Mechanochromic behavior of aryl-substituted buta-1,3-diene derivatives with aggregation enhanced emission

Article abstract of DOI:10.1002/chem.201403132

Three tetra-aryl substituted 1,3-butadiene derivatives with aggregation enhanced emission (AEE) and mechanochromic fluorescence behavior have been rationally designed and synthesized. The results suggest an effective design strategy for developing diverse materials with aggregation induced emission (AIE) and significant mechanochromic performance by employing D-π-A structures with large dipole moments. Mechanochromic luminescence: Three tetra-aryl substituted 1,3-butadiene derivatives with distinct electronic push and/or pull substituents have been prepared. All three derivatives show typical aggregation enhanced emission (AEE) features owing to the restriction of intramolecular rotations. The results demonstrate that chromophores with both large dipole moments and AEE characteristics are of benefit for mechanochromic applications with efficient solid-state emissions.

Full text of DOI:10.1002/chem.201403132

DOI: 10.1002/chem.201403132
Communication
&
Mechanochromic Luminescence
Mechanochromic Behavior of Aryl-Substituted Buta-1,3-Diene
Derivatives with Aggregation Enhanced Emission
Yijia Zhang,[a] Ting Han,[a] Shangzhi Gu,[a] Tianye Zhou,[a] Chuanzhen Zhao,[a] Yuexin Guo,[b]
Xiao Feng,*[b] Bin Tong,[a] J. Bing,[a] Jianbing Shi,[a] Junge Zhi,[b] and Yuping Dong*[a]
posed and investigated. For example, Araki et al. described the
Abstract: Three tetra-aryl substituted 1,3-butadiene deriv-
atives with aggregation enhanced emission (AEE) and me-
switching of fluorescence in a hexyl amide-substituted tetra-
phenylpyrene derivative relative to intermolecular hydrogen-
chanochromic fluorescence behavior have been rationally
bond interactions;[5] Ito et al. demonstrated the mechanochro-
designed and synthesized. The results suggest an effective
mic behavior of a gold complex undergoing a meta-stable to
design strategy for developing diverse materials with ag-
stable phase change;[6] Fraser et al. reported morphology-de-
gregation induced emission (AIE) and significant mecha-
pendent fluorescence for solid states of difluoroboron avoben-
zone.[7] Nevertheless, most studies have focused on regulating
nochromic performance by employing D-p-A structures
with large dipole moments.
molecular packing modes and consequent intermolecular in-
teractions;[8] examination of the effect of molecular structure
on the mechanochromic behavior is rather limited.[9] In this re-
Mechanochromic luminogens, a class of responsive materials
the emission colors of which can be altered in response to ex-
ternal mechanical stimuli such as shearing, grinding, or elonga-
tion,[1] have received tremendous attention owing to their po-
tential applications in stress sensors, indicators, luminescence
switches, security inks, optical data storage systems and light-
emitting devices.[2] A series of mechanochromic luminogens
based on organic molecules, organometallic compounds, and
dye-doped polymers have been recently developed.[3] It is cru-
cial to understand the underlying mechanisms of mechano-
chromism from a molecular level and to establish a general
method to change and control solid-state luminescence. Dy-
namic control of solid-state photoluminescent properties by
mechanical forces can be achieved by either chemical or physi-
cal structural change. Mechanical force induced chemical struc-
tural change,[4] involving chemical bond breaking or forming, is
highly designable yet suffers from insufficient conversion or ir-
reversible reactions. By contrast, physical structural change is
more efficient for attaining mechanochromic behaviors owing
to the ease of adjusting molecular stacking modes in the solid
state.
spect, it is essential to establish a correlation between molecu-
lar structure and mechanochromic solid-state photophysical
properties.
Mechanochromic luminogens with efficient solid-state emis-
sion and large contrast are particularly desirable for practical
applications. The development of aggregation induced/en-
hanced emission (AIE/AEE) materials,[10] showing no emission
or little emission in dilute solutions but becoming brightly flu-
orescent in nanoparticles or films, paves the way for designing
highly emissive solid-state fluorophores. The restriction of in-
tramolecular rotations (RIR) in aggregated states, which blocks
the non-radiative pathway and opens up the radiative channel,
is proposed to account for the exceptional AIE/AEE phenomen-
on.[11] Limited examples of AIE-active mechanochromic lumino-
gens have been reported.[12]
We have recently described a tetraphenylbutadiene (TABD)
derivative with two carboxyl groups as substituents, the pho-
toluminescent color of which can be altered by gentle grinding
and recovered upon exposure to solvents.[13] We also clarified
the role of the hydrogen bonds in the mechanochromic re-
sponse. In this report, three compounds derived from the
TABD molecule, namely dimethyl 4,4’-((1Z,3Z)-1,4-diphenylbu-
ta-1,3-diene-1,4-diyl)dibenzoate (TABDE), dimethyl 4,4’-((1Z,3Z)-
1,4-bis(4-(trifluoromethyl)phenyl)buta-1,3-diene-1,4-diyl)diben-
zoate (TABDE-CF3), and dimethyl 4,4’-((1Z,3Z)-1,4-bis(4-(diphe-
Up to now, several mechanisms of the mechanochromic pro-
cess that relies on physical structural change have been pro-
[a] Y. Zhang,+ T. Han,+ S. Gu, T. Zhou, C. Zhao, Prof. B. Tong, Dr. J. Bing, J. Shi,
Prof. Y. Dong
nylamino)phenyl)buta-1,3-diene-1,4-diyl)dibenzoate
(TABDE-
NPh2), are employed for the systematic and comparative study
of the structural effect on mechanochromic performance. In
these three compounds, ÀCF3 and ÀCOOCH3 groups act as
electron acceptors (A), whereas the ÀNPh2 and TABD moieties
serve as electron donors (D) and conjugation bridges, respec-
tively. All of these TABD derivatives are found to possess AEE
features as well as mechanochromic properties. The results
show that the mechanochromic performance follows the se-
quence of TABDE-NPh2 >TABDE>TABDE-CF3. This order can be
School of Materials Science & Engineering, Beijing Institute of Technology
5 South Zhongguancun Street, Beijing, 100081 (P.R. China)
E-mail: chdongyp@bit.edu.cn
[b] Y. Guo, Dr. X. Feng, Prof. J. Zhi
School of Chemistry, Beijing Institute of Technology
5 South Zhongguancun Street, Beijing, 100081 (P.R. China)
E-mail: fengxiao86@bit.edu.cn
[+] Y. Zhang and T. Han contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403132.
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solution of TABDE-NPh2 triggers
not only a clear increase of in-
tensity, but also
a significant
shift of fluorescent wavelength.
This suggests that TABDE-NPh2
favors intramolecular charge
transfer (ICT) in the higher polar-
ity solvent mixture. The fluores-
cence quantum yields (FF) of
TABDE-CF3, TABDE, and TABDE-
NPh2 increase by 6.4, 2.4, and 8.5
fold, respectively, when going
from their THF solution to the
solid state (Table S1 in the Sup-
porting Information).
Clearly, all of these three com-
pounds are typical AEE lumino-
gens. The formation of aggre-
gates upon addition of water to
their THF solutions is supported
by dynamic light scattering
(DLS) analysis (Figure S2 in the
Supporting Information) and
scanning electron microscopy
(SEM; Figure S3in the Supporting
Information). The results of the
size distribution by intensity
analysis indicate that the
amount of larger nanoparticles
Scheme 1. Synthesis routes to the three TABD derivatives. a) 2-methylbut-3-yn-2-ol, Pd(PPh3)2Cl2, CuI, NEt3, 508C;
b) NaOH, CaH2, toluene, reflux; c) CuBr2, PdCl2, toluene, CH3CN, r.t.; d–f) Pd(PPh3)4, K2CO3, CH3OH, toluene, reflux.
attributed to the distinctions in the molecular polarity of these
three compounds, as indicated by exploration of their solvato-
chromic properties and through theoretical calculations.
in the THF/water mixtures is significantly increased when the
fw is higher than 50%. Because of the space constraint in these
aggregates, the rotations and motions of the multiple aromatic
rings and C=C bonds in the TABDE derivatives are restricted.
Therefore, the nonradiative channels are blocked and the emis-
sions are enhanced.
The synthetic routes to these three luminogens are shown
in Scheme 1. Intermediate 3 was prepared according to the lit-
erature,[14] and was subsequently dimerized to produce 4 with
high stereoselectivity by using PdCl2 and CuBr2 as catalysts in
MeCN/Toluene (v/v, 1:50). Finally, TABDE, TABDE-CF3, and
TABDE-NPh2 were obtained through Suzuki coupling reaction
of 4 with corresponding boronic acid compounds by using
Pd(PPh3)4 as a catalyst. Detailed synthetic procedures and char-
To reveal the mechanism of the AEE processes for TABDE de-
rivatives, their molecular geometries and packing arrange-
ments in the solid state were studied. Single crystals of TABDE
and TABDE-CF3 were obtained by the diffusion process and
characterized by single-crystal X-ray crystallography (SXRD).
Unfortunately, we were not able to obtain single crystals of
good quality for TABDE-NPh2 owing to the existence of bulky
ÀNPh2 groups in the twisted backbone, which is unfavorable
for the highly ordered alignment of molecules. As shown in
Figure 2, the molecules have twisted conformations maintain-
ing large torsion angles (688 for TABDE and 608 for TABDE-CF3)
in the crystal. The adjacent molecules in a single molecular
column are packed by CÀH···p hydrogen-bond interactions re-
sulting in a distance of less than 2.8 ꢁ, rather than p–p interac-
tions for both derivatives. This distance results in the RIR pro-
cess and minimizes the likelihood of the formation of excimers
in the crystal. These results demonstrate that the formation of
aggregates imposes physical restraints on the intramolecular
rotations, and thus populates the radiative excitons.
1
acterization data of the compounds from IR, H NMR, 13C NMR,
and MS spectroscopic studies are available in the Supporting
Information. The decomposition temperatures (Td) of these
three compounds, determined by thermal gravimetric analysis
(TGA), range from 300 to 4008C (Figure S1 in the Supporting
Information), demonstrating that all compounds are thermally
stable, which is a required property for further device applica-
tions.
Figure 1 shows the fluorescent (FL) spectra of TABDE,
TABDE-NPh2, and TABDE-CF3 in THF/water mixtures. For these
three TABD derivatives, the fluorescence intensities of their sol-
utions are negligibly small when the water fraction (fw) is lower
than 70%. However, in solvent mixtures with fw ꢀ70%, their
emissions are drastically intensified. A slight redshift of emis-
sion maximum are observed in the FL spectra of TABDE and
TABDE-CF3, a shift that is attributed to the change in the polar-
ity of the solvent mixture. The addition of water into the THF
Besides AEE properties, we also observed that the powders
of TABD derivatives exhibit mechanochromic behavior. The
fluorescence of the pristine TABDE-NPh2 powders can change
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ducible and can be repeated for
many cycles almost without any
deterioration (Figure 4), indicat-
ing the excellent reversibility of
their mechanochromism.
To gain insight into the me-
chanochromic behavior of the
solid-state TABDE-NPh2, TABDE,
and TABDE-CF3, powder X-ray
diffraction (PXRD) experiments
of the fumed, ground, and
heated samples were conducted
(Figure S4 in the Supporting In-
formation). The diffraction pat-
terns of the fumed and heated
solids show sharp and intense
reflection peaks, indicative of
a well-ordered structure. In con-
trast, the PXRD intensity of the
ground samples displays rela-
tively broad and featureless re-
flection along with a series of
weakened peaks, indicating par-
tial deformation of an ordered
molecular packing structure.
These results confirm that the
mechanochromic
nature
of
TABDE-NPh2, TABDE, and TABDE-
CF3 is caused by the disruption
of the molecular packing. That
is, in the fumed and heated
solids, molecules are well organ-
ized, a fact that is reliant on in-
termolecular interactions. Apply-
ing high pressure can destroy
these interactions and leads to
Figure 1. a–c) FL spectra of TABDE, TABDE-CF3, and TABDE-NPh2 in THF/water mixtures with different fw. Concen-
tration: 10 mm; excitation wavelength: 350, 360, and 320 nm, respectively. d) Plots of maximum intensity of
TABDE, TABDE-CF3, and TABDE-NPh2 versus fw in THF/water mixtures.
from green to yellow under irradiation with UV light by grind-
ing with a mortar or spatula (Figure 3), and this change could
be found only at the pressed area. Similar to TABDE-NPh2,
TABDE and TABDE-CF3 likewise exhibit a pressure-induced lumi-
nescence change. However, the variation of the emission
change of TABDE and TABDE-CF3 is not as significant as TABDE-
NPh2 (Figure 3). Upon grinding, the fluorescent colors gradually
redshifted from sky blue to cyan for the TABDE solid-state
powders, and from dark blue to cyan for the TABDE-CF3. The
maximum emission wavelength for the pristine and ground
samples of TABDE-CF3, TABDE, and TABDE-NPh2, are shifted
from 459, 455, and 520 nm to 465, 465, and 540 nm, respec-
tively (Figure 4). The FF of the pristine and ground powder of
TABDE-CF3, TABDE, and TABDE-NPh2 are listed in Table S1 in
the Supporting Information. Usefully, the emission colors of
the ground samples can be restored to almost their original
colors by adding a drop of polar solvent (e.g., MeOH, EtOH,
THF) or heating at 1808C for 2 h. These remarkable changes in
fluorescence upon grinding are characteristic mechanochromic
phenomena. Moreover, it is confirmed that the mechanochro-
mic behavior of these three TABD-based molecules are repro-
the formation of a less ordered phase, where close packing
governs the molecular arrangement. The permeation of polar
solvents into the powder or heating the samples promotes the
regeneration of intermolecular interactions, and thus recovers
the original emissions.
TABDE-NPh2 exhibits more evident fluorescence alterations
than the other two samples in the fuming–grinding cycles. We
consider that the electronic structures of these samples have
great influence on their mechanochromic behavior. Therefore,
we checked the solvatochromic behavior and calculated the
frontier molecular orbitals of these three TABD derivatives.
The electron-accepting ÀCF3 and ÀCOOCH3 groups or elec-
tron-donating ÀNPh2 groups endow different kinds of push–
pull molecular structures on the TABD derivatives. Especially
for TABDE-NPh2 with a D-p-A structure, it is envisioned as
being capable of forming ICT interactions. To characterize the
environmental sensitivity of these three compounds, the maxi-
mal wavelengths for absorbance and emission were measured
in solvents with varying polarity. For TABDE-NPh2, the UV ab-
sorption spectra changed only a little, but the FL spectra
changed significantly with increasing solvent polarity parame-
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effect. It suggests that the excit-
ed state of TABDE-NPh2 has
a larger dipole moment than the
ground state owing to substan-
tial
charge
redistribution
through the strong polarity. The
lines of best fit for TABDE and
TABDE-CF3 are negative with
small slopes, À724 and À1023
(Figure S6 in the Supporting In-
formation), showing
a
slight
negative solvatochromism effect.
The results indicate that among
these three compounds, TABDE-
NPh2 is more favorable for the
ICT process, and consequently
verifying that it possesses the
highest molecular polarity.
To better understand the elec-
tronic structure effect on the
mechanochromic behavior of
the three TABD derivatives, den-
sity functional theory (DFT) cal-
culations (B3LYP/6-31G) were
Figure 2. Molecular packing arrangements in single crystals of a) TABDE and b) TABDE-CF3.
ter (Df). An evident redshift with Df from light green (lem
=
carried out. The highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) plots are
shown in Figure S7 in the Supporting Information. Unlike the
509 nm) to pink (lem =585 nm) was observed when the solvent
changed from a nonpolar one (e.g., hexane) to highly polar
ones (e.g., DMF). In contrast, the emissions of TABDE and
TABDE-CF3 display only a small bathochromic shift.
The influence of Df on the Stokes shift (Dv) was further ex-
plored by the Lippert–Mataga equation in the Supporting In-
formation. From the plots of Du versus Df, we can find that
the Stokes shift of TABDE-NPh2 with the increase from 1.30ꢂ
10À2 cmÀ1 in hexane to 1.53ꢂ10À2 cmÀ1 in DMF gives a large
positive slope of 6442, exhibiting significant solvatochromism
Figure 4. a) Maximum emission wavelength change upon repeated fuming–
grinding cycles. Squares: after grinding; triangles: after fuming. b) Maximum
emission wavelength change upon repeated heating–grinding cycles.
Squares: after grinding; triangles: after heating.
Figure 3. Images of the TABD derivatives upon grinding and fuming taken
at room temperature under ambient light and 365 nm UV light.
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partially move to the methyl benzoate moieties from HOMO to
LUMO. For TABDE-NPh2, the HOMO is primarily localized over
the ÀNPh2 moieties, whereas the LUMO is concentrated on the
methyl benzoate moieties. These results are consistent with
the substituent effects.
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Acknowledgements
The work reported in this paper was partially supported by the
National Basic Research Program of China (973 Program;
2013CB834704), the National Science Foundation of China
(51073026, 51061160500, 21074011, and 51328302) and the
Basic Research Foundation of the Beijing Institute of Technolo-
gy (20131942002).
Keywords: 1,3-butadiene · aggregation enhanced emission ·
chromophores · fluorescence · mechanochromic luminescence
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Received: April 16, 2014
Published online on && &&, 0000
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Communication
COMMUNICATION
Mechanochromic luminescence: Three
tetra-aryl substituted 1,3-butadiene de-
rivatives with distinct electronic push
and/or pull substituents have been pre-
pared. All three derivatives show typical
aggregation enhanced emission (AEE)
features owing to the restriction of in-
tramolecular rotations. The results dem-
onstrate that chromophores with both
large dipole moments and AEE charac-
teristics are of benefit for mechanochro-
mic applications with efficient solid-
state emissions.
&
Mechanochromic Luminescence
Y. Zhang, T. Han, S. Gu, T. Zhou, C. Zhao,
Y. Guo, X. Feng,* B. Tong, J. Bing, J. Shi,
J. Zhi, Y. Dong*
&& – &&
Mechanochromic Behavior of Aryl-
Substituted Buta-1,3-Diene Derivatives
with Aggregation Enhanced Emission
Chem. Eur. J. 2014, 20, 1 – 7
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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