Excited-state intramolecular single and double proton transfer emission of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

Article abstract of DOI:10.1016/j.dyepig.2016.03.002

In this work, excited-state intramolecular single proton transfer emission (S-PTE) and double proton transfer emission (D-PTE) are presented by employing 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (1) as a prototype. 1 has been strategically designed and synthesized in an aim to explore single proton transfer and double proton transfer in the excited state. In solution 1 exhibits the lowest lying S₀ → S₁ absorption at ～395 nm. Upon excitation, two large Stokes shifted emission bands maximized at 475 nm and 550 nm are resolved, which are ascribed to the tautomer emission resulting from single proton transfer and double proton transfer isomers, respectively. It is found that solvents have greatly influenced on excited-state intramolecular proton transfer emission.

Full text of DOI:10.1016/j.dyepig.2016.03.002

Dyes and Pigments 129 (2016) 186e190
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Excited-state intramolecular single and double proton transfer
emission of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol
Yongchao Hao, Yi Chen*
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences,
Beijing, 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, excited-state intramolecular single proton transfer emission (S-PTE) and double proton
transfer emission (D-PTE) are presented by employing 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (1) as
a prototype. 1 has been strategically designed and synthesized in an aim to explore single proton transfer
and double proton transfer in the excited state. In solution 1 exhibits the lowest lying S0 / S1 absorption
at ~395 nm. Upon excitation, two large Stokes shifted emission bands maximized at 475 nm and 550 nm
are resolved, which are ascribed to the tautomer emission resulting from single proton transfer and
double proton transfer isomers, respectively. It is found that solvents have greatly influenced on excited-
state intramolecular proton transfer emission.
Received 19 January 2016
Received in revised form
29 February 2016
Accepted 2 March 2016
Available online 3 March 2016
Keywords:
Excited state intramolecular proton transfer
emission
© 2016 Elsevier Ltd. All rights reserved.
Single proton transfer
Double proton transfer
2,5-Bis(benzoxazol-2-yl)thiophene-3,4-diol
Synthesis
1. Introduction
protons is, however, of paramount interest, and attracts more
attention due to its intrinsic property in mimicking proton relay in
Proton transfer is one of the most important chemical reactions
in chemistry and biology [1e3]. Excited state intramolecular proton
transfer (ESIPT) [4] is one class of the proton transfer reactions that
can be initiated by a light pulse [5,6]. Significant interest has existed
in the fundamental investigation [7e10] and the application
[11e15] of organic molecules exhibiting ESIPT because of their four-
level photophysical scheme, spectral sensitivity to the surrounding
medium and a large Stoke's shifted fluorescence. It has been
vital bio-systems. For instance, prototypes include ESIPT of green
fluorescence protein, photo-induced mutation of DNA AꢀT and
GꢀC base pairs with double and triple hydrogen bonds [20e22].
Unlike the ubiquitous single-proton transfer fluorescence, however,
double or multiple proton-transfer fluorescence is rarely reported
[23e26]. In this paper, we employ 2,5-bis(benzoxazol-2-yl)thio-
phene-3,4-diol (1) (Scheme 2) as a model compound. 1 contains
two enol units, and is expected to be able to go intramolecular
single and double proton transfer emission upon excitation. To
better identify single and double proton transfer emission of 1, two
analogues (2) and (3) (Scheme 2) which represent none proton
transfer emission and single proton transfer emission, respectively
are synthesized for control experiments, and their fluorescence
properties are also performed.
demonstrated that the ESIPT process involves
a
photo-
tautomerization reaction, the enol isomer converts to keto isomer
via an intramolecular hydrogen bond involving the transfer of hy-
droxyl proton to the nitrogen atom in excited state (Scheme 1)
[16,17]. The excited keto isomer relaxes radiatively to give a large
Stokes shifted fluorescence or nonradiatively to the ground-state
keto isomer, which then undergoes thermal re-enolization to give
the original enol isomer. The transformation from enol isomer to
keto isomer occurs extremely fast in the subpicosecond or the pi-
coseconds time scale [18,19].
2. Experimental
2.1. General
Most ESIPT reactions refer to the single proton transfer. Excited-
state proton transfer incorporating the transfer of two or more
1H and 13C NMR spectra are recorded at 400 and 100 MHz,
respectively, with TMS as an internal reference. MS spectra are
recorded with TOF-MS spectrometer, respectively. All the fluores-
* Corresponding author.
E-mail address: yichen@mail.ipc.ac.cn (Y. Chen).
cence spectral measurements were done at
1
ꢁ
10ꢀ5
M
http://dx.doi.org/10.1016/j.dyepig.2016.03.002
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

Y. Hao, Y. Chen / Dyes and Pigments 129 (2016) 186e190
keto isomer
O
187
enol isomer
H
O
*
*
H
N
N
ESIPT
O
O
H
O
O
H
N
O
N
O
HBO
Scheme 1. Schematic of the ESIPT of HBO.
Fig. 1. Absorption and fluorescence spectra of 1 (black) together with fluorescence
spectra of 2 (blue) and 3 (red) in solution (CHCl3, 10
pretation of the references to colour in this figure legend, the reader is referred to the
m
M), lex ¼ 394 nm. (For inter-
web version of this article.)
resolved intensity decay by the method of time-correlated single-
photon counting (TCSPC) technique.
2.2. Synthesis of 1, 2 and 3
The synthetic routes for compounds 1e3 are outlined in Scheme
3, and the detailed procedures are presented as follows: (a) To a
solution of 2-aminophenol (16.35 g, 150 mmol) and chloroacetyl
chloride (16.95 g, 150 mmol) in dry chlorobenzene (200 mL) is
added pyridine (0.5 ml). The solution is stirred at ambient tem-
perature for 2 h. To above solution is added p-toluenesulfonic acid
(2.58 g, 15 mmol), the solution is refluxed till no water is dis-
charged. After cooling to room temperature, the solution is washed
with water (100 mL ꢁ 3) and a saturated solution of NaCl (50 mL),
respectively. The organic solution is collected and dried over
Na2SO4, after evaporation of the solvent, the crude 2-(chlor-
omethyl)benzoxazole (yellow oil, 23.1 g, 93% yield) is obtained for
next step without purification. (b) To a solution of 2-(chloromethyl)
Scheme 2. Chemical structures of target 1 and analogues 2 and 3.
concentrations of solute to avoid aggregation and self-quenching.
The steady state absorption spectra are recorded on Hitachi U-
3010 spectrophotometer. All the emission spectra are recorded on a
F-2500 fluorescence spectrophotometer equipped with a 10 mm
quartz cell. Fluorescence lifetimes are measured from time-
Scheme 3. Synthesis of target compound 1 and analogues 2 and 3. Reagents and conditions: (a) pyridine cat., dry cholorbenzene, rt, 2 h; p-toluenesulfonic acid cat., reflux, 6 h, 93%;
(b) tetrabutylammonium bromide cat., DCM/H2O (v/v ¼ 3/2), rt, 4 h, 82%; (c) Na, dry MeOH, rt, 1 h; dry DMF, reflux, 6 h, 70%; (d) K2CO3 base, KI cat., DMF, 100 ꢂC, 5 h, 70% for 2 and
10% for 3.

188
Y. Hao, Y. Chen / Dyes and Pigments 129 (2016) 186e190
3
10
2
10
1
10
0
10
0
5
10
15
Time/ns
20
25
3.3
0.0
-3.3
Fig. 4. The fluorescence decay of 1 at 550 nm emission in CHCl3.
Fig. 2. Excitation spectra of 1 in solution (CHCl3, 10 mM) at different wavelength.
solution is refluxed for 6 h until no starting material is detected
(TLC detection). After cooled to room temperature, the solution is
acidified with HCl (12 N, 50 ml) till pH ¼ 3, the solid is filtered off
and washed successively with EtOH (30 ml), saturation NaHCO3
(30 ml) and H2O (30 ml). After dried in vacuo, the target compound
1 is obtained (yellow solid, 2.45 g). 1. Yield: 70%. 1H NMR (DMSO-
Table 1
Optical date of 1, 2 and 3 in CHCl3 solution.
Compd
labs (nm)
lem (nm)
Dn(cmꢀ1
)
d6):
d
(ppm) 7.45e7.40 (m, 4H), 7.82e7.75 (m, 4H), 5.01 (s, 2H). 13
¼ 157.7, 151.6, 149.2, 142.7, 127.3,
C
1
2
3
354, 372, 394
356, 370, 392
358, 374, 395
412, 430, 475, 550
420, 438
415, 436, 485
1108, 2168, 4284, 7200
1700, 2678
1220, 2380, 4592
NMR (100 MHz, DMSO-d6):
d
126.5, 120.8, 116.5, 111.8. MS (EI) C18H10N2O4S. calcd. 350.0361,
found: 350.0368. Anal. Calcd for C18H10N2O4S: C, 61.71; H, 2.88.
Found: C, 61.68; H, 2.91. (d) To a solution of 1 (1.05 g, 3 mmol) in
DMF (10 ml) is added bromoethane (0.98 g, 9 mmol), K2CO3 (1.24 g,
9 mmol) and KI (0.15 g, 0.9 mmol). The mixture solution is heated at
100 ꢂC for 5 h, after cooled to room temperature, the solution is
acidified with HCl (12 N, 50 ml) till pH ¼ 3. The product is extracted
with DCM, the combined organic phase is dried with anhydrous
Na2SO4. After evaporation of the solvent, the crude product is pu-
rified by flash column chromatography with petroleum/ethyl ace-
tate (1:2) as eluent to afford compound 2 and 3.
benzoxazole (23.1 g, 139.5 mmol) and tetrabutylammonium bro-
mide (4.49 g, 13.95 mmol) in dichloromethane (150 ml) is added
Na2S$9H2O (40.2 g, 167.4 mmol) in H2O (100 ml), the mixture so-
lution is stirred at ambient temperature for 4 h until the starting
material disappeared (TLC detection). The mixture solution is
washed with water (100 ml ꢁ 3), the washed organic solution is
then dried over Na2SO4, after evaporation of the solvent, the crude
bis(benzoxazol-2-ylmethyl) sulfane (brown oil, 33.8 g, 82% yield) is
obtained for next step reaction without purification. (c) Sodium
(1.15 g, 50 mmol) is added to anhydrous MeOH, the solution is
stirred for 1 h at ambient temperature. To above solution is added
dropwise bis(benzoxazol-2-ylmethyl)sulfane (2.98 g, 10 mmol) and
diethyl oxalate (1.46 g, 10 mmol) in dry DMF (20 ml). The mixture
2. Yield: 70%. 1H NMR (CDCl3):
d (ppm) 7.80e7.77 (m, 2H),
7.60e7.58 (m, 2H), 7.38e7.36 (m, 4H), 4.46e4.41 (q, 4H), 1.52 (t,
J1 ¼ 6.8 Hz, J2 ¼ 7.2 Hz, 6H). 13C NMR (100 MHz, DMSO-d6):
d
¼ 159.8, 153.4, 152.6, 149.7, 127.3, 124.5, 120.8, 116.5, 112.8, 65.6,
14.9. HRMS (EI) C22H18N2O4S, calcd. 406.0987 found: 406.0973.
Anal. Calcd for C22H18N2O4S: C, 65.01; H, 4.46. Found: C, 65.07; H,
4.42.
3. Yield: 10%. 1H NMR (DMSO-d6):
d (ppm) 7.82e7.76 (m, 4H),
7.45e7.40 (m, 4H), 5.31 (s, 1H), 4.37e4.32 (q, 2H), 1.43 (t, J1 ¼7.2 Hz,
3
J2 ¼ 6.8 Hz, 3H). 13C NMR (100 MHz, DMSO-d6):
d
¼ 159.8, 157.4,
10
153.2, 152.3, 149.5, 127.3, 124.4, 120.5, 116.3, 112.4, 65.9, 14.9. HRMS
(EI) C20H14N2O4S, calcd. 378.0674 found: 378.0693. Anal. Calcd for
2
C20H14N2O4S: C, 63.48; H, 3.73. Found: C, 63.51; H, 3.69.
10
3. Results and discussion
1
10
3.1. Synthesis
0
10
Target compound 1 and analogues 2 and 3 are prepared from
0
2
4
6
8
10 12 14 16 18 20
Time/ns
Table 2
3.3
0.0
Fluorescent lifetime of 1 and 2 in DCM solution.
Compd
lem (nm)
tf (ns)
c2
-3.3
1
1
2
475
550
438
0.67, 2.40
2.44, 3.63
0.68
0.971
0.969
1.237
Fig. 3. The fluorescence decay of 1 at 474 nm emission in CHCl3.

Y. Hao, Y. Chen / Dyes and Pigments 129 (2016) 186e190
189
Scheme 4. Schematic of the single and double proton transfer of 1.
commercially available starting material 2-aminophenol following
the depiction in Scheme 3. Bis(benzoxazol-2-ylmethyl) sulfane is
produced by the condensation of 2-aminophenol with 2-
chloroacetyl chloride in chlorobenzene (93% yield), followed by
the reaction with Na2S in H2O-DCM (v/v ¼ 1:1) using tetrabuty-
lammonium bromide (TBAB) as catalyst (80% yield). Condensation
of bis(benzoxazol-2-ylmethyl) sulfane with diethyl oxalate in EtOH
using sodium methanolate (NaOCH3) as base affords 2,5-
bis(benzoxazol-2-yl)thiophene-3,4-diol (1) in 70% yield. Treat-
ment of 1 with bromoethane in the ratio of 1:2 equiv. in EtOH gives
2 (70% yield) and 3 (10% yield), respectively.
excitation of 1. Two small Stokes shifted emission maximized at
412 nm (Dn ¼ 1108 cmꢀ1) and 432 nm (Dn ¼ 2168 cmꢀ1), and two
large Stokes shifted emission maximized at 475 nm
(
Dn ¼ 4284 cmꢀ1), and 550 nm (Dn ¼ 7200 cmꢀ1), respectively, are
resolved versus the lowest lying absorption (394 nm). The iden-
tical excitation and absorption spectra (Fig. 2) conclude the same
ground-state origin for the emission. Both small and large differ-
ence in Stokes shift indicates that not only occurred normal
electron transition from S0 / S1 but also proton transfer in the
excited state.
3.2. Absorption spectra of 1.
3.4. Control experiments
Fig. 1 depicts the steady-state absorption and emission spectra
of 1 in chloroform (CHCl3) at room temperature. Solution absor-
bance study of 1 is performed to determine the appropriate exci-
tation wavelength for fluorescence studies. According to Fig. 1, 1 is
characterized by virtually identical features, the absorption spectra
exhibits three well-defined vibronic peaks at 354 nm
(ε ¼ 3.45 ꢁ 104 cm L Mꢀ1), 372 nm (ε ¼ 4.84 ꢁ 104 cm L Mꢀ1), and
394 nm (ε ¼ 3.95 ꢁ 104 cm L Mꢀ1), respectively, which are assigned
To identify the correspondence of emission peaks and to eluci-
date the proton transfer in 1, control experiments have been carried
out. First, we probe this issue via chemical modification. Com-
pounds 2 (used for none proton transfer emission) and 3 (used for
single proton transfer) as prototypes are employed. Both 2 and 3
exhibit similar absorption profile and absorption bands (Table 1)
with that of 1. However, fluorescence spectra show distinct differ-
ence among them. As shown in Fig. 1, 2 exhibits a normal Stokes
shifted emission (lem ¼ 438 nm, Dn ¼ 2678 cmꢀ1) versus the lowest
lying absorption (394 nm), and no large Stokes shifted emission is
observed. 3 exhibits, however, both normal Stokes shifted emission
to both the
3.3. Fluorescence of 1
As shown in Fig. 1, four emission peaks are observed upon
p / p* and n / p* type of electronic transition.
(
lem ¼ 436 nm, Dn ¼ 2380 cmꢀ1) and a large Stokes shifted emis-
sion (lem ¼ 485 nm, Dn ¼ 4592 cmꢀ1) versus the lowest lying
Fig. 5. Fluorescence of 1 in non-polar solution (10
mM), lex ¼ 394 nm.
Fig. 6. Fluorescence of 1 in polar solution (10
mM), lex ¼ 394 nm.

190
Y. Hao, Y. Chen / Dyes and Pigments 129 (2016) 186e190
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absorption (394 nm) (Fig. 1). The large Stokes shifted emission
suggests the occurrence of proton-transfer tautomer in 3.
Second, fluorescence lifetime of 1 is also performed. It is found
that the decay curves of both 474 nm and 550 nm are well fitted by
biexponential decay pattern (Fig. 3 and Fig. 4). The decay at 474 nm
exhibits two lifetimes t1 ¼ 0.67 ns and t2 ¼ 2.40 ns (Table 2), which
indicates that there are two decay components. As compared to the
fluorescence lifetime of 2 (
t
¼ 0.69 ns), it can be assumed that the
short lifetime of 1 (t1 ¼ 0.67 ns) corresponds to normal Stokes
shifted emission, and the long lifetime (t2 ¼ 2.40 ns) attributes to
proton transfer emission. Measurement of fluorescence decay at
550 nm exhibits two long lifetimes t1 ¼ 2.44 ns and t2 ¼ 3.63 ns,
the similar lifetime (t1 ¼ 2.44 ns) makes it reasonable to ascribe the
t1 ¼ 2.44 ns to single proton transfer emission, and t2 ¼ 3.63 ns to
double proton transfer emission.
The above spectroscopic results led to the suggestion that
excited-state intramolecular proton transfer takes place in 1,
resulting in two types of proton transfer tautomers (Scheme 4)
corresponding to emission maximized at 475 and 550 nm. The
molecular structure of each isomer can be inferred from the control
compound. Single and double proton transfer may proceed via
synchronous or asynchronous types of double proton-transfer or
even more complicated multiple pathways. Unfortunately, because
of the lack of ultrafast time-resolved spectrometer, the mechanism
of double proton transfer which undergoes one-step or stepwise
process is unclear.
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3.5. The effect of solvents on S-PT emission and D-PT emission
It is well known that excited state proton transfer emission is of
great sensitivity to the surrounding medium [27e29]. The fluo-
rescence of 1 in different solvents is conducted. It is found that 1
exhibits both S-PT emission and D-PT emission in either non-polar
solvents such as THF and toluene (Fig. 5) or polar solvents such as
DCM and MeCN (Fig. 6). Comparing the spectra in Fig. 5 with that in
Fig. 6 finds that the D-PT emission in Fig. 5 is not as significance as
that in Fig. 6, which suggests that the D-PT emission is influenced
by solvents. In addition, it is also found that no or very weak
emission is detected when 1 is in protic solvents such as MeOH, the
quenched emission of 1 in MeOH probably results from the inter-
molecular H-bond between 1 and MeOH.
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4. Conclusions
In summary, a symmetric molecule 2,5-bis(benzoxazol-2-yl)
thiophene-3,4-diol which contains two enol units to explore single
proton transfer and double proton transfer in the excited-state has
been strategically designed and synthesized. It has demonstrated
that 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol exhibits excited-
state intramolecular single and double proton transfer emission.
It has also demonstrated that solvents play a role in excitied-stated
intramolecular proton transfer emission.
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hydrogen bond as a switch for inducing ground and excited state intra-
molecular double proton transfer in doxorubicin: an excitation wavelength
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This work was supported by the National Natural Science
Foundation of China (No 21572241 and No 21302197).
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