A new organic far-red mechanofluorochromic compound derived from cyano-substituted diarylethene

Article abstract of DOI:10.1016/j.tet.2013.10.066

A new cyano-substituted diarylethene derivative (R-NH₂) with reversible far-red mechanofluorochromic property was synthesized and confirmed by standard spectroscopic methods. To the best of our knowledge, the 684 nm red-shifted wavelength of the ground R-NH₂ is the longest wavelength that has appeared in the literature. The mechanofluorochromic mechanism was investigated by small and wide-angle X-ray scattering and was ascribed to the destruction of crystalline structure. More in-depth study by infrared spectra and time-resolved emission-decay behaviors showed that the changes of C-H out-of-plane bending vibrations in aryl group of the compound and the obvious increase of fluorescence lifetime might be the fundamental reasons. The synthetic strategy reported here can be extended to prepare more and more long-wavelength emission mechanofluorochromic materials, which can broaden the scope of application of such materials and for thoroughly understanding the mechanofluorochromic mechanism.

Full text of DOI:10.1016/j.tet.2013.10.066

Tetrahedron 69 (2013) 10552e10557
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A new organic far-red mechanofluorochromic compound derived
from cyano-substituted diarylethene
a
,
b
c
a
b
a,
*
*
Xiqi Zhang , Zhiyong Ma , Meiying Liu , Xiaoyong Zhang , Xinru Jia , Yen Wei
a
Department of Chemistry and the Tsinghua Center for Frontier Polymer Research, Tsinghua University, Beijing 100084, China
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of
b
Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
c
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Laboratory of New Materials, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 August 2013
Received in revised form 30 September 2013
Accepted 14 October 2013
Available online 25 October 2013
A new cyano-substituted diarylethene derivative (R-NH ) with reversible far-red mechanofluorochromic
property was synthesized and confirmed by standard spectroscopic methods. To the best of our
knowledge, the 684 nm red-shifted wavelength of the ground R-NH is the longest wavelength that has
2
2
appeared in the literature. The mechanofluorochromic mechanism was investigated by small and wide-
angle X-ray scattering and was ascribed to the destruction of crystalline structure. More in-depth study
by infrared spectra and time-resolved emission-decay behaviors showed that the changes of CeH out-of-
plane bending vibrations in aryl group of the compound and the obvious increase of fluorescence lifetime
might be the fundamental reasons. The synthetic strategy reported here can be extended to prepare
more and more long-wavelength emission mechanofluorochromic materials, which can broaden the
scope of application of such materials and for thoroughly understanding the mechanofluorochromic
mechanism.
Keywords:
Far-red fluorescence
Mechanofluorochromic compound
Cyano-substituted diarylethene derivative
Mechanism
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
unique AIE materials have attracted great research interest because
they could overcome the notorious issues in most planar conju-
Mechanofluorochromic materials have advanced fluorescent
properties that change in response to external force stimuli and
expected to generate new intelligent materials based on their su-
perior high efficiency and reproducibility compared with conven-
tional fluorescent materials, and have got wide applications in
sensors, memory chips, and security inks.1,2 Chemical structure and
molecular assembly structure changes are considered as two major
mechanofluorochromic ways. Between them, molecular assembly
structure change is believed easier to realize the dynamic control of
fluorescent process. In 2002, Weder et al. reported on two cyano-
substituted diphenylethene derivatives-doped linear low-density
gated organic dyes named aggregation-caused quenching (ACQ)
effect. Different AIE architectural frameworks involving siloles,
triphenylethene, tetraphenylethene, distyrylanthracene, cyano-
substituted diarylethene derivatives conjugated molecules have
been developed and utilized for chemosensor and bioimaging
10e30
applications.
Since 2010, the Xu’s group synthesized and reported a number
31e37
of new mechanofluorochromic compounds with AIE feature,
and proposed a ‘molecular conformational planarization’ mecha-
nism for mechanofluorochromic compounds to establish a com-
mon structureeproperty relationship of the mechanofluoro
chromic materials and AIE materials, which would be helpful in
identifying and synthesizing more novel mechanofluorochromic
3
polyethylene as the first two mechanofluorochromic materials.
Since then, the appeared organic mechanofluorochromic mate-
1
rials were still extremely rare until 2010 due to the lack of estab-
materials. According to this relationship, a number of new organic
4
e8
lished propertyestructure relationship.
In 2010, Xu et al.
mechanochromic fluorescent compounds were synthesized, which
greatly broadened the field of mechanofluorochromic materi-
reported on a compound derived from tetraphenylethylene and
3
8e56
divinylanthracene, which exhibited obvious mechanofluoro
als.
In view of this situation, the very critical thing is either to
9
chromic and aggregation-induced emission (AIE) properties. The
discover new structures of mechanofluorochromic materials or to
broaden the range of the fluorescent emission wavelength. Among
the existing reported mechanofluorochromic materials, the fluo-
rescent emission of them mainly located at short wavelength from
blue to yellow due to their inherent conjugated groups. Rare
*
Corresponding authors. E-mail addresses: sychyzhang@126.com (X. Zhang),
weiyen@tsinghua.edu.cn (Y. Wei).
0
040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.10.066

X. Zhang et al. / Tetrahedron 69 (2013) 10552e10557
10553
mechanofluorochromic materials emit fluorescence with longer
wavelength. Although Ooyama et al. reported on a dye SO with red
625 nm) mechanofluorochromic property, only a 7 nm redshift
was observed for the fluorescent emission wavelength after
2. Results and discussion
The fluorogen R-NH
2
(
2
was prepared following the synthetic
route shown in Scheme 2. Its structure was characterized and
confirmed by standard spectroscopic methods. To gain the lowest
energy spatial conformation of the compound, we conducted
quantum mechanical computations by using the Gaussian 03
57
grinding the as-recrystallized dyes SO
Herein, we report a new organic far-red fluorescent cyano-
substituted diarylethene-based compound (R-NH ) (derivatized
2
.
2
5
8
from 4, 8-bis[5-(2-ethylhexyl)-2-thienyl]benzodithiophene core
structure with bis[2-(4-aminophenyl)acrylonitrile] side chains)
with reversible mechanofluorochromic property (Scheme 1). Solid-
state fluorescent spectra and UVevis spectra were studied to reveal
the mechanofluorochromic property. Then, the mechano-
fluorochromic mechanism was investigated by differential scan-
ning calorimetry (DSC), infrared spectra (IR), small and wide-angle
X-ray scattering (SWAXS), and time-resolved emission-decay
behaviors.
2
software. The HOMO and LUMO of R-NH were obtained (Fig. 1)
according to the density functional method at the B3LYP/6-31 G
level after the structural optimization. The HOMO of the compound
showed dispersed electron cloud distributions on all of the aryl
groups and amino groups on the molecule, whereas the electron
cloud of the LUMO exhibited the migration from both ends of the
amino and thienyl groups to aryl core and cyano groups. Moreover,
the compound adopted a twisted spatial conformation, as shown in
the optimized structure (Scheme 1, right). The dihedral angles of
2
Scheme 1. The chemical structure (left) and the optimization geometry (right) of R-NH .
2
Scheme 2. Synthetic route of R-NH .

10554
X. Zhang et al. / Tetrahedron 69 (2013) 10552e10557
grinding, solid-state fluorescent spectra of the compound in origi-
nal and grinding states were carried out, which realized a redshift
of 24 nm from 660 nm in the original state to 684 nm in the
grinding state (Fig. 2C). To the best of our knowledge, the red-
shifted wavelength at 684 nm is so far the longest wavelength
that has appeared in the literature. The absolute fluorescent
quantum yield for the original and ground samples were de-
termined as 3.4% and 1.6%, respectively. The solid-state UVevis
spectra also revealed the change before and after grinding the
sample, which was shown in Fig. 2D. As the original sample was
ground, the absorption area ranging from 600 to 800 nm signifi-
cantly increased, causing prodigious changes in the color of the
sample. This result demonstrated that R-NH
chromic material as well. Fluorescent spectra of R-NH
2
is also a mechano-
in toluene
2
Fig. 1. Calculated spatial electron distributions of HOMO and LUMO of R-NH .
2
and DMF were determined and shown in Fig. S1, indicating a blue-
shift in solution with respect to the solid samples. Meanwhile, the
AIE characteristic of R-NH2 has been conducted (Fig. S2), demon-
strating the existence of AIE feature for R-NH2.
ꢀ
ꢀ
the adjacent aryl planes were calculated as 52 and 28 for planes
AeB and AeC, respectively. While this twisted conformation may
bring about mechanofluorochromism to the compound.1
Therefore we ground the compound in a mortar with a pestle
directly, the color of the compound was changed from bright red to
dark brown immediately (Fig. 2B), meanwhile, the fluorescent light
tuned from blood-red into dark red under 365 nm UV light (Fig. 2A),
which showed obvious mechanofluorochromism. To quantitatively
To determine the mechanism of the mechanofluorochromic
compound, DSC measurements and IR spectra were conducted.
Reproducible heating DSC result (Fig. 3A) showed that the original
ꢀ
state of R-NH2 had an endothermic peak located at 176 C with
ꢁ
1
enthalpy value 3.0 kJ mol , which indicated a melting transition.
Whereas for the grinding state of the compound, the endothermic
measure the fluorescent wavelength change of R-NH
2
after
Fig. 2. A. Fluorescent image of R-NH
2
before and after grinding when taken at room temperature under 365 nm UV light; B. image of R-NH
2
before and after grinding when taken at
room temperature under daylight; C. normalized solid-state fluorescent spectra of R-NH
2
in original and grinding states; D. normalized solid-state UVevis spectra of R-NH in
2
original and grinding states.
2 2
Fig. 3. A. DSC curves of R-NH in original and grinding states; B. IR spectra of R-NH in original and grinding states, the absorption is normalized according to the CeH stretching
ꢁ
1
band at 2924 cm
.

X. Zhang et al. / Tetrahedron 69 (2013) 10552e10557
10555
ꢀ
peak located at 176 C greatly decreased, with enthalpy value re-
ꢁ1
duced to 1.37 kJ mol . This result suggested that destruction of the
crystalline structure was a direct cause of mechanofluorochromism
9
of R-NH
(
2
.
More detailed information is provided by IR spectra
ꢁ
1
Fig. 3B). The strong absorption signals at 839, 828, and 801 cm
emerged in the original sample of R-NH . When the original sample
was ground, both IR absorption signals at 839 and 828 cm de-
creased, whereas the absorption position at 801 cm dramatically
increased, which were ascribed to the changes of CeH out-of-plane
bending vibration bands in aryl group of the compound. The IR
results further indicated the mechanism of mechanofluorochrom-
ism was caused by the destruction of aggregation state owing to the
2
ꢁ1
ꢁ1
2
changes of CeH bending vibrations in aryl groups of R-NH .
To test the reproducibility of the ground state compound, the
ground sample was fumed by dichloromethane solvent in a sealed
beaker for 10 min to obtain the resulting recovered sample. Then
solid-state fluorescent spectrum of the recovered sample was car-
ried out (Fig. 4A). The fluorescent wavelength moved from 684 nm
of the ground sample to 660 nm, indicating good reversibility. This
result demonstrated that the destruction of crystalline structure of
Fig. 5. Time-resolved emission decay curves of R-NH
covered state.
2
in original, grinding, and re-
2
R-NH could be easily recovered by solvent fuming, which exhibi-
ted important advantage for practical applications of the material
with good feasibility. SWAXS measurements were employed to
elucidate the microstructures of R-NH
recovered states (Fig. 4B). The sharp scattering peaks were ob-
2
in original, grinding, and
Table 1
Solid-state fluorescence lifetime data of R-NH
state
2
in original, grinding and recovered
ꢁ
1
served for the original sample located at 2.54, 3.73, and 18.0 nm
q), indicating an ordered crystalline structure. After grinding,
(
Sample
s
1
(ns)a
A
1
b
s
2
(ns)a
A
2
b
<s
> (ns)c
a transition from crystalline structure to amorphous phase oc-
curred with the above sharp peaks attenuated or even disappeared.
Finally the recovered sample by solvent fuming led to the recovery
of the crystalline structure by accompanying the appearance of
peaks that coincided with the original sample. The results indicated
that the reversible transition between the ordered and disordered
molecular aggregations was crucial for the mechanofluorochromic
properties.31
Original
Grinding
Recovered
0.35
0.49
0.47
0.64
0.75
0.78
1.25
2.16
1.75
0.36
0.25
0.22
0.67
0.91
0.75
a
Fluorescence lifetime.
Fractional contribution.
Weighted mean lifetime.
b
c
2 2
Fig. 4. A. Normalized solid-state fluorescent spectra of R-NH in grinding and recovered states; B. SWAXS patterns of R-NH in original, grinding, and recovered states.
The time-resolved emission-decay behaviors of R-NH
2
in origi-
indicated that the obvious increasement of lifetime gave rise to the
significant changes of fluorescent wavelength.
3
2
nal, grinding, and recovered states were investigated to obtain
further information of the mechanofluorochromism. The time-
resolved fluorescence curves and the lifetime data are illustrated
in Fig. 5 and Table 1, respectively. Two relaxation pathways were
shown for the fluorescence decay, indicating that the time-resolved
PL spectra of the compound included independent emissions from
3. Conclusions
In summary, we have synthesized a new cyano-substituted
diarylethene derivative (R-NH ) with reversible far-red mechano-
fluorochromic property. To the best of our knowledge, the
684 nm red-shifted wavelength of the ground R-NH is the longest
2
the segments with different
p
-conjugation extent due to the
detected multiple lifetimes. The weighted mean lifetimes < > of
the original and grinding sample were significantly different:
.67 ns (original) and 0.91 ns (grinding). After recovered by solvent
fuming, the < > of the sample reduced to 0.75 ns. The results
3
2
s
2
wavelength that has appeared in the literature. The mechano-
fluorochromic mechanism was investigated and ascribed to the
destruction of crystalline structure causing the changes of CeH out-
0
s

10556
X. Zhang et al. / Tetrahedron 69 (2013) 10552e10557
of-plane bending vibrations in aryl group of the compound and the
obvious increasement of fluorescence lifetime. The synthetic strat-
egy reported here can be extended to prepare more and more long-
wavelength emission mechanofluorochromic materials, which can
broaden the scope of application of such materials and for thor-
oughly understanding the mechanofluorochromic mechanism.
room temperature. Then tetrabutylammonium hydroxide solution
(0.8 M, 10 drops) was added and the mixture was heated to reflux
for 2 h precipitating a dark red solid. The reaction mixture was
cooled to room temperature and filtered, washed with ethanol for
several times obtaining a dark red solid R-NH
2
(0.34 g, yield 71%).
(ppm): 0.88e0.98 (m, 12H, eCH ),
e), 1.65e1.74 (m, 2H, (methylene) CeH),
2.86 (d, 4H, J¼6.4 Hz, thienyleCH e), 6.68 (d, 4H, J¼8.8 Hz, aryleH),
.90 (d, 2H, J¼3.2 Hz, aryleH), 7.33 (d, 2H, J¼3.2 Hz, aryleH),
1
H NMR (400 MHz, CDCl
.28e1.42 (m, 16H, eCH
3
)
d
3
1
2
3
4
4
. Experimental procedure
2
6
13
.1. Materials and characterization
7.42e7.53 (m, 6H, aryleH), 7.96 (s, 2H, aryleH); C NMR (100 MHz,
CDCl (ppm): 147.8, 146.6, 139.9, 137.0, 136.1, 130.9, 129.2, 128.4,
127.4,125.9,124.5,123.9,115.2,111.2, 41.5, 34.4, 32.5, 29.0, 25.7, 23.1,
3
) d
Thiophene, 3-(bromomethyl)heptane, thiophene-2-carboxylic
ꢁ1
acid, thionyl chloride, dimethylamine, n-butyllithium, stannic
chloride, 2-(4-aminophenyl)acetonitrile, tetrabutylammonium hy-
droxide (0.8 M in methanol) purchased from Alfa Aesar were used
as received. All other agents and solvents were purchased from
commercial sources and used directly without further purification.
Tetrahydrofuran (THF) was distilled from sodium/benzophenone.
Ultra-pure water was used in the experiments. The synthetic route
14.3, 11.0; IR (cm ): 3375, 2955, 2924, 2856, 2330, 2212, 1672,
1604, 1581, 1516, 1456, 1377, 1294, 1268, 1234, 1182, 1143, 1076, 839,
2
þ
828, 801; HRMS calcd. for C52
432.1683.
H
54
N
4
S
4
, [Mþ2H] : 432.1688, found
Acknowledgements
of R-NH
2
was showed in Scheme 2. Intermediates 1, 2, and 3 were
This research was supported by the National Science Foundation
of China (No. 21134004, 21201108), and the National 973 Project
(No. 2011CB935700), China Postdoctoral Science Foundation
(2013T60100, 2012M520243, 2012M520388, 2013T60178).
5
9,60
prepared according to the literature methods.
1
13
H NMR and C NMR spectra were measured on a JEOL
00 MHz spectrometer [CDCl as solvent and tetramethylsilane
TMS) as the internal standard]. HRMS was obtained on Shimadzu
4
(
3
LCMS-IT-TOF high resolution mass spectrometry. Fluorescence
spectra and lifetime were measured on FLS 920 lifetime and steady
state spectrometer. The absolute fluorescent quantum yield for the
solid samples were measured on a MK-301 EL/PL Measurement
Program (Bunkoukeiki Co., Ltd, Japan) equipped with an integrating
sphere and a CCD spectrometer (Andor Tech, CCD-6685). Solid-
state UVevis spectra were recorded on a Hitachi U4100 UVevis-NIR
spectrophotometer. Differential scanning calorimetry (DSC) curves
were performed on TA Instruments DSC Q2000 at a heating rate of
Supplementary data
Electronic Supplementary data (ESI) available: fluorescent
2
spectra of R-NH in toluene and DMF; the AIE characteristic of R-
NH . Supplementary data related to this article can be found at
2
http://dx.doi.org/10.1016/j.tet.2013.10.066.
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