Simultaneous modification of N-alkyl chains on cyclometalated and ancillary ligands of cationic iridium(III) complexes towards efficient piezochromic luminescence properties

Article abstract of DOI:10.1039/c4tc02879a

In this study, we have designed and synthesized a series of multifunctional cationic iridium(iii) complexes with different lengths of N-alkyl chains on 2-phenyl-1H-benzimidazole-based cyclometalated ligands and phenyl-pyridine type ancillary ligands. The

Full text of DOI:10.1039/c4tc02879a

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Simultaneous modification of N-alkyl chains on
cyclometalated and ancillary ligands of cationic
iridium(III) complexes towards efficient
Cite this: DOI: 10.1039/c4tc02879a
piezochromic luminescence properties†
a
a
a
a
Yi Han, Hong-Tao Cao, Hai-Zhu Sun, Guo-Gang Shan, Yong Wu,a
*
a
b
*
*
Zhong-Min Su and Yi Liao
In this study, we have designed and synthesized a series of multifunctional cationic iridium(III) complexes
with different lengths of N-alkyl chains on 2-phenyl-1H-benzimidazole-based cyclometalated ligands
and phenyl-pyridine type ancillary ligands. The photophysical properties of each complex have been
investigated in detail and also ascertained by comprehensive density functional theory calculations.
Despite showing negligible influence on their emission spectra and excited-state characteristics in
solution, altering the N-alkyl chain length can efficiently modify their photophysical properties in the
solid state. All complexes exhibit fascinating visible piezochromic luminescence (PCL) behaviour. Most
interestingly, these iridium(III)-based luminophores with longer N-alkyl chains display significant emission
colour changes and unique reversible features by mechanical grinding and solvent fuming. The powder
X-ray diffraction (PXRD), 1H NMR and MALDI-TOF/TOF mass spectrometry data demonstrate that the
phase transitions between crystalline and amorphous states are crucial to the present piezochromism.
Moreover, with the merit of high quantum efficiency in aggregate states, the studied iridium(III) complex
can serve as an efficient sensor for the sensitive and selective detection of the explosive, 2,4,6-
trinitrophenol (TNP).
Received 15th December 2014
Accepted 13th January 2015
DOI: 10.1039/c4tc02879a
www.rsc.org/MaterialsC
liquid crystals, polymers as well as metal–organic frameworks.5
Nevertheless, organometallic complexes with PCL behaviour are
Introduction
rarely reported, although they exhibit preferable photophysical
properties. Moreover, the lack of adequate reports on their
structure–property relationships has made the controllable
design and synthesis of PCL materials difficult. Therefore, it is
still a great challenge to develop a new series of organometallic
complexes with excellent PCL features.
Stimuli-responsive luminescent materials presenting colour
changes under external stimuli such as heat, light, electricity,
pH and mechanical forces are considered as “smart” materials.1
Such materials have attracted extensive attention due to their
academic importance and promising applications.2 Among
them, piezochromic luminescent (PCL) materials showing
dynamic optical switching properties under mechanical stim-
ulus environments possess potential applications in the eld of
optoelectronics, such as optical recordings, sensors, memory
devices and switches.3 A commonly pursued approach for
changing their emission characteristics is to control the
molecular stacking between stable crystalline states and meta-
stable amorphous states without the use of chemical reactions.4
Recently, a number of materials displaying PCL behavior have
been reported, including organic and inorganic complexes,
Organometallic iridium(III) complexes have attracted much
interest owing to their excellent photophysical and photo-
chemical properties, i.e., high quantum efficiency, as well as
tunable emission wavelengths.6 In recent years, Huang et al.,7
Talarico et al.,8 and our group9 have developed a set of iridiu-
m(III) complexes with intriguing PCL behaviour. Iridium(III)-
based rewritable data records and storage devices have been
achieved, which will open up a new avenue for the development
of multifunctional iridium(III) complexes in the future.7 Most
recently, introducing different alkyl groups into PCL materials
has proven to be effective in constructing novel PCL dyes and
investigating the relationship between the molecular structures
and PCL behaviours.4b,10 Taking these into consideration, we
have designed and synthesized two series of iridium(III)
complexes with different N-alkyl chains on cyclometalated or
ancillary ligands, respectively.11 The results demonstrate that
aInstitute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal
University, Changchun, 130024 Jilin, People's Republic of China. E-mail:
shangg187@nenu.edu.cn; 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: Computational details
and corresponding photophysical results. See DOI: 10.1039/c4tc02879a
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such chromophores exhibit the opposite PCL effect. For
example, the complexes with shorter N-alkyl chains on the
ancillary ligand display more signicant PCL features but more
hindered reversibility. Nevertheless, the ones containing
shorter N-alkyl chains on the cyclometalated ligand show
insignicant PCL properties but better reversibility. It is
generally known that both signicant PCL features and revers-
ibility are highly required for real-world practical applications.
Therefore, the development of new iridium(III)-based phosphors
with efficient PCL behaviour and excellent reversibility is
strikingly desirable. Based on our previous studies, we speculate
that such materials can be constructed by simultaneously
modifying both ligands of iridium(III) complexes with N-alkyl
chains.
Keeping this in mind, we herein report the design and
synthesis of a series of iridium(III) complexes with the same N-
alkyl chains on cyclometalated and ancillary ligands, as shown
in Scheme 1. All complexes exhibit obvious piezochromism with
chain length-dependency. More importantly, complex 4 with
the longest N-alkyl chain length simultaneously showed the
most signicant PCL properties and reversibility when
compared to others. Upon fuming with organic solvents, its
emission colour can recover to the original one and this process
can be reiterated several times without any deterioration. The
obtained results clearly demonstrate that the present mecha-
nochromic behaviour is attributed to the crystal-to-amorphous
phase transformation. Additionally, all synthesized complexes
are efficient luminophores in their solid states. With the merit
of high luminescence efficiency, complex 4 can also be used as
an effective chemosensor for sensitive and selective detection of
nitroaromatic explosives on the ppm scale.
Experimental section
General information and materials
All reagents and solvents employed were commercially available
and used as received without further purication. Solvents for
syntheses were freshly distilled over appropriate drying
reagents. All experiments were performed under a nitrogen
atmosphere by using standard Schlenk techniques. NMR
spectra were recorded on a Bruker Avance 500 MHz (1H: 500
MHz, 19F: 470 MHz). The molecular weights of the complexes
were measured by using electrospray-ionization mass spec-
troscopy (MS) and matrix-assisted laser desorption-ionization
time-of-ight (MALDI-TOF) mass spectrometry. The emission
spectra were recorded by using a F-4600 FL spectrophotometer.
The excited-state lifetimes were measured on a transient spec-
trouorometer (Edinburgh FLS920) with a time-correlated
single-photo counting technique. Powder X-ray diffraction
(PXRD) patterns of the samples were collected on a Rigaku
Dmax 2000.
Synthesis
Synthesis of the cyclometalated and ancillary ligands.
Scheme 1 depicts the synthetic routes to the cyclometalated and
ancillary ligands. As shown in Scheme 1, the ligands used in this
work can be easily synthesized by the condensation of 2-phenyl-
1H-benzo[d]imidazole or 2-phenyl-1H-benzo[d]imidazole with
corresponding aryl bromide in acetone solvent in the presence
of KOH.9b,11b
Synthesis of the ancillary ligands
2-(1-Ethyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Ethtaz). The
precursor 2-(3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (1.11 g, 5.00
Scheme 1 Chemical structures of the reported Ir(III)-based phosphors and synthesis of the ligands and the iridium(III) complexes studied in this
work.
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mmol) was synthesized by a previously reported procedure.11a,12 water mixture (v/v ¼ 3 : 1, 32 mL) for 24 h. The mixture was
Under a nitrogen atmosphere, the precursor was dissolved in treated with water (30 mL) to induce precipitation of the off-
acetone. An equivalent amount of KOH was added and the white solid. The product was ltered and washed with diethyl
mixture was stirred for 30 min. Then, bromoethane (0.65 g, ether followed by with ethanol, and dried (yield: 74%). Other
6.00 mmol) was added into the reaction mixture and the chloride-bridged complexes, [Ir(Bebd)2Cl]2, [Ir(Hebd)2Cl]2, and
mixture was heated to reux overnight. Aer cooling to room [Ir(Oebd)2Cl]2, were synthesized using a method similar to that
temperature, the mixture was quenched with ice water and then for [Ir(Eebd)2Cl]2. The chloro-bridged dimers were used in the
extracted with dichloromethane. The organic layer was dried subsequent reactions without further purication.
with Na2SO4 and the solvent was removed. The obtained resi-
Synthesis of the complexes
dues were puried by column chromatography on silica gel with
Synthesis and characterization of complex 1. A solution of
ethyl acetate/petroleum ether (v/v ¼ 1 : 2) as the eluent to yield a ligand Ethtaz (0.21 g, 0.63 mmol) and the chloro-bridged dimer
light-yellow oil (60%). 1H NMR (500 MHz, CDCl3, ppm): d 8.64– [Ir(Eebd)2Cl]2 (0.56 g, 0.30 mmol) in a solvent mixture of
8.65 (m, 1H), 8.30 (d, J ¼ 8.0 Hz, 1H), 8.17–8.19 (m, 2H), methanol (15 mL) and dichloromethane(30 mL) was reuxed
7.79–7.81 (m, 1H), 7.44–7.46 (m, 2H), 7.38–7.40 (m, 1H), 7.28– for 24 h in the dark. Aer cooling to room temperature, the
7.30 (m, 1H), 4.87 (t, J ¼ 8.0 Hz, 2H), 1.52–1.55 (m, 3H). The mixture was ltered, and then an excess of solid KPF6 was added
related ancillary ligands (Buttaz, Hextaz, and Octtaz) were and stirred for another 0.5 h at room temperature. The solvent
prepared using similar procedures.
was removed under reduced pressure. The crude product was
2-(1-Butyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Buttaz). puried by silica gel column chromatography using dichloro-
1H NMR (500 MHz, CDCl3, ppm): d 8.66–8.67 (m, 1H), 8.31 methane/ethyl acetate (v/v ¼ 5 : 1) and the resulting powders
(d, J ¼ 8.0 Hz, 1H), 8.16–8.18 (m, 2H), 7.81–7.84 (m, 1H), 7.43– were recrystallized from a dichloromethane and petroleum
7.46 (m, 2H), 7.37–7.40 (m, 1H), 7.31–7.34 (m, 1H), 4.85 (t, J ¼ ether mixture to give complex 1 as a light-yellow solid (yield
7.5 Hz, 2H), 1.90–1.96 (m, 2H), 1.37–1.42 (m, 2H), 0.93 (t, J ¼ 7.5 58%). 1H NMR (500 MHz, d6-DMSO, ppm): d 8.60 (d, J ¼ 8.0 Hz,
Hz, 3H).
1H), 8.34 (d, J ¼ 8.0 Hz, 1H), 7.98 (d, J ¼ 8.0 Hz, 1H), 7.88 (d, J ¼
2-(1-Hexyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Hextaz). 8.5 Hz, 1H), 7.82 (d, J ¼ 8.0 Hz, 1H), 7.72–7.79 (m, 2H), 7.38 (t,
1H NMR (500 MHz, CDCl3, ppm): d 8.66–8.67 (m, 1H), 8.30 (d, J ¼ J ¼ 7.5 Hz, 1H), 7.29–7.32 (m, 2H), 7.18–7.21 (m, 1H), 7.12–7.14
8.0 Hz, 1H), 8.16–8.18 (m, 2H), 7.81–7.84 (m, 1H), 7.43–7.46 (m, (m, 3H), 7.05–7.08 (m, 1H), 6.87 (t, J ¼ 8.0 Hz, 1H), 6.83–6.86 (m,
2H), 7.37–7.40 (m, 1H), 7.31–7.34 (m, 1H), 4.83 (t, J ¼ 7.5 Hz, 2H), 2H), 6.76–6.79 (m, 2H), 6.70 (t, J ¼ 7.5 Hz, 1H), 6.60 (t, J ¼ 7.5 Hz,
1.92–1.96 (m, 2H), 1.25–1.38 (m, 6H), 0.86 (t, J ¼ 7.5 Hz, 3H).
1H), 6.26 (d, J ¼ 8.0 Hz, 1H), 5.94 (d, J ¼ 7.5 Hz, 1H), 5.68 (d, J ¼
2-(1-Octyl-3-phenyl-1H-1,2,4-triazol-5-yl)pyridine (Octtaz). 1H 8.0 Hz, 1H), 4.77–4.95 (m, 4H), 4.65–4.68 (m, 1H), 4.55–4.58 (m,
NMR (500 MHz, CDCl3, ppm): d 8.66–8.67 (m, 1H), 8.31 (d, J ¼ 8 1H), 1.51 (t, J ¼ 7.5 Hz, 3H), 1.22–1.31 (m, 6H). 19F NMR (470
Hz, 1H), 8.16–8.18 (m, 2H), 7.81–7.85 (m, 1H), 7.43–7.46 (m, 2H), MHz, d6-DMSO, ppm): ꢀ67.35 (d, J ¼ 711.11 Hz, 6F). MS
7.37–7.40 (m, 1H), 7.31–7.34 (m, 1H), 4.83 (t, J ¼ 7.5 Hz, 2H), (MALDI-TOF): m/z 885.3 (M ꢀ PF6).
1.92–1.96 (m, 2H), 1.22–1.37 (m, 10H), 0.86 (t, J ¼ 7.5 Hz, 3H).
Synthesis and characterization of complex 2. Light-yellow solid,
1
Synthesis of the cyclometalated ligands
(yield 62%). H NMR (500 MHz, d6-DMSO, ppm): d 8.62 (d, J ¼
Ethyl-2-phenyl-1H-benzo[d]imidazole (Eebd). 1H NMR (500 8.0 Hz, 1H), 8.35 (d, J ¼ 8.0 Hz, 1H), 7.98 (d, J ¼ 8.0 Hz, 2H), 7.89
MHz, CDCl3, ppm): d 7.84–7.86 (m, 1H), 7.73–7.74 (m, 2H), 7.52– (d, J ¼ 8.5 Hz, 1H), 7.74–7.79 (m, 2H), 7.38 (t, J ¼ 8.0 Hz, 1H),
7.54 (m, 3H), 7.43 (t, J ¼ 4.5 Hz, 1H), 7.31–7.33 (m, 2H), 4.26– 7.24–7.30 (m, 2H), 7.13–7.19 (m, 4H), 7.05–7.08 (m, 1H), 6.96 (t,
4.29 (m, 2H), 1.45 (t, J ¼ 7 Hz, 3H).
J ¼ 8.0 Hz, 1H), 6.86–6.88 (m, 3H), 6.74–6.75 (m, 1H), 6.68 (t, J ¼
Butyl-2-phenyl-1H-benzo[d]imidazole (Bebd). 1H NMR (500 8.0 Hz, 1H), 6.59 (t, J ¼ 7.5 Hz, 1H), 6.17 (d, J ¼ 8.0 Hz, 1H), 5.99
MHz, CDCl3, ppm): d 7.84–7.86 (m, 1H), 7.72–7.74 (m, 2H), (d, J ¼ 7.5 Hz, 1H), 5.64 (d, J ¼ 8.0 Hz, 1H), 4.88–4.97 (m, 3H),
7.52–7.54 (m, 3H), 7.42 (t, J ¼ 4.5 Hz, 1H), 7.31–7.33 (m, 2H), 4.51–4.73 (m, 3H), 1.92 (t, J ¼ 7.5 Hz, 2H), 1.19–1.69 (m, 10H),
4.21 (t, J ¼ 7.5 Hz, 2H), 1.79 (t, J ¼ 7.5 Hz, 2H), 1.25–1.29 (m, 2H), 0.68–0.93 (m, 6H), 0.66–0.68 (m, 3H), 1.19–1.69 (m, H). 19F NMR
0.84 (t, J ¼ 7.5 Hz, 3H).
(470 MHz, d6-DMSO, ppm): ꢀ67.35 (d, J ¼ 711.11 Hz, 6F). MS
Hexyl-2-phenyl-1H-benzo[d]imidazole (Hebd). 1H NMR (500 (MALDI-TOF): m/z 969.4 (M ꢀ PF6).
MHz, CDCl3, ppm): d 7.84–7.86 (m, 1H), 7.71–7.73 (m, 2H), 7.51–
Synthesis and characterization of complex 3. Light-green solid,
1
7.54 (m, 3H), 7.40 (t, J ¼ 4.5 Hz, 1H), 7.29–7.31 (m, 2H), 4.19 (t, (yield 69%). H NMR (500 MHz, d6-DMSO, ppm): d 8.63 (d, J ¼
J ¼ 7.5 Hz, 2H), 1.79 (t, J ¼ 7.0 Hz, 2H), 1.21–1.25 (m, 6H), 0.84 (t, 8.0 Hz, 1H), 8.34 (t, J ¼ 8.0 Hz, 1H), 7.98 (d, J ¼ 8.0 Hz, 2H), 7.89
J ¼ 6.5 Hz, 3H).
(t, J ¼ 8.5 Hz, 1H), 7.73–7.79 (m, 2H), 7.38 (t, J ¼ 8.0 Hz, 1H),
Octyl-2-phenyl-1H-benzo[d]imidazole (Oebd). 1H NMR (500 7.23–7.28 (m, 2H), 7.12–7.17 (m, 4H), 7.04–7.08 (m, 1H), 6.96 (t,
MHz, CDCl3, ppm): d 7.84–7.86 (m, 1H), 7.71–7.72 (m, 2H), J ¼ 8.0 Hz, 1H), 6.84–6.89 (m, 3H), 6.69–6.74 (m, 2H), 6.59 (t, J ¼
7.50–7.52 (m, 3H), 7.41 (t, J ¼ 3.0 Hz, 1H), 7.30–7.32 (m, 2H), 7.5 Hz, 1H), 6.17 (d, J ¼ 7.5 Hz, 1H), 6.00 (d, J ¼ 7.5 Hz, 1H), 5.64
4.20 (t, J ¼ 7.5 Hz, 2H), 1.79 (t, J ¼ 7 Hz, 2H), 1.19–1.26 (m, 10H), (d, J ¼ 8.0 Hz, 1H), 4.87–4.99 (m, 3H), 4.51–4.69 (m, 3H), 1.91–
0.84 (t, J ¼ 6.5 Hz, 3H).
1.97 (m, 2H), 1.50–1.70 (m, 4H), 0.97–1.39 (m, 17H), 0.75–0.86
Synthesis of the chloro-bridged dimer. The organo- (m, 10H). 19F NMR (470 MHz, d6-DMSO, ppm): ꢀ67.35 (d,
metallated dimer [Ir(Eebd)2Cl]2 was synthesized by the reaction J ¼ 711.11 Hz, 6F). MS (MALDI-TOF): m/z 1053.5 (M ꢀ PF6).
of IrCl3$3H2O (0.51 g, 1.43 mmol) with ethyl-2-phenyl-1H-benzo
Synthesis and characterization of complex 4. Light-green solid
[d]imidazole (Eebd 0.66 g, 3.15 mmol) in a 2-ethoxyethanol and (yield 74%). 1H NMR (500 MHz, d6-DMSO, ppm): d 8.63 (d,
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J ¼ 7.5 Hz, 1H), 8.34 (t, J ¼ 7.5 Hz, 1H), 7.95–7.99 (m, 2H), 7.89– emission spectra compared to those at RT (Fig. 1). It is note-
7.91 (m, 1H), 7.77 (d, J ¼ 8.0 Hz, 1H), 7.73 (d, J ¼ 7.5 Hz, 1H), worthy that although different alkyl chains are attached to these
7.38 (t, J ¼ 7.5 Hz, 1H), 7.22–7.28 (m, 2H), 7.12–7.19 (m, 4H), complexes, they display a similar emission pattern at RT and 77
7.04–7.07 (m, 1H), 6.94 (t, J ¼ 8.0 Hz, 1H), 6.83–6.89 (m, 3H), K. These results suggest that altering the N-alkyl chains on both
6.74 (t, J ¼ 7.5 Hz, 1H), 6.68 (t, J ¼ 7.5 Hz, 1H), 6.58 (t, J ¼ 7.5 Hz, ligands shows a negligible impact on their phosphorescence
1H), 6.16 (d, J ¼ 7.0 Hz, 1H), 6.00 (d, J ¼ 7.5 Hz, 1H), 5.62 (d, J ¼ spectra as well as excited-state characteristics in solution. To
8.0 Hz, 1H), 4.90–5.00 (m, 3H), 4.51–4.68 (m, 3H), 1.92–1.95 (m, verify this issue, the density functional theory (DFT) and time-
2H), 1.49–1.70 (m, 4H), 0.97–1.37 (m, 29H), 0.69–0.87 (m, 10H). dependent DFT (TD-DFT) calculations have been performed on
19F NMR (470 MHz, d6-DMSO, ppm): ꢀ67.35 (d, J ¼ 711.11 Hz, them to ascertain the nature of their excited states. As depicted
6F). MS (MALDI-TOF): m/z 1137.6 (M ꢀ PF6).
in Fig. 2, the highest-occupied molecular orbital (HOMO) of
complex 1 mainly resides on the benzimidazole group of the
cyclometalated ligands and the iridium ion, while the lowest
unoccupied molecular orbital (LUMO) primarily delocalizes
over the pyridine and 1,2,4-triazole moieties of the ancillary
ligands. Complexes 2–4 show almost identical HOMO and
LUMO distributions (see Fig. S1, ESI†). Clearly, the N-alkyl
chains in 1–4 do not contribute to either the HOMOs or LUMOs.
The TD-DFT data show that the lowest-lying triplet (T1) states of
the complexes mainly originate from the excitation of HOMO
/ LUMO (see Table S2, ESI†). Therefore, the T1 states have
predominantly mixed 3MLCT and 3LLCT characteristics. The
charge transfer nature of the emitting triplets is in agreement
with the experimental emission spectra. In addition, the
unpaired-electron spin density distribution perfectly matches
the topology of the HOMO / LUMO excitation from which the
T1 excited-states originate (see Fig. 2). These results further
conrm that the mixed 3MLCT and 3LLCT excited-state char-
acters are attributed to their T1 states and the different N-alkyl
chains have little effect on their emission in solutions.
Despite 1–4 exhibit identical emission spectra as well as
luminescent colours in solution, their solid-state emissions are
obviously different. They display bright luminescence with
emission colours ranging from yellow (542 nm for 1) to green
(513 nm for 4), respectively. The relevant solid-state emission
data are given in Table 1. It is generally recognized that the
solid-state emission properties of luminophores strongly
depend on the molecular arrangement of motifs and intermo-
lecular interactions.16 As a result, we conjecture that adjusting
N-alkyl chains attached to iridium(III) complexes can induce
different molecular packing and hence emission colours in
solid-states. Longer N-alkyl chains might impede close packing,
which results in loose molecular packing and blue-shied
emission spectra. Similar to previous reports, simultaneously
modifying the N-alkyl chain on both ligands of cationic
Theoretical calculations
All calculations on the ground and excited electronic states of
the complexes were carried out at the PBE0 level using the
Gaussian 09 soware package13 together with 6-31G* basis sets
for C, H, N atoms and the LANL2DZ for the Ir atom. An effective
core potential (ECP) replaces the inner core electrons of iridium
leaving the outer core (5s)2(5p)6 electrons and the (5d)6 valence
electrons of Ir(III). The geometry of the lowest-lying triplet (T1)
states was fully optimized and calculated at the spin-unre-
stricted UPBE0 level with a spin multiplicity of 3. All expectation
values calculated for S2 were smaller than 2.05.
Results and discussion
Photophysical properties in solution and solid state
Fig. 1 shows the emission spectra of complexes 1–4 in CH3CN
solution at room temperature (RT) and 77 K, and the relevant
data are summarized in Table S1.† The sub-microsecond ranges
of emissive lifetimes clearly demonstrate their phosphores-
cence emission nature.14 It is well known that three excited
states can contribute to the observation of light emission for
cationic iridium(III) complexes, namely spin-forbidden metal-to-
ligand charge transfer (3MLCT), ligand-to-ligand charge transfer
(3LLCT) and ligand-centered (LC 3p–p*) characters.15 As shown
in Fig. 1, at room temperature, complexes 1–4 exhibit broad and
featureless emission spectra in solution, indicating that their
3
3
emissions mainly originated from MLCT and/or LLCT states
rather than from LC 3p–p*, since the emissions from LC 3p–p*
states always are vibronically structured.6c Aer cooling the
solution to 77 K, all complexes display largely blue-shied
Fig. 2 Electronic density contours calculated for the HOMO (a) and
Fig. 1 Emission spectra of complexes 1–4 in CH3CN (1 ꢁ 10ꢀ5 M) at the LUMO (b) of complex 1. (c) Spin density distribution calculated for
298 and 77 K.
complex 1 in the T1 excited state.
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Table 1 Emission maxima (l, in nm) of as-synthesized, ground and
fumed complexes 1–4
ground states were studied and are shown in Fig. 3b and c. It is
clear that grinding results in broadened emission spectra,
showing obvious emission wavelength changes of 21 nm for 1
and 33 nm for 4, respectively. It is generally recognized that the
PCL behavior for a given luminophore is related to the
arrangement of its molecular motifs as well as the intermolec-
ular interactions. Usually, the PCL materials with weak inter-
molecular interactions, especially the materials functionalized
with different alkyl chains, exhibit more signicant PCL prop-
erties than those with stronger ones.10b,10c Moreover, the exis-
tence of a large dipole moment for a D–p–A PCL dye is also
a
Complex
las-synthesized
lground
DlPCL
lfumed
1
2
3
4
542
536
513
519
563
559
550
552
21
23
37
33
548
553
544
520
a
DlPCL ¼ lground ꢀ las-synthesized
.
iridium(III) complexes is also an effective approach to control favorable for signicant PCL behaviors.3f In this work, however,
their photophysical properties in the solid state.11
there are no D–p–A structures in the studied systems. There-
fore, it is speculated that introducing long alkyl chains into
both cyclometalated and ancillary ligands can weaken the
intermolecular interactions more effectively. As a result, the
grinding may result in more signicant PCL features for iri-
dium(III) complexes with longer alkyl chains.
Solid-state piezochromic properties
Organic solid-state materials with tunable light-emitting
behaviours have attracted much attention due to their potential
applications as sensors, logicgate units, memory devices, etc.17
Developing such efficient materials and deeply investigating the
relationship between the structure and the PCL behaviour will
be valuable for further molecular design. In our previous work,
the effect of N-alkyl chains on PCL properties was successfully
accomplished, in which the N-alkyl chains were graed onto
either cyclometalated or ancillary ligands.11 To evaluate the PCL
behaviours of iridium(III) complexes studied in this work, their
solid-state emissions were investigated. As shown in Fig. 3a and
Table 1, the as-synthesized solid powders of complexes 1–4 emit
intense yellow or green light at 542, 536, 513 and 519 nm,
respectively. Aer grinding the original powders in agate
mortars with pestles, noticeable colour changes in lumines-
cence were observed (see Fig. 3a). Meanwhile, as visualized by
the naked eye, it is found that the complexes with longer N-alkyl
chains show more remarkable colour changes in comparison
with those of the shorter ones. In other words, the grinding
induced spectral shis (DlPCL, DlPCL ¼ lground ꢀ las-synthesized) of
these iridium(III) luminophores are length-dependent, which
are similar to recently reported studies. Herein, we take 1 and 4
as examples. The emission spectra of 1 and 4 in original and
As mentioned above, the cationic iridium(III) complexes
possessing longer N-alkyl chains exhibit more efficient PCL
behaviour with larger emission wavelength changes. However,
it is not known whether they can also exhibit excellent colour-
change reversibility triggered by external stimuli, such as
grinding and solvent fuming. To check their reversibility, the
grinding–fuming switching experiment was performed. All
ground samples can be stable for months at room temperature.
Upon treating the ground samples with ether solvent, however,
the emissions are obviously blue-shied to different degrees
compared to those of ground ones (see Fig. 4, S2 and S3†). It is
worth mentioning that only complex 4 exhibits more excellent
reversibility, the emission of the ground sample of which can
perfectly revert to the original one with the same spectral prole
and emission maximum (Fig. S2 and S3†). Further grinding the
fumed sample 4 leads to the orange-emitting form as seen in
Fig. 4a. Its emissions can thus be reversibly switched between
green-emitting and orange-emitting states several times by
grinding–fuming cycles with almost no fatigue. Nevertheless,
the solvent-fuming cannot entirely induce the emission colour
Fig. 4 (a) Emission spectra of complex 4 in various states at room
Fig. 3 (a) Photographs of complexes 1–4 taken under UV illumination temperature. (b) Maximum emission wavelength change versus
(365 nm). Emission spectra of complexes 1 (b) and 4 (c) in the solid repeating cycles. (c) Photographs of the reversible writing/erasing
state at room temperature, respectively.
cycles using complex 4 as medium.
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of ground samples 1–3 to change to their original ones. For representative example, the as-synthesized powder shows many
instance, the emission colour of fumed-2 was nearly invariant sharp and intense diffraction peaks, which suggest that the
with only 6 nm spectral shi with respect to its ground sample. solid powders obtained by column chromatography are well-
Simultaneously attaching the long N-alkyl chains onto the ordered crystalline structures.18 In contrast, aer grinding, the
ligands of cationic iridium(III) complexes may more effectively XRD prole shows only a few broad and weak signals, which are
decrease the intermolecular interactions, which endows them indicative of the transition from a crystalline to an amorphous
with signicant colour-switchable features as well as good phase.19 Similar observations are also found in other complexes
reversibility. Hence, it is believed that introducing specic alkyl (see Fig. S4†). Aer careful treatment of the ground samples of
chains into cationic iridium(III) complexes to control their complex 4 with drops of diethyl ether or long time solvent-
molecular packing is a feasible way to design and construct fuming, clear reection peaks appear again, which t well with
effective iridium(III)-based PCL materials.
those of the original as-prepared solid sample, demonstrating
It is envisioned that complex 4 can be used as an appropriate reversal from the amorphous to the crystalline phase. Although
optical memory device by judicious utilization of its mechano- some fumed solids show different diffraction peaks from those
chromism. To demonstrate its useful practical applications, a of as-synthesized ones (1–3), it is also certain to say that the
small quantity of solid powder 4 was carefully spread on a lter crystalline structures have formed upon fuming. Consequently,
paper as a thin lm and then subjected to solvent fuming the PXRD results indicate that the emission colour changes
(Fig. 4c). The strong green-emitting lm is observed under 365 observed upon grinding are ascribed to the phase transition
nm UV irradiation. The letter “A” was written on the lm using a between crystalline and amorphous states. It is proved that the
metal spatula, and a clear orange-emitting pattern “A” with ground samples are amorphous metastable states, which can
large colour contrast appears, which can be readily distin- recover to the crystalline ones under the inuence of external
guished from the bright green background. Then, upon fuming stimuli. Once the amorphized powders are fumed by the
the lm with diethyl ether or dropping a spot of diethyl ether solvent, they can spontaneously arrange themselves into a
onto the lm, the letter “A” is immediately erased by converting crystalline form. For complex 4, the longer alkyl chains in it will
the luminescent colour back to the background emission. Then, provide higher molecular mobility in comparison to others.
a new letter “B” with an orange-emitting colour can be re- Upon solvent fuming, the ground sample 4 can effectively
written on this lm and erased using the same method as recover to the original crystalline powder, which is further
mentioned above, demonstrating a reproducible recording– conrmed by similar PXRD proles of the original and the
erasing process. Such fascinating features make complex 4 a fumed powders. This is a possible reason why complex 4
promising material for recyclable optical recording and security exhibits better reversibility than others.
inks.
Moreover, the time-resolved emission decay behaviours aer
and before grinding, and the excited-state lifetimes of
complexes 1–4 in various states were studied. The lifetime data
are illustrated in Table S2.† As shown in Table S2,† the weighted
mean lifetimes hsi of all ground samples are signicantly
different from those of the original ones. Longer lifetimes were
achieved aer grinding. The reported cationic iridium(III)
without PCL behaviours, however, shows almost identical life-
times under various conditions. Therefore, it can be concluded
that the changes in the emission colour of these iridium(III)-
based PCL materials are associated with alteration of the mode
of solid-state molecular packing and/or the intermolecular
interactions and hence changes in their excited-state lifetime.
The mechanism for the present piezochromic behaviour
Since the solid-state emission of luminophores strongly
depends on the molecular structure as well as the solid packing,
alteration of the molecular arrangement of motifs or phase
transition between crystalline and amorphous states in
response to pressure was commonly considered as the main
reason for their piezochromic luminescence properties. To gain
more insight into the PCL mechanism of these iridium(III)
complexes, powder X-ray diffraction (PXRD) analysis was
carried out, as shown in Fig. 5 and S4.† Taking complex 4 as a
1
Their same H NMR spectra in various states further conrm
this point. The mechanochromic effects presented herein are
thus caused by physical processes during the grinding rather
than by a chemical reaction.3a
Selective detection of 2,4,6-trinitrophenol TNP
Recently, detection of high explosives has become important
due to homeland security, environmental and humanitarian
implications.20 Most high explosives are nitro-substituted
organic compounds, such as 2,4,6-trinitrophenol (TNP), 2,4,6-
trinitrotoluene (TNT), and 2,4-dinitrotoluene (2,4-DNT). Among
them, TNP exhibits superior power to the well-known TNT and
is widely used in reworks, dyes and so on, which usually results
in the contamination of soil and aquatic systems.20b,21 For the
Fig. 5 PXRD patterns of complex 4 in various states.
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emission quenching experiments for other nitro aromatics,
including TNT, 2,4-dinitrotoluene (2,4-DNT), nitrobenzene
(NB), 2,6-dinitrotoluene (2,6-DNT), 1,3-dinitrobenzene (1,3-
DNB), 2-nitrotoluene (oNT) and 3-nitrotoluene (mNT), were
studied. As shown in Fig. 7, each nitroaromatic compound
exhibits relatively less effect on emission quenching under the
same conditions. Evidently, the emissive nanoaggregates of
complex 4 display both high sensitivity and selectivity toward
TNP, exploiting the potential applications of the cationic iri-
dium(III) complexes in future. The LUMO energy of 4 is higher
than those of nitroaromatic analytes, which facilitates the jump
of electrons to the lower ones of analytes, leading to photo-
induced electron transfer-caused (PET) emission quenching of
4 (see Fig. S6†).25
Fig. 6 (a) Photoluminescence spectra of complex 4 in acetonitrile–
water (v/v ¼ 1 : 9) mixtures containing different amounts of TNP. (b)
Corresponding Stern–Volmer plots of TNP.
purpose of highly sensitive and selective detection of TNP in
aqueous media, various chemosensors based on organic lumi-
nescent materials have been developed to date. Nevertheless,
the traditional aggregation caused quenching effect in aquatic
systems always results in drastically negative effects on the
efficiency and sensitivity of sensors. Recent reports indicate that
efficient luminophores with appropriate emission wavelengths
can be used as sensitive sensors to detect TNP.22 Therefore, we
herein chose complex 4 to explore its utility as a chemosensor
for the detection of explosives due to its obvious aggregation-
induced emission enhancement features (see Fig. S5†).
As shown in Fig. 6a, the nanoaggregates of complex 4 exhibit
intense emission in the acetonitrile–water (v/v ¼ 1 : 9, by
volume) mixture. However, a visible emission quenching is
observed with addition of a small amount of TNP into the
mixtures. The luminescence intensity obviously decreases
below 50% at a TNP concentration of 2 ppm. When the TNP
concentration reached 9 ppm, negligible emission has been
observed with a quenching efficiency of nearly 92%. The Stern–
Volmer (SV) plot, as depicted in Fig. 6b, shows curves sloping
upwards, which indicates that emission quenching becomes
more efficient upon increasing the TNP concentration. The
quenching constant is evaluated to be 1.6 ꢁ 106 Mꢀ1 by using
the SV equation (I0/I) ¼ KSV[A] + 1, where I and I0 are the
emission intensities with and without the quencher, respec-
tively, KSV is the quenching constant (mꢀ1), [A] is the molar
concentration of the quencher.23 Such a quenching constant is
higher than those of the reported uorescent sensors for the
detection of explosives, demonstrating that 4 shows a high
sensitivity to TNP.24 Moreover, selectivity is another key factor
for a sensor. To check the sensing selectivity of complex 4,
Conclusions
In summary, we have designed and synthesized a series of
cationic iridium(III) complexes with similar structures but
different N-alkyl chains on both ligands. In sharp contrast to the
same emission in solution, their solid-state photophysical
properties exhibit N-alkyl chain length dependency, showing
obviously blue-shied emission upon extending their chain
lengths. Aer grinding, each complex exhibits visible PCL.
Among them, complex 4 with the longest chains features more
efficient PCL properties with a larger grinding-induced emis-
sion colour change. More importantly, aer treatment of
ground powders with organic solvents, the emission colour of
ground sample 4 can perfectly revert to its original one, but this
is not the case for the other complexes. A reproducible two-
colour emission writing/erasing process for 4 was thus ach-
ieved. It is believed that simultaneously introducing an appro-
priate N-alkyl chain into cationic iridium(III) complexes is a
feasible design strategy for constructing effective iridium(III)-
based PCL materials with good reversibility. As supported by
XRD data, the present piezochromism is attributed to the
transformation between crystalline and amorphous states.
Inspired by intrinsic characteristics of these phosphors, a
highly sensitive and selective TNP sensor based on complex 4
has also been demonstrated. The present study will provide a
new insight into the design of multifunctional cationic iridiu-
m(III) complexes for optical devices and sensors in the near
future.
Acknowledgements
The authors gratefully acknowledge the nancial support from
the National Natural Science Foundation of China (21273030,
21303012, 51203017 and 21131001), the SRFDP and RGC ERG
Joint Research Program (20120043140001), the China Post-
doctoral Science Foundation funded project (2013M540239 and
2014T70269), the Science and Technology Development Plan-
ning of Jilin Province (20130522167JH and 20130204025GX)
and the Fundamental Research Funds for the Central Univer-
Fig. 7 Quenching percentage obtained for different analytes (9 ppm). sities (12QNJJ012).
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