A new solvatochromic linear π-conjugated dye based on phenylene-(poly)ethynylene as supersensitive low-level water detector in organic solvents

Article abstract of DOI:10.1016/j.dyepig.2016.09.055

A novel class of phenylene-(poly)ethynylene-based dye Sensor 1 with remarkable properties of solvatochromism has been designed and synthesized. A long, coplanar and asymmetry electron-rich π-conjugated structure endows Sensor 1 with maximum solvatochromic

Full text of DOI:10.1016/j.dyepig.2016.09.055

Accepted Manuscript
A new solvatochromic linear π-conjugated dye based on phenylene-(poly)ethynylene
as supersensitive low-level water detector in organic solvents
X.L. Guan, T.M. Jia, D.H. Zhang, Y. Zhang, H.C. Ma, D.D. Lu, S.J. Lai, Z.Q. Lei
PII:
S0143-7208(16)30375-8
10.1016/j.dyepig.2016.09.055
DYPI 5507
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 2 August 2016
Revised Date: 21 September 2016
Accepted Date: 23 September 2016
Please cite this article as: Guan XL, Jia TM, Zhang DH, Zhang Y, Ma HC, Lu DD, Lai SJ, Lei ZQ, A new
solvatochromic linear π-conjugated dye based on phenylene-(poly)ethynylene as supersensitive low-
level water detector in organic solvents, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.09.055.
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ACCEPTED MANUSCRIPT
Graphical abstract
A novel solvatochromic dye Sensor 1 was successfully used for qualitative and
quantitative detection of trace water in five organic solvents.

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A new solvatochromic linear π-conjugated dye based on
phenylene-(poly)ethynylene as supersensitive low-level water
detector in organic solvents
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X. L. Guan,a* T. M. Jia, a D. H. Zhang,a Y. Zhang,a H. C. Ma,a D. D. Lu,a S. J. Lai b and
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Z. Q. Leia*
a Key Laboratory of Eco-Environment-Related Polymer Materials Ministry of Education, Key
Laboratory of Polymer Materials Ministry of Gansu Province, College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou, Gansu 730070, P.R. China.
b School of Chemical Engineering, Lanzhou University of Arts and Science, Lanzhou, Gansu
730000, P.R. China.
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12 Abstract
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A novel class of phenylene-(poly)ethynylene-based dye Sensor 1 with remarkable properties of
solvatochromism has been designed and synthesized. A long, coplanar and asymmetry
electron-rich π-conjugated structure endows Sensor 1 with maximum solvatochromic effect by
intramolecular charge-transferstates (ICT). More importantly, The dye is found to serve as
supersensitive fluorescent indicator for the quantitative detection of low-level water content in five
common organic solvents with low detection limits. Furthermore, the development of
easy-to-prepare test papers provides a convenient and reliable approach to qualitatively monitor
water in organic solvents by naked eye. Results in this work not only provide a successful
molecular design concept of the novel sensitive solvatochromic dye sensor for water in organic
solvents, but also develop practical applications of the linear π-conjugated molecule bearing
phenylene-(poly)ethynylene in chemosensor field.
24 Keywords
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π-conjugated dye; phenylene(poly)ethynylene; solvatochromic sensor; water detector; organic
solvents
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*corresponding author ; fax: 8609317971989
E-mail address:guanxiaolin@nwnu.edu.cn.( X. L. Guan)
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29 1. Introduction
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Fluorescence sensing is the key detection method in chemistry and cell biology due to the
advantage of rapidity, sensitivity and low background noise[1-5]. Therefore, the development of
new photoluminescent (PL) responsive compounds based sensing platform is of great significance.
Of the fluorescent probes, solvatochromic probes, which exhibits different emission and
absorption spectra depending on solvent polarity, represents a fascinating class of organic dyes
and particularly useful for chemical and biochemical research in complex systems[6-8]. For
example, some solvatochromic dyes had been potentially used in the investigation of solvent
properties[9,10] and protein folding and protein-protein interactions in the field of proteomics[11]
as well as been applied in vitro and in vivo as sensitive sensors of lipid organization and dynamics
in cellular membranes[12]. During the past few decades, some topical dyes and their derivatives
have been studied as a solvatochromic fluorophore, such as naphthalimide, thioxanthone
fluorophores and Prodan[13-15]. However, reports of new family of dyes displaying excellent
solvatochromism are still quite limited. So, the development of novel solvatochromic dyes with
probably higher sensitivity is a huge challenging task nowadays.
To induce the maximum sensitivity and response to external stimuli of solvatochromic
fluorophore, there are several molecular key features to be considered, including degree of
polarization, coplanar π-conjugated bridges and push-pull charge-transfer(CT) character[7, 16-18].
It is essential to possess-high degree of polarization during the design of suitable small molecules
for solvatochromic probe. Introduction of polar functionality groups in molecule, such as amino,
cyano, carbonyl, nitro, or heteroaryl groups helps to acquire high polarization. In addition, a high
level of conjugated π-system with backbone coplanarity is a central design principle for
developing top-performing solvatochromic dye. The use of phenylene-(poly)ethynylene as spacer
is an effective means to improve the spectral sensitivity of solvatochromic dye due to its
π-conjugated rigid rod-like conformation, structural planarity, high thermal stability and good
optical properties. Meanwhile, the coplanar π-conjugated bridges bearing arylene ethynylene
could lead to extended charge separation, which is beneficial to enhance the efficiency of
intramolecular charge-transfer (ICT)[19,20]. Recently, commercially available naphthalene-based
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dyes are widely used as solvatochromic probe due to their desirable photophysical properties such
as microenvironment polarity-dependent emission wavelength changes, high quantum yield, and
good photostability[6, 13, 15]. For these aforementioned reasons, we conceived that the covalent
linkage of phenylene-(poly)ethynylene with naphthalene groups would produce top-performing
solvatochromic probe. Nonetheless, the application of long π-conjugated dyes with
naphthalene-containing phenylene-(poly)ethynylene strand has been mainly limited to
optoelectronic devices, with very few attempts of the implementation in solvatochromic probe
field.
Determination of trace water content in organic solvents is very important due to its crucial
applications in routine chemical processes and environmental monitoring industries[21-24]. For
example, trace amounts of water in organic solvents is able to quench or inhibit some reactions,
such as Friedel-Crafts reaction, Claisen Condwnsation reaction and Witting reaction. In the
laboratory scale, Karl-Fischer coulometric titration method has been widely used for measuring
water content of organic solvents[25]. However, the traditional method is of some disadvantages
such as the requirements for skilled personnel, long measuring time and interference from
co-existing species. By comparison, the use of optical water probes based on solvatochromic dye
has been indicated to determinate trace water content in organic solvents successfully with the
greater sensitivity, lower detection limits and non-destructive properties [26-29].
Herein, aiming at obtaining novel fluorescent sensor that exhibits an excellent
solvatochromic effect and possesses high sensitivity and a large response to external stimuli, we
designed and synthesized the first reported organic dye Sensor 1 bearing electron-rich
π-conjugated naphthalene and phenylene-(poly)ethynylene strands using palladium-mediated
(Sonogashira) coupling chemistry. Our strategy was based on the following design principles:
modication of naphthalene with amino-substituted phenylene-(poly)ethynylene would help to
acquire excellent optical performance based on the long, coplanar and asymmetry electron-rich
π-conjugated structure and bestow the dye with a maximum solvatochromic effect by a ready
intramolecular charge-transferstates (ICT) process[19, 20]. Meanwhile, the addition of a long
alkyl chain on molecule stucture can optimize its solubility. Spectral investigations showed that
Sensor 1 exhibited apparent solvatochromism effect that originates from ICT character, which is
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supported by theoretical calculations. More importantly, Sensor 1 could also be used as
supersensitive fluorescence indicator for the detection of low-level water content in five common
organic solvents with low detection limits of 0.059%-0.400% (v/v). Moreover, some test papers
loaded by Sensor 1 were also prepared to emphasize the practical application of qualitative
detection of low-level water content in five common organic solvents. To the best of our
knowledge, there are almost no reports on phenylene-(poly)ethynylene-based dye acting as
suitable water indicators in common organic solvents.
94 2. Experimental
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2.1. Materials and methods
5-bromo-2-iodotoluene, 6-hydroxy-2-naphthaldehyde, dichlorobis(triphenylphine)palladium
[Pd(PPh3)2Cl2], 4-ethynylaniline, CuI and triphenylphosphine(PPh3), N-bromosuccinimide(NBS),
tetrabuthyl ammonium bromide ((C4H9)4NBr), 2-methyl-3-butyn-2-ol were purchased from Alfa
Acesar and used without further purification. Triethylamine (Acros, 99%) was heated under reflux
over calcium hydride (CaH2) for at least 8 h and distilled before use. Benzoyl peroxide (BPO) was
purified by recrystallization from methanol. Toluene and tetrahydrofuran (THF) were pre-dried
over 4 Å molecular sieves and distilled under nitrogen atmosphere from sodium and
benzophenone immediately prior to use; N,N-dimethyl formamide (DMF) was pre-dried over 4 Å
molecular sieves and distilled at reduced pressure; Dichloromethane and acetonitrile were heated
at reflux over CaH2 and phosphorus pentoxide (P2O5), respectively, and then distilled prior to use;
Acetone was pre-dried over calcium sulphate (CaSO4), then decanted and distilled prior to use.
Other solvents were HPLC grade and dried with 4 Å molecular sieves prior to use. Doubly
distilled water was used throughout. The water contents in the prepared pure organic solvents
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were standardized with the Karl-Fischer method. 1H NMR and 13C NMR spectra were recorded on
a Brucker AM 600MHz spectrometer at room temperature, using deuterated chloroform (CDCl3)
and dimethyl sulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as the internal
standard. Mass spectral data were obtained using a Hewlett-Packard 5989A mass spectrometer.
High-resolution mass were recorded with a Kratos MS50TC instrument. The UV-vis absorption
spectra were recorded on a Shimadzu UV-2550 spectrophotometer. The fluorescence
measurements were carried out using an F97Pro fluorescence spectrophotometer equipped with a
Xenon lamp, a recorder, and dual monochromators detector at room temperature.
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2.2. Synthesis
2.2.1 Synthesis of (5-bromo-2-iodophenyl)methanol (compound 4)
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A mixture of 5-bromo-2-iodotoluene (25.0 g, 0.1 mol), NBS (36.6 g, 0.1mol), and BPO (0.6
g, 2.6 mmol) were dissolved in 200 mL of CCl4 and refluxed. The mixture was allowed to stir
until the completion of the reaction, which was confirmed by thin layer chromatography (TLC)
analysis (7 h). When the reaction ended, a large amount of the floating succinimide produced in
the reaction systems was filtrated off. The filtrate was evaporated to remove the solvent. The
crude product was purified by column chromatography on silica with petroleum ether as the eluent
1
to give 16.3 g of a white solid 5-bromo-2-iodobenzyl bromide. Yield:50% ; H NMR (600 MHz,
CDCl3): δ= 4.52 (s, 2H, Ph-CH2), 7.21-7.59(m, 3H, Ar-H) ppm.
The mixture solution of 5-bromo-2-iodobenzyl bromide (16.0 g, 50.0 mmol), NaAc (4.9 g,
60.0 mmol), tetrabutylammonium bromide (1.6 g, 5.0 mmol), and 300 mL distilled water were
stirred and heated to 110 ◦C. After refluxing for 20 h, 30% NaOH (8 mL) was added into the above
solution (pH value was controlled at 9-10). The reaction was continued at 110 ◦C for 24 h. A
precipitate was produced when the reaction mixture was cooled. After washed with water, the
crude product was purified by column chromatography on silica with as the petroleum ether/ethyl
1
acetate (3:1) as eluent to give 12.5 g of a white solid. Yield:95% ; H NMR (600 MHz, CDCl3):
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δ=2.03 (s, 1H, -OH), 4.63 (s, 2H, Ph-CH2-), 7.12-7.67 (m, 3H, Ar-H) ppm (Fig. S1). HRMS(EI,
DIP); m/z calcd. for C7H6BrIO [M+H]) +: 314.86, found 314 (Fig. S2).
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2.2.2 Synthesis of (2-(2-(4-aminophenyl)ethynyl)-5-bromophenyl)methanol
(compound 3)
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Solution of compound 3 (5.00g, 0.04 mol) dissolved in dry triethylamine (100 mL) was
added dropwise to a suspension of compound 2 (12.52 g, 0.04 mol), Pd(PPh3)2Cl2 (0.63 g, 0.9
mmol), triphenylphosphine (0.89 g, 3.4 mmol), and copper(I) iodide (0.29 g, 1.7 mmol) in dry
triethylamine (200 mL) at room temperature under nitrogen. The mixture was stirred at near 80 ◦C
for 24 h. The mixture was cooled to room temperature and filtered to remove the insoluble salt
(triethylamine hydrobromide) formed during the reaction. The filtrate was washed with ether.
After concentrated, the product was purified by column chromatography [silica gel, petroleum
ether: ethyl acetate (v:v) = 2:1] to give 6.3 g of a yellowish solid. Yield:55% ; 1H NMR (600 MHz,
DMSO-d6): δ= 7.62 (s, 1H), 7.41 (dd, J = 8.2, 1.9 Hz, 1H), 7.32 (d, J = 8.2 Hz, 1H), 7.19 (d, J =
8.4 Hz, 1H), 6.55 (d, J = 8.5 Hz, 1H), 5.58 (s, 1H), 5.42 (t, J = 5.7 Hz, 1H), 4.65(d, J = 5.6 Hz,
1H)ppm (Fig. S3). HRMS(EI,DIP); m/z calcd. for C15H12BrNO [M+H]+: 301.01, found 301 (Fig.
S4) .
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2.2.3 Synthesis of (2-(2-(4-aminophenyl)ethynyl)-5-ethynylphenyl)methanol
(compound 2)
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Solution of 2-methyl-3-butyn-2-ol (0.67 g, 7.97 mmol) dissolved in dry triethylamine (30
mL) was added dropwise to a suspension of compound 4 (2.4 g, 7.97 mmol), Pd(PPh3)2Cl2 (0.33 g,
0.47 mmol), triphenylphosphine (0.24 g, 0.93 mmol), and copper(I) iodide (0.12 g, 0.69 mmol) in
dry triethylamine (180 mL) at room temperature under nitrogen. The mixture was stirred at near
◦
80 C for 24 h. The mixture was cooled to room temperature and filtered to remove the insoluble
salt (triethylamine hydrobromide) formed during the reaction. The filtrate was washed with ether.
After concentrated, the product was purified by column chromatography [silica gel, petroleum
ether: ethyl acetate (v:v) =1:1] to give 1.8 g of a yellowish solid. Yield :60% ; 1H NMR (600 MHz,
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CDCl3): δ=7.55 (d, J = 7.8 Hz, 3H), 7.44 (s, 3H), 7.40 (s, 1H), 6.68 (d, J = 7.8 Hz, 2H), 5.45 (s,
1H), 3.66 (s, 2H), 3.14(s, 1H)ppm (Fig. S5). 13C NMR (150 MHz, CDCl3): δ= 153.15, 144.34,
138.76, 135.68, 132.02, 128.99, 124.81, 121.41, 119.28, 115.15, 97.83, 83.51, 78.04, 74.19ppm
(Fig. S6). HRMS(EI,DIP); m/z calcd. for C17H13NO [M+H]+: 247.10, found 247 (Fig. S7).
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2.2.4 Synthesis of 2-(2-bromoethynyl)-6-(hexyloxy)naphthalene
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2-(2-bromoethynyl)-6-(hexyloxy)naphthalene was prepared following the reported
procedures[30]. 1H NMR (600 MHz, CDCl3): δ= 7.89 (s, 1H), 7.67 (d, J = 8.9 Hz, 1H), 7.63 (d, J
= 8.5 Hz, 1H), 7.43 (d, J = 8.5 Hz, 1H), 7.15 (dd, J = 8.9, 2.4 Hz, 1H), 7.08 (s, 1H), 4.06 (t, J =
6.6 Hz, 2H), 1.90-1.79 (m, 2H), 1.50 (dt, J = 14.6, 7.2 Hz, 2H), 1.37 (dd, J = 8.7, 5.4 Hz, 4H),
0.92(s, 3H)ppm (Fig. S8). 13C NMR (150 MHz, CDCl3): δ=157.97, 134.35, 131.91, 129.18,
128.88, 128.14, 126.69, 119.76, 117.37, 106.28, 80.60, 68.11, 48.88, 31.58, 29.15, 25.75, 22.59,
14.01ppm (Fig. S9).
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2.2.5 Synthesis of
(2-(2-(4-aminophenyl)ethynyl)-5-(4-(2-(hexyloxy)naphthalen-6-yl)buta-1,3-diynyl)phe
nyl)methanol (Sensor 1).
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Compound 6 (4.80 g, 14.5 mmol), triphenylphosphine (PPh3, 0.477 g, 1.82 mmol), CuI
(0.270 g, 1.42 mmol), and PdCl2(PPh3)2 (0.633 g, 0.902 mmol) in 100 mL of dry triethylamine
was degassed with Ar. Then 3.00 g (12.2 mol) of compound 5 was added and refluxed for 24 h.
The mixture was cooled to room temperature and filtered to remove the insoluble salt
(triethylamine hydrobromide) formed during the reaction. The filtrate was washed with ether.
After concentrated, the crude product was purified by column chromatography on silica with as
the petroleum ether/dichloromethane (1:1) as eluent to give 1.87g of a yellowish solid. Yield
40% ;1H NMR(600MHz,CDCl3): δ= 7.99 (s, 1H), 7.68 (dd, J = 22.9, 8.6 Hz, 3H), 7.56 (d, J = 8.4
Hz, 1H), 7.49 (d, J = 8.1 Hz, 2H), 7.46 (d, J = 11.3 Hz, 2H), 7.17 (d, J = 9.8 Hz, 1H), 7.09 (s, 1H),
6.69 (d, J = 8.4 Hz, 2H), 5.48 (s, 2H), 4.08 (t, J = 6.5 Hz, 2H), 1.88-1.80 (m, 2H), 1.51 (s, 2H),
1.37 (s, 4H), 0.92(s, 3H)ppm (Fig. S10). 13C NMR (150 MHz, DMSO-d6): δ=158.50, 151.92,
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147.36, 139.49, 136.62, 135.08, 133.88, 133.29, 132.85, 131.88, 129.94, 129.43, 129.05, 127.79,
126.09, 120.34, 120.02, 115.37, 114.34, 107.29, 99.44, 83.72, 82.11, 74.38, 68.18, 31.37, 28.91,
25.60, 22.49, 14.28ppm (Fig. S11). HRMS(EI, DIP); m/z calculated. for C35H31NO2 [M+Na]+:
519.24, found 519.26 (Fig. S12).
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2.3. Computational methods
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We employed density functional theory (DFT) calculations to optimize the molecule geometry
at B3LYP/6-31G (d, p)[31, 32] computational level. At the optimized geometry, frequency
analysis was performed to confirm that the geometry is true minima on the potential energy
surface. The geometry of the first excited state (S1) was optimized by time-dependent (TD) DFT
calculations, using the hybrid B3LYPfunctional and the 6-31+G(d,p) basis set. Solvent effects of
methanol were taken into account by the polarizable continuum model (PCM)[33] in TD-DFT
calculations. All calculations were carried out using the Gaussian09 program package.
2.4. Fluorescent Operating procedure
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Stock solutions of fluorescence probes (1.00 × 10-3 mol/L) were prepared by dissolving 5 mg
of sensor 1 in 10 mL of dichloromethane. The sample solutions with different water contents in
the pure organic solvents were freshly prepared prior to each measurement according to the
following procedure. A 50 µL portion of stock solutions (1.00 ×10-3 mol/L) was poured into a 5
mL volumetric flask and evaporated to dryness. After addition of some specified organic solvent,
a certain bulk volume of doubly distilled water was added by micro or macro transfer pipettes.
The solution was then diluted with the above organic solvent to volume and mixed evenly.
2.5. Preparation of the dyestuff test paper
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The dyestuff test paper was prepared according to the following procedure. Some pieces of
filter paper were cut into fixed shape (3.5 cm × 1.2 cm rectangular), and then soaked in the stock
solutions of fluorescence probes (1.00 × 10-3 mol/L) for 10 minutes. These test papers which
absorbed a small amount of Sensor 1 were dried in an infrared drying oven and stored for use.
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219 3. Results and discussion
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3.1. Synthesis route
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The synthetic route of Sensor 1 is illustrated in Scheme 1. Firstly, compound 4 was
synthesized by two steps of the bromination reaction by using NBS and the alcoholization reaction
in the presence of base by using 5-bromo-2-iodotoluene as the intial material. Then, compound 3
and compound 2 were obtained by the Sonogashira coupling reaction and alkynylation reaction,
respectively. Finally, Sensor 1 was obtained by Sonogashira coupling reaction between compound
2 and 2-(2-bromoethynyl)-6-(hexyloxy)naphthalene in Et3N. The structures of compound 4,
1
compound 3, and compound 2 and Sensor 1 were confirmed by H NMR, 13C NMR and HRMS
spectroscopy (Supporting information, Figs. S1-S12).
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OH
NBS, BPO (C4H9)4NBr
Br
I
+
Br
I
NH2
CCl4
NaOH,NaAc
4
OH
CuI,PPh3,Et3N
Pd(Ph3)2Cl2
CH CC(CH3)2OH
Pd(Ph3)2Cl2
KOH
Br
NH2
Toluene
3
OH
Br
+
CuI,PPh3,Et3N
Pd(Ph3)2Cl2
NH2
OC6H13
2
OH
C6H13
O
NH2
Sensor 1
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Scheme 1 Chemical structures and synthetic routes of fluorescence Sensor 1
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3.2. Solvatochromism of Sensor 1
Since the photophysics of amino-substituted phenylene-(poly)ethynylene in solution is
strongly dependent on the solvent polarity, the absorption and fluorescence spectra of Sensor 1 in
different solvents were carried out and some parameters of frequently-used organic solvents and
the solution spectral properties are listed in Table 1. As shown in Fig. S13, the absorption
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spectrum of Sensor 1 display negligible changes in different solvents. In contrast, as the solvent
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polarity increased from toluene to DMF, the fluorescence emission peaks of Sensor 1 gradually
red-shifted from 486 nm to 589 nm, exhibiting a remarkable bathochromic effect (Fig. 1a). At the
same time, a broadening of the emission bands and an evident reduction in fluorescence quantum
yield of Sensor 1 from 32.0 % in apolar toluene to 1.6% in highly polar DMF were observed in
Table 1. Obviously, Sensor 1 exhibited dramatic fluorescent color variations under UV irradiation
with 365 nm light from blue to orange by changing the solvent from toluene to DMF (Fig. 1b),
which were consistent with the fluorescence spectra. Results shown in the absorption and
fluorescence spectra can be explained with stabilization of the charge-separated excited states of
aminoacetylenes by irradiation under UV-light. A positive charge on the amino group and a
negative charge on Ph-(C≡C)2- and naphthalene moieties of Sensor 1 in the excited state cause
intramolecular charge transfer and the charge-separated excited state can be stabilized more
efficiently than the ground state in polar solvent, which leads to the bathochromic shift of
emission maximaz [19,20].
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Table 1. Spectral properties of Sensor 1 in various solvents
Solvent
Toluene
TETN
ε a
n b
∆f c
λem/nmd
486
496
516
519
539
543
550
535
589
572
564
583
QY/%e
32.0
2.37
1.496
1.401
1.447
1.353
1.371
1.372
1.404
1.424
1.431
1.358
1.361
1.344
0.0126
0.0494
0.1482
0.1668
0.2005
0.2020
0.2107
0.2276
0.2749
0.2846
0.2884
0.3055
2.44
15.3
19.0
39.0
19.3
3.0
Chloroform
Ether
4.81
4.34
EtOAc
Acetic acid
THF
6.02
6.15
7.58
25.1
19.2
1.6
DCM
10.37
36.70
20.70
24.30
37.50
DMF
Acetone
Ethanol
MeCN
3.8
1.4
3.1
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MeOH
32.70
1.328
0.3086
567
1.6
b
c
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aε, solvent dielectric constant. n, index of refraction. ∆f = (ε-1)/(2ε+1)-(n2-1)/(2n2+1) accounts for the spectral
d
shifts due to reorientation of the solvent molecules, called the orientation polarizability. λem, emission maximum
e
wavelength. QY, fluorescence quantum yield [estimated by using Rhodamine B as standard in ethanol]. MeOH,
methyl alcohol; MeCN, acetonitrile; DMF, dimethylformamide; DCM, dichloromethane; THF, tetrahydrofuran;
EtOAc, ethyl acetate; TETN, triethylamine.
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Fig. 1. (a) Fluorescence spectra of Sensor 1 in organic solvents of different polarity. (b) Fluorescence photographs
of Sensor 1 in organic solvents of different polarity. (c) Fluorescence photographs of test papers after being soaked
in organic solvents of different polarity. MeOH, methyl alcohol; MeCN, acetonitrile; DMF, dimethylformamide;
DCM, dichloromethane; THF, tetrahydrofuran; EtOAc, ethyl acetate; HAc, acetate acid; TETN, triethylamine.
Concentration: 10 µM ; λex = 403 nm
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The solvatochromism of Sensor 1 is evaluated by using Lippert-Mataga equation[34], which
relates the Stokes shift (∆υ) to solvent orientation polarizability (∆f ), in eleven solvents with a
broad range of polarity. It is found that the fluorescence emission shows strong solvent-polarity
dependence with a slope of 12827.1 cm-1 in the ∆υ-∆f plots (Fig. 2). The extra large slope of the
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fitting line implies that Sensor 1 possesses strong solvatochromism, which provides a strong
evidence for Sensor 1 as a suitable solvent polarity sensor.
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Fig. 2 Stokes shifts (νabs-νfl) of Sensor 1 vs ∆f . The results of linear fit for Sensor 1: νabs-νfl =38115+12827∆f
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(R2=0.9595)
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3.3. Theoretical study
To correlate the influence of the geometric and electronic structure of Sensor 1 on
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solvatochromism well, theoretical calculation has also been conducted by the density functional
theory (DFT) method using Gaussian 09 programs[35]. As shown in Fig. 3, the highest occupied
molecular orbitals (HOMO) for Sensor 1 is delocalized over the whole molecule except the alkyl
chain part and the lowest unoccupied molecular orbital (LUMO) is localized predominantly on the
Ph-(C≡C)2- unit. What's more, a visible separated charge state can be observed from HOMO to
LUMO +1, which indicated that Sensor 1 has an obvious tendency to ICT. As a result, the
excitation of the ICT state induced a charge shift from the aminobenzene to the
phenylene-diethynylene part. Meanwhile, the estimated dipole moment of first excited state (3.37
Debye) is the double of the ground-state (1.58 Debye) of Sensor 1 (Table 2), which could cause
significant solvatochromism. Because the energy of the excited state can be reduced by the
interaction between dipole moment and the polar solvent molecules, the emission of Sensor 1 is at
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lower energies or longer wavelengths with the solvent polarity increased. TDDFT calculation
suggests that the oscillator strength of HOMO → LUMO is very high and the second lowest
excited state (S2) of Sensor 1 exhibits slightly larger excitation energy than the S1 state (Table 3).
The conclusion of calculation proves that the vertical transitions at optimized S1 geometry take
place between HOMO and LUMO, which conforms the experimentally observed the energy of the
emission wavelengths.
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Fig. 3. HOMO and LUMO orbitals for Sensor 1 at optimized geometries
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Table 2. Computed energy levels of frontier molecular orbitals and HOMO-LUMO energy gap at the ground state
(S0) and the first excited state (S1) geometries.
State
S0
HOMO / eV
-5.24
LUMO / eV
-2.04
△E / eV
3.20
Dipole moment / Debye
1.58
3.37
S1
-4.94
-2.20
2.74
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Table 3. Excitation energy, oscillator strength and molecular orbital (MO) compositions from TDDFT calculations
at the S1 geometry.
State
S1
Excitation energy
Oscillator strength
MO composition
2.65 eV, 467.36 nm
2.8819
H-0 → L+0 (97%)
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S2
3.21 eV, 385.74 nm
0.0732
H-1 → L+0 (97%)
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3.4. Quantitative detection of water content in organic solvents
It is interesting to note that the fluorescence emission of Sensor 1 is highly sensitive to solvent
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polarity according to the above discussions. To gain insight into the general signaling behaviors of
the prepared Sensor 1 to detect water content, we measured spectroscopic behaviors in five
frequently-used water-miscible organic solvents, including tetrahydrofuran (THF),
dimethylformamide (DMF), acetone, ethanol and acetonitrile (MeCN). Herein, THF was
considered as the representative for discussing the spectral changes induced by the presence of
water because of the similar curve trends of the above five solvents. In Fig. S14, the absorption
spectra of Sensor 1 in THF underwent slight changes from pure THF to THF containing 10% (v/v)
water, indicating that both solvent polarity and hydrogen bonding render no significant influence
on the absorption spectra. In comparison, the fluorescence intensities of Sensor 1 decreased
dramatically with small bathochromic shift upon the addition of different amounts of water to
THF (Fig. 4a and Fig. 4a’). The observed solvatochromic shift and quenching of fluorescence
intensities could be attributed to the general solvent effects and the hydrogen bonding interaction
of hydroxyl and amino groups of the Sensor 1 with water molecules[36, 37]. For DMF, acetone,
ethanol and MeCN, the changes in emission behaviors in response to water content were quite
similar to THF and the solvatochromic behaviors of Sensor 1 in the four solvents are shown in Fig.
4.
M
ore importantly, all of the ratio of blank fluorescence intensity to quenched fluorescence
intensity (I0/I) were linearly proportional to water concentrations from 0% to 10% (v/v) with the
detection limits of 0.059% in THF, 0.081% in ethanol, 0.092% in MeCN, 0.210% in DMF and
0.400% in acetone, respectively (Table 4). Compared with the detections of water in above five
organic solvents reported in references [26-29, 38-40], the detection limit of Sensor 1 was low or
moderate, indicating that Sensor 1 is sensitive to low-level water content in frequently used
water-miscible organic solvents.
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Fig. 4. Fluorescence emission spectrum of Sensor 1: (a) THF solution (λex= 418 nm), (b) acetone solution (λex=
413 nm), (c) MeCN solution (λex= 408 nm), (d) DMF solution (λex= 420 nm), (e) Ethanol solution ((λex= 400 nm)
in the presence of increasing amounts of water (0-10%, v/v). Fluorescence emission spectrum and A plot of
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fluorescence intensity change of Sensor 1 (10µM) against varied concentrations of H2O (0-1.0%, v/v) in: (a') THF
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solution (λex = 346 nm), (b') acetone solution (λex = 413 nm), (c') MeOH solution (λex = 408 nm), (d') DMF
solution (λex = 420 nm), (e') MeOH solution((λex = 400 nm).
Table 4. Detection limits of Sensor 1 for water determination in various solvents
Solvent
THF
Detection limit (%)
0.059
R2
0.9909
0.9922
0.9908
0.9936
0.9971
MeCN
Acetone
Ethanol
DMF
0.092
0.400
0.081
0.210
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The detection limits were calculated by the formula (3σ/|k|) and gave results. Standard
deviation (σ) was obtained by fluorescence responses (10-times of consecutive scanning on
the Varian Cary Eclipse Fluorescence Spectrophotometer) and k was slope of calibration curve.
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3.5. Qualitative detection of water content in organic solvents
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To develop a fast detection method of water content in organic solvents for the practical
application of Sensor 1, several test papers, which were loaded a small amount of Sensor 1, were
prepared and put into the organic solutions containing various contents of water. Herein, THF was
also considered as the representative for the discussing of the application of test papers. The
fluorescence emissions of THF solutions containing different amounts of water were changed
dramatically and immediately once the test papers were immersed into these solutions under
illumination with 365 nm light. As shown in Fig. 5, a gradual red-shift of emission of the solutions
with a significant decrease in the fluorescence intensity was clearly observed when the water
content increased from 0% to 20% (v/v). The phenomena in the other four solvents (DMF, acetone,
ethanol and MeCN) were similar to the THF solutions and the changes of fluorescence color are
shown in Fig. 5. Because the changes could be observed directly by naked eye, it is more
convenient to detect low-level water content roughly and quantificationally in several common
organic solvents by using the test papers.
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Fig. 5 Fluorescence photographs of Sensor 1 in THF, acetone, DMF, MeCN, ethanol in the presence of increasing
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amounts of water (0%, 1%, 2%, 5%, 10% and 20%) after the dyestuff test papers were soaked in these solutions
373 2. Conclusions
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We have designed and successfully synthesized a phenylene-(poly)ethynylene-based strong
fluorescent dye to develop a new family of highly sensitive solvatochromic probe, whose
fluorescence emission is strongly dependent on solvent polarity. An intriguing bathochromic shift
in the emission spectrum of Sensor 1 from 486 nm to 589 nm was found as the solvent polarity
was raised due to the ICT effect. Meanwhile, an amazing multicoloured tunable performance from
blue to orange by changing the solvent from apolar (toluene) to polar (DMF) under illumination
with 365 nm was observed, which indicates the potential value as sensor for the empirical
determination of solvent polarity. Furthermore, a pronounced fluorescence quenching is observed
with increasing water content in five water-miscible organic solvents due to the general solvent
effects and the hydrogen bonding interaction. In particular, the low detection limit (less than 0.1%
in THF, ethanol and MeCN; less than 0.5% in DMF and acetone) enables Sensor 1 to be utilized
as a supersensitive water sensor for the qualitative detection of low-level water in organic solvents.
At the same time, after the dyestuff test papers were soaked, the fluorescence color change of
organic solvents containing different amounts of water could be observed directly by naked eye.
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Thus, the proposed new sensing system would provide an alternative and convenient approach for
qualitative and quantitative detection of a trace amount of water in common organic solvents of
THF, acetone, MeCN, DMF, and ethanol. Our results in this work not only provide a successful
molecular design concept of the novel sensitive solvatochromic sensor for water in organic
solvents, but also develop practical applications of the linear -π-conjugated molecule bearing
phenylene-(poly)ethynylene in chemosensor field.
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Acknowledgements
Funding was partially provided by National Natural Science Foundation of China (51363019)
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and Specialized Research Fund for the Doctoral Program of Higher Education of China
(20136203120002).
399 Reference
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Highlights
1. A new family of highly sensitive solvatochromic dye based on
phenylene-(poly)ethynylene was firstly developed in the field of determination of
trace water content in organic solvents.
2. The fluorescence color change of organic solvents containing different amounts of
water could be observed directly by the naked eye by using the dyestuff test
papers.
3. The proposed new sensing system would provide an alternative and convenient
approach for qualitative and quantitative detection of a trace amount of water in
different organic solvents.
4. This work not only provides a successful molecular design concept of the novel
sensitive solvatochromic dye for water in organic solvents, but also develops
practical applications of the linear π-conjugated molecule bearing
phenylene-(poly)ethynylene in chemosensor field.