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.