Effects Of Intermolecular Interactions On The Luminescence Of Metal-Organic Frameworks

Luminescent properties of luminescent metal-organic frameworks (LMOFs) can be greatly affected when interacting with guest molecules, making LMOFs potential candidates for light emitting devices and luminescent sensors. However, a clear understanding of this guest-molecule-based luminescent mechanism is lacked. Uncovering the underlying mechanism would be helpful for scientists to design and synthesis LMOFs with higher efficiency and selectivity. Herein, the mechanisms of guest-molecule-based luminescence of three different LMOFs are carefully studied. The main results are listed below.(1) Luminescence intensity of LMOF Cu4(L)4·2EtOH is closely related to the polarity of the solvents. In polar solvents namely ethanol and acetone, the luminescence is enhanced greatly. In non-polar solvents such as cyclohexane, the luminescence is barely affected. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are applied to fully study the luminescent mechanism of LMOF in solvent ethanol, acetone and cyclohexane. Hydrogen bond, which forms between polar solvents and LMOF, is found to play essential role in the enhancement of luminescence. Hydrogen bond between polar solvent and LMOF is quite strong. During the excitation process, it can affect the frontier molecular orbital as well as the charge distribution all across the LMOF structure. Consequently, the redistribution of electrons in excited state affected the strength of hydrogen bond. By analyzing the hydrogen bond length and vibration frequency, the hydrogen bond is confirmed to be weakened in excited state. Hydrogen bond is known to lower the energy level of an electronic state which can tune the energy gap between two states. In this case, the weakening of hydrogen bond in the excited state expands the energy gap between So and S1 states. This greatly inhibits the internal conversion process, which enhances the luminescence of this LMOF. In the case of non-polar solvent cyclohexane, the interaction between solvent and LMOF is quite weak. The molecular orbitals involved in the excitation process cannot be affected, which cannot affect the luminescence of LMOF.(2) Based on the strong luminescence quenching phenomenon when exposed to nitrobenzene, LMOF [Zn2(L)(bipy)(H2O)2]·(H2O)3(DMF)2 can serve as an excellent explosives-detecting candidate. DFT and TDDFT methods are applied to fully investigate the underlying detecting mechanism of this LMOF. Interactions between LMOF and three analytes namely nitrobenzene, benzene, acetone are studied. We find that the nitrobenzene-induced luminescence quenching is generated from intermolecular electron transfer from LMOF to nitrobenzene. The orbital coupling between valence bands (VB) of LMOF and occupied orbitals of nitrobenzene is so strong that the electrons can be directly excited from the VB of LMOF to the LUMO of nitrobenzene. Hydrogen bond and ?-? stacking are proved to enhance the orbital coupling and serve as bridges in the electron transfer process. Besides, hydrogen bond can partially neutralize the repulsive force between aromatic rings, which enhances the ?-? stacking and induces strong luminescence quenching.(3) With the aids of DFT and TDDFT, the explosives-detecting mechanism of LMOF sensor Zn(L)(HDMA)2(DMF)(H2O)6 has been comprehensively studied. The interactions between the framework and two analytes, namely, benzene and nitrobenzene are investigated. By studying both the periodic models and cluster models, intermolecular electron transfer from conduction bands (CB) of the framework to LUMO of nitrobenzene is demonstrated to be the inducement for the luminescence quenching phenomenon. Two stable binding patterns with large binding energies between LMOF and nitrobenzene are found. Intermolecular electron transfer is only observed in the binding pattern with both hydrogen bond and ?-? stacking. ?-? stacking is found to enhance the orbital coupling between LMOF and nitrobenzene greatly, which serves as the high efficient electron transfer bridge. By contrast, hydrogen bond hardly enhances the orbital coupling which cannot serve as an efficient electron transfer bridge. Thus, intermolecular electron transfer cannot occur in the binding pattern with hydrogen bond alone. Nevertheless, hydrogen bond can enhance the binding strength between LMOF and nitrobenzene. This can reinforce the ?-? stacking interaction. The cooperation of the two interactions induces facile intermolecular electron transfer which strongly quenches the luminescence of the LMOF sensor.