Hierarchical self-assembly of all-organic photovoltaic devices

Article abstract of DOI:10.1016/j.tet.2005.09.150

Photovoltaic devices built by a hierarchical self-assembly process using hydrogen-bonding terminated self-assembled monolayers (SAMs) on gold and the combination of a hydrogen-bonding barbituric acid appended fullerene and a complementary melamine termina

Full text of DOI:10.1016/j.tet.2005.09.150

Tetrahedron 62 (2006) 2050–2059
Hierarchical self-assembly of all-organic photovoltaic devices
Chih-Hao Huang,a Nathan D. McClenaghan,a Alexander Kuhn,b
Georges Bravicc and Dario M. Bassania,
*
a
´ ´
LCOO/CNRS UMR 5802, Centre de Recherche en Chimie Moleculaire, Universite Bordeaux 1, 33405 Talence, France
b
´
LACReM, ENSCPB Universite Bordeaux 1, 33607 Pessac, France
´
c
´ ´
Institut de Chimie de la Matiere Condensee de Bordeaux-CNRS, Universite Bordeaux 1,
87 Avenue du Dr. Schweitzer, F-33608 Pessac, France
Received 14 June 2005; revised 22 September 2005; accepted 24 September 2005
Available online 16 November 2005
Abstract—Photovoltaic devices built by a hierarchical self-assembly process using hydrogen-bonding terminated self-assembled
monolayers (SAMs) on gold and the combination of a hydrogen-bonding barbituric acid appended fullerene and a complementary melamine
terminated p-conjugated thiophene-based oligomer are presented. The incorporation of these electron donor (oligomer) and electron
acceptor (methanofullerene) assemblies into simple photovoltaic (PV) devices as thin films leads to a 2.5 fold-enhancement in photocurrent
compared to analogous systems comprising non-hydrogen-bonding C60–oligomer systems, which is ascribed to higher molecular-level
ordering. The modification of the gold electrode surface with self-assembled monolayers bearing hydrogen-bonding molecular recognition
endgroups was seen to further enhance the PV response of the corresponding functional supramolecular device. This superposition of two
types of self-assembly facilitates the generation of binary supramolecular fullerene-containing architectures. Importantly, all functional
materials are accessible in a direct fashion.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
proposed that the fullerence component acts as the
electron transport material, increasing the efficiency of
charge separation and transport. However, the introduction
of high proportions of C60 results in phase separation,
creating unwanted heterogeneous domains, which
adversely affect the photovoltaic response.4 A successful
strategy to enhance compatibility between the organic
polymer and fullerene components involves appending
solubilizing groups to the latter. This allows a higher
fullerene loading to be used, which results in improved
performance (up to 3%) and increased device stability.5
At the same time, the synthesis and photophysical
investigation of covalently-linked fullerene p-conjugated
oligomer dyads, triads, and more complex dendritic
assemblies has led to important breakthroughs in the
comprehension of the fundamental energy and electron
transfer processes leading to the generation of long-lived
charge separated states. Thus, through covalent modifi-
cation of C60, introduction of a plethora of electron-rich
oligomers, including oligo(p-phenylene vinylene)s,6
oligo(thiophenes)7 and others8 has been achieved and
used to construct increasingly complex covalent
architectures, including several doubly fullerene-termi-
nated oligomer ‘dumbbells’.9 The construction of solid-
state devices based on these covalent adducts is underway
and bodes well for future applications.10
Silicon-based photovoltaic devices have played a major
role in the development of light energy-to-electrical
energy conversion, and multi-junction solar cells have
been reported with efficiencies of 31%.1 Portability,
durability (lack of moving parts), and zero emission
during operation are attractive features that are countered
only by their relatively high manufacturing costs and
fragility. For these reasons, as well as concerns about the
use of rare-earth heavy metals in the active layers, hybrid
inorganic/organic and all-organic polymer alternatives
have been actively pursued in recent years as viable
alternatives amenable to large-scale deployment.2 Based
on current organic polymer technology and of additional
interest for the construction of o-LED and o-FET devices,
the polymer approach has benefited from increased
interest since the report by Sariciftci and co-workers
demonstrating that a substantial improvement in the
photovoltaic properties of such materials could be
obtained by blending a small amount of C60 into the
polymer mixture.3 In such composite devices, it has been
Keywords: Oligomer; Fullerene; Electron donor; Hydrogen-bonding;
Photovoltaic; Self-assembly.
Corresponding author. Tel.: C335 4000 2827; fax: C335 4000 6158;
e-mail: d.bassani@lcoo.u-bordeaux1.fr
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.150

C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
2051
Photoinduced charge separation in composite devices
takes place at a heterojunction between the electron donor
and electron acceptor materials. However, highly ordered
materials are required for efficient charge transport, and
the conciliation of these apparently contradictory require-
ments is a major challenge.11 The effect of ordering on charge
mobility was recently illustrated with sexithiophene whose
charge carrier mobility increases on passing from
an amorphous film (10K5 cm2 VK1 sK1) to a partially
organized film (2!10K3 cm2 VK1 sK1) to the single
crystal (10K1 cm2 VK1 sK1).12 Although covalent tethering
surmounts the phase separation problem, a high degree of
long-range ordering conducive to charge transport is more
difficult to obtain. For these reasons, the induction of long-
range ordering through self-assembly, using non-covalent
supramolecular interactions, is particularly appealing. Hydro-
gen-bonding systems offer the possibility to generate
preordained structures in which the binding strength can be
modulated and the relative geometry between active units
controlled.13 Additionally, in certain cases hydrogen-bonding
has been shown to promote photoinduced electron transfer
processes.14
combination of features compatible with the construction of
functional self-assembled devices: (i) the hydrogen-bonding
motif, capable of forming up to six hydrogen bonds, is non-
self-complementary and can be introduced in a single
synthetic step; (ii) the symmetry of 6, which presents two
identical edges, is conducive with the formation of extended
arrays in the presence of conjugated oligomers bearing
complementary hydrogen-bonding units; (iii) its compact
nature is adapted to promote close inter-fullerene approach in
a binary system where the other component is a melamine-like
unit. The first two features are important to assure
programmed extended bicontinuous photo-/electroactive
molecular tapes and efficient photoinduced electron transfer,
while the latter two points are prerequisites for charge
movement following the primary electron transfer act.
Molecule 6 was recently reported to form supramolecular
assemblies in solution17 with simple melamine units, in a
manner related to unfunctionalized barbituric acid,18 and to
play the role of an electron acceptor in ultra-fast
photoinduced electron transfer processes with oligothieny-
lenevinylene electron donors in discrete 1:1 supramolecular
complexes.19 Clearly, the presence of barbituric acid does
not adversely affect the electronic properties of the fullerene
cage, and the fullerene–grafted barbiturate is capable of
forming hydrogen-bonded structures, even if a degree of
distortion appears to be present on the barbiturate unit due to
non-bonding interactions between the carbonyl groups and
the fullerene cage.16,20 Molecule 6 was therefore deemed a
Several examples of hydrogen-bonding systems combining
fullerenes and p-conjugated oligomers have recently
appeared in the literature, for example using ammonium
groups and crown ethers,15 or self-complementary ureidopyr-
imidinone units.16 However, the recently described fullerene–
barbituric acid dyad 617 (see Scheme 1) presents a unique
O
R
O
R
N
N
1a, n = 5, R = H,
92%
O
1b, n = 6, R = CH3, 71%
O
R
O
R
O
N
N
1. Br(CH2)nBr,
acetone
n = 5, 58%;
n = 6, 68%
O(CH2)nSAc
CHO
CHO
OH
NH2
N
N
H2N
NH2
2. KSAc, acetone
n = 5, 99%,
n = 6, 91%
O(CH2)nSAc
n = 1, 2
2,4,5,6-tetraamino-
pyrimidine sulfate
N
2a, n = 5, 90%
2b, n = 6, 27%
i-PrOH
O(CH2)nSAc
O
O
NH2
N
H2N
S
S
O
O
N
N
R
N
N
R
S
S
S
2,4,5,6-tetraamino-
H2N
N
NH2
S
S
pyrimidine sulfate
S
S
S
3 : R = H
NH2
POCl3, DMF
1,2-dichloroethane
82 %
H2N
i-PrOH, 61%
4 : R = CHO
5
O
HN
NH
O
O
5-bromobarbituric acid
C60
DBU, DMSO/toluene, 45%
6
Scheme 1. Synthesis and structural formulae of 1–6.

2052
C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
good candidate for the pursuit of functional supramolecular
devices.
2. Results and discussion
2.1. Synthesis
To be of interest for future development, the electron donor
(oligomer) and acceptor (fullerene) components should be
easily accessible in high yields with direct synthetic
approaches (Scheme 1). The functionalized fullerene is
obtained in one step,17 while the oligomer can be
functionalized with hydrogen-bonding molecular recog-
nition units, either melamine or barbituric acid, starting
from the same dialdehyde also in one step.21 These strategies
in themselves constitute a widely applicable methodology to
introduce complementary molecular recognition motifs,
which is currently lacking. Thus the possibility to incorporate
complementary, yet non-self-complementary hydrogen-
bonding motifs onto conjugated oligomers (potentially
different oligomers) opens the door to the construction of
molecular fabrics and libraries of mixed chromophore layers,
where the judicious choice of suitably-decorated chromo-
phores assures a better matching of absorption properties
with the solar spectrum.
Pertinent synthetic procedures and structural formulae of the
molecules used in this study are shown in Scheme 1.
Fullerene–barbituric acid dyad 6 was prepared by a
Bingel reaction between 5-bromobarbituric and C60 in a
toluene:DMSO mixture.17,24 The synthesis of the oligomers
and introduction of molecular recognition motifs was
recently reported.21 Briefly, the 5T chromophore (3; a
symmetric linear pentathiophene oligomer with a central
EDOT unit) was obtained via Suzuki cross-coupling reaction
between the boron ester of dithiophene and 2,5-dibromo-
EDOT. This parent unit was subsequently functionalized bya
double Vilsmeier-Haack formylation giving dialdehyde 4,
which can be considered as a versatile starting point for the
introduction of terminal molecular recognition motifs.
Introduction of the melamine unit was achieved using a
Schiff base reaction to give 5. As Schiff base formation is an
equilibrium process, good yields of the most stable adduct
can be obtained on reaction with an excess of 2,4,5,6-
tetraaminopyrimidine. Equally, the symmetric complemen-
tary barbituric acid-terminated oligomer (not shown) could
be prepared in one step in similarly high yield by a
Knoevenagel reaction with barbituric acid. The alkylthio-
acetate terminated barbituric acid or cyanurate were
prepared by adapting the above methodologies.25 Structu-
rally-related barbiturate species were previously reported by
Ringsdorf, albeit in lower yields.26 4-Hydroxybenzaldehyde
was alkylated with the appropriate a,u-dibromoalkane27
followed by condensation with potassium thioacetate28
giving an intermediate, which could be subsequently
condensed with either barbituric acid or tetraminopyrimi-
dine to give 1 or 2, respectively.
In the current study, thefocus is directed at thecombination of
a melamine-appended oligothiophene 5 with the complemen-
tarybarbiturate-appended fullerene6, andthedevelopment of
self-assembled optoelectronic devices based on the resulting
assemblies. Different microscopic hydrogen-bonding struc-
tures may be anticipated using these molecular building
blocks bearing complementary units, in line with findings on
simpler binary melamine:barbiturate structures described
previously, which are shown schematically in Figure 1.18
Although rosettes, crinkled tapes, and linear tapes are
possible, only the linear tape architectures can satisfy all the
hydrogen-bonding sites present in the complementary
components and their formation is expected to be thermo-
dynamically favored.18 Preliminary photovoltaic measure-
ments have been reported,21 and we now report the effect of
elaborating the gold electrode surface with the appropriate
bifunctional hydrogen-bonding motif and thiol-terminated
species. The terminal thiol-group interacts with the gold
surface resulting in the formation of self-assembled mono-
layers (SAMs) that are proposed to better accommodate the
subsequent formation of the photo-/electroactive fullerene-
containing thin films due to complementary hydrogen-
bonding interactions. Several interesting all-organic systems
have been recently reported in the context of photovoltaic
devices, including examples based on photoactive SAMs22
and chromophore-appended nanotubes,23 as well as
fullerene–oligomer dyads/triads.10 The incorporation of
hydrogen-bonding motifs giving photo-/electroactive mole-
cules programmed to assemble into hierarchical extended
photovoltaic arrays has received less attention.
2.2. X-ray crystal structure of 3
Conjugation—and hence electron–hole pair delocalization—
over adjacent heterocycles in oligo- and polythiophene
derivatives is highest when the structure adopts a planar
geometry maximizing p-overlap. This is indeed the case
in most oligothiophenes (with the exception of some 3,
4-disubsitituted thiophene-containing derivatives), for
which a large number of solid-state structures have been
reported.12 Substitution in the thiophene C-3 and C-4
position generally introduces sufficient steric repulsion to
induce a deviation from planarity. Although only a few
crystal structures of EDOT-containing oligothiophenes have
been reported, these do not indicate that the EDOT moiety
introduces a substantial deviation from planarity. To
investigate whether this is the case in 3, its solid-state
structure was determined by X-ray diffraction. Single
crystals were grown from THF solution, crystallizing in a
monoclinic p21/n unit cell.† The structure of 3, shown in
evaporation
† Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC 275009. Copies of
the data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: C44 1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk].
( I )
( II )
( III )
5 + 6
Figure 1. Possible hydrogen-bonding architectures formed with com-
pounds 5 (red) and 6 (green). Compared to the crinkled tape (II) and rosette
(III), only the hydrogen-bonded tape structure (I) satisfies all the hydrogen-
bonding requirements of both components.

C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
2053
Figure 2, indicates that the planarity of the pentameric
oligomer is indeed retained even upon introduction of the
functionalized central unit, although one of the terminal
thiophenes shows a torsion angle of 24.78 in the
S(11)–C(15)–C(22)–(S21) plane, presumably due to crystal
packing forces. The extended crystal packing structure is a
herringbone type as observed with related oligothiophene
species.12
architectures proposed above, they do indicate that the
compatibility between inherently incompatible compounds
such as oligothiophenes and C60 can be significantly
enhanced by the presence of complementary molecular
recognition groups. In practical terms, this implies that the
electron-rich oligomers are mostly located in the vicinity of
a fullerene moiety, which can translate into enhanced charge
separation efficiency.
Figure 3. Representative fluorescence microscopy images of thin films of
oligomer 5 (left); 5CC60, 1:2 equiv (center); and 5C6, 1:2 equiv (right).
2.3.2. Electrochemical and photoelectrochemical studies
of thin films on a gold surface. Thin films analogous to
those described above were subsequently deposited from
solution onto a gold electrode and introduced into a standard
three-electrode photoelectrochemical cell, with methylvio-
logen in water acting as an electron carrier, sodium sulfate
as the supporting electrolyte and a platinum wire counter
electrode.21 Irradiation was performed using a high-pressure
Figure 2. X-ray crystal structure of 3 and crystal packing.
2.3. Thin films comprising photo-/electroactive oligomer
and fullerene units
2.3.1. Fluorescence microscopy of thin films. Blends of
p-conjugated oligomers and fullerenes have the tendency to
undergo phase separation, and high-speed spin-coating of
these species onto the appropriate substrate from volatile
solvents is often used to overcome this process. In the
current study, drop-casting of thin films from solution
(0.5 mM 5 in DMSO or DMSO/THF solutions) onto a solid
substrate followed by slow evaporation of the solvent is
anticipated to favor the formation of thermodynamically
equilibrated non-covalent architectures (see Fig. 1). As a
preliminary test to see if homogeneous films would result
from this drop-casting technique, mixtures of the pertinent
species were separately deposited on microscope slides in
order to visualize the residual fluorescence from the films.
Bearing in mind that the oligomer fluorescence will be
quenched in the presence of fullerene (either via energy or
electron transfer), an estimation of bulk homogeneity can be
made by examining the intensity and distribution of the
residual fluorescent areas. The fluorescence microscopy
images of three different thin films are shown in Figure 3. In
the first case, the intense fluorescence of oligomer 5 can be
clearly seen. In the second case (b), 5 and C60 were
co-deposited in a 1:2 stoichiometry from DMSO solution.
Though a portion of the intrinsic oligomer fluorescence is
quenched by the presence of C60, localized regions of
fluorescence are visible showing that the film exhibits
considerable heterogeneity, presumably due to phase
separation during the deposition process. In the third case
(c), films of 5 and 6 (in a 1:2 stoichiometry) were deposited.
Here, the oligomer fluorescence is comprehensively
quenched, indicating that the fullerene and oligomer
components are more homogeneously dispersed than in
(b). While these results per se do not provide irrefutable
evidence as to the formation of the hydrogen-bonding
mercury lamp equipped with
a band-pass filter
(400–500 nm; intensityZ150 mW cmK2).29 In line with
the idea of higher organization and better compatibility of
the two different electron donor and acceptor materials
giving the highest photocurrent, the best performance was
measured for a mixture of 5 and 6, which was anticipated to
be the most organized thin film comprising the hydrogen-
bonding fullerene and the complementary p-conjugated
oligomer.21 This response was 2.5 times higher than that
obtained on combining 5 with unsubstituted C60, while the
lowest conversion was obtained for the film of oligomer 5
alone with a value, which was half that obtained with 5:C60.
From the I/V graph of photocurrent generated versus
applied potential, the open-circuit voltage conversion
efficiency F(Voc), and short-circuit quantum conversion
efficiency F(Isc) of the photovoltaic device could be
estimated. Respectively, these values express the fraction
of light energy that is converted into voltage, and the
number of photoelectrons generated for every photon of
light that is absorbed30 (film thickness was estimated at ca.
200 nm using AFM, which also showed that the films were
even and reproducible).
FðVocÞ Z Voc=Eoo
(1)
(2)
FðIscÞ Z ðdne=dtÞ=ðdnhn=dtÞ
where ne and nhn are the number of photoelectrons and
number of photons absorbed, respectively, while Eoo is the
energy of the S1)S0 transition in eV. The incident light
intensity of 150 mW cmK2 corresponds to an average
photon flux of 5.6!10K8 Einstein (at 450 nm). Considering

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C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
the device comprising an oligomer film without fullerene, it
is estimated that ca. 2.7% of incident light is absorbed
(based on the measured thickness of the film). From Eqs. 1
and 2 above and values of VocZ0.39 V and Isc Z6.8 mA,
one can estimate the efficiency of open-circuit voltage
conversion, F(Voc)Z0.14, and the quantum conversion
efficiency, F(Isc)Z0.05. These values are comparable to
other photovoltaic devices based on similar technology
(although experimental error, associated mainly with the
determination of film thickness, must be taken into
consideration). Thus, comparing the relative photocurrents
generated for the thin films, the quantum conversion
efficiencies were estimated at 5% for 5 alone; 9% for
5:C60; 1:2 stoichiometryj and ca. 25% for 5:6; 1:2
stoichiometry. This latter value compares favorably with
recent photovoltaic devices based on the layer-by-layer
construction of conjugated polymers and C60 and with IPCE
values of 15%.31 In all cases it can be assumed that
absorption of light is effected almost exclusively by the
oligomer, as fullerene derivatives possesses low molar
absorptivities above 400 nm. Therefore, despite uncertain-
ties in the absolute values for the efficiency of these simple
devices, these results show that the films comprising two
complementary hydrogen-bonding electroactive units are
more performant. An additional factor that became
immediately apparent during this initial testing phase of
the devices is their inherent stability towards air and
moisture. Indeed, the photovoltaic cell set-up uses an
aerated aqueous solution of methylviologen as the electron
transport agent. The response of 5 alone is stable over time,
whereas device B (5Cpristine C60) exhibits a slow (ca. 10%
loss) of efficiency over a period of 1 h. During this same
period, no measurable decrease in efficiency was detected
for the device prepared with 5C6.
ellipsometry measurements. In the first case, the determi-
nation of surface coverage can be obtained by electro-
chemistry while the thickness of the monolayer can be
determined by ellipsometry, and the surface coverage
estimated by rastering the surface with the incident laser
beam.
The surface coverage by self-assembled monolayers of the
gold electrode can readily be evaluated by electrochemistry
when the chemisorbed molecules serve as a barrier to block
a redox process involving a species in solution at the
electrode surface.32 In the current case the reduction of
Fe3C (as the potassium hexacyanoferrate (III) salt) to Fe2C
was employed to check for defects in the SAM. For the
surface coated with 1a, a highly uniform close packing
monolayer layer was formed. As shown in Figure 4,
essentially no FeIII reduction could be measured upon
modification of the gold surface with the barbiturate-
terminated alkane-thiol, compared with the electrochemical
response for bare gold. The situation was somewhat
different when the experiment was repeated with the
corresponding melamine-terminated alkane-thiol (Fig. 4).
A much poorer blocking effect was observed with 2a
suggesting a less ordered SAM. This result was rationalized
by considering the experimentally determined ‘odd–even’
effect reported for simple alkanethiol SAMs where the
number of repeat units (odd /or even) influences close-
packing of the molecules.33 The experiment was thus
repeated with molecule 2b, which is identical with 2a
except for the incorporation of an additional methylene
repeat unit in the alkyl chain. This simple structural
modification resulted in a much better passivation of the
2.3.3. Thin films on a hydrogen-bonding SAM modified
gold surface. To obtain photoactive fullerene-containing
architectures directed by a hierarchical combination of
supramolecular self-assembly processes, gold–thiol inter-
actions can be employed in addition to hydrogen-bonding.
The self-assembly of different nanoparticles (gold and CdS)
was recently demonstrated using nanoparticles whose
surface was decorated with thiol-terminated melamines
and barbiturates, respectively.25 The gold electrode surface
was first modified by reaction with a barbiturate or
melamine unit bearing a thiol-terminated alkyl chain, thus
generating a self-assembled monolayer with the capacity to
bind electroactive units using non-covalent interactions. In
the current case, the choice of hydrogen-bonding building
blocks used to decorate the SAMs would be anticipated to
dictate the chemical nature of the layer adjacent to the
surface self-assembled monolayer. These anticipated
second layers would be a p-conjugated melamine-bearing
oligomer (5) with a barbiturate-decorated surface, and a
barbituric acid bearing fullerene (6) with a melamine-
decorated surface. In other words, the SAM may serve as a
homogeneous starting point for subsequent deposition of the
photoactive binary thin film.
Hydrogen-bonding SAMs were obtained by dipping the
gold-substrate into a solution of the appropriate thiol 1–2,
see Scheme 1 for structural formulae. The surface coverage
was subsequently evaluated by electrochemical and surface
Figure 4. Cyclic voltammograms of the reduction of aqueous K3[Fe(CN)6]
using gold electrodes whose surfaces were modified with 1a or 1b (top) and
2a or 2b (below).

C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
2055
electrode surface. This result is consistent with the
formation of a better monolayer, as shown in Figure 4. A
similar behavior was observed upon investigation of the
N-methylated derivative of 1a, for which compact SAMs
took several days to form (data not shown). Again,
incrementing the pentamethylene linker by one –CH2–
unit (compound 1b) significantly improved the self-
assembly of compact monolayers, which were formed
overnight.
mation on the coverage, and a homogeneous layer, in terms
of height is obtained (Fig. 5a). Using this height profile
(Fig. 5b), an estimation of the geometry adopted can be
obtained and this is represented in Figure 5c for both 1a and
2b. The measured height corresponds well to the dimensions
obtained using molecular modeling. In the case of 1a the
measured height was 1.50 nm while the calculated value
was 1.42 nm. For 2b the measured height was 1.83 nm with
a calculated value of 1.90 nm.
Ellipsometry measurements of surfaces modified with 1a
and 2b are shown in Figure 5, which correspond to the
closest packing SAMs as estimated by electrochemistry,
formed with thiol-terminated barbiturates and melamines,
respectively. Rastering the modified surface gave infor-
7Photoelectronic devices similar to those reported in Section
2.3.2 were prepared using gold electrodes modified with
SAMs prepared from compounds 1a, 1b, and 2b. Comparison
of the response obtained from electrodes modified with 2b
versus 1a provides an indication as to the effect of the
Figure 5. Characterization of SAMs obtained for 1a (left) and 2b (right) modified gold surfaces using ellipsometry: (a) topology of surface; (b) height profile
and (c) calculated height assuming close-packing of the molecules.

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C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
hydrogen-bondingendgroup, whereas the use of1b serves as a
control in which hydrogen-bonding is blocked. Since it was
shown that incorporating the barbiturate-appended fullerene 6
resulted in thin films giving greater photocurrent than pristine
C60, only devices based on 5 and 6 (in a 1:2 stoichiometry) are
considered in this section. The different photocurrent response
for each of the four different devices is shown in Figure 6,
where deviceA isthe film deposited onthe baregold; deviceB
is the thin film deposited on the surface decorated with
melamine 2b; device C is the film deposited on the surface
decorated with 1b; and device D is the film deposited on the
surface decorated with the alkanethiol–barbiturate 1a. It was
immediately clear that device A is the least performant device
when compared to the devices incorporating SAM-decorated
surfaces, all of which gave a higher photocurrent.
which the effect of stirring the solution is much lower or
absent, as in the case of device A. Device D also displayed
exceptional stability, no deterioration in its response being
observed after two week storage under ambient conditions.
3. Conclusions
The formation of SAMs appended with suitable recognition
motifs is shown to be advantageous in the construction of
supramolecular all-organic photovoltaic devices. Based on
the formation of hydrogen-bonded networks, the molecular
constituents in these devices are programmed to self-
assemble into molecular-level heterojunctions. Thus two
levels of self-assembly can be employed to generate multi-
component functional fullerene-containing supramolecular
architectures. Although direct measurement of the degree of
order instilled by the presence of complementary hydrogen-
bonding donor–acceptor sites is difficult, considerable
differences in efficiency and behavior are noted. The
improvement of the photovoltaic response when using the
SAM modified surfaces may in part originate from either
the variations in the surface properties (the increased
hydrophobicity of the SAM-covered surface results in
lower contact angles when exposed to organic solvents)
and/or inhibition of the phonon quenching of the excited
states generated in close proximity of the gold surface.34
However, this reasoning alone is insufficient to explain the
difference in behavior between devices B, C, and D. In the
latter device, it can be anticipated that the presence of a
barbituric acid-appended SAM may induce the preferential
deposition of the complementary melamine-containing
oligothiophene unit 5. In contrast, the use of the
melamine-based SAM (device B) would lead to the
selective deposition of fullerene derivative 6. Due to
the electrical bias applied during the photovoltaic measure-
ments, it is expected that the latter construction should be
less efficient as the presence of the fullerene proximal to the
gold electrode would induce an energy barrier to the
transport of holes towards the cathode. Work is in progress
to further elucidate the morphologies of these films and to
test their photovoltaic response in solid-state devices.
Figure 6. Top: Photocurrent generated with thin films of 5C6, 1:2 equiv
deposited on: bare gold (device A); Au/2b (melamine endgroup, device B);
Au/1b (N,N0-dimethylbarbiturate endgroup, device C); Au/1a (barbiturate
endgroup, device D). Bottom: Effect of stirring on the photocurrent of
devices A and D; with stirring off (I and V), slow stirring (II and IV) and
fast stirring (III).
4. Experimental
4.1. General
1H and 13C NMR spectra were recorded at room
temperature on a Bruker DPS-200 FT, AC-250 FT or
DPX-300 FT spectrometer using residual solvent proton
peaks as internal references unless otherwise indicated.
Chemical shifts are given in ppm (d) with respect to
tetramethylsilane. Abbreviations used are sZsinglet, dZ
doublet, tZtriplet, qZquartet, and brZbroad. Mass spectra
were recorded on a VG Autospec-Q by EI (70 eV); m/z (%).
High resolution mass spectra were recorded on a FTICR
mass spectrometer Bruker 4.7T BioApex II. The elemental
analysis was carried out by the Service Centrale d’Analyse
at Vernaison (CNRS-France). Merck silica gel 60 (70–230
mesh) was used for flash chromatography. Electronic
absorption spectra were obtained using a Hitachi U-3300
spectrophotometer. Fluorescence spectra were recorded on
The behavior of device D is intriguing, displaying a rapid
decrease in the photogenerated current upon each illumina-
tion cycle. We attribute this reproducible behavior to
extension of the diffusion layer due to depletion of the
MV2C in the vicinity of the electrode. To confirm this, we
proceeded to monitor the photovoltaic response of device D
upon mechanical stirring of the solution. The results are
presented in Figure 6, in which it is quite clear that the
photovoltaic response of D increases substantially upon
stirring, in a way that is proportional to the rate of stirring.
Therefore, in the case of device D the observed performance
is limited by the rate of diffusion of the electron-carrying
species in solution rather than the charge carrier mobility in
the material. This is not the case for the other devices, for

C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
2057
a Hitachi F-4500 fluorescence spectrophotometer. Fluor-
escence microscopy images were taken on a Nikon Eclipse
E600 FN instrument equipped with a 365 nm excitation
filter.
resulting white precipitate was filtered off and the filtrate
was concentrated under reduced pressure to give a white
solid (0.97 g, yieldZ99%). H NMR (200 MHz, CDCl3)
d 9.87 (s, 1H, CHO), 7.82 (d, 2H, JZ8.7 Hz, ArH), 6.98
(d, 2H, JZ8.7 Hz, ArH), 4.03 (t, 2H, JZ6.3 Hz, OCH2),
2.90 (t, 2H, JZ7.0 Hz, S–CH2), 2.33 (s, 3H, CH3),
1.87–1.77 (m, 2H), 1.67–1.50 (m, 4H); 13C NMR
(50 MHz, CDCl3) d 195.2, 190.8, 164.1, 132.0, 129.8,
114.7, 104.5, 68.0, 30.6, 29.3, 28.9, 28.5, 25.2.
1
4.1.1. Materials. Commercially available starting materials
and anhydrous DMSO and DMF (Fluka) were used as
received. The preparation of compound 5 was recently
described.21 Tetrahydrofuran (THF) was distilled over
sodium/benzophenone immediately before use. All
reactions were carried out under a nitrogen atmosphere.
4.2.2. 6-(40-Formylphenoxy)hexyl thioacetate.28 4-(6-
Bromohexyloxy)-benzaldehyde (7.78 g, 27.28 mmol) and
potassium thioacetate (3.18 g, 27.28 mmol) in acetone
(20 mL) were stirred at room temperature overnight. The
resulting white precipitate was filtered off and the filtrate
was concentrated under reduced pressure to give a white
˚
The polycrystalline gold electrodes (nickel (80 A), chro-
˚
˚
mium (20 A) and gold (3900 A) on 1.0 mm glass) were
purchased from ACM France. 4-(5-Bromopentyloxy)benz-
aldehyde and 4-(6-bromohexyloxy)benzaldehyde were
prepared following a procedure described in the literature.27
1
solid (6.98 g, yieldZ91%). H NMR (200 MHz, CDCl3)
4.1.2. Cleaning of substrates and preparation of
monolayers and devices. The gold electrodes were cut
into slides (ca. 2.5 cm!1.0 cm) and were cleaned by
immersion in ‘piranha’ solution (H2SO4/30% H2O2, 7:3
(v/v); Caution: ‘Piranha’ is an extremely dangerous
oxidizing agent and should be handled with care using
appropriate shielding) for 30 min, and were then rinsed with
deionized water and absolute ethanol and dried with a
stream of argon. For SAM-based devices, the clean gold
slides were immersed in anhydrous THF–ethanol (1/5)
mixed-solutions of alkylthioacetate terminated 1a, 1b, 2a,
or 2b (1 mM) overnight. To remove any solution-deposited
material, the slides were then rinsed with ca. 5 mL of
anhydrous ethanol, followed by immersing in an ethanol
solution for 5 min, and dried with a stream of argon. In all
devices, deposition was carried out by drop-casting 20 mL of
2.0!10K5 M DMSO solutions of 5 with 2.0 equiv of 6. The
slow evaporation of the solvent under reduced pressure left
a thin film of material on a well-defined area (2.50 cm2) of
the gold electrode. The modified slide was then dried with a
stream of argon and before use in photoelectrochemical
measurements.
d 9.87 (s, 1H, CHO), 7.82 (d, 2H, JZ8.7 Hz, ArH), 6.98
(d, 2H, JZ8.7 Hz, ArH), 4.03 (t, 2H, JZ6.3 Hz, OCH2),
2.87 (t, 2H, JZ7.1 Hz, S–CH2), 2.32 (s, 3H, CH3),
1.82–1.76 (m, 2H), 1.67–1.55 (m, 2H), 1.54–1.43 (m, 4H);
13C NMR (50 MHz, CDCl3) d 195.5, 190.4, 163.9, 131.7,
129.5, 114.4, 104.5, 67.9, 30.3, 29.2, 28.7, 28.6, 28.1, 25.2.
4.2.3. 5-{4-[(Tetrahydro-2,4,6-trioxo-5(2H)-pyrimidiny-
liene)methyl]phenoxy}pentyl thioacetate (1a). 5-(40-For-
mylphenoxy)pentyl thioacetate (0.67 g, 2.52 mmol), and
barbituric acid (0.33 g, 2.52 mmol) in ethanol:water
(14 mL, 5:2; v/v), was stirred under reflux for 10 min, and
then at room temperature overnight. The resulting yellow
suspension was filtered, washed with EtOH (10 mL) and
dried under reduced pressure to give a yellow solid (0.87 g,
1
yieldZ92%). H NMR (200 MHz, d6-DMSO) d 11.29 (s,
1H, NH), 11.18 (s, 1H, NH), 8.36 (d, 2H, JZ9.0 Hz, ArH),
8.24 (s, 1H, CH]C), 7.05 (d, 2H, JZ9.0 Hz, ArH), 4.09 (t,
2H, JZ6.4 Hz, OCH2), 2.86 (t, 2H, JZ6.9 Hz, S–CH2),
2.31 (s, 3H, CH3), 1.80–1.70 (m, 2H), 1.61–1.43 (m, 4H);
13C NMR (50 MHz, d6-DMSO) d 195.3, 163.9, 162.9,
162.2, 154.9, 150.2, 137.5, 125.0, 115.4, 114.3, 67.8, 30.6,
28.9, 28.2, 27.9, 24.6. APCI-MS m/z (%) 377 (MC1, 100);
376 (M, 75). HRMS 376.1088, MC; Calcd 376.1093. Anal.
Calcd for C18H20N2O5S: C 57.43, H 5.36, N 7.44. Found: C
57.28, H 5.40, N 7.43.
4.1.3. Experimental conditions. All photoelectrochemical
studies were performed as previously described.21 The
cyclic voltammograms were recorded in 1 mM
K3[Fe(CN)6] aqueous solution with 0.1 M KNO3 as the
supporting electrolyte. The working electrode was a
modified gold slide, the counter electrode was a platinum
wire, and an Ag/AgCl (saturated KCl) reference electrode
was used. The Fe3C/ Fe2C redox potential was obtained
with a scan speed of 50 mV sK1. The ellipsometer
(Ielli2000, Nanofilm Technologie GmbH, Germany)
equipped with a frequency doubled Nd-Yag laser (lZ
532 nm, PmaxZ50 mW), was used for the thickness
measurements, and the obtained data were analyzed with
the furnished software. The incident angle was set to 658. A
refractive index of 1.50 was assumed for all films in the
calculation of the thickness.
4.2.4. 6-{4-[(1,3-Dimethyl-tetrahydro-2,4,6-trioxo-5-
(2H)-pyrimidinyliene)methyl]phenoxy}hexyl thioacetate
(1b). 6-(40-Formylphenoxy)hexyl thioacetate (1.55 g,
5.54 mmol), and N,N0-dimethylbarbituric acid (0.88 g,
5.54 mmol) in benzene (100 mL) were stirred under reflux
for 4 h using Dean-Stark apparatus. After cooling to room
temperature, the solvent was removed, and the residue was
dissolved in CHCl3 (100 mL), extracted with saturated
aqueous NaHCO3 solution (50 mL), dried over MgSO4, and
concentrated under reduced pressure to give a yellow solid
1
(1.65 g, yieldZ71%). H NMR (200 MHz, CDCl3) d 8.51
(s, 1H, CH]C), 8.32 (d, 2H, JZ9.0 Hz, ArH), 6.95 (d, 2H,
JZ9.0 Hz, ArH), 4.06 (t, 2H, JZ6.4 Hz, OCH2), 3.41 (s,
3H, N–CH3), 3.39 (s, 3H, N–CH3), 2.88 (t, 2H, JZ7.1 Hz,
S–CH2), 2.33 (s, 3H, CH3), 1.87–1.77 (m, 2H), 1.65–1.42
(m, 6H); 13C NMR (50 MHz, CDCl3) d 196.0, 164.0, 163.2,
161.0, 159.0, 151.4, 138.1, 125.3, 114.1, 114.0, 68.2, 30.6,
29.4, 29.0, 28.9, 28.8, 28.4, 28.3, 25.5. HRMS 419.1634,
MC; Calcd 419.1640.
4.2. Synthesis
4.2.1. 5-(40-Formylphenoxy)pentyl thioacetate.28 4-(5-
Bromopentyloxy)benzaldehyde (1.00 g, 3.68 mmol) and
potassium thioacetate (0.43 g, 3.68 mmol) in acetone
(20 mL) were stirred at room temperature overnight. The

2058
C.-H. Huang et al. / Tetrahedron 62 (2006) 2050–2059
4. Yu, G.t; Gao, Y.; Hummelen, J. C.; Wudl, F.; Heeger, A. J.
5-{4-[(2,4,6-Triamino-N5-pyrimidinyliene)-
4.2.5.
Science 1995, 270, 1789–1791. Matsumoto, K.; Fujitsuka, M.;
Sato, T.; Onodera, S.; Ito, O. J. Phys. Chem. B 2000, 104,
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methyl]phenoxy}pentyl thioacetate (2a). 2,4,5,6-Tetra-
aminopyrimidine sulfate (3.21 g, 13.06 mmol), potassium
carbonate (1.79 g, 13.06 mmol), and 5-(40-formylphenoxy)-
pentyl thioacetate (0.87 g, 3.27 mmol) in 2-propanol
(15 mL) were stirred under reflux overnight. After cooling
to room temperature, water (30 mL) was added to the
reaction. The brown suspension solution was filtered,
washed with water (10 mL), EtOH (10 mL), and diethyl
ether (10 mL), then dried under reduced pressure to give a
deep-yellow solid (1.31 g, yieldZ90%). 1H NMR
(200 MHz, d6-DMSO) d 8.49 (s, 1H, N]CH), 7.79 (d,
2H, JZ8.6 Hz, ArH), 6.97 (d, 2H, JZ8.7 Hz, ArH), 5.95 (s,
4H, NH2), 5.65 (s, 2H, NH2), 4.00 (t, 2H, JZ6.3 Hz, OCH2),
2.86 (t, 2H, JZ6.9 Hz, S–CH2), 2.32 (s, 3H, CH3),
1.80–1.67 (m, 2H), 1.65–1.43 (m, 4H); 13C NMR
(50 MHz, d6-DMSO) d 195.4, 160.0, 159.2, 157.1, 152.9,
130.8, 129.0, 114.3, 102.0, 67.4, 30.6, 28.9, 28.3, 28.1, 24.7.
ESI-MS m/z (MC, 387.7). HRMS 388.1680, MC; Calcd
388.1681.
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Delaunay, J.; Roncali, J. Adv. Mater. 2002, 14, 283–287.
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4.2.6.
6-{4-[(2,4,6-Triamino-N5-pyrimidinyliene)-
methyl]phenoxy}hexyl thioacetate (2b). 2,4,5,6-Tetraami-
nopyrimidine sulfate (8.51 g, 34.65 mmol), potassium
carbonate (4.78 g, 34.65 mmol), and 6-(40-formylphenoxy)-
hexyl thioacetate (2.43 g, 8.66 mmol) in 2-propanol
(25 mL) was stirred under reflux overnight. After cooling
to room temperature, water (150 mL) was added to the
reaction. The brown suspension solution was filtered,
washed with water (100 mL), and CH2Cl2 (20 mL), then
dried under reduced pressure to give a deep-yellow solid
(0.92 g, yieldZ27%). 1H NMR (200 MHz, d6-DMSO)
d 8.49 (s, 1H, N]CH), 7.78 (d, 2H, JZ8.6 Hz, ArH),
6.96 (d, 2H, JZ8.6 Hz, ArH), 5.90 (s, 4H, NH2), 5.57
(s, 2H, NH2), 4.00 (t, 2H, JZ6.4 Hz, OCH2), 2.84 (t, 2H,
JZ7.0 Hz, S–CH2), 2.32 (s, 3H, CH3), 1.80–1.60 (m, 2H),
1.60–1.30 (m, 6H); 13C NMR (50 MHz, d6-DMSO) d 195.3,
159.9, 159.8, 157.4, 152.4, 130.9, 128.9, 114.3, 102.2, 67.4,
30.6, 29.1, 28.5, 28.3, 27.9, 25.0. APCI-MS m/z (%) 404
(MC1, 100); 403 (M, 30). HRMS 403.1904, MC; Calcd
403.1916.
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