Crystallization-induced emission of styrylbenzoxazole derivate with response to proton

Article abstract of DOI:10.1016/j.dyepig.2014.07.026

A benzoxazole derivative called BVDA was synthesized. BVDA exhibited an AIE effect although there was H-aggregated dimer in the crystal. According to the spectral results and crystal structure, the presence of excimer and the restriction of intramolecular

Full text of DOI:10.1016/j.dyepig.2014.07.026

Dyes and Pigments 112 (2015) 255e261
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Crystallization-induced emission of styrylbenzoxazole derivate
with response to proton
,
Pengchong Xue a *, Boqi Yao a, Jiabao Sun a, Zhenqi Zhang a, Kechang Li b, Baijun Liu b,
,
*
Ran Lu a
a State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 2519# Jiefang Road, Changchun, PR China
b College of Chemistry, Jilin University, Changchun, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A benzoxazole derivative called BVDA was synthesized. BVDA exhibited an AIE effect although there was
H-aggregated dimer in the crystal. According to the spectral results and crystal structure, the presence of
excimer and the restriction of intramolecular rotation are responsible for the AIE phenomenon. More-
over, BVDA could respond to proton in solution and to volatile acid vapours in film. BVDA exhibited three
colours because its binding sites to proton have different basicities. These binding sites can then be
protonated in turn. These results indicate that AIE fluorophores with H-aggregated dimer is possible.
Furthermore, these fluorophores can exhibit stimuli-responsive characteristics through the introduction
of functional moieties.
Received 17 June 2014
Received in revised form
16 July 2014
Accepted 21 July 2014
Available online 31 July 2014
Keywords:
Enhanced emission
Styrylbenzoxazole
Response
© 2014 Elsevier Ltd. All rights reserved.
Protonation
Crystal
Excimer
1. Introduction
distryrylanthracene, and cyano-diphenylethene derivatives [5]. In
solid or crystal state, intramolecular rotation is suppressed, and the
Highly efficient emission of organic molecules in solid state is
essential in optoelectronic devices, such as organic light-emitting
diodes (OLEDs), organic light-emitting field-effect transistors,
organic solid-state lasers, and organic fluorescent sensors [1]. How-
ever, achieving strong emission for organic molecules in solid or
crystal state is extremely difficult. One cause of the quenching process
is mechanistically associated with “aggregation-caused quenching”
because of the quick de-excitation of excited fluorophores through
internal conversion, intersystem crossing, intramolecular charge
transfer, intermolecular electron transfer, excimer or exciplex for-
mation, or isomerization. In fact, there are some reports to disclose
organic fluorophores with strong emission in solid [2]. More impor-
tantly, the group of Tang discovered a silole derivative in 2001,
exhibiting dramatically enhanced emission in solid state, and named
this phenomenon aggregation-induced emission (AIE) or
aggregation-induced enhanced emission [3]. Many AIE molecules
have been developed and applied in OLEDs, piezochromism, fluo-
rescent sensors and probes, bioimaging, and fluorescent switch [4].
Generally, AIE molecules have twisted configurations and freely
rotated single bonds, such as silole, tetraphenylethene, 9,10-
twisted configuration leads to loose stacking, which promotes strong
emission relative to the solutions. The formation of J-aggregates can
also activate radiative transition and cause fluorophores to exhibit AIE
properties [6]. However, fluorescent H-aggregates that exhibit AIE
behaviour are rarely reported, although some emissive dyes in H-
aggregate have been observed [7].
As we know, BVDA is one fluorescent molecule and have been
applied in fluorescence probe for cell, solvent polarity, and PH [8].
In the present study, we found that BVDA exhibited weak emission
in solutions but emitted strong fluorescence in crystal state. Spec-
tral result and single-crystal structure suggest that the presence of
anti-parallel H-aggregated dimer should be responsible for emis-
sion enhancement. Moreover, BVDA exhibited dual-mode response
to proton and to volatile acid vapour in the solution and crystal
state, respectively.
2. Experimental section
2.1. General information
All the raw materials were used without further purification. All
the solvents as analytical reagent were purchased from Beijing
Chemical Works (Beijing, China), and were used without further
* Corresponding authors.
E-mail addresses: xuepengchong@jlu.edu.cn (P. Xue), luran@mail.jlu.edu.cn
(R. Lu).
http://dx.doi.org/10.1016/j.dyepig.2014.07.026
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

256
P. Xue et al. / Dyes and Pigments 112 (2015) 255e261
was dropwise added into above suspension and stirred for 10 min
at 0 ꢀC. Then, a THF solution of 4-(dimethylamino)benzaldehyde
(1.47 g, 9.8 mmol) was dropped slowly into the above solution at
0
ꢀC. After stirred for 2 h, the mixture was poured into water
Scheme 1. Synthesis route of BDVA.
(200 mL) and yellow solid was collected by filtration. The crude
product was purified by a silica gel column using CH2Cl2 as the
eluent. Yellow platy crystal was obtained in a yield of 81% (2.1 g).
mp: 181e182 ꢀC. FT-IR: 3091, 3064, 3054, 3010, 2982, 2982, 2894,
2816, 1635, 1603, 1555, 1537, 1455, 808, and 746 cme1. Element
analysis (%): calculated for C17H16N2O: C, 77.25; H, 6.10; N, 10.60;
purification. Water used throughout all experiments was purified
with the Millipore system. The UVevis absorption spectra were ob-
tained using a Mapada UV-1800pc spectrophotometer. Photo-
luminescence measurements were taken on
a Cary Eclipse
Fluorescence Spectrophotometer. The fluorescence quantum yields
ofBVDA in solvents were measuredbycomparingto a standard (9,10-
diphenyl anthracene in benzene, FF ¼ 0.85). The excitation wave-
length was 375 nm. The absolute fluorescence quantum yield of
BVDA crystal was measured on an Edinburgh FLS920 steady state
fluorimeter using an integrating sphere. Fluorescence decay experi-
mentwasmeasuredonanEdinburghFLS920steadystatefluorimeter
equipped with an nF900 ns flash lamp. Mass spectra were obtained
with AXIMA CFR MALDI-TOF (Compact) mass spectrometers. C, H,
and Nelementalanalyses were performed with a PerkineElmer 240C
elemental analyzer. Single crystal was obtained in the mixture of
CH2Cl2 and n-hexane by slow solvent diffusion method. The molec-
ular configuration in crystal was used to obtain frontier orbitals of
BVDA by density functional theory (DFT) calculations at B3LYP/6-31G
level with the Gaussian 09W program package [9].
Found: C, 77.20; H, 6.30; N, 10.68. 1H NMR (400 MHz, CDCl3)
d 7.76
(t, J ¼ 16.2 Hz, 1H), 7.72e7.67 (m, 1H), 7.56e7.49 (m, 3H), 7.36e7.30
(m, 2H), 6.88 (d, J ¼ 16.2 Hz,1H), 6.76 (d, J ¼ 8.6 Hz, 2H), 3.06 (s, 6H).
MALDI-TOF MS: m/z: calcd for C17H16N2O: 264.1; found: 265.1
(M þ H)þ.
3. Results and discussion
BVDA as yellow platy crystal was synthesized through a one-
step reaction (Scheme 1) with a yield of 81%. BVDA easily dis-
solved in CH2Cl2, CHCl3, benzene, toluene, THF, DMF, and so on, but
exhibited low solubility in cyclohexane and hexane. Moreover,
BVDA solutions have different absorption and emission spectra
(Table S1). The maximal absorption peak was observed at 378 nm in
cyclohexane, a red-shifted peak (389 nm) for toluene solution
appeared, and the DMF solution possessed an absorption band
Single crystal of BVDA was selected for X-ray diffraction analysis
on in
a
Rigaku RAXIS-RAPID diffractometer using graphite-
radiation (
¼ 0.71073 Å). The crystal
monochromated Mo-K
a
l
(lab ¼ 394 nm) with the lowest energy (Fig. 1a). A structured
was kept at room temperature during data collection. The struc-
tures were solved by the direct methods and refined on F2 by full-
matrix least-square using the SHELXTL-97 program [10]. The C, N, O
and H atoms were easily placed from the subsequent Fourier-
difference maps and refined anisotropically. CCDC 994905 con-
tains the supplementary crystallographic data for this paper.
emission band with a maximum absorption peak at 434 nm and a
shoulder peak at 456 nm was observed in cyclohexane. With an
increase in solvent polarity, the emission wavelength of the solu-
tions shifted to the low energy region, and the emission bands
became structureless. A red shift in the emission wavelength in-
dicates the excited state of BVDA with stronger polarity than that in
ground state [11], and absorption and emission bands in different
solvents could be ascribed to intramolecular charge transfer (ICT)
transition. The quantum-chemical calculation confirmed the ICT
transition. Fig. 1c shows that the dimethylaniline group has a larger
density than benzoxazole in the highest occupied frontier molec-
ular orbital (HOMO). As the electron density of the lowest unoc-
cupied molecular orbital (LUMO) of the dimethylaniline unit
decreases, the benzoxazole and vinyl moieties achieve large
2.2. Synthesis of BVDA
(E)-4-(2-(benzo[d]oxazol-2-yl)vinyl)-N,N-dimethylaniline
(BVDA)
t-BuOK (2.2 g, 19.6 mmol) was added into dry THF (20 mL) and
stirred for 10 min at 0 ꢀC. 2-methylbenzoxazole (0.3 mL,10.8 mmol)
Fig. 1. (a) Absorption and (b) fluorescence spectra of BVDA in different solvents, and (c) frontier orbitals of BVDA.

P. Xue et al. / Dyes and Pigments 112 (2015) 255e261
257
electron density. The stimulated absorption spectrum of BVDA
exhibited a strong absorption band at ca. 375 nm, which attributed
to the HOMO / LUMO transition (Table S2) [12]. Light excitation
clearly induced electron transfer from the donor unit to the
acceptor. This result further proves that absorption in the long
wavelength region and emission are ascribed to ICT transition.
Becasue BVDA consists of two binding sites to proton, the
response of BVDA to proton was expected. When TFA was gradually
added to the CHCl3 solution of BVDA, the solution turned orange.
The further addition of TFA bleached the orange solution (Fig. 2c).
The change in absorption spectra also supports this phenomenon.
Fig. 2a shows that the THF solution possesses one absorption peak
at 390 nm and that the absorption edge ranges from 400 nm to
450 nm, which causes the solution to take on yellowish. When TFA
was added, this absorption peak gradually decreased, and a new
peak at 475 nm emerged and increased along with TFA concen-
tration (Fig. S3). After the addition of 36 equiv TFA, the absorption
peak at 475 nm reached the maximum, at which point the solution
turned orange. Further addition of TFA induced a decrease of the
absorption peak at 475 nm. With 500 equiv TFA, the absorbance in
the visible region was so weak that the solution was almost
colourless.
The 1H NMR spectra before and after the addition of TFA were
measured and compared to verify the cause of the colour trans-
formation (Fig. 3). We found that the addition of TFA caused the
signals to display a low field shift for all protons, which is indicative
of binding to proton. Considering that the N atom of benzoxazole
has stronger basicity than dimethylaniline moiety, proton first
bound to the benzoxazole group to form BVDAH, which enhanced
the electron withdrawing ability of benzoxazole moiety and caused
the red shift in absorption spectrum [13]. When dimethylaniline
started protonation, the sp2 hybridized N of the dimetylamino unit
transformed into sp3 hybridized one and BVDA2H formed, which
resulted in the disappearance of ICT absorption and a hypsochromic
absorption band. Quantum chemical calculation supports our hy-
pothesis (Fig. S4, Fig S5 and Table S2). The absorption peak of BVDA
was calculated at 375 nm, and that of BVDAH appeared at 432 nm.
After bound two protons, the absorption band located 400 and
380 nm.
The fluorescence of BVDA in CHCl3 solution can also respond to
proton. The emission band around 460 nm decreased upon the
addition of TFA, and one new emission band emerged with a
maximum absorption peak of 525 nm (Fig. 2b). The emission colour
changed from blue to green. The emission band at 525 nm reached
its maximum upon the addition of 6 equiv TFA. Further addition of
TFA led to a continuous decrease of this emission band. With excess
TFA, the fluorescence colour shifted to blue again, and the emission
intensity weakened.
Fig. 2. (a) Absorption and (b) emission spectral change in BVDA in CHCl3 (10ꢁ4 M)
from 0 equiv to 500 equiv. Excitation wavelength is 375 nm. The red line is the ab-
sorption spectrum with 500 equiv TFA. Photos of BVDA solution under (c) natural light
and (c) 365 nm light.
Fig. 3. 1H NMR spectra of BVDA in CDCl3 upon addition of TFA.

258
P. Xue et al. / Dyes and Pigments 112 (2015) 255e261
yield
0.009 (Table S1). On the other hand, BVDA crystal with yellow green
emission (lem ¼ 535 nm, Fig. 4) had a value of 0.67. This result
F of 0.034, whereas BVDA in toluene exhibited a low F of
F
suggests that BVDA is AIE active. [14] The mixed solvent system
comprising THF and water was selected to confirm the AIE-active
BVDA [15]. Fig. 5 shows the absorption and emission spectra of
BVDA in mixed solvents. When water content was less than 70%,
the red-shifted absorption spectra emerged as a result of an in-
crease in solvent polarity. When water content increased to 80%,
absorbance started to decrease and undergo blue shifting, which
indicated that the molecule started to form aggregates [16]. How-
ever, emission intensity was independent of solvent even water
content reached 90%. Moreover, it was found that the yellow plate-
like microcrystal with strong emission formed when the suspen-
sion contained 90% water was aged 10 h. This result indicates that
the restriction of intramolecular rotation (RIR) and the ordered
arrangement are responsible for its strong yellow green fluores-
cence. So, BVDA should be a crystallization-induced emission flu-
orophore [17].
Fig. 4. Fluorescence spectra of BVDA in toluene and crystal state. Insets are photos
The single-crystal structure of BVDA was obtained to under-
stand the mechanism of the AIE phenomenon and establish
structureeproperty relationships. The single crystal of BVDA be-
longs to orthorhombic space group P bca with eight molecules per
unit cell (Z ¼ 8, see Table S3 for the crystallographic data). In crystal,
two molecules first formed anti-parallel dimer with a distance of
under 365 nm light.
The BVDA solutions were found to emit weak fluorescence, but
the yellow as-synthesized platy crystal of BVDA exhibited
extremely strong yellow green fluorescence under 365 nm light
(Fig. 4). BVDA in cyclohexane exhibited a fluorescence quantum
3.44 Å (Fig. 6), which implies strong
many dimers arranged into a herringbone structure (Fig. S6). In
addition, one BVDA molecule comprises four weak CHe and
pep interaction [18]. Then,
p
Fig. 6. Crystal packing of BVDA with different intermolecular interactions: (a) CHe
p
Fig. 5. (a) UVevis absorption and (b) fluorescence emission spectra (lex ¼ 375 nm) of
BVDA (1.0 ꢂ 10ꢁ5 M) in THFewater with different amounts of water (% volume).
and CHeN hydrogen bonds; (b) pep interaction for dimer. Inset is single crystal under
natural and 365 nm UV light.

P. Xue et al. / Dyes and Pigments 112 (2015) 255e261
259
emission of BVDA crystal is originated from the excimer induced by
light irradiation [21]. The result of time-resolved fluorescence
spectra also supports our hypothesis. In toluene, the fluorescence
lifetime (t
) is 0.59 ns (Fig. S8), and a radiative rate of 0.015 nsꢁ1 can
be extracted. On the other hand, yellow crystal has a very long
t of
63.2 ns, and a relative slow radiative rate (0.011 nsꢁ1). Slow deac-
tivation of excited state in crystal suggests H-aggregate, and long
lifetime confirms the red-shifted emission of crystal indeed origi-
nates from excimer [22]. Thus, the large
F of yellow crystal should
be attributed to RIR and to the presence of excimer.
The above results indicate that BVDA solutions can respond to
proton in terms of colour and fluorescence. The response of the
BVDA crystal with strong emission to the volatile acid was ex-
pected [23]. The colour of the crystal film was strongly dependent
on the concentration of TFA vapour. The crystal film was yellow in
the absence of TFA and turned red when the TFA concentration
was 228 ppm. The disappearance of the absorption peak at 370 nm
and the emergence of a new absorption band at around 510 nm
also explained the red colour of the film (Fig. 7a). Further
increasing TFA concentration weakened the absorbance in the
visible region. When TFA concentration reached 2736 ppm, the red
film became colourless. The change in the fluorescence spectra
was similar to that in the absorption spectra (Fig. 7b). The yellow
green fluorescence turned to red and gradually decreased in
emission intensity. Emission was completely quenched upon
exposure of the film to TFA with high concentration. We also found
that the colourless film turned red and not yellow after exposing
to air. This result suggests that proton escaped from N of the
dimethylaniline unit and that benzoxazole moiety continued to
undergo protonation. A red film could rapidly revert to its yellow
colour upon exposure to NH3 vapour (Fig. S9). Thus, our emissive
crystal film reversibly responds to volatile acid vapour in three
colours. The test paper, prepared by soaking filter paper in CHCl3
solution of BVDA and then air-drying, also possessed the similar
responsive behaviour to that of the crystal film (Fig. 8). Yellow test
paper changed into red one in TFA vapour with low concentration,
and turned into colourless one with high concentration TFA
vapour. Red and colourless papers could restore original yellow by
exposing their to NH3 vapour, so such test paper was used
repeatedly. Therefore, our crystal film and test film have superi-
ority over common PH indicators because they can be used to
detect acid vapour and be able to recycle.
Fig. 7. Absorption (a) and fluorescence (b) spectra of BVDA film upon exposure to TFA.
lex ¼ 375 nm.
CHeN hydrogen bonds with adjacent four molecules. This structure
suppresses the free rotation of a single bond. The dihedral angles
are small, thus suggesting good conjugation in crystal. The optimal
structure of TD/DFT-B3LYP/6-31G reveals that molecule in vacuum
adopts a planar conformation (Fig. S5). Thus, planarization does not
cause the strong emission of BVDA in crystal state, and cis-trans
isomerization and intramolecular free rotation of single bond may
be reason of weak emission for BVDA in solutions [19]. Fig. S7 in-
dicates that the crystal exhibits a blue-shifted absorption band
relative to that of the solution. Moreover, a large slip angle of 55.8ꢀ
between two molecules in dimer was observed. These results
suggest that the dimer is a face-to-face H-aggregate [20]. Consid-
ering broad emissive band and large Stokes shift, the yellow-green
4. Conclusions
In summary, we discovered that a benzoxazole derivative called
BVDA exhibited an AIE effect although H-aggregated dimer in the
crystal existed. According to the spectral results and quantum-
chemical calculation, the presence of excimer and RIR are respon-
sible for the AIE phenomenon. Moreover, BVDA can respond to
proton in solution and to volatile acid vapour in film. BVDA exhibits
three colours because its binding sites to proton have different
basicities. These results indicate that fluorophores with H-
Fig. 8. Photos of filter paper consisting of BDVA in response to TFA vapour.

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P. Xue et al. / Dyes and Pigments 112 (2015) 255e261
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