Photoswitchable sexithiophene-based molecular wires

Article abstract of DOI:10.1021/ol8028059

(Chemical Equation Presented) Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes. The redox properties in the open state are typical of sexithiophenes. Ring closure of both photochromic units leads to a decrea

Full text of DOI:10.1021/ol8028059

ORGANIC
LETTERS
2009
Vol. 11, No. 3
721-724
Photoswitchable Sexithiophene-Based
Molecular Wires
Jetsuda Areephong,† Johannes H. Hurenkamp,† Maaike T. W. Milder,‡
Auke Meetsma,† Jennifer L. Herek,§ Wesley R. Browne,*,† and Ben L. Feringa*,†
Centre for Systems Chemistry, The Stratingh Institute for Chemistry and Zernike Institute
for AdVanced Materials, Faculty of Mathematics and Natural Sciences, UniVersity of
Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands, FOM Institute for
Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, and
Optical Sciences Group, Department of Science and Technology, MESA+ Institute for
Nanotechnology, UniVersity of Twente, 7500 AE, Enschede, The Netherlands
w.r.browne@rug.nl; b.l.feringa@rug.nl
Received December 4, 2008
ABSTRACT
Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes. The redox properties in the open state
are typical of sexithiophenes. Ring closure of both photochromic units leads to a decrease in the energy of the LUMO orbitals with little affect
on the energy of the HOMO orbitals. The photochemical tuning of the conjugation of a molecular wire is achieved by combining dithienylethene
units with a sexithiophene.
The electronic properties of polythiophenes are remarkable
in comparison to semiconductor and metallic nonmolecular
materials, especially considering the range of conductivities
that can be accessed through synthetic tuning and process-
ing.1 Furthermore, their stability toward dioxygen and water
has facilitated their application in organic electronic devices.2
Understanding the electronic and photonic properties of
polythiophene materials is key to the molecular design of
components for application in electronic devices.3 In this
regard, oligothiophenes offer considerable advantages over
polythiophenes due to the precisely defined structures that
can be prepared and probed electrochemically and spectro-
scopically.4 R,R′,R′′-Sexithiophene and its derivatives5 have
(1) (a) Sonmez, G.; Sonmez, H. B.; Shen, C. K. F.; Jost, R. W.; Rubin,
Y.; Wudl, F. Macromolecules 2005, 38, 669–675. (b) Schendeman, I.;
Hickman, R.; Sonmez, G.; Schottland, P.; Zong, K.; Welsh, D.; Reynolds,
J. R. Chem. Mater. 2002, 14, 3118–3122. (c) Roncali, J. Chem. ReV. 1992,
92, 711–738.
† University of Groningen.
‡ FOM Institute for Atomic and Molecular Physics.
§ University of Twente.
(2) Lorcy, D.; Cava, M. P. AdV. Mater. 1992, 4, 562–564.
(3) Klauk, H. Organic Electronics; Wiley-VCH: Germany, 2006.
10.1021/ol8028059 CCC: $40.75
Published on Web 01/13/2009
2009 American Chemical Society

attracted widespread interest in recent years not only as
models for polythiophenes but also in their own right as
versatile active components in electronic devices such
as electrochromic materials,6 field-effect transistors,7
OLEDs,8 molecular wires,9 and photovoltaic cells.10 Control
over their electronic properties can be achieved through
synthetic modification11 or postsynthetically through process-
ing. Postsynthetic tuning of properties by external stimuli,
specifically light through incorporation of a photoresponsive
unit, is an attractive alternative, but is considerably more
challenging due to the ability of oligothiophenes to quench
excited states efficiently.12 Tsivgoulis and Lehn demonstrated
that when substituted with ter- and tetrathiophene units the
photochromism of the dithienylethene unit is retained and
enables photoswitching of conjugation pathlengths.13
Recently, we demonstrated14 related systems, in which
photochromic units, i.e., dithienylcyclopentenes, that could
be used as switching elements to control the electropoly-
merizability of R,R′-alkene-bridged bis-terthiophene mono-
mers. In the ring-open state, electropolymerization to form
alkene-bridged sexithiophene polymers proceeded smoothly.
However, ring closure of the dithienylethene unit15 of the
monomer with UV light resulted in a complete inhibition of
polymer formation. Subsequent ring opening with visible
light restored the electropolymerizability of the monomer.
The dithienylethene/sexithiophene polymer films obtained,
however, were photochemically inert both in terms of
photoluminescence and photochromism. The key fundamen-
talquestionfacingthedevelopmentofswitchablesexithiophene
systems is: Can we combine multiple dithienylethenes units
with sexithiophene and retain the photochromic properties
of dithienylethenes?
Here, we report the synthesis and spectroscopic and
electrochemical characterization of two sexithiophene-based
compounds 1 and 2 (Scheme 1), which incorporate photo-
Scheme 1. Synthesis of 1oo and 2oo
switchable dithienylhexafluorocyclopentene units, to address this
question. Compounds 1 and 2 can be viewed as either a dimer
of photochromic dithienylethenes bridged by a tetrathiophene
unit or alkene/methyl end-capped sexithiophenes. We demon-
strate that dithienylethene photochromic units can retain their
photochromic functionality when part of an extended π-con-
jugated sexithiophene system. Furthermore, the effect of a
change in a, essentially peripheral, substituent (i.e., phenyl-
vs chloro-), on the properties of these systems is found to
be stronger than would be anticipated. The present systems
are complementary to those of Tsivgoulis and Lehn13 (vide
supra) in that here the sexithiophene unit is retained in all
three photochromic states.
(4) Alberti, A.; Ballarin, B.; Guerra, M.; Macciantelli, D.; Mucci, A.;
Parenti, F.; Schenetti, L.; Seeber, R.; Zanardi, C. Chem. Phys. Chem. 2003,
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The approach taken here to prepare the photochromic
sexithiophene systems is based on the electrochemically
driven coupling of two terthiophene units via their radical
cation oxidation states. R-Dimerization1c,5b,16 offers advan-
tages as a complementary route to preparing sexithiophenes,
in particular to the metal-catalyzed (Sn, Pd, etc.)17 coupling
reactions or chemically induced oxidative coupling with
ferric chloride.18 The terthiophene-based diarylethene pho-
tochromic switches 3o and 4o were obtained using methods
reported earlier for the preparation of asymmetric dithie-
nylethenes (Scheme 1).19 Cyclic voltammetry indicated that
as for the electropolymerizable system reported earlier,14
oxidation did not lead to ring closure but instead to
dimerization.
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Clegg, W. J. Mater. Chem. 2007, 17, 225–231.
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Martini, M.; das Neves, S.; De Paoli, M.-A. Handbook of AdVanced
Electronic and Photonic Materials and DeVices; Nalwa, H. S., Ed; Acadamic
Press: San Diego, CA, 2001.
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J. M. Acc. Chem. Res. 2000, 33, 791–804.
The preparation of 1oo and 2oo through R-dimerization
of 3o and 4o, respectively, was carried out by potentiostatic
(12) Feng, Y.; Yan, Y.; Wang, S.; Zhu, W.; Qian, S.; Tian, H. J. Mater.
Chem. 2006, 16, 3685–3692.
(16) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner,
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1400. (b) Tsivgoulis, G. M.; Lehn, J. M. AdV. Mater. 1997, 9, 39–42.
(14) Areephong, J.; Kudernac, T.; de Jong, J. J. D.; Carroll, G. T.;
Pantorotta, D.; Hjelm, J.; Browne, W. R.; Feringa, B. L. J. Am. Chem. Soc.
2008, 130, 12850–12851.
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Mark, T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547–4551.
(c) Bauerle, P. AdV. Mater. 1992, 4, 102–107. (d) Zang, F.; Bauerle, P.
J. Am. Chem. Soc. 2007, 129, 3090–3091.
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2001. (b) Yao, J. N.; Hashimoto, K.; Fujishima, A. Nature 1992, 355, 624–
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(19) See the Supporting Information.
722
Org. Lett., Vol. 11, No. 3, 2009

oxidative coupling at ca. 1.10 V (vs SCE, 1 mM in 0.1 M
KPF6/CH3CN). The light yellow solutions turned dark green
upon oxidative electrolysis and subsequently deep yellow
upon standing or after the applied potential was switched to
0.1 V vs SCE. 1oo and 2oo were purified by preparative
teristic23 of end-capped sexithiophenes. For both 1oo and
2oo, two sequential reversible (Ep,a - Ep,c ∼ 60-80 mV)
redox processes (e.g., 1oo f 1oo+ and 1oo+ f 1oo2+) are
observed. The absorption spectrum of 1oo is red-shifted
significantly compared with that of 2oo by ∼2700 cm-1. The
shift of the first and second oxidation waves by 190 mV
indicates that the chloro/phenyl-substituted thiophene end
groups affect both the HOMO and LUMO levels, albeit with
significantly more stabilization of the HOMO. Furthermore,
the effect of the substituents on the fluorescence of 1oo and
2oo is significant with a 3-fold decrease in the fluorescence
quantum yield. A more pronounced vibronic progression is
observed for 1oo than for 2oo. Notably, in contrast to their
absorption spectra, the energies of the emission maxima of
1oo and 2oo are similar. Considering the relative remoteness
of the phenyl/chloro substituents from the sexithiophene unit,
the data indicate that the alkene bridge mediates electronic
intercomponent communication effectively.
thin-layer chromatography and characterized by 1H and 13
C
NMR spectroscopy and by MALDI-TOF mass spectrom-
etry.19
X-ray structural analysis (Figure 1)19 reveals that 1oo is
located at a crystallographic inversion center and the
Importantly, bulk oxidation of the open form (e.g., 1oo2+)
does not lead to formation25 of the ring-closed state (e.g.,
1cc2+). Reduction of 1oo2+ and 2oo2+ leads to a full recovery
of the spectrum of the neutral forms 1oo and 2oo, respec-
tively. The absence of a significant influence of the phenyl
and chloro substituents on the absorption spectra of the
monocations and the absence of electrochemical ring closure
suggest that the polaron is localized on the sexithiophene
core. This model is supported by DFT calculations on 1oo
which show that the frontier molecular orbitals are localized
on the sexithiophene.19 Irradiation of the 1oo-4o with UV
light results in the appearance of a new absorption band that
is characteristic of photochemical ring closure of the dithie-
nylperfluorocyclopentene unit.15d The photochromic response
is reversed upon irradiation with visible (>500 nm) light.
For all compounds, the intensity of the fluorescence decreases
by >98%, indicating that at the photostationary state at 365
nm (PSS) essentially complete conversion of 1oo to 1co/
1cc is achieved.
For the terthiophene-substituted diarylethenes 3o and 4o,
irradiation with UV light results in the formation of the ring-
closed forms 3c and 4c, respectively. For 1oo and 2oo there
is the possibility of forming two photoproducts, i.e., where
one or both of the dithienylethene units are in the closed
state. HPLC analysis indicates that the ring closure proceeds
via the closed/open state to the closed/closed state (e.g., 1cc,
see graphical abstract). Isolation of 1co and 1cc was
precluded by the thermal instability of both states toward
ring opening to 1oo.19 Comparison of the UV/vis spectra of
Figure 1. Molecular structure of 1oo.
sexithiophene moiety is essentially planar. It exhibits an all-
anti, anti conformation that reduces steric interaction between
the thiophene units; however, the high conversion to 1cc in
solution (vide infra) confirms that the syn and anti conforma-
tions are in dynamic equilibrium in solution.
Comparison of the FTIR spectra of 1oo/2oo with 3o/4o,
respectively,19 shows that the characteristic absorptions (i.e.,
C-F stretching) of the hexafluorocyclopentene units are
retained.20 Notably for both 3o and 4o the characteristic C-H
oop bending of the terminal thiophene C-H, at 690 cm-1,21
is absent in the R,R′-dimerization products 1oo and 2oo. The
absorption band at 790 cm-1 in the spectra of 1oo and 2oo
is characteristic of a sexithiophene.22
The influence of the sexithiophene unit on the physical
properties of the dithienylethene units, and vice versa, was
examined spectroscopically and electrochemically. The
UV-vis absorption, fluorescence, and electrochemical data
for 1-4 (see the Supporting Information) are summarized
in Table 1. The absorption and emission spectra and the
cyclic voltammetry of 1oo and 2oo (Figure 2) are charac-
Table 1. Electronic Absorption, Fluorescence, and Redox Data
abs λmax/nm
(ε/103 M-1cm-1
em
λmax/nm (Φ)
Ep,c/V
E1/2/V
(vs SCE)
d
)
(20) de Jong, J. J. D.; Browne, W. R.; Walko, M.; Lucas, L. N.; Barrett,
L. J.; McGarvey, J. J.; van Esch, J. H.; Feringa, B. L. Org. Biomol. Chem.
2006, 12, 2387–2392.
1oo 270 (40.2), 435 (41.0)
504 (0.28)c -1.80
0.83, 1.04
307 (23.0), 392 (18.0),
1cc 468 (20.0), 634 (44.0)
-1.47, -1.14,
246 (52.0), 357 (40.0),
-0.96
0.79, 0.85
1.02, 1.22
(21) Akimoto, M.; Furukawa, Y.; Takeuchi, Y.; Harada, I. Synth. Met.
1986, 15, 353–360.
2oo 389 (sh)
2cc 374 (37.0), 567 (36.0)
499 (0.09)b
413a
(22) Zotti, G.; Zecchin, S.; Vercelli, B.; Berlin, A.; Grimoldi, S.; Pasini,
M. C.; Raposo, M. M. M. Chem. Mater. 2005, 17, 6492–6502.
(23) (a) Pappenfus, T. M.; Burand, M. W.; Janzen, D. E.; Mann, K. R.
Org. Lett. 2003, 5, 1535–1538. (b) Mason, C. R.; Skabara, P. J.; Cupertino,
D.; Schofield, J.; Meghdadi, F.; Ebner, B.; Sariciftci, N. J. Mater. Chem.
2005, 15, 1446–1453.
(24) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.; Lucas,
L. N.; Uchida, K.; Van Esch, J. H.; Feringa, B. L. Chem.sEur. J. 2005,
11, 6414–6429.
(25) Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83–89.
3o
3c
260 (32.6), 354 (31.0)
282 (21.7), 385 (26.0),
401 (27.0), 604 (24.0)
1.13 (irr)e
0.78, 0.95
1.34 (irr)e
0.92
4o
4c
250 (18.2), 355 (18.1)
407a
384 (16.0), 565 (13.5)
b
c
a λex ) 350 nm. λex ) 367 nm. λex ) 430 nm. d Measured against
9,10-diphenylanthracene (Φfl ) 0.90 in cyclohexane).24 e irr ) a return
cathodic wave is not observed (Ep,a only), sh ) shoulder.
Org. Lett., Vol. 11, No. 3, 2009
723

each compound obtained by HPLC19 with the spectra
obtained at the PSS indicates that the PSS consists of >90%
of 1cc.
As for the open state, the lowest energy absorption band
of the phenyl-substituted 1cc is red-shifted by ca. 1800 cm-1
with respect to the chloro-substituted 2cc. The effect of the
substituent on the absorption spectrum in this case, however,
is not unexpected considering the contribution of the bis-
trans-butadiene unit of the closed dithienylcyclopentene to
the (HOMO/LUMO) frontier orbitals. The redox chemistry
of 1oo and 1cc is shown in Figure 2c. The effect of ring
cm-1) which corresponds closely to the shift in the lowest
energy absorption band upon ring closing (ca. 7200 cm-1).25
The frontier molecular orbital diagrams obtained from DFT
calculations19 suggested that upon closing of the dithie-
nylethene rings the system can best be described as two
closeddithienylethenesratherthananend-cappedsexithiophene.
That is, the properties are not those of a sexithiophene but
instead are those of a dithienylethene. This provides a
rationalization for the decrease in the separation of the first
and second oxidation steps upon ring closing. For a
hexafluorocyclopentene bridged dithienylethene the separa-
tion between the first and second oxidation is typically less
than 80 mV. Furthermore the ring opening to form 1oo2+
that follows oxidation of 1cc to 1cc2+ explains why
electrochemically driven ring closing is not observed in either
1oo or 2oo.19
In summary, we have shown that end-capped sexithiophenes,
which incorporate the functionality of dithienylethene pho-
tochromic switches, can be prepared conveniently by elec-
trochemical dimerization. These hybrid systems can be
switched between three electronic states, i.e., 1oo, 1co, and
1cc. The photochemical activity of these end-substituted
sexithiophenes demonstrates that the photochromism of the
dithienylcyclopentene motif is retained when part of a
sexithiophene “molecular wire”. This indicates that the
absence of photochemistry in films of a related dithie-
nylethene/sexithiophene-based polymer is due to intermo-
lecular excited-state quenching and not intramolecular quench-
ing of the photochemistry of the dithienylethene units by
the sexithiophene unit.
Finally, it is clear that the hexafluorocyclopentene unit
provides for a remarkable degree of communication between
the peripheral and core units. The ability of a simple electron-
deficient alkene bridge to allow for relatively remote sub-
stituents to be used to tune the properties of the sexithiophene
core opens up new possibilities in the molecular tuning of
this important material for organic electronics. Detailed
photophysical studies of these systems will be reported in
due course.
Figure 2. (a) UV-vis absorption spectra of 1oo (thick solid), 1cc
Acknowledgment. The Zernike Institute for Advanced
Materials (J.H.H., A.M., B.L.F.), Ubbo Emmius (J.A.),
Nederlandse Organisatie voor Wetenschappelijk Onderzoek
Vidi grants (J.L.H., W.R.B.), the Stichting voor Fundamen-
teel Onderzoek der Materie (FOM) (M.T.W.M., J.L.H.), and
Nanoned are thanked for financial support.
(thick dashed), 2oo (light solid), and 2cc (light dashed) in hexane. 1cc
and 2cc were obtained by irradiation of 1oo and 2oo, respectively, at
λexc 365 nm.20 Cyclic voltammetry of (b) 1oo, 0.01 V s-1 and 2oo,
0.1 V s-1 and (c) of 1oo (dotted) and 1cc (PSS 365 nm solid) 0.1 V
s-1 in CH2Cl2/0.1 M TBAPF6.
Supporting Information Available: Synthetic procedures
and NMR, IR, UV/vis-NIR spectroscopic and electrochemi-
cal characterization, and DFT calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
closure on the first oxidation potential is minimal with the
separation between the first and second oxidation process
decreasing upon ring closure. By contrast, the change in the
reduction potential is typical of dithienylethenes with a
positive shift in the first reduction potential of 840 mV (6700
OL8028059
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Org. Lett., Vol. 11, No. 3, 2009