Coordination-based molecular assemblies of oligofurans and oligothiophenes

Article abstract of DOI:10.1002/chem.201300034

Molecular assemblies (MAs) of oligofurans and oligothiophenes were formed from solutions on various substrates. These films were obtained by alternating deposition of organic chromophores (oligofurans or oligothiophenes) and a palladium salt. These coordination-based MAs were characterized by UV/Vis spectroscopy, spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and electrochemistry. The MAs exhibit similar electrochemical behavior and their growth and structure are apparently not affected when different organic template layers are used. The density of the MAs is a function of the structure of the molecular component. The oligothiophene density is approximately 50 % higher than that observed for the oligofuran-based assemblies. The optical and electrochemical properties of the MAs scale linearly with their thickness. The UV/Vis data indicate that upon increasing the film thickness, there is no significant conjugation between the metal-separated organic chromophores. DFT calculations confirmed that the HOMO-LUMO gap of the surface-bound oligofuran and oligothiophene metal oligomers do not change significantly upon increasing their chain length. However, electrochemical measurements indicate that the susceptibility of the MAs towards oxidation is dependent on the number of chromophore units. Coming to the surface: Surface-bound molecular assemblies (MA-1 to MA-3) were constructed by an iterative solution deposition approach. The formed MAs consist of either oligofuran or oligothiophene chromophores coordinated to a palladium salt. Copyright

Full text of DOI:10.1002/chem.201300034

FULL PAPER
DOI: 10.1002/chem.201300034
Coordination-Based Molecular Assemblies of Oligofurans and
Oligothiophenes
Adva Hayoun Barak,[a] Graham de Ruiter,[a] Michal Lahav,[a] Sagar Sharma,[a]
Ori Gidron,[a] Guennadi Evmenenko,[b] Pulak Dutta,[b] Michael Bendikov,*[a] and
Milko E. van der Boom*[a]
Abstract: Molecular assemblies (MAs)
of oligofurans and oligothiophenes
were formed from solutions on various
substrates. These films were obtained
by alternating deposition of organic
chromophores (oligofurans or oligo-
ed when different organic template
layers are used. The density of the
MAs is a function of the structure of
the molecular component. The oligo-
thiophene density is approximately
50% higher than that observed for the
oligofuran-based assemblies. The opti-
cal and electrochemical properties of
the MAs scale linearly with their thick-
ness. The UV/Vis data indicate that
upon increasing the film thickness,
there is no significant conjugation be-
tween the metal-separated organic
chromophores. DFT calculations con-
firmed that the HOMO–LUMO gap of
the surface-bound oligofuran and oligo-
thiophene metal oligomers do not
change significantly upon increasing
their chain length. However, electro-
chemical measurements indicate that
the susceptibility of the MAs towards
oxidation is dependent on the number
of chromophore units.
thiophenes) and
a
palladium salt.
These coordination-based MAs were
characterized by UV/Vis spectroscopy,
spectroscopic
ellipsometry,
atomic
force microscopy (AFM), X-ray reflec-
tivity (XRR), X-ray photoelectron
spectroscopy (XPS), and electrochem-
istry. The MAs exhibit similar electro-
chemical behavior and their growth
and structure are apparently not affect-
Keywords: chromophores
mers · palladium · self-assembly ·
thin films
· oligo-
AHCTUNGTRENNUNG
Introduction
and van der Waals interactions.[7] In addition, the assembly
order can be of critical importance in attaining desirable
thin-film properties.[8] Initial organic layers are essential for
determining device performance and properties.[9]
Organic p-conjugated materials have been studied exten-
sively because of their electronic properties and potential
applications.[1–5] For instance, acene and oligothiophene de-
rivatives play a fundamental role in ongoing research on
new materials for organic electronic devices.[2] In particular,
pentacene and a-sexithiophene are frequently used in or-
ganic light-emitting diodes (OLEDs),[3] organic photovolta-
ics (OPVs),[4] and organic field-effect transistors (OFETs).[5]
The spatial arrangement of organic molecules is paramount
for fabricating functional materials and devices thereof.[6]
The molecular arrangement on surfaces is governed by mo-
lecular geometry and intermolecular interactions such as p–
p stacking, hydrogen bonding, dipole–dipole interactions,
A variety of techniques are used to control the assembly
order and function, including physical vapor deposition
(PVD),[10] spin-coating,[11] microcontact printing,[12] dip-pen
lithography,[13] and other self-assembly methods.[14] Coordi-
nation-based assembly is of particular interest, because it
allows for precision engineering of surfaces, as demonstrated
by the groups of Mallouk,[15] Katz,[16] Rubinstein,[17] Nishi-
hara,[18] and others.[19] For example, the self-assembly of or-
ganic ligands through coordination with metal precursors in
solution results in cagelike structures that can be used for
the templated synthesis of highly monodispersed silica nano-
particles.[20] In addition, Wçll, Fisher et al. created crystalline
metal–organic frameworks on surfaces by using layer-by-
layer (LbL) deposition, and thus avoided the interpenetrat-
ing networks that would have arisen if other assembly meth-
ods had been used.[21] Similarly, Kitagawa and co-workers
used LbL assembly of copper dithiooxamidates to fabricate
a crystalline assembly on the surface, which is not accessible
by other means.[22] For organic (hybrid) electronics, the self-
assembly of alkylphosphonates on inorganic surfaces has
also been used recently.[23] The assembly of organic chromo-
phores or metal complexes with late-transition-metal salts
can result in linear or exponential growing films.[24–27]
[a] A. Hayoun Barak, G. de Ruiter, Dr. M. Lahav, Dr. S. Sharma,
Dr. O. Gidron, Prof. M. Bendikov, Prof. M. E. van der Boom
Department of Organic Chemistry, Weizmann Institute of Science
76100 Rehovot (Israel)
Fax: (+972)8-934-4142
E-mail: michael.bendikov@weizmann.ac.il
milko.vanderboom@weizmann.ac.il
[b] Dr. G. Evmenenko, Prof. P. Dutta
Department of Physics and Astronomy, Northwestern University
Evanston IL 60208-3113 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300034.
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We recently introduced a series of long conjugated oligo-
furans as an alternative to oligothiophenes.[28] Oligofurans
offer several potential advantages: 1) higher fluorescence,
2) greater rigidity, and 3) enhanced solubility and subse-
quent processability. The field-effect mobility of oligofurans
is similar to that of the corresponding oligothiophenes.[28]
Although chemically different oligothiophene-based thin
films have been assembled on various surfaces,[29] the ex-
pected functionality of multilayered systems have not been
explored. Moreover, the LbL self-assembly of oligofurans
and oligothiophenes into MAs is unknown.
strong interchain p–p stacking. DFT calculations are in
agreement with the experimental observations and show
that the energy levels of the highest occupied molecular or-
bitals (HOMO) and lowest unoccupied molecular orbitals
(LUMO) are lower in energy in MA-3 relative to MA-2.
These energy levels are not significantly affected by the
thickness of the assemblies.
Results and Discussion
In this study, we incorporated terfuran 1 and terthiophene
3 into surface-confined molecular assemblies (MAs) and
studied their properties as a function of their chain length
(Scheme 1). The MAs were formed using coordination-
based LbL deposition with the respective oligofurans/oligo-
thiophenes and a palladium salt. The three assemblies con-
sist of terfuran [1n-Pdn] on a 1-based template (MA-1) or
terfuran [1n-Pdn] on a 2-based template (MA-2), and of ter-
Synthesis of chromophore 1 and the formation of the 1-
based template layer: The pyridine groups of terfuran 1
allow for coordination to PdCl2. The three furan rings
ensure solubility while maintaining conjugation. The new
terfuran 1 was prepared using a modification of known pro-
cedures (Scheme 2).[28a,30] Bromination of furan 4, followed
by Stille coupling with tributylACTHNUTRGENUGN(2-furyl)stannane, yielded ter-
furan 6. Lithiation of compound 6 at À788C in THF, fol-
lowed by the addition of tributyltin chloride, afforded the
stannylated compound 7, which was then used in a Stille
coupling reaction with 4-bromopyridine at 1008C to afford
5,5’’-di(pyridine-4-yl)-2,2’:5’,2’’-terfuran (1; 82%).
The new chromophore 1 was used to form a pyridine-ter-
minated interface to allow the formation of MA-1.[24–27,31]
The 1-based template layer was formed by a two-step proc-
ess. Freshly cleaned quartz, silicon, and indium tin oxide
(ITO) substrates were immersed in a dry pentane solution
(1:100 v/v) of 4-(chloromethyl)phenyltrichlorosilane at room
temperature (RT) for 30 min to afford chlorobenzyl-termi-
nated coupling layers.[27,32] These coupling layers were subse-
thiophene [3n-Pdn] on
a
2-based template (MA-3;
Scheme 1). The structure and properties of these MAs are
apparently not affected by the structure of the template
layer used (oligofuran MA-1 versus vinylpyridine MA-2).
We observed regular film growth with linear scaling of the
material properties. Electrochemical measurements indicat-
ed that the redox properties of the MAs are a function of
the assembly thickness, whereby for greater thicknesses the
evolution of a new oxidation peak at 1.70 (MA-1) or 1.65 V
(MA-2 and MA-3) arises. Remarkably, no clear shift in the
maximum absorption intensity was observed, which suggest-
ed that there is no significant conjugation along the chain or
Scheme 1. Solution-based layer-by-layer (LbL) deposition of molecular assemblies (MAs) MA-1 (terfuran 1 on a 1-based template), MA-2 (terfuran 1 on
a 2-based template), and MA-3 (terthiophene 3 on a 2-based template). These molecular assemblies are formed by immersing a pyridine-terminated tem-
plate layer on quartz, silicon, or ITO-coated glass substrates[27] in a 1.0 mm solution of [Pd
ACHTNUGTRENUNG(PhCN)2Cl2] in THF (step i), followed by immersion in 0.5 mm
solutions of chromophores 1 or 3 in THF (step ii). This two-step procedure was repeated seven times to generate MAs of the type [1n-Pdn] or [3n-Pdn]
with n=1–7. The enlargement shows a schematic representation of the molecular structure of the chains.
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Oligofurans and Oligothiophenes
FULL PAPER
ꢀ1 indicates that only one of the pyridine nitrogen atoms
reacted with the chlorobenzyl-terminated coupling layers.
Semicontact-mode atomic force microscopy (AFM) meas-
urements of the 1-based template layer on single-crystal sili-
con (100), with 500 nmꢁ500 nm scan areas, revealed a
smooth surface with a root-mean-square roughness (Rrms) of
0.2 nm, thus indicating the formation of a homogeneous film
(Figure S2 in the Supporting Information).
Scheme 2. Synthesis of chromophore 1. Reagents and conditions: a) Br2,
Spectroscopic ellipsometry revealed a thickness of (12Æ
1) ꢂ for the 1-based template layer (including the coupling
layer). The thickness of approximately 12 ꢂ is in good
agreement with DFT calculations at the B3LYP/6-31G(d)
level of theory. The calculated molecular length of 18.1 ꢂ
for chromophore 1, with an calculated angular orientation
of approximately 178 to the surface plane, corresponds to a
film thickness of 12 ꢂ (Figure S3 in the Supporting Informa-
tion). These findings are also supported by X-ray reflectivity
DMF, 08C then RT; b) tributyl
reflux; c) i) nBuLi, THF, À788C then RT, N2; ii) (Bu)3SnCl, À788C, N2;
d) 4-bromopyridine, [Pd(PPh3)4], toluene, 1008C (82%).
ACHTUNGTERNNUG(2-furyl)stannane, [PdHCATUNGTERN(NUNG PPh3)4], N2, toluene,
ACHTUNGTRENNUNG
quently used in a solution-to-surface SN2 reaction with chro-
mophore 1, thereby giving rise to the 1-based template
layer. To facilitate this reaction, the substrates were heated
at 808C in a solution (0.5 mm) of compound 1 in THF in a
glass pressure reactor for 96 h under N2 with the exclusion
of light.
The 1-functionalized substrates were thoroughly washed
and sonicated in THF and dried under a stream of N2. The
optical absorption of the 1-based template layer exhibits a
significant bathochromic shift (70 nm) of the p–p* band
Figure 2. Representative synchrotron specular X-ray reflectivity (XRR)
data of the 1-based template layer, in which R is the reflectivity normal-
ized to the Fresnel reflectivity RF. The solid line represents the best fit as-
suming a uniform electron density within the film. The inset shows the
XRR-derived electron-density profile of the 1-based template layer.[34]
Figure 1. Left: Molecular structure of chromophore 8. Right: Normalized
optical absorption of 0.01 mm solutions of chromophores 1 (a) and 8 (b)
in THF. The dotted trace shows the optical absorption of the 1-based
template layer on quartz.
(XRR), which shows a thickness of (15Æ1) ꢂ (Figure 2).
This value is comparable to that found by ellipsometry or
those derived from DFT calculations. The XRR-derived
Rrms of 0.4 nm is similar to the roughness derived from the
AFM measurements regardless of the different scales of the
probed areas (AFM: 500 nmꢁ500 nm; XRR: 1 mmꢁ
25 mm). The XRR-determined electron density (1=
0.41 eꢂÀ3) and surface coverage (G=1.8 moleculesnmÀ2) in-
dicate the formation of densely packed MAs. The surface
coverage is consistent with our UV/Vis data (see above).
The experimental XRR data and the derived electron-densi-
ty profile (shown in Figure 2) are based on a bilayer model
with two regions (the coupling layer and chromophore).[33]
In addition to the new 1-based template layer, a known
template layer was also used (Scheme 1). This layer was
(Figure 1; dotted line). Such large shifts indicate the forma-
tion of pyridinium salts.[27] Indeed, model chromophore 8 ex-
hibits a nearly identical absorption spectrum (Figure 1, trace
b) relative to the surface-confined chromophore 1. Chromo-
phore 8 was formed by the benzylation of chromophore 1
with benzyliodide (see the Experimental Section). Assuming
that the extinction coefficient (e) of the 1-based template
layer is similar to that of chromophore 8 in solution (2.2ꢁ
104 mÀ1 cmÀ1), the surface coverage (G) of the 1-based tem-
plate layer is determined to be Gꢀ1.4 moleculesnmÀ2. The
formation of the pyridinium salt was also confirmed by X-
ray photoelectron spectroscopic (XPS) analysis of the 1-
based template layers on ITO. A characteristic signal of the
pyridinium nitrogen (N+) was observed at 402 eV, whereas
another nitrogen signal (N) was observed at 400 eV (Fig-
ure S1 in the Supporting Information).[27,32] The ratio N+/N
formed
from
1,3,5-tris(4-pyridylethenyl)benzene
(2;
TPEB).[27] The properties of the 2-based layer, including its
surface morphology, thickness, roughness, and chromophore
density, are comparable to those of the 1-based template
layer.[27] The 1- and 2-based template layers were subse-
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M. Bendikov, M. E. van der Boom et al.
quently used in fabricating MAs through our coordination
assembly approach.
Molecular assembly with chromophore (1): The MAs on
quartz, silicon, and ITO were formed by a two-step iterative
deposition process (Scheme 1): 1) the 1- or 2-based template
layers were immersed in a solution of [PdACTHNUGRTNEUNG(PhCN)2Cl2] in
THF (1.0 mm) for 15 min, and subsequently sonicated in
THF (2 times) and acetone (1 time); 2) thereafter, the tem-
plate layers were immersed in a solution of chromophore 1
in THF (0.5 mm) for 15 min and afterwards sonicated in
THF (2 times) and acetone (1 time). This iterative proce-
dure was repeated (7 times). The known trans-coordination
mode of PdCl2 with the pyridine moieties[35] ensures the
linear propagation of the MA upon each chromophore dep-
osition step. The UV/Vis spectra of MAs formed by chro-
mophore 1 on a 1-based template layer (MA-1) and on a 2-
based template layer (MA-2) show a regular increase in the
optical absorption upon increasing the number of chromo-
phore deposition steps (Figure 3). A linear increase in the
absorption of the p–p* band at lmax =412 and 422 nm are
shown in Figure 4 (top) for chromophore 1 in MA-1 and
MA-2, respectively. The average lmax of (417Æ5) nm for the
p–p* band is shifted approximately 30 nm relative to the
same absorbance band in solution. Although the bathochro-
mic shift is not as large as that seen upon quaternization of
the pyridine nitrogen, the 30 nm shift represents coordina-
tion of the PdCl2 to the pyridine moiety of chromophore 1.
This observation is in agreement with DFT calculations
Figure 4. Top: Optical absorbance of the p–p* bands at l=412 (lower set
of data points; MA-1) and 422 nm (upper set of data points; MA-2) as a
function of the number of deposition steps for chromophore 1. Bottom:
increases in the ellipsometry-derived thickness of MA-1 (lower set of
data points) and MA-2 (upper set of data points) as a function of the
number of chromophore deposition steps. TL refers to the template
layer. For all fits, R2 >0.94.
(Table 1). With an increasing number of deposition steps,
there is no apparent shift in the absorption maximum of the
p–p* bands of chromophore 1, which is characteristic of
films that are not subjected to 1) structural changes or 2) H-
or J-type aggregation.[24,36] Moreover, it also indicates the
absence of significant conjugation between the oligofurans
or oligothiophenes, although it cannot be completely ruled
out, as indicated by DFT calculations (see below) and the
broad shoulder that evolves at approximately 550 nm
(Figure 3). Interestingly, when an identical assembly ap-
proach is used, MAs constructed with 1,4-bi(pyridylethenyl)-
benzene (BPEB) exhibit a J-type aggregation, whereas films
with 9,10-di(4-pyridylethenyl)anthracene exhibit 3D order-
ing.[24,37] This demonstrates the importance of the molecular
structure on the internal ordering of MAs. For both MA-1
and MA-2, the same linear correlation is observed between
the film thickness and the number of deposition steps. The
average increase of 2.5 ꢂ per chromophore deposition re-
sults from the angular orientation of the chromophores in
the respective 1- and 2-based template layers and the direc-
tional Pd–pyridine coordination (Figure 4, bottom).[26,35]
The XRR-derived thicknesses of 2.2 nm are in good
agreement with the observed values from spectroscopic el-
lipsometry. The specular XRR spectra for MA-1 and MA-2
are shown in Figure S5 of the Supporting Information. The
XRR pattern and electron-density profiles obtained from
these data clearly show that there is no distinction between
Figure 3. Optical absorption of molecular assemblies (MAs) on quartz,
with an increasing number of deposition steps for chromophore 1 with
the 1-based template layer (top; MA-1) and with the 2-based template
layer (bottom; MA-2). The absorption of the template layer has been
subtracted (see Figure S4 in the Supporting Information for details).
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Oligofurans and Oligothiophenes
FULL PAPER
Table 1. Calculated HOMO and LUMO energies of oligothiofuran (1), oligothiophene (3), and their com-
plexes of the type [1n-PdnÀ1], with various chain lengths (n=1–5), at the B3LYP/basis_set_A level using the
M06L/basis_set_A-optimized geometry.
face topologies, clearly indicate
that the structural differences
in the template layers (1- or 2-
based) did not appreciably in-
fluence the structural or materi-
al properties of the MAs.
[a]
[a]
HOMO LUMO
Eg
lmax[b] [nm]
HOMO LUMO
Eg
lmax[b] [nm]
[eV]
[eV]
[eV]
([eV])
[eV]
[eV]
[eV]
([eV])
1
A
À5.15
À5.31
À5.33
À5.33
À5.33
À2.03
À2.37
À2.54
À2.57
À2.58
3.12
2.94
2.79
2.76
2.75
429 (2.89)
470 (2.64)
483 (2.57)
486 (2.55)
488 (2.54)
3
ACHTUNGTRENNUNG
À5.31
À5.46
À5.48
À5.48
À5.48
À2.44
À2.70
À2.86
À2.89
À2.90
2.87
2.76
2.62
2.59
2.58
465 (2.67)
498 (2.49)
512 (2.42)
516 (2.40)
518 (2.39)
N
ACHTUNGTRENNUNG
Molecular assemblies of chro-
mophores 1 and 3 and their
properties: Following our syn-
thesis of long a-oligofurans,[28a]
an interest developed in com-
paring the properties of oligo-
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[a] Eg is the HOMO–LUMO gap. [b] Calculated absorption transition at TD-B3LYP/basis_set_A level using
the M06L/basis_set_A-optimized geometry.
the electron density of these MAs with the 1- and 2-based
template layers, that were deposited by our coordination-
based LbL assembly method (inset of Figure S5 in the Sup-
porting Information). Moreover, the electron-density pro-
files of MA-1 and MA-2 close to the substrate are similar to
the electron-density profile of the 1-based template layer
(Figure 2). The XRR-derived roughness for both MA-1 and
MA-2 is approximately 0.9 nm for a 25 mm2 scan area. The
AFM measurements indicate smooth film topology without
any apparent grains or islands (Figure 5). The roughness for
500 nmꢁ500 nm scan areas of MA-1 and MA-2 on a single-
crystal silicon is approximately 0.3 nm. The nearly identical
growth of MA-1 and MA-2, combined with their similar sur-
furans with those of their thiophene analogues. Therefore,
we also constructed MAs with the thiophene analogue of
chromophore 1 and compared their properties with MA-1
and MA-2. The thiophene analogue 3, 5,5’’-di(pyridine-4-yl)-
2,2’:5’,2’’-terthiophene, was prepared according to a proce-
dure similar to that shown in Scheme 2.[38] Formation of
MA-3 (which comprises chromophore 3 on a chromophore
2 template) was achieved by employing the same coordina-
tion-based assembly strategy that was outlined for the other
MAs (Scheme 1). Although we have shown that the nature
of the template layer does not influence the material proper-
ties of MA-1 and MA-2 (see above), MA-3 was prepared
under conditions identical to MA-2 to enable a direct com-
parison between these two assemblies. Upon formation of
MA-3, the absorption band at l=440 nm does not vary sig-
nificantly (Figure 6). The differences between chromophores
1 and 3 in their respective MAs are expressed by the ab-
sorption of their p–p* bands. The p–p* band of the oligofur-
an-based MA-2 is observed at l=420 nm, whereas this band
is observed at l=440 nm for MA-3. The p–p* band is shift-
ed by 20 nm (0.134 eV) for the oligothiophene-based MA-3,
which is supported by DFT calculations.
The calculated HOMO–LUMO gap of 2.58 eV for MA-3
(n=5) is 0.17 eV smaller than that of MA-2 (2.75 eV), with
the same number of deposition steps (n=5). The HOMO
energy level (À5.15 eV) of terfuran 1 is 0.16 eV higher than
that of terthiophene 3 (À5.31 eV) (Table 1). Relative to
MA-2, the absorbance intensity at l=440 nm is about
1.5 times more intense in MA-3. Although the extinction co-
efficients in solution are similar (1: 5.4ꢁ104 mÀ1 cmÀ1 versus
3: 4.5ꢁ104 mÀ1 cmÀ1), the difference in intensity suggests that
the thiophene chromophores in MA-3 are present at a
higher molecular density. Indeed, the average molecular
densities (estimated from the XRR data) in MA-3 and MA-
2 are determined to be G=0.57 and 0.42 moleculesnmÀ2, re-
spectively. Consequently, the optical intensity differences do
not reflect a different average orientation of the oligomers
on the surface, but instead result from a higher molecular
density in MA-3. The XRR-derived roughness of 0.9 nm for
a 3.0 nm-thick assembly is practically identical to the rough-
ness of MA-2 from chromophore 1 on an identical template
layer. The electron-density profiles for MA-2 and MA-3 are
similarly close to the substrate surface (the template layer
region); however, MA-3 displays a higher electron density
Figure 5. Semicontact AFM images of a 500ꢁ500 nm scan area of MA-1
(top) and MA-2 (bottom) on silicon substrates with thicknesses of 2.5
and 2.7 nm, respectively. The calculated Rrms values for MA-1 and MA-2
are approximately 0.3 nm.
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M. Bendikov, M. E. van der Boom et al.
out on a series of model oligomers of [1n-PdnÀ1] and [3n-
PdnÀ1] with n=2–5 chromophore units (Scheme 1 and Fig-
ure S6A in the Supporting Information; see the Experimen-
tal Section for details). The model takes into account the co-
ordination between Pd and the chromophores present in the
MA; however, it does not take into account the 2-based
template layer with its positive charge. It was reported that
for p-conjugated organic systems, the HOMO–LUMO gap
linearly correlates with the reciprocal of the number of func-
tional units (up to twelve aromatic rings). Convergent be-
havior is observed for higher numbers of aromatic rings.[39,40]
Here, such convergent behavior is indicated as well. Since
each chromophore unit in the MAs is composed of five
rings (three furan or thiophene rings and two pyridine
rings), time-dependent density functional theory (TD-DFT)
shows that convergent behavior predominates for three
chromophore units and up, as is evident from Figure S7 in
the Supporting Information. Consequently, the calculated
UV/Vis spectra of 1- and 3-based oligomers show small
shifts in the wavelength of the absorption maximum with
each additional chromophore unit (Table 1). For instance,
when moving from three to five chromophore units, a shift
of 5 nm is predicted (lmax values for [1n-Pdn]-based oligo-
Figure 6. Top: Optical absorption of molecular assembly 3 (MA-3) on
quartz with an increasing number of deposition steps for chromophore 3.
The absorption of the template layer has been subtracted (Figure S4 in
the Supporting Information). Bottom: Optical absorbance of the p–p*
band at l=440 nm (a) and the molecular assembly thickness (b) of MA-
3 as a function of the number of deposition steps for chromophore 3. TL
refers to the thickness of the 2-based template layer. For all fits, R2 >
0.99.
ACHTUNGTRENmNUGN ers for n=3, 4, and 5 are 483, 486, and 488 nm, respective-
ly). Therefore, from the calculated convergence, it can be
concluded that no significant shift in lmax is expected upon
increasing the number of chromophore units. Indeed, this
was also observed experimentally.
Increasing the chain length of the chromophores resulted
in a decrease of the LUMO energy levels, whereas the
HOMO energy levels remain constant (Table 1). Similar re-
sults were obtained in calculations on MAs previously com-
municated by us.[41] We also calculated the shape of the fron-
tier orbitals for chromophores 1 and 3, as well as their
oligomers (Figure S6 in the Supporting Information). The
shapes of the frontier orbitals for oligomers that contain five
chromophore units are shown in Figure 8. The shapes of the
molecular orbitals reveal that the electron delocalization is
similar for both 1- and 3-based oligomers irrespective of
their chain length. The HOMOÀ1 and HOMO energy
levels are degenerated and are delocalized at the edges,
whereas the LUMO is delocalized over the three chromo-
phore units at the center of the MA.
at a large distance from the surface, that is, the chromo-
phore region (Figure 7 and Figure S5 in the Supporting In-
formation).
DFT calculations for [1n-PdnÀ1] and [3n-PdnÀ1] with n=2–5
chromophore units: To gain a better understanding of the
structure and electronic properties of chromophores 1 and 3
in their corresponding MAs, DFT calculations were carried
Electrochemical properties of MA-1, MA-2, and MA-3: The
CV of chromophore 1 in dry acetonitrile is shown in
Figure 9. The electron-withdrawing pyridine rings make the
furan core more electropositive, which results in a relatively
high oxidation potential relative to other oligofurans.[28a]
The two irreversible peaks at 1.0 and 1.3 V (versus saturated
calomel electrode (SCE)) might be related to oxidation
processes observed for other (poly) furans and thiophene-
s.[28a,43,44] For instance, an oxidative furan-ring opening with
the subsequent formation of a diketone is known.[43] The for-
mation of other products in our system cannot be ruled
out.[45] The CVs of the MAs were measured as a function of
the number of chromophore deposition steps (Figure 10 and
Figure 7. Representative synchrotron specular X-ray reflectivity (XRR)
data of MA-3 with a 27 nm thickness, in which R is the reflectivity nor-
malized to the Fresnel reflectivity RF. The solid line represents the best
fit assuming a uniform electron density within this film. The inset shows
the XRR-derived electron-density profile of MA-3.[34]
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Oligofurans and Oligothiophenes
FULL PAPER
MA-3, similar behavior is ob-
served upon increasing the
thickness of the assembly
(Figure 10). The first oxidation
occurs at 1.15 and 1.28 V for
MA-2 and MA-3, respectively.
No shift is observed upon in-
creasing the assembly thickness.
The second oxidation peak
(Eox2) for MA-2 (1.65 V) and
MA-3 (1.65 V) was observed
after a thickness of 2.1 (MA-2)
or 2.4 nm (MA-3). Coulombic
repulsion might hamper oxida-
tion of the chromophores close
to the positively charged tem-
plate layer. This effect is inver-
sely proportional to the square
root of the distance to the
charge. Therefore, elongation of
the chains results in a more
facile process, as is clearly seen
for the second oxidation step.
The UV/Vis and XRR meas-
urements of MA-2 and MA-3
Figure 8. Calculated frontier orbitals of oligomers 1 and 3, with five chromophores and four PdCl2 units.
Atomic color scheme: C=gray, H=white, N=blue, O=red, S=yellow, Pd=teal (dark greenish-blue), Cl=
green. The shapes of the molecular orbitals were calculated at a B3LYP/basis_set_A level using M06L/basis_-
set_A-optimized geometry.
revealed
a somewhat higher
Figure S8 in the Supporting Information). The observed cur-
rent increase upon increasing the number of deposition
steps is in agreement with the UV/Vis and ellipsometry
data. The MAs also exhibit regular growth on a metal oxide
surface. The electrochemical properties of the thicker mo-
lecular assemblies MA-1, MA-2, and MA-3 on ITO resem-
ble the behavior of the corresponding chromophores in solu-
tion. Interestingly, the second oxidative peak is only clearly
observed for MAs with longer chromophore chain lengths
(Figure 10). For MA-1, two irreversible oxidation peaks are
observed at 1.28 (Eox1) and 1.70 V (Eox2), which are pre-
served upon increasing the assembly thickness from 1.4 to
2.7 nm. Eox2 is dominant at higher thicknesses (Figure S8 in
the Supporting Information; blue trace). For MA-2 and
chromophore density of thiophene derivative 3 in MA-3
than for furan derivative 1 in MA-2. This higher density is
Figure 10. Representative CV scans of (top) MA-2 with 2, 4, and 6 chro-
mophore deposition steps on ITO (a through c, respectively) and (bot-
tom) MA-3 with 2, 4, and 6 chromophore deposition steps on ITO (a
through c, respectively). Curves d denote CVs with bare ITO electrodes.
Fc/Fc+ is set at 0.40 V versus SCE under these conditions.[42] The scan
Figure 9. Cyclic voltammogram of a 0.5 mm solution of chromophore 1
(trace a) in dry acetonitrile with TBAPF6 (0.1m) as supporting electrolyte
and ITO as the working electrode. Trace b is the scan of a bare ITO elec-
trode in the absence of chromophore 1. Fc/Fc+ is set at 0.40 V versus
SCE under these conditions.[42] The scan rate is 100 mVsÀ1
.
rate is 100 mVsÀ1
.
Chem. Eur. J. 2013, 19, 8821 – 8831
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8827

M. Bendikov, M. E. van der Boom et al.
evident and reflected in the CVs of MA-2 and MA-3. For
similar thicknesses of MA-2 and MA-3, the oxidative peak
current (Ipc) of MA-3 is approximately 1.5 times higher than
the peak current observed for MA-2 (Figure 10: top and
bottom, trace c).
The electrochemical/optical properties and the derived
energy levels of the HOMO and LUMO of the MAs are
shown in Table 2. The energy levels of the HOMO and
lecular packing in MA-3 was found to be approximately
1.5 times higher than in MA-2. DFT calculations (Table 1)
and electrochemical measurements indicated that the
HOMO energy level of MA-2 is approximately 0.15 eV
higher than that of MA-3. Increasing the number of chro-
mophore (1 or 3) units resulted in a decrease in the LUMO
energy levels, whereas the HOMO energy levels do not
seem to be affected. Electrochemical measurements re-
vealed that there are no significant differences in the elec-
trochemical behavior among all three MAs. The electro-
chemical properties of the MAs are characterized by two ir-
reversible oxidation peaks, whereas the second oxidation
peak is only observed after a certain assembly thickness is
obtained.
Table 2. Electrochemical and optical properties of molecular assemblies
MA-1, MA-2, and MA-3, with thicknesses of 2.5, 2.7, and 3.0 nm, respec-
tively.
[a]
[a]
op
Eox1
Eox2
DE
[V]
lmax
EHOMO
ELUMO
DE
gap
[V]
[V]
[nm]
[eV][b]
[eV][c]
[eV][d]
In conclusion, although no apparent interactions are ob-
served experimentally between the chromophores, the su-
pramolecular control in the assemblies of chromophores 1
and 3 is of major importance for device performance.[9,46]
Furthermore, we have shown that the molecular assemblies,
independent of the template layer that is used, exhibited
thickness-dependent electrochemical behavior. This behav-
ior can be exploited in different applications since oligofur-
ans and oligothiophenes have many advantages over other
known aromatic or heteroaromatic analogues.[2f,g,28,47] More-
over, the energy levels of the MAs do not change signifi-
cantly upon increasing the chromophore chain length and
match well with the work function of platinum (5.6 eV),
which can be utilized as contacts in future devices.
MA-1
MA-2
MA-3
1.28
1.15
1.28
1.70
1.65
1.65
0.42
0.50
0.37
412
422
440
À5.68
À5.55
À5.68
À2.76
À2.63
À2.90
2.92
2.92
2.78
[a] Versus SCE. [b] According to
E
HOMO =À
ACHTUGNTRENN(UGN Eox(versus SCE)+4.4).
[c] According to ELUMO =EHOMO +DEogapp. [d] According to DE
[eV]=
op
gap
1239.8/lmax [nm].
LUMO were calculated according to ELUMO =EHOMO +DEgoapp
in which ELUMO and EHOMO are the energy levels of the
,
LUMO and HOMO, respectively. The HOMO–LUMO gaps
op
gap
(DE ) are derived from the optical data of the MAs. As-
suming that the energy level of the SCE is 4.4 eV below
vacuum, the EHOMO of the MAs is estimated from the first
oxidation potentials according to EHOMO =À
ACHTUGNERTN(NUGN Eox (versus
SCE)+4.4). For MA-2 and MA-3, the HOMO energy levels
were found to be À5.55 and À5.68 eV, respectively, which is
in good agreement with our DFT calculations on the indi-
vidual chromophore molecules (Table 1).
Experimental Section
Materials and methods: The synthesis of 4-bromopyridine, bis(benzoni-
AHCTUNGTRENNUNG
2,2’:5,2’’-terfuran (6), and 2,5’’-bis(tributylstannyl)-2,2’:5’,2’’-terfuran (7)
are described elsewhere.[28a,38,48] 4-(Chloromethyl)phenyltrichlorosilane
was purchased from Gelest (Morrisville, PA). Deuterated solvents, dry
acetonitrile (99.8%), triethylamine (99.5%), and tributyl-(2-furyl)stan-
nane (95%) were purchased from Sigma–Aldrich and used as received.
Solvents (AR grade) were purchased from Bio-Lab (Jerusalem), Frutar-
om (Haifa), or Mallinckrodt Baker (Phillipsburg, NJ). Pentane, THF, tol-
uene, and diethyl ether were distilled under N2 over Na and degassed
before being introduced into an M. Braun N2-filled glovebox with O2 and
H2O levels of less than 2 ppm. Single-crystal silicon (100) substrates
(2.0 cmꢁ1.0 cm), ITO-coated glass substrates (7.5 cmꢁ0.8 cm), and
quartz substrates (2.0 cmꢁ1.0 cm) were purchased from Wafernet (San
Jose, CA), Delta Technologies (Loveland, CO), and Chemglass (Vine-
land, NJ), respectively. All substrates were cleaned according to a pub-
lished procedure.[49] The silane-based chemistry and formation of the 2-
based template layer were carried out in a N2-filled glovebox or by using
standard Schlenk cannula techniques.[27] 1H and 13C{1H} NMR spectra
were recorded at 500.13 and 125.77 MHz using a Bruker Avance 500
NMR spectrometer or at 300.13 and 75.47 MHz using a Bruker Avance
Conclusion
Three new MAs were formed using chromophores 1 and 3.
Each MA displayed linear growth, as indicated by the good
correlation between the thickness and absorbance intensity
(lmax) of the assembly and the number of chromophore dep-
osition steps. The material properties of oligofuran-contain-
ing MA-1 and MA-2 in terms of absorbance spectra, surface
roughness, and thickness were found to be nearly identical.
The apparent absence of grains or islands suggests that the
films are homogeneous and form structurally well-defined
molecular assemblies. For MA-1 and MA-2, the effect of the
template layer (terfuran 1 versus vinylpyridine 2) was found
to be negligible. Furthermore, XRR analysis revealed identi-
cal surface coverage, and that within each film there is no
distinction between the template layer and the subsequently
deposited layers.
1
300 MHz spectrometer. The H and 13C{1H} NMR spectroscopic chemical
shifts are reported relative to tetramethylsilane or to residual solvent
peaks. UV/Vis spectra were recorded using a Varian Cary 100 spectro-
photometer in the double-beam transmission mode (200–800 nm). Low-
resolution mass spectra were recorded using a Micromass Platform LCZ
4000 mass spectrometer operated in electrospray ionization (ESI) mode.
High-resolution mass spectra (HRMS) were recorded using a Waters Mi-
cromass GCT Premier mass spectrometer using field desorption (FD)
Although no effects were observed from the nature of the
template layers, substituting chromophore 1 for its thio-
phene analogue 3 led to differences in the resulting MAs.
The differences between MA-2 and MA-3 are subtle and
can be found in the molecular packing of the MAs. The mo-
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Chem. Eur. J. 2013, 19, 8821 – 8831

Oligofurans and Oligothiophenes
FULL PAPER
ionization or
a
Synapt high-definition mass spectrometer (HDMS)
did not indicate Pd coordination. However, the reaction of chromophores
system operated in the ESI mode. Spectroscopic ellipsometry was carried
out using an M2000V (J.A. Woollam Co., Inc.) instrument.
1 and 3 with [PdCl2ACTHNGUTERNUN(G PhCN)2] resulted in the formation of coordination-
based polymers.
5,5’’-Di(pyridine-4-yl)-2,2’:5’,2’’-terfuran (1): Freshly prepared 4-bromo-
pyridine (0.60 g, 3.8 mmol) was added to solution of [Pd(PPh3)4]
XRR measurements: Synchrotron XRR studies were performed at beam-
line X18A of the National Synchrotron Light Source (NSLS; Brookhaven
National Laboratory, USA) using a Huber four-circle diffractometer in
the specular reflection mode (i.e., the incident angle is equal to the exit
angle q). The reflected intensity was measured as a function of the scat-
tering vector component qz =(4p/l) sinq, perpendicular to the reflecting
surface. X-rays of energy E=10 keV (l=1.240 ꢂ) were used with a
beam size of 0.3–0.5 mm vertically and 0.5–1.0 mm horizontally. The reso-
lution was 3ꢁ10À3 ꢂÀ1. The samples were placed under a slight overpres-
sure of helium during the measurements to reduce the background scat-
tering from the ambient gas and radiation damage. The off-specular
background was measured and subtracted from the specular counts. De-
tails of the data acquisition and analysis are given elsewhere.[34] The
XRR measurements were performed at 20–258C.
a
ACHTUNGTRENNUNG
(0.08 g, 0.07 mmol) and compound 7 (0.80 g, 1.0 mmol) in dry toluene
(15 mL). The resulting mixture was stirred for 3 h at 1008C. Upon cool-
ing to room temperature, a yellow precipitate formed, which was collect-
ed by filtration. The filtrate was washed with toluene (2ꢁ30 mL) and
pentane (2ꢁ30 mL), and purified by basified (NEt3) silica column chro-
matography (hexane/acetone=20:80 v/v) to yield chromophore 1 as a
1
yellow powder (0.28 g, 78%; Scheme 2). H NMR (CD2Cl2): d=8.61 (dd,
4H) 7.65 (dd, 4H), 7.10 (d, J=3.6 Hz, 2H), 6.90 (s, 2H), 6.87 ppm (d, J=
3.7 Hz, 2H); 13C{1H} NMR (CD2Cl2): d=151.20, 150.11, 147.76, 146.25,
137.79, 118.15, 111.97, 109.24, 108.91 ppm; UV/Vis (THF) l (e)=391 nm
(5.4ꢁ104 mÀ1 cmÀ1); HRMS (ES+): m/z calcd for C22H15N2O3 [M+1]+:
355.1083; found: 355.1091.
1-Benzyl-4-{5’’-(pyridine-4-yl)-[2,2’:5’,2’’-terfuran]-5-yl}pyridine-1-ium
iodide (8): Benzyliodide (0.05 g, 0.23 mmol) was added to a solution of
chromophore 1 (0.10 g, 0.28 mmol) in dry THF (100 mL) under argon.
The reaction mixture was stirred and heated for 30 h at 808C with the ex-
clusion of light. Upon cooling to À48C, a brown precipitate formed,
which was isolated by filtration. The filtrate was dried under high
vacuum and purified by column chromatography on neutral alumina
using methanol as eluent to yield compound 8 as a brown solid (0.012 g,
XPS measurements: A Kratos Analytical AXIS-HS instrument using a
monochromated AlKa source at a base pressure of approximately 2ꢁ
10À9 torr at various takeoff angles was used for XPS measurements. The
charging, which usually developed under the X-ray beam, was substan-
tially reduced by applying a negative voltage on a grid above the sample.
The C(1s) line at 284.8 eV was used for energy calibration. High-resolu-
tion spectra of C(1s), O(1s), Si(2p), and N(1s) were collected on an ap-
proximately 0.5 mm spot diameter, with 20 and 40 eV pass energies and a
resolution of approximately 0.5 eV. Shirley background subtraction and
Gaussian–Lorenzian lineshapes were used for the curve fitting.
1
9.1%; Figure 1). H NMR (CD3OD): d=8.86 (d, J=7.1 Hz, 2H), 8.56 (d,
J=6.3 Hz, 2H), 8.27 (d, J=7.0 Hz, 2H), 7.82 (d, J=3.9 Hz, 1H), 7.79 (d,
J=6.3 Hz, 2H), 7.50 (d, J=2.3 Hz, 5H), 7.32 (d, J=3.7 Hz, 1H), 7.21 (d,
J=3.7 Hz, 1H), 7.13 (d, J=3.9 Hz, 1H), 7.05 (d, J=3.7 Hz, 1H), 7.00 (d,
J=3.6 Hz, 1H), 5.72 ppm (s, 2H); 13C{1H} NMR (DEPT; CD3OD): d=
150.71, 145.52, 130.93, 130.73, 129.95, 121.44, 121.41, 119.25, 113.25,
112.94, 111.18, 110.56, 110.31, 64.49 ppm; UV/Vis (THF): l (e)=465 nm
(2.2ꢁ104 mÀ1 cmÀ1); HRMS (FD): m/z calcd for [C29H21N2O3]+: 445.1552;
found: 445.1551.
Electrochemical measurements: CVs were recorded with a CHI 660A po-
tentiostat. All measurements were performed in a three-electrode cell
configuration that consisted of 1) an MA-functionalized ITO substrate as
the working electrode, 2) a Pt wire as the counter electrode, and 3) an
Ag wire as the reference electrode. All electrochemical measurements
were performed at RT in an M. Braun N2-filled glovebox with O2 and
H2O levels <2 ppm. Dry acetonitrile that contained 0.1m Bu4NPF6 was
used as the electrolyte solution. The ferrocene/ferrocenium (Fc/Fc+)
redox couple was used as an internal standard for all measurements. The
Fc/Fc+ redox couple was set at 0.40 V relative to SCE under these condi-
tions.[42]
Acknowledgements
Computational details: All calculations were carried out using Gauss-
ACHTUNGTRENNUNG
ian 09 software.[50] The geometries of the molecules were fully optimized
This research was supported by the Helen and Martin Kimmel Center
for Molecular Design, the Gerhardt M.J. Schmidt Minerva Center, the
Israel Science Foundation, and the U.S. National Science Foundation
(DMR-1006432). M.v.d.B. is the incumbent of the Bruce A. Pearlman
Professorial Chair in Synthetic Organic Chemistry. M.B. is a member ad
personam of the Lise Meitner-Minerva Center for Computational Quan-
tum Chemistry. We thank Drs. H. Cohen and T. Bendikov (WIS) for car-
rying out the XPS experiments. The XRR measurements were performed
at beamline X18A of the National Synchrotron Light Source, supported
by the U.S. Department of Energy (DE-AC02-98CH10886).
using the M06L functional. With this functional, a combination of two
basis sets were used: the 6-31G(d) basis set was used for the lighter ele-
ments (H, C, N, S), and the SDD basis set with a relativistic effective
core potential was employed for Pd (as implemented in Gaussian 09).
This combination of basis sets is called “basis_set_A” in this work. After
geometry optimization, further single-point calculations were performed
at the B3LYP/basis_set_A level, since it is known that B3LYP provides
good estimates for HOMO–LUMO gaps.[40] The shapes of molecular or-
bitals were calculated at the B3LYP/basis_set_A level using M06L/basis_-
set_A-optimized geometries. To estimate UV/Vis spectra, time-depend-
ent density functional theory (TD-DFT) calculations were carried out at
the B3LYP/basis_set_A level of theory in the gas phase using M06L/ba-
sis_set_A-optimized geometries unless stated otherwise.
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Received: January 5, 2013
Published online: May 16, 2013
Chem. Eur. J. 2013, 19, 8821 – 8831
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8831