DOI: 10.1002/chem.201502087
Communication
&
Ligand Design
Phosphorescent Platinum(II) Complexes with C^C*
Cyclometalated NHC Dibenzofuranyl Ligands: Impact of Different
Binding Modes on the Decay Time of the Excited State
Alexander Tronnier,[a] Gerhard Wagenblast,[b] Ingo Münster,[b] and Thomas Strassner*[a]
and, thus, blueshifted emission maxima, addressing the still un-
Abstract: Two C^C* cyclometalated platinum(II) N-hetero-
cyclic carbene (NHC) complexes with the general formula
[(C^C*)Pt(O^O)] (C^C*=1-dibenzofuranyl-3-methylbenz-
imidazolylidene; O^O=dimesitoylmethane) have been
synthesized and extensively characterized, including solid-
state structure determination, 195Pt NMR spectroscopy, and
2D NMR (COSY, HSQC, HMBC, NOESY) spectroscopy to elu-
cidate the impact of their structural differences. The two
regioisomers differ in the way the dibenzofuranyl (DBF)
moiety of the NHC ligand is bound to the metal center,
which induces significant changes in their physicochemi-
cal properties, especially on the decay time of the excited
state. Quantum yields of over 80% and blue emission
colors were measured.
fulfilled demand for stable blue emitters. The resulting com-
plexes feature imidazole or benzimidazole moieties, allowing
for well-directed fine-tuning of the electronic properties by
modifying the N-bound substituents or incorporating electron-
donating or -withdrawing groups in the NHC backbone.[7]
Our group has reported different studies on bidentate C^C*
ligands, wherein the influence of modifications in the cyclome-
talated fragment, of extensions of the NHCs p-system, the use
of triazole-based carbenes, and the impact of ancillary ligands,
was investigated.[7b–f,8] A complex (Scheme 1A) bearing a diben-
zofuranyl (DBF)-based NHC ligand together with the common
monoanionic acetylacetonato (acac) ancillary ligand showed an
outstanding quantum yield of 90%, a blue emission maximum
(463 nm), but a relatively long decay lifetime of 23 ms in poly-
(methyl methacrylate) (PMMA) at room temperature (RT).[9]
First device tests showed very good external quantum efficien-
cies of 6.2% at 300 cdmÀ2 and a maximum luminance of
6750 cdmÀ2 considering that the stack design was not opti-
mized. In contrast, similar complexes featuring pyridinyl in-
stead of 3-methylimidazol-ylidene showed green emission
(516 nm) and quantum yields of 43% (Scheme 1B).[10]
The concept of employing transition-metal complexes as phos-
phorescent emitter materials in organic light-emitting devices
(OLEDs), utilizing their strong spin-orbit coupling (SOC) to en-
hance quantum yields and emission decay properties, has
been extensively explored over the past decade.[1] Quantum
yields approaching unity and the ability to tune the emission
color from the blue to near infrared region of the spectrum
have triggered a rush in research towards cyclometalated com-
plexes with heavy transition metals, especially iridium and plat-
inum complexes.[2] In 1998, Baldo et al. demonstrated the first
device with a porphyrin-based platinum complex that exhibit-
ed red electrophosphorescence.[3] Following these findings, re-
search efforts shifted to heteroleptic cyclometalated PtII emit-
ters with bidentate[2b–e,4] or tridentate[5] ligands. As such, pyri-
dinyl, imidazolyl, or other five-membered heterocycles are a re-
peatedly found fragment used as neutral N-coordinated s-
donor ligands. Recently, these fragments were replaced by N-
heterocyclic (NHC) ligands,[6] leading to stronger ligand fields
Replacing the acac ligand with a more bulky congener,
namely, the dimesitoylmethanato (mesacac) ligand, led to in-
creased solubility, reduced aggregation behavior, and an im-
provement of the photoluminescence properties to 91%,
466 nm, and 18.9 ms (Scheme 1C).[7c] Furthermore, we recently
showed that an extension of the p-system in the rigid NHC
backbone could increase the quantum yield.[7e] In the present
study we combine these earlier results in one system and in-
vestigate the influence of different ways of attaching the DBF
fragment to the heterocyclic system and thus the metal center
with the goal to reduce the decay lifetime, which remains
a very critical aspect in overall device stability. In particular, the
latter may inspire new ideas of how the established DBF frag-
ment can be used in other cases.
[a] Dr. A. Tronnier, Prof. Dr. T. Strassner
Physikalische Organische Chemie
Technische Universität Dresden
01069 Dresden (Germany)
[b] Dr. G. Wagenblast, Dr. I. Münster
BASF SE
67056 Ludwigshafen (Germany)
Supporting information for this article is available on the WWW under
Scheme 1. Platinum(II) complexes with cyclometalated DBF-NHC (A, C) and
DBF-pyridinyl (B) ligands.
Chem. Eur. J. 2015, 21, 12881 – 12884
12881
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