Photoswitchable molecular wires: From a sexithiophene to a dithienylethene and back

Article abstract of DOI:10.1016/j.cplett.2009.08.013

Photoswitchable dithienylethene groups, added to both α-ends of a sexithiophene molecular wire, switch reversibly between their open and closed forms upon irradiation. The open form has an excited state lifetime of 500 ps and is highly fluorescent, as is

Full text of DOI:10.1016/j.cplett.2009.08.013

Chemical Physics Letters 479 (2009) 137–139
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Photoswitchable molecular wires: From a sexithiophene to
a dithienylethene and back
Maaike T.W. Milder a, Jetsuda Areephong b, Ben L. Feringa b, Wesley R. Browne b, Jennifer L. Herek c,
*
a FOM Institute for Atomic and Molecular Physics, Science Park 113, 1098 XG, Amsterdam, The Netherlands
b Centre for Systems Chemistry, 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
c Optical Sciences Group, Department of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE, Enschede, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2009
In final form 4 August 2009
Available online 7 August 2009
Photoswitchable dithienylethene groups, added to both
a
-ends of a sexithiophene molecular wire, switch
reversibly between their open and closed forms upon irradiation. The open form has an excited state life-
time of 500 ps and is highly fluorescent, as is typical for oligothiophenes. In contrast the closed form is
non-fluorescent and the excited state lifetime is a shorter by a factor of 100, reminiscent of a dithieny-
lethene. In this system, we control the fundamental character of a molecular system through optical
switching; toggling between a sexithiophene molecular wire and a dithienylethene reversibly.
Ó 2009 Published by Elsevier B.V.
1. Introduction
ing temperature were ascribed to the formation of aggregates at
temperatures below 260 K.
In the bottom–up approach to molecular electronics, highly
conjugated molecules with multiple functionalities provide impor-
tant properties, including high charge mobility and the possibility
to form rigid structures, together with the possibility to modify
materials reversibly after synthesis and even after incorporation
into devices. Within the context of functional molecular materials
dithienylethenes and oligothiophenes stand out. Photochromic
dithienylethene switches are especially interesting because of their
often excellent fatigue resistance and the thermal stability in both
photochromic states [1,2]. Oligothiophenes form materials that
have already proven their versatility in electronic devices such as
FETs, OLEDs, photovoltaic cells and molecular wires [3–5]. They
provide for high charge mobilities and emission quantum yields,
regular morphologies, and show excellent stability. The combina-
tion of these two functional units allows for interesting new mate-
rials, such as switchable molecular wires.
A fundamental challenge facing the design of multifunctional
systems arises when the two functional units in a molecular sys-
tem overlap structurally (Fig. 1); can both units retain their indi-
vidual characteristics. In the present case, for example, will the
sexithiophene unit or the molecular dithienylethene unit dominate
the properties of this multi-component system. These questions
are of key importance in the development of switchable molecular
devices. When two functional components are combined to form a
single molecular system, often its properties are determined
primarily by only one of the functional units with the second
component serving to modify and/or tune these properties. How-
ever, it would be highly interesting to design systems in which
the fundamental nature of a multi-component system can be chan-
ged through reversible switching. Here, steady state and ultrafast
time-resolved spectroscopies are employed to explore the behavior
of these dithienylethene–sexithiophene multi-component com-
pounds. We show that the fundamental character of the molecule
can be toggled between two states: in the open form the molecule
Recently, we introduced photoswitchable units at both
a-ends
of a sexithiophene (Fig. 1). These units allow for tuning of the prop-
erties of these systems after the compounds are synthesised [6]. In
these systems the absorption and fluorescence spectra of the open
form exhibit a remarkable dependence on temperature [7]. In the
open form the UV/Vis spectrum at 298 K shows a broad unstruc-
tured absorption band in solution that becomes increasingly struc-
tured below 260 K and is accompanied by a near complete loss in
fluorescence intensity. The changes in the absorption spectrum
and the dramatic decrease in fluorescence observed with decreas-
behaves as
a sexithiophene and in the closed form as a
dithienylethene.
2. Experimental
Compound 1oo (Fig. 1) was available from earlier studies [6]. All
chemicals used were of spectroscopic grade or better and were
used as received. Steady state experiments were performed in hex-
ane. Absorption spectra were recorded using a HP8453 diode array
spectrophotometer or a JASCO V570 dual beam UV/Vis NIR spec-
trophotometer. Fluorescence spectra were recorded on a JobinYvon
* Corresponding author.
E-mail address: J.L.Herek@tnw.utwente.nl (J.L. Herek).
0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V.
doi:10.1016/j.cplett.2009.08.013

138
M.T.W. Milder et al. / Chemical Physics Letters 479 (2009) 137–139
the open form (1oo). Such a shift is typical for dithienylethene pho-
tochromic switches and is due to changes in the energies of the
frontier orbitals [1,8]. The absorption spectrum undergoes a red
shift of ꢀ40 nm over a 180 K temperature range (see supporting
online information) and is broadened slightly compared with that
at room temperature. Even at 120 K, the absorption spectrum does
not show structure that would indicate aggregation, in contrast to
1oo. Furthermore in the open form, the compound is highly fluo-
rescent (Ufl = 0.2), whereas the closed form is non-fluorescent
(
Ufl < 10À5). In summary, the steady state spectra show that the
behavior of the closed form does not resemble that of the open
form; 1cc is non-fluorescent and there is no evidence for H-aggre-
gate formation at low temperature.
Fig. 1. Molecular structure of compound 1 in the fully open (1oo) and fully closed
(1cc) state. Compound 1 is end-capped with phenyl groups. The suffixes oo and cc
indicate that both photochromic units are in the open and closed states, respec-
tively. The dotted lines between the open and closed form indicate the overlap in
structure of the sexithiophene and the dithienylethene unit.
We now proceed to the analysis of the ultrafast dynamics with
particular focus on selective excitation of the closed form. The
quantum yield of photochemical ring opening for dithienylethenes
is low, typically 1% or less [9]. Therefore, the transient spectra and
kinetic traces are virtually free of contributions from the photo-
chromic reaction and the formation of 1oo. Furthermore, perma-
nent bleaching is not observed in the transient data, allowing for
unencumbered analysis of the photophysics of 1cc. Fig. 3 shows
the room temperature transient spectra and decay kinetics of
1cc. The negative signal (minimum at 614 nm) corresponds in
shape to the steady state absorption spectrum. Hence, this signal
is assigned to ground state bleach. Between 670 nm and 750 nm,
the end of the detection window, a positive signal is observed, that
can be assigned to excited state absorption (ESA) from S1 to Sn. The
kinetic traces at 298 K can be fitted globally with a three level
sequential model with time constants of 140 fs, 1.6 ps and 6.3 ps.
The shapes of the transient spectra of 1cc at 125 K and 298 K are
similar (see Supporting online information Fig. B). The kinetic
traces at 125 K are fitted globally with the same model as at room
temperature. Time constants resulting from this fit are 90 fs, 1.5 ps
and 5.0 ps. Fitting the data at 125 K with the same parameters as
the room temperature data resulted in a decrease of the v2 of
the fit by less than 4%. The errors in the time constants from the
global fits are ꢀ10%. Because of the close resemblance in spectral
shape of the levels involved in the model, the different time con-
stants cannot be directly linked to a physical process, however,
comparison with previous studies is strongly suggestive of certain
underlying processes [10–14]. The subpicosecond time constant
observed in our data corresponds well with a time constant as-
signed to the initial decay of the excited state by level crossing
(1B–2A) in cyclohexadiene, the core of the dithienylethene molec-
ular switches. From this point onwards the decay proceeds to a
minimum on the potential energy surface from which the molecule
returns to its electronic ground state. This process is followed by
vibrational cooling to allow for relaxation to the minimum of the
ground state.
Fig. 3B shows a kinetic trace for 1oo as a comparison [7]. The
closed form (1cc) decays by a factor of a hundred times faster than
the open form (1oo); within the lifetime of 1cc the signal of the
open form is virtually static. Previously we assigned the properties
of the 1oo – the shape and intensity of the fluorescence spectra and
the temperature dependent aggregation – to behavior typical of a
sexithiophene [7]. However, the properties of the closed state
examined here are markedly different from those of sexithioph-
enes. Instead, the results of both steady state and time resolved
spectroscopy are consistent with previous studies of ring closed
dithienylethenes [9,15]. Hence, the state – open (1oo) or closed
(1cc) – of the dithienylethene functional units has a profound
influence on the nature of this multi-component compound: in
the open form it behaves as a sexithiophene and in the closed form
as a dithienylethene. This observation is supported by electro-
chemical measurements reported earlier [6], where it was shown
that cyclic voltammetry of 1oo is typical of end-capped sexithioph-
Fluorolog Max or a Jasco 7200 fluorimeter. An Oxford instruments
Optistat DN was used for temperature dependent measurements.
Details of the transient-absorption acquisition system are de-
scribed elsewhere [7]. Briefly, for 1oo the pump pulse was centered
at 475 nm (45 fs time resolution, 100
was tuned to 620 nm (33 fs time resolution, 30
l
W). For 1cc the pump pulse
W). Broadband
l
probe pulses (ꢀ 450–750 nm) were generated by focusing part of
the output of a Ti:sapphire amplifier into a 1 mm sapphire disk.
Build-up of photoproducts was prevented in room temperature
measurements by using a 2 mm flow cell with a 30 ml reservoir
of the compound dissolved in cyclohexane. Low temperature mea-
surements were recorded at 125 K in isopentane, in a 1 mm cuv-
ette using a cryostat (Optistat CF, Oxford Instruments liq. N2
cooled cryostat).
Ring-closure of the compound was achieved by irradiation with
a UV lamp (Spectroline 312 nm) for approximately 1 min. HPLC
analysis showed that full ring closure to the cc state proceeded
through the co state [6]. However, isolation of co and cc was pre-
cluded by thermal instability with respect to ring opening. Tran-
sient spectra show that during the experiments on the closed
state the system is in the cc state.
3. Results and discussion
Fig. 2 summarizes the results of steady-state spectroscopy. The
absorption spectrum of the sample closed photochemically shows
a pronounced bathochromic shift of ꢀ180 nm compared to that of
1.2
0.9
0.6
0.3
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Absorption spectra of 1oo (black, solid) and 1cc (blue, dots) and the
fluorescence spectrum of 1oo (red, dashed). The closed form is non-fluorescent. The
individual spectra are normalized to their maxima in the region 430–620 nm. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)

M.T.W. Milder et al. / Chemical Physics Letters 479 (2009) 137–139
139
0.02
0.01
0.00
-0.01
0.02
0.01
0.00
-0.01
550
600
650
700
750
0
5
10
15
20
25
30
35
Wavelength (nm)
Delay (ps)
Fig. 3. Transient absorption spectra of the closed form in cyclohexane at 298 K. Pump 620 nm, 33 fs, 30 lW. Panel A shows the spectral evolution of the closed form at 1 ps
(black squares), 5 ps (red circles), 10 ps (green triangles), and 20 ps (blue stars). The symbols represent the spectra obtained from global analysis of the data. Panel B shows
the temporal evolution of the transient signals at 600 nm (black circles), and 750 nm (red triangles). The steady state absorption spectrum is presented scaled and in grey
shading. As a comparison panel B shows the time evolution of the open form at 545 nm (green squares). The solid lines are global fits of the data. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
enes. However, upon ring closing to 1cc, the reduction potential
observed is characteristic of dithienylethenes in the closed form.
The shift in reduction potential corresponds closely to the shift of
the lowest absorption in the UV/Vis absorption spectrum.
Research on Matter) with financial support from the Nederlandse
Organisatie voor Wetenschappelijk Onderzoek (Netherlands Orga-
nization for the Advancement of Research) for Vidi grants (W.R.B.
and J.L.H.) and Nanoned.
4. Conclusion
Appendix A. Supplementary material
The behavior of this molecular wire can be changed reversibly
between that of a sexithiophene with photochromic end groups
and that of an oligothiophene bridged bis-dithienylethene. This
change is manifested by a dramatic difference in fluorescence
intensity and excited state lifetime between 1oo and 1cc. Thus,
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2009.08.013.
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