Unexpected organic hydrate luminogens in the solid state

Developing organic photoluminescent materials with high emission efficiencies in the solid state under a water atmosphere is important for practical applications. Herein, we report the formation of both intra- and intermolecular hydrogen bonds in three tautomerizable Schiff-base molecules which comprise active hydrogen atoms that act as proton donors and acceptors, simultaneously hindering emission properties. The intercalation of water molecules into their crystal lattices leads to structural rearrangement and organic hydrate luminogen formation in the crystalline phase, triggering significantly enhanced fluorescence emission. By suppressing hydrogen atom shuttling between two nitrogen atoms in the benzimidazole ring, water molecules act as hydrogen bond donors to alter the electronic transition of the molecular keto form from nπ* to lower-energy ππ* in the excited state, leading to enhancing emission from the keto form. Furthermore, the keto-state emission can be enhanced using deuterium oxide (D2O) owing to isotope effects, providing a new opportunity for detecting and quantifying D2O.

W ater is essential for regulating physiological and biochemical processes in living organisms [1][2][3][4][5] . Water can form intermolecular hydrogen bonds with inorganic compounds, organic molecules, and biological macromolecules, such as proteins and enzymes, inducing fascinating functionalities [6][7][8][9][10][11][12][13] . For example, water can help remove damaged adenine residues from oligonucleotides in the presence of a DNA glycosylase 14 . The structural participation of bound water in biomolecular recognition provides effective predictions for drug design 15 . However, water also causes some problems and limits the application of certain devices, such as perovskite solar cells and organic light-emitting diodes (OLEDs). For example, the performance of perovskite solar cells under an air atmosphere dramatically decreases over time because water in the air destroys the perovskite structure. Hydrogen bonding is among the most important noncovalent intermolecular interactions 16 , determining molecular conformation, molecular aggregation, and the function of numerous macroscopic substances. For example, bound water molecules exhibit liquid properties rather than gas properties owing to the existence of multiple intermolecular hydrogen bonds. In addition, hydrogen bond can also be used to design smart materials, such as self-healing polymers 17,18 , artificial muscle 19 , and organic or inorganic fluorescent/phosphorescent materials 20,21 . Hydrogen bond-assisted emission behaviour has mostly been attributed to restricted intramolecular rotation and vibration 22,23 .
Smart (stimuli-responsive) organic materials are able to reversibly respond to external stimuli, allowing their properties to be controlled by environmental stimuli, including temperature, electricity, mechanical, magnetic fields, chemicals, and stress [24][25][26][27][28] . Organic photoluminescent (PL) materials, as a class of smart organic materials, have gained considerable attention owing to their highly sensitive signal response and practicality-no sophisticated or expensive instruments are required [29][30][31][32][33][34] . However, in polar solvents, such as water, many organic PL materials show weak or quenched luminescence owing to polar interactions between the electronic structure and water, limiting their practical applications. Recently, certain organic PL materials have shown strong luminescence in the solid state without any solvent. In contrast, the emission behaviour of organic PL materials in the solid state under a water atmosphere remains in its infancy, but is highly important because most practical applications, such as OLEDs, are performed under an air atmosphere. Therefore, we are interested in designing a type of molecule to achieve high emission efficiencies in the solid state under a water atmosphere as an important development in this field.
Herein, we report a series of organic PL materials that can form hydrated crystals with water to produce organic hydrate luminogens (OHLs; Fig. 1, Supplementary Figs. 1, 5, 9, and 13). These OHLs are tautomerizable Schiff-base materials (2- imidazol-2-yl)imino)methyl)-5-methoxyphenol (named as 1c)) in which water plays a crucial role in regulating molecular stacking modes and effectively induces turnon fluorescence. Hydrated single crystals (microcrystals) and anhydrous single crystals of the compounds were obtained in the presence and absence, respectively, of water (H 2 O) or deuterium oxide (D 2 O). The three Schiff-base compounds in anhydrous organic solvents, the anhydrous polycrystalline powder state, and the anhydrous single crystal state all showed weak fluorescence. However, their water-containing microcrystals and single hydrate crystals exhibited enhanced fluorescence due to the formation of a hydrogen-bonding network, which hindered excited state tautomerization and thus minimized energy dissipation. Intercalated water molecules altered the molecular reorganization of these small organic compounds in the hydrated crystalline state. Strong affinity between organic molecules and water blocked the in situ structural rearrangement due to the formation of intermolecular hydrogen bonds in the hydrated crystals. Furthermore, reversible "on-off" fluorescence switching of the three organic compounds was achieved by hydration and dehydration of their polycrystalline samples. Compounds 1a, 1b, and 1c in anhydrous polycrystalline and crystalline phases induced strong light emission through in situ assembly with water ( Fig. 1). The mechanism of enhanced luminescence in these compounds was attributed to the inhibited tautomer formation of benzimidazoles, which altered the nπ* to ππ* electronic transition of their keto form, and increased molecular rigidity of their assemblies in the solid state.

Results
Synthetic target compounds. Target compounds 1a, 1b, and 1c were prepared in high yields by one-step aldimine condensations of commercially available materials (Supplementary Figs. 1, 5, and 9). These compounds were characterized by 1  Single crystals of target compounds. The single-crystal structures of the small organic compounds showed that all structures possessed intramolecular and intermolecular hydrogen bonds. Molecules of 1a interlinked to form a one-dimensional (1D) supramolecular chain ( Supplementary Fig. 17). Two molecules of 1b interlinked to form a dimer via two N-H···O hydrogen bonds, with these dimers forming a 1D supramolecular chain (Fig. 2a). Four molecules of 1c packed together to form a supramolecular ring via N-H···O and N-H···N intermolecular hydrogen bonds ( Supplementary Fig. 20). The molecular configuration of 1a single crystals showed a dihedral angle between R 1 (benzimidazole ring) and R 2 (phenyl ring) of about 18.56°( Supplementary Fig. 18). An intramolecular hydrogen bond between the hydrogen atom of a hydroxyl group and the nitrogen atom of a C=N bond (N•••H distance, 1.892 Å) and one intermolecular hydrogen bond between two adjacent molecules (N-H•••N distance, 2.133 Å or 2.072 Å) supported the aforementioned organization of the 1a molecules ( Supplementary Fig. 17).
Single crystals of target compounds with H 2 O or D 2 O. In organic crystal growth, the presence of H 2 O or D 2 O molecules induces reorganization of the supramolecular array of organic compounds. Although the supramolecular structures of the three compounds were distinctive, the water-containing single-crystal structures of 1a and 1c were similar. The stoichiometry of water molecules in the unit cell of hydrated single crystals was 1:1. The unit cell dimensions of hydrated single crystals of 1a and 1c were larger than those of their anhydrous counterparts. For compound 1b, the unit cell dimensions of hydrated single crystals were smaller than those of their anhydrous counterparts (Supplementary Tables 1-3). Taking compound 1a as an example, a water molecule acted as a tridentate H-bonding moiety to link three surrounding molecules into a 1D ladder-shaped superstructure ( Supplementary Fig. 17). The dihedral angles between aromatic segments R1 and R2 in the Optical properties. Hydrated single crystals and microcrystals of 1a, 1b, and 1c showed significantly enhanced fluorescence compared with their solutions and anhydrous powders (Fig. 2c,  Supplementary Figs. 28 and 29). We investigated the photophysical properties of all synthesized compounds in different states, namely, in solution, as anhydrous crystals, and as hydrated crystals. Supplementary Figs. 26a-26c show UV-visible absorption spectra of the chromophores in different anhydrous organic solvents (methanol, ethanol, tetrahydrofuran, and dichloromethane). The absorptions of the three compounds in the region of 320-450 nm was assigned to π-π* excitation of the enol ground state (E) to the first enol excited state (E*) (S 0 → S 1 ). We also investigated the PL properties of these compounds in different organic solvents ( Supplementary Fig. 27). The formation of the keto excited state (K*) occurred through the excited-state intramolecular proton transfer (ESIPT) process following photoexcitation (E → E*) of the enol ground states, as shown in Supplementary Fig. 33a. All chromophores showed almost negligible fluorescence with dual emission peaks (Supplementary Table 4) upon excitation. The position of the methoxy group significantly influenced the proton transfer event in the excited state. The short-wavelength emission of the compounds at 459 nm was attributed to the excited state enol form (E*) (enol emission), while peaks at 594 nm (1a), 620 nm (1b), and 572 nm (1c) were attributed to the excited state keto form (keto emission) through a four-level photocycle (Supplementary Fig. 33a). Methoxy group substitution at different positions had a negligible impact on enol emission wavelength, but significantly affected the keto emission wavelengths. In the K* state, the methoxy group at the 5-position in 1b further increased the electron density of the highest occupied molecular orbital (HOMO), which decreased the energy gap of the keto tautomer. Therefore, the ESIPT emission peak of 1b was red-shifted compared with that of 1a. However, when the methoxy group was located at the 4-position in 1c, the conjugation effect was hindered and an inductive effect occurred. This increased the energy level of the lowest unoccupied molecular orbital (LUMO) and increased the energy gap of the keto tautomer. Therefore, the ESIPT emission of 1c was blueshifted compared with that of 1a. These molecules were all typical ESIPT chromophores. C=N isomerization is the predominant nonradiative decay process of excited states in compounds with an unbridged C=N structure, such that solutions of the three compounds were nonfluorescent [35][36][37] .
Compared with UV-visible absorption spectra of solutions, the absorption spectra of microcrystals dispersed in aqueous solution were blue-shifted due to the formation of hydrated microcrystals ( Supplementary Figs. 26d-26f). This blue-shift in the absorption spectra of hydrated microcrystals indicated the formation of Haggregation, which was caused by cofacial stacking of the molecules. Subsequently, the emission spectra of hydrated microcrystals dispersed in water were investigated. A dramatic increase in the fluorescence intensity of hydrated microcrystals (microcrystals of  Table 5), accompanied with a red-shift to 593 nm. Steady-state fluorescence emission spectra of the hydrated single crystals (Supplementary Fig. 28) showed similar emission behaviour to the corresponding microcrystals. Steady-state fluorescence emission spectra of the molecular polycrystalline powders were also investigated ( Supplementary Fig. 28). All compounds were weakly emissive with fairly low fluorescence QYs. The emission wavelengths of polycrystalline powders of 1a, 1b, and 1c were blue-shifted compared with their individual hydrated single crystals and microcrystals.
As 1b showed the fastest water-binding rate among the three molecules, 1b was used to explore the in situ formation of new hydrogen bonds with guest water molecules. To avoid the influence of the OH-stretching vibration in H 2 O, D 2 O was used for in situ XRD and Fourier transform infrared (FT-IR) spectroscopy analyses. When the polycrystalline powder of anhydrous 1b was dipped in D 2 O, in situ XRD measurements showed new Bragg reflection peaks assigned to 1b·D 2 O (Fig. 3a). For example, (002), (012), (122), and (131) lattice directions appeared when 1b polycrystalline powder was dipped in D 2 O for 1 h. When the sample was dipped for almost 24 h, the XRD pattern matched well with that of the aforementioned 1b·D 2 O microcrystals, indicating successful in situ transformation from the anhydrous state to the hydrated state. Interestingly, new peaks for the (002) lattice direction occurred when anhydrous 1a and 1b polycrystalline powders were dipped in D 2 O. The (002) lattice direction is the orientation plane in which water and organic molecules form new intermolecular hydrogen bonds in the anhydrous crystal lattices. In situ FT-IR spectra (Fig. 3c) of 1b polycrystalline powder with D 2 O showed a stretching band at 1577 cm −1 that diminished gradually and a new stretching band at 1560 cm −1 that appeared simultaneously. This change suggested that the N-H chemical environment in the benzimidazole ring changed after being dipped in D 2 O. Combined with single-crystal data analysis, the disappearance of the stretching band at 1577 cm −1 was attributed to breakage of the N-H···O (-OH) hydrogen bond between organic molecules in the anhydrous structure (Fig. 3e), while the new stretching band at 1560 cm −1 suggested the formation of N-H···O (-OCH 3 ) hydrogen bonds between molecules in the hydrated structure (Fig. 3c). Another new stretching band at 1025 cm −1 was attributed to the formation of a N···D (D 2 O) hydrogen bond (Fig. 3f). The location of the methoxy group of 1b in Fig. 3e and f has changed due to the formation new hydrogen bonds between 1b molecules.
Possible mechanism. To understand the fluorescence turn-on properties of the hydrated crystals, theoretical calculations were conducted. The emission wavelength of hydrated crystals (1a·H 2 O (D 2 O), 1b·H 2 O (D 2 O), and 1c·H 2 O (D 2 O)) was similar to that of the keto emission in an anhydrous organic solvent, while the enol emission was not observed. This showed that intramolecular hydrogen bonding was more effective in the solid state with the ESIPT process. The active hydrogen atom in the benzimidazole ring was responsible for tautomer formation and shuttled between its two N atoms (Fig. 4a), resulting in fluorescence quenching. Using the polycrystalline powder of 1a, the radiation rate (kr~E 2 f) in the powder state was very low owing to the low f value (f = 0.0123). Therefore, the fluorescence quantum efficiency of ESIPT emissions (K*) from polycrystalline powders of 1a, 1b, and 1c was low owing to the nonradiative deactivation pathways (nπ*) of the tautomer (Supplementary Table 7 and Supplementary Fig. 33b). Water acting as a hydrogen bond donor formed strong intermolecular hydrogen bonds (N•••H-O or N•••D-O) with the nitrogen atom of the molecule (Fig. 4a), inhibiting tautomer formation. When the tautomer was inhibited by intermolecular hydrogen bond formation in the excited states, the electronic transition of geometry-B (S1) ( Supplementary  Fig. 34) changed from nπ* to lower-energy ππ*, resulting in enhanced keto (K*) emission in the solid state. The radiation rate (kr~E 2 f) of 1a·H 2 O was higher than that of 1a polycrystalline powder owing to the higher f value (f = 0.1934) (Supplementary Table 9 and Supplementary Fig. 34). Therefore, hydrated crystals  Table 5) 38 . The fluorescent images also showed fluorescence intensity of 1a·D 2 O microcrystals grown on silicon dioxide substrate was stronger than that of 1a·H 2 O microcrystals grown on silicon dioxide substrate ( Supplementary Fig. 39).
A 1-ns molecular dynamics simulation was also conducted using the COMPASS force field to investigate the molecular stacking mode and hydrogen-bonding network. Quantitative mean square displacements of dynamic trajectories, as shown in Fig. 4b, indicated a larger thermal fluctuation in the 1a molecular crystal than in 1a·H 2 O. The corresponding equilibrium structures are also shown in Fig. 4c and d, respectively. This verified that a supramolecular ring was present via N-H···O and N-H···N hydrogen bonding in the single crystal of 1a. This interaction induced a twist between R1 and R2, but also formed a dimer structure between the organic molecules that exhibited certain flexibility with high freedom. However, for the single crystal of 1a·H 2 O, water acted as a "glue" to connect the organic molecules, with a well-organized three-dimensional hydrogen network lowering the fluctuation displacement and protecting the planarity of the organic molecule. Therefore, aided by water, the hydrogenbonding network in 1a·H 2 O single crystals was prone to forming a relatively confined space to suppress the vibration of 1a molecules, which effectively decreased loss from electronic transitions between orbitals and further enhanced molecular photoluminescence.
To further confirm the existence of hydrogen bonds in 1a·H 2 O, solid-state 1D 1 H NMR double quantum filtered (DQ-filtered) spectra and 1H one-pulse spectra were recorded. Figure 4e shows the 1 H one-pulse spectrum of 1b. The resonances at 12.8 and 14.8 ppm were associated with the -OH and -NH units, respectively. However, these two signals were not observed in the 1 H DQ-filtered spectrum (Fig. 4f), indicating that the -OH and -NH protons of 1a exhibited high local mobility. In contrast to 1a, 1a·H 2 O showed only one peak at 12.0 ppm in the 1 H onepulse spectrum. This was in agreement with the single crystal structure, in which hydrogen bonding with water induced identical local structures for the -OH and -NH protons. The signal at 12.0 ppm was also observed in the 1 H DQ-filtered spectrum, implying that -OH and -NH protons in 1a·H 2 O had limited local mobility, which might be induced by hydrogen bonding with water. To further verify the mechanism, compound (2-(((1-methyl-1H-benzo[d]imidazol-2-yl)imino)methyl)phenol (named as 1d)) ( Supplementary Fig. 32) was prepared, in which the active hydrogen atom on the N atom present in 1b was replaced by a methyl group (Supplementary Fig. 13). Compound 1d strongly fluoresced in the solid state owing to the lack of tautomer formation after replacing the active hydrogen in the benzimidazole moiety. The hydrated microcrystals were also found to be metastable structures. Using 1a·H 2 O microcrystals as an example, the XRD pattern changed significantly after heat treatment, becoming similar to that of bulk 1a polycrystalline powder, indicating a rearrangement of molecules in the crystal lattice ( Supplementary  Fig. 37). Furthermore, the surfaces of the hydrated microcrystals were covered with small dots after thermal treatment at 60°C for 25 min, which might have been caused by water loss (Supplementary Figs. [21][22][23]. Upon thermal treatment, a 1a·H 2 O microcrystal film deposited on a polytetrafluoroethylene (PTFE) membrane ( Supplementary Fig. 34) showed gradually decreasing fluorescence with prolonged heating. The fluorescence quenching of the 1a·H 2 O film deposited on a PTFE membrane after heat treatment was turned on by adding water. Other microcrystals have shown similar fluorescence on-off switching behaviour. Owing to the large gap in quantum yield between 1b·H 2 O microcrystals and 1b·D 2 O microcrystals (Supplementary Table 5), 1b was used as a fluorescent probe to investigate different ratios of H 2 O and D 2 O (Fig. 2d).

Discussion
Organic hydrate luminogens were prepared and their waterpromoted fluorescence enhancement properties were investigated. Intercalated water molecules played an important role in triggering molecular reorganization in the hydrated crystalline state by forming new intermolecular hydrogen bonds between the organic molecules and guest water molecules. This water-induced rearrangement of the organic compounds in the crystalline state caused significant enhancement of their molecular emissions in the solid state by suppressing shuttling of the active hydrogen atom between N atoms in the benzimidazole ring. Water molecules acting as hydrogen bond donors not only changed the molecular stacking mode in the hydrated crystals, but also markedly altered the electronic transition of the molecular keto form from nπ* to ππ* through the ESIPT process. This process enabled these small organic compounds to optically sense heavy water and quantify the ratio of H 2 O to D 2 O. This study provides a basis for the design and development of functional organic hydrate luminogens that exhibit water-induced fluorescence enhancement for solid-state device applications.