Piezochromic luminescent (PCL) behavior and aggregation-induced emission (AIE) property of a new cationic iridium(iii) complex

Article abstract of DOI:10.1039/c2dt31013a

A new cationic Ir(iii) complex based on a dendritic ancillary ligand has been designed and synthesized, which simultaneously exhibits piezochromic luminescent (PCL) behavior and aggregation-induced emission (AIE) property for the first time.

Full text of DOI:10.1039/c2dt31013a

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Piezochromic luminescent (PCL) behavior and aggregation-induced emission
(AIE) property of a new cationic iridium(III) complex†
Guo-Gang Shan,a Hai-Bin Li,a Jun-Sheng Qin,a Dong-Xia Zhu,a Yi Liao*b and Zhong-Min Su*a
Received 9th May 2012, Accepted 31st May 2012
DOI: 10.1039/c2dt31013a
A new cationic Ir(III) complex based on a dendritic ancillary
ligand has been designed and synthesized, which simul-
taneously exhibits piezochromic luminescent (PCL) behavior
and aggregation-induced emission (AIE) property for the
first time.
The design and synthesis of solid-state organic luminescent
materials have been attracting increasing attention in both scien-
tific research and practical aspects, due to their potential appli-
cations in photonic and optoelectronic devices.1 In the solid
state, the intermolecular interactions control the molecular
packing motifs of organic luminophors. In this regard, the emis-
sion properties of organic luminophors are primarily governed
Scheme 1 Chemical structure of complex 1.
by the whole molecular aggregate rather than by the individual
molecule.2 Upon external stimulus such as pressure and mechan-
We demonstrated from our density functional theory (DFT) cal-
ical grinding, alternation of solid-state arrangement and/or phase
transition may occur, thus leading to a change in emission color.
This process is defined as “piezochromism”.3 Since lumines-
cence can be detected with high sensitivity, the materials with
piezochromic luminescent (PCL) behavior have a wide variety
of applications.2,3 Although compounds exhibiting PCL proper-
ties could be liquid-crystalline, polymer, organic and inorganic
molecules, PCL materials are rather sparse.4 Further design of
PCL materials and investigation of the relationship between the
molecular packing and the piezochromic behavior also remain a
considerable challenge.
culations that the weak emission of these complexes in the dilute
solution might arise from the intramolecular rotation of the den-
dritic ancillary ligands, which induces an efficient radiationless
decay pathway.9 A number of photoactive Ir(III) complexes have
been synthesized to date.10 Zhao and coworkers also presented
detailed studies on the photophysical properties of Ir(III) sys-
tems.10e,f They indicated that Ir(III) complexes characterized as
3ILCT excited-state usually show very weak emission or non-
emission in solution. Recently, a successful example of an Ir(III)
compound (Ir[(dfppz)2(Mptz)]PF6) exhibiting reversible piezo-
chromic behavior has been reported by us.4b It is natural to ques-
tion whether this Ir(III) complex could simultaneously show the
AIE behavior if a dendritic ligand, such as carbazole based den-
drons, is introduced into triazole-pyridine as the ancillary ligand
(See Scheme 1).
Inspired by this idea, herein, we designed and prepared
a novel cationic Ir(III) complex, Ir[(dfppz)2(L)]PF6 (1), with
1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) as the cyclometalated
ligand and a dendritic ligand (L) as the ancillary ligand
(Scheme 1). The quantum chemical calculations reveal that the
T1 state of complex 1 shows a predominant 3ILCT character (see
Table S1 and Fig. S1, ESI†), which is different from that of our
previously reported Ir(III) complex Ir[(dfppz)2(Mptz)]PF6, a
piezochromic luminescent complex. Considering the presence of
3ILCT excited-state and dendritic ancillary ligand, we expect that
complex 1 might be non-emissive in solution, but show strong
emission in the solid state due to molecular stacking.8a,b,d The
photophysical properties of complex 1 have been investigated in
detail. As expected, it is non-emissive when dissolved in good
solvents, but displays enhanced emission in the solid state.
Aggregation-induced emission (AIE),5 on the other hand, is
an unusual phenomenon, which has no emission in the dilute
solution but enhanced emission in the solid state. Since the first
report of AIE material by Tang and co-workers in 2001, great
efforts have been devoted to the design and synthesis of organic
fluorophores with AIE properties.6,7 Only recently, the phosphor-
escent AIE phenomenon based on transition metal complexes
has also been detected.8 Among the phosphorescent AIE com-
pounds, the Ir(III) phosphors have been synthesized through
chemical modification of the ligands by our group and others.8,9
aInstitute of Functional Material Chemistry, Faculty of Chemistry,
Northeast Normal University, Changchun, 130024 Jilin, People’s
Republic of China. E-mail: zmsu@nenu.edu.cn
bDepartment of Chemistry, Capital Normal University, Beijing, People’s
Republic of China. E-mail: liaoy271@nenu.edu.cn
†Electronic supplementary information (ESI) available: 1H NMR
spectra, thermogravimetric analysis and detailed experimental procedure.
CCDC 870135. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31013a
9590 | Dalton Trans., 2012, 41, 9590–9593
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Additionally, the as-prepared powder of 1 obtained through
recrystallization exhibits bright yellow emission and its emission
color can change to orange upon grinding. In other words,
complex 1 is both a PCL and AIE material. To the best of our
knowledge, it is the first example of an Ir(III) complex simul-
taneously displaying piezochromism and aggregation-induced
emission.
induced luminescence change. As a matter of fact, the ground
orange-emitting powder (1G) can be reverted to the original
yellow-emitting one through recrystallization in dichloro-
methane–petrol ether. Moreover, after heating the ground sample
1G at 325 °C for 5 min, the obtained powder (1H) also exhibits
similar emission color to sample 1A and the recrystallized
sample (1R) (Fig. S6, ESI†). Note that the changes in emission
color are visible to the naked eye. When the powder 1A was
spread on a filter paper and the letters “AIPE” were written on
that with a spatula, the marked “AIPE” can be observed clearly
on the yellow “paper” under the UV lamp (Fig. 1b), implying
that complex 1 has potential applications in optical recording
and pressure-sensing fields.
The ancillary ligand L was readily synthesized by C–N
coupling of corresponding bromide with 2-(5-phenyl-2H-1,2,4-
triazol-3-yl)pyridine catalyzed by copper(I) iodide. Complex 1
was prepared by reacting the dimeric iridium(III) intermediate
[Ir(dfppz)2Cl]2 with ligand L, followed by a counter-ion
exchange reaction (Schemes S1 and S2, ESI†). Detailed
procedures and characterization data are given in the ESI.† The
crystals of 1 were obtained by recrystallization from dichloro-
methane–petrol ether. Similar to reported cationic Ir(III) com-
plexes,4b,11 complex 1 exhibits a distorted octahedral geometry
around the Ir atom coordinated by two cyclometalated ligands
and one ancillary ligand. Here, the two dfppz ligands adopt
mutually an eclipsed configuration with two nitrogen atoms
residing at trans locations (Fig. S4, ESI†). Close investigation of
the molecular packing structure of 1 reveals that multiple inter-
molecular C–H⋯π interactions exist in the single crystal struc-
ture of 1 (Fig. S5, ESI†). Due to these weak interactions, the
crystal structure of 1 would be easily destroyed, which may be
triggered by packing rearrangement or phase transition under
external pressure. Thus, the emission color of complex 1
changed upon grinding, which can be considered as one of the
possible reasons for the present PCL behavior.2a,12 In addition,
the ancillary ligand possesses intrinsic intramolecular rotation
units. In solutions, the rotation is active and can serve as a relax-
ation channel, resulting in an efficient radiationless decay
pathway. As a result, complex 1 would be non-luminescent in
dilute solution due to the efficient radiationless decay as well as
To elucidate the PCL behavior of complex 1A, NMR spec-
troscopy, differential scanning calorimetry (DSC) and powder
X-ray diffraction (PXRD) studies have been performed. As
shown in Fig. 2, the PXRD patterns show distinctly different
patterns for the samples 1A and 1G. In the case of 1A, it dis-
plays clear and sharp reflection peaks which are in good
agreement with the simulated XRD from the single crystal,
indicating that the 1A molecules are packed in a relatively well-
ordered structure (crystalline state). In contrast, the diffractogram
of the ground sample 1G does not show any noticeable
reflections. These results demonstrated that the crystalline
state 1A was converted into an amorphous state (1G) after grind-
ing. Upon heating 1A and 1G to 350 °C, the DSC curve of 1A
exhibited a clear endothermic melting peak, while a broad
exothermic recrystallization peak at ca. 320 °C was detected in
the case of 1G (Fig. S7, ESI†). This exothermic peak in the
DSC curve of 1G is similar to the temperature at which thermal
recrystallization starts to take place. Furthermore, 1H NMR
spectra and 13C magic-angle spinning nuclear magnetic
resonance (13C CP/MAS-NMR) analysis have also been carried
out to further determine whether the chemical reaction
3
1
the ILCT excited-state character. In contrast, the intramolecular
and isomer process happen. It is found that the H NMR spectra
rotational motions should be effectively suppressed in the aggre-
gation state, thereby the radiation decay would be enhanced and
induce it to emit.5,7,13
Under UV light irradiation of the as-prepared powder 1 (for
clarity, hereafter abbreviated as the letter 1A), the sample 1A
shows strong yellow emission (537 nm), as shown in Fig. 1.
When the powder 1A is gently ground using a pestle in the
ceramic mortar, however, a significant bathochromic shift (λmax
= 564 nm) occurs in the orange-emitting ground sample 1G
(Fig. 1). This result indicates that complex 1 exhibits a pressure-
for complex 1 in various states are almost identical (Fig. S8,
ESI†). Additionally, the sharp resonance lines are observed in
13C CP/MAS NMR of 1A (Fig. 2b), but the ground sample 1G
exhibits broader distribution compared with that of 1A, and
keeps its original conformation at the molecular level.14 Based
on our experimental results, we can confidently propose that the
ground sample, obtained by mechanical grinding, represents the
same compound in the state of a highly amorphous powder. No
chemical reaction and isomer process take place during the
grinding process.
Fig. 1 (a) Emission spectra of the samples 1A and 1G. (b) The powder
1A was cast on the filter paper and the letters “AIPE” were written with
a spatula under UV light at room temperature.
Fig. 2 (a) XRD patterns and (b) 13C CP/MAS-NMR spectra of the
samples 1A and 1G.
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Fig. 4 (a) TEM image of nanoaggregates of complex 1 formed in
CH3CN–H2O mixtures with 90% water fraction. (b) Electron diffraction
pattern of the nanoaggregates.
Fig. 3 Emission spectra of complex 1 in CH3CN–H2O mixtures with
different water fractions (0–90%).
that of the ground sample 1G (Fig. S11, ESI†). This indicates
that there are some amorphous molecular aggregates formed in
the mixtures, which is confirmed by the results of transmission
electron microscopy (TEM) and confocal luminescence exper-
iments (Fig. 4b and Fig. S12, ESI†).16 Motivated by the existing
“on–off” luminescence behavior of complex 1, it is believed that
complex 1 might serve as a chemical sensor for volatile organic
vapors. As shown in Fig. S13,† the bar-like luminescence
pattern formed from the CH3CN solution of complex 1 can be
observed clearly on a TLC plate. After exposure of CH2Cl2
solvent, the luminescence was completely turned off, demon-
strating its ability to detect organic vapors.
In summary, a novel cationic Ir(III) complex has been designed
and successfully prepared by employing a dendritic ancillary
ligand, whose emission color of its as-prepared powder can
change obviously upon grinding, and revert to the original emis-
sion via heating and/or recrystallization. Interestingly, it exhibits
no emission in solution. This is the first example of a cationic Ir(III)
complex simultaneously exhibiting PCL and AIE properties. The
crystalline–amorphous phase transformation is responsible for
the altered emission of the ground samples by carefully analyz-
ing our experimental data. The design concept, the judicious
modification of dendritic ancillary ligand for tuning the character
of excited states, should provide valuable information for the
development of multifunctional materials based on Ir(III)
complexes.
In our previous work,4b the cationic Ir(III) complex, Ir-
[(dfppz)2(Mptz)]PF6 also shows an intense emission in the solid
state as well as PCL behavior, which is similar to that of
complex 1. Generally, for cationic Ir(III) complexes, three excited
states contribute to the observation of light emission, namely,
LC 3π–π*, 3MLCT and 3LLCT. The emission spectra from
3MLCT and 3LLCT states are usually broad and featureless,
while the obvious vibronic emission bands are always attributed
to LC 3π–π* state. In addition, the emission spectra from
3MLCT and 3LLCT states depend on the polarity of solvents due
to the charge transfer character of the excited state. Strong and
broad emissions in CH2Cl2, CH3CN, THF and DMF solutions
are observed at room temperature for complex Ir[(dfppz)2(Mptz)]-
PF6 (Fig. S9, ESI†). Moreover, the emission spectrum of
complex Ir[(dfppz)2(Mptz)]PF6 has a dependence on the polarity
of solvents, suggesting that its luminescence mainly originates
from 3MLCT mixed with 3LLCT states.15 This conclusion is
consistent with our calculated results shown in Table S1 and
Fig. S1.† In sharp contrast, complex 1 can be easily dissolved in
common organic solvents, such as CH2Cl2, CHCl3, CH3CN and
DMF, it is not emissive in these solutions (Fig. S10, ESI†), but
become highly emissive in the solid state as mentioned above.
The absolute quantum yields of complex 1 in CH3CN solution
and in the solid state (as-prepared powder) using an integrating
sphere were estimated to be 0% and 20%, respectively. This
phenomenon is similar to the previously reported AIE mo-
lecules, indicating that 1 might feature the AIE property.
To probe the AIE property of complex 1, the photolumines-
cence (PL) spectra of 1 in CH3CN–H2O mixtures with various
water contents were investigated. Since 1 dissolves readily in
CH3CN solution but is insoluble in water, addition of large
amounts of water into the CH3CN solution of 1 will cause
complex 1 to aggregate. If complex 1 possesses AIE activity, the
PL intensity of 1 in CH3CN–H2O mixtures should increase due
to the aggregation of 1. As shown in Fig. 3, complex 1 in pure
CH3CN solvent exhibits faint emission, but the intensity of emis-
sion remarkably increases when the water content exceeds 60%.
Adding further water up to 90%, the PL intensity is significantly
enhanced, which is about 120-fold greater than that in pure
CH3CN solvent. This indicates that complex 1 has a significant
AIE effect. In addition, it is noteworthy that the emission spec-
trum of 1 in CH3CN–H2O (1 : 9) mixtures is nearly identical to
The authors gratefully acknowledge the financial support from
NCET-10-0011, NSFC (20971020, 20903020 and 21131001),
973 Program (2009CB623605), the Science and Technology
Development Planning of Jilin Province (20100540 and
201101008) and NENU-STC0812.
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