Figure 4. Molecular orbital energy diagram for Zn-MOF-nitrobenzene showing the vertical electronic transitions for the maxima absorption band at B3LYP/def2-TZVPP theoretical level.
The radiative deactivation of the excited state can occur by two mechanisms, fluorescence or phosphorescence, after photoexcitation. It is well-known since 1950, from Michael Kasha’s works that “the emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity” 49, which is known as the kasha’s rule. In this sense, fluorescence is the emission processes, due to radiative deactivation from first excited singlet states to the ground state. In the particular case of phosphorescence, it is necessary to populate an excited triplet (T1) with less energy than the first singlet excited S1 state, this mechanism is known as an intersystem cross (ISC), for relaxes to its ground state.50-51 The importance of the precise description of these states, the S1 state or T1state, to understand the mechanisms by which activate or deactivate the luminescence in chemosensors optical has been emphasized by Briggs and Besley in 2015.52 In recent studies, our group proposed and verified a theoretical protocol, based on this statement, from the elucidation of the sensing mechanism in chemosensor selective to metal ions.
A precise description, both of the S0 state and of the S1 state, in terms of energy and structure, allowed us to explain in detail the turn-on fluorescent mechanism of the two chemosensor luminescent based in Schiff basis selective to metal ions.53-54
Considering the importance of knowing the emissive state of chemosensor is optimized S1 state of the Zn-MOF to understand the origin of fluorescence in this system. Thus, the optimized geometry of the S1 state was taken as input data to calculate the electronic transitions that constitute emission spectrum of Zn-MOF by means of TD-DFT methods. These calculations confirmed that the molecular orbitals involved in the emission band are linkers-localized. This result indicates that fluorescence is originated by a transition π-π* linker-centered from the LUMO to HOMO, whit 97% of contributions corresponding to this electronic transition. It also shows that the electron density of LUMO is located on the BYP linker, while the HOMO is localized on the OBA linkers see Figure 5. This result is consistent with those observed in other LMOFs based on metals ions d10 recently reported, which origin of emission is assigned to ligand-to-ligand charge transfer. 2212 This emission pathway is observed mainly in LMOFs based Zn2+ and Cd2+ since these ions with oxidation state 2+, tend to retain the d10 configuration.5556