Monomeric models

Monomeric models of [M(C^N^N)CN] (C^N^N = pyridinyl-phenylpyridine, M = Pd Pd-1-mono, Pt 1-mono; indazolyl-phenylpyridine, M = Pd Pd-2-mono, Pt Pt-2-mono; benzyltriazol-phenylpyridine, M = Pt Pt-3-mono; pyrazolyl-phenylpyridine, M = Pt Pt-4-mono) were optimized in the ground state S⁰ and low-est lying singlet and triplet excited states. All calculations were performed using DCM (dichloromethane) as the solvent. The optimized structures are presented in Figures 11 and 12 and the key structural parameters are collected to Table 1. For the palladium monomers, two different minima were found on the triplet potential energy surface. Starting from the optimized ground state, the triplet optimizations lead to a structure corresponding to a ³MLCT/ILCT (Pd-1-mono) or ³(d, π-π*) (Pd-2-mono) state, whereas by manually distorting the starting geometry we were able to locate also the ³MC minimum structures. For the platinum models, attempts to lo-cate the ³MC minima were unsuccessful.

Figure 11. Optimized structures of Pd-1-mono and Pd-2-mono in DCM in the ground state S0 and lowest lying singlet S1 and triplet T1 excited states. * Two differ-ent minima were located for the triplet states, which correspond to ³MLCT/ILCT (Pd-1-mono) or ³(d, π-π*) (Pd-2-mono) and ³MC structures.

29 Figure 12. Optimized structures of Pt-1-mono, Pt-2-mono, Pt-3-mono and Pt-4-mono in DCM in the ground state S⁰ and lowest lying singlet S¹ and triplet T¹ excit-ed states.

In the ground state, all optimized structures feature somewhat distorted square planar coordination environments as the C-M-N and N-M-N bite angles vary from 81.6° to 82.5° and from 76.8° to 78.4°, respectively (Table 1). Worth observing is the distinct difference in the metal-nitrogen bond lengths, the M-Ntrans (trans to the

cy-30

clometalating carbanion) bond being noticeably longer than the M-Ncis bond length.

This can be attributed to the strong trans-influence induced by the cyclometalating carbanion.^181,182

Table 1. Selected structural parameters for the studied monomers in DCM at the optimized ground state.

Pd-1-mono

Pd-2-mono

Pt-1-mono

Pt-2-mono

Pt-3-mono

Pt-4-mono bonds [Å]

M-Ncis 1.992 1.997 1.993 1.997 2.012 2.000

M-N^trans 2.152 2.132 2.132 2.110 2.126 2.129

M-C 1.983 1.978 1.989 1.986 1.987 1.985

M-CCN 1.960 1.960 1.947 1.948 1.946 1.946

angles [°]

C-M-Ncis 82.2 82.5 82.0 82.2 81.6 81.7

N^cis-M-N^trans 78.1 76.8 78.4 77.4 78.0 77.4

For the excited state structures, only very minor geometric alterations are observed, except for the S¹ state structure of Pd-1-mono and the ³MC structures of Pd-1-mono and Pd-2-mono. In the S¹ state structure of Pd-1-mono, the phenyl moiety is dis-torted slightly below the molecular plane. The ³MC state structures, however, fea-ture severely displaced strucfea-tures as all the metal-ligand bond lengths are elongat-ed by roughly 10 %. Moreover, the central pyridine fragments are being twistelongat-ed above the plane spanned by the normally planar C^N^N ligand, as well as the MCN fragments being twisted below the plane (Figure 11).

To study the excitation and emission characteristics and resolve the important low lying excited states of the solvated Pd and Pt complexes, TD-DFT calculations were performed. The calculated data for the two lowest lying singlet-singlet and the low-est lying singlet-triplet vertical excitations are collected to Table 2.

31 Table 2. Calculated two lowest lying singlet and the lowest lying singlet-triplet vertical excitations for the monomeric models in DCM (λ = excitation wavelength, f = oscillator strength).

Complex Transition λ [nm] f Major contributions* Assignment

Pd-1-mono

S₀ → S₁ 381 0.0118 94 % H → L MLCT/ILCT

S₀ → S₂ 359 0.0143 97 % H-1 → L MLCT

S₀ → T₁ 424 - 84 % H → L MLCT/ILCT

Pd-2-mono

S₀ → S₁ 356 0.0614 94 % H → L d, π → π*

S₀ → S₂ 331 0.0218 97 % H-2 → L MLCT

S₀ → T₁ 409 - 65 % H → L, 25 % H → L+1 d, π → π*

Pt-1-mono

S₀ → S₁ 422 0.0042 95 % H → L MLCT/ILCT

S₀ → S₂ 361 0.0166 97 % H-2 → L MLCT

S₀ → T₁ 462 - 83 % H → L MLCT/ILCT

Pt-2-mono

S₀ → S₁ 387 0.0131 96 % H → L d, π → π*

S₀ → S₂ 335 0.4160 81 % H-1 → L, 15 % H → L+1 d, π → π*

S₀ → T₁ 432 - 76 % H → L, 14 % H → L+1 d, π → π*

Pt-3-mono

S₀ → S₁ 392 0.0082 98 % H → L d, π → π*

S₀ → S₂ 335 0.0262 99 % H-2 → L MLCT

S₀ → T₁ 437 - 87 % H → L d, π → π*

Pt-4-mono

S₀ → S₁ 385 0.0155 98 % H → L d, π → π*

S₀ → S₂ 329 0.0257 99 % H-2 → L MLCT

S₀ → T₁ 432 - 83 % H → L d, π → π*

[*] H = HOMO, L = LUMO

For the complexes incorporating the pyridinyl-phenylpyridine ligand (i.e., Pd-1-mono and Pt-1-mono), the lowest lying S⁰ → S¹ transitions (estimated at 381 nm and 422 nm) can be assigned as a mixed MLCT/ILCT transitions with the electron density shifting from the metal and phenyl moieties over to the bipyridine system (Figures 13 and 14). The S⁰ → T¹ excitations have virtually the same character with ca. 40 nm higher excitation wavelength (Figures 13 and 14). For every other Pd and Pt monomer, the S⁰ → S¹ and S⁰ → T¹ transitions have a distinct π-π* character and are assigned as metal perturbed local excitations within the cyclometalating C^N^N ligand (Figures 13 and 14). The second lowest singlet-singlet transition, S0 → S², is assigned as a pure MLCT transition for each studied monomer, except for Pt-2-mono, for which it has a distinct metal perturbed π-π* character (Figures 13 and 14).

32

Figure 13. Natural transition orbital hole-particle pairs for the low-lying vertical excitations for the monomeric Pd models in DCM.

Figure 14. Natural transition orbital hole-particle pairs for the low-lying vertical excitations for the monomeric Pt models in DCM.

As stated above, for Pd-1-mono and Pd-2-mono, two different minima were located on the triplet potential energy surface. Unrestricted DFT as well as TD-DFT calcula-tions at the optimized minimum structures suggest that the ³MC structure is about 120 meV more stable than the ³MLCT/ILCT structure for Pd-1-mono and 235 meV more stable than the ³(d, π-π*) structure for Pd-2-mono. Thus, the ³MC state is ex-pected to represent the global minimum within the triplet manifold for both palla-dium monomers, providing an efficient non-radiative pathway back to the ground state, resulting in completely quenched emission.

33 As for the monomeric platinum complexes, the S1 and T1 states are separated by an energy difference of 0.36 eV, 0.58 eV, 0.53 eV and 0.58 eV for Pt-1-mono, Pt-2-mono, Pt-3-mono and Pt-4-mono, respectively. The smaller ΔE(S1-T1) value for Pt-1-mono is explained by the increased charge transfer character, and thus smaller quantum mechanical exchange energy. The small energy differences, combined with calcu-lated spin-orbit coupling matrix elements of 34.1 cm^-1, 63.2 cm^-1, 49.8 cm^-1 and 44.1 cm^-1 between the S¹ and T¹ states should result in fast ISC rate and efficient popula-tion of the T1 states. The energy difference is, however, far too great to be overcome by thermal energy, rendering any reverse ISC implausible. Phosphorescence emis-sion stemming from a ³MLCT/ILCT (Pt-1-mono) or ³(d, π-π*) (Pt-2-mono, Pt-3-mono and Pt-4-mono) is thus predicted by the TD-DFT calculations (Figure 15).

Calculated estimates for the radiative rate range from 3.8 · 10⁴ s^-1 to 7.5 · 10⁴ s^-1, cor-responding to radiative lifetimes between 26 and 13 microseconds (Table 3), with the predicted emission wavelengths falling into the green-yellow region. The pre-dicted rates are in line with previously reported radiative rates for other Pt(II) com-plexes.10,106,183-185

Table 3. Calculated radiative rates, lifetimes, emission wavelengths and emissive state characters for the phosphorescence emission in monomeric Pt complexes in DCM.

Complex kr [s^-1] τ [μs] λ [nm] Assignment

Pt-1-mono 4.4 · 10⁴ 23 578 MLCT/ILCT

Pt-2-mono 3.8 · 10⁴ 26 542 d, π → π*

Pt-3-mono 6.2 · 10⁴ 16 537 d, π → π*

Pt-4-mono 7.5 · 10⁴ 13 537 d, π → π*

Figure 15. Natural transition orbital hole-particle pairs for the phosphorescence emission for the monomeric Pt models in DCM obtained at the optimized T¹ state geometries.

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