Multi-stimuli responsive fluorescence switching: The reversible piezochromism and protonation effect of a divinylanthracene derivative

Article abstract of DOI:10.1039/c3tc31553c

A novel divinylanthracene derivative 9,10-bis((E)-2-(pyridin-4-yl)vinyl) anthracene (BP4VA) was synthesized and its two polymorphs with different crystal structures were obtained. The introduction of pyridine in BP4VA leads to multi-stimuli responsive fluorescence. An investigation of the photophysical and stimuli responsive luminescent properties of BP4VA, including the piezochromism and protonation effect, demonstrates that the piezochromic luminescence originates from changes in the molecular aggregation state. Additionally, protonation-deprotonation of the pyridine moieties in BP4VA has a significant effect on the frontier molecular orbitals, resulting in distinct green and red emissions under acid and base stimuli. This study on BP4VA provides a comprehensive insight into the mechanisms within this type of stimuli-responsive luminescent material, and suggests that BP4VA may be a potential candidate for applications in sensing, detection and display devices with remarkable color-changing properties.

Full text of DOI:10.1039/c3tc31553c

Journal of
Materials Chemistry C
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PAPER
Multi-stimuli responsive fluorescence switching: the
reversible piezochromism and protonation effect of a
divinylanthracene derivative†
Cite this: J. Mater. Chem. C, 2013, 1,
554
7
a
a
b
a
a
a
a
Yujie Dong, Jibo Zhang, Xiao Tan, Lijuan Wang, Jinlong Chen, Bao Li, Ling Ye,
a
b
a
Bin Xu,* Bo Zou and Wenjing Tian*
A novel divinylanthracene derivative 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) was synthesized
and its two polymorphs with different crystal structures were obtained. The introduction of pyridine in
BP4VA leads to multi-stimuli responsive fluorescence. An investigation of the photophysical and stimuli
responsive luminescent properties of BP4VA, including the piezochromism and protonation effect,
demonstrates that the piezochromic luminescence originates from changes in the molecular
aggregation state. Additionally, protonation–deprotonation of the pyridine moieties in BP4VA has a
significant effect on the frontier molecular orbitals, resulting in distinct green and red emissions under
acid and base stimuli. This study on BP4VA provides a comprehensive insight into the mechanisms
within this type of stimuli-responsive luminescent material, and suggests that BP4VA may be a potential
candidate for applications in sensing, detection and display devices with remarkable color-changing
properties.
Received 9th August 2013
Accepted 12th September 2013
DOI: 10.1039/c3tc31553c
www.rsc.org/MaterialsC
8
conformations or frontier molecular orbitals. Y. Wang et al.
investigated the vapochromic and piezochromic properties of a
Introduction
Stimuli-responsive luminescent materials that demonstrate a donor–acceptor structured compound and demonstrated that
switch in emission upon external stimuli are of great impor- the uorescence emission could be changed from blue to
9
tance for their potential applications in the eld of optoelec- orange using different stimuli. Z. G. Chi et al. developed a
1
tronics, such as in sensors, memory devices and switches.
Recently, the tuning and switching of organic solid-state uo-
rescence based on environmental factors have been intensively
uorescence-switching compound with aggregation-induced
10
emission properties based on tetraphenylethene. However,
reports on multi-stimuli responsive uorescent materials are
quite rare, owing to the lack of clear guidelines for the design of
studied both in the eld of fundamental research and potential
2
applications. Several organic molecules with efficient and molecules that possess all of the features of different smart
switchable solid state uorescence have been developed, which materials, such as piezochromism, vapochromism and ther-
3
4
respond to external stimuli such as light, mechanical force,
mochromism. In addition, a detailed investigation into the
5
vapour, etc. As for uorescent molecules in an aggregated state, related origins of these properties is also required.
the changes in uorescence arise not only from intramolecular
effects, such as the molecular conguration and extent of
conjugation, but also intermolecular effects such as, for
Recently, we reported a series of divinylanthracene deriva-
11
tives with fascinating properties. Due to their twisted confor-
mation and multiple supramolecular interactions, these
molecules possess high solid state uorescent quantum effi-
6
example, p–p overlap. Piezochromic luminescent materials
have been obtained through the manipulation of molecular ciencies and special molecular aggregation states. By modifying
7
packing modes by grinding and heating treatments. Thermo- their molecular structures, interesting phenomena have been
and acid-dependent luminescent materials have also been observed, such as aggregation-induced emission, self-assembly
developed based on the control of their molecular behaviour and amplied spontaneous emission. In particular,
when pyridyl groups were introduced into the molecular
a
structure, more supramolecular interactions such as N–H
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Qianjin street No. 2699, Changchun, China. E-mail: wjtian@jlu.edu.cn; xubin@jlu. hydrogen bonds were formed, resulting in different crystal
edu.cn; Fax: +86 431 85193421
polymorphs. Such polymorphs can be used to study structural
changes in molecular aggregates under mechanical force, so as
to reveal the intrinsic process of the piezochromic uorescence.
Additionally, the basicity of the pyridyl group also provides an
b
State Key Laboratory of Superhard Materials, Jilin University, Qianjin street No. 2699,
China
†
Electronic supplementary information (ESI) available: Crystal structure
renements and Fig. S1–S3 (CCDC reference numbers 943524-943525). See DOI:
0.1039/c3tc31553c
1
ideal model to see how the uorescent properties can be altered
7
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11
using the stimulus of protonation. Herein, we report a structures. Anisotropic thermal parameters were rened for all
combined study based on a novel derivative 9,10-bis((E)-2-(pyr- of the non-hydrogen atoms. The hydrogen atoms were added at
idin-4-yl)vinyl)anthracene (BP4VA), which possesses multi- the idealized positions and rened using isotropic displace-
stimuli responsive uorescent properties. Two polymorphs with ment. Crystallographic data for the structures reported in this
high uorescence efficiencies were successfully obtained. High paper are given in Table S1.†
pressure-PL experiments conrm that the different polymorphs
of BP4VA can be used to explain its piezochromic properties.
Additionally, uorescence changes based on controlling
protonation–deprotonation have been investigated, which show
that different acidic/basic atmospheres can induce a switch in
the emission from green to red.
High pressure-PL experiments
Powder and crystal samples were placed in the 200 mm diameter
holes of a T301 steel gasket, which was pre-indented to a
thickness of 50 mm. A small ruby chip was inserted into the
sample compartment for in situ pressure calibration, utilizing
the R1 ruby uorescence method. A 4 : 1 mixture of methanol
and ethanol was used as a pressure-transmitting medium
(PTM). The hydrostatic-pressure conditions around the powders
were determined by monitoring the widths and separation of
the R1 and R2 lines. Photoluminescence measurements under
high pressure were performed using a QuantaMaster 40 spec-
trometer in reection mode. The 405 nm line of a violet diode
laser with a spot size of 20 mm and a power of 100 mW was used
as the excitation source. The diamond anvil cell (DAC) con-
taining the sample was put under a Nikon uorescence micro-
scope in order to focus the laser on to the sample. Emission
spectra were obtained using a monochromator equipped with a
photomultiplier. All of the experiments were conducted at room
Experimental section
Materials
All starting materials were purchased from Acros or Aldrich
Chemical Co. and used without further purication. THF was
dried by distillation from sodium/benzophenone under
nitrogen.
Instruments
1
H NMR spectra were obtained using a Bruker AVANCE
500 MHz spectrometer with tetramethylsilane used as an
internal standard. Time of ight mass spectra were recorded
using a Kratos MALDI-TOF mass spectrometer. UV-vis absorp-
tion spectra were recorded using a Shimadzu UV-3100 spectro-
photometer. Photoluminescence spectra were collected with a
Shimadzu RF-5301PC spectrophotometer and Maya 2000Pro.
14
temperature.
Results and discussion
optical ber spectrophotometer. The crystalline state PL effi- Synthesis
12
ciencies were measured using an integrating sphere (C-701,
Labsphere Inc.), with a 405 nm Ocean Optics LLS-LED as the
excitation source. The excitation light was introduced into the
sphere through an optical ber.
BP4VA was synthesized using the procedure shown in Scheme 1
and characterized by H NMR, C NMR and mass spectroscopy
as well as elemental analysis.
1
13
Compound
compound
1
(9,10-bis(chloromethyl)anthracene) and
2
(tetraethyl anthracene-9,10-diylbis(methylene)
Preparation of the crystals and the emission spectroscopy
measurements
diphosphonate) were synthesized according to the previously
reported methods. Compound 2 (0.500 g, 1.04 mmol) was stir-
red with t-BuOK (0.468 g, 4.17 mmol) in THF (70 mL) under
nitrogen. Picolinaldehyde (0.24 mL, 2.53 mmol) in THF (70 mL)
was added to the solution, which was kept in an ice-bath, and
then the mixture was stirred for 12 h at room temperature. The
resulting precipitate was successively washed with MeOH and
Single crystal C1 was prepared by vacuum sublimation, while
single crystal C2 was prepared by slowly evaporating a mixture
of chloroform and petroleum ether (3 : 1) at room temperature
under the rigorous exclusion of light. The emission spectra of
the crystals were obtained using the following procedure: the
large-scale crystal samples were stuck on to the surface of
substrates using non-emissive silica grease, and then placed
into the optical path of the exciting light source. Quartz cells
were used to measure the solution emission spectra.
11
then ltered off to give BP4VA as a yellow powder (50% yield).
1
H NMR (500 MHz CDCl ) d 6.893–6.926 (d, J ¼ 16.5 Hz, 2H,
3
CH]CH), 7.514–7.534 (m, 4H, Ar), 7.544–7.556 (d, 4H, Ar),
8
.153–8.186 (d, J ¼ 16.5 Hz, 2H, CH]CH), 8.321–8.341 (m, 4H,
Ar), 8.697–8.709 (d, 4H, Ar); VMALDI-TOF MS: calcd for
C
C
28
28
H
H
20
N
N
2
: 384.16, found: 383.89. Elemental anal. calcd for
: C, 87.47; H, 5.24; N, 7.29%; found: C, 86.90; H, 5.49;
X-ray crystallography
20
2
Diffraction experiments were carried out using a Rigaku RAXIS
RAPID diffractometer equipped with
N, 7.21%.
a
graphite-mono-
chromator (Mo Ka radiation) and control soware, while in
RAPID AUTO mode at 293 K. Empirical absorption corrections
Crystal structures
were applied automatically. The structures were resolved with Single crystals with highly ordered molecular packing struc-
direct methods and rened by a full-matrix least-squares t of tures can provide direct and denitive evidence of the rela-
2
13
F , using SHELX v. 5.1 soware. The space groups were tionship between the molecular aggregation state and
determined from systematic absences and their correctness was luminescence properties. By either the slow diffusion of petro-
conned by the successful solution and renement of the leum ether vapour into a chloroform solution of BP4VA or
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Scheme 1 Synthesis route of BP4VA.
˚
vacuum sublimation, two single crystals (C1 and C2) suited for which have an interaction distance and angle of 2.57 A and
ꢀ
X-ray structural analysis were obtained. Through a systematic 149.7 , respectively.
analysis of the phases of the two crystals, we found that the
molecular conformations of both crystals maintained a large
torsion angle (64.5 for C1 and 67.5 for C2), while the packing
Photophysical properties
ꢀ
ꢀ
The UV-vis and PL spectra of BP4VA in THF solution and the PL
spectra of the two single crystals are shown in Fig. 2. In THF
solution, BP4VA has a main absorption peak at 410 nm and a
maximum PL peak (lmax) around 584 nm. In contrast to the
solution, the PL spectra of the two crystals demonstrate large
blue-shis of 52 nm (C1) and 32 nm (C2), with lmax values of
around 532 nm and 552 nm, respectively. Moreover, both of the
structures were completely different.
ꢀ
The unit cell of C1 is triclinic with the space group P1. As
shown in Fig. 1a, the main interactions between adjacent
molecules in the C1 crystal were found to be CH–p interac-
tions. In a single molecular column, a CH–p hydrogen bond
(
interaction 1) is formed between two molecules with an
˚
ꢀ
interaction distance of 2.79 A and an angle of 145.8 , in which
the pyridyl moiety along the long axis of one molecule acts as
the H donor and the corresponding anthrylene core of an
adjacent molecule acts as the H acceptor. For C1, the mole-
cules adopt a stacking mode with J-type aggregation along the
X axis and there is practically no overlap between the central
anthracene planes, indicating that almost no p–p interactions
are formed in C1.
crystals exhibited quite high uorescence quantum yields (FC1
:
ꢁ
0.54 and FC2: ꢁ0.33), determined through integrating sphere
measurements. According to our previous work, the inhibition
of vibrational relaxation in the aggregation state should be the
origin of the high quantum yield and blue-shi in the crystals of
11
such a molecular system.
Additionally, the two crystals show a red-shi in their uo-
rescence emission and there is a decrease in the uorescence
quantum yield from C1 to C2, which could be related to the
differences in their packing structures. It is worth noting that
the overlap of anthracene planes between adjacent molecules is
increased from C1 to C2. As described in Fig. 7a, the low-lying
intramolecular electronic transition from HOMO / LUMO in
The C2 crystals belong to the monoclinic system with the
molecules adopting
a stacking mode similar to H-type
aggregation along the Y axis in each column. The distance
between two molecules within one column in C2 is measured
˚
to be 4.02 A, and the vertical distance between two anthra-
˚
cene planes is measured to be ꢁ3.60 A, as shown in Fig. 1b.
11
BP4VA is mainly derived from its anthracene core. Therefore,
the packing mode of the anthracene planes should play a key
role in the luminescence change of the molecular aggregation
state. As mentioned above, there are almost no p–p interactions
in C1, while relatively strong p–p stacking exists in C2, which
induces a red-shi in the PL spectrum of C2 relative to that of
C1. This suggests that the PL spectrum of the aggregation state
of BP4VA can be changed by adjusting its molecular packing
Moreover, the adjacent anthracene planes overlap with each
other by around 40%. In fact, there is relatively strong p–p
stacking due to the large overlap between adjacent anthra-
15
cene planes. Each molecule interacts with four neighbour-
ing molecules via weak CH–N interactions (interaction 2),
16
structure.
Fig. 1 The aggregation state of BP4VA in the two single crystals: (a) C1 and
(
b) C2.
Fig. 2 Normalized absorption and PL spectra of BP4VA samples.
7
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Piezochromic behavior
As shown in Fig. 3, the emission spectrum of the initial BP4VA
powder (lmax z 523 nm) shows green uorescence. Once the
powder has been ground using a pestle and mortar, a signi-
cant red-shi (lmax z 555 nm) occurs and the emission band
ꢀ
broadens. Aer being heated to 170 C for about 20 min, the
samples fully recover their original luminescence. This cycle
can be repeated, allowing the powder to switch between the two
different emission colours. In its absorption spectra, the ground
powder demonstrates a weak blue-shi relative to the initial
sample, suggesting that the aggregation structure changed
during the grinding process (Fig. S1†).
In order to gain further insight into the origin of the uo-
rescence change of the BP4VA powder, powder X-ray diffraction
(PXRD) experiments were conducted (Fig. 4). The diffraction
pattern of the initial powder exhibits sharp and intense reec-
tions that are consistent with the simulated XRD pattern from
the crystal data of C1, suggesting that the initial sample adopts
the same molecular arrangement as that of the C1 polymorph
with J-type aggregation along the molecular long axis and
effectively no p–p interactions between the central anthracene
planes. Although some of the resolvable peaks of the ground
sample are consistent with those of the unground sample, they
are weaker in intensity and have broader peak shapes, indi-
cating that the initial aggregation state was changed by
grinding. This suggests that the crystal structure of BP4VA can
be readily inuenced by an external stimulus.
In order to further understand the piezochromic behavior,
the effect of applied pressure on the luminescence of BP4VA was
investigated. As shown in Fig. 5, as the applied pressure was
increased, the uorescence emission of the BP4VA powder
demonstrated a gradual red-shi. Compared to the grinding
method, the application of pressure from 0 to 6 GPa caused a
Fig. 4 The PXRD patterns of BP4VA: the ground powder, initial powder and
simulated pattern obtained from the crystal data of BP4VA–C1.
more noticeable luminescence colour change of the BP4VA
powder from green (523 nm) to red (650 nm). It is worth noting
that the spectra exhibit multiple uorescence peaks during the
process of increasing the pressure. In particular, for the crystal
sample an obvious transitional uorescence emission can be
observed, which means that it changes into a different packing
polymorph as the pressure increases (Fig. S2†). Such an obvious
change in the luminescent colour of the BP4VA powder and
crystal samples under the applied pressure could be ascribed to
changes in their molecular aggregation states under the high
11,16
external pressure.
Protonation effect
Another notable feature of the present compound is that it
shows a pronounced protonation effect. Fumigation with acid
vapour has a signicant effect upon the uorescence of the
BP4VA powder, turning it from green to red, while using a
volatile base can lead to partial recovery of the original emis-
sion. The PL spectrum of the BP4VA powder, shown in Fig. 6,
Fig. 3 (a) Photographs of the ground powder and heated powder under UV
Fig. 5 Normalized solid-state PL spectra of BP4VA powder after the application
light (365 nm). (b) Absorption and PL spectra of the initial and ground samples.
of pressure (GPa).
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exhibits a strong green emission peaking at 523 nm. Interest-
ingly, aer being fumed with hydrochloride (HCl) vapour, the
powder gives off a red emission and the peak is shied to
637 nm under 365 nm UV illumination. The fumed powder
was also treated with alkali vapour. Aer being fumed with
triethylamine (TEA), the original green light exhibited by the
powder was recovered.
The 110 nm shi in the uorescence and subsequent recovery
towards the initial state are a demonstration of the signicant

uorescence switching properties of the BP4VA powder under
protonation–deprotonation stimuli. As shown in Fig. 7, when
excess HCl was added into the THF solution, red-shis were
observed in both absorption (410 nm to 507 nm) and emission
(584 nm to 629 nm). According to the theory of frontier molecular
orbits, for a BP4VA molecule, the Highest Occupied Molecular
Orbital (HOMO) is mainly located on the divinylanthracene
group, while the Lowest Unoccupied Molecular Orbital (LUMO)
is partially distributed over the whole molecule. In contrast, we
calculated the orbitals of the protonated BP4VA molecule, which
showed that the LUMO was spread over the whole molecule
with an increased electron cloud density on pyridine. Since
the protonation of its nitrogen atom enhances the electron-
withdrawing ability of pyridine, the LUMO should be more
delocalized in the protonated molecules. Generally, the delo-
calization of the electron cloud is benecial for stabilizing the
17
molecular excited state, resulting in a decreased band gap.
Consequently, the delocalization of the LUMO stabilizes the
excited molecule at a lower band gap, leading to red-shis in the
emission and absorption of the BP4VA samples.
Considering that there is a small offset (about 8 nm) between
the spectra of the HCl fumed BP4VA powder and that of BP4VA
in THF/HCl solution, we think that the uorescence switching
of the fumed powder arises mostly from the change in the
Fig. 7 (a) The molecular structures and frontier orbital contributions calculated
from theory for BP4VA and its protonated form. (b) Normalized UV-vis absorption
and PL spectra of BP4VA in THF and in a THF solution with excess HCl.
electron cloud density of the BP4VA molecule, which inuences
its frontier molecular orbitals. Meanwhile, some new super-
molecular forces may be formed during this protonation
process, leading to a different aggregation state and a variation
in the emission.
Conclusions
We present a multi-stimuli responsive uorescence molecule,
BP4VA, which demonstrates remarkable uorescence changes
under pressure, heat, acid and base stimuli. According to the
single crystal structures and high pressure PL experiments, the
piezochromic behavior of the BP4VA samples originates from
changes in their aggregation state under high external pressure.
Photophysical and computational studies denitively conrm
the protonation process of BP4VA. The protonation–deproto-
nation of the BP4VA powder indicates that fumigation with HCl/
TEA vapour leads to a transformation of the compound’s fron-
tier molecular orbitals between its protonated and unproto-
nated forms, resulting in distinct green and red emissions. This
study on BP4VA provides a comprehensive insight into the
origins of the properties in this type of stimuli-responsive
luminescent material, and suggests that it may be a potential
candidate for applications in sensing, detection and display
devices with remarkable color-changing properties.
Fig. 6 (a) Photographs of the fumed powder under UV light (365 nm). (b) PL
spectra of the initial, HCl fumed and TEA fumed powders.
7
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This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 7554–7559 | 7559