New fluorene-substituted TTF derivatives as photofunctional materials

Article abstract of DOI:10.1016/j.physb.2009.11.053

We have investigated photoinduced conductors and photoelectric conversion materials using new tetrathiafulvalene (TTF) derivatives containing a fluorescent part. In this study, we focused on new TTF molecules 1-3 in which a fluorene molecule was used as a

Full text of DOI:10.1016/j.physb.2009.11.053

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Physica B 405 (2010) S12–S14
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New fluorene-substituted TTF derivatives as photofunctional materials
Ã
H. Fujiwara , K. Tsujimoto, Y. Sugishima, S. Takemoto, H. Matsuzaka
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
We have investigated photoinduced conductors and photoelectric conversion materials using new
tetrathiafulvalene (TTF) derivatives containing a fluorescent part. In this study, we focused on new TTF
molecules 1-3 in which a fluorene molecule was used as a fluorescent part and was connected to the
TTF moiety through alkyl or alkene spacers. We discuss the synthesis, electrochemical properties, and
optical properties of molecules 1-3. We also investigated the photoelectric conversion functionality
by photoelectrochemical method using the thin films of molecules 1-3 that were spin-coated on
ITO-coated glass substrate. We observed electric current generations that depend on the absorption
spectra of thin films, suggesting that absorbed photons are converted to electric currents on the thin
film/ITO electrode.
Keywords:
Organic conductors
Photofunctional materials
Electrochemical properties
Optical properties
Photoelectric conversion
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
reported method [11]. The protecting group of 4 was removed
with 28 wt% NaOMe in methanol at 0 1C, and the resultant
TTF-thiolate was reacted with 2-bromomethyl-fluorene 5. After
column-chromatography on silica gel with CHCl3 as an eluent,
compound 1 was obtained as yellow powder in 53% yield. [12] On
the other hand, the Wittig reagent 6 was prepared from 5 by
adding triphenylphosphine under benzene reflux. Then,
compound 6 was reacted successively with NaOMe and the
corresponding formyl-substituted TTF derivatives 7, 8 [13] in
benzene at room temperature. After the separation by column-
chromatography on silica gel with CHCl3/n-hexane (1:1, v/v) as an
eluent, compounds 2, 3 were obtained as reddish-orange powders
in 53 and 31% yields, respectively [14].
The cyclic voltammograms of 1–3 were measured in benzoni-
trile at 25 1C. All the donors showed two pairs of one-electron
reversible redox waves as summarized in Table 1. Because these
redox potentials are almost the same to those of TTF (E1= +0.41 V
and E2= +0.80 V under the identical condition), the fluorene-
containing donors 1–3 have a good electron-donating ability.
UV-Vis absorption spectra of the 10ꢀ4 M CHCl3 solution of
compounds 1–3 were measured at room temperature as shown in
Fig. 1. Compound 1 showed a strong absorption maximum at
In these decades, photofunctional materials have been received
much attention in the fields of material sciences such as
fluorescence-based chemical sensors, photovoltaic and NLO appli-
cations, electroluminescence devices [1–6]. Among them, the
development of donor-acceptor type dyads using tetrathiafulva-
lene (TTF) frameworks has been extensively studied to investigate
the photoinduced intramolecular charge-transfer and the resultant
formation of charge-separated state [7,8]. Recently, we have
reported new TTF-based dyads containing a 2,5-diphenyl-1,3,4-
oxadiazole (PPD) as a fluorescent part because PPD derivatives are
well-known as electron-transport materials in electroluminescence
devices, and cleared that the TTF-PPD dyads can be a candidate for
photoelectric conversion materials [9,10]. On the other hand, in
this study, we focused on TTF derivatives containing a fluorene
molecule used as
a fluorescent part because of its smaller
molecular size and higher planarity than those of PPD. In this
paper, we report the synthesis, electrochemical properties, optical
properties of new TTF derivatives 1–3 containing the fluorene part.
We also investigated the photoelectric conversion functionality by
a photoelectrochemical method using thin films of molecules 1–3
that were spin-coated on ITO-coated glass substrate (Chart 1).
276 nm and
a weak maximum around 390 nm, whereas
absorption maxima were observed at 312 and 369 nm in the
case of TTF and at 266 nm in the case of fluorene. Thus, compound
1 showed almost the same absorption maxima as those of TTF and
fluorene and the shape of absorption spectrum of 1 corresponds to
the overlap of those of TTF and fluorene, suggesting that the
intramolecular interaction between the TTF and fluorene moieties
is quite small in the case of 1. On the other hand, the molecules
containing the alkene spacer 2, 3 indicated strong absorption
maxima at longer wavelength region of 340 and 366 nm,
respectively, than that of 1 (276 nm). Furthermore, they showed
2. Results and discussion
Synthesis of 1–3 was performed as described in Scheme 1.
Thus, 4-(20-cyanoethylthio)-TTF (4) was prepared according to the
Ã
Corresponding author. Tel./fax: +81 72 254 9818.
E-mail address: hfuji@c.s.osakafu-u.ac.jp (H. Fujiwara).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2009.11.053

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H. Fujiwara et al. / Physica B 405 (2010) S12–S14
S13
relatively strong charge-transfer absorption bands around
400–500 nm. These results suggest that the intramolecular
interactions between TTF and fluorene parts in molecules 2, 3
are stronger than those in molecule 1. Emission spectra of 1–3 and
fluorene were also measured in CHCl3 solutions at room
temperature. When the solution of fluorene was irradiated by
the excitation light of 266 nm that corresponds to the absorption
maximum, already-known quite strong fluorescence was
observed around 317 nm. However, in the case of the CHCl3
solution of 1–3, almost no fluorescence was observed under
irradiation with the light of strong absorption maxima that may
correspond to the excitation of the fluorene part. This result
suggests that the fluorescence from the excited fluorene part is
almost quenched by the intramolecular electron transfer process
from the electron-donating TTF part to the fluorene part because
the fluorene part possesses electron-accepting ability upon
excitation [15–17].
Xe lamp, generated photocurrents between the working and
counter electrodes were recorded in the wavelength range from
300 to 600 nm. The photocurrent action spectra for the thin film
of 1–3 on ITO electrode were measured under bias voltage vs.
Ag/AgCl. Cathodic photocurrent maxima of 60 nA around 330 nm,
780 nA around 320 nm and 300 nA around 345 nm for 1, 2 and 3
respectively, were observed under ꢀ0.30 V bias voltage, corre-
sponding to the absorption maxima of the identical thin-films on
ITO electrode (see Fig. 2). These results suggest that the absorbed
photons were converted to electric currents on the thin-films of
molecules 1–3 on ITO electrode as a result of charge transfer
Table 1
Redox potentials of 1–3 and TTFa.
Compound
E1/V
E2/V
E2ꢀE1/V
1
+0.44
+0.40
+0.41
+0.41
+0.82
+0.80
+0.80
+0.80
0.38
0.40
0.39
0.39
Such photoinduced intramolecular electron transfer can be
expected to cause transitional charge-separated state. To inves-
tigate the photoelectric conversion functionality of the synthe-
sized molecules 1–3 at the photoinduced charge-separated state,
photocurrent generation was measured by a photoelectrochem-
ical method using thin films of molecules 1–3. The thin films of
1–3 were prepared on an ITO-coated soda-lime glass substrate by
2
3
TTF
V vs. Ag/AgCl, 0.1 mol Lꢀ1 n-Bu4NClO4 in benzonitrile at 25 1C, Pt electrodes,
a
scan rate of 50 mV s-1
.
a spin-coating method using 5 m
L of 1 g Lꢀ1 solution of the donors
in CHCl3 at 2000 rpm for 60 s [9]. Photoelectrochemical measure-
ments were performed using this thin film-coated ITO electrode
as a working electrode in 0.1 mol L–1 aqueous KCl solution.[18]
Platinum and Ag/AgCl electrodes were used as counter and
reference electrodes, respectively. Under irradiation from 150 W
Fig. 1. UV-Vis absorption spectra of 1–3, TTF and fluorene in 10ꢀ4 M CHCl3
Chart 1. Molecular structure of 1–3.
solution.
Scheme 1. Synthesis of 1–3.

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H. Fujiwara et al. / Physica B 405 (2010) S12–S14
the electron-donating TTF part to the fluorene part occurred when
the fluorene part was irradiated. The photoelectrochemical
measurements indicate that cathodic photocurrents can be
generated from the spin-coated thin film of 1–3 on ITO electrode.
We think that the synthesized donors can be considered as
candidates for photoelectric current conversion materials.
Acknowledgments
This work was financially supported in part by Grants-in-Aid
for Scientific Research (No. 20110006 and No. 21750150) from the
Ministry of Education, Culture, Sports, Science and Technology of
Japan, and by Grant for Young Researcher from JGC-S Scholarship
Foundation.
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Fig. 2. Photocurrent action spectrum under ꢀ0.30 V bias voltage vs. Ag/AgCl
reference electrode and absorption spectrum of the thin film of 2 spin-coated on
ITO electrode.
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