Tunable trimers: Using temperature and pressure to control luminescent emission in gold(i) pyrazolate-based trimers

Article abstract of DOI:10.1002/chem.201404058

A systematic investigation into the relationship between the solid-state luminescence and the intermolecular AuAu interactions in a series of pyrazolate-based gold(I) trimers; tris(μ₂-pyrazolato-N,N')-tri-gold(I) (1), tris(μ₂-3,4,5- trimethylpyrazolato-N,N')-tri-gold(I) (2), tris(μ₂-3-methyl-5- phenylpyrazolato-N,N')-tri-gold(I) (3) and tris(μ₂-3,5-diphenylpyrazolato- N,N')-tri-gold(I) (4) has been carried out using variable temperature and high pressure X-ray crystallography, solid-state emission spectroscopy, Raman spectroscopy and computational techniques. Single-crystal X-ray studies show that there is a significant reduction in the intertrimer AuAu distances both with decreasing temperature and increasing pressure. In the four complexes, the reduction in temperature from 293 to 100 K is accompanied by a reduction in the shortest intermolecular AuAu contacts of between 0.04 and 0.08 The solid-state luminescent emission spectra of 1 and 2 display a red shift with decreasing temperature or increasing pressure. Compound 3 does not emit under ambient conditions but displays increasingly red-shifted luminescence upon cooling or compression. Compound 4 remains emissionless, consistent with the absence of intermolecular AuAu interactions. The largest pressure induced shift in emission is observed in 2 with a red shift of approximately 630 cm^-1 per GPa between ambient and 3.80 GPa. The shifts in all the complexes can be correlated with changes in AuAu distance observed by diffraction.

Full text of DOI:10.1002/chem.201404058

DOI: 10.1002/chem.201404058
Full Paper
&
Gold Complexes
Tunable Trimers: Using Temperature and Pressure to Control
Luminescent Emission in Gold(I) Pyrazolate-Based Trimers
Christopher H. Woodall,[a] Sara Fuertes,[a] Christine M. Beavers,[b] Lauren E. Hatcher,[a]
Andrew Parlett,[a] Helena J. Shepherd,[a] Jeppe Christensen,[a] Simon J. Teat,[b]
Mourad Intissar,[c] Alexandre Rodrigue-Witchel,[c] Yan Suffren,[c] Christian Reber,*[c]
Christopher H. Hendon,[a] Davide Tiana,[a] Aron Walsh,[a] and Paul R. Raithby*[a]
Abstract: A systematic investigation into the relationship be-
tween the solid-state luminescence and the intermolecular
Au···Au interactions in a series of pyrazolate-based gold(I)
trimers; tris(m2-pyrazolato-N,N’)-tri-gold(I) (1), tris(m2-3,4,5-
trimethylpyrazolato-N,N’)-tri-gold(I) (2), tris(m2-3-methyl-5-
phenylpyrazolato-N,N’)-tri-gold(I) (3) and tris(m2-3,5-diphenyl-
pyrazolato-N,N’)-tri-gold(I) (4) has been carried out using var-
iable temperature and high pressure X-ray crystallography,
solid-state emission spectroscopy, Raman spectroscopy and
computational techniques. Single-crystal X-ray studies show
that there is a significant reduction in the intertrimer Au···Au
distances both with decreasing temperature and increasing
pressure. In the four complexes, the reduction in tempera-
ture from 293 to 100 K is accompanied by a reduction in the
shortest intermolecular Au···Au contacts of between 0.04
and 0.08 ꢀ. The solid-state luminescent emission spectra of
1 and 2 display a red shift with decreasing temperature or
increasing pressure. Compound 3 does not emit under
ambient conditions but displays increasingly red-shifted
luminescence upon cooling or compression. Compound 4
remains emissionless, consistent with the absence of inter-
molecular Au···Au interactions. The largest pressure induced
shift in emission is observed in 2 with a red shift of approxi-
mately 630 cmÀ1 per GPa between ambient and 3.80 GPa.
The shifts in all the complexes can be correlated with
changes in Au···Au distance observed by diffraction.
bonds.[1d] Aurophilic interactions are generally considered to
be less directional than conventional hydrogen bonds. Exten-
sive synthetic, structural, theoretical and photophysical studies
have been performed on a variety of gold(I) trimers including
pyrazolates, pyridinates, imidazolates, carbeniates and triazo-
lates, all of which consistently demonstrate the importance of
aurophilic interactions in the solid-state for the observations of
their photoluminescent properties.[2,3]
Introduction
Homoleptic gold(I) trimers are an intriguing class of photoem-
issive materials. They frequently possess bright phosphores-
cence and “hexamer” style molecular geometries that are com-
monly associated with the presence of Au···Au “aurophilic in-
teractions” that dominate the solid-state chemistry of gold(I).[1]
As a group of compounds they demonstrate many desirable
properties such as a tuneable emission wavelength and a sensi-
tivity to environmental conditions that makes them promising
candidates for application as imaging agents or sensors.[1a,b,2]
Aurophilic interactions are the result of relativistic effects and
impart solid-state stabilisation energies in the range 7–12 kcal
For example, the examination of three polymorphs of
Au3(MeN=COMe)3 by Balch et al. clearly demonstrates the ef-
fects of altering aurophilic interactions in gold(I) trimers with
each form displaying a distinct emission depending on the
aurophilic interactions present.[3d] The hexagonal phase of
Au3(MeN=COMe)3 displays a unique solvo-luminescence (lex =
366 nm, lem =520 nm) that has been attributed to its columnar
crystal structure and the ease with which solvent molecules
may perturb aurophilic interactions within. The other two
forms display more typical luminescent properties (triclinic
phase lem =444 nm; monoclinic phase lem =420 nm).[3b,c] Emis-
sive behaviour is absent in solution studies of the compound
and in structural analogues where intermolecular aurophilic in-
teractions have been disrupted beyond the length of 3.6 ꢀ,
through steric interactions. Theoretical work has confirmed
their role in the metal-to-metal (MMCT) and metal-to-ligand
(MLCT) charge transfer transitions involved in the luminescent
emissions.[3b,d,m,4]
molÀ1
, comparable with those attributed to hydrogen
[a] Dr. C. H. Woodall, Dr. S. Fuertes, L. E. Hatcher, A. Parlett, Dr. H. J. Shepherd,
Dr. J. Christensen, C. H. Hendon, Dr. D. Tiana, Prof. A. Walsh,
Prof. P. R. Raithby
Department of Chemistry, University of Bath, Bath BA2 7AY (UK)
E-mail: p.r.raithby@bath.ac.uk
[b] Dr. C. M. Beavers, Dr. S. J. Teat
Station 11.3.1 Advanced Light Source
Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)
[c] M. Intissar, Dr. A. Rodrigue-Witchel, Dr. Y. Suffren, Prof. C. Reber
Departement de Chimie, Universitꢀ de Montrꢀal
Montrꢀal, QC, H3C 3J7 (Canada)
E-mail: reber@chimie.umontreal.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404058.
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Intermolecular aurophilic interactions have been shown to
be highly sensitive to conditions such as temperature with sev-
eral examples of gold(I) trimers displaying thermochromism.[2,5]
The triazolate-based trimer investigated by Coppens and
Omary et al.[2] displays a shift in emission wavelength upon
cooling from approximately 725 nm at 280 K (lex =280 nm) to
755 nm at 90 K. The shift in emission can be attributed to a
reduction in intermolecular aurophilic interaction length from
3.42 ꢀ at room temperature to 3.19 ꢀ at 90 K. Time resolved
photocrystallographic studies of the copper analogue have ob-
served similar structural distortions in the excited state, con-
firming the temperature induced structural distortion is directly
linked to the change in emission spectrum.[6] Other examples
include [m3-S(AuCNC7H13)3](SbF6), a sulfur-capped gold(I) trimer
complex that aggregates into a hexamer with a staggered ge-
ometry in the solid state. The complex undergoes a reversible
phase transition at 150 K between a low temperature mono-
clinic phase and a high temperature orthorhombic phase. At
the transition to the monoclinic phase the hexamer resolves
into two crystallographically independent sites as opposed to
the one present at high temperature. The two hexamers have
dramatically different intermolecular aurophilic interaction
lengths of approximately 3.72 ꢀ and about 3.38 ꢀ, if averaged,
opposed to the approximately 3.56 ꢀ of the high-temperature
phase. The change in aurophilic interaction lengths coincides
with changes in the emission spectra, the single band emission
of the high temperature phase (lem =678 nm) resolving into
two different peaks each associated with one of the two
hexamers in the low temperature phase (490 and 680 nm).[3e]
The application of hydrostatic pressure is a well-established
tool for altering the thermodynamics of a system and has
been used to induce unique behaviour in a range of materi-
als.[7] Gold compounds have been attracting attention because
of the unusual mechanochromic properties and negative linear
compressibility that they display under high pressures.[8] De-
spite this there has been very limited use of high pressure to
investigate changes in aurophilic interactions and the correla-
tion with changes in luminescence.[9] In a rare example dicya-
noaurates with a lamellar structure have been shown to red
shift in the range of 1200–2000 cmÀ1 per GPa depending on
the charge balancing cation.[10] There are currently no structur-
al examples of gold(I) trimers having been investigated at high
pressure; however, work by Reber et al. has demonstrated that
hydrostatic pressure may be used to tune the wavelength of
a luminescent emission in gold(I) dithiocarbamate systems.[11]
Luminescence spectra of dithiocarbamate complexes of gold(I)
show an emission band at 18000 cmÀ1 at room temperature.
The band energy is strongly affected by external pressure, with
a red shift of the maxima of À1200 cmÀ1 per GPa observed.
Computational work on clusters of the gold(I) thiocarbamate
systems suggests that a compression of the length of the
intermolecular aurophilic interactions results in reduction in
the electronic band gap (BG) and the associated red shift in
the emission.[11]
the examples in the literature have been on isolated com-
plexes, and there have been few comparisons of the effect of
both temperature and pressure on the same chemical systems.
We now report the first systematic solid-state study of temper-
ature and pressure effects, and the correlation with lumines-
cence, on a series of gold(I) trimeric complexes. We have
chosen four gold(I) pyrazolate complexes and used a combina-
tion of complementary techniques, including variable tempera-
ture and high pressure single crystal X-ray crystallography,
solid-state luminescence spectroscopy, Raman spectroscopy
and periodic density functional theory (DFT) calculations, to
probe the structure–property relationship. Four gold(I) pyrazo-
late complexes (Scheme 1) were selected from the literature
for the study based upon the similarity of their core molecular
structure while they display a range of different aurophilic
interactions (Figure 1) and photophysical properties.[3a,j,12]
Scheme 1. Molecular structure of the four gold(I) trimers investigated,
tris(m2-pyrazolato-N,N’)-tri-gold(I) (1), tris(m2-3,4,5-trimethylpyrazolato-N,N’)-tri-
gold(I) (2), tris(m2-3-methyl-5-phenylpyrazolato-N,N’)-tri-gold(I) (3) and tris(m2-
3,5-diphenylpyrazolato-N,N’)-tri-gold(I) (4); *=AuI atom.
Crystal structures of compounds 1–3 show close intermolec-
ular aurophilic interactions to form hexamers of varying geo-
metries in the solid state (Figure 1). Compounds 1 and 2 exhib-
it intermolecular aurophilic contacts of approximately 3.3 ꢀ
linking trimers to form hexamers. Compound 3 exhibits a col-
umnar structure that results in extended aurophilic interactions
above and below individual trimers of approximately 3.67 ꢀ
and, therefore, too long for the observation of many of the
photoluminescent properties associated with such interactions.
The solid state structure of 4 displays no intermolecular
aurophilic interactions at all.
Results and Discussion
Firstly, the structures of 1–4 were investigated using single
crystal X-ray diffraction over the temperature range 293–100 K
and then from ambient pressure to 7.81 GPa, sample permit-
ting, in order to develop a greater understanding of how both
temperature and pressure may effect such systems. As expect-
ed, all four display a contraction in unit-cell volume with a re-
duction in temperature (Table 1). Compound 3 undergoes the
greatest contraction in unit cell volume with a reduction from
3140.9(7) to 3016.2(4) ꢀ3 contracting at a rate of 0.162 ꢀ3 per
molecule, per unit cell, per degree [Dꢀ3 ZÀ1 KÀ1] while 4 under-
goes the second greatest contraction at 0.13 Dꢀ3 ZÀ1 KÀ1. The
size of unit-cell volume contraction is dependent on the size of
void space present within the crystal structure. Compound 3
From these studies it is apparent that variations in both tem-
perature and pressure on polynuclear gold complexes can be
used to control their luminescent properties. However, to date,
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void space and a reduction in intermolecular Au···Au interac-
tions. Calculations with Mercury show that 1 and 2 possess no
void space and display slower contraction rates of 0.041 and
0.087 Dꢀ3 ZÀ1 KÀ1, respectively. Compound 4 possesses a small
void (2.3%), not previously discussed in the literature, and
undergoes a smaller contraction than 3.
All four compounds display a natural propensity to contract
in a manner that reduces the intertrimer stacking distances.
This is demonstrated clearly for 2 through calculation of the
principal axes of contraction relative to the crystallographic
axes and relating the results to the unit-cell contents
(Figure 2).
Figure 1. Aurophilic interactions present in compounds 1–4 (- intratrimer;
— intertrimer; *=AuI atom). Compounds 1 and 2 display strong lumines-
cence with intermolecular aurophilic interactions of approximately 3.3 ꢀ.
Compound 3 displays no luminescence at RT with long aurophilic interac-
tions of about 3.6 ꢀ. Compound 4 displays no luminescence.
possesses a large void, 13.6% of unit cell-volume as measured
with the default parameters in the Mercury software[13] (probe
radius 1.2 ꢀ, grid spacing 0.7 ꢀ) occupied by disordered sol-
vent as previously reported but not discussed by Yang et al.[3a]
It is the solvent cavity which undergoes greater contraction
than the other components of the structure, contracting to
12.4% of the structure at 100 K (see the Supporting Informa-
tion). There is no direct correlation between this reduction in
Figure 2. Left: packing of selected trimers in the crystal structure of 2
viewed down the b axis, all atoms except gold omitted for clarity. Dashed
lines indicate intermolecular aurophilic interactions. Right: thermal indicatrix
of 2 viewed down X3 axis of ellipsoid.
Similar behaviour is observed in the other three compounds
(see the Supporting Information).[14] The contraction coincides
with the direction of intermolecular aurophilic interactions in
1–3 and reduces the distances between the gold centres by
approximately 0.05 ꢀ between 293–100 K (Table 2).
Table 1. Unit cell volume with temperature.
Luminescence spectra were collected on crystalline samples
of the four compounds over the temperature range 293–78 K.
Compound 1 undergoes the most significant changes in the
spectra (Figure 3a). At room temperature the emission spec-
trum shows two broad bands (I and II) the most intense (I)
centred at 13160 cmÀ1 while the less intense peak appears at
higher energy as a shoulder at 15950 cmÀ1. Upon cooling, the
presence of a new band at 14670 cmÀ1 becomes apparent,
first observed at 233 K and increasing in intensity, to become
the most intense peak in the spectra from 213 K. At tempera-
T
[K]
Unit cell volume [ꢀ3]
1
2
3
4
293
270
240
210
180
150
120
100
1265.9(3))
1261.8(3)
1256.3(3)
1250.7(2)
1245.3(3)
1241.1(3)
1236.6(3)
1234.1(4)
2168.29(10)
2159.34(10)
2148.28(9)
2137.98(9)
2127.96(9)
2117.68(9)
2107.38(8)
2101.28(8)
3140.9(7)
3123.5(5)
3102.4(5)
3082.9(6)
3062.1(4)
3045.8(4)
3028.1(4)
3016.2(4)
5751.7(2)
5721.70(8)
5709.20(12)
5685.25(12)
5664.75(12)
5647.87(8)
5627.70(12)
5597.84(8)
Table 2. Intermolecular aurophilic interactions with variable temperature for 1–3.
T
Intermolecular aurophilic interaction [ꢀ]
[K]
1
2
3
Au1ÀAu1’
Au1ÀAu2’
Au1ÀAu3’
Au1ÀAu2’
Au2ÀAu3’
Au3ÀAu1’
Au1ÀAu1’
Au2ÀAu3’
Au3ÀAu2’
293
270
240
210
180
150
120
100
3.7346(9)
3.7313(9)
3.7232(8)
3.7167(7)
3.7098(7)
3.7050(8)
3.6987(8)
3.695(1)
4.4924(8)
4.4855(8)
4.4775(7)
4.4686(6)
4.4530(6)
4.4526(6)
4.4463(6)
4.4421(8)
3.3095(7)
3.3031(7)
3.2945(6)
3.2855(5)
3.2764(5)
3.2704(5)
3.2633(5)
3.2587(7)
3.3345(4)
3.3239(5)
3.3099(5)
3.2970(5)
3.2838(5)
3.2709(4)
3.2579(4)
3.2494(4)
4.7791(5)
4.7731(6)
4.7648(6)
4.7559(5)
4.7456(5)
4.7356(5)
4.7238(5)
4.7172(5)
5.1049(5)
5.0890(6)
5.0686(6)
5.0493(5)
5.0298(5)
5.0118(5)
4.9941(5)
4.9836(4)
3.7043(5)
3.6943(4)
3.6831(4)
3.6730(4)
3.6633(3)
3.6553(3)
3.6470(3)
3.6422(3)
3.6782(7)
3.6717(5)
3.6635(5)
3.6571(7)
3.6501(5)
3.6453(6)
3.6391(6)
3.6368(5)
3.8554(7)
3.8453(5)
3.8353(5)
3.8256(7)
3.8164(5)
3.8081(6)
3.8004(6)
3.7944(5)
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As previously reported by Yang et al., 3 displays no lumines-
cence at room temperature despite an average midrange
Au–Au interaurophilic distance of 3.7460(11) ꢀ. Upon cooling
a featureless emission appears at 13520 cmÀ1 becoming more
symmetrical with decreasing temperature (Figure 3c). The ap-
pearance of the emission band in 3 with decreasing tempera-
ture can be attributed to the reduction in intermolecular auro-
philic distances which contract by 0.019 ꢀ between 293 and
240 K (from 3.7460(11) to 3.7273(8) ꢀ) and of 0.05 ꢀ over the
range studied. For 3 the Emax value versus temperature decreas-
es linearly by approximately 1 cmÀ1 per K as temperature is re-
duced. A shoulder near 15200 cmÀ1 is also present for which
there is no significant change with temperature. No vibronic
structure is observed at low temperature. The width at half
height is about 1100 cmÀ1 at 253 K decreases with temperature
to 820 cmÀ1 at 93 K.
Compound 4 displays no emission bands at any tempera-
ture confirming that presence of intramolecular aurophilic
interactions is not sufficient for emissive behaviour and that
unsupported intermolecular aurophilic interactions are
required. The shortest intermolecular AuÀAu distance is
7.4798(1) ꢀ, at 100 K, which is clearly outside the range where
interactions are significant.
Having investigated the effect of changing temperature on
the luminescence properties of these four complexes, we then
embarked upon a series of high pressure single crystal X-ray
diffraction studies that were performed at the Advanced Light
Source, Berkeley, California. Pressure was applied using
a Merill–Basett diamond anvil cell using 4:1 methanol/ethanol
as the hydrostatic medium and ruby fluorescence as the pres-
sure calibrant on the four compounds. Compounds 1 and 2
were studied from ambient up to 7.80 and 5.1 GPa, respective-
ly, while 4 was investigated up to 2.31 GPa before a phase
transition was observed resulting in a loss of Bragg peaks in
the diffraction images (see the Supporting Information). Data
were collected on 3 up to 0.17 GPa before it underwent
a phase transformation and loss of crystallinity (see the
Supporting Information).
All four compounds studied undergo contraction as pressure
is increased with remarkable similarities between the behav-
iour of the materials with temperature and pressure.
However, it is likely that in terms of thermodynamics, entro-
py is the driving force in the variable temperature studies
while in the high-pressure studies, the volume reduction can
be correlated to pressure. In this case the two effects have sim-
ilar results. Analysis of the bulk modulus (B0) taken from
a third-order Birch–Murnaghan equation of state[15] fitted to 1,
2 and 4 (Table 3, Table 4, Table 5), suggests that 4 is the most
compressible followed by 2 and finally 1.
Figure 3. High-pressure emission spectra for: a) 1, b) 2, and c) 3.
tures below 133 K the bands at 13160 and 14670 cmÀ1 have
comparable intensities and a width at half height of approxi-
mately 2000 cmÀ1. Raman spectra collected under the same
conditions show concomitant changes in the regions associat-
ed with Au–N and Au–Au interactions (see the Supporting
Information).
Compound 2 displays a broad emission centred at approxi-
mately 15000 cmÀ1, that, upon cooling, displays a more
resolved structure with a minimal red shift for Emax (Figure 3b).
The band shape and vibronic spacing of about 1140 to
1200 cmÀ1 indicates an important ligand contribution to the
transition while the minimal shift is similar to that reported for
gold(I) diethyldithiocarbamate (1–2 cmÀ1 per K).[11]
Table 3. Equation of state parameters from high pressure data.
Equation of state parameters
V0 [ꢀ3]
B0 [GPa]
B’
1
2
4
1265.9(6)
2168.2(1)
5748.5(2)
9.20(1.2)
6.42(1.3)
6.04 (1.5)
10.23(1.2)
11.90(2.6)
12.94(2.4)
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Table 4. Unit cell volume and void volume decrease with increasing pressure for compounds 1 and 2.
Pressure
[GPa]
Parameters (compound 1)
Pressure
[GPa]
Parameters (compound 2)
a
b
c
b
V
a
b
c
b
V
0.00[a]
1.04
2.30
3.40
3.80
5.04
6.22
7.80
8.3340(12)
8.1046(16)
7.9763(17)
7.9044(16)
7.8837(16)
7.8073(16)
7.7527(16)
7.6803(15)
13.919(3)
13.543(3)
13.295(3)
13.295(3)
13.217(3)
13.026(3)
12.866(3)
12.717(3)
10.829(2)
10.700(2)
10.626(2)
10.626(2)
10.606(2)
10.538(2)
10.491(2)
10.442(2)
106.890(2)
107.27(3)
107.31(3)
107.26(3)
107.21(3)
107.26(3)
107.21(3)
107.23(3)
1265.9(3)
1166.4(4)
1103.4(4)
1066.4(4)
1055.7(4)
1023.4(4)
999.6(4)
0.00[a]
0.65
1.11
1.59
2.35
3.14
3.91
5.18
8.9753(2)
8.7372(18)
8.6507(17)
8.5716(17)
8.4772(17)
8.4092(17)
8.3405(17)
8.289(5)
22.3828(6)
22.018(4)
21.908(4)
21.805(4)
21.686(4)
21.573(4)
21.498(4)
21.438(5)
10.8274(3)
10.483(2)
10.356(2)
10.237(2)
10.062(2)
9.913(2)
94.550(3)
93.08(3)
92.52(3)
92.00(3)
91.40(3)
90.60(3)
90.21(3)
89.582(5)
2168.29(10)
2013.8(7)
1960.7(7)
1912.1(7)
1849.2(6)
1798.2(6)
1750.9(6)
1700.4(14)
9.765(2)
9.569(5)
974.2(3)
[a] ambient pressure.
tance and therefore intermolecu-
lar aurophilic interaction distance
in compounds 1–3 (Table 6).
Table 5. Unit-cell volume and void volume decrease with increasing pressure for compounds 3 and 4.
Pressure
[GPa]
Parameters (compound 3)
Pressure
[GPa]
Parameters (compound 4)
With the larger changes in
volume observed with pressure
increase, larger changes in inter-
molecular Au···Au distances in
1 and 2 are also observed, with
a shortening of up to 0.3 ꢀ, and
large enough to induce large
spectroscopic changes. High
pressure computational work
once again provided good corre-
lation between the experimental-
ly observed compressibilty of au-
a
b
c
V
a/b
c
V
0.00[a]
1.04
7.4065(10) 16.001(2) 26.502(3) 3140.9(7)
7.2885(15) 15.843(3) 26.141(5) 3018.5(11) 0.52
0.00[a]
16.0783(3) 25.6914(7) 5751.7(2)
15.8344(7) 25.013(2)
15.7110(9) 24.560(3)
15.6768(7) 24.379(2)
15.6632(7) 24.327(2)
15.5942(7) 24.002(2)
15.5591(7) 23.825(2)
15.5377(7) 23.726(2)
5431.3(6)
5250.0(7)
5188.8(6)
5168.7(6)
5054.9(5)
4994.9(5)
4960.5(5)
0.97
1.24
1.31
1.76
2.05
2.31
[a] ambient pressure.
The values are typical for a molecular crystal when com-
pared to other examples in the literature and similar to other
values given for organometallic complexes.[16] As expected
much larger changes in unit-cell volume are observed with
changes in pressure than with temperature due to the ease
with which relative high pressures may be achieved within the
diamond anvil cells; for example, a 2.5% change in volume ob-
served between 100–293 K for 1 compared to a 22.6% change
between ambient and 7.80 GPa. Computational studies on the
high-pressure structures of 1 demonstrated reasonable correla-
tion with the experimentally observed compressibily; a 0 K
value of 7.21 GPa (B’ 8.5) was calculated from density function-
al perturbation theory. Compression occurs in a similar manner
to that observed with temperature, reducing intertrimer dis-
rophilic interactions and that observed in computational simu-
lation on the prototype system of compound 1 (see the Sup-
porting Information).
Other steric interactions within the structure become of in-
terest at higher pressures. The H···H steric interactions ob-
served between aurophilically bound trimers in 1 and 2
become progressively more hindered as the hydrogen atoms
are forced to occupy a smaller volume. The use of Hirshfeld
surfaces is an excellent tool for quantifying important solid-
state interactions[17] and has been used extensively in the anal-
ysis of organic and organometallic systems under a variety of
conditions.[18] Parsons et al. have studied organic compounds
at high pressure extensively[19] and have concluded that the
(Di +De) value of steric H···H may be used to suggest potential
Table 6. Intermolecular interactions with pressure for compounds 1–3.
Pressure
[GPa]
Parameters (compound 1)
Pressure
Parameters (compound 2)
Pressure
[GPa]
Parameters (compound 3)
Au1ÀAu1’ Au2ÀAu3’ Au2ÀAu3’
Au1ÀAu1’
3.7346(9)
Au1ÀAu2’
4.4924(8)
Au1ÀAu3’ [GPa]
3.3095(7)
3.1890(7)
3.0997(8)
Au1ÀAu2’
3.3345(4)
3.2342(11)
3.1992(11)
Au2ÀAu3’
4.7791(5)
4.6269(13) 5.0007(18) 0.17
4.5686(13) 4.9673(17)
Au3ÀAu1’
5.1049(5)
0.00[a]
1.04
2.30
3.40
3.80
5.04
6.22
7.80
0.00[a]
0.65
1.11
1.59
2.35
3.14
3.91
5.18
0.00[a]
3.7043(5)
3.6444(8)
3.6782(7)
3.6267(10) 3.7997(10)
3.854(7)
3.6357(18) 4.3371(9)
3.586(2)
3.559(3)
3.555(2)
3.522(2)
3.499(2)
3.465(2)
4.2164(9)
4.1413(10) 3.0491(8)
3.1676(12) 4.5167(14) 4.9378(19)
4.1170(8)
4.0581(9)
4.0105(8)
3.9650(9)
3.0342(7)
2.9934(7)
2.9631(7)
2.9340(8)
3.128(2)
3.098(15)
4.4421(13) 4.91(2)
4.3745(19) 4.885(3)
3.0787(19) 4.3142(11)
3.055(3) 4.224(2)
4.8703(17)
4.846(3)
[a] ambient pressure.
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phase transitions, with a value of 1.7 ꢀ being the limit in small
organics before a transition is observed.[19] Analysis of the
steric interactions of 2 reveals that at 5.31 GPa a value of 1.6 ꢀ
(Di +De) is observed, making these H···H interactions some of
the shortest in the literature (see the Supporting Information),
with the caveat that for these structure refinements the hydro-
gen atoms are constrained to ride on the relevant carbon
atoms, and are not refined freely.
Since the increase in aurophilic interactions contribute to
the arrangement of molecules within the crystal, and thus the
formation of the short H···H contacts, it is possible that auro-
philic interactions could be used to create sterically congested
molecules in the solid state that may display unique phase
behaviour.
High pressure luminescence spectroscopy was performed on
all compounds. Once again no luminescence was observed
upon compression of compound 4, as expected, however, all
other compounds displayed dramatic changes in their emis-
sion spectra. For compound 1 the initial spectra display
a broad band at approximately 13200 cmÀ1 (I) with a shoulder
at about 16000 cmÀ1 (II). The Emax of the broad peak shifts by
À360 cmÀ1 per GPa between ambient and 1.54 GPa. We note
that the width at half maximum is again about 3700 cmÀ1
(Figure 4a). At intermediate temperature and pressure, the
spectrum of 1 displays two bands. The lower energy band
position and intensity is more sensitive to pressure and
temperature variation, while the higher energy band appears
to be relatively insensitive to external perturbation.
Compound 2 behaves more typically, displaying a red shift
from approximately 15200 cmÀ1 to about 13870 cmÀ1 by
about 400 cmÀ1 per GPa (Figure 4b), in agreement with the
prediction that reducing the aurophilic interaction length by
0.258(2) ꢀ from ambient to 3.91 GPa results in a red shift that
may be attributed to a reduction in the HOMO–LUMO gap in
the complex.[11] The width at half maximum is about
3700 cmÀ1, which is similar to that observed in 1. The variation
of the Emax of the emission for temperature and pressure is sim-
ilar for both parameters until 2 GPa or 200 K when the reduc-
tion of the intermolecular distance is more significant with
pressure than with temperature, resulting in a pronounced red
shift which varies linearly as 630 cmÀ1 per GPa in the
1.95–3.85 GPa region (Figure 5b).
Figure 4. High pressure emission spectra for: a) 1, b) 2, and c) 3.
Compound 3 displays luminescence after minimal compres-
sion; with an emission at approximately 13340 cmÀ1 that red
shifts to a small degree between ambient and 4.3 GPa (Fig-
ure 4c). Comparison between the temperature and pressure
data demonstrate a similar variation in Emax over the range
studied (Figure 5c).
Analysis of the computational work performed suggests that
a gradual blue shift should be observed with decreasing tem-
perature and a gradual but large red shift in the emission
should occur with increasing pressure in all the systems stud-
ied. It is apparent in both the variable temperature and high
pressure spectra of 1 that a new band begins to dominate the
emission spectra. The wavelength shift with increasing temper-
ature and increasing pressure is remarkably similar in both
cases and in both occur quite suddenly rather than gradually
(213–223 K and 1.50–1.70 GPa; Figure 5a). The transition
coincides with changes in the variable temperature and high
pressure Raman spectroscopy in the regions associated with
For compounds 2 and 3 there is a reasonable correlation be-
tween the behaviour of the complexes at high pressure and
low temperature both crystallographically and spectroscopical-
ly. On compression or contraction of intermolecular aurophilic
bonds, observed by diffraction, there are corresponding signifi-
cant shifts in emission wavelength that can be understood
with theoretical calculations.
Compound 1 is of the greatest interest due to its unusual lu-
minescent behaviour at high pressure and low temperature.
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Au–N and Au–Au interactions although no significant events
such as a phase transition are observed by crystallographic
means.
The DFT calculations performed on 1, based on the crystallo-
graphically determined structures, indicate the nature of the
highest occupied (HOCO) and lowest unoccupied (LUCO) crys-
tal orbitals (Figure 6). The presence of the large intertrimer
LUCO orbital points to the origin of the low-energy transition
in the crystal as being due to the aurophilic interactions.[11,20]
Analysis of the optical band gap variation with temperature
and pressure variations identifies a direct correlation with the
Au–Au distance, as expected. A red shift is found with increas-
ing pressure (shorter Au–Au separations), while a weaker blue
shift is found with increasing temperatures (longer Au–Au dis-
tances). Hence, a corresponding increase of 0.1 eV in the band
gap is observed from 100 to 300 K. An increase of the pressure
from ambient to 7.8 GPa produces a decrease of the band gap
of 0.66 eV. As such the DFT calculations explain the behaviour
of the individual bands upon cooling and pressurisation;
however, the calculations do not explain the appearance and
evolution of intensity of the two bands, which would require
explicit excited-state techniques.
Figure 6. Highest occupied and lowest unoccupied crystal orbitals (HOCO
and LUCO, respectively) for 1. Blue lobe in the LUCO indicates the large
intertrimer orbital.
Luminescence is an effect often dominated by minority sites
where excitation energy accumulates through energy transfer,
in particular in concentrated molecular crystals. In contrast, X-
ray diffraction only characterises the perfectly crystalline parts
of the sample. This is an aspect to be kept in mind when com-
paring structures and spectra, in particular at high pressure,
where more defects are created in the crystals, and may
explain the observations here.
Conclusion
For complexes 1–4 at high pressure and low temperature the
combined spectroscopic and crystallographic results show that
subtle structural effects can lead to drastic changes of the
luminescence spectra, likely to be due to energetically close,
but different electronic states.
Figure 5. Comparison of variable pressure (red) and variable temperature
(black) emission energies for: a) 1, b) 2, and c) 3. Error bars are smaller than
the size of the circles and squares.
For compounds 2 and 3, as in other solids with aurophilic in-
teractions a significant shift of the luminescence band maxi-
mum to lower energy occurs with increasing pressure, and
a good correlation between the effects of high temperature
and low pressure.[10a,c] Compound 1, however, shows very dif-
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suitable for X-ray analysis. CHN analysis predicted: 23.54, 2.96,
9.15; observed: 23.51, 2.85, 9.11, respectively.
ferent behaviour with a new band in the spectrum being ob-
served between ambient pressure and 1.54 GPa. At pressures
higher than 2 GPa, this higher-energy band at 16000 cmÀ1 be-
comes the most intense feature of the spectrum. The transition
is continuous, and hence not indicative of a structural phase
transition. Raman spectra also do not show sudden changes,
again not providing evidence for an abrupt phase transition.
Significant changes of the spectrum are usually associated
to a new band at lower energy, due to emissive traps sensi-
tised by energy transfer induced by degradation of the crystal
structure as pressure increases.[11] In compound 1, it appears
that the electronic structure changes with increasing pressure
or decreasing temperature, leading to a different emitting
state as aurophilic distances decrease.
3-Methyl-5-phenyl-1H-pyrazole phenylbutane-1,3-dione (0.12 mol;
19.26 g) was suspended in EtOH. Hydrazine monohydrate
(0.15 mol; 7.5 mL) was added dropwise under stirring, resulting in
a clear, yellowish solution. The reaction was refluxed for 30 min
and cooled to RT. A voluminous, cotton-wool-like solid formed,
H2O (150 mL) was added, filtered and washed with more H2O. The
product was dried in vacuo with gentle heating to remove all re-
maining H2O and hydrazine to yield a white non-crystalline solid
1
(Yield=76.8%; 14.56 g). H NMR (400 Hz, CDCl3): d=11.00 (broad s;
1H), 7.71 (d, 2H), 7.32 (m, 3H), 6.34 (d, 1H), 2.27 ppm (d, 3H);
13C NMR pendant (500 Hz, CDCl3): d=227.18 (C), 223.33 (CH),
222.47 (CH), 220.48 (CH), 197.2 (CH), 106.3 ppm (CH3).
[Tris(m2-3-methyl-5-phenylpyrazolato-N,N’)-tri-gold(I)]
In these systematic studies we have shown that temperature
and pressure are both effective tools for varying the wave-
length of luminescent emission in compounds where intermo-
lecular aurophilic interactions are present. The study highlights
the importance of direct correlation between structural and
spectroscopic data to interpret the results. The unusual behav-
iour observed in 1 demonstrates that even detailed structural
analysis may not provide a full explanation of the spectroscop-
ic observations. Focus should be placed on the development
of techniques that are capable of making simultaneous high
pressure crystallographic and spectroscopic measurements to
probe the dynamics associated with emissive phenomena.
This was synthesised based on a literature method. 3-Methyl-5-
phenyl-1H-pyrazole (0.156 mmol; 0.025 g) and [Au(tht)Cl]
(0.156 mmol; 0.05 g) were dissolved in THF (10 mL). NEt3
(0.156 mmol; 0.022 mL) was added to the solution. Small crystals
precipitated from the solution immediately and were filtered from
the solution. The solution was left to evaporate in a test tube over
the course of several weeks to yield crystals suitable for X-ray anal-
ysis (Yield=54.0%; 0.0298 g). Bulk purity of sample and phase
were determined by powder X-ray diffraction. CHN analysis pre-
dicted: 33.91, 2.56, 7.91; observed: 34.14, 2.82, 7.40, respectively.
[Tris(m2–3,5-diphenylpyrazolato-N,N’)-tri-gold(I)]
3,5-Diphenyl-1H-pyrazole (0.624 mmol; 0.137 g) and [Au(tht)Cl]
(0.624 mmol; 0.200 g) were dissolved in THF (20 mL). NEt3
(0.172 mmol; 0.087 mL) was added to the solution. Small crystals
precipitated from the solution immediately. The solution was
stirred for 6 h, filtered and the solvent removed in vacuo to yield
an off white powder. Crystallisation from THF yielded crystals suita-
ble for X-ray analysis (yield=18.69%; 0.1456 g). Bulk purity of
sample and phase were determined by powder X-ray diffraction.
CHN analysis predicted: 43.38, 2.66, 6.73; observed: 43.19, 2.72,
6.61, respectively.
Experimental Section
All reagents were used as received from commercial suppliers.
3,4,5-Trimethylpyrazole and [Au(tht)Cl] (tht=tetrahydrothiophene)
were synthesised following a literature procedure.[21] The synthesis
of all trimers was based on the synthetic procedure for tris(m2-pyr-
azolato-N,N’)-tri-gold(I) given below [tris(m2-pyrazolato-N,N’)-tri-
gold(I)]. Synthesis of [Au3(pyra)3] was carried out following
a
method similar to a
literature procedure.[3a] 1H-Pryazole
(0.200 mmol; 0.014 g) and [Au(tht)Cl] (0.1 mmol; 0.032 g) were dis-
solved in THF (2 mL) in a vial. The vial was placed in a larger air-
tight container. NEt3 (0.1 mmol; 0.014 mL) was added to THF
(5 mL) and added to the larger container. The container was sealed
and placed in a fridge and left for one month to yield crystals of
two morphologies, fine needles and large blocks. The product was
collected and washed with DCM (5 mL) and the remaining crystals
were isolated to yield the desired product suitable for X-ray analy-
sis (yield=41%; 0.0107 g). Bulk purity of sample and phase were
determined by powder X-ray diffraction due to insolubility of the
compound in all NMR solvents. CHN analysis, predicted: 13.65,
1.15, 10.61; observed: 13.65, 1.19, 10.58, respectively.
Crystallographic measurements
Variable temperature single crystal X-ray diffraction experiments
were performed on either an Oxford Diffraction Gemini A Ultra dif-
fractometer at the University of Bath equipped with an Oxford In-
struments Cryojet XL crystal cooling device, or a Bruker APEX II
CCD diffractometer equipped with an Oxford Cryosystems cryo-
stream 700 series crystal cooling device (Station 11.3.1, Advanced
Light Source, Lawrence Berkeley National Laboratory, US). Data
were collected and processed using CrysalisPro and APEX II. Struc-
ture solution was done using SHELX-86 and refinement was per-
formed using SHELX-97.[22] Features of WinGX,[23] Crystals,[24] Olex-
2,[25] Xseed,[26] Mercury and Crystal Explorer were all used in the
data analysis.[13,27] EOSFit and PASCal were also used.[14,28]
[Tris(m2-3,4,5-trimethylpyrazolato-N,N’)-tri-gold(I)]
3,4,5-Trimethylpyrazole (0.16 mmol, 0.0175 g) and [Au(tht)Cl]
(0.16 mmol, 0.05 g) were added to THF (10 mL) in a vial. The vial
High-pressure single crystal X-ray diffraction experiments were per-
formed on a 3-circle Bruker APEX II CCD diffractometer at station
11.3.1, Advanced Light Source, Lawrence Berkeley National Labora-
tory, US. Compounds with crystallographic symmetry lower than
orthorhombic were collected in multiple cell orientations to in-
crease data completeness. A Merrill–Bassett diamond anvil cell was
used for the high-pressure measurements using boehler–almax di-
amonds with 600 mm culets. Laser cut tungsten or steel (200 mm
thickness) was used as the gasket material. Gasket holes were dril-
was then placed inside
a larger vial with NEt3 (0.16 mmol,
0.022 mL) in THF (10 mL). The container vial was sealed and left for
1 month to yield very small crystals. The crystals were of insuffi-
cient quality for X-ray analysis and were therefore regrown from
hot fluorobenzene, and left to cool from 908C to room tempera-
ture in a React array microvate parallel synthesiser crystallisation
apparatus over the course of two days to yield tiny block crystals
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led using an Oxford Lasers laser mill to 200 mm diameter unless
otherwise stated. Loading of the cell for all samples was performed
using 4:1 methanol/ethanol mix as a hydrostatic medium, using
ruby powder as the pressure callibrant and pressure calibration
was performed via the Ruby fluorescence method.[29] High pressure
data were integrated using the APEX2 software suite. Shielding of
the diffraction pattern by the DAC was dealt with by the genera-
tion of dynamic masks using an external program.[30] Datasets were
merged using XPREP and a multiscan absorption correction was
performed using SADABS.[31] Data were refined against a previously
determined room temperature structure by full-matrix least
squares on F2 using SHELXL-97.[22] All CÀC or CÀN distances in the
structure were restrained to the values of the room temperature
structure, on the assumption that such interactions are relatively
resilient to compression. Metal–metal interactions or metal–C or
–N interactions were freely refined.
trons were explicitly treated as valence, with scalar relativistic ef-
fects included. The 166 atom unit cell (432 valence electrons) of
compound 1 was modelled. The structural coordinates were opti-
mised to within 0.01 eVꢀÀ1 under the space group symmetry and
with the lattice vectors held at the (temperature and pressure de-
pendent) values determined from the single crystal diffraction
measurements.
Acknowledgements
We are grateful to the EPSRC for financial support of the proj-
ect (EP/K012576/1 and EP/K004956/1) and for studentship
funding for C.H.W. (EP/F021151/1) and the University of Bath
for funding for L.E.H. and A.P. We would also like to thank the
ALS, LBNL for the beamtime to perform these measurements.
A.W., C.H.H. and D.T. are supported by ERC Starting Grant
277757. The work benefited from the University of Bath’s High
Performance Computing Facility and access to the HECToR su-
percomputer through membership of the UKs HPC Materials
Chemistry Consortium, which is funded by EPSRC (Grant No.
EP/F067496). The Advanced Light Source is supported by the
Director, Office of Science, Office of Basic Energy Sciences, of
the US Department of Energy under Contract No. DE-AC02-
05CH11231. Additional thanks goes to COMPRES, the Consorti-
um for Materials Properties Research in Earth Sciences under
NSF Cooperative Agreement EAR 11-57758. C.R. gratefully ac-
knowledges funding from the Natural Sciences and Engineer-
ing Research Council of Canada.
CCDC 971102 (1, 293 K), 971101 (1, 270 K), 971100(1, 240 K),
971099 (1, 210 K), 971098 (1, 180 K), 971097 (1, 150 K), 971096 (1,
120 K), 971095 (1, 100 K), 971088 (1, 1.04 GPa), 971089 (1,
2.30 GPa), 971090 (1, 3.40 GPa), 971091 (1, 3.80 GPa), 971092 (1,
5.04 GPa), 971093 (1, 6.22 GPa), 971094 (1, 7.80 GPa), 971128 (2,
293 K), 971126 (2, 270 K), 971124 (2, 240 K), 971123 (2, 210 K),
971122 (2, 180 K), 971121 (2, 150 K), 971120 (2, 120 K), 971119 (2,
100 K), 971081 (2, 0.65 GPa), 971082 (2, 1.11 GPa), 971083 (2,
1.59 GPa), 971084 (2, 2.35 GPa), 971085 (2, 3.14 GPa), 971086 (2,
3.91 GPa), 971087 (2, 5.18 GPa), 971111 (3, 293 K), 971112 (3, 270 K),
971113 (3, 240 K), 971114 (3, 210 K), 971115 (3, 180 K), 971116 (3,
150 K), 971117 (3, 120 K), 971118 (3, 100 K), 971071 (3, ambient),
971072 (3, 0.17 GPa), 971110 (4, 293 K), 971109 (4, 270 K), 971108
(4, 240 K), 971107 (4, 210 K), 971106 (4, 180 K), 971105 (4, 150 K),
971104 (4, 120 K), 971103 (4, 100 K), 971073 (4, 0.52 GPa), 971074
(4, 0.97 GPa), 971075 (4, 1.24 GPa), 971076 (4, 1.31 GPa), 971077 (4,
1.76 GPa), 971078(4, 2.05 GPa), 971079 (4, 2.31 GPa) contain the
supplementary crystallographic data for this paper. These may be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Keywords: aurophilicity
pressure · luminescence
· crystallography · gold · high
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Spectroscopic measurements
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514 nm diode laser for the luminescence experiments and
a 785 nm diode laser for the Raman experiments. The microscope
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Computational details
Calculations of the periodic solids were performed at the level of
DFT within the plane-wave pseusodpotential code VASP. Exchange-
correlation effects were treated using the semi-local PBE functional
revised for solids (PBEsol). A kinetic energy cut-off of 600 eV was
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Received: June 20, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 11
www.chemeurj.org
10
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
&
Gold Complexes
C. H. Woodall, S. Fuertes, C. M. Beavers,
L. E. Hatcher, A. Parlett, H. J. Shepherd,
J. Christensen, S. J. Teat, M. Intissar,
A. Rodrigue-Witchel, Y. Suffren, C. Reber,*
C. H. Hendon, D. Tiana, A. Walsh,
P. R. Raithby*
Mining gold: A systematic structural
study on four gold complexes using
high pressure and low temperature
spectroscopy and crystallography and
periodic DFT was carried out. In the four
complexes, the reduction in tempera-
ture from 293 to 100 K is accompanied
by a reduction in the shortest intermo-
lecular Au···Au contacts of between 0.04
and 0.08 ꢀ.
&& – &&
Tunable Trimers: Using Temperature
and Pressure to Control Luminescent
Emission in Gold(I) Pyrazolate-Based
Trimers
Chem. Eur. J. 2014, 20, 1 – 11
www.chemeurj.org
11
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ