Study of Red-Emission Piezochromic Materials Based on Triphenylamine

Article abstract of DOI:10.1002/cplu.201600104

Two conjugated molecules based on triphenylamine and 1,3-indandione have been synthesized by employing Knoevenagel condensation. Both materials demonstrated aggregation-induced emission behavior, and solvato- and piezochromic properties. These red lumines

Full text of DOI:10.1002/cplu.201600104

DOI: 10.1002/cplu.201600104
Full Papers
Study of Red-Emission Piezochromic Materials Based on
Triphenylamine
Chunxuan Qi, Hengchang Ma,* Hongting Fan, Zengming Yang, Haiying Cao, Qiaojuan Wei,
and Ziqiang Lei*[a]
Two conjugated molecules based on triphenylamine and 1,3-
indandione have been synthesized by employing Knoevenagel
condensation. Both materials demonstrated aggregation-in-
duced emission behavior, and solvato- and piezochromic prop-
erties. These red luminescence materials are able to respond
to Fꢀ and Iꢀ sensitively. The emission wavelength changed by
almost 100 nm in the presence of Fꢀ and Iꢀ, and could be ob-
served by the naked eye under daylight and UV light.
Introduction
Organic luminescent materials with unique and excellent opto-
electronic properties have attracted much attention as a result
of their wide applications as biological[1] or chemical probes,[2]
photoelectric materials,[3] or piezoluminescence sensors.[4] In
most cases, the emission is quenched when the fluorescent
molecules aggregate, as caused by the aggregation-caused
quenching (ACQ) effect. According to reports in the literature,
molecules in the aggregated state suffer from short-range in-
teractions, such as p–p stacking, which leads to fluorescence
quenching, and thus, restricts further applications.[5] Fortunate-
ly, in 2001, Tang’s group reported two new fluorogen species:
2,3,4,5-tetraphenylsilole (TPS) and tetraphenylethene (TPE).[6]
These molecules are weakly emissive in dilute solution, but
have a strong emission as aggregates or in the solid state. This
phenomenon is called the aggregation-induced emission (AIE)
effect. This effect is mainly ascribed to restricted intramolecular
rotation (RIR): in solution, phenyl rings in TPS or TPE can freely
rotate. However, in the aggregated state, intramolecular rota-
tions are greatly restricted, so TPS and TPE cannot accumulate
through a p–p stacking process. Thus, fluorescence emission
emerges.[7]
stituted with 1,3-indandione (IND-TPE).[4a] Because TPA is a core
with ACQ properties, most TPA-based derivatives exhibit ACQ
properties, which limits their applications.[10] However, it
should be noted that several examples of decorated TPA deriv-
atives show emission behavior that has changed from ACQ to
AIE; this provides the possibility of designing and preparing
AIE-active TPA derivatives.[11]
Although Fꢀ is an essential element for dental health and
the treatment of osteoporosis,[12] excessive ingestion of fluo-
ride may cause kidney disorders, fluorosis, urolithiasis, and
even cancer or death.[13] Therefore, it is very meaningful to de-
velop fluorescent probes that are able to show high sensitivi-
ty,[12c,14] rapid response,[12c,15] and easy to observe[12b,14b,16]
toward Fꢀ ion detection. Recently, there were three representa-
tive strategies for Fꢀ recognition: 1) fluoride-ion-induced de-
protonation through hydrogen bonding,[14a,15b,17] 2) forming Bꢀ
F compounds,[12b,17b] and 3) destroying SiꢀO or SiꢀC bonds
through desilylation.[15a,16a,18] The desilylation reaction needs ex-
cessive Fꢀ and BꢀF compounds are sensitive to oxygen.[19] Tet-
rabutylammonium fluoride (TBAF) is a common organic base
and promotes the hydrolysis reaction of olefins, which are re-
placed by electron-withdrawing substituents. At the same
time, iodide is of great interest owing to its essential role for
thyroid gland function.[20] Herein, we report two kinds of red-
emission materials, which have AIE and piezochromic proper-
ties (Scheme 1). Both of them can be used to recognize Fꢀ and
Iꢀ ions in THF, which could be easily detected by the naked
eye under visible- and UV-light illumination.
Piezochromic (mechanochromic) luminescent materials have
been explored in recent years,[8] because of great potential ap-
plications in mechanical sensors, security systems, deformation
detectors, optical memories, and some kinds of display devi-
ces.[4b,e,9] However, mechanochromic luminescent materials are
seldom discussed; this may be attributed to the fact that
a common synthetic protocol for mechanochromic compounds
is not clear and the ACQ effect. TPE and triphenylamine (TPA)
are two kinds of ideal luminescent cores. Some kinds of me-
chanochromic molecules have been reported, such as TPE sub- Results and Discussion
Solvatochromism
[a] C. Qi, Prof. H. Ma, H. Fan, Z. Yang, H. Cao, Q. Wei, Prof. Z. Lei
Chemistry Department, Northwest Normal University
967 Anning East Road, Lanzhou 730070 (P. R. China)
E-mail: mahczju@hotmail.com
It is known that TPA is usually an electron donor and IND is re-
garded as an electron acceptor. So, 2-(4-Diphenylaminobenzyli-
dene)indan-1,3-dione (IND-TPA) or 2-[5-(4-diphenylaminophe-
nyl)thiophen-2-ylmethylene]indan-1,3-dione (IND-TPAT) have
electron pull–push properties and may result in intramolecular
leizq@nwnu.edu.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cplu.201600104.
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Scheme 1. Synthetic routes to IND-TPA and IND-TPAT.
Figure 1. a) Absorption spectra of IND-TPAT in different solvents. b) PL spectra of IND-TPAT in different solvents. c) Plot of Stokes shift (Dn) of IND-TPAT in
each solvent versus Df of the respective solvent. Concentration: 20 mm. Solvents: toluene, chloroform, ethyl acetate (EA), THF, 1,4-dioxane, acetone, acetoni-
trile, dimethylformamide (DMF).
2Df
hca2
charge transfer (ICT). Thus, the absorption and emission behav-
ior of IND-TPA and IND-TPAT in solution are probably related
to the solvent polarity.[21]
2
ð1Þ
Dv ¼ va ꢀ ve ¼
ðmE ꢀ mGÞ þconst
in which Dv is the Stokes shift; va is the maximum absorbance
wavenumber; ve is the emission wavenumber; mG and mE are
the dipole moments in the ground and excited states, respec-
tively; and h, c, and a are the Planck constant, the speed of
light, and the Onsager solvent cavity radius, respectively. The
solvent polarity, Df, is calculated by using Equation (2):
To study the solvatochromic behavior of IND-TPAT and IND-
TPA, the UV/Vis absorption and fluorescence spectra are per-
formed in different polarity solvents. From Figure 1, we can
see that the UV/Vis absorption changes negligibly with sol-
vents of different polarities. However, under the same mea-
surement conditions, the IND-TPAT and IND-TPA fluorescence
spectra changed clearly from less polar toluene to more polar
DMF (Figure 1b and Figure S1 in the Supporting Information);
these results demonstrate a typical solvatochromic effect. The
photoluminescence (PL) intensity is gradually reduced with in-
creasing solvent polarity. The fluorescence quantum yield (FF)
decreases from 0.89 to 0.01% for IND-TPA and from 1.4 to
0.01% for IND-TPAT when the solvent is changed from toluene
to DMF (Tables S1 and S2 in the Supporting Information). The
large redshift, combined with the decreased value of FF, could
be related to stabilization of the excited state. This behavior is
mainly attributed to a strong ICT effect.[22]
e ꢀ 1
n2 ꢀ 1
ð2Þ
Df ¼
ꢀ
2e þ 1 2n2 þ 1
in which e and n are the static dielectric constant and the re-
fractive index of the solvent, respectively.
The relationship between Dv with Df is given in Figure 1c
and Figure S1c in the Supporting Information. Both figures
show that, with increasing solvent polarity (Df), the Stokes
shift (Dv) increases linearly. The results prove that IND-TPAT
and IND-TPA exhibit solvatochromism effects. The steeper
slope of IND-TPAT (5183.9 cmꢀ1) compared with that of IND-
TPA (1263.7 cmꢀ1) indicates that the thiophene group of IND-
The Lippert–Mataga equation [Eq. (1)] is used to estimate
the relationship between solvent polarity and the nature of
the emission:
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TPAT exhibits more remarkable solvatochromic effects than
that of IND-TPA.
Theoretical calculations
To gain a deeper insight into the photophysical properties of
IND-TPAT and IND-TPA, the electron-density distribution of the
HOMO and LUMO of IND-TPAT and IND-TPA were calculated at
the CAM-B3LYP/6-31G level (Gaussian 09). The simulation re-
sults of IND-TPAT and IND-TPA are shown in Figure 3. For both
molecules, the HOMO electron clouds are mainly focused on
the TPA core or 5-(4-diphenylaminophenyl)thiophene-2-carbal-
AIE properties
TPA is a typical ACQ core.[10] Decorating TPA can probably
change its emission behavior from ACQ to AIE,[11] so it is neces-
sary to investigate the AIE properties of IND-TPA and IND-TPAT.
Thus, the fluorescent behavior of IND-TPA and IND-TPAT is re-
corded in a mixture of MeOH and water, both of which act as
good and poor solvents for IND-TPA and IND-TPAT, respectively.
As displayed in Figure 2a and c, in a dilute solution of MeOH
(10 mm), IND-TPA and IND-TPAT show weak emissions and the
PL spectroscopic lines are nearly parallel to the abscissa (Fig-
ure 2b and d). This could be ascribed to two reasons: hydro-
gen-bond formation with protic methanol,[23] and the free rota-
tion of carbon–nitrogen single bonds between the three phe-
nyls and the nitrogen atom; these possibly eliminated the
entire excited luminogens and eventually led to fluorescence
quenching in dilute solutions in MeOH.[24] If a large amount of
water was added to the solution, the fluorescence was en-
hanced dramatically. When the water fraction (fw) reached to
70 and 30%, respectively, the solutions of IND-TPA and IND-
TPAT became more emissive. When the water fraction in-
creased to 90%, the PL intensity was almost 200- and 14-fold
stronger than that in pure MeOH. The strong fluorescence
emission can be attributed to the aggregation of fluorogenic
molecules: intramolecular rotations are restricted and hydro-
gen bonds are destroyed; this results in the AIE effect.[5–7,25]
Figure 3. Calculated energy-level diagram of compounds IND-TPA and IND-
TPAT.
Figure 2. PL spectra of IND-TPA (a) and IND-TPAT (c) in mixtures of MeOH/water with different fractions of water (fw); c=10 mm, lex =468 nm. Plot of the rela-
tive PL intensity (I/I0) of IND-TPA (b) at l=623 nm and IND-TPAT (d) at l=662 nm versus the composition of the mixture of MeOH/water (fw). I0 =PL intensity
of IND-TPA and IND-TPAT in MeOH, respectively.
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dehyde (TPAT) segment, whereas the LUMO electron clouds
are mainly concentrated on the electron-accepting IND section.
Such electron distributions reveal the nature of the ICT phe-
nomenon for IND-TPAT and IND-TPA. The energy gaps are cal-
culated as DEg =HOMO–LUMO. The DEg values of IND-TPAT
and IND-TPA are 0.108 and 0.0958 eV, respectively. As exhibited
in Figures S2 and S3 in the Supporting Information, there are
planar configurations between the thienyl ring and IND (2 and
3 in Figure S2) of IND-TPAT and between the phenyl ring and
IND (1 and 2 in Figure S3) of IND-TPA. Both dihedral angles are
08, which is mainly due to the formation of an intramolecular
hydrogen bond between the hydrogen atom of the phenyl
ring and the carbonyl group of IND. In IND-TPAT, the phenyl
and thienyl rings (1 and 2 in Figure S2 in the Supporting Infor-
mation) have a less distorted configuration owing to steric re-
pulsion between the hydrogen atom of the phenyl and thienyl
rings, so the dihedral angle is 13o. Thus, relative to IND-TPA,
IND-TPAT has greater conjugation and a smaller DEg value.
From the slope and Onsager cavity radius, dipole moment
changes, between the ground and excited states of IND-TPA
and IND-TPAT, are determined according to Equation (1) and
the optimized structures in Figure 3. Assuming that the Onsag-
er cavity radius, a, is given as a=(3Vvdw/4p)1/3, in which Vvdw is
the van der Waals volume of the solute,[26] the cavity radius is
determined to be 5.15 ꢁ for IND-TPA and 5.38 ꢁ for IND-TPAT.
Thus, the dipole moment changes are calculated to be 4.05 D
for IND-TPA and 8.75 D for IND-TPAT. The dipole moments of
the excited state are 7.67 and 11.55 D, respectively. Such high
dipole moments for IND-TPA and IND-TPAT at the excited state
must be important for solvatochromic fluorescence.
Piezochromic properties
We find that the emission colors of IND-TPAT and IND-TPA
solids change when an external pressure exists. To evaluate
the fluorescence emission performances of the powders before
and after grinding, PL spectroscopy is applied (Figure 4). The
emission wavelengths of powders as-prepared and after grind-
ing are l=629 and 677 nm, respectively, for IND-TPAT (Fig-
ure 4a). This result demonstrates that IND-TPAT possesses a typ-
ical mechanochromism property. To further study the chromic
effects of IND-TPAT, we determined the recyclability between
two emission powders (Figure 4c and d). IND-TPAT and IND-
TPA exhibit excellent recyclability after three grinding and
heating cycles. In contrast to IND-TPAT, IND-TPA shows incon-
spicuous mechano- and thermochromatic properties. The as-
prepared and ground powders are orange and light red under
UV irradiation. The PL spectra (Figure 4b) show that after
grinding the emission wavelength redshifts by only 21 nm
(from l=611 to 632 nm). Although IND-TPA exhibits good re-
peatability between grinding and heating (Figure 4d), consid-
ering its inconspicuous colors, we conclude that the IND-TPA
molecule is not a typical mechano- and thermochromism ma-
terial. Meanwhile, the results prove that TPA, 4-formyltriphenyl-
amine (TPAF), and TPAT have slight wavelength changes, only
3–4 nm redshifts occur after grinding (Figure S4 in the Sup-
Figure 4. PL spectra of powders of IND-TPAT (a) and IND-TPA (b) as-prepared and after grinding, lex =468 nm. Reversible switching of the emission wave-
lengths of IND-TPAT (c) and IND-TPA (d) after repeated grinding and heating cycles. The heating temperature was 1308C.
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porting Information). Thus, TPA, TPAF, and TPAT are not mecha-
no- and thermochromism materials.
zoic acid, trifluoroacetic acid (TFA), acetic acid, sodium ethox-
ide, sodium dodecyl benzene sulfonate (SDBS), potassium tert-
butoxide (KTB), benzylamine, TBAF, tetrabutylammonium chlo-
ride, tetrabutylammonium bromide, and TBAI. Analytes are
added to solutions of IND-TPAT and IND-TPA (2ꢂ10ꢀ5 m) in THF,
and the emissions of IND-TPAT and IND-TPA are measured 1 h
after the addition of analytes. As shown in Figure 6 and Fig-
To further understand the piezochromic phenomenon, XRD
analysis was performed on powders of IND-TPAT and IND-TPA.
As shown in Figure 5, both solids display intensified and sharp
diffraction peaks as-prepared, whereas after grinding most of
the sharp peaks become weak, which reveals that the mecha-
Figure 6. Fluorescence spectra of IND-TPAT (2ꢂ10ꢀ5 m, in THF) upon the ad-
dition of different analytes.
ure S6 in the Supporting Information, among the analytes
studied, clear fluorescence quenching is observed upon the
addition of five equivalents of TBAF and TBAI to IND-TPAT and
IND-TPA at l=625 and 589 nm, respectively. Thus, the sensing
behavior of IND-TPAT and IND-TPA to two kinds of tetrabuty-
lammonium salts in THF were further investigated.
The fluorescence spectroscopy titrations were performed in
THF at room temperature. As depicted in Figure 7 and Fig-
ure S7 in the Supporting Information, the fluorescence of IND-
TPAT and IND-TPA changed in the presence of TBAF (a and b)
and TBAI (c and d). The emission bands of IND-TPAT and IND-
TPA at lmax =625 and 589 nm decrease gradually, whereas new
emission bands at lmax =500 and 450 nm emerge and are en-
hanced gradually at the same time (Figure 7a,c and Fig-
ure S7a,c in the Supporting Information). From the results in
Figure 7b,d and Figure S7b,d in the Supporting Information,
the detection limits are calculated by the concentration of the
sharp change in fluorescence intensity multiplied by the con-
centrations of IND-TPAT and IND-TPA.[27] The following equation
was used for to calculate the detection limit: DL =CLCT (CL: con-
centrations of IND-TPAT and IND-TPA; CT: concentrations of ti-
trant at which the change is observed). Thus, IND-TPAT and
IND-TPA possess the same detection limits of 2 and 1.2 ppb for
Fꢀ and Iꢀ, respectively.
Figure 5. XRD patterns of IND-TPAT (a) and IND-TPA (b); blue lines indicate
the as-prepared powders and the red lines indicate the ground samples.
nochromism phenomenon results from the transformation be-
tween crystalline and amorphous states.[4a] Thermal annealing
is a helpful technique to reconstruct the crystalline structure.
As shown in Figure 4c and d, both materials can be recycles
many times. Thus, we infer that color changes under different
conditions are induced by packing of the molecules. The pat-
tern changes for IND-TPAT and IND-TPA were also studied by
differential scanning calorimetry (DSC). The results in Figure S5
in the Supporting Information indicate that there are endo-
thermic peaks at approximately 194 and 1668C in the DSC
curve of the initial powder. A new exothermic peak appears in
the DSC curve of the ground powder. This new peak indicates
the metastable amorphous state of the ground powder.
In addition, the colors of IND-TPAT and IND-TPA (in THF)
changed from red to green and yellow to blue (Figures S8 and
S9 in the Supporting Information), and could be detected by
the naked eye. Take the detection of TBAF as an example,
a clear relationship between PL intensity and time is recorded
in Figure 8 and Figure S10 in the Supporting Information. As
shown in Figure 8 and Figure S10 in the Supporting Informa-
tion, solutions of IND-TPAT and IND-TPA emitted at l=625 and
Responses to TBAF and TBAI
Highly selective detection of TBAF and tetrabutylammonium
iodide (TBAI) over other potentially competing species is nec-
essary. The selectivity of IND-TPAT and IND-TPA for different an-
alytes was determined by fluorescence tests. Under the same
conditions, we analyzed the fluorescence responses of IND-
TPAT and IND-TPA towards analytes including adipic acid, ben-
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Figure 7. Fluorescence spectra of IND-TPAT upon titration with solutions of TBAF (a) and TBAI (c) in THF. A plot of changes of fluorescence intensity upon the
addition of TBAF (b) and TBAI (d) at l=625 nm.
589 nm in the first stage, and as the reaction time increased,
the florescence was clearly quenched. The relative fluorescence
quantum yields of IND-TPAT and IND-TPA are 1.4 and 0.8%, re-
spectively. After the addition of TBAF (5 equiv), the relative
fluorescence quantum yields of IND-TPAT and IND-TPA are en-
hanced to 72 and 14%, respectively.
To further understand the practical applicability of IND-TPAT
and IND-TPA as TBAF and TBAI probes, interference experi-
ments were performed by fluorescence spectroscopy. The
emission intensities of IND-TPAT and IND-TPA in the presence
of five equivalents of Fꢀ and Iꢀ remained unaffected upon the
addition of five equivalents of competing molecules. (Figures 9
and 10 and Figures S11 and S12 in the Supporting Informa-
tion).
To further verify the reaction mechanism, 1H NMR titration
analysis of IND-TPAT and IND-TPA was performed in CDCl3 with
varying equivalents of TBAF (Figure 11 and Figure S13 in the
Supporting Information) and TBAI (Figures S14 and S15 in the
Supporting Information). Notably, with the addition of TBAF
(0–4 equiv) to solutions of IND-TPAT and IND-TPA, the reso-
nance corresponding to the aldehyde hydrogen emerged; this
was probably produced in the hydrolysis of IND-TPAT and IND-
TPA by TBAF[4a] (Scheme 2 and Scheme S2 in the Supporting In-
formation). Meanwhile, the resonances of aromatic protons
Figure 8. a) Time-dependent PL intensity changes to IND-TPAT in THF. b) Var-
iation of the band intensity of the emission of IND-TPAT versus reaction
time. The data are extracted from a). I0: the initial intensity of IND-TPAT in
THF; I: the PL intensity of the addition of TBAF (5 equiv) in solution at differ-
ent reaction times. Concentration of IND-TPAT: 20 mm; lex =468 nm. The so-
lution was recorded every 20 min.
continually shift upfield with the addition of fluoride ions. Con-
sequently, the results of the 1H NMR titration provide strong
evidence for the proposed mechanism depicted in Scheme 2.
For TBAI, the resonance protons continually shift upfield with
the addition of iodide ions. As previously reported,[29] the
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Figure 9. Competitive selectivity of IND-TPAT towards TBAF in the presence of other analytes (5 equiv) at l=625 nm.
Figure 10. Competitive selectivity of IND-TPAT towards TBAI in the presence of other analytes (5 equiv) at l=625 nm.
Scheme 2. The hydroxyl mechanism of IND-TPAT in THF in the presence of
TBAF.
Figure 11. 1H NMR spectroscopic changes of IND-TPAT in CDCl3 in the pres-
ence of TBAF (0–4 equiv; d/ppm).
the spin-orbit coupling. Here, the heavy-atom effect is caused
by the formation of a weak, transient, probably charge-transfer
complex between the TPA moieties in IND-TPAT (or IND-TPA)
and the iodide ions.
charge-transfer complex may be responsible for this phenom-
enon. The heavy-atom interaction between the excited state of
IND-TPAT (or IND-TPA) and iodides leads to an enhancement of
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Z. M. Yang, H. Y. Cao, Z. W. Yang, J. H. Tong, X. Q. Yao, Z. Q. Lei, ACS
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It was demonstrated that two representative red-emitting TPA-
based derivatives, IND-TPAT and IND-TPA, were synthesized.
Both derivatives demonstrated evident AIE properties, solvato-
chromism effects, and piezochromic behavior. The XRD and
cycle measurements reveal that the piezochromic property is
mainly caused by a transition from crystalline to an amorphous
phase. In addition, IND-TPAT and IND-TPA could be used as
fluorescence probes to detect Fꢀ and Iꢀ sensitively; detection
could even be performed by the naked eye. Furthermore, our
work explored a new strategy to synthesize red and near-infra-
red materials with piezochromic performances. Further re-
search into synthetic protocols and applications is underway.
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Experimental Section
Synthesis and characterization of IND-TPAT and IND-TPA
5-(4-Diphenylaminophenyl)thiophene-2-carbaldehyde (TPAT) was
synthesized by 4-bromophenyl)diphenylamine Suzuki coupling
with 5-formylthiophene-2-boronic acid. TPAF was prepared accord-
ing to the Vilsmeier–Haack reaction. IND-TPAT and IND-TPA were
synthesized from the precursor TPAT and TPAF through Knoevena-
gel condensation with IND (Scheme 1).[28] The obtained com-
pounds were characterized by H NMR, 13C NMR, and FTIR spectros-
1
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Instrumentation
1H (600 MHz) and 13C NMR (100 MHz) spectra were recorded on
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Acknowledgements
This study was supported by the National Natural Science Foun-
dation of China (nos. 21202133 and 21361023). We thank the
Key Laboratory of Eco-Environment-Related Polymer Materials
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Keywords: aggregation
piezochromism · sensors
·
conjugation
·
luminescence
·
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Manuscript received: April 25, 2016
Accepted Article published: May 25, 2016
Final Article published: && &&, 0000
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FULL PAPERS
C. Qi, H. Ma,* H. Fan, Z. Yang, H. Cao,
Q. Wei, Z. Lei*
Seeing red: Two conjugated molecules
based on triphenylamine (TPA) and 1,3-
indandione (IND) are prepared by em-
ploying Suzuki cross-coupling reaction
and Knoevenagel condensation. Both
materials demonstrate red emission, ag-
gregation-induced emission behavior,
and solvatochromic and piezochromic
properties. These red luminescence ma-
terials are able to respond to fluoride
and iodide in THF sensitively (see
&& – &&
Study of Red-Emission Piezochromic
Materials Based on Triphenylamine
figure; TPAT=5-(4-diphenylaminophe-
nyl)thiophene-2-carbaldehyde).
&
ChemPlusChem 2016, 81, 1 – 10
www.chempluschem.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!