Abstract
The three complexes [M(Me2dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me2dpb−) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal dyz, phenyl C4, C2, C1, and C6, and Cl pz orbitals were found. As expected, the dz2 (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding dx2−y2 orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced dz2-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the dz2 orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(dyz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me2dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called “dark”, i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the “dark” states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.
Highlights
Luminescent transition metal complexes have gained enormous importance in the last 20 years with potential applications in fields such as photocatalysis [1,2], sensing [3,4,5,6], and optoelectronic devices [5,6,7,8,9,10,11,12,13,14]
Efficient intersystem crossing (ISC) and otherwise spin-forbidden phosphorescence is favoured through large spin-orbit coupling (SOC) of the heavy metal centres [5,6,7,8,9,10,11]
We completed the series of homologous complexes [M(Me2 dpb)Cl]
Summary
Luminescent transition metal complexes have gained enormous importance in the last 20 years with potential applications in fields such as photocatalysis [1,2], sensing [3,4,5,6], and optoelectronic devices [5,6,7,8,9,10,11,12,13,14]. Phosphorescent metal complexes are of particular interest in the field of OLED (organic light emitting diode) applications, as these materials are able to harvest all generated excitons in the operating electroluminescent devices [11]. Efficient intersystem crossing (ISC) and otherwise spin-forbidden phosphorescence is favoured. Efficient intersystem crossing (ISC) and otherwise spin-forbidden phosphorescence is favoured through large spin-orbit coupling (SOC) of the heavy metal centres [5,6,7,8,9,10,11]. Through large spin-orbit coupling (SOC) of the heavy metal centres [5,6,7,8,9,10,11]. Centres [3,4,8,9,10,11,12], or the [3,4,8,9,10,11,12], d8 -configured Pt(II)
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