D-π-A benzo[c][1,2,5]selenadiazole-based derivatives via an ethynyl bridge: Photophysical properties, solvatochromism and applications as fluorescent sensors

Article abstract of DOI:10.1016/j.dyepig.2014.06.035

A series of novel D-π-A benzo[c][1,2,5]selenadiazole (BSe)-based derivatives 7a-i including electron-donating/electron-withdrawing groups were prepared by Pd-catalyzed Sonogashira-Hagihara reaction of 4,7- diethynylbenzoselenadiazole with various diiodo a

Full text of DOI:10.1016/j.dyepig.2014.06.035

Accepted Manuscript
D-π-A benzo[c][1,2,5]selenadiazole-based derivatives via an ethynyl bridge:
photophysical properties, solvatochromism and applications as fluorescent sensors
Hui Li, Yang Guo, Yunxiang Lei, Wenxia Gao, Miaochang Liu, Jiuxi Chen, Yuefei Hu,
Xiaobo Huang, Huayue Wu
PII:
S0143-7208(14)00277-0
10.1016/j.dyepig.2014.06.035
DYPI 4443
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 May 2014
Revised Date: 27 June 2014
Accepted Date: 29 June 2014
Please cite this article as: Li H, Guo Y, Lei Y, Gao W, Liu M, Chen J, Hu Y, Huang X, Wu H, D-π-
A benzo[c][1,2,5]selenadiazole-based derivatives via an ethynyl bridge: photophysical properties,
solvatochromism and applications as fluorescent sensors, Dyes and Pigments (2014), doi: 10.1016/
j.dyepig.2014.06.035.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
D-π-A benzo[c][1,2,5]selenadiazole-based derivatives via an ethynyl
bridge: photophysical properties, solvatochromism and applications
as fluorescent sensors
a
b
a
a
a
a
Hui Li , Yang Guo , Yunxiang Lei , Wenxia Gao , Miaochang Liu , Jiuxi Chen ,
c
a,
a,
Yuefei Hu , Xiaobo Huang *, Huayue Wu *
a
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China
b
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, P. R. China
c
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
*Corresponding authors. Tel.: +86 577 88368280; fax: +86 577 88368280
E-mail addresses: xiaobhuang@wzu.edu.cn (X. Huang); huayuewu@wzu.edu.cn (H. Wu)
ABSTRACT: A series of novel D-π-A benzo[c][1,2,5]selenadiazole (BSe)-based derivatives
7
a-i including electron-donating/electron-withdrawing groups were prepared by Pd-catalyzed
Sonogashira- Hagihara reaction of 4,7-diethynylbenzoselenadiazole with various diiodo aryl
compounds. Photophysical properties of these compounds were investigated by absorption and
emission spectra analyses. Compound 7c containing methoxy group exhibited positive
solvatochromism and its solvatochromic properties were analyzed by the Lippert-Mataga equation
and Kamlet-Taft equation, respectively. The resulting compounds showed tunable band gaps in
the range of 2.21-2.70 eV using theoretical calculations, which was in good agreement with the
results derived from the absorption of UV-vis spectra. In addition, fluorescent sensors 7c and 7d
2
+
showed highly sensitive response for Hg and traces of water using BSe as an acceptor or a
fluorophore in real-time detection, respectively. These findings indicate that BSe-based molecules
can be developed as excellent fluorophores for fluorescent material applications.
Keywords: Benzo[c][1,2,5]selenadiazole, photophysical properties, solvatochromism, fluorescent
sensor
1
. Introduction
Over the two decades, donor-π-acceptor (D-π-A) type organic fluorescent sensors have attracted
much interest owing to their high sensitivity, selectivity and ease of measurement, and another
fascinating advantage of such materials is that the π conjugation, band gap, and electro-optical
properties can be effectively tuned by judicious choice of the fluorophores, donor and acceptor
substituents. As a result, a lot of organic molecules have been developed as fluorophores in this
kind of system for the fluorescent detection of ions [1], small organic molecules detection [2], and
those structures with larger absorption and emission wavelengths also can be applied in biological
and medical study [3]. Due to strong and stable fluorescence, good thermal stability and good
electron-withdrawing ability, benzochalcogendiazoles, such as 2,1,3-benzothiadiazole (BT) plays
1

ACCEPTED MANUSCRIPT
a very important position in the synthesis and application field of π-conjugated structure
fluorescent sensors as an excellent fluorophore [4]. 2,1,3-Benzoselenadiazole (BSe) has a similar
chemical structure as BT, but compared with BT, since the selenium atom has a much larger size
and less electronegativity than the sulfur atom, it would have a more important influence for the
heteroatoms on the optical properties of the BSe-based derivatives [5]. Recent years, comparison
of effects caused by the heteroatom S and Se is becoming very common, it have been reported that
BSe-based compounds showed obvious absorption and emission redshift than the BT-based
compounds because BSe can more effectively lower the band gaps of these compounds and some
of them exhibited interesting photoelectric properties [5a-e, 6].
To the best of our knowledge, there have few reports of the synthesis of D-π-A compounds
containing BSe and aryl units join by an ethynyl bridge. The most possible reason is that dihalo
compounds of BSe are hard to participate in the Sonogashira-Hagihara coupling reaction due to
the poor reactivity and solubility influenced by the selenium atom. Recently, Bunz’ group [7] and
our group [8] successfully synthesized the key intermediate diethynylbenzoselenadiazole from BT
by a strategy of firstly introducing the trimethylsilylacetylene flexible group, and then introducing
the selenium atom. And then, this intermediate has been applied to the synthesis of a series of
novel D-π-A polymers with low energy band gap incorporating a BSe moiety by us [8,9]. These
polymers displayed obvious absorption peaks at the region from 477 to 510 nm, narrow
orange-red or red fluorescence in the range of 576-595 nm, and their band gaps were in the range
of 1.37-1.76 eV. Just as pointed out in the reported literatures [5a-e], these BSe-based polymers
also showed outstanding absorption and emission redshift in comparison with the corresponding
BT-based polymers. And one of them has been developed as a highly selective and sensitive
chemosensor for heavy metal ions detection based on colorimetry and fluorometry [9]. In view of
the very promising applications of these π-conjugated molecules based on BSe, their design,
synthesis and photophysical properties are very attractive topics for researchers.
As a further extension of our research on the design of novel fluorophores for fluorescent
materials [10], in this work, we described a series of D-π-A BSe-based derivatives 7a-j by
introducing appropriate electron donors and acceptors to BSe skeleton via an ethynyl linkage and
systematically investigated their photophysical properties, solvatochromism, structure-property
relationship and applications as fluorescent sensors. These compounds displayed obvious
absorption peaks at the region from 429 to 488 nm and the fluorescence from green to red in the
range of 512-669 nm in CHCl . Compound 7c containing methoxy group exhibited positive
3
solvatochromism and its solvatochromic properties were analyzed by the Lippert-Mataga equation
and Kamlet-Taft equation, respectively. The HOMO-LUMO energy gaps of the resulting
compounds derived from theoretical calculations has been compared with the results calculated by
the photophysical measurements. In addition, fluorescent sensors 7c and 7d showed highly
sensitive response for specific metal ion and traces of water using BSe as an acceptor or a
fluorophore in real-time detection, respectively.
2

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2
. Experimental part
2
.1. Materials
All solvents and reagents were commercially available and analytical-reagent-grade. THF and
Et N were purified by distillation from sodium in the presence of benzophenone. 4,7-Dibromo-
3
benzo[c][1,2,5]selenadioazole (2) and 3,6-dibromobenzene-1,2-diamine (3) could be prepared
from BT (1) according to the literature reported by Myashi and coworkers [11], 3,6-bis(2-
(trimethylsilyl)ethynyl)benzene-1,2-diamine (4), 4,7-bis(2-(trimethylsilyl)ethynyl)benzo[c][1,2,5]-
selenadiazole (5) and 4,7-diethynylbenzoselenadiazole (6) could be prepared according to a
reported procedure [8].
2
.2. Measurements
1
Melting points were recorded on a Buchi Melting Point B-540 (uncorrected). The H NMR and
1
3
C NMR spectra were recorded in solution of CDCl or DMSO-d on a Bruker DRX 500 NMR
3
6
spectrometer with tetramethylsilane (TMS) as the internal standard. The chemical shift was
recorded in ppm and the following abbreviations were used to explain the multiplicities: s =
singlet, d = doublet, m = multiplet, br = broad. EI mass spectra were recorded on Agilent 5975C
DIP/MS mass spectrometer. C, H and N of elemental analyses were performed on an Elementar
Vario MICRO analyzer. UV-vis absorption was recorded on a Shimadzu UV-1700 spectrometer
and fluorescence spectra were recorded on a RF-5301PC fluorometer. Thermogravimetric analysis
(
TGA) was performed on a Perkin-Elmer Pyris-1 instrument under a N atmosphere.
2
2
.3. Preparation of BSe-based derivatives 7
General procedure for synthesis of BSe-based derivatives 7: A mixture of 4,7-diethynyl-
benzoselenadiazole (6) (27.72 mg, 0.12 mmol), Pd(PPh ) Cl (4.21 mg, 0.006 mmol), CuI (2.30
3
2
2
mg, 0.012 mmol), PPh (3.15 mg, 0.012 mmol) and diiodo aryl compound (0.36 mmol) was
3
dissolved in 10 mL of Et N and 10 mL of THF. The reaction mixture was degassed and stirred at
3
o
6
0 C for 12 h under a N atmosphere. The solvent was removed under reduced pressure, and the
2
residue was extracted with CH Cl . The organic layer was washed with water and then saturated
2
2
sodium chloride solution, dried over anhydrous Na SO , and then evaporated in vacuum to
2
4
dryness. The residue was purified by silica gel column chromatography with petroleum
ether/chloroform (30:1, v/v) as an eluent to give the target compounds.
4
,7-Bis(2-phenylethynyl)benzo[c][1,2,5]selenadiazole (7a): Yield: 60%; yellow solids; m. p.
o
1
1
41-142 C; H NMR (CDCl , 500 MHz): δ 7.71 (s, 2H), 7.68-7.66 (m, 4H), 7.41-7.38 (m, 6H);
3
1
3
C NMR (CDCl , 125 MHz): δ 159.3, 132.7, 132.0, 129.0, 128.4, 122.6, 119.0, 97.4, 85.7. FT-IR
3
-
1
+
(
KBr, cm ): 2921, 2848, 2359, 1653, 1490, 839. MS (EI): m/z: 383.9 [M ]. Anal. Calcd for
C H N Se: C, 68.94; H, 3.16; N, 7.31, found C, 69.05; H, 3.11; N, 7.24.
2
2
12
2
3

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4
2
7
1
,7-Bis(2-p-tolylethynyl)benzo[c][1,2,5]selenadiazole (7b): Yield: 65%; yellow solids; m. p.
o
1
56-257 C; H NMR (DMSO-d , 500 MHz): δ 7.79 (s, 2H), 7.52 (d, J = 7.5 Hz, 4H), 7.30 (d, J =
6
1
3
.5 Hz, 4H), 2.37 (s, 6H); C NMR (DMSO-d , 125 MHz): δ 158.3, 139.2, 132.2, 131.4,
6
-1
29.5,119.1, 118.1, 96.6, 86.0. FT-IR (KBr, cm ): 2914, 2372, 1649, 1540, 1504, 839, 809. MS
+
(
EI): m/z: 411.9 [M ]. Anal. Calcd for C H N Se: C, 70.07; H, 3.92; N, 6.81, found C, 69.89; H,
2
4
16
2
3
.89; N, 6.85.
4
,7-Bis(2-(4-methoxyphenyl)ethynyl)benzo[c][1,2,5]selenadiazole (7c): Yield: 75%; orange solids;
o
1
m. p. 234-235 C; H NMR (CDCl , 500 MHz): δ 7.66 (s, 2H), 7.60 (d, J = 8.5 Hz, 4H), 6.92 (d, J
3
1
3
=
9.0 Hz, 4H), 3.85 (s, 6H); C NMR (CDCl , 125 MHz): δ 160.2, 159.4, 133.6, 132.4, 118.9,
3
-
1
1
14.8, 114.1, 97.6, 84.9, 55.4. FT-IR (KBr, cm ): 2921, 2848, 2354, 1600, 1507, 826. MS (EI):
+
m/z: 443.9 [M ]. Anal. Calcd for C H N O Se: C, 65.02; H, 3.64; N, 6.32, found C, 64.97; H,
2
4
16
2
2
3
.61; N, 6.39.
4
,7-Bis(2-(4-aminophenyl)ethynyl)benzo[c][1,2,5]selenadiazole (7d): Yield: 62%; red solids; m. p.
o
1
2
50-251 C; H NMR (CDCl , 500 MHz): δ 7.56 (s, 2H), 7.39 (d, J = 7.0 Hz, 4H), 6.61 (d, J = 6.5
3
1
3
Hz, 4H); C NMR (CDCl , 125 MHz): δ 159.3, 147.5, 133.3, 131.8, 118.6, 114.5, 111.4, 98.4,
3
-1
+
8
4.2. FT-IR (KBr, cm ): 3431, 2915, 2842, 2346, 1597, 1507, 833. MS (EI): m/z: 413.9 [M ].
Anal. Calcd for C H N Se: C, 63.93; H, 3.41; N, 13.55, found C, 64.02; H, 3.38; N, 13.60.
2
2
14
4
4
,7-Bis(2-(4-fluorophenyl)ethynyl)benzo[c][1,2,5]selenadiazole (7e): Yield: 70%; yellow solids;
o
1
m. p. 267-268 C; H NMR (CDCl , 500 MHz): δ 7.70 (s, 2H), 7.66-7.64 (m, 4H), 7.11-7.08 (m,
3
1
3
4
2
8
6
H); C NMR (CDCl , 125 MHz): δ 163.0 (d, J = 249.4 Hz), 159.2, 133.9, 132.6, 118.8 (d, J =
3
-1
5.3 Hz), 115.8 (d, J = 22.1 Hz), 114.1, 96.3, 85.5. FT-IR (KBr, cm ): 2924, 2848, 2359, 1507,
+
35. MS (EI): m/z: 419.9 [M ]. Anal. Calcd for C H F N Se: C, 63.02; H, 2.40; N, 6.68, found C,
2
2
10
2
2
2.90; H, 2.46; N, 6.79.
4
,7-Bis(2-(4-chlorophenyl)ethynyl)benzo[c][1,2,5]selenadiazole (7f): Yield: 66%; yellow solids;
o
1
m. p. 251-252 C; H NMR (CDCl , 500 MHz): δ 7.71 (s, 2H), 7.59 (d, J = 8.5 Hz, 4H), 7.37 (d, J
3
1
3
=
8.0 Hz, 4H); C NMR (CDCl , 125 MHz): δ 159.1, 135.1, 133.2, 130.9, 128.8, 125.0, 118.9,
3
-1
+
9
6.4, 86.8. FT-IR (KBr, cm ): 2915, 2849, 2352, 1649, 1490, 824. MS (EI): m/z: 451.8 [M ]. Anal.
Calcd for C H Cl N Se: C, 58.43; H, 2.23; N, 6.19, found C, 58.51; H, 2.19; N, 6.05.
2
2
10
2
2
4
,7-Bis(2-(4-nitrophenyl)ethynyl)benzo[c][1,2,5]selenadiazole (7g): Yield: 65%; yellow solids; m.
o
1
p. 210-211 C; H NMR (CDCl , 500 MHz): δ 8.28 (d, J = 8.5 Hz, 4H), 7.82 (d, J = 8.5 Hz, 4H),
3
1
3
7
.79 (s, 2H); C NMR (CDCl , 125 MHz): δ 159.0, 147.5, 133.2, 132.7, 129.2, 123.7, 118.9, 95.5,
3
-
1
+
9
0.2. FT-IR (KBr, cm ): 2914, 2848, 2365, 1590, 1507, 859. MS (EI): m/z: 473.8 [M ]. Elemental
Anal. Calcd for C H N O Se: C, 55.83; H, 2.13; N, 11.84, found C, 55.90; H, 2.06; N, 11.78.
2
2
10
4
4
4

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4
,7-Bis(2-(4-(trifluoromethyl)phenyl)ethynyl)benzo[c][1,2,5]selenadiazole (7h): Yield: 60%;
o
1
yellow solids; m. p. 253-254 C; H NMR (CDCl , 500 MHz): δ 7.77 (d, J = 8.0 Hz, 4H), 7.75 (s,
3
1
3
2
3
2
1
H), 7.66 (d, J = 8.0 Hz, 4H); C NMR (CDCl , 125 MHz): δ 159.1, 133.0, 132.2, 130.7 (d, J =
3
-1
2.6 Hz), 126.2, 125.4, 123.8 (d, J = 270.6 Hz), 118.9, 95.9, 87.6. FT-IR (KBr, cm ): 2928, 2855,
+
358, 1613, 1504, 841. MS (EI): m/z: 519.9 [M ]. Anal. Calcd for C H F N Se: C, 55.51; H,
2
4
10
6
2
.94; N, 5.39, found C, 55.42; H, 1.89; N, 5.46.
4
2
7
,7-Bis(2-(pyridin-2-yl)ethynyl)benzo[c][1,2,5]selenadiazole (7i): Yield: 62%; yellow solids; m. p.
o
1
07-208 C; H NMR (CDCl , 300 MHz): δ 8.69-8.68 (m, 2H), 7.79 (s, 2H), 7.74-7.67 (m, 4H),
3
1
3
.31-7.26 (m, 2H); C NMR (CDCl , 125 MHz): δ 159.1, 150.3, 142.9, 136.2, 133.3, 127.6, 123.3,
3
-1
1
18.8, 96.4, 85.1. FT-IR (KBr, cm ): 2921, 2848, 2359, 1646, 1576, 1454, 844, 832. MS (EI): m/z:
+
3
1
85.9 [M ]. Anal. Calcd for C H N Se: C, 62.35; H, 2.62; N, 14.54, found C, 62.47; H, 2.70; N,
20 10 4
4.42.
2
.4. Metal ion titration of 7c and the detection for traces of water of 7d
Each metal ion titration experiment was started with a 3.0 mL 7c in THF solution with a known
-5
-1
concentration (1.0 × 10 mol·L ). Mercury perchlorate salt and other various metal salts (nitrate,
-3
-1
1
.0 × 10 mol·L , CH CN) were used for the titration. The concentration of 7d was fixed at 1.0 ×
3
-5
-1
1
0 mol·L in anhydrous THF and the aqueous THF (water:THF = 1:9, v/v) for titration was
deionized water. Because the minimum response time that can be measured is limited in the
manipulation time, all kinds of measurements were monitored 10 s after the addition of the metal
salt or water to the host molecule solutions for the application of real-time detection.
3
. Results and discussion
3
.1 Synthesis of BSe-based derivatives
Scheme 1 illustrated the synthetic routes to target BSe-based derivatives 7a-i. The key
intermediate 4,7-diethynylbenzo[c][1,2,5]selenadiazole (6) was designed to be synthesized by a
strategy of firstly introducing the trimethylsilylacetylene flexible group, and then introducing the
selenium atom from the starting material BT (1). And then BSe derivatives 7a-i was synthesized
by Pd-catalyzed Sonogashira-Hagihara reaction from compound 6 with various commercially
available diiodo aryl compounds in 60-75% yields. These target compounds were fully
1
13
characterized by H NMR, C NMR, EI mass spectra and elemental analysis, etc. Thermal
stability is one of the key requirements for practical applications of organic fluorophores. The
thermal properties of 7a-i were investigated by TGA, which were carried out under a N2
atmosphere at a heating rate of 10°C/min. As shown in Fig. 1, although these compounds have a
similar structure, their TGA plots are different. The TGA curves reveal that the degradation
temperature (T ) of 5% weight loss of 7a-i was in the range of 211-378°C. There is a total loss of
d
5

ACCEPTED MANUSCRIPT
about 32-50% for 7a-i when heated to 700°C, respectively. The results indicate these BSe
derivatives can provide a desirable thermal property for various fluorescent materials.
3
.2 Optical properties of compounds 7a-i
The resulting compounds 7a-i are soluble in common organic solvents such as CHCl3,
dichloromethane (DCM), THF, Acetone, DMSO, dimethylformamide (DMF), CH CN and toluene,
3
etc. Their UV-vis absorption and emission spectra were measured in CHCl as shown in Fig. 2 and
3
Fig. 3, respectively, and the data are presented in Table 1. These compounds have well-resolved
absorption peaks in the range of 235 to 500 nm, and the band situated at a short wavelength
3
00-350 nm region is assigned to the localized π-π* transition [12]. And in view of maximum
max
absorption wavelengths (λabs ) of these compounds, all of them have strong absorption at a long
wavelength 420-500 nm region compared to compound 6 (393 nm) due to its coplanarity of BSe
unit and aryl group via triple bond as well as the charge transfer from aryl group to BSe unit [13].
The fluorescence quantum yields (Ф ) of 7a-i were determined in CHCl at room temperature
F
3
using the quinine sulfate solution as a fluorescence reference (Ф = 0.55 in 0.5 mol/L H SO ). The
r
2
4
2
Ф values could be calculated using the equation [14]: Ф =Ф (F /F )(A /A )(η /η ) , where s and r
F
F
r
s
r
r
s
s
r
denote the sample and reference, respectively, F is the integrated fluorescence intensity, A is the
absorbance at the excited wavelength, and η is the refractive index of the solvent. As shown in
Table 1, it can be found that all these compounds have high quantum yields (Ф = 0.17-0.37)
F
owing to D-π-A structure, with the exception of 7d (Ф = 0.04) containing a NH group which
F
2
quenches fluorescence. As shown in Fig. 3 and Fig. 4, it can be found that these compounds show
tunable emission wavelengths by introducing appropriate electron donors and acceptors to BSe
skeleton via an ethynyl linkage, and emit such as green, yellow, orange and red fluorescence under
max
a 365 nm UV lamp, respectively. The maximum fluorescence emission wavelengths (λem ) of
compounds 7b-d including electron-donating groups have obvious redshift relative to 7a (546 nm)
containing only phenyl group. And among these compounds, the strongest donor substituted 7d
appears emission peak situated at 669 nm, and has as high as 123 nm red shift and shows red
fluorescence, whereas, compounds 7g-i containing strong electron-withdrawing groups show
obvious blue shift (19-33 nm). All these compounds show large Stokes shifts (77-181 nm) which
is beneficial for the detection of emission wavelength by avoiding the interference from the
excitation wavelength, especially for 7c and 7d, the large Stokes shifts indicate a very efficient
intramolecular charge transfer (ICT) in the excited state between the terminal methoxy group or
NH group and the BSe unit [3h]. Compared with those corresponding BT-based compounds
2
which have the some structure with 7a and 7c [15], 7a and 7c exhibit about 20-60 nm redshift in
absorption and emission spectra. These results indicate that the π-conjugated organic compounds
are good candidates for wavelength-shifting material development.
3
.3 Fluorescence change of compounds 7a-i toward solve polarity
6

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D-π-A organic compounds often show solvatochromism which is based on the interaction
between the solvent molecules and a fluorophore, and solvent-dependent changes in the spectra of
these solvatochromic compounds provide a sensitive response to changes in the surrounding
environment [16]. Based on the polarity of organic solvent varied, the solvatochromic changes of
these compounds were studied. Herein, we firstly studied the solvatochromism of 7c. The UV-vis
and fluorescence spectra of 7c in various solvents, such as toluene, CHCl , THF, DCM, DMF,
3
acetone and CH CN, are shown in Fig. 5 and Fig. 6, and the absorption and emission data have
3
been listed in Table 2, respectively. The results indicate that compound 7c containing strong
electron-donating methoxy group presents similar absorption spectra in different polar solvents,
whereas, displays obvious red shift changes of fluorescence spectra from 553 nm to 604 nm as
increasing solvent polarity from toluene to CH CN. Visible fluorescence of 7c in various solvents
3
appears visual color change from green to red, which can be clearly observed under a 365 UV
lamp (Fig. 7). The results indicate 7c shows solvent polarity dependence.
There are several independent modes of fluorophore-solvent interaction identified for
solvatochromism, and one of the most used quantitative treatments of the solvent effects is the
E (30) scale introduced by Reichardt [17]. E (30) values were derived from the long-wavelength
T
T
vis/near-IR absorption band of a negatively solvatochromic pyridinium N-phenolate betaine dye.
The dependence of the emission maxima of 7c on E (30) solvent polarity parameter [18] can be
T
fitted to linear function and the correlation coefficients R = 0.89 (Fig. 8). As the solvent polarity
increased, a positive solvatochromic shift can be observed.
The influence of solvent polarity on the optical properties of a fluorophore can be studied by
2
3
Lippert-Mataga equation: ∆ν = νabs-νem = 2(µ -µ ) ∆f/hca +C, a model that describes the
e
g
interactions between the solvent and the dipole moment of a fluorophore [19]. In the equation, µe
and µ are the dipole moments in the excited and ground states, respectively, h is the Planck
g
constant, c is the speed of light, a is the radius of the fluorophore and ∆ν is Stokes shifts of
fluorescence spectra. ∆f is called the orientation polarizability, which can be expressed as
2
2
(
ε-1)/(2ε+1)-(n -1)/(2n +1), herein, ε and n are the dielectric constant and refractive index of
solvent, respectively. ∆f values for the various solvents could be calculated from known values of
ε and n (Table 2). The Lippert-Mataga plot of 7c is depicted in Fig. 9. It can be found that the plot
of ∆ν against ∆f in these different polar solvents has a positive correlation and is nearly linear with
correlation coefficient R = 0.92. The positive linear dependence suggests that the excited-state of
7
c has a larger dipole moment than the ground-state. The obvious solvatochromic redshift of 7c
can be attributed to ICT effect on the fluorescent emission, which leads to the decrease of the
energy level and solvent stabilization of excited state. This effect becomes larger as the solvent
polarity increases, resulting in the emission at a longer wavelength [20]. As a result, emissions of
7
c depend on the solvent polarity and the fluorescence color changes can be directly detected by
naked eyes under UV-lamp. We further studied the plots of ∆ν of the other compounds vs ∆f. The
UV-vis and fluorescence spectra of the other compounds in various solvents are shown in Fig. S1
7

ACCEPTED MANUSCRIPT
and Fig. S2 (See Supporting Information), and the absorption and emission data are listed in Table
S1 and Table S2 (See Supporting Information), respectively. The results indicate that the optical
properties of all of these compounds also show different degrees of positive solvent dependence
(
Fig. S3, see Supporting Information), and in these compounds, 7d exhibits the steepest slope,
indicating the largest fluorescence solvatochromism which could be due to the strongest electron
donating ability of NH group. Thus, we can construct colorful fluorophores based on BSe
2
derivatives which can form an ICT state that is normally sensitive to the solvent polarity, which
also implies these kinds of compounds might be applied to the development of efficient sensors
for the detection of volatile organic compounds (VOCs).
Considering not only the polarity but also the acidity and the basicity of the solvents, the
solvatochromic Stokes shifts and emission bands of 7c can be calculated based on Kamlet-Taft
*
equation [21]: Y = Y +aα+bβ+cπ . Herein, Y is the property of substance in the absence of
0
0
*
solvent e.g. in the gas-phase, α, β and π are the solvent parameters that characterize the acidity,
the basicity and polarity of the solvent, respectively. a, b and c are the corresponding coefficients.
*
The corresponding α, β and π values of various solvents used in this study for calculating the
emission wavenumbers at the maximum (ν ) and the Stokes shifts (∆ν) are given in Table S3 (See
em
Supporting Information). The Kamlet-Taft coefficients for νem and ∆ν can be obtained by
multilinear regression analysis. According to the regression analysis for νem and ∆ν, the
relationships can be established as follows:
-1
*
νem (cm ) = (19271±168)-(4181±436)α-(1173±167)β-(2150±289)π
-1
*
∆
ν (cm ) = (2642±404)+(5074±1052)α+(2087±404)β+(1508±697)π
For the sake of better visualization of the experimental and the fitted data, the calculated values
of νem and ∆ν are listed in Table 2 and plotted against measured values in Fig. 10, respectively. As
shown in Fig. 10, it can be found that the measured values for νem and ∆ν are correlated well with
those of the calculated by Kamlet-Taft equation, and the correlation coefficients R = 0.99 and 0.98,
respectively. As a result of these calculations, it is now evident that the Kamlet-Taft theory is
capable of describing the solvatochromic behavior of 7c in a wide variety of solvents.
3
.6. Molecular orbital calculations
We performed density functional theoretical (DFT) calculations to gain a better understanding
of the geometric, electronic structure and optical properties of the molecules presented in the study.
All calculations were carried out with the Gaussian 03, Revision C. 02 program [22], using the
Becke’s three-parameter set with Lee-Yang-Parr modification (B3LYP) with 6-31+G** basis set.
Fig. 11 displays the LUMO and the HOMO diagrams of compounds 7a-i and the calculated
HOMO, LUMO and E are listed in Table 1.
g
According to Fig. 11, the LUMO distributions of these compounds mainly reside on the central
BSe core, and the HOMO distributions are predominantly located on the BSe core and substituted
phenyl rings. Thus, the excited states of these compounds can be assigned to π-π* transition mixed
8

ACCEPTED MANUSCRIPT
with ICT process in this system. This is in accordance with the experimental results of the
dependence on the solvent polarity for the fluorescence spectra of these compounds. As seen from
Table 1, it can be found that the HOMO energy levels of 7b-d containing electron donor group are
higher than 7e-i containing electron acceptor group because of the electron-donating ability of the
donor unit, such as 7d and 7g show the highest HOMO energy (-4.86 eV) and the lowest HOMO
energy (-6.18 eV), respectively. It also can be found 7e-i have lower HUMO energy levels
compared to 7b-d because strong electron acceptor group can effectively reduce the energy of
LUMO, and in these compounds, 7g has lowest LUMO energy (-3.60 eV). The band gaps (E ) of
g
7
a-i are in the range of 2.21 eV and 2.70 eV, and 7d has the lowest band gap (2.21 eV) which can
be mainly attributed that NH group with strongest electron-donating ability greatly increases the
2
HOMO energy of the compound. The energy gaps obtained from the theoretical calculations and
UV-vis absorption maxima are described in Fig. 12. Apart from a slight variation for each
compound, there is a satisfactory correlation of the data from the theoretical calculations and
UV-vis absorption maxima in that all compounds show a similar trend except 7i.
2
+
3
.4. The fluorescence response of 7c on Hg
BT unit has been reported as a good receptor for the recognition of specific metal ion, such as
2+
2+
Hg and Ni in some previous reports [4]. In comparison with BT, BSe used as recognition
moiety might weaken the binding capacity to other metal ions except a specific metal ion, and
then improve the selectivity of the BSe-based fluorescent sensor. Herein, we studied the ability of
compound 7c to identify metal ions using BSe as recognition site. Compound 7c displayed orange
fluorescence centered at 575 nm when excited at λex = 458 nm in THF. In order to provide a
potential ‘‘zero-wait’’ detecting method for metal ions, we investigated the fluorescence response
behaviors of 7c upon the addition of various metal ions in real-time detection, all kinds of
measurements were monitored 10 s after addition of the metal salt to the host molecule solutions.
-5
-1
2+
Fig. 13 shows the fluorescence spectra of 7c in THF solution (1.0 × 10 mol·L ) with Hg in
-5
-1
CH CN concentrations from 0 to 4.5 × 10 mol·L . As is evident from Fig. 13, upon the addition
3
2+
2+
of Hg , obvious fluorescence quenching can be observed. Hg led to about 94% quenching of 7c
-5
-1
fluorescence at a concentration of 4.0 × 10 mol·L . In addition, the orange color of the
2
+
compound fluorescence disappeared after the addition of Hg , which could be easily detected by
naked eyes (Fig. 13, inset). The quenching effect could be attributed to the intramolecular
2
+
photoinduced charge transfer between 7c and the BSe-Hg complex. Notably, there exists a good
2
+
linear relationship between the changes in fluorescence intensity of 7c and the Hg concentration,
2
+
-5
-1
a curve equation is got in the range of Hg concentrations from 0 to 4.0×10 mol·L is F =
4
8
2+
2+
3
.43×10 -7.85×10 [Hg ], R = 0.997, N = 13, which indicates that Hg can be quantitative
detection (Fig. 14). According to the equation and the standard deviation of the blank, the
2
+
-7
-1
fluorescence detection limit of 7c for Hg was determined to be 5.0×10 mol·L . The quenching
efficiency of a sensor can be described by the Stern-Volmer equation: F /F = 1 + K [Q], herein,
0
SV
9

ACCEPTED MANUSCRIPT
F0 and F are the fluorescence intensities in the absence and presence of a quencher, respectively,
KSV is the quenching constant and [Q] is the quencher concentration. At low concentrations of
2+
-5
-1
Hg (0 to 1.0×10 mol·L ), a linear Stern-Volmer plot was obtained with the K value of
SV
5
-1
2
.46×10 mol ·L, however, at high concentrations, the plot was found to be non-linear, showing
positive curvature as show in Fig. 15. The nonlinearity suggests the mechanism of the quenching
process may involve a combination of complexation and collisional processes [23].
2
+
To test the selectivity of the sensor for Hg , the fluorescence response behaviors of 7c on other
-4
-1
important metal ions at a concentration of 2.0 × 10 mol·L were performed (Fig. 16). All these
metals induced little fluorescence change, the degree of fluorescence quenching (1-F/F ) of 7c
0
+
2+
2+
2+
2+
3+
2+
2+
were no more than 6% by other metal ions, such as K , Pb , Cu , Co , Cd , Fe , Mn , Ni ,
2
+
+
Zn and Ag at a high mole ratio of 4:1, which could be attributed to the poor coordination ability
of the compound with these metal ions and the unmatched radius of the metal ions. The results
2
+
2+
indicate 7c exhibits excellent selectivity for Hg sensing. For effective Hg detection, another
2
+
essential requirement is no or less interference from other metal ions. The Hg detection
-5
-1
experiments of the 7c solution were performed in the presence of 4.0 × 10 mol·L of other metal
ions (Fig. 17). The deviation of the fluorescence quenching in the presence of other metals ions
2
+
was less than 5%. The results demonstrated the Hg detection showed no significant interference
2
+
from other metal ions. The high sensitivity and selectivity toward Hg of the compound may be
attributed to several factors, such as the structural rigidity of the BSe unit, the larger radius of the
2+
2+
Hg ion and preferred coordination arrangement of Hg , soft acid/base characteristics of BSe and
2+
2+
Hg [24]. These results indicate that BSe unit could function as recognition moiety for Hg
probe with high selectivity and sensitivity.
3
.5. The fluorescence response of 7d on water
BSe unit also can be used as a fluorophore of D-π-A BSe-based fluorescent sensors for
detecting the other species by introducing some specific recognition sites on the parent structure.
Brown and Yuan reported that the fluorescent quantum yield and lifetime of
4
-aminonaphthalimide both decreased with the increased water content of the system, which can
be attributed to the formation of a hydrogen-bonded cluster between the probe and the water
molecules by NH group [25]. This fact indicates that 7d might be used as an efficient water
2
sensor, using amino group as an acceptor and BSe unit as a fluorophore. Herein, we investigated
the effects of the fluorescence response behaviors of 7d on water in real-time detection. Fig. 18
-5
-1
and Fig. 19 show the fluorescence spectra and intensity of 7d (1.0×10 mol·L in anhydrous THF)
upon the water content from 0 to 1.83% in THF, respectively. It could be found that the spectra
exhibited significant fluorescence quenching with an obvious redshift in the emission maxima as
the water content increased. When the water content was increased to be 1.83% (v/v), the degree
of fluorescence quenching was 81%, and the red color of 7d fluorescence disappeared (Fig. 18,
inset). When 7d was used to determine the water content, the changes of fluorescence intensity fit
1
0

ACCEPTED MANUSCRIPT
well with a Stern-Volmer equation as a function of the water content: F /F = 1 + 2.38 [H O],
0
2
herein, [H O] was the percentage of the volume of water and THF, and KSV was 2.38 (Fig. 20).
2
The results indicate that compound 7d containing amino group can be developed to be a good
fluorescent sensor for the quantitative detection of traces of water in organic solvent.
4
. Conclusion
A series of novel D-π-A BSe-based derivatives 7a-i with tuneable absorption and emission
wavelengths over a wide range by introducing strong electron-donating/electron-withdrawing
groups into a BSe unit were designed and synthesized. These compounds showed obvious
absorption peaks at the region from 429 to 488 nm, the fluorescence from green to red in the range
of 512-669 nm and high quantum yields. The HOMO-LUMO energy gaps of 7a-i obtained using
theoretical calculations were in the range of 2.21-2.70 eV, which were in good agreement with
those derived from the absorption of UV-vis spectra. Compound 7c containing methoxy group
exhibited positive solvatochromism, which indicates these kinds of arylethynylene materials
containing BSe might be used for solvent polarity sensors. In addition, fluorescent sensors 7c and
7
d showed highly sensitive response for mercury ion and traces of water using BSe as an acceptor
or a fluorophore in real-time detection, respectively. These findings indicate that BSe-based
molecules can be developed as excellent fluorophores for fluorescent material applications.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21204066),
Commonweal Project of Science and Technology Department of Zhejiang Province (No.
2
012C23030, 2013C33188) and the Bureau of Science and Technology of Wenzhou (No.
S20100007).
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Figure Captions
Fig. 1. TGA curves of compounds 7a-i.
-5
Fig. 2. UV-vis absorption spectra of compounds 7a-i (1×10 mol/L in CHCl ).
3
Fig. 3. Fluorescence spectra of compounds 7a-i in CHCl3.
Fig. 4. Fluorescence photos of 7a-i in CHCl under a 365 nm UV lamp.
3
Fig. 5. UV-vis spectra of 7c in various solvents.
Fig. 6. Fluorescence spectra of 7c in various solvents.
Fig. 7. Fluorescence photos of 7c in different solvents under a 365 nm UV lamp: from left to right:
toluene, CHCl , THF, DCM, DMF, Acetone and CH CN.
3
3
Fig. 8. Polt of emission maxima of 7c vs E (30) solvent polarity parameter of various solvents.
T
Fig. 9. Polt of Stokes shifts (∆ν) of 7c vs orientation polarizability (∆f ) of various solvents.
Fig. 10. Plots of the measured and calculated emission maxima (a) and Stokes shifts (b) for 7c
obtained by the Kamlet-Taft equation.
Fig. 11. Molecular orbital diagrams for the LUMO (up) and HOMO (down) of compounds 7a-i
from DFT calculations.
Fig. 12. Comparison of experimental optical energy gaps (from UV-vis spectra) with theoretical
values.
-5
-1
Fig. 13. Fluorescence spectra of 7c (1.0 × 10 mol L ) in THF with increasing concentration of
2+
-5
-1
Hg (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 × 10 mol·L in CH CN) (λ = 458
3
ex
2
+
-5
nm). Inset: visible fluorescence of 7c before (left) and after (right) the addition of Hg (4.0 × 10
-1
mol·L ) under a 365 nm UV lamp.
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Fig. 14. Calibration curve was obtained from the plot of fluorescence intensity of 7c with the
2
+
-5
-1
added Hg from 0 to 4.5 × 10 mol·L .
2
+
Fig. 15. Stern-Volmer plots of 7c emission quenched by Hg . Inset: a linear Stern-Volmer plot
2
+
-5
-1
was obtained at low concentrations of Hg (0 to 1.0×10 mol·L ).
-5
-1
2+
Fig. 16. Fluorescence quenching degree of 7c (1.0 × 10 mol·L ) in the presence of Hg (4.0 ×
-5
-1
-5
-1
1
0 mol·L ) and other metal ions (each 4.0 × 10 mol·L ).
-5
-1
2+
Fig. 17. Metal specificity: the concentration of 7c was 1.0×10 mol·L , the concentration of Hg
-5
-1
-4
-1
was 4.0×10 mol·L and the other metal ions were used at 4.0×10 mol·L . Mix: the mixture of
+
2+
2+
2+
2+
3+
2+
2+
2+
+
K , Pb , Cu , Co , Cd , Fe , Mn , Ni , Zn and Ag .
-5
-1
Fig. 18. Fluorescence spectra of 7d (1.0×10 mol·L ) in anhydrous THF with increasing water
content (0, 0.033, 0.067, 0.1, 0.133, 0.167, 0.233, 0.333, 0.5, 0.667, 0.083, 1, 1.17, 1.5, 1.83%, v/v)
(λex = 502 nm, slit width (EX/EM): 5 nm/5 nm). Inset: Visible fluorescence of 7d before (left) and
after (right) the addition of water (water content: 1.83%) under a 365 nm UV lamp.
Fig. 19. Fluorescence intensity of 7d vs the water content.
Fig. 20. Stern-Volmer plot of 7d in THF emission quenched by water.
Scheme 1. Synthesis procedures of BSe derivatives 7a-i.
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Table Captions
Table 1. Photophysical properties of compounds 7a-i in CHCl3.
Table 2. UV-vis absorption maxima and fluorescence emission maxima of 7c and solvent polarity
parameter in different solvents.
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Table 1. Photophysical properties of compounds 7a-i in CHCl3.
max
max
em
op
[c]
Compound
6
λ
abs
λ
(λex
)
Stoke’s
E
g
HOMO
LUMO
E
g
Ф
PL
[a]
[b]
[b]
[b]
(
nm)
(nm)
shift (nm)
(eV)
(eV)
(eV)
(eV)
-
393
444
452
465
488
443
443
429
432
435
464(373)
546(448)
563(450)
595(460)
669(489)
546(445)
543(446)
527(438)
518(434)
512(428)
71
102
111
130
181
103
100
98
2.85
2.48
2.42
2.34
2.20
2.48
2.49
2.55
2.57
2.53
-
-
-
7
a
-5.65
-5.50
-5.29
-4.86
-5.77
-5.90
-6.18
-6.17
-5.87
-3.08
-2.99
-2.90
-2.65
-3.21
-3.32
-3.60
-3.52
-3.17
2.57
0.20
0.37
0.17
0.04
0.25
0.27
0.31
0.22
0.27
7
b
2.51
2.39
2.21
2.56
2.58
2.58
2.65
2.70
7
c
7
d
7
e
7
f
7
7
g
h
86
7
i
77
op
[
[
[
a] Eg was calculated from the absorption of UV-vis spectra: band gap energy [eV] = 1240/wavelength [nm].
b] DFT quantum mechanical calculations (B3LYP/6-31+G**).
c] Quantum yields were determined with the quinine sulfate solution as a fluorescence reference (Ф = 0.55 in 0.5
r
mol/L H SO ).
2
4
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Table 2. UV-vis absorption maxima and fluorescence emission maxima of 7c and solvent polarity
parameter in different solvents.
Toluene
CHCl
3
THF
DCM
DMF
Acetone
3
CH CN
λ
abs/nm
459
21786
553
465
457
21882
575
460
21739
586
455
21978
604
452
22124
583
451
22173
604
-1
νabs/cm
21505
584
λ
em/nm
-
1
ν
em/cm
18083
18088
3703
3610
0.0135
33.9
16807
16834
4698
4906
0.155
39.1
17391
17443
4491
4619
0.210
37.4
17065
17041
4674
4611
0.218
40.7
16556
16570
5422
5409
0.276
43.2
17153
17040
4971
4985
0.284
42.2
16556
16588
5617
5436
0.305
45.6
-
1 [a]
ν
em/cm
-1
∆
ν/cm
-
1 [a]
∆
ν/cm
∆
f
-1 [b]
T
E (30)/kcal mol
[
a] Obtained by the Kamlet-Taft equation.
b] The values were taken from the reference [18].
[
3

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