Figure 1 . Solvent-induced isomerization and theoretical calculation. (A) Fluorescence spectra (10 μM) of HBT-DPI in different solvents, green lines: with strong emission, blue lines: with weak emission, n -Hex: n -hexane, TOL: toluene, DCM: dichloromethane, TCM: chloroform, EtOAc: ethyl acetate, THF: tetrahydrofuran, DIO: dioxane, ACE: acetone, EtOH: alcohol, MeOH: methanol, ACN: acetonitrile, DMF: N , N -dimethylformamide, DMSO: dimethyl sulfoxide, λex = 365 nm, slit: 5 nm/5 nm. (B) Statistics of fluorescence and wavelength changes in various solvents. (C) Chemical structure and the energy of OH-BS andOH-DPI in different solvents. (D) The schematic representation of the ESIPT state of OH-DPI in THF solvents. (E) Potential energy surfaces of OH-BS in TCM solvents. The relative energies (in eV) and oscillator strengths (f ) were evaluated at the level of (TD)DFT/O3LYP/def2-SVP.
Multimodal H-Bonds induced multiple optical behaviors in crystals.
To further demonstrate the multi-dimensional regulation of intra- and intermolecular H-bonds in HBT-DPI and the resulting diverse photochemical properties, we prepared single crystals ofHBT-DPI in various solvents. As expected, we successfully obtained four types of HBT-DPI single crystals (HBT-DPI-N , HBT-DPI-Y , HBT-DPI-G , andHBT-DPI-O ) with distinct photophysical behaviors by the solvent-induced method. Through the single-crystal X-ray diffraction (SXRD) analysis, we found that the crystals exhibit varieties in the contents of HBT-DPI isomers and the molecular packing behaviors (Figure 2, Figures S5-8 and Tables S9-10), which are largely influenced by the strong H-bonds (distance between hydrogen atom and H-bond acceptor < 2.2 Å, bond energy around -4 to -15kcal/mol).17 The single crystal ofHBT-DPI-N was first obtained from a gas (n -pentane)-liquid (TCM) diffusion system. Due to the absence of competitive intermolecular H-bonds between imidazole moiety and solvent molecules, two types of strong intramolecular H-bonds including O1-H···N1 (1.859 Å) and N3-H···O1 (2.119 Å) were constructed inHBT-DPI-N corresponding to OH-BS isomer (Figure 2C). The specific double intramolecular H-bonds system endows considerable reorganization energy, suppressing the emission of the OH-BSisomer (Figures 2B and 2F). Moreover, the severe π-π stacking (r1 = 3.541 Å) of crossing packing mode further consumes the excited state energies (Figure S5), thus quenching the fluorescence emission in the crystalline state (Φ F < 0.001) (Figures 2J-K). While an enhanced fluorescence quantum yield (Φ F = 10.54%) can be obtained inHBT-DPI-Y cocrystals prepared by evaporating n -Hex and DCM mixture (Figures 2C and 2J-K). The HBT-DPI-Y cocrystals formed a new intramolecular H-bond (N2···H-O1, 1.793 Å), which corresponds to the OH-DPI isomer (Figure 2G), facilitating the activation of ESIPT process. Unlike TCM, DCM molecules were trapped in the HBT- DPI-Y cocrystals, providing multiple interactions containing halogen bond (Cl···π 4.007 Å), H-bond (C-H···N2 2.637 Å, C-H···Cl 2.852 Å, C-H···Cl 2.915 Å) and C-H···π (2.807 Å) (Figure S6A), along with the intermolecular H-bonds (N3-H···N1, 2.192 Å) amongOH-DPI isomers, triggering the formation of an interlock crossing packing mode, which contributes a yellow fluorescence emission at 549 nm (Figures 2C and S6).
In response to THF that offers H-bond acceptors, a type of green fluorescence cocrystals HBT-DPI-Gem = 505 nm) was collected (Figure 2D). The THF molecule formed an intermolecular H-bond with HBT-DPI between N3-H and the oxygen atom (O2) of THF (Figure 2H), benefiting the formation of the OH-DPI isomer. The antiparallel packing mode found in the cocrystals HBT-DPI-Genlarges the centroid distance of adjacent molecules to avoid the π-π stacking (Figures S7B-C), rendering the promotedΦ F of 26.07% (Figures 2J-K). Intriguingly, a triad cocrystal species HBT-DPI-O was obtained from the protic EtOH. As shown in Figure 2I, both the OH-BS and OH-DPIisomers were found in HBT-DPI-O at a 1:1 molar ratio. Possessing both the H-bond donor and acceptor, EtOH tethers a pair ofOH-BS and OH-DPI via two intermolecular H-bonds (N3-H···O3 2.168 Å, O3-H···N2 1.994 Å) (Figure 2I); and the paired units pack organized, leading to red-shifted fluorescence emission at 564 nm with a reduced Φ F (6.02 %) (Figures 2E and 2J-K), which could be due to the existence of OH-BS isomers, π-π stacking and other concomitantly abundant intermolecular interactions (Figure S8). The discovery and characterizations of the four crystals demonstrated the success of tuning the photophysical behaviors of HBT-DPI via multi-dimensional regulation of intra- and intermolecular H-bonds.
Further attempts revealed that HBT-DPI-N can also grow in the solvent without H-bond acceptor, such as TOL (Figures S9 and S11A, Table S11); in contrast, the solvents with H-bond acceptor, including ACN, DIO, and ACE, favor the formation of HBT-DPI-G (Figures S10 and S11B-D, Tables S11-12). The orange emission similar toHBT-DPI-O was also observed in powders obtained from MeOH, which was confirmed to have a similar structure with HBT-DPI-Oby X-ray diffraction (XRD) (Figures S12 and S13), suggesting the packing mode of HBT-DPI-O might be favorable in alcohols. Whereas the distinct packing mode of HBT-DPI-Y was only found in the crystal prepared in DCM. Overall, these results prove that the multi-dimensional regulation of intra-/intermolecular H-bonds offers extra diversity in constructing multi-emissive environment sensitive materials.