Cation-anion interaction-directed molecular design strategy for mechanochromic luminescence

Article abstract of DOI:10.1002/adfm.201302086

Mechanofluorochromic materials have great potential for a wide variety of applications such as sensors, memory devices, motion systems, security systems, and so forth. However, only few design principles have been disclosed, which greatly impedes the growth of mechanofluorochromic dyes. Here, a strategy of molecular design for mechanochromic luminescence is reported, based on the cation-anion interaction-directed switching of molecular stacking. On the basis of this strategy, a series of common N-heteroaromatic onium fluorophores such as imidazolium, 1,2,4-triazolium, triazolopyridinium, benzoimidazolium, γ-carbolinium, and pyridinium salts have been designed and proved to have striking reversible mechanofluorochromic behaviors. The simple attachment of a non-fluorescent imidazolium unit to the pyrene scaffold through a flexible carbon chain can even trigger the mechanofluorochromic phenomenon, which gives a consummate interpretation that the cation-anion interaction can be considered as an important general tool to design organic mechanochromic luminescent materials. Molecular Design: A cation-anion interaction-directed molecular design strategy for mechanochromic luminescence is proposed. On the basis of this strategy, a series of common N-heteroaromatic onium fluorophores such as imidazolium, 1,2,4-triazolium, triazolopyridinium, benzoimidazolium, γ-carbolinium, and pyridinium salts are designed and prove to have striking reversible mechanofluorochromic behaviors.

Full text of DOI:10.1002/adfm.201302086

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Cation–Anion Interaction-Directed Molecular Design
Strategy for Mechanochromic Luminescence
Gaocan Li, Feijie Song, Di Wu, Jingbo Lan, Xingyan Liu, Jie Wu, Shuaijun Yang,
Dan Xiao, and Jingsong You*
induced luminescence changes have been
observed mainly in certain metal (Au,
Mechanofluorochromic materials have great potential for a wide variety
of applications such as sensors, memory devices, motion systems, secu-
rity systems, and so forth. However, only few design principles have been
disclosed, which greatly impedes the growth of mechanofluorochromic
dyes. Here, a strategy of molecular design for mechanochromic lumines-
cence is reported, based on the cation–anion interaction-directed switching
of molecular stacking. On the basis of this strategy, a series of common
N-heteroaromatic onium fluorophores such as imidazolium, 1,2,4-triazolium,
triazolopyridinium, benzoimidazolium, γ-carbolinium, and pyridinium salts
have been designed and proved to have striking reversible mechanofluoro-
chromic behaviors. The simple attachment of a non-fluorescent imidazolium
unit to the pyrene scaffold through a flexible carbon chain can even trigger
the mechanofluorochromic phenomenon, which gives a consummate inter-
pretation that the cation–anion interaction can be considered as an important
general tool to design organic mechanochromic luminescent materials.
[
3–9]
Cu, Ag, Zn, Pt, Al, and Pd)
and boron
[10]
complexes,
based molecules,
vinylene) derivatives,
anthracene and pyrene-
[
11–14]
oligo(p-phenylene
and others.
[
15,16]
[17–21]
Undoubtedly, it is highly desirable, yet
very challenging, to establish fundamental
principles of molecular design that can
be applied to assemble a large number of
libraries of mechanofluorochromic dyes.
Mechanochromic materials commonly
rely on the changes in physical molecular
[1,2]
packing modes.
From this perspec-
tive, mechanochromic phenomenon is
ascribed to a reversible switching between
two different stable or metastable mech-
ano-dependent molecular stacking models
in the solid state. Generally, molecular
stacking may be controllable by various
secondary intermolecular interactions.
1
. Introduction
Metal-metal interaction, π–π interaction, hydrogen bonding
and dipole-dipole interaction have been proposed as possible
Mechanochromic luminescent materials, as a kind of “smart”
stimuli-responsive materials, have great potential for various
applications such as sensors, memory devices, motion sys-
[
1]
parameters to trigger an alteration of the stacking modes.
However, the cation–anion interaction, an important type of
interaction, has been almost entirely ignored for the rational
design of organic mechanochromic luminescent materials.
To enable a reproducible switching of mechano-dependent
molecular stacking modes, a subtle balance of diverse intra-
and intermolecular interactions appears to represent a cru-
[
1,2]
tems, security systems, and so forth.
Although many cases
have been reported in the literature, the understanding of
the relationship between molecular structures and mechano-
responsive behaviors still remains obscure. Up to now, only
few examples demonstrate the rational molecular designs of
mechanochromic luminescence.[ Such hurdles have greatly
impeded the growth of mechanofluorochromic dyes. In par-
ticular, the number of mechanical-stimuli-responsive organic
[1,11,15]
cial prerequisite.
It is well-known that the cation–anion
2]
interaction is one of the important interactions to govern the
molecular packing modes of onium salts and, thus, can lead to
changes in the properties such as melting point, density, vis-
small molecule materials is still limited despite the popu-
[
22–25]
cosity, and electrical conductivity.
Given that the molecular
larity of organic fluorophores.[
1,2]
Currently, mechanically
assemblies of organic cations with a rich set of anions can offer
a great abundance of cation-anion interactions, we envisioned
that the cation-anion interactions and other intra- and inter-
molecular interactions could be regarded as ideal competing
factors to reach such a balance through the variation of coun-
teranions. Thus, onium salts would be promising candidates
for mechanochromic luminescent materials.
G. Li, Dr. F. Song, Prof. Dr. D. Wu, Prof. Dr. J. Lan,
X. Liu, J. Wu, Dr. S. Yang, Prof. Dr. D. Xiao,
Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology
of Ministry of Education
College of Chemistry
Another important requirement to achieve these goals men-
tioned above is that the organic cation exhibits significant lumi-
nescent properties in the solid state. Imidazolium salts have
been shown to be the most common type of ionic liquids (ILs),
typically bearing two alkyl groups at the two nitrogen atoms of
the imidazolium core. Unfortunately, these imidazolium salts
and State Key Laboratory of Biotherapy
West China Medical School
Sichuan University
2
9 Wangjiang Road, Chengdu, 610064, PR China
E-mail: jsyou@scu.edu.cn
DOI: 10.1002/adfm.201302086
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to be an ideal counteranion to enable excellent luminescent
properties (Table 1). Moreover, the emission wavelengths varied
with different counteranions, which implied that different
molecular packing modes could be gained by tuning anions.
Owing to the low emission quantum yield of 1·I in the
−
−
solid state, we chose five counteranions (i.e., NTf , PF ,
2
6
Scheme 1. Molecular modification of imidazolium salts.
−
−
−
OTf , BF , and N(CN) ) to illustrate the correlation between
4
2
the cation–anion interaction and the mechanochromic per-
formance of 1·X (X = counteranion). After three minutes
of pressing under 6 MPa using an oil press, 1·NTf , 1·PF ,
generally exhibit very low emission intensities in the visible
2
6
1
·OTf, 1·BF , and 1·N(CN) underwent redshifts of the peak
region (Φ < 0.05) probably due to their relatively small chromo-
4 2
f
26]
phores.[
emission wavelengths (approximately 102 nm, 90 nm, 78 nm,
6 nm, and 7 nm, respectively) (Table 1 and Figure 1a), which
was in good agreement with the known order of the strength
Consequently, it becomes difficult to observe the
3
mechanofluorochromic phenomenon. Recently, a new genera-
tion of aryl alkyl ionic liquids (TAAILs) containing alkyl and aryl
−
−
[
27]
of the cation-anion interaction as following: NTf < PF4 <
substituents at the nitrogen atoms has been developed. Their
physicochemical and electronic properties may be tuned by
altering anions or substituents at the nitrogen atoms. Herein,
we disclose that the modular modification of the phenylimida-
zolium core can offer minimal fluorophores that not only emit
intense visible light in the solid state, but also exhibit reversible
mechanochromic luminescence through a simple adjustment
of anion (Scheme 1).
2
−
−
− [22–25]
OTf < BF4 < N(CN)2 .
It hinted that the strength of the
cation-anion interaction could determine the mechanochromic
−
luminescence property to a certain extent. The NTf2 anion
associated with the delocalization of negative charge and the
shielding effect of the sulfonyl oxygens and trifluoromethane
groups is widely used to reduce the capacity of the cation-
[
29]
anion interaction to develop low melting ionic liquids.
In
this context, the NTf2− anion was well demonstrated as an
ideal counteranion to induce mechanochromic luminescence.
We rationalized that the weak interactions between the imida-
zolium cation and the NTf2− anion made molecular stacking
relatively loose. The loose molecular stacking was more
susceptible to tiny slip by mechanical stimuli. It is worth noting
2
. Results and Discussion
2
.1. Influence of Anions on Mechanochromic Luminescence
that 1·NTf is one of the smallest fluorophore exhibiting an
2
The phenylimidazolium salts, as a parent core, do not show
luminescence emission in the visible region. Considering
that the electron-deficient imidazolium ring is the electron-
accepting moiety, the proper introduction of an electron donor
such as dialkylamino on the benzene ring would present a
obvious mechanofluorochromic behavior, which constitutes
one of the longest emission wavelength redshifts reported so
far.
[
2,14]
“push–pull” π-electron mode, offering an unlocking opportu-
2.2. Mechanochromic Luminescence of 1·NTf2
nity for visible photoluminescence. The synthesis of aryl alkyl
imidazolium salts with different counteranions included the
copper-catalyzed N-arylation of imidazoles, followed by the
quaternization of the imidazole ring at the N3-position with
alkyl halide, and subsequent anion exchange.[ The initially
prepared imidazolium salt 1·I with an iodine anion displayed
To gain further understanding of the mechanochromic
performance, the influence of applied pressure on the
mechanochromic luminescence of 1·NTf2 was investi-
gated. As the operating pressure increased from 0 to 6 MPa,
a gradual redshift of the fluorescence emission of the 1·NTf2
powder appeared with a peak wavelength change from 407 nm
to 509 nm (Figure 1b), which implied that different stress
conditions could lead to the tiny dissimilarity in the molecular
aggregation. However, after the pressure exceeded 6 MPa, no
further redshifts of the emission wavelengths were observed.
28]
very weak fluorescence (Φ = 0.002) in the solid state, which
f
was presumably attributed to the external heavy atom effect
of a halogen atom. After surveying a series of common coun-
−
teranions such as NTf (bis((trifluoromethyl)sulfonyl)amide),
2
−
−
−
−
−
PF , OTf , BF and N(CN) , the NTf anion was proven
6
4
2
2
Table 1. Physical and photophysical properties of onium salts 1·X.
(nm)b)
λem (nm)b)
(under 6.0 MPa pressure)
1
·X
M.p.
°C]
λabs
λ
Emission wavelength
shift [nm]
em
c)
[nm]a) (in CHCl3)
(Φf) (pristine)
407 (0.76)
371 (0.12)
423 (0.59)
433 (0.45)
418 (0.53)
[
1
1
1
1
1
·NTf2
82-83
133-134
58-60
299
299
300
302
298
509
461
501
469
425
102
90
78
36
7
·PF6
·OTf
·BF4
156-157
108-110
·N(CN)2
a)Only the wavelengths of maximum absorption are shown; b)Excited at 370 nm (excited at 350 nm for 1·PF6); c)Absolute emission quantum yields estimated by calibrated
integrating sphere system.
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The imidazolium salt 1·NTf2 is a white
crystalline powder as prepared and displayed
a blue emission (λem = 407 nm) with a fluo-
rescence quantum yield (Φ = 0.76) in the
f
solid state (Figure 2a,b,n). After being heated
to the melting state and cooled down to
room temperature, the pristine white powder
switched to a pale yellow solid emitting a yel-
lowish-green luminescence (λem = 518 nm,
Φ = 0.25), which could remain for several
f
months at room temperature (Figure 2c,d,n).
Upon a gentle grinding with a spatula, the
blue emission was converted to the yel-
lowish-green luminescence (Figure 2e,f,n).
Figure 1a,c clearly indicated that the grinding
process exhibited a more redshifted light
emission than the pressing operation (111 nm
versus 102 nm), which hinted that these two
different operations might have different
effect on molecular packing or conformation
variations. As illustrated in the confocal laser-
scanning microscopy (CFLSM) images, even
a tiny scratch on the surface of colorless crys-
tals also resulted in a yellow emission color (as
Figure 1. a) Mechanochromic performance of onium salts 1·X and C·X with different coun-
teranions upon grinding and under pressure (3 min, 6 MPa). b) Normalized emission spectra
of the powder under the external pressure and yellow crystals of 1·NTf excited at 370 nm. the emission below 500 nm was filtered out)
2
c) Chromaticity analysis of 1·NTf in different states shown in (a,b): a linear trajectory of chro- under irradiation at 405 nm (Figure 2j–m).
2
maticity coordinates plotted on a 1931 CIE diagram.
The perturbed yellowish-green-emitting
Figure 2. Photographs of 1·NTf under ambient light and UV irradiation at 365 nm: a) the pristine crystalline powder under ambient light, and b) under
2
UV light; c) the solid under fast cooling from its melting state under ambient light, and d) under UV light; e) the pristine solid after grinding the right-
half with a spatula under ambient light, and f) under UV light; g) the entirely ground solid, and h) the sample after thermal annealing under UV light;
and i) the regeneration of the yellowish-green emission demonstrated by the letters “SCU” and the Chinese characters “Sichuan University” written
with a spatula. CFLSM images of colorless single crystal of 1·NTf : j) the image of unscratched crystal under ambient light, and k) under irradiation at
2
4
05 nm; and l) the image of crystal with a tiny scratch under ambient light, and m) under irradiation at 405 nm (as the emission below 500 nm was
filtered out). n) Normalized emission spectra of 1·NTf in different states excited at 370 nm. o) Normalized UV-visible absorption spectra of 1·NTf in
2
2
thin film before and after grinding. p) Reversible switching of emission of 1·NTf by repeated grinding-heating cycles (λ = 370 nm).
2
ex
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tion peaks, which disclosed the possible
formation of new aggregates (Figure 3g and
Figure S5, Supporting Information). The
colorless crystals displayed blue emission
very similar to the emission spectra of the
pristine powder and the annealed sample
with the emission wavelength of about 407
nm (Figures 3b, 2n). The yellow ones emitted
blue-green fluorescence, which was neither
consistent with the ground sample nor the
pristine powder (Figures 3d, 2n, 1b). There-
fore, one could suggest that the mechanism
was differentfrom the well-known typical
models, in which obvious changes in the
[1,2]
XRD patterns are observed.
Subsequently, absorption and fluorescence
decay measurements of the pristine and
ground samples of 1·NTf were carried out.
2
The absorption spectra clearly showed a new
Figure 3. Photographs of single crystals of 1·NTf : a) the colorless single crystal under ambient
2
light, and b) under UV light (λex = 365 nm); and c) the yellow single crystal under ambient light, small absorption peak at a longer wavelength
and d) under UV light (λ = 365 nm). Molecular stacking of the colorless single crystals of of 341 nm after grinding (Figure 2o), which
ex
1
·NTf : e) side view; and f) partial view of the crystal packing with anions omitted for clarity.
2
suggested the generation of a new aggregate
during the grinding process. The phenomena
g) Powder X-ray diffraction patterns of 1·NTf in different states.
2
could also be explained by the different fluo-
sample was treated with thermal annealing at approximately
rescence emissions and lifetimes of the white pristine powder
(λem = 407 nm, τ = 3.5 ns) and the yellowish ground sample
(λem = 518 nm, τ = 4.2 ns) (Figure S6, Supporting Informa-
8
0 °C to revert back to the blue emission (Figure 2g,h,n), and
recovered again by subsequent regrinding (Figure 2i). The
emission of 1·NTf could be reversibly switched between the
tion). Furthermore, chromaticity analysis of 1·NTf illustrated
2
2
two different phases by repeated grinding and annealing treat-
ments with no obvious fatigue response (Figure 2p).
that the samples in different states were probably binary blends
of the blue-emitting aggregate (absorption at 321 nm) and the
new aggregate (absorption at 341 nm) (Figure 1c, 2o). The con-
centration of the new aggregate in the yellow crystals was too
low to be detected by single crystal XRD measurements. Dif-
ferential scanning calorimetry (DSC) analysis for the pristine
2
.3. Investigation of Plausible Mechanism of Piezochromic
Luminescence of 1·NTf2
and ground samples of 1·NTf showed that the ground sample
2
To gain insight into the mechanism of piezochromic lumines-
generated a new, very small endothermic peak at approximately
71 °C before melting in the first heating curve (Figure 4),
which further suggested the formation of new aggregate with
cence of 1·NTf , both single crystal X-ray diffraction (XRD)
2
and powder X-ray diffraction (PXRD) measurements were
conducted. After numerous attempts, two different forms
[
13]
a low concentration. However, despite very small amounts
of the new aggregate, its luminescence efficiency reached a
high enough level to cause the pronounced change in fluores-
of crystals of 1 NTf were fortunately isolated from the dilute
2
and concentrated ethyl acetate solutions in refrigerator, respec-
tively (Figure 3a–d). X-ray single crystal structural analysis of
the colorless crystals revealed that the molecules stacked in a
face-to-face orientation with a slip angle of approximately 32°
along the long molecular axis, and a dihedral angle of approxi-
mately 15° between the phenyl ring and the
[
3]
cence emission. We assumed that a tiny slip of the molecular
stacking after grinding or pressure led to the generation of
new aggregates ascribed to the local distortions in the whole
crystalline structure or the formation of the local amorphous
imidazolium ring (Figure 3e,f). Furthermore,
only weak π–π interactions in the sheet-like
stacking pattern were observed through
analysis with the program PLATON. Inter-
estingly, the yellow crystals did exhibit the
stacking pattern and the cell parameters
identical to the colorless ones (Supporting
Information, Figures S3,S4, and Table S2).
Although the XRD patterns of the pristine
powder agreed well with those simulated
from the single crystal data in both crystal-
line forms, the diffraction data of the ground Figure 4. DSC curves (first heating) of 1·NTf in different states: a) pristine solid and b) ground
2
solid showed tiny changes in the diffrac- solid.
7
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Table 2. Physical and photophysical properties of onium salts 1·NTf -8·NTf .
2
2
Compd.
M.p.
λabs
λ
λem
Emission wavelength shift
[nm]
em
d)
a)
[nm]b) (in CHCl3)
[nm]c) (Φf) (pristine)
407 (0.76)
[nm]c) (ground)
[
°C]
1·NTf2
2·NTf2
3·NTf2
4·NTf2
5·NTf2
6·NTf2
7·NTf2
8·NTf2
86
299
293
302
307
290
289
299
253
518
–
111
–
– e)
– e)
– e)
70
479 (0.05)
481 (0.03)
–
–
484 (0.10)
–
–
430 (0.45)
472
504
492
532
42
90
67
79
86
414 (0.45)
80
425 (0.38)
122
453 (0.13)
a)
Melting points were obtained from the first heating run using DSC under N2, rate = 5 °C min−1; b)Only the maximum absorption wavelengths are shown; c)Emission of
d)
e)
samples excited at the corresponding maximum excitation wavelength; Absolute emission quantum yields estimated by calibrated integrating sphere system; Room
temperature ionic liquid.
state with the enhancement of aromatic stacking interactions or
dipole-dipole interaction, which resulted in a red-shifted emis-
sion. In addition, other aspects such as molecular conformation
variation after grinding or pressure might also have great effect
on luminescence of fluorophore. Upon grinding or enhancing
the external pressure, an increase in the amount of the new
aggregates induced a gradual redshift of the emission wave-
length from 407 nm to 518 nm (Figure 1b,c). Upon thermal
annealing treatment, the metastable new aggregate was recov-
ered to the more stable blue-emitting aggregate.
phenylimidazolium fluorephore core could influence the cat-
ion-anion interactions and thus resulted in the different mecha-
nochromic properties of these imidazolium salts.
To further test our hypothesis, we designed and synthesized
a series of other N-heteroaromatic onium fluorophores such as
[1,2,4]triazolo[4,3-a]pyridin-2-ium salts (Pz·X), benzoimidazol-
3-ium salts (Bz·X), γ-carbolinium salts (C·X), and pyridinium
salts (Py·X). As expected, these common organic onium salts
were found to display significant mechanofluorochromic behav-
iors through a choice of suitable anion (Table 3 and Table S1,
Supporting Information). Similar to imidazolium salts, other
azolium salts such as [1,2,4]triazolo[4,3-a]pyridin-2-ium salts
2
.4. Application of Molecular Design Strategy
(
Pz·NTf ) and benzoimidazol-3-ium salts (Bz·NTf ) exhib-
2
2
ited approximately 40 nm emission wavelength redshifts after
This molecular design strategy is simple, yet capable of devel-
oping a broad range of onium materials showing piezochromic
grinding. The emission of the pyridinium salt Py·NTf was red-
2
shifted from 648 nm to 712 nm (the near infrared region) after
luminescence. For example, the aryl alkyl imidazolium-based
grinding. Notably, the mechanochromic performance of C·X
−
salts 1·NTf -8·NTf with the NTf counteranion were modu-
−
−
−
−
−
2
2
2
(X: NTf , PF , OTf , BF , and N(CN) ) was also in accord-
2 6 4 2
larly synthesized through judicious choice of the substituents
of the “phenylimidazolium” parent core (Table 2). The alkyl
chain length of the substituents (even one more carbon atom
in the chain) could drastically influence the melting point of
ance with the order of the strength of the cation-anion interac-
tion (Figure 1a). Upon thermal annealing or recrystallization,
the ground samples of these onium salts were easily recovered
to the original state. As known, no obvious mechanochromic
luminescence was observed for the fluorescent pyrene. How-
ever, the simple attachment of a non-fluorescent imidazolium
unit to the pyrene scaffold through a flexible carbon chain
imidazolium salts. When the methyl substituent in 1·NTf was
2
replaced by the ethyl group, the room temperature ionic liquid
2
·NTf was obtained. Compounds 3·NTf and 4·NTf bearing
2
2
2
the butylmethylamino and diethylamino substituents on the
phenyl ring respectively were also ionic liquids. The imida-
zolium salt with a more electron-donating substituent on the
(
P·X) could induce the reversible mechanochromic lumines-
cence, which demonstrated that the cation–anion interaction
could be considered as a general strategy in designing organic
mechanochromic luminescent materials.
phenyl ring (5·NTf ) and the salts with electron-withdrawing
2
substituents on the imidazolium ring (7·NTf , and 8·NTf )
2
2
emitted redshifted fluorescence compared with 1·NTf . Except
2
ionic liquids 2·NTf , 3·NTf and 4·NTf , all the prepared imi-
dazolium salts exhibited significant reversible mechanochromic
luminescence with various emission wavelength redshifts (42–
2
2
2
3
. Conclusions
We have established the cation-anion interaction-directed
strategy for the rational design of mechanochromic
1
11 nm). It demonstrated that the modification of the parent
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Table 3. Physical and photophysical properties of onium salts.
Compd.
M.p.
°C]
λabs
λ
λem
Emission wavelength shift
[nm]
em
c)
[nm]a) (in CHCl3)
[nm]b) (Φf) (pristine)
415 (0.07)
[nm]b) (ground)
[
P·NTf2
59-61
65-68
375
316
345
478
338
486
511
472
712
485
71
36
38
64
29
Bz·NTf2
Pz·NTf2
Py·NTf2
C·NTf2
475 (0.55)
92-93
434 (0.06)
146-147
163-164
648 (0.09)
456 (0.23)
a)Only the maximum absorption wavelengths are shown; b)Excited at the corresponding maximum excitation wavelength; c)Absolute emission quantum yields estimated
by calibrated integrating sphere system.
luminescence. This strategy has been successfully harnessed
to discover a wide range of onium piezochromic luminescent
materials such as imidazolium salts, 1,2,4-triazolium salts, tria-
zolopyridinium salts, benzoimidazolium salts, γ-carbolinium
salts, and pyridinium salts. The possible mechanism of
piezochromic luminescence of 1·NTf2 has been proposed,
but each onium salt itself may have an idiographic model.
Although a deeper understanding of the relationship between
molecular structures and mechano-responsive behaviors is still
required, these first results have already opened a new avenue
of investigation into stimuli-responsive luminescent materials,
and may enable organic onium salts to find an array of novel
applications in sensors, memory devices, liquid crystals, optical
displays, and rewritable optical media.
pressure and the residue was purified by column chromatography on
silica gel.
General Procedure for the Synthesis of Onium Salts: A Schlenk tube
with a magnetic stir bar was charged with neutral N-arylheterocycles,
alkyl halide and solvent under N . The resulting mixture was stirred at
2
indicated temperature for 24 h and the solvent was then removed. The
residue was purified by column chromatography on silica gel and the
product was further purified by recrystallization.
Crystallographic data for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-912422 and no. CCDC-912423 for
colorless crystal and yellow crystal of 1·NTf , respectively.
2
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
4
. Experimental Section
Instrumentation: NMR spectra were obtained on a Bruker AV II-400
or Bruker AV II-600. Fluorescence emission spectra were obtained using
a Horiba Jobin Yvon-Edison Fluoromax-4 fluorescence spectrometer.
Absorption spectra were obtained on a HITACHI U-2910 spectrometer.
Fluorescence lifetime data were determined in 1 mm cell on a Hamamatsu
FL920S instrument. Differential scanning calorimetry (DSC) data was
performed using a TA instrument DSC-Q200 1474 (rate = 5 °C min ,
range = –40 to 200 °C). The ESI-TOF mass spectra were recorded with
a Waters Q-Tof premier instrument. X-Ray single-crystal diffraction data
were collected on an Oxford Xcalibur E CCD area-detector diffractometer.
Melting points were determined with XRC-1 and are uncorrected.
Acknowledgements
This work was supported the National High Technology Research and
Development Program of China (863 Program) (No. 2013AA031901) and
the National NSF of China (Nos 21025205, 21272160, and 21021001).
–1
Received: June 19, 2013
Revised: August 20, 2013
Published online: September 3, 2013
General Procedure for the Synthesis of Neutral N-Arylheterocycles: A
flame-dried Schlenk tube with a magnetic stir bar was charged with aryl
iodide (5.0 mmol), free (N–H) heterocycle (6.0 mmol), CuI (0.380 g,
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system was then evacuated and backfilled with N for twice. After stirring
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at the indicated temperature for 36 h, DMF was removed under reduced
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