Communication
with consequent reduction of the performance and lifetime of
a PhOLED device.
Abstract: Platinum(II) complexes with an N-heterocyclic
carbene and a cyclometalating phenyl ligand (C^C*) are
excellent candidates as efficient blue triplet emitters for
OLED applications. The electronic and photophysical prop-
erties of these complexes can be fine-tuned with the ob-
jective to increase the quantum yields and lower the
phosphorescence decay times. We found that platinum
complexes with an imidazopyrazine C^C* ligand and
bulky acetylacetonates are sky-blue triplet emitters, char-
acterised by an almost unitary quantum yield and short
phosphorescence decay times.
To overcome these shortcomings, the scientific community
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proposed some strategies to prevent the population of MC
states.[9b,c,12] Efficient blue triplet emitters can be obtained
using strong-field ligands which provide strong metal–ligand
bonds.[12–13] Also, the rigidity of the complexes is impor-
tant.[8b,14] Strong-field ligands (those ligands with excellent s-
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donor and p-acceptor properties) destabilise MC states.[11b,14d]
Strong metal–ligand bonds prevent the breaking of bonds,[15]
and rigid systems prevent the distortions in the excited state.
In this perspective, N-heterocyclic carbenes with a cyclometa-
lated fragment[16] (C^C* ligands) distinguished themselves as
excellent systems for photophysical applications.[17] These ex-
ceptional ligands form strong metal–ligand bonds, and their
complexes are, in general, rigid. Also, the electronic properties
of both the cyclometalated and the carbene fragments can be
fine-tuned. For example, modifying the p-conjugation on the
C^C* ligand, the phosphorescent decay time can be changed
between 8 and 400 ms.[18] Recently we also reported[19] that the
introduction of a heteroatom in the backbone of the N-hetero-
cyclic carbene changes the photophysical properties of the
C^C* platinum(II) complexes. For example, platinum(II) com-
plexes with an imidazopyridine ligand (Figure 1c, and d),
Introduction
Current sources of illumination are not efficient. For instance,
the incandescent bulb, which is the most widely used form of
artificial illumination, dissipates more than 90% of the energy
as heat.[1] Powering these sources of illumination requires
more than 20% of the overall energy production.[2] This energy
demand is responsible for more than one-third of the world’s
greenhouse gas production.[3] Considering that we are facing
both an energy and an environmental crisis, it is imperative to
find more efficient sources of illumination. A promising alterna-
tive to the incandescent bulb is offered by the PhOLED (phos-
phorescent organic light emitting diode) technology. In a
PhOLED an organometallic molecule, called triplet emitter, is
electrically excited. After the excitation, the triplet emitter re-
laxes from the excited triplet state to the ground state, emit-
ting a photon. The emission has a well-defined wavelength,
which depends on the energy gap between the excited triplet
and the ground state. Hence, by accurate molecular design of
the triplet emitters, different emission colours can be obtained.
During the last decade, there has been great interest from
both academia and industry to develop new triplet emitters
with improved photophysical properties. Efficient and stable
triplet emitters have been obtained for red,[4] orange,[5] yel-
low,[5a,6] and green[7] emissions; however, the blue colour is still
an open challenge.[8] Efficient blue triplet emitters are difficult
to obtain because the energies of the emissive and metal-cen-
tred (3MC) states are comparable. If the energy difference be-
Figure 1. Selected C^C* platinum complexes reported by our group a,[13c]
d
b,[18], c,[19a] [19b]. The measured quantum yields are given for comparison.
acac=pentane-2,4-dione.
showed higher quantum yields and at the same time shorter
decay times than structurally related phenyl imidazole (Fig-
ure 1a) and benzimidazole (Figure 1b) platinum(II) C^C* com-
plexes. Additionally, we have shown that the use of bulky aro-
matic acetylacetonates improves the photophysical properties
of the emitters. For example, using bulky aromatic acetylaceto-
nates we observed an increase in phosphorescence quantum
yields.
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tween an excited triplet state and a MC state is small, then
3
the MC state can be accessible at room temperature.[9] The
These results recently prompted us to study the electronic
and photophysical properties resulting from the introduction
of a second nitrogen atom. In the literature imidazopyrazine
complexes of gold(I),[20] silver(I),[21] rhodium(I),[21] copper(I),[22]
and platinum(II)[23] are known. We compared the electronic and
spectroscopic properties of the imidazopyrazine complex with
those observed for related systems (Figure 2, complexes 1–3).
Then, we investigated the electronic and photophysical prop-
erties of platinum(II) complexes with an imidazopyrazine and
bulky aromatic acetylacetonates. We found that the imidazo-
pyrazine complexes with bulky aromatic acetylacetonates are
among the most efficient and fastest platinum sky-blue triplet
emitters.
3
population of MC states leads to a geometrical distortion of
the triplet emitter,[10] which in some cases may lead to a bond-
breaking process.[9a,10–11] Therefore, the population of 3MC
states is associated with the degradation of the triplet emitter
[a] P. Pinter, J. Soellner, Prof. Dr. T. Strassner
Physikalische Organische Chemie
Technische Universitꢀt Dresden, 01069 Dresden (Germany)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
&
&
Chem. Eur. J. 2019, 25, 1 – 6
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Pinter, Piermaria
