Revealing the charge-transfer interactions in self-assembled organic cocrystals: two-dimensional photonic applications

Insights into the Control of Optoelectronic Properties in Mixed-Stacking Charge-Transfer Complexes

Although cocrystallization has provided a promising platform to develop new organic optoelectronic materials, it is still a big challenge to purposely design and achieve specific optoelectronic properties. Herein, a series of mixed-stacking cocrystals (TMFA, TMCA, and TMTQ) were designed and synthesized, and the regulatory effects of the acceptors on the co-assembly behavior, charge-transfer nature, energy-level structures, and optoelectronic characteristics were systematically investigated. The results demonstrate that it is feasible to achieve effective charge-transport tuning and photoresponse switching by carefully regulating the intermolecular charge transfer and energy orbitals. The inherent mechanisms underlying the change in these optoelectronic behaviors were analyzed in depth and elucidated to provide clear guidelines for future development of new optoelectronic materials. In addition, due to the excellent photoresponsive characteristics of TMCA, TMCA-based phototransistors were investigated with varying light wavelength and optical power, and TMCA shows the best performance among all reported cocrystals under UV illumination.

Structure of Imidazolium-N-phthalolylglycinate Salt Hydrate: Combined Experimental and Quantum Chemical Calculations Studies

An organic supramolecular salt hydrate (imidazolium:N-phthalolylglycinate:H2O; IM+-NPG−-HYD) has been examined for its charge-transfer (CT) characteristics. Accordingly, IM+–NPG−–HYD has been characterized thoroughly using various spectroscopic techniques. Combined experimental and quantum chemical studies, along with wave function analysis, were performed to study the non-covalent interactions and their role in CT in the supramolecular salt hydrate. Notably, IM+–NPG−–HYD crystalizes in two configurations (A and B), both of which are held together via non-covalent interactions to result in a three-dimensional CT supramolecular assembly. The through-space CT occurs from NPG– (donor) to IM+ (acceptor), and this was mediated via non-covalent forces. We demonstrated the role of π–π stacking interactions (mixed-stacking donor-acceptor interactions) in the presence of charge-assisted hydrogen bonds in the regulation of CT properties in the self-assembly of the IM+–NPG−–HYD salt hydrate.

5,6,12,13-Tetraazaperopyrenes as Unique Photonic and Mechanochromic Fluorophores

5,6,12,13-Tetraazaperopyrenes with different number of tert-butyl groups (c-TAPP-T, c-TAPP-H) were synthesized, via four-fold Bischler-Napieralski cyclization as the key step. As deduced from the single-crystal structures and optical properties, N-doping and substitution type allow for a precise control of intermolecular interactions. Compared to the reported 1,3,8,10-tetraazaperopyrenes, significantly different packing modes were found in 5,6,12,13-tetraazaperopyrenes. Going from c-TAPP-T to c-TAPP-H, two additional tert-butyl groups lead to different preferential growth directions, affording 1D and 2D microcrystals, respectively. Most importantly, both microcrystals exhibit excellent optical waveguide properties with extraordinarily low loss coefficients and unique polarization features. Although c-TAPP-H possesses a rigid and planar core, its crystals display an exceptional mechanochromic fluorescence, which, again, depends on the mode of molecular packing.

One-Dimensional Fullerene/Porphyrin Cocrystals: Near-Infrared Light Sensing through Component Interactions

Recently, organic donor-acceptor (D-A) cocrystals have attracted special interest as functional materials because of their unique chemical and physical properties that are not exhibited by simple mixtures of their components. Herein, we report the preparation of one-dimensional novel D-A cocrystals from C₆₀ and 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin (3,5-TPP); these cocrystals have near-infrared (NIR) light-sensing abilities, despite each of their component molecule individually having no NIR light-sensing properties. Micrometer-sized rectangular columnar C₆₀-3,5-TPP cocrystals were produced by a simple liquid-liquid interfacial precipitation method. The cocrystals exhibit a new strong transition in the NIR region indicative of the existence of charge-transfer interactions between C₆₀ and 3,5-TPP in the cocrystals. The C₆₀-3,5-TPP cocrystals showed n-type transport characteristics with NIR light-sensing properties when the cocrystals were incorporated in bottom-gate/bottom-contact organic phototransistors, revealing that organic cocrystals with suitable charge-transfer interaction are useful as functional materials for the creation of novel NIR-light-sensing devices.

Design, synthesis, structure, and photophysical features of highly emissive cinnamic derivatives

New cinnamic derivatives 1a–c were designed starting from the chromophores working in polybenzofulvene derivatives poly-6-DMFL-BF3k, poly-6-MCBZ-BF3k, and poly-6-TPA-BF3k endowed with outstanding optoelectronic performances.

Multifunctional molecular charge-transfer thin films

We report the controlled interfacial interaction in crystallized organic charge transfer thin films, consisting of bis(ethylenedithio)tetrathiafulvalene and C60. The induced broad-band absorption from the UV to near-infrared region leads to a wavelength dependent ambipolar (negative/positive) photoresponse, while multi-stimuli responsive behavior is achieved through charge-transfer interactions. In addition, by coupling with the tetrathiafulvalene-(7,7,8,8-tetracyanoquinodimethane) charge transfer complex, a significantly increased conductivity is achieved. The controlled interfacial charge transfer interaction provides an efficient approach to obtain multifunctional molecular crystallized thin films with a superior external stimuli response.

The Emergence of Organic Single-Crystal Electronics

Organic semiconducting single crystals are perfect for both fundamental and application-oriented research due to the advantages of free grain boundaries, few defects, and minimal traps and impurities, as well as their low-temperature processability, high flexibility, and low cost. Carrier mobilities of greater than 10 cm²  V^-1  s^-1 in some organic single crystals indicate a promising application in electronic devices. The progress made, including the molecular structures and fabrication technologies of organic single crystals, is introduced and organic single-crystal electronic devices, including field-effect transistors, phototransistors, p-n heterojunctions, and circuits, are summarized. Organic two-dimensional single crystals, cocrystals, and large single crystals, together with some potential applications, are introduced. A state-of-the-art overview of organic single-crystal electronics, with their challenges and prospects, is also provided.

Consistency and variability of cocrystals containing positional isomers: the self-assembly evolution mechanism of supramolecular synthons of cresol-piperazine

The disposition of functional groups can induce variations in the nature and type of interactions and hence affect the molecular recognition and self-assembly mechanism in cocrystals. To better understand the formation of cocrystals on a molecular level, the effects of disposition of functional groups on the formation of cocrystals were systematically and comprehensively investigated using cresol isomers (o-, m-, p-cresol) as model compounds. Consistency and variability in these cocrystals containing positional isomers were found and analyzed. The structures, molecular recognition and self-assembly mechanism of supramolecular synthons in solution and in their corresponding cocrystals were verified by a combined experimental and theoretical calculation approach. It was found that the heterosynthons (heterotrimer or heterodimer) combined with O-H⋯N hydrogen bonding played a significant role. Hirshfeld surface analysis and computed interaction energy values were used to determine the hierarchical ordering of the weak interactions. The quantitative analyses of charge transfers and molecular electrostatic potential were also applied to reveal and verify the reasons for consistency and variability. Finally, the molecular recognition, self-assembly and evolution process of the supramolecular synthons in solution were investigated. The results confirm that the supramolecular synthon structures formed initially in solution would be carried over to the final cocrystals, and the supramolecular synthon structures are the precursors of cocrystals and the information memory of the cocrystallization process, which is evidence for classical nucleation theory.

Cocrystal Engineering: A Collaborative Strategy toward Functional Materials

Cocrystal engineering with a noncovalent assembly feature by simple constituent units has inspired great interest and has emerged as an efficient and versatile route to construct functional materials, especially for the fabrication of novel and multifunctional materials, due to the collaborative strategy in the distinct constituent units. Meanwhile, the precise crystal architectures of organic cocrystals, with long-range order as well as free defects, offer the opportunity to unveil the structure-property and charge-transfer-property relationships, which are beneficial to provide some general rules in rational design and choice of functional materials. In this regard, an overview of organic cocrystals in terms of assembly, containing the intermolecular interactions and growth methods, two functionality-related factors including packing structure and charge-transfer nature, and those advanced and novel functionalities, is presented. An outlook of future research directions and challenges for organic cocrystal is also provided.

Solvation-Enhanced Intermolecular Charge Transfer Interaction in Organic Cocrystals: Enlarged C-C Surface Close Contact in Mixed Packing between PTZ and TCNB

The mixed π-π packing of the donor (D) and acceptor (A) molecules is the highlighting feature of the intermolecular interactions following charge transfer (CT) issues in organic cocrystal systems. There is an inverse relationship between the D-A interplanar distance and the intermolecular CT interaction. However, the D-A C-C surface close contact (relative areas) on the intermolecular CT interactions in organic cocrystal systems is rarely investigated. Herein, we designed and constructed a novel cocrystal and its solvate cocrystal. The structural and electrostatic potential analyses suggest that the solvation destroys the N-H···N hydrogen bond interaction between phenothiazine (PTZ) and 1,2,4,5-tetracyanobenzene (TCNB), which causes the TCNB molecules to have a 90° rotation along the normal axis of the PTZ plane. Thus, the D-A C-C surface close contact is enlarged, strengthening the intermolecular π-π stacking interactions and intermolecular CT interaction between PTZ and TCNB, which are further evidenced by the absorption and Raman spectroscopic analyses. This study provides rare evidence of the enlarged C-C surface close contact in the mixed packing between D and A that greatly contributes to the intermolecular CT interaction in a D-A cocrystal system. It also provides a deeper understanding of the role of solvation in the structure-property relationship of organic cocrystal materials.

Short- and Long-Range Solvation Effects on the Transient UV-Vis Absorption Spectra of a Ru(II)-Polypyridine Complex Disentangled by Nonequilibrium Molecular Dynamics

Evidence of subtle effects in the dynamic reorganization of a protic solvent in its first- and farther-neighbor shells, in response to the sudden change in the solute's electronic distribution upon excitation, is unveiled by a multilevel computational approach. Through the combination of nonequilibrium molecular dynamics and quantum mechanical calculations, the experimental time evolution of the transient T₁ absorption spectra of a heteroleptic Ru(II)-polypyridine complex in ethanol or dimethyl sulfoxide solution is reproduced and rationalized in terms of both fast and slow solvent re-equilibration processes, which are found responsible for the red shift and broadening experimentally observed only in the protic medium. Solvent orientational correlation functions and a time-dependent analysis of the solvation structure confirm that the initial, fast observed red shift can be traced back to the destruction-formation of hydrogen bond networks in the first-neighbor shell, whereas the subsequent shift, evident in the [20-500] ps range and accompanied by a large broadening of the signal, is connected to a collective reorientation of the second and farther solvation shells, which significantly changes the electrostatic embedding felt by the excited solute.

Facile synthesis of 1D organic-inorganic perovskite micro-belts with high water stability for sensing and photonic applications

The development of low-dimensional perovskite micro/nanostructures with high water stability for novel photonic/electronic applications is highly desirable. Herein, one-dimensional (1D) organic-inorganic hybrid perovskite micro-belts [(AD)Pb2Cl5] (OIHP-AD, AD = acridine) were facilely synthesized through fast precipitation in aqueous solution at room temperature without any organic solvent and expensive alkyl halide. Luminescent properties and water stability are efficiently enhanced due to the highly regular arrangement of the protonated AD dyes with larger steric hindrance distributed in the perovskite host-guest system, which can afford denser crystal packing to prevent water erosion. The OIHP-AD micro-belts present upconversion fluorescence, polarized photoemission and optical waveguide performances with a low loss coefficient (0.004 dB μm-1) during propagation, thus extending the applications of 1D perovskite micro/nanostructures to potential optical communication micro-devices.

Directed Functionalization Tailors the Polarized Emission and Waveguiding Properties of Anthracene-Based Molecular Crystals

Organic semiconducting crystals are characterized by anisotropic optical and electronic properties, which can be tailored by controlling the packing of the constituent molecules in the crystal unit cell. Here, the synthesis, structural characterization, and emission of anthracene derivatives are focused to correlate directed functionalization and optical properties. These compounds are easily and scalably prepared by standard synthesis techniques, and alterations in functional groups yield materials with either exclusive edge-to-face or face-to-face solid-state interactions. The resulting crystals feature either platelet or needle shapes, and the emission exhibits polarization ratios up to 5 at room temperature. In needle-shaped crystals, self-waveguiding of the emission is also observed with propagation loss coefficients as low as 1.3 dB mm^-1. Moreover, optical coupling between crossing crystalline microwires is found and characterized. The combination of optical anisotropy and emission self-waveguiding opens interesting routes for the exploitation of these active materials in photonic applications, including optical integrated circuits and microscale light sources.

1D versus 2D cocrystals growth via microspacing in-air sublimation

Organic cocrystals possess valuable properties owing to the synergistic effect of the individual components. However, the growth of molecular cocrystals is still in its primary stage. Here we develop a microspacing in-air sublimation method to grow organic cocrystals, and furthermore to realize morphology control on them, which is essential for structure–property relations. A series of polycyclic aromatic hydrocarbon (PAH)‒1,2,4,5-tetracyanobenzene (TCNB) complexes cocrystals are grown directly on the substrate, with the morphology tunable from 1D needle-like to 2D plate-like on demand. Spatially resolved photoluminescence spectra analyses on different cocrystals display morphology dependent and anisotropic optical waveguiding properties. In situ observation and energy calculations of the crystallization processes reveal the formation mechanism being from a competition between growth kinetics-defined crystal habit and the thermodynamics driving force. This growth technique may serve the future demand for tunable morphology organic cocrystals in different functional applications.

Organic cocrystals have interesting properties owing to synergistic effects of the individual components but development of effective growth methods for molecular cocrystals is still in its primary stage. Here the authors demonstrate a sublimation method which allows growing organic cocrystals with controlled morphology.

Living in the salt-cocrystal continuum: indecisive organic complexes with thermochromic behaviour

The thermochromic behaviour of the haloaniline – 3,5-dinotrobenzoic acid cocrystals over the temperature range from 30–108 °C.

Unravelling substitution effects on charge transfer characteristics in cocrystals of pyrene based donors and 3,5-dinitrobenzoic acid

Ambipolar to p-type semiconductivity switching with the change of the ⋯DADADA⋯ to ⋯ADDADD⋯ packing arrangement in charge transfer cocrystals of pyrene based donors is discussed.

Three-dimensional directed assembly of organic charge-transfer heterostructure

Multicomponent crystalline heterostructures are a powerful approach to integrate different functional materials into the ordered structures. Here we describe three-dimensional spherical assembly of binary organic solids that consist of electron donor and acceptor molecules. A mechanistic study of heterostructure formation reveals that the dewetting and drying-mediated assembly processes are responsible for the spherical crystallite formation. The assembled spherical heterostructures are highly tunable, crystalline and chemically stable, exhibiting phase separation controlled optoelectronic behavior. This simple, generalizable three-dimensional assembly can be modified for the formation of ordered functional organic multicomponent heterostructures for emerging applications.

Analysis of charge transfer transitions in stacked π-electron donor-acceptor complexes

Charge-transfer (CT) interactions have recently attracted enhanced interest in creating ordered ferroelectric networks of electron donors (D) and acceptors (A) and in providing the basis of functional photonic devices. In the present work, the CT character of three distinct donor-acceptor (D-A) complex interactions (pyromellitic diimide (PDI)/1,5-diaminonaphthalene (DAN), para-chloranil (pClA)/tetramethyl-para-phenylenediamine (TMPD) and tetracyanobenzene (TCNB)/1,2-di(4-pyridyl)ethylene (Bpe)) has been investigated in their ground and excited states using high-level quantum chemical methods (second-order algebraic diagrammatic construction (ADC(2)) and time-dependent density functional theory (TD-DFT) using a long-range corrected functional (ωB97xD)). The calculations show that the lowest electronic excitation has pronounced CT character in all the three dimers investigated. On the contrary, the ground states possess only smaller amounts of CT degree except for the pClA/TMPD complex, which has a strong amount of CT of 0.4 e. Optimization of the S1 state and calculation of the resulting vertical fluorescence transitions led to the interesting finding of a zero-energy gap for pClA/TMPD. The next smallest energy gap is computed for PDI/DAN (1 eV) followed by TCNB/Bpe (2.6 eV). The analysis of the electronic charge distribution of the D-A complex in the excited state shows a significant variation even though practically a full electron charge has been transferred from D to A.

Organic 2D Optoelectronic Crystals: Charge Transport, Emerging Functions, and Their Design Perspective

2D organic semiconductor crystals are emerging as a fascinating platform with regard to their applications in organic field-effect transistors (OFETs), attributed to their enhanced charge transport efficiency and their new optoelectronic functions, based on their unique morphological features. Advances in material processing techniques have not only enabled easy fabrication of few-monolayered 2D nanostructures but also facilitated exploration of the interesting properties induced by characteristic 2D morphologies. However, to date, only a limited number of representative organic semiconductors have been utilized in organic 2D optoelectronics. Therefore, in order to further spur this research, an intuitive crystal engineering principle for realizing organic 2D crystals is required. In this regard, here, not only the important implications of applying 2D structures to OFET devices are discussed but also a crystal engineering protocol is provided that first predicts molecular arrangements depending on the molecular factors, which is followed by realizing 2D supramolecular synthon networks for different molecular packing motifs. It is expected that 2D organic semiconductor crystals developed by this approach will pave a promising way toward next-generation organic 2D optoelectronics.

Design of two series of 1:1 cocrystals involving 4-amino-5-chloro-2,6-dimethylpyrimidine and carboxylic acids

Two series of a total of ten cocrystals involving 4-amino-5-chloro-2,6-dimethylpyrimidine with various carboxylic acids have been prepared and characterized by single-crystal X-ray diffraction. The pyrimidine unit used for the cocrystals offers two ring N atoms (positions N1 and N3) as proton-accepting sites. Depending upon the site of protonation, two types of cations are possible [Rajam et al. (2017). Acta Cryst. C73, 862-868]. In a parallel arrangement, two series of cocrystals are possible depending upon the hydrogen bonding of the carboxyl group with position N1 or N3. In one series of cocrystals, i.e. 4-amino-5-chloro-2,6-dimethylpyrimidine-3-bromothiophene-2-carboxylic acid (1/1), 1, 4-amino-5-chloro-2,6-dimethylpyrimidine-5-chlorothiophene-2-carboxylic acid (1/1), 2, 4-amino-5-chloro-2,6-dimethylpyrimidine-2,4-dichlorobenzoic acid (1/1), 3, and 4-amino-5-chloro-2,6-dimethylpyrimidine-2-aminobenzoic acid (1/1), 4, the carboxyl hydroxy group (-OH) is hydrogen bonded to position N1 (O-H...N1) of the corresponding pyrimidine unit (single point supramolecular synthon). The inversion-related stacked pyrimidines are doubly bridged by the carboxyl groups via N-H...O and O-H...N hydrogen bonds to form a large cage-like tetrameric unit with an R₄²(20) graph-set ring motif. These tetrameric units are further connected via base pairing through a pair of N-H...N hydrogen bonds, generating R₂²(8) motifs (supramolecular homosynthon). In the other series of cocrystals, i.e. 4-amino-5-chloro-2,6-dimethylpyrimidine-5-methylthiophene-2-carboxylic acid (1/1), 5, 4-amino-5-chloro-2,6-dimethylpyrimidine-benzoic acid (1/1), 6, 4-amino-5-chloro-2,6-dimethylpyrimidine-2-methylbenzoic acid (1/1), 7, 4-amino-5-chloro-2,6-dimethylpyrimidine-3-methylbenzoic acid (1/1), 8, 4-amino-5-chloro-2,6-dimethylpyrimidine-4-methylbenzoic acid (1/1), 9, and 4-amino-5-chloro-2,6-dimethylpyrimidine-4-aminobenzoic acid (1/1), 10, the carboxyl group interacts with position N3 and the adjacent 4-amino group of the corresponding pyrimidine ring via O-H...N and N-H...O hydrogen bonds to generate the robust R₂²(8) supramolecular heterosynthon. These heterosynthons are further connected by N-H...N hydrogen-bond interactions in a linear fashion to form a chain-like arrangement. In cocrystal 1, a Br...Br halogen bond is present, in cocrystals 2 and 3, Cl...Cl halogen bonds are present, and in cocrystals 5, 6 and 7, Cl...O halogen bonds are present. In all of the ten cocrystals, π-π stacking interactions are observed.

Cu-Thienoquinone Charge-Transfer Complex: Synthesis, Characterization, and Application in Organic Transistors

A facile and unusual reaction between thienoquinone compound QDTBDT2C and copper is reported. The formation of Cu-QDTBDT2C complex is proved by absorption spectra, IR spectra, Raman spectra, and X-ray photoelectron spectroscopy data. This complex can serve as a doping layer at the interface of Cu/QDTBDT2C and greatly improve the performance of organic transistors in which the copper electrode is source/drain electrodes and QDTBDT2C is an active layer. The transistors display an electron mobility of 0.95 cm² V^-1 s^-1, to our knowledge, the highest electron mobility reported for copper electrode-based n-type transistors and nearly two times higher than that of the Au electrode-based devices. These results demonstrate the potential applications of Cu-QDTBDT2C complex in organic electronics, and the unique properties of QDTBDT2C (spontaneously reacting with copper) provide a new insight into the design of n-type organic semiconductors for copper electrode-based organic transistors.

Tuning Light-Driven Motion and Bending in Macroscale-Flexible Molecular Crystals Based on a Cocrystal Approach

Flexible molecular crystals with stimuli-responsive properties are highly desirable; however, uncovering them is still a challenging goal. Herein, we report a cocrystal approach to obtain elastic molecular crystals that exhibit light-induced fluorescence changes and dynamic mechanical responses at the macroscale level. Cocrystals of naphthylvinylpyridine and tetrafluoroterephthalic acid were fabricated in different stoichiometry ratios (2:1 and 1:1), which present different shapes [two-dimensional (2D) and one-dimensional (1D) morphologies], photoemission, and mechanical properties (rigidity and flexibility). Moreover, obviously different photomechanical energy conversions (light-driven cracking/popping and bending/motion) occur for the 2D and 1D cocrystals, respectively. Nuclear magnetic resonance (NMR) spectra show the occurrence of photoinduced [2 + 2] cycloaddition in both cocrystals, which is the primary mechanism for their photoactuating behaviors. Crystal structure analysis and theoretical calculation further reveal that protonation and the hydrogen-bonding network play important roles in light-stimulus-bendable 1D cocrystal. Thus, the transformation from rigidity to flexibility based on cocrystallization with different stoichiometry may offer an effective means to tune the dynamic light-driven responses for smart crystalline materials.

Precise Patterning of Laterally Stacked Organic Microbelt Heterojunction Arrays by Surface-Energy-Controlled Stepwise Crystallization for Ambipolar Organic Field-Effect Transistors

Ambipolar organic field-effect transistors (OFETs) combining single-crystalline p- and n-type organic micro/nanocrystals have demonstrated superior performance to their amorphous or polycrystalline thin-film counterparts. However, large-area alignment and precise patterning of organic micro/nanocrystals for ambipolar OFETs remain challenges. Here, a surface-energy-controlled stepwise crystallization (SECSC) method is reported for large-scale, aligned, and precise patterning of single-crystalline laterally stacked p-n heterojunction microbelt (MB) arrays. In this method, the p- and n-type organic crystals are precipitated via a stepwise process: first, the lateral sides of prepatterned photoresist stripes provide high-surface-energy sites to guide the aligned growth of p-type organic crystals. Next, the formed p-type crystals serve as new high-surface-energy positions to induce the crystallization of n-type organic molecules at their sides, thus leading to the formation of laterally stacked p-n microbelts. Ambipolar OFETs based on the p-n heterojunction MB arrays exhibit balanced hole and electron mobilities of 0.32 and 0.43 cm2 V-1 s-1 , respectively, enabling the fabrication of complementary-like inverters with large voltage gains. This work paves the way toward rational design and construction of single-crystalline organic p-n heterojunction arrays for high-performance organic, integrated circuits.

2D Organic Materials for Optoelectronic Applications

The remarkable merits of 2D materials with atomically thin structures and optoelectronic attributes have inspired great interest in integrating 2D materials into electronics and optoelectronics. Moreover, as an emerging field in the 2D-materials family, assembly of organic nanostructures into 2D forms offers the advantages of molecular diversity, intrinsic flexibility, ease of processing, light weight, and so on, providing an exciting prospect for optoelectronic applications. Herein, the applications of organic 2D materials for optoelectronic devices are a main focus. Material examples include 2D, organic, crystalline, small molecules, polymers, self-assembly monolayers, and covalent organic frameworks. The protocols for 2D-organic-crystal-fabrication and -patterning techniques are briefly discussed, then applications in optoelectronic devices are introduced in detail. Overall, an introduction to what is known and suggestions for the potential of many exciting developments are presented.

Molecular cocrystals: design, charge-transfer and optoelectronic functionality

This perspective article primarily focuses on the research work related to optoelectronic properties of organic charge transfer cocrystals.

Highly Luminescent 2D-Type Slab Crystals Based on a Molecular Charge-Transfer Complex as Promising Organic Light-Emitting Transistor Materials

A new 2:1 donor (D):acceptor (A) mixed-stacked charge-transfer (CT) cocrystal comprising isometrically structured dicyanodistyrylbenzene-based D and A molecules is designed and synthesized. Uniform 2D-type morphology is manifested by the exquisite interplay of intermolecular interactions. In addition to its appealing structural features, unique optoelectronic properties are unveiled. Exceptionally high photoluminescence quantum yield (Φ_F ≈ 60%) is realized by non-negligible oscillator strength of the S₁ transition, and rigidified 2D-type structure. Moreover, this luminescent 2D-type CT crystal exhibits balanced ambipolar transport (µ_h and µ_e of ≈10^-4 cm² V^-1 s^-1 ). As a consequence of such unique optoelectronic characteristics, the first CT electroluminescence is demonstrated in a single active-layered organic light-emitting transistor (OLET) device. The external quantum efficiency of this OLET is as high as 1.5% to suggest a promising potential of luminescent mixed-stacked CT cocrystals in OLET applications.