Mechanochromism, thermochromism, protonation effect and discrimination of CHCl3 from organic solvents in a Et2N-substituted Salicylaldehyde Schiff base

Article abstract of DOI:10.1016/j.dyepig.2021.109708

Multi-stimuli-responsive materials, which can show color/fluorescence change in response to external chemical or physical stimuli, have received increasing attention due to the capacities for facile realization of multifunctional sensing. Herein, we presented a Et₂N-substituted salicylaldehyde Schiff base compound (DDHAC) with multi-stimuli-responsive behaviors. An investigation of the photophysical properties in solution demonstrated DDHAC was an excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) active molecule. Owing to structural flexibility from salicylaldehyde Schiff base, the compound displayed reversible mechanochromic and thermochromic behavior, which were associated with the crystalline-to-amorphous and crystalline-to-crystalline, respectively. Moreover, the Et₂N group endowed DDHAC with the capability that response of protonation. Due to its protonation effect, DDHAC could serve as a unique optical probe for discriminating CHCl₃ from organic solvents assisted by UV irradiation. The selectivity was attributed to the interaction of DPPTP with HCl gas from photodecomposition product of chloroform.

Full text of DOI:10.1016/j.dyepig.2021.109708

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Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Mechanochromism, thermochromism, protonation effect and
discrimination of CHCl3 from organic solvents in a Et2N-substituted
Salicylaldehyde Schiff base
,
*
Jun Shua, Tong Nia, Xiaoqiang Liua, Bin Xua, Lang Liub, Wendao Chuc, Kaiming Zhanga
,
,
**
Weidong Jianga
a Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, School of Chemistry and Environmental Engineering, Sichuan University of Science &
Engineering, Sichuan Zigong, 643000, PR China
b College of Chemistry, Xinjiang University, Urumqi, 830046, PR China
c Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University,
Nanchong, 637002, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Multi-stimuli-responsive materials, which can show color/fluorescence change in response to external chemical
or physical stimuli, have received increasing attention due to the capacities for facile realization of multifunc-
tional sensing. Herein, we presented a Et2N-substituted salicylaldehyde Schiff base compound (DDHAC) with
multi-stimuli-responsive behaviors. An investigation of the photophysical properties in solution demonstrated
DDHAC was an excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE)
active molecule. Owing to structural flexibility from salicylaldehyde Schiff base, the compound displayed
reversible mechanochromic and thermochromic behavior, which were associated with the crystalline-to-
amorphous and crystalline-to-crystalline, respectively. Moreover, the Et2N group endowed DDHAC with the
capability that response of protonation. Due to its protonation effect, DDHAC could serve as a unique optical
probe for discriminating CHCl3 from organic solvents assisted by UV irradiation. The selectivity was attributed to
the interaction of DPPTP with HCl gas from photodecomposition product of chloroform.
Multi-stimuli response
Salicylaldehyde schiff base
Mechanochromism
Thermochromism
Protonation effect
Discrimination of CHCl3
1. Introduction
incorporation of diethylamino (pH/CO2-responsive group) and acryl-
amide (thermoresponsive moiety) into homopolymer [41]. By tagging
Organic stimuli-responsive molecules can change their chemical or
physical structures upon external stimuli and exhibit tunable emission
and/or absorption properties. The smart materials have aroused enor-
mous interest owing to their promising practical applications in detec-
tion of mechanical force [1–14], heat [15–22], acid/base [23–30],
organic solvent [31–35] and so on. To date, a series of stimuli-responsive
materials have been developed, and whereas most of which typically
exhibit only single environmental stimuli response. Samples possessing
multi-stimuli-responsive (MSR) capabilities are much less frequently
reported. Normally, the integration strategy that installing distinct
stimuli-responsive moieties and fluorophore into one system is
employed to construct MSR polymers [36–43]. For example, Song and
multi-function unit of cyanoethylene to a polymer, a smart mechano-
and thermoresponsive material was achieved by Lavrenova and
co-workers [42]. In most instances, however, the MSR polymeric ma-
terials always raise additional synthetic challenges [39]. By contrast,
small organic molecules with simple structure could be preferred
[44–48].
Salicylaldehyde-based Schiff base compound is a type of potential
stimuli-responsive material because (1) it is easily synthesized and pu-
rified; (2) it often shows AIE attribute [49–54], which is suitable for
monitoring stimuli-induced chromism in solid state; (3) more impor-
tantly, its structural flexibility [55,56] is beneficial to phase transition
from one to another states and resultant tunable emission upon external
stimuli. Indeed, its derivatives have been extensively used for detection
co-workers developed
a
novel MSR macromolecule through
* Corresponding author.
** Corresponding author.
E-mail addresses: susezkm@163.com (K. Zhang), jwdxb@suse.edu.cn (W. Jiang).
https://doi.org/10.1016/j.dyepig.2021.109708
Received 4 May 2021; Received in revised form 5 August 2021; Accepted 6 August 2021
Available online 8 August 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

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J. Shu et al.
Dyes and Pigments 195 (2021) 109708
of mechanical force, heating, cations, anions and enzymes [12–16,
57–62]. However, little effort has been made to exploit their capacity in
multi-parameter sensing.
77.37, 77.16, 76.95, 45.65, 12.49. IR (KBr pellet): 2979(w), 1745(s),
1629(s), 1507(m), 1326(m), 1255(m), 823(w) cmꢀ 1. HRMS (ESI): m/z
[M+H]+ calcd. for [C13H15N2O4]+ 263.1032, found 263.1019.
Synthesis of 3-amino-7-diethylaminocoumarin. Stannous chloride
dihydrate (11.36 g, 50.33 mmol) was added in conc. HCl (35 mL). To
this suspension 7-diethylamino-3-nitrocoumarin (1.76 g, 6.7 mmol) was
added at 0 ◦C in small portions, over a period of 1 h. The mixture was
stirred for 4 h at room temperature and then diluted in iced water (300
mL). The pH was adjusted to 8 with NaOH and the mixture was extracted
three times with ethyl acetate (100 mL) at a time. The combined organic
layers were dried over Na2SO4 and the solvent was evaporated in vac-
uum to obtain 3-amino-7-diethylaminocoumarin as pale yellow solid
(1.29 g, 83 %), m. p. 90.4–91.2 ◦C. 1H NMR (600 MHz, CDCl3) δ 7.09 (d,
J = 8.7 Hz, 1H), 6.69 (s, 1H), 6.56 (d, J = 8.5 Hz, 1H), 6.51 (s, 1H), 3.90
(s, 2H), 3.35 (q, J = 7.1 Hz, 4H), 1.16 (t, J = 7.1 Hz, 6H). 13C NMR (151
MHz, CDCl3) δ 160.42, 151.66, 147.48, 127.65, 126.05, 114.52, 109.83,
109.47, 98.10, 44.75, 12.55. IR (KBr pellet): 3372 (double, w), 2970(w),
1697(s), 1623(s), 1589(m), 1326(m), 1129(m), 816(w) cmꢀ 1. HRMS
(ESI): m/z [M+H]+ calcd. for [C13H17N2O2]+ 233.1290, found
233.1277.
In this study, we reported a multi-stimuli-responsive compound
DDHAC with Et2N-substituted salicylaldehyde Schiff base skeleton
(Scheme 1). Due to the structural flexibility from salicylaldehyde Schiff
base, this molecule showed mechanochromism and thermochromism
behavior. Moreover, the introduction of the Et2N group endowed
DDHAC with the capability that response of protonation, further
allowed it to discriminate CHCl3 from organic solvents assisted by UV
irradiation.
2. Experimental
2.1. Measurements and material
All reagents and solvents were purchased from commercial sources
and used as received without further purification. 1H and 13C NMR
spectra were recorded on a Bruker AV III HD 600 spectrometer. LRMS
and HRMS were obtained on Bruker micrOTOF-Q-II and Bruker SolariX-
70FT mass spectrometer, respectively. UV–Vis spectra were recorded on
a Purkinje TU-1950 spectrophotometer. Photoluminescence (PL) spectra
were recorded on an Agilent Cary Eclipse fluorescence spectrophotom-
eter. The fluorescence quantum yields in solutions were measured by
comparing a standard (fluorescein in 0.1 N NaOH aqueous solution, QF
= 0.79). The absolute fluorescence quantum yields of solids were
measured on a Hamamatsu C9920-02G spectrometer. The X-ray
diffraction (XRD) measurements were carried out on a Skyray DX-2600
X-ray diffractometer. The emission spectra were measured under exci-
tation at 415 nm. The photographic images were taken under a 365 nm
UV lamp.
Synthesis of DDHAC. To a stirred solution of 3-amino-7-diethylami-
nocoumarin (464 mg,
2
mmol), 4-(diethylamino)-2-hydrox-
ybenzaldehyde (425 mg, 2.2 mmol) in EtOH (16 mL) were added
CH3COOH (0.5 mL). The mixture was stirred for 2 h at room tempera-
ture and then filtered off to obtain red solid. Further purification was
performed by recrystallization from THF to yield DDHAC as yellow solid
(627 mg, 77 %), m. p. 238.2–238.8 ◦C. 1H NMR (600 MHz, DMSO‑d6) δ
13.63 (s, 1H), 8.94 (s, 1H), 7.81 (s, 1H), 7.44 (d, J = 8.9 Hz, 1H), 7.26 (d,
J = 8.9 Hz, 1H), 6.73 (dd, J = 8.9, 2.3 Hz, 1H), 6.57 (d, J = 2.2 Hz, 1H),
6.31 (dd, J = 8.9, 2.3 Hz, 1H), 6.06 (d, J = 2.2 Hz, 1H), 3.47–3.35 (m,
8H), 1.12 (td, J = 7.0, 4.6 Hz, 12H). 13C NMR (151 MHz, DMSO) δ
163.41, 160.61, 158.79, 154.14, 151.49, 149.67, 133.92, 129.22,
129.05, 126.94, 109.41, 108.82, 108.22, 103.95, 96.93, 96.47, 44.06,
43.92, 12.57, 12.37. IR (KBr pellet):3441 (br, m), 2971(w), 1700(s),
1615(s), 1511(m), 1343(m), 1238(m), 1129, 819(w) cmꢀ 1. HRMS (ESI):
m/z [M+H]+ calcd. for [C24H30N3O3]+ 408.2287, found 408.2259.
Preparation of the samples for photophysical properties study. The
UV–Vis absorption and PL spectra of DDHAC in various organic solvents
were measured using freshly prepared 1.0 × 10ꢀ 5 M solutions. The
CH3CN/water (or THF/CH3OH) mixtures (concentration of 1.0 × 10ꢀ 5
M) with different water (or CH3OH) fractions were prepared by slowly
adding distilled water into the CH3CN (or THF) solution of sample under
ultrasound at room temperature.
3. Results and discussion
3.1. Synthesis and characterization
2.2. Synthesis
The synthetic routes were illustrated in Scheme S1. The reaction of
starting compound 4-diethylamino salicylaldehyde with ethyl nitro-
acetate in the presence of piperidine as catalyst gave 7-diethylamino-3-
nitro coumarin, which was further reduced by stannous chloride to 3-
amino-7-diethylamino coumarin [63]. Compound 3-amino-7-diethyla-
mino coumarin could be easily converted to target compound DDHAC
by condensation reaction with 4-diethylamino salicylaldehyde. 1H NMR,
13C NMR and MS-ESI were carried out to confirm the molecular
structures.
Synthesis of 7-diethylamino-3-nitrocoumarin. 4-diethylamino-2-
hydroxybenzaldehyde (2.50 g, 13 mmol) was dissolved in n-BuOH (40
mL), ethyl nitroacetate (2.39 g, 18 mmol) and piperidine (0.4 mL) was
added, and the mixture was refluxed for 12 h. After cooling to room
temperature, orange crystals precipitated out from the mixture and they
were filtered off and washed with cold n-BuOH and diethyl ether to yield
7-diethylamino-3-nitrocoumarin as orange solid (1.89 g, 56 %), m. p.
205.4–206.1 ◦C. 1H NMR (600 MHz, CDCl3) δ 8.67 (s, 1H), 7.43–7.40
(m, 1H), 6.69 (dd, J = 9.1, 2.4 Hz, 1H), 6.44 (d, J = 2.3 Hz, 1H), 3.48 (q,
J = 7.2 Hz, 4H), 1.25 (t, J = 7.2 Hz, 6H). 13C NMR (151 MHz, CDCl3) δ
158.82, 154.71, 153.57, 143.43, 132.71, 126.69, 111.35, 106.29, 96.81,
3.2. ESIPT properties
Owing to the rotation of central C–C single bond connecting the
phenyl and imino group, there were two species for compound DDHAC
in solution including open-enol and hydrogen-bonded enol forms [64]
(Scheme 2). The former displayed normal emission; the latter could
undergo the excited-state intramolecular proton transfer (ESIPT) pro-
cess after photoexcitation, and exhibited keto-form emission based on a
keto-enol tautomerization. Therefore, the dual emission bands were
observed for DDHAC in solution (Table 1). For example, clearly
double-peaked bands were found in PL spectra in n-hexane, toluene,
tetrahydrofuran, ethyl acetate and chloroform (Fig. 1b). One was the
normal emission in short-wavelength region, and the other was the
keto-form emission after the ESIPT process in long-wavelength region.
However, only single-peaked broad emission bands were observed in
Scheme 1. The molecular structure and multi-stimuli responses of DDHAC.
2

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Dyes and Pigments 195 (2021) 109708
Scheme 2. General representation of the ESIPT Process.
n-hexane, toluene, tetrahydrofuran, ethyl acetate and chloroform
(Fig. 1a), where the shorter absorption band was ascribed to coumarin
structure and the longer absorption band was assigned to the coupling
between coumarin and the hydroxyphenyl ring.
Table 1
Photophysical data of DDHAC in various solvents.
Solvent
λAbs (nm)
λPL (nm)
ΦF (%)
n-hexane
425, 451
437, 460
436, 455
439, 459
442, 464
458
468, 500
490, 508
493, 509
495, 516
513, 550
513
3.87
8.28
7.88
9.65
21.24
8.11
7.51
36.28
toluene
3.3. AIE properties
ethyl acetate
tetrahydrofuran
chloroform
acetone
To investigate the fluorescent behavior of DDHAC in aggregation
state, the emission spectra in CH3CN/water mixtures with different
fractions of water (fw) were performed (Fig. 2a). DDHAC emitted a weak
emission in strong polar solvent CH3CN. However, the fluorescence was
enhanced gradually by increasing the fw, and reached the intensity
maximum at 0.4 of water content, revealing the AIE characteristic of
DDHAC. From 0.4 to 0.9 of fw, DDHAC displayed reduction of emission
intensity, which could be attributed to the formation of nanoparticles in
a higher water faction. The aggregation of DDHAC in CH3CN/water
system was supported by absorption spectra and dynamic light scat-
tering (DLS) measurements. As shown in Fig. 2c, in the absorption
spectra, a level-off tail in the long-wavelength region could be obviously
observed in 40 % water/THF (v/v) solution, which was caused by light
scattering effect of aggregate suspensions. DLS measurements revealed
that the particle size was around 100 nm (Fig. 2c inset). Additionally,
acetonitrile
methanol
458
536
469
561
a Only the absorption/emission peaks wavelengths were shown.
b Emission spectra were performed upon excitation at 415 nm.
acetone and acetonitrile. It was because the intramolecular hydrogen
bonding (-OH… N-) could be interrupted by polar solvents [64], and the
open-enol form of DDHAC was the dominant species. It was noted that
DDHAC emitted weak fluorescence in the majority of organic solvents,
and non-radiative relaxation of excited states from the intramolecular
rotation [51] or solvent relaxation [65] could account for the phe-
nomenon. Furthermore, DDHAC displayed dual absorption peaks in
Fig. 1. (a) Normalized absorption spectra and (b) PL spectra and photographic images (inset) of DDHAC in various solvents.
3

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Dyes and Pigments 195 (2021) 109708
Fig. 2. Fluorescence spectra and emission images (insets) of DDHAC in (a) CH3CN/water and (b) THF/CH3OH mixtures; absorption spectra and DLS results (insets)
of DDHAC in (c) 40 % CH3CN/water (v/v) mixture and (d) CH3OH.
AIE properties of DDHAC were examined in THF-CH3OH systems with
different methanol fractions (fM) (Fig. 2b and d). In a good solvent of
THF, DDHAC exhibited very weak fluorescence. After the addition of
MeOH, a poor solvent for Schiff base dye, into the THF solution, the
emission intensity enhanced gradually, and reached the maximum at
0.90 of fM. Similarly, absorption spectra and DLS of CH3OH solution also
indicated that the aggregates were generated. The AIE properties could
be attributed to restriction of the intramolecular rotation (RIR) and
torsion effect [53,66].
3.4. Mechanochromism and thermochromism behavior
Considering that structural flexibility of salicylaldehyde imine, the
mechanochromism and thermochromism behavior of DDHAC were
performed. As shown in Fig. 3a, the pristine powders obtained by
crystallization in THF solution emitted bright yellow emission (ΦF
=
11.7 %) with double peaks at 566 nm and 587 nm (black line). Once
original powders were ground using a pestle and mortar, the resultant
samples showed weak fluorescence (Φ = 2.8 %) with single peak
located at 597 nm (red line). The fluoresFcence could be recovered to its
original state by fuming with THF (pink line), suggesting the reversible
mechanical stimuli-response behavior. Furthermore, by heating pristine
Fig. 3. (a) Emission spectra and (b) XRD patterns of the corresponding samples for DDHAC. Inset: natural and fluorescent images of DDHAC in different states.
4