Solvent-induced isomerization and photophysical properties of HBT-DPI.
We first investigated the intramolecular H-bond switching in solution. Although HBT-DPI exhibits similar absorption bands (280-400 nm) in the tested solvents (Figure S1), the fluorescent emission spectra ofHBT-DPI were found to be much stronger in the solvents containing O or N atoms that can act as H-bond acceptor (including ethyl acetate (EtOAc), tetrahydrofuran (THF), dioxane (DIO), acetone (ACE), alcohol (EtOH), methanol (MeOH), acetonitrile (ACN), N ,N -dimethylformamide (DMF), dimethyl sulfoxide (DMSO)) than in those without H-bond acceptor (including n -hexane (n -Hex), toluene (TOL), dichloromethane (DCM) and chloroform (TCM)) (Figures 1A and 1B), implying different molecular status in the two types of environments. Quantum chemical calculations reveal that, in TCM (without an H-bond acceptor), HBT-DPI tends to form the isomerOH-BS which is stabilized by two intramolecular H-bonds to achieve the lowest potential energy (Figure 1C). In contrast, the O atom in THF competes for the free hydrogen of DPI , forming a strong N-H···O intermolecular H-bond (Figure S2); while the imidazole, a stronger H-bond acceptor than BT , preferentially forms the intramolecular H-bond with the hydroxyl in phenol core, representing the isomer OH-DPI (Figure 1C). After demonstrating the two isomers with switched intramolecular H-bonds, further analysis shows that the keto form of OH-DPI has a significantly larger oscillator strength (f ) of 0.8695 than the enol form of OH-DPI and the vertical keto*→keto photoluminescence peak is calculated to be 513 nm for OH-DPI (Figure 1D), which is consistent with the experimental data, indicating the emissive ESIPT process ofOH-DPI isomer. Whereas OH-BS possesses a weak emission mainly due to the fast non-radiative decay process caused by the large reorganization energy of 0.964 eV (Figure 1E). Besides, we also found that the energy barrier of possible transforming routes betweenOH-DPI and OH-BS is relatively small in TCM (Figure S3). The small energy barrier would allow a partial transition fromOH-BS to OH-DPI , contributing to the retention of fluorescence emission in TCM and possibly other solvents without H-bond acceptor. Additionally, when increasing the content of THF in then -Hex solution, the fluorescence intensity of HBT-DPIsignificantly increased (Figure S4), indicating the occurrence of isomerization from OH-BS to OH-DPI . These results demonstrate the success of our molecular design on intramolecular H-bond switching, which can be controlled via the hydrogen accepting capability of solvent.