Pyridazine–carbazole based fluorescent probes for volatile acid detection

Article abstract of DOI:10.1016/j.dyepig.2021.109613

Stimuli-responsive materials based on donor–acceptor systems have great potential for sensing applications, including in the solid state. Herein, two push–pull-type molecules, 3,6-di(9H-carbazol-9-yl)pyridazine (CzPyr–H) and 3,6-bis(3,6-dimethoxy-9H-carbazol-9-yl)pyridazine (CzPyr–OMe), each of which contain two carbazole donors and a pyridazine acceptor, were designed and synthesized. The photophysical properties of CzPyr–H and CzPyr–OMe were systematically explored with the aim of switching the absorption and emission properties using trifluoroacetic acid (TFA) as an external analyte, thus realizing volatile acid sensing. The protonation of the central pyridazine unit led to a broad absorption band at longer wavelengths owing to intramolecular charge transfer (ICT), and the emission intensity gradually decreased with increasing TFA concentration. Density functional theory calculations of the HOMO–LUMO energy gaps and orbital distributions of CzPyr–H and CzPyr–OMe as well as their protonated forms, CzPyr–H(H+) and CzPyr–OMe(H+), confirmed that charge transfer occurred in the protonated species. In their aggregated solid states, both compounds showed interesting emission properties including aggregation-enhanced emission (AEE). Utilizing their TFA sensitivity and the AEE phenomenon, CzPyr–H and CzPyr–OMe were successfully applied as fluorescent probes for volatile acid detection and as security ink for information encryption on paper.

Full text of DOI:10.1016/j.dyepig.2021.109613

Dyes and Pigments 194 (2021) 109613
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Pyridazine–carbazole based fluorescent probes for volatile acid detection
a
,
1
b
,
1
a
a
b
Sohee Lee , Seung-Hwan Kim , Sunhee Lee , Yeeun Lee , Yong Sup Lee ,
c
d
,
**
a,*
Kwang-Hwan Yang , Kang-Kyun Wang
, Won-Sik Han
a
Department of Chemistry, Seoul Women’s University, Seoul, 01797, South Korea
Department of Pharmacy, College of Pharmacy, Kyung Hee University, Seoul, 02447, South Korea
b
c
Department of Water Campus, Korea Water Cluster, Korea Environment Corporation, Daegu, 43008, South Korea
Bi Biophotonics Co., Ltd., Gimpo-si, Kyunggi-do, 10090, South Korea
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Stimuli-responsive materials based on donor–acceptor systems have great potential for sensing applications,
including in the solid state. Herein, two push–pull-type molecules, 3,6-di(9H-carbazol-9-yl)pyridazine (CzPyr–H)
and 3,6-bis(3,6-dimethoxy-9H-carbazol-9-yl)pyridazine (CzPyr–OMe), each of which contain two carbazole
donors and a pyridazine acceptor, were designed and synthesized. The photophysical properties of CzPyr–H and
CzPyr–OMe were systematically explored with the aim of switching the absorption and emission properties using
trifluoroacetic acid (TFA) as an external analyte, thus realizing volatile acid sensing. The protonation of the
central pyridazine unit led to a broad absorption band at longer wavelengths owing to intramolecular charge
transfer (ICT), and the emission intensity gradually decreased with increasing TFA concentration. Density
functional theory calculations of the HOMO–LUMO energy gaps and orbital distributions of CzPyr–H and
CzPyr–OMe as well as their protonated forms, CzPyr–H(Hþ) and CzPyr–OMe(Hþ), confirmed that charge
transfer occurred in the protonated species. In their aggregated solid states, both compounds showed interesting
emission properties including aggregation-enhanced emission (AEE). Utilizing their TFA sensitivity and the AEE
phenomenon, CzPyr–H and CzPyr–OMe were successfully applied as fluorescent probes for volatile acid
detection and as security ink for information encryption on paper.
Pyridazine
Carbazole
Acid sensing
Aggregation enhanced emission
Sensor
1
. Introduction
the most basic with a relatively large pK
owing to the two nitrogen atoms in its conjugated skeleton, pyridazine is
a good
-acceptor ligand for transition metals [20–22]. Owing to these
a
value of 2.3 [10]. Furthermore,
Diazine derivatives are aromatic dinitrogen heterocycles found in
π
natural products. Because of the importance of diazines as building
blocks for many chemical and biological investigations [1–3], consid-
erable advances have been made in the development of preparation
methods [4–6] and structural modification strategies [7–9]. Diazines
can be classified as pyridazines, pyrimidines, or pyrazines depending on
the position of the nitrogen atoms (1,2-, 1,3-, and 1,4-positions,
advantages, pyridazine derivatives have been reported to possess bio-
logical activities [23,24] and a wide range of applications [25], such as
organic semiconductors for organic field-effect transistors [26,27], host
materials [28,29], efficient ligands for strong phosphorescent iridium
complexes in OLEDs [30–32], and solvatochromic/fluorescent sensors
[33–36].
respectively). All three isomers exhibit a highly
π
-deficient nature [10]
Volatile acid detection is important for determining acidity in envi-
ronmental [37,38], medicinal [39,40], and chemical processes [41,42].
In this regard, many efforts have been made to develop fluorescent
materials that show acidochromism [43–45]. In particular, because of
and thus can be used as electron acceptor units in
π
-conjugated
donor–acceptor dyad systems. Accordingly, diazine moieties have been
utilized in organic electronic materials, such as organic light-emitting
diodes (OLEDs) [11–13], liquid crystals [14,15], and nonlinear optics
the lone pair electrons on its nitrogen atoms and its relatively large pK
a
[
16–18] with significant ICT character. Among diazine moieties, the
value, pyridazine has been incorporated as an electron acceptor unit for
proton binding and functionalized with different electron donor units
pyridazine ring has the strongest dipole moment (
μ
= 4.22 D) [19] and is
** Corresponding author
*
Corresponding author
E-mail address: wshan@swu.ac.kr (W.-S. Han).
These authors contributed equally.
1
https://doi.org/10.1016/j.dyepig.2021.109613
Received 10 May 2021; Received in revised form 28 June 2021; Accepted 28 June 2021
Available online 30 June 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

S. Lee et al.
Dyes and Pigments 194 (2021) 109613
[
13,36,46–48]. Incorporating the donor and acceptor units in the same
DMF while stirring. Continued stirring for 1 h at RT under nitrogen as a
protective gas converted 3,6-dimethoxy-9H-carbazole to the corre-
sponding sodium salt. The sodium salt solution was then added in por-
tions to 3,6-dichloropyridazine (0.18 g, 1.2 mmol) in 10 mL of DMF
while stirring and cooling in an ice bath. After stirring the mixture at RT
12 h, the solvent was distilled off in using oil pump vacuum (~0.5 mm
Hg) and the residue was redissolved in 30–40 mL of ice water. The
aqueous phase was extracted using 10–20 mL of dichloromethane, and
the organic phase was washed several times with water. The mono-
substituted byproduct was removed from the crude mixture by column
molecular structure can aid in ICT owing to electron delocalization,
which affects the electronic behavior of the system.
In this work, we prepared two pyridazine–carbazole-based push–pull
compounds, 3,6-di(9H-carbazol-9-yl)pyridazine (CzPyr–H) and 3,6-bis
(
3,6-dimethoxy-9H-carbazol-9-yl)pyridazine (CzPyr–OMe), and tested
their ability to detect volatile acids. Although pyridazine–carbazole
dyad moieties have been developed as host materials for OLEDs [29,49]
and pyridazine-fused carbazole moieties exhibit antitumor activity [50],
to the best of our knowledge, the volatile acid sensing ability of this type
of molecule has not yet been studied. The photophysical properties of
CzPyr–H and CzPyr–OMe were systematically characterized, and their
fluorescence emission was found to be sensitive to trifluoroacetic acid
chromatography (silica gel; dichloromethane:n-hexane = 1:2). Yellow
1
powder; yield: 71% (0.45 g); H NMR (300 MHz, CDCl
3
, δ): 8.02 (s, 2H),
7.94 (d, 4H, J = 9.0 Hz), 7.55 (d, 4H, J = 2.4 Hz), 7.11 (dd, 4H, J = 9.0,
2.4 Hz), 3.98 (s, 12H); 13C NMR (75 MHz, CDCl
, δ): 155.7, 155.2, 152.8,
134.1, 130.1, 126.1, 123.7, 115.5, 112.8, 103.4, 56.2. HRMS calcd for
: 530.1954, found 530.1981. Elemental analysis: calculated
(
TFA). Further, in the aggregated solid state, each compound showed
3
interesting emission properties, including aggregation-enhanced emis-
sion (AEE). Finally, we fabricated TFA-sensing films using CzPyr–H and
CzPyr–OMe to test their applicability as fluorescent probes for volatile
acid detection and as security ink for encrypting information.
32 26 4 4
C H N O
for C32
H
26
N
4
O
4
: C, 72.44; H, 4.94; N, 10.56; Found: C, 72.43; H, 4.96; N,
◦
◦
◦
10.57. T
d
= 288 C, T
g
= 126 C, T
m
= 177 C.
2
. Experimental Section
2.1.2. X-ray diffraction
The single-crystal structure of CzPyr–H was determined at 223 K
using an X-ray diffraction system (Bruker AXS) at the Korea Basic Sci-
ence Institute, Seoul Western Centre. The system was equipped with a
2
.1. General information
All experimental procedures were performed under a dry nitrogen or
sealed-tube X-ray source (50 kV, 30 mA); monochromatic Mo-K radi-
α
argon atmosphere using standard Schlenk techniques. All solvents were
freshly distilled and used under dry nitrogen or argon purging. 9H-
Carbazole, sodium hydride, and 3,6-dichloropyridazine were purchased
from Aldrich and used without further purification. The starting mate-
ation (λ = 0.71073 Å) corresponding to graphite was obtained from the
X-ray source. At this preliminary stage, the unit cell constants were
◦
determined from a set of 45 narrow-frame (
ω
= 0.3 ) scans. A double-
pass scanning method was used to exclude any noise. The collected
frames were integrated using an orientation matrix, which was devel-
oped using narrow-frame scans. The SMART software package was used
for data collection, and the SAINT software package [53] was used for
frame integration. The final cell constants were determined by global
refinement of the xyz centroids of the reflections, which were harvested
from the entire data set. The structural solution and refinement were
carried out using the SHELXTL PLUS software package [54]. Crystallo-
graphic data for CzPyr–H is deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC-2082659.
rials, 3,6-dibromo-9H-carbazole [51] and 3,6-dimethoxy-9H-carbazole
1
[
52], were prepared according to previously reported procedures.
H
1
3
and C nuclear magnetic resonance (NMR) spectra were recorded in
CDCl using a Bruker Fourier 300 MHz spectrometer operated at 300.1
and 75.4 MHz, respectively. The H and C NMR chemical shifts were
3
1
13
1
referenced relative to CDCl
3
(7.26 ppm for H NMR and 77.16 ppm for
1
3
C NMR). Elemental analyses were performed using a Carlo Erba In-
struments CHNS-O EA 1108 analyzer. A high-resolution mass spec-
trometry (HR-MS) analysis was performed using a highly sensitive liquid
n
chromatography–multistage MS (LC/MS/MS ) (n = 10) spectrometer
n
(
Thermo Fisher Scientific, LCQ Fleet Hyperbolic Ion Trap MS/MS
2.1.3. Absorption and fluorescence measurements
Spectrometer).
Absorption spectra were recorded using a Shimadzu UV-3101PC
scanning spectrophotometer. Emission and excitation spectra were
measured using a Varian Cary Eclipse fluorescence spectrophotometer.
2
.1.1. Synthesis
,6-Di(9H-carbazole-9-yl)pyridazine (CzPyr–H). Sodium hydride
0.71 g, 30 mmol) was added in portions to 9H-carbazole (4.41 g, 26
3
(
2.2. Fluorescence quantum yield
mmol) in 100 mL of dimethylformamide (DMF) while stirring.
Continued stirring for 1 h at room temperature (RT) under nitrogen as a
protective gas converted 9H-carbazole to the corresponding sodium salt.
The sodium salt solution was then added in portions to 3,6-dichloropyr-
idazine (1.79 g, 12 mmol) in 10 mL of DMF while stirring and cooling in
an ice bath. After stirring the mixture at RT for 12 h, the solvent was
distilled off using an oil pump vacuum (~0.5 mm Hg) and the residue
was redissolved in 300–400 mL of ice water. Subsequently, the aqueous
phase was extracted using 100–200 mL of dichloromethane, and the
organic phase was washed several times with water. The mono-
substituted byproduct was removed from the crude mixture by column
The absolute PL quantum yields (PLQYs, Φem) in toluene solution
and solid states were obtained at 298 K using an integrating sphere (F-
3029) installed on a Fluorolog-3 with TCSPC (HORIBA) in Chungnam
University.
2.3. Density functional theory (DFT) calculations
Theoretical calculations for all complexes were conducted using the
Gaussian16 package [55]. The ground-state geometries of CzPyr–H and
CzPyr–OMe were optimized using B3LYP DFT with the 6-31G(d,p) basis
set.
chromatography (silica gel; toluene:n-hexane = 10:1). Pale orange
1
powder; yield: 36% (1.77 g); H NMR (300 MHz, CDCl
3
, δ): 8.18 (dt, 4H,
J = 7.5, 0.9 Hz), 8.12 (s, 2H), 8.02 (d, 4H, J = 8.1 Hz), 7.53 (td, 4H, J =
2.4. Cyclic voltammetry measurements
1
3
7
.2, 1.2 Hz), 7.41 (td, 4H, J = 7.8, 0.9 Hz); C NMR (75 MHz, CDCl
53.5, 139.3, 126.9, 125.1, 124.6, 122.2, 120.7, 111.5. HRMS calcd for
3
, δ):
1
Cyclic voltammetry was performed using CHIE electrochemical
ꢀ 1
C
C
28
H
H
18
18
N
N
4
4
d
: 410.1531, found 410.1552. Elemental analysis: calculated for
workstation (CH Instruments, Inc.) at a scan rate of 0.2 V s in an
28
: C, 81.93; H, 4.42; N, 13.65; Found: C, 81.91; H, 4.43; N,
electrolytic solution consisting of 0.1 M tetra(n-butyl)ammonium hex-
◦
1
3.66. T
= 378 C, T
g
= not detected, T
m
= not detected.
4 6
afluorophosphate (n-Bu NPF ) in dichloromethane at RT under an argon
3
,6-Bis(3,6-dimethoxy-9H-carbazole-9-yl)pyridazine
CzPyr–OMe). Sodium hydride (0.072 g, 3.01 mmol) was added in
portions to 3,6-dimethoxy-9H-carbazole (0.61 g, 2.7 mmol) in 30 mL of
atmosphere. Glassy carbon, platinum wire, and Ag/AgCl were used as
the working, counter, and reference electrodes, respectively. All po-
(
+
tentials were calibrated to the ferrocene/ferrocenium (Fc/Fc ) redox
2

S. Lee et al.
Dyes and Pigments 194 (2021) 109613
couple.
3.2. Photophysical properties
2
.5. Thermal properties
The steady-state UV–vis absorption and fluorescence spectra of
CzPyr–H and CzPyr–OMe (10 M in toluene) were recorded at RT
(Fig. 2 and Table 1). CzPyr–H showed intense absorption bands at
approximately 280 nm, which can be attributed to the * transitions of
both the carbazole and pyridazine moieties. Additional absorption
μ
Thermogravimetric analysis (TGA) was measured using a Scinco Co.
◦
◦
Ltd – STA S-650 system. The TGA was heated from 25 C to 900 C under
air condition. Decomposition temperature was determined when the y-
axis of the graph changes to around 95% by 5% weight loss. Differential
scanning calorimetry (DSC) was measured using Ta Instruments Dis-
π–π
1
1
bands in the range 300–350 nm were attributed to the
L
b
←
A tran-
sition of the carbazole unit [57] and ICT in the ground state from the
carbazole unit (donor) to the pyridazine unit (acceptor). For
CzPyr–OMe, in which electron-donating methoxy groups were
substituted at the 3- and 6-positions of carbazole, the absorption bands
were red-shifted to 310 and 345 nm.
◦
covery DSC 25/RCS 90 System. The DSC was heated from 25 C to
◦
1
70 C under air condition.
3
. Results and discussion
The excited states of CzPyr–H and CzPyr–OMe were investigated
3
.1. Synthesis and crystal structure
using photoluminescence (PL) spectroscopy at RT in 10 M toluene so-
μ
lution and solid states, as shown in Figs. 2 and 3, respectively. Similar to
the absorption properties, the substitution of electron-donating methoxy
groups in CzPyr–OMe significantly shifted the emission to longer
wavelengths. The emission of CzPyr–H and CzPyr–OMe originates from
the charge-transfer state, as evidenced by the solvent-dependent emis-
sion profiles. As shown in Fig. S6, the emission of both compounds was
red-shifted as the solvent polarity increased from toluene to dimethyl
sulfoxide, whereas the absorption spectra remained unchanged in sol-
vents with different polarities. It should be noted that CzPyr–OMe
showed bidirectional solvatofluorochromism [58,59]. In highly polar
solvents, such as acetonitrile and dimethyl sulfoxide, CzPyr–OMe ex-
hibits dual emission with the appearance of new emission at shorter
The synthetic procedures for CzPyr–H and CzPyr–OMe are shown in
Scheme 1, and the detailed experimental procedures and chemical
characterizations are summarized in the Experimental Section. The re-
action of 9H-carbazole with 3,6-dichloropyridazine in the presence of
sodium hydride resulted in the formation of CzPyr–H in a modest yield.
For CzPyr–OMe, 3,6-dimethoxy-9H-carbazole was prepared by bromi-
nation of 9H-carbazole using NBS followed by conversion of the bro-
mines to methoxy groups in the presence of sodium and copper(I)
iodide. Subsequent treatment of 3,6-dimethoxy-9H-carbazole with 3,6-
dichloropyridazine in the presence of sodium hydride gave
CzPyr–OMe in good yield. Both compounds, CzPyr–H and CzPyr–OMe
are soluble in the common organic solvent, such as toluene, dichloro-
methane, THF, acetonitrile, and DMSO. The structure and purity of the
wavelength. When the CT state is slightly below but close enough to S
state, mixing between CT and S states may cause bathochromic spectral
1
1
1
13
final products were confirmed by H and C NMR spectroscopy (Figs. S3
and S4), HRMS, and elemental analysis, as summarized in the Experi-
mental Section.
shifts, for example, from toluene to THF. Further polarity-induced
lowering of the CT state in a more polar medium (acetonitrile and
1
dimethyl sulfoxide) decreases the ratio of its mixing with S state,
The single-crystal of CzPyr–H was grown by slow evaporation
technique from the dichloromethane/n-hexane mixed solution and its
crystal structure was elucidated by single-crystal X-ray analysis (CCDC
causing a rise of hypsochromically shifted emission which results in dual
emission bands.
In solid state, CzPyr–H showed dual emission bands at 427 and 557
nm, while CzPyr–OMe showed similar emission band within solution
state, as shown in Fig. 3. When they are aggregate, CzPyr–H showed
similar fluorescence quantum yield (PLQY), while CzPyr–OMe showed
significantly enhanced PLQY as summarized in Table 1. Optical prop-
erties in the aggregated state are further studied in acetone/water so-
lutions with different water fractions (vide infra).
2
082659). Fig. 1 shows the crystal structure of CzPyr–H (the corre-
sponding ORTEP III diagram is shown in Fig. S1 (Supplementary Mate-
rial) with anisotropic displacement parameters at 30% probability) and
its packing structure. Tables S1–S3 summarize the X-ray structural pa-
rameters, bond lengths, and bond angles. In the crystal structure, the
two carbazole rings are slightly tilted relative to the central pyridazine
◦
◦
unit with ϕ
(
1
= 33.85 and ϕ
2
= 44.97 (Fig. 1a). The two carbazole rings
rings 1 and 2, Fig. 1a), which are twisted in the same direction, have a
3.3. Electrochemical properties
◦
horizontal position with a mean plane angle of 0.91 . In the unit cell, the
intermolecular hydrogen bonds between the nitrogen atoms and Ar–H
atoms in the central pyridazine units have an average length of 2.543 Å
To investigate the effect of the different substituents in CzPyr–H and
CzPyr–OMe on the electrochemical properties, cyclic voltammograms
ꢀ 1
(
Fig. 1b). Because of N∙∙∙Hꢀ C interactions, the carbazole rings have
were recorded at a scan rate of 0.2 V s in dichloromethane containing
+
face-to-face
π
∙∙∙
π
interactions with a separation of 3.76 Å.
0.1 M n-Bu
4
NPF
6
. The potentials were calibrated using Fc/Fc . The
Scheme 1. Synthetic procedures for CzPyr–H and CzPyr–OMe.
3

S. Lee et al.
Dyes and Pigments 194 (2021) 109613
Fig. 1. (a) Single-crystal structure and (b) packing structure of CzPyr–H obtained using the Olex2 program [56].
onset in the absorption spectra, the energy levels of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(
LUMO) were estimated using equations (1) and (2):
ꢀ
)
ox
E
HOMO(eV) = ꢀ E
+ 4.8 eV
(1)
(2)
onset
(
ꢁ
opt
E
LUMO(eV) =
E
HUMO + Eg eV
The HOMO levels of CzPyr–H and CzPyr–OMe were ꢀ 5.74 and
5.36 eV, respectively, and the LUMO levels were ꢀ 2.64 and ꢀ 2.44 eV,
ꢀ
respectively. CzPyr–OMe showed a more positive oxidative potential
because of the electron-donating nature of the methoxy groups.
Accordingly, the energy band gap of CzPyr–OMe (2.92 eV) was smaller
than that of CzPyr–H (3.10 eV).
3
.4. Acid titrations
Fig. 2. The absorption and emission spectra of CzPyr–H and CzPyr–OMe in 10
To confirm the ability of the pyridazine unit (pK
= 2.3) to act as an
a
μ
M toluene at RT. The star symbols at 680 and 700 nm indicate the second-
ꢀ 5
efficient proton acceptor, CzPyr–H and CzPyr–OMe (3 × 10 M in
toluene) were titrated with TFA. Upon increasing the concentration of
TFA (0–300 mM) in the CzPyr–H solution, the absorption band at 335
nm gradually decreased with a slight bathochromic shift and a new red-
shifted broad absorption band appeared at 375–500 nm (Fig. 5a).
Correspondingly, the color of the solution changed from colorless to
order contribution of the excitation light at 340 and 350 nm, respectively.
results are shown in Fig. 4 and the oxidation potentials are summarized
in Table 1.
From the onset points of the first oxidation waves and the optical
Table 1
Photophysical and electrochemical properties of CzPyr–H and CzPyr–OMe.
a
a
ꢀ 1
a
b
onset
ox (V)
c
λ
abs (nm)
λ
em (nm)
Stokes shift (cm
)
PLQY (%)
solution
1.3
E
HOMO (eV)
LUMO (eV)
g
E (eV)
solid
2.1
CzPyr–H
323 (23,000)
35 (28,400)
310 (37,900)
435
509
6862
8365
0.94
0.56
ꢀ 5.74
ꢀ 5.36
ꢀ 2.64
ꢀ 2.44
3.10
2.92
3
CzPyr–OMe
1.7
12.9
3
3
41 (14,900)
57 (14,500)
a
ꢀ 1
ꢀ 1
Measured in 10
μ
M toluene solution at RT and values in parentheses are molar absorption coefficients (M cm ).
b
Obtained at 298 K using an integrating sphere.
c
Measured in dichloromethane solution (1 mM) at RT.
Fig. 3. The emission spectra of (a) CzPyr–H and (b) CzPyr–OMe in solid state (inset: photographs taken under daylight and 365 nm UV irradiation from a hand-held
UV lamp).
4

S. Lee et al.
Dyes and Pigments 194 (2021) 109613
3
.5. DFT calculations
To investigate the changes in the geometries and electronic proper-
ties of CzPyr–H and CzPyr–OMe upon protonation, DFT calculations
were performed at the B3LYP/6-31G* level using the Gaussian16 pro-
gram [55]. The optimized structures and orbital distributions of the
HOMO and LUMO energy levels in the neutral/protonated states are
shown in Figs. S7 and 8, respectively. In the optimized structure
(
Fig. S7), the torsion angles between the carbazole rings and the central
◦
◦
pyridazine ring increased in the protonated state from 36.4 to 38.8 for
CzPyr–H and from 32.8◦ to 37.3 for CzPyr–OMe, which could be
attributed to the steric hindrance induced by protonation. In addition,
the N–N bond distances increased in the pyridazine ring of both com-
pounds upon protonation owing to electron sharing with the added
proton. The orbital distributions in the neutral and protonated states are
◦
shown in Fig. 8. In the neutral state, the
π
electrons of the HOMO were
fully delocalized over the entire molecule, whereas the LUMO was
mainly located on the central pyridazine ring. Upon protonation, the
HOMO of CzPyr–H(Hþ) became localized on one of the carbazole rings,
whereas the LUMO was primarily localized on the pyridazine core.
CzPyr–OMe(Hþ) showed a similar orbital distribution with an addi-
tional small contribution from the pyridazine ring and the peripheral
methoxy groups to the HOMO. The electron-donating nature of the
methoxy group affected the HOMO energy level of CzPyr–OMe(Hþ),
resulting in significant destabilization as compared to the HOMO of
CzPyr–H(Hþ). It should be noted that the HOMO and LUMO distribu-
tions of both compounds were almost completely separated in the pro-
tonated state. These changes in the orbital distribution indicate that
charge transfer occurs in the protonated species. Furthermore, the
calculated bandgap of each protonated species is narrower than that of
the corresponding neutral species, in accordance with the red shift of the
absorbance band following TFA addition.
ꢀ
1
Fig. 4. Cyclic voltammograms (scan rate: 0.2
V
s
)
of CzPyr–H and
CzPyr–OMe (1 mM) in dichloromethane containing 0.1 M n-Bu
4 6
NPF as an
electrolyte.
yellow. The origin of this new band is the protonated species,
þ
CzPyr–H , which is expected to exhibit an enhanced charge-transfer
transition because the electron-withdrawing character of the pyr-
idazine ring increases upon protonation [60]. An isosbestic point was
observed at 342 nm, which indicates a potential equilibrium between
þ
CzPyr–H and CzPyr–H .
The absorption spectra of CzPyr–OMe showed similar changes upon
TFA addition (Fig. 5b), namely, the absorption band at 310 nm gradually
decreased while the absorption bands at 342 and 355 nm increased, and
a new red-shifted broad absorption band appeared at 375–550 nm. For
CzPyr–OMe, an isosbestic point was observed at 362 nm. More distinct
changes were observed in the PL spectra. For both compounds,
increasing the concentration of TFA caused a steady decrease in the
fluorescence intensity and ultimately led to total quenching, as shown in
Fig. 6.
3.6. Optical properties in the aggregated state
To study the optical behavior of CzPyr–H and CzPyr–OMe in the
aggregated state, fluorescence spectra were obtained in acetone/water
To gain a better understanding of the sensitivities of CzPyr–H and
w
solutions with different water fractions (f ). Both compounds exhibited
CzPyr–OMe toward TFA, the Stern–Volmer quenching constant (KSV
)
intriguing optical behavior in the aggregated state. For CzPyr–H,
was calculated using the Stern–Volmer equation, I
0
/I = 1 + KSV[Q],
gradual quenching of the emission with a bathochromic shift occurred
where I is the initial fluorescence intensity in the absence of TFA, I is the
0
until f
w
= 50%, as shown in Fig. 9a. This phenomenon probably origi-
intensity in the presence of TFA, [Q] is the molar concentration of TFA,
and KSV is the quenching constant. As shown in Fig. 7, the KSV values for
CzPyr–H and CzPyr–OMe were estimated from the slopes of the
nates from a twisted intramolecular charge transfer (TICT) state [61,62],
which agrees well with the red shift of the emission of CzPyr–H with
increasing solvent polarity. Namely, in polar solvents, the
charge-separated conformation resulting from intramolecular twisting is
stabilized by the solvation effect and the emission shifts to the longer
wavelength. In this case, the excited state often decays non-radiatively
[63,64], which results in a decrement of the emission intensity. Subse-
5
5
ꢀ 1
M , respectively,
Stern–Volmer plots as 0.57 × 10 and 3.22 × 10
which indicates that CzPyr–OMe has higher sensitivity as a TFA sensor.
For both compounds, the Stern–Volmer plot was nearly linear at lower
TFA concentrations, but deviations from linearity were observed at
higher concentrations, with a nearly exponentially increase demon-
strating an amplified quenching process.
w
quently, when f was more than 60%, the fluorescence intensity began
to be restored with a hypsochromic shift. At this point, the molecules
start to aggregate and form a less polar microenvironment. The polarity
Fig. 5. UV–vis absorption spectra of (a) CzPyr–H and (b) CzPyr–OMe in 30
μ
M toluene solution with different concentrations of TFA.
5

S. Lee et al.
Dyes and Pigments 194 (2021) 109613
Fig. 6. Fluorescence spectra of (a) CzPyr–H and (b) CzPyr–OMe in 30
and 350 nm for CzPyr–H and CzPyr–OMe, respectively.
μ
M toluene solution with different concentrations of TFA. The excitation wavelengths are 340
Fig. 7. Stern–Volmer plots for (a) CzPyr–H and (b) CzPyr–OMe in response to TFA (insets: linear ranges of Stern–Volmer plots at lower TFA concentrations).
Fig. 8. HOMOs and LUMOs of CzPyr–H and CzPyr–OMe in neutral and protonated states.
of the microenvironment continues to decrease as the water fraction
increases, resulting in more severe aggregation that causes blue-shifted
and enhanced emission intensity [65]. In contrast, CzPyr–OMe showed
typical AEE properties, in which a species that is weakly luminescent
displays remarkably enhanced emission in an aggregated state [66]. At
3.7. Solid-state fluorescence sensing
As the emission of CzPyr–H and CzPyr–OMe was very faint under
daylight but significantly enhanced in the solid state, their applicability
to acid sensing in the solid state was investigated. First, “SWU” was
written on a piece of filter paper using CzPyr–H and CzPyr–OMe in
acetone solution. As shown in Fig. 10a, the naked-eye detection of
“SWU” under daylight is difficult, whereas the word can be clearly
observed under UV light. After fuming with TFA vapor (Fig. 10b),
“SWU” on the paper disappeared under UV irradiation owing to pro-
tonation of CzPyr–H and CzPyr–OMe. Subsequent fuming of the paper
with triethylamine (TEA) vapor resulted in full conversion to the “on”
state, with a dramatic increase in the emission from “SWU” (Fig. 10c).
The emission could be turned on and off, and this switching cycle could
f
w
< 60%, the emission intensity decreased with increasing f
w
and was
completely quenched at f
w
= 60% (Fig. 9b). This phenomenon is typical
of many fluorophores with TICT properties, where the emission is
red-shifted and weakened in highly polar solvents. However, a dramatic
increase in intensity (~8 fold) and blue shift of the emissions occurred at
w
f > 70%, indicating a remarkable AEE effect. Namely, radiative decay
process from the excited state is enhanced due to the restriction of
intramolecular motion via intermolecular interactions of CzPyr–OMe in
aggregated state [67].
6

S. Lee et al.
Dyes and Pigments 194 (2021) 109613
Fig. 9. Emission spectra of (a) CzPyr–H and (b) CzPyr–OMe in 30
μ
M acetone/water mixtures with different water fractions (f
w
). Photographs of (c) CzPyr–H and
(
w
d) CzPyr–OMe in acetone/water mixtures with different water fractions (f /vol%) taken under 365 nm UV irradiation from a hand-held UV lamp.
Fig. 10. Photographs of sensors consisting of CzPyr–H and CzPyr–OMe deposited on filter paper under daylight and under illumination by 365 nm UV light: (a) as-
written, (b) after fuming with TFA vapor, and (c) after fuming with TEA vapor.
be repeated several times. Therefore, it is expected that this system could
be used as a fluorescent probe for volatile acid detection or as a security
ink for encrypting information on paper.
Investigation. Yeeun Lee: Investigation, Formal analysis. Yong Sup
Lee: Supervision. Kwang-Hwan Yang: Formal analysis, Funding
acquisition. Kang-Kyun Wang: Supervision, Writing – original draft.
Won-Sik Han: Supervision, Writing – review & editing.
4
. Conclusions
Declaration of competing interest
In this work, push–pull-type pyridazine compounds functionalized
with carbazole (CzPyr–H) and 3,6-dimethoxy-carbazole (CzPyr–OMe)
were prepared and their luminescence and acid-responsive behaviors
were systematically studied. Owing to the ICT characteristics of their
excited states, both compounds showed significant solvatochromic ef-
fects. The DFT calculation results strongly indicated that charge transfer
occurs in the protonated species. In addition, both compounds exhibited
acidochromism, with significant decreases in emission intensity in the
presence of increasing concentrations of TFA. The estimated
Stern–Volmer quenching constants indicated that CzPyr–OMe has
higher sensitivity than CzPyr–H as a TFA sensor. Furthermore, an
evaluation of the emission properties in the solid state revealed that the
photochromic activity of CzPyr–H and CzPyr–OMe can be reversibly
switched off and on by controlling the pH during the AEE process. Thus,
CzPyr–H and CzPyr–OMe have potential as fluorescent probes for vol-
atile acid detection and as security ink for information encryption on
paper.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
This work was supported by a research grant from Seoul Women’s
University (2021-0184) and Korea Water Cluster through the supporting
program for water-tech as “Mini-Cluster project”.
Appendix B. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109613.
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