A proton and optic dual-control molecular switch based on photochromic diarylethene bearing a rhodamine unit

Article abstract of DOI:10.1016/j.tet.2011.04.023

A novel fluorescent switch based on rhodamine B and photochromic diarylethene, 1-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4- rhodamine B hydrazine-Schiff base-phenyl)-3-thienyl]perfluorocyclopentene (1), has been successfully synthesized through the condensation of rhodamine B hydrazine and 1-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4- formylphenyl)-3-thienyl]perfluorocyclopentene. UV and FL measurements reveal that the compound exhibits good photochromic properties responsive to proton and optic dual inputs. Upon irradiation with 297 nm light, the colorless solution of compound 1 turns blue, while the blue solution becomes colorless after irradiated with visible light (λ>450 nm). Furthermore, upon an addition of H⁺, the fluorescence resonance energy transfers from the rhodamine unit (FRET donor) to the closed-ring diarylethene unit (FRET acceptor), although no energy transfer occurs when the diarylethene is in the open-ring form. The emission intensity of the rhodamine can be modulated with proton and UV/vis light and molecular-level signal communication has been constructed, indicating high potentials of the compound in molecular switches or naked eye recognition systems.

Full text of DOI:10.1016/j.tet.2011.04.023

Tetrahedron 67 (2011) 4236e4242
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A proton and optic dual-control molecular switch based on photochromic
diarylethene bearing a rhodamine unit
*
Weijun Liu, Shouzhi Pu , Shiqiang Cui, Gang Liu, Congbin Fan
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel fluorescent switch based on rhodamine B and photochromic diarylethene, 1-[2-methyl-5-(4-me-
Received 13 January 2011
Received in revised form 29 March 2011
Accepted 8 April 2011
thoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4-rhodamine
B hydrazine-Schiff base-phenyl)-3-thienyl]per-
fluorocyclopentene (1), has been successfully synthesized through the condensation of rhodamine
B hydrazine and 1-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4-formylphenyl)-3-thienyl]
perfluorocyclopentene. UV and FL measurements reveal that the compound exhibits good photochromic
properties responsive to proton and optic dual inputs. Upon irradiation with 297 nm light, the colorless so-
lution of compound 1 turns blue, while the blue solution becomes colorless after irradiated with visible light
Available online 14 April 2011
Keywords:
Photochromism
Diarylethene
Rhodamine
(l
>450 nm). Furthermore, upon an addition of Hþ, the fluorescence resonance energy transfers from the
rhodamine unit (FRET donor) to the closed-ring diarylethene unit (FRET acceptor), although no energy
transfer occurs when the diarylethene is in the open-ring form. The emission intensity of the rhodamine can
be modulated with proton and UV/vis light and molecular-level signal communication has been constructed,
indicating high potentials of the compound in molecular switches or naked eye recognition systems.
Ó 2011 Elsevier Ltd. All rights reserved.
Molecular switch
1. Introduction
and rhodamine, their combination is promising in developing
a dual-control fluorescence resonance energy transfer (FRET) sys-
Recently, photochromic compounds have attracted more and
more attention due to their unique characteristics in simulating
functionalelectronic components. Open-ringand closed-ring isomers
of photochromic compounds, corresponding to colorless and colored
modes when stimulated with UV/vis light, can represent ‘0’ and ‘1’ of
digital codes homologous to ‘on’ and ‘off’ states, respectively.1 Among
various photochromic compounds, diarylethenes bearing two thio-
phene/benzothiophene rings are regarded as the best candidates for
photonics applications, such as optical memories2 and photo-
switches,3 owing to their excellent thermal stability, remarkable fa-
tigue resistance, and rapid response.4 In the field of photochromic
complex molecular systems, integrating several switchable groups
within a single molecule to form molecular switches is especially at-
tractive. Inthisregard,variousapproachestoswitchingsystemsbased
on diarylethenes have been proposed in recent years.5
Because of their high fluorescence quantum yields, large molar
extinction coefficients, and visible wavelength excitation, rhoda-
mine dyes are widelyemployed to construct fluorescent probes6 and
chemosensors.7 Rhodamine derivatives can undergo equilibrium
between a colorless spirolactam (no fluorescence) form and a red
open-ring form (strong fluorescence) when protons or metal ions
are bound to the host compound. In view of virtue of diarylethenes
tem. FRET is a typical process where fluorescence energy transfers
from an excited fluorophore group (donor) to another chromophore
unit (acceptor) via a link distance of 1e10 nm. Such occurrence,
however, requires that the acceptor characteristic absorption band
overlaps the emission of the donor. Therefore, reports about pho-
tochromic diarylethene and fluorophore functionalized to form
FRET molecular are still lacking. Zheng et al. reported a dual-control
perhydrogencyclopenteneediarylethene switching molecule using
rhodamine as a fluorophore donor and the closed-ring form of dia-
rylethene as an acceptor, and, however, the stability and fatigue
resistance were found to be unsatisfactory.8 Similarly, Tian et al.
employed perhydrogencyclopentene as a control molecule and
a naphthyl moiety as a fluorescence donor to construct a multi-
stimulus that responded to UV irradiation, proton, and copper
ions.9 Giordano et al. studied a series of photochromic FRET mole-
cules based on perfluorocyclopentene as acceptors and naphthyl
fluorophorederivatives as donors, butonlyfound themtopossess no
proton and light control response, presumably because naphthyl
fluorophore has no proton capture capability.10
Herein we synthesize a novel proton and optic dual-control
fluorescent switch containing photochromic perfluorocyclopentene
and rhodamine B moieties, 1-[2-methyl-5-(4-methoxylphenyl)-3-
thienyl]-2-[2-methyl-5-(4-rhodamine B hydrazine-Schiff base-phe-
nyl)-3-thienyl]perfluorocyclopentene (1), and explore its proton and
light dual-control responses.
* Corresponding author. E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.04.023

W. Liu et al. / Tetrahedron 67 (2011) 4236e4242
4237
2. Results and discussion
2.1. Photochromism of compound 1
Diarylethene 1 canundergoa reversible photochromism reaction
in DMSO upon alternating irradiation with UV and visible light
(Scheme 1). Changes in the absorption spectra of compound 1 in-
duced by photoirradiation at room temperature in DMSO
(C¼2.0ꢀ10ꢁ5 mol/L) are shown in Fig.1. The absorption maximum of
compound 1O0 (see Scheme 1) is observed at 276 nm
(
3
¼4.12ꢀ104 L molꢁ1 cmꢁ1). Upon irradiation with 297 nm light,
the colorless solution of 1O0 turns blue due to the appearance of
a new visible absorption band centered at 618 nm (
3
¼1.11ꢀ104
L molꢁ1 cmꢁ1) attributable to the formation of the closed-ring form
1C. After irradiation for 30 min under UV light, a photostationary
state (PSS) is reached and about 66% of the open-ring form (1O0) is
converted to the closed-ring form (1C0) as evidenced by 1H NMR
results. For open-ring 1O0, a peak at 3.83 ppm is assigned to the 1H
resonance of eOCH3. After the formation of PSS, a noticeable new
peak at 3.86 (H signal shift of eOCH3) appears, indicative of the
formation of a new closed-ring compound 1C0. On the other hand,
the blue solution turns colorless upon irradiation with visible light
Fig. 1. Changes in the absorption spectra of compound 1 in DMSO at room tempera-
ture (C¼2.0ꢀ10ꢁ5 mol/L). Inset: color change of compound 1 upon alternating irradi-
ation with UV/vis light in DMSO.
1.0
0.8
0.6
(l
>450 nm) for 2 min, indicating that 1C0 returns to its initial form
1O0, and a clear isosbestic point is observed at 364 nm. Such colo-
rationedecoloration cycles can be repeated more than 50 times. In
ordertoclarify the difference beforeand afterthe introduction of the
rhodamine dye moiety into the diarylethene, compound 2 (Scheme
1) was studied under the same measurement conditions. As
revealed in Fig. 2, the absorption maximum in the open- and closed-
UV
Vis
0.4
0.2
0.0
Vis
UV
ring forms are 314 (
3
¼1.78ꢀ104 L molꢁ1 cmꢁ1) and 618 nm
300
400
500
600
700
800
(3
¼9.68ꢀ103 L molꢁ1 cmꢁ1), respectively, while a clear isosbestic
Wavelength (nm)
point is observed at 348 nm. Apparent differences exist between the
photochromic properties of compounds 1 and 2. Diarylethene 1
exhibits a larger molar coefficient in either the open- or closed-ring
forms than those of 2, while displays a smaller absorption maximum
in the open-ring form than that of 2. This observation could be at-
tributed to the substituent effect induced by the rhodamine unit.
Similar substitution effects on the photochromic properties of dia-
rylethene derivatives have been reported in our previous studies.11
Fig. 2. Changes in the absorption spectra of compound 2 in DMSO at room tempera-
ture (C¼2.0ꢀ10ꢁ5 mol/L).
spirolactam form is non-fluorescent and non-absorbing, while the
open-ring form has strong fluorescent emission. The emission in-
tensity of compound 1 can thus be tuned with proton, and the
emission spectra of 1 (C¼2.0ꢀ10ꢁ5 mol/L) are shown in Fig. 3A with
excitation at 520 nm in DMSO after alternating additions of tri-
fluoroacetic acid (TFA) and triethylamine (TEA). Initially the 1O0
DMSO solution is non-fluorescent with excitation at 520 nm. Upon
2.2. Photon-control photochromism of compound 1
Rhodamine derivatives undergo equilibrium between the color-
less spirolactam form and the red open-ring form. The two forms
feature completely different spectrophotomatric properties when
protons or metal ions are bound to the host compounds: the
addition of TFA gradually (0.1 mL each time) up to 30 equiv (w1.0 mL),
the color of the 1O0 solution becomes red progressively and an ob-
vious increase is found in fluorescent emission for 1O at about
586 nm, due to the formation of the open-ring amide form of 1O,
F
F
F
F
F
F
F
F
A
F
F
F
F
N
N
UV
Vis
S
S
S
S
O
N
O
O
N
O
H3CO
N
H3CO
N
N
N
1O'
1C'
F
F
F
F
F
F
F
F
B
F
F
F
F
UV
Vis
S
S
S
S
O
O
H3CO
H3CO
2C
2O
Scheme 1. Photochromism of diarylethenes 1 (A) and 2 (B).

4238
W. Liu et al. / Tetrahedron 67 (2011) 4236e4242
and fatigue resistance of perfluorocyclopenteneediarylethene
A
have been improved markedly compared with a perhydrogen-
3000
2500
2000
1500
1000
500
cyclopenteneediarylethene molecular switch.8
TFA TEA
2.3. Optic-control photochromism of compound 1
Diarylethene derivates usually afford different color changes
upon alternating irradiation with UV/vis light. Because of these
features, the combination of diarylethene with a chemo-responsive
dye offers a promising approach to the design of optical molecular
switches.8,9 In this study, we record spectral modulation of com-
pound 1O (1O0 in the presence of TFA) using UV/vis light and FL
techniques. Fig. 5A depicts the fluorescent changes of compound 1O
(C¼2.0ꢀ10ꢁ5 mol/L, the addition quantity of TFA is 30 equiv
0
550
600
650
Wavelength (nm)
(w1.0 mL)) with excitation at 520 nm in DMSO under alternating
B
1.0
0.8
0.6
0.4
0.2
0.0
irradiation with UV/vis light. The results show that the fluorescent
emission of 1O can also be modulated reversibly and undergo re-
versible photochromism upon alternating irradiation with UV/vis
light. Under acidic condition, 1O exhibits a strong fluorescent
emission at 586 nm ascribed to the formation of the open-ring
rhodamine amide. The emission intensity is greatly dependent on
the status of the diarylethene whether in the open- or closed-ring
forms. When the photocyclization reaction is carried out on the
diarylethene unit upon irradiation with UV light, the formation of
the closed-ring form results in decreased emission intensity at
586 nm with the increasing UV irradiation time. After reaching the
photostationary state, the emission intensity of 1C is quenched to
only w 17% of its original intensity and the FRETefficiency is close to
100%.10,13 Such fluorescence quenching is likely due to the intra-
molecular fluorescence resonance energy transfer from the open-
ring rhodamine amide unit to the closed-ring diarylethene unit,
since FRET is a typical process where fluorescence energy transfers
TFA/TEA
Fig. 3. Fluorescent changes (A) and switch cycles (B) of compound 1 (C¼2.0ꢀ10ꢁ5 mol/
L) induced by alternating additions of TFA and TEA in DMSO excited at 520 nm and
detected at 586 nm at room temperature.
whose characteristic absorption is centered at 565 nm. Its ab-
sorption intensity increases with increasing amount of TFA. As the
absorption intensity reaches the maximum, the fluorescence in-
tensity also peaks with a fluorescent quantum yield of 0.103 using
anthracene as a reference.12 The gradual back addition of TEA up
A
3000
Vis
UV
2500
2000
1500
1000
500
to 50 equiv (w3.0 mL) regenerates the spirolactam form of rho-
damine (see Fig. 4 for changes in the absorption spectra upon
alternating addition of TFA and TEA, where the addition amount of
the latter is more than that of the former), while the fluorescent
intensity decreases as the original emission spectra recover si-
multaneously. Shown in Fig. 3B is the reversible modulation of the
fluorescent emission intensity of compound 1 with alternating
additions of TFA and TEA; the results indicate that the fluorescent
switch cycles can be repeated many times with only a slight
degradation observed. The results also suggest that the stability
0
550
600
650
Wavelength (nm)
B
1.00
0.75
0.50
0.25
0.06
TFA TEA
0.04
0.02
0.00
0
3
6
9
12 15 18 21 24
Time (min)
500
550
600
Fig. 5. Fluorescent changes (A) and switch cycles (B) of compound 1 (C¼2.0ꢀ10ꢁ5 mol/L)
Wavelength (nm)
in the presence of 30 equiv (w1.0 mL) of TFA upon alternating irradiation with 297 nm
Fig. 4. Changes in the absorption spectra of compound 1O in DMSO (C¼2.0ꢀ10ꢁ5 mol/L)
ultraviolet light and visible light (
586 nm at room temperature.
l>400 nm), in DMSO excited at 520 nm and detected at
with alternating additions of TFA and TEA at room temperature.

W. Liu et al. / Tetrahedron 67 (2011) 4236e4242
4239
from an excited fluorophore group (donor) to another chromophore
back to red upon irradiation with visible light (l>450 nm), in-
unit (acceptor) via a link distance of 1e10 nm. The process requires
that the characteristic absorption band of the acceptor overlaps the
emissionof the donor.10,14 Becausethe emissionband(550e650 nm)
of rhodamine in the open-ring state has certain overlap with the
absorption band (550e750 nm) of the closed-ring form of diary-
lethene, the FRET process occurs. Subsequent back irradiation with
dicating that 1C has returned to the initial state of 1O.
2.4. Logic circuit with compound 1
If compound 1O0 in acidic condition is regarded as the initial
state, the fluorescence changes of compound 1 can be combined as
a molecular switch with three inducing inputs: I1 (297 nm UV
light), I2 (450 nm visible light), and I3 (TEA). The responding output
signal is the emission at 586 nm. The output is off when the fluo-
rescent intensity at 586 nm is quenched to 17% of the original value,
with the original value signaling on. The molecular switch in
compound 1 can read a string of three inputs and give a specific
output. For example, when the input string 001 is corresponding to
off, off, and on in I1, I2, and I3, respectively, the output of compound
1 is off and the fluorescence is quenched. Similarly, when the string
is 010, 000, 100, 101, 110, 111, and 011, the output is 1, 1, 0, 0, 1, 0, and
0, respectively. The corresponding fluorescent and molecular
structure change schematics are shown in Fig. 7. Upon an addition
of Hþ to compound 1O0, the molecule exhibits strong red fluores-
cence at 586 nm when excited at 520 nm, which is ascribed to the
characteristics of the open-ring rhodamine amide. The subsequent
irradiation with 297 nm UV light weakens the fluorescence sharply,
owing mainly to the formation of the closed-ring diarylethene and
thus resulting in fluorescence transfers from the dye donor to the
closed-ring diarylethene acceptor. In other words, the emission
intensity of the compound can be modulated with proton and UV/
vis light with good stability and fatigue resistance. At the same
time, the color change responsive to both light and chemical inputs
is desired to be visible to naked eye. For example, upon irradiation
with 297 nm UV light, the colorless open-ring isomer 1O0 induces
a ring-closure reaction and evolves into the blue color of the closed-
ring isomer 1C0. The blue solution returns to colorless upon irra-
diation with visible light because 1C0 goes back to the initial state
1O0. Furthermore, upon addition of Hþ, the colorless 1O0 solution
becomes red due to the formation of the open-ring amide 1O, and
the subsequent irradiation with UV light yields the blue 1C solution,
visible light (l>450 nm) regenerates the open-ring form of diary-
lethene and recovers the original emission spectra. Furthermore, the
modulation of the emission intensity of 1O upon alternating irra-
diation with UV/vis is reversible and the fluorescent switch cycles
can be repeated several times as shown in Fig. 5B. Compound 1O is
also found to have good stability and fatigue resistance. Absorption
changes of compound 1O are illustrated in Fig. 6 corresponding to
the repeated irradiation. Due to the appearance of a new visible
absorption band centered at 618 nm, the red solution of 1O turns
blue upon irradiation with 297 nm light. The blue solution changes
Fig. 6. Absorption changes of compound 1O (1O0 in the presence of 30 equiv (w1.0
mL)
of TFA, C¼2.0ꢀ10ꢁ5 mol/L) in DMSO upon alternating irradiation with 297 nm ultra-
violet light and visible light (l>400 nm). Inset: color change of 1O upon alternating
irradiation with UV/vis light in DMSO.
F
F
F
F
F
F
F
F
FRET-ON
FRET-OFF
F
F
F
F
UV
Vis
S
S
S
S
OCH3
N
OCH3
CONH
N
CONH
N
O
N
N
O
N
1C
1O
H
H
+ H
+ H
F
F
F
F
F
F
F
F
F
F
F
F
N
N
UV
Vis
S
S
S
S
O
OCH3
O
N
OCH3
N
N
N
O
O
N
N
1O'
1C'
Fig. 7. Schematics of molecular structures and fluorescence changes with proton and optic stimuli.

4240
W. Liu et al. / Tetrahedron 67 (2011) 4236e4242
which further changes to red upon back irradiation with visible
light. These observations demonstrate that the compound has great
potentials as a molecular switch.15
thienyl]perfluorocyclopentene (1), using rhodamine B as a fluo-
rophore and perfluorodiarylethene as a photochromic group. The
compound exhibits good photochromic, and proton and optic dual-
control properties. The fluorescent emission intensity of compound
1 can be modulated with proton and UV/vis light stimuli, and the
fluorescent switch cycles can be repeated many times. A molecular-
level signal communication system is constructed using three
external inputs and one optical output. These results demonstrate
that compound 1 has great potentials as a molecular switch and
3. Conclusion
In the current paper, we have synthesized a novel fluorescent
molecular switch, 1-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]-
2-[2-methyl-5-(4-rhodamine B hydrazine-Schiff base-phenyl)-3-
°C
°C
°C
Scheme 2. Synthetic route to compound 1.

W. Liu et al. / Tetrahedron 67 (2011) 4236e4242
4241
a naked eye recognition system in virtue of its color changes upon
light or proton stimuli.
138.93, 140.25, 140.49, 142.44, 143.37, 159.62, 191.24. MS (ESI) m/z
578.4 (MþH).
4.5. Synthesis of compound 3
4. Experimental
Compound 3 was synthesized according to the reference.7b,17 In
a 250 mL flask equipped with a condenser and a stirrer, rhodamine
B (3.00 g) was dissolved in 150 mL of ethanol. Hydrazine hydrate
(85%, 7.0 mL) dissolved in 10 mL of ethanol was added dropwise
with vigorous stirring at room temperature. After the addition, the
stirred mixture was refluxed for 4 h, while the solution color
changed from dark purple to light orange. The mixture was then
cooled and ethanol was removed under reduced pressure. To the
mixture was slowly added HCl (1.00 M) in the flask to generate
a clear red solution. Subsequently, NaOH (1.00 M) was introduced
slowly with stirring until the pH of the solution reached 9e10. The
resulting precipitate was filtered, washed with water, and then
dried in vacuum. A product of compound 3 (2.22 g) was obtained as
a pink solid in 73% yield. Mp 176e177 ꢂC. 1H NMR (CDCl3, 400 MHz),
4.1. General methods
NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer using CDCl3 as a solvent and tetramethylsilane as an
internal standard. UV/vis spectra were recorded on a PerkineElmer
Lambda 900 spectrometer. Photoirradiation was carried out using
an SHG-200 UV lamp, a CX-21 ultraviolet fluorescence analysis
cabinet, and a BMH-250 visible lamp. Lights of appropriate wave-
lengths were isolated using different light filters. Luminescence
spectra were measured on a HITACHI 4500 fluorescence spectro-
photometer. The luminescence quantum yield in solution was
measured using anthracene (V¼0.27 in DMSO) as a reference. All
solvents used were of spectro-grade and purified by distillation
prior to use.
d
(ppm): 1.09 (t, 12H, J¼6.8 Hz), 3.24e3.29 (q, 8H, J¼6.7 Hz), 3.53 (s,
2H), 6.22 (d, 2H, J¼8.0 Hz), 6.34e6.40 (m, 4H), 7.03(d, 1H, J¼8.0 Hz),
4.2. Synthesis
7.38 (t, 2H, J¼4.0 Hz), 7.86 (d, 1H, J¼4.0 Hz). 13C NMR (CDCl3,
100 MHz),
d (ppm): 12.62, 44.37, 65.91, 98.00, 104.63, 108.05,
The rhodamine-based perfluorodiarylethene derivative, 1-[2-
methyl-5-(4-methoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4-rho-
122.99, 123.83, 128.09, 130.06, 132.49, 148.90, 151.57, 153.86, 166.14.
MS (ESI) m/z 456.3 (MþH).
damine
B hydrazine-Schiff base-phenyl)-3-thienyl]perfluoro-
cyclopentene (1), was synthesized from rhodamine B hydrazide
and perfluorodiarylethene in 75% yield. Compound 1 and other
Acknowledgements
intermediate products were confirmed structurally by 1H and 13
C
This work was supported by the National Natural Science
Foundation of China (20962008), New Century Excellent Talents in
University (NCET-08-0702), Project of Jiangxi Academic and Tech-
nological Leader (2009DD00100), Natural Science Foundation of
Jiangxi Province (2009GZH0034, 2009GQH0036, 2010GQH0038),
and Project of the Science Funds of Jiangxi Education Office
(GJJ11026).
NMR spectroscopy. Their synthetic route is summarized in
Scheme 2 and experimental details were carried out as follows.
4.3. Synthesis of compound 1
To a stirred solution of 1-[2-methyl-5-(4-methoxylphenyl)-3-
thienyl]-2-[2-methyl-5-(4-formylphenyl)-3-thienyl]perfluorocyclo
pentene (2, 0.80 g) and rhodamine B hydrazine (3, 0.65 g) in 80 mL
of chloroform, a few drops of acetic acid were added to catalyze the
reaction at room temperature and under a nitrogen atmosphere.
After stirring for 10 min, the reaction mixture was heated to 85 ꢂC
and refluxed for 72 h; it was then allowed to warm to room tem-
perature. The mixture solution was subsequently washed with di-
lute NaOH solution and extracted with CHCl3, and the organic
phase was dried over anhydrous magnesium sulfate, filtered, and
evaporated. The crude product was purified by column chroma-
tography on silica gel (ethyl acetate/petroleum ether, v/v¼1/6) to
give pure product 1.05 g in 75% yield. Mp 170e171 ꢂC. 1H NMR
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(CDCl3, 400 MHz),
d
(ppm): 1.15 (t, 12H, J¼6.0 Hz), 1.91 (s, 3H), 1.95
(s, 3H), 3.29e3.35 (q, 8H, J¼8.0 Hz), 3.83 (s, 3H), 6.23e6.26 (m, 2H),
6.44e6.53 (m, 4H), 6.89 (s, 1H), 6.91 (s, 1H), 7.12 (s, 1H), 7.14 (s, 1H),
7.24 (s,1H), 7.42e7.55 (m, 8H), 7.99 (d,1H, J¼8.0 Hz), 8.72 (s,1H). 13C
NMR (CDCl3, 100 MHz),
d (ppm): 12.62, 14.40, 14.50, 44.32, 55.38,
66.24, 97.97, 106.30, 108.04, 114.41, 121.29, 122.79, 123.34, 123.93,
125.30, 125.65, 126.20, 126.93, 128.03, 128.30, 129.63, 133.29,
134.25, 141.71, 142.21, 146.68, 148.98, 151.56, 153.30, 159.55, 164.87.
MS (ESI) m/z 1017.2 (MþH).
4.4. Synthesis of compound 2
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d (ppm): 14.47,
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