Intramolecular π-stacking in cationic iridium(iii) complexes with a triazole-pyridine type ancillary ligand: Synthesis, photophysics, electrochemistry properties and piezochromic behavior

Article abstract of DOI:10.1039/c2jm30480e

To make Ir(iii)-based complexes potentially multifunctional materials, two new cationic Ir(iii) complexes with a 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3- yl)pyridine (Phtz) ancillary ligand were designed and synthesized. By introducing the pendant phenyl ring into the ancillary ligand, the two complexes possess desired intramolecular π-π stacking between the pendant phenyl ring of the Phtz ligand and one of the phenyl rings of the cyclometalated ligand, which renders the complexes more stable. Density functional theory calculation indicates that the intramolecular π-π interactions in both complexes can reduce the degradation reaction in metal-centered (³MC) states to some extent, which further implies their stability. With these results in combination with their reversible oxidation and reduction processes as well as excellent photophysical properties, the stable light-emitting cells (LECs) would be expected. Furthermore, the two synthesized complexes exhibit reversible piezochromism. Their emission color can be smartly switched by grinding and heating, which is visible to the naked eye. In light of our experimental results, the present piezochromic behavior is due to interconversion between crystalline and amorphous states.

Full text of DOI:10.1039/c2jm30480e

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PAPER
Intramolecular p-stacking in cationic iridium(III) complexes with
a triazole–pyridine type ancillary ligand: synthesis, photophysics,
electrochemistry properties and piezochromic behavior†
a
Guo-Gang Shan, Hai-Bin Li, Dong-Xia Zhu, Zhong-Min Su and Yi Liao
a
a
a
b
*
*
Received 25th January 2012, Accepted 8th April 2012
DOI: 10.1039/c2jm30480e
To make Ir(III)-based complexes potentially multifunctional materials, two new cationic Ir(III)
complexes with a 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (Phtz) ancillary ligand were
designed and synthesized. By introducing the pendant phenyl ring into the ancillary ligand, the two
complexes possess desired intramolecular p–p stacking between the pendant phenyl ring of the Phtz
ligand and one of the phenyl rings of the cyclometalated ligand, which renders the complexes more
stable. Density functional theory calculation indicates that the intramolecular p–p interactions in both
complexes can reduce the degradation reaction in metal-centered (3MC) states to some extent, which
further implies their stability. With these results in combination with their reversible oxidation and
reduction processes as well as excellent photophysical properties, the stable light-emitting cells (LECs)
would be expected. Furthermore, the two synthesized complexes exhibit reversible piezochromism.
Their emission color can be smartly switched by grinding and heating, which is visible to the naked eye.
In light of our experimental results, the present piezochromic behavior is due to interconversion
between crystalline and amorphous states.
ligand-structure control.8 In addition, white LECs were realized
Introduction
via combination of blue-green and red emitting cationic Ir(III)
complexes in a host–guest system.9 Nowadays, LECs are regar-
ded as good candidates for potential flat-panel displays and low-
cost solid-state lighting sources.
Phosphorescent transition-metal complexes (TMCs), particu-
larly Ir(III) complexes, have attracted much attention due to their
high emission efficiency, relatively long excited-state lifetime,
photo- and thermal-stabilities, as well as easy tunability of the
emission wavelength.1 Therefore, Ir(III) complexes are promising
for the various applications such as chemosensors,2 biological
probes,3 photocatalytic water reduction,4 organic light-emitting
diodes (OLEDs)5 and light-emitting electrochemical cells
(LECs).6 Ir(III)-based LECs, a new type of organic electrolumi-
nescent devices, possess several advantages over conventional
OLEDs such as simple device architecture, low turn-on voltage,
and independent of the work function of the electrode material,
which are initial requirements for achieving the unencapsulated
devices. Since Slinker reported the first LECs based on cationic
Ir(III) complexes,7 Ir(III)-based LECs with green, yellow, orange
and red emitting color have been achieved through careful
However, the water-assisted ligand-exchange reactions are
believed to occur during the operations of LECs, resulting in
luminescence quenching of emitting molecules, which remark-
ably decrease the efficiency and stability of the devices.10 Recent
studies have demonstrated that the intramolecular p–p interac-
tions between coordinated ligands within a single cationic Ir(III)
complex significantly increase the stability of devices.11 Although
this approach has been presented, limited efforts have been
devoted to designing and synthesizing the cationic Ir(III)
complexes with intramolecular p-stacking up to now. To
construct this kind of supramolecular interactions, the pendant
phenyl ring was usually introduced into the appropriate position
of the ancillary ligand. Recently, we reported two cationic Ir(III)
complexes based on the 2-(5-methyl-2-phenyl-2H-1,2,4-triazol-
3-yl)pyridine (Mptz) ancillary ligand.12 By carefully analyzing the
structure of the reported complex [Ir(dfppz)2Mptz]PF6 (dfppz:
1-(2,4-difluorophenyl)-1H-pyrazole), and replacing the methyl
group of the ancillary ligand with a phenyl moiety, the intra-
molecular p-stacking conformation might be achieved in the
designed complex. Furthermore, the complex B1 also exhibited
fascinating piezochromic property, which opens up the new
opportunities for Ir(III) complexes in optical recording and
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: H NMR and
1
MS spectra of complexes 1 and 2 and the calculational informations.
CCDC 862865 (1) and 862866 (2). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2jm30480e
12736 | J. Mater. Chem., 2012, 22, 12736–12744
This journal is ª The Royal Society of Chemistry 2012

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temperature- and pressure-sensing fields.13 Thereby, it is of great
interest to design and synthesize an Ir(III) complex capable of
possessing the intramolecular p-stacking structure and simulta-
neously exhibiting the reversible piezochromic property.
syntheses were freshly distilled over appropriate drying reagents.
All experiments were performed under a nitrogen atmosphere by
using standard Schlenk techniques. Elemental analyses (C, H, and
N) were performed on a Perkin-Elmer 240C elemental analyzer.
TG analyses were performed on a Perkin-Elmer TG-7 analyzer
heated from 30 to 800 ꢀC in a flow of nitrogen at the heating rate of
10 ꢀC minꢁ1. Powder X-ray diffraction (XRD) patterns of the
samples were collected on a Rigaku Dmax 2000. 1H NMR spectra
were measured on Bruker Avance 500 MHz with tetramethylsi-
lane as the internal standard. The molecular weights of ligands
and complexes were tested by using electrospray-ionization mass
spectroscopy and matrix-assisted laser desorption-ionization
time-of-flight (MALDI-TOF) mass spectrometry, respectively.
UV-vis absorption spectra were recorded on a Hitachi U3030
spectrometer. The emission spectra were recorded using the
F-7000 FL spectrophotometer. The excited-state lifetime was
measured on a transient spectrofluorimeter (Edinburgh FLS920)
with a time-correlated single-photo-counting technique. The
photoluminescence quantum yields (PLQYs) of the neat film were
measured in an integrating sphere.
In this contribution, we designed and successfully synthesized
two novel cationic Ir(III) complexes with the 2-(5-phenyl-2-
phenyl-2H-1,2,4-triazol-3-yl)pyridine (Phtz) ancillary ligand,
namely [Ir(dfppz)2(Phtz)](PF6) (1) and [Ir(ppy)2(Phtz)](PF6) (2)
(Scheme 1) in which ppy was 2-phenylpyridine. Their X-ray
crystal structures clearly demonstrated that they possessed the
desired intramolecular p–p stacking between the pendant phenyl
ring of the Phtz ligand and one of the phenyl rings of the cyclo-
metalated ligand. The photophysical properties and their stability
caused by these intramolecular p–p interactions were rationalized
by theoretical calculations based on the density functional theory
(DFT) approach. In particular, the complexes 1 and 2 also
exhibited an interesting piezochromism, and the color changes in
solid state emission could be repeated many times, indicative of an
excellent reversibility in the switching processes.
Experimental section
Materials and measurements
Synthesis
All reagents and solvents employed were commercially available
and used as received without further purification. The solvents for
The cyclometalated chloride-bridged dimers were synthesized
according to a procedure described in the literature by refluxing
Scheme 1 Synthesis of the ancillary ligand (Phtz) and the cationic Ir(III) complexes 1 and 2.
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IrCl3$3H2O with the corresponding cyclometalated ligands in the
mixture of 2-ethoxythanol and water.14 The ancillary ligands
Phtz were synthesized as depicted in Scheme 1. Complexes 1
and 2 could be readily achieved from the reaction of ancillary
ligands with the organometallated dichloro-bridged dimer
[Ir(dfppz)2Cl]2 and [Ir(ppy)2Cl]2, respectively, by a bridge-split-
ting reaction. The molecular structures of these complexes were
52.17; H, 3.20; N, 8.90. Found: C, 52.20; H, 3.25, N, 8.92%. MS
(MALDI-TOF): m/z 799.2 (M ꢁ PF6).
X-ray crystallography study
Data collection of complexes 1 and 2 was performed on a Bruker
Smart Apex II CCD diffractometer with graphite-mono-
ꢀ
1
chromated Mo Ka radiation (l ¼ 0.71069 A) at 293 K.
characterized by H NMR, mass spectrometry and elemental
Absorption corrections were performed by using the multi-scan
technique. The crystal structure was solved by Direct Methods of
SHELXTL-97 (ref. 16) and refined by full-matrix least-squares
techniques using SHELXTL-97 within WINGX.17 The hydrogen
atoms of aromatic rings were included in the structure factor
calculation at idealized positions by using a riding model.
Anisotropic thermal parameters were used to refine all non-
hydrogen atoms except for some of the nitrogen and carbon
atoms. The detailed crystallographic data and structure refine-
ment parameters are summarized in Table S1 (ESI†).
analysis (Fig. S1–S6, ESI†).
2-(5-Phenyl-2H-1,2,4-triazol-3-yl)pyridine
(Phtz).
The
precursor 2-(5-phenyl-2H-1,2,4-triazol-3-yl)pyridine (HPhtz)
was synthesized by a previously reported procedure.15 HPhtz
(1.69 g, 7.6 mmol), iodobenzene (2.45 g, 12 mmol), 1,10-phe-
nanthroline (0.96 g, 4.8 mmol), copper(I) iodide (0.48 g, 2.5
mmol), and caesium carbonate (3.72 g, 11 mmol) were dissolved
in DMF (40 mL). The mixture was stirred vigorously for10 min
at room temperature (RT) and subsequently refluxed under an
argon atmosphere for another 24 h. After cooling to RT, the
solvent was removed under vacuum, then the residue was
extracted with CH2Cl2 (200 mL). The product was then obtained
by column chromatography on silica gel with petroleum ether
(b.p. 60–90 ꢀC)–ethyl acetate (2 : 1) as the eluent to yield a white
solid (75%). 1H NMR (500 MHz, CDCl3, ppm): d 8.52–8.53 (m,
1H), 8.25–8.27 (m, 1H), 7.93–7.94 (m, 1H), 7.77–7.81 (m, 1H),
7.42–7.50 (m, 8H), 7.30–7.33 (m, 1H). Anal. calcd for C19H14N4:
C, 76.49; H, 4.73; N, 18.78. Found: C, 76.52; H, 4.81, N, 18.83%.
ESI-MS (m/z): 299.1307 [M + H]+.
Electrochemical measurements
Cyclic voltammetry was performed on a BAS 100 W instrument
with a scan rate of 100 mV sꢁ1 in CH3CN (10ꢁ3 M) with the three-
electrode configuration: a glassy-carbon electrode as the working
electrode, an aqueous saturated calomel electrode as the pseudo-
reference electrode, and a platinum wire as the counter-electrode.
A 0.1 M solution of tetrabutylammonium perchlorate (TBAP) in
CH3CN was used as the supporting electrolyte and ferrocene was
selected as the internal standard.
[Ir(dfppz)2(Phtz)](PF6) (1). A solution of ligand Phtz (0.63 g,
2.1 mmol) and the dichloro-bridged diiridium complex [Ir(dfppz)
Cl]2 (1.2 g, 1.0 mmol) in dichloromethane (30 mL) and methanol
(15 mL) was refluxed for 24 h in the dark. After cooling to room
temperature, the mixture was filtrated, and then an excess of
solid KPF6 was added and stirred for another 0.5 h at room
temperature. The solvent was removed under reduced pressure
and the residue was purified by silica gel column chromatog-
raphy using ethyl acetate to yield 1. (72%). 1H NMR (500 MHz,
d6-DMSO, ppm): d 8.66 (d, J ¼ 2.5 Hz, 1H), 8.55 (d, J ¼ 2.5 Hz,
1H), 8.15 (t, J ¼ 8 Hz, 1H), 8.05 (d, J ¼ 2.5 Hz, 1H), 8.01 (d, J ¼
5.5 Hz, 1H), 7.92 (d, J ¼ 6.5 Hz, 2H), 7.79–7.83 (m, 3H), 7.69 (t,
J ¼ 6 Hz, 1H), 7.57 (d, J ¼ 2.5 Hz, 1H), 7.35 (d, J ¼ 8 Hz, 1H),
7.27–7.30 (m, 1H), 7.08–7.15 (m, 5H), 6.90 (t, J ¼ 3 Hz, 1H), 6.86
(t, J ¼ 3 Hz, 1H), 6.68–6.73 (m, 1H), 5.55–5.57 (m, 1H), 5.12–
5.14 (m, 1H). Anal. calcd for IrC37H24N8F10P: C, 44.72; H, 2.43;
N, 11.28. Found: C, 44.76; H, 2.51, N, 11.39%. MS (MALDI-
TOF): m/z 849.2 (M ꢁ PF6).
Theoretical calculations
The ground and excited electronic states of complexes were
investigated by performing DFT and TD-DFT calculations at
the B3LYP level.18 The 6-31G* basis sets were employed for
optimizing the C, H, N atoms and the LANL2DZ basis sets for
the Ir atom. An effective core potential (ECP) replaces the inner
core electrons of iridium leaving the outer core (5s)2(5p)6 elec-
trons and the (5d)6 valence electrons of Ir(III). The geometry of
the metal-centered triplet (3MC) was fully optimized and was
calculated at the spin-unrestricted UB3LYP level with a spin
multiplicity of 3. All calculations reported here were carried out
with the Gaussian 09 software package.19
Results and discussion
X-ray structure characterization
Crystal structures of 1 and 2 were obtained by the slow evapo-
ration of diethyl ether into the CH2Cl2 solution. As depicted in
Fig. 1, both complexes reveal the distorted octahedral geometry
around the Ir atom with two cyclometalated ligands and one
ancillary ligand. Both the cyclometalated ligands adopt a mutu-
ally eclipsed configuration with two nitrogen atoms residing at
trans locations. The overall structural arrangement is similar to
several previously reported ones.8b,9b,11a,20 Moreover, the bond
length Ir–Nancillary ligand is significantly longer than that of Ir–
Ncyclometalated ligand, which might be attributed to the strong trans
influence of the carbon donors.21 For example, in the case of
complex 1, the Ir–N bond lengths in the ancillary ligand Ir1–N5
[Ir(ppy)2(Phtz)](PF6) (2). The synthesis of complex 2 was
similar to that of complex 1 except that the organometallated
dimer 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) was replaced
with 2-phenylpyridine (ppy). The crude product was also purified
by silica gel column chromatography using ethyl acetate as
1
eluent. H NMR (500 MHz, d6-DMSO, ppm): d 8.54 (d, J ¼ 5
Hz, 1H), 8.31 (d, J ¼ 8 Hz, 1H), 7.98–8.09 (m, 3H), 7.77–7.95 (m,
9H), 7.63–7.66 (m, 1H), 7.46 (d, J ¼ 6.5 Hz, 1H), 7.31–7.34 (m,
2H), 7.16–7.25 (m, 4H), 6.98–7.01 (m, 1H), 6.93 (t, J ¼ 8 Hz, 1H),
6.83–6.86 (m, 1H), 6.63–6.71 (m, 2H), 6.07 (d, J ¼ 7.5 Hz, 1H),
5.95 (d, J ¼ 7.5 Hz, 1H). Anal. calcd for IrC41H30N6F6P: C,
12738 | J. Mater. Chem., 2012, 22, 12736–12744
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Fig. 1 Crystal structure of complexes 1 (a) and 2 (b). Thermal ellipsoids
are drawn at 30% probability. The PF6 counter-anion is omitted for
clarity.
Fig. 2 The intramolecular p–p interactions are shown in 1 (a) and 2 (b),
respectively.
ꢀ
and Ir1–N6 are 2.161(3) A and 2.145(4) A, respectively, which
ꢀ
are longer than those in the cyclometalated ligand Ir1–N2
character with reference to that reported for other Ir(III)
complexes.6,8,23 Generally, for cationic Ir(III) complexes, three
excited states contribute to the observation of light emission,
ꢀ
ꢀ
(2.028(4) A) and Ir1–N4 (2.018(4) A) (see Table S2, ESI†). In
addition, the bite angles of the Phtz ligands (75.98(14)ꢀ for 1 and
76.00(3) and 75.50(3)ꢀ for 2) are smaller than those of the
cyclometalated ligands (80.02(19) and 80.26(19)ꢀ for 1 and
80.30(3), 79.60(3), 81.40(4) and 79.30(4)ꢀ for 2). These observa-
tions have also been made in related cationic Ir(III) complexes.22
As expected, the robust p–p stacking between the phenyl ring of
the Phtz ligand and phenyl ring of one of the cyclometalated
ligands is observed in complex 1, with a centroid–centroid
3
3
3
namely, LC p–p*, MLCT and LLCT. The emission spectra
3
from the MLCT state are usually broad and featureless, while
the obvious structured emission bands are always attributed to
the LC 3p–p* state.24 At room temperature, both complexes
display almost featureless emission spectra in solutions and neat
film, indicating that their emissive excited states take on much
3
3
3
more MLCT or LLCT characters than LC p–p* characters
(Fig. 3, S8 and S9, ESI†). In the neat film, emission peaks of 1
and 2 occurred at 499 and 564 nm, respectively, with a quantum
efficiency of 0.18 for 1 (s ¼ 0.57 ms) and 0.25 for 2 (s ¼ 0.20 ms).
ꢀ
distance of ca. 3.6 A (Fig. 2a). Although there are two isolated
complexes 2 in the crystal structures, the intramolecular p–p
interactions are also constructed (see Fig. 2b and S7, ESI†). This
observation confirms our hypothesis that with replacement of the
methyl group of complex [Ir(dfppz)2Mptz]PF6 with a phenyl
substituent, the supramolecular cage configuration is formed.
The inherent intramolecular p–p interactions in both complexes
should reduce the possibility for nucleophilic attack, and thus
stable LECs could be obtained via such complexes.11
Photophysical properties
The absorption and emission spectra recorded for 1 and 2 in
degassed CH3CN at room temperature are shown in Fig. 3. The
detailed photophysical characteristics are summarized in Table 1.
As shown in Fig. 3, both complexes exhibit two major absorption
bands in their absorption spectra. The dominant absorption
band in the range of 200–350 nm is assigned to spin-allowed p–
p* transitions from the ligands. The low-energy absorption from
350 nm extending to the visible region is ascribed to both spin-
allowed, spin-forbidden metal-to-ligand charge transfer
(3MLCT) and ligand-to-ligand charge transfer (1LLCT)
Fig. 3 Normalized absorption and emission spectra of complexes 1 and
2 in the CH3CN solvent at room temperature.
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Table 1 Photophysical and electrochemical characteristics of complexes
1 and 2
pyridine and 1,2,4-triazol groups of ancillary ligand. Two phenyl
rings in ancillary ligand have not contributed to the LUMO
orbitals. Their HOMOs mainly localize on the cyclometalated
ligands and d orbital of iridium atom. On the basis of the TD-
DFT calculation and orbital diagrams, the T1 states of the two
complexes originate from the excitation of HOMO / LUMO
(91% and 95% for complexes 1 and 2, respectively), with char-
Emission
at 77 K
Electrochemical
datac
Emission at room temperature
a
Solution
l [nm]
Neat film Fem
l [nm] (s [ms])
lb [nm] Eox (V) Ered (V)
1/2
1/2
3
acter of mixed MLCT (Ir atom / pyridine and 1,2,4-triazol
1 513 (CH3CN) 499
504 (CH2Cl2)
2 570 (CH3CN) 564
568 (CH2Cl2)
0.18 (0.57) 479
0.25 (0.20) 511
1.27
0.89
ꢁ1.41
ꢁ1.42
groups of ancillary ligands) and 3LLCT (cyclometalated
ligands / pyridine and 1,2,4-triazol groups of ancillary ligands).
According to the photophysical results, it is believed that the
observed emission bands of both complexes should mainly
a
b
c
Measured in the neat film. In CH3CN glass. The data were versus
Fc+/Fc (Fc is ferrocene).
3
3
originate from the MLCT (Ir / ancillary ligands) and LLCT
(cyclometalated ligands ancillary ligands) states. The
/
quantum chemical calculations therefore indicate that the nature
of the emissive excited state of complexes 1 and 2 could be
attributed to the mixed 3MLCT and 3LLCT characters with
different contributions.
The relatively high PLQY in neat film is beneficial for the
application in optical devices. Upon cooling the CH3CN solu-
tions to 77 K, their emission spectra undergo rigidochromic blue
shifts, but still are broad and featureless (Fig. S8, ESI†). These
emission properties at 77 K further suggest that their emissions
mainly originate from MLCT mixed with LLCT states.25 It is
noticeable that after replacing the cyclometalated ligand 1-(2,4-
difluorophenyl)-1H-pyrazole (dfppz) with the 2-phenylpyridine
(ppy) ligand, the emission of 2 is red-shifted compared to the
green emission of the complex 1, which can be attributed to the
deceased HOMO levels. This viewpoint is firmly supported by
their electrochemical behaviors.
In addition, the metal-centered states (3MC) result from the
excitation of an electron from the occupied t2g(dp) HOMO to
the unoccupied e2g(ds*), which is assigned to be the origin of the
degradation process for the Ru(II)- and Ir(II)-based LECs.10b,11b,27
In light of the experimental and theoretical results, Bolink et al.
have demonstrated that the metal–ligand bond length in the 3MC
state played a key role for the stability of the Ir(III)-based LECs. 11c
The rupture of metal–ligand bonds can open the structure of
the complexes and enhance the reactivity of complexes in the
3MC state, facilitating the degradation process and resulting in
unstable LECs. However, the cage effect caused by the intra-
molecular p–p interaction effectively restricts the opening of the
3
3
Electrochemical properties
The electrochemical behaviors of complexes 1 and 2 were
investigated by cyclic voltammetry in CH3CN solution, and the
redox potentials are listed in Table 1. Both complexes exhibit
reversible oxidation and reduction processes in CH3CN solution.
The redox reversibility indicates that both the holes (upon
oxidation) and electrons (upon reduction) can be easily trans-
ported and is beneficial for the application of the complexes in
LECs.26 Because the reduction of the cationic Ir(III) complexes
usually occurs on the ancillary ligands, the reduction potential of
complex 1 (ꢁ1.41) is almost identical to that of 2 (ꢁ1.42). As the
p–p* energy gap of the cyclometalated ligand decreases from
complex 1 to complex 2, the oxidation potentials are shifted
anodically (Table 1), indicating that the HOMO energy level of 1
was stabilized with respect to that of complex 2. Thus, the energy
gap of 1 should be larger than that of 2, which is in agreement
with our photophysical data (vide supra).
3
structure of complexes in the MC state, leading to the virtual
decoordination of only one of the Ncyclometalated ligand atoms. This
supramolecular cage effect makes the cationic Ir(III) complexes
more robust, reducing the possibility of the ligand-exchange
degradation reaction. To determine whether the intramolecular
p–p interactions in complexes 1 and 2 can reduce the opening of
structures in the 3MC state or not, the geometry optimization of
their 3MC states has been performed following the methodology
illustrated in the reported works.10b,28 The metal-centered
Quantum chemical calculations
To gain insight into the nature of the emissive excited state
involved in the emission process of complexes 1 and 2, the
quantum chemical calculations were performed (see Experi-
mental section). The low-lying triplet states of both complexes
are calculated based on their optimized ground geometry
through a time-dependent density functional theory approach
(TD-DFT). The excitation energies and molecular orbitals
involved in the excitations are summarized in Fig. 4 and
Table S3 (ESI†).
According to our quantum chemical calculation results, the
LUMOs of 1 and 2 are almost identical which reside on the
Fig. 4 Calculated HOMO and LUMO orbitals for complexes 1 and 2.
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characters of the triplet state for both complexes 1 and 2 were
confirmed by the spin densities calculated for the optimized 3MC
states geometries (see Fig. S10, ESI†). The spin densities are
mainly concentrated on the iridium atom, and hold 1.50 and 1.50
3
unpaired electrons for complexes 1 and 2 in MC states, respec-
tively. As shown in Fig. 5, the Ir–N bond (R1) of the cyclo-
metalated ligands in complexes 1 (2) involved in the p–p
ꢀ
ꢀ
interaction only lengthens from 2.08 A (2.08 A) in ground
states (S0) to 2.17 A (2.23 A) in the 3MC state. In contrast,
ꢀ
ꢀ
electron promotion results in the elongation of another two Ir–
ꢀ ꢀ
cyclometalated ligand lengths (R2), from 2.05 A in S0 state to 2.50 A
N
3
3
ꢀ
ꢀ
in MC state for 1, and from 2.08 A in S0 state to 2.52 A in MC
state for 2, respectively. Consequently, the p–p interactions in
both complexes (1 and 2) would be in favor for reducing the
3
degradation reaction in MC to some extent and precluding the
Fig. 6 Stimuli-responsive behaviors of as-synthesized complexes 1A and
2A upon grinding and heating treatments (under UV lamp, lex ¼ 365
nm).
attack by the nucleophiles, which is very beneficial for LECs.
Piezochromic behavior
Interestingly, the solid state powders of complexes 1 and 2
obtained through column chromatography exhibit unusual pie-
zochromic behaviors. For clarity, the as-synthesized samples,
ground and heated ground samples, were hereafter abbreviated
as the letters ‘‘A’’, ‘‘G’’ and ‘‘H’’, respectively (e.g., the sample 1A
is the as-synthesized complex 1 through column chromatog-
raphy; 1G is the ground sample of 1A). Upon grinding, the
emission color changes of samples 1A and 2A are depicted in
Fig. 6, showing a significant emission red-shift from sky-blue to
blue-green for 1A and from yellow to orange for 2A, respectively.
In addition, the experimental results show that the solid-state
absorption spectra are different between the as-synthesized
samples and ground samples. As shown in Fig. S11†, the
absorption spectra of the ground samples 1G and 2G were
obviously red-shifted and broadened compared with those of the
as-synthesized samples 1A and 2A, respectively, indicating that
the changes in the emission color may be caused by changing the
mode of the molecular packing. Here, we take complex 1 as an
example. When the sky-blue emitting (lmax ¼ 471 nm, 0.94 ms)
solid state powder 1A was ground, the obtained sample 1G
emitted blue-green luminescence at 499 nm with the lifetime
being 1.0 ms (Fig. 7a). Similar to 1A, the samples 2A also exhibit
a pressure-induced luminescence change feature, with the emis-
sion wavelength changing from 542 nm (0.70 ms) to 563 nm (0.50
ms) upon grinding treatment. The change in emission maximum
of 1A and 2A before and after grinding are 28 and 21 nm,
respectively. Evidently, the complexes 1 and 2 are piezochromic
materials. Interestingly, the emission color of the ground samples
1G and 2G can easily be reverted to the original unground state
(1A and 2A) upon heating at 230 ꢀC for 5 min (Fig. S12, ESI†).
When the heated samples 1H and 2H were further ground, the
blue-green and orange-emitting powders were obtained again.
These color changes in solid state emission of 1A and 2A are thus
repeated many times, indicative of an excellent reversibility in the
switching processes (Fig. 8). To verify the change in the chemical
structure of samples 1A and 2A in different states, their 1H NMR
and MALDI-TOF spectra have been determined. The spectra in
different states are almost identical (Fig. S13 and S14, ESI†),
indicating that their different emission properties might be
caused by physical processes, such as changing the
3
Fig. 5 The minimum-energy structure calculated for the MC state of
complexes 1 (a) and 2 (b), respectively. Ir–Ncyclometalated
ꢀ
lengths are give in A.
bond
Fig. 7 The emission spectra of unground and ground samples 1A (a) and
2A (b).
ligand
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intermolecular interactions and/or the mode of the molecular
packing during the grinding and heating processes. It is impor-
tant to point out that a strong shift in emission color was also
observed by Bolink et al.29 for a cationic Ir(III) complex in thin
films with different concentrations and the associated LECs,
which is detrimental for cationic Ir(III) complexes as blue-emit-
ting LECs. It is believed that the reported reversible piezochro-
mic effect will be helpful for preparing rare blue-emitting Ir(III)-
based LECs through appropriate treatments and further
developing white-light emitting applications. In addition, when
the powders 1A and 2A were spread on the filter paper, the letters
‘‘IFMC’’ were written on them with a spatula, respectively.
Under the UV lamp, the marked ‘‘IFMC’’ can be observed
clearly on the sky-blue and yellow ‘‘paper’’ with the naked eye
(Fig. S15, ESI†), implying that both 1A and 2A have potential
application in the fields of optical recording and pressure-
sensing.
Fig. 9 Power X-ray diffraction patterns of 1A (a) and 2A (b) in different
state, and the DSC curves of the corresponding samples of 1A (c) and 2A
(d) in different state.
The mechanism for the present piezochromic behavior
To determine the possible mechanism of the present piezochro-
mic behavior, the powder X-ray diffraction (PXRD) measure-
ments were performed on each sample (Fig. 9a and b). PXRD
patterns measured for 1A and 2A are in agreement with the
simulated results from the respective single crystal XRD data.
These intensive and sharp reflection peaks were observed in the
samples 1A and 2A, indicating that 1A and 2A should be the well-
ordered aggregates.30 In contrast, the obtained ground samples
1G and 2G show very weak and broad diffraction signal inten-
sities, indicating that the amorphous states formed instead of the
original ordered structures. The small peaks in the XRD patterns
of 1G and 2G with the 2q values consistent with those of the
peaks for 1A and 2A implied that a part of unground samples 1A
and 2A remained after grinding. This result suggests that their
piezochromic behavior is generated via changing the model of
molecular packing upon grinding, such as from crystalline state
to amorphous state. Generally, the amorphous solid will become
less viscous and the molecules may obtain enough freedom of
motion to spontaneously arrange themselves into a crystalline
form upon heating. Herein, the ground samples were heated at
230 ꢀC for 5 min, some sharp diffraction peaks appeared and
these peaks coincided with those of corresponding unground
crystals (Fig. 9a and b). It suggests that the difference in XRD
patterns from samples 1G and 2G to samples 1H and 2H is
ascribed to the transition from amorphous ground samples to the
crystalline states after heat treatment. Except heating treatment,
recrystallization is another feasible approach to make the
amorphous phase samples (1G and 2G) revert to the crystalline
state samples. Similar results were also observed for other
reported piezochromic material.31 As shown in Fig. S16 and S17
(ESI†), the XRD patterns and emission spectra of recrystallized
samples perfectly reverted to the original crystalline states.
In addition, the thermal properties of all samples obtained
from different treatments are carefully analyzed by differential
scanning calorimetry (DSC). In the DSC curves of the heating of
unground (1A and 2A) and heated samples (1G and 2G), there
are no endo- and/or exo-thermic peaks prior to 300 ꢀC (see
Fig. 9c and d). In contrast, upon heating amorphous states 1G
and 2G, their DSC curves exhibitꢀan exothermic recrystallization
ꢀ
peak at temperatures about 192 C and 217 C for 1G and 2G,
respectively. The observed recrystallization peaks of ground
samples and the transition enthalpy (see Table S4†) suggest that
the ground samples are a metastable state which can be restored
to thermodynamically stable crystals (1A and 2A) from the
powders (1G and 2G) through an exothermic recrystallization
process.
Conclusion
In summary, two cationic complexes possessing the intra-
molecular p–p stacking between Phtz ancillary ligands and one
of the cyclometalated ligands and exhibiting interesting piezo-
chromic property were designed and successfully prepared.
Studies on their photophysical properties in solution and films
have been carried out. Both complexes showed intense emission
in both solution and neat film, and their emission mechanism has
been supported by the quantum chemical calculations. Our
theoretical results also indicated that this supramolecular cage
effect made both complexes more robust, which reduced the
possibility of the ligand-exchange degradation reaction and
3
resulted in stable complexes even in MC states. Furthermore,
both complexes exhibited the fascinating piezochromic phos-
phorescence properties in solid state with an obvious change in
emission color which was visible to the naked eye. The reversible
Fig. 8 Emission wavelengths of the repeated piezochromic behaviors of
1A (a) and 2A (b).
12742 | J. Mater. Chem., 2012, 22, 12736–12744
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Chem., 2005, 44, 8723; (e) H. Su, C. Wu, F. Fang and K. Wong,
Appl. Phys. Lett., 2006, 89, 261118.
piezochromic phosphorescence nature is attributed to a crystal-
line-amorphous phase transformation by carefully analyzing
9 (a) L. He, L. Duan, J. Qiao, G. F. Dong, D. Q. Zhang, L. D. Wang
and Y. Qiu, Adv. Funct. Mater., 2009, 19, 2950; (b) H. C. Su,
H. F. Chen, F. C. Fang, C. C. Liu, C. C. Wu, K. T. Wong,
Y. H. Liu and S. M. Peng, J. Am. Chem. Soc., 2008, 130, 3413; (c)
L. He, L. Duan, J. Qiao, G. F. Dong, L. D. Wang and Y. Qiu,
Chem. Mater., 2010, 22, 3535.
1
their PXRD, DSC and H NMR data in different states. We
believe that this reversible color change feature and robust p–p
intramolecular interactions can make them useful in fabricating
stable LECs and optical recording applications in future.
€
ꢁ
10 (a) H. J. Bolink, L. Cappelli, E. Coronado, M. Gratzel, E. Ortı,
R. D. Costa, P. M. Viruela and M. K. Nazeeruddin, J. Am. Chem.
ꢁ
Acknowledgements
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S. Graber, C. E. Housecroft and E. C. Constable, Adv. Funct.
Mater., 2010, 20, 1511.
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Chem. Commun., 2011, 47, 6467; (b) H. J. Bolink, E. Coronado,
The authors gratefully acknowledge financial support from
NSFC (20971020, 20903020 and 21131001), 973 Program
(2009CB623605), and the Science and Technology Development
Planning of Jilin Province (20100540 and 201101008).
ꢁ
R. D. Costa, E. Ortı, M. Sessolo, S. Graber, K. Doyle,
M. Neubuger, C. E. Housecroft and E. C. Constable, Adv. Mater.,
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12 G. G. Shan, H. B. Li, H. T. Cao, D. X. Zhu, P. Li, Z. M. Su and
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