Homoleptic platinum(ii) complexes with pyridyltriazole ligands: Excimer-forming phosphorescent emitters for solution-processed OLEDs

Article abstract of DOI:10.1039/c9tc00768g

Two new homoleptic platinum(ii) complexes are reported that feature aryl-appended 5-(2-pyridyl)-1,2,4-triazole chelates acting as N^N^- ligating ions, PtL¹₂ and PtL²₂. Readily prepared from easily accessible proligands, they offer good solubility in organic solvents, allowing them to be incorporated into OLEDs through solution processing. Crystal structures reveal staggered, face-to-face packing of the π systems in adjacent complexes, but with no close Pt?Pt interactions. The complexes display bright unimolecular phosphorescence: for PtL¹₂ and PtL²₂ respectively, λ_max = 502 and 514 nm; Φ = 0.21 and 0.48; τ = 5.1 and 4.6 μs in deoxygenated CH₂Cl₂ at 295 K. Both complexes show a strong propensity to form intensely emissive excimers at higher concentrations: λ_max = 585 and 625 nm for PtL¹₂ and PtL²₂. The photophysical properties in doped and neat thin films have been investigated using steady-state and time-resolved methods. These studies highlight the presence of different environments of bimolecular excited states with different lifetimes, those emitting at lowest energy apparently having the longest lifetimes, contrary to what is normally found for unimolecular emitters through the effects of vibrational deactivation. The prototype solution-processed OLEDs gave EQEs of 9.6-12.5% for PtL¹₂ and 8.8-11.4% for PtL²₂, impressive values for solution-processed devices incorporating such simple complexes and only a little inferior to the EQE of 15% achieved using PtL¹₂ in a device prepared by evaporation. Compounds of this type have potential to provide the red and green components for white light OLEDs, due to their tunable, uni- and bimolecular excited state emission.

Full text of DOI:10.1039/c9tc00768g

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Materials Chemistry C
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Homoleptic platinum(II) complexes with pyridyltriazole
ligands: excimer-forming phosphorescent emitters
for solution-processed OLEDs†‡
Cite this:DOI: 10.1039/c9tc00768g
a
b
a
b
Melissa T. Walden,§ Piotr Pander, § Dmitry S. Yufit, Fernando B. Dias * and
a
J. A. Gareth Williams
*
Two new homoleptic platinum(II) complexes are reported that feature aryl-appended 5-(2-pyridyl)-1,2,4-
ꢀ
1
2
triazole chelates acting as N^N ligating ions, PtL
2 2
and PtL . Readily prepared from easily accessible
proligands, they offer good solubility in organic solvents, allowing them to be incorporated into OLEDs
through solution processing. Crystal structures reveal staggered, face-to-face packing of the p systems
in adjacent complexes, but with no close Ptꢁ ꢁ ꢁPt interactions. The complexes display bright unimolecular
1
2
phosphorescence: for PtL
2
and PtL
Cl
2
respectively, lmax = 502 and 514 nm; F = 0.21 and 0.48; t = 5.1
and 4.6 ms in deoxygenated CH
2
2
at 295 K. Both complexes show a strong propensity to form
1
2
intensely emissive excimers at higher concentrations: lmax = 585 and 625 nm for PtL
2 2
and PtL . The
photophysical properties in doped and neat thin films have been investigated using steady-state and
time-resolved methods. These studies highlight the presence of different environments of bimolecular
excited states with different lifetimes, those emitting at lowest energy apparently having the longest
lifetimes, contrary to what is normally found for unimolecular emitters through the effects of vibrational
1
Received 9th February 2019,
Accepted 13th March 2019
deactivation. The prototype solution-processed OLEDs gave EQEs of 9.6–12.5% for PtL
2
and 8.8–11.4%
2
for PtL
2
, impressive values for solution-processed devices incorporating such simple complexes and
1
only a little inferior to the EQE of 15% achieved using PtL
2
in a device prepared by evaporation.
DOI: 10.1039/c9tc00768g
Compounds of this type have potential to provide the red and green components for white light OLEDs,
due to their tunable, uni- and bimolecular excited state emission.
rsc.li/materials-c
interest in them for the purpose of harnessing otherwise wasted
Introduction
triplet excited states that form in electroluminescent OLED devices
3
Research into the design and development of phosphorescent in ratios of up to 3 : 1 relative to singlets. But, whilst inter-
1
platinum(II) complexes currently attracts a great deal of attention.
molecular interactions in Ir(III) complexes are usually detrimental
Square-planar complexes of the d Pt(II) ion offer features that are to luminescence, leading to quenching and loss of efficiency,
not open to pseudo-octahedral complexes of d Ir(III). Both of these specific face-to-face interactions between planar Pt(II) complexes
8
6
3rd-row transition metal ions have high spin–orbit coupling can give rise to excited states that are localised over two or more
constants: they can promote efficient T - S phosphorescence molecules, either in pre-existing dimers and aggregates or in
1
0
4
from charge-transfer excited states in suitably designed complexes excimers. The resulting low-energy emission bands may offer a
featuring conjugated ligands. This property has led to widespread route to efficient deep-red and near-infrared (NIR) emitting
2
5
phosphors. Moreover, when mixed with the higher-energy emission
of discrete molecules, it provides an attractive pathway to single-
dopant white-light-emitting devices (WOLEDs).
Pt(II) complexes with cyclometallating ligands predominate
amongst the most brightly luminescent systems, particularly those
a
Department of Chemistry, Durham University, Durham, DH1 3LE, UK.
E-mail: j.a.g.williams@durham.ac.uk
6
b
Department of Physics, Durham University, Durham, DH1 3LE, UK.
E-mail: f.m.b.dias@durham.ac.uk
†
Invited contribution to the special issue celebrating the career and numerous based on ligands such as 2-phenylpyridine (ppy) and derivatives
contributions of our esteemed colleague Professor Martin Bryce to the field of
with combinations of other N-heterocycles and C-metallated aryl
rings. The strong s-donation associated with the C–Pt carbon
bond, coupled with the p-acceptor nature of the heterocycle, leads
to large ligand field splittings, which in turn often ensure that
potentially deactivating metal-centred d–d states are pushed up
functional organic materials.
7
‡
Electronic supplementary information (ESI) available. CCDC 1894904–1894906.
For ESI and crystallographic data in CIF or other electronic format see DOI:
1
0.1039/c9tc00768g
§
M. T. W. and P. P. contributed equally to this work.
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to thermally inaccessible energies (at ambient temperature).
Meanwhile, the frontier orbitals in such complexes tend to be based Pt(N^N )
In the present work, we sought to prepare 1,2,4-triazole-
ꢀ
2
complexes that might offer improved solubility,
localised on different parts of the molecule, the LUMO typically in order to evaluate their photophysical properties in solution. We
on the heterocycle and the HOMO on the metal and metallated were also motivated to prepare organically soluble, aryl-appended
ring, allowing tuning of emission through introduction of derivatives in order to probe their utility as phosphors in solution-
substituents at these distinct parts of the molecule. Numerous processed – as opposed to vacuum sublimed – devices. The target
8
9
cyclometallated complexes with tri- and tetradentate ligands complexes proved to be readily prepared, displaying bright lumi-
complement the many examples comprising bidentate N^C- nescence in solution: they offer broad-band emission across the
coordinating ligands in combination with L^X and L^L ancillary green and red regions of the spectrum from unimolecular and
ligands.
excimeric excited states. Here, we describe the synthesis, structural
Nevertheless, cycloplatination often requires quite forcing and photophysical properties of two such complexes (Fig. 2),
conditions, whilst in the resulting complexes, the very strong including a detailed examination of their photoluminescence in
trans influence associated with metallated carbon atoms can doped and neat films and electroluminescence from solution-
1
0
result in labilisation of trans related ligands, which may be processed devices.
undesirable in systems such as OLEDs where robust stability is
required. From both points of view, azole rings such as pyrazoles,
triazoles and tetrazoles, represent interesting alternatives to the
Results and discussion
Synthesis of proligands and complexes
aryl rings of conventional cyclometallating ligands. Chi and
co-workers pioneered the use of such ligands with Pt(II), examining
selected 5-(2-pyridyl)-pyrazoles and 5-(2-pyridyl)-1,2,4-triazoles (e.g., We chose to target the two complexes shown in Fig. 2, with a
structures B and C respectively in Fig. 1) wherein deprotonation view to assessing the effects of aryl substitution on both the triazole
ꢀ
5f,11
of the azole leads to bidentate, anionic:N^N coordination.
and the pyridine ring. tert-Butyl substituents in the aryl rings were
ꢀ
Whilst the pyrazolate-based homoleptic Pt(N^N )
type B were strongly luminescent in solution, non-radiative previously reported complexes such as Pt(ptp)
decay processes predominated in the triazole-based analogues C, triazole proligands were synthesised as shown in Scheme 1. HL
2
complexes of employed in the hope of improving upon the very poor solubility of
1
2
2
.
The requisite
1
leading to low quantum yields in solution. Emission in the was synthesised using a procedure analogous to that described for
15a
solid state was, however, brighter. Subsequently, Omary and the related tridentate 2,6-disubstituted pyridine proligand. Treat-
co-workers employed the homoleptic complex of 3,5-bis(2-pyridyl)- ment of 2-cyanopyridine 1a with hydrazine monohydrate gave the
1
,2,4-triazole, Pt(ptp) (D in Fig. 1), 30% doped into CBP, as the amidrazone 1b, which, upon reaction with tert-butylbenzoyl chloride,
2
1
2a,b
emissive layer in a vacuum-sublimed WOLED.
The poor led to the intermediate 1c. Thermal cyclisation of 1c at an elevated
solubility of this complex prohibited investigation of photo- temperature of 185 1C in ethylene glycol gave the desired N^NH
1
2
physical properties in solution. A trifluoromethyl-substituted proligand HL . The bis-aryl-appended analogue HL was prepared
analogue was used by Wang et al. in preparing OLEDs with using a similar sequence from 2-cyano-4-(p-tert-butylphenyl)pyridine
EQE up to 31%; the role of emission from aggregate states 2a. This compound was prepared readily by Pd-catalysed cross-
1
2c
was highlighted.
Heteroleptic complexes, comprising one coupling of 2-cyano-4-bromopyridine with p-tert-butylbenzene
pyridyltriazole ligand and a second – different – ligand, have boronic acid under standard Suzuki conditions. The proligands
1
3,14
1
2
1
13
meanwhile been explored by others.
More recently, dianionic HL and HL were characterised by H and C NMR spectro-
ꢀ
ꢀ
1
tridentate ligands based on N^N^N -coordinating 2,6-bis(1,2,4- scopy and by mass spectrometry; HL was additionally studied
triazol-5yl)pyridine have been used to prepare an extensive range in the crystalline state by X-ray diffraction (see below).
1
of complexes, whose photophysical properties have been put
The homoleptic Pt(II) complexes of the two ligands PtL
2
and
15
2
forward for various applications.
2
PtL were prepared by reaction with the standard platinum(II)
ꢀ
2
Fig. 1 The structural relationship between complexes featuring cyclometallated N^C -coordinating arylpyridines, {e.g. cis-Pt(ppy) }, A; 5-(2-pyridyl)-
11
11,12
pyrazolates B, and 5-(2-pyridyl)triazolates C and D.
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1
2
Fig. 2 New homoleptic complexes PtL
2
2
and PtL prepared and studied in this work.
1
2
Scheme 1 Synthesis of proligands HL and HL and their homoleptic Pt(II) complexes.
1
3
precursor salt K
2
PtCl
4
, in refluxing ethanol/water (3 : 1) or
C NMR spectroscopy, high-resolution mass spectrometry, and
acetonitrile/water (3 : 1), respectively, for 18 h. No base was by X-ray crystallography.
required, in contrast to methods described for complexes with
Structures in the crystal: X-ray diffraction analysis
related ligands that employed strong bases such as NaH or
Na CO to deprotonate the azole NH. Both complexes precipitated Crystals of the proligand HL suitable for X-ray diffraction
1
2
3
1
2
from solution; PtL was obtained in an analytically pure state in analysis were obtained by slow evaporation of a solution in
2
6
7% yield by a series of simple washings, whereas PtL
additional purification by column chromatography on alumina close to being coplanar in the solid state, with torsion angles of
with CH Cl , somewhat compromising the yield to 22%. The 9.11 and 7.91 between the triazole ring and the pyridyl and
identity and purity of the complexes was confirmed by H and phenyl rings respectively (Fig. 3). The molecules are arranged in
2
2 2
required CH Cl . The molecular structure shows that the three rings are
2
2
1
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1
Fig. 3 Molecular structure of HL and the packing of molecules in the crystal.
dimers through hydrogen-bonding between the triazole N–H in bond lengths and angles). Despite the absence of base in the
2
one molecule and the triazole:N of the partner molecule; they synthesis, each triazole has bound as an anion, through deproto-
1
4
are oriented in a head-to-tail fashion, which evidently mini- nation of N , rather than as a neutral ligand through N (which is
n
mises steric repulsion between the tert-butyl groups.
what is observed for N-alkylated ligands). In each PtL 2 unit, the
were obtained by recrystallisation two ligands are found be bound in a head-to-tail arrangement; i.e.
1
2
Crystals of PtL
2
and PtL
2
from DMF and from a mixture of CH
both cases, the molecular structures confirm the homoleptic
1
2
Cl
2
/C
6
H
14 respectively. In with pyridine trans to pyridine and triazole trans to triazole giving
2h as opposed to C2v local symmetry. The same arrangement has
: 2 (M : L) formulation (see Fig. 4 and 5, and Table 1 for selected been observed in the previously reported pyrazolate and triazolate
D
1
2
Fig. 4 Molecular structure and crystal packing of PtL .
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2
2
Fig. 5 Molecular structure and crystal packing of PtL .
1
2
2
2
1
2
Table 1 Selected bond lengths and angles in PtL
by X-ray diffraction
and PtL
determined proligand HL ; the value in PtL
is similar (12.31). The angle
2
2
between the pyridyl ring and its substituent phenyl in PtL
2
is 32.41, the larger torsion angle being typical for adjacent
-membered rings, with more steric hindrance disfavouring a
conformation closer to coplanarity.
Interestingly, the packing of the molecules in the two crystals
is such that they are staggered off-centre relative to one another.
Complex Bond length/Å
Bond angles/1
Npy(1)–Pt–Npy(2)
Npy(1)–Pt–Ntrz(1) 100.6(2)
Npy(1)–Pt–Ntrz(2) 79.4(2)
6
1
py
PtL
2
Pt–N
Pt–N
2.033(4)
1.992(4)
180.0
trz
1
py
PtL
2
Pt–N
Pt–N
1.996(3), 1.997(3) Npy(1)–Pt–Npy(2)
2.024(3), 2.024(3) Npy(1)–Pt–Ntrz(1) 79.1(1)
Npy(1)–Pt–Ntrz(2) 101.0(1)
180.0(1)
trz
This arrangement leads to no close contacts between the Pt
1
centres: the PtꢁꢁꢁPt distances are 5.582 and 6.595 Å in PtL
2
and
2
PtL
2
respectively. The structure contrasts with that of the pre-
11,12
complexes of Fig. 1.
This disposition of the ligands contrasts viously characterised pyridyl-triazole complex Pt(ptp) , where the
2
with that found in bis-cyclometallated Pt(II) complexes of the form molecules stack in an eclipsed fashion to form infinite chains
Pt(N^C) (e.g. N^C = phenylpyridine, thienylpyridine), where the with short PtꢁꢁꢁPt distances of 3.289 Å, indicative of metallophilic
2
16
12
aryl rings are cis to one another. In that case, the strong trans interactions through orbital overlap. The pyrazolate complex
influence of the strongly s-donating metallated rings disfavours structurally characterised by Chi and co-workers also displayed
11
the trans arrangement. In the present case of the triazolates, the eclipsed packing with similarly short PtꢁꢁꢁPt distances of 3.442 Å.
trans arrangement of the ligands leads to short Nꢁ ꢁ ꢁH contacts
between the H atom of the C–H ortho to the N atom of the
coordinated pyridine and the uncoordinated N atom of the
Ground-state properties: UV-visible absorption and
electrochemistry
1
2
1
pyrazolate: 2.32 and 2.34 Å for PtL and PtL respectively. The absorption spectrum of PtL shows a broad absorption
2
2
2
Correspondingly short distances were also observed in the band or bands over the range 380 to 460 nm that has no counter-
ꢀ
1
ꢀ1
previously described pyrazolate complexes, with values in the part in the proligand, with emax around 4000 M cm (Fig. 6).
1
1
range 2.25–2.30 Å.
The appearance of such bands is typical of related complexes and
The torsion angle between the triazole and its phenyl sub- of cyclometallated analogues comprising Pt(N^C) units, and
1
2
stituent is slightly increased to 13.71 in PtL
compared to reflects the introduction of relatively low-energy charge-transfer
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1
5
.75 and 3.09 eV, respectively, for PtL
2
, and 5.81 and 3.25 eV
2
2
2 2
for PtL . The introduction of the aryl substituent in PtL is
thus seen to decrease the electrochemical frontier orbital energy
1
2
gap by 0.1 V relative to PtL , due primarily to stabilisation of the
LUMO. The smaller gap is consistent with the modest red-shift in
2
1
the lowest-energy absorption band in PtL
2 2
relative to PtL .
Density functional theory (DFT) calculations
Further support for this interpretation and the significance of
HOMO and LUMO energies in determining the absorption
energy is obtained from DFT and time-dependent DFT (TD-DFT).
18
Calculations were performed using the B3LYP functional and
LANL2DZ basis set for all atoms. The DFT results reveal that the
pyridyl rings make the major contribution to the LUMO, whilst the
HOMO spans the triazole, its phenyl substituent and the metal
(
Fig. 8 and Table 2). Such a picture is similar to that typically found
from DFT calculations on cyclometallated arylpyridine complexes,
Fig. 6 UV-visible absorption spectra and normalised photoluminesence where the aryl and pyridyl rings contribute to HOMO and LUMO
1
2
2
2
7,19
2
spectra of PtL
(black lines) and PtL
(red lines) in CH
2
Cl
2
at 295 K.
respectively.
2
In the case of PtL , the LUMO also shows a
contribution from the phenyl ring appended to the pyridyl rings,
whereas these rings make no significant contribution to the
1
,7
excited states upon metallation.
The more intense set of
HOMO. This observation is consistent with the observed greater
absorption bands at l o 350 nm are typical of ligand-based
p–p* transitions. The spectrum of PtL
2
2
2
effect of the pendent rings in PtL
on the reduction as opposed to
2
shows similar features,
oxidation potential discussed above. TD-DFT calculations carried
out at the optimised ground-state geometry support the assign-
ment of the first excited spin-allowed state as having predominant
HOMO–LUMO character, i.e., dPt|ptrz - ppy*, whilst the calculated
energies qualitatively reproduce the observed red shift on going
but the charge-transfer bands in the visible region are somewhat
red-shifted and increased in intensity. An additional strong band
appears at around 350 nm, and the intensity of all of the shorter-
wavelength ligand-centred bands is substantially increased, con-
sistent with the introduction of an additional aryl ring on each
ligand and the resulting substantial extension of the p-conjugated
system. Further evidence for the origin of the additional band is
provided by calculations discussed in the next section.
Electrochemically, both complexes undergo irreversible oxidation
in dichloromethane solution at a similar onset potential of
around +0.65 V relative to the ferrocene|ferricenium couple:
cyclic voltammograms are shown in Fig. 7. In contrast, the
2
quasi-reversible reduction of PtL
2
appears at a significantly
1
less negative potential than that of PtL
respectively). Using the commonly accepted relationship, the
ionisation potential and electron affinity (corresponding to
2
(ꢀ1.85 and ꢀ2.01 V
17
ꢀEHOMO and ꢀELUMO respectively) can be estimated to be
1
2
2
2
Fig. 8 Frontier orbitals of PtL
and PtL
from DFT calculations.
1
2
Table 2 Lowest-energy spin-allowed transitions for PtL
2 2
and PtL
calculated by TD-DFT
1
2
Complex
PtL
2
PtL
2
Energy/eV
2.53
489
0.0296
2.49
499
Wavelength/nm
Oscillator strength
Main contribution
0.0453
HOMO - LUMO (70)
1
2
Fig. 7 Cyclic voltammograms of PtL
2
and PtL
2
(black and red lines
NBF
HOMO - LUMO (70)
respectively) in CH Cl at 295 K, in the presence of 0.1 M Bu
2
2
4
4
as the Assignment
d |p - ppy
*
d |p - ppy*
Pt trz
Pt trz
supporting electrolyte.
LLCT/MLCT
LLCT/MLCT
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1
2
2
2
from PtL
to PtL
(further details of TD-DFT results are given in
As the concentration in CH
2
Cl
2
is increased, a broad, structure-
Tables S3 and S4 of the ESI‡). The TD-DDT results also predict an less band at longer wavelength in the red region grows in (lmax
2
2
1
2
2
2
intense band ( f = 0.374) at 355 nm for PtL , the S - S transition approx. 585 and 625 nm for PtL and PtL respectively, Fig. 9).
0
5
which has HOMOꢀ2 - LUMO character. The HOMOꢀ2 orbital is This concentration dependence is indicative of emission from a
shown in Fig. S35 (ESI‡). It can be seen that the transition heavily bimolecular excited state, which is stabilised relative to that of
involves the pyridyl-appended phenyl ring, and probably thus the green-emitting unimolecular excited state. Such bimolecular
accounts for the appearance of the additional band at 352 nm in excited states are well-established in Pt(II) chemistry, where the
2
1
PtL
2
, not present in PtL
2
(Table 2).
square-planar geometry favours face-to-face interactions that
can lead to the formation of dimers and aggregates in the
ground state, and/or to excimers wherein the interaction occurs
Photoluminescence in solution
5
–7
Both complexes are luminescent in deoxygenated solution, exclusively in the excited state. Typically, the former may be
emitting brightly in the green region of the spectrum (Fig. 6). accompanied by the appearance of a low-energy absorption
Some vibrational structure is evident with a progression of band, whereas the excitation spectrum of an excimer should
about 1200 cm . The photoluminescence quantum yields were be identical to that of the corresponding unimolecular emission.
determined to be 0.21 and 0.48 for PtL
ꢀ
1
1
2
2 2
and PtL respectively, In the present instance, there is no change in the absorption
with emission lifetimes in dilute solution of around 5 ms (Table 3). spectrum with concentration, whilst the appearance of the excimer
This behaviour is in marked contrast to the very weak or negligible is accompanied by a progressive decrease in the lifetime of the
11
emission displayed by systems such as C in solution (Fig. 1). The unimolecular emission. These observations suggest that the
luminescence is quenched quite efficiently by dissolved oxygen: in species responsible forms from the unimolecular excited state,
air-equilibrated solution, the lifetimes and intensities are reduced rather than it pre-existing in the ground state, and thus an
by an order of magnitude and bimolecular rate constants for the excimer seems likely.
9
ꢀ1 ꢀ1
quenching by O
2
are determined to be around 10 M
s
.
Apparent rate constants of self-quenching, kSQ, could be
Such lifetimes and sensitivities to oxygen are typical of many determined from the linear variation of the observed emission
luminescent cyclometallated Pt(II) complexes.
decay rate constant kobs (=1/t) as a function of the concentration
1
2
2
Table 3 Photophysical data for complexes PtL
2
and PtL
in CH
2
Cl
2
solution at 295 K. The photoluminescence data refer to the unimolecular emission
1
2
PtL
2
PtL
2
ꢀ
1
ꢀ1
Absorption lmax/nm (e/M cm
)
275 (79 900), 290 (86 300), 310 (58 900),
25 (27 200), 425 (4100)
502, 535
5100 [300]
0.21
5.1
14
270 (118 000), 304 (110 000), 352 (85 100),
3
420 (5900)
514, 538sh
4600 [500]
0.48
3.3
8.1
Emission lmax/nm
a
b
t/ns degassed [air-equilibrated]
a,c
lum
F
8
ꢀ1
ꢀ1 d
ꢀ1
k
k
a
SQ/10 M cm
.
O2
Q
8
ꢀ1
e
10 M cm
b
c
d
In degassed solution. Values in [parenthesis] refer to air-equilibrated solution. Determined using [Ru(bpy)
quenching rate constant determined from the gradient of a plot of 1/t versus concentration. Bimolecular rate constant for quenching by O
estimated on the basis of the lifetimes in degassed and air-equilibrated solution, and assuming that [O
atmospheric pressure at 295 K.
3
]Cl2(aq) as the standard. Self-
e
2
at
ꢀ3
2
] = 2.2 mmol dm in CH
2
Cl
2
1
2
Fig. 9 Concentration dependence of the emission spectra of PtL
2
(left) and PtL
2
(right) in CH
2
Cl
2
at 298 K. Spectra are normalised to lmax for the
ꢀ
3
unimolecular emission (500 and 513 nm respectively); legends give concentrations in mol dm
.
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0
of the complex (eqn (1), where t is the lifetime at infinite
dilution).
kobs = 1/t + kSQ[Pt]
(1)
0
8
The values of kSQ thus obtained (Table 3) are 5.1 ꢂ 10 and
8
ꢀ1 ꢀ1
1
2
3
.3 ꢂ 10 M
s
2 2
for PtL and PtL respectively, implying a
modestly reduced propensity of the latter to form excimers
compared to the former, perhaps due to a greater degree of
steric hindrance from the additional tert-butyl groups to the
necessary close face-to-face contacts. Indeed, the excimer starts
1
to dominate the emission spectrum of PtL
2
at concentrations
ꢀ4
2
above about 2 ꢂ 10 M, whereas higher concentrations of PtL
2
are required before the excimer becomes significant (Fig. 9).
Steady-state photoluminescence in neat and doped films
The complexes were doped into films of a typical OLED emitter
comprising a carbazole, mCP, and oxadiazole, OXD7, in 80/20
ratio by mass (the systematic names of the materials used are
given in the Experimental section). Films containing the complexes
at 5, 20, and 30% by mass were prepared by spin-coating from a
chloroform/chlorobenzene solution (95/5 by volume). Neat (100%)
films of the complexes were prepared similarly. We also examined
films comprising a very low loading of complex (0.005%) in
polystyrene (PS). PS was selected as the host for the very dilute
samples, as it has no emission of its own (unlike the OLED host
material), is photostable, and solubilises the complexes well
1
2
Fig. 10 Photoluminescence spectra of PtL
2 2
and PtL in neat films, in
polystyrene film at a loading of 0.0055% by mass, and in an OLED host
material (MCP : OXD7, 80 : 20) at the % by mass indicated. Spectra are
normalised to the global lmax
.
(unlike Zeonex, for example). The inevitably weak emission from
these very dilute samples could be compensated for by spinning on the other hand, pre-exists in the ground state, as may often
thicker films. The photoluminescence spectra of these various be the case in the solid state. However, solids are not necessarily
films are shown in Fig. 10.
sufficiently rigid to rule out the possibility of thermally activated
, the spectrum from the ‘‘dilute’’ PS film resembles movement of molecules relative to one another in the excited state,
2
1
For PtL
that in dilute solution, lmax = 500 nm, with the vibrational to give an ‘‘excimer-like’’ excited state. Such a distinction between
structure rather more well-resolved in the more rigid environment ‘‘excimer-like’’ and ground-state aggregate/dimer formation has
of the polymer. In the neat film, a single emission band centred at been put forward previously in cyclometallated Pt(II) complexes
20
about 600 nm is observed, a longer wavelength than the excimer in that emit from low-energy bimolecular excited states.
solution. At 30% in the OLED host, a single band at 582 nm is
observed, more similar to that of the excimer in solution. In the
Time-resolved photoluminescence in neat and doped films
spectrum at 5% loading, contributions from both the red and The time-resolved luminescence in the series of films was
vibrationally-structured green bands are evident. A similar trend to investigated using a gated iCCD camera in order to probe not
1
2
2
that found for PtL
with, again, a significant red-shift in the neat film (centred around Such measurements may help to confirm or refute the presence
45 nm) relative to that at 30% or the excimer in solution. of an additional emitting species. The decay of luminescence is
Although the different environment of the molecules in the reconstructed from the integrated emission over time, and thus
is observed for the films containing PtL
2
,
only the lifetimes but also the temporal evolution of the spectra.
6
neat films compared to the 30% doped films might conceivably reflects the presence of all kinetic species in the system; i.e.,
account for the observed red shifts in the spectrum, another unimolecular, bimolecular excimer-like and aggregate. It is clear
possible explanation is the formation of an additional, new that the photoluminescence lifetime is reduced upon increasing
species in the neat film, that emits at slightly longer wavelength the concentration of the complex (Fig. 11 and Fig. S3–S11 in the
than the ‘‘excimer-like’’ emission. This interpretation in terms ESI‡). The excimer and/or aggregate species that form at higher
of the involvement of a third species – most likely associated concentration have shorter lifetimes than the unimolecular
with aggregate states – is supported by time-resolved measure- excited state. Only at high dilution in PS is the decay mono-
ments as discussed below. It may first be helpful to note that exponential, reflecting the presence of discrete unimolecular
the term ‘‘excimer’’ normally refers to the diffusion-controlled excited states only, under these conditions. The lifetimes are
1
2
formation of a bimolecular species in solution, whose bimolecular 6.0 ꢃ 0.2 ms and 3.3 ꢃ 0.1 ms for PtL
2 2
and PtL respectively,
excited state is stabilised relative to the unimolecular excited state, quite similar to the values in deoxygenated solution. At the
yet which has a repulsive ground state. A dimer or aggregate, higher concentrations, the emission becomes shorter, following
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2
Fig. 11 (a) Photoluminescence decay of PtL
2
in neat film, polystyrene
(
PS) at 0.005%, and in the OLED host material (mCP : OXD7, 80 : 20) at the
%
of PtL
loadings indicated, at 295 K. (b) Photoluminescence decay of a neat film
1
2
at 295 K and at 80 K.
bi- or multi-exponential decay, reflecting the contributions of
bimolecular as well as unimolecular excited states. The average
2
1
1
2
1
lifetime decreased to 0.15 and 0.35 ms for PtL
2
and PtL
2
.
2
Fig. 12 Time-resolved photoluminescence spectra of PtL at 20% loading
Detailed values of fitted photoluminescence decay time constants in mCP : OXD7 (80: 20) at (a) 295 K and (b) 80 K.
can be found in Fig. S2–S11 (ESI‡).
At 80 K, the lifetimes in PS increase to 8.0 ꢃ 0.3 ms and 3.8 ꢃ
1
1
2
of the PtL
2
complex, a higher propensity to aggregation seems
0
.1 ms and in neat films to 1.1 ms and 1.4 ms for PtL and PtL
2 2
reasonable. At 80 K, the emission band from the red-emitting
species sharpens and red-shifts, reflecting the stabilisation of
bimolecular excited states with decreasing temperature (Fig. 12).
In contrast, the unimolecular emission is essentially unchanged.
respectively, due to the suppression of non-radiative decay. The
1
proportionately larger increase in neat films of PtL
to PtL
2
compared
2
suggests that non-radiative decay is more efficient for the
2
former. Though at first sight, this appears contrary to expectations
from the energy gap law given the lower energy emission from
1
2
2
Solution-processed OLED devices using PtL and PtL 2 as
emitters
2
PtL
2
, it should be noted that an excimer-like excited state is
dissociative in the ground state. The energy gap law describes
the conversion of electonically excited energy to ground-state Previous studies of pyridyl-triazolate complexes as phosphorescent
vibrations, and thus should not apply to an excimer. It may be emitters were confined to OLEDs prepared by vacuum sublimation,
2
2
that the more extended nature of the conjugated system in PtL
due to the low solubility of the complexes. In the present instance,
renders the resulting excimer more resistant to non-radiative the good solubility of the complexes allowed us to fabricate OLEDs
deactivation. by solution processing methods. Owing to the good solubility of
ꢀ1
In the time-resolved emission spectra (Fig. 12 and Fig. S12–S20, both complexes (5–10 mg mL ) in chloroform/chlorobenzene
ESI‡), the unimolecular emission can be distinguished as the (95 : 5, v/v), the compounds were employed to prepare emitting
structured band to high energy of the main red band arising from layers containing the Pt(II) complex in a blend of mCP and
the bimolecular species. However, inspection reveals that the latter OXD-7 (80 : 20 w/w). The device architecture was ITO|HIL 1.3N
red-shifts with increasing time delay, supporting the notion that (45 nm)|mCP : OXD-7 (80 : 20) co Pt complex x% (60 ꢃ 5 nm)|TPBi
more than one type of bimolecular species emits in the red region, (50 nm)|LiF (0.8 nm)|Al (100 nm). Devices (Dev) 1, 2, and 3
1
and that they have somewhat different lifetimes. Based on the contained PtL
2
(x = 30, 20 and 5% respectively) whilst Dev 4, 5,
2
wavelengths, we tentatively assign the higher energy species to an and 6 contained PtL
excimer-like excited state and the lower to an aggregate. Further Dev 1–3 containing PtL
2
at the corresponding loadings.
1
2
all show a similar yellowish EL
inspection of excitation spectra (Fig. S21 and S22, ESI‡) indeed colour, CIE coordinates from (0.50, 0.49) to (0.41, 0.55), reflecting
reveals the aggregates to be present in the case of both emitters at the propensity of this complex to form bimolecular excited states
higher concentration. Related findings have been reported for (as observed also in solution) which dominate the EL spectrum
2
cyclometallated Pt(II) complexes with cyclometallating, tridentate (Fig. 13 and Fig. S23–S26 in ESI‡). On the other hand, PtL 2
20a
ligands. Interestingly, a closer look at the excitation spectra of offers different colours according to loading: the EL of Dev 6
1
PtL
2
suggests an inhomogeneity of the films as aggregates seem (5%) shows almost exclusively the green, unimolecular emission
to be visible also at lower concentrations. Given the lower solubility profile, whilst increasing the doping gives yellow-orange Dev 5
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1
2
2
2
Fig. 13 Characteristics of OLED devices prepared using PtL
(top) and PtL
(bottom): electroluminescence spectra normalised to lmax (left) and EQE
versus current density (right). The % in devices 1–6 indicates concentration of the emitter by weight. The 30% value for device 7 represents the
contribution based on % evaporation rate in co-evaporation.
(0.47, 0.51) and orange-red Dev 4 (0.52, 0.47). The devices show (0.8 mm)|Al (100 nm). This device has a low turn-on voltage
external quantum efficiencies (EQE) in the range 8.8–12.5%, of 3.8 V, an EQE of 15.0% and a maximum brightness of
ꢀ
2
impressive values for solution-processed devices, with very low 39 000 cd m . Its EL spectrum is quite similar to that of the
ꢀ2
roll-off and high maximum luminance of up to 28 700 cd m
solution-processed device at 30% loading, the small differences
(Dev 2). The relatively high EQE of these devices can be traced to observed (e.g. slightly more contribution from unimolecular
the high photoluminescence quantum yields found in thin excited states) being attributed to potential subtly different
films, in the range 0.55–0.85 (Table S2 in ESI‡). The lower roll- packing of the molecules in the vacuum-deposited film compared
off of the 20% and 30%-loaded devices compared to the 5% to solution-processed film.
(Fig. 13b and d) probably reflects the shorter emission lifetimes
associated with the bimolecular species as compared to the
unimolecular emission, the latter being substantially longer-
lived as discussed in the previous section (see Fig. 11). Long
Summary and conclusions
lifetimes, of the order of several microseconds, typically lead to The synthetic work reported here reveals that the solubility of
ꢀ
more roll-off at high current density.
simple, square-planar, homoleptic Pt(II) complexes with N^N -
Nevertheless, the turn-on voltages of all six devices are coordinating pyridyltriazole ligands can be readily enhanced
rather high, at 9–11 V, suggesting the existence of an energy through incorporation of aryl pendents. Such substituents
barrier to carrier injection/transport in the device. The use of a apparently influence the crystal structures, eliminating close
different electron transport material with a higher LUMO, Ptꢁ ꢁ ꢁPt contacts found in a previously reported system of poor
mCP : PO-T2T (70 : 30), led to a significant reduction in turn-on solubility, though retaining close interfacial distances of the
voltage to 7–8 V. Devices 8–13 (Fig. S27–S34, ESI‡) were prepared aromatic ligands in adjacent molecules. The complexes phos-
with an architecture ITO|HIL 1.3N (45 nm)|mCP : PO-T2T phoresce strongly in deoxygenated solution, in doped films, and
(
70 : 30) PtL
2
x% (70 ꢃ 5 nm)|PO-T2T (50 nm)|LiF (0.8 nm)|Al in neat films. Green emission under dilute conditions is accom-
(100 nm). However, the desirable reduction in turn-on voltage panied by intense red emission at higher concentrations, both
was accompanied by compromised efficiency, probably due to in solution and films, attributed to bimolecular excited states.
the improved electron mobility in the emitting layer causing The time-resolved studies suggest the presence of different
current leakage to the hole injection layer.
environments of the bimolecular excited states. It is interesting
Finally, to demonstrate the versatility of the Pt(II) complexes to note that those which appear to emit at lowest energy appear to
as emitters, a vacuum evaporated device Dev 7 was also fabri- have the longest lifetimes. This contrasts with most unimolecular
cated, with the structure: ITO|NPB (35 nm)|TSBPA (10 nm)|mCP emitters, where non-radiative decay through vibrational deactivation
(5 nm)|mCP Pt complex 30% (20 nm)|TPBi (50 nm)|LiF normally becomes more efficient as the excited state energy
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2
2
decreases (quantified through the ‘‘energy gap law’’). The parameters of refinement are listed in Table S1 (ESI‡). CCDC
observation can be rationalised owing to the potential energy 1894904–1894906.‡
surface of the excimer becoming repulsive upon emission. It
Solution-state photophysics
highlights the intriguing potential of the use of excimers in
potentially circumventing the consequences of the energy gap UV/vis-electronic spectra were recorded on a Biotek Instruments
law, and so access to efficient red/NIR emitters.
UVIKON XS spectrometer operating with LabPower software.
The complexes are amenable to incorporation into OLED Solution-based emission were acquired on a Jobin Yvon Spex
devices through solution processing. The resulting devices perform Fluoromax-2 spectrometer. All samples were contained within
well, reaching an EQE of 12.5% and a maximum luminance of up to 1 cm pathlength quartz cuvettes modified for connection to a
ꢀ
2
2
8 700 cd m . The high efficiency of these devices at higher dopant vacuum line. Degassing was achieved by three freeze–pump–thaw
concentrations is thanks to the short photoluminescence lifetimes cycles whilst connected to the vacuum manifold: final vapour
ꢀ
2
of the excimer/aggregate species that successfully compete with pressure at 77 K was o5 ꢂ 10 mbar. Emission was recorded at
non-radiative decay, inhibiting quenching mechanisms and 901 to the excitation source, and spectra were corrected after
ensuring low device efficiency roll-off.
acquisition for dark count and for the spectral response of the
detector. The quantum yields were determined relative to an
aqueous solution of [Ru(bpy) ]Cl ; Flum = 0.028.
3 3
25
Experimental
General
Luminescence lifetimes of the complexes were measured by
time-correlated single-photon counting method, using an EPL405
pulsed-diode laser as excitation source (405 nm excitation, pulse
length of 60 ps, repetition rate 20 kHz). The emission was
detected at 901 to the excitation source, after passage through a
monochromator, using a Peltier-cooled R928.
Reagents were obtained from commercial sources and used
without further purification unless stated otherwise. All solvents
used in preparative work were at least Analar grade and water
was purified using the PuriteSTILL plust system. Dry solvents
were obtained from HPLC grade solvent that had been passed Solid state photophysics
through a Pure Solv 400 solvent purification system and stored
Solid films were fabricated using similar conditions to OLED
over activated 3 or 4 Å molecular sieves. For procedures invol-
ving dry solvent, glassware was oven-dried for at least 8 h prior
to use. Oxygen-free argon cylinders (BOC, UK) were used to
devices (see below) except for polystyrene films, which were
ꢀ1
deposited by drop casting from 100 mg mL solutions in chloro-
form and dried at room temperature. All films were dried in vacuo
after preparation for at least 1 h. The photoluminescence spectra
of the films were recorded using a QePro spectrometer (Ocean
Optics) coupled with an integrating sphere (Labsphere) and a
1
13
provide an inert atmosphere where required. H and C NMR
spectra were recorded on a Bruker Avance-400 spectrometer.
Two-dimensional NMR (COSY, NOESY, HSQC and HMBC)
spectra were acquired on Varian VNMRS-600 (600 MHz) or
VNMRS-700 (700 MHz) instruments. Chemical shifts (d) are in
ppm, referenced to residual protio-solvent resonances, and coupling
constants are given in hertz. Mass spectra were obtained by electro-
spray ionisation (positive and negative ionisation modes) on a
Waters TQD mass spectrometer interfaced with an Acquity UPLC
system with acetonitrile as the carrier solvent. Measurements
requiring the use of an atmospheric solids analysis probe (ASAP)
for ionisation were performed on Waters Xevo QToF mass
spectrometer.
365 nm LED light source (Ocean Optics) for excitation. Time-
resolved spectra and photoluminescence decays in the films were
recorded using nanosecond gated luminescence and lifetime
measurements (from 400 ps to 1 s) using either the third harmonic
of a pulsed Nd:YAG laser emitting at 355 nm (EKSPLA) or a N2
laser emitting at 337 nm. Emission was focused onto a spectro-
graph and detected by a gated iCCD camera (Stanford Computer
Optics) with sub-nanosecond resolution. PF/DF time-resolved
measurements were performed by exponentially increasing gate
and integration times. Temperature-dependent experiments were
conducted using a continuous flow liquid nitrogen cryostat (Janis
Research) under a nitrogen atmosphere. Details of the specific
X-ray crystallography
26
The X-ray single crystal data have been collected using lMoKa experimental set-up used have been reported elsewhere.
radiation (l = 0.71073 Å) on a Bruker D8Venture (Photon100
CMOS detector, ImS-microsource, focusing mirrors) diffractometer
Electrochemistry
equipped with a Cryostream (Oxford Cryosystems) open-flow Electrochemical measurements were performed by cyclic voltam-
s
nitrogen cryostat at the temperature 120.0(2) K. The structures metry in solution in dichloromethane (Chromasolv , 99.9%
4 4
were solved by direct method and refined by full-matrix least Sigma Aldrich) in the presence of Bu NBF at a concentration
2
23
24
squares on F for all data using Olex2 and SHELXTL software. of 0.1 M (99%, Sigma Aldrich, dried). Solutions were purged with
All non-disordered non-hydrogen atoms were refined anisotropi- argon prior to measurement. The working electrode was a Pt disc
1
cally; hydrogen atoms in structure HL were refined isotropically; (1 mm diameter), the counter electrode was Pt wire, and the
+
ꢀ1
hydrogen atoms in the two complexes were placed in the reference was an Ag |Ag electrode; a scan rate of 50 mV s was
calculated positions and refined in riding mode. Disordered used. Potentials were calibrated against ferrocene. The ionization
2
atoms in structure PtL
2
were refined isotropically with fixed potential (IP) and electron affinity (EA) were estimated from the
SOF = 0.5 and restrained C–C bond lengths. Crystal data and onset oxidation (Eox) and reduction (Ered) potentials, respectively,
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0
1
7
4
6
4
using following equations: IP = Eox + 5.1, EA = Ered + 5.1.
(176 MHz, chloroform-d) d 153.8 (C ), 151.4 (C ), 149.6 (C ),
0
0
0
3
2
1
2
3
Detailed description of the experimental technique can be found 134.5 (C ), 132.9 (C ), 126.7 (C ), 126.5 (C ), 126.2 (C ), 124.3
t
1
0
t
2
2
7
5
CN
Bu
Bu
+
elsewhere.
(C ), 117.4 (C ), 34.8 (C
), 31.2 (C ); MS (ES ) m/z = 237.3
+
+
+
[
[
M + H] ; HRMS (ES ) m/z = 237.1394 [M + H] ; calculated for
OLED devices
+
C H N ] 237.1392.
1
6
17 2
OLEDs were fabricated by a hybrid spin-coating/evaporation
method. The hole injection layer (Heraeus Clevios HIL 1.3N)
and emitting layer (mCP:OXD-7 or mCP:PO-T2T + dopant)
were spin-coated, whereas the electron transport layer (TPBi
or PO-T2T) and cathode (LiF/Al) were deposited by evaporation.
Devices of 4 ꢂ 2 mm pixel size were fabricated. 2,4,6-Tris[3-
(
diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T, sublimed,
LUMTEC), 1,3-bis(carbazol-9-yl)benzene (mCP, sublimed, LUMTEC),
,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7,
1
0
00
sublimed), 2,2 ,2 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimid-
azole) (TPBi, sublimed, LUMTEC), LiF (99.995%, Sigma Aldrich),
Compound 1b. To a solution of 2-cyanopyridine 1a (1.0 g,
and aluminium wire (99.9995%, Alfa Aesar) were purchased from 9.6 mmol) in ethanol (30 mL) was added hydrazine monohydrate
the companies indicated in parentheses. OLED devices were (4.6 mL, 96.0 mmol). The reaction mixture was stirred at room
fabricated using pre-cleaned glass substrates coated with indium temperature for 18 h, yielding a pale off-white solid. The precipitate
tin oxide (ITO) after ozone plasma treatment, with a sheet was filtered, washed with cold ethanol and dried under vacuum
ꢀ
2
+
+
+
resistance of 20 O cm and ITO thickness of 100 nm. Heraeus (1.2 g, 92%); (ES ) m/z = 136.7 [M + H] ; HRMS (ES ) m/z =
+
+
Clevios HIL 1.3N was spin-coated and annealed onto a hotplate at 137.0822 [M + H] ; calculated for [C H N ] 137.0827. Other
6
9 4
2
00 1C for 3 min to give a 45 nm film. The emitting layer was experimental data were consistent with literature data for this
2
9
spun from a chloroform:chlorobenzene (95 :5 v/v) solution of compound.
mCP : OXD-7 (80 : 20 w/w) or mCP : PO-T2T (70 : 30 w/w) with total
concentration of host + dopant kept at 20 mg mL . The dopant
was dissolved in the host solution in order to obtain final 5–30%
ꢀ1
(w/w) concentration in the emitting layer. The solution was spun
onto the HIL 1.3N layer and then annealed at 50 1C for 5 min
giving a 60 ꢃ 5 nm (mCP : OXD-7) and 70 ꢃ 5 nm (mCP: PO-T2T)
film. All solutions were filtered directly before application using a
PVDF or PTFE syringe filter with 0.45 mm pore size. All other
organic and cathode layers were thermally evaporated using a
Compound 2b. As for 1b; to a solution of 2a (1.0 g, 4.2 mmol)
in ethanol (40 mL) was added hydrazine monohydrate (2.1 mL,
1
4
(
(
2.3 mmol). A white solid was obtained (1.1 g, 96%); H NMR
ꢀ6
Kurt J. Lesker Spectros II deposition system at 10 mbar. All
organic materials and aluminum were deposited at a rate of
6
700 MHz, chloroform-d) d 8.52 (dd, J = 5.2, 0.8 Hz, 1H, H ), 8.26
0
3
2
5
s, 1H, H ), 7.64 (d, J = 8.2 Hz, 2H, H ), 7.50–7.46 (m, 3H, H
ꢀ
1
ꢀ1
1
Å s . The LiF layer was deposited at 0.1–0.2 Å s . Characterisa-
tion of OLED devices was conducted in a 10 inch integrating sphere
Labsphere) connected to a Source Measure Unit. Instrumental
0
3
a
b
d
and H ), 5.31 (s, 2H, H and H ), 4.58 (s, 2H, H ), 1.35 (s, 9H,
t
2
0
4
Bu 13
2
H
); C NMR (176 MHz, chloroform-d) d 152.4 (C ), 151.2 (C ),
(
0
0
0
4
6
1
2
3
1
1
2
48.6 (C ), 148.3 (C ), 134.9 (C ), 126.7 (C ), 126.0 (C ),
28
t 1 t 2
details have been reported elsewhere.
5
3
Bu
Bu
+
21.6 (C ), 117.2 (C ), 34.7 (C ), 31.3 (C ); MS (ES ) m/z =
69.4 [M + H] ; HRMS (ES ) m/z = 269.1774 [M + H] ; calculated
+
+
+
Synthetic details and characterisation of compounds
+
for [C H N ] 269.1766.
1
6
21 4
Compound 2a. 4-Bromopyridine-2-carbonitrile (1.00 g,
5.46 mmol), 4-tert butyl phenylboronic acid (1.07 g, 6.01 mmol)
and Na CO (4.6 g, 43.7 mmol) were added to a Schlenk with
2
3
DME (15 mL) and H O (15 mL). The mixture was degassed via
2
three freeze–pump–thaw cycles and then Pd(PPh
0
2
3
)
4
(789 mg,
.68 mmol) was added under argon. After heating at 85 1C for
4 h, water was added and the organic phase was extracted into
and the solvent removed in vacuo. The
DCM, dried over MgSO
4
resulting oil was purified by chromatography on silica using a
gradient of 100 : 0 to 80 : 20 hexane/ethyl acetate as eluant to
yield a white solid (1.2 g, 91%); R = 0.53 (silica, 80 : 20 hexane/
f
1
ethyl acetate); H NMR (700 MHz, chloroform-d) d 8.71 (dd,
6
3
J = 5.2, 0.8 Hz, 1H, H ), 7.90 (dd, J = 1.8, 0.8 Hz, 1H, H ),
Compound 1c. Compound 1b (877 mg, 6.4 mmol) and
CO (750 mg, 7.1 mmol) were added to a dry Schlenk with
.54 (d, J = 8.4 Hz 2H, H ), 1.36 (s, 9H, H ); C NMR dry DMF (25 mL) and cooled to 0 1C. A solution of 4-tert butyl
0
2
5
7
7
.70 (dd, J = 5.2, 1.8 Hz, 1H, H ), 7.58 (d, J = 8.4 Hz, 2H, H ), Na
2
3
0
t
2
3
Bu
13
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benzoyl chloride (1.4 mL, 6.4 mmol) in dry DMF (9.4 mL) was cooling to ambient temperature, the suspension was filtered
added drop-wise. The mixture was stirred at 0 1C and then and the resulting colourless solid was dried under vacuum
1
allowed to warm to ambient temperature. Water was added and (706 mg, 94%); H NMR (600 MHz, chloroform-d) d 8.73
6
3
the resulting suspension was filtered. The yellow solid obtained (d, J = 10.3 Hz, 1H, H ), 8.31 (dd, J = 8.0, 5.0 Hz, 1H, H ), 8.12
0
b
4
was washed with water and dried under vacuum (1.6 g, 83%); (d, J = 8.3 Hz, 2H, H ), 7.88 (dd, J = 7.9, 2.2 Hz, 1H, H ), 7.50
0
1
c
c
5
6
H NMR (700 MHz, DMSO-d ) d 10.11 (s, 1H, H ), 8.57 (d, J = (d, J = 8.5 Hz, 2H, H ), 7.40 (ddt, J = 7.6, 4.8, 1.4 Hz, 1H, H ), 1.36
0
t 2
6
3
Bu
13
e
4
1
.5 Hz, 1H, H ), 8.15 (d, J = 8.0 Hz, 1H, H ), 7.88 (td, J = 7.8, (s, 9H, H0
); C NMR (151 MHz, chloroform-d) 163.1 (C ),
0
0
4
b
d
6
2
4
a
.7 Hz, 1H, H ), 7.79 (d, J = 8.0 Hz, 2H, H ), 7.49–7.43 (m, 3H, 152.8 (C ), 149.3 (C ), 146.3 (C ), 137.4 (C ), 127.8 (C ), 126.2
0
0
2
0
0
t
2
0
0
t
1
t
5
c
a
d
Bu
b
c
5
3
Bu
Bu
H
and H ), 6.89 (s, 2H, H and H ), 1.29 (s, 9H, H
C NMR (176 MHz, DMSO-d
); (C ), 125.4 (C ), 124.7 (C ), 121.8 (C ), 34.8 (C
) 233.19 (C ), 163.55 (C ), 154.15 (ESI ) m/z = 279.8 [M + H] ; HRMS (ES ) m/z = 279.1611 [M + H] ;
), 31.3 (C
);
1
3
b
e
+
+
+
+
6
0
0
d
2
6
4
a
+
(
(
3
2
C ), 151.08 (C ), 148.50 (C ), 137.31 (C ), 132.38 (C ), 127.90 calculated for [C H N ] 279.1610.
t
2
0
17 19 4
0
0
b
c
5
3
Bu
C ), 125.36 (C ), 125.10 (C ), 121.09 (C ), 31.42 (C
5.05 (C ); (ES ) m/z = 296.9 [M + H] ; HRMS (ES ) m/z =
97.1716 [M + H] ; calculated for [C17
),
t
1
0
Bu
+
+
+
+
+
21 4
H N O] 297.1715.
2
Proligand HL . This compound was prepared similarly to
1
HL , starting from 2c (570 mg, 1.3 mmol) in ethylene glycol
(
6 mL), and giving the product as a colourless solid (414 mg,
6%); H NMR (700 MHz, chloroform-d) d 8.79 (dd, J = 5.2,
1
7
0
8
1
2
Compound 2c. This compound was prepared in the same
way as 1c, starting from compound 2b (418 mg, 1.6 mmol) and
Na CO (198 mg, 1.9 mmol) in dry DMF (13 mL), to which was
added a solution of 4-tert butyl benzoyl chloride (304 mL,
.6 mmol) in dry DMF (5 mL). After work-up as for 1c, the
6
3
.8 Hz, 1H, H ), 8.60 (dd, J = 1.9, 0.8 Hz, 1H, H ), 8.16 (d, J =
0
0
2
b
.4 Hz, 2H, H ), 7.74 (d, J = 8.4 Hz, 2H, H ), 7.64 (dd, J = 5.2,
0
3
2
3
5
.9 Hz, 1H, H ), 7.55 (d, J = 8.4 Hz, 2H, H ), 7.50 (d, J = 8.4 Hz,
0
0
t
2
t
2
c
Bu
Bu
13
H, H ), 1.38 (s, 9H, H ), 1.36 (s, 9H, H
176 MHz, chloroform-d) d 153.1 (C ), 152.6 (C ), 150.3 (C ),
); C NMR
1
0
0
4
d
4
(
1
product was obtained as a yellow powder (600 mg, 90%);
0
0
0
2
6
2
1
a
49.4 (C ), 146.7 (C ), 134.2 (C ), 127.8 (C ), 126.9 (C ), 126.4
1
d
H NMR (599 MHz, DMSO-d ) d 10.15 (s, 1H, H ), 8.62 (d,
0
0
0
6
b
3
c
3
5
0
(C ), 126.2 (C ), 125.6 (C ), 122.4 (C ), 119.6 (C ), 34.8–34.7
t 1 t 1 t 2 t 2
0 0
(C
6
3
b
J = 5.2 Hz, 1H, H ), 8.39 (s, 1H, H ), 7.79 (d, J = 8.0 Hz, 2H, H ),
7
Bu
Bu
Bu
Bu
+
0
5
2
and C ), 31.3 (C
), 31.2 (C ); MS (ES ) m/z = 411.0
.76 (d, J = 5.1 Hz, 1H, H ), 7.73 (d, J = 8.0 Hz, 2H, H ), 7.56
0
0
+
+
+
3
c
[M + H] ; HRMS (ES ) m/z = 411.2543 [M + H] ; calculated for
(d, J = 8.1 Hz, 2H, H ), 7.48 (d, J = 8.1 Hz, 2H, H ), 6.93
t 2 t 2
0
+
a
c
Bu
Bu
[C
27
H
30
N
4
] 411.2544.
(
s, 2H, H and H ), 1.31 (s, 9H, H ), 1.29 (s, 9H, H
);
1
3
6
C NMR (151 MHz, DMSO-d
6
) d 154.1, 152.6, 149.1 (C ), 134.6,
27.9 (C ), 127.0 (C ), 126.6 (C ), 125.3 (C ), 122.4 (C ), 117.9
0 0
t 1 t 1 t 2 t 2
0
0
0
0
c
b
2
3
5
1
3
Bu
Bu
Bu
Bu
(C ), 35.1 (C
), 34.9 (C ), 31.5–31.4 (C
and C ); MS
+
+
+
+
(ES ) m/z = 429.8 [M + H] ; HRMS (ES ) m/z = 429.2651 [M + H] ;
+
calculated for [C H N ] 429.2654.
1
6
17 2
1
1
Complex PtL
2
. HL (100 mg, 0.36 mmol) and K
O/EtOH
1 mL : 3 mL). The mixture was heated at reflux for 24 h and
2 4
PtCl
(
(
68 mg, 0.16 mmol) were added to a Schlenk with H
2
then cooled to RT. Water (3 mL) was added and the suspension
was filtered to isolate a solid which was washed with water,
MeOH and Et O before being dried under vacuum. An orange
2
1
solid was obtained (80 mg, 67%); H NMR (600 MHz, chloro-
Proligand HL . A suspension of 1b (800 mg, 2.7 mmol) in form-d) d 9.61 (s, 2H, H ), 7.83 (d, J = 7.7 Hz, 4H, H ), 7.54
0
1
6
b
0
c
3
ethylene glycol (8 mL) was heated to 100 1C in an open round (d, J = 6.9 Hz, 4H, H ), 7.44 (d, J = 7.8 Hz, 4H, H ), 7.15 (d, J =
t
2
4
5
Bu
bottom flask to allow water evaporation. On turning clear, the 6.5 Hz, 2H, H ), 6.71 (t, J = 6.4 Hz, 2H, H ), 1.41 (s, 18H H );
1
3
6
resulting solution was set to reflux at 185 1C for 1 h. After
C NMR (151 MHz, chloroform-d) d 213.8 (C ), 172.1, 167.4,
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0
0
5
b
c
4
1
1
[
51.0, 139.1 (C ) 134.8, 125.9 (C ), 125.2 (C ), 123.97 (C ) References
t
1
t
2
3
Bu
Bu
+
20.2, 119.5 (C ), 34.7 (C ), 31.4 (C ); (ESI ) m/z = 750.7
M + H] ; HRMS (ES ) m/z =749.2593 [M + H] ; calculated for
+
+
+
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94
+
[C H N
35 8
Pt] 749.2611.
3
4
2596–2611; (c) S. Huo, J. Carroll and D. A. K. Vezzu, Chem. –
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2
Complex PtL
56 mg, 0.13 mmol) were added to a Schlenk with H
1 mL : 3 mL). The mixture was heated at reflux for 24 h and
2
. HL (100 mg, 0.24 mmol) and K
2 4
PtCl
(
(
2
O/MeCN
then cooled to RT. Water (3 mL) was added and the suspension
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2
013, ch. 3, pp. 77–113.
MeOH and Et O before being columned on alumina 100% DCM
2
4
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1
2
2%); R
f
= 0.9 (alumina, 100% DCM); H NMR (700 MHz,
6
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chloroform-d) d 9.70 (s(broad), 2H, H ), 7.90 (d, J = 7.6 Hz, 4H,
H ), 7.49 (d, J = 7.6 Hz, 4H, H ), 7.44 (d, J = 7.6 Hz, 4H, H ), 7.32
0
0
0
3
b
2
0
c
3
5
(
d, J = 7.6 Hz, 4H, H ), 7.25 (s, 2H, H ), 7.12 (d, J = 5.7 Hz, 2H, H ),
t
2
t
2
0
Bu
Bu
13
1.40 (s, 18H, H ), 1.37 (s, 18H, H
); C NMR (176 MHz,
t 1
Bu
chloroform-d) d 127.35, 126.34, 126.19, 125.39, 34.90 (C ),
), 31.26 (C ); (ASAP ) m/z = 1014.4
M + H] ; HRMS (ES ) m/z = 1013.4474 [M + H] ; calculated for
t
1
0
t
2
0
t
2
Bu
Bu
Bu
+
34.72 (C
), 31.39 (C
+
+
+
[
[
1
94
+
C
54
H
59
N
8
Pt] 1013.4489.
1801930.
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(
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M. T. W. thanks EPSRC for a PhD studentship. P. P. acknowl-
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