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-G (λem = 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.