New Fluorescent Conjugates Displaying Solvatochromic Properties

Article abstract of DOI:10.1002/cjoc.201600732

Four new fluorescent dyes based on bis(phenyl-ethynyl-)-2-naphthyl (BPEN) were designed and synthesized. To improve the solvatochromic property and enhance the brightness of the fluorescent BPEN, an electron-donating unit of azetidine and/or an electron deficient group of -NO₂ was introduced. The corresponding fluorophores are denoted as T1, T2, T3 and T4, respectively. Moreover, to facilitate derivatization of the probes, two ethoxy carbonyl residues were grafted onto the side positions of BPEN. Spectroscopic studies demonstrated that introduction of azetidine leads to superior solvatochromic properties and largely enhanced fluorescence quantum yields as evidenced by the fact that T3 shows more than 150 nm shift in its maximum emission when dissolved in solvents of very different polarities and displays high fluorescence quantum yields in the solvents studied. However, T2, which is the one bearing a -NO₂ group, is non-fluorescent. Theoretical analysis and Lippert-Mataga modeling revealed the intra-molecular charge transfer (ICT) nature of the solvatochromic behavior of the compounds. Further test reveals that the fluorophores, in particular T3, are sensitive to the presence of trace water in less polar solvents, such as THF and 1,4-dioxane. Moreover, it is believed that the new fluorophores may serve as building blocks for creating environment-sensitive fluorescent sensors.

Full text of DOI:10.1002/cjoc.201600732

FULL PAPER
DOI: 10.1002/cjoc.201600732
New Fluorescent Conjugates Displaying Solvatochromic
Properties
,b
Huijing Liu,a Rongrong Huang,b and Yu Fang
a Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Materials Science
and Engineering, Shaanxi Normal University, Xi'an, Shaanxi 710062, China
b School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710062, China
Four new fluorescent dyes based on bis(phenyl-ethynyl-)-2-naphthyl (BPEN) were designed and synthesized.
To improve the solvatochromic property and enhance the brightness of the fluorescent BPEN, an electron-donating
unit of azetidine and/or an electron deficient group of －NO2 was introduced. The corresponding fluorophores are
denoted as T1, T2, T3 and T4, respectively. Moreover, to facilitate derivatization of the probes, two ethoxy carbonyl
residues were grafted onto the side positions of BPEN. Spectroscopic studies demonstrated that introduction of
azetidine leads to superior solvatochromic properties and largely enhanced fluorescence quantum yields as evi-
denced by the fact that T3 shows more than 150 nm shift in its maximum emission when dissolved in solvents of
very different polarities and displays high fluorescence quantum yields in the solvents studied. However, T2, which
is the one bearing a －NO2 group, is non-fluorescent. Theoretical analysis and Lippert-Mataga modeling revealed
the intra-molecular charge transfer (ICT) nature of the solvatochromic behavior of the compounds. Further test re-
veals that the fluorophores, in particular T3, are sensitive to the presence of trace water in less polar solvents, such
as THF and 1,4-dioxane. Moreover, it is believed that the new fluorophores may serve as building blocks for creat-
ing environment-sensitive fluorescent sensors.
solvents of very different properties.[6]
Introduction
As well known, a solvatochromic probe is structur-
ally characterized by a conjugated -system functional-
ized with an electron-withdrawing and/or an electron-
donating unit. For example, 2-propionyl-6-dimethyla-
minonaphthalene (Prodan), one of the best-known po-
larity probes developed during the last few decades,
takes a typical donor--acceptor structure, where the
naphthalyl residue, the propionyl unit and dimethyla-
mino group are the bridge, the electron-withdrawing and
the electron-donating structures, respectively.[7] Howev-
er, even for this widely recognized probe, it is still hard
to meet all the requirements as mentioned since it ab-
sorbs at wavelengths shorter than 400 nm. Similar limi-
tations occur for other reported probes. For example, the
two well-known families of polarity probes, dansyl
chloride and its derivatives as well as 4-(N,N-dimethyl-
aminonaphthal)imide and its analogues, are generally
showing low absorption coefficients.[8] Commercially
available Nile Red (9-diethyl-amino-5H-benzo-[alpha]-
phenoxazine-5-one), another fluorescent probe, is rela-
tively weak in recognition of solvents of different polar-
ities.[9] For anthracene and its analogues, their fluores-
cence quantum yields are usually low.[10] DNS (4-dime-
thylamino-4'-nitro-stilbene) and TNS (2-(p-toluidinyl)-
In the last few decades, various kinds of fluores-
cence probes have been developed and used as sensitive
tools for the detection of physiologically or environ-
mentally important analytes.[1] In particular, the probes
displaying solvatochromic properties could show re-
sponse to changes in their microenvironment induced by
the conformational or structural changes of proteins,
synthetic polymers, and assemblies of amphiphilic
compounds, such as cell membranes.[2] Fast recognition
of unknown liquids is another direct application of the
solvatochromic probes.[3] In addition, the fluorophores
as emphasized can also be utilized as emitting materials
for optoelectronic devices.[4] It is of the reasons, a large
number of solvatochromic probes have been developed
during the last few years.[5] However, from the view-
point of practical uses, there is still a big room for con-
ducting research in this area. This is because as an ideal
solvatochromic probe, the fluorophore needs to meet a
few criterions, including but not limited to: (1) absorp-
tion in the visible region (＞400 nm), (2) high absorp-
tion coefficients (＞30000 mol1•L•cm1), (3) great flu-
orescence quantum yields, (4) good photochemical sta-
bility, (5) strong solvatochromism, and (6) adaptive to
*
E-mail: yfang@snnu.edu.cn; Tel.: 0086-29-81530786. Fax: 0086-29-81530787
Received October 29, 2016; accepted January 3, 2017; published online April 5, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600732 or from the author.
In Memory of Professor Enze Min.
Chin. J. Chem. 2017, 35, 707—715
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Liu, Huang & Fang
FULL PAPER
naphthalene-6-sulfonic acid), which are highly applica-
ble for fluorescent imagine and probing studies, are also
not satisfying due to their short wavelength absorptions
and weak fluorescence in common solvents.[11]
isomers (o-xylenes, m-xylenes and p-xylenes), and even
mono-alkyl-substituted benzenes can also be recognized.
However, the probes as mentioned are hard to be deri-
vatized due to lack of reactive groups, which may limit
their practical applications. In addition, the probes as
developed are poor to resist photochemical degradation.
Accordingly, in the present work, two ethoxy carbonyl
structures were introduced instead of n-hexadecyl resi-
dues which were taken as grafts in the earlier studies.
Moreover, －NO2 was employed as an electron-with-
drawing group, and the well-known azetidine as an
electron-donating structure. In this way, four new deriv-
atives of BPEN with push-pull structures, T1, T2, T3
and T4 (Scheme 1), were designed, and their solvato-
chromic properties were studied. This paper reports the
details.
Because of the shortcomings of the known probes
and the increasing demand for them with specific char-
acteristics, recent years were experienced by great pro-
gress in the development of new versatile solvato-
chromic probes. As examples, Brummond and co-
workers reported a variety of Prodan-like fluorophores,
and they appear promising since they overcome the
drawbacks of the short wavelength absorptions of
Prodan.[12] Another representative solvatochromic
probes are push-pull substituted fluorene which have
superior solvatochromic properties and have already
been applied for labelling of biomolecules.[6a,13] Very
recently, Marx,[14] Lavis and co-workers[15] reported that
replacement of a general electron donation structure of
Experimental
N,N-dialkylamino groups in
a
fluorophore with
Synthetic procedure
four-membered azetidine ring not only improves the
quantum yield of the compound, but also enhances its
photochemical stability. Similar finding was also re-
ported by Xu, Liu, and co-workers.[16]
All reagents were purchased from commercial sup-
pliers and used directly unless otherwise noted. Prior to
use, CH2Cl2, toluene, and diisopropylamine were puri-
fied by distillation from CaH2. Reactions were con-
ducted under a dry N2 atmosphere using standard vac-
uum line techniques. Column chromatographic purifica-
tion was performed with silica gel (300－400 mesh) as
Recently, we focused on the studies of 1,4-bis(phe-
nylethynyl)-benzene (BPEB) and bis(phenylethynyl)-
2-naphthyl (BPEN)-based fluorophores.[17] Some of
them show typical solvatochromic properties, of which
different electron-withdrawing and electron-donating
units were introduced. It was found that solvents with
very similar structures or properties, such as methanol,
ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol,
n-heptanol, n-octanol and n-decanol could be discrimi-
nated with no difficulty. Moreover, ethylbenzene and its
1
the stationary phase. H NMR and 13C NMR spectra
were recorded on Bruker 600 MHz spectrometer in
CD2Cl2 and CDCl3 with tetramethylsilane (TMS) as an
internal standard. Mass spectrometry measurements
were performed on an AXIMA-CFR in MALDI-TOF
mode by using α-cyano-4-hydroxycinnamic acid (CCA)
as the matrixes.
Scheme 1 Molecular structures of T1, T2, T3 and T4
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Chin. J. Chem. 2017, 35, 707—715

New Fluorescent Conjugates Displaying Solvatochromic Properties
Photophysical characterization
The UV-vis absorption spectra of the compounds
were also measured in other solvents of different polari-
ties. The results are provided in Figure S-1 and the rel-
evant parameters are listed in Table 1. It is seen that the
positions of the absorption maxima of the compounds in
the solvents vary from 364 to 429 nm depending on the
exact structures of them. Further concentration-de-
pendent absorption tests in the solvents revealed that the
molar extinction coefficients (ε) (S1←S0 transition) of
the four compounds are greater than 3×104 mol1•L•
cm1 (c.f. Table 1), which is favorable for them to be
used as fluorescent probes. With reference to the two
sets of the data shown in Table 1, it is seen that the ab-
sorption maxima and the extinction coefficients slightly
change with increasing solvent polarity, but the change
can not be simply correlated with their ET(30) values.[18]
Careful examination of the data shown in the Table and
the spectra depicted in Figure S-1 demonstrates that the
spectra of the compounds in toluene are slightly
red-shifted if compared to the absorptions recorded in
other solvents, which could be a result of stabilization of
the excited state due to specific - interaction between
the solvent and the compounds.[19]
UV-vis absorption spectra were recorded on an
UV-vis spectrophotometer (U-3900, Hitachi), and the
molar absorption coefficients for the compounds were
calculated using the Lambert-beer law. Steady state flu-
orescence measurements were performed at room tem-
perature on a time correlated single photon counting
Edinburgh Instruments FLS 920 fluorescence spectrom-
eter. All the solvents used for fluorescence measure-
ments were freshly distilled prior to use. Absolute fluo-
rescence quantum yields were determined on the same
system equipped with an integral sphere at a concentra-
tion of 1.0×106 mol/L. With this instrument, the
measured f values may be subject to an error of ±5%.
Fluorescence lifetimes for the four compounds in dif-
ferent solvents were measured on the same system using
a picosecond pulsed light emitting diode (EPLED-375,
EPLED-405 or EPLED-445) as excitation source at a
concentration of 5.0×106 mol/L.
Results and Discussion
UV-vis absorption
To understand the structure and structural differences
of the four compounds as synthesized, their UV-vis ab-
sorption spectra in 1,4-dioxane were determined and the
results are depicted in Figure 1. It is seen that all the
spectra as obtained are characterized by two bands,
which must be related to the transitions to different ex-
cited states. Further inspection of the spectra reveals
that from T1 to T4 the absorption maxima of the low
energy bands (S1S0) significantly red-shifted as evi-
denced by the value changing from 365 to 372, then to
404, and finally to 421 nm, respectively, suggesting the
differences in the energy gaps between the Franck-
Condon (FC) excited states and the ground states of the
relevant compounds. However, for the maximum ab-
sorptions of the transitions to higher excited state, little
shift is observed.
Fluorescence properties
The fluorescence emission spectra of the 1,4-dioxane
solutions of T1, T2, T3 and T4 are, respectively, shown
in Figure 2d. In accordance with the absorption spectra,
the emission maximum exhibits great red-shift with the
compound being changed from T1 to T3, and then to T4.
Specifically, T1 is around 417 nm, T3 and T4 are around
551 and 599 nm, respectively. But the fluorescence
quantum yields of them decrease in the solvent in the
same order. In particular, T2 is almost non-fluorescent,
which is the reason why it is not listed in the Table. Ac-
tually, for most of the nitro-aromatics, the aromatic
rings are lacking electronic density because of the elec-
tron withdrawing effect of the nitro-groups, which must
change the electronic structures of the compounds, re-
sulting in decreased fluorescence quantum yields. In-
troduction of azetidine (T3), however, enhances the
emission of the mother compound, and co-existence of
the two structures also favors the emission of the com-
pound as evidenced by the high fluorescence quantum
yield of T4.
0.30
0.25
T1
T2
0.20
T3
Fluorescence lifetime (τf) and quantum yield (Φf) are
fundamental properties of fluorophores, and therefore
the parameters for the three compounds as obtained
were measured in different solvents. The results are also
shown in Table 1. It is to be noted that all the fluores-
cence decays as recorded could be fitted quite well with
a mono-exponential function (as examples, see Figure
S-2). Based upon the lifetimes and the quantum yields
as measured, the radiative decay rate constants (kr) for
each compound in different solvents can be calculated
by using kr＝f/τf, and moreover, the relevant non-ra-
diative decay rate constants (knr) by knr＝(1/f1)kr.
T4
0.15
0.10
0.05
0.00
300
350
400
450
500
550
Wavelength/nm
Figure 1 UV-vis absorption spectra of the 1,4-dioxane solutions
of T1, T2, T3 and T4 recorded at a concentration of 5.0×106
mol/L and at 25 ℃.
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Liu, Huang & Fang
FULL PAPER
Table 1 Photophysical data for T1, T3 and T4 in selected solvents: solvent polarity parameter ET(30), molar absorption coefficient (ε),
maximum absorption wavelength (λabs) and emission wavelength (λem), Stokes shifts (ν), fluorescence quantum yield (Φf), lifetimes (τf),
radiative decay rate constant (kr) and non-radiative decay rate constant (knr) for the four compounds recorded in different solvents. For
fluorescence quantum yield measurements, the probe’s concentration was kept at 1.0 mol/L. For other fluorescence measurements, the
concentrations were set at 5.0 mol/L
ε/
λabs
/
λem/
nm
△ν/
cm1
Φf/
%
τf/
ns
kr/
MHz
knr/
MHz
Fluorescent probe
Solvent
ET(30)
(mol1•L•cm1) nm
Hexane
Toluene
31.0
33.9
36725
37520
46510
48020
45806
38265
39765
46510
365
370
365
367
370
367
368
364
405
418
417
421
426
427
446
442
2706
3104
3416
3495
3553
3829
4752
4849
99
97
90
77
92
73
58
57
1.24
1.43
1.42
1.43
1.10
1.30
1.38
1.27
798
678
634
538
836
562
413
449
8
21
1,4-Dioxane 36.0
70
THF
37.4
39.1
40.7
45.1
55.4
161
73
T1
CHCl3
CH2Cl2
DMSO
MeOH
208
312
339
Hexane
Toluene
31.0
33.9
31068
32675
34256
34927
35153
33586
34595
35658
398
411
405
408
404
409
409
408
467
529
551
618
613
596
622
548
3712
5427
6604
8329
8439
7671
8373
6262
66
90
69
11
23
19
69
16
1.51
2.91
3.16
1.65
1.76
1.37
1.53
1.36
437
309
219
66
225
34
1,4-Dioxane 36.0
98
THF
37.4
38.1
39.1
40.7
48.1
539
438
591
203
618
T3
EA
131
139
451
118
CHCl3
CH2Cl2
OOH
Hexane
Toluene
CCl4
31.0
33.9
32.4
32561
31680
32489
38326
418
429
426
421
492
522
577
599
3598
4153
6143
7086
96
68
45
15
1.80
2.20
0.50
2.64
533
309
900
57
22
145
1100
322
T4
1,4-Dioxane 36.0
Clearly, similar with the BPEN-based fluorophores re-
ported earlier,[17d,17e] the fluorescence quantum yields of
the three compounds as synthesized in the present study
are generally high in less polar solvents, and low in po-
lar solvents.
The photo-chemical stability of the three compounds
was also studied as it is often a key factor affecting the
practical applications of fluorescent probes.[20] An ex-
ample result is shown in Figure S-3. It was shown that
two-hour continuous illumination with a xenon lamp
(450 W) as the light source did not result in any signifi-
cant deduction in the fluorescence emission, indicating
that the compounds under study possess exceptional
photochemical stability.
spectra become less structured or even structure-less.
Further interrogating of the spectra shown in Figure
2, and the data depicted in Table 1, it is also seen that
the three compounds display different solvatochromic
properties. Specifically, for T1, its emission maximum
appeared around 405 nm in apolar n-hexane, but the
emission shifted to 427 nm when the solvent was
changed to a slightly polar solvent of CH2Cl2, a 22 nm
shift due to the polarity increase, which is a typical pos-
itive solvatochromic behavior as expected from the
weak push-pull structure of the fluorophore (naphthyl
structure to the phenyl residue). With introduction of
azetidine structure at the sixth position of the naphthyl
residue of the core structure, both the solvatochromic
property of the compound is significantly improved. For
example, the emission maximum of T3 appeared at
~467 nm in n-hexane, but shifted to 622 nm in CH2Cl2,
a 155 nm shift due to the same polarity increase,
demonstrating the effectiveness of the structural modi-
fication. Further introduction of －NO2 group also en-
hances the solvatochromic properties of the relevant
compounds as evidenced by their increased emission
maximum shift from n-hexane to dioxane, which is 30,
85, and 107 nm, respectively. However, it is to be noted
that introduction of nitro-group significantly inhibits
Solvatochromic properties
The fluorescence emission spectra of T1, T3 and T4
were recorded in solvents of different polarities, and the
results are shown in Figure 2. Entirely different from the
absorption spectra, the emission positions of the three
compounds studied are highly dependent upon the na-
ture, in particular the polarity of the solvents tested.
Further inspection of the spectra shows that the emis-
sion profiles of them are structured in n-hexane. With
increasing the polarity of the solvents, however, the
710
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Chin. J. Chem. 2017, 35, 707—715

New Fluorescent Conjugates Displaying Solvatochromic Properties
a
1.0
1.0
0.8
0.6
0.4
0.2
0.0
b
HEX
TOL
DIO
THF
EA
CHF
DCM
OOH
HEX
TOL
DIO
THF
CHF
DCM
DMSO
MeOH
0.8
0.6
0.4
0.2
0.0
400
c
450
500
550
600
650
450 500 550 600 650 700 750 800
Wavelength/nm
Wavelength/nm
5
4
3
2
1
0
d
1.0
0.8
0.6
0.4
0.2
0.0
T1
T2
T3
T4
HEX
TCM
TOL
DIO
400 450 500 550 600 650 700 750 800
Wavelength/nm
450 500 550 600 650 700 750 800
Wavelength/nm
Figure 2 Normalized fluorescence emission spectra of T1 (a), T3 (b) and T4 (c) recorded at a concentration of 5.0×106 mol/L in dif-
ferent solvents with 365, 420 and 430 nm as the excitation wavelength of the respective compound, and comparison of the fluorescence
emission spectra of the 1,4-dioxane solutions of T1, T2, T3 and T4 (d). The inset of d is the fluorescence pictures of the four compounds
in 1,4-dioxane under UV light illumination (365 nm). All spectroscopic measurements were conducted at 25 ℃. HEX: hexane, TCM:
carbon tetrachloride, TOL: toluene, DIO: dioxane, THF: tetrahydrofuran, DCM: dichloromethane, CHF: chloroform, and OOH: o-octanol.
the fluorescence emission of the relevant compounds as
demonstrated by the fact that the fluorescence quantum
yield of T4 in dioxane is only 16%, but the values for
T1 and T3 in the same solvent are 90% and 69%, re-
spectively. Moreover, nitro-containing T2 is non-fluo-
rescent as indicated earlier. Introduction of the group
also decreases the solubility of the compound in the
solvents studied, which makes T4 less adaptive to sol-
vents of different polarities. However, the negative ef-
fect induced by nitro-group does not mean that the
compound, T4, can not find uses in solvent discrimina-
tion.
The emission positions and the remarkable shifts in
the emission maxima of the fluorophores, in particular
T3 and T4, as developed imply that they have the poten-
tial to be used for visualization discrimination of some
organic solvents. Figure 3 compares the solvatochromic
performances of T1 and T3. It is seen clearly that struc-
Figure 3 The fluorescence images of T1 (a) and T3 (b) in dif-
ferent solvents (5.0×10-6 mol/L), which were taken under the
illumination of UV light (365 nm) and at room temperature.
tural modification endows the core structure, T1, excep-
tional solvatochromic properties. Based on the findings,
study the molecular structures and their photophysical
properties relevant to the ground- and excited-dynamics.
The ground-state structures of T1 to T4 were first opti-
mized in the gas-phase using the CAM-B3LYP/6-31g*
level of theory (Figure 4). From the results as depicted,
it can be seen that for the four compounds as obtained
the π-π conjugate, BPEN, takes a flat and rigid structure.
it is anticipated that T3 and possibly T4 as developed
may find uses in the discrimination of solvents/liquids
with similar properties and in the determination of trace
water in organic liquids.
Theoretical calculations
Several quantum chemical methods were used to
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Liu, Huang & Fang
T4
FULL PAPER
T1
T2
T3
Optimized Structures
LUMO
HOMO
1.14 eV
6.75 eV
1.67 eV
7.01 eV
1.00 eV
6.20 eV
1.57 eV
6.34 eV
Figure 4 Molecular orbital energies and iso-density surface plots of HOMO and LUMO for T1, T2, T3 and T4 depicted in different
columns.
In addition, for T3 and T4, the azetidine is almost in the
co-planar position with BPEN. The molecular orbital
diagrams of the compounds as generated from TD-DFT
calculations are in support of the statement. For the
highest occupied molecular orbitals (HOMOs) of the
compounds, the electron densities are largely localized
on the azetidine-(naphthalene ethynyl)-benzene part of
the molecules. In contrast, the electron densities of the
LUMO orbitals of the four compounds gradually move
to another end with the fluorophore changing from T1 to
T4. The change in the electron density distribution from
the HOMO orbital to the LUMO orbital must be the
origin of the difference between the ground and the ex-
cited state dipole moments (μ) of the compounds as
studied. The results from the calculations are listed in
Table 2, which are 1.6, 7.8, 9.0 and 16.4 D, respectively,
for the four compounds under study. In addition, Table 2
also shows the absorption maxima (λmax), the main or-
bital transitions, and the oscillator strengths () of the
compounds from the calculations. It is seen that the the-
oretical λmax values are in good agreement with the ex-
perimental results considering the fact that solvent effect
was not considered during the calculations.
Rationalization based on experimental interrogation
From the experimental results shown in Table 1, it is
seen that the emission position of T4 is more sensitive
to the change of solvent polarity if compared to that of
T3 and T1, and similarly, T3 is more sensitive than T1,
which is a result in accordance with the change in dipole
moment between the ground and excited states as de-
picted in Table 2. To further validate the nature of the
solvatochromic characteristics of the fluorophores as
created, Lippert-Mataga theory was employed to con-
duct further experimental studies.[21] The difference in
dipole moment between the two states (μ) was esti-
mated by using the famous Lippert-Mataga equation (eq
1):
2f
＝A－F＝
 2 ＋cons tant
(1)


hca3
In the equation, A and F are the wavenumbers of the
relevant absorption and emission maxima, respectively.
h is the Planck’s constant, c is the light velocity, a is the
Table 2 TD-DFT calculations at the long-range corrected CAM-B3LYP function and experimentally obtained absorption properties
maxa/nm
353

maxb/nm
365
(eg)/Debye
1.52
Compound
Transition
S1S0
S2S0
S1S0
S1S0
S1S0
S2S0
Main orbital transition
HOMOLUMO
1.45
0.18
1.74
1.67
1.85
0.06
T1
298
HOMOLUMO＋1
HOMOLUMO
327
1.36
T2
T3
361
370
7.73
369
405
8.94
HOMOLUMO
383
421
16.38
HOMOLUMO
T4
309
HOMOLUMO＋1
323
14.12
a Calculated absorption maximum; b Observed absorption maximum in 1,4-dioxane.
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© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2017, 35, 707—715

New Fluorescent Conjugates Displaying Solvatochromic Properties
radius of the cavity in which the fluorophore resides,
and f is the orientation polarizability of the solvent
system defined as follows (Eq. 2):
fluorophore is considered to exist within a cavity with
an ideal spherical shape. But for the compounds under
study, the situation is different and in fact they take a
linear shape. Accordingly, it should be better to modify
the model into an ellipsoidal one. Moreover, the cavity
radius is further supposed to be 40% of its long axis. In
this way, the Onsager radii of T1, T3 and T4 can be
calculated, and the results are 6.21, 6.35 and 6.51 Å,
respectively. Based on the modified model and the
Lippert-Mataga plots, the dipole moment changes (μ)
of the three probes were calculated, and the results are
14, 37 and 39 D, respectively. Obviously, these values
are much larger than those estimated from theoretical
calculations (Table 2), suggesting that there might be
some specific interactions between the probes and the
solvents, which did not take into account during the
theoretical calculations. Anyway, the orders of the solv-
atochromic abilities of the relevant compounds predi-
cated from the two methods are the same, and fit with
the experimental observations.
-1
n2 -1
f＝
－
(2)
2＋1 2n2＋1
where  and n are the dielectric constant and refractive
index of the solvent. Specifically, n-hexane/1,4-dioxane
was chosen as a class of typical solvent mixtures to ad-
just the orientation polarizabilities (f) of the mixture
solvents through regular variation of their compositions.
The emission spectra of T1, T3 and T4 in the mixtures
were acquired and the results are shown in Figure S-4a,
Figure 5a, and Figure 5b, respectively. The plots of the
relevant compound’s Stokes shift ν vs. f were shown
in Figure S-4b, Figure 5c and Figure 5d, respectively.
With reference to the Figures, it can be observed that the
linear correlations between the Stokes shifts of the
compounds in the mixtures and the solvent orientation
polarizabilities are perfect. In addition, the slops of the
plots are positive, suggesting that the compounds as
studied possess the ability to discriminate solvents of
different polarities.
Detection of water
As predicted in the discussions, the fluorescence
emission of T3 and/or T4 should be sensitive to the
presence of water in organic solvents. Accordingly, the
fluorescence emission spectra of T3 in two example
organic solvents, 1,4-dioxane and THF, with the pres-
It is to be noted, however, that the Lippert equation
was derived on the basis of many assumptions, and it is
only an approximation of real situations. For example,
in the development of the equation, the molecule of a
a
b
1.0
1.0
0%
0%
10%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0.8
0.8
0.6
0.4
0.2
0.0
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.6
0.4
100%
0.2
0.0
450 500 550 600 650 700 750 800
Wavelength/nm
450 500 550 600 650 700 750 800
Wavelength/nm
7500
7000
6500
6000
5500
5000
4500
4000
7500
d
7000
c
6500
6000
5500
5000
4500
4000
0.000
0.005
0.010
0.015
0.020
0.000
0.005
0.010
0.015
0.020
f
f
Figure 5 Fluorescence emission spectra of T3 (a) and T4 (b) in mixture solvents of n-hexane and DIO of different ratios at a concentra-
tion of 5.0×106 mol/L and at 25 ℃, of which excitation wavelength is 420 nm. Lippert plots for T3 (c) and T4 (d) in the set of mixture
solvents.
Chin. J. Chem. 2017, 35, 707—715
© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
713

Liu, Huang & Fang
FULL PAPER
6
5
4
3
2
1
0
8
7
a
b
0
0
6
0.04%
0.1%
0.2%
0.4%
0.6%
0.8%
1%
0.04%
0.1%
0.2%
0.4%
0.6%
0.8%
1%
5
4
3
2%
4%
2
2%
4%
1
0
-1
450 500 550 600 650 700 750 800
Wavelength/nm
450 500 550 600 650 700 750 800
Wavelength/nm
Figure 6 Fluorescence emission spectra of T3 in 1,4-dioxane-water (a) and THF-water (b) mixtures with different water contents (vol.%)
recorded at a concentration of 5.0×106 mol/L and at 25 ℃, of which excitation wavelength is 420 nm. The pictures shown in (c) and (d)
are the fluorescence images of T3 in the two sets of solvents under UV light illumination (365 nm).
compounds (T3, T4) bearing an azetidine structure are
superior than others in the discrimination of solvents of
different polarities. But T4 is poor than T3 considering
the facts (1) it is hard to be dissolved in most of com-
mon organic solvents, and (2) its fluorescence quantum
yields in some polar solvents are significantly lower
than that of T3. T2 behaviors very differently from oth-
ers as it is fluorescence inactive in nature. The possible
reason behind is the introduction of －NO2 group. To
our luck, the fluorophores are all stable under long time
light illumination. Theoretical calculations and Lip-
pert-Mataga model-based experimental studies revealed
that the nature of the solvatochromic properties origi-
nates from the excitation related intra-molecular charge
transfer (ICT) of their BPEN-based push-pull structures.
Considering the long wavelength absorption (＞400 nm),
large molar absorption coefficient (＞32 000 cm1•
mol1•L), great color change (400－620 nm), and ex-
ceptional photochemical stability of T3 and T4, it is be-
lieved that the two compounds as developed are attrac-
tive as they may find important applications due to their
superior solvatochromic properties. As an example,
trace amount of water in aprotic solvents could be de-
termined with no difficulty.
ence of different amount of water were recorded, and
the results are shown in Figure 6a and Figure 6b, re-
spectively. With reference to the spectra, it is clearly
seen that water shows significant quenching effect upon
the fluorescence emission of the compound in the sol-
vents (Figures 6c and 6d), and the quenching efficien-
cies are linearly related to the concentration of water in
the systems. Quantitative analysis of the data (Support-
ing Information) shows that detection limits for water in
the two solvents could be lower than 0.015% and
0.019% (V/V), respectively.
The Stern-Volmer plots of the data shown in Figure
S-5 are up-curved, which is an indication of the com-
plex nature of the quenching process. As an example,
Figure S-6 shows one of the two plots (1,4-dioxane). To
reveal the nature of the quenching, fluorescence lifetime
measurements were conducted for the T3/1,4-dioxane
system. The results are also presented in Figure S-6. It is
clearly seen that the quenching is a combined static and
dynamic process, and the contributions from them are 1
to 1.
Conclusions
In conclusion, four fluorescent conjugates with po-
tential solvatochromic properties were designed and
synthesized. In the design, an extended and rigid π sys-
tem of bis(phenyl-ethynyl-)-2-naphthyl (BPEN) was
taken as the core structure, a newly reported structure,
azetidine, was chosen as an electron-donating group,
and －NO2 as an electron-withdrawing unit. In addition,
two ethoxy carbonyl moieties were grafted onto the side
positions of the core structure in order to endow the as
to be developed fluorophores reactivity, which lays
foundation for them to be grafted or labelled onto other
structures. Spectroscopic studies demonstrated that the
Acknowledgements
This work was supported by NSF of China (Nos.
21273141, 21527802, 21673133), the 111 project (No.
B14041), and the Program for Changjiang Scholars and
Innovative Research Team in University (No. IRT1070).
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