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Highly Stable and Strongly Emitting N‑Heterocyclic Carbene Platinum(II) Biaryl Complexes
journal contribution
posted on 2018-06-22, 15:24 authored by Dominik Suter, Luuk T. C. G. van Summeren, Olivier Blacque, Koushik VenkatesanC^C cyclometalated
platinum(II)
triplet emitters bearing electronically different N-heterocyclic carbenes(1,3-diisopropyl-4-(trifluoromethyl)-imidazol-2-ylidene
(d), 1,3-diisopropyl-benzimidazol-2-ylidene (e), and 1,3-diisopropyl-imidazol-2-ylidene (f))as
neutral ligands and biphenyl (bph) as well as its fluorinated derivative
octafluorobiphenyl (oFbph) as dianionic cyclometalating ancillary
ligand were synthesized and structurally characterized by 1H, 13C, 19F, and 195Pt NMR, single
crystal X-ray diffraction, and HR-ESI-MS studies. Detailed photophysical
investigations carried out reveal a strong influence on the excited-state
properties exerted by the electronic nature of the N-heterocyclic carbenes as well as the fluorine functional groups
on the ancillary biphenyl moiety. The solid-state structures of all
complexes reveal a nearly planar and slightly distorted square planar
geometry around the platinum center. Introduction of fluorine groups
into the ligand framework leads to a less structured emission centered
at 513 nm in poly(methyl methacrylate) (PMMA) thin films, compared
to the highly structured emission profile of the bph analogues. Additionally,
a hypsochromic shift of approximately 10–12 nm was found in
the absorption as well as in the emission profiles and is attributed
to the electron deficient nature of the oFbph ligand. Three wt % of
the compounds doped in PMMA exhibit photoluminescence efficiencies
as high as 92% in thin films. DFT and TD-DFT calculations on selected
molecules revealed the charge transfer to be an admixture of intraligand
(3ILCT) and metal-to-ligand charge transfer (3MLCT) and the frontier orbitals corresponding to the emission to
be mainly localized on the bph and oFbph ligands, which is consistent
with the observations from the photophysical investigations. The thermal
stability of the complexes evaluated by thermogravimetric analysis
(TGA) shows an enhanced thermal stability for the complexes bearing
fluorine functional groups.