Thiophene-inserted aryl-dicyanovinyl compounds: The second generation of fluorescent molecular rotors with significantly redshifted emission and large stokes shift

Article abstract of DOI:10.1002/ejoc.201100891

Fluorescent molecular rotors can be used as molecular sensors for the viscosity of a microenvironment. However, these molecular rotors are limited to 9-(dicyanovinyl)julolidine (DCVJ) and a few derivatives. Furthermore, these traditional rotors show short absorption/emission wavelengths and small Stokes shifts. To address these drawbacks, we have developed a small library of new molecular rotors for viscosity sensing, prepared by incorporating a thiophene unit into the conventional fluorescent molecular rotors with the aim of accessing molecular rotors with redshifted excitation/emission wavelengths and larger Stokes shifts compared with the known rotors. The new rotors show substantially improved photophysical properties. For example, rotor 4 shows absorption/emission wavelengths of 559/697 nm, respectively, and a very large Stokes shift of 138 nm compared with the absorption/emission wavelengths (465/503 nm) and very small Stokes shift (38 nm) of the traditional fluorescent molecular rotor DCVJ. The photophysical properties of the rotors were rationalized by DFT calculations. Thiophene-inserted aryl-dicyanovinyl molecular rotors were prepared that show substantially improved photophysical properties for the absorption/emission wavelengths and the Stokes shift. For example, rotor 4 shows absorption/emission wavelengths of 559/697 nm, respectively, and a very large Stokes shift of 138 nm compared with the traditional molecular rotor DCVJ.

Full text of DOI:10.1002/ejoc.201100891

FULL PAPER
DOI: 10.1002/ejoc.201100891
Thiophene-Inserted Aryl–Dicyanovinyl Compounds: The Second Generation of
Fluorescent Molecular Rotors with Significantly Redshifted Emission and
Large Stokes Shift
Jingyin Shao,[a] Shaomin Ji,[a] Xiaolian Li,[a] Jianzhang Zhao,*[a] Fuke Zhou,[a] and
Huimin Guo*[a]
Keywords: Cyanides / Fluorescence / Molecular devices / Viscosity / Density functional calculations
Fluorescent molecular rotors can be used as molecular sen-
sors for the viscosity of a microenvironment. However, these
molecular rotors are limited to 9-(dicyanovinyl)julolidine
(DCVJ) and a few derivatives. Furthermore, these traditional
rotors show short absorption/emission wavelengths and
small Stokes shifts. To address these drawbacks, we have de-
veloped a small library of new molecular rotors for viscosity
sensing, prepared by incorporating a thiophene unit into the
conventional fluorescent molecular rotors with the aim of ac-
cessing molecular rotors with redshifted excitation/emission
wavelengths and larger Stokes shifts compared with the
known rotors. The new rotors show substantially improved
photophysical properties. For example, rotor 4 shows absorp-
tion/emission wavelengths of 559/697 nm, respectively, and
a very large Stokes shift of 138 nm compared with the ab-
sorption/emission wavelengths (465/503 nm) and very small
Stokes shift (38 nm) of the traditional fluorescent molecular
rotor DCVJ. The photophysical properties of the rotors were
rationalized by DFT calculations.
and a small Stokes shift (38 nm).[1] A short emission wave-
length makes it difficult for these sensors to be used for in
vivo analysis due to the significant auto-fluorescence of the
biological samples by excitation with UV/blue light,
whereas the small Stokes shift is detrimental to the fluores-
cence signal, because the optical filter, which removes the
excitation light from the collected emission signal, also
blocks parts of the emission (due to significant overlap of
the excitation and emission spectra of dyes with a small
Stokes shift).[11] To our surprise, however, there have been
no reports so far on the tailoring of the structures of the
aryl–dicyanovinyl molecular rotors (for example, 2) to
tackle the aforementioned photophysical drawbacks.
Recently, a BODIPY-based molecular rotor was used for
in vivo fluorescent lifetime imaging of viscosity, but it still
suffers from short absorption/emission wavelengths and an
especially small Stokes shift.[14] Porphyrin-based rotors have
been reported, but the rotors are synthetically de-
manding.[12,16] Therefore, simple rotors with redshifted exci-
tation/emission and a large Stokes shift are highly desired
as alternatives to DCVJ for molecular viscosity sensing.
We have been interested in the photophysics of lumino-
phores for a while.[18–20] Usually, the excitation/emission
wavelengths of a fluorophore can be redshifted by extension
of the π conjugation.[11,18,19] A small Stokes shift usually
arises from the rigidity of the fluorophore, and thus the
vibration relaxation from the Franck–Condon excited state
to the vibrationally relaxed S1 state is not significant.[11]
Introduction
Recently, aryl–dicyanovinyl fluorescent molecular rotors
have attracted considerable attention due to their ability as
fluorescent viscosity sensors (1 and 2, Scheme 1).[1–11] These
rotors emit weakly in fluid solution (low viscosity) due to
the free rotation of the C–C/C=C bonds in the excited state,
which serves as an efficient drain pipe for the photoexcited
molecules. In a viscous environment, however, the intramo-
lecular rotation is restricted, and thus the emission is greatly
intensified.[1] Well-known molecular rotors are 1-(dicya-
novinyl)-4-(dimethylamino)benzene (1) and 9-(dicyanovi-
nyl)julolidine (2, DCVJ; Scheme 1).[1] DCVJ and a few
other rotors, based on different mechanisms,[12–14] have
been used to measure the viscosity or shear stress of a mi-
croenvironment,[1] reaction medium,[15] or intracellular
plasma.[16,17]
However, the conventional aryl–dicyanovinyl molecular
rotors for viscosity sensing [for example, DCVJ (2)] suffer
from fundamental photophysical drawbacks, such as short
absorption/emission wavelengths (465/503 nm, respectively)
[a] State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology,
Dalian 116024, P. R. China
Fax: +86-411-8498-6236
E-mail: zhaojzh@dlut.edu.cn
guohm@dlut.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100891.
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Thiophene-Inserted Aryl–Dicyanovinyl Compounds
Scheme 1. Synthesis of thiophene-containing fluorescent molecular rotors 3–6 and control compound 7. The known rotors 1 and DCVJ
(2) are also shown.
Thus, a large Stokes shift can be achieved by increasing which can be used to label biomolecules).[8] We also demon-
the flexibility of the fluorophore skeleton. On this basis, we strate that our approach can be extended to other fluoro-
inserted a thiophene moiety into the aryl–dicyanovinyl ro- phores (rotors 6). The specific effect of thiophene insertion
tors to extend the π conjugation of the molecules and at the on the photophysical properties of the rotors is demon-
same time increase their vibrational flexibility (Scheme 1) strated by a control rotor with a phenylene linker (7).
and thus redshift the excitation/emission wavelengths and,
more importantly, increase the Stokes shift of the molecular
rotors.
Herein we demonstrate the greatly improved photophysi-
cal properties, that is, the redshifted excitation/emission
wavelengths and increased Stokes shifts, observed for the
Results and Discussion
Design of the Fluorescent Molecular Rotors
The main goal of our study was to prepare molecular
new molecular rotors 3–6 (Scheme 1). Furthermore, we rotors with redshifted absorption/emission wavelengths and
prove that a rotor with an attached carboxylic acid can be Stokes shifts larger than those of the known molecular
readily constructed by our strategy (for example, rotor 5, probes 1 and 2 (Scheme 1). In principle, this goal can be
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J. Shao, S. Ji, X. Li, J. Zhao, F. Zhou, H. Guo
FULL PAPER
achieved by extension of the π-conjugated framework of the small Stokes shift of 38 nm for DCVJ (2).[1] These greatly
chromophore of the rotors. Recently, we showed that the improved photophysical properties of 4 are beneficial for in
acetylene bond can be used to extend the absorption/emis- vivo molecular viscosity sensing. Similar results were found
sion wavelengths of the rotors.[21] However, we found that for 3 (λab/λem = 513/661 nm, respectively). A Stokes shift of
the sensitivity of the resulting rotors to viscosity was re- 148 nm was observed for 3 (see the Supporting Infor-
duced. Thus, we set out to explore another strategy to im- mation).
prove the photophysical properties of the rotors. The thio-
phene moiety is extensively used in materials chemistry (for
example, in sensitizers in dye-sensitized solar cells),[22,23]
very often as a π-conjugation linker. Recently, we proved
that the thiophene moiety can be used as an efficient π-
conjugation linker in fluorescence boronic acid sensors to
move the absorption/emission to the red end of the spec-
trum.[24] Thus, in this work we devised new molecular ro-
tors containing thiophene (Scheme 1). Insertion of the thio-
phene moiety into the traditional rotors (compounds 1 and
2) leads to new rotors 3 and 4. Rotor 5 was designed with
a carboxylic group attached, and thus this rotor can be used
for labeling.[6,25,26] Furthermore, we used a different chro-
mophore to construct the molecular rotors, that is, the phe-
nothiazine chromophore, which is a known electron-donat-
ing moiety.[27] Rotor 7 was prepared as a control compound
to prove that the thiophene moiety can be used as an ef-
ficient π-conjugation linker, but not the phenyl moiety.
Table 1. Photophysical parameters of rotors 1–7 [the data of re-
ported molecular rotors 1 and DCVJ (2) are included for compari-
son].
[a]
[b]
Rotor
Solvent
λabs
[nm] [nm]
λem
Stokes shifts τ[c]
Φ[d]
[nm]
[ns]
1
DCM
ethylene glycol 441
glycerol
DCM
ethylene glycol 465
glycerol
DCM
ethylene glycol 513
glycerol
DCM
ethylene glycol 559
glycerol
DCM
ethylene glycol 530
glycerol
DCM
ethylene glycol 485
glycerol
DCM
ethylene glycol 501
glycerol 512
433
474
482
485
491
503
505
620
661
655
664
697
689
631
670
664
734
669
652
683
603
580
41
41
42
33
38
2.44
0.16
0.22
0.55
0.18
0.28
0.95
0.39
1.24
2.11
0.71
0.89
1.52
0.54
1.26
0.36
0.0059
0.0078
0.0951
0.0058
0.0082
0.0755
0.0804
0.0235
0.0695
0.1229
0.0066
0.0279
0.1091
0.0280
0.0656
0.0014
0.0001
0.0062
0.0037
0.0007
0.0077
443
458
2
3
4
5
6
7
472
508
33
102
148
133
111
138
117
107
140
128
246
184
155
181
102
68
522
553
572
524
536
488
[e]
–
497
502
1.29
0.17
0.30
1.41
UV/Vis Absorption and Fluorescnece Spectra of the New
Rotors
The UV/Vis absorption and fluorescence emission of the
new rotors were studied and compared with those of the
traditional rotors 1 and 2 (Scheme 1). Redshifted absorp-
tion and emission wavelengths were observed for the new
rotors. For example, 4 shows absorption/emission at 559/
697 nm, respectively (Figure 1, Table 1), which are greatly
redshifted compared with DCVJ (2; λab/λem = 465/503 nm,
respectively).[1] The emission of 4 is redshifted by around
194 nm compared to that of DCVJ. The broad excitation/
emission bands are probably due to the intramolecular
charge-transfer nature of 4. In particular, a large Stokes
shift of 138 nm is observed for 4, which compares with the
[a] UV/Vis absorption maximum. [b] Emission maximum. [c] Fluo-
rescence lifetime. [d] Fluorescence quantum yield, with quinine sul-
fate as standard (Φ = 54.7% in 0.05 m H2SO4). [e] Not determined
due to the weak emission.
Viscosity Dependency of the Fluorescence Emission of the
Rotors
First we investigated the emission spectra of rotor 4 in
methanol, ethylene glycol, and glycerol to study the effect
of solvent viscosity on the emission intensity of rotor 4
(Figure 2). The emission of rotor 4 in methanol is weak,
Figure 1. Normalized fluorescence excitation and emission spectra
of DCVJ (2; λex = 465 nm, λem = 503 nm) and rotor 4 (λex
=
Figure 2. Emission spectra of 4 in methanol, ethylene glycol, and
glycerol. The emission intensity at 690 nm in glycerol is 17.5-fold
that in methanol (c = 1.0ϫ10–5 mol/L, 20 °C).
577 nm, λem = 697 nm; c = 1.0ϫ10–6 mol/L in ethylene glycol,
20 °C).
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Thiophene-Inserted Aryl–Dicyanovinyl Compounds
but can be greatly enhanced in solvents with much higher sion wavelengths (530/670 nm), large Stokes shift (140 nm)
viscosity, such as ethylene glycol and glycerol. These results and x value (0.305; see the Supporting Information). With
indicate that the fluorescence intensity of rotor 4 can be the carboxylic ester moiety, 5 can be used for labeling bio-
enhanced by increasing the viscosity of the solvent (micro- molecules.[6,25,26]
environment).
The molecular structures of the thiophene-containing
The sensitivity of 4 to viscosity was studied quantita- molecular rotors are characterized as D–π–A, and thus we
tively (Figure 3a). The emission intensity of 4 at 697 nm expected that the fluorescence emissions of these molecular
was increased by around 4.5-fold by increasing the viscosity rotors may be sensitive to solvent polarity. Note that this is
of the solvent (13.5–945 cP), which is a comparable increase not necessarily the case, because the traditional molecular
to that of DCVJ (ca. 4.0-fold, 49–631 cP).[1] Notably, the rotors, such as rotors 1 and 2, do not show polarity-depend-
emission enhancement of 4 by increasing the viscosity from ent emission. We found that the UV/Vis absorption and
0.59 to 945 cP from MeOH to glycol, is around 17-fold (see fluorescence emission of rotor 4 are sensitive to solvent po-
the Supporting Information). The emission intensity of the larity (Figure 4). The sensitivity was quantitatively evalu-
rotors and the viscosity can be correlated by the Förster– ated by the Lippert–Matage relationship and the correlation
Hoffmann equation [Equation (1)] in which η is the vis- between the Stokes shift and ET(30) values (Figure 5).[28]
cosity, I is the emission intensity, C is a constant, and x is Similar results were found for the other molecular rotors
the sensitivity of the rotor to viscosity.
described herein (see the Supporting Information). The
traditional rotors do not show solvent-polarity-dependent
emission wavelengths. Thus, we propose that the changes in
the dipole moments of the new rotors upon photoexcitation
are more significant than traditional rotors such as DCVJ.
logI = C + xlogη
(1)
Figure 3. Fluorescence spectra of 4. (a) Emission of 4 in ethylene
glycol/glycerol mixtures of different viscosity. (b) Fitting of the data
to the Förster–Hoffmann equation (c = 1.0ϫ10–6 mol/L, 20 °C).
Figure 4. (a) UV/Vis absorption of rotor 4 in different solvents (c
= 1.0ϫ10–5 mol/L, 20 °C). (b) Fluorescence emission spectra of ro-
tor 4 in different solvents (c = 1.0ϫ10–5 mol/L, 20 °C).
A perfect linear fitting was observed with a value of x of
0.35 for 4 (Figure 3b) and a large value of x (0.468) for
rotor 4 in the low-viscosity range (0.59–13.5 cP, see the Sup-
porting Information). These values are comparable to the x
value of DCVJ (0.59). Interestingly, the emissions of the
new rotors are sensitive to the polarity of the solvent, which
is valuable for dual-functional in vivo molecular sensing of
polarity and viscosity.[1,2]
Our thiophene insertion approach can be readily ex-
tended to more functionalized derivatives such as 5
(Scheme 1). Compound 5 shows similar absorption/emis- L, 20 °C).
Figure 5. (a) Lippert–Mataga regressions and (b) plot of Stokes
shifts vs. ET(30) values for rotor 4 in solvents (c = 1.0ϫ10–5 mol/
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J. Shao, S. Ji, X. Li, J. Zhao, F. Zhou, H. Guo
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Note that the sensitivity of the rotors to polarity is not crease in the viscosity of the liquid crystal and not to the
a disadvantage; previously, it has been demonstrated that reduced temperature. A similar result was found for rotor 6
these kinds of viscosity molecular sensors can be used as (Figure 7b).
dual-responsive molecular sensors of viscosity and po-
larity.[2]
Our approach can be extended to other electron-donat-
ing chromophores [for example, phenothiazine (6;
Scheme 1)], which shows absorption/emission wavelengths
of 485/669 nm, respectively, and a Stokes shift of 184 nm
(Figure 6)]. The emission intensity of rotor 6 is enhanced in
viscous solvents (Figure 6); an x value of 0.609 was ob-
served for rotor 6.
Figure 6. (a) Emission spectra of rotor 6 in mixtures of ethylene
glycol/glycerol and (b) dependency of the emission intensity on the
viscosity of the solvent (λex = 497 nm, λem = 652 nm, c = 1.0ϫ
10–5 mol/L, 20 °C). The solid line in part (b) is the linear fitting
according to the Förster–Hoffmann equation.
Figure 7. Use of molecular rotors to monitor the phase transitions
of liquid crystal DYLC 7077-050 (LC) by monitoring the changes
in fluorescence intensity vs. time (decreasing temperature of the
liquid crystal). (a) Rotor 3 was dissolved in the liquid crystal (λex
= 510 nm, λem = 658 nm, c = 3.0ϫ10–6 mol/L). A control experi-
ment was carried out in DMF. (b) Use of the viscosity probe 6 to
monitor the phase transitions of liquid crystal 761 (LC) by observ-
ing the changes in the fluorescence signal intensity vs. time. The
For a control rotor with phenyl but not the thiophene
linker (7), the emission is poor (see the Supporting Infor-
mation). Thus, thiophene insertion is crucial to improve the
emission properties.
probe 6 was dissolved in the liquid crystal (λex = 375 nm, λem
389 nm). For the control experiment, DMF was used (λex
485 nm, λem = 778 nm, c = 3.0ϫ10–6 mol/L).
=
=
We noted that some new molecular rotors have been re-
ported, such as those based on BODIPY. However, these
BODIPY-based rotors give emission at 510 nm, and the
Stokes shift is small (typical small Stokes shift of BODIPY
fluorophores).[13] In comparison, our new rotors give emis-
sions at 600–700 nm, and the Stokes shifts are in the range
of 100–200 nm.
Density Functional Theory (DFT) Calculations
DFT calculations have recently been used to study the
photophysical properties of fluorescent dyes,[29] molecular
probes,[18–20,30–34] and luminescent materials.[35–39] In this
work we used DFT calculations to study the absorption and
emission properties of the new molecular rotors to investi-
Application of the Rotors to the Monitoring of the Phase
Transition of Liquid Crystals by Fluorescence Intensity
The phase transition of liquid crystals is accompanied by gate the role of the thiophene moiety in the π conjugation
significant viscosity variation. Thus, we used the new rotors of the molecular framework.
to monitor the phase transition of a liquid crystal by fluo-
rescence emission, because the emission of the rotors will was optimized. The molecule adopts a coplanar geometry.
be enhanced by the increase of the viscosity. The UV/Vis absorptions of the rotors were calculated on
First, the ground-state geometry of the known rotor 2
First, the molecular rotors were dissolved in the liquid the basis of the optimized ground-state geometry, and the
crystal, and the mixture was heated to 80 °C, well beyond results are presented in Table 2. The calculated absorption
the clearing point of the liquid crystal (68 °C). With a de- of 403 nm is close to the experimental value of 454 nm. The
crease in the temperature, we observed a sharp increase in absorption maxima of other rotors were also calculated,
the emission intensity (Figure 7a), which is believed to be and the results are close to the experimental values. For
due to the transition from the liquid phase to the liquid example, the absorption of rotors 3 and 4 were predicted to
crystal phase. A solution of rotor 3 in DMF was used as a be 521 and 558 nm, respectively. These values are close to
control experiment. No enhanced emission was observed in the experimental observations of 498 and 540 nm, respec-
the DMF solution upon lowering the temperature; thus, the tively (Table 4). Thus, we can see that DFT calculations can
enhancement of the emission of the rotor is due to the in- be used to predict the absorptions of the rotors. The red-
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Thiophene-Inserted Aryl–Dicyanovinyl Compounds
Table 2. Selected parameters of the vertical excitation (UV/Vis absorptions) of compounds 1–4. Electronic excitation energies, oscillator
strengths (f), and configurations of the low-lying excited states of the probes and its fluorescent precursors calculated at the TDDFT//
B3LYP/6-31G(d) level of theory based on the optimized ground-state geometry (the solvent methanol was considered in all the calcula-
tions).
Compound
Electronic transition[a]
TDDFT//B3LYP/6-31G(d)
Excitation energy[b] [eV]
f[c]
Composition[d]
CI[e]
1
S0 Ǟ S1
S0 Ǟ S2
3.18 (390)
4.24 (292)
1.0080
0.0041
H Ǟ L
H – 2 Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
H Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
H Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
H Ǟ L
H – 2 Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
0.7081
0.1175
0.6381
0.2747
0.7085
0.6787
0.1883
0.7495
0.7495
0.2154
0.7108
0.1802
0.6296
0.2610
2
3
4
S0 Ǟ S1
S0 Ǟ S2
3.08 (403)
3.97 (312)
1.0166
0.0102
S0 Ǟ S1
S0 Ǟ S2
2.38 (521)
3.56 (348)
1.0841
0.2282
S0 Ǟ S1
S0 Ǟ S2
2.22 (558)
3.47 (358)
1.2029
0.3172
[a] Only selected excited states were considered. [b] The numbers in parentheses are the excitation energies in wavelength [nm]. [c] Oscillator
strength. [d] H represents the HOMO, and L represents the LUMO. Only the main configurations are presented. [e] Coefficient of the
wavefunction for each excitation. The CI coefficients are absolute values.
Table 3. Emission-related electronic excitation energies, oscillator strengths (f), configurations of the low-lying excited states of the rotors
1–4 calculated at the TDDFT//B3LYP/6-31G(d) level of theory based on the optimized excited-state geometries (the solvent methanol
was considered in all the calculations).
Compound
Electronic transition[a]
TDDFT//B3LYP/6-31G(d)
Excitation energy[b]
f[c]
Composition[d]
CI[e]
1
S0 Ǟ S1
S0 Ǟ S2
2.82 (439)
4.10 (302)
1.1592
0.0109
H Ǟ L
H – 2 Ǟ L
H Ǟ L + 1
H Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
H Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
H Ǟ L
H – 1 Ǟ L
H Ǟ L + 1
0.7076
0.6710
0.2071
0.7077
0.6926
0.1324
0.7076
0.6860
0.1460
0.7085
0.6735
0.1991
2
3
4
S0 Ǟ S1
S0 Ǟ S2
2.73 (454)
3.80 (326)
1.1750
0.0177
S0 Ǟ S1
S0 Ǟ S2
2.10 (589)
3.39 (366)
1.3053
0.1986
S0 Ǟ S1
S0 Ǟ S2
1.98 (628)
3.39 (366)
1.3873
0.2792
[a] Only selected excited states were considered. [b] The numbers in parentheses are the excitation energies in wavelength [nm]. [c] Oscillator
strength. [d] H represents the HOMO, and L represents the LUMO. Only the main configurations are presented. [e] Coefficient of the
wavefunction for each excitation. The CI coefficients are absolute values.
shifted absorptions and emissions of the new rotors can be the calculated emission wavelength is 589 nm, which is close
attributed to the extended π-conjugation frameworks of the to the experimental value of 647 nm. The calculated Stokes
rotors due to the π-conjugation linker of the thiophene moi- shifts are in line with the experimental results.
ety. For example, the frontier molecular orbitals of rotor 4
are distributed over the whole molecular framework (see
Figure 9).
Table 4. Theoretical values calculated at the TDDFT//B3LYP/6-
31G(d) level of theory and measured values of the absorption and
The calculated excitation energies of the rotors are sum-
emission wavelengths (the solvent methanol was considered in all
marized in Table 2. To study the fluorescence emission of
the rotors, the singlet excited states of the rotors were opti-
mized, and the emission wavelengths were calculated by the
TDDFT method based on the optimized S1 excited-state
geometries (Table 3).
The calculated emission wavelengths of the probes are
close to the experimental values (Table 4). For example, the
calculated emission wavelength for rotor 2 is 454 nm, which
the calculations).
Compound
Theoretical value
Measured value
λab [nm] λem [nm]
[a]
[b]
λab [nm]
λem [nm]
1
2
3
4
390
403
521
558
439
454
589
628
438 480
454
498
540
499
647
681
[a] Maximum UV/Vis absorption wavelength. [b] Maximum fluo-
is close to the experimental value of 499 nm. For rotor 3, rescence emission wavelength.
Eur. J. Org. Chem. 2011, 6100–6109
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The frontier molecular orbitals involved in the singlet ex- ground and S1 excited states of the rotors show that for
cited states of the rotors are presented in Figures 8 and 9 the traditional rotors, the ground-to-excited-state transition
(for other rotors, see the Supporting Information). From electric dipole moments (ca. 2 D) are much smaller than
these MOs, it is clear that their energy levels are different those of the thiophene rotors (ca. 10 D). Larger dipole mo-
to those at the ground-state geometries. This discrepancy is ment changes in the S1 state than in the S0 state will induce
reasonable, because the conformation relaxation will follow larger Stokes shifts.[40]
the Frank–Condon excitation. The DFT calculations on the
Conclusions
Thiophene-inserted aryl–dicyanovinyl molecular rotors
have been prepared, and the absorption/emission wave-
lengths and Stokes shifts were substantially improved com-
pared with those of the traditional molecules [for example,
rotor 4 shows absorption/emission wavelengths of 559/
697 nm, respectively, and a very large Stokes shift of
138 nm]. For the conventional molecular rotor 2, these pho-
tophysical parameters are 465/503 nm and 38 nm, respec-
tively. We also demonstrated that the strategy of the thio-
phene-linked donor–acceptor can be used to prepare more
functionalized rotors and can be extended to other fluoro-
phores such as phenothiazine. The role of π conjugation of
the thiophene moiety was proved by DFT calculations,
which successfully predicted the UV/Vis absorption and
Figure 8. Frontier molecular orbitals (MOs) of rotor 2 involved in
fluorescence emission wavelengths. The molecular rotors
can be used to monitor the phase transition of liquid crys-
tals by monitoring the fluorescence intensity enhancement
upon decreasing the temperature of the liquid crystal. We
believe these results will inspire the design of new fluores-
cent rotors for molecular viscosity sensing.
the vertical excitation (i.e., UV/Vis absorption, left column) and
emission (right column). For the DFT calculations methanol was
employed as the solvent. The vertical excitation calculations are
based on the optimized ground-state geometry (S0 state) and the
emission calculations are based on the optimized excited state (S1
state) performed at the B3LYP/6-31G(d) level of theory by using
Gaussian 09W. CT represents the conformation transformation.
Excitation and radiative processes are marked as solid lines, and
non-radiative processes are marked by dotted lines.
Experimental Section
General Methods and Analytical Measurements: NMR spectra were
recorded with a 400 MHz Varian Unity Inova spectrometer. Mass
spectra were recorded with a Q-TOFMicro MS spectrometer. UV/
Vis spectra were recorded with an HP8453 UV/Vis spectrophotom-
eter. Fluorescence spectra were recorded with JASCO FP-6500 and
Sanco 970 CRT spectrofluorimeters. Fluorescence quantum yields
were measured with quinine sulfate as reference (Φ = 0.547 in
0.05 m H2SO4). Fluorescence lifetimes were measured with a Hor-
iba Jobin Yvon Fluoro Max-4 (TCSPC) instrument. p-Iodo-N,N-
dimethylaniline (3a),[41] methyl 6-iodo-3,4-dihydro-1(2H)-quin-
oline-1-propionate (5a),[41] N-[2-(methoxycarbonyl)ethyl]-1,2,3,4-
tetrahydroquinoline (precursor of compound 5a),[42] 2-formyl-5-[4Ј-
(dimethylamino)phenyl]thiophene (3b), julolidine (4a),[43] and 4-
bromojulolidine (4b)[44] were synthesized according to literature
procedures. The liquid crystal used in the viscosity sensing was
DYLC 7077-050 (nematic-phase liquid crystal, clearing point
68 °C).
5-[4-(Dimethylamino)phenyl]thiophene-2-carbaldehyde (3b): 5-For-
mylthiophene-2-boronic acid (187 mg, 1.2 mmol) and p-iodo-N,N-
dimethylaniline (247.0 mg, 1.0 mmol) were dissolved in a mixture
of toluene (5.0 mL), aqueous K2CO3 (1.0 mL, 2 m) and ethanol
(5 mL) under argon, and then [Pd(PPh3)4] (58.0 mg, 0.05 mmol)
was added. The mixture was stirred at 85 °C for 3 h. After removal
of the solvent under reduced pressure, water was added to the mix-
ture, and the product was extracted with dichloromethane. The or-
ganic layer was collected and dried with anhydrous Na2SO4. Then
the residue was purified by column chromatography (silica gel;
Figure 9. Frontier molecular orbitals (MOs) of rotor 4 involved in
the vertical excitation (i.e., UV/Vis absorption, left column) and
emission (right column). For the DFT calculations methanol was
employed as the solvent. The vertical excitation calculations are
based on the optimized ground-state geometry (S0 state) and the
emission calculations are based on the optimized excited state (S1
state) performed at the B3LYP/6-31G(d) level of theory by using
Gaussian 09W. CT represents the conformation transformation.
Excitation and radiative processes are marked as solid lines, and
the non-radiative processes are marked by dotted lines.
6106
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 6100–6109

Thiophene-Inserted Aryl–Dicyanovinyl Compounds
dichloromethane) to give the product as a yellow solid. Yield:
182.0 mg, 79.0%. H NMR (400 MHz, CDCl3): δ = 9.82 (s, 1 H,
CHO), 7.67 (d, J = 4.0 Hz, 1 H, thiophene-CH), 7.55 (d, J =
8.0 Hz, 2 H, Ar-CH), 7.24 (d, J = 4.0 Hz, 1 H, thiophene-CH),
6.70 (d, J = 8.0 Hz, 2 H, Ar-CH), 3.02 (s, 6 H, CH3) ppm. MS
(TOF EI): calcd. for [C13H13NOS + H]+ 231.0718; found 231.0723.
CH2Cl2) to give a red solid. Yield: 41.0 mg, 89.2 %. 1H NMR
(400 MHz, CDCl3): δ = 7.62 (s, 1 H, CH=CH), 7.54 (d, J = 4.0 Hz,
1 H, thiophene-CH), 7.39 (d, J = 12.0 Hz, 1 H, Ar-CH), 7.25 (s, 1
H, Ar-CH), 7.18 (d, J = 4.0 Hz, 1 H, thiophene-CH), 6.54 (d, J =
8.0 Hz, 1 H, Ar-CH), 4.09 (t, J = 12.0 Hz, 2 H, CH2), 3.63 (t, J =
12.0 Hz, 2 H, CH2), 3.34 (t, J = 12.0 Hz, 2 H, CH2), 2.73 (t, J =
12.0 Hz, 2 H, CH2), 2.57 (t, J = 12.0 Hz, 2 H, CH2), 1.91–1.94 (m,
2 H, CH2), 1.22 (t, J = 12.0 Hz, 3 H, CH3) ppm. 13C NMR
(100 MHz, CDCl3): δ = 172.0, 159.2, 150.1, 146.8, 141.2, 131.9,
127.8, 126.3, 123.2, 121.8, 119.8, 115.2, 114.3, 110.5, 72.8, 60.9,
49.7, 47.1, 31.7, 28.0, 21.8, 14.3 ppm. HRMS (ESI): calcd. for
[C22H21N3O2S + Na]+ 414.1252; found 414.1253.
1
({5-[4-(Dimethylamino)phenyl]thiophen-2-yl}methylidene)propane-
dinitrile (3): Compound 3b (69.0 mg, 0.30 mmol) and malononitrile
(60.0 mg, 0.36 mmol) were dissolved in CH2Cl2 (5 mL), and Al2O3
(92.0 mg, 0.9 mmol) was added as catalyst and desiccator. The mix-
ture was stirred at 20 °C for 1 h. The solvent was evaporated under
reduced pressure, and the crude product was purified by column
chromatography (silica gel; CH2Cl2/CH3OH = 50:1, v/v) to give a
10-Butyl-10H-phenothiazine (6a): n-C4H9Br (1.64 g, 12 mmol) was
added to a stirred solution of phenothiazine (1.99 g, 10 mmol),
hexadecyltrimethylammonium bromide (CTAB) (0.1 g), and
NaOH (0.6 g) in acetone (10 mL). The mixture was heated at reflux
for 6 h. Then the solvent was removed. The residue was extracted
with dichloromethane (DCM) and washed with water. The organic
phase was dried with anhydrous Na2SO4. The solvent was removed,
and the residue was purified by column chromatography (silica gel;
DCM/petroleum ether = 1:3, v/v) to give the product as a light-
1
red solid. Yield: 63.0 mg, 75.2%. H NMR (400 MHz, CDCl3): δ
= 7.69 (s, 1 H, CH=CH), 7.60 (d, J = 4.0 Hz, 1 H, thiophene-CH),
7.57 (d, J = 12.0 Hz, 2 H, Ar-CH), 7.27 (d, J = 4.0 Hz, 1 H, thio-
phene-CH), 6.69 (d, J = 12.0 Hz, 2 H, Ar-CH), 3.06 (s, 6 H, CH3)
ppm. 13C NMR (100 MHz, CDCl3): δ = 159.1, 151.8, 150.3, 141.1,
132.1, 128.1, 122.1, 120.0, 115.2, 114.3, 112.2, 73.4, 40.3 ppm. MS
(TOF EI): calcd. for [C16H13N3S + H]+ 279.0830; found 279.0836.
5-(2,3,6,7-Tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)thiophene-
2-carbaldehyde (4c): The reaction was carried out according to the
procedure used for the preparation of 3b. The crude product was
purified by column chromatography (silica gel; CH2Cl2) to give the
product as a yellow solid. Yield: 60.0 mg, 42.4 %. 1H NMR
(400 MHz, CDCl3): δ = 9.78 (s, 1 H, CHO), 7.63 (d, J = 4.0 Hz, 1
H, thiophene-CH), 7.16 (d, J = 4.0 Hz, 1 H, thiophene-CH), 7.11
(s, 2 H, Ar-CH), 3.21 (t, J = 12.0 Hz, 4 H, CH2), 2.76 (t, J =
12.0 Hz, 4 H, CH2), 1.94–2.00 (m, 4 H, CH2) ppm. MS (TOF EI):
calcd. for [C17H17NOS + H]+ 283.1031; found 283.1035.
1
green liquid. Yield: 1.2 g, 47.0%. H NMR (CDCl3, 400 MHz): δ
= 7.10–7.14 (m, 4 H, Ar-CH), 6.86 (t, J = 8.0 Hz, 2 H, Ar-CH),
6.83 (d, J = 8.0 Hz, 2 H, Ar-CH), 3.81 (t, J = 12.0 Hz, 2 H, CH2),
1.72–1.80 (m, 2 H,CH2), 1.40–1.46 (m, 2 H, CH2), 0.92 (t, J =
12.0 Hz, 3 H, CH3) ppm.
3-Bromo-10-butyl-10H-phenothiazine (6b): A solution of NaOH
(0.165 g, 1.61 mmol) in glacial acetic acid (10 mL) was added to a
solution of 6a (0.35 g, 1.37 mmol) in chloroform (5 mL) followed
by dropwise addition of a solution of bromine (0.07 mL,
1.37 mmol) in glacial acetic acid (3 mL) at 0 °C. The mixture was
stirred at 0–5 °C for 2 h. The solvents were removed, and, after
addition of water (15 mL) and dichloromethane (25 mL), the or-
ganic layer was separated and dried with Na2SO4. The solvent was
removed, and the residue was purified by column chromatography
(silica gel; DCM/petroleum ether, 1:3, v/v) to give the product as a
light-yellow liquid. Yield: 0.31 g, 67.8 %. 1H NMR (CDCl3,
400 MHz): δ = 7.17 (d, J = 8.0 Hz, 2 H, Ar-CH), 7.06–7.13 (m, 2
H, Ar-CH), 6.86 (t, J = 8.0 Hz, 1 H, Ar-CH), 6.80 (d, J = 8.0 Hz,
1 H, Ar-CH), 6.60 (t, J = 8.0 Hz, 1 H, Ar-CH), 3.75 (t, J = 12.0 Hz,
2 H, CH2), 1.68–1.75 (m, 2 H, CH2), 1.35–1.43 (m, 2 H, CH2), 0.90
(t, J = 12.0 Hz, 3 H, CH3 ) ppm. MS (APCI): calcd. for
C16H16BrNS [M + H]+ 335.3; found 335.1.
{[5-(2,3,6,7-Tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)thio-
phen-2-yl]methylidene}propanedinitrile (4): The reaction was carried
out according to the procedure used for the synthesis of 3. The
crude product was purified by column chromatography (silica gel;
CH2Cl2) to give a red solid. Yield: 36.0 mg, 85.3 %. 1H NMR
(400 MHz, CDCl3): δ = 7.62 (s, 1 H, CH=CH), 7.53 (d, J = 4.0 Hz,
1 H, thiophene-CH), 7.18 (d, J = 4.0 Hz, 1 H, thiophene-CH), 7.14
(s, 2 H, Ar-CH), 3.26 (t, J = 8.0 Hz, 4 H, CH2), 2.76 (t, J = 12.0 Hz,
4 H, CH2), 1.94–2.00 (m, 4 H, CH2) ppm. 13C NMR (100 MHz,
CDCl3): δ = 160.0, 149.9, 145.2, 141.4, 131.5, 125.7, 121.5, 121.4,
118.7, 115.5, 114.6, 71.8, 50.0, 27.8, 21.6 ppm. MS (TOF EI): calcd.
for [C20H17N3S + H]+ 331.1143; found 331.1153.
Ethyl 3-[6-(5-Formylthiophen-2-yl)-3,4-dihydroquinolin-1(2H)-yl]-
propanoate (5b): The reaction was carried out according to the pro-
cedure used for the synthesis of 3b. The crude product was purified
by column chromatography (silica gel; CH2Cl2) to give a yellow
solid. Yield: 85.0 mg, 78.2%. 1H NMR (400 MHz, CDCl3): δ =
9.80 (s, 1 H, CHO), 7.65 (d, J = 4.0 Hz, 1 H, thiophene-CH), 7.39
(d, J = 4.0 Hz, 1 H, Ar-CH), 7.37 (s, 1 H, Ar-CH), 7.19 (d, J =
4.0 Hz, 1 H, thiophene-CH), 6.58 (d, J = 8.0 Hz, 1 H, Ar-CH),
4.14 (t, J = 12.0 Hz, 2 H, CH2), 3.65 (t, J = 16.0 Hz, 2 H, CH2),
3.35 (t, J = 12.0 Hz, 2 H, CH2), 2.77 (t, J = 12.0 Hz, 2 H, CH2),
2.60 (t, J = 12.0 Hz, 2 H, CH2), 1.95–1.99 (m, 2 H, CH2), 1.26 (t,
J = 16.0 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz, CDCl3): δ =
182.5, 172.2, 156.2, 145.9, 140.0, 138.3, 127.4, 125.8, 123.0, 121.4,
120.7, 110.5, 60.9, 49.6, 47.1, 31.6, 28.1, 21.9, 14.3 ppm. It should
be noted that ethanol was used as solvent for the reaction; we
found that the methyl ester was transformed into the ethyl ester.
5-(10-Butyl-10H-phenothiazin-3-yl)thiophene-2-carbaldehyde (6c):
Compound 6b (668.5 mg, 2.0 mmol), 2.0 m aqueous K2CO3 solu-
tion (10 mL), and [Pd(PPh3)4] (115 mg, 0.1 mmol) in THF (15 mL)
were heated to 50 °C and stirred under argon for 0.5 h. 5-For-
mylthienyl-2-boronic acid (376 mg, 2.4 mmol) in THF (15 mL) was
added, and the mixture was heated at reflux for 24 h. After comple-
tion of the reaction, water (15 mL) was added, and the product was
extracted with DCM. The organic layer was collected and dried
with anhydrous Na2SO4. Then the residue was purified by column
chromatography (silica gel; DCM/petroleum ether, 1:1, v/v) to give
the product as a yellow solid. Yield: 211.0 mg, 28.9%. 1H NMR
(CDCl3, 400 MHz): δ = 9.81 (s, 1 H, CHO), 7.63 (d, J = 4.0 Hz, 1
H, thiophene-CH), 7.37 (d, J = 8.0 Hz, 1 H, Ar-CH), 7.34 (s, 1 H,
Ar-CH), 7.21 (d, J = 4.0 Hz, 1 H, thiophene-CH), 7.09–7.16 (m, 2
H, Ar-CH), 6.92 (t, J = 16.0 Hz, 1 H, Ar-CH), 6.84 (d, J = 16.0 Hz,
Ethyl 3-{6-[5-(2,2-Dicyanoethenyl)thiophen-2-yl]-3,4-dihydro- 1 H, Ar-CH), 6.79 (t, J = 8.0 Hz, 1 H, Ar-H), 3.82 (t, J = 16.0 Hz,
quinolin-1(2H)-yl}propanoate (5): The reaction was carried out ac-
cording to the procedure used for the synthesis of 3. The crude
2 H, CH2), 1.73–1.80 (m, 2 H, CH2), 1.41–1.47 (m, 2 H, CH2),
0.93 (t, J = 16.0 Hz, 3 H, CH3) ppm. MS (TOF EI): calcd. for
product was purified by column chromatography (silica gel; [C21H19NOS2 + H]+ 365.0908; found 365.0918.
Eur. J. Org. Chem. 2011, 6100–6109
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6107

J. Shao, S. Ji, X. Li, J. Zhao, F. Zhou, H. Guo
FULL PAPER
{[5-(10-Butyl-10H-phenothiazin-3-yl)thiophen-2-yl]methylidene}-
propanedinitrile (6): The reaction was carried out according to the
procedure used for the synthesis of 3. The crude product was puri-
fied by column chromatography (silica gel; DCM/petroleum ether,
2:1, v/v) to give a red solid. Yield: 37.0 mg, 87.3 %. 1H NMR
(CDCl3, 400 MHz): δ = 7.67 (s, 1 H, Ar-CH), 7.59 (d, J = 4.0 Hz,
1 H, Ar-CH), 7.41 (t, J = 8.0 Hz, 1 H, Ar-CH), 7.34 (s, 1 H,
CH2=CH2), 7.22 (d, J = 4.0 Hz, 1 H, thiophene-CH), 7.08–7.16
(m, 2 H, Ar-CH), 6.92 (d, J = 16.0 Hz, 1 H, Ar-CH), 6.84 (d, J =
8.0 Hz, 1 H, thiophene-CH), 6.79 (d, J = 12.0 Hz, 1 H, Ar-CH),
3.83 (t, J = 16.0 Hz, 2 H, CH2), 1.72–1.80 (m, 2 H, CH2), 1.41–
1.48 (m, 2 H, CH2), 0.92 (t, J = 16.0 Hz, 3 H, CH3) ppm. 13C
NMR (100 MHz, CDCl3): δ = 156.1, 150.4, 147.3, 144.2, 140.4,
133.5, 127.8, 127.7, 126.5, 126.1, 126.0, 125.2, 123.8, 123.6, 123.3,
117.0, 115.9, 115.6, 114.6, 113.7, 47.6, 29.1, 20.3, 13.9 ppm. MS
(TOF EI): calcd. for [C24H19N3S2 + H]+ 413.1020; found 413.1031.
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[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
4-(2,3,6,7-Tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)benz-
aldehyde (7a): The reaction was carried out according to the pro-
cedure used for the synthesis of 3b. The crude product was purified
by column chromatography (silica gel; DCM/petroleum ether, 3:1,
v/v) to give a yellow solid. Yield: 69.0 mg, 86.3 %. 1H NMR
(400 MHz, CDCl3): δ = 9.98 (s, 1 H, CHO), 7.84 (d, J = 8.0 Hz, 2
H, Ar-CH), 7.66 (d, J = 8.0 Hz, 2 H, Ar-CH), 7.13 (s, 2 H, Ar-
CH), 3.21 (t, J = 12.0 Hz, 4 H, CH2), 2.82 (t, J = 16.0 Hz, 4 H,
CH2), 1.97–2.03 (m, 4 H, CH2) ppm. MS (TOF EI): calcd. for
[C19H19NO + H]+ 277.1467; found 277.1468.
[10]
[11]
[12]
[13]
[4-(2,3,6,7-Tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)benzyl-
idene]propanedinitrile (7): The reaction was carried out according
to the procedure used for the synthesis of 3. The crude product was
purified by column chromatography (silica gel; DCM) to give a red
[14]
[15]
[16]
1
solid. Yield: 42 mg, 89.5%. H NMR (400 MHz, CDCl3): δ = 7.86
(d, J = 8.0 Hz, 2 H, Ar-CH), 7.66 (s, 1 H, CH=CH), 7.64 (d, J =
4.0 Hz, 2 H, Ar-CH), 7.15 (s, 2 H, Ar-CH), 3.24 (t, J = 12.0 Hz, 4
H, CH2), 2.81 (t, J = 12.0 Hz, 4 H, CH2), 1.97–2.02 (m, 4 H,CH2)
ppm. 13C NMR (100 MHz, CDCl3): δ = 159.2, 147.9, 144.1, 131.8,
128.1, 126.0, 125.9, 124.7, 121.8, 114.8, 113.6, 79.1, 50.0, 28.0,
21.9 ppm. MS (TOF EI): calcd. for [C22H19N3 + H]+ 325.1579;
found 325.1589.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[45]
DFT Calculations:
All the calculations were based on density
functional theory (DFT) with B3LYP functional and 6-31G(d) ba-
sis set. Solvent methanol was considered in all the calculations
(CPCM model). The UV/Vis absorption of the compounds (verti-
cal excitation) were calculated with the TDDFT methods based on
the optimized ground state geometry (S0 state). For the fluores-
cence emission, the emission wavelength were calculated based on
the optimized excited states (in most cases S1 state). All these calcu-
lations were performed with Gaussian 09W.
F. Silvestri, A. Marrocchi, M. Seri, C. Kim, T. J. Marks, A.
Facchetti, A. Taticchi, J. Am. Chem. Soc. 2010, 132, 6108–6123.
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5685–5695.
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Bioorg. Chem. 2005, 33, 415–425.
Supporting Information (see footnote on the first page of this arti-
cle): More synthesis details, structural characterization data, and
photophysical properties of the rotors.
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
Y. Wu, H. Guo, J. Shao, X. Zhang, S. Ji, J. Zhao. J. Fluoresc.
2011, 21, 1143–1154.
Acknowledgments
F. Han, L. Chi, W. Wu, X. Liang, M. Fu, J. Zhao, J. Pho-
tochem. Photobiol. A: Chem. 2008, 196, 10–23.
G. Zhao, J. Liu, L. Zhou, K. Han, J. Phys. Chem. B 2007, 111,
8940–8945.
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H. Guo, Y. Jing, X. Yuan, S. Ji, J. Zhao, X. Li, Y. Kan, Org.
Biomol. Chem. 2011, 9, 3844–3853.
We thank the National Natural Science Foundation of China
(NSFC) (20972024 and 21073028), the Fundamental Research
Funds for the Central Universities (DUT10ZD212 and
DUT11LK19), the Royal Society (UK) and NSFC China-UK
Cost-Share Science Networks (21011130154), the Ministry of Edu-
cation (SRFDP-200801410004 and NCET-08-0077), the Education
Department of Liaoning Province (2009 T015) and the State Key
Laboratory of Fine Chemicals (KF0802) for financial support.
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