Palladium(0) NHC complexes: A new avenue to highly efficient phosphorescence

Article abstract of DOI:10.1039/c4sc03914a

We report the first examples of highly luminescent di-coordinated Pd(0) complexes. Five complexes of the form [Pd(L)(L′)] were synthesized, where L = IPr, SIPr or IPr? NHC ligands and L′ = PCy₃, or IPr and SIPr NHC ligands. The photophysical properties of these complexes were determined in degassed toluene solution and in the solid state and contrasted to the poorly luminescent reference complex [Pd(IPr)(PPh₃)]. Organic light-emitting diodes were successfully fabricated but attained external quantum efficiencies of between 0.3 and 0.7%.

Full text of DOI:10.1039/c4sc03914a

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Palladium(0) NHC complexes: a new avenue to
highly efficient phosphorescence†
Cite this: Chem. Sci., 2015, 6, 3248
Adam F. Henwood,‡a Mathieu Lesieur,‡b Ashu K. Bansal,‡c Vincent Lemaur,d
David Beljonne,d David G. Thompson,e Duncan Graham,e Alexandra M. Z. Slawin,b
c
b
a
*
*
*
Ifor D. W. Samuel, Catherine S. J. Cazin and Eli Zysman-Colman
We report the first examples of highly luminescent di-coordinated Pd(0) complexes. Five complexes of the
form [Pd(L)(L0)] were synthesized, where L ¼ IPr, SIPr or IPr* NHC ligands and L0 ¼ PCy3, or IPr and SIPr NHC
ligands. The photophysical properties of these complexes were determined in degassed toluene solution
and in the solid state and contrasted to the poorly luminescent reference complex [Pd(IPr)(PPh3)].
Organic light-emitting diodes were successfully fabricated but attained external quantum efficiencies of
between 0.3 and 0.7%.
Received 17th December 2014
Accepted 2nd April 2015
DOI: 10.1039/c4sc03914a
www.rsc.org/chemicalscience
simultaneously satisfy these two key requirements of phos-
phorescence and color tunability.
Introduction
By far the most popular and successful class of metal
complexes that have been studied so far are based on iridiu-
m(III). The high spin–orbit coupling (SOC) constant of iridium3
gives concomitantly short emission lifetimes (se) and high
photoluminescence quantum yields (FPL). Facile functionali-
sation of the ligand scaffold of the prototypical fac-Ir(C^N)3
complex (where C^N is a monoanionic cyclometalating bis-
chelate such as 2-phenylpyridinato) employed for these devices
can lead to emission spanning the visible spectrum with FPL
near unity.4 However, in spite of these advantages, iridium is
incredibly scarce – it is the third rarest transition metal in the
Earth's crust5 – making the search for alternatives to iridium
crucial if large scale implementation of OLED lighting tech-
nology is to be realised.
A number of crucial design features need to be considered in
order to obtain high performance organic light-emitting diodes
(OLEDs). Chief amongst these is the development of bright
emitters capable of recruiting 100% of the excitons in an
operational device. Phosphorescent molecules satisfy this
criterion although emission lifetimes need to be sufficiently
short in order to mitigate undesired quenching processes that
can reduce the overall efficiency of these types of compounds in
the bulk solid.1 Color tunability of the material across the visible
spectrum is required in order to achieve white light emission,
which is a necessary feature for large area lighting applications.2
Only a few types of phosphorescent materials have been
extensively explored in the context of solid-state lighting that
With this scarcity in mind, alternative emitters have been
explored in light-emitting devices, including those based on
Re(I),6 Ru(II),7 and Os(II).7c,8 However, until now these materials
have failed to match iridium on device stability,6 color
tunability9 or excited state lifetimes.7c So far, only platinum(II)
and copper(I) complexes have proven to be competitive with
iridium as phosphorescent materials for solid-state lighting
(SSL).
aOrganic Semiconductor Centre, EaStCHEM School of Chemistry, University of St
Andrews, St Andrews, Fife, KY16 9ST, UK. E-mail: eli.zysman-colman@st-andrews.
ac.uk; Web: http://www.zysman-colman.com; Fax: +44-(0)1334-463808; Tel: +44-(0)
1334-463826
bEaStCHEM School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST,
UK. E-mail: cc111@st-andrews.ac.uk; Fax: +44-(0)1334-463808; Tel: +44-(0)1334-
464808
cOrganic Semiconductor Centre, SUPA School of Physics and Astronomy, University of
The greater abundance of copper and its signicantly lower
cost than most other photophysically active metals make it an
attractive option, and experimental results both in solution10
and in devices11 have been encouraging, particularly for green
emitters.12 Two broad classes of Cu(I) complexes have been
explored: multinuclear Cu(I) dimers and clusters;13 and more
recently, sterically congested mononuclear three- or four-coor-
dinate Cu(I) complexes bearing a combination of N^N, N^P, P^P
and N-heterocyclic carbene (NHC) ligands.12,14 Apart from the
obvious benet of using copper for its low cost, interest is
St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK. E-mail: idws@st-andrews.
ac.uk
d
´
Service de Chimie des Materiaux Nouveaux & Centre d'Innovation et de Recherche en
´
`
´
Materiaux Polymeres, Universite de Mons – UMONS/Materia Nova, Place du Parc, 20,
B-7000 MONS, Belgium
eWestCHEM Department of Pure and Applied Chemistry and Centre for Molecular
Nanometrology, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK
† Electronic supplementary information (ESI) available. CCDC 1021492 and
1021493. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4sc03914a
‡ These authors contributed equally to the work.
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burgeoning in the use of Cu(I) for device applications owing to
its capacity to recruit triplet excitons in spite of its small SOC
constant (x ¼ 829 cmꢀ1).14c,15 This is made possible by the
capacity of most (but not all)14a Cu(I) complexes to thermally
populate the S1 state from the T1 state in a fashion that allows
for thermally activated delayed uorescence processes to occur
(TADF).
However, the stability of mononuclear Cu complexes can be
an issue. For example, it is common for these complexes to
undergo ligand exchange as a result of their lability. Addition-
ally, tetrahedral four-coordinate complexes can distort to a
square planar geometry in the excited state, allowing for
nucleophilic attack and the subsequent formation of a “penta-
coordinated exciplex” species that undergoes rapid emission
quenching and reduces device stability.11 Additionally, the lack
of bright red copper emitters still remain a major issue.11
Besides Cu(I), there has been considerable research into
cyclometalated platinum(II) complexes. These complexes are
highly stable, and show comparable microsecond se values and
color tuning properties to Ir(III). Synthetic versatility in the
design of ligand scaffolds for decoration about the platinum
center16 has sought to take advantage of these properties,
leading to reports of highly luminescent complexes bearing
multidentate ligands,17 NHC-cyclometalating ligands,18 azolato-
based pseudo-cyclometalating ligands,19 among others,20 for
OLED applications. However, square planar Pt(II) compounds
have the propensity to aggregate through metallophilic inter-
actions that induce undesirable deactivation pathways or exci-
mer formation that reduce color purity.7c
Despite its analogous coordination chemistry with platinum,
one metal severely underrepresented in terms of its photo-
physical properties is palladium. In the case of Pd(II), this is
likely attributable to the much smaller ligand eld splitting for
Pd(II) than that of Pt(II), which results in facile population of the
dx2–y2 antibonding orbital and thus efficient deactivation of the
excited state by twisting into a tetrahedral-type D2d excited state
geometry.21 Accordingly, studies into the luminescence of Pd(II)
are sparse and there is only one example of which we are aware
where a Pd(II) complex has been adopted as the emitter in an
OLED using vacuum deposition techniques (Fig. 1), although
even in this case, the se of this complex (121 ms) is likely to be too
long for widespread commercialisation of this material for
OLED applications.21d
Fig. 2 Selection of luminescent Pd(0) complexes bearing phosphines
and/or NHCs previously reported.
On the other hand, Pd(0) complexes, while receiving vast
attention for their impressive catalytic properties,22 have been
explored even more sparingly than Pd(II) in a photoluminescent
context. This is perhaps surprising, since Pd(0), bearing a d10
electronic structure, has no accessible d–d metal centered (MC)
states23 that might rapidly deactivate the excited state in a
manner similar to Pd(II). However, the instability of Pd(0)
compounds with respect to oxygen24 has perhaps dissuaded
exploration of the photoluminescence properties of these
compounds. Work has largely focussed on luminescence
studies of Pd(0) phosphine complexes such as [Pd(PPh3)3],24
[Pd(PPh3)4],25 and [Pd2(dppm)3] (dppm ¼ 1,1-bis(diphenyl-
phosphino)methane), which turn out to be poorly emissive.25
Tsubomura and co-workers have reported a series of tetra-
coordinated Pd(0) disphosphine complexes that emitted
strongly (12 # FPL # 38%) in the IR region.26 More recently, the
same group reported a series of poorly luminescent di- and
tricoordinated Pd(0) phosphine complexes with emission
Fig. 1 Example of a Pd(II) complex incorporated into an OLED.21d
Fig. 3 Complexes under investigation in this study.
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maxima spanning from blue to orange (lmax ¼ 485–700 nm).27
Replacement of one of the phosphine ligands with a bulky IPr
N-heterocyclic carbene (NHC) in [Pd(IPr)(PAr3)] also resulted in
very weakly emissive complexes (IPr ¼ N,N0-bis(2,6-(diisopropyl)
phenyl)imidazol-2-ylidene).28 A dimeric palladium complex
bridged by two quinone-type ligands and capped by an NHC was
found to be non-emissive at room temperature and photo-
chemically unstable.29 Consequently, no Pd(0) complexes as of
yet have been reported to be operational in a lighting device.
Fig. 2 summarises the photophysics of a selection of these
complexes.
Unsurprisingly,
a large number of luminescent Pd(0)
complexes reported contain NHC ligands, given their suitability
as ligand motifs for luminescent applications has been
demonstrated extensively across a variety of other metals. Their
exceptionally strong s-donating capabilities, where applicable,
serve to push deleterious metal-centered (3MC) states to very
high energies, making radiative decay pathways more favour-
able and thus resulting in brightly emissive complexes, while
their reasonable p-accepting properties also facilitate metal-to-
ligand charge transfer (MLCT) type transitions that are char-
acteristic of many luminescent complexes.30 The myriad exam-
ples of NHC complexes of other metals includes Cu(I),31 Ag(I),31a
Re(I),32 Ir(III),33 Pt(II),34 and Au(I).35
Fig. 4 Crystal structures of 4 (left) and 6 (right). Hydrogen atoms have
been omitted for clarity. Selected bond lengths [A˚] and angles [ꢁ]: for 4
Pd1–C1 2.027(4), Pd1–P1 2.2380(11), C1–Pd1–P1 170.91(10); for 6
Pd1–C1 2.030(6) and Pd1–C31 2.015(6), C1–Pd1–C31 178.1(3).
isopropanol) at ꢀ35 ꢁC (Fig. 4).39 Both structures slightly deviate
from
a linear geometry with NHC–Pd–L bond angle of
170.91(10)ꢁ and 178.1(3)ꢁ for complexes 4 and 6 respectively.
These data are in accordance with the structures reported for
related complexes (1: 169.49(2)ꢁ; 2: 170.88(2)ꢁ; 5: 175.98(14)ꢁ).40
The NHC–Pd and NHC–PR3 bond lengths for complexes 4
The issue we explore in this contribution is whether efficient
photoluminescence can be obtained from Pd(0) complexes. The
present report builds on our previous work on Pd(0) NHC
complexes for catalysis22e,23b,36 and our recent observations23b
that many of these complexes (Fig. 3) showed unprecedented
intense emission in toluene solution; exceptionally, preliminary
absorption and emission data for weakly luminescent
[Pd(IPr)(PAr3)], 1, has been reported although not quantied
(Ar ¼ Ph, o-Tol).28 Herein, we report a detailed solution state
photophysical study of the rst examples of strongly emissive
Pd(0) complexes, 2–6 (Fig. 3), and compare these to the refer-
ence complex 1. This study showcases the exciting potential of
palladium – being more than een times more abundant than
iridium37 – as a potential new class of OLED materials that could
substitute for Ir complexes.
˚
˚
˚
(2.027(4) A and 2.2380(11) A, respectively) and 6 (2.030(6) A) are
similar to those reported in the literature for congeners
(1: 2.0547(8) A and 2.2100(2) A, respectively; 2: 2.0292(9) A and
˚
˚
˚
˚
˚
2.2212(3) A, respectively; 5: 2.026(4) A). In order to better
understand the spatial occupation about the palladium center,
percent buried volumes (%VBur
41
) were determined (Fig. S3–S4,
S7–S8, S14–S15 and S17†). The range of buried volumes lies
between 37.8–48.0% for the NHC and between 30.7–33.3% for
the phosphine ligand. There is no correlation between the
solution state photophysical properties of these complexes and
this structural parameter, however a strong correlation does exist
with respect to thin lm photophysical properties (vide infra).
UV-vis absorption
The absorption spectra for the six complexes are shown in Fig. 5
and the molar absorptivity data summarised in Table 1. These
complexes are highly reactive towards molecular oxygen, rapidly
forming Pd(II) peroxo adducts upon exposure to O2.22e,23b In
Results and discussion
Complex synthesis and characterisation
The synthesis of complexes 1–5 was carried out according to the order to rigorously avoid oxygen exposure, all samples were
method reported by Nolan.36a Reaction of [Pd(h3-allyl)- prepared in the glovebox, with dry degassed toluene used as the
(Cl)(NHC)] (allyl ¼ C3H5) with PR3 or NHC in the presence of solvent of choice to mitigate any solvent coordination of the
KOtBu gave the desired complexes in 81–85% yield. Complex 6 vacant palladium sites, which has been shown to be possible for
was synthesized in 66% yield as described in the literature38 by linear [Pd(NHC)(PR3)] complexes dissolved in solvents such as
reaction of [Pd(m-Cl)(h3-crotyl)]2 (crotyl ¼ 3-MeC3H5) in THF dichloromethane.28
with two equivalents of free SIPr (SIPr ¼ N,N0-bis[2,6-(diiso-
The assignment of the individual absorption bands of each
of spectrum is made more difficult by the complex nature of the
propyl)phenyl]imidazolin-2-ylidene).
The
syntheses
complexes 1–3 and 5–6 have been previously reported, while electronic structure of d10 complexes, for which any observed
that of 4 is novel. Complexes 1–3 and 5–6 were characterized by MC states are symmetry-allowed. The high-energy bands are
1H and, where applicable, 31P{1H} NMR, while 4 was charac- usually assigned as typical 1p–p* transitions, but the nature of
terized by 1H, 13C{1H} and 31P{1H} NMR and elemental analysis. the lower energy bands is less obvious, especially when
Suitable crystals for single crystal X-ray diffraction studies were compared with literature complexes. For example, Tsubomura
obtained for complexes 4 and 6 aer slow diffusion (toluene/ assigns the lowest energy band for the bent42 complex
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[Pd(PCy)2], found at 350 nm, as an MC dz2–pz transition.27
Similarly, a number of low energy excited states observed for the
[Pd2(dppm)3] dimer (trigonal planar about the palladium
Table 1 Absorption data for complexes 1–6 at 298 K
Complex
Absorbancea 298 K (nm) [3 (ꢃ103 Mꢀ1 cmꢀ1)]
1
*
centre)25 are assigned as a variety of MLCT states: ds–ps
1
2
3
4
5
6
303 [13.1], 344 [5.5], 421 [2.9], 465 [0.24]
305 [11.1], 338 [5.1], 422 [1.8], 471 [0.07]
301 [11.7], 342 [4.8], 437 [1.6], 491 [0.06]
318 [12.3], 353 [6.4], 440 [3.4], 491 [0.29]
340 [12.3], 378 [6.0], 414sh, 478 [2.5], 529 [0.22]
332 [13.0], 386 [5.6], 416sh, 500 [2.4], 562 [0.15]
3
3
*
(440 nm), dd–ps (500 nm) and ds–ps (574 nm).24 The presence
of delocalised NHC ligands with low-lying p-states is likely to
result in pronounced MLCT-type transitions taking place for
these complexes as well.
Comparing the spectra of 1 with 2 demonstrates that
changing the phosphine from PPh3 to PCy3 does not modulate
the electronics in any appreciable fashion, in spite of the
expected increased s-donating capability of the aliphatic
phosphine in comparison with the aromatic phosphine. The
principal absorption features of 1 (303, 344 and 421 nm) virtu-
ally overlap with that of 2 (305, 338, and 422 nm). However, two
distinct differences are worth noting: rstly, the magnitude of
the extinction coefficients for each of the three bands for 1 are
greater than for 2, suggesting that the increased conjugation of
the PPh3 ligand contributes to the overall absorptivity of 1.
Secondly, there are weakly absorbing bands at low energy,
ascribed as direct population of the triplet state, which are
observable for 1 (465 nm) and 2 (471 nm). This band is well
resolved in the case of 2, but much more poorly dened for 1.
Despite the insensitivity of the electronics of the absorption
spectra to phosphine, it is likely that the phosphine ligand
contributes to the process, especially when observing the
drastic change in the form of the spectra upon replacement of
the phosphine for a second NHC. For example, while the molar
absorptivity of the band at 340 nm for 5 falls within the regime
of the corresponding bands for 1 and 2, there is a large bath-
ochromic shi of ca. 3600 cmꢀ1 (35 nm) compared to the cor-
responding band for 1 (303 nm) and 2 (305 nm). Similar red-
shiing of the lower energy absorption bands is also observed
for 5, with the bands at 378 nm and 478 nm red-shied by
ꢂ3100 cmꢀ1 (ꢂ40 nm) and 2800 cmꢀ1 (ꢂ55 nm), respectively. In
a
All measurements were carried out in anhydrous toluene from
samples prepared in a glovebox.
addition, the form of the bands at 344 and 338 nm for
complexes 1 and 2 resemble shoulders, rather than the full
band at 378 nm observed for complex 5, suggesting a change in
the nature of the transition. Similar changes in the absorption
prole of 6 are present when compared to 3: the band at 332 nm
in 6 is moderately more absorptive, consistent with an
increased p-network, while there are large red-shis in three
low energy bands in 6 of ꢂ3100 (31 nm), 3300 (34 nm) and
2883 cmꢀ1 (63 nm) compared to those, respectively, found in 3.
Finally, complexes 5 and 6 possess shoulders at 414 and
416 nm, respectively, that are absent in the other complexes,
suggesting a new CT transition in these two complexes.
By comparison to the limited change in the absorption
spectra observed from 1 to 2, modifying the NHC leads to small
but distinct changes in their spectral prole. For example, the
highest energy band for complex 3, found at 301 nm, is blue-
shied compared to 2 by a modest 436 cmꢀ1 (4 nm) upon
saturation of the imidazolium NHC backbone, due to reduced
delocalisation across the p-network. However, a red-shi of
814 cmꢀ1 (15 nm) is observed for the lowest energy band at 437
nm. The red-shi of this 1MLCT band is probably due to a lower
lying LUMO in 3 compared to 2 as a result of the change in the
Fig. 5 Absorption spectra of complexes explored in this study. Measurements were carried out at 298 K in degassed toluene. Inset shows low
energy absorption spectra illustrating the weakly absorbing bands ascribed as direct population of the T1 state.
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Fig. 6 Energy of the frontier electronic levels ranging from HOMO ꢀ 9 to LUMO + 9 for all complexes. The pictures correspond to the
localization of the HOMO (bottom) and LUMO (top) orbitals.
nature of the NHC ligand from IPr to SIPr. When the steric (TD)-DFT, calculations were performed to examine the nature
demand of the NHC ligand is increased signicantly in 4, the (spatial localization) and energetics of the frontier molecular
absorption spectrum is red-shied relative to 2 and 3; the orbitals as well as the nature of the low-lying singlet and triplet
energy differences between the three bands of 2 and of 4 all fall
within similar regimes: ꢂ1000 to 1300 cmꢀ1 (7 to 18 nm).
Similar changes in spectral prole are observed from 5 to 6
as were found with 2 to 3. The magnitude of the energy
differences is also quite similar for the two pairs of
complexes. For example, the peak at 340 nm for complex 5 is
blue shied by 709 cmꢀ1 (8 nm) upon saturation of both NHC
backbones (compared with 436 cmꢀ1, 4 nm, between 2 and 3).
Similarly, the lower energy band (478 nm for complex 5) is
red-shied by 921 cmꢀ1 (22 nm), which is only a small
increase from the 813 cmꢀ1 (15 nm) difference between the
corresponding bands in 2 and 3. No noticeable change in
molar absorptivity is observed across the spectra when
comparing 2 to 3 or 5 to 6 as might be expected for such
similar structures.
In order to assist the interpretation of the experimental
absorption spectra, time-dependent density functional theory,
Fig. 7 TD-DFT simulated absorption spectra of 1–6.
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Fig. 8 Normalized emission spectra of complexes explored in this study. Measurements were carried out at 298 K in degassed toluene and result
from excitation at 360 nm.
excited states. Unless otherwise specied, all electronic struc- signicantly in 1 compared to all other complexes, since the
ture calculations have been performed at the optimized geom- former involve PPh3.
etry of the ground state.
The simulated absorption spectra of 1–6 are displayed in
Fig. 6 and Table S1† summarize the energy and localization Fig. 7. The TD-DFT calculations (i) indicate that all electronic
of the ten highest occupied and ten lowest unoccupied molec- excitations wavelengths longer than 250 nm involve excited
ular orbitals (MOs). Regardless of the nature of the ligands, the states with dominant MLCT character (from the occupied Pd
ve highest-energy occupied MOs are palladium-centered. Their d-centred orbitals to the unoccupied ligand-centred orbitals)
energies can however be modulated by the nature of the ligands. (Fig. 6 and Table S1†) and (ii) support the view that the weakly
For example, replacing PPh3 by PCy3 (from 1 to 2, 3, or 4) leads absorbing bands at low energy are due to direct excitation into
to a ꢂ0.2 eV destabilization of the metal-centred orbitals as a triplet states (see below). For 1, all the absorption bands rep-
result of the increased s-donating character of the aliphatic resented in Fig. 6 correspond mainly to the promotion of one
phosphine. For complexes with two NHC ligands (5 and 6), the electron from the Pd centre to the PPh3 group, though there is a
energies of the HOMOs are further destabilized. As for the minor contribution of the peripheral aryl moieties of the NHC
lowest unoccupied energy levels, they are similar for complexes ligand to the band simulated at 305 nm and a very weak
2, 3, 5 and 6 and localized over the whole NHC ligand for the contribution for the band at 366 nm. In line with the measured
LUMO yet only on the peripheral aryl rings for LUMO + 1, LUMO results, the simulated absorption spectra of 2 looks similar to 1
+ 2 and LUMO + 3. Interestingly, complex 4 exhibits LUMO but the nature of the bands is totally different, as expected from
levels that are likewise localized on the NHC ligand but at lower the change in the nature of the frontier molecular orbitals. The
energy compared to complexes 2, 3, 5 and 6 because of an rst two bands at low energy (444 nm and 382 nm) only involve
increased delocalization across the p-network present in the the NHC ligand while PCy3 and NHC contribute to the band at
IPr* ligand. The nature of the rst unoccupied orbitals differs 320 nm. For complex 3, the measured red-shis of the lowest
two optical absorption bands (+15 nm and +4 nm) and the
Table 2 Relevant photophysical data for 1–6 in toluene solution
kr (ꢃ105 sꢀ1
)
knr (ꢃ105 sꢀ1
)
c
Complex
lema (nm)
FPLb (%)
se (ms)
1
2
3
4
5
6
512
508
543
515
555
608
1.0
16.4
69.0
13.9
70.1
69.8
0.014
1.56
5.46
7.14
1.05
1.26
—
2.39
1.24
707
5.36
0.57
—
1.02
0.54
0.36 (15%), 1.59 (85%)
2.93
5.62
a
Measured at 298 K in degassed toluene and excited at 360 nm. b Using quinine sulfate as the standard (FPL ¼ 54.6% in 0.5 M H2SO4 at 298 K).43
c
Excited at 379 nm.
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Table 3 TD-DFT calculated (column 2) and experimental (column 3)
emission wavelengths together with the relative deviation with respect
to the calculated values (column 4)
centred occupied orbitals. In both compounds, the lowest energy
band features a large contribution over the imidazolium unit
while the higher energy bands involve the entirety of the NHC
ligands (except for the second band of 6 for which the imidazo-
lium segment does not contribute signicantly). These results
are consistent with those obtained when saturating the imida-
zolium ring of complex 2. Overall, the predicted singlet electronic
transitions reproduce the measured optical absorption spectra
remarkably well, except at low energy where we hypothesize that
direct absorption into triplet states occurs. To check this
hypothesis, we have calculated the vertical S0 / T1 transition
energy for each of the complexes; these match well the weak low-
energy spin-forbidden absorption bands (calculated [measured]
maxima at 499 nm [465 nm], 500 nm [471 nm], 516 nm [491 nm],
522 nm [491 nm], 580 nm [529 nm] and 594 nm [562 nm] for 1, 2,
3, 4, 5 and 6, respectively). It is interesting to point out that the
lowest triplet excited state is in all cases MLCT-like, thus
reecting the nature of the corresponding singlet states.
Complex
lem,th (nm)
lem,exp (nm)
Deviation (%)
1
2
3
4
5
6
549
553
600
561
619
679
512
508
543
515
555
608
7
9
10
9
12
12
corresponding blue-shi of the third band (ꢀ4 nm) as
compared to complex 2 are well-reproduced by the calculations
(with predicted shis of +11 nm, +2 nm, and ꢀ7 nm, respec-
tively). The nature of the main optical transitions in 3 and 2 is
similar, except for the band at 384 nm for which the contribu-
tion of the imidazolium ring is drastically reduced in 3. Satu-
rating the imidazolium ring has therefore only a limited impact
on the optical properties of the complexes. Incorporating
phenyl rings in IPr* in 4 leads to a systematic red-shi of all the
absorption bands compared to 2; the nature of each of the
transitions is however preserved. The red-shis originate from
the stabilization of the rst unoccupied levels stemming from
an increased p-delocalization. With two NHC ligands on the Pd
(complexes 5 and 6), there is a large red-shi of the absorption
bands, which originates from the destabilization of the Pd-
Solution state photophysical behavior
The normalized emission spectra of the six complexes are
shown in Fig. 8, while the relevant photophysical data are
summarised in Table 2. All measurements were carried out on
samples prepared in the glovebox using anhydrous toluene.
Emission across the family of complexes ranges from 508 to
608 nm. In spite of the relatively small Stokes shis observed for
these complexes, emission is ascribed to phosphorescence on
the basis of the microsecond lifetimes observed for all the
complexes except 1 and the presence of the heavy palladium
center.
On the basis of the unstructured emission proles, ligand
centred (LC) emission, which is common for many other d10
complexes, can be ruled out.44 The proles of the emission
spectra are broad and unstructured and are thus assigned to
metal-to-ligand charge transfer (MLCT) states, similar to other
Pd(0) systems and in line with the TDDFT calculations.24
The emission energies are virtually unchanged with respect
to the nature of the phosphine ligand, with 1 emitting at 512 nm
and 2 at 508 nm. This strongly suggests that the phosphine is
not signicantly involved in the emission. The susceptibility of
the electronics to changes in the NHC is much more
pronounced. For instance, there is a signicant energy differ-
ence (1268 cmꢀ1, 35 nm) between the emission maxima of 2 and
3. The increased electronic density conferred by the SIPr NHC
serves to red-shi emission, suggesting that it is raising the
energy of the metal-localised frontier orbital and reducing the
HOMO–LUMO gap. By comparison, the adoption of the bulky
Table 4 Relevant photophysical data for 5 and 6 in thin filmsa
Complex
lem (nm)
FPL (%)
s1 (ms)
A1
s2 (ms)
A2
5
6
550
604
10.1
20.3
0.269
0.698
0.40
0.23
0.645
2.28
0.60
0.77
Fig. 9 (a) Normalized excitation (dashed) and emission (solid) spectra
of 5 (black) and 6 (red) in thin films. (b) Time-resolved PL intensity of
materials 5 (blue) and 6 (red) in thin films. The samples were excited at
379 nm.
a
Measured at RT in encapsulated lms and excited at 379 nm.
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Fig. 10 Stacked cyclic voltammograms of 2, 3, 5 and 6 in degassed THF using a platinum disk working electrode, platinum wire counter-
electrode, a Ag/Ag+ pseudo-reference electrode and referenced vs. SCE using Fc/Fc+ as an internal standard (0.56 V in THF)].46 Scan rate:
50 mV sꢀ1
.
IPr* ligand in 4 confers only a modest 268 cmꢀ1 (7 nm) red-shi porphyrins (FPL ¼ 47%)45 or O^N^C^N cyclometalating ligands
in the emission compared with 2.
(FPL ¼ 22%)21d whereby the rigidity imparted by these ligands
Substitution of the phosphine for a second NHC ligand restricts the capacity for these complexes to undergo distortion
results in a dramatic red-shi of the emission. The emission of 5 in the excited state, facilitating radiative decay as a result.
is red-shied by 1667 cmꢀ1 (47 nm) with respect to 2 – illustra-
These high FPL values can be rationalized from the excited
tive of the increased s-donation ability of the second NHC ligand states kinetics. For example, the parent complex 1 is barely
compared to phosphine. Interestingly, this observed red-shi is emissive, with an overall FPL value of just 1.0%, and has a
of similar magnitude to that identied between 2 and 3, sug- notably short excited state lifetime (se) of just 14 ns. The dele-
gesting that any p-back-bonding effected by the IPr imidazolium teriously large contribution from knr (two orders of magnitude
ring is mitigating the overall s-donating properties of the NHC. larger than kr) leads to drastically shortened se values and thus
Like the structure-property differences between 2 and 5, the inefficient emission. By comparison, 2 exhibits a knr that is two
emission energy of 6 is red-shied compared to 3. However, this orders of magnitude smaller versus 1: se increases to 1.56 ms and
red-shi is larger (1968 cmꢀ1, 65 nm) than the IPr couple, FPL increases to 16.4% in 2.
suggesting a cumulative effect from the increased s-donation of
Despite very little difference in emission energy between 1
the second NHC and the decreased p-back-bonding of SIPr and 2, there is a marked increase in FPL observed for 2 over 1. It
compared to IPr. Analogously, 6 is red-shied compared to 5 by is possible that the bulkier PCy3 ligand may more effectively
1571 cmꢀ1 (53 nm).
conformationally lock the orientation of the NHC to minimize
Interestingly, the experimentally observed emission wave- eclipsing interactions, effectively rigidifying the system such
lengths match extremely well with the theoretical results that non-radiative decay becomes less efficient.
calculated on the basis of the lowest triplet state optimized
Complex 3 (FPL ¼ 69.0%) emits even more efficiently than 2,
geometries (Table 3). The red-shis upon saturation of the which is explained by the order of magnitude reduction in knr
imidazolium ring and for complexes with two NHC ligands are for 3 over 2. Accordingly, the value for se is larger (5.46 ms).
well-reproduced. Quantum-chemical calculations also conrm Interestingly, in spite of the added steric bulk present in 4, this
that the emission process has strong MLCT character involving complex is less emissive than both 2 and 3, with FPL ¼ 13.9%.
the NHC ligands, except for 1 for which PPh3 is also contrib-
uting (Fig. S32†).
Finally, the bis-NHC complexes 5 and 6 are also very efficient
emitters. Counter-intuitively to what might be expected from
The most important result is that the photoluminescence the energy gap law, virtually no change in FPL is observed for
quantum yields (FPL) of 2–6 are very high, ranging from complex 6 (FPL ¼ 69.8%) in comparison to 5 (FPL ¼ 70.0%), in
14–70%. To the best of our knowledge, 3, 5 and 6 display the spite of a red-shi in emission of 1571 cmꢀ1
.
highest quantum yields for any palladium complex to date, be
they Pd(0) or Pd(II). Previously, the highest reported FPL for a
Pd(0) complex is 8% for the complex [Pd2(dpam)3] (dpam ¼
bis(diphenylarsino)methane), while the brightest Pd(II)
complexes rely on the use of tetradentate ligands such as
Solid state photophysical behavior
Although solution measurements give valuable insight into the
photophysical properties of these materials it is the thin lm
properties that are pertinent for identifying lead candidates for
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Table 5 Relevant electrochemical data for complexes 2, 3, 5 and 6a
ox
1/2
ox
1/2
b
Compound
E
(V) [DEp (mV)]
E
(V) [DEp (mV)]
EHOMO (eV)
ELUMOcd (eV)
2
3
5
6
ꢀ0.17 [114]
ꢀ0.29 [119]
ꢀ0.39 [149]
ꢀ0.45 [236]
0.10 [119]
0.00 [115]
ꢀ4.07
ꢀ3.95
ꢀ3.85
ꢀ3.79
ꢀ1.39
ꢀ1.38
ꢀ1.46
ꢀ1.53
a
All measurements were performed at 50 mV sꢀ1 in deaerated THF solution using Fc/Fc+ as an internal standard, and are referenced with respect to
SCE (Fc/Fc+ ¼ 0.56 V in THF).46 EHOMO ¼ ꢀ½EvosxFc=Fcþ þ 4:8ꢄeV; where E
.
ELUMO ¼ EHOMO + E0,0 eV. d E0,0 estimated from the intersection
b
c
47
ox
vs Fc=Fcþ
point of the absorption and emission spectra at 298 K in toluene.
electroluminescent devices. We therefore investigated thin lm phosphine ligand for a second NHC ligand partially mitigates
PL behavior next. The thin lm excitation and PL spectra are this undesired non-radiative pathway. For 5 there is a decrease
shown in Fig. 9a for complexes 5 and 6. The thin lm spectra for in FPL in moving from solution (70%) to the lm (10%);
these two complexes are similar to their respective toluene however, it is a less pronounced decrease than that observed
solution spectra. However, for 5, the solid state emission spec- with 3. Finally, the lm FPL of 6 was found to be 20.1%. The
trum is broader than that in solution with a low energy tail signicant thin lm FPL values correlate with the high %VBur of
evident, which is attributed to aggregate or excimer emission the NHC ligand (5: 42.6%; 6: 37.8%) indicating the importance
arising from inter-chromophore interactions.
To probe the photophysical properties more closely and the bright emitters in the solid state for this family of complexes.
role of inter-chromophore interactions, we measured the FPL Similar trends are seen in the time-resolved PL (TRPL)
of an extremely congested ligand sphere in order to produce
and PL lifetimes in the solid-state (Table 4) for complexes 3, 5 measurements. As the light-emitting chromophores are very
and 6, which showed the highest FPL in solution (cf. Table 2). similar for 5 and 6, differences in emission lifetime are likely to
We noticed an important difference in their properties in arise from differences in non-radiative decay rate; i.e., faster
moving from the solution to the solid state. For 3, the neat lm decay implies more quenching of PL as a result of inter-chro-
is almost nonluminescent with a FPL of only 1.3%. Despite the mophore interactions. The TRPL results for 5 and 6 in thin lms
bulkiness of the ligands, the low FPL is due to strong inter- are shown in Fig. 9b and the tted parameters are collected in
chromophore interactions in the lm. Substitution of the Table 4. The PL decay of 5 is signicantly faster than that of 6.
Fig. 11 (a) Device architecture for OLEDs using 5 or 6 as the emissive layer. (b) Normalized electroluminescent emission spectra of 5 (black) and
6 (red) in thin films. Photograph of the working OLED with (c) 5 (d) 6. (e) External quantum efficiency and power efficiency versus applied voltage
for OLEDs. (f) Current density and luminance versus applied voltage for OLEDs made with 5 (black) and 6 (red).
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Table 6 OLED performance data
Complex
Turn on voltage (V)
Current density (mA cmꢀ2
)
Luminance (cd mꢀ2
)
EQE (%)
Power efficiency (lm Wꢀ1
)
5
6
8.2
6.2
35.2
54.5
342
766
0.28
0.70
0.20
0.28
This is consistent with the observed thin lm FPL of these deposited by solution processing methods. Device performance
materials, showing strong quenching in the solid state.
is summarized in Table 6.
The EL spectra are very similar to the PL spectra of the neat
lms as shown in Fig. 11b, meaning that in both cases the
complexes are emitting from the same excited state. Given that
in a device the charge carriers are most likely to recombine on
the lowest energy sites, the fact that the EL is not red-shied
relative to the PL is a further indication that the Pd complexes
bearing two NHC ligands do not suffer from strong aggregation
effects. Fig. 11e shows the external quantum efficiency and
power efficiency of the devices. An upper limit on the potential
EQE of a device can be estimated from the PLQY of the lm and
the out-coupling of light (normally taken to be 20% given the
refractive index of typical organic materials) giving 2% for 5 and
4% for 6.50 For the OLED using 5 the measured EQE is less than
0.3% in comparison to the OLED using 6 where the EQE reaches
to 0.7%, both of which are much lower than materials of their
solid state PLQY could achieve. This is likely to be due to an
imbalance of charge injection and transport in the device.
Fig. 11f shows the current–voltage–luminance characteristics of
devices. The turn on voltages for both devices is similar. The key
parameters of the devices are highlighted in Table 6. These
results suggest the performance of the devices can be optimized
further by improving the charge balance.
Cyclic voltammetry
Given the unprecedented photophysical properties demon-
strated by these Pd(0) complexes, their electrochemistry was
studied by cyclic voltammetry (CV) in order to discern their
suitability as materials for OLEDs. Measurements were carried
out on the four most promising candidates (2, 3, 5 and 6) and
the relevant data are given in Fig. 10 and in Table 5.
The CV traces are divided into two families. The heteroleptic
complexes 2 and 3 show two closely spaced waves in the region
of 0.1 to ꢀ0.4 V. Based on the pronounced reversibility of these
waves, they are attributed to sequential one electron oxidations
of the palladium centre from Pd(0) to Pd(I) and then Pd(II),
which has been a phenomenon observed in several Pd(L)2
systems, where L is a phosphine ligand.48 Taking into account
the dependency of the low energy absorption bands on the
nature of the NHC ligand, the rst oxidation wave, and hence
the HOMO, is assigned to the Pd0/I redox couple with contri-
bution from the NHC ligand. The second oxidation wave is
assigned to the PdI/II redox couple with contribution from the
phosphine ligand based in part on similar oxidation potentials
reported for Pd(0)(PR3)n systems.48a,49 Indeed, the higher HOMO
energy observed for the more electron rich 3 over 2 corroborates
these assignments.
Conclusions
In contrast to the heteroleptic complexes, the homoleptic
complexes 5 and 6 show only a single reversible oxidation wave
in the region of ꢀ0.4 V. Based on the larger peak-to-peak
separation, DEp, and greater currents, we tentatively assign
these waves as two non-resolved single electron oxidations
oxidation waves. In certain cases – particularly when the ligand
scaffold about the palladium center is the same – resolving the
two single-electron oxidation waves from Pd(0) to Pd(II) is non-
trivial, requiring careful control of thermodynamic and kinetic
parameters of the system to distinguish these events.48c Once
again, the SIPr containing complex, 6, possesses a higher
HOMO level than the IPr analog 5. The CV data are consistent
with the structure–property relationships established by
absorption spectroscopy (vide infra).
In summary, six di-coordinated neutral palladium(0) complexes
bearing a combination of NHC and PR3 ligands have been
synthesized and had their full solution photophysics charac-
terized. In spite of their inherent reactivity, their exceptional
photophysics makes them exciting candidates for potential
OLED applications. In particular, complexes 3, 5 and 6, with FPL
values close to 70% in solution, are the most efficient emitters
of any palladium complexes reported to date. This is likely
attributable to the lack of d–d MC states that tend to plague the
emissive properties of conventional Pd(II) complexes. Just as
signicantly, these complexes also satisfy the criteria of color
tunability and short lifetimes, with emission spanning from
blue-green to orange, and se values of the most emissive
complexes ranging from 1.56 to 5.62 ms. By fullling the three
main criteria of bright, tunable and short-lived emission, this
OLED devices
With the most promising candidates identied in terms of their work has shown that palladium(0) NHC complexes are attrac-
high thin lm FPL, OLED devices using 5 and 6 were fabricated tive candidates for OLED applications. OLEDs were successfully
and tested. The device architecture is shown in Fig. 11a, having fabricated using 5 and 6 and in these initial studies the EQEs
ITO/PEDOT:PSS (40 nm)/PVK (35 nm)/active layer (40 nm)]/TPBI were 0.3 and 0.7%, respectively. The EL study shows that further
(60 nm)/Ca (20 nm)/Al (100 nm) structure where 5 or 6 act as the renement in charge balance is necessary in order to improve
active layer. Except for TPBI and the contacts, all the layers were the performance of the OLED devices.
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widely used B3LYP functional52 and the basis set for the
description of the electrons of the non-metallic atoms is
6-31G**53 while the LANL2DZ basis set54 is used for the metallic
centre.55 The PCM (Polarizable Continuum Model)56 scheme has
been coupled to all (TD)-DFT calculations to account for solvent
(toluene) effects. Within this model, the solute is embedded in a
shape-adapted cavity surrounded by the solvent implicitly
described by a dielectric continuum which characterized by a
dielectric constant (3 ¼ 2.37 for toluene). The characterization
of the nature of the lowest-lying singlet and triplet excited states
involved in the absorption spectra relies on time-dependent
density functional theory (TD-DFT) performed on the basis of
the singlet optimized ground-state geometries and using the
same functional and basis set. The emission wavelengths were
also calculated at the TD-DFT level but on the basis of the triplet
ground-state geometry. All (TD)-DFT calculations were per-
formed within the Gaussian09 package.57 The isosurface plots
in Fig. S32† were generated by the Jmol program58 by combining
for each atom the LCAO coefficients in all unoccupied molec-
ular orbitals involved in the TD-DFT description of the excited
state and their weights in the wavefunctions.
Experimental section
Photophysical measurements
All samples were prepared in the glovebox using dry toluene
with varying concentrations on the order of mM. Samples were
sealed within an in-house made Schlenk-cuvette containing a
Young tap, and were placed under an argon atmosphere.
Absorption spectra were recorded at RT using a Shimadzu
UV-1800 double beam spectrophotometer. Molar absorptivity
determination was veried by linear least-squares t of values
obtained from at least three independent solutions at varying
concentrations with absorbance ranging from 1.26 ꢃ 10ꢀ4 to
3.43 ꢃ 10ꢀ5 M.
As with the samples for absorption, the sample solutions for
the emission spectra were prepared in toluene solution in the
glovebox and sealed within an in-house made Schlenk-cuvette
containing a Young tap, before placing under an argon atmo-
sphere. Steady-state emission, excitation and time-resolved
emission spectra were recorded at 298 K using an Edinburgh
Instruments F980. All samples for steady state measurements
were excited at 360 nm while samples for time-resolved
measurements were excited at 378 nm. Photoluminescence
quantum yields were determined using the optically dilute
method.51 Four dilutions were prepared with absorbances of ca.
0.1 0.075, 0.05 and 0.025, respectively. The Beer–Lambert law
was found to be linear at the concentrations of the solutions.
The emission spectra were then measured and for each sample,
linearity between absorption and emission intensity was veri-
ed through linear regression analysis and additional
measurements were acquired until the Pearson regression
factor (R2) for the linear t of the data set surpassed 0.9. Indi-
vidual relative quantum yield values were calculated for each
solution and the values reported represent the slope value. The
equation Fs ¼ Fr(Ar/As)(Is/Ir)(ns/nr)2 was used to calculate the
relative quantum yield of each of the sample, where Fr is the
absolute quantum yield of the reference, n is the refractive index
of the solvent, A is the absorbance at the excitation wavelength,
and I is the integrated area under the corrected emission curve.
The subscripts s and r refer to the sample and reference,
respectively. A solution of quinine sulfate in 0.5 M H2SO4
(Fr ¼ 54.6%) was used as the external reference.43 Solid-state
PLQY measurements were measured in an integrating sphere
under a nitrogen purge using a Hamamatsu C9920-02 lumi-
nescence measurement system.
Electrochemical measurements
Cyclic voltammetry (CV) measurements were performed on an
Electrochemical Analyzer potentiostat model 600D from CH
Instruments. Solutions for cyclic voltammetry were prepared in
the glovebox using degassed THF. Tetra(n-butyl)ammo-
niumhexauorophosphate (TBAPF6; ca. 0.1 M in THF) was used
as the supporting electrolyte. An Ag/Ag+ electrode (silver wire in
a solution of 0.1 M KCl in H2O) was used as the pseudo-refer-
ence electrode; a Pt electrode was used for the working electrode
and a Pt electrode was used as the counter electrode. The redox
potentials are reported relative to a saturated calomel electrode
(SCE) electrode with a ferrocene/ferrocenium (Fc/Fc+) redox
couple as an internal reference (0.56 V vs. SCE).46
Devices fabrication
The organic light emitting diode was prepared on 12 mm ꢃ
12 mm glass substrates with indium tin oxide (ITO) top anode
layer of 120 nm. The substrates were carefully cleaned by
ultrasound-assisted cleaning in water, acetone and isopropanol
for 15 minutes. All the organic layers on top of the anode were
prepared by spin coating inside the glovebox where the oxygen
and water content was reported less than 0.1 ppm. All the
organic materials and solvents were used without further
purication. All the solvents used were purchased from Sigma
Aldrich. The buffer layer poly(3,4-ethylenedioxythiophene) poly-
(styrenesulfonate) (PEDOT:PSS) was purchased from Clevios
and was used without further purication. Polymer poly
(9-vinylcarbazole) (PVK) was purchased from Sigma Aldrich and
Organic small molecule 2,20,200-(1,3,5-benzenetriyl)-tris(1-
phenyl-1-H-benzimidazole) (TPBI) was purchased from Lumi-
Time-resolved luminescence measurements were performed
using the time-correlated single photon counting technique,
with excitation at 375 nm from a pulsed Picoquant GaN laser
diode and an instrument response of 250 ps. The PL decay in
each case was measured at a wavelength corresponding to the
peak of the emission spectrum.
Quantum-chemical calculations
The singlet and triplet ground-state geometries of the Pd(0) nescence Technology Corp.
complexes have been optimized at the Density Functional
The organic light emitting diode structure was: ITO coated
Theory (DFT) level, starting from the X-ray structures when glass, a 40 nm layer of PEDOT:PSS by spin coating at 4000 rpm
available. The chosen exchange correlation functional is the for 60 seconds and baked on a hot plate at 120 ꢁC for
3258 | Chem. Sci., 2015, 6, 3248–3261
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˜
15 minutes. A hole-transport layer of 35 nm-thick poly(N-vinyl-
carbazole) (PVK) was spin-coated on the PEDOT:PSS layer and
baked at 80 ꢁC for 2 h in a nitrogen glovebox. An emissive layer
of the Pd complex was spin coated on top of PVK layer by
spinning at 2000 rpm. An electron-transport layer of 60 nm-
thick TPBI was deposited without a shadow mask in a thermal
evaporator. A cathode of Ca/Al (20 nm/100 nm) was then
deposited on the TPBI layer in the same vacuum system using a
shadow mask. The concentration of polymer PVK in chloro-
benzene was 10 mg mlꢀ1 and of compound complex 6 was
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5 mg mlꢀ1 in degassed toluene solvent. OLED consisted of three 10 D. M. Zink, D. Volz, T. Baumann, M. Mydlak, H. Flugge,
¨
¨
pixels of dimensions 2 mm ꢃ 4 mm. The active area of the
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curable epoxy (Norland 68) by shining 365 nm from low power 11 N. Armaroli, G. Accorsi, F. Cardinali and A. Listorti, Top.
UV lamp for 3 minutes.
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Acknowledgements
The authors are grateful to the Royal Society (University
Research Fellowship to CSJC and Wolfson Research Merit
Award for IDWS) and to EPSRC (grant: EP1J01771X) for nancial
support. EZ-C thanks the University of St Andrews for funding.
We thank David Nelson for the crystal structure of complex 6.
We also thank Umicore for the generous gi of Pd starting 14 (a) V. A. Krylova, P. I. Djurovich, B. L. Conley, R. Haiges,
materials. The work in Mons was supported by the European
M. T. Whited, T. J. Williams and M. E. Thompson, Chem.
Commun., 2014, 50, 7176–7179; (b) N. Armaroli, G. Accorsi,
M. Holler, O. Moudam, J. F. Nierengarten, Z. Zhou,
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Trans., 2013, 42, 9826–9830.
´
Commission/Region Wallonne (FEDER – Smartlm RF project),
the Interuniversity Attraction Pole program of the Belgian
Federal Science Policy Office (PAI 7/05), and the Programme
´
d’Excellence de la Region Wallonne (OPTI2MAT project). D.B. is
FNRS Research Director.
15 Z. A. Siddique, Y. Yamamoto, T. Ohno and K. Nozaki, Inorg.
Chem., 2003, 42, 6366–6378.
16 (a) J. A. G. Williams, S. Develay, D. Rochester and L. Murphy,
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