Photoluminescent gold(I) complex with biphenyl acetylene ligand showing stable nematic liquid-crystalline phase

Article abstract of DOI:10.1080/15421406.2014.940510

A novel liquid-crystalline (LC) gold(I) complex (Biph6) having biphenyl acetylene ligand was designed and synthesized, and its LC behavior and photoluminescence properties were compared with those of a previously reported gold complex (Ph6) with phenyl acetylene ligand. Biph6 exhibited an enantiotropic nematic phase in a temperature range wider than that of Ph6. Ph6 and Biph6 showed intense photoluminescence with different colors in the crystalline phase: blue and yellowish green luminescence, respectively. We demonstrated that the color of luminescence of the gold complexes could be changed by changing the structure of the molecules and molecular aggregates.

Full text of DOI:10.1080/15421406.2014.940510

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Photoluminescent Gold(I) Complex with
Biphenyl Acetylene Ligand Showing
Stable Nematic Liquid-Crystalline Phase
Nana Sugimotoa, Sho Tamaia, Kaori Fujisawaa & Osamu Tsutsumia
a Department of Applied Chemistry, College of Life Sciences,
Ritsumeikan University, Kusatsu, Japan
Published online: 06 Dec 2014.
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To cite this article: Nana Sugimoto, Sho Tamai, Kaori Fujisawa & Osamu Tsutsumi (2014)
Photoluminescent Gold(I) Complex with Biphenyl Acetylene Ligand Showing Stable Nematic
Liquid-Crystalline Phase, Molecular Crystals and Liquid Crystals, 601:1, 97-106, DOI:
10.1080/15421406.2014.940510
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Mol. Cryst. Liq. Cryst., Vol. 601: pp. 97–106, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.940510
Photoluminescent Gold(I) Complex with Biphenyl
Acetylene Ligand Showing Stable Nematic
Liquid-Crystalline Phase
NANA SUGIMOTO, SHO TAMAI, KAORI FUJISAWA,
AND OSAMU TSUTSUMI∗
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan
University, Kusatsu, Japan
A novel liquid-crystalline (LC) gold(I) complex (Biph6) having biphenyl acetylene
ligand was designed and synthesized, and its LC behavior and photoluminescence
properties were compared with those of a previously reported gold complex (Ph6) with
phenyl acetylene ligand. Biph6 exhibited an enantiotropic nematic phase in a temper-
ature range wider than that of Ph6. Ph6 and Biph6 showed intense photoluminescence
with different colors in the crystalline phase: blue and yellowish green luminescence,
respectively. We demonstrated that the color of luminescence of the gold complexes
could be changed by changing the structure of the molecules and molecular aggregates.
Keywords Gold complex; liquid crystal; photoluminescence
Introduction
Complexes containing Au(I) atoms show strong luminescence due to intermolecular inter-
action between Au atoms (aurophilic interaction) [1]. Since luminescence results from the
intermolecular interaction, gold complexes show much stronger luminescence in the solid
state than in solution. Thus, they are expected to be potential materials for organic light-
emitting diodes, chemical sensors, and so forth. Because the luminescence exhibited by
the aggregates of Au(I) complexes is due to the aurophilic interactions, the luminescence
properties, including intensity and color, depend on the structure of aggregates of these
complexes [2,3]. For example, mechanical grinding of the crystals of the Au(I) complex
(C6F5Au)2 (μ-1,4-diisocyanobenzene) induced a change in the photoluminescence color
because of a change in the crystal packing structure [3]. This suggests that it is important
to control the aggregate structure of Au(I) complexes, for their utilization as luminescent
materials with desired color and high efficiency [4].
To control the structure of aggregates, gold complexes showing liquid crystalline
(LC) properties have been developed by several groups [5]. The LC materials have both
fluidity and self-organizing nature, and they show responsivity to external fields such as
∗Address correspondence to Prof. Osamu Tsutsumi, Department of Applied Chemistry, College
of Life Sciences, Ritsumeikan University, 1-1-1, Nojihigashi, Kusatsu 525–8577, Japan. Tel.: (+81)
77-561-8577; Fax: (+81) 77-561-2659. E-mail: tsutsumi@sk.ritsumei.ac.jp
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
97

98
N. Sugimoto et al.
Figure 1. Molecular structures of gold complexes, Ph6 and Biph6, used in this study.
electric fields, magnetic fields, and light [6]. Therefore, the structures of aggregates of gold
complexes may be controlled by means of their LC nature. In our previous study, gold(I)
complexes having phenyl acetylene ligand exhibited an enantiotropic LC phase and showed
strong photoluminescence in the condensed phases [7]. However, the temperature range
over which the LC phase is maintained was very narrow (range of around 4 ◦C). For wider
applications, gold complexes that exhibit the LC phase over a wider temperature range are
needed. Generally, biphenyl derivatives tend to exhibit LC phases over a wide temperature
range [8]. Therefore, in the present study, we designed and synthesized a new gold complex
with biphenyl group (Biph6) instead of the phenyl group (Ph6). In this paper, we report the
LC behavior and photoluminescent properties of the gold complex with biphenyl acetylene
ligand.
Experimental
Materials
The structure of the gold complexes used in the study is shown in Fig. 1. The complex
Biph6 was first synthesized through a method similar to that used for preparing Ph6 [7], as
shown in Scheme 1. All solvents and reagents used in the preparation of the complex, were
of reagent grade and commercially available. They were used without further purification,
except as noted otherwise. 1H NMR spectra were recorded on a JOEL ECS-400 (400 MHz)
spectrometer. The chemical shifts were reported in parts per million (ppm), using the
residual proton in the NMR solvent as an internal reference.
Scheme 1.

Photoluminescent Liquid-Crystalline Au(I) Complex
99
Table 1. Crystal data of complex Biph6
Empirical formula
C26H32AuNO
571.49
Formula weight
Temperature (K)
Color, Habit
Crystal size (mm)
Crystal system
R[F2 > 2σ(F2)]
wR(F2)
93.0(2)
colorless, plate
0.15 × 0.15 × 0.05
monoclinic
0.0439
0.0976
Space group
Z
P21/n
8
a (Å)
b (Å)
c (Å)
α (degree)
β (degree)
γ (degree)
V (Å3)
9.6606(13)
14.538(2)
33.229(5)
—
93.054(2)
—
4660.2(11)
To confirm the structure of Biph6, single-crystal X-ray structural analysis was per-
formed. The obtained crystallographic data are summarized in Table 1. The data in Table 1
have been indexed and are included in the Cambridge Crystallographic Centre (CCDC)
database with the reference number of CCDC 959978. The indexed database contains ad-
ditional supplementary crystallographic data for this paper and may be accessed without
charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html. The CCDC may contacted by
mail at 12, Union Road, Cambridge CB2 1EZ, U.K., by fax at (44)1223–336–033, or by
e-mail at deposit@ccdc.cam.ac.uk.
Synthesis of 1. 4-Bromo-4ꢀ-hydroxybiphenyl (300 mg, 1.2 mmol), potassium carbonate
(330 mg, 2.4 mmol), and 1-bromohexane (198 mg, 1.2 mmol) were added to 5 mL of
acetone, and the resulting mixture was refluxed for 24 h. The solid in the reaction mixture
was filtered off, and the filtrate was evaporated. The residue was dissolved in ethyl acetate
and then sequentially washed with water and brine. The organic layer was dried with
anhydrous sodium sulfate, and then, the solvent was removed by evaporation. The residual
crude product was dissolved in dichloromethane and then purified by reprecipitation from
ethanol to obtain 114 mg (0.34 mmol) of 1 in 68% yield. 1H NMR (400 MHz, CDCl3, δ):
7.52 (dd, J = 6.7 and 2.1 Hz, 2H, 3,5-H in biphenyl), 7.47 (dd, J = 6.9 and 2.3 Hz, 2H,
3ꢀ,5ꢀ-H in biphenyl), 7.41 (dd, J = 6.7 and 3.0 Hz, 2H, 2,6-H in biphenyl), 6.96 (dd, J =
6.9 and 2.3 Hz, 2H, 2ꢀ,6ꢀ-H in biphenyl), 3.99 (t, J = 6.6 Hz, 2H, OCH3), 1.80 (m, 2H,
OCH2CH2), 1.51–1.30 (m, 6H, CH2), 0.91 (t, J = 7.2 Hz, 3H, CH3).
Synthesis of 2. 1 (114 mg, 0.34 mmol), 2-methyl-3-butyn-2-ol (0.14 mL, 1.4 mmol),
bis(triphenylphosphine)palladium chloride (8.8 mg, 0.01 mmol), triphenylphosphine
(1.6 mg, 0.007 mmol), and copper iodide (2.0 mg, 0.007 mmol) were dissolved in tri-
ethylamine (2.0 mL). The resultant solution was refluxed for 5 h under Ar. After the solid
in the reaction mixture was removed by filtration, the filtrate was evaporated. The residue
was dissolved in ethyl acetate, and then washed sequentially with water, saturated aqueous

100
N. Sugimoto et al.
ammonium chloride, and brine. The organic layer was dried over anhydrous sodium sulfate,
and the solvent was removed by evaporation. The crude product obtained was purified by
silica gel column chromatography (eluent: dichloromethane) to obtain 84 mg (0.25 mmol)
1
of 2 in 74% yield. H NMR (400 MHz, CDCl3, δ): 7.52–7.26 (m, 6H, 2,3,5,6,3ꢀ,5ꢀ-H in
biphenyl), 6.96 (dd, J = 6.9 and 2.3 Hz, 2H, 2ꢀ,6ꢀ-H in biphenyl), 4.00 (t, J = 6.6 Hz,
2H, OCH3), 2.01 (s, 1H, OH) 1.82–1.34 (m, 14H, CH2, C≡C–C(CH3)2OH), 0.91 (t, J =
6.8 Hz, 3H, O(CH2)5CH3).
Synthesis of 3. 2 (84 mg, 0.25 mmol) and potassium hydroxide (44 mg, 0.75 mmol) were
added to toluene (8.0 mL), and the mixture was refluxed for 5 h. After the solid in the reaction
mixture was removed by filtration, the filtrate was evaporated. The obtained residue was
dissolved in ethyl acetate and was then washed sequentially with water, saturated aqueous
ammonium chloride, and brine. The organic layer was dried over anhydrous sodium sulfate,
and the solvent was removed by evaporation. The crude product obtained was purified
by silica gel column chromatography (eluent: dichloromethane/hexane (1/1)) to obtain
39 mg (0.14 mmol) of 3 in 56% yield. 1H NMR (400 MHz, CDCl3, δ): 7.55–7.50 (m, 6H,
2,3,5,6,3ꢀ,5ꢀ-H in biphenyl), 6.97 (dd, J = 6.9 and 2.3 Hz, 2H, 2ꢀ,6ꢀ-H in biphenyl), 4.00
(t, J = 6.4 Hz, 2H, OCH3), 3.11 (s, 1H, C≡CH), 1.78–1.35 (m, 14H, CH2), 0.91 (t, J =
6.8 Hz, 3H, O(CH2)5CH3).
Synthesis of Biph6. CH3COONa (57 mg, 0.70 mmol) in
a
solution of
dichloromethane/methanol (4.5/1.7 mL) was added to a solution of 3 (39 mg, 0.14 mmol)
and (tht)AuCl (49 mg, 0.15 mmol) in dichloromethane/methanol (0.3/1.3 mL) under Ar.
After the reaction mixture was stirred for 24 h, the precipitate was collected by filtration
and then sequentially washed with methanol, water, methanol, and dichloromethane. The
green solid obtained was suspended in dichloromethane (5 mL), and 1-pentylisocyanide
(21 μL, 0.17 mmol) was added to the resultant suspension. By stirring the suspension
for 3 h at room temperature, a purple homogeneous solution was obtained. The reaction
mixture was filtered, and the solvent was evaporated. The solid obtained was recrystallized
from the dichloromethane/heptane solution, and thus, 52 mg of Biph6 (0.091 mol) was
1
obtained as pale yellow needles in 73% yield. m.p. 146.7–147.5◦C. H NMR (400 MHz,
CHCl3, δ): 7.50–7.45 (m, 6H, 2,3,5,6,3ꢀ,5ꢀ-H in biphenyl), 6.94 (dd, J = 6.9 and 2.3 Hz,
2H, 2ꢀ,6ꢀ-H in biphenyl), 3.98 (t, J = 6.9 Hz, 2H, OCH2), 3.63 (t, J = 6.9 Hz, 2H, NCH2),
1.83–1.76 (m, 4H, NCH2CH2, OCH2CH2), 1.60–1.34 (m, 8H, CH2), 0.97–0.89 (m, 6H,
CH3). MS-MALDI-TOF (m/z): 573 (4%, M + 2), 572 (25%, M + 1), 571 (100%, M+).
HRMS-ESI-TOF (m/z): [M + Na]+ calcd for C26H32AuNNaO, 594.2047; found, 594.2038.
Phase Transition Behavior
The LC behavior of the complex was observed by polarizing microscopy (POM) using an
Olympus BX51 equipped with a hot stage (Instec, HCS302 hot-stage and mK1000 con-
troller). Thermogravimetric-differential thermal analysis (TG-DTA) using a DTG-60AH
unit (Shimadzu) at heating rate of 1.0◦C/min was performed to check the thermal stability
of the complexes. Thermodynamic properties of LCs were determined on a differential
scanning calorimeter (DSC, SII X-DSC7000) at a heating and cooling rate of 5.0◦C/min.
At least three scans were performed to check reproducibility.

Photoluminescent Liquid-Crystalline Au(I) Complex
101
Figure 2. ORTEP view of the molecular structure of Biph6 obtained by single-crystal X-ray struc-
tural analysis. Color legends: gray, C; white, H; red, O; purple, N; yellow, Au.
Photophysical Property
UV–visible absorption and steady-state photoluminescence spectra were recorded on a
JASCO V-500 absorption spectrophotometer and on a Hitachi F-7500 fluorescence spec-
trophotometer, respectively.
Figure 3. Thermodynamic behavior of Biph6: (A) TG (red) and DTA (blue) thermographs (heating
rate = 1◦C/min); (B) DSC thermogram (scan rate = 5◦C/min).

102
N. Sugimoto et al.
Figure 4. Polarizing optical micrograph of complex BiPh6 observed in the heating process at 148◦C.
Results and Discussion
Synthesis and Characterization of Complexes
The analytical data given in the Experimental section confirm that the desired compounds
were obtained. The molecular structure obtained is shown in Fig. 2, and the key crystallo-
graphic data are summarized in Table 1. The compound was crystallized in the monoclinic
space group P21/n, with eight formula units per unit cell. The Au atoms are linearly coor-
dinated by the isocyanide and alkynyl ligands; thus, the core unit of the molecule exhibits
a rod-like structure. Full characterization data for Ph6 have been reported elsewhere [7].
Single-crystal X-ray analysis of Ph6 suggested the existence of an aurophilic interaction
between neighboring molecules, as the interatomic distance between Au atoms was 3.5
Å, which is much shorter than the sum of their van der Waals radii (3.8 Å) [9]. Thus,
the complex may have formed a dimer in the Ph6 crystal. On the other hand, the inter-
atomic distance between Au atoms in neighboring molecules of the crystal BiPh6 is 3.9 Å,
excluding the existence of an interaction between Au atoms. Thus, in contrast to Ph6, the
complex did not form a dimer in the BiPh6 crystal.
The prepared complexes were stable in air at room temperature. Thermogravimetric
analysis (TGA) was performed to assess their thermal stability (Fig. 3A for Biph6). We
confirmed that Biph6 was stable up to 165◦C, and we defined the thermal decomposition
temperature as the temperature corresponding to 5% weight loss. The thermal decompo-
sition temperature of Ph6 was 148◦C. Above the decomposition temperature, Ph6 and
Biph6 showed approximately 18% and 13% weight loss, respectively, which nearly cor-
respond to the weight percentage of isocyanide ligands (20 wt% for Ph6 and 17 wt% for
Biph6). In addition, the thermal decomposition temperature is close to the boiling point of
pentylisocyanide (137–138◦C); thus, on heating to 140◦C, the coordination bond between
the Au atom and the isocyanide ligand may be cleaved, thereby liberating pentylisocyanide.
Maximum weight loss was observed after the complex was heated to 400◦C.
LC Behavior of the Au(I) Complex
The LC behavior of Au complexes was observed by POM, TGA, and differential scanning
calorimetry. On the basis of their optical textures, the LC phase of both complexes was
identified as nematic (N) by POM (Fig. 4). The phase-transition behaviors of the complexes

Photoluminescent Liquid-Crystalline Au(I) Complex
103
Table 2. Phase transition behavior of gold complexes Ph6 and Biph6a
Complex
Phase transition temperatureb (◦C)
Ph6
Heating
Cooling
Heating
Cooling
Cr 89 N 93 I
Cr 70 N 94 I
Cr 146 N 165 Dec.
Cr 117 N 165 Dec.
Biph6
aAbbreviations: Cr, crystalline; N, nematic; I, isotropic; Dec., decomposition. bData refer to DSC
scan. Phase transition temperatures were determined as peak onset.
are summarized in Table 2. In the DSC thermogram of Biph6 (Fig. 3B), the endothermic
peak appearing at 146◦C in the heating process could be assigned as the melting point. It
was observed by POM that the N-to-isotropic phase transition of Biph6 occurred at 165◦C.
However, thermal decomposition of the complex was also induced simultaneously with
the phase transition, so the clearing temperature could not be determined exactly. Notably,
Biph6 showed a N phase in a temperature range of 19◦C, which is considerably wider
than that in the case of Ph6 (4◦C). As we previously reported, Ph6 forms dimers in the N
phase by intermolecular aurophilic interaction, and the dimer of the complex acts as a unit
mesogen in the N phase [7]. However, the molecules in Biph6 showed no intermolecular
interaction in the crystal; thus, they might have existed as the monomeric form in the N
phase as well as in the crystalline phase, and the monomer acted as the unit mesogen.
Since the structures of the unit mesogen are quite different, the aspect ratio of the mesogen
is different in both complexes: it should be much larger in Biph6. We believe that the
difference in the aspect ratio of the unit mesogen results in a difference in the temperature
range of the N phase.
Figure 5. Normalized absorption and photoluminescence spectra of the complex Biph6. Green:
absorption spectrum in dichloromethane solution (10−5 mol/L), blue: normalized excitation spectrum
of the crystal (λem = 504 nm), and red: normalized photoluminescence spectrum of the crystal (λex
=
360 nm). Inset: photograph of luminescent crystals of Biph6 taken under UV irradiation at 365 nm.

104
N. Sugimoto et al.
Figure 6. Comparison of luminescence properties between Ph6 and Biph6. (A) Normalized photo-
luminescence spectra of both complexes in crystalline form: black, Ph6 (λex = 340 nm); red, Biph6
(λex = 360 nm). (B) CIE diagrams for photoluminescence of both complexes: ◦, Ph6 (λex = 340 nm);
ꢀ, Biph6 (λex = 360 nm).
Photophysical Property of the Au(I) Complex
Fig. 5 shows absorption spectra of Biph6 in a dichloromethane solution (2.8 × 10−5 mol/L).
An absorption band for the dilute solution appeared at 314 nm. Photoluminescence and
excitation spectra of Biph6 in the crystalline phase are also shown in Fig. 5. As shown
in the inset of the figure, this complex showed relatively strong photoluminescence in the
crystalline phase upon excitation at 360 nm. In the luminescence spectrum of the crystalline
phase, the major and minor luminescence bands are at ∼550 and ∼400 nm, respectively.

Photoluminescent Liquid-Crystalline Au(I) Complex
105
However, the complex showed only the minor photoluminescence band at 400 nm in a dilute
solution (10−5 mol/L). Therefore, the minor luminescence can be attributed to emission
from the monomeric form of Biph6, and the major emission observed in the crystalline
phase can be attributed to aggregation of the complexes. The results clearly indicate that the
Au complexes synthesized in the study did not exhibit concentration quenching, but rather
demonstrated aggregation induced emission (AIE). The same trends in photoluminescence
were observed for Ph6 in the crystalline phase (Fig. 4) [7].
Fig. 6 shows a comparison of photoluminescence properties between Ph6 and Biph6
in the crystalline phase at 25◦C. As shown in Fig. 6A, the shapes of photoluminescence
spectra of both complexes are very similar, but the emission maximum of Biph6 is at
approximately 100 nm above that of Ph6. Moreover, the photoluminescence colors were
quite different for Ph6 and Biph6: blue and yellowish green, respectively. We evaluated the
photoluminescence color quantitatively using a Commission Internationale de l’Eclairage
(CIE) chromaticity diagram (Fig. 6B). The CIE coordinates (x, y) were (0.16, 0.11) for
Ph6 and (0.35, 0.56) for Biph6. Since it is already known that the luminescent properties
of gold complexes, such as color and intensity, depend on the structure of molecular
aggregates, as well as the structure of the single molecules [2,3], the different properties of
Ph6 and Biph6 indicate their different structures. The X-ray structural analysis indicated
the difference in the crystal packing structure between them; Ph6 was crystallized in the
triclinic space group P-1, but Biph6 was crystallized in the monoclinic space group P21/n,
as mentioned above. Thus, we confirm that the color of photoluminescence of these LC
gold complexes can be controlled by changing both structures of aggregates and of single
molecules.
Conclusion
In this study, we developed a new LC gold complex having biphenyl acetylene ligand.
It showed an enantiotropic N phase over a temperature range that is wider than that in
the case of previously reported analog complex, which had a phenyl acetylene ligand.
The gold complexes exhibited intense photoluminescence in the crystalline phase but no
luminescence in dilute solutions. Therefore, we can safely conclude that these materials
exhibit AIE and that the complexes are potential materials for use in practical light-emitting
devices. We demonstrated that the color of luminescence from the gold complexes depended
on both structures of molecules and molecular aggregates. To obtain a gold complex emitting
light of the desired color, we thus need to design and control not only the molecular structure
but also the structure of the molecular aggregates.
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