Excited State Intramolecular Proton Transfer in Ethynyl-Extended Regioisomers of 2-(2′-Hydroxyphenyl)benzothiazole: Effects of the Position and Electronic Nature of Substituent Groups

Article abstract of DOI:10.1002/asia.201601340

Although the organic dyes based on excited state intramolecular proton transfer (ESIPT) mechanism have attracted significant attention, the structure-property relationship of ESIPT dyes needs to be further exploited. In this paper, three series of ethynyl-extended regioisomers of 2-(2′-hydroxyphenyl)benzothiazole (HBT), at the 3′-, 4′- and 6-positions, respectively, have been synthesized. Changes in the absorption and emission spectra were correlated with the position and electronic nature of the substituent groups. Although 4′- and 6-substituted HBT derivatives exhibited absorption bands at longer wavelengths, the keto-emission of 3′-substituted HBT derivatives was found at a substantially longer wavelength. The gradual red-shifted fluorescence emission was found for 3′-substituted HBT derivatives where the electron-donating nature of substituent group increased, which was opposite to what was observed for 4′- and 6-substituted HBT derivatives. The results derived from the theoretical calculations were in conformity with the experimental observations. Our study could potentially provide experimental and theoretical basis for designing novel ESIPT dyes that possess unique fluorescent properties.

Full text of DOI:10.1002/asia.201601340

DOI: 10.1002/asia.201601340
Full Paper
Substituent Effects
Excited State Intramolecular #ProtonTransfer in Regioisomers of
HBT: Effects of the Position and Electronic Nature of Substituents
Qin Wang, Longfei Xu, Yahui Niu, Yuxiu Wang, Mao-Sen Yuan,* and Yanrong Zhang*[a]
Abstract: Although the organic dyes based on excited state
intramolecular proton transfer (ESIPT) mechanism have at-
tracted significant attention, the structure-property relation-
ship of ESIPT dyes needs to be further exploited. In this
paper, three series of ethynyl-extended regioisomers of 2-(2’-
hydroxyphenyl)benzothiazole (HBT), at the 3’-, 4’- and 6-posi-
tions, respectively, have been synthesized. Changes in the
absorption and emission spectra were correlated with the
position and electronic nature of the substituent groups. Al-
though 4’- and 6-substituted HBT derivatives exhibited ab-
sorption bands at longer wavelengths, the keto-emission of
3’-substituted HBT derivatives was found at a substantially
longer wavelength. The gradual red-shifted fluorescence
emission was found for 3’-substituted HBT derivatives where
the electron-donating nature of substituent group increased,
which was opposite to what was observed for 4’- and 6-sub-
stituted HBT derivatives. The results derived from the theo-
retical calculations were in conformity with the experimental
observations. Our study could potentially provide experi-
mental and theoretical basis for designing novel ESIPT dyes
that possess unique fluorescent properties.
Introduction
the 4’- and 5’-positions of HBT with tertiary amino groups, and
studied the electronic effects of different electron-donating
amino groups on the photophysical properties.[18] However,
they did not synthesize any other regioisomers of amino HBT
derivatives and their synthetic strategy has not yet been con-
veniently exploited to introduce other substituents. Han et al.
introduced an electron-donating group (triphenylamine) in the
6-position and an electron-withdrawing group (trifluoromethyl)
in the 4’-position simultaneously, to obtain an HBT derivative,
which can generate pure white light emission in mixed sol-
vents.[19] Zhao et al. conjugated the naphthalimide at the 5’-po-
sition of HBT through CꢀC linker and observed a long-lived
triplet state for this HBT derivative.[20] Zhao et al. also reported
another HBT derivative with a boron-dipyrromethene substitu-
ent in the 5’-positon, which showed no ESIPT effect.[21] These
studies demonstrated that subtle structural changes on HBT
might bring about dramatic impact on its photophysical prop-
erties. Despite the potential practical application of the HBT
derivatives,[22–26] the structure-property relationship of the
ESIPT dyes is still obscure and needs to be further exploited.
Establishing this relationship could provide experimental and
theoretical guidance to meet specific applications through fur-
ther chemical transformations.
Proton-transfer reactions play very important roles in many
chemical and biological processes.[1] The excited state intramo-
lecular proton transfer (ESIPT) process refers to a photoinduced
tautomerization of the enolform to the ketoform in the excited
state through a pre-existing intramolecular hydrogen bond in
the subpicosecond time scale,[2–7] followed by a relaxation of
the excited keto tautomer to its ground state accompanied by
fluorescence emission. It subsequently returned to its energeti-
cally favored ground-state enol tautomer via ground state in-
tramolecular proton transfer (GSIPT).[8–10] Because of the signifi-
cant difference between the absorbing species (enol tautomer)
and the emitting species (keto tautomer) in terms of structure
and electronic configuration, ESIPT fluorophores usually exhibit
fluorescence emission with larger Stokes shifts than common
fluorophores.[11–13]
2-(2’-Hydroxyphenyl)benzothiazole (HBT),
a typical ESIPT
dye, has been extensively studied experimentally and theoreti-
cally due to its structural simplicity, facile preparation and
chemical stability.[13–17] Chemical modifications of the existing
ESIPT dyes are often efficient strategies to precisely control
ESIPT and to meet specific applications. Wang et al. modified
Herein, it is aimed to develop a facile method to synthesize
three series of ethynyl-extended regioisomers of HBT, at the 3’-,
4’- and 6-positions, respectively (molecular structures shown in
Scheme 1), and to study the influence of the position and elec-
tronic nature of substituents on the ESIPT process by using
steady-state fluorescence in different solvents as well as theo-
retical calculations.
[a] Dr. Q. Wang, Dr. L. Xu, Dr. Y. Niu, Dr. Y. Wang, Dr. M.-S. Yuan, Dr. Y. Zhang
College of Science
Northwest A&F University
Yangling, Shaanxi 712100 (P. R. China)
E-mail: yuanms@nwsuaf.edu.cn
zhangyr@nwsuaf.edu.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201601340.
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Scheme 1. Synthetic routes and molecular structures of ethynyl-extended regioisomers of HBT. (a) 2-aminothiophenol, C2H5OH, H2O2, HCl, RT, 0.5 h; (b) Ac2O,
pyridine, CH2Cl2, RT; (c) KOH, H2O, reflux; (d) salicylaldehyde, C2H5OH, H2O2, HCl, RT, 0.5 h; (e) PdCl2(PPh3)2, CuI, NEt3, Toluene, nitrogen atmosphere, 908C, 12 h;
(f) K2CO3, MeOH/THF, RT.
We have chosen commercially available 2-amino-6-bromo-
Results and Discussion
benzothiazole as the starting material to synthesize 6-substitut-
ed HBT derivatives 9. 2-Amino-6-bromobenzothiazole was hy-
Design and synthesis of ethynyl-extended regioisomers of
drolyzed under strong alkaline condition to provide 5-bromo-
HBT
2-aminothiophenol 4.[32] 6-Bromo-substituted HBT 5 was ob-
Various groups with different Hammett substituent constants
(s) were introduced via Sonogashira cross-coupling reactions
between terminal alkynes and aryl halides.[27–29] The commer-
cially available regioisomers of brominated salicylaldehyde
could make further chemical modification feasible. Synthetic
strategies of the 3’-, 4’- and 6-substituted regioisomers of HBT
are outlined in Scheme 1. Reaction between 3-bromosalicylal-
dehyde or 4-bromosalicylaldehyde and 2-aminothiophenol in
the presence of HCl/H2O2 in methanol yielded 3’- and 4’-
bromo-substituted HBT 1 or 3, which could be readily moni-
tored by thin layer chromatography (strong yellow color).
When the aryl halide has a nucleophilic substituent group
(e.g., OH) in the ortho position, cyclization to the correspond-
ing heterocyclic compound occurs exclusively under Sonoga-
shira reaction conditions.[30,31] Therefore, 1 was protected using
acetic anhydride in a large excess of pyridine to provide 2.
After workup, the crude reaction mixture was directly subject-
ed to Sonogashira cross-coupling conditions (PdII, CuI, NEt3, tol-
uene),[31] with aromatic acetylenes to yield acetate-protected
HBT 6. By removing the acetate protecting group under alka-
line condition (K2CO3, MeOH/THF), the target 3’-substituted
HBT derivatives 7 were obtained. Under the same Sonogashira
cross-coupling conditions, 4’-bromo-substituted HBT derivative
3 reacted directly with aromatic acetylenes to yield 4’-substi-
tuted HBT derivatives 8.
tained by reacting 4 with salicylaldehyde in the presence of
HCl/H2O2, in methanol. Under Sonogashira conditions, 6-substi-
tuted HBT derivatives 9a, 9b and 9c were obtained. These
new compounds were unambiguously characterized by NMR
spectroscopy and HRMS (Supporting Information, Figures S18–
S48). Single-crystal X-ray analysis was also performed for 8a
(Figure 1), which further confirmed the connection of the phe-
nylacetylene fragment at the 4’-position of HBT and clearly re-
vealed the presence of an intramolecular hydrogen bond be-
tween the hydroxyl proton and the nitrogen atom of benzo-
thiazole moiety (1.900 ꢁ). Suitable crystals of other compounds
could not be obtained.
Figure 1. Crystal structure of 4’-substituted HBT derivative 8a.
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The effect of substituent position on the fluorescence spec-
tra of HBT derivatives
Table 1. Steady-state photophysical properties of the ethynyl-extended
regioisomers of HBT in toluene solution.
7a 8a 9a 7b 8b 9b 7c 8c 9c
In order to study the photophysical properties of ethynyl-ex-
tended regioisomers of HBT derivatives bearing the same sub-
stituent group at the 3’-, 4’- or 6-position of HBT, we have
compared the absorption and fluorescence emission spectra of
phenylethynyl-substituted HBT derivatives (7a, 8a and 9a), 4-
methoxyphenylethynyl-substituted HBT derivatives (7b, 8b
and 9b) and 4-methylbenzoate ethenyl-substituted HBT deriv-
atives (7c, 8c and 9c) in toluene. Previous studies have re-
vealed that the existence of a stable intramolecular H-bond be-
tween the phenolic hydroxyl proton and nitrogen atom in the
benzothiazole ring is a prerequisite for the proton transfer
from the O atom to the N atom to occur, in order to complete
the tautomerization from enol to keto form at excited state.[33]
The non-H-bonding non-polar solvents cannot interfere with
the H-bond between the N atom of the benzothiazole ring
and the phenolic hydroxyl proton, the closed cis-enol form
being the most stable conformer for these dyes. This confor-
mer undergoes ESIPT upon excitation to form the keto tauto-
mer and gives rise to keto emission, as shown in Scheme 2.[34]
Figure 2 depicts the steady-state absorption and fluores-
cence emission spectra of the examined HBT derivatives in tol-
uene. The fluorescence excitation spectra of these compounds
(Figure S1, Supporting Information) closely correspond to their
respective absorption spectrum, which indicate that the keto
emission originated from the group state absorbing spe-
cies.[25,35] The photophysical data of these three series of HBT
derivatives including their maximum absorption wavelength
(labs), maximum emission wavelength (lem), molar extinction
coefficient (emax), Stokes shift values (Dl) and fluorescence
quantum yields (Ff), are summarized in Table 1. 3’-Substituted
HBT derivatives (7a, 8a and 9a) have labs values of 358 nm,
374 nm and 371 nm, respectively (Figure 2a), and lem values of
[a]
labs [nm]
358 374 371 364 382 379 361 378 379
551 522 522 557 520 520 549 524 525
1.57 2.85 3.48 1.74 3.78 3.52 2.18 3.41 3.75
193 148 151 193 138 141 188 146 146
[b]
lem [nm]
emax ꢂ104[c] [mÀ1 cmÀ1
Dl[d] [nm]
]
[a] The maximum absorption wavelength; [b] The maximum fluorescence
emission wavelength; [c] The molar absorption coefficient; [d] Strokes
shift.
551 nm, 522 nm and 522 nm, respectively (Figure 2b). In com-
parison with the parent compound HBT (labs =347 nm, lem
=
516 nm) (Figure S2), compounds 7a, 8a and 9a exhibit ab-
sorption bands and emit fluorescence from the excited keto-
tautomer K* at longer wavelengths, which indicates that the
introduction of an aromatic ethynyl moiety indeed enlarges
the p-conjugation and decreases the energy gap between the
ground and excited states of the keto-tautomer, regardless of
the connected positions. In the absorption spectra (Figure 2a),
the magnitude of bathochromic shift is much higher for 4’-
(27 nm) and 6-substituted (24 nm) HBT derivatives 8a and
9athan that of 3’-substituted HBT derivative 7a (11 nm). More-
over, the emax value is also much higher for 4’- (2.85ꢂ
104 mÀ1 cmÀ1) and 6-substituted (3.48ꢂ104 mÀ1 cmÀ1) HBT deriv-
atives 8a and 9a than for 3’-substituted HBT derivative 7a
(1.57ꢂ104 mÀ1 cmÀ1). The difference in labs and emax values
might be associated with the different planarity of the dyes,
since less planar molecular structures usually show a lower
probability for the p–p* transition.[36,37] However, in the fluores-
cence emission spectrum (Figure 2b), the red-shifted magni-
tude for 3’-substituted HBT 7a (35 nm) is much higher than
that of 4’- and 6-substituted HBT derivatives 8a and 9a (5 nm).
Scheme 2. Schematic representation of the photophysical process for the ethynyl-extended regioisomers of HBT.
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Figure 2. The absorption and emission spectra of ethynyl-extended regioisomers of HBT in toluene solution. The top row for phenylethynyl modified HBT de-
rivatives (7a, 8a and 9a), the middle row for 4-methoxyphenylethynyl-substituted HBT derivatives (7b, 8b and 9b), and the bottom row for 4-methylbenzoa-
teethyny-substituted HBT derivatives (7c, 8c and 9c).
Therefore, the Stokes shift with respect to the absorption peak
wavelength for 3’-substituted HBT derivative 7a (193 nm) is
larger than those of 4’- and 6-substituted HBT derivatives 8a
and 9a (148 and 151 nm, respectively), these large Stokes
shifts also supporting the occurrence of the ESIPT process. A
similar trend was also observed for 4-methoxyphenylethynyl-
substituted regioisomers of HBT 7b, 8b and 9b (Figures 2c–
2d), as well as for 4-methylbenzoate ethenyl-substituted re-
gioisomers of HBT 7c, 8c and 9c (Figures 2e–2 f). Therefore, it
could be concluded that the 3’-position of HBT is preferred for
designing ESIPT fluorescent compounds that are capable of
possessing red-shifted emission spectra.
The electronic effect of substituent groups on the fluores-
cence spectra of HBT derivatives
In the previous section, the position effect of the substituent
on the fluorescence spectra of HBT derivatives has been ex-
plored. In this section, we examined the electronic effect of
substituents connected at the different positions of HBT on the
fluorescent spectra. All fluorescent spectra were recorded at
room temperature with excitation at the maximum absorption
wavelength and showed solvent-dependent behavior (see Fig-
ures 3–5). The main emission wavelengths have been summar-
ized in Table 2. The absorption spectra recorded in different
solvents resembled their corresponding excitation spectra (Fig-
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Figure 4. Fluorescence spectra of 4’-substituted HBT derivatives in various
solvents. a) for 8a, b) for 8b, c) for 8c.
Figure 3. Fluorescence spectra of 3’-substituted HBT derivatives in various
solvents. a) for 7a, b) for 7b, c) for 7c.
The electronic effect of substituent groups on the fluores-
cence spectra of the 3’-substituted HBT derivatives
ures S3–S11), which demonstrated that the fluorescence emis-
sion stemmed from the ground state absorbing species.[25]
3’-Substituted HBT derivatives 7a, 7b and 7c show only the
keto emission in non-H-bonding solvents such as PhMe and
Table 2. The main emission wavelengths of ethynyl-extended regioisomers of HBT in various solvents.
7a
7b
7c
8a
8b
8c
9a
9b
9c
Substituent group
H
OMe
CO2Me
H
OMe
CO2Me
H
OMe
CO2Me
s
0
À0.27
13.324
1.40
557
552
558
0.45
13.413
4.27
549
546
553
0
À0.27
12.614
5.67
520
514
521
0.45
12.652
8.67
524
518
529
0
À0.27
12.403
4.43
520
517
404
0.45
12.664
8.20
525
519
527
Chemical shift
Ff [%] in PhMe
lem [nm] in PhMe
lem [nm]in CH2Cl2
lem [nm] in THF
lem [nm] in MeCN
lem [nm] in MeOH
13.362
2.69
551
547
550
544
542
12.646
6.40
522
515
523
520
514
12.414
5.96
522
518
396
483
399
551
484
546
547
519
502
521
517
411
413
521
518
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emission (482 and 542 nm), which stems from anion species
and keto tautomer, respectively; 7c shows dual emission (547
and 387 nm), which originates from keto tautomer and enol
tautomer, respectively. The different fluorescent spectra in
MeOH further imply that the intramolecular H-bond in the HBT
derivative with larger s value is reinforced, which facilitates the
ESIPT process to produce keto emission. Actually, the strength
1
of the intramolecular H-bond can also be observed by H NMR
spectroscopy, with the chemical shift of the phenolic hydrogen
proton gradually moving downfield for the HBT derivatives
with large s (Table 2 and Figures S19, S22, S25)[29,39]
.
The electronic effect of substituent groups on the fluores-
cence spectra of the 4’-substituted HBT derivatives
Similar to 3’-substituted HBT derivatives 7, 4’-substituted HBT
derivatives 8a, 8b and 8c also show only the keto emission in
non-H-bonding solvents such as PhMe and CH2Cl2. However,
the change trend is different from that of the 3’-substituted
HBT derivatives. In comparison with 8a, HBT derivative 8b
bearing an electron-donating OMe group shows the blue-shift-
ed fluorescence with a decrease in Ff, and the HBT derivative
8c with the electron-withdrawing CO2Me group shows the op-
posite trend. For example, in PhMe, the lem of 8a is 522 nm
and its Ff is 6.40%; the lem of 8b with À0.27 s blue-shifts to
520 nm and its Ff decreases to 5.67%; the lem of 8c with 0.45
s red-shifts to 524 nm and its Ff increases to 8.67%. In THF
and MeCN, both keto and enol emissions are detected for all
three 4’-substituted HBT derivatives. In MeOH, unlike the 3’-
substituted HBT derivatives 7, compounds 8a, 8b and 8c
show dual emission, originating from the keto tautomer and
enol tautomer; the ratio is increasing for the 4’-substituted
HBT derivative with larger s, which also indicates that the in-
tramolecular H-bond in the HBT derivative with larger s is rein-
forced. The change in the chemical shift of the phenolic hydro-
gen proton can reflect the relative strength of the intramolecu-
lar H-bond of the 4’-substituted HBT derivatives (Table 2 and
Figures S29, S32, S35), which are similar to the 3’-substituted
HBT derivatives.
Figure 5. Fluorescence spectra of 6-substituted HBT derivatives in various
solvents. a) for 9a, b) for 9a, c) for 9a.
CH2Cl2. In comparison with 7a, 7b bearing the electron-donat-
ing group OMe showed a red-shifted fluorescence with a de-
crease in Ff, while 7c, bearing the electron-withdrawing group
CO2Me showed the opposite. For example, in PhMe, the lem of
7a is 551 nm and its Ff is 2.69%, while the lem of 7b with
À0.27 s red-shifts to 557 nm and its Ff decreases to 1.4%; the
lem of 7c with 0.45 s blue-shifts to 549 nm and its Ff increases
to 4.27%. These results are consistent with our previously re-
ported change trend for 5’-substituted HBT derivatives.[29] In
THF and MeCN, which can act as H-bond acceptors, only the
keto emission was detected for 7c, while both keto and enol
emission were detected for 7a and 7b, which is a typical char-
acteristic of ESIPT dyes. The difference of fluorescence emission
in THF and MeCN indicates that the intramolecular H-bond in
the HBT derivative with smaller s is weaker,[38] therefore, more
likely to be destroyed by solvent molecules.[29] In the polar
protic solvent MeOH, 7b has emission at 484 nm, which could
be assigned to anion species (A*–A, Scheme 2); 7a shows dual
The electronic effect of substituent groups on the fluores-
cence spectra of the 6-substituted HBT derivatives
6-Substituted HBT derivatives 9a, 9b and 9c also show only
the keto emission in PhMe and CH2Cl2. Compared to 9a, HBT
derivative 9b, bearing an electron-donating OMe group,
shows blue-shifted fluorescence with a decrease in Ff, while
HBT derivative 9c, bearing the electron-withdrawing CO2Me
group, shows the opposite trend. Therefore, the electronic
effect of substituent groups on the fluorescence spectra of the
6-substituted HBT derivatives is the same as that of 4’-substi-
tuted HBT derivatives, but is opposite to that of 3’-substituted
HBT derivatives. For example, in PhMe, the lem of 9a is 522 nm
and its Ff is 5.96%; the lem of 9b with À0.27 s blue-shifts to
520 nm and its Ff decreases to 4.43%; the lem of 9c with 0.45
s red-shifts to 525 nm and its Ff increases to 8.2%. In THF and
MeCN, both 9a and 9b show enol emission; 9c shows dual
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emission in both solvents, which could be assigned to the keto
emission and enol emission, respectively. In MeOH, 9a and 9b
emit in the enol form, while 9c emits as both enol and keto
tautomer.
the parent compound HBT. From S0 to S1 state, the bond
lengths of O–H are all elongated (Tables S2–S37), which indi-
cates that there is an increase in the acidity of the phenolic hy-
droxyl proton in the excited state, a prerequisite for ESIPT.[45]
However, the change in magnitude for 3’-substituted HBT de-
rivatives is very significant, ranging between of 0.0423 and
0.0593 ꢁ, whereas the change in magnitude for 4’- and 6-sub-
stituted HBT derivatives is relatively smaller, with values of
0.0069–0.0076 ꢁ and 0.0039–0.0089 ꢁ, respectively. This differ-
ence between ethynyl-extended regioisomers of HBT is also
supported by the change of electronic distribution from S0 to
S1 state, which is shown in Figure 7 and Figures S14–S15. For
Considering the fluorescent spectra of the ethynyl-extended
regioisomers of HBT in various solvents together, a red-shifted
keto emission could be detected when the electron-donating
nature in 3’-position of HBT increased; on the other hand, the
keto emission was blue-shifted when an electron-donating
group at the 4’- or 6-position of HBT is introduced, manifesting
an opposite electronic effect. The experimentally observed
trends are well in line with the theoretical calculations (vide
infra). Regardless of the connected position, increasing the
electron-withdrawing nature of substituents is highly impor-
tant to observe keto emission of the substituted HBT, as it
could also be inferred from previous results.[40] Moreover, the
conjugated substituents with large s value on HBT can im-
prove the Ff, which does not show any relationship with the
connected position.[41,42] Therefore, the tunable ESIPT fluores-
cence could be observed by changing the electronic nature
and position of substituents.
Theoretical calculation on the ethynyl-extended regioisom-
ers of HBT
Theoretical investigations were then conducted for the enol
and ketotautomers of these HBT derivatives to understand the
effects of the electronic nature and position of substituents on
the photophysical properties of the ethynyl-extended re-
gioisomers of HBT in toluene.[43,44] The optimized S0 and S1
state geometries of the enol form of HBT derivatives 7a, 8a
and 9a are shown in Figure 6, while those of the other HBT
Figure 7. The calculated HOMO/LUMO and electronic contribution of enol-
form of phenylethynyl-substituted HBT derivatives (7a, 8a and 9a).
3’-substituted HBT derivatives, in the S0 state, the HOMO is lo-
cated on the phenol ring and the aryl ethynyl moiety, while in
the S1 state, the LUMO is distributed on the phenol ring and
the benzothiazole ring, respectively. This obvious electron mi-
gration from S0 to S1 state provides the prerequisites for ESIPT.
However, for 4’- and 6-substituted HBT derivatives, the elec-
trons are evenly distributed throughout the molecule in the S0
state, but showing a slight migration toward the HBT moiety
in the S1 state. Based on the above-mentioned results, it could
be concluded that the chemical modification on the 3’-position
of HBT is favorable to design novel ESIPT dyes.
Theoretical results of the keto tautomers of HBT derivatives
are shown in Figures 8, S16 and S17. For the 3’-substituted
HBT derivatives, the HOMO and is mainly located on the
phenol ring and the aryl alkynyl moiety, while the LUMO is on
the phenol ring and the benzothiazole ring. Therefore, the
electronic nature of the substituent influences the HOMO. For
the 4’- and 6-substituted HBT derivatives, the HOMO is mainly
located on the phenol ring and the benzothiazole ring, and
not on the aryl alkynyl moiety; however, the electronic distri-
bution extends to the aryl alkynyl moiety for the LUMO. There-
fore, the substituent effect is more significant for the LUMO
than the HOMO. According to previous reports, the s value
could be exploited to interpret the substituent effect on the
emission properties.[41–43] Thus, the calculated energy levels of
the HOMO and LUMO of the keto tautomers of 3’-,4’- and 6-
substituted HBT were plotted as a function of s values, as
shown in Figure 9a. For HBT derivatives, regardless of the sub-
Figure 6. The energy-optimized structures of enol tautomer of phenylethyn-
yl-substituted HBT derivatives 7a, 8a and 9a. In the ball-and-stick represen-
tation, carbon, nitrogen, oxygen and sulfur atoms are colored in gray, blue,
red and yellow, respectively.
derivatives are shown in Figures S12–S13. In both S0 and S1
states, the aryl ethynyl moiety is coplanar with the HBT part,
which indicates that the aryl ethynyl moiety could indeed
extend the p-conjugation of HBT and that these HBT deriva-
tives present red-shifted absorption bands in comparison with
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energy gap between HOMO and LUMO. Again, the opposite
effect was observed when electron-donating groups were
present. Therefore, these theoretical calculations were consis-
tent with the experimentally observed results.
Conclusions
Syntheses of ethynyl-extended regioisomers of HBT at its 3’-,
4’- and 6- positions were described and the effect of the posi-
tion and electronic nature of substituents on their ESIPT fluo-
rescence properties was studied. Introducing electron-donating
and electron-withdrawing groups at the 3’-position of HBT led
to red and blue shifts of the ESIPT fluorescence, respectively.
Introducing such groups at the 4’ or 6-position of HBT resulted
in fluorescence shifts in the opposite direction. Theoretical cal-
culations demonstrated that for 3’-substituted HBT derivatives,
the HOMO–LUMO energy gap became larger when the Ham-
mett constant of substituent increased, since the HOMO was
more affected by the electronic nature of substituted group.
For the 4’- and 6-substituted HBT derivatives, the HOMO–
LUMO energy gap became larger with decreasing Hammett
constants with a more pronounced substituent effect on the
LUMO. These theoretical calculations support the experimen-
tally observed results and our study could provide an experi-
mental and theoretical basis for the chemical modification on
the ESIPT dyes that possess unique fluorescent properties.
Figure 8. The calculated HOMO/LUMO and electronic contribution of keto-
form of phenylethynyl-substituted HBT derivatives (7a, 8a and 9a).
Experimental Section
Materials
2-Amino-6-bromobenzothiazole, 4-iodobenzoic acid methyl ester,
4-methoxyphenylacetylene were purchased from Heowns (Tianjin,
China). PdCl2(PPh3)2, CuI, PPh3, phenylacetylene, salicylaldehyde, 2-
aminothiophenol, 3-bromo-2-hydroxybenzaldehyde and 4-bromo-
2-hydroxybenzaldehyde were purchased from Energy Chemical
(Shanghai, China). All other materials were purchased from local
commercial suppliers and were of analytical reagent grade, unless
otherwise stated. Solvents were purified by using standard proce-
dures. All reactions were monitored by thin-layer chromatography
(TLC).
Instruments
Figure 9. (a) Plot of the calculated energy levels of HOMO and LUMO of HBT
derivatives against s; (b) Plot of the calculated energy gap between HOMO
and LUMO of HBT derivatives against s.
1H NMR and 13C NMR spectra were recorded at room temperature
on a Bruker 500 MHz NMR spectrometer in CDCl3 using the residual
protonated solvents signals as the internal references, while chemi-
cal shifts were reported in ppm and coupling constants (J) in Hz.
High resolution mass spectra (HRMS) were recorded on a Bruker in-
strument using standard conditions (electrospray ionization, ESI).
UV-Vis absorption spectra were recorded on a U-3310 spectropho-
tometer. The fluorescence emission and excitation spectra were
measured in a fluorospectrophotometer model F-7000. The fluores-
cence quantum yield (Ff) was determined using quinine sulfate
stituted positions, the energy levels of both HOMO and LUMO
decreased when the electron-donating nature of the substitu-
ent group decreased. The calculated energy gap between
HOMO and LUMO of HBT derivatives against s value is shown
in Figure 9b. Adding an electron-withdrawing substituent in
the 3’-position of HBT showed a decrease in energy magnitude
for the HOMO, and also for the LUMO, leading to a bigger
energy gap between HOMO and LUMO. The opposite effect
was observed in the case of electron-donating substituents.
When adding an electron-withdrawing substituent at the 4’- or
6-position, the decrease in magnitude of the energy of the
LUMO was larger than that of the HOMO, resulting in a smaller
(Ff =0.55 in 0.1m H2SO4) as a standard[44]
.
Theoretical calculations
All calculations were performed using Gaussian 09 package.[25,29] To
evaluate the solvent effect, toluene was employed as the solvent
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in a self-consistent reaction field (SCRF) calculation using a polar-
ized continuum model (PCM). The ground state (S0) and the first
singlet excited state (S1) geometries of the enol and keto tauto-
mers of ethynyl-extended regioisomers of HBT were optimized
using density functional theory (DFT) and time-dependent density
functional theory (TDDFT) at the B3LYP/6-31+G(d) level, respec-
tively.
7.59–7.56 (m, 1H), 7.50–7.47 (m, 1H), 7.43–7.38 (m, 3H), 7.02 ppm
(t, J=8.0 Hz, 1H). 13C NMR (126 MHz, CDCl3): d=168.94, 158.73,
151.64, 136.07, 132.69, 131.80, 128.40, 128.31, 126.90, 125.84,
123.43, 122.30, 121.58, 119.26, 116.91, 113.13, 94.40, 84.96 ppm.
2-(Benzo[d]thiazol-2-yl)-6-((4-methoxyphenyl)ethynyl)phenol
(7b). Light yellow solid, yield 72% (118.2 mg). HRMS (ESI) m/z
calcd for C22H16NO2S+ [M+H]+ 358.08963, found 358.08960.
1H NMR (500 MHz, CDCl3): d=13.32 (s, 1H), 8.02 (d, J=8.5 Hz, 1H),
7.95 (d, J=8.0 Hz, 1H), 7.71 (dd, J=8.0, 1.5 Hz, 1H), 7.64–7.59 (m,
3H), 7.56 (t, J=8.0 Hz, 1H), 7.47 (t, J=7.5 Hz, 1H), 6.99 (t, J=
8.0 Hz, 1H), 6.96–6.92 (m, 2H), 3.89 ppm (s, 3H). 13C NMR (126 MHz,
CDCl3): d=169.00, 159.72, 158.55, 151.63, 135.91, 133.26, 132.69,
128.08, 126.86, 125.79, 122.26, 121.57, 119.25, 116.84, 115.58,
113.99, 113.44, 94.46, 83.64, 55.33 ppm.
Synthesis and characterizationSynthesis and characteriza-
tion of 2-(benzo[d]thiazol-2-yl)-6-bromophenol (1)
In a 100 mL round-bottomed flask equipped with a magnetic stir
bar, 2-aminothiophenol (4 mmol, 501 mg) and 3-bromo-2-hydroxy
benzaldehyde (4 mmol, 992 mg) were dissolved in ethanol (30 mL).
Aq. 37% HCl (12 mmol, 1 mL) was gradually added to the mixture,
followed by aq. 30% H2O2 (24 mmol, 2.5 mL). Then, the mixture
was stirred at room temperature for 30 min. 20 mL water were
added to quench the reaction. The mixture was extracted with
ethyl acetate three times. The organic layer was washed with brine
and then dried over anhydrous Na2SO4. The solvent was evaporat-
ed in vacuo, and the crude reaction mixture was subjected to
column chromatography (ethyl acetate/petroleum ether=1:4) to
Methyl
4-((3-(benzo[d]thiazol-2-yl)-2-hydroxyphenyl)ethynyl)-
benzoate (7c). Light yellow solid, yield 52% (92.1 mg). HRMS (ESI)
m/z calcd for C23H16NO3S+ [M+H]+ 386.08454, found 386.08456.
1H NMR (500 MHz, CDCl3): d=13.41 (s, 1H), 8.08 (d, J=7.5 Hz, 2H),
8.03 (d, J=8.0 Hz, 1H), 7.96 (d, J=8.0 Hz, 1H), 7.75 (dd, J=8.0,
1.5 Hz, 1H), 7.71 (d, J=8.5 Hz, 2H), 7.65 (dd, J=7.5, 1.5 Hz, 1H),
7.59–7.55 (m, 1H), 7.50–7.46 (m, 1H), 7.02 (t, J=8.0 Hz, 1H),
3.98 ppm (s, 3H). 13C NMR (126 MHz, CDCl3): d=168.80, 166.65,
158.90, 151.56, 136.10, 132.65, 131.66, 129.51, 128.88, 128.17,
126.94, 125.90, 122.30, 121.60, 119.32, 117.00, 112.53, 93.56, 88.10,
52.23 ppm.
1
provide the product as white solid. Yield 28% (342.9 mg). H NMR
(500 MHz, CDCl3): d=13.70 (s, 1H), 8.02 (d, J=8.5 Hz, 1H), 7.96 (d,
J=8.0 Hz, 1H), 7.92 (dd, J=7.5, 1.5 Hz, 1H), 7.75 (dd, J=7.5,
1.0 Hz, 1H), 7.60–7.56 (m, 1H), 7.51–7.47 (m, 1H), 6.80 ppm (t, J=
7.5 Hz, 1H).
Synthesis and characterization of 2-(benzo[d]thiazol-2-yl)-5-
bromophenol (3)
Synthesis and characterization of 3’-substituted HBT deriva-
tives 7
Using 4-bromine-2-hydroxy benzaldehyde as a starting material,
the procedure for 2-(benzo[d]thiazol-2-yl)-6-bromophenol 1 was
adopted to synthesize 2-(benzo[d]thiazol-2-yl)-5-bromophenol 3.
White solid, yield 56% (685.8 mg). 1H NMR (500 MHz, CDCl3): d=
12.76 (s, 1H), 8.03 (d, J=8.0 Hz, 1H), 7.95 (d, J=8.0 Hz, 1H), 7.60–
7.55 (m, 2H), 7.50–7.46 (m, 1H), 7.34 (d, J=2.0 Hz, 1H), 7.14 ppm
(dd, J=8.5, 2.0 Hz, 1H).
2-(Benzo[d]thiazol-2-yl)-6-bromophenol 1 (140 mg, 0.46 mmol) was
dissolved in 0.74 mL of acetic anhydride, after which 0.37 mL of
pyridine (dried over sodium) were slowly added under a nitrogen
atmosphere, and the resulting mixture was stirred at room temper-
ature for 2 h. The mixture was extracted with CH2Cl2 three times.
The organic layer was washed with HCl 0.1m and brine successive-
ly, then dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure to provide the acetyl-protected compound
2 quantitatively. Compound 2 was dissolved in 5 mL of anhydrous
toluene, then PdCl2(PPh3)2 (42 mg, 0.06 mmol) and triethylamine
(0.65 mL) were added successively. The resulting mixture was de-
gassed with a stream of nitrogen for 30 min before aryl alkynyl
(0.45 mmol) and CuI (5.7 mg, 0.03 mmol) were added. The mixture
was stirred at 908C overnight under a nitrogen atmosphere. After
cooling down, the mixture was extracted with CH2Cl2 three times.
The organic layer was washed with brine and then dried over an-
hydrous Na2SO4. The solvent was removed under reduced pressure,
after which the crude reaction mixture was subjected to column
chromatography to give acetyl-protected 3’-substituted HBT deriv-
atives 6. Compound 6 was dissolved in 10 mL of a mixture of
MeOH/THF (50/50 v:v). Potassium carbonate (193 mg, 1.4 mmol)
was added and the resulting mixture was stirred at room tempera-
ture for 1 h. The crude solution was extracted with CH2Cl2 three
times, saturated NaCl aqueous solution and then dried over anhy-
drous Na2SO4. The solvent was evaporated in vacuo, the crude re-
action mixture being subjected to column chromatography to pro-
vide the pure product.
Synthesis and characterization of 4’-substituted HBT deriva-
tives 8
2-(Benzo[d]thiazol-2-yl)-5-bromophenol 3 (0.15 mmol, 55 mg) was
dissolved in 5 mL of anhydrous toluene, then PdCl2(PPh3)2 (42 mg,
0.06 mmol) and triethylamine (0.65 mL) were added successively.
The resulting mixture was degassed with a stream of nitrogen for
30 min before aryl alkynyl (0.45 mmol) and CuI (5.7 mg, 0.03 mmol)
were added. The mixture was stirred overnight at 908C under a ni-
trogen atmosphere. After cooling down, the mixture was extracted
with CH2Cl2 three times. The organic layer was washed with brine
and then dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure then the crude reaction mixture was sub-
jected to column chromatography to afford 4’-substituted HBT de-
rivatives 8.
2-(Benzo[d]thiazol-2-yl)-5-(phenylethynyl)phenol (8a). Yellow
solid, yield 60% (29.4 mg). HRMS (ESI) m/z calcd for C21H14NOS+
[M+H]+ 328.07906, found 328.07907. 1H NMR (500 MHz, CDCl3):
d=12.65 (s, 1H), 8.04 (d, J=8.0 Hz, 1H), 7.96 (d, J=8.0 Hz, 1H),
7.71 (d, J=8.0 Hz, 1H), 7.61 (dd, J=7.5, 2.0 Hz, 2H), 7.57 (t, J=
8.0 Hz, 1H), 7.47 (t, J=8.0 Hz, 1H), 7.44–7.40 (m, 3H), 7.32 (d, J=
0.5 Hz, 1H), 7.16 ppm (dd, J=8.0, 1.0 Hz, 1H). 13C NMR (126 MHz,
CDCl3): d=168.68, 157.71, 151.85, 132.72, 131.82, 128.69, 128.44,
128.31, 127.52, 126.85, 125.73, 122.90, 122.84, 122.28, 121.57,
120.67, 116.77, 91.79, 88.93 ppm.
2-(Benzo[d]thiazol-2-yl)-6-(phenylethynyl)phenol
(7a).
Light
yellow solid, yield 61% (91.8 mg). HRMS (ESI) m/z calcd for
C21H14NOS+ [M+H]+ 328.07906, found 328.07907. 1H NMR
(500 MHz, CDCl3): d=13.36 (s, 1H), 8.04 (d, J=8.0 Hz, 1H), 7.97 (d,
J=8.0 Hz, 1H), 7.75 (dd, J=8.0, 1.5 Hz, 1H), 7.67–7.64 (m, 3H),
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2-(Benzo[d]thiazol-2-yl)-5-((4-methoxyphenyl)ethynyl)phenol
(8b). Light yellow solid, yield 53% (28.4 mg). HRMS (ESI) m/z calcd
for C22H16NO2S+ [M+H]+ 358.08963, found 358.08966. 1H NMR
(500 MHz, CDCl3): d=12.61 (s, 1H), 8.04 (d, J=8.0 Hz, 1H), 7.96 (d,
J=8.0 Hz, 1H), 7.70 (d, J=8.5 Hz, 1H), 7.58–7.53 (m, 3H), 7.49–7.45
(m, 1H), 7.29 (d, J=1.5 Hz, 1H), 7.14 (dd, J=8.0, 1.5 Hz, 1H), 6.96–
6.93 (m, 2H), 3.89 ppm (s, 3H). 13C NMR (126 MHz, CDCl3): d=
168.76, 160.01, 157.71, 151.87, 133.33, 132.70, 128.29, 127.94,
126.83, 125.68, 122.70, 122.26, 121.56, 120.42, 116.49, 114.97,
114.12, 92.01, 87.78, 55.36, 29.72 ppm.
Methyl 4-((2-(2-hydroxyphenyl)benzo[d]thiazol-6-yl)ethynyl)ben-
zoate (9c). Light yellow solid, yield 53% (30.6 mg). HRMS (ESI) m/z
calcd for C23H16NO3S+ [M+H]+ 386.08454, found 386.08456.
1H NMR (500 MHz, CDCl3): d=12.66 (s, 1H), 8.10–8.08 (m, 2H), 8.05
(d, J=8.0 Hz, 1H), 7.97 (d, J=8.0 Hz, 1H), 7.73 (d, J=8.0 Hz, 1H),
7.68–7.65 (m, 2H), 7.60–7.56 (m, 1H), 7.50–7.46 (m, 1H), 7.33 (d,
J=1.5 Hz, 1H), 7.18 (dd, J=8.0, 1.5 Hz, 1H), 3.99 ppm (s, 3H).
13C NMR (126 MHz, CDCl3): d=168.55, 166.52, 157.72, 151.82,
132.73, 131.71, 129.88, 129.59, 128.38, 127.55, 126.91, 126.81,
125.83, 122.90, 122.35, 121.59, 120.85, 117.19, 91.71, 90.80, 52.28,
29.72 ppm.
Methyl
4-((4-(benzo[d]thiazol-2-yl)-3-hydroxyphenyl)ethynyl)-
benzoate (8c). Light yellow solid, yield 58% (33.5 mg). HRMS (ESI)
m/z calcd for C23H16NO3S+ [M+H]+ 386.08454, found 386.08450.
1H NMR (500 MHz, CDCl3): d=12.65 (s, 1H), 8.09 (d, J=8.5 Hz, 2H),
8.05 (d, J=7.0 Hz, 1H), 7.97 (d, J=7.5 Hz, 1H), 7.72 (d, J=8.0 Hz,
1H), 7.66 (d, J=8.5 Hz, 2H), 7.57–7.55 (m, 1H), 7.50–7.46 (m, 1H),
7.33 (d, J=1.5 Hz, 1H), 7.17 (dd, J=8.0, 1.5 Hz, 1H), 3.98 ppm (s,
3H). 13C NMR (126 MHz, CDCl3): d=168.54, 166.52, 157.71, 151.82,
132.73, 132.49, 131.70, 129.87, 129.59, 128.37, 127.55, 126.86,
125.83, 122.89, 122.34, 121.59, 120.85, 117.18, 91.71, 90.80,
52.28 ppm.
Acknowledgements
This study was supported by the National Natural Science
Foundation of China (21405122), the Natural Science Founda-
tion of Shaanxi (2016JM2009) and the Fundamental Research
Funds for the Central Universities (2452015428).
Keywords: density functional calculations
·
electronic
Synthesis and characterization of 2-(6-bromobenzo[d]thia-
zol-2-yl)phenol 5
structure · excited state intramolecular proton transfer ·
regioisomers · solvent effects
2-Amino-5-bromobenzenethiol 4 was synthesized according to
a previously reported method.[29] Light yellow solid, yield 67%
(780.3 mg). 1H NMR (500 MHz, CDCl3): d=7.52 (d, J=2.5 Hz, 1H),
7.22 (dd, J=8.5, 2.5 Hz, 1H), 6.66 (dd, J=8.5, 4.5 Hz, 1H), 4.21 (s,
2H), 3.02 ppm (s, 1H). Using 2-amino-5-bromobenzenethiol 4 and
salicylaldehyde as starting materials, the procedure for 2-(benzo[d]-
thiazol-2-yl)-6-bromophenol 1 was adopted to synthesize 5. White
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2-(6-(Phenylethynyl)benzo[d]thiazol-2-yl)phenol (9a). Yellow
solid, yield 67% (32.8 mg). HRMS (ESI) m/z calcd for C21H14NOS+
[M+H]+ 328.07906, found 328.07904. 1H NMR (500 MHz, CDCl3):
d=12.41 (s, 1H), 8.13 (s, 1H), 8.00 (d, J=8.5 Hz, 1H), 7.75–7.69 (m,
2H), 7.62–7.60 (m, 2H), 7.47–7.39 (m, 4H), 7.16 (d, J=8.5 Hz, 1H),
7.02 ppm (t, J=7.5 Hz, 1H). 13C NMR (126 MHz, CDCl3): d=170.36,
158.02, 151.50, 133.08, 132.84, 131.69, 130.34, 128.57, 128.50,
128.47, 124.62, 123.01, 121.95, 120.78, 119.67, 117.99, 116.69, 90.66,
88.88 ppm.
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2-(6-((4-Methoxyphenyl)ethynyl)benzo[d]thiazol-2-yl)phenol
(9b). Light yellow solid, yield 59% (31.6 mg). HRMS (ESI) m/z calcd
for C22H16NO2S+ [M+H]+ 358.08963, found 358.08957. 1H NMR
(500 MHz, CDCl3): d=12.401 (s, 1H), 8.09 (d, J=1.5 Hz, 1H), 7.98 (d,
J=8.5 Hz, 1H), 7.73 (dd, J=8.0, 1.5 Hz, 1H), 7.68 (dd, J=8.5,
1.5 Hz, 1H), 7.56–7.53 (m, 2H), 7.46–7.42 (m, 1H), 7.16 (dd, J=8.5,
0.5 Hz, 1H), 7.03–7.00 (m, 1H), 6.97–6.93 (m, 2H), 3.99 ppm (s, 3H).
13C NMR (126 MHz, CDCl3): d=170.15, 159.91, 158.00, 151.30,
134.34, 133.16, 133.02, 132.84, 130.23, 128.48, 124.38, 121.93,
121.19, 119.66, 117.97, 116.74, 115.08, 114.15, 90.76, 87.62,
55.36 ppm.
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Manuscript received: September 29, 2016
Final Article published: && &&, 0000
Chem. Asian J. 2016, 00, 0 – 0
www.chemasianj.org
11
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Substituent Effects
Theory and practice hand in hand:
Three series of ethynyl-extended re-
gioisomers of 2-(2’-hydroxyphenyl)ben-
zothiazole (HBT), at the 3’-, 4’- and 6-po-
sition, respectively, have been synthe-
sized. Changes in the absorption and
emission spectra were correlated to the
position and electronic nature of the
substituent groups. Compared with
others, 3’-substituted HBT showed red-
shifted emission with smaller Ff with
large s. The theoretical calculations
were in accord with the experimentally
observed trends.
Qin Wang, Longfei Xu, Yahui Niu,
Yuxiu Wang, Mao-Sen Yuan,*
Yanrong Zhang*
&& – &&
Excited State Intramolecular
#ProtonTransfer in Regioisomers of
HBT: Effects of the Position and
Electronic Nature of Substituents
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&
Chem. Asian J. 2016, 00, 0 – 0
www.chemasianj.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!