Synthesis of fused 2-(2′-hydroxyphenyl)benzoxazole derivatives: the impact of meta-/para-substitution on fluorescence and zinc binding

Article abstract of DOI:10.1016/j.tetlet.2016.06.098

A fused bis[2-(2′-hydroxyphenyl)benzoxazole] bis(HBO) 3 was synthesized via reaction of 4,6-dihydroxyisopthalic acid with 2-amnophenol in the presence polyphosphoric acid (PPA). In bis(HBO) 3, two benzoxazol-2-yl groups are connected via meta-phenylene, i

Full text of DOI:10.1016/j.tetlet.2016.06.098

Tetrahedron Letters xxx (2016) xxx–xxx
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Synthesis of fused 2-(2 -hydroxyphenyl)benzoxazole derivatives: the
impact of meta-/para-substitution on fluorescence and zinc binding
a
a,b,
⇑
Chathura S. Abeywickrama , Yi Pang
a
Department of Chemistry, University of Akron, Akron, OH 44325, USA
Maurice Morton Institute of Polymer Science, University of Akron, Akron, OH 44325, USA
b
a r t i c l e i n f o
a b s t r a c t
0
Article history:
A fused bis[2-(2 -hydroxyphenyl)benzoxazole] bis(HBO) 3 was synthesized via reaction of 4,6-dihydrox-
Received 6 June 2016
Revised 15 June 2016
Accepted 22 June 2016
Available online xxxx
yisopthalic acid with 2-amnophenol in the presence polyphosphoric acid (PPA). In bis(HBO) 3, two ben-
zoxazol-2-yl groups are connected via meta-phenylene, in comparison with para-phenylene in its isomer
5
, in order to examine the effect of regiochemistry on optical properties. UV–vis and fluorescence spectra
show that bis(HBO) 3 gave emission nearly exclusively from its keto tautomer, indicating more effective
excited-state intramolecular proton transfer (ESIPT)’ than mono HBO. Bis(HBO) 3 gave blue ESIPT
‘
Keywords:
Fluorescence
Excited-state intramolecular proton transfer
Zinc binding
Sensor
fluorescence (kem ꢀ 480 nm), while 5 gave orange-red emission at 597 nm, showing a large effect of
2
+
regiochemistry on emission. The effect was further evaluated through binding with Zn cation, with 3
showing spectral blue shift upon zinc binding while 5 did not. In addition, 3 revealed ability to detect dif-
ferent stages of zinc binding, giving different spectral response to (3)
ratio) and 3–Zn species (with 1:1 ligand-to-metal ratio).
2
–Zn (with 2:1 ligand-to-metal
Ó 2016 Elsevier Ltd. All rights reserved.
Introduction
It should be noted that the metal complex of a simple mono
6
HBO, such as 1–Zn (kmax ꢀ 376 nm, kem ꢀ 443 nm), can no longer
Fluorescent chemical sensors play increasingly important roles
gives ESIPT, as metal binding event removes the phenolic proton
1
in biological studies, including detection of metal ions and biolog-
(necessary for ESIPT). Recently, bis(HBO) 2 has shown to be capable
2
2+
ical tissues. In principle, a fluorescence chemical sensor is a
of binding Zn cation to form 2–Zn while still being able to give
1
2,13
molecular device that transforms a chemical binding event into a
discernible fluorescence signal. To avoid the self-absorption prob-
lem, the emission of the probes should be away from the absorp-
ESIPT emission.
Different from 1, bis(HBO) 2 has two hydroxy
groups that are associated with two different HBO fragments. Upon
binding to metal cation, only one HBO is used while the other is
preserved for ESIPT. Such metal ion binding can generate a large
fluorescence turn-on and induce a new emission band at the near
0
tion or excitation wavelengths. 2-(2 -Hydroxyphenyl)benzoxazole
(
HBO) 1 and its derivatives have emerged to be a very useful sys-
tem for chemical sensor applications, because the probes can exhi-
bit large Stokes shift as consequence of excited-state
1
3
infrared wavelength (e.g., ꢁ710 nm). The zinc complex can exist
a
a
in a dimer, i.e., (2–Zn) , which is found to give intense red emission
2
3,4
14
intramolecular proton transfer (ESIPT). Upon excitation, HBO 1
can produce two distinguishable emissions from both enol 1a
and keto 1b tautomers (Scheme 1).5 By utilizing the ESIPT event
as a switching mechanism, different HBO derivatives have been
developed for various sensor applications, including metal
(kem ꢁ 630 nm) upon interaction with an alcohol. In addition to
its response to metal cation, 2 is also capable of sensing anions
ꢂ
ꢂ
ꢂ
15
(OH , F and OAc ), which increases emission by up to 20 folds.
The findings from 2 show that bis(HBO) is an attractive molecular
frame for chemosensor design, as two HBO units can be used for
different purpose (e.g., one for metal ion binding and the other
for ESIPT). In this manuscript, we report the synthesis of bis
(HBO) 3, in which the two hydroxyl groups are placed in
meta-position, in contrast to 2 where the two hydroxyl groups
are located in para-position. Through comparison between the iso-
meric pair 3 and 5, the study provides the first example to evaluate
the effect of regiochemistry on the ESIPT properties of bis(HBO).
6
,7
8–10
pH sensor, temperature.9 Rational design
11
cations, anions,
of ESIPT sensors is typically achieved via effective control of the
emission ratio from the excited enol/keto tautomers. A successful
design is dependent on our fundamental knowledge about various
factors that control the ESIPT emission.
⇑
Corresponding author.
E-mail address: yp5@uakron.edu (Y. Pang).
http://dx.doi.org/10.1016/j.tetlet.2016.06.098
040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
0
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2
C. S. Abeywickrama, Y. Pang / Tetrahedron Letters xxx (2016) xxx–xxx
O
N
* ESIPT
O
*
HO
O
OH
O
O
N
HO
OH
N
H
1b
KHCO
3
HO
OH
H
O
O
O
L
L
L
210°C
72%
Zn++
1-Zn
1
a
HBO enol tautomer
max~330 nm; λem~375 nm
keto tautomer
6
7
λ
λ
~490 nm
em
R
NH
OH
2
HBO
PPA
HBO
HO
O
OH
O
2
20-250°C
O
H
O
H
O
N
N
O
O
N
N
O
Zn2+
N
N
26%
8
a: R=H; b: R= tert-Bu
O
O
H
L
L
Zn
2-Zn
λmax = ~480 nm
λem = ~535, 712 nm;
3
HBO
2
λmax = ~410 nm;
λem = ~533 nm; f
Scheme 3. Synthesis of bis(HBO) 3.
Scheme 1. Structures of mono HBO 1 and bis(HBO) 2, and their zinc complexes,
where L represents a hydrogen atom or a ligand group.
t-Bu
t-Bu
N
O
H+
H+
N
O
Ar
H
Ar
-
Results and discussion
10
9
0
H
4
,6-Bis(benzoxazol-2 yl)benzene-1,3-diol, i.e., bis(HBO) 3, was
N
O
+
-
4 9
C H
synthesized via a two-step reaction sequence using resorcinol 6
as the starting material. The desired intermediate product,
Ar
1
1
4
,6-dihydroxyisopthalic acid 7, was obtained conveniently by di-
carboxylation of neat 6 with anhydrous KHCO at elevated temper-
atures in 72% isolation yield (Scheme 3). The reaction proceeded
3
Scheme 4. Possible mechanism of eliminating tert-butyl from benzoxazole
derivatives.
1
6
via direct carboxylation of readily available resorcinol 6 in solid
state. The desired product bis(HBO) 3 was synthesized in 26% iso-
lation yield via condensation of 7 with 2-aminophenol 8 in the
presence of polyphosphoric acid (PPA), which is often the reagent
attributing to keto emission on the basis of large Stokes shift. The
assumption was supported by lack of spectral overlap between
absorption and fluorescence in the 370–430 nm region. In DMF,
an additional emission peak was observed at ꢁ439 nm, which
could be attributed to deprotonated phenolic anion 12. The emis-
sion peak at 439 nm was likely from the enol form of the mono
anion 12a, on the basis of small Stokes shift as excess negative
charge on the phenol ring is known to interrupt and disable the
ESIPT (observed from the 1,4-dihydroxy compound 5). Fluores-
cence quantum yield of 3 was calculated to be /
DCM, which was higher than its isomeric bis(HBO) 5b
= 0.021) and mono HBO 1 (/
The impact of regiochemistry on bis(HBO) was further evalu-
ated by comparison of the absorption and emission spectra of 3,
its isomer 5a and mono HBO 1 in DMF (Fig. 2). The absorption of
(kmax = 353 nm) was at a slightly longer wavelength than 1
1
7,18
of choice for synthesis of benzoxazole from carboxylic acids.
In order to examine the general utility of the synthetic
approach, 4,6-dihydroxyisophthalic acid was also reacted with
2
-amino-4-(tert-butyl)phenol (8b) under the same condition.
Repeated attempts, however, only gave a trace amount of 4.
Unexpectedly, 3 was identified as the major product (not the antic-
ipated 4), on the basis of 1H NMR and mass spectral data. It was
likely that the formed benzoxazole fragment 9 could be protonated
to give allylic cation 10, which might be further stabilized by the
electron-rich oxazole ring. tert-Butyl cation was then eliminated
from 10 to give 11 (Scheme 4).
15
fl
= 0.078 in
15
19
(/
fl
fl
= 0.00125) (Scheme 5).
Spectroscopic properties. UV–vis absorption of 3 showed two
absorption bands at about 338 nm and 353 nm in common organic
solvents such as hexane and THF (Fig. 1a). Interestingly, an addi-
tional absorption band was observed in DMF ꢁ 388 nm, suggesting
3
(k
max = 331 nm), partially attributing to the second –OH group on
HBO (see Scheme 2). The absorption of the isomer 5a
max = 410 nm), however, occurred at a much longer wavelength,
ꢂ
that deprotonation of a phenolic group (Ph-OH ? Ph-O ) might
occur partially due to strong solvent–solute interaction.
Fluorescence of 3 revealed an emission band at about 480 nm in
common organic solvents except DMF (Fig. 1b), which was
(k
as the regiochemistry permits the through conjugation between
the two benzoxazole groups on the para-phenylene. In addition,
the phenolic proton in 3 appeared to be more acidic than that in
both 1 and 5a, as the deprotonated species was only detected in
the 3 (absorption kmax ꢁ 388 nm). A possible reason is that the
shared 1,3-dihydroxy-
meta-phenylene
2
nd hydroxyl group in 3 exhibits mainly inductive effect (electron
withdrawing) on the first hydroxy group, in contrast to the isomer
O
H
N
5a where the 2nd hydroxyl exhibits mainly resonance effect
(i.e., electron donating).
H
O
HO
O
OH
B
O
N
R
20
B
O
H
It should be noted that the absorption (kmax ꢀ 353 nm) and keto
N
N
O
R
O
emission (kem ꢀ 480 nm) of 3 was similar to HBO (kmax ꢀ 331 nm;
H
O
5
R
R
k
em ꢀ 490 nm), but quite different from the 1,4-dihydroxy bis
λmax = ~410 nm;
= ~430 (enol), ~600 nm (keto)
a: R=H; b: R= t-Bu
15
(
HBO) 5 (kmax ꢀ 410 nm; kem ꢀ 600 nm) (Fig. 2). Since the effec-
3
(R=H)
(R= tert-Bu)
λem
4
tive chromophore in 3 resembled that of mono HBO, the structure
of 3 could be viewed as two mono HBO units fused together by
sharing a common 1,3-dihydroxyphenyl ring (shown in Scheme 2).
The two benzoxazole fragments are connected via a 1,3-phenylene
Scheme 2. Comparison between 1,3-dihydroxy bis(HBO) 3–4 and 1,4-dihydroxy
bis(HBO) 5.
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C. S. Abeywickrama, Y. Pang / Tetrahedron Letters xxx (2016) xxx–xxx
3
Figure 1. UV–vis (a) and fluorescence spectra (excited at 355 nm) (b) of 3 in different solvents at room temperature.
O
O
O
O
N
N
H
N
N
H
O
O
O
O
1
2a (enol form)
12b
(keto form)
(
λ
~439 nm)
(λ ~480 nm )
em
em
Scheme 5. The enol and keto forms of mono anion 12, where the electron flow from the oxygen anion (blue arrow) interrupts the electron flow in ESIPT (red arrows).
0
0
0
0
0
0
0
0
0
.40
.35
.30
.25
.20
.15
.10
.05
.00
(a)
1.2
(b)
70
1
5
3
480
602
3
331
1
5
3
a
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
a
3
53
410
keto
4
39
keto
enol
4
90
3
88
451
3
00
350
400
450
350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Wavelength (nm)
Figure 2. UV–vis (a) and normalized fluorescence spectra (b) of 1 (excited at 331 nm), 5a (excited at 405 nm) and 3 (excited at 355 nm) in DMF at room temperature.
(
or meta-phenylene) bridge, which is known to effectively inter-
was not formed in other solvents (Fig. 1). The results clearly indi-
cated that bis(HBO) 3 had a higher tendency to give ESIPT, and
the ESIPT emission was switched off upon formation of mono
anion. This observation prompted us to examine the binding of
zinc ions, as the metal binding to phenol could re-enable the ESIPT
rupt the
p
-conjugation,21 thereby leading to a shorter conjugation
length in 3. The difference in absorption kmax between 1 and 3
could be attributed to the substitute effect.
As noted in previous discussion, one of the phenolic protons in
bis(HBO) 3 could be partially dissociated in DMF solvent, giving
two emission peaks (kem ꢀ 439, 480 nm). If the emission at
1
3
in bis(HBO) complex 2–Zn.
Titration of 3 in acetonitrile with Zn(OAc)
2
revealed a new
4
80 nm was assigned to keto tautomer, the emission at 439 nm
absorption band at ꢁ380 nm (Fig. 3a), whose intensity increased
2+
was not likely from the enol tautomer of 3, because of the following
reasons: (a) the enol and keto emissions are usually well separated
in HBO (e.g., by >100 nm); (b) there is no spectral overlap between
the absorption (ending at 370 nm, see Fig. 1) and emission of 3
with Zn (0–1 equiv). The UV–vis spectra revealed a clear isobestic
point at ꢁ359 nm, indicating the formation of a new species in the
solution, which was assumed to be 3–Zn (a mono anion). The job
plot also revealed the 1:1 ligand-to-metal ratio in the zinc complex
2
+
(
starting at ꢁ390 nm). Since the emission at 439 nm was not from
(Fig. S17) Further addition of Zn beyond 1 equiv (by up to
2 equiv) did not cause any notable change in the absorption of
3–Zn, as the formation of a dianion is much more difficult than
the mono anion.
the enol tautomer of 3, it was attributed to the enol emission of
mono anion 12. The assignment was consistent with the spectral
data, as the emission at 439 nm was absent when the mono anion
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4
C. S. Abeywickrama, Y. Pang / Tetrahedron Letters xxx (2016) xxx–xxx
Figure 3. UV–vis (a) and fluorescence titration (b) of 3 (2 ꢃ 10ꢂ6 M in acetonitrile) with Zn(CH
COO)
2
(1 ꢃ 10ꢂ3 M in MeOH). Excitation wavelength at 355 nm.
3
Figure 4. UV–vis (a) and fluorescence titration (b) of 5a (2 ꢃ 10ꢂ6 M in acetonitrile) with Zn(CH
COO)
3 2
(1 ꢃ 10ꢂ3 M in MeOH). Excitation wavelength at 405 nm.
wavelength (i.e., no ESIPT emission from the keto tautomer). The
optical characteristics of 3 were further examined by comparison
with zinc binding of isomer 5a. Upon addition of Zn(OAc) to 5a,
2
O
O
O
O
Zn2+
O
N
H
N
Zn2+
H
3
O
the absorption gave a broad shoulder at ꢁ500 nm without showing
a distinct band (Fig. 4a). While the absorption showed an isobestic
point at 425 nm (indicating the zinc binding), the fluorescence was
N
N
O
2
+
O
consistently decreasing with Zn addition (Fig. 4b), in contrast to
(
3) - Zn
2
2
+
Zn
ꢁ
binding to the derivative of 5 (e.g., new emission at
1
2,13
700 nm from 2–Zn).
These results clearly pointed out that
Zn2+
the 1,3-bis(HBO) 3 had quite different optical characteristics from
the 1,4-bis(HBO) 5a, revealing a large impact of the regiochemistry
on the ESIPT.
+
O
O
Fluorescence response of 3 to Zn2+ blue-shifted the emission to
N
H
N
Zn2+
2+
4
21 nm when about 0.5 equiv of Zn was added (Fig. 3b). Further
addition of Zn resulted in the fluorescence decreasing. It was pos-
X
2+
O
O
sible that the initially formed complex 3–Zn was reacting with free
3
- Zn
2+
2
ligand 3 to give (3) –Zn, in which two ligands were binding to Zn
cation to give the four-coordinated complex. The emission at
21 nm was likely from (3) –Zn, as this peak was disappearing
when the complex was predominantly 3–Zn after addition of
Scheme 6. Possible structures of zinc complexes when bis(HBO) 3 was mixed with
Zn(OAc) . In (3) –Zn, the 2nd ligand (shown in blue color) is perpendicular to the
first ligand.
4
2
2
2
2
+
1
equiv of Zn (Scheme 6).
In order to further evaluate the characteristics of mono anion,
Addition of Zn2+ caused the decrease of fluorescence of 3
one equiv of LiOH (a strong base) was added to remove one of the
phenolic protons of 3, which was assumed to form mono anion
3–Li (with a similar structure as in 3–Zn). The resulting mono anion
gave a new absorption band (kmax = 390 nm) and a blue-shifted
fluorescence (kem ꢁ 421 nm), in consistence with the mono anion
formation. The emission intensity of 3–Li, however, was increased
(k
em = 481 nm), which was accompanied with a new emission peak
2+
at 421 nm (Fig. 3b). The Zn binding also decreased the fluores-
cence intensity of 3. On the basis of small Stokes shift, the emission
at 421 nm was attributed to the enol tautomer of 3–Zn. Therefore,
2
+
Zn -binding to 3 did not induce a new emission band at a longer
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C. S. Abeywickrama, Y. Pang / Tetrahedron Letters xxx (2016) xxx–xxx
5
Figure 5. UV–vis (a) and fluorescence response (b) of 3 (2 ꢃ 10ꢂ6 M in acetonitrile) upon addition of 1 equiv. LiOH (1 ꢃ 10ꢂ2 M in EtOH).
associated with the metal complex composition. Thus, formation
of (3) –Zn complex (in 2:1 ligand-to-metal ratio) gives notable
2
spectral blue shift, and subsequent transformation to 3–Zn com-
plex (in 1:1 ligand-to-metal ratio) causes the fluorescence quench-
2
+
ing. This is in contrast to the Zn binding of 5a, where its
fluorescence shows constant quenching without notable spectral
shift. These findings point that bis(HBO) 3 could be a useful molec-
ular frame for new sensor design.
Acknowledgments
We thank the Coleman endowment from the University of
Akron for partial support. We also thank Nicholas Alexander from
the mass spectrometry laboratory from University of Akron.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.06.098.
Figure 6. Stability of mono anions formed from 3 (in acetonitrile) by addition of
one equiv of LiOH, (Bu) OH, or Zn(OAc) (in EtOH).
4
2
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1
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(Dk
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em ꢀ 60 nm), thus enabling ratiometric measurement. The
2+
dynamic fluorescence response of 3 to Zn binding appears to be
Please cite this article in press as: Abeywickrama, C. S.; Pang, Y. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.06.098