Influence of alkyl chain lengths on the properties of iridium(III)-based piezochromic luminescent dyes with triazole-pyridine type ancillary ligands

Article abstract of DOI:10.1016/j.dyepig.2013.08.015

To investigate the relationship between structures and piezochromic luminescence behavior and to develop excellent materials for efficient light-emitting electrochemical cells, a series of iridium(III) complexes with different length of alkyl chains have been designed and synthesized. The results show that all complexes exhibit not only naked-eye visible and reversible piezochromic behavior, but chain length-dependent emission properties: the shorter the alkyl chain is, the more remarkable mechanochromic behavior and higher recrystallization temperature will be. In light of powder X-ray diffractometry and differential scanning calorimetric data, the interconversion between crystalline and amorphous states upon external stimuli is response for the present piezochromism. Additionally, these complexes show high quantum yields of 55-65% in neat films as well as excellent redox reversibility. Despite changed alkyl chains are introduced into these complexes, the negligible effect on emitting colors and excited-state characters of them in both solutions and neat films have been observed. Such photophysical properties are also interpreted with the help of theoretical calculations. Moreover, the theoretical results suggest that the intrinsic intramolecular π-π stacking confirmed by the crystal structures can effectively restrict the opening of the structures in metal-centered excited-state, demonstrating their stability. The structure-property relationships and the results demonstrated here will help researchers develop and design more promising iridium(III)-based piezochromic materials and attractive phosphors for optical devices.

Full text of DOI:10.1016/j.dyepig.2013.08.015

Accepted Manuscript
Influence of alkyl chain lengths on the properties of iridium(III)-based piezochromic
luminescent dyes with triazole-pyridine type ancillary ligands
Guo-Gang Shan, Hai-Bin Li, Hong-Tao Cao, Hai-Zhu Sun, Dong-Xia Zhu, Zhong-Min
Su
PII:
S0143-7208(13)00313-6
10.1016/j.dyepig.2013.08.015
DYPI 4062
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 July 2013
Revised Date: 17 August 2013
Accepted Date: 19 August 2013
Please cite this article as: Shan G-G, Li H-B, Cao H-T, Sun H-Z, Zhu D-X, Su Z-M, Influence of alkyl
chain lengths on the properties of iridium(III)-based piezochromic luminescent dyes with triazole-pyridine
type ancillary ligands, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.08.015.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
A series of iridium(III)-based phosphorescent dyes showing naked-eye visible and
reversible piezochromic luminescent behavior are successfully synthesized. Moreover,
they are promising luminescent materials with high quantum yields of 55~65%, excellent
redox reversibility and relative stability.

ACCEPTED MANUSCRIPT
Highlights
►
►
►
►
►
Synthesis of Ir(III)-based dyes with different N-alkyl chains is described.
They show naked-eye visible and reversible piezochromism.
The chain length-dependent emissions are presented and investigated.
The dyes exhibit high quantum yields, redox reversibility and good stability.
The experimental results are rationalized by the theoretical calculations.

ACCEPTED MANUSCRIPT
Influence of alkyl chain lengths on the properties of
iridium(III)-based
piezochromic
luminescent
dyes
with
triazole-pyridine type ancillary ligands
Guo-Gang Shan, Hai-Bin Li, Hong-Tao Cao, Hai-Zhu Sun, Dong-Xia Zhu,
1
Zhong-Min Su*
Institute of Functional Material Chemistry, Faculty of chemistry, Northeast Normal
University, Changchun, 130024 Jilin, People’s Republic of China.
*
To whom correspondence should be addressed. E-mail: zmsu@nenu.edu.cn
Fax: +86-431-85684009, Tel.: +86-431-85099108
1

ACCEPTED MANUSCRIPT
Abstract
To investigate the relationship between structures and piezochromic luminescence
behavior and to develop excellent materials for efficient light-emitting
electrochemical cells, a series of iridium(III) complexes with different length of alkyl
chains have been designed and synthesized. The results show that all complexes
exhibit not only naked-eye visible and reversible piezochromic behavior, but chain
length-dependent emission properties: the shorter the alkyl chain is, the more
remarkable mechanochromic behavior and higher recrystallization temperature will be.
In light of powder X-ray diffractometry and differential scanning calorimetric data,
the interconversion between crystalline and amorphous states upon external stimuli is
response for the present piezochromism. Additionally, these complexes show high
quantum yields of 55 ~ 65% in neat films as well as excellent redox reversibility.
Despite changed alkyl chains are introduced into these complexes, the negligible
effect on emitting colors and excited-state characters of them in both solutions and
neat films have been observed. Such photophysical properties are also interpreted
with the help of theoretical calculations. Moreover, the theoretical results suggest that
the intrinsic intramolecular π-π stacking confirmed by the crystal structures can
effectively restrict the opening of the structures in metal-centered excited-state,
demonstrating their stability. The structure-property relationships and the results
demonstrated here will help researchers develope and design more promising
iridium(III)-based piezochromic materials and attractive phosphors for optical
devices.
2

ACCEPTED MANUSCRIPT
Keywords
Iridium(III) complex; Piezochromic luminescence; Chain length-dependent emission;
High quantum efficiency; Density functional theory
3

ACCEPTED MANUSCRIPT
1
. Introduction
Organic luminophores that exhibit clear and vivid luminescence in the solid states
have attracted tremendous attention due to their great potential in various
photofunctional applications [1, 2]. The photophysical properties of these solid-state
organic luminophores, for example emission color, strongly depend on the mode of
molecular stacking and the intermolecular interactions [3, 4]. Accordingly, dynamic
tuning and switching of solid-state luminescence can be achieved by means of the
external stimulus that triggers the alteration of molecular arrangement motif [5-7].
Recently, the organic luminophores undergo a significant pressure-induced change in
the emission color, termed piezochromic luminescent (PCL) materials, are growing
rapidly [8-18]. Such materials exhibit reversible and obvious color change upon
external mechanical forces but without chemical reactions, making them highly
attractive alternative for both fundamental research and practical application. To this
date, PCL behaviors for a few materials have been observed, including organic and
inorganic complexes, liquid crystal materials, and polymer-based assemblies [19-27].
However, the number of such materials is still rare, especially for metal-contained
complexes. To develop excellent PCL materials and establish clear structure-property
relationship, the in-depth understanding of PCL phenomena at molecular level are
essential. Thereby, it remains a great challenge to design new PCL materials and
investigate the relationship between molecular packing and piezochromic behavior.
Organometallic iridium(III) complexes exhibiting high phosphorescence
efficiency and rich photophysical properties as well as superior optical stability, have
4

ACCEPTED MANUSCRIPT
emerged as promising candidates for high-tech applications such as organic
light-emitting diodes (OLED), light-emitting electrochemical cells (LECs), chemical
sensor, and biological probes, etc [28-32]. Recently, our group and others have
developed a series of cationic iridium(III) complexes showing an interesting
piezochromic behavior, and found that their emission color switch is generally
attributed to a crystalline–amorphous phase transformation [33-36]. Based on this
mechanistic proposal, the PCL behavior can be ingeniously switched by simply
adjusting the substitution on ancillary ligands of the iridium(III) complexes [37]. In
addition, iridium(III) complexes can provide a better structural control on
excited-state properties and the optical output in different medias. Very recently, the
multifunctional
iridium(III)
complexes
simultaneously
exhibiting
aggregation-induced emission (AIE) and PCL properties have been realized by
attaching the dendritic group into complexes or modifying ancillary ligand with
carbazole unit [37, 38]. Despite these advances in iridium(III)-based PCL material,
such materials are still in their infancy and more clear structure-property relationship
has not been well addressed. Recent works on organic fluorescent PCL materials
demonstrates that different long alkyl chains grafted on the materials not only can
construct a series of new PCL materials with comparable molecular structure, but also
provide an efficient way to study the relationship between the structure and PCL
behavior [39-41]. Whereas, such great successes obtained through the flexible
strategy in organic fluorescent PCL materials are still limited for metal complexes.
On the other hand, to realize efficient and long-lifetime light-emitting
5

ACCEPTED MANUSCRIPT
electrochemical cells (LECs), cationic iridium(III) complexes with fascinating
characteristics, such as high quantum efficiency especially in the neat film, excellent
stability and redox reversibility, are highly desired [42-45]. And the exploitation of
novel cationic iridium(III) complexes for the efficient and stable LECs is also rising
significant interest in recent years [46-50]. If the different length of alkyl chains are
attached to the ligands to construct a series of comparable molecular PCL cationic
iridium(III) complexes that can meet the above-mentioned requirement for efficient
LECs, which may be a “one stone two bird” design strategy. Keeping this in mind, we
herein designed and prepared a new cationic iridium(III) complex [(dfppz) (L1)]PF
2
6
(complex 1) as shown in Scheme 1 and its analogues 2, 3, 4, and 5 with different
length of alkyls. Their photophysical properties in the solid-states as well as solution
have been investigated systematically. The results indicate that all complexes exhibit
naked-eye visible PCL behavior with a color change from sky-blue to blue-green
emitting and alkyl chain length dependent PCL properties. The emission color can be
easily switched by grinding and heating. Comprehensive density functional theory
(DFT) approach has been performed to gain insights into their photophysical
properties in solutions as well as to evaluate the stability accommodated by
intramolecular π-π stacking. The theoretical results suggest that the intrinsic
intramolecular π-stacking in the complexes can effectively restrict opening of the
3
structure in the MC state, indicative of their good stability. Combining with the high
quantum efficiency in neat film and excellent redox reversibility, it is anticipated that
these complexes are also attractive candidates for LECs.
6

ACCEPTED MANUSCRIPT
Insert Scheme 1.
Scheme 1. Synthetic procedure and the chemical structures of complexes 1–5.
2
2
. Experimental section
.1. Materials and measurements
All reagents and solvents employed were commercially available and used as received
without further purification. The solvents for syntheses were freshly distilled over
appropriate drying reagents. All experiments were performed under a nitrogen
1
atmosphere by using standard Schlenk techniques. H NMR spectra were measured on
Bruker Avance 500 MHz with tetramethylsilane 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 Hitachi U3030 spectrometer. The
emission spectra were recorded using the F-4600 FL spectrophotometer. The
excited-state lifetime were measured on a transient spectrofluorimeter (Edinburgh
FLS920) with time-correlated single-photon counting technique. The
photoluminescence quantum yields (PLQYs) of the neat film were measured in an
integrating sphere. Cyclic voltammetry was performed on a BAS 100 W instrument
-1
with a scan rate of 100 mV s in CH
3
CN solutions with the three-electrode
7

ACCEPTED MANUSCRIPT
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, respectively. A 0.1 M solution of tetrabutylammonium
perchlorate (TBAP) in CH CN was used as the supporting electrolyte, and the
3
+
ferrocene/ferrocinium (Fc /Fc) potential was measured and selected as the internal
standard.
2
.2. Synthesis
2
2
.2.1. Synthesis of ancillary the liangds
-(1-methyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Metaz)
A mixture of 2-(3-phenyl-1H-1,2,4-triazol-5-yl)pyridine prepared according to
previous report [51] (1.1 g, 5.0 mmol) and NaH (0.46 g, 20 mmol) in dry DMF (15
o
mL) was stirred at 0 C for 10 min. Iodomethane (1.1 g, 7.5 mmol) in 5 mL DMF was
added dropwise to the reaction mixture, and then the solution was stirred at room
temperature for an additional 12 h. After that, the mixture was poured into the ice
water, and extracted with CH
bicarbonate solution and water for several times. Then, the organic layer was dried
over anhydrous MgSO , filtered and then concentrated. The residue was purified by
2 2
Cl . The organic layer was washed with sodium
4
column chromatography (silica, ethyl acetate/petroleum ether = 1: 2) to yield a white
1
powder Metaz (yield: 66%). H NMR (500 MHz, CDCl , ppm): δ 8.70 (d, J = 4.5 Hz,
3
1
4
H), 8.33 (d, J = 8 Hz, 1H), 8.15–8.17 (m, 2H), 7.84–7.87 (m, 1H), 7.35–7.48 (m,
H), 4.42 (s, 3H).
8

ACCEPTED MANUSCRIPT
2
-(3-phenyl-1-propyl-1H-1,2,4-triazol-5-yl)pyridine (Protaz)
The precursor 2-(3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (1.1 g, 5.0 mmol) was
dissolved in acetone, and the equivalent KOH was added and the mixture was stirred
for 0.5 h. Then, 1-bromopropane (0.73 g, 6.0 mmol) was added into the reaction
mixture and the mixture was heated to reflux for overnight. After cooling to room
temperature, the mixture was quenched by water and then extracted by
2 4
dichloromethane. The organic layer was dried with Na SO and the solvent was
removed. The obtained residues was purified by column chromatography on silica gel
with ethyl acetate/petroleum ether (1:2) as the eluent to yield a light-yellow oil (60%).
1
3
H NMR (500 MHz, CDCl , ppm): δ 8.62–8.63 (m, 1H), 8.30 (d, J = 8 Hz, 1H),
8
7
3
2
.18–8.20 (m, 2H), 7.75–7.78 (m, 1H), 7.43–7.46 (m, 2H), 7.38–7.39 (m, 1H),
.24–7.27 (m, 1H), 4.82 (t, J = 7.5 Hz, 1H), 1.95–1.99 (m, 2H), 0.96 (t, J = 7.5 Hz,
H).
-(1-pentyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Pentaz)
The synthetic procedure was similar to that of Protaz except that 1-bromopropane
1
was used instead of 1-bromopentane. H NMR (500 MHz, CDCl
3
, ppm): δ 8.66–8.67
(m, 1H), 8.31 (d, J = 8 Hz, 1H), 8.17–8.18 (m, 2H), 7.81–7.84 (m, 1H), 7.41–7.46 (m,
2
1
H), 7.37–7.40 (m, 1H), 7.31–7.34 (m, 1H), 4.85 (t, J = 8 Hz, 2H), 1.92–1.98 (m, 2H),
.33–1.37 (m, 4H), 0.88–0.89 (m, 3H).
2
-(1-heptyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Heptaz)
1
3
H NMR (500 MHz, CDCl , ppm): δ 8.67–8.68 (m, 1H), 8.30–8.32 (m, 1H),
8
.16–8.18 (m, 2H), 7.82–7.85 (m, 1H), 7.43–7.46 (m, 2H), 7.39–7.41 (m, 1H),
9

ACCEPTED MANUSCRIPT
7
.32–7.35 (m, 1H), 4.85 (t, J = 7 Hz, 2H), 1.92–1.96 (m, 2H), 1.23–1.37 (m, 8H), 0.87
(t, J = 7 Hz, 3H).
2
-(1-nonyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Nontaz)
1
H NMR (500 MHz, CDCl , ppm): δ 8.66–8.68 (m, 1H), 8.30–8.31 (m, 1H),
3
8
.16–8.18 (m, 2H), 7.82–7.85 (m, 1H), 7.43–7.47 (m, 2H), 7.37–7.41 (m, 1H),
.32–7.35 (m, 1H), 4.85 (t, J = 7.5 Hz, 2H), 1.93–1.96 (m, 2H), 1.24–1.37 (m, 12H),
.87 (t, J = 6 Hz, 3H).
7
0
2
2 2
.2.2. Synthesis of the chloro-bridged dimer [Ir(dfppz) Cl]
The organometallated dimer [Ir(dfppz) Cl] was synthesized from reaction of
2
2
3 2
IrCl ·3H O (0.81 g, 2.32 mmol) with 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz, 0.92
g, 5.10 mmol) in 2-ethoxyethanol and water mixture (V:V = 3:1, 40 mL) for 24 h. The
mixture was treated with water (30 mL) to induce precipitation of the off-white solid.
The product was filtered out and washed with diethyl ether followed by ethanol, and
dried (yield: 79%).
2
.2.3 Synthesis of the complexes
Synthesis and characterization of complex 1
A solution of ligand Metaz (0.50 g, 2.1 mmol) and the dichloro-bridged diiridium
2 2
complex [Ir(dfppz) Cl] (1.2 g, 1.0 mmol) in 1,2-ethandiol (30 mL) was heated at 150
o
C for 15 h in the dark. After cooling to room temperature, the 20 mL methanol
PF (2.0 g in
00 mL of deionized water) was slowly added into the reaction mixture under stirring.
The obtained suspension was filtrated, and washed with water and methanol. The
solution was added into the mixture. Then, the aqueous solution of NH
4
6
1
1
0

ACCEPTED MANUSCRIPT
crude product was purified by silica gel column chromatography using
dichloromethane/ethyl acetate (6:1) and the resulting powders were recrysatallized
from dichloromethane and petroleum ether mixture to give complex 1 as a light-green
o
1
solid, m.p. 347 C (yield 81%). H NMR (500 MHz, d –DMSO, ppm): δ 8.64 (d, J =
6
8
1
.5 Hz, 2H), 8.51 (d, J = 2.5 Hz, 1H), 8.38 (t, J = 8.0 Hz, 1H), 8.01 (d, J = 5.5 Hz,
H), 7.80 (d, J = 2.5 Hz, 1H), 7.74 (t, J = 7.0 Hz, 1H), 7.34 (d, J = 2.5 Hz, 1H), 7.27
(t, J = 7.5 Hz, 1H), 7.05–7.10 (m, 3H), 6.99 (d, J = 8.0 Hz, 2H), 6.84 (t, J = 2.5 Hz,
1
1
d
H), 6.80 (t, J = 2.5 Hz, 1H), 6.66–6.67 (m, 1H), 5.56 (dd, J
1
= 8.0 Hz, J
2
=2.0 Hz,
19
H), 5.06 (dd, J = 8.0 Hz, J = 2.0 Hz, 1H), 4.51 (s, 3H). F NMR (470 MHz,
1
2
6
-DMSO, ppm): -66.57 (d, J = 710.64 Hz, 6F), -109.54 (dd, J
= 5.17 Hz, J = 8.46 Hz, 1F), -119.87 (dd, J
1.28 Hz, 1F), -121.74 (dd, J = 5.17 Hz, J = 11.75 Hz, 1F). MS (MALDI-TOF): m/z
87.1 (M–PF6). The related complexes (2, 3, 4 and 5) were prepared using the similar
1
= 8.46 Hz, J
2
= 14.57
Hz, 1F), -111.23 (dd, J
1
2
1
= 5.17 Hz, J =
2
1
7
1
2
procedures.
Synthesis and characterization of complex 2
o
1
Light-green solid; m.p. 281 C (Yield 78%). H NMR (500 MHz, d
.62–8.64 (m, 2H), 8.49 (d, J = 2.5 Hz, 1H), 8.38 (t, J = 8.0 Hz, 1H), 8.01 (d, J = 5.5
Hz, 1H), 7.86 (d, J = 2.0 Hz, 1H), 7.74 (t, J = 7.5 Hz, 1H), 7.42 (d, J = 2.5 Hz, 1H),
.26 (t, J = 7.5 Hz, 1H), 7.05–7.13 (m, 3H), 7.02 (d, J = 7.5 Hz, 2H), 6.85 (t, J = 2.5
Hz, 1H), 6.78 (t, J = 2.5 Hz, 1H), 6.62–6.67 (m, 1H), 5.57 (dd, J = 8.0 Hz, J = 2.5
Hz, 1H), 5.07 (dd, J = 8.0 Hz, J = 2.0 Hz, 1H), 4.74–4.89 (m, 2H), 2.01–2.06 (m,
H), 1.02 (t, J = 7.5 Hz, 3H). F NMR (470 MHz, d -DMSO, ppm): -66.57 (d, J =
6
–DMSO, ppm): δ
8
7
1
2
1
1
19
2
6
11

ACCEPTED MANUSCRIPT
7
10.64 Hz, 6F), -109.57 (dd, J
Hz, J = 8.93 Hz, 1F), -119.81 (dd, J
.64 Hz, J = 12.22 Hz, 1F). MS (MALDI-TOF): m/z 815.2 (M–PF6).
Synthesis and characterization of complex 3
1
= 7.89 Hz, J
2
= 14.10 Hz, 1F), -111.24 (dd, J
1
= 5.17
2
1
= 6.11 Hz, J
2
= 12.22 Hz, 1F), -121.72 (dd, J =
1
5
2
o
1
Light-green solid; m.p. 293 C (Yield 80%). H NMR (500 MHz, d
.60–8.64 (m, 2H), 8.49 (d, J = 2.5 Hz, 1H), 8.39 (t, J = 8.0 Hz, 1H), 8.01 (d, J = 5.5
Hz, 1H), 7.85 (d, J = 2.5 Hz, 1H), 7.74 (t, J = 6.5 Hz, 1H), 7.42 (d, J = 2.0 Hz, 1H),
.27 (t, J = 7.5 Hz, 1H), 7.05–7.13 (m, 3H), 7.02 (d, J = 8.0 Hz, 2H), 6.85 (t, J = 2.5
Hz, 1H), 6.77 (t, J = 2.5 Hz, 1H), 6.62–6.67 (m, 1H), 5.57 (dd, J = 8.0 Hz, J =2.5
6
–DMSO, ppm): δ
8
7
1
2
Hz, 1H), 5.07 (dd, J
H), 1.33–1.37 (m, 4H),0.89 (t, J = 7.5 Hz, 3H). F NMR (470 MHz, d
ppm): -66.57 (d, J = 710.64 Hz, 6F), -109.58 (dd, J = 8.46 Hz, J = 14.57 Hz, 1F),
= 4.70 Hz, J = 12.22 Hz,
1 2
= 8.0 Hz, J = 2.0 Hz, 1H), 4.77–4.90 (m, 2H), 2.00–2.01 (m,
1
9
2
6
-DMSO,
1
2
-111.23 (dd, J
1
= 5.64 Hz, J
2
= 8.46 Hz, 1F), -119.80 (dd, J
1
2
1
F), -121.71 (dd, J = 5.17 Hz, J = 12.22 Hz, 1F). MS (MALDI-TOF): m/z 843.2
1
2
(
M–PF6).
Synthesis and characterization of complex 4
o
1
Light-green solid; m.p. 285 C (Yield 72%). H NMR (500 MHz, d
.60–8.64 (m, 2H), 8.49 (d, J = 2.5 Hz, 1H), 8.39 (t, J = 8.0 Hz, 1H), 8.01 (d, J = 5.5
Hz, 1H), 7.86 (d, J = 2.0 Hz, 1H), 7.74 (t, J = 6.5 Hz, 1H), 7.42 (d, J = 2.0 Hz, 1H),
.26 (t, J = 7.5 Hz, 1H), 7.05–7.13 (m, 3H), 7.02 (d, J = 7.5 Hz, 2H), 6.86 (t, J = 2.5
Hz, 1H), 6.77 (t, J = 2.5 Hz, 1H), 6.62–6.66 (m, 1H), 5.57 (dd, J = 8.0 Hz, J =2.5
Hz, 1H), 5.08 (dd, J = 8.0 Hz, J =2.0 Hz, 1H), 4.75–4.91 (m, 2H), 1.99–2.03 (m,
6
–DMSO, ppm): δ
8
7
1
2
1
2
1
2

ACCEPTED MANUSCRIPT
1
9
2
H), 1.25–1.40 (m, 8H), 0.86 (t, J = 7.0 Hz, 3H). F NMR (470 MHz, d
ppm): -66.57 (d, J = 711.11 Hz, 6F), -109.57 (dd, J = 6.11 Hz, J = 8.46 Hz, 1F),
= 8.46 Hz, 1F), -119.81 (dd, J = 4.70 Hz, J = 12.22 Hz,
F), -121.72 (dd, J = 4.70 Hz, J = 12.22 Hz, 1F). MS (MALDI-TOF): m/z 871.3
6
-DMSO,
1
2
-111.23 (dd, J
1
= 6.11 Hz, J
2
1
2
1
1
2
(M–PF6).
Synthesis and characterization of complex 5
o
1
Light-green solid; m.p. 246 C (Yield 62%). H NMR (500 MHz, d
6
–DMSO, ppm): δ
.60–8.64 (m, 2H), 8.49 (d, J = 2.0 Hz, 1H), 8.36–8.39 (m, 1H), 8.01 (d, J = 4.5 Hz,
H), 7.85 (d, J = 2.0 Hz, 1H), 7.74 (t, J = 6.5 Hz, 1H), 7.42 (d, J = 2.0 Hz, 1H), 7.27
8
1
(t, J = 7.5 Hz, 1H), 7.05–7.14 (m, 3H), 7.02 (d, J = 7.0 Hz, 2H), 6.85 (t, J = 2.5 Hz,
1
1
1
H), 6.76 (t, J = 2.5 Hz, 1H), 6.63–6.67 (m, 1H), 5.56 (dd, J
H), 5.08 (dd, J = 8.0 Hz, J = 2.0 Hz, 1H), 4.76–4.90 (m, 2H), 1.98–2.01 (m, 2H),
.23–1.40 (m, 12H), 0.82–0.85 (m, 3H). F NMR (470 MHz, d
1 2
= 8.0 Hz, J = 1.5 Hz,
1
2
1
9
6
-DMSO, ppm):
-
(
-
66.57 (d, J = 711.11 Hz, 6F), -109.57 (dd, J = 9.40 Hz, J = 14.10 Hz, 1F), -111.22
1
2
dd, J
1
= 0.94 Hz, J
2
=14.10 Hz, 1F), -119.82 (dd, J
1
2
= 5.17 Hz, J = 11.28 Hz, 1F),
121.74 (dd, J
1
2
= 4.70 Hz, J = 9.40 Hz, 1F). MS (MALDI-TOF): m/z 899.3 (M–PF6).
2
.2.4. Theoretical calculations
Calculations on the ground and excited electronic state of complexes were
investigated by performing density functional theory (DFT) and time-dependent
density functional theory (TD-DFT) at B3LYP level [52]. The 6-31G* basis sets were
employed for optimizing the C, H, N atoms and the LANL2DZ basis sets for Ir atom.
1
3

ACCEPTED MANUSCRIPT
An effective core potential (ECP) replaces the inner core electrons of iridium leaving
2
6
6
the outer core (5s) (5p) electrons and the (5d) valence electrons of Ir(III). The
3
geometry of the metal-centered triplet ( MC) were fully optimized and was calcluated
at the spin-unrestricted UB3LYP level with a spin multiplicity of 3. All expectation
2
values calculated for S were smaller than 2.05. All calculations reported here were
carried out with the Gaussian 09 software package [53].
2
.2.5. X-ray crystallography study
Data collection of complexes 1 and 2 were performed on a Bruker Smart Apex II
CCD diffractometer with graphite-monochromated Mo K radiation (λ = 0.71069 Å)
α
at 293 K. Absorption corrections were performed by using the multi-scan technique.
The crystal structure was solved by Direct Methods of SHELXTL-97 [54] and refined
by full-matrix least-squares techniques using SHELXTL-97 within WINGX. 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 part of nitrogen and carbon atoms. The
detailed crystallographic data and structure refinement parameters are summarized in
Table S1 (see supporting information). Further details of the crystal structure
determination have been deposited to the Cambridge Crystallographic Data Centre as
supplementary publication. CCDC 951185 (1) and 951186 (2) contain the
supplementary crystallographic data for this paper.
3
. Results and discussion
1
4

ACCEPTED MANUSCRIPT
3
.1. Synthesis and Crystal Structure
The ancillary ligands were easily prepared by the reaction of
-(3-phenyl-1H-1,2,4-triazol-5-yl) pyridine with corresponding halohydrocarbon in
2
the presence of the strong base condition. Simple heating of the mixture of the
dimeric iridium(III) intermediates and the ancillary ligands in 1,2-ethandiol and
subsequent the counter-ion exchange reaction produced the target complexes with
1
19
high yields (70% ~ 81%). All complexes were fully characterized by H NMR and F
NMR spectrometry as well as MADIL-TOF. In addition, the molecular structures of
complexes 1 and 2 were further determined by X-ray crystallography. Both complexes
reveal the distorted octahedral geometry around the Ir atom with two cyclometalated
ligands and one ancillary ligand, as shown in Fig. 1. The two cyclometalated ligands
both adopt a mutually eclipsed configuration with two nitrogen atoms residing at
trans locations. It is noted that the bond lengths Ir–Nancillary ligand are significantly
longer than those of Ncyclometalated ligand, as a result of the strong trans influence of the
carbon donors. For example, in the case of complex 1, the Ir–N bond lengths in the
ancillary ligand Ir1–N5 and Ir1–N6 are 2.167(11) Å and 2.158(10) Å, respectively,
which are longer than those in cyclometalated ligand Ir1–N2 1.985(12) Å and Ir1–N4
2
.017(11) Å (see Table S2). These observations have also been made in complex 2
and the related cationic Ir(III) complexes [55, 56]. In addition, the robust π-π
intramolecular interactions between the phenyl ring of the corrsponding ancillary
ligands and phenyl ring of one of cyclometalated ligands are observed in complex 1
1
5

ACCEPTED MANUSCRIPT
and 2, with a centroid-centroid distance of 3.864 and 3.685 Å, respectively (Fig. 1).
The inherent intramolecular π-π stacking in such cationic iridium(III) complexes can
3
suppress the opening of the structure in metal-center ( MC) excited-state effectively,
leading to reduction of the degradation reaction to some extent when they are used as
emitting materials in LECs, and hence the efficient and stable devices [57-59].
Insert Fig. 1.
Fig. 1. Crystal structures of complexes 1 (a) and 2 (b). Thermal ellipsoids are drawn
at 30% probability. The PF counter-anion and hydrogen atoms are omitted for clarity.
6
3
.2. Photophysical properties in the solid-state and pizochromic behavior
The emission spectra of all as-synthesized solid powders are measured (see Fig. 2 and
Fig. S1) and the numerical data are summarized in Table 1. All complexes exhibit
strong sky-blue emission with peak wavelength of 461–472 nm, as shown in the inset
of Fig. 2. When these solid state powders obtained through column chromatography
(namely, as-synthesized powders) were ground in a ceramic mortar, naked-eye visible
luminescent color change could be clearly observed. Herein, we take complex 1 as an
example. For clearly, the as-synthesized samples and ground samples were hereafter
abbreviated as the letters “a” and “g”, respectively (e.g., the sample 1a is the
as-synthesized complex 1 through column chromatography; 1g is the ground sample
of 1a). Upon grinding, the emission color change of sample 1a is depicted in Fig. 2a,
1
6

ACCEPTED MANUSCRIPT
showing a significant emission red-shift from sky-blue (λem = 466 nm) to blue-green
(λem = 483 nm). The similar phenomenon can be observed for other samples 2a–5a. It
is noted that the grinding-induced spectral shifts (ꢀλPCL = λground – λas-synthesized) of
samples 1a–5a are alkyl length-dependent, although they all exhibit obvious
piezochormic luminescence behavior. As shown in Fig. 2, 5a shows much smaller the
grinding-induced spectral shift of 8 nm compared with that of 1a (ꢀλPCL = 17 nm).
The ꢀλPCL for 2a, 3a and 4a are 20, 21, 10 nm, respectively. In other words, the
sample 5a with a longest alkyl chain among these samples exhibits the smallest
emission red shift. The result also suggests that alkyl chains play a functional role in
turning the PCL behavior and the order of color changes induced by grinding is
similar to the previously reported fluorescent PCL materials. It is known the PCL
materials are potential candidates for optical recording application. To primarily test
the optical recording potential for them, the solid state sample 5a was then outspread
on a filter paper and the word “Dye” was written on the filter paper using a spatula.
The blue-green ‘‘Dye’’ can be observed clearly on the blue ‘‘paper’’ with the naked
eye under the UV light (Fig. S2).
Insert Fig. 2.
Fig. 2. Emission spectra of complexes 1 and 5 in the solid states. The letters “a” and
“
g” represent as-synthesized and ground samples, respectively.
Insert Table 1.
1
7

ACCEPTED MANUSCRIPT
Table 1䟏Solid-state photophysical data and DSC results of complexes 1–5 in various
states.
Interestingly, the emission color of ground samples can easily be reverted to the
o
sky-blue emitting samples upon heating at about 220 C for 5 min. The corresponding
emission spectra of the heated sample, 1h–5h, are revealed in Figs. S3 and S4. Further
grinding the sky-blue emitting heated samples can result in blue-green emitting
powders again. The color changes in solid state can repeat many times, suggesting
that these samples also possess an excellent reversibility in the switching processes. In
addition, the emission profile of the heated samples with relatively long alkyl chain,
such as 3h, 4h and 5h exactly match those of the original unground states compared
1
with 1h and 2h (see Figs. S3 and S4). The H NMR and MALDI-TOF spectra of them
in various solid state have been also measured. Almost no changes were found for
these spectra in different states, indicating that the emission color changes of them
might be caused by physical processes during the grinding and heating process [60,
6
1].
Moreover, the photoluminescence decays of the complexes in the solid powders
before and after grinding have been also investigated and the excited-state lifetimes
for the different samples in various states are listed in Table 1. It can be seen that, for
all complexes, the weighted mean lifetimes <τ> of as-synthesized samples 1a–5a and
their ground samples are significantly different, respectively. However, in the case of
cationic iridium(III) complex without PCL behavior, the lifetimes of as-synthesized
1
8

ACCEPTED MANUSCRIPT
and ground sample have almost the same <τ> values. These results suggest that the
PCL materials based on iridium(III) complexes show significantly different
time-resolved emission-decay between the as-synthesized and ground samples. Thus,
the changes in emission color in association with <τ> values of PCL samples 1a–5a
should be caused by altering the mode of solid-state molecular packing and/or the
intermolecular interactions after grinding.
Insert Fig. 3.
Fig. 3. PXRD patterns of as-synthesized samples 1a and 2a and ground samples 1g
and 2g.
3
.3. Powder X-ray diffraction and differential scanning calorimetry of solid
samples
A possible mechanism was previously proposed to explain PCL behavior of transition
metal-based complexes, that is, efficient grinding results in the amorphous powders
instead of original crystalline structures (as-synthesized samples). To gain an insight
into the present PCL behavior of theses iridium(III) complexes, the powder X-ray
diffraction (PXRD) experiment was performed on both the as-synthesized and ground
samples. As shown in Fig. 3, the measured PXRD peaks of as-synthesized samples,
1
a and 2a, are consistent with their single-crystal diffraction data, showing sharp and
intense reflections. Such intense reflection peaks are also observed for other samples
1
9

ACCEPTED MANUSCRIPT
3
a–5a. The results indicate that the as-synthesized samples should be well-ordered
aggregates, i.e. crystalline structure. In contrast, the diffractograms of the ground
samples exhibit weak and broad PXRD pattern, suggesting that the amorphous states
formed instead of the original ordered structures. The existence of a small amount
crystalline feature in PXRD curves of ground samples is possibly caused by
incomplete grinding. In addition to PXRD investigations, the differential scanning
calorimetry (DSC) study was carried out to further confirm the formation of the
amorphous powders upon grinding. No endo- and/or exothermic peaks are detected
before the isotropic metal transition of the as-synthesized samples, except for 1a that
o
shows higher melt point than 300 C, as shown in Fig. 4a. However, for ground
samples, each DSC curves firstly appears an exothermic recrystallization peak, and
then endothermic melting peaks are detected, which are similar to the melting points
of corresponding crystalline state samples. It is known that the amorphous solid will
become less viscous and the molecules may obtain enough freedom of motion to
spontaneously arrange themselves into crystalline forms upon heating. Thus, the
results suggest that the amorphous ground samples investigated here are all
metastable states. If the ground samples are heated above their recrystallization
temperatures, they will restore to thermodynamically stable crystals through the
exothermic recrystallization process. Noticeably, 5g with long alkyl chains exhibits
lower recrystallization temperatures compared with those of 1g–4g, which may
attribute to the much weaker intermolecular interactions in 5g than others. The results
show that the recrystallization temperatures for ground samples are also chain
2
0

ACCEPTED MANUSCRIPT
length-dependent; that is the longer the alkyl chains, the lower recrystallization
temperatures, as revealed in Fig. 4b. These findings are similar to recently reported
PCL materials based on organic small molecules [41].
Insert Fig. 4.
Fig. 4. DSC curves of as-synthesized samples (a) and ground samples (b).
Photophysical properties in the soultion and film
The obtained complexes 1-5 were easily solvent in common organic solvent,
such as CH Cl , THF and CH CN. The UV-vis absorption and emission spectra
2
2
3
recorded for all cationic iridium(III) complexes 1–5 in CH
3
CN solution at room
temperature (RT) are presented in Fig. 5a and the numerical results are tabled in Table
2
. They exhibit almost indential absorption spectra in same solutions. The intense
1
absorption bands below 300 nm can be assigned to the ligand-centered π–π*
transition, and the low-energy absorption bands that occur in the region longer than
1
3
3
00 nm are ascribed to the MLCT, MLCT (metal-to-ligand charge transfer) and
3
π–π* transition. Upon photoexcitation, all the investigated complexes exhibit bright
CN solutions with the peaks at about 505 nm. The significant
green emission in CH
3
variations of emission intensity and excited-state lifetime observed between degassed
and aerated CH CN solution and microsecond time scale lifetime highlight their
3
phosphorescent nature. It is noted that their emission profiles are amlost indentical
despite the different alkyl chains attached on the acllicary ligands, showing broad and
2
1

ACCEPTED MANUSCRIPT
featureless emission spectra. Generally, for cationic iridium(III) complexes, three
3
3
excited states contribute to the observation of light emission, namely, MLCT, LLCT
3
3
3
and LC π-π*, The emission spectra from MLCT and LLCT states are usually broad
and featureless, while the obvious structured emission band are always attributed to
3
the LC π-π* one. Thus, the emissive excited states of them take on much more
3
3
3
MLCT and LLCT character than LC π-π*, which is further confirmed by the
low-temperature emission and TD-DFT calculations (vide infra). Upon cooling the
CH CN solution to 77 K, the emission spectra of them are obviously blue-shifted
3
compared with those at RT but are still borad and featureless [42, 49]. These emission
properties at 77 K further suggest that the emissions of them mainly originate from
3
3
MLCT mixed with some content of LLCT states. To gain insight into the nature of
the emissive excited state involved in the emission process of all complexes, the
quantum chemical calculations employing DFT were performed (see experimental
section). The low-lying triplet states of them are calculated based on their optimized
ground geometry through TD-DFT. The corresponding excitation energies and
molecular orbitals involved in the excitations are summarized in Tables S3 and S4. On
1
the basis of the TD-DFT calculation, the T states of all complexes originate from the
excitation of HOMO→LUMO with different contribution (see Table S4). As shown in
Tables S3, the HOMOs of them mainly reside on the cyclometalated ligands and d
orbital of iridium atom, while the LUMOs locates on the ancillary ligands.
Accordingly, the observed emission bands of all complexes should mainly originate
3
3
from the
MLCT (Ir→ancillary ligands) and
LLCT (cyclometalated
2
2

ACCEPTED MANUSCRIPT
ligands→ancillary ligands) states, which is compliance with our experimental data.
Moreover, the alkyl chains on the ancillary ligand have no contributed to the HOMO
and LUMO orbitals, rationally explaining that the alkyl chains have little influence on
emission spectra. The PL spectra in neat film are also recorded at RT and the
corresponding emission spectra are placed in the Fig. S6. It is observed that the neat
film emission spectra are also board and show a common red-shift with respect to
3
those in CH CN solutions, which may caused by the molecular aggregation and
triplet–triplet interaction when going from solutions to neat film. These complexes
exhibit considerable quantum efficiency of 54.9% ~ 65.3%, indicating that they would
be excellent emitting materials used in the optical devices.
Insert Fig. 5.
3
Fig. 5. Absorption and emission spectra of complexes 1–5 in CH CN solution with
-5
concentration of 1×10 M at room temperature (a) and 77 K (b).
Insert Table 2.
Table 2. Photophysical and electrochemical properties of complexes 1–5.
3
.4. Electrochemical properties
The electrochemical behaviors of complexes 1–5 have been determined by cyclic
voltammertry (CV) and the redox potentials are listed in Table 2. According to the
previous reports, the reduction of cationic iridium(III) complexes usually occurs on
2
3

ACCEPTED MANUSCRIPT
the ancillary ligands [45, 49]. In addition, all complexes process almost identical
emission energy as mentioned above. Thus, it can be proposed that these complexes
exhibit similar oxidation and reduction processes in CV curves. As expected, the
electrochemical data show that each complex exhibits a reversible redox processes in
CH
3
CN solution with oxidation peaks at ~ +1.23 V and reduction peaks at ~ –1.81 V
+
versus Fc/Fc , respectively (see Fig. S7). The different alkyl chains play a functional
role in their PCL behavior but have neglectable influence on the photophysical
properties of them and even the electrochemical behaviors. Moreover, these
complexes showing redox reversibility indicates that both the holes (upon oxidation)
and electrons (upon reduction) can be easily transported when employing them as
emission materials, which is beneficial for the application in LECs [30, 57].
Insert Fig. 6.
3
Fig. 6. The minimum-energy structure calculated for the MC state of complex 1.
3
3
.5. Metal-centered states ( MC)
Cationic iridium(III) complex as an active component layer in LECs offer several
advantages over conventional OLEDs such as simple device structure, low turn-on
voltage as well as independence of the work function. Recently, a great deal of effort
has been invested into optimization of LECs inculding emission color, luminescent
quantum and trun-on time, etc. Although these advances in iridium(III)-based LECs,
the relatively short lifetime of the devices that mainly originates from the degradation
2
4

ACCEPTED MANUSCRIPT
3
process of the complexes in metal-centered ( MC) states [57], has been observed.
3
Essentially, MC state is defined as the excitation of an electron from the occupied
t2g(dπ) HOMO to the unoccupied e2g(dσ*). The rupture of metal-ligand bonds would
open the structures of the complexes and enhance the reactivity of complexes in the
3
MC state, leading to the detrimental degradation process. Nevertheless, the intrinsic
intramolecular π-π stacking has been proved to be a feasible way to enhancing the
3
stability of LECs, which can restrict the opening of the structure in the MC state
effectively. In addition to the high quantum efficiency and reversible redox processes,
these complexes as displayed in the crystal structures exhibit robust intramolecular
3
π-π interactions. To evaluate the stability of the complexes, the MC states of them
have been performed using the simimar methodology illustrated in the reported works.
The key bond lengths between iridium atom and N atom of cyclometalated ligands of
3
the complexes in MC states have been determined and marked in Fig. 6. As shown in
Fig. S4, the Ir–N bond (R1) of complex 1, of which N atom is in cyclometalated
ligand bearing intramolecular π-π stacking with ancillary ligand, only lengthens from
3
2
.08 Å in ground states (S
0
) to 2.17 Å in the MC state. In contrast, electron
promotion results in the elongation of another Ir-N length (R2), from 2.08 Å in S
0
3
state to 2.49 Å in the MC state for 1, leading to the virtual decoordination. Similar
phenomenon are also observed in complexes 2–5. This indicates that the opening of
3
the structure of the complex in the MC state for these complexes 1–5 are effectively
restricted due to the robust intramolecular π-π interactions, and the complexes studied
here will be attractive materials for LECs. The relevant work on this aspect is
2
5

ACCEPTED MANUSCRIPT
underway in our laboratory.
4
. Conclusion
In summary, a series of multifunctional cationic iridium(III) complexes with different
length of alkyl chains have been successfully synthesized. They all undergo
remarkable and reversible PCL behavior, and the emission color can be switched upon
grinding and heating. Moreover, their PCL properties are chain length-dependent,
showing that the complex with shorter alkyl chain possesses more remarkable PCL
behavior. The simple adjustment of the length of N-alkyl chains of piezochromic
iridium(III) complexes with triazole-pyridine ligand could endow them with tunable
solid-state emission and thermodynamics properties. The transformation between
crystalline and amorphous states is found to be the main reason for the present
piezochromism by carefully analyzing the experimental data (PXRD and DSC). The
photophysical properties in solutions and neat films demonstrate that introducing
different alkyl chains into the complexes has a little effect on their emitting colors and
excited-state characters, which are supported by the theoretical calculations.
Furthermore, the high quantum yields in neat films, excellent redox reversibility as
well as the robust intramolecualer π-π interactions that effectively restrict the opening
3
of the structures in MC states, suggest that they would be promising candidates for
applications in LECs. The results obtained here will help researchers to further design
and synthesize more promising iridium(III)-based PCL materials and attractive
phosphors for LECs.
2
6

ACCEPTED MANUSCRIPT
Acknowledgements
The authors gratefully acknowledge the financial support from NSFC (20903020,
1273030, 51203017 and 21131001), 973 Program (2009CB623605), the Science and
Technology Development Planning of Jilin Province (20100540, 201101008 and
0130522167JH), and the Fundamental Research Funds for the Central Universities
12QNJJ012).
2
2
(
References
1] Yang Y, Zhao Q, Feng W, Li F. Luminescent chemodosimeters for bioimaging.
Chem Rev 2013;113:192-270.
2] Hong Y, Lam JWJ, Tang BZ. Aggregation-induced emission. Chem Soc Rev
011;40:5361-88.
3] Zhang HY, Zhang ZL, Ye KQ, Zhang JY, Wang Y. Organic Crystals with
[
[
2
[
Tunable Emission Colors Based on a Single Organic Molecule and Different
Molecular Packing Structures. Adv Mater 2006;18:2369-72.
[
4] Zhang Z, Xu B, Su J, Shen L, Xie Y, Tian H. Color-Tunable Solid-State Emission
of 2,2′-Biindenyl-Based Fluorophores. Angew Chem Int Ed 2011;50:11654-7.
5] Han M, Hara M. Intense fluorescence from light-driven self-assembled aggregates
of nonionic azobenzene derivative. J Am Chem Soc 2005;127:10951-5.
6] Kunzelman J, Kinami M, Crenshaw BR, Protasiewicz JD, Weder C.
[
[
2
7