Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh₃ and SbPh₃

Abstract

The introduction of heavy ancillary ligands L = AsPh3 and SbPh3 in [Pt(C^N^C)(L)] complexes was explored to enhance the luminescence efficiency through increased spin orbit coupling.

Molecular Rigidification of Cyclometalated N∧C∧N Pt(II)- and Pd(II)-Based Triplet Emitters

“…In accordance with literature reporting on similar cyclometalated complexes, the luminescence originates from a metal-perturbed LC triplet state ( 3 MP-LC). 18,28,41,42,45,47 Overall, the photoluminescence spectra of the [M(N ∧ C ∧ N)-Cl] complexes at 77 K strongly resemble those of the recently reported complexes [M(Me 2 dpb)Cl] (M = Pt or Pd), including the blue-shift of the Pd complex with its maximum at 468 nm compared with the Pt derivative peaking at 489 nm. 42 However, as discussed above for the parent [Pt(dpb)Cl] complex, the [M(N ∧ C ∧ N)Cl] derivatives emit markedly redshifted.…”

“…(E = P, Sb, or As), we recently found electrochemical HOMO−LUMO gaps larger than the optical gaps by up to 0.5 eV. 18 As from our TD-DFT calculations we can rule out direct singlet → triplet transitions S15. contributing to the bands around 400 nm (Tables S13 and S14), we assumed hampered electrochemical responses to be responsible for this phenomenon.…”

“…A closer look reveals that the optical HOMO–LUMO gaps (∼3 eV) are slightly smaller than the electrochemical HOMO–LUMO gaps (3.22 and 3.31 eV); normally, this is the opposite way, as electrochemistry probes the ground state. For the C ∧ N ∧ C-coordinated Pt­(II) complexes [Pt­(C ∧ N ∧ C)­(EPh 3 )] (E = P, Sb, or As), we recently found electrochemical HOMO–LUMO gaps larger than the optical gaps by up to 0.5 eV . As from our TD-DFT calculations we can rule out direct singlet → triplet transitions contributing to the bands around 400 nm (Tables S13 and S14), we assumed hampered electrochemical responses to be responsible for this phenomenon.…”

“…1−7,10−15,17−26 The involved excited states are labeled either as mixed LC/MLCT (ligand-centered/metal-to-ligand charge transfer) or as metalperturbed ligand-centered (MP-LC) states. [17][18][19][20][21]26 However, there are several examples of cyclometalated Pt(II) complexes with seemingly suitable cyclometalating ligands, such as [Pt(C ∧ N ∧ C)(CO)] 22 and [Pt(C ∧ N ∧ C)(benzoisonitrile)] 23,24 (HC ∧ N ∧ CH = 2,6-diphenyl-pyridine) that are non-emissive, as well as [Pt(C ∧ N ∧ N)Cl] (HC ∧ N ∧ N = 6-phenyl-2,2′-bipyr-idine), which is only weakly emissive at ambient temperature. 25 The lack of emission from the [Pt(C ∧ N ∧ C)(L)] complexes at 298 K, despite the fact that C ∧ N ∧ C provides a stronger ligand field than N ∧ N ∧ C or N ∧ C ∧ N coordination modes, is due to significant structural distortions in their geometries, as the aromatic rings of the C ∧ N ∧ C ligand adopt a bent conformation in the triplet (T 1 ) excited state rather than the coplanar configuration found in the singlet (S 0 ) ground state, while the metal coordination geometry changes from square planar toward a tetrahedral configuration.…”

“…These states are responsible for nonradiative decay paths via conical intersections to the ground state. ,,− Second, such ligands provide a rigid coordination environment to the metal which prevents radiationless decay from the excited triplet state. ,,, Further, heteroaromatic systems show intense light absorption mediated by excited π–π* states. The heavy metal cations provide strong spin–orbit coupling (SOC), hence promoting singlet–triplet intersystem-crossing (ISC) and phosphorescence as a consequence. − Among the group 10 metals, many cyclometalated Pt­(II) and a significant number of Pd­(II) complexes have been reported as triplet emitters. − ,− ,− The involved excited states are labeled either as mixed LC/MLCT (ligand-centered/metal-to-ligand charge transfer) or as metal-perturbed ligand-centered (MP-LC) states. − , However, there are several examples of cyclometalated Pt­(II) complexes with seemingly suitable cyclometalating ligands, such as [Pt­(C ∧ N ∧ C)­(CO)] and [Pt­(C ∧ N ∧ C)­(benzoisonitrile)] , (HC ∧ N ∧ CH = 2,6-diphenyl-pyridine) that are non-emissive, as well as [Pt­(C ∧ N ∧ N)­Cl] (HC ∧ N ∧ N = 6-phenyl-2,2′-bipyridine), which is only weakly emissive at ambient temperature . The lack of emission from the [Pt­(C ∧ N ∧ C)­(L)] complexes at 298 K, despite the fact that C ∧ N ∧ C provides a stronger ligand field than N ∧ N ∧ C or N ∧ C ∧ N coordination modes, is due to significant structural distortions in their geometries, as the aromatic rings of the C ∧ N ∧ C ligand adopt a bent conformation in the triplet ( T 1 ) excited state rather than the coplanar configuration found in the singlet ( S 0 ) ground state, while the metal coordination geometry changes from square planar toward a tetrahedral configuration. , Such structural deformations can lead to the vibronic coupling of the T 1 and S 0 states and thus to rapid radiationless relaxation from the T 1 state.…”