Theoretical Studies And Molecular Simulations On The Structure Of Azobenzene Sulfonate Derivatives Intercalated Layered Double Hydroxides

The structures and properties of azobenzene sulfonate derivatives intercalated into layered double hydroxides (LDH) have been studied by using density functional theory (DFT). The host-guest and guest-guest interaction models of the intercalated structures have been built based on the frontier molecular orbitals of the different intercalated azobenzene sulfonate anions (AbS-) and the lamella cluster with D3d symmetry of Mg/Al-LDH, so the supramolecular structures of AbS--LDH have been obtained. The interaction essence of the AbS--LDH is revealed from the microcosmic structures and energies point of view. Furthermore, the crystal structures of AbS--LDH have been built by their microcosmic structures, and the simulated powder X-Ray diffraction (PXRD) patterns of their crystal structures have also been obtained.The host-guest interaction model of AbS--LDH can be built based on the investigation of electronic structures for Mg/Al-lamella and AbS-anion by using B3LYP and B3PW91 methods at the levels of LANL2DZ, 6-31G(d,p), and 6-311++G(d,p). The calculated results show that the combination of AbS- anions and lamella is favored according to their energies and frontier molecular orbitals. The AbS- anions with di-sulfonic acid groups in the para-and meta-positions, respectively, should be vertical-arranging in the interlayer space and formed a single intercalation layer, and the calculated interlayer distances of p,p'-AbS--LDH and m,m'-AbS"-LDH is 2.07 and 1.82 nm, respectively. An arrangement of anions in the interlayer may be revealed from the host-guest interaction model. The monosubstituted AbS- anions in the same positions are also arranged perpendicular to the lamella, forming an interdigitated structure with double-layer in the interlayer space, while this structure cannot display the arrangement of anions.For the double-layer structure of monosubstituted AbS- anions intercalated into Mg/Al-LDH, there are two major interactions, the host-guest and guest-guest, respectively. The guest-guest interaction of AbS--LDH are studied by the same methods as the host-guest model, which has been modeled as the aggregation of AbS- anions. In order to eliminate the electrostatic repulsion effect, a proton is introduced into the sulfonic acid group to form a charge-neutral molecule, so a dimeric structure with parallel-displaced aromatic stacking interactions is obtained. Based on the relation between the host-guest and guest-guest interaction models, the intercalated structure with the interdigitated double-layer anion is obtained by using combinational calculation of molecular fragments (CCMF). The interlayer distances of p-AbS--LDH and m-AbS--LDH obtained from the theoretical calculated is 2.37 and 1.86 nm, respectively.The perfect crystal structures of AbS--LDH are constructed according to the above optimized geometries and the crystal structure of LDH with the pace group R-3m and R-3. In the simulated PXRD patterns, the characteristic peaks of each crystal structures for AbS--LDH can be clearly observed. Through a comparing between the simulated PXRD patterns of AbS--LDH and experimental data of interlayer anions with the similar to AbS- anion structure, it is confirmed that the crystal structure of AbS--LDH obtained from the theoretical model is reasonable.The photo-and thermal-stabilities of supramolecular structures of AbS--LDH are also investigated by using DFT method. Compared with the characters of AbS molecules, the influence of lamellae for AbS--LDH supramolecular structures is discussed. For AbS--LDH, the potential energy profile alone the thermal isomerization pathway can be described as the one-step concerted process of inversion and rotation, and the trans-and cis-isomers are separated by only one transition state duo to the strong electrostatic interaction between host and guest. The cis-trans isomerization form of AbS molecule concerns the complex pathway that is characterized by the inversion combined with certain degree of rotation, and it is worthy to notice that the potential energy profile from the transition state connects not to a reactant or to a product but to another transition state, which in turn connects two symmetrically structures with lower symmetry. The energy barrier of AbS--LDH is slight higher than that of AbS, and it indicates that there is some influence for the ismerization of AbS- anion in the interlayer space. The S1<←S0 (n→π*) and S2←S0 (π→π*) transitions for both AbS--LDH and AbS molecule appear exceedingly similar:the S1←S0 transition is forbidden and the S2←S0 transition is allowed. It is found a blue shift in S2←S0 transition for AbS--LDH because the energy for its transition increases. From the theoretically aspect, it can explain that AbS--LDH exhibits advanced photo-and thermal-stabilities compared with the individual AbS- anion.Finally, thermal and photochemical isomerization mechanisms of the AbS molecule with di-sulfonic acid groups have been studied. There are two pathways for thermal isomerization. One is the inversion combined with rotation, and the potential energy profile from the transition state connects not to a reactant or to a product but to another transition state, which in turn connects two symmetrically structures with lower symmetry. The other pathway is the rotation involved inversion, while the two isomers are connected through the only one transition state. The calculation results indicate that the molecular structures at the highest points on the potential energy profiles of two pathways are identical, which shows that two mechanisms (inversion and rotation) operate simultaneously for thermal isomerization. For the photochemical isomerization pathways, the results show that the rapid energy redistribution among the various vibrations renders the concentration of such amounts of energy in the inversion coordinate very improbable, while the isomerization can easily occur through the S0/S1 conical intersection and the S0-T1-S0 crossing to reach the product by the rotation pathway.