New heterocyclic tetrathiafulvalene compounds with an azobenzene moiety: Photomodulation of the electron-donating ability of the tetrathiafulvalene moiety

Article abstract of DOI:10.1021/jo070651e

(Chemical Equation Presented) New heterocyclic TTF compounds 1a-c and 2 with an azobenzene moiety were described. The oxidation potential of 1a could be reversibly modulated by alternating UV and visible light irradiation. As a result, a molecular switch

Full text of DOI:10.1021/jo070651e

New Heterocyclic Tetrathiafulvalene Compounds
with an Azobenzene Moiety: Photomodulation of
the Electron-Donating Ability of the
The electron-donating ability of the TTF moiety is related to
its conformation that can be affected by the peripheral substi-
tuted groups, in particular, for the heterocyclic tetrathiafulvalene
compounds. These peripheral substituted groups may induce the
conformational change of the central fulvalene core, and
accordingly, it is possible to modulate the electron-donating
ability. For example, the deformation of the TTF moieties in
Tetrathiafulvalene Moiety
Guoyong Wen, Deqing Zhang,* Yanyan Huang, Rui Zhao,
Lingyun Zhu, Zhigang Shuai, and Daoben Zhu*
6
tetrathiafulvalenophanes enhanced their electron-donating abil-
ity. It should be noted that it is important to find ways to tune
the electron-donating ability for designing new functional
molecular systems with TTF moieties.
Beijing National Laboratory for Molecular Sciences, Organic
Solids Laboratory, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100080, China
It is well-known that the reversible transformation between
the trans- and cis-forms of azobenzene can occur under UV/
visible light irradiation. By taking advantage of this property
of azobenzene, a great deal of interesting molecules containing
azobenzene moieties have been described for the studies of
molecular switches, molecular machines, etc. These molecular
systems also show potential applications in optical data storage.
By combining the features of TTF and azobenzene moieties,
new heterocyclic TTF compounds (1a-c, Scheme 1) are
dqzhang@iccas.ac.cn
ReceiVed March 30, 2007
7
8
(
3) (a) Bryce, M. R. AdV. Mater. 1999, 11, 11-23 and further references
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H.; Jeppesen, J. O.; Levillain, E.; Becher, J. Chem. Commun. 2003, 846-
New heterocyclic TTF compounds 1a-c and 2 with an
azobenzene moiety were described. The oxidation potential
of 1a could be reversibly modulated by alternating UV and
visible light irradiation. As a result, a molecular switch with
UV/visible light as the inputs and the electrochemical signal
as the output was achieved. Moreover, it was found that the
influence of the azobenzene photoisomerization on the
electronic property of the TTF unit became stronger with
shorter spacers in compounds 1a-c.
8
47. (f) Zhang, G.; Zhang, D.; Guo, X.; Zhu, D. Org. Lett. 2004, 6, 1209-
212. (g) Xiao, X.; Xu, W.; Zhang, D.; Xu, H.; Lu, H.; Zhu, D. J. Mater.
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Since the synthesis of tetrathiafulvalene (TTF) in the 1970s,
TTF and its derivatives have been intensively investigated as
1
building blocks of organic conductors and superconductors.
TTF and its derivatives are strong electron donors, and they
can be reversibly transformed into the corresponding radical
cations and dications. By making use of this unique property
of the TTF moiety, a number of electron donor-acceptor
molecules and supramolecules have been designed and studied
2
004, 126, 16296-16297. (e) Li, X.; Zhang, G.; Ma, H.; Zhang, D.; Li, J.;
Zhu, D. J. Am. Chem. Soc. 2004, 126, 11543-11548. (f) Zhang, G.; Li,
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2
3
4
for use as molecular shuttles, molecular switches, logic gates,
and chemical sensors.5
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1
0.1021/jo070651e CCC: $37.00 © 2007 American Chemical Society
Published on Web 07/06/2007
J. Org. Chem. 2007, 72, 6247-6250
6247

SCHEME 1. Synthetic Approach to Compounds 1a-c and
2
-
4
FIGURE 1. Absorption spectra of 1a (1.0 × 10 M in CH
2 2
Cl ) after
UV light irradiation for different periods; inset shows the reversible
absorbance variation at 359 nm for 1a after alternating UV light
irradiation (for 8.0 min) and visible light irradiation (for 15 min); a
UV lamp (365 nm, 100 W) and a tungsten lamp (>460 nm, 150 W)
were used for the UV and visible light irradiation experiments,
respectively.
designed, aimed at developing approaches to modulate the
electron-donating ability of the TTF moiety by UV/visible light
irradiation. The design rationale is simply described as fol-
lows: The transformation of the trans-azobenzene to the
corresponding cis-azobenzene unit would alter the steric strain
within the cyclic compounds 1a-c; as a result, the conformation
of the TTF moiety and therefore its electron-donating ability
would be changed if the spacers linking the azobenzene and
TTF moieties are relatively short. The results show that (1)
reversible tuning of the electron-donating ability, demonstrated
by variation in oxidation potential upon alternating UV and
visible light irradiation, is achieved for compounds 1a and 1b
with -(CH2)3- and -(CH2)5- as the spacers, respectively; (2)
the variation degree of the oxidation potential is dependent on
the lengths of spacers. When the spacers are extended to
azobenzene also has two possible isomers (cis and trans).
Therefore, compounds 1a-c would have four possible isomers.
Again, it was not possible to separate the four isomers with
column chromatography. For comparison studies later, com-
pound 2 was prepared similarly by reaction of compound 6
with compound 4a in diluted solution.
10
It is widely known that, at thermal equilibrium, azobenzene
11
1
exists mainly as the trans isomers. The H NMR data of
compounds 1a-c are consistent with this; the signals in the
aromatic region (7.0-8.0 ppm) due to the trans isomer are
predominant (Figures S12, S14, and S16 of the Supporting
Information). Based on the analytical HPLC data (Figures S4-
S7 of the Supporting Information), 81.8% of 1a has a trans
(azo) configuration, and 18.2% adopts the cis configuration at
-
(CH2)12-, the oxidation potential remains unchanged upon
thermal equilibrium. Similarly, the molar percentages of trans
and cis configurations were estimated to be 85.6% and 14.4%
for compound 1b, 90.1% and 9.9% for compound 1c, and 76.2%
and 23.8% for compound 2 at thermal equilibrium.
light irradiation; (3) variation of the oxidation potential was
found to be negligible for compound 2 in which the azobenzene
moiety is linked to TTF moiety in a side manner. These results
are also consistent with DFT calculations. From the application
perspective, the reversible modulation of the oxidation potentials
of by alternating UV and visible light irradiation can be
considered as a new molecular switch performing the electronic
transduction of optical signals.
Photoisomerization. As anticipated, photoisomerization can
occur to compounds 1a-c and 2 upon UV and visible light
irradiation. As an example, Figure 1 shows the absorption
spectra of 1a after UV light irradiation at 365 nm. Obviously,
the absorption band around 360 nm gradually decreased, while
the absorption bands around 450 nm increased after UV light
Synthesis. Reaction of compound 3 (Scheme 1) with the
dibromoalkane in the presence of K2CO3 led to compounds 4a-
11a,b,12
irradiation. According to previous studies,
the absorption
9
c. Reaction of compound 5 with compounds 4a-c in diluted
band around 360 nm is due to the trans form of azobenzene
moiety while that around 450 nm is ascribed to the cis form.
Therefore, the absorption spectral variation of 1a after UV light
irradiation indicates the transformation of the trans-azobenzene
into the cis-azobenzene moiety. The photostationary state was
reached after UV light irradiation for about 8.0 min. Based on
solution afforded compounds 1a-c, respectively, in acceptable
yields. As reported previously, compound 5 has two possible
isomers which cannot be separated by column chromatography;
(8) See, for example: (a) Muramatsu, S.; Kinbara, K.; Taguchi, H.; Ishii,
N.; Aida, T. J. Am. Chem. Soc. 2006, 128, 3764-3769. (b) Muraoka, T.;
Kinbara, K.; Aida, T. Nature 2006, 440, 512-515. (c) Muraoka, T.; Kinbara,
K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003, 125, 5612-5613. (d)
Qu, D.; Wang, Q.; Tian, H. Angew. Chem., Int. Ed. 2005, 44, 5296-5299.
(10) Binet, L.; Fabre, J. M.; Montginoul, C.; Simonsen, K. B.; Becher,
J. J. Chem. Soc., Perkin Trans. 1 1996, 783-788.
(
e) Qu, D.; Wang, Q.; Ren, J.; Tian, H. Org. Lett. 2004, 6, 2085-2088. (f)
(11) See, for example: (a) Ruslim, C.; Ichimura, K. J. Phys. Chem. B
2000, 104, 6529-6535. (b) Aemissegger, A.; Kr a¨ utler, V.; van Gunsteren,
W. F.; Hilvert, D. J. Am. Chem. Soc. 2005, 127, 2929-2936. (c) Jousselme,
B.; Blanchard, P.; Gallego-Planas, N.; Delaunay, J.; Allain, M.; Richomme,
P.; Levillain, E.; Roncali, J. J. Am. Chem. Soc. 2003, 125, 2888-2889.
(12) Tamai, N.; Miyasaka, H. Chem. ReV. 2000, 100, 1875-1890 and
further references cited therein.
Norikane, Y.; Tamaoki, N. Org. Lett. 2004, 6, 2595-2598. (g) Murakami,
H.; Kawabuchi, A.; Kotoo, K.; Kunitake, M.; Nakashima, N. J. Am. Chem.
Soc. 1997, 119, 7605-7606.
(9) (a) Jia, C.; Zhang, D.; Xu, W.; Zhu, D. Org. Lett. 2001, 3, 1941-
1
944. (b) Jia, C.; Zhang, D.; Guo, X.; Wan, S.-H.; Xu, W.; Zhu, D. Synthesis
2
002, 15, 2177-2182.
6
248 J. Org. Chem., Vol. 72, No. 16, 2007

TABLE 1. Molar Percentages (%) of the Trans and Cis Isomers of
Compounds 1a-c and 2 at Thermal Equilibrium and after UV
Light (365 nm) Irradiation for 8.0 min, Followed by Visible
Irradiation for 15.0 min Based on the Analytical HPLC Results
thermal
equilibrium
(trans/cis)
UV light
irradiation
(trans/cis)
further vis light
irradiation
(trans/cis)
compd
1
1
1
2
a
b
c
81.8/18.2
85.6/14.4
90.1/9.9
39.8/60.2
26.6/73.4
9.0/91.0
80.4/19.6
83.5/16.5
88.9/11.1
76.2/23.8
13.9/86.1
the reversed-phase HPLC analysis, the photostationary state of
1
3
1a contained 39.8% trans isomer and 60.2% cis isomer.
Further visible light irradiation led to the absorption intensity
enhancement for the band around 360 nm and simultaneously
induced the decrease of the absorption intensity around 450 nm.
The initial absorption spectrum was restored after visible light
irradiation for 15 min. The analytical HPLC result showed that
the molar ratio between the trans isomer and cis isomer for the
sample of 1a after sequential UV and visible light irradiation
is almost the same as that for the original sample of 1a before
UV light irradiation (see Table 1). The reversible absorption
spectral change observed for 1a can be performed for several
cycles as displayed in the inset of Figure 1, where the absorbance
variation at 359 nm after alternating UV and visible light
irradiations is displayed. But, the photoisomerization process
of 1a exhibits a little fatigue since the absorbance change at
FIGURE 2. Cyclic voltammograms of compound 1a before (black
line) and after UV light (365 nm) irradiation for 8.0 min (red line);
1
p
inset shows the reversible variation of the peak potential (E ) under
alternating UV light (8.0 min) and visible light irradiations (15.0 min);
a UV lamp (365 nm, 100 W) and a tungsten lamp (>460 nm, 150 W)
were used for the UV and visible light irradiation experiments,
respectively.
3
59 nm decreases from 1.43 for the first light irradiation cycle
FIGURE 3. Optimized geometries for compound 1a: trans isomer
to 1.29 for the fourth light irradiation cycle.
(left) and cis isomer (right); the theoretical calculations were performed
1
The H NMR spectrum of 1a was measured before and after
at the density functional theory (DFT) level with the hybrid B3LYP
functional and the 6-31G* basis set, as implemented in the Gaussian-
1
UV light irradiation. Before UV light irradiation, H NMR
signals at 7.0-6.7 ppm for the aromatic protons due to the cis
isomer were rather weak compared to those ascribed to the trans
0
3 package.
1
isomer. After UV light irradiation, the H NMR signal intensities
Electrochemical and Theoretical Studies. The electron-
at 7.0-6.7 ppm (due to the cis isomer, see Figure S8 of the
Supporting Information) increased. This observation is in
agreement with the results of absorption spectral studies as
discussed above.
donating abilities of compounds 1a-c and 2 can be represented
by their oxidation potentials, which are related to the corre-
sponding HOMO energies. Thus, oxidation potentials for
compounds 1a-c and 2 after UV and visible light irradiation
will provide information about their electron-donating abilities.
As an example, the cyclic voltammogramms of compound 1a
before and after UV irradiation is shown in Figure 2. Two quasi-
Similar absorption spectral variation was also observed for
compounds 1b, 1c, and 2 (Figures S1-S3 of the Supporting
Information). This indicated that the reversible transformation
between the corresponding trans isomers and cis isomers could
also occur for compounds 1b, 1c, and 2. The molar percentages
of the trans and cis isomers of compounds 1a-c were listed in
Table 1, at thermal equilibrium and after UV light irradiation,
followed by visible light irradiation. For compounds 1a-c, by
prolonging the length of the spacer the percentage of the trans
isomer increases at thermal equilibrium. Our explanation is that
the disfavored electronic interaction between the nitrogen atom
of the trans-azobenzene moiety and TTF unit in the case of
short spacers favors cis-azobenzene instead. At the photosta-
tionary state formed by UV light irradiation, the percentage of
cis isomer increases by prolonging the length of the spacer for
compounds 1a-c. The initial composition of trans and cis
isomers of compounds 1a-c can be restored by further visible
light irradiation. The molar percentages of the trans and cis
isomers of compound 2 at thermal equilibrium and photosta-
tionary state after UV light irradiation were also estimated by
analytical HPLC as listed in Table 1.
1
2
reversible redox waves at Eox ) 0.572 and Eox ) 1.081 (vs
Ag/AgCl) were observed at thermal equilibrium, corresponding
to the redox reactions TTF/TTF and TTF /TTF , respec-
·+
·+
2+
tively. After the solution of 1a was exposed to UV light (365
1
nm) for 8.0 min, the peak potential (Ep ) of the first anodic
wave was shifted to the positive region by ca. 46 mV; the peak
2
potential (Ep ) of the second anodic wave was also slightly
shifted. Interestingly, the initial cyclic voltammogram was
almost restored after visible light irradiation for another 15 min.
The inset of Figure 2 demonstrates the reversible variation of
1
the Ep upon alternating UV and visible light irradiations. This
can be employed to construct a molecular switch with UV/
visible light as the inputs and the electrochemical signal as the
1
output. But, the variation of Ep decreases from 46 mV for the
th
first light irradiation cycle to 41 mV for the 3 light irradiation
cycle. This is likely due to the little fatigue exhibited by the
photoisomerization process of 1a.
The mechanism underlining the variation of the oxidation po-
tential of 1a was the conformational change of TTF unit result-
ing from the trans and cis transformation of the azobenzene
(13) Both the trans and cis isomers are related to the azobenzene moiety.
J. Org. Chem, Vol. 72, No. 16, 2007 6249

moiety, according to our DFT study. As shown in Figure 3, the
visible light as the inputs and the electrochemical signal as the
output. Moreover, it was found that the influence of the
azobenzene photoisomerization on the electronic property of
the TTF unit became attenuated by prolonging the spacer for
compounds 1a-c. It is also interesting to note that it may not
be probable to modulate the electronic property of TTF unit by
modifying the TTF unit in the side manner.
TTF unit in the trans isomer of 1a adopts a nonplanar
1
4
conformation, whereas that in the cis isomer is planar. It was
6
reported early that the conformational change of TTF unit
would affect the HOMO energy and therefore the electron-
donating ability. Our calculation results indicated that HOMOs
of both the trans and cis isomers of 1a were mainly located on
the TTF unit, and the HOMO energy of the trans isomer was
found to be higher than that of the cis isomer (EHOMO ) -4.75
eV for the trans isomer, and EHOMO ) -4.77 eV for the cis
isomer). This is consistent with our experimental results as
discussed above.
Experimental Section
Synthesis of Compound 1a. To a solution of 5 (0.10 g, 0.27
mmol) in anhydrous degassed DMF (50 mL) was added a solution
of CsOH‚H O (0.10 g, 0.53 mmol) in anhydrous degassed MeOH
2
(5 mL) over a period 30 min. The mixture was stirred for an
additional 30 min. The above solution and a solution of compound
The oxidation potentials of compounds 1b-c and 2 were also
1
measured after UV and visible light irradiation. The Ep of 1b
was anodically shifted by ca. 30 mV after UV light irradiation
4
a (0.12 g, 0.27 mmol) (see the Supporting Information) in
anhydrous degassed THF (50 mL) were added simultaneously over
h at room temperature to 200 mL of anhydrous degassed DMF
2
for 8.0 min, and the Ep remained almost unchanged (Figure
S9 of the Supporting Information). For compound 1c in which
8
1
the spacer was further extended, the variation of both Ep and
under stirring. The solution was then stirred overnight, and the
reaction mixture was concentrated in vacuum. The residue was the
2
Ep was found to be rather negligible after UV light irradiation
(
Figure S10 of the Supporting Information). These results may
dissolved in CH
mL). The orange phase was dried (Na
afford orange solid. Then, the residue was purified by column
chromatography using ethyl acetate/CH Cl /petroleumether (1:1:
0) as eluant, affording compound 1a as a yellow powder (0.04 g,
6.5%): mp 160-161 °C; H NMR (400 MHz, CDCl , trans
3
isomer) δ 7.92 (d, J ) 8.68 Hz, 4H), 7.08 (d, J ) 8.76 Hz, 4H),
2
Cl
2
(150 mL) and washed with water (3 × 100
be due to the fact that a longer spacer will reduce the influence
of the trans and cis transformation of the azobenzene unit on
the electronic property of the TTF unit. It is interesting to note
that almost no oxidation potential change was detected after
UV light irradiation for compound 2, in which the azobenzene
moiety was linked to the TTF unit in a side manner (Figure
S11 of the Supporting Information). This result implies that it
may not be probable to modulate the electronic property of TTF
unit by modifying the TTF unit in the side manner.
New heterocyclic TTF compounds 1a-c and 2 with azoben-
zene moiety were synthesized and characterized, with a view
to modulating the electronic property of TTF unit by light
irradiation and developing new molecular switches that can carry
out the electronic transduction of optical signals. Absorption
2
SO ) and concentrated to
4
2
2
1
2
1
5
2
1
.56 (s, 2H), 4.41 (t, J ) 5.6 Hz, 4H), 2.69 (t, J ) 6.0 Hz, 4H),
13
.04 (m, 4H); C NMR (100 MHz, CDCl
3
) δ 159.1, 147.7, 125.8,
24.8, 124.6, 118.8, 118.0, 117.6, 66.3, 30.9, 29.7; IR (KBr pellet)
-1
2923, 2854, 1597, 1494, 1234, 1144, 1023, 847 cm ; HRMS calcd
for C24H N O S [M ] 562.0006, found 562.0004.
22 2 2 6
+
The synthesis and characterization of compounds1b,c and 2 were
provided in the Supporting Information.
Acknowledgment. The present research was financially
supported by NSFC, Chinese Academy of Sciences, and State
Key Basic Research Program.
1
and H NMR spectral studies show that photoisomerization of
azobenzene units in compounds 1a-c and 2 can occur easily.
The oxidation potential of 1a can be shifted anodically after
UV light irradiation, and the variation of the oxidation potential
can be reversibly realized by alternating UV and visible light
irradiation. As a result, a molecular switch is achieved with UV/
Supporting Information Available: Experimental procedures
1
and characterization data (absorption spectra, HPLC, CV, H NMR,
13
C NMR, and computational data). This material is available free
of charge via the Internet at http://pubs.acs.org.
(14) The calculation details are provided in the Supporting Information.
JO070651E
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250 J. Org. Chem., Vol. 72, No. 16, 2007