Solvent dependent photophysical properties and near-infrared solid-state excited state intramolecular proton transfer (ESIPT) fluorescence of 2,4,6-trisbenzothiazolylphenol

Article abstract of DOI:10.1016/j.dyepig.2015.10.002

A new excited-state intramolecular proton transfer (ESIPT) fluorophore, 2,4,6-trisbenzothiazolylphenol (4), was synthesized and its photophysical properties were studied with steady state absorption and emission spectroscopy as well as time-dependent density functional theory calculation. It was found that 4 showed solvent dependent absorption and fluorescence emission. In the nonpolar solvents, ESIPT occurred and only keto tautomer emission at 570 nm was observed, whereas an emission at 510 nm from the deprotonated anion species was observed in polar solvents. With addition of fluoride, the keto emission was quenched, whereas the anion emission was drastically enhanced, making a ratiometric fluorescence sensing of fluoride achievable. In solid state, 4 showed a fluorescence emission at 605 nm, which is longer than those of 2,4-dibenzothiazolylphenol (575 nm) and 4-methyl-2,6-dibenzothiazolylphenol (592 nm) respectively. Thus, the color tuning of solid state ESIPT emission were achieved from green to yellow and near-infrared by extending of the π-conjugation.

Full text of DOI:10.1016/j.dyepig.2015.10.002

Dyes and Pigments 125 (2016) 80e88
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Solvent dependent photophysical properties and near-infrared
solid-state excited state intramolecular proton transfer (ESIPT)
fluorescence of 2,4,6-trisbenzothiazolylphenol
Xuan Zhang*, Jing-Yun Liu
College of Chemistry, Chemical Engineering & Biotechnology, National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua University,
Shanghai 201620, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new excited-state intramolecular proton transfer (ESIPT) fluorophore, 2,4,6-trisbenzothiazolylphenol
(4), was synthesized and its photophysical properties were studied with steady state absorption and
emission spectroscopy as well as time-dependent density functional theory calculation. It was found
that 4 showed solvent dependent absorption and fluorescence emission. In the nonpolar solvents,
ESIPT occurred and only keto tautomer emission at 570 nm was observed, whereas an emission at
510 nm from the deprotonated anion species was observed in polar solvents. With addition of fluoride,
the keto emission was quenched, whereas the anion emission was drastically enhanced, making a
ratiometric fluorescence sensing of fluoride achievable. In solid state, 4 showed a fluorescence emission
at 605 nm, which is longer than those of 2,4-dibenzothiazolylphenol (575 nm) and 4-methyl-2,6-
dibenzothiazolylphenol (592 nm) respectively. Thus, the color tuning of solid state ESIPT emission
Received 27 August 2015
Accepted 5 October 2015
Available online 23 October 2015
Keywords:
Benzothiazole
Excited-state intramolecular proton transfer
Near-infrared fluorescence emission
Solvent effect
Ratiometric
were achieved from green to yellow and near-infrared by extending of the
p-conjugation.
Anion sensing
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Fig. 1) have been reported to show a yellow ESIPT fluorescence
emission in CHCl3 solution (ca. 560 nm), which is red-shifted about
45 nm compared to the green emission (515 nm) of 1 [32]. It is
Excited-state intramolecular proton transfer (ESIPT) has
received considerable attention due to its importance on funda-
mental reaction process and promising applications on organic
optoelectronic materials as well as fluorescence sensing [1e20]. In
this regard, several ESIPT prototype chromophores, for example,
methyl salicylate (MS) [21,22], 3-hydroxyflavone (3HF) [23,24], 2-
(20-hydroxyphenyl)benzoxazole (HBO) [25,26], and 2-(2-
hydroxyphenyl) benzothiazole (HBT) [27‒31], have been exten-
sively investigated on both fundamental photophysics and poten-
tial application on luminescent materials. However, it is still a
challenge to design new ESIPT fluorophores with predetermined
photophysical properties. One popular strategy used for design of
therefore interesting to try for further extending of the
p-conju-
gation by introducing the third benzothiazole group to investigate
the substituent effect on ESIPT fluorescence emission of HBT de-
rivatives. The aim of this study was to synthesize exemplary tris-
HBT derivative 2,4,6-trisbenzothiazolylphenol (4) and investigate
its photophysical properties, in comparison with those of HBT 1 and
bis-HBT 2 and 3. The tris-HBT 4 exhibits a high efficient near-
infrared (NIR) solid state ESIPT emission around 605 nm with a
fluorescence quantum yield of 30%,which is longer than those of
HBT 1 (525 nm), bis-HBT 2 (575 nm) and 3 (592 nm) respectively.
Thus, the ESIPT emission color tuning of HBT chromophore from
new fluorophores is to extending the
p
-conjugation [3]. HBT (1,
green to yellow and red was achieved by properly extending the p-
Fig. 1) is the well-known ESIPT dye that usually showed both a blue
emission and a green emission from enol and keto tautomers
respectively [27,28]. Recently, bis-HBT derivatives 2,4-
dibenzothiazolylphenol (2, Fig. 1), 6-dibenzothiazolylphenol (3,
conjugation. The photophysical properties of the tris-HBT 4 were
studied in detail with steady state spectroscopy and DFT/TDDFT
computations, and its ratiometric sensing property for fluoride
anion as well as creation of white emission was also investigated.
These findings will contribute to develop highly emissive NIR
fluorescent materials and ratiometric fluorescent probes based on
the ESIPT chromophores.
* Corresponding author.
E-mail address: xzhang@dhu.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.dyepig.2015.10.002
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

X. Zhang, J.-Y. Liu / Dyes and Pigments 125 (2016) 80e88
81
Fig. 1. Molecular structures of 1‒5 investigated in this work.
2. Experimental section
134.72, 132.75, 131.90, 127.50, 126.89, 126.44, 125.91, 125.34, 125.09,
122.89, 122.31, 121.67, 121.59, 118.64, 117.21. MALDI-TOF-MS: m/z
calcd for C20H12N2OS2, 360.04; found 361.10 (M þ H)þ.
2.1. Materials and measurements
All the chemicals are analytical grade (Sinopharm Chemical
Reagents Co.) and were used as received. Fluorescence and ab-
sorption spectra were measured on a Hitachi F-7000 (Ex/Em slit
widths: 2.5 nm) and a Persee TU-1901 spectrometer with a 1 cm
quartz cuvette, respectively. The fluorescence quantum yield in
solution was determined by using quinine sulfate (Фf ¼ 0.546 in
0.1 M H2SO4) as a standard [33]. The solid fluorescence spectra and
absolute quantum yields were obtained on Edinburgh FS5 spec-
trofluorometer equipped with an integrating sphere (EI-FS5-SC-
30). Fluorescence micrographs were recorded on Nikon ECLIPSE 80i
microscope. 1H NMR and 13C NMR spectra were recorded on a
Bruker AVANCE III 400 MHz spectrometer with TMS as standard.
Mass spectra were collected on AB Sciex MALDI-TOF/TOFTM MS
spectrometer. Elemental analysis data were obtained on Elmentar
Vario EL III system.
2.2.3. 2,6-Dibenzothiazolyl-4-methylphenol (3)
2,6-diformyl-4-methylphenol was firstly synthesized according
to the reported procedure [35]. Briefly, under N2 atmosphere, 4-
methylphenol
(10
mmol)
and
hexamethylenetetramine
(40 mmol) were dissolved in TFA (15 mL) and refluxed at 110 ꢀC for
48 h. The mixture was cooled to room temperature, poured into 4 M
HCl solution (80 mL), and extracted with chloroform (3 ꢁ 20 mL).
The organic layer was combined and washed with 4 M HCl (80 mL),
water (80 mL), then dried over MgSO4 and evaporated to dryness
under reduced pressure, respectively. The crude product was
further purified by column chromatography (silica gel, hexane/
ethyl acetate ¼ 20:1 v/v). Yield: 60.9%. 1H NMR (400 MHz, CDCl3),
d
(ppm): 11.48 (s, 1H), 10.24 (s, 2H), 7.79 (s, 2H), 2.41 (s, 3H); 13C
NMR (100 MHz, CDCl3),
d (ppm): 192.22, 161.77, 137.99, 129.53,
122.92, 20.09. Then 3 was prepared by employing the similar pro-
cedure described for 2. The pure 3 was isolated as pale yellow solid
by
column
chromatography
(silica
gel,
hexane/
2.2. Synthesis
dichloromethane ¼ 4:1 v/v). Yield: 8%. 1H NMR (400 MHz, CDCl3),
d
(ppm): 8.13 (d, J ¼ 8 Hz, 4H), 7.99 (d, J ¼ 8 Hz, 2H), 7.57 (t, J ¼ 8 Hz,
2.2.1. 2-(2-Hydroxyphenyl)benzothiazole (1)
2H), 7.46 (t, J ¼ 8 Hz, 2H), 2.51 (s, 3H); 13C NMR (100 MHz, CDCl3),
1 was prepared from salicylic acid and 2-aminothiophenol ac-
cording to the similarly reported procedure with some modifica-
tions [34]. Briefly, under N2 atmosphere, salicylic acid (5 mmol) and
2-aminothiophenol (5 mmol) were dissolved in polyphosphoric
acid (8 mL) and heated at 180 ꢀC for 5 h. The mixture was cooled to
room temperature and poured into 80 mL ice water. The obtained
solid product was further purified by column chromatography
(silica gel, hexane/dichloromethane ¼ 10:1 v/v) to give 1 as white
d
(ppm): 154.25, 151.43, 132.01, 126.56, 125.35, 122.37, 121.51, 29.71,
20.53. MALDI-TOF-MS: m/z calcd for C21H14N2OS2, 374.05; found
375.12 (M þ H)þ. Elemental Anal: calcd for C21H14N2OS2, C 67.35%,
H 3.77%, N 7.48%; found C 67.50%, H 4.63%, N 6.42%.
2.2.4. 2,4,6-Tribenzothiazolylphenol (4)
Firstly, 3,5-diformyl-4-hydroxybenzoic acid was synthesized
according to the reported procedure [36]. Briefly, under N2 at-
mosphere, 4-hydroxybenzoic acid (10 mmol) and hexamethy-
lenetetramine (40 mmol) were dissolved in TFA (15 mL) and
refluxed at 110 ꢀC for 72 h. After cooling to room temperature, the
mixture was poured into ice water (80 mL), and the product was
obtained by filtration. Yield: 55%. 1H NMR (400 MHz, DMSO-d6),
solid. Yield: 34.7%. 1H NMR (400 MHz, CDCl3),
d (ppm): 12.42 (s, 1H),
8.03 (d, J ¼ 8 Hz, 1H), 7.94 (d, J ¼ 8 Hz,1H), 7.74 (d, J ¼ 8 Hz, 1H), 7.55
(t, J ¼ 10 Hz, 1H), 7.43 (m, 2H), 7.14 (d, J ¼ 8 Hz, 1H), 6.98 (t, J ¼ 8 Hz,
1H); 13C NMR (100 MHz, CDCl3),
d (ppm): 169.38, 157.95, 151.76,
132.79, 132.59, 128.42, 125.56, 122.16, 121.52, 119.53, 117.89, 116.77,
29.72. MALDI-TOF-MS: m/z calcd for C13H9NOS, 227.04; found,
228.09 (M þ H)þ.
d
(ppm): 13.34 (s, 1H), 10.30 (s, 2H), 8.55 (s, 2H); 13C NMR
(100 MHz, DMSO-d6), (ppm): 192.42, 165.99, 165.30, 138.05,
d
124.07, 123.06. Then 4 was prepared by employing the similar
procedure described for 2. The pure 4 was isolated as pale yellow
solid by column chromatography (silica gel, hexane/
dichloromethane ¼ 1:1 v/v). Yield: 5.2%. 1H NMR (400 MHz,
2.2.2. 2,4-Dibenzothiazolylphenol (2)
2 was prepared as previously described with some modifica-
tions [32]. Briefly, under an N2 atmosphere, 5-formylsalicylic acid
(5 mmol) and 2-aminobenzenethiol (10 mmol) were dissolved in
DMSO (2 mL) and heated at 180 ꢀC for 2 h. After cooling to room
temperature, the mixture was poured into water and the crude
product was collected by filtration. The obtained solid product was
further purified by column chromatography (silica gel, hexane/
dichloromethane ¼ 1:1 v/v) to give 2 as pale yellow solid. Yield:
CDCl3),
d
(ppm): 9.16 (s, 2H), 8.20 (t, J ¼ 8 Hz, 3H), 8.03 (d, J ¼ 8 Hz,
2H), 7.99 (d, J ¼ 8 Hz, 1H), 7.61 (m, 3H), 7.51 (m, 3H). MALDI-TOF-
MS: m/z calcd for C27H15N3OS3, 493.04; found 494.12 (M þ H)þ.
Elemental Anal: calcd for (C27H15N3OS3 þ hexane), C 68.36%, H
5.04%, N 7.25%; found: C 68.10%, H 5.33%, N 6.73%.
19.9%. 1H NMR (400 MHz, CDCl3),
d (ppm): 13.08 (s, 1H), 8.51 (s, 1H),
8.07 (d, J ¼ 8 Hz, 1H), 8.02 (m, 1H), 7.96 (d, J ¼ 8 Hz, 1H), 7.92 (d,
2.2.5. 1,3,5-Trisbenzothiazolylbenzene (5)
Control compound 5 was synthesized according to the re-
ported procedure [37]. Briefly, under N2 atmosphere, 1,3,5-
J ¼ 8 Hz, 1H), 7.47 (m, 5H), 7.23 (d, J ¼ 12 Hz, 1H); 13C NMR
(100 MHz, CDCl3),
d (ppm): 168.64, 166.91, 160.36, 153.89, 151.57,

82
X. Zhang, J.-Y. Liu / Dyes and Pigments 125 (2016) 80e88
benzenetricarboxylic acid (5 mmol) and 2-aminothiophenol (15
mmol) were dissolved in polyphosphoric acid (8 mL)and heated at
230 ꢀC for 12 h. The mixture was cooled to room temperature,
poured into ice water (100 mL), and neutralized with sodium bi-
carbonate, respectively. The obtained brown solid was
recrystallized from chloroform to give 5 as a grey-white solid.
Yield: 2.7%. 1H NMR (400 MHz, CDCl3),
(ppm): 9.02 (s, 3H), 8.24
d
(d, J ¼ 8 Hz, 3H), 8.01 (d, J ¼ 8 Hz, 3H), 7.60 (t, J ¼ 8 Hz, 3H), 7.50 (t,
J ¼ 8 Hz, 3H); 13C NMR (100 MHz, CDCl3),
d (ppm): 166.12, 153.36,
135.12, 135.06, 128.57, 126.85, 125.98, 123.47, 121.88.
Fig. 2. Absorption (b, d, f, h, j) and fluorescence (a, c, e, g, i) spectra of 2‒4 in various solvents. Excitation wavelength is 345 nm.

X. Zhang, J.-Y. Liu / Dyes and Pigments 125 (2016) 80e88
83
Table 1
2.3. DFT calculations
Photophysical Properties of Compounds 2e4 in various solvents.
εb (Mꢂ1 cmꢂ1
)
lFluc (nm) FF
a
d
All calculations were performed using Gaussian 09 package [38].
The ground state (S0) and the first singlet excited state (S1) geom-
etries of the compounds were optimized in the gas phase using
density functional theory (DFT) and time-dependent density
functional theory (TDDFT) at the B3LYP/6-31 þ G(d) level, respec-
tively. The absorption and fluorescence emission were calculated
with TDDFT based on the optimized S0 and S1 state geometries,
respectively.
Compounds Solvents lAbs (nm)
2
3
4
Hexane 356
11,240
12,330
29,980
22,960/23,430
27,320/1570
12,420
12,996
17,820/13,650
8676/30,250
541
537
401/472
397/472
393/466
568
560
518/566
516
0.051
0.066
0.301
0.212
0.122
0.029
0.086
0.066
0.381
0.069
0.047
0.056
0.068
0.162
0.302
CHCl3
DMSO
DMF
360
316
378/441
303/400
MeOH
Hexane 369
CHCl3
DMSO
DMF
370
301/375
325/491
MeOH
Hexane 378
295/372/439 23,090/13,030/3012 560
3. Results and discussion
7172
7562
570
560
497/560
497
502
CHCl3
DMSO
DMF
380
3.1. Absorption and fluorescence spectra
318/382/480 20,480/8510/2160
387/477
440
19,510/19,650
10,980
MeOH
The absorption and fluorescence spectra of HBT derivatives 1e4
were studied in hexane, CHCl3, DMSO, DMF and MeOH (Fig. 2 and
Figure S1). As shown in Figure 2, 4 exhibits a solvent-dependent
absorption and emission behavior. In nonpolar solvents hexane
and CHCl3, 4 shows an absorption band at 350e450 nm, accom-
panied by shoulders around 290e350 nm (Fig. 2b and d), respec-
tively. The low energy absorption band are due to the S0eS1 (pp*)
transition, which is assigned to the intramolecular hydrogen-
bonding closed enol conformer (EeE*, Scheme 1, vide infra DFT
calculation section). 2 and 3 present similar absorption behaviors in
these solvents, but their wavelengths of low energy absorption
bands are slightly short than that of 4 (Fig. 2 and Table 1), implying
a larger conjugation in tri-HBT 4. In polar solvents DMSO, DMF and
MeOH, a new low energy absorption band appeared around
400e500 nm for 4 (Fig. 2f, h and j), which can be attributed to the
deprotonated anion conformer (AꢂA*, Scheme 1, vide infra F
titration section) due to the solvent-assisted deprotonation inter-
action. In contrast, the absorption spectra of HBT 1 only showed a
a
b
c
The maximum absorption wavelength.
The molar absorption coefficient.
The maximum fluorescence emission wavelength (lex ¼ 345 nm).
d
The fluorescence quantum yield measured by using quinine sulfate (Фf ¼ 0.546)
as a standard.
slight change with variation of solvents (Figure S1), suggesting the
presence of a weak influence of inter- and intramolecular hydrogen
bonding on the absorption spectra.
On the other hand, 4 shows a single emission band at about
500e650 nm in nonpolar solvents hexane and CHCl3 with a large
stokes-shift of ca. 9000 cmꢂ1 (Fig. 2a and c), which is a typical
characteristic of ESIPT emission from keto tautomer (K*ꢂK,
Scheme 1). As shown in Scheme 1, the K* state was derived from E*
state via an ESIPT process, relaxed to ground state (K) through
ESIPT emission and followed by back ground state intramolecular
proton transfer (GSIPT) to the more stable ground state enol
Scheme 1. Schematic representation of the photophysical processes for 4.

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X. Zhang, J.-Y. Liu / Dyes and Pigments 125 (2016) 80e88
tautomer [13,26]. Such long ESIPT emission was also observed in
1‒3 derivatives, respectively (Fig. 2a, b and S1, Table 1). In DMSO, 4
gives a dual emission, where a new weak emission appeared
around 500 nm, ascribed to the deprotonated anion conformer
(A*ꢂA, Scheme 1), besides the longer keto tautomer emission
around 565 nm (Fig. 2e). And in DMF and MeOH, 4 only gives a
strong emission from the deprotonated anion conformer (A*ꢂA,
Scheme 1) with the maximum wavelength of 500 nm (Fig. 2g and
i), which is in well agreement with anion bands observed in the
absorption spectra (Fig. 2h and j). While 3 exhibits the similar
emission behavior with that 4, both 1 and 2 show a different dual
fluorescence, where two new emissions appeared around 400 nm
and 475 nm in these polar solvents (Fig. 2e, g and i and S1),
ascribed to the neutral enol and anion conformers, respectively
[28,32].
The excitation spectra of 1‒4 recorded in different solvents
resemble their corresponding absorption spectra (Figure S2),
revealing that the fluorescence emission originates from the
ground state absorbing species. The fluorescence emission and
quantum yields of HBT derivatives 1e4 in different solvents are
summarized in Table 1. The absence of enol emission for 3 and 4 in
these solvents suggests that their enol forms in the excited state are
not stable, probably resulted from their two symmetric benzo-
thiazole moieties of proximity to phenolic OH that makes the
closed H-bonding conformer predominant. In contrast, strong enol
emission was observed for 1 and 2 in polar solvents due to the
intramolecular hydrogen bond between phenolic OH and N atom of
benzothiazole moiety is disrupted by solvent molecule, made an
open form conformer predominant [26,28]. The control compound
5 that could not undergo ESIPT due to the absence of phenolic OH,
showed a near solvent-independent absorption and fluorescence
spectra (Figure S3), indicating that the phenolic OH is crucial in 4
for its solvent-dependent photophysical properties.
Fig. 4. Solid state fluorescence spectra of 1‒4 (up) and their corresponding fluores-
cence microscopic images (bottom). Excitation wavelength is 345 nm.
appear at the same time (Fig. 3a). The CIE chromaticity diagram
(Fig. 3b) clearly shows the fluorescence color change with the ratio
of 2 and 4, where near pure white coordinates (0.33, 0.33) were
obtained at 2/4 ratios of 1:1 (0.33, 0.34) and 4:3 (0.31, 0.32),
respectively.
3.2. White emission and solid state fluorescence
The solid state fluorescence of 4 was investigated by measuring
powder samples. As shown in Fig. 4, a NIR emission with the
maximum around 605 nm and a fluorescence quantum yield of 30%
was observed in 4, whereas the corresponding value is 525 nm
(60%), 575 nm (40%) and 592 nm (40%) in 1‒3, respectively. The
fluorescence microscopic images clearly demonstrated the solid
state emission color change from green to yellow and red of 1‒4
The solvent-dependent multiple emission from blue to green,
yellow and red of HBT derivatives 1e4 covers a broad visible region,
making it promising to be utilized for developing color tunable
fluorescence even white by simply mixing them together [39]. For
example, in DMSO, 4 gives two emission bands located around
500 nm and 600 nm, whereas 2 presents dual emission around
400 nm and 500 nm, and color tunable fluorescence from blue to
yellow was obtained by gradually mixing 2 with 4. More interest-
ingly, a white emission was successfully created at a ratio of 0.5,
where three emission bands around 400 nm, 500 nm and 600 nm
(Fig. 4). This reveals that the extending of
p-conjugation could
create the red emitting HBT derivatives, which will contribute to
develop highly emissive NIR fluorescent materials based on the
ESIPT chromophores [40].
Fig. 3. Fluorescence spectrum (lex ¼ 345 nm) of the mixture of 2 and 4 at a ratio of 1:1 in DMSO (a), and CIE chromaticity diagram of the mixture of 2 and 4 at various ratios, where
the ratio of 2/4 changed from 10:0 to 0:10 from dot 1 to 11 (b). Inset in (a) is photographs of 2, 1:1 mixture of 2 þ 4 and 4 in DMSO under a UV lamp (365 nm) irradiation.

X. Zhang, J.-Y. Liu / Dyes and Pigments 125 (2016) 80e88
85
Fig. 5. Optimized molecular structure of 4 in S0 (a) and S1 (b) states.
3.3. DFT calculations
occurs and the molecule becomes more planar at the S1 state
(Fig. 5). Regarding to the structural parameters involved in the
hydrogen bond, it is noted that the bond length of OeH elongated
from 0.9963 Å (in S0) to 1.0493 Å (in S1), the distance of N/H
became shorter from 1.7356 Å (in S0) to 1.5398 Å (in S1), and the
angle of OeH/N enlarged from 146.82ꢀ (in S0) to 152.02ꢀ (in S1).
These structural changes indicate that the intramolecular
hydrogen bond (OeH/N) is substantially strengthened in the S1
state, facilitating the occurrence of ESIPT [41]. The calculated ab-
sorption and emission maximum wavelength of 4 are 383.78 nm
and 618.85 nm, respectively, which are close to the experimental
results (376 nm and 570 nm in hexane, Fig. 2). By comparison with
others HBT derivatives 1e3, it is noted that both the absorption
To further investigate the photophysical property of tri-HBT
derivative 4, DFT/TDDFT calculations have been performed in the
gas phase. The ground and exited state geometries of the HBT de-
rivatives were optimized with DFT and TDDFT methods at the
B3LYP/6-31 þ G(d) level, respectively. The UVevis absorption and
the fluorescence emission of the compounds were calculated with
TDDFT methods based on optimized S0 and S1 state geometries,
respectively.
The optimized S0 and S1 state geometries of 4 are shown in
Fig. 5. In the ground state, two benzothiazole moieties (2 and 4
position) are near coplanar with phenol ring, but the third ben-
zothiazole unit (6-position) is twisted about 45ꢀ from phenol
plane (Fig. 5). The twisting of the non-hydrogen bonded benzo-
thiazole from the phenol ring could rationalized by considering
the phenolic OH steric hindrance, which is supported by the
planar molecular geometry of the control compound 5 optimized
at the same calculation method. Upon excitation, the dihedral
angle between the twisted benzothiazole moiety (6-position) and
phenol ring is around 15ꢀ, indicating a structural reorganization
and emission maximum undergo red-shift with extending the
p-
conjugation (Table S1). The frontier molecular orbitals of 1‒4 were
further calculated and the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) are
depicted in Fig. 6. Notably, the HOMO delocalized on whole
molecule and the LUMO mainly distributed on hydrogen bonding
involved HBT moiety (Fig. 6), suggesting a predominant
transition character of EeE* (Scheme 1).
pꢂp*
Fig. 6. HOMO (up) and LUMO (bottom) orbitals of 1‒4 in the ground state.

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X. Zhang, J.-Y. Liu / Dyes and Pigments 125 (2016) 80e88
3.4. Ratiometric fluorescence sensing of anions
decreased with an isobestic point at 345 nm (Fig. 7h), indicating the
Fꢂ-induced a ground state deprotonation of phenolic OH in 4
[44e46]. Similarly, the Fꢂ-induced enhancements of anion emis-
sion and absorption were also observed in others HBT derivatives
1e3 (Fig. 7aef). However, both the absorption and fluorescence
spectra of control compound 5 did not show any noticeable change
with addition of Fꢂ, further confirming that the phenolic OH is
crucial in 4 for ratiometric fluorescence sensing property toward Fꢂ.
It should be noted that the emitting species involved in 3 and 4 are
keto conformer and anion, whereas in 1 and 2, enol conformer and
anion. Furthermore, 4 showed the highest sensitivity toward Fꢂ
sensing, where the fluorescence response observed even 5 equiv Fꢂ
addition and reaches the maximum upon 40 equiv Fꢂ addition, but
others derivatives 1e3 only showed the obvious response upon
more than 40 equiv Fꢂ addition (Fig. 7). The difference on the
The solvent dependent multiple emission of 4 makes it prom-
ising to be utilized for developing ratiometric fluorescent probes
[20,42,43]. Therefore, the sensing property for anions such as
fluoride (Fꢂ) was investigated in DMSO. As shown in Fig. 7, all HBT
derivatives showed a ratiometric fluorescence response toward Fꢂ.
For example, without Fꢂ addition, 4 shows dual fluorescence,
originating from both keto conformer (K*ꢂK, Scheme 1) and anion
(A*ꢂA, Scheme 1) respectively; upon introduction of Fꢂ, the keto
emission (560 nm) decreased and anion emission (500 nm)
increased with an iso-emissive point at 550 nm (Fig. 7g). At the
same time, the absorption spectra also showed an obvious change
upon addition of Fꢂ, where the anion absorption band around
475 nm enhanced and the absorption band around 300 nm
Fig. 7. Change of fluorescence (a, c, e, g) and absorption (b, d, f, h) spectra of 1‒4 (10 m
M) in DMSO with addition of various amounts of Fꢂ.

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sensitivity reveals that the anion sensing property could be tuned
by proper modification of molecular structure. These results may
contribute to develop the ratiometric fluorescent probes based on
the ESIPT chromophores.
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4. Conclusions
In summary, 2,4,6-trisbenzothiazolylphenol (4) was prepared as
a new excited state intramolecular proton transfer (ESIPT) chro-
mophore. The photophysical properties were studied with ab-
sorption and emission spectroscopies as well as theoretical
calculations. It was found that 4 showed solvent dependent ab-
sorption and fluorescence emission. In the nonpolar solvents
(hexane etc.), ESIPT occurred and only keto tautomer emission at
570 nm was observed, whereas an emission at 510 nm from the
deprotonated anion species was observed in polar solvents (DMF
etc.). With addition of fluoride into DMSO solution of 4, the keto
emission was quenched, whereas the emission from anion form
was drastically enhanced, making
a ratiometric fluorescence
sensing of fluoride achievable. In solid state, 4 showed a near-
infrared (NIR) ESIPT fluorescence emission at 605 nm, which is
longer than those of parent molecule 2-(2-hydroxyphenyl)benzo-
thiazole (HBT, 1) (525 nm), 2,4-dibenzothiazolylphenol (2, 575 nm)
and 4-methyl-2,6-dibenzothiazolylphenol (3, 592 nm) respectively.
Thus, the color tuning of solid state ESIPT emission of the HBT
chromophore was achieved from green to yellow and near-infrared
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methyl salicylate. J Am Chem Soc 1978;100:7472e4.
by properly extending the
p-conjugation that was further
confirmed by TDDFT calculations. The white emission was also
created by simply mixing two HBT derivatives in DMSO. These in-
vestigations will be useful for designing new ESIPT chromophore
and developing ESIPT-based highly emissive NIR fluorescent ma-
terials and ratiometric fluorescent probes.
[22] Zhou P, Hoffmann MR, Han K, He G. New insights into the dual fluorescence of
methyl salicylate: effects of intermolecular hydrogen bonding and solvation.
J Phys Chem B 2015;119:2125e31.
[23] Klymchenko AS, Demchenko AP. Electrochromic modulation of excited-state
intramolecular proton transfer: the new principle in design of fluorescence
sensors. J Am Chem Soc 2002;124:12372e9.
Acknowledgment
[24] Ghosh D, Batuta S, Das S, Begum NA, Mandal D. Proton transfer dynamics of
40-N,N-Dimethylamino-3-hydroxyflavone observed in hydrogen-bonding
solvents and aqueous micelles. J Phys Chem B 2015;119:5650e61.
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azole. J Phys Chem A 2002;106:3665e72.
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and emission sensitivity of 2-(2’-Hydroxyphenyl)benzoxazoleto solvents and
impurities. Photochem Photobiol 2015;91:586e98.
This work was financially supported by the Research Fund
for the Doctoral Program of Higher Education of China (No.
20120075120018), Fundamental Research Funds for the Central
Universities (Nos. 2232014D3-11, 2232015G1-61), and the National
Natural Science Foundation of China (No. 51203018).
Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.10.002.
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