronic devices

Growth, linear and nonlinear optical property and dielectric studies of an efficient organic ionic single crystal: 2-amino-5-methyl-pyridinium trifluoroacetate for optoelectronic devices

Dielectric switching from a high temperature plastic phase transition in two organic salts with chiral features

Two organic ionic crystals were successfully synthesized, namely [N(CH2CH3)3(CH2Cl)][ClO4] (1) and [N(CH2CH3)3(CH2Cl)][BF4] (2), with high-temperature reversible phase transitions. In addition, compounds 1 and 2 also show obvious CD signals.

Fascinating Supramolecular Assembly through Noncovalent Interactions Involving Anions in Organic Ionic Crystals

Fascinating Supramolecular Assembly through Noncovalent Interactions Involving Anions in Organic Ionic Crystals

Charge Storage Mechanism and Structural Evolution of Viologen Crystals as the Cathode of Lithium Batteries.

Although organic ionic crystals represent an attractive class of active materials for rechargeable batteries owing to their high capacity and low solubility in electrolytes, they generally suffer from limited electronic conductivity and moderate voltage. Furthermore, the charge storage mechanism and structural evolution during the redox processes are still not clearly understood. Here we describe ethyl viologen iodide (EVI2 ) and ethyl viologen diperchlorate (EV(ClO4 )2 ) as cathode materials of lithium batteries which crystallize in a monoclinic system with alternating organic EV2+ layers and inorganic I- /ClO4 - layers. The EVI2 electrode exhibits a high initial discharge plateau of 3.7 V (vs. Li+ /Li) because of its anion storage ability. When I- is replaced by ClO4 - , the obtained EV(ClO4 )2 electrode displays excellent rate performance with a theoretical capacity of 78 % even at 5 C owing to the good electron conductivity of ClO4 - layers. EVI2 and EV(ClO4 )2 also show excellent cycling stability (capacity retention >96 % after 200 cycles).

Charge Storage Mechanism and Structural Evolution of Viologen Crystals as the Cathode of Lithium Batteries

AbstractAlthough organic ionic crystals represent an attractive class of active materials for rechargeable batteries owing to their high capacity and low solubility in electrolytes, they generally suffer from limited electronic conductivity and moderate voltage. Furthermore, the charge storage mechanism and structural evolution during the redox processes are still not clearly understood. Here we describe ethyl viologen iodide (EVI2) and ethyl viologen diperchlorate (EV(ClO4)2) as cathode materials of lithium batteries which crystallize in a monoclinic system with alternating organic EV2+ layers and inorganic I−/ClO4− layers. The EVI2 electrode exhibits a high initial discharge plateau of 3.7 V (vs. Li+/Li) because of its anion storage ability. When I− is replaced by ClO4−, the obtained EV(ClO4)2 electrode displays excellent rate performance with a theoretical capacity of 78 % even at 5 C owing to the good electron conductivity of ClO4− layers. EVI2 and EV(ClO4)2 also show excellent cycling stability (capacity retention &gt;96 % after 200 cycles).

Molecular salt crystals with bis(head-to-tail) interionic complementary assembly for efficient organic THz generators

New ionic organic crystals with bis(head-to-tail) complementary cation–anion assembly result in extremely efficient THz wave generation due to strong interionic binding interactions and parallel alignment of nonlinear optical cationic chromophores.

Organic Ionic Crystals: Solid‐Phase Structure and Thermal Properties of Dicationic α,ω‐Bis[N,N′‐(4‐alkylpyridinium)]alkane Hexafluorophosphate Salts

AbstractAs organic ionic crystals, C2‐, C3‐, and C4‐alkylene α,ω‐bis[N,N′‐(4‐alkylpyridinium)] PF6− salts are synthesized to investigate their thermal properties and phase behaviors along with chemical structure changes. The X‐ray crystal structure of 1,2‐bis[N,N′‐(4‐methylpyridinium)]ethane 2PF6− (2–1) showed that two pyridinium rings in one cation molecule and two pyridinium ring planes of neighboring molecules were exactly parallel. Multiple C−H⋅⋅⋅F hydrogen bonds were observed for all protons of the bis‐pyridinium with PF6− anions. Interestingly, most of the prepared C2‐bridged dicationic bis‐pyridinium PF6− salts showed solid–solid phase transitions, which are typical features of soft crystalline materials, such as plastic and liquid crystals. Colorfully aligned crystalline textures of the polarized optical microscope images are formed with strong birefringence for the soft crystalline phase of 1,2‐bis[N,N′‐(4‐n‐propylpyridinium)]ethane PF6−, showing a low entropy of fusion (ΔSf=25.0 J mol−1 K−1). The C2‐bridged n‐undecyl (2–11) and n‐dodecyl (2–12) salts exhibit a very soft solid phase, showing large heat absorptions and entropy changes during their multiple solid‐solid phase transitions. Additionally, the C3‐ and C4‐bridged dicationic (bis)pyridinium PF6− salts are prepared to determine the property changes with different bridge lengths. The longer alkylene connected salts are thermally more stable than the C2‐bridged compounds and show only one melting phase transition.

Fluorinated Organic Electro‐Optic Quinolinium Crystals for THz Wave Generation

AbstractFluorinated electro‐optic crystals with state‐of‐the‐art second‐order nonlinear optical response and excellent characteristics for terahertz (THz) wave generation are reported. The fluorinated organic ionic crystals consist of optically highly nonlinear fluorinated HM6FQ (6‐fluoro‐2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium) or HM7FQ (7‐fluoro‐2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium) cations and 4‐methylbenzenesulfonate (T) counter anions. Compared to benchmark electro‐optic crystals based on nonfluorinated HMQ (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium) cations, introducing fluorine substituent on HM6FQ cations creates additional hydrogen bonds (ArF···HC). Such a molecular engineering leads to an enhanced thermal stability and significant modulations of phonon vibrational modes of crystals in THz frequency region, while excellent π–π stacking and space filling characteristics of HM6FQ cations in crystals lead to state‐of‐the‐art diagonal component of the macroscopic nonlinear optical susceptibility, similar to the case of HMQ cations. HM6FQ‐based crystals exhibit a very high optical‐to‐THz conversion efficiency, comparable to benchmark HMQ‐based crystals. In addition, due to additional hydrogen bonds induced by fluorine substituents, the spectral shape of the generated THz wave is remarkably modified; e.g., the largest spectral gap is near 1.5 and 2.0 THz for HM6FQ‐ and HMQ‐based crystals, respectively. The fluorinated cationic engineering on nonlinear optical crystals having benchmark nonlinear optical response is, as far as is known, reported for the first time.

A simple and efficient phosphorescent probe for iodide-specific detection based on crystallization-induced phosphorescence of organic ionic crystals

A novel time-gated detection method for iodide ions based on the unique crystallization-induced phosphorescence of tetraphenylphosphonium iodide was developed.

Anion-regulated transient and persistent phosphorescence and size-dependent ultralong afterglow of organic ionic crystals

Regulation and the transient and persistent room-temperature phosphorescence of organic ionic crystals by the alteration of anions, and unique size-dependent ultralong afterglow of specific organic crystals under ambient conditions were reported.