Results and discussion
The geometrical parameters upon geometry optimization of the Zn-MOF are in good agreement with experimental X-ray data.31 The results showed that the computed [Zn(28)–Zn(93)], [Zn(28)–N(34)] and [Zn(28)–O(29)] bond lengths are Å 2.84 Å, 2.08 Å, and 2.07 Å, while the experimental values for Zn-MOF are 2.89 Å 2.02 Å, and 2.04, respectively.31 These are also consistent with theoretical reported 2.23 Å, 2.25 Å and 2.06 Å respectively values, computed at the PBE-D3/ Triple-zeta (TZP) theoretical level and used a truncated structural model, for this MOF by Ce Hao et. al. 24 in 2016.
Besides, the calculations reveal similar results regarding the [N(34)–Zn(28)–O(4)], and [O(29)–C(35)–O(30)] bond angles which are 97.8˚and 123.5˚, see the structure on Figure 1. These values agree with the experimental X-ray data which are 97.9˚ and 124.2˚ respectively31 and reported 96.9˚and 124.2˚ theoretical values.24.
These results suggest that the proposed structural truncated model is consistent with models previously reported and based on a fragmentation scheme. Based on this fact, the optical properties of the Zn-MOF were explored on the minimum optimized structure of the ground state (S0) by means of TDDFT approaches. The simulated UV-vis spectrum using both CAM-B3LYP and B3LYP functional (with maxima of 248 and 281 nm respectively) showed an excellent agreement with the experimental absorption wavelength value31, 280 nm. The TDDFT(B3LYP) computed values are closer to the experimental one, presumably due to the absence of the charge transfer between the metals and ligands. Table 1 reports the maximum absorption wavelength (in nanometer, nm), oscillator strengths (f ) as well as the assignments to the electronic transitions with the largest f . The molecular orbitals involved in the excitations are localized along with the OBA linker as can be seen from Figure 2.
Table 1. Singlet - Singlet electronic transitions for Zn-MOF.