A reversible vapor-responsive fluorochromic molecular platform based on coupled AIE–ESIPT mechanisms and its applications in anti-counterfeiting measures

Article abstract of DOI:10.1016/j.dyepig.2020.108535

In this work, a series of 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives with an aggregation-induced emission–excited state intramolecular proton transfer (AIE-ESIPT) dual mechanism was synthesized. We systematically investigated their optical properties in solution, aqueous suspension, and the solid state. The fluorescence properties of these five compounds were highly dependent on the solvent environment. Particularly, DMSO and DMF were found to significantly promote deprotonation of the phenolic hydroxyl and favor fluorescence emission from the phenolic anion. The Keto or Enol forms generally could be transformed to phenolic anions with the aid of base in solution. The results from aggregation-induced emission enhancement (AIEE) and red-shifted AIE clearly illustrated the evolution process of the fluorescence spectra, indicating that the three luminescent species in the solution were transformable. The systematic investigation demonstrated that the desired fluorescence could be generated for HBT derivatives by varying the solvents or adding additives, such as base or water, to the solution. Because of the synergistic effect of AIE and ESIPT, these fluorophores exhibited high solid-state quantum yields and large Stokes shifts. These dyes also featured high photostability and tunable emission spectra covering most of the visible light region. Single crystal studies and theoretical calculations elucidated the luminescent properties. When loaded on filter paper, the HBT-based dyes exhibited high-efficiency fluorescence visualization and reversible solid-state luminescence switching under alternating amine and acid vapor treatments. These dyes were used on banknotes for anti-counterfeiting measures, demonstrating the practical applications of these molecules as security inks.

Full text of DOI:10.1016/j.dyepig.2020.108535

Journal Pre-proof
A reversible vapor-responsive fluorochromic molecular platform based on coupled
AIE–ESIPT mechanisms and its applications in anti-counterfeiting measures
Bin Li, Datong Zhang, Yiran Li, Xiaobin Wang, He Gong, Yue-zhi Cui
PII:
S0143-7208(19)33045-1
DOI:
https://doi.org/10.1016/j.dyepig.2020.108535
DYPI 108535
Reference:
To appear in: Dyes and Pigments
Received Date: 30 December 2019
Revised Date: 24 April 2020
Accepted Date: 9 May 2020
Please cite this article as: Li B, Zhang D, Li Y, Wang X, Gong H, Cui Y-z, A reversible vapor-responsive
fluorochromic molecular platform based on coupled AIE–ESIPT mechanisms and its applications in anti-
counterfeiting measures, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108535.
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Graphical abstract

A reversible vapor-responsive fluorochromic molecular platform based on coupled AIE–ESIPT
mechanisms and its applications in anti-counterfeiting measures
Bin Li, Datong Zhang*, Yiran Li, Xiaobin Wang, He Gong, Yue-zhi Cui
School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong
Academy of Sciences), 3501 Daxue Road, Jinan 250353, China
* Correspondence: dtzhang@qlu.edu.cn;
Abstract: In this work, a series of 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives with an
aggregation-induced emission–excited state intramolecular proton transfer (AIE-ESIPT) dual
mechanism was synthesized. We systematically investigated their optical properties in solution,
aqueous suspension, and the solid state. The fluorescence properties of these five compounds were
highly dependent on the solvent environment. Particularly, DMSO and DMF were found to
significantly promote deprotonation of the phenolic hydroxyl and favor fluorescence emission from the
phenolic anion. The Keto or Enol forms generally could be transformed to phenolic anions with the aid
of base in solution. The results from aggregation-induced emission enhancement (AIEE) and
red-shifted AIE clearly illustrated the evolution process of the fluorescence spectra, indicating that the
three luminescent species in the solution were transformable. The systematic investigation
demonstrated that the desired fluorescence could be generated for HBT derivatives by varying the
solvents or adding additives, such as base or water, to the solution. Because of the synergistic effect of
AIE and ESIPT, these fluorophores exhibited high solid-state quantum yields and large Stokes shifts.
These dyes also featured high photostability and tunable emission spectra covering most of the visible
light region. Single crystal studies and theoretical calculations elucidated the luminescent properties.
When loaded on filter paper, the HBT-based dyes exhibited high-efficiency fluorescence visualization
and reversible solid-state luminescence switching under alternating amine and acid vapor treatments.
These dyes were used on banknotes for anti-counterfeiting measures, demonstrating the practical
applications of these molecules as security inks.
Keywords: HBT dyes; AIE-ESIPT dual mechanism; reversible solid-state luminescence;
anti-counterfeiting ink; vapor response;
1

1. Introduction
Fluorescent dyes based on excited state intramolecular proton transfer (ESIPT) feature a large Stokes
shift, unique dual emission, and special sensitivity to the external environment [1-3]. Over the past few
decades, extensive spectroscopic and theoretical research has been performed to elucidate the
photophysical processes and the structure-property relationships of ESIPT molecules [4-6]. As a
representative ESIPT molecule, 2-(2-hydroxyphenyl)benzothiazole (HBT) possesses an intramolecular
hydrogen bond between the phenolic hydroxyl and the benzothiazole ring [7,8]. The basic four-level
photophysical cycle (E-E*-K*-K) of the system is illustrated in Figure 1. In the electronic ground state,
HBT exists exclusively in the enol form (Enol) as a result of stabilization of the intramolecular
hydrogen bond. Upon photoexcitation, an excited enol form (Enol*) is generated. Then, a fast proton
transfer reaction accompanied by a tautomeric transformation from the Enol* to the excited keto form
(Keto*) occurs because of the redistribution of electronic charge. The Keto* can radiatively decay to its
ground state (Keto), followed by reverse proton transfer to revert to the Enol form. This fast four-level
cycle in the HBT molecule usually brings about two emission bands: a short wavelength emission
caused by the decay of Enol* to Enol (normal emission, Enol FL) and a longer one resulting from the
Keto* form (ESIPT emission, Keto FL) [1,9,10].
< Insert Figure 1>
According to the ESIPT mechanism, the fluorescence properties of the HBT skeleton are highly
dependent on the solvent environment because of the alterable intramolecular hydrogen bond. Some
research has demonstrated that acids or bases in the solvent can change their fluorescence [11,12].
Reportedly, luminescent phenolic anions also could be generated as the deprotonation product of the
phenolic hydroxyl when some HBT derivatives were dissolved in protic solvents like ethanol or
encounter bases [13,14], which means that a new emission band would be observed. Therefore, it is
possible for some HBT compounds to exhibit triple fluorescence under certain conditions. According
to related literature reports, it could be concluded that compounds containing an ESIPT moiety
generally would be expected to have three emission patterns (single emission, dual emission, and triple
emission), depending on the molecular structure, solvent, and external additives. Recognizing how the
transformation of several luminescent species in the solution of HBT derivatives occurs and
2

understanding how to adjust the multiple fluorescence emission have not been well explored up to now
[14,15].
Recently, organic solid-state fluorescent molecules based on ESIPT have drawn significant attention
because of their wide applications in optoelectronic devices [16, 17], chemical sensors [18,19], and
bioimaging [20]. As ESIPT molecules possess both proton donor and acceptor groups, typical ESIPT
molecules can be protonated or deprotonated depending on the pH of the solution. We envisioned that
alkaline vapor could also react with the hydroxyl group of the HBT derivatives, leading to a
fluorescence change. Such vapor-induced fluorescence responses are useful for detection of volatile
organic compounds (VOCs) [21-23], anti-counterfeiting labels [24], and rewritable printing media [25].
For HBT-based dyes, however, investigation of their fluorescent response to alkaline or acidic vapor as
well as their potential applications in a solid state have been rather limited.
In developing stimuli-responsive luminescent organic materials, a series of fluorescent molecules
based on the HBT scaffold was designed and synthesized (Scheme 1). The phenol ring of HBT was
modified with simple substituents, including hydroxyl, chloride, and bromine. Phenylcarbazole and
triphenylamine moieties were also connected on the phenol ring to afford two HBT derivatives with
extended aromatic conjugated systems. By careful modification of the phenol ring of the HBT core, a
broad tuning range of fluorescence emission from 450 to 700 nm was achieved. Evaluation of the
solvent effect demonstrated that DMSO and DMF more significantly facilitated normal enol emission
than protic solvents. DMSO and DMF also were found to markedly promote deprotonation and to favor
fluorescence emission from the phenolic anion. In our experiments, we succeeded in transforming the
Keto or Enol forms to phenolic anions with the aid of a base. In a tetrahydofuran (THF) solution of
these compounds, the addition of water contributed to aggregation-induced emission enhancement
(AIEE), whereas in the DMSO solution, an increase in the water content caused an
aggregation-induced red-shift emission. Through the AIE experiments, the transformation processes
among the three luminescent species in the HBT derivative solutions were clearly observed—that is,
the multiple fluorescence emission was adjustable. Thus, fluorescence color adjustment of a solution of
HBT derivatives could be achieved simply by varying the solvents or introducing additives such as
base or water to the solution.
HBT-based compounds, which exhibit highly emissive and photostable properties, provide a unique
opportunity for solid-state sensing. The solid state of these fluorophores shows a sensitive fluorescent
3

response to amines and acetic acid vapor. Upon exposure of a dye-loaded filter paper strip to amine
vapor, the fluorescence of the paper changed color. Removal of the amine gas or exposure to acetic acid
vapor generated the original emission. The intensity ratios of the two fluorescence emission peaks
remained nearly constant after at least five cycles, indicating a good fatigue resistance. HBT-based dyes
have been shown to work as anti-counterfeiting fluorescent inks on banknotes. Therefore, we
developed a vapor-triggered reversible luminescence-switching platform.
2. Experimental
2.1 Materials and instruments
All reagents were of analytical grade and obtained from commercial suppliers. Thin layer
chromatography (TLC) silica gel plates (GF-254) and silica gel (200-300 mesh) for column
chromatography were obtained from Qingdao Marine Chemicals, China. Deionized water was
1
employed throughout all experiments. H NMR and 13C NMR spectra were acquired on a Bruker
AVANCE II 400 spectrometer (Bruker, Switzerland), respectively. High resolution mass spectra
(HRMS) spectra were measured with a 6510-Q-TOF spectrometer (Agilent, USA). Single-crystal
X-ray diffraction intensity data were recorded using a Bruker SMART 1000 CCD diffractometer
(Bruker, Switzerland) with Mo-Ka radiation (λ = 0.71073 Å) at room temperature. The absolute
fluorescence quantum yield (Φf) values of the solid were calculated on an Edinburgh Fluorescence
Spectrometer FLS1000 using an integrating sphere (Edinburgh, England). The solid ultraviolet-visible
(UV–visible) absorption spectra were recorded on a SHIMADZU UV-3600 spectrometer (Japan).
UV–visible absorption spectra of the solution were recorded by a TU-1901 spectrometer (Beijing,
China). Fluorescent measurements of the solution were performed on a Lengguang F97Pro FL
Spectrophotometer (Shanghai, China). All spectra were recorded at room temperature. All theoretical
calculations were carried out with GAUSSIAN 03 package [26].
2.2 Synthesis and characterization of compounds
Two types of HBT derivatives were synthesized. As shown in Scheme 1, three simple derivatives
were prepared by directive condensation of the aromatic aldehydes with 2-aminothiophenol. Two
derivatives with bulky aromatic amines, which were connected to the HBT core through a phenyl
moiety, were synthesized by Suzuki coupling.
Synthesis of OHBT
4

OHBT was synthesized using the same procedure described in literature [27]. In a round-bottomed
flask (50 mL) equipped with a magnetic stirrer, a solution of 2-Aminothiophenol (0.22 ml, 2 mmol),
2,4-dihydroxybenzaldehyde (304 mg, 2.2 mmol) and sodium metabisulfite (Na2S2O5, 465 mg, 2.4
mmol) in dry DMF (10 mL) was prepared. After stirring with reflux for 2 hr, the reaction mixture was
cooled and added slowly into water (20 mL). The precipitate was filtered off, washed with water (20
cmL×3) and dried to give the crude product, which was further purified by recrystallization from
1
methanol to afford a white solid (405 mg, yield 83.3%). Mp: 199-201°C. H NMR (400 MHz,
DMSO-d6) δ 11.67 (s, 1H), 10.17 (s, 1H), 8.08 (d, J = 7.6 Hz, 1H), 7.94 (dd, J = 8.7 Hz, 2H), 7.50 (t, J
= 7.2 Hz, 1H), 7.39 (t, J = 7.5 Hz, 1H), 6.49 – 6.43 (m, 2H). The melting point and 1H NMR spectrum
were identical with that reported in the literature [27].
Synthesis of CHBT
CHBT was synthesized using the same method described in the literature [28]. In a round-bottomed
flask (25 mL) equipped with a magnetic stirrer, a solution of 2-aminothiophenol (1.0 mmol, 126 mg)
and 5-chlorosalicylaldehyde (1.1 mmol, 171 mg) in ethanol (10 mL) was prepared. Aq. 37% HCl (3
mmol, 0.5 mL) was gradually added to the mixture, followed by aq. 30% H2O2 (6 mmol, 0.63 mL). The
mixture was then stirred at room temperature. When the starting materials had completely disappeared
as monitored by TLC (eluent: petroleum ether /EtOAc = 30:1), the mixture was quenched by H2O (10
mL), extracted with EtOAc (10 mL×3), and the combined extracts were dried over anhydrous Na2SO4.
After removal of solvents, the crude product was purified by silica gel chromatography (eluent:
petroleum ether/EtOAc = 30:1) to give the desired compound as a white solid (210 mg, yield 79.8%).
Mp: 152–154 . 1H NMR (400 MHz, DMSO-d6) δ 11.69 (s, 1H), 8.25 (d, J = 2.4 Hz, 1H), 8.15 (d, J =
7.9 Hz, 1H), 8.07 (d, J = 8.1 Hz, 1H), 7.55 (t, J = 7.6 Hz, 1H), 7.49 – 7.40 (m, 2H), 7.11 (d, J = 8.8 Hz,
1H). The 1H NMR spectrum was identical with that reported in the literature [29].
Synthesis of DHBT
DHBT was synthesized according to a procedure similar to CHBT. White solid (270 mg, yield 63.8%).
Mp: 149-151 °C. 1H NMR (400 MHz, DMSO-d6) δ 12.80 (s, 1H), 8.23 (d, J = 7.9 Hz, 1H), 8.19 – 8.07
1
(m, 2H), 7.96 (m, 1H), 7.62 (t, J = 7.2 Hz, 1H), 7.54 (t, J = 7.5 Hz, 1H). The H NMR spectrum was
identical with that reported in the literature [29].
Synthesis of HCP
In a round-bottomed flask (25 mL) equipped with a magnetic stirrer, a solution of
5

2-(benzo[d]thiazol-2-yl)-4-bromophenol (306 mg, 1 mmol) in THF (10 mL) was prepared. Then,
K2CO3 (2 mM in water, mL) was added under nitrogen atmosphere. Next,
2
a
[4-(9H-carbazol-9-yl)phenyl]boronic acid (288 mg, 1 mmol) was put into the mixture, and after
stirring at room temperature for 10 minutes, Pd(PPh3)4 (58 mg, 0.05 mmol) was finally added to the
solution. The mixture was stirred at 80 for 8 hr. After completion of the reaction (monitored by
TLC), the mixture was quenched by adding H2O (5 mL). The aqueous layer was extracted three times
with dichloromethane. The combined organic layers were washed with water and dried over
anhydrous Na2SO4. The organic layer was concentrated under vacuum and allowed to crystallize. The
precipitate was collected to obtain an orange solid (270 mg, 57.6% yield). 1H NMR (400 MHz,
DMSO-d6) δ 11.75 (s, 1H), 8.63 (d, J = 2.2 Hz, 1H), 8.28 (d, J = 7.8 Hz, 2H), 8.18 (d, J = 7.8 Hz, 1H),
8.12 (d, J = 8.1 Hz, 1H), 8.00 (d, J = 8.4 Hz, 2H), 7.88 (dd, J = 8.5, 2.2 Hz, 1H), 7.75 (d, J = 8.3 Hz,
2H), 7.57 (t, J = 7.3 Hz, 1H), 7.47 (dd, J = 9.1, 4.6 Hz, 5H), 7.38 – 7.22 (m, 3H). 13C NMR (100 MHz,
DMSO-d6) δ 64.81, 156.53, 151.97, 140.61, 139.05, 136.24, 135.24, 131.44, 131.26, 128.41, 127.65,
126.94, 126.78, 125.57, 123.28, 122.77, 122.49, 121.04, 120.59, 119.64, 118.24, 110.22. HRMS (ESI)
calcd for [M+H]+ 469.1375, found: 469.1370.
Synthesis of HTPA
HTPA was synthesized according to a procedure similar to HCP. The crude product was purified by
silica gel chromatography (eluent: petroleum ether/EtOAc = 40:1) to give the desired compound as a
1
yellow solid (423 mg, 90% yield): H NMR (400 MHz, DMSO-d6). δ 11.61 (s, 1H), 8.43 (s, 1H), 8.16
(d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.1 Hz, 1H), 7.70 (d, J = 8.6 Hz, 1H), 7.62 (d, J = 8.5 Hz, 2H), 7.55 (t,
J = 7.7 Hz, 1H), 7.46 (t, J = 7.5 Hz, 1H), 7.33 (t, J = 7.8 Hz, 4H), 7.17 (d, J = 8.6 Hz, 1H), 7.08 (t, J =
9.0 Hz, 8H). 13C NMR (100 MHz, DMSO-d6) δ 169.52, 165.05, 155.98, 151.94, 147.62, 146.81,
135.12, 134.17, 131.91, 130.85, 130.06, 127.80, 126.92, 126.28, 125.53, 124.41, 124.31, 123.56,
122.70, 122.46, 119.37, 118.06. HRMS (ESI) calcd for [M+H]+471.1531, found: 471.1529.
2.3 UV–visible absorption and fluorescence measurements
Stock solutions of HBT derivatives (10 mM) were prepared in analytical grade tetrahydrofuran
(THF). The test solutions (50 µM) were prepared by adding aliquots (50 μL) of the stock solution to 10
mL volumetric flasks, followed by filling volumetric flasks with different solvents. All spectra were
6

recorded at room temperature.
2.4 Aggregation-induced emission enhancement (AIEE) measurements
Stock solution of HBT derivatives (10 mM) were prepared in analytical grade THF. Aliquots (50 μL)
of the stock solution were added to 10 mL volumetric flasks. After appropriate amounts of THF were
5
added, water was added dropwise under shaking to give 5 × 10− M solutions with different water
contents (0-99 vol %). The UV-visible and photoluminescent measurements of the resultant solutions
were then performed immediately at room temperature.
2.5 Measurement of the fluorescence switching on a filter paper strip loaded with fluorescence
dyes
2.5.1 Preparation of the filter paper strip loaded with fluorescence dyes
The dye-loaded filter paper was prepared by dropping 15 µL of a pre-mixed THF solution of tested
compounds (50 μM) on a small piece of filter paper and drying under air for 24 hr.
2.5.2 Measurement of fluorescence switching
The amine vapors were prepared by charging amines (15 µL) in a quartz cell (O.D = 1 cm, height =
4 cm), and then saturating the cell with a tightly closed lid over 0.5 hr. The acetic acid (AcOH) vapor
was prepared according to the same procedure.
The dye-loaded filter strips were fastened on a small glass plate that was put in a quartz cell. The cell
with the plate was positioned on a solid sample holder of the fluorescence spectrometer. The strips
were fumed in amine vapor for 30 s and then their fluorescence spectra were measured. The filter strips
were taken out, dried for 1 min with a hair dryer, and then put in the AcOH vapor for 30 s. The
fluorescence spectra were then recorded and this procedure was repeated several times.
< Insert Scheme 1>
3. Results and discussion
3.1 Ultraviolet-visible absorbance properties and fluorescence characteristics
The optical properties of the five HBT derivatives in the solid state and in solvents of different
polarities were investigated by ultraviolet-visible (UV-visible) and fluorescence spectroscopy. All of
the compounds showed a strong absorption band between 280 and 370 nm in various solvents. In the
7

solid state, compounds HCP and HTPA exhibited absorption maxima at 396 and 418 nm, respectively,
which were red shifted compared with the absorption peaks in solvents (Table 1 and Figure S1 in the
Supporting Information).
After studying the absorption properties of the HBT derivatives, their emission behaviors in the solid
state were examined. All of the compounds were highly emissive in the solid state (Table 1). Figure 2a
illustrates the emission spectra of these compounds in the solid state. The emission maxima of OHBT,
CHBT, and DHBT were located at 502, 526, and 536 nm, respectively. As expected, HCP and HTPA
showed red shifts in the emission spectra because of the extended aromatic conjugation system and the
emission maxima appeared at 560 nm for HCP and 588 nm for HTPA. The emission spectra of these
solid samples covered the visible light region from 450 nm to 700 nm. Figure 2a presents the
solid-state luminescence images of the crystalline powders illuminated under UV light (λex = 365 nm).
These samples have emission colors ranging from cyan to orange red.
The photostability of the solid HBT derivatives were then evaluated by time-progression
fluorescence measurements. As shown in Figure 2b, the fluorescence intensity of these five compounds
remained almost constant under continuous in situ UV irradiation (1 hr), which was indicative of the
good solid-state photostability exhibited by these compounds.
< Insert Figure 2>
As mentioned, the ESIPT process is quite sensitive to the solvent environment. In a preliminary test,
HTPA had a highly solvent-dependent fluorescence and single, dual, or triple emission could be
achieved by varying the solvent. For compounds containing an ESIPT moiety, having two emission
patterns (single broad emission and dual emission) generally was expected. A few HBT derivatives
also have been reported to show the triple fluorescence phenomenon, which has indicated the
coexistence of the enol, keto, and phenolic anions. The spectral evolution process of the multiple
fluorescence, as well as the solvent effect on the generation of phenolic anions, have not been explored
very well [13, 14]. Therefore, a thorough fluorescence spectral investigation of HTPA in various
solvents was first performed to obtain a detailed elucidation of the process (Figures 3, Figure S2 and
Figure S3). As shown in Figures 3a and 3b and Figure S2, HTPA showed weak fluorescence emission
in all of the organic solvents except DMSO and DMF. HTPA exhibited dual fluorescence at ~408 nm
8

and ~585 nm in non- and weakly polar solvents (hexane, toluene, and dichloromethane), which were
ascribed to the Enol FL and Keto FL, respectively; the Keto forms were more prominent in the spectra
in these solvents. In THF, two fluorescence peaks at 440 nm and 580 nm could be attributed to the Enol
FL and Keto FL, respectively, and the Enol form was dominant. A triple fluorescence (centered at ~412,
~500, and ~583 nm) phenomenon was observed for HTPA in ethyl acetate, acetonitrile, methanol, and
ethanol; the two emission peaks at ~412 and ~583 nm were attributed to the corresponding Enol and
Keto forms, respectively. In sharp contrast, HTPA emitted a very strong bluish green fluorescence
around 507 nm in polar aprotic solvents, such as DMSO and DMF. With increased solvent polarity,
generally, the fluorescence spectral profiles gradually evolved from dual fluorescence (at ~420 and
~580 nm) to triple fluorescence (at ~412, ~500, and ~583 nm), and finally a strong single fluorescence
at 507 nm dominated the entire emission spectrum. Compared with the other solvents, DMSO and
DMF showed high stabilization (ε ~ 47 for DMSO; ε ~ 36.7 for DMF). The basic natures of DMSO
and DMF are also beneficial to the deprotonation process. Therefore, we believe that the strong
emission around 507 nm for HTPA originated from the phenolic anion form of HTPA that was
generated because of the deprotonation induced by the intermolecular H-bond between the phenolic
hydroxy and the solvent molecules. To present solid evidence of the generation of the anion, the
ultraviolet absorption and fluorescence emission spectra of HTPA when titrated with piperidine (from
0 to 3 equiv.) in DMSO were monitored. As shown in Figures 4a and 4b, with the addition of the base,
the very weak original absorption band at 440 nm became strong and gradually moved to 431 nm,
which could be assigned to the absorption of the phenolic anion. Meanwhile, in the fluorescence
spectra, the emission peak at 507 nm increased significantly and reached its maximum intensity when
the amount of piperidine was 3 equivalents. The changes in the fluorescence spectra indicated that the
presence of base was favorable for phenolic anion formation. Based on the experimental results and
related literature, it would be reasonable to assign the fluorescence emission at 507 nm to the phenolic
anion of HTPA.
For DHBT, a moderate emission at ~535 nm in hexane, toluene, dichloromethane, and acetonitrile
was observed, which was attributed to the Keto FL of DHBT (Figure 3e). In THF, a dual fluorescence
at 410 nm and 540 nm appeared; the fluorescence peak at 540 nm was attributed to the Keto FL and the
very weak emission at 410 nm was ascribed to the Enol FL. In ethanol and DMSO, DHBT exhibited a
dual fluorescence at ~462 nm and ~535 nm, which originated from the phenolate anion and Keto forms,
9

respectively. DHBT emitted a single fluorescence at ~462 nm in methanol and DMF, which arose from
the phenolic anion form. To verify the generation of the phenolic anion, the emission spectra of DHBT
when titrated with piperidine (from 0 to 5 equiv.) in acetonitrile (CH3CN) was recorded. As shown in
Figure 4c, upon the addition of the base, the peak at 530 nm decreased gradually and a new
fluorescence band centered at 470 nm emerged and increased progressively with a well-defined
isosbestic point at ~520 nm. When 3 equivalents of piperidine were added, the emission intensity at
470 nm reached its maximum value and a single fluorescence peak was formed.
For HCP, an emission band at ~550 nm was observed in hexane, toluene, and dichloromethane,
which was assigned to the Keto FL (Figure 3f). For the ethanol solution of HCP, a strong emission at
417 nm appeared, accompanied by a weak peak at 540 nm, and the peak at 417 nm belonged to the
Enol FL of HCP. HCP exhibited intense dual fluorescence at ~417 nm and ~483 nm in DMSO and
DMF, which were attributed to the Enol and phenolic anion forms, respectively. Furthermore, titration
experiments with piperidine in DMSO were conducted to demonstrate the generation of the phenolic
anion. As the amount of base increased, the peak at 418 nm consistently decreased and, simultaneously,
the peak at 483 nm gradually was enhanced with a clean isosbestic point at 460 nm. Finally, a strong
emission band centered at 487 nm dominated the entire spectrum (Figure 4d).
OHBT showed almost no fluorescence in the visible region in all solvents. A weak emission band
appeared at around 500 nm in dicholoromethane and toluene (Figure 3c). In DMSO and DMF, a strong
single fluorescence peak was observed at ~383 nm. The two emissions were ascribed to Enol FL and
Keto FL, respectively. The generation of the phenolic anion was also verified by titration experiments
in DMSO (Figure S4a). A new peak at ~447 nm emerged and became stronger at the expense of the
band at 384 nm upon the addition of piperidine. Because of the weak acidic property of OHBT, the
band at 384 nm could not be transformed completely to the new band, even if a large amount of base
was added. As a result, the dual fluorescence was retained.
For CHBT in hexane, toluene, and dichloromethane, an emission was observed at ~530 nm, which
could be attributed to Keto FL. CHBT exhibited dual fluorescence at ~390 nm and ~470 nm in polar
solvents (DMSO, DMF, methanol, and ethanol), which was ascribed to the Enol and phenolic anion
forms, respectively (Figure 3d). The generation of the phenolic anion was also demonstrated by
titration experiments in DMF (Figure S4b). As the amount of piperidine was raised, the peak at 390 nm
showed a consistent decrease; meanwhile, the peak at 470 nm was gradually elevated and a
10

well-defined isosbestic point at 451 nm was observed. When 60 equivalents of piperidine were added,
the band at 390 nm was almost totally consumed and a strong emission band centered at 473 nm
dominated the whole spectrum.
< Insert Figure 3>
< Insert Figure 4>
< Insert Figure 5>
The systematic investigation of the luminescent properties of the five compounds in a variety of
solvents showed that non- or weakly polar solvents favored the ESIPT process and long-wavelength
emission from the Keto form, which agreed with literature reports [13]. Most of the compounds
dissolved in DMSO and DMF were found to exhibit a much more significant hindering of the ESIPT
process, facilitating normal emission from the Enol form. DMSO and DMF also significantly enhanced
deprotonation, promoting the phenolic anion fluorescence. For example, the fluorescence emission
originating from the phenolic anion of HTPA contributed to strong single peaks in DMSO and DMF.
In our experiments, the alkaline additive facilitated the dissociation of the phenolic hydroxyl group and
consequently gave rise to a remarkable enhancement of the emission from the phenolate form. Notably,
we succeeded in transforming Keto or Enol forms to phenolic anions with the aid of a base. A possible
mechanism is proposed in Figure 5, which shows the excited state structures generating different
emission wavelengths.
In contrast to the very solvent-dependent fluorescence properties in solution, these compounds all
exhibited a strong emission in the solid state, which was assigned to the Keto form fluorescence. As
shown in Table 1, the solid-state samples showed higher fluorescence quantum yields than those in
solution. To understand the relationship between the aggregated state and the fluorescence quantum
efficiencies, aggregation-induced emission (AIE) studies were performed.
3.2 Aggregation-induced emission enhancement properties
Normally, AIE luminophores exhibit stronger emission in the aggregated state than in the dispersed
state. Because all of these HBT derivatives showed weak fluorescence in pure THF, the AIE features in
11

a THF/H2O mixture were examined first. The fluorescence characteristics of OHBT in the mixed
water/THF solvents (water fraction (fw) ranging from 0% to 99%, v/v) were studied (Figure 6a). In pure
THF, OHBT had very weak fluorescence at 375 nm. When water was added, the emission intensity
increased rapidly, accompanied with gradual red shifts, which reached its maximum at 70% water
content. When the fw ≥ 80%, the emission peak at 385 nm weakened continually and, simultaneously, a
new emission at ~438 nm emerged and consequently dominated the spectra, thus resulting in a dual
fluorescence (385 and 438 nm). Based on the previous conclusions about the solution phase, the two
peaks (385 and 438 nm) were ascribed to the Enol and phenolic anion forms, respectively.
Changes in the fluorescence spectra of CHBT in the THF/H2O mixture were also recorded (Figure
6b). CHBT exhibited very weak green fluorescence in THF. When water was added, the intensity of
blue fluorescence emission at 471 nm increased rapidly and reached its maximum at 60% water content.
Once the fw was beyond 70%, the peak at 471 nm decreased sharply, a new emission around 521 nm
emerged and, finally, green fluorescence dominated the spectrum. The two peaks (471 and 521 nm)
were attributed to the phenolic anion and Keto forms, respectively.
DHBT showed a medium fluorescence emission at 538 nm in THF. As shown in Figure 6c, when
water was added, the emission peak at 538 nm blue shifted slightly (521 nm), and a new fluorescence
emission around 471 nm appeared. Thus, a dual fluorescence (471 and 521 nm) was observed. When
the fw > 50%, the emission peak at 471 nm was dramatically elevated. Interestingly, as the water
fraction increased to 80%, the fluorescence peak at 538 nm was recovered, accompanied by a decrease
of the emission band at 471 nm. As the water content increased further, the yellow green fluorescence
was obviously enhanced, and a single peak was formed. The two peaks (471 and 538 nm) were
attributed to the phenolic anion and Keto forms, respectively.
For HCP, there was a weak dual fluorescence (402 and 556 nm) in THF (Figure 6d). When the fw =
10%, the emission at 402 nm increased significantly and red shifted to 411 nm; meanwhile, the peak at
556 nm was replaced by a new strong emission band at 480 nm. With an increase in the water fraction
(20% to 50%), the peak at 411 nm gradually decreased. Simultaneously, the peak at 480 nm was
continually enhanced, and a well-defined isosbestic point at 440 nm was observed. When the fw
increased further (60% and 70%), the fluorescence peak at 556 nm reappeared, yielding a triple
fluorescence (411, 480 and 556 nm). The three peaks (411, 480, and 556 nm) were attributed to the
Enol, phenolic anion, and Keto forms, respectively. When the fw ranged from 80% to 99%, a strong
12

single emission at 556 nm was obtained.
HTPA exhibited a weak dual fluorescence (440 and 580 nm) in THF (Figure 6e). In the mixtures
with a low water fraction (0 to 20%), the emission at 440 nm red shifted to 480 nm and increased
dramatically. Then, the peak continued to decrease as the water content was increased further (30 to
60%). Once the fw was beyond 70%, the mixture exhibited a consistent increase in the peak at 580 nm
and, finally, a strong single emission around 580 nm was observed. The two peaks (480 and 580 nm)
were attributed to the phenolic anion and Keto forms, respectively. When the fw = 99%, the
fluorescence intensity decreased because of aggregate precipitates.
In the THF/H2O mixtures, the addition of water caused the fluorescence emission intensity of the
tested solutions to increase significantly. When the fw was higher than 70%, the aggregated states of the
HBT derivatives typically were preferred because of their insolubility in water, in which the
intramolecular hydrogen bonds were stable. As a result, an ESIPT process occurred, leading to a
fluorescence emission from the Keto form (i.e., an AIE emission). During the AIE process, all of the
compounds experienced a dual or triple fluorescence process and the emission bands gradually red
shifted with the elevation of water content. Finally, the aggregation of dye molecules in the aqueous
solution resulted in emission spectra similar to that in the solid state, except for OHBT.
More evidence for the aggregation of HBT derivatives in aqueous solution was obtained from the
absorption spectra. As shown in Figure S5, level-off tails can be observed in the UV absorption spectra
as the fw values increase, which were due to light scattering of the aggregated suspensions.
The AIE study was also carried out in DMSO/H2O mixtures. All of the compounds exhibited
aggregation-induced red-shift emission. For OHBT, with the increase of the water fraction, a strong
single fluorescence at 384 nm (Enol FL) gradually decreased and a new emission at ~440 nm (phenolic
anion form) emerged. Then, a third emission band at ~480 nm (Keto FL) was detected. Finally, triple
fluorescence with a wide spectrum was generated (Figure S6a). Thus, in DMSO/H2O mixtures, OHBT
exhibited red-shift AIE emission and simultaneously experienced an emission evolution from single, to
dual, and finally to triple fluorescence as the water content increased. The evolution processes among
the enol, phenolic anion, and Keto forms were also observed for CHBT, DHBT, and HCP (Figure S6b,
Figure S6c, and Figure S6d) and, finally, a strong single emission in the long wavelength region was
obtained with high water content. HTPA showed a strong single fluorescence at 507 nm (phenolic
anion form) in DMSO. For HTPA, the complete transformation from the phenolic anion to the Keto
13

form was attained by progressively increasing the water content in DMSO, thus resulting in a single
emission at 580 nm (Figure S6e).
The AIEE and aggregation-induced red-shift emission indicated that the three luminescent species in
the solutions of the HBT derivatives were transformable—that is, the multiple fluorescence emission
was adjustable. Based on this inspiring phenomenon, it would be very convenient to generate the
desired fluorescence simply by adding a certain amount of water to a solution of the HBT derivatives.
< Insert Figure 6>
3.3 Structural analysis
To understand the relationship between the solid-state fluorescence properties and the structural
characteristics, single-crystal studies were performed. A single crystal of OHBT was obtained by slow
evaporation of an OHBT solution in a petroleum ether/ethyl acetate mixture at room temperature.
Suitable CHBT and HTPA single crystals were produced through a crystallization process in a
refrigerator in a similar solvent system. Several attempts were made to develop single crystals of the
other two compounds, but suitable samples for X-ray analysis could not be obtained.
The single-crystal X-ray diffraction studies showed that OHBT and CHBT were almost planar with
small dihedral angles (1.33° and 1.20°) between the benzothiazole heterocycle and phenol ring (Figure
7a and Figure 8a). Both compounds had strong intramolecular H-bond interactions with the same
distance (1.88 Å), which accounted for the coplanar conformation. In contrast to OHBT and CHBT,
HTPA had a larger dihedral angle (15.04°) between the benzothiazole and phenol ring. In the HTPA
crystal, the length of the intramolecular H-bond between the hydroxyl and the benzothiazole ring was
1.92 Å, which was slightly longer than that of OHBT and CHBT (Figure 9a). The strong hydrogen
bonding was beneficial to the stabilization of the enol form in the ground state. In the ESIPT process,
the excited enol form transformed to the excited keto tautomer.
In the OHBT crystal, the molecules aggregated into molecular chains based on a strong
intermolecular H-bond (2.29 Å) (Figure 7b) between the 4-hydroxy groups on the benzenes of two
molecules. The chains were connected into a molecular sheet by C-H···O interactions (2.60 Å) (Figure
7c), along with π-π stacking interactions with a contact distance of 3.67 Å (Figure 7d). The molecular
14

sheets were held together by three strong C-H···π interactions (2.67 Å, 2.77 Å and 2.95 Å) to form the
3D structure (Figures 7e and 7f, and Figure S7).
< Insert Figure 7>
< Insert Figure 8>
The planar CHBT molecules was arranged into molecular columns through three weak
intermolecular face-to-face π-π stacking interactions with contact distances of 3.82 Å, 3.75 Å, and 3.70
Å (Figure 8b). In the columns, molecules were parallel with slight twisting and the orientations of
adjacent molecules were essentially opposite. The benzothiazole moieties of adjacent molecules did not
completely overlap. The molecular columns were linked together by C-H···O (2.73 Å) and C-H···Cl
(2.89 Å) interactions, resulting in the formation of a 3D structure (Figure 8c and Figure S8).
The noncovalent intermolecular interactions could be a factor affecting the low fluorescence
quantum efficiency. OHBT and CHBT exhibited different packing modes in the single crystals. In
OHBT crystals, various types of intermolecular interactions existed (O-H···O, C-H···O, C-H···π and
π-π) and the O-H···O (2.29 Å) and C-H···O (2.60 Å) interactions were particularly strong. The three
C-H···π interactions played an essential role in linking the two-dimensional molecular sheets together.
As compared with OHBT, the molecular packing structure feature of CHBT was based on weak π-π
stacking and modest intermolecular H-bonds. The orientations of neighboring molecules avoided
maximum face-to face overlap, causing the quenched emission. These observations could be
responsible for the higher quantum efficiency (Φf = 82%) of CHBT in the solid state.
< Insert Figure 9>
Two strong C-H···π interactions (2.89 Å) between H15 and the benzothiazole cycle were observed
in the crystal lattice of HTPA that led to the formation of a dimer with exactly opposite dipoles (Figure
9c). The terminal benzene of the triphenylamine unit formed two C-H···π interactions (2.91 Å and 3.20
Å) with two neighboring molecules from different dimers, which played an important role in holding
the adjacent molecules together (Figures 9c and 9d).
15

Incorporation of a triphenylamine moiety on the HBT core triggered a marked effect on the
molecular structure. In the HTPA molecule, the dihedral angle between the triphenylamine moiety and
the HBT plane was 41.56° in the aggregated state (Figure 9b). As noted previously, HTPA exhibited a
large dihedral angle (15.04°) between the benzothiazole and phenol ring, while it was close to 0° for
OHBT and CHBT. The twisted structure of HTPA led to a lower fluorescence quantum yield (Φf =
21%) as compared with OHBT and CHBT.
3.4 Computational studies
To gain further insights into the luminescent behaviors of the compounds, density functional theory
calculations were performed at the B3LYP/6-31G (d, p) level [26]. The molecular orbitals of the
compounds are shown in Figure 10 (only HOMO and LUMO are presented). The LUMO distributions
of the five compounds were very similar, whereas those of the HOMO were different as a result of the
different substituents and substituted positions on the phenol ring. OHBT, CHBT, and DHBT were
molecules with simple substituents on the phenol ring. Compared with OHBT, which had a hydroxyl
group, the halogen-substituted molecules showed smaller energy gaps, resulting in a red shift of the
emission spectra.
For HCP and HTPA, incorporation of the phenylcarbazole and triphenylamine moieties to the
4-position of benzene extended the length of the π-conjugation system, which gave rise to significantly
increased HOMO energy levels. Consequently, the energy gaps decreased, and the emission spectra
exhibited a red shift compared with the three simpler compounds.
< Insert Figure 10>
3.5 Tunable solid-state emission by external stimuli
The fact that the deprotonation products of the HBT derivatives are highly emissive in solution has
been demonstrated. However, the response of the solid-state HBT compounds to alkali vapor, thus far,
has received little attention [20]. The products were exposed to amine vapor and the fluorescence of the
solid sample blue shifted and emitted another color. The vapor-fumed samples could regain their
original emission state when exposed to ambient air as the vapors volatized. The strong solid state and
switchable fluorescence prompted us to conduct vapor-responsive experiments using a filter
16

paper-supported sample to explore the practical applications. OHBT was investigated first. The
dye-loaded filter paper strip was prepared by dropping the OHBT solution on filter paper and
air-drying. The change and recovery of the OHBT fluorescence spectra are shown in Figures 11a and
11b. OHBT on filter paper emitted cyan light centered at 485 nm under UV illumination (365 nm),
which arose from the Keto form. Upon exposure to piperidine vapor for 30 s, the emission color of the
paper strip changed to blue centered around 430 nm, which was close to that of the phenolic anion
formed in the alkaline solution, thus replicating the solution behavior. When exposed to ambient air, the
blue-shifted fluorescence returned to its original state within 4–6 hr. Note that fuming with acetic acid
vapor could reduce the recovery time to 30 s. The peak wavelength difference of the two emission
spectra was about 55 nm. The fatigue resistance of the sample-loaded filter papers was evaluated by
repeatedly switching between the cyan and blue emissive states. The fluorescence intensity ratios
remained nearly unaltered, without obvious degradation after five cycles, demonstrating a good fatigue
resistance (Figures 11a and 11b).
The four other HBT derivatives displayed similar vapor-responsible reversible
fluorescence-switching behavior on the filter paper strips. The peak wavelength differences of the two
emission spectra ranged from 60 nm to 85 nm. For all compounds, the fluorescence changes provided
clear signal and high contrast (insets in Figure 12b to Figure 15b). Such vapor-induced fluorescence
switching also could be reproduced without apparent fatigue (Figures 12–15). Notably, the
HTPA-loaded on filter paper exhibited a clear selectivity in response to a variety of amine vapors. As
can be seen in Figure 16, under the same conditions, only piperidine and pyrrolidine triggered the blue
shift of the initial fluorescence. Other common aliphatic amines, aniline, pyridine, and ammonia only
caused quenching of the fluorescence intensity at 585 nm, indicating its potential for use in basic vapor
sensors owing to the obvious fluorescence visualization recognition.
< Insert Figure 11>
< Insert Figure 12>
< Insert Figure 13>
< Insert Figure 14>
< Insert Figure 15>
< Insert Figure 16>
17

3.6 Application as an anti-counterfeiting ink
Fluorescent materials have been incorporated onto banknotes and trademarks and are used widely as
anti-counterfeiting labels. Because of the light color, high fluorescent brightness, and photostability of
the HBT derivatives, developing these dyes into anti-counterfeiting fluorescent inks is attractive. To
illustrate that HBT-based dyes can be used as fluorescent inks, the letters “HBT” were written on a
practice banknote with a THF solution of HCP (10 mM), which was invisible under daylight but highly
emissive under UV light (Figure 17). Reversible fluorescence-switchable dyes further increased
security. Amine vapors have long been employed to reveal anti-counterfeiting ink by inducing
fluorescence or color formation [24,30]. Exposure to the amine vapor triggered an obvious
fluorescence color change on the practice banknote, which could be distinguished with the naked eye.
Leaving the banknote under ambient air for several minutes allowed the original fluorescent color to be
recovered. There were no obvious color changes in the letters “HBT” in daylight after the recovery.
The amine vapor-induced fluorescence switching was quick and could be reproduced many times,
which increased the complexity of the anti-counterfeiting performance compared with the common
single-color fluorescence dyes. Recently, biluminescent or multicolor luminescent materials have been
developed for anti-counterfeiting measures [31-33]. Two or three excitation sources, however, were
required for these materials. Thus, the cost of excitation sources and the complexity of the preparation
process limited the scope of their potential applications. To the best of our knowledge, amine
vapor-responsive reversible fluorescence-switchable anti-counterfeiting dyes seldom exist in the
literature. In addition to HCP, the other molecules showed similar fluorescence-switchable behavior on
practice banknotes (Figures S9–S12). These experiments demonstrated that HBT-based dyes as
fluorescent inks have significant potential for use in high-security anti-counterfeiting labels.
< Insert Figure 17>
4. Conclusions
A series of AIE-ESIPT dual mechanism–based HBT derivatives were synthesized. A systematic
comparison in a variety of solvents showed that the fluorescence properties of these compounds were
18

quite sensitive to the solvent environment. DMSO and DMF greatly influenced the deprotonation of the
phenolic hydroxyl, thus favoring fluorescence emission from the phenolic anion. In solution, the Keto
or Enol forms could generally be transformed to the phenolic anion with the aid of a base. These
molecules exhibited high solid-state quantum yields and large Stokes shifts, and the emission spectra of
these solids covered most of the visible light region. The HBT-based dyes showed AIEE in THF/H2O
mixtures, as well as aggregation-induced red-shift emission in DMSO/H2O mixtures. Through the
AIEE and red-shift AIE experiments, the fluorescence spectral evolution process was clearly observed.
Based on these results, we concluded that the three luminescent species in the HBT derivative solutions
were transformable and, therefore, the fluorescent emission was adjustable. Single crystal studies and
theoretical calculations were conducted to elucidate the luminescent properties. HBT-based dyes
exhibited vapor-responsible reversible fluorescence-switching behavior on filter paper strips. Upon
exposure to amine and acid vapors, the distinct fluorescence color changes were clear enough to be
recognized by the naked eye. Finally, we successfully demonstrated the applications of these molecules
as anti-counterfeiting fluorescent inks on banknotes.
Acknowledgement
This work was financially supported by the Natural Science Foundation of Shandong Province
(ZR2018MB025), a Project of Shandong Province Higher Educational Science and Technology
Program (NO. J18KZ004), and the Funds for Research Leader in Jinan (2018GXRC028).
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24

Table
Table 1. Summary of the optical properties of HBT derivatives
Compounds
Medium
λAbs
λex
λem
Stokes Stokes Quantum
[nm]
[nm]
[nm]
shift
[nm]
125
214
41
shift
[cm-1]
6604
efficiency
Φf [%]
25[a]
OHBT
Solid
377
291
344
291
292
340
396
288
345
294
342
378
299
356
294
346
295
348
396
295
295
350
325
325
502
505
385
Toluene[b]
EtOH[b]
14562
3095
2.8[c]
17.3[c]
94
8390
DMSO[b]
Solid
325
360
360
360
385
526
532
534
93
8273
68[c]
82[a]
6.2[c]
9.7[c]
CHBT
130
136
246
189
180
132
148
239
182
240
188
176
123
164
258
108
257
128
191
170
10400
6456
Toluene[b]
EtOH[b]
15995
10259
12917
8142
DMSO[b]
360
474
65[c]
31[a]
DHBT
Solid
355
355
536
538
7798
Toluene[b]
14857
9503
24.1[c]
12.9[c]
EtOH[b]
355
355
534
471
15287
10175
12667
7504
DMSO[b]
11.8[c]
HCP
Solid
365
365
365
560
553
403
552
420
483
588
7395
70[a]
Toluene[b]
EtOH[b]
15815
9084
9.6[c]
15782
10437
13542
6916
18.1[c]
65 [c]
DMSO[b]
292
418
365
360
HTPA
21[a]

Solid
319
360
449
585
582
505
130
266
264
184
9076
14254
14264
11350
Toluene[b]
7.4[c]
7.5[c]
53[c]
318
321
360
360
EtOH[b]
DMSO[b]
[a] Absolute quantum yields in the solid state. [b] Measured from 5×10-5 M solution. [c] Quantum
yields measured by relative method using quinine sulphate standard (10-5 M).

A reversible vapor-responsive fluorochromic molecular platform based on coupled AIE–ESIPT
mechanisms and its applications in anti-counterfeiting measures
Bin Li, Datong Zhang*, Yiran Li, Xiaobin Wang, He Gong, Yue-zhi Cui
School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong
Academy of Sciences), 3501 Daxue Road, Jinan 250353, China
* Correspondence: dtzhang@qlu.edu.cn;
Figures and Schemes
Figure 1 Schematic representation of four-level ESIPT photocycle of HBT.
Figure 2 (a) Fluorescence-emission spectra of HBT derivatives in their crystalline states and images of
the corresponding crystals under 365 nm UV light. (b) Photostability evaluation of HBT derivatives in
their crystalline states.
Figure 3 Fluorescence spectra and image of HBT derivatives (50 µM) in different solvents. (a) HTPA,
λex = 360 nm.(b) Fluorescence image of HTPA, λex = 360 nm. (c) OHBT, λex = 325 nm. (d) CHBT, λex
= 360 nm. (e) DHBT, λex = 355 nm. (f) HCP, λex = 365 nm.
Figure 4 Changes of (a) UV-vis absorption of HTPA in DMSO, (b) fluorescence spectra of HTPA in
DMSO, (c) fluorescence spectra of DHBT in CH3CN and (d) fluorescence spectra of HCP in DMSO
with the addition of piperidine. The concentrations are 5 × 10-5 mol/L.
Figure 5 Excited state structures proposed for the multifluorescence emissions of HBT derivatives in
different solvents.
Figure 6 Fluorescence spectra of (a) OHBT (λex = 325 nm), (b) CHBT (λex = 360 nm), (c) DHBT (λex
= 355 nm), (d) HCP (λex = 355 nm) and (e) HTPA (λex = 360 nm) in THF/H2O mixtures with different
water fractions (fw). The concentrations of compounds are 5 × 10-5 mol/L. Insets: Fluorescence images
of HBT derivatives in THF/H2O mixtures with different water fractions (fw) (λex = 365 nm).
Figure 7 Views of the intra- or intermolecular interactions and packing modes in single crystal OHBT:
(a) intramolecular H-bonding interaction, (b) a molecular chain connecting with hydrogen bonds, (c)
and (d) a molecular sheet connecting by intermolecular hydrogen bonds and π-π stacking, (e) 3D
structure connected by C-H···π interactions and (f) cross stacked molecules connected by C-H···π
interactions. The non-covalent interactions are indicated by dashed lines.
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Figure 8 Views of the intra- or intermolecular interactions and packing modes in single crystal CHBT:
(a) intramolecular H-bonding interaction, (b) a molecular column connected by π-π stacking and (c) 3D
structure formed by columns connected by hydrogen bonds. The non-covalent interactions are
indicated by dashed lines.
Figure 9 Views of the intra- or intermolecular interactions and packing modes in single crystal HTPA:
(a) intramolecular H-bonding interaction, (b) dihedral angle of triphenylamine unit and HBT, (c) cross
stacked molecules connected by C-H···π interactions and (d) 3D structure of HTPA. The non-covalent
interactions are indicated by dashed lines. Hydrogen atoms except H23 are omitted for clarity.
Figure 10 Frontier molecular orbitals analysis for HBT derivatives in the ground and excited states.
Figure 11 (a) The change and recovery of fluorescence spectra and (b) fluorescence ratio changes of
OHBT-loaded filter paper by fuming with piperidine and AcOH vapors repeatedly. Inset: Fluorescence
images of OHBT-loaded filter paper upon piperidine and AcOH fuming (λex = 365 nm).
Figure 12 (a) The change and recovery of fluorescence spectra and (b) fluorescence ratio changes of
CHBT-loaded filter paper by fuming with piperidine and AcOH vapor repeatedly. Inset: Fluorescence
images of CHBT-loaded filter paper upon piperidine and AcOH fuming (λex = 365 nm).
Figure 13 (a) The change and recovery of fluorescence spectra and (b) fluorescence ratio changes of
DHBT-loaded filter paper by fuming with isopropylamine and AcOH vapor repeatedly. Inset:
Fluorescence images of DHBT on a filter paper upon isopropylamine and AcOH fuming (λex = 365
nm).
Figure 14 (a) The change and recovery of fluorescence spectra and (b) fluorescence ratio changes of
HCP-loaded filter paper by fuming with isopropylamine and AcOH vapor repeatedly. Inset:
Fluorescence images of HCP-loaded filter paper upon isopropylamine and AcOH fuming (λex = 365
nm).
Figure 15 (a) The change and recovery of fluorescence spectra and (b) fluorescence ratio changes of
HTPA-loaded filter paper by fuming with piperidine and AcOH vapor repeatedly. Inset: Fluorescence
images of HTPA-loaded filter paper upon piperidine and AcOH fuming (λex = 365 nm).
Figure 16 Changes of fluorescent spectra of HTPA loaded on filter papers after fumed by different
amines.
Figure 17 Illustration of HCP as an anti-counterfeiting ink on a practice banknote with letters of
“HBT”. Images taken (a) under daylight, (b) under 365 nm UV light, (c) under daylight after fumed
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