Regulating the piezofluorochromism of 9,10-bis(butoxystyryl)anthracenes by isomerization of butyl groups

Article abstract of DOI:10.1039/c4cc10196k

Isomers of 9,10-bis(butoxystyryl)anthracene (DSA₄), including n-butyl, i-butyl and t-butyl at ortho or para positions, were designed and synthesized. All of them display an aggregation-induced emission phenomenon. Remarkably, it was found that isomerization of butyl endgroups presents significant influences on their piezofluorochromic properties. Thus, an alternative approach to design and obtain piezofluorochromic compounds is proposed here.

Full text of DOI:10.1039/c4cc10196k

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Regulating piezofluorochromism of 9,10-
bis(butoxystyryl)anthracenes by isomerization of butyl
group
Yao Xiong, a, b Xilong Yan, a, b Yawen Ma, a, b Yang Li, a, b Guohui Yin, * c and Ligong
Chen*a, b
for DSAn, varying alkoxy-chains’ linking positions from ortho, meta
to para at peripheral aryl rings has been found to greatly influence
their PFC properties.3 However, branched alkyl chains modified AIE
compounds including 9,10-bis(alkoxystyryl)anthracenes are rarely
reported. Considering that branched alkyl chains would affect
molecular backbone conformation and stacking structure, so
different PFC properties should be expected.
Isomers
including
of
9,10-bis(butoxystyryl)anthracene
(DSA4),
n-butyl,
i
-butyl and
t
-butyl at ortho or para
positions, were designed and synthesized. All of them display
aggregation-induced emission phenomenon. Remarkably, it
was found that isomerization of butyl endgroups presents
significant influences on their piezofluorochromic properties.
Thus, an alternative approach to design and obtain
Herein, five isomers (n-butyl, i-butyl, and t-butyl at ortho or para
positions) of 9,10-bis(butoxystyryl)anthracenes (DSA4, Scheme 1)
were designed and synthesized, their optical properties and PFC
performances are intensively investigated. All these DSA4 are AIE-
active, whereas they exhibit obviously different pressing-induced
spectral shifts (ΔλPFC = 3~53 nm), signifying that isomerization of
alkyl chains can be chosen as an alternative approach to alter PFC
properties of DSAn compounds. It should be noted that this strategy
is of great possibility to regulate other AIE-active compounds’
piezofluorochromism.
piezofluorochromic compounds is proposed here
.
Piezofluorochromic materials exhibiting variable colors in response
to external forces are attracting intensive interest due to their
potential applications in optical recording and fluorescence sensors.1
This scientific trend has been boosted by the finding of aggregation-
induced emission (AIE) phenomena (Tang et al, 2001).2 Generally,
AIE molecules are characterized by strongly twisted conjugation
skeletons bearing rotatable aryl moieties,3 which enable them to emit
more efficiently in the aggregated or solid state. Since then, AIE
materials have been found to be promising stimuli-responsive
materials.4-7 As the first piezofluorochromic aggregation-induced
emission (PAIE) compound was reported by Park et al. in 2010,4 a
number of novel PAIE compounds were synthesized and extensively
studied.8
To date, 9,10-bis(arylvinyl)anthracene derivatives are one of the
most popularly investigated categories among PAIE compounds
owing to their fundamental importance in understanding the
relationship between molecular packing modes and photophysical
behaviors.9 Based on current researches, it has been demonstrated
that the piezofluorochromic (PFC) behaviors of 9,10-
Scheme 1 Chemical structures of five DSA4’s isomers
The aggregation-induced emission properties of 9,10-
bis(butoxystyryl)anthracenes were firstly examined. By dissolving
the isomers in THF aqueous solutions with different water fractions,
their photoluminescence (PL) were recorded respectively. It was
found that all of them show obvious aggregation-induced emission
properties, weak emission in pure THF, whereas enhanced
fluorescence with the increase of water fractions. Taking DSA-pt4
as an example (Fig.1), weak emissions were observed until a water
fraction of up to 70% was added to the THF solution. As water
fraction continues to rise, the PL intensity begins to dramatically
increase to its peak at 124.6, which is almost 25.4 times as that in
bis(arylvinyl)anthracenes could be significantly affected by the
peripheral aryl units connected to the 9,10-anthylene core.10-13 On
one hand, controlling length of linear alkyl chains attached to the
peripheral aryl units has been reported as an effective way to tune
PFC activities of some AIE compounds, such as 9,10-
bis(alkoxystyryl)anthracenes (DSAn),10 9,10-bis[(N-
alkylphenothiazin-3-yl)vinyl]anthracenes (PT-Cn)11 and 9,10-bis[(N-
alkylcarbazol-3-yl)vinyl]anthracenes (ACZn).14 On the other hand,
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pure THF solution. This observation clearly demonstrates that DSA-
pt4 is of AIE activity. It can be ascribed to the molecule aggregation
with the increase of water fraction, in which the twisted conjugated
molecular structure will hamper intramolecular torsion motion and
inhibit the non-radiative decay, resulting in higher PL intensity.
When water fraction continues to increase to 90%, a PL intensity
reduction and a slight red-shifted emission have also been noticed.
This phenomenon was commonly observed in the determination of
AIE effects, but the reasons remain unclear.15 Similar AIE behaviors
were also observed for the other isomers of DSA4. (ESI† Fig. S1)
Fig.1 PL spectra of DSA-pt4 in different concentrations of THF
aqueous solutions
Then, piezofluorochromic behaviors of all the isomers were
investigated. As shown in Fig.2, DSA-pn4 and DSA-pi4 emit
yellow fluorescence in pristine states as prepared, and display slight
color changes in ground states, but obvious color changes from
green to yellow luminescence before and after grinding were
witnessed for DSA-pt4, DSA-on4 and DSA-oi4, which illustrated
these three isomers are sensitive to external force stimuli with
significant color changes. Moreover, the ground samples recovered
to green colors after fuming with dichloromethane vapor, and turned
yellow again by regrinding. This process is reproducible and can be
repeated several times, indicating that DSA-pt4, DSA-on4 and
DSA-oi4 exhibit excellent piezofluorochromic properties.
Fig. 3 Fluorescence spectra of DSA4 in pressed and fumed states
recorded spectra are shown in Fig.3 and the corresponding
spectroscopic data are summarized in Table 1. The pristine solids
emit with peak wavelengths of 497–519 nm, in which DSA-pt4,
DSA-on4 and DSA-oi4 present similar emission wavelengths around
500 nm, while the emissions of DSA-pn4 (512nm) and DSA-pi4
(519 nm) are more bathochromic. After pressing, DSA-pn4 and
DSA-pi4 exhibit slight wavelength changes (ΔλPFC) with only
negligible 8 nm and 3 nm compared to the emission wavelengths of
samples at original state, respectively. As for DSA-pt4, DSA-on4
and DSA-oi4, significant piezofluorochromisms are observed (DSA-
pt4: ΔλPFC = 24 nm; DSA-on4: ΔλPFC = 31 nm; DSA-oi4: ΔλPFC
=
53 nm). It is further found that pressing exhibits the same
effectiveness as grinding, also, DSA-pt4, DSA-on4 and DSA-oi4
turn out to be more piezofluorochromic. These results, well
consistent with the fluorescence color changes observed under an
UV-lamp, clearly demonstrate that isomerization of alkyl chains
have an remarkable effect on the PFC behaviors of 9,10-
bis(alkoxystyryl)anthracenes. Moreover, it’s clear that ortho-
substituted 9,10-bis(butoxystyryl)anthracenes display more
prominent piezofluorochromic properties than the para-substituted
ones. Meanwhile, the pressing-induced spectral shift of DSA-oi4
increases over 20 nm than that of DSA-on4 and almost reaches the
biggest ΔλPFC among all the reported 9,10-
bis(alkoxystyryl)anthracenes derivatives, which incline to denote the
greater effect of isomerization of alkyl chains on piezofluorochromic
properties of ortho-substituted DSAn.
Table 1 Peak emission wavelengths (λ/nm) of DSA4 under various
external stimuli
Fig. 2 Fluorescence images of pristine, ground, fumed and reground
samples of five DSA4’s isomers under a 365 nm UV lamp
λ repressed/nm
samples
DSA-pn4
DSA-pi4
DSA-pt4
DSA-on4
DSA-oi4
λ pressed/nm
520
λ fumed/nm
512
ΔλPFC/nm
521
524
523
527
556
8
522
519
3
Meanwhile, photoluminescence spectra of the five samples in
pressed, fumed and repressed states have been conducted to
quantitatively investigate their optical properties responding to
external forces. The
524
500
24
31
53
528
497
557
504
Pressing-induced spectral shift, ΔλPFC = λ pressed -λ fumed
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To further understand the mechanism of PFC behaviors displayed by DSA-pn4, molecules adopt a stacking mode with J-type aggregation
DSA4, wide-angle X-ray diffraction (WAXD) and differential along the long axis of the molecule with an inclination angle of
scanning calorimetry (DSC) were employed to examine their pristine 39.54° and there is practically no overlapping between the central
and ground samples. The WAXD curves of five DSA4 (ESI† Fig. S2) anthracene planes, denoting that almost no π–π interaction is formed
present similar sharp peaks before and after grinding, signifying that
within DSA-pn4. The tight intermolecular packing, indicated by the
both of pristine and ground DSA4 samples are all in crystalline states. relatively short vertical distance (3.476 Å) between two close
In comparison with the sharp and intense reflections showed by
pristine solids, the ground samples display low-intensity reflections
along with some peaks missing, indicating the changes in molecular
packing structures of crystals and somewhat amorphous features.14
neighbouring molecules, might account for its poor
piezofluorochromism (ΔλPFC = 8 nm). As for DSA-pt4 and DSA-oi4
crystals, they are assumed to stack with H-type aggregation in view
of their inclination angles along the long axis of the molecules
The remained crystalline features in the ground samples occurring in (54.87° for DSA-pt4 and 64.04° for DSA-oi4). The vertical
the WAXD patterns can be probably attributed to the incomplete
phase transition. The differences and variations in the crystalline
patterns implied that piezofluorochromic properties of DSA4 are
closely related to the structural changes in molecular aggregation.
DSC measurements show that DSA-pn4 and DSA-pi4 have two
endothermic peaks upon heating (ESI† Fig. S3), while the rest three
isomers only have one peak. The peaks in the low-temperature zone
for DSA-pn4 and DSA-pi4 could be ascribed to the phase transition
(TL) from crystalline to liquid crystal,3 and these TL peaks become
slightly flat and broad after grinding. The high-temperature peak,
recognized as the isotropic melt transition (TH), is found to remain
unchanged before and after grinding. It should be noted that phase
transition and its change upon grinding are irrelevant to PFC
behaviors, DSA-pt4, DSA-on4 and DSA-oi4 have no phase
transition features but exhibit significant PFC properties.
distances between two adjacent molecules are 3.637 Å and 3.546 Å
for DSA-pt4 and DSA-oi4, respectively. These loose molecular
aggregations, rendering the crystals sensitive to external pressure,
are thought to result in the significant piezofluorochromism of DSA-
pt4 and DSA-oi4.
Conclusions
Five isomers of 9,10-bis(butoxystyryl)anthracene DSA-pn4
,
DSA-pi4 DSA-pt4 DSA-on4 and DSA-oi4 were designed and
,
,
synthesized. All of them are AIE active, in which, DSA-oi4
displays the largest PFC spectral shift (ΔλPFC = 53 nm). It was
found that bulky branched-chains such as i-butyl and t-butyl
will greatly enhance piezofluorochromism of DSA4 with alkoxy
chains linked at para- and ortho-positions of phenyl rings and
this isomeric effect of alkyl chains is more influential for the
ortho-substituted DSA4. Investigation on the relationship
between their photophysical properties and the structure of
butyl substituted DSA4 demonstrated the isomeric effect of alky
endgroups on DSA4’s piezofluorochromism. Furthermore, X-
ray crystallographic analyses clearly reveal that the structures
of alkyl chains at peripheral aryl rings of 9,10-
bis(arylvinyl)anthracenes could alter molecular backbone
conformations and solid-state packing structure, inducing
different optical properties. These findings suggest that
isomerization of alkyl chains can be chosen as an alternative
way to tune PFC properties of DSAn compounds and this
strategy is of great possibility to regulate other AIE-active
compounds’ piezofluorochromism.
Single crystals with highly ordered molecular packing structures can
provide direct and definitive evidence for the relationship between
the molecular aggregation state and luminescence properties. By
slowly vaporizing petroleum ether into the saturated THF solution of
each sample at room temperature, single crystals of DSA-pn4, DSA-
pt4, and DSA-oi4 suitable for X-ray structural analysis were
obtained.
Notes and references
a
School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, P. R. China. Fax: +86-022-27406314; Tel: +86-022-
27406314; E-mail: lgchen@tju.edu.cn
b
Collaborative Innovation Centre of Chemical Science and Engineering
(Tianjin), Tianjin 300072, P.R. China
Fig. 4 Stacking modes of DSA-pn4, DSA-pt4 and DSA-oi4
molecules in crystals
c
School of Science, Tianjin University, Tianjin 300072, P. R. China
yingh21@aliyun.com
Systematic analysis on X-ray spectra (Fig. 4) revealed that all the
isomers’ conformations maintained torsion angles between
anthracene core and peripheral aryl rings (68.25° for DSA-pn4,
73.24° for DSA-pt4, and 78.63° for DSA-oi4 ), coinciding with our
speculation that the larger dihedral angle (DHA) of the molecule
presents the shorter PL wavelength (Table 1). It is easy to
understand that larger DHA makes the molecule exist in a more
twisted pattern, which could hinder the internal molecular
conjugation and lead to induce blue-shifted emission. Moreover, the
packing structures of all the crystals are different. In the case of
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