Effect of substituent position on aggregation-induced emission, customized self-assembly, and amine detection of donor-acceptor isomers: Implication for meat spoilage monitoring

Article abstract of DOI:10.1016/j.saa.2018.07.021

We synthesized a class of positional isomers by attaching electron donor and acceptor units in different sites of a conjugated core. These isomers exhibit both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) effects, which are

Full text of DOI:10.1016/j.saa.2018.07.021

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Effect of substituent position on aggregation-induced emission,
customized self-assembly, and amine detection of donor-acceptor
isomers: Implication for meat spoilage monitoring
a,
Jingdan Hou a, Jiaorui Du b, Yue Hou a, Peijun Shi a, Yang Liu c, Yuai Duan a, , Tianyu Han
⁎
⁎
a
Department of Chemistry, Capital Normal University, Beijing 100048, China
School of Chemistry, Beijing Institute of Technology, Beijing 100081, China
b
c
Beijing Key Laboratory of Radiation Advanced Materials, Beijing Research Center for Radiation Application, Beijing 100015, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 May 2018
Received in revised form 20 June 2018
Accepted 7 July 2018
Available online 09 July 2018
We synthesized a class of positional isomers by attaching electron donor and acceptor units in different sites of a
conjugated core. These isomers exhibit both aggregation-induced emission (AIE) and intramolecular charge
transfer (ICT) effects, which are proved by adequate spectroscopic analysis. Their structure-property relation-
ships were systematically studied. We found that relocation of the D/A units would have remarkable impact
on the intermolecular dipole-dipole interaction, further controlling the shape and color of the self-assembled ar-
chitectures. With D/A units shifting to different sites, four types of the structures appear sequentially, including
quadrate microsheets, microrods, nanofilaments and nanowires. Furthermore, the A unit (benzoic acid moiety)
of the AIE isomers is easy to adsorb amines, leading to changes in both emission wavelength and intensity.
Then a portable sensor is prepared on solid support based on the self-assembled architecture of HMBA-4,
which has been proved to be the most sensitive to amines. It affords fast spectral responses as well as a low de-
tection limit of 186 Pa (vapour pressure). The sensing mechanism was revealed by density functional theory
(DFT) calculation, which indicates that the spectral responses stem from the weakened ICT effect. The sensor is
able to detect amine vapours generated by meat, and thus succeeds in detecting the spoiled pork samples, offer-
ing high potential for meat spoilage monitoring in real-world applications.
Keywords:
Aggregation-induced emission
Self-assembly
Position effect
Amine sensor
Meat spoilage
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
generated in degradation of proteins, they could be regarded as an indi-
cator for food spoilage. Inspired by this fact, some examples of AIE-
As huge amount of meat products is stored and shipped all over the
world every year, food safety has attracted increasing attention because
it is closely related to human health and economic issue. Relevant food
monitoring techniques have been developed, which are mostly based
on mass spectrometry [1,2], electrochemical method [3,4], and nuclear
magnetic resonance (NMR) spectrometer [5]. By contrast, optical sen-
sors or techniques are easier to use, owing to their fast response, on-
line visual inspection and low-cost measurement [6–8]. In search of op-
tical sensors, aggregation-induced emission (AIE) molecular systems
have emerged as a quickly growing research hotspot [9–14]. The beauty
of AIE stems from the fact that the molecules are designed to be highly
twisted to ensure high emission efficiency at solid state, in which the in-
termolecular π-π stacking have been avoided to suppress the formation
of excimer/exciplex [15]. Thus the AIE-based sensors are compatible
with various solid supports such as thin films, nano-carriers, and poly-
mer substrates [16–18]. Given that some specific amines would be
based sensors for food spoilage detection have been reported recently
[19,20]. And the mechanism of the above-mentioned AIE sensors
mainly involves a specific chemical reaction between AIE sensor and
amines. However, change in chemical structure may suffer from insuffi-
cient conversion or prolonged reaction time, and these disadvantages
could be even more serious in solid-gas reactions [20].
AIE materials capable of emission switching by changing molecular
packing mode are believed to be an alternative way to fabricate fluores-
cent sensors including those for meat spoilage detection. This design
philosophy focuses on the changes in aggregation structure instead of
that in molecular structure, and relies heavily on the self-assembly tech-
niques. Nano/micro-sized sensor systems built via self-assemblies have
emerged as a highly growing field, and a large number of studies have
revealed that the self-assemblies will inherited the sensing perfor-
mances or optical properties of the isolated molecules, and benefit a
great deal from their high-ordered molecular packing [21–23]. Just as
the application of fluorescent sensors based on molecular self-
assembled structure took several years longer to optimize than simply
on molecular structure, so too has the application of AIE sensor lagged
⁎
Corresponding author.
E-mail addresses: duanya@cnu.edu.cn (Y. Duan), phanw@163.com (T. Han).
https://doi.org/10.1016/j.saa.2018.07.021
1386-1425/© 2018 Elsevier B.V. All rights reserved.

2
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

J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
3
Scheme 1. Synthetic route to HBMA-1, -2, -3 and -4. D and A units are highlighted in blue and red color, respectively. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article).
undergoes an even bigger red-shift (from 545 nm to 574 nm) as seen in
HMBA-3 (Fig. 1). When attaching both D and A units to the meta-
position (HMBA-4), the emission wavelength changes to 556 nm. The
results demonstrate that the ICT emission can be well controlled by
the position of D-A units.
The first way to test whether they are AIE-active is to confirm their
quantum efficiency in both solution and solid state. The absolute ΦF
values of these molecules in solution and the solid state were obtained
using the calibrated integrating sphere, and then summarized in
Table 1. All quantum yields of the THF solution are low and fall in the
range of 0.22% to 1.41%. In contrast, they are remarkably increased at
the solid state. When comparing the solution of HMBA-1, 2, 3 and 4 to
their solid powder, there is an approximate 38, 25, 10 and 2.34-fold in-
crease in ΦF, respectively. Presumably the lack of emission in solution is
due to the vibrational and torsional relaxation, while the significant in-
crease of ΦF in solid state suggests the nonradiative relaxations are
inhibited by the motionless rigid matrix.
The emission behavior of the four compounds was investigated
through typical AIE experiments, which is done by dissolving each
luminogen in pure THF or DMSO and then adding hexane or water as
a poor solvent. The PL spectrum was collected at every increment of
poor solvent. Typically, HMBA-1 was almost non-emissive in THF solu-
tion as shown in Fig. 2A. Increase of the hexane fraction (fh) from 10% to
90% exerts no change in PL spectra, because HMBA-1 is still solvated.
When fh reaches 99%, there is a significant boost of the PL intensity at
535 nm, affording approximately a 688-fold enhancement relative to
that in pure THF solution. HMBA-2 and HMBA-3 shows a similar spec-
tral change as displayed in Fig. 2B and C. They exhibit 87 and 227-fold
enhancement in emission intensity, respectively. In comparison, the
AIE effect of HMBA-4 is much weaker: there is still weak emission in
DMSO solution and the enhancement in emission intensity is
HMBA-1
HMBA-2
HMBA-3
HMBA-4
Table 1
Photophysical data of the HMBAs, abbreviation: λab,DMSO = maximum absorption wave-
length in DMSO solution (10 μM), λem,P = maximum emission wavelength of the solid
powders. λem,aggr = maximum emission wavelength of the aggregates formed in poor sol-
vents. λem,F = maximum emission wavelength of the self-assembly films. ФF,S = absolute
quantum yield in THF solution (1 mM), ФF,P = absolute quantum yield of the solid
powders.
λab,DMSO
λem,P
λem,aggr
λem,F
ФF,S
ФF,P
ΔE
Dipole
(nm)
(nm)
(nm)
(nm)
(%)
(%)
(eV) moments
(Debye)
500
550
600
650
Wavelength (nm)
HBMA-1 284
545
552
574
556
535
551
567
552
541
550
566
544
0.22 8.46 3.80
0.17 4.23 3.94
0.35 3.58 3.37
1.41 3.30 4.03
2.79
3.73
2.27
3.00
HBMA-2 277
HBMA-3 305
HBMA-4 281
Fig. 1. PL spectra of the solid powders of HBMAs. Excitation wavelength: 350 nm (HMBA-
1), 340 nm (HMBA-2), 375 nm (HMBA-3 and HMBA-4). Photographs of the solid powders
were taken under UV irradiation from a hand-held UV lamp.

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J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
100
80
700
600
A
B
fh (%)
fh (%)
99
500
60
400
40
99
300
200
100
0
20
0
0-90
0-90
0
20 40 60 80 100
(%)
0
20
40
60
80
100
f
h
f
(%)
h
450
500
550
600
650
700
480
530
580
630
680
Wavelength (nm)
Wavelength (nm)
250
200
150
100
50
3.0
2.5
2.0
1.5
1.0
0.5
0.0
C
D
fw (%)
fh (%)
99
99
...
10
0
0
0
20 40 60 80 100
(%)
0
20
40
60
80 100
0-90
f
f
(%)
w
h
500
550
600
650
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 2. PL spectra of HBMA-1 (A), -2 (B), and -3 (C) in THF/hexane mixtures. (D) PL spectra of HBMA-4 in DMSO/water mixture. Excitation wavelength: 350 nm (HMBA-1), 340 nm (HMBA-
2), 375 nm (HMBA-3 and HMBA-4). Concentration: 1 mM. Plots of their relative PL intensity (I/I0 − 1) at peak position versus fh or fw are shown in the insets of each figure.
indistinctive (2.6-fold) after forming aggregates at 99% water fraction
(fw). The above PL spectra indicate that the AIE performances depend
highly on the position of D-A units. The excellent AIE property of
HMBA-1 stems from the highly symmetrical para-position of its D-A
units, which would favor the compact and well-ordered molecular
packing. The free volume in the aggregation is thus shrunken to further
suppress the intramolecular motion to enable higher emission effi-
ciency. On the contrary, molecular asymmetry is easy to induce loose
packing which allows limited rotational and/or vibrational motion and
thus lowers the emission efficiency.
that the most remarkable bathochromic-shift was found with HMBA-
3, which is as large as 30 nm. This result suggests the strongest ICT effect
of HMBA-3, and agrees well with its longest emission wavelength of the
solid-state spectra as seen in Fig. 1. The spectral changes including
bathochromic shift and the gradually enhanced shoulder peak clearly
demonstrate the formation of the CT state, especially in polar solvents.
In the resulting CT state, solvent relaxation occurs with the intramolec-
ular rotation of the D-A groups to a certain angle, resulting in a much
weaker conjugation relative to its locally-excited state (plane conforma-
tion). Such twisted conformation will be stabilized by the polar solvent
surrounded, giving rise to a much narrowed energy gap and the conse-
quent red-shifts in the absorption bands [37,38].
3.2. Intramolecular Charge Transfer
DFT calculations on their energy levels were performed to prove the
ICT process of the HMBAs. The HOMO and LUMO molecular orbital am-
plitude plots are shown in Fig. 4. Indeed from molecular simulation the
distribution in the HOMO and LUMO electron clouds is asymmetrical for
all the involved molecules. For instance, HOMO orbital of HMBA-1 is
mainly located on the methyl benzene moiety and partially located on
the other benzene ring, carboxylic group are excluded from electron de-
localization due to its electron-deficiency. In contrast, the benzoic acid
group has attracted a large proportion of LUMO electron, owing to its
strong electronic withdrawing ability. The other compounds also ex-
hibit asymmetrical electronic cloud distribution and their locations are
similar to HMBA-1. In short, HOMO electron would be held due to the
electronegativity of methyl benzene moiety, and it tends to shift to the
benzoic acid in the LUMO orbital. Although the energy levels of both
The sensitivity of HMBAs to solvent polarity was studied to probe
their ICT effect. Take HMBA-1 for example, its absorption spectrum in
hexane shows that the π-π* absorption band peaks at around 276 nm
(Fig. 3A). Moreover, it has a shoulder peak at 356 nm, which is ascribed
to the CT transition. From non-polar solvents (i.e., hexane) to polar sol-
vents (i.e., DMSO),
a bathochromic-shift of π-π* transition
(276–284 nm), is recorded. Simultaneously, the CT band, which is rela-
tively weak in hexane, EA and CHL, has increased remarkably in high-
polar solvent such as DMF and DMSO. The π-π* and CT absorption of
the other compounds (including HMBA-2, 3 and 4) fall into the similar
region as that of HMBA-1. In addition, their absorption spectra show a
similar response toward solvent polarity: red-shift in π-π* absorption
band and enhancement in CT absorption band. It is worth mentioning

J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
5
A
B
HE
HE
EA
EA
THF
CHL
DMF
DMSO
THF
CHL
DMF
DMSO
260
310
360
410
460 260
310
360
410
460
Wavelength (nm)
Wavelength (nm)
C
D
HE
EA
HE
EA
THF
CHL
DMF
DMSO
THF
CHL
DMF
DMSO
260
360
460
560
260
360
460
560
Wavelength (nm)
Wavelength (nm)
Fig. 3. Normalized UV–vis spectra of HBMA-1 (A), -2 (B), -3 (C) and -4 (D) in organic solvents with different polarities. Concentration: 10 μM. Abbreviation: ethyl acetate (EA),
tetrahydrofuran (THF), chloroform (CHL), N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO).
Fig. 4. Molecular orbital amplitude plots of the HOMO and LUMO energy levels of HBMA-1 (A), -2 (B), -3 (C) and -4 (D), calculated by using the B3LYP/6-31G* basis set.

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J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
HOMO and LUMO varies slightly with the position of D/A units, the re-
sult is consistent with that from spectral analysis and demonstrates
that the lowest energy gap (3.37 eV) was obtained with HMBA-3
(Table 1), which has been proved to have the strongest ICT effects. It
is clear that these solid emitters could change the emission color by sim-
ply varying the position of D/A groups to fine-tune the HOMO/LUMO
energy levels.
two-dimensional square structure with kelly green fluorescence. SEM
images in Fig. 5B and C reveal the fine structure of the squares, which
are actually formed by closely stacked multilayer sheets with nano-
sized thickness and glaze surfaces. After natural evaporation of a few
drops of chloroform solution (2 mM), HMBA-2 assembles into micro-
sized rods with yellow fluorescence (Fig. 5D). As displayed in the SEM
images (Fig. 5E and F), the diameter of the rod-like assemblies ranges
from 0.5 to 2 μm. It is difficult to estimate the aspect ratio because the
micro rods are mostly interlaced. As expected, the longest emission
wavelength (orange color) was obtained with HMBA-3 assemblies,
which is formed by drop-casting its chloroform solution (2 mM) as
well. As shown in Fig. 5G–I, HMBA-3 assembles into one-dimensional
nanofilaments with diameter down to 100–500 nm and length up to
hundreds of micrometers. Ordered assemblies are also formed by
drop-casting technique (chloroform, 10 mM) using HMBA-4 molecules
as the building blocks. The assemblies seem to be one-dimensional and
emit yellow fluorescence upon excitation with a UV beam (Fig. 5J). We
only got an obscure view of the HMBA-4 assemblies with fluorescence
microscope due to the fact that they have fallen into nanoscale level.
The ambiguity of the structural details was resolved by SEM, which ex-
hibits that HMBA-4 forms flexible nanowires with their average width
down to dozens of nanometers (Fig. 5K and L).
3.3. Color-tuned and Shape-controlled Self-assembly
Molecular self-assembly is the process in which disordered mole-
cules arrange themselves into an ordered system or even well-defined
architectures as a consequence of specific intermolecular weak forces
within the “building blocks”. These driving forces, including electro-
static interactions, hydrogen bonding, π-π stacking, dipole-dipole inter-
action, and hydrophilic/hydrophobic effect, have enabled diversiform
supramolecular structures such as spheres, fibers, rods and helical struc-
tures [39–42]. Impressive as it is, many sensors would only function
based on specific self-assembled structures [24,43]. For the HMBAs, D/
A units not only adjust the direction of dipole moment but also serve
as functional group, which would induce molecular self-assembly in to-
tally different ways. In this section, we aim to illustrate the position ef-
fect of D/A units in self-assembly behavior.
The above results indicate that a small change in D/A position would
have large effects in the shape of self-assembly structures. If HMBA-1 is
identified as the starting material, repositioning the carboxylic acid and
methyl group could hence built the other three isomers. Quadrate
The assembly of HMBA-1 was fabricated by drop-casting using chlo-
roform solution (50 mM). It was then observed with a fluorescence mi-
croscopy as shown in Fig. 5A. It reveals that HMBA-1 has assembled into
Fig. 5. Fluorescence microscopy images of self-assembly structures of HMBA-1 (A), -2 (D), -3 (G) and -4 (J) prepared by drop-casting using chloroform solution at room temperature.
Excitation wavelength: 365 nm. SEM images showing the fine structure of HMBA-1 (B and C), -2 (E and F), -3 (H and I) and -4 (K and L).

J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
7
sheets would turn into micro-rods when shifting carboxylic acid to
meta-position. And changing the position of methyl groups would cre-
ate nanofilaments. With carboxylic acid and methyl group both
changed, flexible nanowires are obtained. Among the four compounds,
the assembly of HMBA-4 shows the minimum scale, which endows it
with the largest specific surface area. Therefore, its performances may
be improved if it is fabricated into a sensor device. Besides, the diversi-
fied shapes of the molecular assemblies could be obtained through facile
drop-casting process, in which no surfactants, patterned templates, or
additives are needed. Such simplified technology makes the molecular
system a reliable candidate for real application.
The shape-controllability of the self-assemblies stems from the in-
ductive effect of D/A units. We anticipate that the direction of dipole-
dipole interaction is the main driving force to organize molecules
when self-assembling. When molecules start to aggregate from their so-
lution, D unit (methyl group) would prefer to couple with A unit (car-
boxylic acid) rather than itself due to dipole-dipole interaction, as the
former is electron-rich whereas the latter is electron-deficient. Such
D-A coupling would correspond to a lowest energy state, compared
with the other packing modes. Polarized by D-A units, the dipole mo-
ments of these molecules range from 2.27 Debye to 3.73 Debye as listed
in Table 1. Molecules with meta-position substituents evidently show
much higher dipole moment than those with para-position substituents
due to molecular asymmetry. On the other hand, the molecules ar-
ranged in the assemblies also prefer to adopt the conformation with
the minimal steric hindrance. Taking both sides into consideration, D/
A units doubtlessly plays a vital role in governing the molecular self-
assembly, as their location will directly influence the dipole-dipole in-
teraction as well as the steric hindrance. As a result, D/A units in this
molecular series can be visually regarded as “rotary knobs”: When the
knobs rotate, i.e., D/A units relocate, the four states of the architec-
ture will appear in sequence, including quadrate microsheets,
microrods, nanofilaments and nanowires. In short, the rational mo-
lecular design of the HMBAs contributes to the nice controllability
of their assembly structures, and further paves ways for developing
novel smart materials with customized morphology to meet the
needs of a given application.
experiment, suggesting the highest sensitivity. Additionally, an obvious
hypochromatic shift from 545 nm to 534 nm was detected. The final
quenching efficiencies of the four sensors are approximately 24.4,
−4%, 49% and 96%, respectively. As listed in Table 1, the maximum
emission wavelength of each molecule shows slight differences among
three forms, i.e., solid powder, aggregates and film. This is owing to
the ICT nature of the isomers, as the CT state is susceptible to multiple
factors including chemical environment, steric hindrance and packing
mode [37,38].
From the point of view of aggregation structures, such different
spectral response can be partially ascribed to their characteristic mor-
phologies with specific surface area differing from each other. HMBA-
4 in possession of the largest specific surface area allows for stronger
solid-gas interaction compared with the other three competitors. Fur-
thermore, the self-assemblies of HMBA-4 is the most vulnerable to am-
monia due to the nano-sized fine structure, it shows the most distinct
visual change with continuous NH3 fumigation under fluorescence mi-
croscope (Fig. 7A–D). In addition, the assemblies of HMBA-4 actually be-
come obscured after fumigation as seen in Fig. 7D. This implies that the
architecture of HMBA-4 is easy to be destructed upon fumigation. The
disassembly of the HMBA-4 with the exposure of other organic amines
including n-butylamine (Fig. S9A), diisopropylamine (Fig. S9B) and
triethylamine (Fig. S9C) was also observed with fluorescence micro-
scope. The reduced fluorescence together with the hypochromatic
shift could be readily detected by naked eyes. The quenching efficiency
is probably owing to the continuous collision between the HMBA-4 and
ammonia, which would consume the energy of the excited molecule.
And the hypochromatic shift may be ascribed to the changes in energy
level of the molecule binding with ammonia. Besides, the disassembled
structure (Fig. S10) together with the corresponding emission spectrum
(Fig. S11) is unable to recover after thermal treatment, implying the sin-
gle use of the sensing device.
The molecular stacking mode is also of great importance, as loose
packing will help the analyte to enter the assembly structure while
close packing is just the opposite. Stacked microsheet structure belongs
to HMBA-1 is less sensitive as indicated in Figs. 6A and 7A, not only be-
cause it offers the smallest surface area for sensor-ammonia interaction,
but also it is probably closely packed to inhibit the invasion of ammonia
due to better molecular symmetry. Furthermore, relocating D/A units
would vary the ICT process as discussed previously. If the A moiety (car-
boxylic acid) is attacked by amines, electron withdrawing ability would
be changed to affect the spectrum, which may involve both intensity
change and wavelength shifting. Besides, the emission of AIE com-
pounds is sensitive to steric hindrance, therefore inserting ammonia
molecule into the self-assembled architectures will further block the in-
tramolecular motion to boost stronger emission. In conclusion, there are
multiple factors that contribute to the spectral responses, including sur-
face area, ICT effect and intermolecular volume, making the sensing per-
formances vary with the type of the AIE isomers.
DFT calculations on electron cloud distribution and energy levels
were performed in order to verify the mechanism of the spectral
changes. As depicted in Fig. 8, the HOMO energy of HMBA-4-ammonia
association is dominated by the orbital from the whole molecule except
for carboxylic moiety. And there is no obvious change in the location of
LUMO electron cloud, the energy level of both HOMO (−5.85 eV) and
LUMO (−1.82 eV) was elevated when HMBA-4 was attacked by ammo-
nia, suggesting a less stable state than that of the starting molecule
(Fig. 4D). In that way, the electron deficiency of the carboxylic acid
group would be remitted by the external ammonia molecule carrying
lone pair electrons. This will weaken the electron withdrawing ability
of the carboxylic acid group. As displayed in Fig. 8, we observed that
electronic cloud exerts no shifting along the D-A direction when com-
paring HOMO to LUMO, implying that the strength and efficacy of the
ICT process are reduced. The spectral changes including decrease in in-
tensity and blue-shift in wavelength can be well explained by the calcu-
lation results.
3.4. Sensing Amine Gas
Benzoic acid moiety shows strong affinity to amines [44], and if its
electron-withdrawing ability was disrupted by amines, the ICT process
would be disturbed, offering the potential to cause fluorescent re-
sponses. A fluorescent sensor for amines can be created based on such
mechanism. The necessity to innovate new amine sensors based on
fluorescent signal is due to their high sensitivity, portability, fast re-
sponse and easy visualization [45–47]. These advantages are exactly
highly desired by food industry. In view of this, we fabricated a fluores-
cent sensor by depositing HMBA-1 assemblies on a quartz plate using
the aforementioned methods, and then fixed it in the inner wall of a
quartz cell. Ammonia water (14 M) was added to provide sufficient am-
monia gas for fumigation. The PL spectrum of the sensor was collected
once every 1 min for 25 min. As displayed in Fig. 6A, the emission inten-
sity at 541 nm becomes progressively lower with an increase in the ex-
posure time, and remains basically unchanged within a short time down
to 10 min. HMBA-2 was also fabricated into a self-assembly film for the
time-dependent measurement. In contrast, the spectrum peaked at
550 nm exhibits a slight decrease in the first minute and then shows
slight growth (Fig. 6B). The rising tendency stops after 3 min, indicating
a full interaction between ammonia gas and the sensor. The HMBA-3
self-assembly film was continuously fumed by ammonia under the
same condition. Its time-dependent emission spectrum undergoes a re-
markable decrease upon fumigation, suggesting a stronger interaction
between the sensor and the analyte (Fig. 6C). As depicted in Fig. 6D, a
much stronger emission induction relative to other three sensors was
obtained with HMBA-4-loaded film in this time-dependent fumigation

8
J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
0.25
0.20
0.15
0.10
0.05
0.00
0.08
0.04
0.00
-0.04
B
A
0
1
2
3
4
5
6
0
2
4
6
8 10 12
Time (min)
Time (min)
500
550
600
650
500
550
600
650
Wavelength (nm)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
0.5
0.4
0.3
0.2
0.1
0.0
C
D
0
5
10 15 20 25
Time (min)
0
5
10 15 20 25 30 35 40
Time (min)
500
550
600
650
700 500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 6. Time-dependent PL spectra of the self-assembly film of HBMA-1 (A), -2 (B), -3 (C) and -4 (D) exposed to ammonia gas. Time interval: 1 min. Excitation wavelength: 350 nm (HMBA-
1), 340 nm (HMBA-2), 375 nm (HMBA-3 and HMBA-4). Plots of their relative PL intensity (I/I0 − 1) at peak position versus time are shown in the insets of each figure.
The sensitivity analysis was estimated by a titration experiment, in
which a self-assembly film of HMBA-4 was fixed in a quartz cell and
then fumed by ammonia water with gradually increased concentra-
tions. As shown in Fig. 9, the emission spectrum of self-assembly film
undergoes a sustainable growth with the vapour pressure of ammonia
ranging from 0 to 652 Pa, and then level off. The final-state emission
has increased by 75% as plotted in the inset of Fig. 9. A linear equation
is obtained via data fitting by using growth rate (I/I0 − 1) as a function
of the square of vapour pressure. The detection limit (DL) of the sensor
is determined to be 186 Pa according to the equation expressed as DL =
3σ/k (σ stands for the standard deviation of the blank signal; k repre-
sents the slope of the linear equation). It is nice to see that signal of
the sensor is enhanced rather than quenched, indicating that the am-
monia gas adsorbed on the film is unable to disassemble the well-
ordered structure of the sensor. Thus the AIE mechanism still dominate
the emission process, in which the ammonia molecules have inserted
into the self-assembly architecture to further block the intramolecular
motion.
the freshness of the meat products. Our aim is to test the response of the
self-assembled film to amine vapours generated by meat samples. Fresh
lean pork was purchased from a local supermarket and then cut into
slices to make four samples for testing: Sample 1 is fresh pork, Sample
2, 3 and 4 were stored at room temperature for 24 h, 48 h and 72 h, re-
spectively. All the samples were sealed in the quartz cell to produce
amines before being measured. The HMBA-4 self-assembled films for
meat monitoring were fabricated using the aforementioned method,
with the average thickness of 4.6 μm. Then the films were continuously
fumed by pork samples and their in-situ time-dependent spectra was
recorded (Figs. S12–15). Fresh sample (Sample 1) exerts almost no
change as seen in the tendency chart (Fig. 10). The fluorescence inten-
sity varies by approximately 2%, which can be ascribed to instrumental
error. In contrast, the pork sample stored for 24 h (Sample 2) causes in-
tensity growth of 17% relative to the blank spectrum. This indicates that
the amount of the amine gas generated by Sample 2 was too small to
disassemble the film structure and thus the AIE mechanism of the
probe would be strengthened to boost higher fluorescence. In sharp
contrast, the emission intensity was reduced by 14% when the film
was placed beside Sample 3, which has been stored for 48 h prior to
use. Similarly, the film losses 40% of its starting intensity with the pork
sample storing for 72 h (Sample 4). The quenching effect of the latter
two samples implies the disassembly of the films and further indicates
the spoilage of the pork, as a large amount of amine gas will be released.
In addition, the spectra of the sensing films show an immediate change
with putrid pork samples, and have completed the full spectral response
3.5. Sensing Meat Spoilage
Microorganisms could break down amino acids to yield various
kinds of amines. In the meat spoilage process, amino acids can either de-
grade to ammonia through deaminisation, or to biogenic amines by de-
carboxylation, such as cadaverine, spermidine, putrescine and spermine
[48]. Therefore, the abnormal high level of amines can be used to signal

J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
9
Fig. 7. Fluorescence microscopy images of the self-assembled film of HBMA-1 (A), -2 (B), -3 (C) and -4 (D) exposed to ammonia, taken at different time. Scale bar: 100 mm. Excitation
wavelength: 365 nm.
within a short time down to 10 min. This suggests a fast detection which
is highly desired in real-world application. This experiment demon-
strates that HMBA-4 offers great potential to serve as a sensor for food
spoilage monitoring, and its diversified signal responses would help to
establish the standard for meat freshness.
4. Conclusion
The incorporation of D-A units to the synthesized positional isomers
endows them with both AIE and ICT effect. Our systematic comparative
0.8
0.6
0.4
0.2
0.0
0
500
1000
1500
Vapour pressure Pa
0.8
0.6
0.4
0.2
0.0
R2 = 0.92
0
100000
200000
[Vapour pressure]2 (Pa2)
500
550
600
650
700
Wavelength (nm)
Fig. 9. PL spectra of HMBA-4 self-assembled film upon fumigation with ammonia water
with different concentrations (0 mM, 0.1 mM, 0.2 mM…0.8 mM). Inset: relative change
in maximum emission intensity versus [vapour pressure] and [vapour pressure]2.
Fig. 8. Molecular orbital amplitude plots of the HOMO and LUMO energy levels of HMBA-4
combining with ammonia. Basis set: B3LYP/6-31G*.

10
J. Hou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 205 (2018) 1–11
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study leads to the discovery of their high controllability in the
photophysical properties: their AIE properties, ICT effects and energy
levels can be well controlled by the position of D-A units. Furthermore,
HMBAs show the color-turned and shape-controlled self-assembly. The
D/A units play a key role in guiding the molecular self-assembly, as their
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units repositioning, four types of the self-assembled architectures ap-
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impact of a different degree in the D-A efficacy of the HMBAs, which is
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nanowire structure with largest specific area, HMBA-4 has the most dis-
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Acknowledgments
Support is acknowledged from the National Natural Science Founda-
tion of China (Grant Nos. 51703135, 21503139 and 11474204) and Pro-
ject of Construction of Scientific Research Base under Beijing Municipal
Education Commission (Grant Nos. KM201610028006 and
KM201610028007). The corresponding author is also grateful for the
support from Beijing Key Laboratory of Optical Materials and Photonic
Devices.
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