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J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
behind on self-assembly [24–26]. An elegant way to create the building
blocks is incorporating the electron donor (D) and acceptor
(A) substituents to AIE cores, which may yield highly polarized
luminogens with high solid-state emission [27–29]. D-A type AIE sys-
tems would enable the intramolecular charge transfer (ICT) phenomena
[30–32], and further provide strong dipole-dipole interactions for self-
assembly [33]. Another advantage of D-A pairs stems from their high
degree of freedom, which means the species, numbers and spatial posi-
tion of the D-A units are highly controllable, making the molecular de-
signing extremely diversified. This open-choice principle undoubtedly
facilitates the systematic study on the relationships between supramo-
lecular structures and optical properties, by which the working mecha-
nisms of the functional subjects would be revealed. This would pave
ways for the development of tailor-made optical sensors and smart ma-
terials for a given application, including AIE-based ones [34–36].
In this work, we present a new AIE molecular system based on posi-
tional isomers. The relationship between the D-A position and the cor-
responding self-assembly structure was investigated. These AIE
isomers exhibit obvious solvatochromic effect in solution and emit
long-wavelength fluorescence ranging from yellow to orange in the
solid state. The AIE molecules show fluorescent responses to amine
gas, and hence we aimed to find an applicable AIE compound showing
the best performance to serve as an indicator for meat spoilage.
HMBA-4 were prepared according to the procedure of HMBA-1 with
some modifications, including an extended reaction time (15 h) and
an elevated reaction temperature (60 °C). Yields of HMBA-2, HMBA-3
and HMBA-4 were 45%, 60% and 50%, respectively. The structures of
the four compounds were well characterized by 1H NMR and 13C NMR
spectroscopy with satisfactory analysis results (Figs. S1–S8). Below are
data obtained from their structural characterization. 1H NMR of
HMBA-1 (600 MHz, DMSO d6), δ (ppm): 12.92 (s, 1H), 12.38 (s, 1H),
8.90 (s, 1H), 8.00–7.97 (m, 2H), 7.47 (d, 1H), 7.45–7.43 (m, 2H),
7.25–7.23 (dd, 1H), 6.87–6.86 (d, 1H), 2.26 (s, 3H). 13C NMR of HMBA-
1 (151 MHz, DMSO d6), δ (ppm): 167.30, 165.00, 158.58, 152.78,
135.04, 132.67, 131.16, 129.12, 128.30, 121.90, 119.44, 116.98, 20.36.
ICP-Mass of HMBA-1: calcd 256.10 [(M + H)+, found 255.90]. 1H
NMR of HMBA-2 (600 MHz, DMSO d6), δ (ppm): 13.12 (s, 1H), 12.51
(s, 1H), 8.94 (s, 1H), 7.87 (t, 1H), 7.85–7.84 (dt, 1H), 7.63–7.61 (dq,
1H), 7.57–7.55 (t, 1H), 7.48–7.47 (d, 1H), 7.23–7.21 (dd, 1H),
6.86–6.85 (d, 1H), 2.25 (s, 3H). 13C NMR of HMBA-2 (151 MHz,
DMSO d6), δ (ppm): 167.36, 164.64, 158.52, 149.11, 134.75, 132.78,
132.57, 130.22, 128.19, 127.87, 126.39, 122.16, 119.43, 116.91, 20.38.
ICP-Mass of HMBA-2: calcd 256.10 [(M + H)+, found 256.20]. 1H
NMR of HMBA-3 (600 MHz, DMSO d6), δ (ppm): 13.29 (s, 1H), 12.93
(br s, 1H), 8.98 (s, 1H), 8.01–7.99 (m, 2H), 7.51–7.47 (m, 3H),
7.33–7.32 (d, 1H), 6.90–6.88 (t, 1H), 2.20 (s, 3H). 13C NMR of HMBA-3
(151 MHz, DMSO d6), δ (ppm): 167.28, 166.16, 159.28, 151.96, 135.05,
131.42, 131.16, 129.32, 125.63, 121.97, 119.21, 118.64, 15.62. ICP-Mass
of HMBA-3: calcd 256.10 [(M + H)+, found 256.20]. 1H NMR of
HMBA-4 (600 MHz, DMSO d6), δ (ppm): 13.37 (s, 1H), 13.14 (s, 1H),
9.01 (s, 1H), 7.92 (t, 1H), 7.87–7.85 (dt, 1H), 7.67–7.66 (dq, 1H),
7.59–7.56 (t, 1H), 7.50–7.49 (dd, 1H), 7.32–7.30 (d, 1H), 6.90–6.87 (t,
1H), 2.21 (s, 3H). 13C NMR of HMBA-4 (151 MHz, DMSO d6), δ (ppm):
167.34, 165.64, 159.16, 148.47, 134.80, 132.62, 131.35, 130.24, 128.04,
126.56, 125.54, 122.12, 119.15, 118.71, 109.99, 15.65. ICP-Mass of
HMBA-4: calcd 256.10 [(M + H)+, found 255.90]. (Abbreviations: br
= broad, s = singlet, d = doublet, t = triplet, q = quartet).
2. Experimental Section
2.1. Instruments and Methods
Reagents and chemicals were purchased from Tokyo Chemical In-
dustry (TCI) and Alfa Aesar and used as received without further purifi-
cation. 1H NMR and 13C NMR spectra were measured on a Varian
VNMRS 600 MHz spectrometer using tetramethylsilane as an internal
reference. The molecular weights were determined on an Agilent
7500Ce inductively coupled plasma source mass spectrometer (ICP-
mass). Emission spectra were recorded on a HITACHI F-7000 FL spectro-
photometer. UV–vis spectra were measured on a SHIMADZU UV-2550
spectrophotometer. The fluorescence quantum yields (ΦF) were deter-
mined using a calibrated integrating sphere on a HORIBA FluoroMax-4
(relative error: 0.017). The ground-state geometries were optimized
using density functional theory (DFT) with the B3LYP hybrid functional
at the basis set level of 6-31G*. The calculations were performed with
Gaussian 05 package. Fluorescence images were taken under an Olym-
pus CKX41 phase contrast microscope (excitation wavelength:
365 nm). Powder X-ray diffraction (PXRD) patterns were collected
with monochromatized Cu-Kα1 (λ = 1.54178 Å) incident radiation
by a Shimadzu XRD-6000 instrument. The images of the morphology
were captured using a Hitachi SU-8010 scanning electron microscope
(SEM). The thickness of the sensing film was measured by a Smart-
Sensor Coating/Filming thickness gauge.
3. Results and Discussion
3.1. Aggregation-induced Emission
Luminogens are commonly used in solid state, such as thin films,
nano-aggregates, crystals and self-assembled architectures. Thus it
would be necessary to increase the emission efficiency at solid state. It
has been reported frequently that AIE luminogens are able to overcome
the emission quenching effect by twisting their molecular conformation
to break π–π stacking. Such mechanism, well known as “restriction of
intramolecular motion (RIM)” is supported by plenty of evidence, led
to an explosion of new AIE systems [15,18]. The first step of this study
was to merge the electron donor-acceptor (D-A) pairs to form the de-
sired luminogens HMBA-1, 2, 3 and 4 (HMBAs), by using a facile Schiff
base reaction. Obviously the propeller-shaped molecular design con-
tributing to AIE mechanism facilitates the creation of the HMBAs, in
which benzylideneaniline moiety serves as conjugated chromophore,
C\\C and C\\N single bonds offer the possibility of both intramolecular
rotation in solution and conformation twisting in aggregation. Addition-
ally, the incorporation of D-A units could fine tune the energy of ground
and exited state, by which the energy gap is narrowed to generate
2.2. Synthesis and Structural Characterization
The synthetic route to the AIE isomers including (E)-4-((2-hydroxy-
5-methylbenzylidene)amino)benzoic acid (HMBA-1), (E)-3-((2-
hydroxy-5-methylbenzylidene)amino)benzoic acid (HMBA-2), (E)-4-
((2-hydroxy-3-methylbenzylidene)amino)benzoic acid (HMBA-3) and
(E)-3-((2-hydroxy-3-methylbenzylidene)amino)benzoic acid (HMBA-
4) are illustrated in Scheme 1. Detailed synthetic procedure of HMBA-
photo-excited
emission
with
longer
wavelength.
The
photoluminescence (PL) spectra of the HMBAs in the solid state to-
gether with their photos under UV irradiation are shown in Fig. 1. For
HMBA-1, the emission peak is located at around 545 nm, which is
assigned to the intramolecular charge transfer (ICT) emission. When
the carboxylic group was changed from para-position to meta-
1
is described as follows. To a solution of 2-hydroxy-5-
methylbenzaldehyde (1.00 g, 7.34 mmol, 1 equiv) in anhydrous ethanol
(150 mL) was added 4-aminobenzoic acid (1.01 g, 7.34 mmol, 1 equiv).
0.1 mL acetic acid was added to the reaction mixture before it was
heated to 50 °C. The reaction mixture was then stirred for 2 h, yielding
HMBA-1 as yellow crystalline powder. The resulting products were col-
lected by vacuum filtration using ethanol to rinse. No further purifica-
tion was needed. Yield: 62%. The synthesis of HMBA-2, HMBA-3 and
position, the corresponding compound HMBA-2 exhibits
a
bathochromic-shift and peaked at 552 nm in the emission spectrum.
Similarly, if methyl group was shifted to another meta-position,
i.e., ortho-position of phenolic hydroxyl, the emission wavelength