Reversible thermal-mode control of luminescence from liquid-crystalline gold(i) complexes

Article abstract of DOI:10.1039/c3tc31973c

The synthesis and characterisation of liquid-crystalline (LC) gold complexes designed to have a rod-like structure in a dimeric form are described, and the relationship between their photophysical properties and aggregated structure is discussed. The luminescence intensities of the complexes were enhanced in the condensed phase, meaning that the complexes showed aggregation-induced emission. Observed photoluminescence in the condensed phase could be assigned to a monomer emission; however, luminescence properties were strongly affected by the aggregated structures of the complexes. A reversible on-off switching of the luminescence induced by the phase transition between LC and isotropic phases is demonstrated. Moreover, complex 1b showed thermochromic photoluminescence controlled by the aggregated structure; the colour of luminescence could be reversibly controlled by the phase transition between crystalline and LC phases. These LC gold complexes show potential application as materials for novel light-emitting devices. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3tc31973c

Showing research from the Tsutsumi group in Ritsumeikan
University, Japan.
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Title: Reversible thermal-mode control of luminescence from
liquid-crystalline gold(I) complexes
Liquid-crystalline gold complexes showed thermochromic
photoluminescence in condensed phases; the color of
luminescence could be reversibly controlled by the phase
transition between crystalline and liquid-crystalline phases.
See Osamu Tsutsumi et al.,
J. Mater. Chem. C, 2014, 2, 3549.
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Reversible thermal-mode control of luminescence
from liquid-crystalline gold(I) complexes†
Cite this: J. Mater. Chem. C, 2014, 2,
3549
Kaori Fujisawa,‡ Yuki Okuda, Yuichi Izumi, Akira Nagamatsu, Yuki Rokusha,
*
Yusuke Sadaike and Osamu Tsutsumi
The synthesis and characterisation of liquid-crystalline (LC) gold complexes designed to have a rod-like
structure in a dimeric form are described, and the relationship between their photophysical properties
and aggregated structure is discussed. The luminescence intensities of the complexes were enhanced in
the condensed phase, meaning that the complexes showed aggregation-induced emission. Observed
photoluminescence in the condensed phase could be assigned to a monomer emission; however,
luminescence properties were strongly affected by the aggregated structures of the complexes. A
reversible “on–off” switching of the luminescence induced by the phase transition between LC and
isotropic phases is demonstrated. Moreover, complex 1b showed thermochromic photoluminescence
controlled by the aggregated structure; the colour of luminescence could be reversibly controlled by the
phase transition between crystalline and LC phases. These LC gold complexes show potential application
as materials for novel light-emitting devices.
Received 8th October 2013
Accepted 20th February 2014
DOI: 10.1039/c3tc31973c
www.rsc.org/MaterialsC
reason, gold complexes have emerged as promising candidates
for AIE materials with potential applicability in practical
devices.3 In addition, the properties of the luminescence
Introduction
Organic materials showing strong luminescence in the
condensed phase have attracted great interest in view of their
potential application in light-emitting devices.1 In general,
however, the luminescence of such materials is strongly
reduced in the condensed phase because of the concentration
quenching effect; namely, luminescence is quenched by the
aggregation of luminescent organic molecules, preventing them
from being used in practical devices. Recently, materials
showing aggregation-induced emission (AIE) have been devel-
oped,2 whereby materials exhibiting AIE properties show
enhanced luminescence via molecular aggregation—in contrast
to quenching at high concentrations—in the condensed phase.
Gold complexes are known to exhibit photoluminescence
with quantum yields (F) of 0.5–0.9 in cases of intermolecular
interactions between gold atoms (aurophilic interaction);3 that
is, luminescence generated by these complexes is enhanced by
molecular aggregation in the condensed phases. For this
derived from gold complexes are structure-dependent4 such
that colour and/or luminescence intensity can be manipulated
by control of the aggregate structure.5 So far, several types of
liquid-crystalline (LC) gold complexes have been developed with
the aim of controlling their aggregate structure.6 Some useful
characteristics of LCs that lend themselves to efficient aggregate
control include their self-assembled nature, uidity, and
responsivity to external stimuli.7 In this study, we examine the
aggregation and photoluminescence behaviours of gold
complexes in both crystalline and LC phases, providing a
discussion of the relationship between the aggregated structure
and luminescence properties. In particular, we report that a
reversible change in the luminescence colour can be induced by
precise control of the aggregate structure.
Our strategy toward molecular design to provide a strong
intermolecular interaction for facile formation of molecular
aggregates was the introduction of simple and non-bulky
ligands onto the gold atom to suppress the effects of steric
hindrance. Thus, we synthesised Au(I) complexes having 4-
alkoxyphenylisocyanide and chlorine atoms as ligands, as
shown in Fig. 1 (1a–c; n ¼ 5–7). Molecules showing calamitic LC
phases are generally designed to have rod-like structures; for
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
1-1-1 Nojihigashi, Kusatsu 525-8577, Japan. E-mail: tsutsumi@sk.ritsumai.ac.jp;
Fax: +81-77-561-2659; Tel: +81-77-561-5966
† Electronic supplementary information (ESI) available: Full details of synthesis,
characterisation of all the complexes, crystallographic data in CIF format, XRD
data of crystalline and Sm phases, DSC and TGA thermograms, and details of
photophysical characterisations including photoluminescence spectra of all
complexes. CCDC 965148–965150. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3tc31973c
‡ Present address: Department of Organic Chemistry, University of Geneva,
Switzerland.
Fig. 1 Structure of Au complexes used in this study.
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this reason, most calamitic LC molecules usually consist of a
rigid multi-ring system, e.g., biphenyl, phenyl benzoate,
diphenylacetylene, azobenzene, etc. However, since we intro-
duced simple phenyl isocyanide ligands to avoid steric
hindrance, the molecular structures of gold complexes
designed in this study are quite different from those of
conventional calamitic mesogens. It has been reported that gold
complexes tend to form dimers in the LC phase, with the dimer
acting as a unit of the mesogen;8 accordingly, we designed the
molecules so as to incorporate a two-ring system into the rod-
like structure by dimer formation.
Results and discussion
Synthesis and characterisation of gold complexes
Complexes 1a–c were synthesised by the method previously
reported by Coco et al.9 The isocyanide ligands were rst
prepared (see ESI†), followed by synthesis of Au complex 1 by
reaction with (tht)AuCl (tht ¼ tetrahydrothiophene). The
complexes were puried by recrystallisation from acetonitrile
and were obtained as colourless crystals. The complexes were
fully characterised by 1H NMR, HRMS, IR, and elemental
analysis. All analytical data are given in the Experimental
section and indicate that the desired compounds were
obtained.
Thermal stabilities of the complexes were evaluated by
thermogravimetric analysis (TGA). It was conrmed by TGA that
the compounds were thermally stable up to 248–262 ꢀC, with
the thermal decomposition temperature dened as the
temperature at which 5% weight loss occurs (Fig. S1†). To clarify
the actual molecular structures of complexes 1a–c in the
condensed phase, single-crystal X-ray structural analyses were
performed. The crystal packing structures of the complexes are
shown in Fig. 2, and selected interatomic distances between
neighbouring molecules are listed in Table 1. The key crystal-
lographic data are also summarised in Table S1.† In the crystals
of 1a–c, the intermolecular Au/Au distance was approximately
Fig. 2 Crystal structures of complexes 1a–c at room temperature: (A)
1a, (B) 1b, and (C) 1c. Existence of intermolecular Au/Au and Au/p
interactions is indicated by broken lines. For clarity, only the major
components are shown and protons are omitted. Atom colour legend:
grey, C; green, Cl; purple, N; red, O; yellow, Au; pink, centroid of the
phenyl ring.
the geometric parameters obtained from single-crystal struc-
tural analysis of 1a, we can also conclude the presence of an
intermolecular Au/p interaction, i.e. in addition to an auro-
philic interaction, the polymer chains were additionally cross-
linked through a Au/p contact. A similar intermolecular Au/
p interaction was also found in complexes 1b and 1c. In
contrast, the aurophilic contact was not extended in these two
complexes; the interatomic distances between the second
˚
3.3 A, substantially shorter than the sum of van der Waals radii
10
˚
(3.8 A) of two gold atoms; this result is suggestive of the
existence of an aurophilic interaction between neighbouring
molecules. The stabilisation energy of the aurophilic interac-
tion is as large as that of the hydrogen bond;10,11 we can there-
fore consider that this interaction provides further stability to
the dimeric complex.
˚
neighbouring Au atoms were longer than 3.8 A and they did not
form linear polymer structures, instead existed as isolated
tetramers in the crystals (Fig. 2). The melting point of 1a
(134 C) was signicantly higher than those of the others (1b:
121 ꢀC; 1c: 124 ꢀC). The difference in melting points between 1a
ꢀ
As shown in Fig. 2, the aurophilic interaction of complex 1a
was extended; as a result, crystal 1a formed a linear polymer
structure without covalent bonds between each repeating unit.
Furthermore, the Au atom appeared close to the phenyl ring of a
neighbouring molecule that was involved in the formation of
another polymer chain via the Au/Au interaction (Fig. S3†); the
distance between the Au atom and the centroid of the phenyl
Table 1 Selected distances (A˚) and angles (ꢀ) between neighbouring
molecules in the crystals of 1a–c
1a
1b
1c
Au1/Au10
Au10/Au100
Au/Pha
qb
3.342(1)
3.342(1)
3.516
3.3810(5)
—
3.592
6.0
3.3092(6)
—
3.538
5.0
˚
ring was found to be 3.516 A, and the angle between the vector
normal to the aromatic ring and that passing through the
centroid to the Au atom (q) was 3.3ꢀ. Generally, Au(I) complexes
form aggregates based on the interaction between Au atoms and
an aromatic p-electron system when the distance between them
3.3
a
Distance between the Au atom and the centroid of the phenyl ring in a
b
neighbouring molecule. The angle between the vector normal to the
is less than 4.0 A, with angle q equal to or less than 20 .12 From
ꢀ
˚
aromatic ring and that passing through the centroid to the Au atom.
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Table 2 Phase transition behaviour of complexes 1a–c
Phase sequences and transition temperaturea (ꢀC)
and the other complexes can be explained by the presence of the
cross-linked polymer structure formed through both aurophilic
and Au/p interactions.
1a
1b
1c
Heating
Cooling
Heating
Cooling
Heating
Cooling
Cr1 100 Cr2 134 Sm 158 I
Cr1 83 Cr2 120 Sm 157 I
Liquid crystalline behaviour
Cr1 59 Cr2 79 Cr3 121 SmC 166 I
b
Crx 110 SmC 166 I
LC phase transition behaviours of 1a–c were studied using
polarised optical microscopy (POM), differential scanning
calorimetry (DSC), and powder X-ray diffractometry (XRD). All
complexes used in this study showed enantiotropic LC phases.
POM observations at the LC temperature revealed a typical fan
shape (Fig. 3), indicating that the complexes form a smectic
(Sm) phase. Detailed LC phase structural analyses were deter-
mined by powder XRD (Fig. 4 for 1b as a representative example,
and Figs S4 and S5†). The phase sequences and phase transition
temperatures (dened as the onset temperature of DSC peaks)
of 1a–c are summarised in Table 2. All complexes showed
polymorphism in the crystalline phase. Especially, 1b showed
many unidentied complex crystalline phases in the cooling
process.
The molecular structures of complexes 1a–c appeared
different from those of typical rod-like calamitic LC molecules;
however, as mentioned above, complexes 1a–c formed dimers
with antiparallel orientations in the crystalline phase, with the
dimers adopting a symmetric rod-like structure. In analogy to
other LC gold complexes reported previously,8 we can consider
Cr1 87 Cr2 124 SmC 172 I
Cr1 72 Cr2 121 SmC 172 I
a
Abbreviations: Cr, crystalline; Sm, smectic; I, isotropic. b Below 110 ꢀC,
many unidentied crystalline phases were observed (see Fig. S2b in the
ESI†).
that complexes 1a–c form the dimer in the Sm phase, and that
the dimer acts as a unit mesogen. The results of XRD analysis
support the hypothesis that the complexes formed dimers in the
LC phase; the interlayer spacing of the Sm phase of 1b was
estimated to be 27 A by XRD, as shown in Fig. 4, which is
noticeably larger than the molecular length of the monomer of
˚
˚
1b (17.9 A), but slightly smaller than the length of the dimer
(32.2 A) formed by the aurophilic interaction. Therefore, it is
˚
reasonable to assume that the Au complex dimer acts as the unit
mesogen, and we can conclude that the tilted mesogens
(dimers) are packed into the Sm layer to form a smectic C (SmC)
phase with a tilt angle (dened as the angle between the vector
normal to the Sm layer and the long axis of the mesogen) of 33ꢀ,
as schematically illustrated in Fig. 5. A further discussion of
obtained XRD data as it relates to phase identity and lumines-
cence properties of 1b is provided later in this report.
Photophysical properties of the complexes
Photophysical properties of the gold complexes were observed
both in crystalline and dilute solutions; Fig. 6A shows the
absorption, photoluminescence, and excitation spectra of
complex 1b. An absorption band appeared at ꢁ280 nm in
solution (CH2Cl2, 1 ꢂ 10ꢃ4 mol Lꢃ1; dielectric constant ¼ 8.93).
In another solvent, e.g. in CHCl3 (dielectric constant ¼ 4.81), the
complex showed similarly shaped bands at the same wave-
length in the absorption spectrum, meaning that a shi in the
absorption spectrum was not induced by solvent polarity. In
addition, the complex exhibited strong photoluminescence in
the crystalline phase. However, the complex showed no photo-
luminescence in dilute solution (1 ꢂ 10ꢃ6 mol Lꢃ1), while only
very weak photoluminescence with the same spectral shape as
in crystal was observed in a more concentrated solution (1 ꢂ
10ꢃ4 mol Lꢃ1, Fig. 6B).
Fig. 3 Optical texture of complexes observed by POM: (A) 1a at 157 ꢀC,
(B) 1b at 155 ꢀC, and (C) 1c at 147 ꢀC. All photographs were taken during
the first cooling scan.
The above results clearly indicate that photoluminescence
was emitted from the complex aggregates, but that the complex
did not emit luminescence in the monomeric form. The exci-
tation spectrum of the crystalline phase did not show overlap
with the absorption spectrum of the dilute solution (Fig. 6A), an
observation that also supports our hypothesis that the lumi-
nescence was in fact aggregate-induced. An emission band
appeared at around 450 nm with vibronic structures both in the
crystalline phase (Fig. 6A) and in solution (Fig. 6B). It has been
reported that photoluminescence resulting from an aurophilic
Fig. 4 XRD patterns of 1b: Cr1 phase at 25 ꢀC simulated from single
crystal X-ray structural analysis (red); SmC phase at 130 ꢀC (blue); Crx
phase at 40 ꢀC obtained by recrystallisation of the SmC phase after
cooling (black).
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p–p* transition.13 In the present complexes, the electronic
transition for photoluminescence at 450 nm is currently
unclear, but we should consider that this luminescence is
emitted from an excited state localised in a single molecule—
namely, monomer emission. However, since no luminescence
was observed under dilute conditions, i.e., when the species
were isolated as the single molecule in dilute solution, we can
thus conclude that this photoluminescence was the result of
AIE. Internal molecular motion, which results in non-radiative
deactivation of the excited state, may be hindered by aggrega-
tion. As a result, the complex showed AIE. The same photo-
physical properties were observed for the other complexes in
both the crystalline phase and in solution.
The photoluminescence lifetimes (s) of 1a–c were also
measured in the crystalline phase (Table 3 and Fig S10†).
The complexes showed single exponential decay photo-
luminescence proles, except for complex 1c, where a bi-expo-
nential decay prole was observed. The s for all the complexes
were on the microsecond timescale, meaning that the observed
photoluminescence was phosphorescence.
Photoluminescence F values were estimated for the crystals
of complexes 1a–c at room temperature in air with an integra-
tion sphere (Table 3). The complexes exhibited F values of 0.05–
0.18 in the crystal. The photostability of the materials is also
important in terms of practical application. When the materials
were irradiated with UV light at 310 nm (6.5 mW cmꢃ2) at room
temperature in air, the intensity of luminescence gradually
decreased while maintaining the spectral shape as shown in
Fig. S11† for complex 1c as a representative example: the
intensity of the luminescence decreased by half within one
hour. While the F values and photostability may be insufficient
for commercial applications, the Au complexes prepared in this
study show phosphorescence in air at room temperature (ꢁ23
ꢀC), which is very favourable for their use as materials in organic
light-emitting devices.
Fig. 5 Schematic illustration of the unit mesogen structural model and
plausible molecular packing structure in crystalline and LC phases of
complex 1b.
Photoluminescence spectra of 1a–c were also recorded both
in the Sm phase and isotropic (I) phase. Fig. 7A shows the
normalised photoluminescence spectra of complex 1b in the
crystalline and SmC phases as a representative example. The
complexes showed photoluminescence in the Sm phases,
showing a decrease in the absolute intensity of luminescence
with a thermal phase transition to the Sm phase (Fig. S7–S9† for
1a–c). During this experiment, all experimental conditions,
namely all instrumental parameters (band pass of both excita-
tion and emission slits ¼ 5 nm), excitation wavelength (310
nm), and the power of excitation light (6.5 mW cmꢃ2), were
Fig. 6 (A) Absorption, photoluminescence, and excitation spectra of
complex 1b: absorption spectrum in CH2Cl2 solution (1 ꢂ 10ꢃ4 mol Lꢃ1
)
Table 3 Photophysical parameters of complex 1 in the crystalline
phase at 23 ꢀC
(blue); photoluminescence in the crystalline phase (lex ¼ 310 nm) (red);
excitation spectrum in a crystal (lem ¼ 430 nm) (green). (B) Photo-
luminescence spectrum of 1b in CH2Cl2 solution (1 ꢂ 10ꢃ4 mol Lꢃ1).
sa (ms)
Pre-exponential factor (%)
F
1a
1b
1c
33 (13)
40 (15)
15 (59)
41 (82)
—
—
95
5
0.18
0.05
0.11
interaction can be generally observed as a broad luminescence
band in the longer wavelength region.3,13 The emission band at
ꢁ450 nm with a vibronic structure has been suggested to be
either a ligand-to-metal charge transfer (LMCT) or ligand-based
a
95% condence interval is indicated in the parentheses.
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randomly in the I phase, the molecular aggregate structures
become random with dissociated molecules existing as the
isolated monomeric form (Fig. 5); we suggest that it is for this
reason that luminescence completely disappeared in the I
phase.
As shown in Fig. 7A, the photoluminescence spectral shape
and emission maxima of complex 1b in the crystalline phases
(Cr1 and Crx) were the same as those in the SmC phase upon
heating. However, in the crystalline phase (Crx) obtained aer
recrystallisation from the melt aer cooling, the emission
maximum was shied by 35 nm to a longer wavelength, which
was accompanied by a change in the spectral shape. When the
Crx crystal phase was melted again to yield the SmC phase, the
emission maximum and spectral shape returned to the initial
state. The Commission Internationale de l'Eclairage (CIE)
chromaticity diagram allows for quantitative evaluation of
photoluminescence colour as shown in Fig. 7B. The lumines-
cence colour was deep blue in the Cr1 and SmC phases, and was
pale bluish-green in the Crx phase: the CIE coordinates (x, y)
were (0.17, 0.10) in the Cr1 phase at 40 ꢀC on rst heating, (0.18,
0.13) in the SmC phase at 130 ꢀC, and (0.21, 0.29) in the Crx
ꢀ
phase at 40 C. These results indicate that the colour of pho-
toluminescence can also be reversibly controlled by the phase
transition between the Crx and SmC phases.
As previously described (Fig. 4), the XRD pattern of the Cr1
phase of 1b was obtained by simulation from single-crystal X-
ray analysis, which revealed that the molecules were packed in
the lattice with tilting similar to the SmC phase (Fig. 5). Thus, it
is reasonable that a similarity in luminescence spectra, (i.e., the
same spectral shape and emission maxima) exists for both Cr1
and SmC phases (viz. Fig. 7A). However, in the Crx phase, the
Fig. 7 (a) Normalised photoluminescence spectra of 1b in various
phases (lex ¼ 310 nm): Cr1 at 40 ꢀC (red); SmC at 130 ꢀC (blue); Crx at
40 ꢀC (black). (B) CIE chromaticity diagram of the photoluminescence
of 1b: Cr1 (,); SmC (P); Crx (*).
˚
d001-spacing was estimated to be 16 A by XRD (Fig. 4C), which is
very similar to the molecular length of the monomer of 1b
˚
(17.9 A). XRD measurements indicated that the molecules were
packed in the lattice without tilting in the Crx phase (Fig. 5).
strictly kept constant. Moreover, since the optical densities of These results suggest to us that the aggregated structure is
the samples used in photoluminescence spectroscopy in the almost the same in the Cr1 and SmC phases, but is quite
condensed phases were larger than 3, we considered that the different in the Crx phase obtained aer recrystallisation from
samples completely absorbed excitation light in all phases, the molten states. Thus, this difference in the aggregated
meaning that the same amount of the excited states was structure may result in the change in the photoluminescence
produced in all phases by excitation with the same excitation colour of 1b induced by the thermal phase transition between
power. Therefore, we can compare the intensity of lumines- Crx and SmC phases. Although photoluminescence from
cence at least qualitatively in various phases, although the complex 1b should be assigned to a monomer emission, its
quantum yields could not be measured in LC and I phases with luminescence is emitted in the aggregate form, the properties of
our instruments. Generally, the intensity of photoluminescence which are dependent on the aggregated structure. At present,
is reduced at a higher temperature because the thermally acti- the reason for this thermochromic photoluminescence by
vated internal molecular motion deactivates the excited states. control of the aggregated structure of 1b is unclear, owing to a
Thus, it is normal to observe a lower photoluminescence lack of information on the electronic mechanism of lumines-
intensity in the Sm phase than in the crystalline phase. An cence in the present system.
abrupt change in the luminescence intensity was observed in
In contrast, the other Au complexes (1a and 1c) did not show
the phase transition to the I phase in all complexes; lumines- changes in the colour or spectrum shape of photoluminescence
cence was completely quenched in the I phase. Luminescence with the phase transition (Fig. S7 and S9†). The XRD patterns
intensity was recovered when cooling the I phase sample to re- of 1a and 1c are also shown in Fig. S4 and S5;† however, the XRD
obtain the Sm phase, demonstrating that a reversible “on–off” of complex 1a could not be recorded in the LC phase because of
switching of photoluminescence is possible through inducing a the insufficient quantity of the sample. The tilted aggregated
phase transition between Sm and I phases. Since each molecule structures were observed in both Cr2 and LC phases of complex
shows vigorous micro-Brownian motion and is oriented 1c; namely, the aggregated structures in both phases were
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almost the same. Therefore, it is quite reasonable that the
C
12H15AuClNO: C, 34.18; H, 3.59; N, 3.32; Au, 46.71. Found, C,
complexes show the same colour of luminescence in both 33.82; H, 3.15; N, 3.31; ash, 46.5%.
phases.
1b–c. According to the above procedure, 1b and 1c were
obtained from the corresponding phenyl isocyanide in 99% and
63% yields, respectively.
Conclusions
1b (n ¼ 6). M.p. 121 ꢀC. 1H NMR (400 MHz, CD2Cl2): d (ppm)
In this study, we synthesised LC complexes containing Au(I) and 7.44 (dd, J ¼ 7.2, 1.8 Hz, 2H, Ar 2,6-H), 6.94 (dd, J ¼ 7.2, 1.8 Hz,
investigated their phase transition behaviours and photo- 2H, Ar 3,5-H), 3.99 (t, J ¼ 6.8 Hz, 2H, OCH2), 1.80 (quin, J ¼ 6.8
physical properties. The luminescence intensity of the Hz, 2H, OCH2CH2), 1.31–1.49 (m, 6H, CH2), 0.91 (t, J ¼ 6.8 Hz,
complexes was enhanced in the condensed phases, a charac- 3H, CH3). FTIR (KBr, cmꢃ1): 3444, 2934, 2860, 2226, 2204, 1600,
teristic that is very favourable for the practical application of 1579, 1502, 1468, 1400, 1382, 1304, 1262, 1241, 1160, 1123,
those complexes in light-emitting devices. Observed photo- 1056, 1020, 988. MALDI-TOF-MS (m/z): [M + Na]+ calcd for
luminescence behaviour in the condensed phases could be
assigned to a monomer emission. In contrast, the luminescence
C
C
13H17AuClNNaO, 458.06; found, 458.09. Anal calcd for
13H17AuClNO: C, 35.84; H, 3.93; N, 3.21; Au, 45.2. Found, C,
properties were strongly affected by the aggregated structures of 35.35; H, 3.53; N, 3.17; ash, 42.6%.
the complexes; a reversible “on–off” switching of luminescence
1c (n ¼ 7). M.p. 124 ꢀC. 1H NMR (400 MHz, CD2Cl2): d (ppm)
was demonstrated by inducing the transition between ordered 7.44 (dd, J ¼ 6.9, 2.1 Hz, 2H, Ar 2,6-H), 6.95 (dd, J ¼ 6.9, 2.1 Hz,
and disordered aggregates, i.e., the phase transition between 2H, Ar 3,5-H), 3.99 (t, J ¼ 6.8 Hz, 2H, OCH2), 1.80 (quin, J ¼ 6.8
the LC and I phases. Moreover, the colour of luminescence Hz, 2H, CH2CH3), 1.31–1.47 (m, 8H, CH2), 0.89 (t, J ¼ 6.9 Hz, 3H,
could be reversibly controlled via the transformation between CH3). FTIR (KBr, cmꢃ1): 3443, 2955, 2927, 2864, 2853, 2220,
ordered aggregates; namely, the thermal phase transition 1600, 1579, 1502, 1467, 1304, 1261, 1160, 1032, 1009, 998.
between Cr and SmC phases. From the results described above, HRMS-ESI (m/z): [M ꢃ Cl + CH3OH]+ calcd for C15H23AuNO2,
we can expect Au LC complexes to show great potential in 446.13943; found, 446.13934. Anal calcd for C14H15AuClNO: C,
applications involving active colour- and intensity-controllable 37.39; H, 4.26; N, 3.11; Au, 43.80. Found: C, 36.99; H, 3.81; N,
light-emitting materials.
3.10; ash, 44.1%.
Experimental
Phase transition behaviour
Materials
LC complex behaviours were observed using a POM (Olympus,
The Au(I) complexes (1a–c) were synthesised from the corre- BX51) equipped with a hot stage (Instec, HCS302 hot-stage and
sponding isocyanide ligands and (tht)AuCl.9 Unless otherwise mK1000 controller). The thermodynamic properties of the LCs
noted, all solvents and reagents were purchased from were determined using DSC (Perꢀkin Elmer, Diamond DSC) at a
commercial suppliers and were used without further purica- heating and cooling rate of 2.0 C minꢃ1. At least three scans
1
tion. H NMR spectra were recorded on a JEOL ECS-400 spec- were performed to check reproducibility. TGA was performed to
trometer at 400 MHz using the residual proton in the NMR check the thermal stability of the complexes using a DTG-60AH
solvent as an internal reference. Electrospray ionisation mass analyser (Shimadzu) at a heating rate of 5 ꢀC minꢃ1. For the
spectra (ESI-MS) were recorded on a JMS-T1000LC (JEOL) sample showing the Sm phase, the phase structure was further
instrument. MALDI-TOF mass spectra were recorded on a conrmed and the interlayer spacing was estimated by XRD
˚
Bruker Autoex II instrument with dithranol (Aldrich) as a (Rigaku FR-E/R-axis IV) with Cu Ka radiation (l ¼ 1.54184 A).
matrix. Infrared (IR) spectra were recorded on a JASCO FT/IR-
4100 spectrometer using a KBr pellet. Phenyl isocyanide ligands
and (tht)AuCl were synthesised according to a literature
procedure.9,14
X-ray crystallography
Single crystals were obtained by slow evaporation from a
1a (n ¼ 5). 4-Pentyloxyphenyl isocyanide (92 mg, 0.49 mmol) CH2Cl2–hexane (1 : 2) solution, mounted on a glass bre, and
and (tht)AuCl (156 mg, 0.49 mmol) were suspended in 9 mL of reection data were measured using an u-scan technique on a
acetone, and the resulting suspension was stirred for 2 h. The Rigaku AFC-5R automated four-circular-axis diffractometer with
˚
solids in the reaction mixture were ltered off, and the ltrate graphite monochromatised Cu Ka radiation (l ¼ 1.54178 A) or a
was evaporated. The obtained white solid was recrystallised Rigaku R-AXIS RAPID two-circular-axis diffractometer with Mo
˚
from acetonitrile to give the title complex (151 mg, 0.36 mmol) Ka radiation (l ¼ 0.71075 A). The measurements were carried
in 74% yield. m.p. 134 ꢀC. 1H NMR (400 MHz, CD2Cl2): d (ppm) out at room temperature (296 K). The initial structure in the
7.44 (dd, J ¼ 7.1, 1.8 Hz, 2H, Ar 2,6-H), 6.94 (dd, J ¼ 7.1, 1.8 Hz, unit cell was determined by a direct method using SIR92.15 The
2H, Ar 3,5-H), 3.94 (t, J ¼ 6.8 Hz, 2H, OCH2), 1.80 (quin, J ¼ 6.8 structure model was rened by full-matrix least-squares
Hz, 2H, OCH2CH2), 1.37–1.46 (m, 4H, CH2), 0.93 (t, J ¼ 7.0 Hz, methods using SHELXL97.16 All calculations were performed
3H, CH3). FTIR (KBr, cmꢃ1): 3450, 2964, 2934, 2864, 2219, 1598, using the crystallographic soware package WinGX.17 The
1579, 1500, 1392, 1302, 1263, 1242, 1159, 1126, 1072, 1050, obtained crystallographic data are summarised in Table S1.†
1024, 981, 908. MALDI-TOF-MS (m/z): [M + Na]+ calcd for When alkyl chains were disordered, the occupancy of atoms was
C
12H15AuClNNaO, 444.04; found, 444.14. Anal calcd for separated into two parts.
3554 | J. Mater. Chem. C, 2014, 2, 3549–3555
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Journal of Materials Chemistry C
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Langmuir, 2011, 27, 10947; (h) S. H. Lim, J. C. Schmitt,
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Photophysical properties
UV-visible absorption and steady-state photoluminescence
spectra were recorded on a JASCO V-550 absorption spectro-
photometer and on a Hitachi F-7500 uorescence spectropho-
tometer, respectively. For measurements in the crystalline and
LC states, the same crystals prepared for the single-crystal X-ray
structure analysis were used. The crystals were placed between a
pair of quartz plates, and the resultant samples were set on a
homemade heating stage to record the spectra at a controlled
temperature. Photoluminescence quantum yields were deter-
mined using a calibrated integrating sphere system (Hitachi).
Photoluminescence decay proles were measured using a N2
laser (USHO pulsed dye laser, KEC-160; wavelength 337 nm;
pulse width 600 ps; 10 Hz). The emission proles were recorded
with a streak camera (Hamamatsu, C4334; starting delay 55 ms;
integration window 497 ms).
Acknowledgements
This work was partly supported by the MEXT-Supported
Program for the Strategic Research Foundation at Private
Universities, 2012–2016, JSPS KAKENHI (Grant no. 24550217),
and JST A-STEP (AS231Z04286D). This research was also sup-
ported by the Kyoto Advanced Nanotechnology Network (Nara
Institute of Science and Technology) and by the Cooperative
Research Program of Network Joint Research Centre for Mate-
rials and Devices (Tokyo Institute of Technology).
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