Highly ordered organic crystalline materials provide an ideal platform for next-generation flexible electronics.

materials, [9][10][11][12] which are crystalline singlephase materials composed of two or more different compounds generally in a stoichiometric ratio, [13] providing a platform for unveiling the structure-property relationships at a molecular level. [14] The building blocks are assembled via noncovalent interactions, offering the opportunity to achieve noncovalent synthesis of functional molecules. [15] Compared with the traditional covalent synthesis, cocrystal engineering offers numerous advantages, including: 1) avoiding complicated synthesis procedures, and cocrystal assemblies have been successfully fabricated by the vapor methods and solution-processing methods in a facile and low-cost way; 2) manipulating intermolecular interactions through selecting suitable conformers from a plentiful supply of raw materials, resulting in tunable structures, morphologies, and sizes; 3) achieving rare and multifunctional properties through a collaborative strategy in distinct constituent units, which are difficult to realize for individual components. [16][17][18] Cocrystal engineering began to draw much attention since the exploration of the metal conductive tetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ) cocrystals in 1973. [19] The peculiar conductivity in organic materials makes it possible for cocrystals to serve as organic electrodes, which can effectively lower the contact resistance and further improve device performance. Encouraged by this, cocrystals can serve as a breakthrough and provide an opportunity to discover novel physicochemical properties, which are far beyond the performance of single materials. In this respect, dielectric response [20] and nonlinear optics [21,22] are also realized through rational design of each component, which is absent in their constitute components. Even more, the bottom-up supramolecular assembly provides the opportunity to equip them with the individual properties of the donor or acceptor to create multifunctional materials. [23] Typically, ambipolar charge transport can be generated by coassembling p-type semiconductors and n-type ones, and the charge-carrier mobility is now up to 1.57 cm 2 V −1 s −1 for holes and 0.47 cm 2 V −1 s −1 for electrons. [24] To date, these exciting results accelerate the investigations of organic functional cocrystals, such as room-temperature ferroelectricity, [25][26][27] optical waveguide properties, [28,29] pure organic room-temperature phosphorescent properties, [30][31][32] stimulusresponse (e.g., mechanical stress, [33,34] heat, [35,36] or solvent [37] ) characteristics, photovoltaics, [38] and near-infrared photothermal (PT) conversion and imaging, [39] etc.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 a...

Organic cocrystals, formed by a combination of electron-rich donors and electron-poor acceptors, play an important role in tailoring the optoelectronic properties of molecular materials. Charge transfer interactions in cocrystals not only endow them with an ordered three-dimensional (3D) supramolecular network in different constituent units, but also render them ideal scaffolds to control the intermolecular interactions in multicomponent solids. In this perspective, we firstly introduce preparation methods, molecular packing modes and charge transfer in organic cocrystals. Then, we focus on the novel and promising optoelectronic properties of organic cocrystals based on charge transfer interactions. Finally, we briefly discuss the outlook for the future development of these multicomponent crystalline materials.

Cocrystals of 4-styrylpyridine and 1,2,4,5-tetracyanobenzene were successfully prepared by supramolecular self-assembly. Donor-acceptor interactions between the molecular components are the main driving force for self-assembly and contribute to intermolecular charge transfer. The cocrystals possess two-photon absorption properties that are not observed in the individual components; suggesting that two-photon absorption originates from intermolecular charge-transfer interactions in the donor-acceptor system. The origin of two-photon absorption in multichromophore systems remains under-researched; thus, the system offers a rare demonstration of two-photon absorption by cocrystallization. Cocrystal engineering may facilitate further design and development of novel materials for nonlinear optical and optoelectronic applications.

The spectroscopic and photophysical properties of organic materials in the solid-state are widely accepted as a result of their molecular packing structure and intermolecular interactions, such as J- and H-aggregation, charge-transfer (CT), excimer and exciplex. However, in this work, we show that Spe-F DIB cocrystals (SFCs) surprisingly retain the energy levels of photoluminescence (PL) states of Spe crystals, despite a significantly altered molecular packing structure after cocrystallization. In comparison, Npe-F DIB cocrystals (NFCs) with new spectroscopic states display different spectra and photophysical behaviors as compared with those of individual component crystals. These may be related to the molecular configuration in crystals, and we propose Spe as an "intramolecular emissive" material, thus providing a new viewpoint on light-emitting species of organic chromophores. Moreover, the nonlinear optical (NLO) properties of Npe and Spe are firstly demonstrated and modulated by cocrystallization. The established "molecule-packing-property" relationship helps to rationally control the optical properties of organic materials through cocrystallization.

Organic cocrystal engineering refers to two or more organic molecules stoichiometrically combined and held together by noncovalent intermolecular interactions, which differs from standard chemical synthesis involving covalent bond breakage and formation. Organic cocrystals have unique properties and offer a new strategy for creating enhanced organics. First, however, some key questions need to be addressed: How do diverse monomers affect the intermolecular interaction kinetics during cocrystallization? How do the intermolecular forces in cocrystals affect cocrystal functions? In this Perspective, the definition and advantages of organic cocrystal engineering, specifically in the construction of a reliable intermolecular interaction–stacking structure–performance relationship, are outlined. Additionally, recent developments in the field and the questions above are discussed. Finally, a brief conclusion and some hints on likely future developments are provided.

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Organic cocrystals, as an intelligent design strategy of functional materials, have received extensive attention in the scientific community over recent years. This review focuses on recent advances and perspectives of organic charge transfer cocrystals in the area of stimuli-response.