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