Synthesis, structure, and photoelectrochemical properties of new tetrathiafulvalene-diphenyl-1,3,4-oxadiazole dyads

Article abstract of DOI:10.1016/j.tetlet.2008.10.001

New donor-acceptor dyads containing tetrathiafulvalene (TTF) and 2,5-diphenyl-1,3,4-oxadiazole (PPD) moieties were synthesized to develop new photoconducting materials. Crystal structure analysis indicated the highly planar molecular skeleton of the dyad. Fluorescence from the PPD part was almost quenched by the intramolecular electron transfer from the TTF part to the PPD part. Photoelectrochemical measurements indicate that cathodic photocurrents can be generated from a thin film of the dyad spin-coated on ITO electrode.

Full text of DOI:10.1016/j.tetlet.2008.10.001

Tetrahedron Letters 49 (2008) 7200–7203
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, structure, and photoelectrochemical properties of new
tetrathiafulvalene-diphenyl-1,3,4-oxadiazole dyads
*
Hideki Fujiwara , Yasuo Sugishima, Keijiro Tsujimoto
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Gakuen-cho, Sakai 599-8570, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
New donor–acceptor dyads containing tetrathiafulvalene (TTF) and 2,5-diphenyl-1,3,4-oxadiazole (PPD)
moieties were synthesized to develop new photoconducting materials. Crystal structure analysis indi-
cated the highly planar molecular skeleton of the dyad. Fluorescence from the PPD part was almost
quenched by the intramolecular electron transfer from the TTF part to the PPD part. Photoelectrochemical
measurements indicate that cathodic photocurrents can be generated from a thin film of the dyad spin-
coated on ITO electrode.
Received 28 August 2008
Revised 29 September 2008
Accepted 1 October 2008
Available online 2 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Recently, donor–acceptor type dyads using a tetrathiafulvalene
(TTF) framework have received considerable interest as materials
for fluorescence switches, chemical sensors, molecular rectifica-
tion, and photovoltaic and NLO applications.1–6 The photo-induced
interactions between the donor and acceptor parts such as intra-
molecular charge-transfer interactions and the resultant formation
of charge-separated state have played an important role for the
development of optoelectronic devices.7,8 We focused on the
development of photo-induced conducting materials based on
the organic conductors in which their conductivities can be
switched by external lights. 2,5-Diphenyl-1,3,4-oxadiazole (PPD)
and its derivatives are known to show strong fluorescence, and
are used as electron-transport materials in electroluminescence
devices.9,10 We designed new donor–acceptor dyads containing
TTF and PPD moieties (1) to realize novel photo-switchable con-
ducting materials. In this Letter, we report the synthesis, structure
and photoelectrochemical properties of 1a, b.
Synthesis of 1a, b was performed as described in Scheme 1.
Thus, tributylstannyl-substituted TTF (2) was prepared from TTF
by the reported method.11 Then, Pd(PPh3)4-catalyzed Stille cou-
pling reaction of 2 and bromo-substituted PPD derivatives (3a,
b)12,13 was performed under toluene reflux for 24 h. After column
chromatography on silica gel with CHCl3 as an eluent, compounds
1a, b were obtained as red microcrystals in 37% and 53% yields,
respectively.14 The cyclic voltammograms of 1a, b were measured
in benzonitrile at 25 °C. Compound 1 showed two pairs of one-
electron reversible redox waves as summarized in Table 1. Because
these redox potentials are almost similar to those of TTF
(E1 = +0.41 V and E2 = +0.80 V under the identical condition), com-
pounds 1a, b have a good electron-donating ability despite the
substitution of the electron-withdrawing 1,3,4-oxadiazole ring.
In the UV–vis absorption spectra measured in 10ꢀ4 M CHCl3
solution at room temperature, 1a showed a strong absorption max-
imum at 312 nm (loge = 4.55) and a weak maximum at 453 nm
N N
1) n-BuLi (1.1 equiv.)
dry THF, —78 ºC
O
SnBu
Br
R
3
S
S
S
S
S
S
S
S
3a, b
2) Bu SnCl (1.1 equiv.)
3
Pd(PPh ) (10 mol%)
3 4
dry toluene reflux
TTF
2
—78 ºC to rt
N
O
N
R
N N
O
S
S
S
S
1a: R = H (37%)
PPD
1b: R = t-Bu (53%)
Scheme 1. Synthesis of 1a and 1b.
* Corresponding author. Tel.: +81 72 254 9818; fax: +81 72 254 9935.
E-mail address: hfuji@c.s.osakafu-u.ac.jp (H. Fujiwara).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.001

H. Fujiwara et al. / Tetrahedron Letters 49 (2008) 7200–7203
7201
Table 1
An X-ray crystal structure analysis of 1a was performed using a
red plate-like single crystal of 1a recrystallized from CHCl3/metha-
nol.15 In the unit cell, there are two crystallographically indepen-
dent molecules A and B. As shown in Figure 3, both of them have
almost planar molecular skeletons, and adopt only one of two pos-
sible planar conformers. As Figure 4 indicates molecule 1a con-
structs –B–A–A0–B0-type stackings along the a-axis. Donor
molecules B form a dimer (B–B0) with overlapping its PPD part over
the TTF part, whereas only PPD parts are overlapped to each other
in the dimer of A–A0. In the A–B overlap, each TTF part and PPD part
stack to each other in a head-to-head overlap mode. Because of
such a mixed-stacking structure of the TTF and PPD parts, it is con-
sidered to be difficult to realize photo-induced conductivities using
this single crystal. However, the high planarity of this molecular
framework will be favorable to construct effective conducting
pathways if appropriate derivatives having substituents adequate
for the construction of segregated stacking structures of TTF and
PPD moieties can be utilized.
Redox potentials of 1a, b and TTFa
Compound
E1/V
E2/V
E2 ꢀ E1/V
0.40
0.40
0.39
1a
1b
TTF
+0.43
+0.44
+0.41
+0.83
+0.84
+0.80
a
V versus Ag/AgCl, 0.1 mol Lꢀ1 n-Bu4NClO4 in benzonitrile at 25 °C, Pt electrodes,
scan rate of 50 mV sꢀ1
.
(log
and 369 nm in the case of TTF, as shown in Figure 1. PPD also
showed a strong maximum at 282 nm (log = 4.43). Molecular
e = 3.65), whereas absorption maxima were observed at 312
e
orbital (MO) calculation of 1a was performed by the density func-
tional theory, B3LYP/6-31G** method using GAUSSIAN 93. As shown
in Figure 2, the atomic coefficients of HOMO and LUMO orbitals
of 1a are mainly localized on the TTF and PPD moieties, respec-
tively. The energy of HOMO–LUMO gap is estimated to be
2.65 eV (=1240/2.65 = 468 nm) from the energy levels of HOMO
(ꢀ4.72 eV) and LUMO (ꢀ2.07 eV), and is almost equal to the weak
absorption maximum at 453 nm of 1a, suggesting that this absorp-
tion corresponds to the charge-transfer absorption from TTF part to
PPD part. On the other hand, the MO calculation also suggests that
the excitations within each TTF or PPD part occur at 3.68 eV
(=337 nm; HOMO to LUMO+2) and 4.00 eV (=310 nm; HOMOꢀ1
to LUMO), respectively. These excitations may correspond to the
strong and broad maximum around 312 nm of 1a.
Emission spectra of 1, PPD, and TTF were measured in 10ꢀ4 M
CHCl3 solutions at room temperature. When the solution of PPD
was irradiated by the excitation light of 282 nm that corresponds
to the absorption maximum of PPD, already-known quite strong
fluorescence was observed around 350 nm. However, in the case
of the CHCl3 solution of 1a and 1b, only a subtle fluorescence that
may correspond to the fluorescence of TTF part was observed
around 375 nm (Fig. 6). This result suggests that the fluorescence
from the excited PPD (PPD*) part is almost quenched by the intra-
molecular electron-transfer process from the electron-donating
TTF part to the PPD* part because the PPD* part possesses elec-
tron-accepting ability upon excitation.16–18 To examine the fluo-
rescence quenching by the electron transfer process, UV–vis
absorption spectra and emission spectra of 1b were measured on
various oxidation states of 1b as shown in Figures 5 and 6, respec-
tively. Upon step-wise addition of an oxidation reagent (NOBF4) to
the CHCl3 solution of 1b, the color of the solution changed from or-
ange to green with an absorption increase around 630 nm, then to
light yellow with weak shoulder absorption over 400 nm. These
color changes correspond to the oxidation processes from 1b to
1b+Å and to 1b2+ states. Such a clear color change upon oxidation
was also reported in the other TTF derivatives bearing a PPD part
through ethynyl-spacer, which can be used as electrochromic
materials.19 As shown in Figure 6, upon oxidation to 1b+Å state,
the emission intensity around 375 nm was almost unchanged. On
the other hand, further oxidation process to 1b2+ state largely in-
creases the emission intensity around 390 nm. These results indi-
cate that the addition of excess NOBF4 oxidizes the TTF part to
its dication state TTF2+, and suppresses the electron-donating abil-
ity of TTF part that can quench the emission from the PPD* part,
and allows the emission from the PPD* part.20–22 Such a combina-
tion of multi-redox property of the TTF moiety and fluorescence
ability of the PPD part may be used as fluorescence switches and
chemical sensors.23–26
Figure 1. UV–vis absorption spectra of 1a, PPD, and TTF in 10ꢀ4 M CHCl3 solution.
Arrows indicate absorption maxima.
As mentioned above, photo-induced intramolecular electron-
transfer process quenches the emission from PPD*. During this pro-
cess, photo-induced charge-transfer state TTF+Å–PPDꢀÅ can be
achieved. To examine the possibility of photo-induced electric
current generation from such a charge-transfer excited state, mea-
surement of photocurrents was performed by photoelectrochemi-
cal method. Thin film of 1a was deposited on an ITO-coated
soda-lime glass substrate (Aldrich No. 576352; 1.6 cm ꢁ 1.6 cm)
by spin-coating using 5 l
L of 1 g Lꢀ1 solution of 1a in CHCl3 at
2000 rpm for 30 s. These conditions for the preparation of thin
films are optimized by repeating photoconductivity measurements
with changing the volume and concentration of solution, speed and
time of spinning. Photoelectrochemical measurements using this
thin film-coated ITO electrode (spin-coated area: 1.0 cm ꢁ 0.8 cm)
Figure 2. LUMO and HOMO orbitals of 1a calculated by B3LYP/6-31G** method.

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H. Fujiwara et al. / Tetrahedron Letters 49 (2008) 7200–7203
Figure 3. Molecular structures of molecules A and B of 1a.
Figure 5. UV–vis absorption spectra of 1b, 1b+Å, and 1b2+ in 10ꢀ4 M CHCl3 solution
upon oxidation of 1b by NOBF4.
Figure 4. Crystal structures of 1a projected on to the ab-plane.
as working electrode were performed in 0.1 mol Lꢀ1 aqueous KCl
solution.27 Platinum and Ag/AgCl electrodes were used as counter
and reference electrodes, respectively. Photocurrents between the
working and counter electrodes were measured by BAS Electro-
chemical Analyzer Model 612B under irradiation from 150 W Xe
lamp, where monochromatized light in the wavelength range from
300 to 600 nm was produced by holographic monochromators
Figure 6. Emission spectra of 1b, 1b+Å, 1b2+, and TTF in 10ꢀ4 M CHCl3 solution upon
oxidation of 1b by NOBF4 (excitation light of 315 nm).

H. Fujiwara et al. / Tetrahedron Letters 49 (2008) 7200–7203
7203
process on the 1a-modified ITO electrode. The conversion yield of
photo-electric conversion ( ) reached to about 1.6% under the bias
g
voltage of ꢀ0.4 V versus Ag/AgCl reference electrode.
In conclusion, we have synthesized new donor–acceptor type
TTF dyads in which TTF part connects directly to a PPD part. The
studies on emission spectra suggest the photo-induced intramolec-
ular electron transfer between TTF and PPD parts. The photoelect-
rochemical measurements indicate that cathodic photocurrents
can be generated from the thin film of 1a that is spin-coated on
ITO electrode. These results suggest that the synthesized dyads
can be considered as candidates for optoelectronic materials such
as photoconducting applications.
Acknowledgments
This work was financially supported in part by Grants-in-Aid for
Scientific Research (Nos. 18028020 and 19750119) from the Minis-
try of Education, Culture, Sports, Science and Technology of Japan.
Figure 7. Photocurrent action spectrum under zero bias voltage versus Ag/AgCl
reference electrode and absorption spectrum of the thin film of 1a spin-coated on
ITO electrode.
References and notes
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corresponds to the absorption maximum of the identical thin film
of 1a on ITO electrode, suggesting that the absorbed photons were
converted to electric currents. The conversion yield of photo-elec-
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V = 3765.2(7) Å3, Z = 4, T = 296 K, Dcalc = 1.498 g cm–3
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b = 29.580(3),
c = 12.9844(15)
Å,
b = 107.743(3)°,
,
l
(Mo K
a
) = 5.18 cmꢀ1
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Figure 8. Bias voltage (vs Ag/AgCl) dependence of photocurrents of the thin film of
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