Synthesis, characterization and mechanochromic behavior of binuclear gold (I) complexes with various diisocyano bridges

Article abstract of DOI:10.1016/j.dyepig.2012.06.014

A series of binuclear gold (I) complexes were synthesized. Their structures were characterized by elemental analyses, IR spectrometry, UV-Vis spectroscopy and single crystal X-ray diffraction. Their fluorescent mechanochromic (tribochromic) properties were investigated. The results of the mechanochromic studies suggested that the gold complexes with methylsubstituted phenyl bridges exhibited mechanochromism and a 100 nm red-shift of fluorescent spectrum could be observed after grinding. The complex with a diphenylmethane bridge exhibited mechanochromism with a change in fluorescence from green to blue (25 nm red-shift after grinding). The ground complexes reverted to their original states by treatment with CH₂Cl₂. No mechanochromism was observed for either the biphenyl or diphenylethane containing complexes.

Full text of DOI:10.1016/j.dyepig.2012.06.014

Dyes and Pigments 95 (2012) 485e490
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, characterization and mechanochromic behavior of binuclear gold (I)
complexes with various diisocyano bridges
*
*
Jinhua Liang, Fang Hu, Xiaoyi Lv, Zhao Chen, Zhiming Chen, Jun Yin , Guang-Ao Yu, Sheng Hua Liu
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of binuclear gold (I) complexes were synthesized. Their structures were characterized by
elemental analyses, IR spectrometry, UVeVis spectroscopy and single crystal X-ray diffraction. Their
fluorescent mechanochromic (tribochromic) properties were investigated. The results of the mechano-
chromic studies suggested that the gold complexes with methylsubstituted phenyl bridges exhibited
mechanochromism and a 100 nm red-shift of fluorescent spectrum could be observed after grinding. The
complex with a diphenylmethane bridge exhibited mechanochromism with a change in fluorescence
from green to blue (25 nm red-shift after grinding). The ground complexes reverted to their original
states by treatment with CH2Cl2. No mechanochromism was observed for either the biphenyl or
diphenylethane containing complexes.
Received 17 May 2012
Received in revised form
21 June 2012
Accepted 22 June 2012
Available online 29 June 2012
Keywords:
Crystal structure
Mechanochromism
Gold (I) complexe
Stimuli-responsive luminescence
Diisocyano bridges
Effects of structure
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
packing changes of organic compounds, metal complexes can utilize
metalemetal interaction to adjust their optoelectronic properties.
Color chemistry is attracting significant attention as a conse-
quence its wide applications in colorimetric probes, fluorescent
imaging and modern dyes. Color change is very important for many
applications [1]. In solution, the introduction of other species may
affect and change the color of compounds possibly owing to elec-
tron or energy transfer. Additionally, the self-assembling style of
the molecule is also very important factor to change the color of the
compound [2,3]. Temperature, density and solvent of the system
can effectively adjust the molecular configuration. While the
molecular packing is generally steady when the compound is in the
solid state. Conveniently changing the color of the compound in the
solid will remains a challenging project [4e7].
Mechanochromism is an overarching term that describes the
phenomenon of color change caused by mechanical grinding,
crushing or rubbing which is also known as tribochromism [8] or
pressing, also termed piezochromism [8], of the solid sample and
reversion to the original color by, for example, heating or recrys-
tallization. Some studies on organic chromophores [9e20] and dye-
doped polymers [21e28] showed that absorption or emission
spectra could be changed by grinding [29]. In comparison with
Several examples of multinuclear organometallic complexes have
been reported, and the results indicated that some changes on the
absorption and emission could be observed after grinding [30e45].
As early as 2002, Catalano and Horner have reported that the
colorless, binuclear gold (I) complex [Au2(dpim)2](ClO4)2$2 MeCN
(dpim ¼ 2-(diphenylphosphanyl)-1-methylimidazole) exhibited an
orange emission, however, the material showed a stronger blue
emission after grinding [34]. Recently, the gold (I) complex
[(C6F5Au)2(m-1,4-diisocyanobenzene)] based on isocyano ligand
displayed from blue to yellow mechanochromic behavior reported
by Ito et al, owing to closed-shell aurophilic interactions [31]. In
view of this isocyano gold (I) complex, we only had a rudimentary
understanding of the relationship between structure and mecha-
nochromic activity. Herein, we designed and synthesized a series of
diisocyano-based gold (I) complexes, and confirmed their struc-
tures, and investigated the effect of ligand structures towards
mechanochromic activity.
2. Materials and methods
2.1. Experimental
* Corresponding authors. Tel./fax: þ86 27 67867725.
E-mail addresses: yinj@mail.ccnu.edu.cn (J. Yin), chshliu@mail.ccnu.edu.cn
(S.H. Liu).
General: All manipulations were carried out under an argon
atmosphere by using standard Schlenk techniques, unless otherwise
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.06.014

486
J. Liang et al. / Dyes and Pigments 95 (2012) 485e490
stated. The starting materials C6F5Au(tht) (tht ¼ thiophane) [46] and
middle isocyano bridge 1,4-diisocyano-2,5-dimethylbenzene (3a)
[25], 1,4-diisocyano-2,3,5,6-tetramethylbenzene (3b) [47], 4,40-dii-
socyanodiphenylmethane (3c) [48], 4,40-biphenyldiisocyanide (3d)
[48], 4,40-(Ethane-1,2-diyl)dianiline (1e) [49] were prepared by
procedures described in the literature. All other chemicals were
obtained from commercial sources without further purification. 1H
NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were collected on
a Varian MERCURY Plus 400 spectrometer (400 MHz). 1H and 13C
NMR chemical shifts are relative toTMS. Elemental analyses (C, H, N)
were performed by the Microanalytical Services, College of Chem-
istry, CCNU. UVeVis spectra were obtained on U-3310 UV Spectro-
photometer. Fluorescence spectra were recorded on a Hitachi-F-
4500 fluorescence spectrophotometer. XRD studies were recorded
on a Shimadzu XRD-6000 diffractometer using Ni-filtered and
(15 mL) of 2e (670 mg, 2.5 mmol) and triethylamine (3.2 mL,
22.5 mmol) at 0 ꢀC. After refluxing for 3 h, 10% aq. K2CO3 (50 mL)
was added dropwise at room temperature. The mixture was
extracted several times with CH2C12. The combined organic phase
was washed with water, dried over magnesium sulfate. After
removal of the solvent, the residue was purified by column chro-
matography on silica gel (eluent: petroleum ether/CH2C12 ¼ 1:1) to
give a white solid. Yield: 0.23 g, 40%. 1H NMR (400 MHz, CDCl3):
d
2.93 (s, 4H, CH2), 7.12 (d, J ¼ 8.4 Hz, 4H, AreH), 7.28 (d, J ¼ 8.4 Hz,
4H, AreH). 13C NMR (100.6 MHz, CDCl3):
d
37.01 (s, CH2), 126.37,
129.40, 142.39, 163.54 (s, Ar). Anal. Calcd for C16H16N2O2: C, 71.62;
H, 6.01; N, 10.44. Found: C, 71.49; H, 5.95; N, 10.36.
2.2.3. Synthesis of diisocyano-based complexes 4ae4e
General method: A mixture of C6F5Au(tht) (0.52 mmol) and 3
(0.25 mmol) were stirred in CH2Cl2 (27 mL) for over night under an
argon atmosphere at room temperature. The emerging precipitate
was filtered, and washed with CH2Cl2, and dried in vacuo to obtain
4. The NMR data of these compounds were not obtained due to the
poor solubility.
graphite-monochromated Cu K
a
radiation (
l
¼ 1.54 Å, 40 kV, 30 mA).
2.2. Synthesis
2.2.1. Synthesis of diformamide ligand 2e
Compound 4a: White solid, yield: 126 mg, 57%. Anal. Calcd for
C22H8Au2F10N2: C, 29.88; H, 0.91; N, 3.17. Found: C, 29.62; H, 0.95;
4,40-(ethane-1,2-diyl)dianiline (4.7 mmol,1.0 g) was dissolved in
HCOOH (10 mL) and the mixture was heated under reflux for 12 h.
After removal of the solvent, ethane (20 mL) was added to get a pale
solid, which was collected by filtration, and washed with ethane
and dried under vacuum. Yield: 0.78 g, 62%. 1H NMR (400 MHz, d6-
N, 2.91. IR (KBr,
n
in cmꢁ1): 2205 (N^C).
Compound 4b: White solid, yield: 157 mg, 69%. Anal. Calcd for
C24H12Au2F10N2: C, 31.60; H,1.33; N, 3.07. Found: C, 31.42; H,1.19; N,
2.91. IR (KBr,
n
in cmꢁ1): 2209 (N^C).
DMSO):
d
2.80 (s, 4H, CH2), 7.14 (d, J ¼ 7.2 Hz, 4H, AreH), 7.48 (d,
Compound 4c: White solid, yield: 203 mg, 86%. Anal. Calcd for
J ¼ 7.6 Hz, 4H, AreH), 8.25 (s, 1 H, CHOcis), 8.72 (d, J ¼ 10.8 Hz, 1 H,
CHOtrans), 10.04 (s, 1 H, NHtrans), 10.09 (br s, 1 H, NHcis). 13C NMR
C27H10Au2F10N2: C, 34.27; H, 1.07; N, 2.96. Found: 34.13; H, 0.95; N,
2.87. IR (KBr,
n
in cmꢁ1): 2214 (N^C).
(100.6 MHz, d-DMSO): d 36.53 (s, CH2),119.13,128.76,136.13,136.75
(s, Ar), 159.43 (CHO). Anal. Calcd for C16H12N2: C, 82.73; H, 5.21; N,
12.06. Found: C, 82.65; H, 5.16; N, 11.95.
Compound 4d: White solid, yield: 184 mg, 79%. Anal. Calcd for
C26H8Au2F10N2: C, 33.50; H, 0.86; N, 3.00. Found: C, 33.56; H, 0.85;
N, 2.96. IR (KBr,
n
in cmꢁ1): 2220 (N^C).
Compound 4e: White solid, yield: 180 mg, 75%. Anal. Calcd for
2.2.2. Synthesis of diisocyano ligand 3e
A dichloromethane solution (5 mL) of triphosgene (816 mg,
2.75 mmol) was added dropwise to dichloromethane solution
C28H12Au2F10N2: C, 35.02; H, 1.26; N, 2.92 Found: C, 34.88; H, 1,18;
N, 2.85. IR (KBr,
n
in cmꢁ1): 2222 (N^C).
O
O
HCOOH
triphosgene
H
H
H2N
Ln
NH2
N
H
Ln
N
H
Et3N / CH2Cl2
1a - 1e
2a - 2e
Au(C6F5)(tht)
CH2Cl2
C N
Ln
N C
C6F5 Au C N
Ln
N C Au C6F5
3a - 3e
4a - 4e
H3C
H3C
H3C
CH3
Ln
=
CH3
CH3
a
b
c
e
d
Scheme 1. Synthesis of diisocyano-based binuclear gold (I) 4ae4e.

J. Liang et al. / Dyes and Pigments 95 (2012) 485e490
487
Table 2
3. Results and discussion
Solid-State absorption and luminescence data of complexes 4ae4e at ambient
temperature.
3.1. Discussion of the synthetic stragegy
Compound
labs/nm
lem/nm
The diisocyano-based binuclear gold (I) complexes 4ae4e were
synthesized according to Scheme 1. The intermediates 3ae3e as
starting materials were reacted with C6F5Au(tht) to afford the
binuclear gold complexes 4ae4e in good yields, respectively.
Before grinding
After grinding
4a
4b
4c
4e
4f
335
332
374
332
330
432, 459
435, 469
477
405
488
531
528
502
405
488
3.2. X-ray structures
3.2.1. Crystallographic details
information, the bond length of C(39)eN(1) was 1.13 Å, which
suggested that this carbon-nitrogen bond was a triple bond.
According to the angle (176.2ꢀ) of C(39)eAu(1)eC(33), the
compound was not linear. The packing of molecules presented
a head-to-head arrangement pattern, as can be observed in Fig. S13.
According to previous literature, it has been accepted that for such
molecules intermolecular goldegold interactions occur when the
intermolecular distance between the gold atoms was within the
range of 2.7e3.3 Å [52]. In this case, the distance of intermolecular
gold atoms was 5.64 Å, indicating no obvious intermolecular
goldegold interaction. However, the existence of intermolecular
CeH.F (dH.F ¼ 2.46 Å, 2.58 Å) and CeF (dCꢁF ¼ 3.05 Å, 3.14 Å)
interactions promoted the molecular packing.
The single crystals of complexes 4a, 4c and 4e suitable for X-ray
analysis were obtained by recrystallization from chlorobenzene. The
crystal of 4a, 4c and 4e were mounted on a glass fiber for the diffraction
experiment. Intensity data were collected on a Nonius Kappa CCD
diffractometer with Mo K
a
radiation (
l
¼ 0.71073 Å) at room
temperature. The structures were solved by a combination of direct
methods (SHELXS-97) [50] and Fourier difference techniques and
refined by full-matrix least-squares (SHELXL-97) [51]. All non-H atoms
were refined anisotropically. The hydrogen atoms were placed in the
ideal positions and refined as riding atoms. Further crystal data and
details of the data collection are summarized in Table 1. Selected bond
distances and angles were given in Tables S1, S2 and S3 (Table 2).
Because of the difference of structure compared to the structure
of 4a, the single crystal structure of structure 4c compared the
herring-bone shape in Fig. 2 and Fig. S14. And the two benzene
rings were twisted and the dihedral angle was 82.9ꢀ, which was
attributed to the insertion of the methylene unit. In this structure,
the angles of C(16)eAu(1)eC(1) and C(15)eAu(2)eC(22) displayed
similar angles (178.2 and 172.5ꢀ) with complex 4a. Furthermore,
the bond lengths of N(1)eC(1) and N(2)eC(15) were 1.132(7) Å and
1.147(8) Å corresponding to the carbon-nitrogen triple bond, which
was in good agreement with complex 4a. The molecular packing,
depicted in Fig. S14, was arranged in a head-to-tail style and the
distance of gold atoms was 3.23 Å, which clearly indicated inter-
molecular goldegold interaction. Similarly the existence of weak
intermolecular interactions (such as the CeH.F and CeF interac-
tions). On the basis of 4c, further insertion of the methylene group,
viz. 4e, made the molecule more linear than complex 4c. The single
crystal structure of 4e is shown in Fig. 3. The bond lengths of the
carbon-nitrogen triple bonds were similar to the complexes 4a and
4c as described in supporting information. While the distance of
intermolecular gold atoms was 4.56 Å. Similar intermolecular
CeH.F and CeF interactions could be observed. (Fig. S15).
3.2.2. X-ray structures of 4a, 4c and 4e
As shown in Fig. 1, the complex 4a displayed completely
symmetrical configuration. From the data in the supporting
Table 1
Crystal data and refinement parameter of 4a, 4c and 4e.
4a
4c
4e
Formula
C3.67, H1.33,
Au0.33,
C27, H10, Au2,
F10, N2
C14, H6,
Au, F5, N
F1.67 N0.33
147.37
Fw
946.30
480.16
273(2)
Temp(K)
Cryst syst
Space group
a (Å)
b (Å)
c (Å)
292(2)
Triclinic
P-1
292(2)
Monoclinic
P2(1)/c
13.2394(14)
19.366(2)
10.2106(11)
90
98.103(2)
90
Monoclinic
P2(1)/n
4.564(3)
11.342(8)
25.478(16)
90
90.369(10)
90
1318.7(15)
4
5.6470(7)
10.1212(12)
10.8615(13)
113.515(2)
93.893(2)
99.398(2)
555.48(12)
6
a
(deg)
(deg)
b
g
(deg)
V (Åꢁ3
)
2591.8(5)
4
2.425
Z
Dcalcd (Mg cmꢁ3
)
2.643
2.418
Cryst size (mm3)
0.16 ꢂ 0.15
0.16 ꢂ 0.12
0.2 ꢂ 0.2
ꢂ 0.12
ꢂ 0.10
ꢂ 0.2
F(000)
402
1736
884
3.3. Mechanochromism (tribochromism)
Diffractometer
Radiation
KappaCCD
Mo Ka
13.288
KappaCCD
Mo Ka
11.401
KappaCCD
Mo Ka
11.205
0.80 to 23.85
ꢁ5 to 5 ꢁ12
to 12
Subsequently, the mechanochromic properties of 4ae4e were
investigated. Firstly, we investigated their UV/Vis absorption
spectra before and after grinding. The results suggested that no
obvious changes were observed (In supporting information:
Figs. S4eS8).
abs coeff (mmꢁ1
)
q
range (deg)
2.07 to 26.99
ꢁ7 to 7, ꢁ12
to 12
1.55 to 28.00
ꢁ17 to 17, ꢁ25
to 23
hkl range
ꢁ13 to 12
ꢁ13 to 13
ꢁ28 to 28
Reflections
4121
22944
7986
The emission spectra of 4a [(C6F5Au)2(m-1,4-diisocyano-2,5-
collected
dimethylbenzene)] in various states were shown in Fig. 4. The
emission spectra of the unground sample of 4a displayed two
emission bands at 432 nm and 460 nm, which could be attributed
Independent
reflections[R(int)
Data / restraints /
parameters
Final R
2383 (0.0269)
2383/0/164
6234 (0.0389)
6234/0/371
2006 (0.0836)
2006/108/167
]
to the phosphorescence of the intraligand-localized
pep* excited
0.0378
0.1166
0.0469
0.1590
1.221
2.144 and
ꢁ1.998
0.0323
0.0805
0.0580
0.0885
0.992
1.673 and
ꢁ1.014
0.0761
0.1517
0.0886
0.1563
1.166
2.718 and
ꢁ2.887
state [53]. After grinding of the sample, a broad emission band with
a maximum wavelength at 530 nm was observed, in which auro-
philic interactions may be responsible for the new ligand-to-metal
charge transfer (LMCT) excited state [54]. Treatment of ground 4a
with dichloromethane induced reversion to the original state.
Photographic images of the color and luminescence changes for 4a
Rw
R (all date)
Rw (all date)
Goodness of fit/F2
Largest diff peak,
hole (e Åꢁ3
)

488
J. Liang et al. / Dyes and Pigments 95 (2012) 485e490
Fig. 1. The single crystal structure 4a.
Fig. 2. The single crystal structure 4c.
were presented in Fig. 4. The mechanochromic luminescence
behavior of compound 4b was similar to that of 4a (Fig. S1). These
results showed that the mechanochromic luminescence behavior of
However, the emission spectra of 4d containing two ethylene
units between the terminal phenyl rings did not display any
changes after grinding in an agate mortar (Fig. S2). For complex 4e,
the fluorescence spectra showed a broad emission band with
a maximum wavelength at 404 nm (Fig. S3) and displayed a purple
color. It was found that compound 4e, like 4d, did not exhibit any
mechanochromic properties upon grinding. It was possible that the
grinding did not lead to the difference of molecular packing and no
obvious intermolecular goldegold interactions were observed.
Additionally, X-ray powder diffractometry was used to further
study the mechanochromic behavior. As presented in Fig. 6, the
unground sample 4a showed clear reflection peaks. In contrast, the
XRD pattern of the ground sample showed significant changes,
4a and 4b were similar to that of [(C6F5Au)2(m-1,4-
diisocyanobenzene)] reported by Ito et al. [31]. Therefore, the
introduction of methyl groups would not affect the mechano-
chromic behavior of the molecules. When dimethylbenzene in the
bridge of 4a was replaced for diphenylmethane to afford 4c, the
compound was also mechanochromically active. The emission
spectra of compound 4c in various states were indicated in Fig. 5. It
showed a broad emission band with a maximum wavelength at
477 nm. After grinding, the maximum wavelength was red-shifted
to 503 nm, following the blue-to-green emission process. Reversion
to the original blue emission by treating the ground sample with
dichloromethane was noted. Photographic images of the color and
luminescence changes of 4c were presented in Fig. 5, demon-
strating its mechanochromic luminescence behavior. Although the
methylene group was introduced to the molecule of complex 4c,
the molecular configuration remained stable. So complex 4c
possessed mechanochromic properties.
which indicated that the grinding caused
a crystalline-to-
metastable phase conversion. Upon treatment of the ground
sample with dichloromethane, the reflection peaks were restored
and corresponded to the original crystalline phase. The variations
of the XRD patterns of 4b and 4c (Figs. S9 and S10) were similar to
that of 4a. While the non-mechanochromic species 4d and 4e were
thoroughly ground, XRD patterns were essentially unchanged
Fig. 3. The single crystal structure 4e.

J. Liang et al. / Dyes and Pigments 95 (2012) 485e490
489
(Figs. S11 and S12), which implied that no changes from crystalline
to non-crystalline transition took place. According to above
experiments, we concluded that (1) 4a and 4b with methyl-
substituted phenyl bridges revealed mechanochromic properties,
and a 100 nm red-shift of fluorescent spectrum could be observed
after grinding; (2) the complex 4c exhibited mechanochromic
property and fluorescent change from green to blue fluorescence
(25 nm red-shift after grinding); (3) the ground complexes 4a, 4b
and 4c could revert to the original states by treatment of the 4a, 4b
and 4c with CH2Cl2; (4) no obvious color changes were observed
when the complexes 4d and 4e were ground.
4. Conclusions
In this work, we reported a series of new binuclear gold (I)
complexes with various diisocyano bridges and characterized their
structures. Their mechanochromic properties indicated that the
structure of ligand has a large effect on mechanochomic activity. In
which, the binuclear gold (I) complexes 4a, 4b and 4c showed
mechanochromic behavior, but 4d and 4e were mechanochromi-
cally inactive. In addition, our studies described the relationship of
structure with mechanochromic activity. Therefore, further studies
will focus on the functionalization of structures in order to find
mechanochromic materials with excellent properties in the future.
Fig. 4. Emission spectra of 4a in various states. Insets show the fluorescence images of
Au (I) complex 4a powder under 365 nm UV light: a) before grinding, b) after grinding,
c) after treatment with dichloromethane.
Acknowledgments
The authors acknowledge financial support from National
Natural Science Foundation of China (Nos. 20931006, 21072070,
21072071), Program for Changjiang Scholars and Innovative
Research Team in University (No. IRT0953), and the Program for
Academic Leader in Wuhan Municipality (No. 201271130441), and
self-determined research funds of CCNU from the colleges’ basic
research and operation of MOE.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.dyepig.
2012.06.014.
Fig. 5. Emission spectra of 4c in various states. Insets show the fluorescence images of
Au (I) complex 4c powder under 365 nm UV light: a) before grinding, b) after grinding,
c) after treatment with dichloromethane and d) repetition of the green yellow emis-
sion by scratching the powder with a pestle at room temperature. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
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