Multistimuli-Responsive Luminescence of Naphthalazine Based on Aggregation-Induced Emission

Article abstract of DOI:10.1002/open.201500016

Stimuli-responsive luminescent materials, which are dependent on changes in physical molecular packing modes, have attracted more and more interest over the past ten years. In this study, 2,2-dihydroxy-1,1-naphthalazine was synthesized and shown to exhibit different fluorescence emission in solution and solid states with characteristic aggregation-induced emission (AIE) properties. A remarkable change in the fluorescence of 2,2-dihydroxy-1,1-naphthalazine occurred upon mechanical grinding, heating, or exposure to solvents. According to the characterization by solid-state fluorescence spectroscopy, X-ray crystallography, differential scanning calorimetry, and X-ray powder diffraction, the fluorescence change could be attributed to transitions between two structurally different polymorphs. These significant properties could also give 2,2-dihydroxy-1,1-naphthalazine more potential applications as a multifunctional material.

Full text of DOI:10.1002/open.201500016

DOI: 10.1002/open.201500016
Multistimuli-Responsive Luminescence of Naphthalazine
Based on Aggregation-Induced Emission
Xiang Yao, Jia-Xi Ru, Cong Xu, Ya-Ming Liu, Wei Dou, Xiao-Liang Tang, Guo-Lin Zhang,* and
Wei-Sheng Liu*[a]
Stimuli-responsive luminescent materials, which are dependent
on changes in physical molecular packing modes, have attract-
ed more and more interest over the past ten years. In this
study, 2,2-dihydroxy-1,1-naphthalazine was synthesized and
shown to exhibit different fluorescence emission in solution
and solid states with characteristic aggregation-induced emis-
sion (AIE) properties. A remarkable change in the fluorescence
of 2,2-dihydroxy-1,1-naphthalazine occurred upon mechanical
grinding, heating, or exposure to solvents. According to the
characterization by solid-state fluorescence spectroscopy, X-ray
crystallography, differential scanning calorimetry, and X-ray
powder diffraction, the fluorescence change could be attribut-
ed to transitions between two structurally different poly-
morphs. These significant properties could also give 2,2-dihy-
droxy-1,1-naphthalazine more potential applications as a multi-
functional material.
Introduction
Stimuli-responsive luminescent materials have attracted more
and more interest in the field of luminescent organic solids.[1]
Of particular interest are organic molecules that can show
a change in fluorescence color induced by external stimuli to
the solid state.[2] These organic molecules have potential appli-
cations in luminescent switches,[3] mechanosensors,[4] security
paper, optoelectronic devices, and data storage.[5] A variety of
multistimuli-responsive organic molecules, metal complexes,
and multicomponent solids and polymers have been devel-
oped recently that can respond to temperature, metal ions,
mechanical stress, pH, and solvents.[6] Moreover, there are sev-
eral design strategies that have provided a better understand-
ing of structure–luminescence relationships.
tance and potential application.[7b,c,8] Since the fluorescence
color and intensity of these molecules are generally sensitive
to the molecular configuration and arrangements in their crys-
talline state,[7d,e,9] interconversion between crystal polymorphs
due to external stimuli such as heat or pressure can be applied
to design stimuli-responsive luminescent materials. Besides the
previously mentioned design strategies, the construction of
well-organized multicomponent materials by assembly of two
or more units has recently led to the development of other
stimuli-responsive luminescent materials.[10]
Pigment Yellow 101 (2,2-dihydroxy-1,1-naphthalazine) is
a commercially available yellow pigment exhibiting fluores-
cence in the crystalline form. It was first synthesized in 1899
and has been produced industrially for decades. This pigment
is widely used for the mass coloration of viscose, for printing
inks, and for artists’ colors.[11] Although a lot of studies about
2,2-dihydroxy-1,1-naphthalazine have been made in solid- and
solution-state fluorescence,[12] 2,2-dihydroxy-1,1-naphthalazine
has not been investigated as a stimuli-responsive luminescent
material. Herein, we synthesized 2,2-dihydroxy-1,1-naphthala-
zine and found it exhibits different fluorescence emission in so-
lution and solid states, with characteristic AIE properties. A re-
markable change in fluorescence of the dye occurred upon
grinding, heating, or exposure to solvents. Powder X-ray dif-
fraction (PXRD) and differential scanning calorimetry (DSC)
studies indicated the fluorescence change could be a result of
the transition between two structurally different polymorphs.
We also report the mechanism of multistimuli-response in the
solid state based on crystal structure analysis.
In recent years, a variety of stimuli-responsive luminescent
materials based on aggregation-induced emission (AIE) has re-
ceived attention in particular because of their excellent piezo-
chromism, thermochromism, and solvatochromism.[7] In addi-
tion, stimuli-responsive organic molecules based on have also
attracted great attention owing to their fundamental impor-
[a] X. Yao, J.-X. Ru, C. Xu, Y.-M. Liu, Dr. W. Dou, Dr. X.-L. Tang, G.-L. Zhang,
Prof. Dr. W.-S. Liu
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of
Gansu Province and State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering
Lanzhou University, Tianshui Road, Lanzhou (P. R. China)
E-mail: zhanggl@lzu.edu.cn
liuws@lzu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/open.201500016.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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Results and Discussion
AIE properties of the dye molecule in tetrahydrofuran/water
We synthesized 2,2-dihydroxy-1,1-naphthalazine as shown in
Scheme 1. We then studied AIE features of the compound in
Scheme 1. Synthesis of 2,2-dihydroxy-1,1-naphthalazine. Reagents and condi-
tions: a) 2-hydroxy-1-naphthaldehyde, EtOH, hydrazine hydrate, 608C, 2 h,
55%.
mixtures of tetrahydrofuran (THF) with different water frac-
tions. The dye in pure THF and THF/water mixtures with water
fractions lower than 60% exhibited extremely weak fluores-
cence signals, while the fluorescence at 535 nm increased dra-
matically in THF/water mixtures with water fractions from 70
to 80% (Figure 1a). We further verified this by observing a 13-
fold increase in quantum yield from THF solution (Ff =2.5%)
to THF/water with a water fraction of 80% (Ff =32.6%).
In addition, when illuminated with UV light, solutions of the
dye in 80% water exhibited strong fluorescence compared to
that for 10% water (inset in Figure 1b). We then investigated
THF/water solutions of 2,2-dihydroxy-1,1-naphthalazine by UV/
Vis spectroscopy (Figure S1 in the Supporting Information).
This fluorescence could be ascribed to a mechanism involving
free rotation around NÀN and CÀC single bonds in good sol-
vents and restricted rotation in poor solvents due to the for-
mation of molecular aggregates.
Figure 1. a) Fluorescence spectra of 2,2-dihydroxy-1,1-naphthalazine (10 mm,
lex =400 nm) in THF/water with different water percentages. b) Plot of maxi-
mum fluorescence intensity vs. %water. Inset: image of 2,2-dihydroxy-1,1-
naphthalazine in THF/water mixtures with 0% (left) and 80% water (right)
under UV illumination at 365 nm.
Piezofluorochromic and solvatochromic behavior of the dye
Next, we studied solid emission properties of 2,2-dihydroxy-
1,1-naphthalazine. While the as-prepared solid powder of 2,2-
dihydroxy-1,1-naphthalazine (1-Form) exhibited strong yellow
luminescence (Ff =31.5%,) under UV irradiation, after we
ground it gently using a spatula or mortar and pestle, it was
converted to a green luminescent powder (2-Form, Ff =
20.2%). This change occurred only at the pressed area (Fig-
ure 2b). Interestingly, this luminescence change from yellow to
green could also be induced upon the addition of some sol-
vents (e.g. THF, 1,4-dioxane) (Figure 2d). The normalized emis-
sion spectra of these two forms of 2,2-dihydroxy-1,1-naphtha-
lazine are shown in Figure 2a. We observed a hypsochromic
shift of 13 nm in the emission spectra from the 1-Form powder
(lmax =543 nm) to 2-Form powder (lmax =530 nm). In addition,
upon volatilization of the THF, we obtained dye crystals (3-
Form), which also displayed a remarkable change from yellow
to green luminescence under UV upon grinding using a mortar
& pestle (Figure 2c). In the emission spectra, we observed
the Supporting Information, we assign the International Com-
mission on Illumination (CIE 1931) colors for the samples ac-
cording to the fluorescence color change before and after
grinding. 2,2-Dihydroxy-1,1-naphthalazine at various states and
conditions was also investigated by UV/Vis spectrophotometry
(Figure S3 in the Supporting Information).
Interestingly, while the maximum emission of the 3-Form
was a single peak at 566 nm, a second emission peak was de-
tected at 530 nm after it was ground. This remarkable fluores-
cence change of the 3-Form powder due to characteristic pie-
zochromic effects is more prominent compared to 1-Form.
Thermochromic behavior of the dye
Aside from affecting the fluorescence of 2,2-dihydroxy-1,1-
naphthalazine by grinding and changing the solvent, tunable
solid-state emission could also be realized through heat stimu-
lus which could affect the mode of molecular packing. We
found the transformation from yellow to green luminescence
could be realized when the 3-Form was heated to 2858C to
a hypsochromic shift of 36 nm from the 3-Form powder (lmax
=
566 nm) to the 2-Form powder (lmax =530 nm). In Figure S2 in
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Figure 3. Differential scanning calorimetry curves of the 1-Form, 2-Form, and
3-Form of 2,2-dihydroxy-1,1-naphthalazine.
Powder X-ray diffraction analysis of the dye
To obtain insights into the relationship between 2,2-dihydroxy-
1,1-naphthalazine structure and its tunable fluorescence prop-
erties, we performed powder X-ray diffraction (PXRD) experi-
ments. The 1-Form and 3-Form clearly exhibited sharp and in-
tense reflections in the their PXRD profiles, which are indicative
of microcrystalline structures. However, their PXRD profiles ex-
hibited different diffraction peaks (Figure 4). More importantly,
the 2-Form also existed as a crystalline rather than an amor-
phous powder, which did not display reflection peaks similar
to that of the 1-Form and 3-Form. At the same time, the THF-
treated 1-Form powder had the same reflection peaks as the
ground 3-Form powder. The ground 1-Form powder displayed
different reflection peaks compared to the ground 3-Form
powder because surplus reflection peaks of the 1-Form still ex-
isted. Thus, 2,2-dihydroxy-1,1-naphthalazine has two different
polymorphs, namely the 3-Form and the 2-Form. The 1-Form is
a combination of the other two. Moreover, fluorescence spec-
troscopy provided evidence for only two forms (lmax =530 nm
and lmax =566 nm). The tunable fluorescence properties were
connected to different molecular arrangements in two poly-
morphs.
Figure 2. a) The normalized fluorescence spectra of 2,2-dihydroxy-1,1-naph-
thalazine in various states (Forms 1-3) and conditions; lex =360 nm.
b) Photograph of the 1-Form cast on a filter paper before (top) and after
(bottom) writing “LZU” with a metal spatula under UV light (lex =365 nm).
c) Photograph of the unprocessed 3-Form (top) and ground sample
(bottom). d) Photograph of the 1-Form before (top) and after THF treatment
(bottom). e) Photograph of the 3-Form before (top) and after annealing at
2858C (bottom) under UV irradiation (lex =365 nm).
then undergo a slow annealing process. This color change is
shown in Figure 2e. Furthermore, the 2-Form could be re-
stored to its initial state (1-Form) upon heating until it melts
and then cooling quickly. Normalized emission spectra also
verify that the annealing of the 2-Form affords the 1-Form (Fig-
ure 2a). Thus, the transformation between the 1-Form and 2-
Form is reversible.
To further understand the switching between the three
states of 2,2-dihydroxy-1,1-naphthalazine upon thermal treat-
ment, the three forms were then analyzed by differential scan-
ning calorimetry (DSC) (Figure 3). The 3-Form showed a big en-
dothermic peak at 2858C before the melting point at 3208C,
while for the 1-Form and 2-Form one peak appeared at the
melting point of 3208C. This result implied that the endother-
mic peak of the 3-Form that emerged at 2858C was supposed
to be a phase transition point, and annealing at this tempera-
ture might induce molecular reorganization in the polymorph
2-Form as well.
X-ray crystallographic analysis
PXRD experiments revealed two existing polymorphs of 2,2-di-
hydroxy-1,1-naphthalazine, but did not explain the molecule
stacking structure in detail. We therefore carried out X-ray crys-
tallographic analysis. The resolved structures and crystal data
are shown in Figure 5 and Figure S5 in the Supporting Infor-
mation. Firstly, we observed intramolecular hydrogen bonds,
which provides a structural basis for high-efficiency AIE proper-
ties. Secondly, the molecular packing of 2,2-dihydroxy-1,1-
naphthalazine consists certainly of planar molecular sheets
driven by the p–p intermolecular interactions which can be
used to tune solid-state molecular arrangements. We have
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(3-Form: 35%, 1-Form: 32%, 2-
Form: 20%), which also suggests
that p–p stacking interaction is
enhanced from 3-Form to 1-
Form to 2-Form. Therefore, the
different stacking modes in the
two polymorphs reasonably
result in different molecular ag-
gregation states and thus lead
to different fluorescence colors.
Conclusions
We have synthesized 2,2-dihy-
droxy-1,1-naphthalazine
and
demonstrated its aggregation-in-
duced emission (AIE) properties.
Its fluorescence colors can be
switched by grinding, heating,
or exposure to solvents (THF/1,4-
dioxane). Based on powder X-ray
diffraction (PXRD) and differen-
tial scanning calorimetry (DSC)
Figure 4. a) Simulated XRD patterns calculated from the crystallographic data of the 3-Form. b) Measured XRD
pattern of as-prepared 1-Form solid powder; c) Measured XRD pattern of the 1-Form powder after grinding;
d) Measured XRD pattern of the 1-Form powder after treatment with THF; e) Measured XRD pattern of the 3-
Form powder after grinding.
experiments, the fluorescence change could be a result of
a transition between two structurally different polymorphs. We
have also reported the mechanism of multistimuli response in
the solid states based on crystal structure analysis. These sig-
nificant properties could lead to more potential applications
for 2,2-dihydroxy-1,1-naphthalazine as a multifunctional materi-
al.
Figure 5. Crystal structure of the 3-Form. a) Side view and illustration of
pitch angle and interlayer distance. b) Front view and illustration of roll
angle and interlayer distance.
Experimental Section
Materials and instruments: 2-hydroxy-1-naphthaldehyde was pur-
chased from Gracia Chengdu Chemical Technology Co., Ltd.
(Chengdu, China). All other reagents and solvents were purchased
from Tianjin Guangfu Chemical Technology Co., Ltd. (Tianjin, China)
seen that the molecular sheets in the 3-Form crystal were ar-
ranged in slip stacks along the short molecular axis with
a pitch angle of 35.68, whereas that along the long molecular
axis is 37.98, as shown in Figure 5. The interlayer distance be-
tween the adjacent molecular sheets in the 3-Form crystal is
2.873 ꢁ.
1
and used without further purification. H NMR and 13C NMR spectra
were recorded on
a JEOL-ECS-400MHz spectrometer (Tokyo,
Japan). Single-crystal XRD measurements were carried out on
a Bruker APEX-II CCD diffractometer (Billerica, USA) operating at 50
KV and 30 mA using Mo Ka radiation (l=0.71073 ꢁ). PXRD patterns
were determined with a Rigaku-Dmax 2400 diffractometer (Tokyo,
Japan) using Cu Ka radiation over the 2q range of 10–708. Steady-
state luminescence spectra were measured on an Edinburgh Instru-
ments FSL920 fluorescence spectrometer (Livingston, UK). Quan-
tum yields of the solid-state samples were determined by an abso-
lute method using an integrating sphere 31 (150 mm diameter,
BaSO4 coating) on an Edinburgh Instrument FLS920. TGA and DSC
experiments were performed on TGA/DSC 1 Mettler-Toledo thermal
analyzer (Greifensee, Switzerland) up to 4008C at a heating rate of
108C minÀ1 under an N2 atmosphere.
Mechanism of multistimuli response in the solid states
The packing of 2,2-dihydroxy-1,1-naphthalazine consists of
planar molecular sheets driven by p–p intermolecular interac-
tions. This kind of molecule tends to form J-type aggregation,
H-type aggregation, or a dimer. For 2,2-dihydroxy-1,1-naphtha-
lazine, different slip-stack features form two possible poly-
morphs. H-type and J-type aggregations correspond to q>
54.78 and q<54.78, respectively. Because X-ray crystallographic
analysis indicated that the molecular sheets in the 3-Form crys-
tal are arranged in slip stacks along the short molecular axis
with a pitch angle of 35.68, the 3-Form has J-type aggregation,
and the other polymorph has H-type aggregation. Tunable
fluorescence properties can be affected by the different slip-
stack features of the two polymorphs. Moreover, the three
crystals showed a decrease in the fluorescence quantum yield
2,2-Dihydroxy-1,1-naphthalazine:
2-hydroxy-1-naphthaldehyde
(1.72 g, 10 mmol) was dissolved in absolute EtOH (50 mL), followed
by addition of 80% hydrazine hydrate (0.31 mL, 5 mmol), and the
mixture was heated to 608C for 2 h. Precipitates were filtrated and
washed with absolute EtOH thrice then dried. Pure 2,2-dihydroxy-
1,1-naphthalazine was obtained as a yellow solid powder (1.8 g,
1
55%); H NMR (400 MHz, [D6]DMSO): d=12.84 (s, 2H), 9.96 (s, 2H),
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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (grants 91122007, 21371083, and 21001059) and
the Specialized Research Fund for the Doctoral Program of
Higher Education (grant no. 20110211130002).
Keywords: aggregation-induced emission
luminescent materials · multistimuli-responsive materials ·
· fluorescence ·
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Received: January 18, 2015
Published online on && &&, 0000
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FULL PAPERS
X. Yao, J.-X. Ru, C. Xu, Y.-M. Liu, W. Dou,
X.-L. Tang, G.-L. Zhang,* W.-S. Liu*
Grind and make it hot! A remarkable
change in the fluorescence emission of
2,2-dihydroxy-1,1-naphthalazine occurs
upon mechanical grinding, heating, and
exposure to solvents. Its characteristic
aggregation-induced emission (AIE)
properties are reported herein. The fluo-
rescence change could be a result of
a transition between two structurally
different polymorphs. This compound
could have applications as a multistimu-
li-responsive luminescent material.
&& – &&
Multistimuli-Responsive Luminescence
of Naphthalazine Based on
Aggregation-Induced Emission
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 2015, 00, 1 – 6
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These are not the final page numbers!