Hydrophilic Cocrystals with Water Switched Luminescence

Abstract

Utilizing water molecules to regulate the luminescence properties of solid materials is highly challenging. Herein, we develop a strategy to produce water‐triggered luminescence‐switching cocrystals by coassembling hydrophilic donors with electron‐deficient acceptors, where 1,2,4,5‐Tetracyanobenzene (TCNB) was used as the electron acceptor and pyridyl benzimidazole derivatives were used as the electron donors enabling multiple hydrogen‐bonds. Two cocrystals, namely 2PYTC and 4PYTC were obtained and showed heat‐activated emission, and such emission could be quenched or weakened by adding water molecules. The cocrystal structure exhibited the donor molecule that can form multiple hydro bonds with water and acceptor molecules due to the many nitrogen atoms of them. The analyses of the photophysical data, powder X‐ray diffraction, and other data confirmed the reversible fluorescence "on‐off" effects were caused by eliminating and adding water molecules in the crystal lattice. The density functional theory calculations indicate that the vibration of the O‐H bond of water molecules in the cocrystal can absorb the excitation energy and suppress fluorescence. Furthermore, the obtained cocrystals also showed temperature, humidity, and H+/NH4+ responsive emission behavior, which allows their applications as thermal and humidity sensors, and multiple information encryptions. This research paves the way for preparing intelligent hydrophilic organic cocrystal luminescent materials.

Odd–Even Effect in Charge Transfer Complexes through the Regulation of the Alkyl Chain

“…17−21 The electronic properties of the CT complex can range from an insulator to a superconductor. As a typical aggregate behavior, the formation and corresponding property of the CT complex are heavily related to the intermolecular interactions between the donor and acceptor, 22,23 thus a better understanding and controlling intermolecular interactions are badly needed to develop related materials. 24−27 Although the crystal is a simple and intuitive way to explore the interactions of the donor and acceptor in the CT complex, it is challenging to culture.…”

“…Compared with the intramolecular CT within one single molecule, bimolecular donor–acceptor (D-A) type exciplex materials offer numerous advantages, such as avoiding complicated synthesis procedures and more flexible luminescent properties due to their tunable component and intermolecular interactions. , In particular, the intermolecular CT can easily lead to the spatial charge-separated state and the realization of small singlet–triplet splitting energy (Δ E ST ) accompanied by a thermally activated delayed fluorescence (TADF) effect, which has been demonstrated to show a great advantage in the application of high-performance OLEDs. − The electronic properties of the CT complex can range from an insulator to a superconductor. As a typical aggregate behavior, the formation and corresponding property of the CT complex are heavily related to the intermolecular interactions between the donor and acceptor, , thus a better understanding and controlling intermolecular interactions are badly needed to develop related materials. − Although the crystal is a simple and intuitive way to explore the interactions of the donor and acceptor in the CT complex, it is challenging to culture. − Besides, some CT complexes have an unavoidable and unpredictable aggregate structure with relatively disordered arrangements, leading to the limited understanding of the spatial interactions between D and A molecules. , For example, when the CT complex acts as the emissive layer in OLEDs, it is fabricated from a mixture of D and A molecules by solution or vacuum precipitation. , Thus, regulating and understanding the packing and intermolecular interactions of the CT complex in powder or film state are important, while related research is still really scarce.…”