3.1 Optimized density functional theory structures

Some important bond lengths and bond angles of the complexes depicted in Fig. 1 are reported in Table 1 and compared to X-ray values when available (complex 5).

The optimized structures of complexes 1 to 7 are depicted in Fig. 2 (hydrogen atoms are hidden for clarify).

The complexes 1 to 7 show significant π-stacking interactions between one (3, 4, 5, 6) or two (1, 2, 7) mesityl group(s) and the phenanthroline (substituted) -ligands. The double interaction occurs in symmetric complexes with less bulky ligands. In the other complexes substituted by butyl groups in 2,9 position of the phenantroline, the symmetry is destroyed, leading to a single interaction. Preliminary calculations performed at the DFT (B3LYP) level did not show these important π-stacking interactions that stabilize the structures of this class of heteroleptic complexes.

The main geometrical parameters reported in Table 1, optimized at the DFT (PBE-D) and DFT (PBE-D3) levels, are numbered according to Scheme 1. The only X-ray structure available is that of [Cu(Mes-phen)(nBu-phen)]⁺ (5). The experimental data are reported in brackets for comparison.

The analysis of the calculated Cu-N bond distances puts in evidence two distinct behaviours depending again on the presence of butyl groups on the phenantroline-like ligands. In complexes 3, 4, 5, 6, we are in presence of two short bond lengths Cu-N₄ and Cu-N₁ (∼ 2.0 Å) and two long bond lengths Cu-N₃ and Cu-N₂ (2.055–2.075 Å). This dissymmetry is correlated to the steric hindrance of the butyl groups and the π-stacking interactions in these complexes. In contrast, the optimized Cu-N bond distances in 1, 2 and 7 range from ∼ 2.0 Å to ∼ 2.03 Å with less distorted structures. The DFT (PBE-D) and DFT (PBE-D3) results are very similar for the bond distances as well as for the bond angles. In both cases, the errors with respect to the X-ray data do not exceed 5% on the bond distances and 3% on the bond angles and the main trends are well reproduced by the calculations.

The Kohn-Sham (KS) orbitals corresponding to the structures of complexes 1 and 5 are depicted in Schemes 2 and 3, respectively.

The HOMO corresponds to 3d_Cu in both complexes whereas the LUMO is localized on the phen ligand in 1 (π*_phen) and on the L₂ ligand in 5 (π*_L2). In complex 5, with a Mes-phen ligand and butyl groups on L₂, the HOMO-1 is localized on the Cu atom whereas the HOMO-2 (245) and HOMO-3 (244) are localized separately on the two mesityl groups of L₂, reflecting the distorted structure of the complex. In complex 1, with a simple phen ligand, HOMO-1 corresponds to a π* localized on L₁ (218) and HOMO-2 is localized on the metal atom. As shown in our previous paper [4], the electronic properties of these complexes are very sensitive to the ligands coordinated to the Cu atom and their substituants.

This is shown in Table 2 where the HOMO-LUMO gaps are reported for complexes 1 to 6. As expected, a small HOMO-LUMO energy gap is observed for complexes 4 and 5 where the LUMO is localized on the strongly π*_L2 acceptor ligand. Complexes 1, 2 and 6 have similar HOMO-LUMO gaps whereas complex 3 is characterized by the highest energy gap due to an important destabilization of the HOMO, localized on Bu-phen, not compensated by the small destabilization of the LUMO. These features will have important consequences on the electronic spectroscopy as illustrated in the next section.

3.2 Time dependant-density functional theory electronic absorption spectra

The transition energies to the low-lying singlet excited states of the L₁ substituted complexes 1, 2, 3 are reported in Table 3 whereas the transition energies to the low-lying singlet states of the L₂ substituted complexes 4, 5 and of the complex 6 with mesityl and butyl groups on the two phenanthrolines are reported in Table 4.

Only the excited states with significant oscillator strengths (greater than 0.05), excepting in case of necessity, and ranging in the UV/vis energy domain between 600 nm and 250 nm are reported in Tables 3 and 4 as well as their character: MLCT, IL or LLCT. The localization of the charge transfer is given by the indices L₁ (when CT occurs towards the substituted phen of the L₁ ligand) or phen (when CT occurs towards the phen substituted by H in 1, Me in 2 and Bu in 3). The main character of the excited states listed in Tables 3 and 4 are given in percentage.

The theoretical absorption spectra of the complexes 1, 2 and 3 are very similar and the structural deformation of complex 3 due to the presence of butyl ligands is of little influence on the absorption spectroscopy. This is in agreement with the experimental findings [4] (Fig. 3). However, the character of the low-lying states may be slightly different with the occurrence of LLCT states in the lowest part of the spectrum of 3 and a blue shift of the starting absorption at 509 nm (vs. 580 nm for 1 and 2), not observed experimentally and probably due to the strong contribution of the MLCT_phen state. The presence of butyl groups seems to induce more charge transfer towards the phen ligand in the lowest as well as in the highest excited states of 3. The absorption of complexes 1 and 2 starts at about 580 nm (vs. 600 nm experimentally), with low-lying MLCT states corresponding to d_Cu → π*_L1/phen excitations showing a major contribution of the charge transfer to L₁ and a maximum calculated at about 465 nm (vs. 480 nm experimentally). The solvent corrected values for CH₂Cl₂ are nearly identical to those in vacuum as illustrated by the data given in parenthesis for some of the important states of 1 and 2. The same trends are observed for complex 3.

The near-UV region of the absorption spectra is characterized by the presence of IL states localized on the L₁ ligand calculated at 370 nm and 320 nm and characterized by rather large oscillator strengths. They correspond to the absorption at 360 nm and 325 nm observed on the experimental spectrum of 1. The UV region is characterized by a high density of states between 315 nm and 265 nm that contribute to the large band centered at 280 nm experimentally. The most intense peaks possess an important contribution of states localized on L₁, either IL or LLCT (π_phen →π*_L1).

All the calculated TD-DFT excited states with non-zero oscillator strengths have been included in the theoretical spectra represented in Fig. 4.

According to the shape of the calculated absorption spectra of complexes 1, 2 and 3 represented in Fig. 4 and taking into account the relative intensities, the calculations including solvent correction (Fig. 4 (a)) are rather realistic leading to a perfect agreement with the experimental spectra depicted in Fig. 3. The theoretical absorption spectrum of complex 7 with phenyl groups on the Me-subsituted phenanthroline ligand, for which there is no experimental spectrum available, starts at 620 nm and is very similar to the spectrum of complexes 1, 2 and 3 as illustrated in Fig. 4 with a band at 460 nm corresponding to MLCT_L1 transitions and at 376 nm with an important IL_L1 character. A series of states calculated around 320 nm correspond to MLCT/IL/LLCT states of mixed character similarly to complexes 1 to 3. The band calculated at 307 nm is present as well, like in the theoretical spectra of complexes 1 and 2.

The absorption spectroscopy of complexes 1, 2, 3 and 7 is characterized by low-lying MLCT states with more (3) or less (1,2 and 7) contribution of charge transfer to the phen ligand. It seems that the presence of butyl groups increases the charge transfer to the phen. The character of the excited states and the energetics are not dramatically affected by the structural distortions due to the presence of butyl groups and the nature of the π-stacking interactions which are very weak and do not modify visible/UV energetics. However, the study of nuclear relaxation in the low-lying electronic excited states that is beyond the scope of the present study could change this conclusion.

The transition energies to the low-lying singlet excited states of the second series of Cu (I) heteroleptic complexes 4, 5 and 6 are reported in Table 4, together with their character and associated oscillator strengths. The theoretical spectra of 4 and 5 are very similar in agreement with the experimental finding (Fig. 3) with higher intensities for 4 related to the contribution of IL states localized on L₁.

The density of states is more important in 4 than in 5 because of the presence of the two ligands L₁ and L₂ in 4 that induce large mixing between MLCT and LLCT states. The absorption starts at about 500 nm in both complexes with a MLCT state localized on L₁/L₂ in 4 and on the mesityl substituted phen in 5. We retrieve the two peaks observed at 390 nm and 370 nm characteristics of these two complexes on the experimental spectra (Fig. 3) and the shoulder at 325 nm only in the spectrum of 4. This shoulder is attributed to a IL_L1/LLCT_L2 state with 40% of intra-ligand character. Most of the important states in 5 are localized on L₂, either MLCT or LLCT whereas important IL/MLCT/LLCT mixing is observed for 4, leading to stronger intensities and higher densities of states. Beyond 300 nm, the absorption spectra of 4 and 5 are nearly identical with strongly absorbing states of similar characters in both molecules.

According to the experimental data (Fig. 3), the absorption spectrum of 6 is less intense than the other spectra and it does not show any peaks at 390, 370 and 325 nm. The UV region is shifted to the blue with respect to 4 and 5. This is confirmed by the theoretical results reported in Table 4 and the theoretical spectra depicted in Fig. 4. The absorption spectrum of 6 is characterized by IL and MLCT states towards the mesityl substituted phen with weak oscillator strengths and an intense peak calculated at 271 nm corresponding to a mixed MLCT/IL/LLCT state localized on the butyl substituted phen ligand.