A photoswitch based on self-assembled single microwire of a phenyleneethynylene macrocycle

Article abstract of DOI:10.1039/c0cc00739k

Crystalline microwires of a phenyleneethynylene (PE) macrocycle self-assembled from solution exhibited superior photoconductive properties. Photoswitches fabricated with single wires afforded nA-scale photocurrents with on/off ratios of ca. 10³. At a bias of 30 volts highest gain value achieved was up to 4.5. The stable and rapid responses to light qualify these microwire-based devices for excellent photoswitches or photodetectors.

Full text of DOI:10.1039/c0cc00739k

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A photoswitch based on self-assembled single microwire
of a phenyleneethynylene macrocyclew
Jia Luo,ab Qifan Yan,a Yan Zhou,ab Tian Li,a Ningbo Zhu,a Chuanjiang Bai,a Yong Cao,b
Jian Wang,*b Jian Pei*a and Dahui Zhao*a
Received 2nd April 2010, Accepted 8th June 2010
DOI: 10.1039/c0cc00739k
Crystalline microwires of a phenyleneethynylene (PE) macro-
cycle self-assembled from solution exhibited superior photo-
conductive properties. Photoswitches fabricated with single
wires afforded nA-scale photocurrents with on/off ratios of ca.
103. At a bias of 30 volts highest gain value achieved was up to
4.5. The stable and rapid responses to light qualify these
microwire-based devices for excellent photoswitches or
photodetectors.
Macrocycle 1 consists of a conjugated hydrocarbon skeleton
of alternating o-PE8 and p-PE units (Fig. 1).9 While the flexible
alkyl side-chains made the molecule soluble in many organic
solvents at room temperature, lowering the solution temperature
proved effective in promoting the molecular self-assembly. A
solution of 1 in 1,4-dioxane at ca. 10À3 mol LÀ1 was cooled to
below the freezing point of the solution before it was allowed
to gradually warm to about 20 1C when dioxane started to
melt. A cloudy suspension was formed in the resultant melted
dioxane. The suspension was then dispersed in a large mount
of methanol, in which macrocycle 1 is insoluble. Thereby
the self-assembled structure obtained in dioxane at low
temperature was properly preserved at room temperature.
Optical microscopy revealed that the above processing
procedures generated long, uniform microwires. The strong
birefringence observed under the cross-polarized light indicated
the crystalline nature of the wires (Fig. 2a and S2w). Scanning
electron microscopy (SEM) showed that the wires were of a
relatively uniform width of approximately one micrometre,
and their typical lengths were in the range of several hundred
micrometres (Fig. 2b and S3w).
Quasi-one-dimensional (1D) nano- or microstructures of
semiconducting properties have attracted great attention for
fabricating electronic and photonic devices from the bottom-up
approach.1 Photoconductivity is one of the valuable properties
sought in such 1D structures, with applications in photo-
detectors, photoswitches, photovoltaic devices, etc.2 Compared
with the vast research results on inorganic nanowires and
carbon nanotubes of photoconductive properties, organic
molecules exhibiting exceptional photoconductivity are emergent3
and micro-devices based on single wires with good performance
have only recently been realized.3b–d
With well-defined geometries and unique self-assembly
abilities, shape-persistent aryleneethynylene macrocycles
(AEMs) offer special opportunities in forming such 1D micro-
structures.4 Although phenyleneethynylene (PE) (macro)-
molecules are well known to exhibit exceptional fluorescing
properties, they used to be considered less desirable for
semiconductive applications.5,6 Previously, perceptible photo-
conductivity was only observed with AEMs in thin films.7
Here we report the first example of self-assembled AEM
microwires from solution displaying optimal photoconductivity.
Photoswitches based on single microwires of PE macrocycle 1
displayed nA-scale photocurrents and on/off ratios of nearly
103. Moreover, the devices offered tens of switching cycles
without significant attenuation in photocurrent.
The HOMO and LUMO energy levels of the microwires
were estimated to be À5.9 eV and À3.1 eV, respectively, based
on the oxidative potential obtained from cyclic voltammetry
and the energy bandgap from the absorption spectrum
(Fig. S4w). The photoconductor devices were then fabricated
by spin-casting the microwire suspension in methanol onto a
treated silicon substrate, followed by the deposition of Au
electrodes. A gap of roughly 20 mm wide across the wire was
attained using a mask during the deposition (Fig. 3a).3b,c
Under dark conditions, the device produced a very low current
of ca. 10À12 A. As the injection barrier between the Au
electrodes (Fermi level of À5.1 eV) and the wire ought to be
relatively low, the wire is presumably an insulator in the dark.
However, once the microwire was illuminated with
a
laser beam of a wavelength of 405 nm, the current was
instantaneously amplified. The current–voltage correlation
a Beijing National Laboratory for Molecular Sciences,
Department of Applied Chemistry, Key Laboratories of Polymer
Chemistry and Physics and of Bioorganic Chemistry and Molecular
Engineering of the Ministry of Education, College of Chemistry,
Peking University, Beijing 10087. E-mail: dhzhao@pku.edu.cn,
jianpei@pku.edu.cn; Fax: +8610 62751708; Tel: +861062753735
b Institute of Polymer Optoelectronic Materials and Devices,
South China University of Technology, and Key Lab of Specially
Functional Materials, Ministry of Education, Guangzhou 510640,
China
w Electronic supplementary information (ESI) available: Syntheses
and characterization of compounds, experimental procedures and
CIF files for crystal structures. CCDC 772272. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc00739k
Fig. 1 Chemical structure of macrocycle 1.
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Fig. 2 Polarized optical micrograph (a) and SEM image (b) of
self-assembled microwires.
(I–V) of a representative device under illumination exhibited a
quasi linearity. Notably, the photocurrent reached ca. 0.9 nA
at a bias of 30 V, with the illumination intensity of
85 mW cmÀ2 (Fig. 3b). Such a photocurrent corresponds to
an on/off ratio of nearly 103. In order to gauge the stability of
the devices, the photoswitches were tested by repeatedly turning
the laser on and off. The photocurrent value did not attenuate
substantially after a significant number of on/off cycles
(Fig. 3c and S8w). The photocurrent was found to correlate
with the light intensity in a similar fashion as that of a
previously studied system (Fig. 3d).3c The gain value, a key
parameter for describing the number of carriers passing
through the photoconductor per absorbed photon, was calculated
to range from 0.11 to 4.5, depending on the illumination
intensity, at a bias of 30 V (Fig. S9w). This value was comparable
with those of SWCN-based devices.10 Such remarkable
photoconductivity suggested that effective exciton dissociation
occurred and photo-generated carriers migrated through the
microwire under a voltage bias.
Fig. 4 Varied temperature XRD patterns of microwires (a & b, the
stars indicate the reflections from Al foil); (c) a schematic presentation
of the hexagonal columnar stacking of the macrocycles at 140 1C;
(d) molecular structure of macrocycle 1 based on single crystal XRD.
of the microwires at various temperatures (Fig. 4a). The
diffraction pattern recorded for the microwire at room
temperature displayed multiple sharp peaks, which confirmed
the crystalline nature of the microwire and reflected a
sophisticated crystal structure. Nonetheless,
a notable
Subsequently, the photoconductivity was investigated upon
annealing the microwires at 140 1C for 2 h. No discernible
change in the appearance was observed with the annealed
microwire.11 However, the photocurrent of the annealed wire
dropped to a negligible value (Fig. S13w). X-ray diffraction
(XRD) analyses were then conducted to elucidate the structures
reflection at ca. 241 indexed a d spacing of 3.6 A. This value
matches well with the typical distance observed for stacked
aromatic rings under the influence of p–p interactions.
Presumably, intermolecular p–p stacking interactions among
the macrocyles play a role in the charge carrier generation and
transporting processes.
Varied-temperature XRD analysis along with differential
scanning calorimetry results unambiguously revealed a transi-
tion to the hexagonal columnar (colh) phase of the microwires
at the elevated temperature (Fig. 4a–b and S10w). In the XRD
pattern, the multiple sharp peaks observed at room temperature
all disappeared when the temperature reached 112 1C.
Meanwhile, two new broad diffractions corresponding to d
spacings of 2.44 and 1.35 nm emerged and persisted up to 160
1C. These diffractions were attributed to reflections of (100)
and (110) in the colh phase. The lattice constant a of colh was
accordingly determined to be 2.82 nm. This value was in good
agreement with the extended molecular dimension calculated
from the single crystal structure (vide infra, Fig. 4d). Such a
mesophase diffraction pattern remained when the sample was
cooled from 140 1C back to the room temperature. Thus, the
annealed microwires should possess the colh structure and
the photoconductivity of the annealed microwire reflected the
property of the colh phase.
Fig. 3 (a) A SEM image of a single-wire device; (b) I–V characteristics
of a photoconductive device under dark (blue line, with I value near
zero) and illumination (red line, l = 405 nm, intensity = 85 mW cmÀ2);
(c) photoswitching characteristic at a bias of 30 V (illuminating for
30 s at 50 s intervals, intensity = 85 mW cmÀ2); (d) photocurrent
dependency on the light intensity at a bias of 30 V.
Considering the complexity of the diffraction pattern of the
microwire, which cannot be assigned to any common columnar
structures, it is speculated that in the crystalline state the
macrocycles likely assumed a staggered packing motif, rather
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than forming a columnar structure. This assumption was
supported by the emssion characteristics. The emission band
of the microwires exhibited a 14 nm bathochromic shift upon
heating to above 112 1C, when the sameple entered the colh
phase (Fig. S11w). The higher emission energy at lower
temperatures implies less intimate stacking of the macrocycles
in the crystalline state.
Science and Technology of China (2006CB921600
&
2009CB930604), and Fok Ying-Tung Educational Foundation
(No. 114008). JL thanks the CDY scholarship.
Notes and references
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As an additional effort at unraveling the structure of the
photoconductive microwire, single crystals of macrocycle 1
were grown by slow evaporation of a dioxane solution.
Indeed, in the single crystal structure the macrocycles were
found to pack in a non-columnar, staggered fashion, with only
a partial overlap between neighboring macrocycles, although
p–p stacking of phenylene rings from adjacent molecules was
still effective (Fig. S5w).12
As the single crystals of the macrocycle were of a long rod
shape, photoconductor devices were also fabricated with these
rod crystals. Although the average device performance of the
single-crystal rods was somewhat inferior to that of the
microwires, pronounced photoconductivity was detected
(Fig. S12w). The reason for the different photoconductivities
of the microwire and single crystal rod may be complicated.
One influential factor is speculated to be the supramolecular
packing motif. Moreover, according to simulations, the normal
axis of the macrocycle is not in perfect parallel with the long
axis of the rod (Fig. S12w). Comparing the photoconductivity
of the single crystal rods containing staggered macrocycles and
that of the annealed microwires exhibiting columnar structures,
a notable observation is that the columnar architecture
possessing maximally overlapped macrocycles packed in a
face-to-face fashion seems not to be the most favorable
structure for photoconductivity. Whereas more pronounced
photoconductivity was exhibited when the macrocycles were
arranged in an off-set motif in the single crystal. Additionally,
in spite of the colunmar stacking, a less ordered structure of
the mesophase may account for the lower photoconductivity
of the annealed wires.
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6 Nanowires of an AEM were previously demonstrated with a
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In summary, a PE macrocycle was able to self-assemble into
crystalline microwires in solution and exhibited exceptional
photoconductive properties. Photoswitches based on single
wires afforded nA-scale photocurrents with on/off ratios of
approximately 103. The wire was shown stable under illumination
by displaying a large number of on-off cycles without
considerable photocurrent attenuation. The highest gain value
observed was up to 4.5 at a bias of 30 V. Thus, these
microwires of the PE macrocycle may serve as excellent
photoswitch materials. The morphology dependency of the
photoconductivity was also investigated. The results indicated
that the supramolecular assembling motif may be a critical
factor governing the semiconducting properties. As the
chemical structure of the macrocycle can be conveniently
tailored to modify the electronic properties, subsequent
investigations will be carried out to identify structures of
superior semiconductive properties and suitable for fabricating
micro-devices.
9 Macrocycle
1 was synthesized according to the procedure
previously reported for an analogous molecule: N. Zhu, W. Hu,
S. Han, Q. Wang and D. Zhao, Org. Lett., 2008, 10, 4283–4286; its
structure was further confirmed by 1H and 13C NMR, MALDI-
TOF MS, and EA (see the ESIw for details).
10 M. Freitag, Y. Martin, J. A. Misewich, R. Martel and P. Avouris,
Nano Lett., 2003, 3, 1067–1071.
11 The chemical structure of the microcycle was intact during thermal
1
annealing, which was confirmed by H NMR.
12 The simulated powder XRD pattern of the single crystals was also
complex and did not exactly match the powder XRD spectrum
recorded for the microwires, but the structures of the wires and
single crystals are likely bear certain similarity, considering the
same solvent used to obtain them. Elemental analysis showed that
dioxane molecules were contained in the microwires, as in the
single crystals.
This work is supported by the National Natural Science
Foundation of China (50603001 & 20774005), the Ministry of
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