3.2 Optimized geometries of the triplet excited
states
In Ln(III) complexes the so-called “antenna effect” defines their
luminescence efficiency and the intramolecular energy transfer from the
ligand environment to the central ion.[1-9] At the
first stage of photoexcitation process of Ln(III) complexes, singlet
excitation is localized on the ligand. The coincidence of the
experimental absorption and excitation spectra of Ln(III) complexes with
the corresponding spectra of individual
ligands,[1-6,43,58,59] as well as numerous
experimental and theoretical studies[8,11,22-24]confirm that excited states of Ln(III) complexes are localized on
separate ligands. At the next stage, after fast nonradiative relaxation
to the local minimum of the lowest singlet state, molecule deactivates
either by fluorescence or by intersystem crossing to the nearest triplet
state. Then nonradiative relaxation occurs to the local minimum of the
lowest triplet state. Finally, the excitation energy transfers to the
resonance levels of the Ln(III) ion, which nonradiatively relaxes to the
radiative level and emits a photon at a certain wavelength. Structural
relaxation of the excited states of Ln(III) complexes is several orders
of magnitude faster than energy transfer processes. Therefore, it is
necessary to take relaxation into account and optimize the geometry of
the excited states.
In recent works,[60-62] advanced DFT and ab
initio techniques have been used to numerically evaluate the energy
transfer rates constants and demonstrated complex mechanisms of Ln(III)
ions’ sensibilization by surrounding ligands. Such mechanisms may
include charge transfer states that mainly occur in molecules with
electro-donating and electro-accepting units electronically connected
via a conjugated skeleton[60] and strongly depend
on the solvent polarity. It is still difficult to unambiguously relate
the Eu(III) sensitization to the triplet or charge transfer states.
Our previous experimental studies[7-9,30,37,43,56]verified indirect sensitization mechanism in the simulated mesogenic
Eu(III) complexes with saturated coordination sphere and did not show
the presence of charge transfer excited states. Whereby in the studied
complexes prevalent pathway includes intersystem crossing followed by
energy transfer from triplet levels of ligands. It is also worth noting
that the calculated (not for individual ligand molecules) triplet levels
of the ligands in Eu(III) complexes coincide with the experimental
values[8,43,44] obtained from the solvent-free
phosphorescence spectra of vitrified Gd(III) complexes’ films with the
same ligand environment (Table 2).
The energy transfer efficiency and the luminescence quantum yield of
Ln(III) complex are determined by the relative positions of the excited
states of ligands and the Ln(III) ion. As was mentioned earlier, the
greatest contribution to this process is made by the triplet levels of
ligands. Therefore, on the next stage of the work the equilibrium
geometries of the studied Eu(III) complexes in the ground state were
used as the starting point for the calculations of the triplet excited
states. According to our previous studies and literature
data[8,11,22-24] the excited states of Ln(III)
complexes are localized on individual ligands. This statement can be
also confirmed by the coincidence of the experimental absorption and
excitation spectra of Ln(III) complexes with the corresponding spectra
of individual ligands.[1-6,44,58,59]
Optimization of the geometries of the triplet excited states led to
corresponding minima and localization of excitation on individual
ligands. It was found that the first three triplet excited states are
localized on β-diketones, and the last state - on the Lewis base. The
average structural parameters of the studied Eu(III) complexes in the
optimized geometries of their triplet excited states are presented in
Table 1. Figures 3 and 4 show the optimized structures of
Eu(CPDK3-5)3Bpy17-17 and
Eu(CPDK35)3Phen complexes in their
ground and triplet excited states with triplet localization on
βdiketone CPDK3-5 and Lewis base. Optimized geometries
of other complexes can be found in the Supporting Information (Figure
S1).
TABLE 2 Vertical energies (ΔE) in Eu(III) complexes and VDP
characteristics in the optimized ground-state (S0) and
triplet (T1) geometries with the reference of the
excited state localization on separate ligands, intramolecular energy
transfer (WET) and back-transfer (WBT)
rates, theoretical and experimental values of quantum yields (Q)