Synthesis of amphiphilic conjugated diblock oligomers as molecular diodes

Article abstract of DOI:10.1002/1521-3773(20021004)41:19<3598::AID-ANIE3598>3.0.CO;2-U

A diodelike rectifying effect was observed in Langmuir - Blodgett monolayers of a p-n junction molecule, the amphiphilic π-conjugated diblock oligomer shown. A reference molecule without the p-n junction feature shows nonrectifying behavior.

Full text of DOI:10.1002/1521-3773(20021004)41:19<3598::AID-ANIE3598>3.0.CO;2-U

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[8] a)F. Vˆgtle, M. Plevoets, M. Nieger, G. C. Azzellini, A. Credi, L.
De Cola, V. De Marchis, M. Venturi, V. Balzani, J. Am. Chem. Soc.
1999, 121, 6290; b)F. Vˆgtle, S. Gestermann, C. Kauffmann, P. Ceroni,
V. Vicinelli, V. Balzani, J. Am. Chem. Soc. 2000, 122, 10398; c)V.
Balzani, P. Ceroni, S. Gestermann, M. Gorka, C. Kauffmann, M.
Maestri, F. Vˆgtle, ChemPhysChem 2000, 1, 224.
Experimental Section
All syntheses were routinely carried out under an argon atmosphere.
Starting materials (POPAM dendrimer G2, 3,5-dihydroxybenzyl alcohol, 2-
bromomethylnaphthalene, dansyl chloride)were purchased from Aldrich.
3,5-Bis(3’,5’-bis(2’’-oxymethylnaphthyl)benzyloxy)benzyl bromide (ob-
tained from precursor 3)and 2 were synthesized according to literature
procedures.[14,15]
[9] a)T. Fˆrster, Discuss. Faraday Soc. 1959, 27, 7; b)F. Barigelletti, L.
Flamigni, Chem. Soc. Rev. 2000, 29, 1.
1:Cesium carbonate (200 mg)was added to a solution of 2 (50.0 mg,
0.019 mmol)in DMF (10 mL)under argon. 3,5-bis(3 ’,5’-bis(2’’-oxymethyl-
naphthyl)benzyloxy)benzyl bromide (193.0 mg, 0.19 mmol) in DMF
(10 mL)was added to this solution, and the mixture was stirred for seven
days, then filtered and the solvent evaporated. The residue was dissolved in
CH2Cl2, washed with water, saturated NaHCO3 solution, and again with
water, then dried over Na2SO4 and evaporated. Purification by column
chromatography on SiO2 (63 100 mm)with chloroform/methanol (20:1)as
eluent yielded a light-yellow solid in 48% yield, m.p. 103 1088C, Rf ¼ 0.08
(chloroform/methanol 20:1). 1H NMR (400 MHz, CDCl3, 258C): d ¼ 1.59
(brs, 24H, CH2), 2.13 (brs, 32H, NCH2), 2.47 (brs, 52H, N(CH3)2 and
N(CH2)2N), 3.35 (s, 16H, SO2NCH2), 4.30 (s, 16H, SO2NCH2Ar), 4.44 (s,
32H, ArOCH2Ar), 4.97 (s, 64H, ArOCH2Naph), 6.23 (m, 16H, CHAr), 6.35
(m, 8H, CHAr), 6.54 6.74 (brm, 56H, 48 CHAr and 8 CHDans), 7.22 7.51
(brm, 104H, 96 CHNaph and 8 CHDans), 7.59 7.87 (bm, 136H, 128 CHNaph and
8 CHDans), 8.14 (m, 8H, CHDans), 8.29 ppm (m, 16H, CHDans); 13C NMR
(100.6 MHz, CDCl3, 258C): d ¼ 25.1, 44.9, 45.1, 50.1, 50.5, 51.2, 101.6 (2C),
106.6 (2C), 114.9, 119.3, 123.1, 125.3, 125.9, 126.1, 126.2, 127.6, 127.9, 128.2,
129.3, 129.6, 129.9, 130.1, 130.2, 132.9, 133.1, 134.2, 135.0, 138.6, 139.1, 151.6,
159.8, 159.9 ppm; C654H580N22O64S8; Mw ¼ 10028.24.
[10] An energy-transfer rate constant of about 8 î 109 sÀ1 has been
estimated by using the equation for the dipole dipole (Fˆrster)
mechanism:[9] ken ¼ 8.8 î 10À25 K2 FJ/(r6 n4 t)where F and t are the
fluorescence quantum yield and lifetime, respectively, of the fluores-
cence of dendron 3, J is the integral overlap between the fluorescence
spectrum of dendron 3 and the absorption spectrum of the acceptor
dansyl units (1.5 î 10À14 cm3 mÀ1), K2 is a geometric factor (taken as
2/3), n is the refractive index of the solvent (1.424), and r is the
distance between donor and acceptor (assumed to be 1 nm as an
average).
[11] Dynamic quenching can be ruled out because of the short lifetime of
dansyl fluorescence and the very low concentration (less than 10À5 m)
of extracted eosin.
[12] The curvature of the plot (Figure 1c, inset)at high eosin concen-
trations can be related to the fact that each dendrimer can host more
than one eosin molecule, so that even at relatively high [eosin]:[1]
ratios, there are still some ™empty∫ dendrimers, which is in agreement
with the unquenched fluorescence lifetime.
[13] J. F. G. A. Jansen, E. W. Meijer, J. Am. Chem. Soc. 1995, 117, 4417.
[14] G. M. Stewart, M. A. Fox, J. Am. Chem. Soc. 1996, 118, 4354.
[15] a)A. Archut, S. Gestermann, R. Hesse, C. Kauffmann, F. Vˆgtle,
Synlett 1998, 5, 546; b)F. Vˆgtle, S. Gestermann, C. Kauffmann, P.
Ceroni, V. Vicinelli, L. De Cola, V. Balzani, J. Am. Chem. Soc. 1999,
121, 12161.
[16] J. N. Demas, G. A. Crosby, J. Phys. Chem. 1971, 75, 991.
[17] a)I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
Molecules, Academic Press, London, 1965; b)S. R. Meech, D. Phillips,
J. Photochem. 1983, 23, 193; c)G. R. Fleming, A. W. E. Knight, J. M.
Morris, R. J. S. Morrison, G. W. Robinson, J. Am. Chem. Soc. 1977, 99,
4306.
In all experiments, the aqueous phase was buffered at pH 7.0 using a
phosphate buffer. Spectroscopic equipment and techniques have been
described elsewhere.[8b] The experiments were carried out in air-equili-
brated solutions. Fluorescence quantum yields were standardized[16] using
naphthalene (F ¼ 0.23 in deaerated cyclohexane)[17a] for dendron 3, quinine
sulfate (F ¼ 0.55 in 1n H2SO4(aq) solution)[17b] for dendrimers 1 and 2, and
fluorescein (F ¼ 0.90 in NaOH; 0.01m)[17c] for eosin 4.
Received: March 21, 2002
Revised: July 10, 2002 [Z18950]
[1] V. Balzani, A. Credi, M. Venturi, Curr. Opin. Chem. Biol. 1997, 1, 506.
[2] T. Pullerits, V. Sundstrˆm, Acc. Chem. Res. 1996, 29, 381, and
references therein.
[3] G. R. Newkome, C. N. Moorefield, F. Vˆgtle, Dendrimers and
Dendrons, Wiley-VCH, Weinheim, 2001.
Synthesis of Amphiphilic Conjugated Diblock
Oligomers as Molecular Diodes**
[4] Recent reviews: a)V. Balzani, S. Campagna, G. Denti, A. Juris, S.
Serroni, M. Venturi, Acc. Chem. Res. 1998, 31, 26; b)A. Adronov,
J. M. J. Frÿchet, Chem. Commun. 2000, 1701; c)V. Balzani, P. Ceroni,
A. Juris, M. Venturi, S. Campagna, F. Puntoriero, S. Serroni, Coord.
Chem. Rev. 2001, 219, 545.
[5] For recent leading papers: a)L.-Z. Gong, Q. Hu, L. Pu, J. Org. Chem.
2001, 66, 6136; b)F. V. R. Neuwahl, R. Righini, A. Adronov, P. R. L.
Malenfant, J. M. J. Frÿchet, J. Phys. Chem. B 2001, 105, 1307; c)M.-S.
Choi, T. Aida, T. Yamazaki, I. Yamazaki, Angew. Chem. 2001, 113,
3294; Angew. Chem. Int. Ed. 2001, 40, 3194; d)M. Maus, R. De, M.
Lor, T. Weil, S. Mitra, U.-W. Wiesler, A. Herrmann, J. Hofkens, T.
Vosch, K. M¸llen, F. C. De Schryver, J. Am. Chem. Soc. 2001, 123,
7668; e)A. Zhu, P. Bharathi, J. O. White, H. G. Drickamer, J. S.
Moore, Macromolecules 2001, 34, 4606; f)M. Kimura, T. Shiba, M.
Yamazaki, K. Hanabusa, H. Shirai, N. Kobayashi, J. Am. Chem. Soc.
2001, 123, 5636.
[6] M. W. P. L. Baars, E. W. Meijer, Top. Curr. Chem. 2001, 210, 131.
[7] Recent leading papers: a)A. P. H. J. Schenning, E. Peeters, E. W.
Meijer, J. Am. Chem. Soc. 2000, 122, 4489; b)L. Zhou, D. H. Russell,
M. Zhao, R. M. Crooks, Macromolecules 2001, 34, 3567; c)W. Chen,
D. A. Tomalia, J. L. Thomas, Macromolecules 2000, 33, 9169; d)Z.
Sideratou, D. Tsiourvas, C. M. Paleos, Langmuir 2000, 16, 1766; e)F.
Vˆgtle, M. Gorka, V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani,
ChemPhysChem 2001, 2, 769; f)V. Balzani, P. Ceroni, S. Gestermann,
M. Gorka, C. Kauffmann, F. Vˆgtle, Tetrahedron 2002, 58, 629; g)K.
Yamamoto, M. Higuchi, S. Shiki, M. Tsuruta, H. Chiba, Nature 2002,
415, 509.
Man-Kit Ng and Luping Yu*
Nearly three decades ago Aviram and Ratner proposed that
individual molecules of the type donor spacer acceptor (D-s-
A)placed between two electrodes would act as molecular
rectifiers under a suitable bias voltage.[1] Previous attempts to
provide experimental proof of molecular rectifiers were
complicated by difficulty in establishing reproducible elec-
trical contacts between the metal and single molecules, the
junction rectifying effect at the metal molecule interface due
[*] L. Yu, M.-K. Ng
Department of Chemistry and the James Franck Institute
The University of Chicago
5735 South Ellis Avenue, Chicago, IL 60637 (USA)
Fax : (þ 1)773-702-0805
E-mail: lupingyu@midway.uchicago.edu
[**] We thank Dr. Norbert Scherer and Dr. Ka Yee Lee for helpful
discussions, and Dr. Dong-Chan Lee and Cherie H. C. Yueh for help
with LB and STM studies. Financial support from the NSF, NSF-
MRSEC, and AFOSR are gratefully acknowledged.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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to the Schottky barrier and/or salt formation, and the work
function difference (however small)between anode and
cathode in most configurations of metal/molecules/metal
heterostructures. Still, there is uncertainty about the number
of active molecules investigated in these microscopic sys-
manipulated by the Langmuir Blodgett (LB)technique at
the air water interface.
The synthetic approach to the diblock co-oligomer is shown
in Scheme 1. The diblock co-oligomer 20 was synthesized by a
Stille coupling between 18 and 19, which were both obtained
almost quantitatively from the corresponding precursors 9
and 17, respectively. It is noteworthy that electrophilic
iodination of 17 proceeded with complete chemo- and
regioselectivity at the unsubstituted a position. After the
Stille reaction was completed, in situ desilylation by addition
of excess TBAFafforded the amphiphilic head-to-tail coupled
oligothiophene-oligothiazole diblock system 20.
For the purpose of comparison with the p-n junction
molecules, a reference compound, amphiphilic octathiophene
21 with the same alkyl side chains, was also synthesized
(Scheme 1). This reference compound is almost an exact
replica of the diblock molecule except for the replacement of
the nitrogen atoms on the electronegative thiazole ring by
methine groups.
6]
tems.[2 More recently, as the drive towards miniaturizing
electronic components intensifies, the development and
realization of molecular-scale electronic devices based on
assemblies of atoms and/or molecules has become an ultimate
goal.[7] Various basic device components such as molecular
wires, rectifying diodes, field-effect transistors, and single-
electron transistors have been demonstrated in a host of
materials, ranging from semiconductor and metal nanowires,
carbon nanotubes, fullerenes, and small organic molecules to
biomolecules.[2 13] However, there is no report on p-n junction
molecules that are akin to those in semiconductors but with
much more structural versatility. These functional molecular
components are crucial to the development of molecular
electronics.
Just like semiconductor materials, the inspiration for p-n
junction molecules can be found in semiconducting organic
materials, conjugated polymers, and oligomers. These materi-
als have been exploited as key electronic components in the
development of a number of technologically important
applications such as light-emitting
The amphiphilic conjugated diblock molecule was oriented
at the air water interface by the Langmuir Blodgett techni-
que, with the donor part adjacent to the water subphase and
the acceptor block away from it. A typical pressure area
(P A)isotherm of the diblock molecules is shown in
diodes, plastic transistors, photo-
voltaic solar cells, sensory devices,
and diode lasers.[14] These materials
have tremendous structural versa-
tility: p-type and n-type polymers
with semiconducting properties
have been synthesized. We intro-
duce a new and useful concept in
designing p-n junction molecules, a
relatively simple diblock molecule
incorporating two different conju-
gated blocks with opposite elec-
tronic demands. We show that this
conjugated diblock molecule in-
deed exhibits a diodelike rectifying
effect.
Polythiophenes (PT)and oligo-
thiophenes (OT)are well-known p-
type semiconducting materials,
whereas polythiazoles are known
to be electron-deficient com-
pounds.[15] Our first target molecule
was
a regioregular, head-to-tail
coupled diblock co-oligomer com-
prising an electron-rich oligo(3-al-
kylthiophene)as the donor block
and an electron-poor oligo(4-al-
kylthiazole)as the acceptor block.
Two different alkyl side groups
with contrasting hydrophilicity/hy-
drophobicity were selectively in-
corporated in the two blocks. The
surfactant-like amphiphilic conju-
gated diblock oligomer can be
Scheme 1. Synthetic approach to diblock co-oligomer 20 and reference compound 21. Reagents and
conditions: a)LDA, THF, À788C, then I2; b)LDA, THF, À788C, then TIPSOTf); c) nBuLi, TMEDA, THF,
À788C, then Bu3SnCl; d)2.5 mol% [Pd 2(dba)3], 20 mol% PPh3, 50 mol% Cu2O, DMF, 1158C; e)TBAF,
THF, RT. f)NaH, diethyleneglycol monomethyl ether, THF, 0 8C; g) nBuLi, Et2O, À788C, then TIPSOTf;
h)NIS, CHCl 3/HOAc. LDA ¼ lithium diisopropylamide, TIPSOTf ¼ triisopropylsilyl trifluoromethanesul-
fonate, TMEDA ¼ tetramethylethylenediamine, dba ¼ dibenzylidene acetone, TBAF ¼ tetrabutylammoni-
um fluoride, NIS ¼ N-iodosuccinimide; MEEM ¼ methoxyethoxyethoxymethyl.
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Figure 1a. As the molecular monolayer is compressed, the
surface pressure increases slowly in the region between 230
and 130 ä2 molÀ1, followed by a kink at P ~ 14 mNmÀ1 (A ~
130 ä2 molÀ1), which is tentatively assigned as a phase
microscope (STM)and scanning tunneling spectroscopy
(STS).
After a stable STM scan was obtained, the feedback loop
for the STM control was briefly interrupted. The Pt/Ir STM
tip (grounded to zero potential)was then held at a constant
position above the film; the tip sample separation was
determined by the impedance used, which typically ranged
from 60 to 300 GW in our experiments. High impedance was
used throughout the STM experiment to avoid large fluctua-
tions in tip movement and prevent mechanical damage to the
sample surface. The voltage was then ramped from positive to
negative bias while the current tunneling through both the air
gap and the molecule was recorded. These current voltage
(I V)measurements were performed at various setpoint
tunnel currents in order to minimize artifacts resulting from
strong tip sample interactions. As can be seen from Fig-
ure 2a, pronounced asymmetric I V behavior was consistent-
ly observed at various setpoint tunneling currents, indicative
of intrinsic molecular p-n junction character in the diblock
molecule. In the STM convention, the direction of easy
electron flow corresponds to electron tunneling from the STM
Figure 1. a)Surface pressure area isotherms of conjugated diblock
oligomer 20 (red solid line)and the reference model oligomer 21 (blue
dashed line). Monolayers of 20 and 21 were transferred to a hydrophilic
gold substrate at 35 mNmÀ1 and 28 mNmÀ1, respectively, 5 mNmÀ1 below
collapse pressure. b)Plausible orientation of a LB monolayer of diblock
molecules on a hydrophilic gold surface modified with a thioglycolic acid
monolayer.
transition from the initially flat-lying conformation of the
monolayer to an increasingly more perpendicular orientation.
This is followed by a monotonic increase in surface pressure
with a characteristically steeper slope (solid-state region).
Close packing of the molecules continues until collapse of the
monolayer was observed at approximately 80 ä2 molÀ1. Ex-
trapolation of the linear portion of the solid-state region of the
isotherm to zero surface pressure gives an estimate of the area
ideally occupied by an isolated molecule (112 ä2)at infinite
dilution. This value is in fairly good agreement with that
determined (115 ä2)from the space-filling model of the
AM1-optimized geometry in the gas phase (Figure 1b). The
Langmuir monolayer was subsequently transferred onto glass,
silicon wafer, or hydrophilic gold substrates with a transfer
ratio of 1.0 ꢀ 0.05. The LB monolayer was fully characterized
by optical ellipsometry and UV/Vis spectroscopy. After
careful drying of the sample, the electrical properties of the
monolayer were investigated by using scanning tunneling
Figure 2. a)Current voltage plots for a LB monolayer of diblock oligomer
20 measured with the tunneling current setpoint at different values (Iset ¼ 2
(red), 4 (green), 6 (blue), 8 (purple), 10 pA (light blue)) and a bias voltage
of þ600 mV. The inset shows the I V characteristics of a gold surface
modified with a thioglycolic acid monolayer. b) I V curves for a LB
monolayer of the reference oligomer 21 measured with the tunneling
current setpoints at different values (Iset ¼ 1 (red), 2 (green), 4 (blue), 6
(orange), 8 (purple), 10 pA (light blue)) and a bias voltage of þ 600 mV.
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tip to the acceptor part (oligothiazole)of the sample. The
average turn-on voltage in the positive sample bias region is
about 0.65 0.75 eV, whereas only small current signals were
recorded in the reverse bias. The rectification ratio remains
small in the low-bias regimes but increases gradually to about
18 at a bipolar bias of 1.0 V. The slight differences in turn-on
voltages at various current setpoints result mainly from the
difference in tunneling gap resistances between the tip and
molecules, as was previously observed.[16] The differential
conductance (dI/dV)was obtained from the data in Figure 2a,
which is roughly proportional to the local density of states
(LDOS)of the sample. It is a good measure of the rectifying
behavior of the diblock molecules. Large asymmetry is
observed in the corresponding conductance versus voltage
spectra with threshold voltages at a bipolar bias of about 0.7 V
(Figure 3a). It is clear that the direction of easy electron flow
is from the tip to the thiazole block and then to the thiophene
block. This is consistent with the fact that the thiophene block
is electron donating and the thiazole block is electron
accepting. In the excited state the thiazole block would bear
negative charge, the thiophene block positive charge, and the
electrons would flow from the thiazole to the thiophene block.
Many rectifying diodes based on molecular systems have
been reported previously, but few are truly of molecular
6]
origin.[2 In order to exclude contributions to the observed
I Vasymmetry from other effects such as molecule interfacial
effect between the LB monolayer and the underlying
thioglycolic acid monolayer, the difference in the work
functions of the electrodes (Au substrate and Pt/Ir tip), or
other poorly controlled experimental parameters, control LB
films of the reference molecule were prepared and subjected
to STS studies under the same experimental conditions as
described above. The spectroscopic measurements revealed
only symmetric I V behavior at various setpoint currents
(Figure 2b). The corresponding tunneling differential con-
ductance clearly shows the molecular wire behavior, again
with threshold turn-on voltages depending upon the preset
tunneling gap resistance (Figure 3b). The electron-transfer
mechanism (tunneling or through-bond transfer)for the
rectifying effect observed in our diblock system is not
completely clear at this stage; further theoretical treatment
to clarify this point is in progress.
In summary, an amphiphilic conjugated diblock oligomer
exhibiting a molecular p-n junction has been synthesized and
its rectification effect demonstrated. The synthetic approach
developed is rather general for coupling heterocycles and
should lead to the synthesis of other p-n junction molecules.
The clear distinction between the I V behaviors for the
diblock and the reference molecules in the present study
constitutes concrete evidence for the molecular origin of the
observed rectifying effect.[17] The rectifying conjugated mol-
ecule described in this report provides an easy entry to
molecular-scale electronic components for the design of logic
circuits. Many variations in structure and electronic properties
of this diblock system can be envisaged, and optimizations of
these features are the subject of ongoing efforts.
Received: May 6, 2002 [Z19225]
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10 pA (light blue)). Each curve has been offset in the vertical direction for
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