Core/sheath organic nanocable constructed with a master-slave molecular pair for optically switched memories

Article abstract of DOI:10.1002/adma.201102892

An organic photochromic material and an organic semiconducting material are combined to produce a "master-slave-type" core/sheath nanocable material in which by switching the photochromic master sheath, the electrical conductance and fluorescence of the semiconducting slave core material can be reversibly tuned. With such nanocables, non-volatile memory devices with both optical and electrical readouts are demonstrated. Copyright

Full text of DOI:10.1002/adma.201102892

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Core/Sheath Organic Nanocable Constructed with
a Master–Slave Molecular Pair for Optically Switched
Memories
Gang Xu, Qing-Dan Yang, Feng-Yun Wang, Wen-Feng Zhang, Yong-Bing Tang,
Ning-Bew Wong,* Shuit-Tong Lee,* Wen-Jun Zhang, and Chun-Sing Lee*
Organic photochromic molecule, which can undergo reversible
transformation between two different structural forms upon
exposure to light of different wavelengths, is a good example
of a photoresponsive bistable molecular system and has been
suggested for plenty of potential applications ranging from bio-
medical research to information technology.[1–11] The develop-
ment of switches and memories with photochromic molecules
has stimulated intense and highly promising interdisciplinary
research efforts. Ideally, a device designer would like to have
molecules with efficient photochromic switching; switchable
electrical, optical, or magnetic properties between the two
photochromic states; and good stability for long-term opera-
tion. This requirement of all-round properties is a challenge for
synthetic chemists to develop such Holy Grail materials. Even
materials with efficient photochromic switching and good elec-
trical or fluorescent properties are rare.[12,13]
Here, we propose a new approach to solve this problem
by distributing the above functions to two optoelectronically
coupled molecules. Briefly, a molecule with good photochromic
switching properties is used as a “master” molecule. Via opto-
electronic coupling, the master molecule will control a “slave”
molecule that has no photochromic properties but has good
electrical and/or optical properties. The essential optoelectronic
coupling is achieved here with a core/sheath nanocable struc-
ture, which provides intimate contact between the “master”
and the “slave” molecules. Through this design, molecules with
more specialized functions can be developed individually, thus
meeting the need for developing all-round materials.
matched energy levels to enable optoelectronic coupling. The pho-
tochromic master molecule used here is a diarylethene derivative,
1-[2-methyl-5-phenyl-3-thienyl]-2-[2-methyl-5-(p-(methyl)phenyl)-
3-thienyl]-hexafluorocyclo-pentene (MPT–MMPT–HFCP), which
has been widely studied for its excellent photochromic proper-
ties.[1–7,14] Coronene is used as the slave molecule for the following
reasons: 1) it has a large aromatic system and is a good hole-
transporting semiconductor, with electrical properties that have
potential tunability upon combined use with MPT-MMPT-HFCP;
2) its photoluminescence (PL) spectrum has extensive overlap
with the absorption spectrum of MPT-MMPT-HFCP, which may
induce efficient intermolecular energy transfer for the emission
modulation; and 3) the π– π interaction between polycyclic aro-
matic molecules would provide driving force for self-assembling
into 1D nanocable.[15,16] With individual coronene/MPT-MMPT-
HFCP core/sheath nanocables, non-volatile memory devices with
electrical readout have been fabricated and characterized.
Figure 1 shows that sample 1 has a typical cable nanostruc-
ture that is several hundreds of nanometers in diameter and
several micrometers in length, where the core consists of an
aligned nanofibers of diameters of 50–100 nm. The color of
sample 1 can be reversibly switched between green and deep
blue by irradiating with light of wavelength 365 and 520 nm,
respectively (Figure 2c). To clarify the component of the core
and the sheath of the nanocable, sample 1 was rinsed with eth-
anol. After washing, the color of the sample turned from green
to yellow, which is the color of coronene. Photochromism could
no longer be observed after ethanol washing. These observa-
tions hint that the sheath is likely to be MPT–MMPT–HFCP
(the photochromic molecule), while the core is composed of
coronene. This supposition is further supported by X-ray dif-
fraction (XRD) spectra (Figure S1, Supporting Information) of
sample 1, MPT–MMPT–HFCP, and coronene, which show the
coexistence of MPT–MMPT–HFCP and coronene crystalline
phases within sample 1.
Here, we demonstrate the above concept with a core/sheath
nanocable constructed with two small molecular materials with
Dr. G. Xu, Q.-D. Yang, F.-Y. Wang, Dr. W.-F. Zhang, Dr. Y.-B. Tang,
Prof. N.-B. Wong, Prof. S.-T. Lee, Prof. W.-J. Zhang, Prof. C.-S. Lee
Center of Super-Diamond and Advanced Films (COSDAF) City
University of Hong Kong Hong Kong, P. R. China
E-mail: bhnbwong@cityu.edu.hk; apannale@cityu.edu.hk;
apcslee@cityu.edu.hk
In our synthesis, tetrahydrofuran (THF) is a good sol-
vent for both coronene and MPT–MMPT–HFCP. In con-
trast, the 3:7 v/v ethanol:water mixture is a poor solvent
for them. In preliminary experiments, coronene/THF and
MPT–MMPT–HFCP/THF solutions were separately injected
into the ethanol/water mixtures. While nanocrystals were
obtained from the coronene solution, MPT–MMPT–HFCP
can only form an emulsion. Based on these and the above
results, we propose a tentative growth mechanism for the
formation of the core/sheath nanostructures (Figure S2,
Supporting Information).
Dr. G. Xu, Q.-D. Yang, F.-Y. Wang, Prof. N.-B. Wong
Department of Biology and Chemistry
City University of Hong Kong Hong Kong, P. R. China
Dr. W.-F. Zhang, Dr. Y.-B. Tang, Prof. S.-T. Lee, Prof. W.-J. Zhang,
Prof. C.-S. Lee
Department of Physics and Materials
Science, City University of Hong Kong
Hong Kong, P. R. China
DOI: 10.1002/adma.201102892
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over the 590 nm region by the closed-ring
isomer increases with the irradiation dose.
Such a change is similar to that observed
in other diarylethene derivatives and can be
attributed to the cyclization isomerization of
the dithienylethene chromophore in MPT–
MMPT–HFCP.[18–24] Irradiating the closed
isomer with light of 520 nm restores the
original spectrum (Figure S3b, Supporting
Information), which indicates that the photo-
induced cyclization is reversible. The kinetics
studies of MPT–MMPT–HFCP and sample
1 show they have similar rate constants
during the isomerization process (see
Supporting Information).
Figure 1. The scanning electron microscopy (SEM) image (a) and transmission electron micro-
scopy (TEM) image (b) of the core/sheath nanowires of sample 1.
PL of the photochromic compounds has
been utilized as a readout signal in memory applications.
Unfortunately, many photochromic compounds, including
MPT–MMPT–HFCP (Figure S4b, Supporting Information)
do not have strong PL emission. For obtaining PL signals, the
most commonly used approach is to attach fluorophores cova-
lently to the organic photochromic species.[25–33] However, it is
a difficult task to design and synthesize these multifunctional
fluorescent photochromic molecules. Moreover, the tunability
of the active absorption bands corresponding to the writing,
erasing, and reading processes should be carefully considered
such that the reading process will not disturb the memory state.
Here, we distribute the “switching” and the “functional” (i.e.,
the electrical and fluorescent properties) capacities into a pair of
master and slave materials. This new approach is demonstrated
by application of the coronene/MPT-MMPT-HFCP core/sheath
nanocables as non-volatile memory devices. In such devices,
the photochromic MPT-MMPT-HFCP molecule is used as the
master material to provide the “switching” capacity. The non-
photochromic coronene molecule will be used as the slave
molecule to provide fluorescence and elec-
Upon injection of the coronene/THF solution into the poor
solvent, the limited solubility of coronene and the strong inter-
molecular π–π interactions provide driving forces for the cor-
onene molecules to aggregate and self-assemble quickly into 1D
microstructures.[15,16] After that, the MPT–MMPT–HFCP emul-
sion is adsorbed onto the surface of the coronene nanofibers,
which induces de-emulsification at the same time. As a result,
MPT–MMPT–HFCP molecules gradually crystallize on the sur-
face of the nanofiber and form the core/sheath structure, as
shown in Figure S2d (Supporting Information).
The MPT–MMPT–HFCP molecule can undergo photo-
chromic reactions between the open-ring and closed-ring
isomers by irradiation with UV (365 nm) and visible light
(520 nm), respectively (Figure 2b). In the dark, the deep blue
color remains stable at room temperature. The absorption
spectra of the closed MPT–MMPT–HFCP are characterized by
the peak at 590 nm, which is assigned to the S0–S1 transitions of
its closed diarylethene unit.[1,17] (Figure S3a, Supporting Infor-
mation). Upon irradiation with 365 nm light, the absorption
trical conductivities.
Figure 3a show PL of the core/sheath nano-
cables after exposure to alternating cycles of
UV (365 nm) and visible (520 nm) illumina-
tion. Since the PL spectra is the same as that of
pure coronene and MPT–MMPT–HFCP has
no apparent photoluminescence (Figure S4,
Supporting Information), we can reasonably
conclude that PL of the core/sheath nano-
cables is inherited from the coronene slave
molecule. As shown in Figure 3, the PL of the
core/sheath cables can be reversibly switched
between the high intensity (“ON”) and the
low intensity (“OFF”) states upon exposure
to visible and UV light, respectively, corre-
sponding to the photochemical conversion
between the two states of the MPT–MMPT–
HFCP master. Upon exposing to the 365 nm
light, the PL intensity gradually decreased
within 420 s and then increased in the next
420 s under irradiation with 520 nm light,
reversibly (Figure 3b). It can also be seen that
upon irradiation with light of 365 or 520 nm,
Figure 2. The coronene (a) and the photochromism of MPT-MMPT-HFCP (b) and the sample 1
deposited on a 1 × 1 cm2 glass substrate observed by irradiation with light of 365 and 520 nm,
respectively (c).
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Figure 3. The photoluminescence of sample 1 modulated with UV (365 nm) and visible light (520 nm) (a) and the reversibility of the emission at
500 nm of the sample 1 tuned by alternating cycles of UV (365 nm, red area) and visible light (520 nm, blue area) (b).
most of the changes in PL intensity (respectively 85% and 50%
of the total changes) occurs over the initial 60 s. The ON/OFF
ratio for the PL intensity at 500 nm is as high as ≈4.5. No sign
of fatigue was observed after several cycles. These results show
that by switching the state of the master, the fluorescence of the
slave can be turned on and off.
to utilize electrical properties for reading the molecule state. To
the best of our knowledge, electrical studies on photochromic
molecule are still in their infancy.[37–42] In particular, the
attempt to fabricate a rewritable non-volatile memory device by
combining the optical and the electrical properties of a photo-
chromic molecule has not been reported before.
This master–slave action poses a question here on the mech-
anisms by which the master can control the slave. In fact, it
can be seen that PL of coronene slave (Figure S4a, Supporting
Information), overlaps considerably with the absorption of
MPT–MMPT–HFCP master (Figure S3a, Supporting Infor-
mation) over the region between 450 and 600 nm. This hints
that the strong photoswitching of the composite nanocables
might be induced by an efficient intermolecular energy transfer
process.[34–36] Detail mechanisms of the master–slave action
will be discussed later.
For practical applications, non-destructive writing, erasing,
and reading processes are required; therefore each process
should be independently carried out by irradiating in dif-
ferent regions of the spectrum. In the nanocables, the excita-
tion of coronene incidentally overlaps with the absorption of
MPT–MMPT–HFCP for the open isomer transforming to the
closed isomer. Consequently, the reading process might trigger
photoisomerization of the MPT–MMPT–HFCP sheath leading
to signal destruction. Obviously, such an optical readout process
might not be ideal for practical usage. One possible solution is
Interestingly, when examining the influence of the photo-
isomerization of the photochromic sheath on the electrical
properties of the semiconducting core, we found that elec-
trical signal could be used as non-destructible readout in our
memory devices. Figure 4a shows the typical current density–
voltage (I–V) curves obtained when exposing the nanocables to
light with a wavelength of 365 or 520 nm (a schematic of the
device can be found in Figure 5). As shown in Figure 4b, the
photoconductivity of the nanocable was first demonstrated at
the beginning 25 min of the phototunable electrical property
measurement. All data in Figure 4b were measured under a bias
voltage of 2 V. When irradiated with 520 nm light, the photocur-
rent of the cable doubled almost instantly (see first and second
columns of Figure 4b) with respect to the dark current. The
current dropped rapidly back to the dark current after cutting
the irradiation (third column of Figure 4b). In our system the
switchable component is a diarylethene, which has been shown
in previous studies to have poor PL and no photoconductivity;
on the contrary, coronene has shown the above properties.[43]
On the other hand, the closed form of diarylethene has more
Figure 4. The I–V curves measured upon exposure to 365 and 520 nm light (a). The current measured at the first 25 min and the reversibility of erasing,
writing, and reading processes of sample 1 memory device measured under 2 V bias voltage (b); the red area was measured in the dark, the yellow-
green area was measured by exposure to 520 nm light, and the blue area was measured by exposed to 365 nm light.
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Figure 5. A schematic of the device configuration for the memory meas-
urement (a), the prefabricated blank electrode (b), and the nanowires of
sample 1 dispersed on the electrode (c).
Figure 6. Retention characteristic of the sample 1 memory device meas-
ured under 2 V bias voltage. Red and black curves show current versus
time after the device was switched to “1” and “0” states, respectively.
extended π-conjugated system and high conductivity compared
to the open form.[38,40] If the electrical measurement is really
due to the poorly conducting diarylethene, the current should
increase upon UV exposure (i.e., opposite of what is observed
in the present results). In short, switching of the diarylethene
cannot explain the optical read-out as it has no PL properties; it
cannot explain the electrical read-out as it will lead to opposite
changes in current.
On the other hand, while we have successfully shown the possi-
bility of electrical read out using the present system, it is noted
that the electrical performance is still below those reported by
Kronemijer et al. and Whalley et al.[40,44] For practical high-
performance applications, performance enhancement is still
needed for the present device. However, the main purpose of
this work is to present the new approach of using the master–
slave system and the memory device is only used as an example
for proofing the concept.
As a memory device, the writing operation was performed by
photoirradiating the cable with 365 nm light (the fourth column;
blue area in Figure 4b). The reading operation was carried out
by turning off the photoexcitation and measuring the current of
the device (the third and fifth columns; red area in Figure 4b).
The erasing procedure involved the irradiation of the cable with
520 nm (the sixth column). In the writing process, the photo-
current decreases almost by one order of magnitude gradually
within 17 min, with a value changing from 3 pA to 0.3 pA that
could be recognized as signal “1” in the later reading process.
In the erasing procedure, the synergism of the photomodula-
tion of the MPT–MMPT–HFCP master and photoconductivity
of the coronene slave induces a nearly 20 times increase on the
current, which then reduces to one order in the reading process
and can be regard as signal “0”. The reversibility of switching
between the “1” and “0” states of the device based on the nano-
cable was also demonstrated in Figure 4b. The two stable states
can be reproducibly switched by repeating the writing, reading,
and erasing processes with little sign of fatigue after three
cycles. In fact, diarylethene derivatives have shown high dura-
bility; the isomerization between the open ring and closed ring
can be repeated more than 104 times while maintaining satis-
factory photochromic qualities.[1–7,14] Therefore, it is expected
that the memory device based on the nanocable would have a
reasonable durability. The retention ability of the “1” and “0”
states of the device was tested under ambient atmosphere. As
found in Figure 6, the devices can preserve the two states for
hundreds seconds without any significant degradation. Com-
paring to some other systems, the finally switching time of the
device is longer. As shown in Figure 4b, the switching from “1”
and “0” states or from “0” and “1” states are both about 20 min.
While the exact working mechanisms of our device are not
been fully understood, we propose a possible interpretation
based on the literature and our theoretical calculation.[37–42] It is
well known that the transport of charge carriers in a semicon-
ductor is easily influenced by traps. For the trapping of holes
(the core of the cable, coronene, is a hole-transporting mate-
rial), there are two cases: 1) the creation of chemical traps: a
host material combined with a guest material whose ioniza-
tion energy is lower (i.e., highest occupied molecular orbital
(HOMO) level is higher) than that of the host material and 2) the
creation of dipolar traps: the energy of electrostatic interactions
between a charge carrier and its surrounding is locally modified
by the presence of polar species. The energy states of coronene
and open- and closed–MPT–MMPT–HFCP were obtained by
theoretical calculation and illustrated in Figure S5 (Supporting
Information). The HOMO of the open–MPT–MMPT–HFCP
(–6.0 V) is slightly lower than that of coronene (–5.8 V), there-
fore this open isomer cannot efficiently trap the positive car-
rier in coronene. However, after transformation to the closed
isomer, the HOMO of MPT–MMPT–HFCP is raised to
–5.4 V, which is higher than that of coronene. Thus, the closed
form of MPT-MMPT-HFCP can act as a trap to the hole car-
riers of the coronene core. On the other hand, the molecular
dipole moment calculations reveal that the dipole moment
of MPT–MMPT–HFCP increases from 5.1 D to 5.9 D after
isomerizating from the open isomer to closed isomer. The local
augment of the dipole moment of the photochromic sheath
after UV irradiation means the dipolar traps may also be cre-
ated for the hole trapping.
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To summarize, we propose a new approach of distributing
the photochromic “switching” and the “functional” capaci-
ties required for a photochromically controlled device into a
master and slave system. Via this approach, photochromic and
functional properties can be individually optimized. The con-
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volatile memory devices with a coronene/MPT–MMPT–HFCP
core/sheath nanocable, where by switching the photochromic
master sheath, the electrical conductance and fluorescence of
the semiconducting slave core material can be reversibly tuned.
The present approach opens another dimension for the design
of photochromic memory devices. By decoupling the switching
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Supporting Information
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Acknowledgements
This work was supported by Research Grants Council of HKSAR (No.
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