A novel carbazole-based gold(i) complex with interesting solid-state, multistimuli-responsive characteristics

Article abstract of DOI:10.1039/c5dt02035b

A new carbazole-based mononuclear gold(i) complex was designed and synthesized. The novel luminogen shows significative solid-state reversible mechanochromism, dual-responsive thermochromism and sensitive thin-film vapochromism properties. In addition, we

Full text of DOI:10.1039/c5dt02035b

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Cite this: DOI: 10.1039/x0xx00000x
A novel carbazole-based gold(I) complex with
interesting solid-state multi-stimuli-responsive
characteristics†
Received 00th January 2012,
Zhao Chen, Jinhua Liang, Yuting Nie, Xuqingfeng Xu, GuangꢀAo Yu, Jun Yin,* and
Sheng Hua Liu*
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new carbazole-based mononuclear gold(I) complex was
designed and synthesized. The novel luminogen shows
significative solid-state reversible mechanochromism, dual-
great attention. Until now, examples on multiꢀstimuliꢀresponsive
smart fluorescent materials of one chromophore are still inadequate,
which is due to the absence of effective guidelines for the design of
switchable fluorescent molecules possessing synchronously all the
features of various smart materials. Furthermore, luminogen
possessing simultaneously reversible mechanochromism, dualꢀ
responsive thermochromism, and sensitive thinꢀfilm vapochromism
characteristics has yet to be discovered, not to mention metalꢀ
bearing luminescent molecule with these alluring properties. Herein,
we describe a novel monoisocyanoꢀbased mononuclear gold(I)
complex with a carbazoleꢀbased skeleton (Scheme 1). The
existence of carbazole unit provides the possibility for generation
of weak intermolecular CꢀH···F interaction. Complex 1 is a
preeminent luminescent molecule which merits significant attention
due to its significative reversible mechanochromism, dualꢀ
responsive thermochromism, and sensitive thinꢀfilm vapochromism
properties. In addition, we obtained two types of crystals of 1,
which are helpful to the reasonable explanation of these interesting
characteristics.
responsive
thermochromism
and
sensitive
thin-film
vapochromism properties. In addition, we obtained two kinds
of crystals of the luminogen, which are conducive to the
reasonable explanation of these interesting characteristics.
Luminescent organic materials, especially smart fluorescent
materials as a response to environmental stimuli, have attracted
extensive interest due to their potential for applications in some
areas such as fluorescent switches, sensors, data storage, and
1ꢀ4
optical devices. For more than a decade, gold(I) chemistry has
received considerable attention due to the occurrence of intriguing
5
ꢀ8
aurophilic AuꢀAu interactions. Mechanochromic materials, as
one rather important type of smart luminescent materials, have also
9ꢀ12
aroused a great deal of attention. To date, the majority of gold(I)
complexes which exhibit mechanochromic behaviors have been
shown to be dinuclear gold(I) complexes. The mononuclear gold(I)
13
complexes with mechanochromic properties are insufficient and
preparing such complexes based on a mononuclear gold(I) scaffold
structure is urgent and challenging. On the other hand, temperature
is one of the most common environmental stimuli, and therefore
thermochromic materials, which are capable of changing colour
14
with temperature, can be applied to temperatureꢀsensing systems.
In the last few decades, significant breakthroughs have been made
in the field of thermochromism. Dualꢀresponsive thermochromic
material, which exhibits two invertible colour changes over a range
Scheme 1 The molecular structure of complex 1.
15
Compound
1 was obtained in 92% yield by the coordination
of temperatures, is more conducive to practical application.
reaction of intermediate product 1c and C F Au(tht) (tht =
Luminescence vapochromism, which involves changes in the
photoluminescence properties upon exposure to vapors of volatile
6 5
tetrahydrothiophene). Specific procedures of the reasonable
synthetic strategy are shown in Scheme S1 (ESI†)
1
6ꢀ19
.
organic compounds, is a promising phenomenon.
Finding
Mechanochromic property of complex 1 was investigated via
photoluminescence (PL) spectroscopy. As Figure 1 shows, the
fluorescence spectrum of solid sample 1 exhibited very broad
emission covering the total visible spectrum, which corresponds to
white emission under 365 nm UV irradiation. Interestingly, a new
broad emission band with a λmax at 513 nm appeared upon gentle
appropriate sensors for detrimental chemical substances that are
likely to exist in the environment or at our workplaces is very
important. Meanwhile, the vapochromic molecules are practically
applied in solid state commonly as thin films. Thus, the discovery
of thinꢀfilm vapochromic materials suitable for sensing volatile
organic compounds is an attractive research topic and deserves
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grinding of sample 1, and the corresponding luminescence was
metastable state is formed, and generation of intermolecular
aurophilic interactions is responsible for the green emission.
2
0
converted from white to green. This green luminescence possibly
2
0
involves charge transfer from ligand to metalꢀmetal (LMMCT).
Moreover, the green fluorescence reverted to its original white
colour after a 30 s treatment of the ground sample with fuming
dichloromethane solvent vapor. It is noteworthy that the
repeatability of this whiteꢀtoꢀgreen mechanofluorochromic
conversion is superior (Fig. S1, ESI†). It was believed that the
mechanical external stimulation possibly caused a change in the
molecular morphology. In order to gain more insight into this
proposed mechanism of mechanochromism, the structural
transition of the solid sample of 1 was studied via Xꢀray diffraction
unground
ground
treated with CH Cl
2
2
(XRD). As Figure 2 shows, the grinding led to weakening and
broadening of all the reflection peaks, indicating that the use of
mechanical force resulted in a crystalꢀtoꢀamorphous morphology
transformation. Furthermore, the initial intense and sharp
diffraction peaks were restored via molecular repacking upon
treatment of dichloromethane solvent vapor. Thus, the XRD test
results proved that the mechanochromism behavior of 1 can be
attributed to the morphology transformation between the crystalline
and amorphous states.
Moreover, we obtained one type of single crystal of 1 that was
both appropriate for Xꢀray structure analysis and very rare whiteꢀ
emitting, which is consistent with the solid fluorescence of 1.
Details of the crystal information and data collection are
summarized in Table S1 (ESI†).
1
0
20
30
(degree)
40
50
2θ
Fig. 2 XRD patterns of solid sample 1: unground, ground and after
treatment with dichloromethane vapor.
Fig. 3 (a) The structural organization of white lightꢀemitting
complex 1; (b) The fluorescence image of white lightꢀemitting
crystal of 1 under 365 nm UV irradiation.
Next, the thermochromic property of 1 was also determined by
luminescence spectroscopy. As Figure
4 shows, the white
fluorescence of luminogen 1 decreased gradually with increasing
temperature. Intriguingly, the strong whiteꢀemitting ability of 1
recovered with a decrease in temperature to the initial value.
Repeated experiments proved that opening and closing of the white
emission was reversible for multiple times (Fig. S4, ESI†). To our
surprise, a new emission band with a λmax at 550 nm appeared at a
temperature of 132 ℃, namely the melting point of complex 1, and
the emission colour changed to yellow. The yellowꢀemitting
luminogen 1 was subsequently converted to green with a decrease
of temperature to room temperature. The yellowꢀtoꢀgreen emission
change was reversible during consecutive heating and cooling
cycles over a temperature range of 20ꢀ132 ℃ (Fig. S5, ESI†). The
powder XRD test data indicate that the greenꢀemitting solid sample
is amorphous state (Fig. S6, ESI†). Furthermore, the greenꢀemitting
luminogen 1 could be restored to the white emission upon exposure
to dichloromethane solvent vapor thereby resulting in interesting
dualꢀresponsive thermochromism behavior of the novel luminogen
Fig. 1 (a) Luminescence spectra of unground solid sample 1, after
being ground and fumed with dichloromethane solvent vapor. The
excitation wavelength is 365 nm. Fluorescence imaging of 1 taken
under irradiation of a 365 nm UV light: (b) asꢀsynthesized solid
sample; (c) ground sample; (d) sample after treatment with
dichloromethane vapor.
The bond angles and distances are listed in Table S2 (ESI†).
Figure 3 reveals that the shortest intermolecular AuꢀAu distance of
the white lightꢀemitting complex 1 is 4.938 Å, indicating the
2
1
absence of strong significant aurophilic interactions. Nevertheless,
the presence of weak CꢀH···F interactions (dH···F = 2.504 Å, 2.494
Å, 2.668 Å, 2.643 Å, 2.522 Å, 2.646 Å, Fig. S2, ESI†) and πꢀπ
interactions (d = 3.442 Å, Fig. S3, ESI†) contributes to molecular
packing, and facilitates slipping with the application of external
mechanical stimuli. When the solid powder sample is ground, a
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1
. This heatingꢀinduced fluorescence variations of solid sample 1
were possibly associated with the changes in the molecular
22
packing.
Fig. 5 a) Emission spectra changes of 1 thin film, exposing to
VOCs vapors and away from VOCs vapors. Excitation wavelength:
3
65 nm; b) Photographic images of 1 thin film, upon exposure to
vapors of VOCs and away from VOCs vapors under 365 nm UV
light.
Fig. 4 a) PL spectra of luminogen 1 over a temperature range of 20ꢀ
20 ℃. Excitation wavelength: 365 nm. b) PL spectra of 1 initial
1
powder, heated powder (132 ℃), cooled powder and treated with
CH Cl vapor. Excitation wavelength: 365 nm. c) The diagram of
2
2
dualꢀresponsive thermochromism behavior of 1.
Subsequently, the thinꢀfilm vapochromic effect of luminogen 1
was investigated via PL spectroscopy. Complex 1 exhibited high
solubility in most of the common solvents, which allowed the
formation of corresponding thin film. Consistent with the
fluorescence of 1 in DMFꢀH O mixtures with high water content
2
(
Fig. S7, ESI†), the thinꢀfilm of 1 emitted strong green
Fig. 6 a) The structural organization of single crystal with
dichloromethane molecules of complex 1; b) Photographic image
of yellow lightꢀemitting crystal of 1 under 365 nm UV irradiation.
luminescence. The vapoꢀtriggered emission spectral changes of 1
are shown in Figure 5. When the thin film of 1 was exposed to
vapors of volatile organic compounds (VOCs), including benzene,
pyridine, acetone, dichloromethane, toluene, trichloromethane,
tetrahydrofuran, acetonitrile, diethyl ether, ethyl acetate, etc., the
emission band centred at 512 nm redꢀshifted to the yellowꢀemitting
region with λmax at 556 nm, and the bright green luminescence was
converted to yellow emission. Moreover, the yellow emission
reverted to the initial green emission when the thin film of 1 was
removed from the VOC vapors. More importantly, luminogen 1
exhibited an sensitive vapor response (For a selected example: see
Movie S1, ESI†), which is very favorable for 1 to sense volatile
organic compounds. Single crystals, with dichloromethane
molecules of 1, appropriate for singleꢀcrystal Xꢀray structure
analysis were obtained by the recrystallization method involving
tardy diffusion of nꢀhexane into solutions of 1 in dichloromethane.
Detailed information of the corresponding crystal is provided in the
ESI.†
As Figure 6 reveals, the existence of dichloromethane molecules
results in the formation of intermolecular CꢀH···Cl interactions
(dH···Cl = 2.874 Å) and intermolecular CꢀH···O interactions (dH···O
=
2
.479 Å), which alter the molecular stacking and shorten the
intermolecular distance between the shortest Au atoms. These
single crystals with dichloromethane molecules emitted yellow
fluorescence under 365 nm UV irradiation, which is consistent with
the luminescence of luminogen 1 exposed to vapors of VOCs. The
acquisition of the yellowꢀemitting single crystals is very important
in explaining the sensitive thinꢀfilm vapochromism mechanism of 1.
Conclusions
In summary, we have discovered an carbazoleꢀbased mononuclear
gold(I) complex, which exhibits reversible mechanochromic
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behavior, noteworthy dualꢀresponsive thermochromism, and
sensitive thinꢀfilm vapochromism characteristics. Furthermore, two
kinds of crystals of complex 1 were fortunately obtained. On the
basis of singleꢀcrystal structural organizations of the two types of
crystals, it is speculated that the changes of luminogen 1 in the
molecular arrangement and the formation of intermolecular
aurophilic interactions are possibly responsible for its valuable and
remarkable properties. To the best of our knowledge, this novel
gold(I) complex is the first example of fluorescent molecule, which
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Acknowledgements
The authors acknowledge financial support from National Natural
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Hubei Province (2013CFB207) and financially supported by selfꢀ
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Key Laboratory of Pesticide and Chemical Biology, Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan 430079,
PR China. Tel: +86ꢀ27ꢀ67867725; Fax: +86ꢀ27ꢀ67867725; Eꢀmail:
yinj@mail.ccnu.edu.cn, chshliu@mail.ccnu.edu.cn.
1
†
Electronic Supplementary Information (ESI) available: Detailed
1
experimental sections, NMR spectra, mass spectra, details of the crystal
data collection, bond distance and angles. CCDC 1044227 and 1044242
(compound
1), and characterization datas mentioned in the paper. See
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Graphical Abstract
A novel carbazoleꢀbased gold(Ι) complex is reported. The luminogen exhibits significative solidꢀstate reversible mechanochromism, dualꢀ
responsive thermochromism and sensitive thinꢀfilm vapochromism properties.
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