Aggregation-induced emission (AIE) behavior and thermochromic luminescence properties of a new gold(i) complex

Article abstract of DOI:10.1039/c3cc00157a

A new gold complex that shows the AIE effect as well as the thermochromic fluorescence switch is reported. This interesting phenomenon is attributed to changes in the intermolecular Au...Au interactions and the formation of nano-aggregates.

Full text of DOI:10.1039/c3cc00157a

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Aggregation-induced emission (AIE) behavior and
thermochromic luminescence properties of a new
gold(I) complex†
Cite this: Chem. Commun., 2013,
49, 3567
Received 8th January 2013,
Accepted 12th March 2013
Jinhua Liang, Zhao Chen, Jun Yin, Guang-Ao Yu* and Sheng Hua Liu*
DOI: 10.1039/c3cc00157a
www.rsc.org/chemcomm
A new gold complex that shows the AIE effect as well as the including conformational planarization, J-aggregate formation,
thermochromic fluorescence switch is reported. This interesting twisted intramolecular charge transfer (TICT), and restriction
phenomenon is attributed to changes in the intermolecular of intramolecular rotation (RIR), have been proposed.6 AIE
AuÁ Á ÁAu interactions and the formation of nano-aggregates.
luminogens based on transition metal complexes have also
been detected occasionally. To date, however, limited studies
Recently, luminescent organic molecules have attracted great have been conducted on the AIE of cationic iridium(III)
interest because of their potential applications in fields such as complexes,7 zinc(II) ion complexes etc.8 The changes are also
sensors, storages and optical devices.1 Of particular interest are realized by tuning the molecular packing in the solid state.
those luminescent materials with tunable and reversible emis- However, it may be that the metal–metal interaction is already
sion in the solid state, which are brought about by exerting present in the form of adjacent intermolecular interactions,
different external stimuli. The properties of stimuli-responsive which can therefore be used to adjust their optoelectronic
fluorescent materials include photochromism, electrochromism, properties. This observation may enable us to design additional
thermochromism, solvatochromism, mechanochromism, vapo- types of metal materials with the AIE property. Independently,
chromism and biochromism.2 However, studies on stimuli- the exploration and development of gold(I) chemistry repre-
responsive fluorescent materials remain inadequate owing to sents a fascinating and challenging area, and has attracted
the absence of effective guidelines for the design of molecular growing interest over the last two decades.9 Aurophilic Au–Au
structures possessing simultaneously the multiple features of interactions have been one of the most intriguing topics in gold
different smart materials.
chemistry, and have been one of the reasons for driving the
Meanwhile, most organic luminescent materials exhibit very rapid development of both physical and chemical studies of
strong luminescence in their dilute solutions, but this tends to gold chemistry.
be weakened or quenched at high concentrations, a phenom-
In this work, we report a novel gold(I) compound with a
enon widely known as aggregation-caused quenching (ACQ).3 1-(decyloxy)-4-isocyanobenzene bridge (Scheme 1). Compound
In 2001, Tang et al. found that some propeller-shaped mole- 1 has attracted great interest because it exhibits AIE characteri-
cules exhibit the phenomenon of aggregation-induced emis- stics and thermochromic behavior, and its fluorescent proper-
sion (AIE), which is exactly the opposite to the ACQ effect.4 ties show reversible changes from blue to yellow-green in
In 2002, Park et al. reported that a new class of organic CN-MBE response to temperature stimuli. This interesting phenomenon
nanoparticles exhibit strongly enhanced fluorescence emis- may be attributed to the intermolecular packing.
sion, and so these were called aggregation-induced emission
enhancement (AIEE) materials.5 Since then, a variety of AIE and
AIEE materials have been prepared and utilized in various
applications. However, most of these have been organic
compounds. A number of possible mechanistic pathways,
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China.
E-mail: chshliu@mail.ccnu.edu.cn; Fax: +86-27-67867725; Tel: +86-27-67867725
† Electronic supplementary information (ESI) available: Synthesis, PL and UV
spectra, details of the data collection, and selected bond distance and angles.
CCDC 918147. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3cc00157a
Scheme 1 The structure of complex 1 and its single crystal structure.
Chem. Commun., 2013, 49, 3567--3569 3567
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properties and two possible explanations have been proposed.
First, after the aggregation, only the molecules on the surface of
the nanoparticles emit light and contribute to the fluorescence
intensity upon excitation, and this leads to a decrease in the
fluorescence intensity.10 However, the enhancement of the
intermolecular gold–gold interactions could enhance light
emission. The net outcome of these antagonistic processes
depends on which process plays the dominant role in affecting
the fluorescent behavior of the aggregated molecules. Second,
the solute molecules can aggregate into nanoparticle suspen-
sions upon addition of water. However, the amorphous parti-
cles formed in this way are not stable enough, which then leads
to a reduction in PL intensity. The two kinds of nano-aggregates
obtained were characterized by DLS (dynamic light scattering).
The results revealed the presence of real nano-aggregates as a
Fig. 1 (a) PL spectra of the dilute solutions of 1 (2.0 Â 10À5 mol LÀ1) in EtOH–
H2O mixtures with different volume fractions of water (excitation wavelength =
310 nm). The inset shows the emission images of 1 (2.0 Â 10À5 mol LÀ1) in pure
EtOH as well as 40%, and 60% water fraction. (b) PL spectra of compound 1
(2.0 Â 10À5 mol LÀ1) in water–EtOH mixtures with different volume fractions of
water (40%–50%). Excitation wavelength = 310 nm.
Molecule 1 was synthesized as a white solid in 65% yield major component (Fig. S5, ESI†). The size of nano-aggregates
according to the synthetic route shown in Scheme S1 (ESI†). became smaller with an increase in the volume fractions of
Detailed procedures for its synthesis and characterization can water. It largely resulted in a change in the molecular packing
be found in the ESI.†
and changes in the intermolecular gold–gold interactions.
To survey the AIE property of complex 1, the photolumines- Furthermore, we have also investigated the PL intensity of 1
cence (PL) spectra and UV spectra (Fig. S1, ESI†) were studied in in the 40% and 60% EtOH–H2O mixture at the concentration of
EtOH–H2O mixtures with various water contents. Interestingly, 0.5 Â 10À5 M and 1 Â 10À5 M. As shown in Fig. S6 (ESI†), the PL
the PL intensity was not only significantly enhanced with the intensity is weakened with the decrease in the concentration
addition of water, but the emission color also changed from of 1. However, PL intensity is not linear with the concentration
blue (402 nm and 425 nm) to yellow-green (559 nm), as shown of 1, which may be due to the inner-filter effect.
in Fig. 1a. This is very unusual.
Thermochromic materials,11 typically dyes and pigments
Complex 1 in pure EtOH (2.0 Â 10À5 mol LÀ1) exhibited very that are able to change color reversibly with changes in tem-
weak photoluminescence (PL) intensity, as shown in Fig. S2 perature, have been the subject of curiosity-driven research and
(ESI†). However, when the water fraction in the EtOH solution commercial exploitation for decades, mainly because of their
exceeded 30%, an emission band was observed with lmax at appealing color-changing visual effect in specialty inks, paints,
402 nm and 425 nm. The intensity in pure EtOH was 0.88 a.u., plastics and textiles. The emission spectra of the crystals of 1
which increased to B70.59 a.u. in the 40% EtOH–H2O mixture, show two emission bands, at 407 nm and 428 nm, which can be
showing an approximately 80-fold enhancement, due to the attributed to the fluorescence from the intra-ligand localized
formation of nano-aggregates. As the compound is insoluble in p–p* excited state.11 Intriguingly, after gentle heating (>55 1C)
water, increasing the water fraction in the mixed solvent might of the sample in a slender piece of glass, a broad emission
change the form of the compound from a dissolved or well- band, with a maximum wavelength of 530 nm, was observed, in
dispersed state in pure EtOH to aggregated particles in the which aurophilic interactions may be responsible for the new
mixtures with high water content. The emission of 1 is thus ligand-to-metal–metal charge transfer (LMMCT) excited state
caused by aggregation, which is typically AIE active.
[C6F5 - AuÁ Á ÁAu].12 Evidently, complex 1 exhibits temperature-
As shown in Fig. 1b, when the water fraction exceeded 40%, induced luminescence changes. That is, complex 1 is a thermo-
the addition of water to the EtOH solution resulted in a chromic material. When the temperature falls to 25 1C, the
decrease in the intensities of the two emission bands at solid exhibits an emission band similar to that of the initial
402 nm and 425 nm, and an increase in the intensity of the sample. An attempt to repeat this blue-to-yellow-green emission
new emission bands at 559 nm. When the water fraction cycle was successful. The PL intensity of complex 1 was reversible
reached 54%, the blue emission completely disappeared, and during consecutive heating and cooling cycles over the tempera-
the yellow-green emission became very obvious. The intensity ture range 25 1C–59 1C (Fig. S7, ESI†). Photographic images of
at 402 nm in pure EtOH was 0.88 a.u., while the intensity at
559 nm increased to B33.59 a.u. in the 52% EtOH–H2O
mixture, showing an approximately 38-fold enhancement. We
speculate that two kinds of nano-aggregates might be formed,
causing the difference in molecular packing and the change in
the intermolecular gold–gold interactions. However, the PL
intensities of the yellow-green emission bands are weakened
if the water fraction is further increased (Fig. S3, ESI†). The
changes in the PL intensity of compound 1 with the different
water fractions (0%–90%) are shown in Fig. S4 (ESI†). This
Fig. 2 (a) Normalized PL spectra of 1 initial, heated and cooled powder, (b) images
phenomenon is often observed in compounds with AIE of the compound before heating and after heating under 365 nm UV illumination.
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3568 Chem. Commun., 2013, 49, 3567--3569
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It is AIE-active and thermochromic since: (1) the addition of water
to solutions of 1 in common organic solvents (EtOH) formed two
kinds of nano-aggregates, showing blue and yellow-green emission;
(2) the emission of 1 can be switched between ‘‘blue’’ and ‘‘yellow-
green’’ repeatedly using simple heating–cooling cycles. The change
in the intermolecular gold–gold interactions, weak p–p interactions
and intermolecular interactions (such as the C–HÁÁÁF and CÁÁÁF
interactions) were found to play an important role in determining
these desirable properties. Further studies will focus on the func-
tionalization of structures in order to find other AIE and stimuli-
responsive fluorescent materials with desirable properties.
The authors acknowledge financial support from National
Natural Science Foundation of China (no. 20931006, 21072070,
21072071, 21272088), and the Program for Academic Leader in
Wuhan Municipality (no.201271130441).
Fig. 3 The structural organization of complex 1.
the color and luminescence changes for 1 are presented in
Fig. 2. It is possible that the heating resulted in a change in the
molecular packing and in the intermolecular gold–gold inter-
actions. Cooling then made the amorphous phase rearrange
into the more stable crystalline phase.
Fortunately, it is possible to obtain single crystals of
complex 1 suitable for X-ray analysis by slow diffusion of
n-hexane into a dichloromethane solution containing 1, as
shown in Scheme 1b. Further crystal data and details of the
data collection are summarized in Table S1 (ESI†). Selected
bond distances and angles are given in Table S2 (ESI†).
In complex 1, the Au–C(6) and C(7)–Au distances are 2.024(6)
and 1.977(7) Å while the C(6)–Au–C(7) angle is 177.8(3)1. These
distances are consistent with those in (CyNC)AuIBr, where the
Au–Br and C–Au distances are 2.372(8) and 1.972(7) Å, respec-
tively, and the C–Au–Br angle is 178.05(17)1.13 According to
previous literature, it has been accepted that intermolecular
AuÁ Á ÁAu interactions occur when the intermolecular distance
between the gold atoms is within the range 2.7–3.3 Å.14 As
shown in Fig. 3, molecules of 1 are weakly self-associated, with
an AuÁ Á ÁAu distance of 3.782 Å. These self-associated molecules
are situated in such a way that they form extended loose chains
with neighboring AuÁ Á ÁAu distances of 4.83 Å. In this case, no
obvious intermolecular gold–gold interaction is indicated.
However, the existence of weak p–p interactions (d = 3.700 Å)
and intermolecular C–HÁ Á ÁF (dHÁ Á ÁF = 2.459 Å, 2.646 Å) and CÁ Á ÁF
(dCÁ Á ÁF = 3.111 Å) promote molecular packing, as indicated in
Fig. S8 (ESI†). When water is added to the EtOH solution or the
blue luminescent powder is heated, nano-aggregates or an
amorphous phase are formed, with aurophilic interactions that
are responsible for the lower energy yellow-green emission, as
shown in Scheme 2.
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Scheme 2 An illustration of the luminescence changes of complex 1.
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Chem. Commun., 2013, 49, 3567--3569 3569