Min Ji

Luminescent Properties of Metal–Organic Framework MOF-5: Relativistic Time-Dependent Density Functional Theory Investigations

The electronically excited state and luminescence property of metal-organic framework MOF-5 were investigated using relativistic density functional theory (DFT) and time-dependent DFT (TDDFT). The geometry, IR spectra, and UV-vis spectra of MOF-5 in the ground state were calculated using relativistic DFT, leading to good agreement between the experimental and theoretical results. The frontier molecular orbitals and electronic configuration indicated that the luminescence mechanism in MOF-5 follows ligand-to-ligand charge transfer (LLCT), namely, π* → π, rather than emission with the ZnO quantum dot (QD) proposed by Bordiga et al. The geometry and IR spectra of MOF-5 in the electronically excited state have been calculated using the relativistic TDDFT and compared with those for the ground state. The comparison reveals that the Zn4O13 QD is rigid, whereas the ligands BDC(2-) are nonrigid. In addition, the calculated emission band of MOF-5 is in good agreement with the experimental result and is similar to that of the ligand H2BDC. The combined results confirmed that the luminescence mechanism for MOF-5 should be LLCT with little mixing of the ligand-to-metal charge transfer. The reason for the MOF-5 luminescence is explained by the excellent coplanarity between the six-membered ring consisting of zinc, oxygen, carbon, and the benzene ring.

Organic electron-rich N-heterocyclic compound as a chemical bridge: building a Brönsted acidic ionic liquid confined in MIL-101 nanocages

A novel functionalized MOF material, Bronsted acidic ionic liquid (BAIL) confined inside well-defined MIL-101 nanocages, was developed for the first time based on the tandem post-synthetic strategy. The catalytic performance of BAIL confined in MIL-101 material was evaluated by using the acetalization of benzaldehyde with glycol as a probe reaction. The resulting BAIL/MIL-101 material exhibited superior catalytic performance, which is mainly due to the combination of the advantageous merits of ionic liquid catalyst with the desirable microenvironment in the MIL-101 nanocages.

Metal–organic frameworks HKUST-1 as porous matrix for encapsulation of basic ionic liquid catalyst: effect of chemical behaviour of ionic liquid in solvent

Ionic liquid hybrid MOFs composite materials, copper-based metal–organic frameworks HKUST-1 as porous matrix was used to encapsulate amino-functionalized basic ionic liquid (ABIL) catalyst by post-synthetic modification strategy under different solvents such as H2O, ethanol and N,N-dimethylformamide. A series of characterization techniques such as PXRD, SEM, N2 physical adsorption–desorption, ICP-OES, element analysis, FT-IR, DRS UV–Vis, XPS and TGA were employed to probe the textural properties, surface characteristics, variations in coordination environment of metal center and thermal stability of as-synthesized catalysts. Furthermore, the Knoevenagel condensation of benzaldehyde and malononitrile was used as a probe reaction to evaluate their catalytic performances. It was interesting to find that the chemical behaviour of ABIL dissolved in these solvent had a profound impact on synthesis of catalysts and their catalytic performances. In the weak alkaline and neutral environment, ABIL can be dissolved as molecular state and be well confined inside HKUST-1 nanocavities via Cu–NH2 coordination bond. In particular, the alkalinity of ABIL dissolved ethanol solvent was the optimal environment for encapsulation of ABIL organocatalyst, and the as-synthesized heterogeneous catalyst demonstrated favourable structural property and excellent catalytic performance.

PdCl2 immobilized on metal–organic framework CuBTC with the aid of ionic liquids: enhanced catalytic performance in selective oxidation of cyclohexene

Metal–organic framework CuBTC was employed as a support to immobilize PdCl2 with the aid of ionic liquids (ILs). The as-synthesized PdCl2-ILs/CuBTC catalyst was studied in selective oxidation of cyclohexene with molecular oxygen as an oxidant and TBHP as an initiator. It is found that the allylic oxidation and radical oxidation processes are main reaction pathways. More importantly, the enhancement of catalytic activity in the oxidation of cyclohexene is observed over the PdCl2-IL/CuBTC catalyst due to Pd–Cu cooperative catalysis. Furthermore, ionic liquids could have a very favourable role in the stabilization of Pd(II) species and improvement of catalyst reusability.

Role of the Electronically Excited-State Hydrogen Bonding and Water Clusters in the Luminescent Metal–Organic Framework

The electronically excited state and luminescence property of metal-organic framework Zn(3-tzba)(2,2-bipy)(H2O)·nH2O have been investigated using the density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated geometry and infrared spectra in the ground state are consistent with the experimental results. The frontier molecular orbitals and electronic configuration indicated that the origin of luminescence is attributed to a ligand-to-ligand charge transfer (LLCT). We theoretically demonstrated that the hydrogen bond H47···O5═C is weakened in the excited state S1; the weakening of the excited-state hydrogen bonding should be beneficial to the luminescence. To explore the effect of the water clusters on the luminescence, we studied four complexes Zn(3-tzba)(2,2-bipy)(H2O)·3H2O, Zn(3-tzba)(2,2-bipy)(H2O)·2H2O, Zn(3-tzba)(2,2-bipy)(H2O)·H2O, and Zn(3-tzba)(2,2-bipy)(H2O). The results reveal that the presence of water should play an important role in the emission characteristics of the MOF. Also, the UV-vis absorption and emission spectra of Zn(3-tzba)(2,2-bipy)(H2O)·3H2O are in good agreement with the experimental results.

Phosphorized SnO2/graphene heterostructures for highly reversible lithium-ion storage with enhanced pseudocapacitance

Tin dioxide (SnO2) based materials are attractive anode candidates for lithium-ion batteries (LIBs) due to their high capacity, low cost and environmental friendliness. However, their practical applications have been hindered by their poor reversibility, sluggish reaction kinetics and huge volume expansion. This work demonstrates the facile synthesis of a partially phosphorized SnO2/graphene nanocomposite (P-SnO2@G) by a combined hydrothermal and low-temperature phosphorization process. Enhanced Li-ion storage performance has been achieved in such a nanocomposite due to the unique phase hybridization of crystalline SnO2, SnPx and metallic Sn homogeneously anchored on graphene nanosheets. The P-SnO2@G composite also manifests a high initial coulombic efficiency of 79% and delivers a high and reversible capacity of 860xa0mA h g−1 at 0.5 A g−1 with 94% capacity retention after 50 cycles. In addition, P-SnO2@G exhibits an excellent rate capacity of 736 mA h g−1 at 2.0 A g−1. The greatly enhanced Li-ion storage capability stems from the significant pseudocapacitance contribution accounting for ∼82% at 1 mV s−1. These results demonstrate that as-synthesized P-SnO2@G composite is a promising anode material for LIBs. The proposed phase hybridization concept can pave the way for designing and exploring more advanced electrode materials for beyond Li-ion batteries.

Hydrogen bonding and coordination bonding in the electronically excited states of Cu2(L)2 (L = 5-(4-pyridyl)tetrazole)MeOH: A TDDFT study

The luminescent metal organic framework (MOF), Cu2(L)2·MeOH (L=5-(4-pyridyl)tetrazole), was studied using time-dependent density functional theory (TDDFT). A combination of frontier molecular orbitals and electronic configuration analysis revealed that the emission mechanism was a ligand to metal charge transition (LMCT) rather than a metal to ligand charge transfer (MLCT). Hydrogen bonding significantly changed the nature of the frontier orbital and the luminescence. Electronic transition energies predicted that the hydrogen bonding in excited state would become weaker with an electronic spectral blue-shift. The bond lengths, frequencies, and binding energies indicated weakening of the hydrogen bonding in the excited state, which can affect emissions in two ways, including: (i) a decrease in the electronic coupling between methanol and the motif and suppressing the occurrence of the photo-induced electron transfer (PET); and (ii) increasing the energy gap between S1 and S0, leading to radiative transition. Coordination bonding was also investigated in the excited state through bond lengths, frequencies, and bond orders. Coordination bonds were found to become stronger in the excited state leading to an enhancement of the luminescence.

The effect of furcated hydrogen bond and coordination bond on luminescent behavior of metal-organic framework [CuCN·EIN]: A TDDFT study

The hydrogen bonding in electronically excited-state of the metal-organic framework [CuCN·EIN] was studied using time-dependent density functional theory (TDDFT). The representative fragment of [CuCN·EIN] was employed for the computation. The geometric structures, binding energies and IR spectra in both ground state and electronically excited state S(1) of the complex were computed using DFT and TDDFT methods to investigate excited-state hydrogen-bonding and coordination bonding, respectively. Based on the analysis of the frontier molecular orbitals and the electronic configuration of the complex, the ligand-to-metal charge transfer (LMCT) luminescence was confirmed. Furthermore, furcated hydrogen bonds are both strengthened in the S(1) state slightly. And then, the strengthening of the hydrogen bonds in the S(1) state goes against the charge transfer from ligand to metal and then should be in favor of the luminescence. In particular, we also discuss strengthening or weakening behavior of the coordination bonds in the S(1) state for the first time. Based on the results of the bond lengths and vibration frequency of the coordination bond, we can conclude that the coordination bond Cu(7)-N(8) is strengthened in the S(1) state. And the strengthening of the coordination bond Cu(7)-N(8) should also be in favor of the luminescence.

Theoretical study of the interaction between X (H, F) and graphene

This study investigates the interaction between X (X = H and F) and graphene C54H18 (D6 h), and the potential energy surface of the graphene radical. The calculations on the structures and energies are further discussed thermodynamically and kinetically using the density function theory method at the B3LYP/6-31G (d) level. Our findings show that there are four distinct isomers of C54H18–X. C54H18–H2 and C54H18–F4 are the most stable isomers in their own systems. In addition, the transition states, as well as reaction pathways of H transferring between different key points on representative patch, are given to explore the possible reaction mechanism. Finally, the stability of C54H18–X2 is discussed through the density functional theory.