Hexabenzocoronene graphitic nanotube appended with dithienylethene pendants: Photochromism for the modulation of photoconductivity

Article abstract of DOI:10.1002/adma.200902601

A semiconducting organic nanotube composed of a graphite-like bilayer of columnarly assembled hexabenzocoronene units (see figure) changes its photoconductivity in response to photoisomerization of the photochromic dithienylethene pendants on the surface.

Full text of DOI:10.1002/adma.200902601

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Hexabenzocoronene Graphitic Nanotube Appended
with Dithienylethene Pendants: Photochromism for the
Modulation of Photoconductivity
By Yaning He, Yohei Yamamoto,* Wusong Jin,* Takanori Fukushima,
Akinori Saeki, Shu Seki, Noriyuki Ishii, and Takuzo Aida*
Photochromic molecules, capable of reversible transformation
between two isomeric structures by light, can change not only
their color but also their physicochemical properties including
dielectric constant, refractive index, and redox potentials.[1]
Hence, photochromic units are often used as fundamental
components for the development of molecular switches.[2] In this
context, dithienylethene (DTE) derivatives are the most attractive
because of high thermal stabilities of their open and closed
isomeric forms, excellent photoresponsibility, and fatigue
resistance.[3] By taking advantage of these features, DTE
derivatives, at a single-molecular level in solution, enable a
digital-type switching between their on and off states.[4] In the
self-assembled state, such a digital-type response of single DTE
molecules may be translated into an analog-type macroscopic
output.[5] Namely, one can modulate the output level into a
desired value between ‘0’ and ‘1’ by changing the mole fractions
of the open and closed forms of DTE. In this Communication, we
report a novel semiconducting organic nanotube with DTE
pendants, whose photoconducting output can be modulated by
the photochromic nature of DTE. The nanotube investigated
herein was obtained by controlled self-assembly of a Gemi-
ni-shaped amphiphilic hexabenzocoronene (HBC)[6,7] with a DTE
unit at one of the termini of triethylene glycol side chains (1,
Fig. 1). Tubularly assembled 1 possesses a coaxial configur-
ation,[6b,c] where a dense molecular layer of DTE laminates a
graphite-like bilayer of p-stacked HBC units (Fig. 1b).
Compound 1open (Fig. 1a) with an open-form DTE pendant
was synthesized in 95% yield by etherification of phenol-
group-appended DTE 3 with HBC 2[8] carrying an iodine group at
one of the termini of the triethylene glycol side chains (Scheme 1).
This compound was highly soluble in ClCH2CH2Cl and displayed
an absorption band at 362 nm, characteristic of molecularly
dispersed HBC, along with a shoulder band at ꢀ310 nm arising
from the open form of DTE (Fig. S1a of the Supporting Information).
We confirmed that the DTE pendant of 1 in solution can undergo
reversible photoisomerization between its open and closed forms.
For example, upon exposure to UV light at 310 (ꢁ5) nm (light power
density, 2.0 mW cmꢂ2), a yellow-colored ClCH2CH2Cl solution of
1open turned dark blue (Fig. S2), where a new absorption band
appeared at 500–750 nm at the expense of the original absorption
band at 310 nm, indicating that the open form of the DTE pendant
had closed (Fig. S1a). When irradiated with visible light at
580–650 nm (light power density, 18 mW cmꢂ2), the resultant
blue-colored solution containing 1closed (Fig. S1b) turned back to
yellow with a complete recovery of the spectral profile of 1open
(Fig. S1a). The reversibility of the photoisomerization of the
[*] Dr. Y. Yamamoto, Dr. W. Jin
ERATO-SORST Nanospace Project
Japan Science and Technology Agency (JST)
2-41 Aomi, Koto-ku, Tokyo 135-0064 (Japan)
E-mail: yamamoto@nanospace.miraikan.jst.go.jp
jin@nanospace.miraikan.jst.go.jp
1
DTE pendant was further confirmed by H NMR spectroscopy
(Fig. S3).
Prof. T. Aida, Dr. Y. He
Department of Chemistry and Biotechnology
School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: aida@macro.t.u-tokyo.ac.jp
Controlled self-assembly of 1open into nanotubes (1NT/open) took
place at 25 8C on standing of its ClCH2CH2Cl solution (1 g Lꢂ1
;
0.54 mM) for three weeks. Scanning electron microscopy (SEM)
images of a solid sample obtained by air-drying of this suspension
show the presence of hollow fibers with a large aspect ratio
(Fig. 2a). Transmission electron microscopy (TEM) shows that the
nanotubes are very uniform in diameter (20 nm) and wall
thickness (3 nm) (Fig. 2b). Under conditions identical to those for
the assembly of 1open, 1closed, with a closed-form DTE pendant,
self-assembled into nanotubes (1NT/closed) with comparable size
regimes to those of 1NT/open (Fig. S4). Analogous to the
photochemical behavior in solution, the nanotubes in the solid
state underwent reversible ring-opening/closing isomerization at
the DTE pendants. Thus, when a ClCH2CH2Cl suspension of
1NT/open was cast onto a quartz plate and air-dried, a yellow-
colored thin film was obtained, which displayed absorption bands
at 430 and 460 nm, characteristic of tubularly assembled HBC
Dr. T. Fukushima
Advanced Research Institute, RIKEN
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Dr. A. Saeki
Institute of Scientific and Industrial Research, Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567-0047 (Japan)
Prof. S. Seki
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
Dr. N. Ishii
Institute for Biological Resources and Functions
National Institute of Advanced Industrial Science and Technology (AIST)
Tsukuba Central-6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566 (Japan)
DOI: 10.1002/adma.200902601
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Figure 2. a) SEM and b) TEM microscopy images of an air-dried
ClCH2CH2Cl suspension of tubularly assembled 1open.
graphitelike bilayer of the nanotube is intact to the geometrical
change of the DTE pendants (vide infra).[10]
Of interest, photoconducting properties of 1NT/open and 1NT/
closed are remarkably different from one another. A ClCH2CH2Cl
suspension of 1NT/open was cast onto a Si substrate covered with a
200-nm-thick SiO2 layer. Then, Au was thermally deposited onto
the cast film of 1NT/open, forming electrodes with a 50 mm
separation. For evaluating the photoconducting properties, the
resultant film was exposed to visible light at 430–470 nm (light
power density, 7.0 mW cmꢂ2) to excite the p-stacked HBC units
selectively. Under an applied electric field of þ2 V, the film
exhibited a photocurrent, which was 15 times greater than
the dark current (Fig. 3a). Noteworthy is also that, when the
open-form DTE pendants on the surface of nanotube 1NT/open
were fully converted into the closed form (1NT/closed), the
photocurrent was enhanced by a factor of 5 from 0.033 to
0.15 nA. The photocurrent varied with the mole fractions of the
open and closed forms of DTE. As is seen from Figure 3b, when
the irradiation time for the conversion of 1NT/open into 1NT/closed
was longer, the photocurrent gradually became larger and reached
a maximum value. On the other hand, when the irradiation
wavelength was changed from 310 to 580–650 nm, the photo-
current, in turn, decreased with time and eventually recovered the
initial value.
Figure 1. a) Molecular structures of 1open and 1closed. b) Schematic repres-
entations of nanotubes 1NT/open and 1NT/closed with a coaxial configuration,
where a graphitelike bilayer of p-stacked HBC units (blue) is laminated by
molecular layers of open-form (yellow) and closed-form DTE (green),
respectively. c) Electronic absorption spectra of a cast film of 1NT/open
before (left) and after (right) exposure (30 min) to UV light at 310 nm.
units,[6] along with a shoulder band at 310 nm due to the
open-form DTE pendants (Fig. 1c, left). Upon exposure to UV
light at 310 nm, the film of 1NT/open changed in color from yellow
to dark green (Fig. S5) with the appearance of a broad absorption
band at 500–750 nm arising from the closed form of DTE (Fig. 1c,
right). The absorption spectrum of the photoirradiated film was
An analogous difference between 1NT/open and 1NT/closed in the
photoconducting properties was obtained by means of flash-
photolysis time-resolved microwave conductivity (FP-TRMC).[11]
FP-TRMC is an electrodeless method for evaluating photocarrier
behaviors at the nanometer scale. As shown in Figure 4a, a
cast film of 1NT/closed upon laser-pulse irradiation (wavelength,
superimposable with that of a cast film prepared from 1NT/closed
.
On subsequent irradiation with visible light at 580–650 nm, a
spectral profile identical to that observed for the as-cast film of
1NT/open resulted. It should be noted here that, in the forward and
backward photoisomerization processes, the positions and
intensities of the absorption bands due to the p-stacked HBC
units (430 and 460 nm) remained unchanged (Fig. 1c).[9] This
observation indicates that the columnar HBC arrays in the
l ¼ 450 nm; photon density ¼ 8.0 ꢃ 1015 cmꢂ2
) exhibited a
greater maximum transient photoconductivity, fSmmax
,
(fSmmax ¼ 4.9 ꢃ 10ꢂ3 cm2 Vꢂ1
s
ꢂ1) than that of a cast film of
1NT/open (fSmmax ¼ 0.96 ꢃ 10ꢂ3 cm2 Vꢂ1
s
ꢂ1). Photocurrent
action spectra (Fig. 4b), observed under
identical flash photolysis conditions, indicated
that the excitation of the HBC moiety
exclusively contributes to the photocurrent
generation. Given that the isomerization of the
DTE pendants does not cause any structural
change in the columnar HBC arrays in the
graphite-like bilayer (vide ante), one can
assume that the carrier mobilities in 1NT/open
and 1NT/closed are essentially identical to one
Scheme 1. Synthesis of 1open
.
830
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Figure 3. a) Current–voltage (I–V) profiles at 25 8C of a cast film of 1NT/open
in the dark (gray, broken line) and upon irradiation to visible light at
430–470 nm before (gray, solid line) and after (black) exposure (5 min) to
UV light at 310 nm. b) Change in photocurrent at 25 8C upon excitation at
430–470 nm under an applied electric field of þ2 V, where a cast film of
Figure 5. a) Fluorescence spectra of a cast film of 1NT/open before (gray)
and after (black) exposure (30 min) to UV light at 310 nm. b) TAS at 25 8C of
a cast film of 1NT/closed upon laser-pulse irradiation (l ¼ 450 nm, photon
density, 8.0 ꢃ 1015 cmꢂ2). The inset shows TAS of a cast film of 1NT/open
.
1
NT/open was irradiated with UV light at 310 nm for the initial 5 min (gray)
and visible light at 580–650 nm subsequently for the next 6 min (black).
different photoconductivities from one another. Furthermore,
photoisomerization of the DTE pendants on 1NT allows for
modulation of the photoconductivity in the graphitelike bilayer
wall (Fig. 3b). The present work provides a prototype of
multimode photoresponsive soft nanomaterials.
Experimental
Synthesis of 1: To a THF solution (5 mL) of a mixture of 2 [8] (28 mg,
0.02 mmol) and 3 (10 mg, 0.018 mmol) was added KOH (2 mg, 0.04 mmol)
and the resulting suspension was refluxed for 24 h under Ar. The reaction
mixture was allowed to cool to room temperature and evaporated to
dryness. To the residue was added CH2Cl2 and a soluble fraction was
separated, washed with water, dried over anhydrous MgSO4, and filtered off
from an insoluble fraction. The filtrate was evaporated to dryness under a
reduced pressure and the residue was subjected to column chromato-
graphy (SiO2, THF) to allow isolation of 1 as yellow solid (32 mg) in 95%
yield. 1H NMR (500 MHz, THF-d8, 55 8C, d): 8.84 (s, 2H), 8.75 (m, 2H),
8.62 (d, J ¼ 8.0 Hz, 2H), 8.54 (d, J ¼ 8.0 Hz, 2H), 8.44 (s, 2H), 7.89–7.88
(m, 4H), 7.66 (br, 2H), 7.45–7.44 (m, 4H), 7.25–7.14 (m, 9H), 6.95–6.94
(m, 2H), 4.31–4.30 (m, 4H), 4.17–4.16 (m, 2H), 3.97–3.95 (m, 4H),
3.88–3.74 (m, 8H), 3.67–3.63 (m, 6H), 3.07–3.04 (m, 4H), 2.32–2.28
(m, 4H), 1.98–1.96 (m, 4H), 1.44–1.25 (m, 36H), 0.91–0.82 (m, 12H). 13C
NMR (125 MHz, THF-d8, 55 8C, d): 159.3, 158.9, 148.9, 147.9, 142.8, 142.6,
139.1, 136.6, 134.2, 134.1, 133.5, 129.3, 129.0, 128.7, 128.4, 127.7, 126.8,
126.3, 125.4, 124.8, 124.3, 124.1, 123.9, 122.8, 122.4, 122.1, 121.2, 120.5,
118.9, 118.7, 118.6, 118.5, 115.1, 115.0, 72.2, 71.1, 70.9, 70.7, 70.1, 70.0,
69.9, 68.0, 67.9, 58.2, 37.1, 32.1, 30.2, 30.1, 30.0, 29.9, 29.5, 22.7, 22.4,
15.0, 14.9, 13.6. Matrix assisted laser desorption/ionisation time-of-flight
mass spectrometry (MALDI–TOF MS): calcd for C120H120F 6O8S2 [M]þ: m/
z ¼ 1866.35; found: 1866.99.
Figure 4. a) FP-TRMC profiles at 25 8C of a cast film of 1NT/open upon
laser-pulse irradiation (l ¼ 450 nm, photon density, 8.0 ꢃ 1015 cmꢂ2
)
before (gray) and after (black) exposure (30 min) to UV light at 310 nm.
b) Laser-pulse-wavelength dependencies of fSmmax at 25 8C, obtained by
FP-TRMC of a cast film of 1NT/open before (gray) and after (black) exposure
(30 min) to UV light at 310 nm.
another. Hence, the better photoconductivity of 1NT/closed than
1NT/open may indicate that the former provides a greater
photocarrier generation yield than the latter. In relation to this
possibility, the energy diagrams of HBC and DTE suggest that the
electron transfer from photoexcited HBC to the closed form of
DTE is energetically much favored over the electron transfer to its
open form (see Supporting Information). In fact, the fluorescence
of the p-stacked HBC units in 1NT was quenched more efficiently
Measurements of Photoconducting Properties: Photoconducting proper-
ties of cast films of nanotubes 1NT were measured at 25 8C using a Keithley
model 4200-SCS semiconductor parameter analyzer under a reduced
pressure (<10ꢂ3 Pa) in a Nagase Electronic Equipment Service model
GRAIL10-Helips-4-HT prober. A xenon light source (Asahi Spectra model
MAX-301) was used for photoirradiation, where the light power density was
evaluated using an Ophir model PD300-UV silicon photodiode and a
NOVA power meter.
when its DTE pendants were in the closed form (1NT/closed
,
Fig. 5a). Accordingly, transient absorption spectroscopy (TAS) of
1NT/closed displayed absorption at 610 nm, assignable to a cation
radical of HBC (HBCþꢄ), while such an absorption band was
hardly detected for photoexcited 1NT/open (Fig. 5b).
In summary, new Gemini-shaped HBC, bearing a DTE
pendant, successfully self-assembles into a nanotubular object,
where the molecular layer of photochromic DTE densely covers
the graphitelike bilayer of columnarly assembled HBC units
(Fig. 1). Isomerization of the DTE pendants leads to a change in
efficiency of the photoinduced HBC-to-DTE electron transfer,
thereby giving rise to an alternation of the photocarrier-
generation yield. Consequently, 1NT/open and 1NT/closed possess
FP-TRMC, TAS, and Fluorescence Spectroscopy Measurements: FP-TRMC,
TAS, and fluorescence spectra were measured with an identical geometry
using an in situ TRMC–TAS system [11]. In the conductivity measurements,
a resonant cavity was used to obtain a high degree of sensitivity. The
resonant frequency and microwave power were set to ꢀ9.1 GHz and 3 mW,
respectively, so that the electric field of the microwave was small enough
not to disturb the thermal motion of charge carriers. The charge carriers
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were photochemically generated using light pulses from a Continuum
model Surelite II Nd:YAG laser with Surelite optical parametric oscillator
plus (3–5 ns pulse duration) with an incident photon density of 8.0 ꢃ 1015
photons cmꢂ2. The TRMC signal, picked up by a diode (rise time <1 ns),
was monitored by a Tektronics model TDS3052B digital oscilloscope. The
observed conductivities were converted into normalized values, given by a
photocarrier generation yield (f) multiplied by sum of the charge carrier
mobilities (Sm), according to the equation fSm ¼ (1/eAI0 Flight)(DPr/Pr),
where, e, A, I0, Flight, Pr, and DPr denote the unit charge of a single electron,
the sensitivity factor (Sꢂ1 cm), the incident photon density of the excitation
laser (photon cmꢂ2), the filling factor (cmꢂ1), and the reflected microwave
power and its change, respectively. For the fluorescence spectroscopy, the
emission from a sample was guided into a Hamamatsu model C7700
wide-dynamic-range streak camera. Likewise, TAS was measured using
continuous white light from a xenon lamp as a probe. All experiments were
performed at 25 8C in air.
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Acknowledgements
Y.H. thanks the JSPS Postdoctoral Fellowship for Foreign Researchers.
Supporting Information is available online from Wiley InterScience
or from the author. This article is part of a Special Issue on Carbon
Materials.
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Received: August 1, 2009
Published online: November 24, 2009
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832
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2010, 22, 829–832