FIGURE 4 Optimized ground (a) and triplet excited state structures of Eu(CPDK5-Th)3Phen with triplet localization on β-diketone CPDK5-Th (b) and Lewis base Phen (c)
Differences in the structure of the ground and excited states can determine the efficiency of energy transfer process and the emission intensity. It was found that localization of excitation on the same ligand in different Eu(III) complexes leads to similar geometry changes (Figures 3b and 4b). The triplet-localization of excitation on β-diketone (Figures 3b and 4b) resulted in insignificant changes of bond lengths by ~ 0.1 Å (Table 2) compared with the ground state (Figures 3a and 4a). At the same time, the dihedral angles between non-alkyl substituents and C = O bonds in β­diketones changed by 10-13º. In the case of Bpy-localized excitations, the C–H bond in position 5 of the pyridine ring came out of the plane by 20º (Figure 3c). But when triplet excitation was localized on Phen, this ligand remained its planar geometry (Figure 4c). Such significant structural deformations of Eu(III) complexes with Bpy can lead to a noticeable shift in the energy minima of the ground and excited states and even to the crossing of their potential curves. Consequently, during the relaxation of the excited state, the molecule can transfer to the curve of the ground state and relax to its stationary state without any emission. On the contrary, the rigid structure of the Eu(III) complexes with Phen minimizes the nonradiative deactivation and increases their emission efficiency. Thus, it can be assumed that in Eu(III) complexes with β-diketones and Lewis bases, the main role in energy transfer is played by three β-diketones due to their more flexible geometry compared to Lewis bases. Similar structural changes of excited ligands can be found in literature.[11,16,18,63]
Table 2 shows the results of the calculated triplet energies of Eu(III) complexes with excitation localization on individual ligands. Similar values were obtained for triplet localization on the same ligands in different Eu(III) complexes. For instance, the excited levels for triplet excitation of Bpy17-17 in complexes 1and 3 differ by 0.03 eV (Table 2). The same distinction was obtained for CPDK5-Th in complexes 3 and4 , and even smaller differences were observed for other ligands. Therefore, it can be assumed that the excited states localized on the corresponding ligand are practically independent of other ligands in the complex.
The inner 4f shell of Ln(III) is shielded from the influence of the ligand environment by the outer 5s and 5p shells. This leads to quite narrow emission bands of 4f transitions which have the same wavelengths for different complexes of a given Ln(III).[1-6] Therefore, the 4f levels of Eu(III) can be obtained from the experimental data[64,65] due to unaffordable computational costs for their calculation for the studied Eu(III) complexes with huge ligands.
Table 2 also presents the calculated rates of the intramolecular forward and back energy transfer from the averaged triplet levels of ligands to the resonance levels of Eu(III), as well as the theoretically predicted quantum yields in comparison with the experimental ones.[47] As can be seen from Table 2, complexes Eu(CPDK3­5)3Phen and Eu(CPDK3­Ph)3Phen are characterized by the highest rates of forward energy transfer and small contributions of the backward processes, which leads to their remarkable emission efficiency according to the values of luminescence quantum yields (32.6 and 29.7 %, respectively). Complexes with Bpy17-17, on the contrary, are characterized by insubstantial luminescence efficiency and intermolecular energy transfer, which can be explained by the analysis of their energy transfer channels.
The triplet level of Bpy17-17-localized excitation (2.790 eV in complex 1 ) is situated between the5D2 (2.667 eV) and5D3 (3.024 eV) levels of Eu(III).[64,65] Therefore, in complex 1energy transfer will occur from the Bpy17-17 triplet level of 2.797 eV to the 5D2 multiplet of Eu(III). Such position of the triplet level can lead to energy losses due to additional stages of interligand transfer or nonradiative transfer between different multiplet of5Dj state. The triplet levels of β-diketones CPDK5­Th (2.192 and 2.207 eV in complexes3 and 4 ) and DK12-14 (2.294 eV in complex 6 with the experimental value of 2.375 eV) are too close to the 5D1 multiplet (2.359 eV) and can partially transfer energy to the5D0 level (2.141 eV). Unfortunately, the presence of this energy transfer channel and wide energy gap between these two states (ligand’s triplet level and5D0) will increase the probability of energy losses due to energy back transfer, molecule deactivation by ligands’ phosphorescence, energy dispersion and decrease of the Eu(III) luminescence efficiency. Therefore, Eu(CPDK5­Th)3Bpy17-17 is characterized by the lowest quantum yield (10.1%) among the studied objects. On the contrary, the triplet levels of CPDK3-5(2.362 eV in complex 1 ), CPDK3-Ph (2.311 eV in complex 5 ) and Phen (2.606 eV in complex 2 ) are in resonance with the 5D1 multiplet and can participate in efficient energy transfer to Eu(III).
According to experimental works[44,66-68] and general theoretical assumptions,[1-6,11,16]complexes with triplet levels located between5D1 and5D2 multiplets of Eu(III) potentially have the highest emission intensity and smaller energy losses. This can be explained by effective energy transfer from the triplet level of ligand to 5D1 multiplet of Eu(III) which is located above the emitting5D0. In the case of resonant energy transfer from the ligand to Eu(III), the contribution of the back transfer from the ion to ligand is quite large. The process of Eu(III) phosphorescence is relatively slow, and ion can return energy to the ligand. But when energy can be transferred to the upper energy level than the emitting one, back transfer competes with the nonradiative relaxation process of the excited ion due to small energy gaps between5Dj multiplets. After such relaxation, the resonance between the ligand’s triplet level and the central ion is disturbed, therefore energy back transfer becomes impossible. Among the studied Eu(III) complexes, Eu(CPDK3­5)3Phen and Eu(CPDK3­Ph)3Phen meet these requirements.