Reversible piezochromic behavior of two new cationic iridium(iii) complexes

Article abstract of DOI:10.1039/c2cc15855h

We demonstrate that two new cationic Ir(iii) complexes exhibit an interesting piezochromism, and their emission color can be smartly switched by grinding and heating. This is the first example that the Ir(iii) complexes display piezochromic phosphorescenc

Full text of DOI:10.1039/c2cc15855h

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COMMUNICATION
Reversible piezochromic behavior of two new cationic iridium(III)
complexesw
Guo-Gang Shan,a Hai-Bin Li,a Hong-Tao Cao,a Dong-Xia Zhu,a Peng Li,a Zhong-Min Su*a
and Yi Liao*b
Received 21st September 2011, Accepted 21st December 2011
DOI: 10.1039/c2cc15855h
We demonstrate that two new cationic Ir(III) complexes exhibit an
interesting piezochromism, and their emission color can be
smartly switched by grinding and heating. This is the first example
that the Ir(III) complexes display piezochromic phosphorescence.
(2-(5-methyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine, Mptz)
ancillary ligands have been designed and synthesized in our
laboratory. During the investigation of their photophysical
properties, we discovered that the complexes underwent
remarkable emission color changes upon grinding. This interesting
observation prompted us to further study their luminescence
behaviors. It is believed that if the Ir(III) complexes are also
capable of showing reversible piezochromic property, new
opportunities for Ir(III) complexes in optical recording could
be well expected.
Solid-state organic luminophoric molecules have received
tremendous attention due to their potential applications in
molecule-based devices.1 It is also generally recognized that
their emission properties strongly depend on the molecular
arrangement motif and intermolecular interactions. Accordingly,
tuning and switching of luminescence could be realized through
external stimulus affecting the mode of molecular packing.2
Piezochromism,3 as an interesting phenomenon, has been
presented and investigated on the basis of a series of pure
organic small-molecules,4 some metal complexes (AuI, AgI,
ZnII, PtII, AlIII),3b,5–9 liquid crystalline materials and polymers10
recently. As is shown by chemists, difference in the molecular
packing motifs allows materials undergo the color change in
luminescence upon grinding and subsequently reverts to its
original color by heating or recrystallization. This provides an
executable strategy to tune the luminescence of compounds and
is thought to be of importance for various recording and
sensing applications.11
Here, we report our preliminary investigation on two new
cationic iridium complexes with piezochromic property, namely,
[Ir(dfppz)2Mptz]PF6 (B1) and [Ir(ppy)2Mptz]PF6 (Y1), respectively
(Scheme 1). Their emission colors show reversible interconversion
from blue to blue-green for B1 and from green to orange for Y1,
respectively, upon grinding. To the best of our knowledge, this
is the first report on piezochromic phosphorescence of the
cationic iridium(III) complexes.
Treatment of organometallated dimer 1-(2,4-difluorophenyl)-
1H-pyrazole (dfppz) and 2-phenylpyridine (ppy) with corres-
ponding ancillary ligands Mptz by a bridge-splitting reaction in
dichloromethane–methanol (2 : 1), then readily give complexes
B1 and Y1. Their identities have been characterized spectro-
scopically, with satisfactory results obtained (Fig. S1–S4,
ESIw). The crystals of B1 were obtained by recrystallization
from dichloromethane–ether. X-Ray suitable crystals of B1
were obtained as prismatic crystals which show high blue
emission upon irradiation of UV-light. (Fig. S5, ESIw). The
structure of B1 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, and no solvent molecules are found
Phosphorescent Ir(III) complexes are of increasing interest
because of their outstanding photophysical properties and superior
photo- and chemical stability, suggesting that Ir(III)-based
complexes have recently emerged as good candidates for
various applications including chemical sensors,12 organic
light-emitting diodes (OLEDs),13 light-emitting cells (LECs)14
and biological cell imaging.15 Ir(III) complexes with triazole–
pyridine derivatives as ancillary ligands have shown intriguing
properties as the outstanding materials for optical devices.16
Moreover, the desirable blue-emitting Ir(III) could be
realized due to the strong s-donor property of the triazole
ring. Recently, a series of cationic Ir(III) complexes with
a Institute of Functional Material Chemistry, Faculty of Chemistry,
Northeast Normal University, Changchun, 130024 Jilin,
People’s Republic of China. E-mail: zmsu@nenu.edu.cn
b Department of Chemistry, Capital Normal University, Beijing,
People’s Republic of China. E-mail: liaoy271@nenu.edu.cn
w Electronic supplementary information (ESI) available: Experimental
details and figures. CCDC 843779 (B1). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc15855h
Scheme 1 Chemical structures of complexes B1 and Y1.
c
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emission properties of them were caused by physical processes,
such as changing the intermolecular interactions and/or the mode
of the molecular packing (Fig. 1c and d). The blue-green (G1)
and orange (O1) luminescence can be easily reverted to blue and
green ones upon recrystallizing from a dichloromethane–ether
mixture or heating at 230 1C for 10 min (Fig. S10, ESIw). Further
grinding of the heated-samples again forms the blue-green- or
orange-emitting ones. Noticeably, an additional high-energy
peak was observed for the heated sample G1, but its XRD
curves and NMR are identical. The potential reason responsible
for that is under investigation. However, the color changes in
solid state emission can be repeated many times, indicative of an
excellent reversibility in the switching processes (Fig. 1e and f),
which is similar to the reported piezochromic material based on
organic small-molecules.4e,17
The solid state samples B1 and Y1 were then outspread on
the filter paper and the letters ‘‘Ir’’ and ‘‘Su’’ were written on
that with a spatula, respectively. The blue-green ‘‘Ir’’ and
orange ‘‘Su’’ can be observed clearly on the blue and green
‘‘paper’’ with the naked eye under the UV light (Fig. 1g and h),
implying that both of them (B1 and Y1) have potential
application as optical recording materials.
To understand the possible origin of the present piezochromic
behavior, the powder X-ray diffraction (PXRD) of each sample
was studied (Fig. 2a and b). XRD patterns of B1 are consistent
with the single-crystal diffraction data. The intensive and sharp
reflection peaks can also be observed in Y1, indicating that B1
and Y1 should be the well-ordered aggregates. While, the ground
samples G1 and O1 show very weak and broad diffraction
signal intensity, indicating that the amorphous states formed
instead of the original ordered structures. After heating or
recrystallization on the ground samples, some sharp diffraction
peaks appeared (Fig. 2 and Fig. S11, ESIw). It suggests that
heating or recrystallization can convert amorphous ground
samples to the crystalline states through molecular repacking
and the reversible interconversion of piezochromic luminescence
can therefore be achieved (Fig. 1e and f).
Fig. 1 Emitting color upon irradiation of UV-light of B1 and G1 (a);
and Y1 and O1 (b); (c) and (d) the luminescence spectra of complexes
B1, G1, Y1 and O1; (e) and (f) repeated cycles of the piezochromism;
B1 (g) and Y1 (h) were cast on the filter paper and the letters ‘‘Ir’’ and
‘‘Su’’ were written with a spatula under UV-light at room temperature.
The thermal properties of all samples obtained from different
treatments are carefully analyzed by differential scanning
calorimetry (DSC). The DSC curves for the heating of unground
sample B1 and heated G1 only exhibited an almost identical
endothermic peak at 318 1C, corresponding to their melting
in the unit cell (Fig. S6, ESIw). As seen from the crystalline
structure of B1, there are weak C–HÁ Á Áp intermolecular inter-
actions and the dimer-like structures are observed in the
packing diagram (Fig. S7, ESIw). Owing to the weak inter-
molecular interactions, the molecular packing was relatively
loose and the ordered crystal structures might be easily
collapsed by external pressure.4b,e,f,5a The emission property
of B1 would also be changed as a result of the alteration of
solid-state arrangement and/or phase transition.
When the blue-emitting solid state powder B1 was ground in
a ceramic mortar, the obtained light-green powder emits blue-
green luminescence G1 at 493 nm with respect to 461 nm of B1
(Fig. 1a). Evidently, B1 exhibits pressure-induced luminescence
change feature, that is, B1 is a piezochromic material. Similar
to that of B1, after grinding treatment of Y1, the sample
unexpectedly converts into O1 with orange-emitting color
1
(Fig. 1b). Since H NMR, MALDI-TOF spectra and elemental
analysis of B1 (G1) and Y1 (O1) gave the same value corres-
ponding to [Ir(dfppz)2(Mptz)]PF6 and [Ir(ppy)2Mptz]PF6 (Fig. S8
and S9, ESIw), it indicates that no chemical reaction happens
during the grinding process and further implies that different
Fig. 2 Power X-ray diffraction pattern (a and b) and the DSC curves
(c and d) of the corresponding samples.
c
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points (Fig. 2c). In contrast, upon heating amorphous states G1,
their DSC curves firstly exhibit an exothermic recrystallization
peak at relatively lower temperatures of about 150 1C, and then
an endothermic melting peak was detected, which is similar to
the melting points of corresponding crystalline state samples.
It is due to the fact that as the temperature increases, the
amorphous solid will become less viscous and the molecules
may obtain enough freedom of motion to spontaneously arrange
themselves into a crystalline form. In addition, because the
melting point of Y1 in different states is higher than their
decomposition temperature (Fig. S12, ESIw), the DSC of Y1,
O1 and heated O1 was only recorded from RT to 300 1C.
As depicted in Fig. 2d, it is clear that except O1 with a
recrystallization peak at about 220 1C, there are no endo- and/
or exothermic peaks prior to 300 1C. The results indicate that the
ground samples are in a metastable state which can be restored
to thermodynamically stable crystals (B1 and Y1) from the powder
(G1 and O1) through an exothermic recrystallization process.
The 13C magic-angle spinning nuclear magnetic resonance
(CP/MAS-NMR) spectroscopy was also used to provide deep
insight into the piezochromic behaviors of the B1 and Y1 crystal-
line powders. As shown in Fig. S13, ESIw the ground samples G1
and O1 also keep their original conformation at the molecular
level. In crystalline samples, they show clear sharp resonance lines.
By contrast, the resonance lines of ground samples exhibit broader
distribution compared with those of crystalline ones. Based on
our experimental results, we can confidently propose that the
ground samples, obtained by the mechanical grinding, are a
kind of B1 and Y1 amorphous state powders.18 The 15N NMR
data also support these results (Fig. S14, ESIw). Although the
intermolecular interaction property of amorphous states
cannot be determined accurately at the current stage, the
emission properties of these two complexes may be strongly
molecular packing dependent. Moreover, the photolumines-
cence decays of the samples before and after grinding were also
performed, revealing that emission lifetimes for crystalline states
are longer than those for amorphous ones (Fig. S15, ESIw).
In summary, we have successfully prepared and discovered
two cationic complexes exhibiting a fascinating piezochromic
phosphorescence, in which the emission color can be switched
reversibly by grinding and heating. This is the first report that
the Ir(III) complexes display piezochromic phosphorescence.
Based on the PXRD, DSC and CP/MAS NMR studies, we
found that this piezochromic phosphorescence origin could be
attributed to a crystalline–amorphous phase transformation.
This reversible color change feature will be useful in developing
new application for Ir(III) complexes, such as optical recording,
temperature- or pressure-sensing in future.
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The authors gratefully acknowledge financial support from
NSFC (20703008 and 20903020), 973 Program (2009CB623605),
Program for Changjiang Scholars and Innovative Research
Team in University (IRT0714), the Science Foundation for
Young of Jilin Scientific Development Project (No. 20100182)
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2002 Chem. Commun., 2012, 48, 2000–2002
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