Naked eye detection of anions by 2,2¤-bianthracene derivative bearing urea groups in various organic solvents

Article abstract of DOI:10.1246/cl.190924

A highly fluorescent 2,2¤-bianthryl derivative bearing urea groups (2) was prepared as an anion receptor. The UV-vis and fluorescence titrations of 2 in various organic solvents revealed that the association constants (K₁₁) were correlated with acceptor number and Swain acity of the solvents used, and tetrahydrofuran was found to strongly enhance the K₁₁ for Cl¹ and AcO¹ due to enthalpy driven complexation, in which naked eye detection of AcO¹ was achieved.

Full text of DOI:10.1246/cl.190924

Advance Publication Cover Page
Naked Eye Detection of Anions by 2,2’-Bianthracene Derivative Bearing Urea Groups
in Various Organic Solvents
Kohei Osawa, Hideyuki Tagaya, and Shin-ichi Kondo*
Advance Publication on the web January 16, 2020
doi:10.1246/cl.190924
© 2020 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
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Publication manuscripts.

1
Naked Eye Detection of Anions by 2,2’-Bianthracene Derivative Bearing Urea Groups in Various
Organic Solvents
Kohei Osawa,1 Hideyuki Tagaya,1 and Shin-ichi Kondo*2
1Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
2 Department of Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata City, Yamagata 990-8560,
Japan
E-mail: kondo@sci.kj.yamagata-u.ac.jp
1
A highly fluorescent 2,2’-bianthryl derivative
41 Taking this into account, the naked eye anion detection is
42 expected to be achieved by using a bisureido 2,2'-bianthryl
43 receptor (2, Scheme 1) with longer -conjugate system than
44 1. Thus, in this study, we prepared 2 to study the anion
45 recognition ability and fluorescent change upon the
46 recognition in various solvents by UV-vis and fluorescent
47 titrations. In addition, we explored the solvent effect on the
48 K11 of 2 and the complexation mechanism depending on the
49 solvent used from the thermodynamic parameters during the
50 complexations. While there have been the studies relating to
51 the solvent effect on the hydrogen-bonding-anion recognition
52 in mixed solvent or in limited solvents,16–24 the effect of a
53 wide range of solvents have been studied only for a shape-
54 persistent triazolophane macrocycle and a bambusuril
55 macrocycle.25,26 Furthermore, the results of these studies
56 describing the correlation between the binding affinity with
57 TBACl and different solvent parameters including reciprocal
58 of the dielectric constant25 and Swain acity26 show that the
59 solvent effect on the hydrogen-binding anion recognition
60 would be affected by the structure of the receptor. Through
61 the explorations with a wide variety of solvents, we show the
62 anion binding affinity of the relatively flexible acyclic
63 receptor also inversely correlate with the acidity of the
64 solvents as is the case with bambusuril macrocycle.26
2
3
4
5
6
7
8
9
bearing urea groups (2) was prepared as an anion receptor.
The UV-vis and fluorescence titrations of 2 in various organic
solvents revealed that the association constants (K11) were
correlated with acceptor number and Swain acity of the
solvents used, and tetrahydrofuran was found to strongly
enhance the K11 for Cl− and AcO− due to enthalpy driven
complexation, in which naked eye detection of AcO− was
achieved.
10 Keywords: Anion recognition, Naked eye anion sensing,
11 2,2’-Bianthryl anion receptor
12
Anions are ubiquitous and play a wide variety of roles
13 in chemical, biological, environmental, and industrial
14 fields.1–3 The importance of the anions have motivated
15 supramolecular chemists to develop a variety of receptors to
16 bind and sense the anionic species. For achieving high
17 binding affinity and selectivity for the anion, hydrogen bond
18 donor (HBD) group such as urea, amide, and pyrrole have
19 widely been employed as a recognition site of the receptors
20 because of its directionality making the recognition site
21 complementary to various geometries of the anion.2–6
22
Among anion sensing techniques, naked eye anion
23 detection by fluorimetry and colorimetry are quite useful due
24 to their affordability and ease of use.7 Although many naked
25 eye anion sensors have been reported, the naked eye detection
26 by a hydrogen-bond based receptor has been less explored, in
27 which the most of the receptors with HBD group show the
28 selectivity only for the relatively highly basic anions such as
29 F− and AcO− with deprotonation of the acidic HDB group.8–
65
8,8’-Di(3-tert-butylureido)-2,2’-binaphthalene (1) was
66 synthesized according to the procedure we have previously
67 reported.13,27 1-(3-t-Butylureido)naphthalene (3, Figure 1)
68 was synthesized by reaction of t-butylamine with the
69 isocyanate28 prepared from 1-naphthylamine using
70 triphosgene. Synthesis of 9,9’-di(3-t-butylureido)-2,2’-
71 bianthracene (2) was summarized in Scheme 1. 2-
72 Bromoanthracene, which was prepared from bromobenzene
73 according to the literatures,29–32 was reacted with nitric acid
12
30
31
In our group, a 2,2’-binaphthalene-based receptor (1,
32 Figure 1) having high 1: 1 binding constants (K11) for AcO−
33 and Cl− in acetonitrile by cooperative hydrogen bonds with
34 urea groups has been reported.13 However, the naked eye
35 detection with 1 has not yet been achieved due to the
36 fluorescence emission at short wavelength despite the
37 observation of fluorescent quenching of 1 by the addition of
38 AcO−.13 We have described naked eye detection of Ba2+ with
39 a 2,2'-bianthracene bearing aza-15-crown-5 moieties by
40 expanding the -conjugation of 2,2’-banaphthalene.14,15
a
d
b, c
4
5
e
f , g
6
2
Scheme 1. Synthesis of 2. (a) HNO3, AcOH, 50 C. (b) HCl,
rt. (c) NaOHaq, 60 C. (d) Fe, NH4Cl, acetone / H2O, reflux.
(e) Zn, PPh3, Bpy, NiCl2, DMAc, 60 C. (f) Triphosgene,
iPr2EtN, THF, rt. (g) t-Butylamine, THF, 50 C.
1
3
Figure 1. Chemical structures of 1 and 3.

2
1
2
3
4
5
6
7
8
9
to afford 2-bromo-9- and 2-bromo-10-nitoroanthracene,
respectively in 1.00:0.59 ratio from the 1H NMR.33
Recrystallization of the mixture from mixed solvent (CHCl3:
Hexane = 1: 1) afforded yellow needle crystals which was
identified to be desired 2-bromo-9-nitoroanthracene (4) by X-
ray diffraction with the single-crystals grown by vaper
diffusion of hexane into a solution of 4 in CHCl3 (Figure
S28).34 Reduction of 4 with iron and ammonium chloride,35
followed by homocoupling with Ni catalyst gave 9,9’-
53 found to be 61 times smaller than that of 1 (7.9×105 mol−1
54 dm3, Table S1). To reveal further insight into the difference
55 of the K11 for Cl− between 2 and 1, the optimized structures
56 of the complexes were calculated by DFT at B3LYP/6-
57 31+G(d) level. While the dihedral angle of C9a-C9-N1-H of
58 the optimized 2•Cl− was 41°, the corresponding one of C8a-
59 C8-N1-H of the optimized 1•Cl− was 21° (Figure S16).
60 Receptor 2 would adopt the larger dihedral angle to prevent
61 the steric repulsion of C8-H of the anthryl moiety with the
62 carbonyl oxygen of the urea, which would lead to the
63 decrease in the K11 with decreasing acidity of the urea
64 hydrogens due to less overlapping p-orbitals of the anthryl
65 and the urea groups of 2. As is the case with Cl−, similar
66 hyperchromic and hypochromic effect were observed for 2
67 upon the addition of AcO−. Moreover, a trend of K11 for AcO−
68 was similar to that for Cl−; that is, the K11 of 2 for AcO− in
69 MeCN (2.8×104 mol−1 dm3) was 72 times smaller than that of
70 1 (2.0× 106 mol−1 dm3).
10 diamino-2,2’-bianthracene (6).27,36,37 The product was treated
11 with triphosgene, and the produced isocyanate was reacted
12 with tert-butylamine to obtain bisureido 2,2’-bianthracene
13 (2)13 and the product was identified with NMR (1H, 13C,
14 COSY, HMBC, and HMQC) and HRMS.
15
Although the originally reported 2,2’-binaphthalene
16 having n-butyl groups27,37 shows high binding affinity for Cl−,
17 the low solubility in organic solvents is troublesome. The
18 solubility was improved by substitution by t-butyl groups
19 (1).13 Hence, we evaluated the solubility of 2 by means of the
20 same evaluation method as 1. The saturated concentration of
21 2 in DMSO was found to be 25–40 times greater than that in
22 MeCN, THF, and CHCl3 (Table 1). The saturated
23 concentration of 2 in DMSO (7.67×10−3 mol dm−3) was
24 approximately a half of 1 (16×10−3 mol dm−3). Although this
25 lower saturated concentration would be due to more
26 favorable  stacking of 2 having longer conjugated
27 system than 1, the concentration is sufficient to use 2 as an
28 optical sensor with UV-vis and fluorescent titrations, in
29 which concentration of ~10−5 mol dm−3 is usually required.
30
71
UV-vis titrations of 2 in various solvents such as THF,
72 AcOEt, acetone, benzene, 1,4-dioxane, CH2Cl2, DMF, CHCl3,
1.0
(a)
0.8
0.6
0.4
0.2
0.0
31 Table 1. Saturated concentrations of 1 and 2 in various
32 solvents.
Solvent
Saturated concentration / mol dm−3
300 350 400 450 500
Wavelength / nm
1a
3.1×10−4
2.43×10−3
8.0×10−5
16.0×10−3
2
CHCl3
THF
MeCN
DMSO
2.71×10−4
2.90×10−4
1.95×10−4
7.67×10−3
0.00
(b)
-0.01
33 a Ref. 13.
34
-0.02
-0.03
-0.04
-0.05
-0.06
35
UV-vis spectrum of 2 shows characteristic absorbance
36 at 408 and 385 nm, which is significantly longer than the
37 corresponding 1 (Figure S1). Considering a report by Wang
38 and Wu38 that HOMO-LUMO energy gap of 2,2’-
39 bianthracene calculated by density functional theory (DFT)
40 at B3LYP/6-31G(d) level is 3.230 eV (384 nm), absorption
41 around 400 nm of 2 would be derived from absorption
42 between the HOMO and LUMO (* transition). UV-vis
43 titrations of 1 have been performed in MeCN at 298 K in our
44 group,13 therefore, the titrations of 2 was also carried out in
45 the same condition. Upon the addition of TBACl (~50 eq),
46 hyperchromic and hypochromic shift of 2 around 440 nm and
47 400 nm via isosbestic points at 424, 338–330, 304, 287.5, and
48 259.5 nm were observed (Figure 2). Nonlinear curve fitting
49 analysis of the titration with 1:1 complex model showed good
50 agreement between the absorbance change and the theorical
51 curve, which suggests the 1:1 complex formation.
52 Furthermore, the calculated K11 (1.3×104 mol−1 dm3) was
0
20
40
60
[TBACl] / [2]
Figure 2. (a) UV-vis spectral changes of 2 upon the addition
of 0–50 eq of TBACl in MeCN containing 1% DMSO at 298
K. [2] = 1.01×10−5 mol dm−3. (b) Absorbance changes at 410
nm. A theorical 1: 1 binding isotherm indicates as a solid
curve.
73 and DMSO were also performed at 298 K using TBACl as a
74 guest (Figure S6) for evaluating solvent effect on the anion
75 recognition. As is the case in the titrations in MeCN, 1: 1
76 complex formation of 2 with Cl− was confirmed in any

3
1
2
3
4
5
6
7
8
9
solvents by the nonlinear curve fitting analysis, and their K11
were summarized in Table S1. Among them, the K11 in THF
(4.4×106 mol−1 dm3) was the largest and 338 times greater
than that in MeCN. Furthermore, the K11 of 2 for Cl− in THF
(4.4×106 mol−1 dm3) was 5.6 times greater than that of 1 in
MeCN, which suggests strong solvent effect on the anion
recognition. In addition, the K11 of 2 for AcO− was also the
greatest in THF.
42 increase in the K11 for Cl− and AcO− with decreasing the
43 acidity of the solvents, free anions would be weakly solvated
44 in the solvent with smaller acidity, which would lead to less
45 competition between the solvation and the complexation of
46 the anion. Hence, the complexation would be more favorable
47 in the solvent with smaller acidity such as THF.
48
To explore the anion selectivity of 2, UV-vis titrations
49 of 2 in THF at 298 K were performed using TBA salts of Br−,
−
−
−
In order to confirm whether the enhancement of the K11
50 NO3 , HSO4 , and H2PO4 . From the results of the nonlinear
51 curve fitting analysis of the spectral changes, 1:1 complex
52 formation of 2 with any anions was indicated and the
53 calculated K11 was found to be in the order of Cl− > AcO− ≈
10 by THF is unique to 2, UV-vis titrations of 1 and 3 with
11 TBACl in MeCN or THF at 298 K were also performed
12 (Figure S2 and S4). Both receptors were found to form the
13 1:1 complex in MeCN and THF, and their K11 for Cl− were
14 enhanced in THF. This indicates the enhancement of K11 for
15 Cl− by THF is general for such urea based receptors. While
16 the K11 of 3 for Cl− in THF (7.5 × 103 mol−1 dm3) was 21 times
17 larger than that in MeCN, 1 and 2 showed more than 400 and
18 338 times larger binding affinity, respectively. Hence, in
19 addition to the solvation effect, donation of cooperative
20 hydrogen bonds by the bisurea group seems to be necessary
21 to strongly enhance the K11 of the receptors. The K11 of the
22 above receptors for AcO− were also found to be enhanced by
23 THF and the cooperative hydrogen bond donation.
−
−
−
54 H2PO4 > Br− > HSO4 > NO3 (Figure S8 and Table 2) as
55 observed for 2,2’-binaphthalene having n-butyl groups.13
56
57 Table 2. The association constants of 2 for various TBA salts
58 of anions in THF containing 1% DMSO obtained from UV-
59 vis titrations at 298 K.
a
Anion
K11
−
NO3
(1.07 ± 0.07) × 104
(1.73 ± 0.02) × 104
(3.84 ± 0.29) × 105
(1.61 ± 0.04) × 106
(1.67 ± 0.02) × 106
(4.40 ± 0.14) × 106
−
HSO4
Br−
24
When the K11 of 2 for Cl− in MeCN and THF was
AcO−
25 compared to that in other solvents, the K11 became small in
26 order of THF > AcOEt > acetone > benzene > 1,4-dioxane >
27 CH2Cl2 > MeCN > DMF > CHCl3 > DMSO (Table S1).
28 Furthermore, plotting of these K11 against solvent factors
29 including relative permittivity (r), solvent orientation
30 polarizability (f), Dimroth-Reichardt parameter (ETN),
31 Gutmann donor number (DNN), acceptor number (AN), and
32 Swain acity suggests that the K11 of 2 for Cl− had the highest
33 correlation with AN and Swain acity, and the K11 increased
34 with decreasing these parameters (Figure 3 and Figure S12).
35 Although the order of the K11 for AcO− was slightly different
36 form that for Cl−, the binding affinity for AcO− also had the
37 highest correlation with the same parameters (Figure S13).
−
H2PO4
Cl−
60 a [2] = 1.0×10−5 mol dm−3.
61
62
Thermodynamic parameters during the complexation of
63 2 with Cl− and AcO− measured by van’t Hoff analysis of UV-
64 vis titrations in MeCN or THF at various temperatures (290–
65 322 K) should give further information on the solvent
66 effect.41 The complexations of 2 with Cl− and AcO− were
67 found to be entropically less favorable (TS < 0) but
68 enthalpically much favorable (H > 0) in THF than those in
69 MeCN (Table 3). As mentioned above, the anion in THF
70 would be weakly solvated, then enthalpy driven
71 complexation was observed.
38
AN and Swain acity are the solvent parameters which
39 can quantify the acidity of the solvent,39,40 thus, the anion with
40 higher basicity than electron neutral 2 would be more
41 profoundly affected by these parameters. Considering an
72
73 Table 3. Thermodynamic parameters of complexation
74 between 2 and anions in MeCN and THF containing 1%
75 DMSO obtained from UV-vis titrations.
7
6
5
4
3
2
1
Solvent
Anion
G
H
TS a
kJ mol−1 kJ mol−1 kJ mol−1
Cl−
Cl−
AcO−
AcO−
-23.51
-37.91
-25.19
-35.54
-19.48
-41.47
-23.41
-41.83
4.04
-3.57
1.77
MeCN
THF
MeCN
THF
-6.29
76 a at 298 K.
77
0
5
10 15 20 25
AN
Figure 3. Relationship of log K11 of 2 with Cl− vs AN of
solvents used.

4
1
2
3
4
5
6
7
8
9
2 shows characteristic yellow-green fluorescence at
around 520 nm excited at 335 nm (the isosbestic point in the
UV-vis titrations) in MeCN containing 1% DMSO originated
from the 2,2’-bianthryl moiety with 80 nm longer -
conjugated system than 2,2’-binaphthyl13 as shown in Figure
S9. The Fl,max of 2 was shifted to longer wavelength and the
enhancement of the fluorescent intensity via an isoemissive
point at 480 nm was observed upon the addition of ~70 eq of
TBACl. As is the case in UV-vis titrations, 1: 1 complex
10 formation of 2 with Cl− was confirmed by the nonlinear curve
11 fitting analysis of fluorescent changes at 520 nm. Contrary to
12 this, significant decrease in fluorescent intensity was
13 observed for 2 upon the addition of TBAAcO (Figure S10).
14 The calculated K11 of 2 for Cl− and AcO− were almost
15 identical to those obtained by UV-vis titrations.
16
17 also performed at 298 K. In the absence of any anions, the
18 Fl,max of 2 in each solvent were different from that in MeCN.
19 For example, the Fl,max in THF (511 nm) was 9 nm shorter
20 than that in MeCN (Figure S9) due to smaller ICT characters
21 in less polar solvent.42
The fluorescent titrations of 2 in other solvents were
Figure 5. Fluorescence of 2 in THF in the absence (a) or
presence of 6 eq of TBAAcO (b). [2] = 1.0×10−5 mol dm−3.
ex = 365 nm.


41
In summary, bisurea based on 2,2'-bianthracene (2) was
had longer-wavelength
42 successfully prepared and
2
22
The fluorescence intensities of 2 in the solvents other
43 absorption than 1. Receptor 2 was sufficiently soluble in
44 DMSO. Although the K11 of 2 for Cl− was 61-fold smaller
45 than that of 1 due to a decrease in acidity of the urea protons
46 with increasing the dihedral angle between the urea and the
47 anthryl  surface, dramatically enhanced K11 of 2 was
48 observed in THF. The correlation of the K11 with AN and
49 Swain acity of the solvents and van’t Hoff plots revealed that
50 enthalpy driven complexation by weakly solvated anions.
51 Moreover, significant fluorescent intensity changes of 2 by
52 the addition of TBAAcO indicates 2 can be utilized as a
53 naked-eye anionic sensor.
23 than MeCN were also changed by the addition of TBACl
24 (Figure S9). Among them, the increasing in the
25 fluorescence intensities of 2 was observed in relatively
26 polar solvents such as MeCN, acetone, DMF, and
27 DMSO, while the intensities decreased in other
28 solvents upon the addition of Cl−. For more detail of the
29 mechanism, further studies are required in the future
30 work. The association constants determined from
31 fluorescence titrations in such solvents were fairly good
32 agreement to those obtained by UV-vis titrations, and the K11
33 in THF was the highest among all other solvents. Moreover,
34 the decrease of fluorescence intensity by AcO− in THF was
35 clearly checked with naked-eye (Figure 4, 5, and S14). Other
54
55 Acknowledgement
56 The authors would like to thank Professor Hiroshi Katagiri,
57 Mr. Yusuke Shibuya, and Mr. Amane Matsunaga, Yamagata
58 University for the measurements of single crystal X-ray
59 diffraction and HRMS.
36 anions such as Br−, NO3 , HSO4 , and H2PO4 were also
37 employed in the fluorescence titrations of 2 in THF at 298 K,
38 and the K11 for these anions and fluorescence intensity
39 changes were smaller (Figure S11 and Table S2). Therefore,
40 2 would be utilized as a naked-eye sensor for the acetate.
−
−
−
60
61 Supporting
Information
is
available
on
62 http://dx.doi.org/10.1246/cl.******.
4
3
2
1
0
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42 34
43
Crystallographic data for 4: empirical formula C14H8BrNO2,
Mw = 302.12, monoclinic, space group P-1, a = 7.3579(2) Å, b
= 9.0748(2) Å, c = 9.3932(2) Å,  = 68.662(2)°,  =
44
45
80.338(2)°,  = 86.693(2)°, V = 575.91(2) Å3, Z = 2, Dcalcd
=
46
1.742 g cm−3, A total of 7962 reflections were collected, of
which 2620 reflections were unique (Rint 0.0325).
47
=
48
Refinement of 163 parameters led to the final R1 = 0.0261 (I >
2σ(I)), wR2 = 0.0663 (all data), goodness of fit (GOF) = 1.061.
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