Two stimulus-responsive carbazole-substituted D–π–A pyrone compounds exhibiting mechanochromism and solvatochromism

Article abstract of DOI:10.1007/s11164-019-03742-2

Two D–A compounds were designed and synthesized from 2,6-dimethyl-4-pyrone and carbazole compounds. Both of them exhibited unique fluorescence properties in the aggregated state. Red shifts of the spectrum of 102?nm and 130?nm were observed for BCSP and C

Full text of DOI:10.1007/s11164-019-03742-2

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03742-2
Two stimulus‑responsive carbazole‑substituted D–π–A
pyrone compounds exhibiting mechanochromism
and solvatochromism
Dongqi Liu1,2 · Yuqi Cao1,2 · Xilong Yan1,2,3 · Bowei Wang1,2,3
Received: 9 October 2018 / Accepted: 12 January 2019
©
Springer Nature B.V. 2019
Abstract
Two D–A compounds were designed and synthesized from 2,6-dimethyl-4-pyrone
and carbazole compounds. Both of them exhibited unique ꢀuorescence properties
in the aggregated state. Red shifts of the spectrum of 102 nm and 130 nm were
observed for BCSP and CSP on changing the solvent from n-hexane to N,N-dimeth-
ylformamide (DMF), viz. solvatochromism. This phenomenon induced by the intra-
molecular charge transfer (ICT) eꢁect was conꢂrmed by density functional theory
calculations. Interestingly, they both displayed excellent blue-shifted mechanochro-
mic behavior, which was investigated by powder x-ray diꢁraction (PXRD) analysis.
The results of this study provide a new avenue for the design of organic mecha-
nochromic materials that exhibit a blue-shifting chromatic trend. Based on the unu-
sual response of the solid ꢀuorescence to external mechanical forces, we believe that
these materials may be useful in various regards for potential applications.
Keywords Pyrone · Mechanoꢀuorochromic · Solvatochromism · Blue-shifted
Dongqi Liu and Yuqi Cao have contributed equally to this work.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4
-019-03742-2) contains supplementary material, which is available to authorized users.
*
*
Xilong Yan
yan@tju.edu.cn
Bowei Wang
bwwang@tju.edu.cn
1
2
3
School of Chemical Engineering and Technology, Tianjin University, Tianjin,
People’s Republic of China
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin,
People’s Republic of China
Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin,
People’s Republic of China
Vol.:(012
1
3456789)
3

D. Liu et al.
Introduction
Recently, the design and synthesis of smart organic ꢀuorescence materials have
aroused extensive interest. Among them, materials that exhibit the so-called mecha-
noꢀuorochromic (MFC) eꢁect have attracted considerable attention due to their
speciꢂc color change induced by external forces (grinding or pressing). To date,
various kinds of ꢀuorescent sensors [1], optoelectronic materials [2], and viscos-
ity probes [3] have been successively developed based on these outstanding char-
acteristics. However, most traditional organic ꢀuorophores often suꢁer from the
aggregation-caused quenching (ACQ) eꢁect. This deleterious eꢁect can lead to
decreased luminescence at high concentrations and seriously inhibits the applica-
tions of luminophores [4, 5]. Fortunately, in 2001, Tang and colleagues discovered
a unique phenomenon in silole compounds. These molecules show strong emission
in the aggregated state but weak ꢀuorescence in dilute solution, which is termed
aggregation-induced emission (AIE) [6]. To date, various kinds of mechanochromic
materials with this AIE property have been reported, including tetraphenylethene
(
[
(
TPE) derivatives [7], triphenylamine derivatives [8, 9], phenothiazine derivatives
10, 11], etc. Nowadays, investigation of organic molecules with alternating donor
D) and acceptor (A) segments is considered to be a fruitful avenue for design of
MFC materials, often leading to remarkable bathochromic MFC behavior [12–14].
Nevertheless, less attention has been focused on designing hypsochromic mecha-
noꢀuorochromic materials. Enlarging the family of blue-shifted AIE-MFC materi-
als and revealing the mechanism underlying their speciꢂc MFC behavior are still
challenging.
Based on previous study, the 2,6-divinyl-4-pyrone unit has been reported to be
a good building block for construction of bathochromic MFC molecules [15–17].
Previously, we varied the molecular electron donors to regulate the packing pattern,
which was expected to aid development of hypsochromic MFC materials. In the
work presented herein, two D–π–A-type 4-pyrone derivatives, namely 2,6-bis((E)-
4
4
-(9-butyl-9H-carbazol-3-yl)styryl)-4H-pyran-4-one (BCSP) and 2,6-bis((E)-
-(9H-carbazol-9-yl)styryl)-4H-pyran-4-one (CSP), were designed and synthe-
sized [15–20]. Both target compounds displayed speciꢂc ꢀuorescence properties in
dichloromethane (DCM)–hexane mixtures and a remarkable solvatochromic eꢁect
in diꢁerent solvents. Density functional theory (DFT) calculations were carried
out to clarify this eꢁect. To our surprise, a unique hypsochromic mechanochromic
behavior in contrary to expectations was observed. Furthermore, powder wide-angle
X-ray diꢁraction (PXRD) was used to investigate the relationship between the struc-
ture of each molecule and their unique MFC behavior.
1
3

Two stimulus‑responsive carbazole‑substituted D–π–A pyrone…
Experimental
Materials and characterization
All materials and solvents were purchased and used without further puriꢂcation.
1
13
Both target compounds were synthesized as depicted in Scheme 1. H and
C
nuclear magnetic resonance (NMR) spectra were recorded using a Bruker Avance
4
00 MHz spectrometer with CDCl as solvent. Ultraviolet–visible (UV–Vis) spec-
3
tra were obtained on an Evolution 300 UV–Vis spectrophotometer. Liquid photolu-
minescence spectra were obtained using a Hitachi F-2500 spectrophotometer, while
solid photoluminescence spectra and ꢀuorescence quantum yields were recorded
Scheme 1 Synthesis of BCSP and CSP
1
3

D. Liu et al.
on a Horiba Jobin–Yvon Fluorolog-3 spectrophotometer. Dynamic light scattering
(
DLS) measurements were conducted on a Malvern Zetasizer Nano ZS90 size ana-
lyzer. XRD was performed on a Rigaka D/max 2500 X-ray diꢁractometer using Cu
K radiation (40 kV, 40 mA) over the 2θ range from 5° to 60°. Density functional
α
theory (DFT) calculations were performed using the Gaussian 09 package.
Synthesis of A
3
4
-Bromocarbazole (10.00 g, 40.63 mmol), 4-(dihydroxyboryl)benzaldehyde (6.71 g,
4.72 mmol), K CO (16.80 g, 121.82 mmol), and Pd(PPh ) (2.35 g, 2.04 mmol)
2
3
3 4
were added in 80 mL tetrahydrofuran (THF)/water mixture (v/v=3/1) under nitro-
gen atmosphere. After reꢀuxing for 20 h, the mixture was cooled to room tempera-
ture, extracted with chloroform, and washed with brine. After drying over anhydrous
sodium sulfate, the organic layer was evaporated. The resulting orange oil was puri-
ꢂed by column chromatography using dichloromethane and petroleum ether as elu-
ent to aꢁord A as yellow solid (6.15 g, 55.81 %) [18].
Synthesis of B
A mixture of A (2.00 g, 7.37 mmol) and potassium hydroxide (2.07 g, 36.89 mmol)
was dissolved in DMF (20.00 mL) and stirred for 0.5 h. 1-Bromobutane (2.02 g,
1
4.74 mmol) was slowly added to the above mixture. After stirring for 20 h at room
temperature, the mixture was poured into water, and extracted with dichlorometh-
ane. The crude product was puriꢂed by column chromatography on silica gel using
dichloromethane and petroleum ether as eluent to aꢁord product B (2.22 g, 92.12 %)
[
19].
Synthesis of C
Carbazole (4.00 g, 23.92 mmol), 4-bromobenzaldehyde (5.00 g, 27.02 mmol),
K CO (8.30 g, 60.05 mmol), palladium acetate (0.40 g, 1.78 mmol), and tri-tert-
2
3
butylphosphine (0.49 g, 2.42 mmol) were dispersed in 40 ml anhydrous toluene and
reꢀuxed for 24 h under nitrogen atmosphere. After the solvent had evaporated, the
residue was puriꢂed by column chromatography to yield C as yellow solid (3.50 g,
5
3.85 %) [20].
Synthesis of BCSP and CSP
A mixture of 2,6-dimethyl-4-pyrone (0.25 g, 2.01 mmol) and B (5.00 mmol) was
dissolved in ethyl alcohol (20.00 mL). Sodium ethoxide (0.34 g, 5.00 mmol) was
added into the solution slowly. After stirring for 48 h at 40 °C, 5 % hydrochloric acid
was used to adjust the pH to neutral. The crude product was extracted with dichlo-
romethane, washed with brine, and dried over Na SO . After the solvent had evapo-
2
4
rated, the mixture was puriꢂed by column chromatography (dichloromethane/ethyl
1
3

Two stimulus‑responsive carbazole‑substituted D–π–A pyrone…
acetate) and recrystallized in dichloromethane and ethyl acetate to aꢁord the target
compound BCSP (0.86 g, 57.5 %) [15, 16].
CSP was synthesized similarly to BCSP.
1
BCSP H NMR (400 MHz, CDCl ) δ 8.37 (s, 2H), 8.17 (d, J=7.7 Hz, 2H), 7.78
3
(
dd, J=14.2, 8.4 Hz, 6H), 7.70 (d, J=8.1 Hz, 4H), 7.57 (d, J=16.0 Hz, 2H),
7
.50 (t, J=7.6 Hz, 4H), 7.44 (d, J=8.1 Hz, 2H), 7.28 (d, J=7.4 Hz, 2H), 6.80 (d,
J=16.0 Hz, 2H), 6.32 (s, 2H), 4.35 (t, J=7.1 Hz, 4H), 1.95–1.82 (m, 4H), 1.43
1
3
(
dd, J=15.1, 7.5 Hz, 4H), 0.97 (t, J=7.3 Hz, 6H). C NMR (101 MHz, CDCl )
3
δ 180.34, 161.43, 143.53, 140.99, 140.32, 135.71, 133.03, 131.00, 128.18, 127.55,
1
1
26.02, 124.92, 123.50, 122.95, 120.49, 119.21, 119.13, 118.79, 113.83, 109.13,
+
09.00, 43.00, 31.19, 20.61, 13.93. HRMS (ESI): calcd. for 743.3632 (M+H) ,
found 743.3630.
1
CSP H NMR (400 MHz, CDCl ) δ 8.15 (d, J=7.7 Hz, 4H), 7.81 (d, J=8.0 Hz,
3
4
H), 7.65 (d, J=8.1 Hz, 4H), 7.58 (d, J=16.0 Hz, 2H), 7.49–7.40 (m, 8H), 7.31
1
3
(
t, J=7.2 Hz, 4H), 6.84 (d, J=16.0 Hz, 2H), 6.37 (s, 2H). C NMR (101 MHz,
CDCl ) δ 180.26, 161.17, 140.50, 139.07, 134.97, 133.84, 129.06, 127.28, 126.16,
3
1
23.71, 120.60, 120.49, 120.42, 114.44, 109.78. HRMS (ESI): calcd. for 653.2205
+
(
M+Na) , found 653.2199.
Results and discussion
UV–Vis spectra
The absorption spectra of BCSP and CSP in dichloromethane are shown in Fig. S1.
*
The peak at 300 nm for BCSP and at 292 nm for CSP can be attributed to π–π
transitions. Meanwhile, both compounds presented another peak, located at 378 nm
for BCSP and 342 nm for CSP, which can be ascribed to the intramolecular charge
transfer (ICT) process in the D–A skeleton.
Photophysical properties of BCSP and CSP
To investigate the ꢀuorescence properties of BCSP and CSP in the aggregated state,
PL spectra were obtained in dichloromethane and n-hexane as a good and poor
solvent, respectively. As shown in Fig. 1a, BCSP exhibited weak ꢀuorescence in
pure dichloromethane, which can be ascribed mainly to the random framework of
the molecules in this good solvent and the nonradiative pathway accelerated by ran-
dom molecular rotations. However, enhanced ꢀuorescence of BCSP was observed
on addition of n-hexane (f <70 %) due to restriction of intramolecular rotation
n
(
RIR) (Fig. 1c) [21]. Upon addition of a poor solvent (n-hexane), the PL intensity
reached a maximum at f =70 %, with ꢀuorescence quantum yield of 54.68 % for
n
BCSP and 23.63 % for CSP. It is easy for these substituents to rotate freely around
a single bond in dilute solution, which accelerates the nonradiative pathway. With
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3

D. Liu et al.
Fig. 1 PL spectra of compound a BCSP and b CSP in n-hexane/dichloromethane mixtures. Insets show
emission images of the mixtures (365-nm UV lamp). c, d Relative emission intensity at diꢁerent n-hex-
ane fractions, where I is the emission intensity in pure dichloromethane and I is the emission intensity at
0
λ
. e, f Particle size distribution of BCSP in DCM/n-hexane mixture at e 70 % (f ) and f 90 % (f )
max
n
n
addition of n-hexane, such free rotation is impeded due to spatial constraints and the
radiative relaxation pathway is further facilitated, resulting in aggregation of mol-
ecules and gradual enhancement of the emission [22]. Moreover, further addition of
n-hexane induced formation of amorphous aggregates and decreased the emission
intensity, especially for CSP. The size distribution of BCSP and CSP in the mixture
was measured by DLS (Fig. 1e, f; Fig. S10). The average diameter of the nanoparti-
cles presented an increase with increasing n-hexane fraction, further conꢂrming the
aggregation process and unique ꢀuorescence behavior in the mixture [23]. Broadly
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3

Two stimulus‑responsive carbazole‑substituted D–π–A pyrone…
speaking, the whole PL spectra displayed a hyperchromatic tendency, which can be
ascribed to a decreased polarity upon addition of n-hexane.
The photophysical properties of BCSP and CSP were studied in solvents with
diꢁerent polarities. As illustrated in Fig. 2, red-shifts of the spectra of 102 nm and
1
30 nm were observed for BCSP and CSP on changing the solvent from n-hexane
to DMF. This positive linear solvatochromic eꢁect behavior on a Lippert–Mataga
plot provides classical evidence for ICT. In addition, based on the linear response of
the ꢀuorescence to organic solvents, BCSP and CSP might be useful as colorimetric
sensors [24]. In this regard, the charge separation condition in BCSP and CSP could
be further stabilized by surrounding solvent molecules due to dipole–dipole inter-
actions, reducing the energy gap between the highest occupied molecular orbital
(
HOMO) and lowest unoccupied molecular orbital (LUMO) levels [15, 16].
Density functional theory (DFT) calculations
DFT calculations on BCSP and CSP were carried out (Fig. 3) to further clarify
the mentioned ICT eꢁect. The electronic density in the HOMO level was mainly
localized on electron-donating groups. In contrast, the LUMO level was distributed
Fig. 2 Normalized PL spectra of BCSP (a) and CSP (b) in diꢁerent solvents; c Lippert–Mataga plot of
Stokes shift (Δν) of BCSP and CSP versus Δf in the solution
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3

D. Liu et al.
Fig. 3 Electron density distribu-
tion of HOMO and LUMO and
energy gap for BCSP and CSP
mainly on the pyrone and phenyl rings. This prominent charge separation agrees
well with the solvatochromic results described above and indicates the typical ICT
eꢁect [25].
Mechanochromic properties (MFC) and PXRD analysis of BCSP and CSP
The MFC behavior of BCSP and CSP is evaluated in Fig. 4. After grinding, the
emission spectrum of the ground sample exhibited an unusual hypsochromic shift
of 22 nm and 24 nm for BCSP and CSP, respectively. The ꢀuorescence thus exhib-
ited an obvious color change, accompanied by a remarkable change of the solid
ꢀ
uorescence quantum yield (Table 1; Fig. 4) [26]. Unfortunately, the ꢀuorescence
switching was irreversible. To gain insight into the molecular conformation of both
compounds and their MFC behavior, their optimal molecular conꢂguration was
determined by DFT calculations (Figs. S2, S3). As illustrated in Fig. S2, one moder-
ately planar conjugation skeleton was adopted in the molecule of BCSP with a dihe-
dral angle between the bridge-linking phenyl rings and the substituted carbazolyl
unit of 36°. It can therefore be deduced that BCSP might show a compact stacking
Fig. 4 Normalized PL emission spectra and ꢀuorescence images of compounds in diꢁerent solid states
taken under a 365-nm UV lamp: a BCSP, b CSP
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Two stimulus‑responsive carbazole‑substituted D–π–A pyrone…
Table 1 Optical properties of BCSP and CSP in solution and solid state
Compound
Solution state
Solid state
ε (10 M cm−1
3
−1
)
λem (nm)
ϕF (%)
λabs (nm)
BCSP
CSP
378
342
26.9
Original
Ground
Original
Ground
552
530
542
518
18.54
9.11
13.01
14.7
3.08
a
Measured in dilute DCM solution
ε measured at λabs
Fluorescence quantum yield in solid state obtained using a calibrated integrating sphere
b
c
mode with strong intermolecular interactions in the original state due to its relatively
planar molecular conꢂguration [27]. In consequence, the mechanochromic mecha-
nism of BCSP can be ascribed to destruction of such intermolecular interactions and
a twisted conformation of the molecules [28, 29]. After grinding, a distorted molec-
ular conformation might form, which would lead to the decrease of intermolecular
interactions and π-conjugation. This is also demonstrated by the enhanced and blue-
shifted emission after grinding. This interpretation also applies to CSP.
To conꢂrm the change of the aggregation state in the original and ground form
of the present compounds, powder X-ray diꢁraction (PXRD) experiments were per-
formed. As shown in Fig. 5, the PXRD diꢁractograms of the pristine samples exhib-
ited sharp scattering peaks, characteristic of a microcrystalline state. After grinding,
the ground samples of BCSP displayed similarly reꢀections but with low intensity.
The remaining crystalline features of the ground sample of BCSP can be attributed
to an incomplete phase transition due to insuꢃcient grinding force (Fig. 5a). Mean-
while, in the pattern for the ground CSP sample, most of the diꢁraction peaks were
remarkably weakened or disappeared, indicating an amorphous state (Fig. 5b) [30,
Fig. 5 Powder wide-angle X-ray diꢁraction (PXRD) patterns of compounds a BCSP and b CSP in dif-
ferent solid states
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D. Liu et al.
3
1]. Hence, the MFC behavior of BCSP and CSP can be related to the transforma-
tion from a crystalline to amorphous state. During this transformation, the intermo-
lecular interactions are destroyed and the conformation of the molecules becomes
more twisted, leading to the observed hypsochromic shift. Unfortunately, the diꢁrac-
tion signals of BCSP and CSP did not return to their original state after annealing
treatment.
Conclusions
Two D–π–A-type 4-pyrone derivatives, viz. BCSP and CSP, were successfully syn-
thesized and their photophysical properties investigated. The results showed that
both of them exhibited obvious ꢀuorescence properties in solution and a remarkable
red-shifting solvatochromic eꢁect. The speciꢂc ICT eꢁect was conꢂrmed by DFT
calculations. More importantly, BCSP and CSP also displayed obvious blue-shift-
ing MFC properties. The relationship between the MFC character and the molecular
packing mode was revealed by PXRD analysis, and the mechanism can be ascribed
to the transformation between a crystalline and amorphous state. This study pro-
motes a strategy to design blue-shifting MFC materials by structural modulation.
Based on the unusual response of the solid ꢀuorescence to external mechanical
forces, such materials might have potential applications in various directions [32].
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