Organic cocrystals: From high-performance molecular materials to multi-functional applications

2.2.1 CT interaction

CT interaction is a key driving force that plays a crucial role in the formation of cocrystals. These interactions involve a specific amount of CT that occurs in a cocrystal between donor and acceptor molecules.

Mulliken's valence bond theory explains how molecules interact, especially in forming cocrystals.^[15] According to Mulliken's theory, cocrystals are formed when the least unoccupied molecular orbital (LUMO) of a donor interacts with the highest occupied molecular orbital (HOMO) of an acceptor to create new molecular orbitals. The energy required for this process depends on various factors, including the ionization potential and electron affinity of molecules. When two molecules approach each other, their atomic orbitals merge to form molecular orbitals, which can be either bonding or antibonding depending on the phase alignment and energy relationship of the interacting atomic orbitals.

In cocrystals, the interaction between two unit molecules is important. Typically, the donor molecule possesses a relatively low-lying electron-rich molecular orbital, whereas the acceptor molecule has a comparatively high-lying electron-deficient molecular orbital. As these molecules come into proximity, their molecular orbitals interact, and new molecular orbitals are generated. Mulliken proposed that the wave function of a CT complex is a linear combination of nonbonded and bonded states, which involves the classical electrostatic attraction between the electron donor and acceptor and CT between them.

As we have discussed, n-type and n⁻-type donor molecules contain lone pair electrons, while π-type and σ-type donor molecules contain π and σ electrons. These electrons can be transferred to acceptor molecules with low electron affinity energies through intermolecular interactions. For example, π-type electron acceptors, such as aromatic compounds and unsaturated hydrocarbon derivatives, can interact with π-type or n-type donor molecules. On the other hand, σ*-type electron acceptors, such as halides and halogenated compounds, can engage with σ-type donor molecules.^[16]

The degree of CT (DCT) depends on the electron affinity energy of the acceptor, the ionization energy of the donor, and the ionization energy of the donor-acceptor pair. A large DCT results in a cocrystal with strong ionic properties that adopts a disaggregated stacking pattern to stabilize its ionic state. Conversely, smaller DCTs (DCT < 0.5) typically yield neutral/quasineutral cocrystals.^{[17, 18]} Modulating the DCT leads to an alteration in the cocrystal conductivity, consequently enabling the manifestation of diverse material properties, ranging from metallic, to insulating, semiconducting, and even extending to superconducting characteristics.^[19] And in the CT cocrystal, the valence band will be close to the HOMO of the donor while the conduction band will relate more to the LUMO of the acceptor (Figure 3A).

CT in organic cocrystals can be qualitatively determined using electron spin resonance (ESR) spectroscopy,^[22] polarized light microscopy (POM), and solid-state nuclear magnetic resonance (NMR).^[23] The reactive radicals detected using ESR spectroscopy indicate CT properties in the ground state. ESR detects reactive radicals in cocrystals with a resonance signal close to g (2.0023), corresponding to free electrons. Solid-state NMR reveals changes in carbon atom environments, indicating CT shift states. The POM observations of the differences between the morphologies of cocrystal components help to examine cocrystal formation.^[24]

2.2.2 π-π interactions

There are two types of π-π stacking:^[29] the conventional interaction among aromatic rings and the structurally complementary curved π-π force, which exhibits a pronounced directionality. Characterized by van der Waals forces, these interactions promote dense packing of aromatic rings in aggregated forms. Notably, the intensity of π-π interactions greatly impacts the electronic and photoelectric conversion properties of organic substances. In the quest for novel semiconductor materials, minimizing the distance between π-stacked planes is vital, as closer packing tends to enhance electrical conductivity, as evidenced by studies.^{[20, 30]} There are instances of π-π stacking between relatively electron-deficient and electron-rich molecules, which create the weak interactions that are typically observed between aromatic rings.^[31]

The general design strategy for π–π interactions involves selecting molecules with strong electronic abilities, such as pyrene (Pyr)-TCNQ,^[32] or donor-acceptor molecules with similar frameworks,^[31] such as arene-perfluoro aromatic interactions.^[28] These strategies contribute to optimizing the photoelectric properties of organic cocrystals. In a notable example, Gao et al. have made significant advancements in the design of novel fullerene bowl-shaped molecules. By carefully adjusting the size and arrangement of the functional groups on these molecules, they have optimized the intermolecular π-π interactions (Figure 3B). This strategic design approach has enabled the formation of cocrystal with C₇₀ through strong concave-convex interactions. Importantly, these tailored interactions have not only facilitated the self-assembly of the fullerene molecules but also significantly improved their charge transport properties.^[25]

2.2.3 Hydrogen bond interaction

The hydrogen bond (X-H···A) is one of the earliest studied interactions, and it is quite common in functional materials and biological systems. This bond plays a vital role in crystal engineering and has numerous applications because of its unique properties, such as stability and directionality, and its electrostatic nature.^{[33, 34]} In organic cocrystals, element X typically includes C and O, and element Y commonly includes N, O, F, and S.^[35] An obvious aspect of these cocrystals' luminescence properties is their tendency to form O-H···N hydrogen bonds, which is further modulated by parameters such as the number of proton donors and the substituent features (Figure 3C). Electronegativity plays a decisive role in defining the strength of hydrogen bonds, where a higher electronegativity of atoms X (proton donor) and A (proton acceptor) amplifies the hydrogen bond's attractive force. Consequently, manipulating the electronegativities of these atoms allows for tuning the electrostatic interaction's intensity, which directly governs the hydrogen bond's robustness.

In the locked-arm supramolecular ordering strategy, donor, and acceptor units with complementary rigid and flexible side chains are extended in 1D and 2D directions by forming complex hydrogen-bonding networks, which then form binary cocrystals under the synergistic effect of CT interactions.^[36] Hydrogen bonding enhances ground-state CT interactions while maintaining the CT properties of the components and forming p/n-type two-channel organic semiconductor crystals.

The energy of the hydrogen bond is 0.25–40 kcal/mol.^[37] The hydrogen bond strength depends not only on the proton-donating ability of C–H but also on the electron density of the p-donor. Thus, the following order is obtained: sp-CH > sp²-CH > sp³-CH; CHX₃ > CH₂X₂ > CH₃X (X, any electron-withdrawing atom or group).^[38]

2.2.4 Halogen bond interaction

The halogen bond has definite directivity and strength, which is not only conducive for the reasonable and accurate prediction of crystal structures but also widely used in 1D, 2D, or 3D crystal construction.^{[30, 39]} In addition, the migration of protons in a hydrogen-bonded cocrystal can induce ferroelectricity, which is uncommon in organic matter.^{[6, 40]} This is because when the temperature drops below the Curie temperature, the space point group and hydrogen bond length change, proton migration occurs, and a stable dipole moment is generated, leading to ferroelectricity.^[41] Examples of organic cocrystals in ferroelectric materials are described in detail in the subsequent sections.

The halogen bond is formed by the attraction of the electrophilic region of the R-X halogen atom to the nucleophilic region of Y-Z, in the form of R-Y^…X-R, where R-X is the halogen bond donor and Y-Z is the halogen bond acceptor.^{[42, 43]} The cocrystal containing halogen bonds is a σ-cocrystal, where lone pairs of electrons from electron-rich atoms (N, O, S, Se, I⁻, Br⁻, Cl⁻, and F⁻) transfer to the halogen atoms (I, Br, Cl, F) in the acceptor.^[44]

Similar to the hydrogen bond, the halogen bond is strong, stable, and directional. However, it is hydrophobic and more sensitive to steric bulk or secondary interactions. The polarizability of the halogen atoms (I > Br > Cl > F) and the electron-withdrawing ability of the substituent are related to the strength of the halogen bond donor. The sp-hybridization modifies the strength of the halogen bond, and it generally follows this order: C(sp)-X > C(sp²)-X > C(sp³)-X.^{[45, 46]} The high strength and directivity of halogen bonds can promote the construction of stable cocrystal supramolecular structures with various photoelectric properties.^[47] For example, the organic molecules DPEpe and F₄DIB, which constitute the DPEpe-F₄DIB cocrystals, are united by halogen bonding (Figure 3D). This is evidenced by the normalized contact distance of R_N⋅⋅⋅I = 0.79, which falls within the range indicative of a halogen bond. Furthermore, the bright green edges observed in the organic microcrystals signify a pronounced characteristic of a 2D optical waveguide.

2.2.5 AP interactions

In addition to serving as strong acceptors, these conjugated molecular donors, including anthracene and coronene, have the capability to form cocrystals with weak acceptors such as octafluoronaphthalene (OFN). This type of interaction is commonly referred to as the AP interaction.^[48]

The AP interactions in the formation of organic cocrystals are a significant aspect, as discussed by Hu et al. These interactions, where perfluoroaromatics complex with aromatics or their derivatives in a 1:1 ratio, result in an almost parallel and alternating stacking pattern in the solid state.^[28] Unlike CT interactions, AP interactions lack the characteristic absorption bands in the ultraviolet (UV) spectra, indicating a distinct mechanism that can be leveraged to generate novel optical and electronic properties through the pairing of AP components (Figure 3F).

Besides, Huang et al. demonstrated how AP interactions can be utilized to combat aggregation-caused quenching (ACQ), a common issue in CT interactions. By co-assembling the polycyclic aromatic hydrocarbon (PAH) chromophore with OFN, they were able to mitigate the ACQ effect.^[49] The use of OFN as a molecular barrier effectively inhibits π-π stacking and other interactions among PAH chromophores, thereby minimizing luminescence quenching. This straightforward and cost-effective approach significantly enhances the photoluminescence quantum yields (PLQYs) in the solid state. In a related study, Ye et al. introduced a novel concept involving periodic molecular barrier arrays that can impede the transition from singlet to triplet states, ultimately achieving a nearly 100% PL quantum efficiency (PLQE) in π-conjugated organic cocrystals.^[50] These results elucidate the underlying mechanism and open up an exciting new direction for the development of solid-state, highly efficient non-bluing organic luminescent materials, potentially overcoming the fundamental bottleneck in the advancement of organic optoelectronics and photonics.

Furthermore, Zhang et al. introduced the concept of solubility in AP cocrystal formation, highlighting its importance in the synthesis process.^[51] The formation of the tetraphenylethylene (TC)-OFN cocrystal is achieved through a green grinding co-assembly technique. In this method, a solid mixture of TC and OFN is manually ground in the presence of a small amount of tetrahydrofuran (THF) as a catalyst. The low solubility of TC necessitates a substantial excess of OFN, which not only acts as a coformer but also functions as a solid solvent. The surplus OFN, along with the catalytic THF, facilitates the intercalation of OFN molecules into the TC phase. By acting as molecular barriers, OFN molecules prevent the close π-π stacking typical of pure TC crystals, disrupting their close-packing mode. The regular insertion of OFN molecules increases the intermolecular distance, effectively overcoming the ACQ effect observed in pure TC solids. This approach exemplifies how manipulating AP interactions can lead to the development of cocrystals with tailored properties.

In summary, investigating the link between intermolecular interactions, stacking structures, and functions is crucial for designing and developing organic cocrystals with enhanced properties. However, owing to the diversity of noncovalent intermolecular interactions and molecular structures, further in-depth investigations are required to develop more universal techniques.

5.1.1 Conductivity

Organic cocrystals show unique conductive properties when donor and acceptor molecules interact and the energy gap between the HOMO of the donor and LUMO of the acceptor decreases. This interaction facilitates electron excitation and transfer, resulting in partial CT and the formation of a mixed-valence band. This delocalization transforms organic cocrystals from insulators to semiconductors, conductors, and even superconductors. The application of organic cocrystal in electrical conductivity is shown in Figure 8.

Rosenfeld et al. synthesized a TTF-TCNQ cocrystal using TTF as the donor and TCNQ as the acceptor in a 1:1 molar ratio.^[5] Hiraoka et al. used inkjet printing to create a TTF-TCNQ thin film with an electrical conductivity of approximately 10 S/cm and used it in a pentaphene thin film transistor (Figure 8A–E).^[82]

The exceptional conductivity of organic cocrystals opens new avenues for energy materials, particularly in the realm of batteries and photocatalysis. In the field of lithium battery cathodes, Zheng et al. demonstrated that cocrystals could significantly enhance the electrochemical performance of energy storage devices.^[84] They prepared CN-1 and CN-3 cocrystals using a simple solution centrifugation method with naphthalene diamide (NDI) derivatives as the acceptors and coronene as the donor. The resultant cocrystal-based cathode showed a specific capacity of 230 mA h g⁻¹ and maintained good cycling stability. Park et al. further demonstrated the potential of cocrystals in lithium-ion batteries, using a C₆₀ and cHBC cocrystal as an anode material (Figure 8F–I). They achieved a reversible capacity of 330 mA h g⁻¹ with 80% retention after 600 cycles.^[83] These studies highlight the important role of organic cocrystals in energy storage solutions.

5.1.2 Charge transport

Cocrystal engineering provides a practical and simple strategy for systematically controlling the operation mode of transistors (ambipolar, p-type, or n-type) by modifying their components. The design and synthesis of materials have made significant advancements in achieving high-performance charge transport in organic materials for applications in basic electronic devices.^{[85, 86]} The application of organic cocrystal in charge transport is shown in Figure 9.

The method for constructing an organic cocrystal field effect transistor is shown in Figure 9A. The development of ambipolar charge-transporting cocrystals, as reported in 2004, has gathered significant attention, and it is a pivotal point in the exploration of cocrystal properties.^[87] Researchers have made substantial progress in the development of cocrystals with ambipolar charge-transport characteristics. Donors such as TTF derivatives and acceptors such as TCNQ and TCNQ-like derivatives have been instrumental in achieving ambipolar charge-transport properties in cocrystals.

For instance, a team led by Zhu and Hu synthesized a cocrystal named DPTTA-DTTCNQ in 2014 with electron and hole mobilities of 0.24 and 0.77 cm² V⁻¹ s⁻¹, respectively, and the device mobility did not decay significantly after 6 months of storage in the air atmosphere. Crystal structure analysis and quantum simulations showed that electronic coupling and super-exchange interactions simultaneously existed in this cocrystal and formed a quasi-2D electron and hole transport network, which was an effective model that revealed the internal mechanism for efficient carrier transport.^[89] In 2016, the team developed DPTTA-F₂TCNQ cocrystals with electron and hole mobilities of 1.57 and 0.47 cm² V⁻¹ s⁻¹, respectively, which were the highest values observed in complex systems.^[68] Zhu et al.’s study on perylene (Pe) and TCNQ co-crystals shows that cocrystal type and shape depend on the concentrations of donor and acceptor molecules.^[88] Higher Pe levels lead to 3:1 cocrystals (P3T1), and lower levels result in 1:1 co-crystals (P1T1). P1T1 forms n-type wires with electron mobility of 0.05 cm² V⁻¹ s⁻¹, and P3T1 forms ambipolar blocks with a high hole mobility of 0.03 cm² V⁻¹ s⁻¹ and a lower electron mobility of 2.1 × 10⁻⁵ cm² V⁻¹ s⁻¹ in the same conditions (Figure 9B).

Gao et al. successfully synthesized a cocrystal of C₇₀:Buckybowl (Figure 9C), which exhibited ambipolar transport characteristics with electron and hole mobilities of up to 0.40 and 0.07 cm² V^–1 s^–1, respectively. This study represents the first device application of a functionalized C₇₀ fragment, showcasing the high potential of buckybowls for the development of novel semiconductors in organic devices. Additionally, it highlights their potential for future advancements in solution-phase fullerene chemistry.

Hu et al. introduced a novel method for creating high-performance bipolar organic cocrystals. They synthesized ZnPc-F₁₆CuPc cocrystals by pairing molecules with closely aligned π-conjugated backbones. The resulting ZnPc-F₁₆CuPc cocrystals demonstrated remarkable field-effect mobilities of 0.68 cm² V⁻¹ s⁻¹ for electrons and 1.64 cm² V⁻¹ s⁻¹ for holes.^[90] This approach leveraged geometric and electronic factors to facilitate the formation of high-quality microcrystals and nanocrystals with high field-effect mobility for electrons and holes. These advancements in ambipolar charge-transport properties and the ability to control transistor operation modes demonstrate the versatility and potential applications of organic cocrystals in the field of organic circuits.

Overall, cocrystallization can be used to adjust the bandgaps of semiconductors to facilitate energy matching between the frontier orbitals of cocrystals and the work function of injected electrodes, which is beneficial for efficient charge injection to improve the performance of organic field-effect transistors.

5.1.3 Ferroelectricity property

Inorganic ferroelectrics form through mechanisms like i) Order-disorder with polar molecules or ions creating spontaneous polarization at temperatures below the Curie point, and ii) Displacive, where ion displacement leads to permanent polarization due to structural instability. Organic ferroelectrics, despite their scarcity and limitations to polymers like polyvinylidene fluoride and compounds like thiourea, have seen new exploration through co-crystallization, enabling tailored properties via non-covalent interactions. Organic ferroelectrics often involve iii) Proton transfer in hydrogen-bonded systems like KDP, where polarization reversal occurs with proton movement. Supramolecular ferroelectrics, including charge-transfer complexes, form ferroelectricity through distorted stacking, leading to one-dimensional chains with alternating donor-acceptor pairs that can switch polarity under an external field (Figure 10A).^[91]

Recent research has identified it as a method for obtaining organic materials with promising near-room-temperature ferroelectric properties. Cocrystals based on intermolecular hydrogen bonds have demonstrated ferroelectric behavior.^{[92, 93]} Organic ferroelectric cocrystals usually contain CT processes. The CT strategy for cocrystal formation, exemplified by TTF and p-chloranil (QCl₄) cocrystals, leads to π-stacked columns of alternating donor and acceptor molecules in one dimension. The arrangement of molecules in the crystal lattice can result in either a ferroelectric or an antiferroelectric state, depending on the 3D arrangement of polar chains. TTF-QCl₄ exhibits both ordering style, and its dielectric properties can be controlled through chemical and physical modifications.^[94]

Researchers in this field have also explored cocrystals with hydrogen bonding and CT forces. Tayi et al. studied three cocrystals prepared using homotetramethylene diimide as the acceptor and naphthalene, Pyr, and TTF as donors.^[6] The cocrystals exhibited ferroelectric behavior at 5–400 K and polarization hysteresis was observed at room temperature, with residual polarization exceeding 1 µC/cm. The cocrystal with a ratio of 1:4 demonstrated large polarization at low temperatures, which was the highest value reported for organic cocrystal ferroelectrics. Researchers speculated that this was due to the CT process in the cocrystal and proton dynamics.

5.1.4 Dielectric properties

Researchers have investigated the dielectric properties of cocrystals and their responses to temperature changes. Inabe et al. examined the dielectric response, phase transition process, and molecular rotation of a cocrystal consisting of a range of thickened aromatic hydrocarbon donors and a TBPA acceptor molecule at different temperatures.^[95] The single crystal with the TBPA acceptor molecule did not exhibit dielectric response properties, whereas the cocrystal consisting of the same components showed significant molecular reorientation and dielectric response phenomena (Figure 10B). Additionally, a phase transition in one of the crystals led to an abrupt change in the dielectric constant of the cocrystal at the phase transition temperature. This study suggested that cocrystals had the potential to become a class of molecular dielectrics, thereby expanding the range of dielectric-responsive materials.

In a word, organic cocrystals have garnered significant interest for their potential applications in electrical fields due to their unique molecular arrangements and tunable electronic structures. Their conductivity can be enhanced through molecular design, optimizing CT between molecules. Cocrystals with bipolar transport properties support the movement of both electrons and holes, crucial for high-performance electronic devices like transistors and diodes. Certain organic cocrystals exhibit ferroelectricity, enabling them to maintain polarization states even after the removal of an electric field, opening avenues for non-volatile memory devices and sensors. Additionally, their dielectric properties can be modulated by altering molecular polarity and interactions, making them promising for applications in capacitors and insulating materials. The exploration of organic cocrystals continues to pave the way for innovative electrical applications, with ongoing research promising advancements in material performance and device development.

5.2.1 Photoelectric detection

Current research on organic cocrystal-based detectors has focused on UV and visible detectors. In these cocrystal complexes, intermolecular interactions create a new CT state between a donor and acceptor, leading to a red-shift in absorption. The strength of the CT interaction determines the absorption band shift, making it possible to achieve optical responses in the infrared or near-infrared (NIR) regions. Hu et al.^[96] prepared a 9,10-diphenylethynylanthracene-homophthalic diimide cocrystal using the solution self-assembly method and used it to fabricate photoresponsive transistor devices. These devices exhibited a photoresponsivity (R_max) of 1.67 × 10³ A/W, and detection degree (D*_max) of 2.06 × 10¹³ Jones, which demonstrated the high potential of organic cocrystals for photodetector applications Zhu et al. prepared P1T1 and P3T1 cocrystals by adjusting the concentration of Pe and TCNQ in acetonitrile solution in a controlled manner.^[88] Further morphological and structural characterization showed that the P1T1 complexes self-assembled into linear cocrystals by segregated stacking, while the P3T1 complexes aggregated into massive crystals by mixed stacking (Figure 11A). The results indicate that the molecular stacking structure of the CT cocrystal largely affects the charge transport behavior and photoresponsivity.

5.2.2 Organic photovoltaic

Energy scarcity is a significant limitation for rapid economic growth in the modern era. Organic photovoltaic (OPV) devices are novel energy-collection devices that have advanced rapidly. However, there is ongoing research on enhancing the photoelectrical conversion efficiency of OPV devices. Organic cocrystals show promise in this regard because of their ability to enhance exciton diffusion, dissociation, and transport.

Kochi et al. demonstrated that the CT excitons generated in TCNB-based cocrystals could act as highly localized excited ion-pair states, potentially leading to efficient photovoltaic devices. In certain cocrystals, the yield of carrier dissociation is surprisingly close to 50%, implying that organic cocrystals may function as efficient photovoltaic devices.^[98] Zhang et al. prepare C₆₀/C₇₀-DPTTA cocrystal. The C₆₀-DPTTA co-crystal solar cell had an open-circuit voltage (V_OC) of 0.48 V, short-circuit current density (J_SC) of 0.3 mA/cm², and fill factor (FF) of 0.183, leading to a power conversion efficiency (PCE) of 0.27%. Conversely, the C₇₀-DPTTA co-crystal solar cell showed a V_OC of 0.17 V, J_SC of 0.002 mA/cm², and FF of 0.157, resulting in a significantly lower PCE of 0.0005% (Figure 11B). The power conversion efficiency of C₆₀-DPTTA is 540 times greater than C₇₀-DPTTA, primarily attributed to a substantial disparity in charge recombination efficiency.^[97]

5.2.3 Photocatalysis

A donor-acceptor system facilitates the spontaneous transfer of charges from a donor to an acceptor, thereby enhancing exciton separation in organic photocatalysts. Excitons undergo a gradual electron transfer process, resulting in the formation of a charge-separated state. This state prevents their recombination and complexation with the ground state, leading to the generation of more photogenerated carriers that actively participate in the photocatalytic reaction. Consequently, the creation of cocrystals emerges as a logical approach to efficiently generate and separate photogenerated carriers.

Wang et al. prepared cocrystals of OFN-9,10-dimethylanthracene (DMA) and OFN-Pe in large quantities using liquid-assisted milling.^[99] These molecular cocrystals showed AP interactions between donor and acceptor molecules (Figure 12A).^[99] This cocrystal can degrade over 95% of Rhodamine B (RhB) within just 20 min under broad-spectrum illumination (300–780 nm), significantly compassing its individual components, which only achieve degradation efficiencies of 44% and 80%, respectively under the same conditions. The reaction rate constants (k) for OFN, DMA, and the OFN-DMA composite are 0.023, 0.068, and 0.12 min⁻¹, respectively. Moreover, the specific activity per unit area (K) of the OFN-DMA co-crystals is 167.7 times greater than that of TiO₂ and 12.1 times greater than g-C₃N₄, underscoring the effectiveness of molecular co-crystallization engineering in enhancing photocatalytic performance.

Liu et al. developed a novel metal-organic framework (MOF) self-assembled cocrystalline material by embedding an electron-rich guest molecule (DMA into electron-deficient strontium–NDI (Sr-NDI).^[100] The unique porous structure of this material was conducive to increasing the interfacial area for photocatalytic reactions. Each Sr²⁺ ion is coordinated by six oxygen atoms of the ligand and two oxygen atoms of N,N-dimethylformamide. Two guest molecules are integrated into adjacent ligands via π–π stacking, resulting in an orderly ··· A ··· D ··· D ··· A ··· structure with molecule spacing of approximately 3.45 Å (Figure 12B). In photocatalytic tests, the MOF cocrystal degraded over 95.31% of RhB in just 15 min, while the original MOF managed only 6.93% degradation under similar conditions.

5.3.1 Photothermal imaging

Wang et al. developed a cocrystal from dibenzotetrathiafluoromanganese (DBTTF) and TCNB to investigate its use in photothermal imaging.^[23] This photothermal material, when irradiated with NIR light, exhibits a substantial temperature increase, monitored using an infrared thermographic camera (Figure 13A). They prepared a cocrystal pattern “TJU”, which progressively brightened under prolonged exposure to NIR light (808 nm, 0.7 W/cm²).

Zhuo et al. introduced a remarkable large-area photothermal nanofiber membrane designed for photothermal imaging through electrospinning.^[101] The nanofibers utilized the CF–PU CT cocrystal, which was derived from coronene and F₄TCNQ, and exhibited a notable increase in temperature from 26 to 52°C in only 250 s under 808 nm laser irradiation. In addition, they showed a high NIR PTC efficiency of 53.7%. These findings highlighted the significant potential of CF-PU nanofibers in NIR photothermal imaging.

5.3.2 Solar water evaporation device

Wang et al. strategically reduced the intermolecular interactions and enhanced the internal motion of aggregated state molecules, which was crucial for increasing the photothermal conversion efficiency. They constructed an integrated device for water-to-electricity coproduction by utilizing a dendritic triphenylamine derivative as the donor and heterocyclic QCN as the electron acceptor. This resulted in a remarkable water evaporation rate of 0.94 kg m⁻² h⁻¹ under simulated sunlight irradiation, which demonstrated the potential of organic photothermal materials in desalination, waste heat, and power generation applications.^[10]

In a related study, Tang et al. synthesized BTPPA derivatives (BTPAA, BTPEA, and BTPPA) with a D-A structure, where the diazole fragment BTT and triphenylamine derivatives acted as powerful electron acceptor and donor units, respectively.^[102] They achieved a high photothermal conversion efficiency of 60% under laser irradiation at 660 nm and 0.3 W/cm², which demonstrated the potential of customized molecular structures for optimizing photothermal properties.

Tian et al. introduced TTF-TCNQ cocrystals (CTCC-S3) for solar desalination and water purification.^[103] In their study, CTCC-S3 cocrystals were floated on water. After 30 min of solar simulator irradiation at 1 Sun intensity, CTCC-S3 reached 47.4°C, inducing rapid steam generation (Figure 13B). The device showed a solar-water evaporation conversion efficiency of 90% under irradiation of the Sun, which corresponded to a water evaporation rate of 1.67 kg m⁻² h⁻¹. MB-polluted water turned colorless, confirming dye removal. Solar evaporation effectively purified E. coli contamination and removed heavy metal ions (Fe³⁺, Cu²⁺, Hg²⁺, Cd²⁺, Ag⁺, Pb²⁺, Co²⁺, Ni²⁺, Zn²⁺) with ∼99.8% efficiency. The system also achieved ∼99.9% removal of ions (Na⁺, Mg²⁺, K⁺, Ca²⁺) in seawater, meeting the World Health Organization standards.

5.3.3 Biological applications

Duan et al. constructed MOF cocrystals with electron-deficient NDI as the acceptor and Pyr as the donor. The duplex intercalation structure and high-density group distribution of the MOFs contributed to enhanced and stabilized CT, resulting in a remarkable photothermal conversion efficiency of 41.8%. They also reported the development of a host-guest MOF (Pyr@Ca-NDI) cocrystal for in vivo photothermal cancer treatment. This electron-deficient MOF was composed of NDI ligands and Ca²⁺ metal nodes, and it exhibited significant efficacy in restraining tumor growth, as confirmed by a reduction in tumor volume and histological analysis. This highlighted the potential of MOF cocrystals for biomedical applications.

Zeng et al. synthesized the host Ca-NDI naphthalene diimide MOF using a modified solvothermal reaction, employing N,N'-bis(5-isophthalic acid) naphthalene diimide as the ligand and Ca²⁺ as the metal ion nodes.^[104] The Pyr molecule within this framework is self-assembled in parallel to the nearest NDI unit at a distance of 3.47 Å. Post-treatment histological analysis using hematoxylin and eosin of tumor tissues demonstrated a distinct margin between normal and severely necrotic areas in the Py@Ca-NDI + hv group, which reveals the potential application of light-induced cancer photothermal therapy of MOF cocrystal (Figure 13C).

Wu et al. synthesized 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP)-TCNQ nanoparticle (NP) cocrystals and found that under 808 nm laser irradiation, the temperature of the TAPP-TCNQ NPs could reach 28°C after 5 min, while the temperature of pristine TAPP NPs and TCNQ NPs changed only slightly.^[105] The antibacterial ability of TAPP-TCNQ NPs was investigated in vivo using a wound infection model in female Kunming mice. The temperature of the infection areas of the mice in the TAPP-TCNQ NPs + L group could reach 51°C in 5 min and stay above 50°C within 10 min, while the temperature changes of those in the control group were negligible. The mice in the TAPP-TCNQ NPs + L group exhibited a better wound-healing effect compared to the other groups (Figure 13D).

5.3.4 Other photothermal applications

Li et al. developed a TMPD-TCBQ cocrystal through self-assembly and revealed a unique application for laser ignition under NIR laser irradiation.^[107] This cocrystal exhibited high-temperature performance and remote triggering capabilities at distances of up to 4 m. The incorporation of TMPD-TCBQ in a transparent polypropylene tube coupled with 808 nm laser irradiation showed excellent penetration, indicating potential exothermic reactions even when encapsulated. Additionally, the mixture of KNO₃ and TMPD–TCBQ showed rapid deflagration under 808 nm laser irradiation, thereby highlighting the efficacy of the cocrystal as a low-power NIR laser-responsive initiator. This innovative strategy not only allowed for the control of absorption through self-assembly but also provided photo-thermal conversion materials for diverse applications to meet evolving demands in laser ignition.

Huang et al. introduced the concept of photo thermoelectric conversion using organic cocrystals.^[103] The assembly of DBTTF, TTF, TCNB, and TCNQ resulted in the formation of organic cocrystals (DTC, TBC, and TQC), which exhibited effective power generation when combined with thermoelectric modules. Under the illumination of the sun, this combination demonstrated an outstanding open circuit voltage (0.427 V) and output power density (2.21 W/m²), which were higher than those of typical photothermal conversion materials. The integration of organic cocrystals with high-output-power-density thermoelectric devices has the potential to emerge as a viable solar-based photo thermoelectric conversion technology, which is particularly suitable for portable power production in remote rural locations.

In summary, with notable NIR absorption and low toxicity, cocrystals have the potential to be used in biological therapies by seamlessly integrating photothermal imaging with treatments for enhanced precision. Organic cocrystals with broad solar spectrum absorption combined with superior interface materials can be used for efficient seawater desalination. Their versatility extends to photothermal-to-thermoelectric conversion, thereby providing a novel perspective on eco-friendly electricity generation. Cocrystals present multifaceted solutions and show adaptability and potential for use in various fields.

5.4.1 Optical waveguide

The optical waveguide phenomenon refers to the guided propagation of light within a dielectric structure, and it is a pivotal aspect of integrated photonics. This structure typically comprises a core with a high refractive index surrounded by a cladding region with a low refractive index, which confines light through total internal reflection.^[109] Unlike inorganic materials, organic materials provide diversity, cost-effectiveness, and scalability for large-scale preparation. Micro/nanoscale binary crystals, which serve as 1D and 2D optical waveguides and logic devices, expand the range of materials, reduce the energy consumption in integrated photonics, and accelerate advancements in optoelectronics.^[110]

Zhuo et al. showed that a cocrystal (DPEpe–F₄DIB) based on halide bond interactions exhibited asymmetric optical waveguide performance.^[27] Unidirectional internal reflection, which was attributed to asymmetric crystal stacking, resulted in optical loss coefficients of 0.0346 and 0.0894 dB/mm along the (001) crystal plane, showcasing the potential of organic cocrystals in microoptical logic gates. In another research work, he successfully synthesized a NIR emissive CT cocrystal TP-F₄TCNQ (Figure 14A), which exhibited a maximum PL peak at 770 nm, a PLQY of 5.4%, and a radiation constant (kf) of 0.0048 ns⁻¹. The TP-F₄TCNQ single-crystalline microwires prepared in this study demonstrated active optical waveguiding properties in the NIR region,^[27] with a low optical-loss coefficient of 0.060 dB/µm.

Zhao et al. explored the effect of concentration on cocrystal properties.^[111] Two types of cocrystals with different molecular orientations and nanophotonic transmission characteristics were synthesized. M-DFC cocrystals displayed anisotropic photon propagation behavior along the first, second, and third directions of the crystal at 140/520/540 dB/cm, whereas the T-DFC cocrystal exhibited isotropic reabsorption waveguide loss along the A, B, and C directions of the crystal at 1040/1150/1080 dB/cm, highlighting the tunability of optical waveguide properties in organic cocrystals.

Moreover, Zhu et al. fabricated “cocrystal waveguide couplers” using Npe-TCNB and Bpe-IFB cocrystals.^[112] These structures, which were designed for energy transfer, facilitated the construction of photonic “AND” and “OR” gates and had the capacity for implementing complex optical functions. This provided a basis for advanced photonic materials and device technologies. This collective research highlights the potential of organic cocrystals in shaping the future of integrated photonics and optical device applications.

5.4.2 Photoemission

The recent advancement of organic solid fluorescent materials has expanded their use in sensors, multicolor displays, and anticounterfeiting materials. Adjusting the fluorescence characteristics of these materials for practical applications is an active area of research. Molecular aggregation in dispersed fluorescent materials can weaken or eliminate their fluorescence, posing challenges for practical use. Understanding the emission properties depends on both the chemical structure of the molecules and their accumulation in the solid state.^[115]

Cocrystal engineering has emerged as a valuable approach for enhancing the fluorescence properties of materials. Zhu et al. prepared 1,2-di(4-pyridyl)ethylene and TCNB molecules that self-assembled into parallelogram-like cocrystals (BTCs).^[116] Although 2D crystal morphology is ideal for anisotropy studies, researchers have observed that the photon propagation properties in a single BTC are independent of molecular stacking and orientation, indicating the versatility of cocrystal structures.

Further advancements in modulating optical emission were explored by Wu et al., who synthesized (TCNB)_x(OFN)_1-x(coronene)-doped cocrystals.^[117] They utilized CT-induced self-assembly and observed a shift in the emission color from blue to orange as the TCNB doping concentration increased. This demonstrated the potential for optical emission modulation in organic cocrystals. In addition, Zhang et al. successfully modified the intrinsic cocrystals BNT-TCNB by employing a donor/acceptor bipolar doping strategy to create ternary cocrystals doped with BCZ and TCNQ with tunable luminescent colors, lifetimes, and quantum yields.^[118] These cocrystals exhibit luminescence tunability from yellow-green to red and high photothermal conversion efficiencies.

Sang et al. demonstrated the first CT-cocrystal-based organic light-emitting transistor device, which achieved a high external quantum efficiency (EQE) of 1.5% in the pristine device structure (Figure 14B).^[114] The head-to-tail dipolar alignment of the complementary DCT3–ACT3 pair promoted the formation of a highly uniform and flexible 2D-type supramolecular structure. This distinctive assembly significantly influenced the photophysical properties, resulting in ambipolar characteristics within the 2:1 D:A stoichiometry. The CT3 system exhibited bright luminescence (ΦF = 60%) and ambipolar transport (µ_h and µ_e of = 10⁻⁴ cm² V⁻¹ s⁻¹).

Zhang et al. precisely controlled the multicolor PL behavior of microcrystals through CT interactions.^[119] The interaction between the electron donor (CzC₆Cz) and acceptor (FA) resulted in significant redshifts of 56, 104, and 221 nm in fluorescence emissions. This demonstrated the potential for manipulating the PL behavior of microcrystals through CT interactions involving two components.

Yu et al. pointed out that the emission behavior of cocrystals can be adjusted by applying mechanical force, as this leads to changes in intermolecular interactions and stacking patterns. Due to the phase transition caused by the grinding process, the cocrystals composed of donor and acceptor exhibit a blue-shifted emission under anisotropic grinding. In contrast, under isotropic pressure, the cocrystals exhibit a red-shifted emission due to tighter molecular stacking.^[115]

These collective advancements represent the dynamic landscape of research on organic solid fluorescent materials and provide insights into their diverse applications and the potential for further innovations in fluorescence modulation and control.

5.4.3 Room temperature phosphorescence

Organic room-temperature phosphorescent materials have long exciton lifetimes and high utilization, making them valuable for screen display, lighting, information encryption, anticounterfeiting, bioimaging, and photodynamic therapy. Researchers have achieved room-temperature phosphorescence through molecular design and materials engineering, focusing on improving efficiency, and lifetime, and exploring photophysical behavior. Strategies such as introducing heavy atoms, forming host-guest materials, constructing hydrogen aggregates, forming hydrogen bonds, and designing donor-acceptor structures have led to highly efficient organic room-temperature phosphorescent materials with long lifetimes.

Gao et al. designed and synthesized a Pyr-MP organic cocrystal through solution self-assembly (Figure 15A). This crystal exhibited NIR phosphorescence with a peak at 730 nm and a PLQY of 34.2%.^[120] The singlet states of MP molecules with an intermediate energy level effectively bridged the singlet and triplet states of Pyr molecules, leading to efficient NIR phosphorescence. It should be noted that the loose packing pattern of MP allows for versatile assembly with various aromatic molecules, ranging from ternary to six-membered benzene rings and even larger benzene skeletons. Unlike conventional cocrystal systems, cocrystal engineering based on MP molecules with weak π-π interactions eliminates the limitations related to molecular selection, uncertain assembly behavior, and unpredictable properties. This study provides valuable insights into the development of novel NIR-phosphorescent materials using a universal strategy.

Abe et al. explored the potential of halogen-bonding-based cocrystals by examining the connection between crystal stacking arrangements and organic room-temperature phosphorescence (ORTP) characteristics in binary cocrystals.^[124] These cocrystals were composed of 1,4-diiodotetrafluorobenzene (DITFB) and various PAHs such as phenanthrene (Phen), chrylene, and Pyr. To explore the impact of the crystal structure on PLQYs, a Phen-DITFB binary cocrystal was created by partially introducing Pyr into the ternary cocrystal. The stacking motifs in the ternary cocrystal inherited σ-hole–π interactions from the Phen-DITFB cocrystal, with Phen sites randomly substituted with Pyr molecules. In this scenario, the ORTP emission originated from Pyr, with the maximum PLQY exceeding 20%, which was attributed to the suppression of nonradiative decay by the altered crystal stacking pattern.

David et al. employed a co-assembly strategy based on strong hydrogen bonding interactions to successfully prepare nucleic acid base-based photofunctional cocrystal materials with excellent phosphorescent properties.^[121] These materials exhibit stable triplet and high-temperature phosphorescent properties, and can effectively generate long-lived triplet and single-linear states of oxygen. This discovery not only provides a new strategy for the preparation of high-performance phosphorescent materials but also opens up new avenues for information security and antimicrobial applications (Figure 14B).

5.4.4 Two-photon absorption properties

Two-photon-excited fluorescence and single-photon-excited fluorescence are different in terms of the simultaneous interaction of two photons in the former, which presents a fluorescence spectrum intensity with a quadratic relationship with the excited optical density. In contrast, single-photon-excited fluorescence exhibits a linear relationship with the excited optical density. The applications of two-photon-excited fluorescence are widespread and encompass fields such as two-photon microscopy, microfabrication, 3D data storage, optical power limiting, up-conversion laser technology, and photodynamic therapy.

Wang et al. synthesized two cocrystals using a naphthalenediimide-based triangular macrocycle and coronene.^[122] These co-crystals exhibited distinct optical properties. The triangle-shaped co-crystal emitted deep-red fluorescence, while the quadrangle-shaped co-crystal displayed deep-red and NIR emission at 668 nm, which was a significant red shift compared to the precursors (Figure 14C). This red shift was attributed to intermolecular CT interactions within the co-crystals. Furthermore, both co-crystals demonstrated higher calculated two-photon absorption cross-sections compared to their individual constituents.

Sun et al. explored the synthesis of styrylpyridine-TCNB cocrystals (STC) using 4-styrylpyridine as the donor and TCNB as the acceptor, resulting in light-yellow cocrystals.^[125] These cocrystals demonstrated a distinctive two-photon absorption (TPA) effect attributed to donor-acceptor stacking interactions, even though each material was TPA-free. This pioneering achievement marked the first realization of TPA through the cocrystallization of two TPA-free components. The ordered arrangement of the cocrystals enhanced the CT interactions, which were validated through spectroscopic analysis. The electronic communication facilitated by π conjugation in the supramolecular pair elucidated the TPA properties of the STC cocrystals arising from CT interactions. This innovative cocrystallization strategy provides a basis for modifying fluorescent properties through one-photon and two-photon processes, potentially opening avenues for groundbreaking applications in next-generation optoelectronic devices.

5.4.5 Thermally activated delayed fluorescence

Sun et al. investigated a 1:2 cocrystal composed of TCNB and trans-1,2-diphenylethylene and examined TADF.^[123] They employed analytical techniques, including absorption spectroscopy, variable-temperature steady-state and time-resolved PL spectroscopy, single-crystal X-ray diffraction, and first-principles computations, to clarify the intricacies of this cocrystal system (Figure 14D). Their findings revealed that within the cocrystals, intermolecular interactions played a pivotal role in diminishing the singlet-triplet energy gap, thereby facilitating reverse intersystem crossing for TADF. These results can not only improve the understanding of TADF mechanisms but also provide a basis for innovative designs and advancements in TADF materials.

5.5.1 Other application

The presence of unpaired and ordered electron spins within p-orbitals can lead to strong interactions, potentially aligning the spins and resulting in ferromagnetic properties, particularly in materials that contain heteroatoms. However, if the spin alignment is not possible, these materials may default to antiferromagnetic ordering or exhibit paramagnetic behavior.^[131]

The recent strides in this area have significantly motivated researchers to explore the magnetic properties of organic cocrystals. Cocrystal engineering presents a unique opportunity to fine-tune the multifunctional properties of these materials. For instance, Yang et al. have demonstrated that Pyr-F₄TCNQ cocrystals, under the influence of a horizontal electric field, generate a notably higher magnetoelectric coupling coefficient compared to that induced by a perpendicular electric field.^[132] This disparity is attributed to charge-transfer interactions and the anisotropic arrangement of molecules along the perpendicular axis, leading to anisotropic magnetoelectric coupling at room temperature. Such findings are particularly promising for the development of multiferroic memory devices.

Furthermore, these Pyr-F₄TCNQ cocrystals have unveiled additional intriguing properties, such as magneto-optical and Cotton-Mouton effects. These effects refer to the interplay between optical and magnetic properties, offering new avenues for the integration of magnetic and optical functionalities in a single material system. The discovery of these effects in organic cocrystals not only expands the scope of their potential applications but also underscores the vast potential of cocrystal engineering in creating materials with tailored properties for advanced technological applications.