Large scale assembly of ordered donor-acceptor heterojunction molecular wires using the Langmuir-Blodgett technique

Article abstract of DOI:10.1039/c1cc12025e

Unidirectionally aligned photoconductive donor-acceptor heterojunction molecular wires spanning over fifty square microns are fabricated using the Langmuir-Blodgett technique.

Full text of DOI:10.1039/c1cc12025e

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Large scale assembly of ordered donor–acceptor heterojunction molecular
wires using the Langmuir–Blodgett techniquew
Richard Charvet,*a Katsuhiko Ariga,a Jonathan P. Hill,a Qingmin Ji,a Ali Hossain Khanb
and Somobrata Acharya*b
Received 9th April 2011, Accepted 28th April 2011
DOI: 10.1039/c1cc12025e
Unidirectionally aligned photoconductive donor–acceptor hetero-
junction molecular wires spanning over fifty square microns are
fabricated using the Langmuir–Blodgett technique.
The fabrication of highly ordered architectures composed of
electronic donor–acceptor pair molecules using a supra-
molecular approach is a subject of critical importance for
organic photovoltaic devices (OPVs).1,2 Increasing the width
of the photoactive region by forming wide area heterojunctions
consisting of precisely ordered donor and acceptor components
can lead to improved photocarrier generation efficiency while
creating pathways for nontrivial charge migration. Long range
charge carrier separation and transport have been demonstrated
via the formation of photoconductive one dimensional (1D)
nanostructures consisting of supramolecular donor/acceptor
heterojunctions.2,3 Nevertheless, deposition of such structures
Fig. 1 (a) Structure of 1. (b) AFM micrographs of aligned molecular
wires of 1 spanning over B20 ꢀ 20 mm2 area with tight packing
on solid substrates leads to disordered arrays containing large
density. The frame represents a part of B50 ꢀ 50 mm2 aligned area.
void areas, which breaks down long range charge carrier
(c) Higher resolution AFM images showing aligned triangular
domains in the form of molecular wires, monolayer film deposited
onto freshly cleaved mica lifted by the LB method at a surface pressure
of 15 mN mꢁ1 at 19 1C. Inset shows an AFM image of an equilateral
triangular block.
transport. Advantageously, the Langmuir–Blodgett (LB)
technique offers sophisticated processability since the packing,
orientation and long range ordering of a molecular assembly
on a surface can be precisely tailored with perfect control and
reliable thickness.4,5 Here we report the fabrication of large
area close packed photoconductive molecular wires using a
newly designed amphiphilic zinc porphyrin–fullerene dyad
with the aid of the LB technique.6–8 The molecular wire
assembly spans over fifty-square microns retaining tight
spatial registry and alignment, which, to our knowledge, has
not been reported previously for any bottom up technique on
the macroscale.9
prevents ground-state electronic interactions and promotes
charge separation between the photoactive entities. The zinc
porphyrin moiety is substituted with a relatively flat and
cone-like unit containing terminal triethylene glycol (TEG)
chains, which can be regarded as a tapered hydrophilic
segment (or dendritic wedge). Thus, 1 can be seen as a
cone-like amphiphilic molecule with TEG chains forming
hydrophilic tails while C60 acts as a hydrophobic head
(Fig. 1a and see the ESIw).
We used the LB technique to organize the amphiphilic
donor–acceptor pairs into long range macromolecular objects
in a controllable and efficient way. The uniaxial compression
promotes the irreversible formation of an aggregate of 1 as
evidenced by the large hysteresis in the isotherm cycle (see
Fig. S1, ESIw). Compression to higher surface pressure
essentially results in unidirectional alignment in the form of
molecular wires as evidenced by atomic force micrographs
(AFM) (Fig. 1b and 1c). Uniformly aligned and tightly packed
molecular wires spanning over a B50 ꢀ 50 mm2 area are
Zinc porphyrin–fullerene dyad 1 consists of an electron-
acceptor fullerene moiety connected to an electron-donor
porphyrin through a rigid 4,40-substituted-diphenylacetylene
bridge to a 5,15-bis-phenyl porphyrin core, a geometry which
a World Premier International (WPI) Research Center for Materials
Nanoarchitectonics (MANA), National Institute for Materials
Science (NIMS), JST, CREST, 1-1 Namiki, Tsukuba, Ibaraki,
305-0044, Japan. E-mail: racharvet@yahoo.com
b Centre for Advanced Materials (CAM), Indian Association for the
Cultivation of Science, Jadavpur, Kolkata 700032, India.
E-mail: camsa2@iacs.res.in
w Electronic supplementary information (ESI) available: General
experimental details, synthesis of 1 and additional characterization
data. See DOI: 10.1039/c1cc12025e
c
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Chem. Commun., 2011, 47, 6825–6827 6825

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obtained. The optimum alignment has been achieved in the
surface pressure range of 10–18 mN mꢁ1 with full retention of
registry. The aligned layer can be transferred in a single step
onto a variety of substrates (see Fig. S2 and S3, ESIw) to yield
large scale parallel molecular wire arrays.
The higher resolution AFM image (Fig. 1c) shows finely
structured wires that are essentially composed of nearly
equilateral triangular blocks aligned unidirectionally in the
form of molecular wires. The edge length of triangular blocks
is B60 nm leading to an area of around 1.5 mm2 per block
(inset of Fig. 1c). Within each molecular wire, the triangular
blocks are decked in a side-by-side tilted configuration
defining the molecular wire diameter of B60 nm (Fig. 1c
and Fig. S4a, ESIw). The molecular wires are exceeding
30 mm in length defining an aspect ratio of greater than
500 (Fig. 1a). The AFM height profile (see Fig. S4, ESIw) of
the triangular blocks is less than 4 nm, therefore less than the
actual upright length of 1, suggesting a tilted configuration
at the air–water interface as also evidenced from the p–A
isotherms. The triangular structure likely results from the
trigonal packing of the fullerene groups and the cone-like
structure of the hydrophilic TEG chains adjacently packed
together under the proximal surface pressure. TEG chains
remain disordered and in a stretched configuration in the
hydrated state, which accounts for the 4 nm height of the
aggregated triangular blocks.
Fig. 3 (a) A 3D molecular structure of 1 and its equivalent cartoon.
(b) A schematic model of 1 aggregation showing an arm of a triangular
block with porphyrin J-aggregate stacking. (c) Segregated fullerene–
tilted porphyrin J-aggregates and entangled TEG chains forming a
triangular block.
Based on AFM observation and spectral evidences, we
modeled the triangular domain structures and their long range
ordering in the form of molecular wires (Fig. 3b and 3c).
Notably, Z-type deposition of LB films implies that hydrophilic
TEG chains are first attached onto a mica surface, while
hydrophobic fullerene moieties are facing outwards in a
trigonal configuration. Implicit here is the existence of
stronger interactions among hydrophilic–hydrophilic and
hydrophobic–hydrophobic groups and rigidity of the spacer
introduced between donor/acceptor chromophores, leading to
a phase-segregation phenomenon and aggregates at the supra-
molecular level. The triangular structures appear to form with
zinc porphyrin units appended with disordered hydrophilic
TEG chains on the base side, while hydrophobic C60 entities
are exposed at the other extremity. Interestingly, the arms of
the triangular blocks can be realized via porphyrin J-aggregate
stacking (Fig. 3b) while the arm length depends on the number
of 1 (Fig. 3a) and extent of spreading of TEG chains.
Well-known strong p–p electronic interactions between C60
moieties, the J-aggregate formation of zinc porphyrin blocks
and hydrogen bonding between TEG chains are leading to
densely packed aggregates of 1 into triangular blocks.10 Owing
to the directionality nature of hydrogen bonding between the
inter 1 TEG chains and p–p interactions of C60 moieties,
adjacent triangular blocks are coupled and decked at a certain
angle resulting in long range interactions in the form of
molecular wires.10b The preferential orientation perpendicular
to the compression direction originates from the reduced
pressure gradient along this direction.
The electronic absorption spectrum of a solution of 1
shows characteristic fullerene bands at 254 and 310 nm, and
characteristic porphyrin bands at 417 nm (Soret band),
540 and 585 nm (Q-bands) respectively. In contrast, a significant
broadening and overall Stokes shift of the porphyrin absorption
bands are observed in the absorption spectrum of an ordered
LB film (Fig. 2a). Furthermore, appearance of a new band at
463 nm indicates the formation of porphyrin J-aggregates.10a,4a
We followed the dynamics of this aggregation process by
monitoring the on-trough UV-vis absorption of the Langmuir
monolayer with increasing surface pressure (Fig. 2b). Gradual
appearance of the new band at 465 nm with increasing
pressure indicates formation of J-aggregates upon compression.
Increase in absorption intensity implies that a large number of
aggregates are dragged by the surface pressure at the footprint
of the beam while the Langmuir film remains stable.
The characterized heterojunction obtained with precisely
organized donor and acceptor domains on large area
prompted us to perform preliminary studies of the photo-
conductive properties. A complete quenching of the porphyrin
emission in ordered LB films indicates a highly efficient
photoinduced electron transfer (PET) between donor and
acceptor components.11 Furthermore, the unidirectionally aligned
wires display robust photocurrent properties owing to higher
absorption cross sections. A linear current–voltage response
upon white light irradiation was measured (see Fig. S5, ESIw)
Fig. 2 (a) Absorption spectra of solution of 1 (1 mM) in chloroform
(red curve) and a 7-layer LB film (blue curve) lifted at p = 10 mN mꢁ1
at temperature 19 1C. (b) Monitoring of the aggregation process using
on-trough UV-vis absorption by changing the surface pressure.
c
6826 Chem. Commun., 2011, 47, 6825–6827
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may lead to potential OPVs with high photon conversion
efficiencies.
Word financial support under Grant no. SR/S5/NM-47/
2005 and Grant no. SR/NM/NS-49/2009 from DST, India and
World Premier International Research Center Initiative (WPI
Initiative) on Materials Nanoarchitectonics, MEXT, Japan
and Core Research for Evolutional Science and Technology
(CREST) program of Japan Science and Technology Agency
(JST), Japan are gratefully acknowledged.
Notes and references
1 (a) J. L. Segura, N. Martin and D. M. Guldi, Chem. Soc. Rev.,
2005, 34, 31–47; (b) T. S. Balaban, in Light-harvesting Nano-
structures in Encyclopedia of Nanoscience and Nanotechnology,
ed. H. S. Nalwa, American Scientific Publishers, Los Angeles,
2004, vol. 4, pp. 505–559; (c) R. Bhosale, R. S. K. Kishore,
V. Ravikumar, O. Kel, E. Vauthey, N. Sakai and S. Matile, Chem.
Sci., 2010, 1, 357–368; (d) M. D. Kelzenberg, D. B. Turner-Evans,
M. C. Putnam, S. W. Boettcher, R. M. Briggs, J. Y. Baek,
N. S. Lewis and H. A. Atwater, Energy Environ. Sci., 2011, 4,
866–871.
2 (a) Y. Yamamoto, T. Fukushima, Y. Suna, N. Ishii, A. Saeki,
S. Seki, S. Tagawa, M. Taniguchi, T. Kawai and T. Aida, Science,
2006, 314, 1761–1764; (b) E. H. A. Beckers, S. C. J. Meskers, A. P.
H. J. Schenning, Z. Chen, F. Wurthner, P. Marsal, D. Beljonne,
J. Cornil and R. A. J. Janssen, J. Am. Chem. Soc., 2006, 128,
649–657; (c) Y. Hizume, K. Tashiro, R. Charvet, Y. Yamamoto,
A. Saeki, S. Seki and T. Aida, J. Am. Chem. Soc., 2010, 132,
6628–6629.
Fig. 4 (a) Photocurrent generation from molecular wires deposited
on ITO with Al as a counter electrode illuminated with white light at
an illumination flux of 30 mW cmꢁ2 for different duration times. Inset,
a plot of conductance versus illumination time. (b) Dynamic on–off
photocurrent switching response under an irradiance power of
30 mW mmꢁ2 with a frequency of 25 mHz and a duty cycle of 50%.
with linearly increasing photocurrent upon increasing incident
light intensity while the conductance also varied linearly upon
variation of illumination flux intensity (see Fig. S6, ESIw).10
The photocurrent was found to increase gradually with the
illumination time (Fig. 4a). The conductance increased
immediately following commencement of light irradiation
and reached saturation after an extended irradiation time
(inset in Fig. 4a). Repetitive on–off device response to white
illumination flux was found to be sharp and repeatable over
successive on/off irradiation cycles (Fig. 4b). A plot of
conductance versus time response reveals a similar trend over
multiple cycles (see Fig. S7, ESIw).
Donor–acceptor molecules 1 form well-defined triangular
blocks which pack in the form of ordered molecular wire
arrays over large area under applied pressure. The separation
of donor and acceptor entities by the rigid 4,40-diphenyl-
acetylene bridge at the molecular level in 1 leads to high
concentrations of electrons and holes on either side of the
interface, which generates a large chemical potential under
illumination. The chemical potential in combination with
built-in potential and charge delocalization within the densely
packed and precisely ordered domains lead to promising
photoconductive properties. Our results illustrate an intimate
relationship between charge carrier mobility and efficient
supramolecular packing between functional entities favorable
to the delocalization of the photogenerated charges. Our
method is quite flexible since the ordered assembly can be
precisely controlled in the sub-micron range by varying the
applied surface pressure. The presented approach for the
fabrication of large area molecular heterojunction devices
3 R. Charvet, S. Acharya, J. P. Hill, M. Akada, M. Liao, S. Seki,
Y. Honsho, A. Saeki and K. Ariga, J. Am. Chem. Soc., 2009, 131,
18030–18031.
4 (a) S. Acharya, J. P. Hill and K. Ariga, Adv. Mater., 2009, 21,
2959–2981; (b) K. Ariga, J. P. Hill, M. V. Lee, A. Vinu, R. Charvet
and S. Acharya, Sci. Technol. Adv. Mater., 2008, 9, 014109.
5 J. A. DeRose and R. M. Leblanc, Surf. Sci. Rep., 1995, 22, 73–126.
6 (a) D. I. Schuster, K. Li, D. M. Guldi, A. Palkar, L. Echegoyen,
C. Stanisky, R. J. Cross, M. Niemi, N. V. Tkachenko and
H. Lemmetyinen, J. Am. Chem. Soc., 2007, 129, 15973–15982;
(b) D. Kuciauskas, S. Lin, G. R. Seely, A. L. Moore, T. A. Moore,
D. Gust, T. Drovetskaya, C. A. Reed and P. D. W. Boyd, J. Phys.
Chem., 1996, 100, 15926–15932.
7 (a) F. D’Souza, P. M. Smith, M. E. Zandler, A. L. McCarty,
M. Itou, Y. Araki and O. Ito, J. Am. Chem. Soc., 2004, 126,
7898–7907; (b) A. L. Schumacher, A. S. D. Sandanayaka, J. P. Hill,
K. Ariga, P. A. Karr, Y. Araki, O. Ito and F. D’Souza,
Chem.–Eur. J., 2007, 13, 4628–4635.
8 (a) S. Okada and H. Segawa, J. Am. Chem. Soc., 2003, 125,
2792–2796; (b) S. Acharya, A. B. Panda, N. Belman, S. Efrima
and Y. Golan, Adv. Mater., 2006, 18, 210–213; (c) S. Acharya and
S. Efrima, J. Am. Chem. Soc., 2005, 127, 3486–3490.
9 (a) F. S. Kim, G. Ren and S. A. Jenekhe, Chem. Mater., 2011, 23,
682–732; (b) G. Ren, P.-T. Wu and S. A. Jenekhe, ACS Nano,
2011, 5, 376–384.
10 (a) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and A. P. H.
Schenning, Chem. Rev., 2005, 105, 1491–1546; (b) Y. Zhang,
P. Chen, L. Jiang, W. Hu and M. Liu, J. Am. Chem. Soc., 2009,
131, 2756–2757.
11 N. V. Tkachenko, L. Rantala, A. Y. Tauber, J. Helaja,
P. H. Hynninen and H. Lemmetyinen, J. Am. Chem. Soc., 1999,
121, 9378–9387.
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