Synthesis, mesomorphism, photophysics and device performance of liquid-crystalline pincer complexes of gold(iii)

Article abstract of DOI:10.1039/d0tc04839a

Emissive gold(iii) complexes of pincer 2,6-diphenylpyridines also bearing a phenylacetylide ligand have been modified at both the pincer and phenylacetylide to confer liquid crystalline properties, with most complexes showing a columnar hexagonal phase in

Full text of DOI:10.1039/d0tc04839a

Journal of
Materials Chemistry C
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PAPER
Synthesis, mesomorphism, photophysics and
device performance of liquid-crystalline pincer
complexes of gold(III)†
Cite this: J. Mater. Chem. C, 2021,
9, 1287
a
a
Rachel R. Parker,
Weiguo Zhu,
Denghui Liu,b Xiankang Yu,b Adrian C. Whitwood,
J. A. Gareth Williams, *c Yafei Wang, *b Jason M. Lynam
and Duncan W. Bruce
*
b
a
a
*
Emissive gold(III) complexes of pincer 2,6-diphenylpyridines also bearing a phenylacetylide ligand have
been modified at both the pincer and phenylacetylide to confer liquid crystalline properties, with most
complexes showing a columnar hexagonal phase in the condensed phase. Solution NMR studies show a
preferred orientation for self-association, consistent with structural parameters in the liquid crystal phase
obtained by X-ray methods. While the pattern of substitution of the phenylacetylide has no discernible
effect on the photophysics, when two alkoxy chains are attached to the pincer ligands, photo-
luminescence quantum yields (PLQY) of around 3% are found, whereas when four alkoxy chains are
attached the PLQY increases significantly to 36%. Insight from computational chemistry indicated that
the incorporation of the alkoxy donor groups raises the energy of the pincer-based HOMOꢀ1 orbital,
Received 12th October 2020,
Accepted 4th December 2020
with
a
concomitant lowering of the LUMO
’
HOMOꢀ1 transition energy, consistent with an
experimentally observed red shift. The gold complexes were fabricated into OLED devices using solution
processing methods, being doped into the emissive layer at 5%, leading to external quantum efficiencies
of up to 7.14%, values that compare well with those of related complexes in the literature.
DOI: 10.1039/d0tc04839a
rsc.li/materials-c
voltages. In addition, for rod-like molecules, one of the attractive
goals is to use the order of nematic or SmA phases to realise
Introduction
Luminescent metallomesogens, which combine the properties polarised emission, which has been demonstrated in fluorescent
of self-organisation and emissive behaviour, have potential materials18 with some reports also for phosphorescent
applications in the field of semiconductors and are known in systems.19
the literature.1 Most of these studies have been based on
Gold(III) compounds have started to attract attention due to
complexes of platinum(II),2–6 which is amenable to liquid crystal their application in semiconductors and to their interesting
phase formation on the basis of its square-planar geometry, but photophysical properties. Much of this interest arose from the
there have also been reports based on, for example, gallium(III),7 report by Yam et al. of room-temperature phosphorescence in
zinc(II),8,9 silver(I),10 various lanthanides11 and iridium(III),12–17 gold(III) complexes containing a CNC pincer ligand and a
the last two of which are somewhat more challenging given the phenylacetylide in the fourth coordination site (Chart 1) that
3D, octahedral nature of the metal centre. One attraction of began to focus interest in these materials.20 As the acetylide
phosphorescent metallomesogens relates to taking advantage of ligand is a strong s-donor, the d-orbital splitting increases
the long-range order possible in liquid crystal systems to gen- when compared to the halide analogues, so that the evidence
erate ordered conduction layers, so potentially reducing drive from vibronic progression in both absorption and emission
spectra shows the chromophore to be based on the pincer
ligand, with effectively no influence of the acetylide.
a Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
E-mail: duncan.bruce@york.ac.uk; Tel: +44 (0)1904 324085
b School of Materials Science & Engineering, Changzhou University,
Changzhou 213164, P. R. China. E-mail: qiji830404@hotmail.com
c Department of Chemistry, Durham University, University Science Laboratories,
South Road, Durham DH1 3LE, UK
Using this observation as a base, numerous analogues have
been prepared21–46 and solution PLQY values for triplet emis-
sion in acetylide derivatives of up to 61% have been reported.26
More recently values of up to 94% have been recorded based on
singlet emission by TADF from gold(III) complexes with tetraden-
tate ligands,47,48 while in solution-processed devices, external
quantum efficiencies (EQE) of 15–16% have been achieved.28,49
† Electronic supplementary information (ESI) available. CCDC 1991483–1991485.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0tc04839a
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gold-based liquid-crystalline materials51,52 are linear and based
on Au(I) with a C–Au–X or C–Au–C skeleton. As such, the
materials reported here represent a new family of metallomesogens
and present an opportunity to develop an understanding of the
relationship between the molecular structure of the complexes
and both liquid crystal and emission properties. In this con-
tribution, we now report a series of gold(III) complexes which,
by virtue of modified pincer and acetylide ligands, exhibit
liquid crystalline properties. The effects of different ligand
substitution patterns on the photophysical properties of the
complexes are described, notably the raising in energy of the
pincer-based HOMO on incorporation of alkoxy groups. Device
data demonstrate that the complexes exhibit an EQE of up
Chart 1 Parent phosphorescent gold(III) chromophore as reported by
Yam et al.20
More recently, Li et al. considered design strategies that address to 7.14%.
both thermal stability and photophysical efficiency of complexes
of this general type. Based on isomeric CCN pincer ligands with
carbazole donor ligands, they have reported EQE values of up to
Synthesis
11.9% for devices constructed using solution processing, which
rise to 21.6% if vacuum deposition is employed. In both cases, the The target compounds have two principal components, the
devices show long operational half-lives.50
tridentate, pincer-like CNC ‘backbone’ and the monodentate
Given the increasing interest in the gold(III) system just acetylide ligand. The pincer proligands were readily prepared
described and our previous experience with phosphorescent by coupling a mono- or di-methoxyphenylboronic acid with 2,6-
liquid crystals, we have undertaken a study that encompasses dibromopyridine using a Suzuki–Miyaura protocol (Scheme 1, step
the design and synthesis of emissive liquid-crystalline complexes (i)). The bis(methoxyphenyl)pyridine and bis(dimethoxyphenyl)
of gold(III) as well as an evaluation of their photophysical pyridine products, 1, were demethylated using molten pyridinium
response and their performance in solution-processed OLEDs. chloride to give 2, after which a Williamson ether reaction of the
To the best of our knowledge, the vast majority of known phenolic or catecholic product with 1-bromododecane furnished
Scheme 1 Synthetic route to the target complexes: (i) [Pd3(OAc)6]/K3PO4/HOCH2CH2OH; (ii) pyH+Clꢀ/D; (iii) C12H25Br/K2CO3/DMF; (iv) (a) Hg(OAc)2/
EtOH, (b) LiCl/MeOH; (v) K[AuCl4]/MeCN/CHCl3/D; (vi) Ar–CRC–H/CuI/Et3N/CH2Cl2.
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Scheme 2 Synthetic route to the phenylacetylenes: (i) CBr4/PPh3/CH2Cl2; (ii) (a) EtMgBr in THF, (b) NH4Cl.
the desired ligands. Smooth, high-yielding auration of such ligands (including two further complexes incorporating 4-ethynylpyridine
is only possible when they are first reacted with mercury(II) acetate as the monodentate ligand) was obtained.
to give a cyclomercurated intermediate 4, from which transmetalla-
tion with tetrachloroaurate(III) yielded the chlorogold(III)–CNC
X-ray single crystal structures of 5, 6a
complex, with a yield over two steps of 26% for 5 and 13% for
14.53 The required mono-, di- or -tri-alkoxyphenyl-substituted
alkynes were prepared from the corresponding benzaldehyde
using a Corey–Fuchs protocol (Scheme 2). In the case of 4-
alkoxyphenylbenzaldehyde and 3,4-dialkoxybenzaldehyde, the
precursors were the simple hydroxy analogues, whereas 3,4,5-
trialkoxyphenylbenzaldehyde was obtained starting from methyl
3,4,5-trihydroxybenzoate wherein the ester was reduced to the
benzyl alcohol before being re-oxidised to the aldehyde. The gold
acetylides were prepared by the copper-mediated metathesis of the
chloro ligand of complexes 5 and 14 with the alkyne (Scheme 1,
step (vi)). Using this approach, a family of 26 new complexes
and 6b
For complexes 5, 6a and 6b it was possible to obtain crystals of
sufficient quality to allow structure determination by X-ray
methods and the crystallographic data are compiled in Table S1
(ESI†). A brief description of the structure of 5 is given in the ESI,†
while those of 6a and 6b are described here. In both complexes
(Chart 2), the gold sits in an approximately square planar
environment where, for example in 6a, the C–Au–C angles within
the pincer ligand are 162.7(3)1 and the N–Au–C angles are about
177.6(3)1. There is also a small bend in the acetylenic bond such
that the Au–C–C angle is 173.2(6)1, the reason for which appears
evident when packing is taken into consideration. As shown in
Chart 2c there is an interdigitation of the alkyl chains which
apparently enables the space to one side of the phenylacetylide to
be filled by a second molecule of complex (Chart 2c).
Solution studies by NMR spectroscopy
Luminescent gold-acetylide prone to aggregation (e.g. through
gel formation) have been studied in solution by NMR spectro-
scopy at various concentrations, predominantly by Yam and
co-workers, with many of these studies being of complexes
bearing a 3,4,5-trialkoxyphenylacetylide co-ligand.42–44 Similar
Chart 2 (a) Molecular structure of complex 6a, (b) molecular structure of
complex 6b, (c) packing of complex 6a.
Fig. 1 Structure of 13a, showing the labelling system for the H-atoms.
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Fig. 2 1H NMR spectra of 13a at increasing concentration in CD2Cl2, showing the downfield shift of the aromatic protons on the pincer ligand.
effects have been observed by Suleymanova et al. for a cyclometal-
The 1H NMR spectra for 13a recorded in CD2Cl2 at a range of
lated platinum(II) complex studied by both 1H and 195Pt NMR concentrations are shown in Fig. 2 and 3. A downfield shift is
spectroscopy.54 In order to quantify this effect to determine if the observed for the aromatic protons on the pincer ligand (Ha, Hb,
aggregation might influence the photophysical properties of these Hc and Hd) with decreasing concentration. The absolute change
complexes, rigorous, concentration-dependent studies of 13a were in the chemical shift is small, with Dd approximately 0.07–
1
undertaken by H NMR and electronic spectroscopies. The label- 0.06 ppm on increasing the concentration from 5.0 ꢁ 10ꢀ5 mmol
ling of the hydrogen atoms in this complex is shown in Fig. 1.
to 1.4 ꢁ 10ꢀ2 mmol; nevertheless, the trend over this concentration
Fig. 3 1H NMR spectra of 13a at increasing concentrations in CD2Cl2, showing the change in the chemical shift of the O–CH2 hydrogen atoms of the
alkyl chains on both ring systems.
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Table 1 Transition temperatures and enthalpies of complexes 5 to 9,
featuring two chains on the CNC ligand
Complex
Transitionc
T (1C)
DH (kJ molꢀ1
)
5
Cr–Lam
Lam–Iso
Cr–Iso
91.9
159.8
71.2
45.3
53.3
68.2
57.0
69.0
101.8
(90.6)
102.5
95.9
101.8
67.2
75.7
103.8
71.8
37.4
28.6
57.2
1.9
12.0
8.2
6a
6b
Cr–Cr0
Cr0–Cr00
Cr00–Iso
Cr–Colh
Colh–Iso
Cr–Iso
(Iso–Colh)
Cr–Iso
Cr–Iso
Cr–Iso
Cr–Cr
Cr–Colh
Colh–I
Cr–Cr
6c
7a
16.3
1.7
38.2
(3.6)
32.2
34.8
34.4
12.3
45.3
5.3
7b
7c
7d
8a
Fig. 4 Proposed aggregate of 13a in concentrated solution showing a
‘back-to-back’ dimer formed through interfacial interactions. Relative
distances are exaggerated for clarity.
8b
7.9
Cr–Colh1
Colh1–Iso
(Colh2–Colh1
77.9
39.1
5.8
(1.2)
40.0
3.9
21.3
7.2
3.9
3.9
22.9
1.3
13.9
5.7a
2.9a
0.8
16.9
32.7
—
104.9
(50.6)
77.5
101.8
74.9
82.8
100.1
55.0
)
range is clear (Fig. 2). A similar trend is also observed in the
resonances for the O–CH2 protons of the alkyl chains on the
cyclometallating ring, Hf and Hg (Fig. 3), although the shifts were
smaller with Dd between 0.02 and 0.04 ppm.
These data are consistent with a ‘back-to-back’ self-assembly
in solution where there is overlap of the rigid aromatic cores of
the complex, resulting in a shielding of the protons of the
C^N^C ligand, and where the phenylalkynyl moiety is free of
associations (Fig. 4). Similar self-association has been postu-
lated previously in a number of systems.20,55
The resonances for the acetylide ligand showed only a very
small and opposite shift (Hh and Hi) or no concentration
dependence (He). This would appear to indicate that any
intermolecular process involving this region of the complex is
different from that mediated by the C^N^C ligands.
b
8c
Cr–Colh
Colh–Iso
8d
Cr–Cr0
Cr0–Colh
Colh–I
9a
9b
Cr–Colr
Colr–I
Cr–Cr
Cr–Colr
Colr–Colh
Colh–I
80.7
64.6
79.8
88.3a
96.7a
57.8
9c
Cr–Cr
Cr–Colr
Colr–Colh
Colh–Iso
Cr–Cr
Cr–Colh
Colh–Iso
(Colh–Colr)
78.1
95.7
102.1
52.7
77.9
88.0
(77.8)
9d
2.2
53.4
3.0
(2.9)
a
b
Consideration of the data in Fig. 2 and 3 shows clearly that the
change in chemical shift ceases as the concentration decreases
below approximately 1 ꢁ 10ꢀ4 M. Electronic spectroscopy below
this concentration regime (see below) shows no change in the
absorption profile, positions of maxima, or molar extinction
coefficients for any of the observed transitions, nor the appearance
of any new absorption bands. This is consistent with the lack of
Observed on second heating cycle. Overlapping Cr–Cr and Cr–Colh
c
peaks. Cr = crystal; Iso = isotropic fluid; Lam = lamellar; Colh
hexagonal columnar phase.
–
Complex 5 shows an ordered lamellar phase between 91.9 and
159.8 1C as identified by SAXS data (d-spacing of 30.7 Å,
Fig. S3a, ESI†), with microscopy showing a texture lacking in
identifiable defects (Fig. S3b, ESI†). A broad and weak reflection
at 8.9 Å was assigned tentatively as a gold–gold separation, but
the presence of several other wide-angle reflections most likely
indicates a degree of order in the phase consistent with crystal-
line order, a conclusion supported by the large clearing
1
change in the H NMR spectra and therefore under these dilute
conditions the extent of aggregation is minimal.
Liquid-crystalline behaviour of the new
complexes
enthalpy of 26.8 kJ molꢀ1
.
Of complexes 6 and 7, only 6c and 7a show a mesophase
(Colh), which is enantiotropic in the former and monotropic in
the latter, although interestingly it is more stable in 7a than in
6c. However, when the coverage of the 2D surface of the
complex increases with two (8) or three (9) terminal chains on
the phenylacetylide, then all of the complexes show enantio-
tropic Colh and/or Colr phases. In general, the phases of these
complexes did not give rise to good optical textures and in
The liquid crystal properties of the new complexes were studied
using polarised optical microscopy, differential scanning
calorimetry (DSC) and small-angle X-ray scattering (SAXS). They
are considered first as two groups – with two (5 to 9) and then
with four (10 to 13) terminal chains on the pincer ligand.
Complexes with two terminal chains on the pincer ligand (5 to 9)
The transition temperatures for complexes 5 to 9 are given in several cases the SAXS data did not show the higher-order
Table 1, while the SAXS data are collected in Table S2 (ESI†). reflections that would confirm the identity (Fig. S4, ESI†).
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Table 2 Transition temperatures and enthalpies of complexes 10–13,
featuring four chains on the pincer ligand
Complex
Transition
T (1C)
DH (kJ molꢀ1
)
10a
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Colh
Colh–Ia
Cr–Cr
87.7
105.9
74.0
179.8
71.4
185.0
83.9
172.1
77.4
163.2
77.3
147.8
72.2
189.4
83.0
191.2
81.0
179.3
77.0
170.7
80.0
154.5
44.2
53.4
187.2
58.9
195.1
71.2
183.9
(39.4)
89.0
57.7
—
28.8
—
62.8
—
58.7
—
75.6
—
91.2
—
68.0
—
85.3
—
82.1
—
73.3
—
103.2
—
50.8
97.8
—
62.1
4.3
75.1
5.9
(101.2)
118.9
2.8
10b
10c
11a
11b
11c
11d
Fig. 5 Transition temperatures and phases for 6 to 8; the melting point of 12a
7a is shown as a black bar (phase is monotropic).
12b
12c
In these cases, it was necessary to identify the phase through
miscibility with a complex whose mesophases were charac- 12d
terised unequivocally by SAXS.
Complexes of 8, which have a 3,4-dialkoxyphenylethynyl co-
ligand, all show columnar hexagonal phases (d10 and d11
reflections seen in the SAXS data) with the melting and clearing
points varying little with terminal chain length to give meso-
phase ranges of between 18 and E30 1C. Interestingly, complex
8b showed a more complex mesomorphism with up to three
Colh phases being observed, two of which appear monotropic.
13a
Cr–Colh
Colh–Ia
Cr–Colh
Colh–I
Cr–Colh
Colh–I
(Colh–Lam)
Cr–Colh
Colh–I
13b
13c
13d
166.0
For complexes 9, the first three in the series (9a–9c) showed a
a
Not observed by DSC.
Colr phase on heating, above which there was a Colh phase for
9b and 9c; complex 9d showed a Colh phase with a Colr phase
appearing monotropically. In all cases the nature of the phase
was confirmed by SAXS data. Complexes 9b–9d showed some
solid-state polymorphism and all melted to the first mesophase
at around 78 1C, whereas in 9a melting occurred at the much
lower temperature of 55 1C. Clearing points increased signifi-
cantly from 9a to 9c (81 to 102 1C) and then dropped in 9d
(88 1C). The behaviour of these complexes is displayed in Fig. 5.
Complexes with four terminal chains on the pincer ligand (10 to 13)
In contrast to complexes 5 to 9, the phase behaviour of
complexes 10 to 13 with four chains on the cyclometallating
ligand is rather uniform, inasmuch as they all show a columnar
hexagonal phase. Data on transition temperatures are found in
Table 2 and are plotted in Fig. 6. The phases were characterised
by SAXS data, which showed characteristic d10, d11 and, in most
cases, d20 reflections (Fig. 7a and Table S3, ESI†) and by their
characteristic optical textures (Fig. 7b).
Fig. 6 Transition temperatures and phases for complexes 10 to 13.
The first point of note is that the mesophases are signifi-
cantly more stable in complexes 10 to 13 with clearing points in all of the other complexes, the V-like area defined by the
up to 100 1C higher than in complexes 5 to 9 (Fig. S5, ESI†). This pincer ligand and the chains in the 4-positions is filled giving a
is understood by the observation that the periphery of the more disk-like unit that can pack effectively. In general within
complexes is better covered by flexible chains (four chains on complexes 11, 12 and 13, there is a decrease in mesophase
the pincer ligand) which will in turn give a greater ‘cohesion’ of stability as the chains on the phenylacetylide group increase in
the complexes within the columns, so increasing mesophase length, with a surprisingly enhanced stability observed for 11d.
stability. This explains the depressed phase stability of 10a, as Melting points in complexes 11 and 12 vary little, although in
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Fig. 7 (a) SAXS pattern for 10b at 107 1C in the Colh phase; (b) photomicrograph of 10b on cooling at 167 1C in the Colh phase.
13 they increase monotonically with the increasing chain
length. Interestingly, 12c shows a fairly strongly monotropic
phase, identified as lamellar, consistent with the observation of
a single reflection in the SAXS data.
Considering the structural data obtained from SAXS, some
differences between the two families of complexes are evident.
Thus, the hexagonal lattice parameter, a, which represents the
intercolumnar distance, is consistently greater for complexes 8
and 9 than for four-chain analogues 12 and 13. Thus, for 8 and
9, a varies between 36 Å (n = 8) and ca. 38.5 Å (n = 14) whereas
for 12 that range is 29.4 to 31.6 Å and for 13 it is 31 to 33 Å. Note
that in all cases there is little dependence on the terminal chain
length. To account for this difference, it is proposed that
complexes 8 and 9 are more akin to a formal half-disk and so
would be likely to associate in a back-to-back fashion similar to
the arrangement shown in Fig. 4. This would help fill space
better and is a motif that has commonly been proposed for
Fig. 8 SAXS pattern of 12b in the isotropic liquid showing a broad
reflection in the small-angle region.
half-disk materials previously.56–58 However, for 12 and 13, the
extra two chains on the pincer make the complex more intrin-
sically disk-like so that the repeat unit is effectively a single
complex, which is a more compact arrangement, consistent
with the structural data. Thus, the solution behaviour does not
predict the organisation in the mesophase.
cubic and TGB phases as described by Goodby et al.59 where a
very similar situation of lattice melting is described. This topic
has more recently been re-visited.60
An interesting observation was that with the exception of
13b, 13c and 13d, none of the complexes 10–13 showed a
clearing event by differential scanning calorimetry (DSC), even
though by microscopy the transition was sharp and readily
observable. Where a transition enthalpy could be measured it
Photophysical measurements
Absorption spectra
was small, particularly for columnar phases, with values of 4.3, The UV-vis absorption spectra of the complexes were recorded
5.9 and 2.8 kJ molꢀ1 being found for 13b, 13c and 13d, in CH2Cl2 at room temperature; absorption maxima and corres-
respectively, comparable to the data found in complexes 6–9 ponding extinction coefficients are summarised in Table 3. The
(3.9 to 5.8 kJ molꢀ1). These data suggest that the columnar spectra fall into two categories, those with two chains on the
phase is quite disordered. Interestingly, SAXS data recorded in pincer ligand (6 to 9) and those with four (10 to 13). Within
the isotropic phase (Fig. 8) showed a broad reflection at around these two groups, the spectral profiles are effectively indepen-
3.51 2y, which corresponds to a distance of ca. 25 Å. Taken dent of the identity of the arylacetylide ligand.
together, these observations suggest that at the clearing point
The effect of the different number of chains on the pincer
there is a second-order melting of the hexagonal lattice to give a ligand is exemplified in Fig. 9, which shows the spectra of 6c
formally isotropic phase in which the columns persist but lose and 10c as representative examples with two and four chains,
their long-range correlation. This is analogous to the situation respectively. In 6c, there are three main absorptions, each
found at the clearing points of, for example, chiral nematic, apparently comprising of a pair of closely separated bands.
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Table 3 Photophysical data for complexes 6 to 13a
Emission 77 Kf
P
t/nsc degassed
[aerated]
knr /
kQ(O2)e/
d
Emission jlum
l
ꢁ
Complex Absorption lmax/nm (e/Mꢀ1 cmꢀ1
)
max/nm 102b
krd/103 sꢀ1 103 sꢀ1 109 Mꢀ1 sꢀ1
lmax/nm
t/ms
6a
260 (57 600), 272 (55 200), 310 (22 100), 502, 534 1.6
325 (22 800), 411 (8300), 430 (8100)
13000 [380]
9200 [470]
7300 [460]
7000 [410]
8400 [400]
1.2
3.7
4.9
3.3
3.3
76
1.2
0.97
0.93
1.0
1.1
1.3
1.2
1.2
1.2
1.2
507, 544
515, 548
270
6c
263 (61 900), 272 (59 000), 313 (24 600), 501, 533 3.4
324 (25 300), 412 (8720), 430 (8870)
264 (69 000), 272 (68 900), 314 (26 000), 501, 532 3.6
326 (27 800), 411 (6890), 431 (7120)
266 (67 600), 272 (70 400), 312 (25 700), 501, 533 2.3
325 (27 800), 410 (9120), 431 (9040)
266sh (66 400), 272 (69 600), 312 (22 500), 502, 534 2.7
325 (25 500), 410 (6800), 431 (6220)
259 (61 700), 282 (44 900), 300sh (20 400), 553, 581sh 36
340 (17 600), 445 (4880)
259 (62 800), 279sh (44 000), 340 (20 000), 551, 579 19
440 (2590)
260 (57 200), 282sh (43 200), 341 (22 000), 554, 582 34
444 (3530)
262 (65 100), 297sh (38 600), 342 (23 100), 552, 581 34
448 (5080)
262 (59 500), 300sh (36 400), 341 (23 000), 552, 579 20
448 (3560)
100
130
140
120
4.3
14
410/160g
(2 : 1)
7a
511, 548, 691sh 290
8a
511, 552, 594sh 430/160g
(2 : 1)
525, 556
9a
300/120g
(2 : 1)
230
10a
10c
11a
12a
13a
a
150000 [350] 2.4
60000 [390] 3.1
565, 611
565, 610, 661sh 240
565, 612, 661sh 220
100000 [390] 3.4
120000 [390] 2.9
6.6
5.5
10
559, 600, 649 260/120g
(4 : 1)
562, 603, 659 200
77000 [390]
2.6
b
c
In degassed CH2Cl2 at 295 ꢂ 3 K, except where indicated otherwise. Quantum yields measured relative to [Ru(bipy)3]Cl2(aq). In degassed
P
d
solution; values in air-equilibrated solution are given in parenthesis. Radiative kr and non-radiative knr rate constants estimated from quantum
e
f
yield and lifetime: kr = f/t, knr = (1 ꢀ f)/t. Bimolecular Stern–Volmer constant for quenching by molecular oxygen. In diethyl ether–isopentane–
g
ethanol (2 : 2 : 1 v/v). Poor fit to mono-exponential kinetics.
The lowest-energy absorption comprises a band centered at
430 nm (e = 8870 Mꢀ1 cmꢀ1) and a second transition at 412 nm
of a similar intensity. There are no such bands in the visible
region for the proligands, and these bands can be attributed to
charge-transfer transitions arising from the introduction of the
metal. (Insight into the orbital parentage of transitions giving
rise to them is provided by TD-DFT calculations, which are
discussed further below: they show that the transitions can be
considered to be predominantly LLCT in nature owing to a low
contribution of the metal.) At higher energy in the UV region,
there are two further, more intense pairs: 324 and 313 nm (e =
25 300 and 24 600 Mꢀ1 cmꢀ1), and 272 and 263 nm (e = 59 000
and 61 900 Mꢀ1 cmꢀ1). The energy separation within the pairs
is around 1100–1300 cmꢀ1, a value consistent with the CQN
and CQC vibrational stretching frequencies of the diphenyl-
pyridine ligand (i.e. excitation to the first and second
vibrational levels of the excited states being resolved), and in
line with previous studies.20,23,41,53,61
The spectra of the complexes with four chains on the pincer
ligand are subtly different in that the lowest-energy band is red-
shifted, its molar absorptivity is reduced significantly and the
vibrational structure in all the bands is lost, perhaps indicative
of more effective coupling of the excited state with lower-energy
vibrations. For 10c as an example (Fig. 9), the lowest energy
band is red-shifted to 440 nm (e = 2590 Mꢀ1 cmꢀ1) with other,
more intense absorptions again at shorter wavelengths 340 (e =
20 000 Mꢀ1 cmꢀ1), 279sh (e = 44 000 Mꢀ1 cmꢀ1) and 259 (e =
62 800 Mꢀ1 cmꢀ1) nm.
Photoluminescence
The gold(III) precursor complexes 5 and 14 that contain chloride
Fig. 9 UV-visible absorption spectra of (a) 6c and (b) 10c in CH2Cl2 at
room temperature.
as the monodentate ligand showed no detectable luminescence
1294 | J. Mater. Chem. C, 2021, 9, 1287--1302
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at room temperature. This lack of emission is consistent with structure, with the 0,0 component being the most intense, at
literature precedent, which attributes it to deactivation through 501 ꢂ 1 nm, and with a vibrational progression of approxi-
low-lying d–d excited states.53 In contrast, all of the acetylide mately 1200 cmꢀ1. Given that this band is independent of
derivatives 6 to 13 were phosphorescent in dichloromethane substituents in the alkyne, and with reference to the computational
solution at room temperature, appearing green (6–9) or yellow work (below) and literature precedent, the emission is assigned as
(10–13). The promotion of the emission through the replace- arising from a metal-perturbed 3[p - p*] intraligand transition
ment of a chloride ligand by an acetylide is consistent with the based on the diphenylpyridine ligand.20–22,31,41,62 The temporal
d–d states being raised in energy by the strongly s-donating decay of the luminescence fits well to monoexponential
nature of the acetylide, as also found for many platinum(II) kinetics, and the lifetimes are around 8 ms (Table 3). The
complexes such as those of the form [Pt(tpy)X]+. Table 3 lists emission is strongly quenched in air-equilibrated solution,
photophysical data for only a single chain length of each and lifetimes are reduced under these conditions to around
complex type (i.e. the a series where n = 8, substituent(s) = 400 ns. These observations are quite typical of many AuIII and
–OC8H17) as there was no discernible effect of changing the PtII complexes that emit from triplet excited states. The quan-
alkoxy chain length on the emission properties. Spectra and tum yields are around 3%, significantly higher than the related
luminescence lifetimes have also been recorded at 77 K in a unsubstituted complexes reported by Wong et al.,41 and are
transparent glass of composition diethyl ether/isopentane/etha- comparable to those of more recently designed gold(III)
nol (2 : 2 : 1 v/v; referred to as EPA).
complexes.20,23,28,34,36,37,41,43 There was no significant change
Complexes 6 to 9 display essentially identical emission in the emission spectral profiles, nor any significant decrease
spectra to one another in deoxygenated CH2Cl2 solution at in the lifetimes, on increasing the concentration by an order of
295 K (Fig. 10 and Table 3). The spectra show clear vibrational magnitude (to around 2 ꢁ 10ꢀ4 M). Any aggregation or
Fig. 10 Absorption spectra (black lines), excitation (dashed green lines), and emission spectra (red lines) for the two-chain complexes (a) 6c, (b) 7a, (c) 8a
and (d) 9a in CH2Cl2 at 295 K. The emission spectra at 77 K in EPA are also shown (blue lines).
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intermolecular interactions in the ground or excited state are thus red shift. This is unexpected as most metal complexes tend to
apparently minimal over this range. Only weak and poorly defined exhibit a hypsochromic shift on cooling due to the rigidochromic
emission was detectable in the (neat) solid state, as commonly effect, although that is typically most pronounced for complexes
found for many planar aromatic fluorophores, and also for triplet- that emit from MLCT states.63 One possible explanation is the
emitting metal complexes, where microsecond lifetimes facilitate formation of small clusters under low-temperature conditions,
intermolecular excited state annihilation processes.
through metal–metal interactions: such entropically disfavoured
Although the emission properties are similar for all five interactions will tend to stabilise the emissive state and thus lead
complexes (6c and 6a–9a), close inspection of the data in to an apparent red-shift. The temporal decay of emission at 77 K
Table 3 suggests a subtle difference for complex 6a, which shows rather poor fits to mono-exponential kinetics; this observa-
lacks substituents in the alkynylbenzene unit. It has a rather tion would also be consistent with formation of cluster-like
longer lifetime but a lower quantum yield than 6c or 7a–9a. species through intermolecular interactions. However, it might
Assuming that the emissive state is formed with unit efficiency, also reflect emission from non-equilibrated, close-lying excited
P
the radiative and non-radiative rate constants, kr and knr, can states. Nevertheless, the long lifetimes of several hundred micro-
be estimated from the quantum yields and lifetimes (Table 3). seconds for the major components suggest that the emission
These data suggest that kr is increased by a factor of around 2–4 emanates from states that are probably quite heavily localised on
P
upon introduction of the substituents, whilst
modestly increased, leading overall to the superior quantum the metal.
yields displayed by the substituted complexes.
knr is only the ligand, with only a relatively modest contribution from
For the five complexes that incorporate four chains on
At 77 K (Fig. 10 and Table 3), the vibrational structure the diphenylpyridine ligand (10c, 10a–13a), the emission spec-
becomes more pronounced, and there is a small but definitive tra are similar to one another (Fig. 11 and Table 3). Their
Fig. 11 Absorption spectra (black lines), excitation (dashed green lines), and emission spectra (red lines) for the four-chain complexes (a) 10c, (b) 11a, (c)
12a, and (d) 13a, in CH2Cl2 at 295 K. The emission spectra at 77 K in EPA are also shown (blue lines).
1296 | J. Mater. Chem. C, 2021, 9, 1287--1302
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Chart 3 Structures of model complexes with truncated alkoxy substituents used for calculations.
emission is red-shifted compared to the two-chain analogues, Computational chemistry
with lmax(0,0) increased to 553 ꢂ 2 nm. Vibrational structure is
Time-dependent density functional theory (TD-DFT) was employed
to rationalise the nature of the absorption and emission spectra of
the gold acetylide complexes. As the photophysical properties were
essentially independent of alkyl chain length, and to aid with
computational efficiency, these groups were substituted by either
ethyl or methoxy units for the purpose of the calculations. The
structures and numbering for these model complexes are shown
in Chart 3. The geometries of the complexes were optimised at the
BP86/SV(P) level and subsequent single-point energy and TD-DFT
calculations were performed using the PBE0 functional with the
def2-TZVPP basis set. The composition of the two lowest-energy
electronic transitions for each complex are presented in Table 4
and compared to the experimental absorption spectra.
again discernible, though a little less well-resolved than in
the two-chain complexes, as was also noted in the absorption
spectra. The most dramatic difference between the two series
of complexes is, however, the much higher luminescence
quantum yields – an order of magnitude greater, reaching
as much as 36%. These values are amongst the highest
values recorded in the literature for phosphorescent
emitters of this type (i.e. [Au(CNC)-acetylide]). Indeed, the
vast majority of such complexes of this general design
show PLQY values o10%21,30,32,42,64,65 and, very often,
o1%.20,23,28,34,36,37,41,43
The increase in quantum yields is accompanied by longer
P
lifetimes. Indeed, estimation of the kr and knr values (Table 3)
reveals that the enhanced performance is due to a reduction in
the non-radiative decay rate by an order of magnitude com-
pared to the two-chain series. The kr values, in contrast, are
essentially the same for both series. The beneficial effect of
inclusion of two further electron-donating alkoxy chains on the
pincer ligand, in reducing non-radiative decay processes, can
be rationalised readily. As noted, their inclusion decreases the
energy of the emissive excited state by around 2000 cmꢀ1 based
on the lmax(0,0) values. Thus, the activation energy to populate
the deactivating d–d states will increase by approximately this
magnitude, essentially cutting off this pathway of deactivation
at room temperature and leading to the enhanced quantum
yields. Amongst the five complexes, there are some small
variations in j and t, with 11a, 12a and the unsubstituted
10a showing the highest values.
Table 4 Summary of DFT-predicted transitions for simplified versions of
complexes 6 to 13. Only transitions with an oscillator strength 40.001 and
orbital contributions 410% are listeda
Calculated energy/
Oscillator Transition
Complex 103 cmꢀ1 (lcalc|lexp)/nm strength (% coefficient)
6-H
24.7 (406|430)
25.1 (398|411)
24.0 (417|430)
25.1 (398|412)
22.7 (440|431)
25.1 (397|411)
22.7 (441|431)
25.1 (398|410)
22.5 (444|431)
25.1 (398|410)
22.9 (438|445)
0.036
0.148
0.0476
0.148
0.057
0.143
0.056
0.142
0.047
0.141
0.0707
HOMO - LUMO (94)
HOMOꢀ1 - LUMO (96)
HOMO - LUMO (95)
HOMOꢀ1 - LUMO (95)
HOMO - LUMO (96)
HOMOꢀ1 - LUMO (95)
HOMO - LUMO (96)
HOMOꢀ1 - LUMO (95)
HOMO - LUMO (96)
HOMOꢀ2 - LUMO (95)
HOMOꢀ1 - LUMO (50)
HOMO - LUMO (48)
HOMO - LUMO (48)
HOMOꢀ1 - LUMO (46)
HOMOꢀ1 - LUMO (93)
HOMO - LUMO (90)
HOMOꢀ1 - LUMO (92)
HOMO - LUMO (91)
HOMOꢀ1 - LUMO (95)
HOMO - LUMO (93)
HOMO - LUMO (93)
HOMOꢀ1 - LUMO (94)
6-Et
7-OMe
8-OMe
9-OMe
10-H
At 77 K in EPA glass, the vibrational structure becomes
better resolved. Again, there is an unexpected, small but
definite red shift in the spectra compared to those recorded
in solution at 295 K. The emission decay at 77 K for these four-
chain complexes fits monoexponential kinetics well (with the
possible exception of 12a where a minor shorter component is
apparent). The measured lifetimes under these conditions
(Table 3) are similar to those of the two-chain series: at
this temperature, non-radiative decay via higher-lying d–d
states will evidently be ruled out even for the two-chain
complexes.
24.2 (414|445)
0.0352
10-Et
22.5 (445|440)
23.9 (419|440)
0.0545
0.0404
0.0516
0.0419
0.0520
0.0548
0.00290
0.0533
11-OMe 22.5 (445|444)
22.8 (439|444)
12-OMe 22.5 (445|448)
22.9 (437|448)
13-OMe 22.1 (452|448)
22.5 (444|448)
a
Whilst an approximation, the experimental energy of S1 is taken to be
the lowest-energy absorption maximum.
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Fig. 12 Calculated Kohn–Sham orbitals for complex 6H, isosurface set to the 0.005 level.
The resulting calculations of first singlet excited-state ener-
In addition to using TD-DFT to determine the singlet
gies are a good match to the experimental data. For the two- excitations, the geometries and energies of the triplet states
chain complexes 6–9, two discrete, low-energy transitions of complexes 6-H and 10-H were calculated.‡ Subtraction of the
corresponding to LUMO ’ HOMO and LUMO ’ HOMOꢀ1 energy of the corresponding ground state singlet complexes
are predicted, separated by between 400 and 2600 cmꢀ1. This then allowed for the predicted energy of emission to be
might be represented in the observed splitting of the lowest determined. For 6-H, this was 500 nm, which is in excellent
energy band in the experimental spectra, although this agreement with the experimental value of 502 nm. Likewise, for
observed structure could alternatively be due to excitation to 10-H, the calculated and experimental values are 548 and
a vibrationally excited level of S1.
553 nm respectively.
Examination of the Kohn–Sham orbitals (Fig. 12) indicates
that the HOMOꢀ1 is based on the pincer ligand, whereas the
HOMO is centered on the acetylide with some gold character.
The LUMO is located primarily on the pyridyl ring of the pincer
ligand. The energy of the LUMO ’ HOMO transition is
reduced by ca. 2000 cmꢀ1 upon incorporation of donor groups
into the acetylide ligand (7-OMe, 8-OMe and 9-OMe), which
probably arises from an increase in the HOMO energy due to
the influence of the donor groups (Fig. 13), and is consistent
with the experimentally observed red-shift in the lowest-energy
absorption band.
In the case of the four-chain complexes 10–13, the addi-
tional alkoxy substituent to the backbone of the CNC ligand
raises the energy of the HOMOꢀ1 orbital (Fig. 13) resulting in
the LUMO ’ HOMOꢀ1 transition being red shifted by between
2200 and 2700 cmꢀ1 when compared to the two-chain
analogues.
Device measurements
Cyclic voltammograms (CV) were recorded to explore the elec-
trochemical properties of the gold complexes and to allow
evaluation of the HOMO and LUMO levels. Data for complexes
6a and 10 are shown in Fig. S5 (ESI†) although experiments
performed on other complexes in both series gave the same
results; only quasi-reversible reduction potentials were detected
for both complexes. Thus, complex 6a displays a reduction wave
with an onset potential of ꢀ1.70 V, while complex 10a presents
a reduction with an onset potential of ꢀ1.94 V (both vs. Fc/Fc+ =
0.0 V). No oxidation process was observed in the accessible
window. The energy of the LUMO can be estimated directly
from the reduction potential according to the method of Pan
et al.,66 giving values of ꢀ2.72 and ꢀ2.48 V for 6a and 10a,
respectively. In the absence of
a measurable oxidation
potential, an estimate of the HOMO energy can be obtained
by subtracting the optical band gap (in turn estimated from the
absorption onset) giving values of ꢀ5.41 and ꢀ4.96 V. The
higher-lying HOMO of 10a is consistent with the calculations
(Fig. 13) and the electron-donating nature of the additional
alkoxy chains. These data informed the choice of materials for
device fabrication (below).
As noted in the introduction, one potential advantage of
liquid-crystalline emitters is that the ordered mesophase offers
the possibility for preferred conduction pathways in discotic
materials,67 which could have a beneficial effect on device
‡ The nature of the triplet states in gold(III) pincer compounds has been
investigated extensively (Chem. Sci., 2015, 6, 3026–3037, Inorg. Chem., 2015, 54,
3624–3630). Although such an approach is beyond the scope of the current study
there is a good agreement between our experimental and calculated emission
values.
Fig. 13 Energies of frontier molecular orbitals for all species calculated
using the PBE0 functional with the def2-TZVPP basis set.
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Table 6 Optimised device data at complex loadings of 5, 10 and 15% in a
host of PVK : OXD-7 (7 : 3)
Emission peak/
Lmax
/
CEmax/ PEmax
/
nm (CIE
Complex wt% Von/V cd mꢀ2 cd Aꢀ1 lm Wꢀ1 EQEmax coordinates)
6a
5
4.0
1458
1378
1004
22.01 12.51
22.32 12.71
6.72
6.71
4.58
508 (0.29, 0.58)
510 (0.31, 0.57)
510 (0.31, 0.58)
10 4.4
15 4.4
15.23
8.37
6c
5
4.4
1259
1370
734
21.41 12.67
6.58
5.24
2.57
508 (0.31, 0.57)
508 (0.31, 0.56)
508 (0.31, 0.57)
10 4.4
15 6.0
17.31
8.29
9.69
3.34
8d
5
4.0
1233
1229
304
20.14 10.97
6.27
4.25
0.72
506 (0.32, 0.57)
508 (0.31, 0.57)
508 (0.30, 0.58)
10 4.4
15 6.8
13.76
2.38
6.97
0.65
10c
11a
12d
13d
5
3.6
1765
1441
1142
22.53 13.41
21.18 11.73
7.14
6.94
3.14
554 (0.47, 0.52)
556 (0.47, 0.51)
558 (0.48, 0.51)
10 4.0
15 4.8
9.71
4.00
5
4.0
1705
1768
1261
21.31 12.45
21.45 11.21
6.82
6.74
5.57
554 (0.47, 0.51)
556 (0.47, 0.51)
556 (0.47, 0.51)
Fig. 14 Plot of current density as a function of applied voltage for
complexes 8d, 12d and 13d in the as-cast state (open circles) and after
annealing in the liquid crystal phase (closed diamonds).
10 4.4
15 4.8
17.59
8.92
5
4.4
1401
867
494
17.14
7.62
4.45
8.94
3.17
1.62
5.52
2.50
1.46
554 (0.47, 0.51)
556 (0.47, 0.51)
556 (0.47, 0.51)
10 5.2
15 6.4
characteristics such as drive voltage. In order to test that
assertion, the electron mobilities of three complexes (8d, 12d
and 13d) were evaluated both in the pristine state and after
annealing in the liquid-crystalline state. The measurements
were made using the SCLC method (see ESI†) in a device
structure of ITO/PEDOT (30 nm)/Au complex (ca. 100 nm)/
MoO3 (8 nm)/Al (100 nm) and TIO/ZnO (30 nm)/Au complex
(ca. 100 nm)/PFN-Br (5 nm)/Al (100 nm). The resulting current–
voltage (IV) characteristics under pulse conditions are shown in
Fig. 14 and the mobilities are found as Table 5.
The first important point about these data is that they show
that following annealing the electron mobility increases. The
process of annealing moved the material into the liquid crystal
state and hence into a columnar arrangement giving better
overlap of the complex p-systems. Of course, the annealing
process does not create a monodomain sample and so the
alignment is by no means perfect, but nonetheless the data
demonstrate unequivocally that the mobility is enhanced as a
result of liquid crystal organisation. It is then noteworthy that
the drive voltage required for complex 8d is significantly greater
than those for 12d and 13d. This observation is consistent with
the intercolumnar separations obtained from the SAXS data,
which imply that in complexes such as 8d, where there are only
5
4.4
1191
1138
911
18.04 10.10
5.69
4.06
3.66
554 (0.46, 0.51)
554 (0.47, 0.52)
554 (0.46, 0.52)
10 4.4
15 4.4
13.14
11.75
7.28
6.58
two alkoxy chains on the pincer ligand, there is back-to-back
overlap of the pincer ligands in the columnar structure. The
data also imply that for more inherently disc-like materials
such as 12d and 13d, there is greater overlap of the complexes,
which would be consistent with the lower drive voltage.
Then to evaluate the electroluminescent (EL) properties of
these gold complexes, selected complexes were evaluated as the
emitter in OLED devices, chosen to represent the range of
complexes studied. Using solution processing, devices with
the structure ITO/PEDOT:PSS/emitters/DPEPO/TmPyPB/CsF/
Al§ were fabricated, with the emissive layer being a blend of
polymer host (PVK : OXD-7 = 7 : 3) and the gold complex, the
latter at concentrations of 5, 10 and 15 wt%. The device
configuration and the molecular structures of the materials
in the device are shown in Fig. S6 (ESI†), and the electrolumi-
nescence data obtained are collected in Table 6.
After optimisation, devices with 5 wt% of gold complex were
found to give the best results (Fig. S8, ESI†) and so the
electroluminescent (EL) properties are discussed in detail for
this loading. Thus, all the devices exhibit intense emission in
the range 400–800 nm (Fig. 15a) and it is of note that the EL
emission is dependent on the number of alkoxy chains on the
Table 5 Electron mobility data for 8d, 12d and 13d in the as-cast state
(open circles) and after annealing in the liquid crystal phase
Mobility/
Thickness/
nm
Processing
temp./1C
Sample
Condition
cm2 Vꢀ1 sꢀ1
§ Anode
= indium tin oxide; hole injection layer = PEDOT:PSS (poly(3,4-
8d
As cast
Annealed
As cast
Annealed
As cast
Annealed
1.69 ꢁ 10ꢀ4
4.05 ꢁ 10ꢀ4
4.68 ꢁ 10ꢀ4
6.19 ꢁ 10ꢀ4
3.95 ꢁ 10ꢀ4
5.89 ꢁ 10ꢀ4
120
100
80
25
90
25
120
25
120
ethylenedioxythiophene):polystyrene sulfonate); host = PVK (poly(9-vinylcarbazole))
and OXD-7 (1,3-bis[2-tert-butylphenyl)-1,3,5-oxadiazo-5-yl]benzene); hole/exciton
blocking layer = DPEPO (bis[2-(diphenylphosphino)phenyl]ether oxide); electron-
transport layer = TmPyPB (1,3,5-tris(3-pyridyl-3-phenyl)benzene); electron-injecting
layer = cesium fluoride; metal cathode = aluminium.
12d
13d
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Fig. 15 Device performance: (a) EL spectra with inset photograph of working device; (b) external quantum efficiency (EQE) as a function of current
density (c) CIE coordinates; (d) current density and luminance as a function of voltage.
pincer ligand. Devices based on complexes 6a, 6c and 8a show 10c (four-chain pincer ligand) showed the best device perfor-
emission maxima at about 508 and 538 nm, in addition to mance for each ‘family’. Thus, devices containing complex 6a
which there is an emission feature at ca. 458 nm attributed to gave an EQE of 6.7% (Fig. 15b), a luminance of 1458 cd mꢀ2
the host matrix and arising from incomplete energy transfer (Fig. 15d) and a current efficiency 22.0 cd Aꢀ1 (Fig. S8, ESI†),
between host and complex. On the other hand, the devices while for complex 10c the same parameters were EQE = 7.1%,
based on 10c, 11a, 12d and 13d show emission maxima at luminance = 1765 cd mꢀ2 and current efficiency = 22.5 cd Aꢀ1
.
554 nm with a shoulder at about 582 nm and here there is Device performance clearly decreases with the increased
complete energy transfer between host and the emitters as dopant concentrations because of the emission quenching at
evidenced by the absence of any higher-energy emissions the higher concentrations. This device performance can be
(Fig. 15a). Compared to complexes 6a, 6c and 8a, the presence considered to be competitive with similar gold(III) complexes
of the two additional chains on the pincer ligand leads to a red- for which values ranging from 0.9%32,36 to 15.3%28 are found,
shifted EL emission due to destabilisation of the HOMO, which the latter of which is the highest reported for a solution-
helps the energy matching with the host matrix.
processed device based on gold(III) emitters.
The CIE coordinates (Fig. 15c) of these devices place them in
the green and yellow region of the spectrum, as shown in Fig. 15a.
With the increased dopant concentration, there is negligible
change in the EL emission, implying the lack of formation of
excimers or aggregate that might emit at lower energy.
Conclusions
The incorporation of long alkoxy chains into the structure of
As shown in Table 6 all devices exhibit relatively low turn-on Au(III) pincer compounds has a profound effect on their physi-
voltages (at 1 cd mꢀ2) in the range of 3.6–6.2 V, and satisfactory cal properties, notably the nature and stability of the LC phases
performance was achieved for the devices. With fewer that are induced are tuned by the substitution pattern of these
flexible chains, complexes 6a (two-chain pincer ligand) and chains on the pincer ligand. Further, the same structural
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modifications to the both the primary and secondary coordina- for the preparation of some precursor ligands and Alice McEllin
tion sphere of the metal complex impart favourable photophy- for recording some cyclic voltammetry data.
sical properties. Thus, having two or four alkoxy chains on the
backbone of the pincer ligand results in a bathochromic-shift
in the absorption and emission wavelengths, reflecting the
influence of the alkoxy groups on the ligand’s electronic
structure. However, analysis of the photophysical data indicates
References
that modification of the secondary coordination sphere of the
metal by placing four alkoxy groups on the pincer ligand also
has a significant effect in reducing non-radiative decay pro-
cesses, reflected in the high quantum yield for emission (up to
36%) and the longer lifetimes in this series.
Electron mobility measurements demonstrate that, as
expected for disc-like materials in columnar mesophases, the
mobility is enhanced in the liquid crystal phase, but in this case
the optimum level of the emitter in the device matrix is low
(5%) so that that the beneficial conduction pathways are not
accessed. Nonetheless, coupled with the observation that elec-
troluminescent properties are certainly competitive with lead-
ing materials in this field, then modification of the primary and
secondary coordination sphere of the Au(III) pincer complexes
represents an important and dual-pronged strategy to develop
and control the materials and photophysical properties of this
class of complex.
1 X. Wu, M. Zhu, D. W. Bruce, W. Zhu and Y. Wang, J. Mater.
Chem. C, 2018, 6, 9848–9860.
2 V. N. Kozhevnikov, B. Donnio and D. W. Bruce, Angew.
Chem., Int. Ed., 2008, 47, 6286–6289.
3 V. N. Kozhevnikov, B. Donnio, B. Heinrich and D. W. Bruce,
Chem. Commun., 2014, 50, 14191–14193.
4 V. N. Kozhevnikov, B. Donnio, B. Heinrich, J. A. G. Williams
and D. W. Bruce, J. Mater. Chem. C, 2015, 3, 10177–10187.
5 C. T. Liao, H. H. Chen, H. F. Hsu, A. Poloek, H. H. Yeh,
Y. Chi, K. W. Wang, C. H. Lai, G. H. Lee, C. W. Shih and
P. T. Chou, Chem. – Eur. J., 2011, 17, 546–556.
6 A. Santoro, A. C. Whitwood, J. A. G. Williams, V. N.
Kozhevnikov and D. W. Bruce, Chem. Mater., 2009, 21,
3871–3882.
7 D. Pucci, I. Aiello, A. Bellusci, A. Crispini, I. De Franco,
M. Ghedini and M. La Deda, Chem. Commun., 2008,
2254–2256.
8 D. Pucci, G. Barberio, A. Crispini, O. Francescangeli, M. Ghedini
and M. La Deda, Eur. J. Inorg. Chem., 2003, 3649–3661.
9 N. S. S. Kumar, M. Z. Shafikov, A. C. Whitwood, B. Donnio,
P. B. Karadakov, V. N. Kozhevnikov and D. W. Bruce,
Chem. – Eur. J., 2016, 22, 8215–8233.
10 D. Pucci, G. Barberio, A. Bellusci, A. Crispini, B. Donnio,
L. Giorgini, M. Ghedini, M. La Deda and E. I. Szerb, Chem. –
Eur. J., 2006, 12, 6738–6747.
Experimental
All experimental data relating to synthesis, single-crystal X-ray
structures (cif and cifcheck files), physical measurements,
computational chemistry (including methodology and Carte-
sian coordinates) and device fabrication are found in the ESI.†
11 K. Binnemans, J. Mater. Chem., 2009, 19, 448–453.
12 A. M. Prokhorov, A. Santoro, J. A. G. Williams and
D. W. Bruce, Angew. Chem., Int. Ed., 2012, 51, 95–98.
Dedication
It is with great pleasure that this paper is dedicated to Professor 13 A. Santoro, A. M. Prokhorov, V. N. Kozhevnikov,
Emeritus Pablo Espinet from Valladolid, Spain. Liquid crystals
and gold have been two of Pablo’s abiding interests where he
A. C. Whitwood, B. Donnio, J. A. G. Williams and
D. W. Bruce, J. Am. Chem. Soc., 2011, 133, 5248–5251.
has made seminal contributions over many years. The subject 14 E. I. Szerb, A. M. Talarico, I. Aiello, A. Crispini, N. Godbert,
matter of this paper could not then be more appropriate as he
moves to his own golden era, within which we wish him good
health and great contentment, free from disorder!
D. Pucci, T. Pugliese and M. Ghedini, Eur. J. Inorg. Chem.,
2010, 3270–3277.
15 A. M. Talarico, M. Ghedini, C. O. Rossi and E. I. Szerb, Soft
Matter, 2012, 8, 11661–11669.
16 Y. Wang, C. P. Cabry, M. Xiao, L. Male, S. J. Cowling,
D. W. Bruce, J. Shi, W. Zhu and E. Baranoff, Chem. – Eur.
J., 2016, 22, 1618–1621.
17 X. Wu, G. Xie, C. P. Cabry, X. Xu, S. J. Cowling, D. W. Bruce,
W. Zhu, E. Baranoff and Y. Wang, J. Mater. Chem. C, 2018, 6,
3298–3309.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
18 M. O’Neill and S. M. Kelly, Adv. Mater., 2011, 23, 566–584.
´
´
We thank the University of York (RRP) and the EPSRC (compu- 19 A. Dıez, S. J. Cowling and D. W. Bruce, Chem. Commun.,
tational equipment used in this study, grants EP/H011455/1 2012, 48, 10298–10300.
and EP/K031589/1) for funding, Johnson Matthey for generous 20 V. W.-W. Yam, K. M.-C. Wong, L.-L. Hung and N. Zhu,
loans of gold salts, the Royal Society and the National Natural Angew. Chem., Int. Ed., 2005, 44, 3107–3110.
Science Foundation of China (51911530197, 51773021) for an 21 V. K.-M. Au, D. P.-K. Tsang, K. M.-C. Wong, M.-Y. Chan, N. Zhu
International Exchange Award (DWB & YW), Rebecca Howarth
and V. W.-W. Yam, Inorg. Chem., 2013, 52, 12713–12725.
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Journal of Materials Chemistry C
22 V. K.-M. Au, D. P.-K. Tsang, Y.-C. Wong, M.-Y. Chan and 46 C.-H. Lee, M.-C. Tang, F. K.-W. Kong, W.-L. Cheung, M. Ng,
V. W.-W. Yam, J. Organomet. Chem., 2015, 792, 109–116.
M.-Y. Chan and V. W.-W. Yam, J. Am. Chem. Soc., 2020, 142,
23 V. K.-M. Au, K. M.-C. Wong, D. P.-K. Tsang, M.-Y. Chan,
520–529.
N. Zhu and V. W.-W. Yam, J. Am. Chem. Soc., 2010, 132, 47 D. Zhou, W.-P. To, Y. Kwak, Y. Cho, G. Cheng, G. S. M. Tong
14273–14278. and C.-M. Che, Adv. Sci., 2019, 6, 1802297.
24 V. K.-M. Au, N. Zhu and V. W.-W. Yam, Inorg. Chem., 2013, 48 D. Zhou, W.-P. To, G. S. M. Tong, G. Cheng, L. Du,
52, 558–567.
D. L. Phillips and C.-M. Che, Angew. Chem., Int. Ed., 2020,
59, 6375–6382.
25 K. T. Chan, G. S. M. Tong, W.-P. To, C. Yang, L. Du,
D. L. Phillips and C.-M. Che, Chem. Sci., 2017, 8, 2352–2364. 49 H. Beucher, S. Kumar, E. Merino, W.-H. Hu, G. Stemmler,
´
26 G. Cheng, K. T. Chan, W.-P. To and C.-M. Che, Adv. Mater.,
2014, 26, 2540–2546.
S. Cuesta-Galisteo, J. A. Gonzalez, J. Jagielski, C.-J. Shih and
C. Nevado, Chem. Mater., 2020, 32, 1605–1611.
27 W.-L. Cheung, S.-L. Lai, M.-C. Tang, C.-H. Lee, M.-Y. Chan 50 L.-K. Li, M.-C. Tang, S.-L. Lai, M. Ng, W.-K. Kwok,
and V. W.-W. Yam, J. Mater. Chem. C, 2019, 7, 8457–8464.
28 C.-H. Lee, M.-C. Tang, W.-L. Cheung, S.-L. Lai, M.-Y. Chan
and V. W.-W. Yam, Chem. Sci., 2018, 9, 6228–6232.
29 C.-H. Lee, M.-C. Tang, Y.-C. Wong, M.-Y. Chan and
V. W.-W. Yam, J. Am. Chem. Soc., 2017, 139, 10539–10550.
30 S. K.-L. Siu, C. Y.-S. Chung and V. W.-W. Yam, J. Organomet.
Chem., 2017, 845, 177–188.
M.-Y. Chan and V. W.-W. Yam, Nat. Photonics, 2019, 13,
185–191.
51 S. Coco and P. Espinet, in Gold Chemistry, ed. F. Mohr,
Wiley, 2009, ch. 8, pp. 357–396.
´
52 R. Chico, E. de Domingo, C. Domınguez, B. Donnio,
B. Heinrich, R. Termine, A. Golemme, S. Coco and
P. Espinet, Chem. Mater., 2017, 29, 7587–7595.
31 S. K.-L. Siu, C. Po, K.-C. Yim, V. K.-M. Au and V. W.-W. Yam, 53 K.-H. Wong, K.-K. Cheung, M. C.-W. Chan and C.-M. Che,
CrystEngComm, 2015, 17, 8153–8162. Organometallics, 1998, 17, 3505–3511.
32 M.-C. Tang, C. K.-M. Chan, D. P.-K. Tsang, Y.-C. Wong, 54 A. F. Suleymanova, O. S. Eltsov, D. N. Kozhevnikov,
M. M.-Y. Chan, K. M.-C. Wong and V. W.-W. Yam, Chem. –
Eur. J., 2014, 20, 15233–15241.
A. O. Lantushenko, M. P. Evstigneev and V. N.
Kozhevnikov, ChemistrySelect, 2017, 2, 3353–3355.
33 M.-C. Tang, C.-H. Lee, M. Ng, Y.-C. Wong, M.-Y. Chan and 55 L. Ciano, N. Fey, C. J. V. Halliday, J. M. Lynam, L. M. Milner,
V. W.-W. Yam, Angew. Chem., Int. Ed., 2018, 57, 5463–5466.
N. Mistry, N. E. Pridmore, N. S. Townsend and
34 M.-C. Tang, M.-Y. Leung, S.-L. Lai, M. Ng, M.-Y. Chan and
A. C. Whitwood, Chem. Commun., 2015, 51, 9702–9705.
V. Wing-Wah Yam, J. Am. Chem. Soc., 2018, 140, 56 R. Atencio, J. Barbera, C. Cativiela, F. J. Lahoz, J. L. Serrano
13115–13124.
and M. M. Zurbano, J. Am. Chem. Soc., 1994, 116,
35 M.-C. Tang, D. P.-K. Tsang, M. M.-Y. Chan, K. M.-C. Wong
11558–11559.
and V. W.-W. Yam, Angew. Chem., Int. Ed., 2013, 52, 446–449. 57 C. K. Lai, A. G. Serrette and T. M. Swager, J. Am. Chem. Soc.,
36 M.-C. Tang, D. P.-K. Tsang, M.-Y. Chan, K. M.-C. Wong and
V. W.-W. Yam, Mater. Chem. Front., 2017, 1, 2559–2568.
37 M.-C. Tang, D. P.-K. Tsang, Y.-C. Wong, M.-Y. Chan,
K. M.-C. Wong and V. W.-W. Yam, J. Am. Chem. Soc., 2014,
136, 17861–17868.
1992, 114, 7948–7949.
58 F. Morale, R. W. Date, D. Guillon, D. W. Bruce, R. L. Finn,
C. Wilson, A. J. Blake, M. Schroder and B. Donnio, Chem. –
Eur. J., 2003, 9, 2484–2501.
59 J. W. Goodby, D. A. Dunmur and P. J. Collings, Liq. Cryst.,
1995, 19, 703–709.
¨
38 W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma, W.-M. Kwok,
X. Guan, K.-H. Low and C.-M. Che, Angew. Chem., Int. Ed., 60 J. W. Goodby, R. J. Mandle, E. J. Davis, T. Zhong and
2013, 52, 6648–6652. S. J. Cowling, Liq. Cryst., 2015, 42, 593–622.
39 W.-P. To, D. Zhou, G. S. M. Tong, G. Cheng, C. Yang and 61 V. K.-M. Au, K. M.-C. Wong, N. Zhu and V. W.-W. Yam, J. Am.
C.-M. Che, Angew. Chem., Int. Ed., 2017, 56, 14036–14041. Chem. Soc., 2009, 131, 9076–9085.
40 B. Y.-W. Wong, H.-L. Wong, Y.-C. Wong, M.-Y. Chan and 62 K. M.-C. Wong, X. Zhu, L.-L. Hung, N. Zhu, V. W.-W. Yam
V. W.-W. Yam, Angew. Chem., Int. Ed., 2017, 56, 302–305. and H.-S. Kwok, Chem. Commun., 2005, 2906–2908.
41 K. M.-C. Wong, L.-L. Hung, W. H. Lam, N. Zhu and 63 A. J. Lees, Comments Inorg. Chem., 1995, 17, 319–346.
V. W.-W. Yam, J. Am. Chem. Soc., 2007, 129, 4350–4365.
42 K.-C. Yim, V. K.-M. Au, L.-L. Hung, K. M.-C. Wong and
V. W.-W. Yam, Chem. – Eur. J., 2016, 22, 16258–16270.
43 K.-C. Yim, V. K.-M. Au, K. M.-C. Wong and V. W.-W. Yam,
Chem. – Eur. J., 2017, 23, 5772–5786.
44 K.-C. Yim, E. S.-H. Lam, K. M.-C. Wong, V. K.-M. Au,
C.-C. Ko, W. H. Lam and V. W.-W. Yam, Chem. – Eur. J.,
2014, 20, 9930–9939.
64 J. Fernandez-Cestau, B. Bertrand, A. Pintus and
M. Bochmann, Organometallics, 2017, 36, 3304–3312.
65 M.-C. Tang, C.-H. Lee, S.-L. Lai, M. Ng, M.-Y. Chan and
V. W.-W. Yam, J. Am. Chem. Soc., 2017, 139, 9341–9349.
66 L. Pan, B. Hu, X. Zhu, X. Chen, J. Shang, H. Tan, W. Xue,
Y. Zhu, G. Liu and R.-W. Li, J. Mater. Chem. C, 2013, 1,
4556–4564.
¨
67 T. Wohrle, I. Wurzbach, J. Kirres, A. Kostidou, N. Kapernaum,
45 D. Zhou, W.-P. To, Y. Kwak, Y. Cho, G. Cheng, G. S. M. Tong
and C.-M. Che, Adv. Sci., 2019, 0, 1802297.
J. Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giesselmann
and S. Laschat, Chem. Rev., 2016, 116, 1139–1241.
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