Self-assembly of a fluorescent galunamide derivative and sensing of acid vapor and mechanical force stimuli

Article abstract of DOI:10.1039/c7tc03752j

A galunamide derivative consisting of a fluorophore with benzoxazole and a cyano unit was designed and synthesized to realize a dual response to acid vapor and mechanical force. Photophysical properties and self-assembly were investigated. The dilute solution could respond to the acid stimulus, and the emission peak at 472 nm shifted to 535 nm with the addition of trifluoroacetic acid (TFA). Moreover, the derivative could self-assemble into thin nanofibers in its gel phase, and the fluorescence of the xerogel film is sensitive to TFA vapor. The emission color gradually changed from yellow to orange red upon exposure to acid vapor, and the detection limit is as low as 70 ppb for TFA. The solid exhibited a reversible fluorescence color change under the stimuli of mechanical force and solvent annealing. The UV-vis absorption and IR spectra indicate that the fluorescence color conversion can be attributed to the destruction of intermolecular hydrogen bonds and pristine π-π interaction induced by mechanical force.

Full text of DOI:10.1039/c7tc03752j

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Self-assembly of fluorescent galunamide derivative and sensing of
acid vapor and mechanical force stimuli
Received 00th January 20xx,
Accepted 00th January 20xx
a,b,*
b
b
b
Pengchong Xue,
Boqi Yao Yanbing Shen, and Hongqiang Gao
DOI: 10.1039/x0xx00000x
A galunamide derivative consisting of a fluorophore with benzoxazole and cyano unit was designed and synthesized to
realize dual response to acid vapor and mechanical force. Photophysical properties and self-assembly were investigated.
The dilute solution could respond to acid stimulus, and the emission peak at 472 nm shifted to 535 nm with the addition of
trifluoroacetic acid (TFA). Moreover, the derivative could self-assemble into thin nanofibers in its gel phase, and the
fluorescence of xerogel film is sensitive to TFA vapor. The emission color gradually changed from yellow to orange red
upon exposure to acid vapor, and the detection limit is as low as 70 ppb for TFA. The solid exhibited reversible
fluorescence color change under the stimuli of mechanical force and solvent annealing. The UV–vis absorption and IR
spectra indicate that the fluorescence color conversion can be attributed to the destruction of intermolecular hydrogen
bonds and pristine π–π interaction induced by mechanical force.
www.rsc.org/
π–π interaction between luminescent gelator and long-range
exciton migration within gel nanofibers can amplify
fluorescence quenching with the help of surface-adsorbed
Introduction
Stimuli-responsive smart materials can change their chemical
and physical properties under external stimuli, such as light,
heat, anions, cations, mechanical force, various gases,
1
4
analytes. To date, several fluorescent or phosphorescent
organogels have been developed for sensing gases, such as,
1
5
16
organic amines, volatile acids,
17
18
explosives and
1
O
2
,
magnetic field, and so on. These stimuli sources may induce
1
9
humity.
molecular structure or intermolecular interaction changes in
materials. Among these materials, luminescent organic
materials that respond to various stimuli sources have been
Recently, luminescent materials in response to mechanical
force have received significant attention because these
materials can change luminescent color, intensity, or lifetime
2
widely developed. External stimuli always induce these
2
0
under external force stimuli.
These materials include
and phosphorescent
mechanochormic materials and may be widely used in
luminescent organic molecules to alter their emission intensity,
emission wavelength, or luminescence lifetimes. Thus far,
luminescent organic molecules act as sensor to detect ions in
mechanofluorochromic
(MFC)
2
1
22
3
sensors, and security inks. Numerous kinds of organic
molecules have been found to exhibit MFC behaviour, such as
solution state and quantitatively sense gases, such as organic
4
5
6
7
8
9
10
amines, O , CO2, SO2, HCl, NH3, trinitrotoluene, and
2
1
23
24
1
tetraphenylethene,
2
triphenylamine,
9,10-divinylanthracene,
picric acid in film state. For obtaining rapid response to
gaseous analytes and low detection limits, large surface area is
5
26
phenothiazine,
and β-diketone boron
2
7
1
2
complexes, and so on. However, the development of organic
molecules with both MFC properties and sensing
characteristics to gases remains a challenge.
needed for sensing films. 3D networks consisting of a large
number of thin self-assemblies can be easily supported in
supramolecular gels constructed by low molecular mass
1
3
gelators. Their xerogel film may provide large surface area
and porous microstructure, which can enhance the adsorption
and accumulation of gaseous molecules. Moreover, the strong
Scheme 1. Molecular structure of 3C16PBCP with hydrogen bonding unit,
proton-binding site and large cyano group.
In this study, we attempt to design a fluorescent organic
molecule that can self-assemble into thin fibers, respond to
acid vapor and exhibit MFC activity. The benzoxazole group is
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confirmed to be excellent proton-binding unit and has been The glass slide was placed in a cell (optical path = 1 cm) and
successfully used as building blocks to prepare sensors for pH the cell was sealed by sealing film. The gaseous acids at
2
8
and acid vapors. Moreover, several studies verified that the different concentrations were simply obtained by injecting
cyano group with a large steric hindrance may suppress saturated vapor with certain volume into the cell. Then, the
aggregation-caused quenching and promote molecule to fluorescence of xerogel film was measured after 5 min.
2
9
possess MFC behavior. In addition, the galunamide unit is a Synthesis, procedures, and characterization
proven efficient functional group to induce the gelation and
self-assembly of molecule and solvents into one-dimensional
3
0
nanofibers. Therefore, a galunamide derivative consisting of
a fluorophore with cyano and benzoxazole moieties was
designed and synthesized. This compound can gelate certain
solvents and self-assemble into one-dimensional nanofibers
with strong yellow fluorescence. The nanofibers could respond
to acids not only in solution but also in gel phase. Interestingly,
the xerogel film also emitted strong fluorescence, which can
be quenched by volatile acid vapors. The detection limit for
trifluoroacetic acid (TFA) reaches 70 ppb. As expected,
mechanical force stimulus can induce fluorescence change
from green to orange. Moreover, solvent annealing leads to
fluorescence recovery, showing a reversible MFC behavior.
Scheme 2. Synthesis route of 3C16BPCP
.
Experiment section
1
13
3C16BPCP was easily obtained by a one-step reaction from
and in the presence of triethylamine, as shown in Scheme 2.
Compounds and were synthesized by the reported
The detailed experimental procedure was as
1
Measurements and instruments
spectra were recorded using a Mercury Plus instrument at 400
MHz and 100 MHz in CDCl in all cases. FT-IR spectra were
recorded using Nicolet-360 FT-IR spectrometer by
:
H NMR and C NMR
2
1
3
3
3
1,32
methods.
follows:
a
incorporation of samples into KBr disks or casting sample in a
NaCl crystal. UV− vis spectra were obtained on a Mapada UV-
N-(4-((Z)-2-(4-((E)-2-(benzo[d]oxazol-2-yl)vinyl)phenyl)-1-
cyanovinyl)phenyl)-3,4,5-tris(hexadecyloxy)benzamide
1
800pc spectrophotometer. Fluorescence emission spectra
(
3C16BPCP
(2.0 g, 2.37 mmol) was dissolved in SOCl
mixture was refluxed for 5 h. After removing the excess SOCl
corresponding acyl chloride was obtained and used without
further purification. was dissolved in dry CH Cl (20 mL) with
triethylamine (0.66 mL). The mixture was cooled in an ice-
water bath. The THF solution of (0.86 g, 2.37 mmol) was
)
were obtained on a Cary Eclipse fluorescence
1
2
(5 mL) and the
spectrophotometer. C, H, and N elemental analyses were
performed on a PerkinElmer 240C elemental analyzer. Mass
spectra were obtained with AXIMA CFR MALDI-TOF (Compact)
mass spectrometer. Transmission electron microscopy (TEM)
images were observed with a Hitachi H-8100 apparatus by
wiping the samples onto a 200-mesh carbon coated copper
grid followed by naturally evaporating the solvent. Scanning
electron microscopy (SEM) was performed on HITACHI SU8020
2
2
2
2
2
3
added dropwise and then the mixture was stirred for 12 h at
room temperature. Water was added and then organic layer
2 4
was separated and dried over Na SO . After the solvent was
(
operating at 3.0 kV). The samples were prepared by casting
removed, the residue was purified by column chromatography
using petroleum ether/CH Cl (v/v = 1/2) as an eluent. Yellow
the wet gels onto a clean silicon wafer followed by drying in air.
Before observation, the samples were coated by a gold film.
Fluorescence microscopy (FM) images were obtained on
fluorescence microscope (Olympus Reflected Fluorescence
System BX51, Olympus, Japan).
2
2
1
solid was obtained in a yield of 82 %. H NMR (400 MHz, CDCl )
3
δ 7.97 (d, J = 8.4 Hz, 2H), 7.85 (d, J = 16.4 Hz, 1H), 7.84 (s, 1H),
7
1
4
.79 – 7.74 (m, 3H), 7.71 (m, 4H), 7.60 – 7.54 (m, 1H), 7.53 (s,
H), 7.42 – 7.32 (m, 2H), 7.19 (d, J = 16.3 Hz, 1H), 7.06 (s, 2H),
.13 – 3.96 (m, 6H), 1.90-1.26 (m, 84H), 0.88 (t, J = 6.7 Hz, 9H)
Gelation Ability Investigation: The solution containing 5.0 mg
gelator in selected organic solvent (1.0 mL) was heated in a
sealed bottle with 1 cm diameter by a heating panel until the
solid was dissolved. After the solution was cooled to room
temperature and placed for 6 h, the gel state was evaluated by
the “stable to inversion of a test tube” method.
1
3
(Fig. S9). C NMR (101 MHz, CDCl ) δ 165.70, 162.44, 153.30,
3
1
1
1
3
1
50.47, 142.15, 141.81, 139.89, 139.24, 138.13, 137.00, 134.91,
30.06, 129.82, 129.41, 128.01, 126.79, 125.53, 124.68, 120.37,
20.01, 117.92, 115.41, 111.57, 110.42, 105.91, 73.61, 69.52,
1.94, 30.36, 29.74, 29.68, 29.61, 29.44, 29.39, 26.11, 22.71,
Preparation and measurement of sensing film
mg) was dissolved in DMSO (1.5 mL) under heating to form
clear solution at 160 C. A hot solution of 10.0 μl was casted
uniformly on the glass slide (1.0 2.5 cm) at 90 C, and then
: 3C16BPCP (1.0
4.13 (Fig. S10).
+
MALDI-TOF MS: m/z: calcd: 1187.9; found: 1187.7 M (Fig. S11).
Chemical Formula: C H N O Elemental Analysis: C, 79.82; H,
º
7
9 117 3 5
×
º
-1
9
.92; N, 3.53; O, 6.73. FT-IR (KBr, cm ): 3280, 2915, 2850, 2213,
655, 1578, 1520, 1492, 1467, 1342, 1120.
the sample was aged at room temperature until a gel film was
formed. After 2 h, the solvent was removed in a vacuum oven.
1
2
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TFA. Based on the absorption, emission and NMR spectra, the
Results and discussion
response of 3C16BPCP to TFA is due to the binding of
4
benzoxazole unit to proton (Fig. S2a).
3
Self-assembly in gel.
a
Table 1. Gelation ability of 3C16BPCP in organic solvents.
b
Solvent
Cyclohexane
n-Hexane
Status (CGC)
Solvent
THF
Status (CGC)
S
P
P
P
S
S
S
I
Ethyl acetate
Acetone
G (0.84)
G (0.56)
G (2.1)
n-Octane
Toluene
Acetophenone
3-Pentanone
Acetic acid
DMSO
o-Dichlorobenzene
G (4.2)
CHCl
Ethanol
CH Cl
3
G (0.84)
G (0.21)
G (0.70)
2
2
S
DMF
b
a
G: gel; I: insoluble; S: soluble; P: precipitate; critical gelation concentration
(mmol/L).
First, the gelation ability in different solvents was
investigated, and the results are listed in Table 1. The gel could
dissolve in nonpolar solvents, such as cyclohexane, n-hexane,
and n-octane, under heating, but only yellow precipitate
appears when the hot solutions were cooled to room
temperature. 3C16BPCP is highly soluble in aromatic solvents
(toluene and o-dichlorobenzene), CH
2 2
Cl , and THF. However,
3
C16BPCP forms yellow gels in certain organic solvents, such
Fig. 1 Absorption and emission spectra of 3C16BPCP in different solvents.
3
ex
λ =
80 nm.
as ethyl acetate, acetone, acetophenone, 3-pentanone, acetic
acid, DMSO, and DMF. Moreover, certain solvents possess low
critical gelation concentrations. A 1.0 mg portion of 3C16BPCP
Photophysical properties in solution.
The absorption and emission spectra of 3C16PBCP were first
investigated. As shown in Fig. 1, the maximal absorption peak
is located at 388 nm in toluene. The absorption spectra in THF
and DMF with large polarities are similar to that in toluene.
The maximal fluorescence peak in toluene is located at 471 nm
with two shoulder peaks at 450 and 500 nm, implying a
vibrational structure. The shoulder peaks disappear, and the
emission bands broaden in polar solvent. Moreover, the
maximal emission peaks show a bathochromic shift. For
example, the THF solution presents a maximal emission peak
at 478 nm that further shifts to 490 nm in DMF. These spectral
changes suggest that 3C16PBCP in the excited state bears a
larger polarity relative to that in ground state because of the
−4
can gelate 4.0 mL DMSO, corresponding to 2.1 × 10 mol/L.
That is, one gelator may prevent more than 67000 DMSO
molecules from flowing. This result shows that 3C16BPCP is an
excellent gelator for certain solvents.
Thereafter, the self-assembly of 3C16BPCP was investigated
by SEM, TEM, FM, IR, and absorption and emission spectra. Fig.
2
shows SEM, TEM and FM images. From the SEM and TEM
images, we can find that the DMSO gel was composed by
numerous long, thin fibers with diameters of 50–200 nm. The
small diameter provides large surface area, which favors the
adsorption of analytes and decreases detection limit. The FM
image of DMSO gel shows that gel fibers emit bright yellow
fluorescence (Fig. 2c) when excited by the light at 330–385 nm,
and numerous fibers construct a 3D network. The results show
that gelators form one-dimensional self-assemblies in gel
phase with the help of the intermolecular non-covalent
interaction. Then, IR, absorption, and emission spectra were
used to study the driving force of molecular self-assembly.
In the IR spectrum of xerogel, the vibration peak of C=O
3
3
molecular weak donor–acceptor characteristics.
Owing to the existence of the benzoxazole unit, the
C16BPCP solution is expected to respond to acids. As shown
3
in Fig. S1, the absorption peak is located at 382 nm, which
gradually decreases after TFA addition. Simultaneously, the
absorbance at ca. 450 nm increases upon the application of
TFA stimulus. Moreover, the addition of TFA results in changes
in the emission spectra. The emission peak at 472 nm becomes
weak upon the addition of 2.0 equiv. TFA, and the intensity
around 500–600 nm increases. With further addition of TFA,
the peak at 472 nm gradually disappears, and a new peak at ac.
−1
group is located at 1666 cm . A wide and weak peak at 3287
−1
cm corresponding to the vibration of N-H is also found, which
suggests a weak intermolecular hydrogen bond between
35
aromatic amide groups. In addition, two strong vibration
−1
peaks at 2918 and 2850 cm ascribed to long alky chains
530 nm appears. These results indicate that 3C16BPCP
((CH
2
)n) indicates is the existence of van der Waals
responds to acid in solution. NMR spectra were used to further
understand the interaction between 3C16BPCP and the acid.
Evident downfield shifts were observed for three vinyl group
upon the addition of TFA (Fig S2b), suggesting that electron
densities around these protons decrease upon the addition of
36
interactions of aliphatic alkyl chains.
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shifts to 389 nm after cooling for 1 min probably because of
molecular planarization. After 2 min, this peak almost
disappears, and a new absorption peak at 356 nm appears
owing of molecular aggregation and the formation of H-
3
7
aggregates. Gelation also results into fluorescence spectral
change. The hot solution emits green fluorescence, but yellow
fluorescence is observed for DMSO gel (Fig. 3b). During
gelation, the maximal emission peak at 490 nm first arises and
then exhibits a red shift to 530 nm. As a result, a gel can emit
enhanced fluorescence. These spectral changes illustrate
strong exciton interaction between aromatic groups, which
may actually be beneficial to amplify the signal of analytes
owing to exciton diffusion. In addition, we found that the gel in
acetic acid emits weak orange fluorescence. During gelation,
the hot solution exhibits yellow fluorescence with a maximum
at 475 nm; however, this peak almost disappears, and a peak
at 581 nm emerges in the gel phase (Fig. S4). This finding
indicates that the fluorescence of the gel fibers may respond
to acid stimuli. Next, the responsive property of xerogel film to
acid vapor will be investigated.
Moreover, XRD pattern of DMSO gel was obtained to
understand the molecular packing in gel phase. As shown in Fig.
S5a, there are several wide diffraction peaks from 5
suggesting the ordering in gel phase is poor.
Response of xerogel film to acid vapour.
º to 30º,
Fig. 2 (a) SEM, (b) TEM and (c) FM images of 3C16BPCP DMSO gel.
Xerogel films were easily prepared by casting of hot solution
on the glass plate followed by solvent evaporation. The xerogel
film possesses strong yellow fluorescence, which could rapidly
be quenched by TFA vapor and emit orange fluorescence (Fig.
4
a, inset). Then, fluorescence spectra were used to investigate
the relation between fluorescence intensity and TFA vapor
concentration. As shown in Fig 4a, the maximal emission peak
is located at 553 nm. A continuous decrease in emission
intensity and a gradual red shift in emission peak were
observed with increasing TFA concentration. When the
concentration of TFA reaches 384 ppm, the emission peak
shows a maximum at 594 nm rooted in the gelator bound with
proton. The concentration-dependent fluorescence quenching
0
efficiency (QE, 1–I/I ) was plotted to demonstrate the
sensitivity of the xerogel film for sensing gaseous TFA in Fig.
4
b. The QE is saturated at TFA vapor concentrations exceeding
56 ppm. Moreover, a noticeable quenching of 44% in the
2
fluorescence emission can be observed by the TFA vapor of 32
ppm. A detection limit of the xerogel film is determined as 70
ppb for the TFA vapor. Moreover, the sensing property of
xerogel film to acetic acid vapor as weak acid was studied (Fig.
S6). The fluorescence of xerogel film possesses lower
sensitivity to acetic acid than that to TFA. An acetic acid
concentration of 240 ppm only induces a QE of 5.6 % probably
because the weak acidity of acetic acid induces gelator can
Fig. 3 (a) Absorption and (b) fluorescence spectral change during gelation from 180 to
2
0 ºC. Concentration is 0.5 mg/mL, λex=380 nm. Insets in Fig. 3a are the photos of sol
and gel and the change of absorbance at 385 nm during gelation, and insets in Fig. 3b hardly be protonated. To further clarify the distinct response
are the photos of sol and gel under 365 nm.
of gel film to TFA and acetic acid, we checked the pK of
b
gelator as base by measuring absorption spectra in different
Absorption and emission spectra were utilized to study π–π
interaction between aromatic moieties during gelation. As
shown in Fig. 3a, the maximal absorption peak appears at 386
nm in DMSO at 180 °C. This absorption peak increases and
pH solutions. It is 11.3, implying that the pK
acid is 2.7. Because pK s of TFA and acetic acid are 0.23 and
.76, TFA can protonate gelator, but acetic acid cannot do it.
a
of its conjugated
a
4
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Therefore, the gel film exhibited poor sensitivity to acetic acid Furthermore, restoration of the solid with green fluorescence
vapor. Thus, our xerogel film may act a sensing material to by grinding will result in a fluorescence color change from
quantitatively detect TFA vapor.
green to orange. This fluorescence color switch could be
repeatedly observed by grinding and then solvent annealing.
These results show that the MFC behavior of 3C16BPCP is
Mechanofluorochromism of 3C16BPCP in solid state.
3
9
reversible (Fig 5a, inset).
Fig. 4 (a) Changes in the spectral emission of the xerogel film exposed to
different concentrations of TFA vapours for 10 s. (b) Concentration-dependent
fluorescence QE of the film. The intensity was monitored at 525 nm.
We found that the as-synthesized solid from THF emitted Fig. 5 (a) Normalized fluorescence spectra of 3C16BPCP in the solid state under
external stimuli. Inset: photos under 365 nm light.
λ
ex = 400 nm.
green fluorescence and fluorescence color changed into
orange when lightly pressed,
indicating For an insight into the MFC mechanism, absorption, excitation,
mechanofluorochromism.
Thereafter, the fluorescence and IR spectra were conducted on the as-synthesized and
3
8
spectra were utilized to monitor MFC behavior of 3C16BPCP in ground material. As shown in Fig. 5b, the maximal absorption
the solid state. As shown in Fig. 5a, a maximal emission peak at peak emerges at 360 nm, which is similar to that in the gel
5
10 nm appears in as-synthesized solid, allowing it to have phase, suggesting that face-to-face H-aggregates exist in the
green fluorescence with light irradiated at 365 nm. The ground as-synthesized solid. However, the emission spectrum of as-
powder emitted orange fluorescence centered at 550 nm. synthesized solid is at 510 nm, which is shorter than that of
When ground solid was exposed to gaseous common solvents, xerogel (553 nm). XRD pattern of as-synthesized solid was
2 2
such as CH Cl
, THF, toluene, DMSO, and DMF, fluorescence obtained to understand the emission difference. As-
color was not recovered. Moreover, thermal annealing did not synthesized solid has strong and sharp diffraction peaks in
induce the recovery of the fluorescence spectrum. However, small and wide angle region and these diffraction peaks are
green fluorescence reappeared and fluorescence spectrum different from those of gel (Fig. S5b), indicating that a different
was consistent with that of as-synthesized solid when a drop packing model from that in gel and more ordered packing in
2 2
of CH Cl
was added to the surface of ground powder (Fig. 5a). as-synthesized solid relative to in gel. This result illustrates that
In addition, the addition of other solvents, such as THF, different molecular stacking result in their distinct emission
toluene and cyclohexane could result in green fluorescence wavelength. When the as-synthesized solid was ground for 2
recovering, implying that recrystallization in the help of min, the absorption peak emerged at 384 nm, which shows a
solvents should be responsible for fluorescence restoration. slight blue shift relative to that in toluene solution (388 nm).
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This finding suggests that mechanical force stimulus leads to A fluorescent gelator with cyano and benzoxazole moieties
the disappearance of H-aggregates. Then, the excitation was synthesized. The gelator could form gels in certain
spectra before and after grinding were compared (Fig. S7). The solvents and self-assemble into nanofibers in the gel phase.
maximal excitation peak of as-synthesized solid is located at The spectral investigations suggest that the driving forces of
355 nm, which is similar to that in its absorption spectrum (360 gel formation are the hydrogen bonds between amide groups,
nm). This excitation peak becomes weak while the solid is π–π interaction between aromatic moieties, and van der Waals
being ground, indicating that some H-aggregates were broken. interactions of aliphatic alkyl chains. Moreover, the
This result is in good agreement with those obtained by fluorescence of the xerogel film could be quenched, and the
absorption spectral change.
fluorescence color changed from yellow to orange-red upon
Fig. S8 shows the IR spectra of 3C16BPCP solid before and exposure to TFA vapor, and a low detection limit of 70 ppb was
after mechanical force stimulus. as the as-synthesized solid achieved. More interestingly, mechanical force grinding led to
−
1
presents two sharp vibration peaks at 3276 and 1655 cm , fluorescence color change from green to orange because
which are ascribed to N-H and C=O stretching vibrations and intermolecular hydrogen bonds and pristine π–π interaction
indicate the existence of intermolecular hydrogen bonds was destroyed by mechanical force. As a result, this molecule
between amide groups in the as-synthesized solid. However, possesses dual responses to acid vapor and mechanical force.
grinding of the solid induces the weakening of primal two This implies that rational structure design may make molecules
peaks and the emergence of two new peaks at 3385 and 1679 possess multiple responses to external stimuli.
−1
cm . Given that the stronger hydrogen bonds are correlated
with lower wavenumber shift of N-H and C=O stretching
vibration peaks, the IR spectral change clearly illuminates that
mechanical force partially destroys hydrogen bonds between
Acknowledgements
4
0
amide moieties. XRD pattern of ground solid was also
measured. It was found that ground solid had weaker
diffraction peaks relative to as-synthesized solid, implying
grinding destroyed molecular stacking ordering.
This work was financially supported by the National Natural
Science Foundation of China (21103067), the Youth Science
Foundation of Jilin Province (20130522134JH), the Open
Project of State Laboratory of Theoretical and Computational
Chemistry (K2013-02).
Based on the above observations, a brief MFC mechanism for
3C16BPCP is given as follows (Fig. 6). In as-synthesized solid,
hydrogen bonds between amide groups and H-aggregate exist.
However, poor stacking order between fluorophores promotes Notes and references
short emission wavelength. After grinding, hydrogen bonds
and H-aggregate are destroyed, but the distance between
fluorophores becomes short owing to the mechanical force
stimuli, and then ground solid emits longer wavelength
fluorescence. After treatment by solvent, hydrogen bonds and
H-aggregates form again, and fluorescence is recovered.
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DOI: 10.1039/C7TC03752J
Graphic abstract
A galunamide derivative can quantitatively sense acid vapor in xerogel film state and change its
fluorescence color under mechanical force stimuli.