Triisocyano-based trinuclear gold(I) complexes with aggregation-induced emission (AIE) and mechanochromic luminescence characteristics

Article abstract of DOI:10.1016/j.ica.2015.04.016

Abstract Three novel trinuclear gold(I) complexes have been designed and synthesized. These gold(I) complexes exhibit aggregation-induced emission characteristics. Furthermore, the photoluminescence properties of these luminogens show off-on green fluores

Full text of DOI:10.1016/j.ica.2015.04.016

Accepted Manuscript
Triisocyano-based trinuclear gold(I) complexes with aggregation-induced emis-
sion (AIE) and mechanochromic luminescence characteristics
Zhao Chen, Peng-Shou Huang, Zheng Li, Jun Yin, Guang-Ao Yu, Sheng Hua
Liu
PII:
S0020-1693(15)00204-2
http://dx.doi.org/10.1016/j.ica.2015.04.016
ICA 16509
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
10 December 2014
14 April 2015
15 April 2015
Please cite this article as: Z. Chen, P-S. Huang, Z. Li, J. Yin, G-A. Yu, S.H. Liu, Triisocyano-based trinuclear gold(I)
complexes with aggregation-induced emission (AIE) and mechanochromic luminescence characteristics,
Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.04.016
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Triisocyano-based
trinuclear
gold(I)
complexes
with
aggregation-induced emission (AIE) and mechanochromic
luminescence characteristics
Zhao Chen‡a, Peng-Shou Huang‡a, Zheng Lia, Jun Yina, Guang-Ao Yua,*, Sheng
Hua Liua,*
a Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College
of Chemistry, Central China Normal University, Wuhan, 430079, China.
*Corresponding author. Tel.: +86 27 6786 7725; fax: +86 27 6786 7725.
E-mail address: yuguang@mail.ccnu.edu.cn (G.-A. Yu); chshliu@mail.ccnu.edu.cn (S.
H. Liu).
‡ These two authors made equal contributions.
ABSTRACT
Three novel trinuclear gold(Ι) complexes have been designed and synthesized. These
gold(Ι) complexes exhibit aggregation-induced emission characteristics. Furthermore,
the photoluminescence properties of these luminogens show off-on green
fluorescence in response to mechanical grinding. The changing of weak multiple
intermolecular C-H···F or π-π interactions, or the formation of aurophilic interactions
are possibly responsible for their mechanochromism phenomena.
1. Introduction
Recently, researchers have begun to pay more and more attention to the
exploitation of effective luminescent organic materials due to their potential
application in photoelectronic devices and fluorescent switches [1-5]. Of particular
interest are those stimuli-responsive, smart luminescent materials. Mechanochromic
materials, as one type of stimuli-responsive fluorescent materials, have also aroused
substantial attention [6-8]. However, many chromophoric organics and polymers
exhibit very strong emissions in their dilute solutions, but tend to be partially or
wholly weakened or quenched due to intermolecular interactions and relevant
1

non-radiative decay when fabricated into thin films [9], a widely known phenomenon
referred to as aggregation-caused quenching (ACQ) [10]. The notorious ACQ effect
limits the efficient practical applications of fluorescent materials to a great extent.
Fortunately, in 2001, Tang et al. found that some propeller-shaped molecules show
an unusual and interesting phenomenon known as aggregation-induced emission (AIE)
[11]. Subsequently, in 2002, Park et al. reported a new class of CN-MBE
nanoparticles, which exhibited aggregation-induced emission enhancement (AIEE)
phenomenon [12]. Both AIE and AIEE are very useful methodologies used to
overcome ACQ and achieve bright aggregative-state emission. Highly aggregate state
emissive organic compounds are regarded as promising candidates for practical
application as effective emissive materials [13-17]. In addition, the study of gold(I)
chemistry is indeed fascinating, and the interesting aurophilic (Au···Au) interaction
between gold centres has fascinated chemical researchers for decades [18-25]. It is
important and significant, therefore, to synthesize novel gold(I) complexes with AIE
and mechanochromic fluorescent properties. Herein, we report three novel
triisocyano-based trinuclear gold(I) complexes. These luminogens are very interesting
fluorescent molecules because they exhibit AIE behaviors and mechanochromic
luminescence properties.
2. Materials and methods
2.1. Experimental
General: All manipulations were carried out under an argon atmosphere by using
standard Schlenk techniques, unless otherwise stated. DMF and CH2Cl2 were
purchased as analytical-grade from Shen Shi Hua Gong Company (China). DMF was
dried with magnesium sulfate then distilled under vacuum, CH2Cl2 was dried with
CaH2 then distilled, and ultra-pure water was used in the experiments. The starting
material benzene-1,2,3-triol, benzene-1,2,4-triol and benzene-1,3,5-triol purchased
from Alfa Aesar was used as received. All other starting materials and reagents were
purchased as analytical-grade from commercial suppliers and used without further
purification. Compounds 1a, 2a, 3a were prepared by procedures described in the
2

corresponding literature [26], [Au(C6F5)(tht)] (tht = tetrahydrothiophene) was
prepared by procedures described in the corresponding literature [27],
N-(4-hydroxyphenyl)formamide was prepared by procedures described in the
corresponding literature [28].
2.2. Compounds characterization
1H and 13C NMR spectra were collected on American Varian Mercury Plus 400
1
spectrometer (400 MHz) or 600 MHz. H NMR spectra are reported as followed:
chemical shift in ppm (δ) relative to the chemical shift of TMS at 0.00 ppm,
integration, multiplicities (s = singlet, d = doublet, t = triplet, m = multiplet), and
coupling constant (Hz). 13C NMR chemical shifts reported in ppm (δ) relative to the
central line of triplet for CDCl3 at 77 ppm. Elemental analyses (C, H, N) were
performed by the Microanalytical Services, College of Chemistry, CCNU. UV-Vis
spectra were obtained on U-3310 UV Spectrophotometer. Fluorescence spectra were
recorded on a Hitachi-F-4500 fluorescence spectrophotometer and Fluoromax-P
luminescence spectrometer (HORIBA JOBIN YVON INC.). XRD studies were
recorded on a Shimadzu XRD-6000 diffractometer using Ni-filtered and
graphite-monochromated Cu Kα radiation (λ = 1.54 Å, 40 kV, 30 mA). The absolute
fluorescence quantum yields of thin-films were measured by Edinburgh Instruments
FLS900. Dynamic light scattering (DLS) measurements were performed on the
Zetasizer instrument ZEN3600 (Malvern, UK) with a 173°back scattering angle and
He-Ne laser (λ = 633 nm). IR spectra were measured by the Bruker Tensor FT-IR
spectrometer. The DMF/water mixtures with various water fractions were prepared by
slowly adding ultra-pure water into the DMF solution of samples.
2.3. Synthesis
2.3.1. Synthesis of 1b, 2b and 3b
A
mixture of compound 1a, 2a or 3a (2 mmol, 1.06 g) and
N-(4-hydroxyphenyl)formamide (6.6 mmol), K2CO3 (13.2 mmol) was stirred in DMF
3

(50 ml) for overnight at 60 ℃. After completion of present reaction, DMF was
removed from reaction system by vacuum distillation. the residual mixtures were
extracted with dichloromethane (3 × 20 mL). The combined organic layers were
washed with brine, dried (Na2SO4), and concentrated in vacuo. The residues were
purified by column chromatography, affording the expected solid products 1b, 2b and
3b in a yield of 50%, 51% and 45% respectively. Compound 1b: 1H NMR (600 MHz,
DMSO): δ (ppm) = 10.00 (s, 3H), 8.18 (s, 3H), 7.47 (d, J = 12.0 Hz, 6H), 7.06 (s, 1H),
6.93-6.83 (m, 6H), 6.64 (d, J = 6.0 Hz, 2H), 3.99-3.88 (m, 12H), 1.84-1.74 (m, 12H).
13C NMR (100 MHz, DMSO): δ (ppm) = 162.67, 159.23, 155.63, 155.02, 152.94,
137.43, 131.59, 131.33, 123.59, 120.85, 119.90, 115.31, 114.69, 106.62, 72.33, 68.19,
67.52, 29.75, 26.62, 25.85. Anal. Calcd. for C39H45N3O9: C, 66.94; H, 6.48; N, 6.00.
1
Found: C, 67.01; H, 6.43; N, 5.96. Compound 2b: H NMR (400 MHz, DMSO): δ
(ppm) = 9.99 (s, 3H), 8.19 (s, 2H), 7.49-7.46 (m, 5H), 7.07 (d, J = 8.0 Hz, 1H), 6.88 (t,
J = 10.0 Hz, 8H), 6.56 (s, 1H), 6.42-6.39 (m, 1H), 3.98-3.89 (m, 12H), 1.83 (s, 12H).
13C NMR (100 MHz, DMSO): δ (ppm) =162.64, 159.17, 155.55, 154.96, 153.73,
149.90, 142.63, 131.54, 131.29, 120.77, 119.83, 115.87, 115.30, 114.66, 104.80,
102.36, 69.50, 68.19, 67.44, 25.70. Anal. Calcd. for C39H45N3O9: C, 66.94; H, 6.48; N,
6.00. Found: C, 66.98; H, 6.41; N, 6.04. Compound 3b: 1H NMR (400 MHz, CDCl3):
δ (ppm) = 8.50 (d, J = 12.0 Hz, 2H), 8.33 (d, J = 4.0 Hz, 2H), 7.67 (d, J = 12.0 Hz,
2H), 7.44-7.41 (m, 3H), 7.02 (d, J = 12.0 Hz, 3H), 6.89-6.85 (m, 6H), 6.06 (d, J = 12.0
Hz, 3H), 4.00 (d, J = 12.0 Hz, 12H), 1.95 (s, 12H). 13C NMR (100 MHz, DMSO): δ
(ppm) = 162.67, 160.67, 159.20, 155.61, 155.00, 131.60, 131.35, 120.82, 119.88,
115.33, 114.69, 93.89, 67.32, 25.60. Anal. Calcd. for C39H45N3O9: C, 66.94; H, 6.48;
N, 6.00. Found: C, 66.97; H, 6.43; N, 6.06.
2.3.2. Synthesis of 1c, 2c and 3c
A CH2Cl2 suspension (15 ml) of 1b, 2b or 3b (0.35 g, 0.5 mmol) and triethylamine
(5 ml) was cooled to 0 ℃. To the mixture was added dropwise a CH2Cl2 solution (10
ml) of triphosgene (1.65 mmol). The mixture was refluxed for 3 h, then 10% aq.
Na2CO3 (50 ml) was added dropwise at room temperature. the mixture was extracted
4

with dichloromethane (3 × 20 mL). The combined organic layers were washed with
brine, dried (Na2SO4), and concentrated in vacuo. The residues were purified by
column chromatography, affording the expected solid products 1c, 2c and 3c in a yield
1
of 80% respectively. Compound 1c: H NMR (400 MHz, CDCl3): δ (ppm) =
7.29-7.25 (m, 6H), 6.95 (t, J = 8.0 Hz, 1H), 6.84-6.80 (m, 6H), 6.57 (d, J = 8.0 Hz,
2H), 4.05–4.00 (m, 12H), 2.04-1.87 (m, 12H). 13C NMR (100 MHz, CDCl3): δ (ppm)
= 162.49, 159.22, 159.10, 152.99, 137.69, 127.57, 123.46, 119.16, 114.86, 106.53,
72.50, 68.26, 68.02, 67.74, 26.63, 25.88, 25.78. Calcd. for C39H39N3O6: C, 72.54; H,
6.09; N, 6.51. Found: C, 72.46; H, 6.13; N, 6.57. Compound 2c: 1H NMR (400 MHz,
CDCl3): δ (ppm) = 7.30-7.27 (m, 6H), 6.86-6.81(m, 7H), 6.49 (s, 1H), 6.39 (d, J = 4.0
Hz, 1H), 4.05–3.97 (m, 12H), 2.05-1.94 (m, 12H). 13C NMR (100 MHz, DMSO): δ
(ppm) = 170.32, 162.97, 159.14, 153.66, 149.79, 142.58, 127.75, 118.53, 115.25,
104.65, 102.26, 69.35, 67.77, 59.83, 25.54, 25.28, 20.74, 14.07. Calcd. for
C39H39N3O6: C, 72.54; H, 6.09; N, 6.51. Found: C, 72.49; H, 6.05; N, 6.46.
Compound 3c: 1H NMR (600 MHz, CDCl3): δ (ppm) = 7.30 (d, J = 6.0 Hz, 6H), 6.86
(d, J = 6.0 Hz, 6H), 6.04 (s, 3H), 4.03 (d, J = 6.0 Hz, 6H), 3.98 (d, J = 6.0 Hz, 6H),
1.96 (m, 12H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 162.82, 160.78, 159.27,
127.69, 115.05, 93.99, 67.86, 67.37, 29.63, 25.87, 25.80. Calcd. for C39H39N3O6: C,
72.54; H, 6.09; N, 6.51. Found: C, 72.50; H, 6.15; N, 6.47.
2.3.3. Synthesis of 1, 2 and 3
A mixture of [Au(C6F5)(tht)] (0.47 mmol) and 1c, 2c or 3c (0.1 g, 0.15 mmol) was
stirred in CH2Cl2 (20 ml) over night at room temperature. After completion of present
reaction, the solvent was evaporated. A small amount of CH2Cl2 (5 ml) was added,
and then 50 ml of n-hexane was added. Collecting the white solid products 1, 2 and 3
by suction filtration in a yield of 75% respectively. Complex 1: 1H NMR (400 MHz，
CDCl3): δ (ppm) = 7.43 (t, J = 10.0 Hz, 6H), 6.98-6.90 (m, 7H), 6.59 (d, J = 8.0 Hz,
2H), 4.08 (s, 12H), 2.01-1.89 (m, 12H). IR (KBr, cm-1): 2213 (N≡C). Anal. Calcd for
C57H39Au3F15N3O6: C, 39.39; H, 2.26; N, 2.42. Found: C, 39.35; H, 2.21; N, 2.48.
1
Complex 2: H NMR (400 MHz, CDCl3): δ (ppm) = 7.45 (t, J = 10.0 Hz, 5H),
5

6.97-6.82 (m, 8H), 6.50 (d, J = 4.0 Hz, 1H), 6.41-6.39 (m, 1H), 4.10-3.99 (m, 12H),
2.01 (s, 12H). IR (KBr, cm-1): 2214 (N≡C). Anal. Calcd for C57H39Au3F15N3O6: C,
39.39; H, 2.26; N, 2.42. Found: C, 39.37; H, 2.23; N, 2.46. Complex 3: 1H NMR (600
MHz, CDCl3): δ (ppm) = 7.46 (d, J = 12.0 Hz, 6H), 6.96 (d, J = 6.0 Hz, 6H), 6.04 (s,
3H), 4.09 (t, J = 6.0 Hz, 6H), 3.99 (t, J = 6.0 Hz, 6H), 2.01-1.96 (m, 12H). IR (KBr,
-1
≡C). Anal. Calcd for C
cm ): 2214 (N
57H39Au3F15N3O6: C, 39.39; H, 2.26; N, 2.42.
Found: C, 39.45; H, 2.20; N, 2.50.
Scheme 1. Synthesis routes of complexes 1, 2 and 3.
3. Results and discussion
3.1. Synthesis
Three triisocyano-based trinuclear gold(I) complexes were synthesized according to
the synthetic routes presented in Scheme 1. The intermediate products 1c, 2c and 3c
as starting materials were reacted with [Au(C6F5)(tht)] to provide the target gold(I)
complexes 1-3 in high yields.
3.2. AIE characteristics
To study the aggregation-induced emission behaviors of complexes 1, 2 and 3, the
UV-visible absorption spectra of 1, 2 and 3 (10 µM) in DMF-H2O mixtures with
6

various water fractions (fw) were investigated (supporting information, Fig. S1-S3).
Their absorptions red-shifted at first, then blue-shifted as the fw values were increased.
In addition, the level-off tails in the visible region were observed in the UV/Vis
absorption spectra of the luminogens 1, 2 and 3 in DMF-H2O mixtures with high
water content. Such tails are associated with the Mie scattering effect, which is
generally observed in nanoparticle suspensions [29]. Subsequently, the
photoluminescence (PL) spectra of 1, 2 and 3 (10 µM) in DMF-H2O mixtures with
various fw were determined. As shown in Fig. 1, when a dilute DMF solution of
luminogen 1 was excited at 330 nm, the PL signals were rather weak. In addition, the
solution was hardly emissive under UV light with a wavelength of 365 nm. However,
when the fw in the DMF solution was increased to 30%, a new green emission was
observed, with λmax at 500 nm. As the fw in the DMF solution was increased to 90%,
the emission intensity of 1 at 500 nm was approximately 98-fold higher than that of
pure DMF and the emission became a strong green color. This is because water is a
non-solvent for 1, and an augment in the fw value of mixed solvent led to the
conversion of the luminogen from the dissolved and well dispersed state in pure DMF
to aggregated particles in DMF-H2O mixtures with high water content. In fact, the
aggregated particles obtained were characterized by dynamic light scattering. As can
be observed in Fig. 4, the results demonstrated the real presence of nanoaggregates as
a main constituent. The generation of nanoaggregates largely gave rise to a variation
in the molecular packing and a change in the intermolecular gold-gold interactions.
Thus, the green light emission of luminogen 1 was triggered by aggregate formation.
Obviously, luminogen 1 is an AIE-active species. Similarly, as presented in Fig. 2 and
Fig. 3, the emission intensity of complexes 2 and 3 was significantly enhanced as the
fw increased. Therefore, 2 and 3 are also AIE-active fluorescent molecules.
7

Fig. 1. (a) PL spectra of the dilute solutions of luminogen 1 (1.0×10-5 mol L-1) in
DMF-H2O mixtures with different volume fractions of water. Excitation wavelength =
330 nm. (b) The changes in emission intensity of 1 at 500 nm in DMF-H2O mixtures
with various volume fractions of water (0%-90%). The inset shows the emission
images of 1 (1.0×10-5 mol L-1) in pure DMF as well as 90% water fraction.
Fig. 2. (a) PL spectra of the dilute solutions of luminogen 2 (1.0×10-5 mol L-1) in
DMF-H2O mixtures with different volume fractions of water. Excitation wavelength =
330 nm. (b) The changes in emission intensity of 2 in DMF-H2O mixtures with
various volume fractions of water (0%-90%). The inset shows the emission images of
2 (1.0×10-5 mol L-1) in pure DMF as well as 90% water fraction.
8

Fig. 3. (a) PL spectra of the dilute solutions of luminogen 3 (1.0×10-5 mol L-1) in
DMF-H2O mixtures with different volume fractions of water. Excitation wavelength =
330 nm. (b) The changes in emission intensity of 3 in DMF-H2O mixtures with
various volume fractions of water (0%-90%). The inset shows the emission images of
3 (1.0×10-5 mol L-1) in pure DMF as well as 90% water fraction.
Fig. 4. (a) Size distribution curve of complex 1 (1.0×10-5 mol L-1) in DMF-water
mixtures with 50% volume fraction of water; (b) Size distribution curve of complex 2
(1.0×10-5 mol L-1) in DMF-water mixtures with 50% volume fraction of water; (c)
Size distribution curve of complex 3 (1.0×10-5 mol L-1) in DMF-water mixtures with
50% volume fraction of water.
3.3. Thin-film fluorescence behaviors
9

Thin-film fluorescence spectra were used to further study the aggregation
characteristics of luminogens 1, 2 and 3. Luminogens 1, 2 and 3 exhibited good
solubility in dichloromethane or trichloromethane solvents, which allowed the
formation of corresponding thin films. As evident from Fig. 5, the thin-film
fluorescence spectra of 1, 2 and 3 showed a single emission band with a λmax at
around 498 nm. Correspondingly, thin-films of these luminogens emitted strong green
fluorescence under illumination with a UV lamp at 365 nm. Among these luminogens,
the thin-film of 1 exhibited the strongest green fluorescence. The thin-film absolute
fluorescence quantum yield of 1 was 15.1%. The quantum yield of thin-film 2 was
8.1%, which was lower than thin-film 1. The thin-film quantum yield of 3 was 8.3%,
which was similar to that of 2.
3
1
2
400
450
500
550
600
Wavelength (nm)
Fig. 5. Thin-film fluorescence spectra of luminogens 1, 2 and 3 (excitation
wavelength = 365 nm).
3.4. Mechanochromic luminescence properties
Next, the mechanochromic fluorescence phenomena of luminogens 1, 2 and 3 were
explored by photoluminescence spectroscopy. As shown in Fig. 6, the solid sample of
1 was hardly emissive under UV illumination at 365 nm. Interestingly, upon grinding
of solid power 1 using a spatula or a pestle, a new emission band with a λmax at 496
nm was observed and the corresponding emission was converted into the strong green
10

luminescence. However, upon treatment of the ground powder with dichloromethane
solvent, the green emitting solid cannot be reverted to its original color. In order to
understand the possible mechanism of this mechanochromic phenomenon, the
structural transformation of the solid sample of 1 was examined by X-ray diffraction
(XRD) measurements (supporting information, Fig. S4). The XRD pattern of the
unground sample 1 exhibited clear and intense diffraction peaks, indicative of its
crystalline nature. In contrast, after grinding, the sharp reflection peak vanished,
which implied that the grinding process led to a crystal-to-amorphous phase
conversion. Upon treatment of the ground sample with dichloromethane solvent, the
intense and sharp reflection peaks did not reappear, suggesting that mechanical
stimulation possibly resulted in an irreversible change in the molecular morphology.
Consistent with the mechanochromic behavior of luminogen 1, luminogens 2 and 3
exhibit similar mechanochromic properties. It was speculated that the alteration of
weak multiple intermolecular C-H···F or π-π interactions, or the formation of
aurophilic interactions are possibly responsible for their mechanochromic phenomena
[7].
Fig. 6. (a) PL spectra of luminogen 1 before grinding, after grinding. Excitation
wavelength: 365 nm. The inset shows photographic images of unground solid sample
and entire ground solid sample of 1 under the irradiation of UV light at 365 nm; (b)
11

PL spectra of luminogen 2 before grinding, after grinding. Excitation wavelength: 365
nm. The inset shows photographic images of unground solid sample and entire ground
solid sample of 2 under 365 nm UV light; (c) PL spectra of luminogen 3 before
grinding, after grinding. Excitation wavelength: 365 nm. The inset shows
photographic images of unground solid sample and entire ground solid sample of 3
under UV light with a wavelength of 365 nm.
4. Conclusion
With the aim of enlarging the family of gold(I) complexes with novel fluorescence
properties, three triisocyano-based trinuclear gold(I) complexes were synthesized.
These gold(Ι) complexes exhibit excellent AIE characteristics. Correspondingly,
thin-films of these luminogens exhibited strong green emission. Furthermore, these
luminogens show mechanochromic luminescence behaviors. The mechanism of their
mechanochromic phenomena probably involves the changing of weak intermolecular
C-H···F or π-π interactions, or the formation of aurophilic interactions. The study
results will be beneficial for the design of novel AIE and mechanical-stimuli
responsive gold(I) fluorescent materials.
Acknowledgments
The authors acknowledge financial support from National Natural Science
Foundation of China (21272088, 21472059 and 21402057) and the Program for
Academic Leader in Wuhan Municipality (201271130441). The work was also
supported by the Scientific Research Foundation for the Returned Overseas Chinese
Scholars, Ministry of Education, and the Natural Science Foundation of Hubei
Province (2013CFB207) and financially supported by self-determined research funds
of CCNU from the colleges’basic research and operation of MOE (CCNU14A05009
and CCNU14F01003).
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Highlights
• Three novel trinuclear gold(Ι) complexes have been designed and synthesized.
• These gold(Ι) complexes exhibit aggregation-induced emission characteristics.
• The photoluminescence properties of these luminogens show off-on green
fluorescence in response to mechanical grinding.
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Graphical Abstract
Three novel gold(I) complexes have been synthesized, these luminogens exhibit
aggregation-induced emission (AIE) behaviors and mechanochromic luminescence
properties.
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