Synthesis, optical, and electrochemical properties of a new family of dendritic oligothiophenes

Article abstract of DOI:10.1021/jo0619581

(Figure Presented) A new class of semi-flexible dendrimers with oligothiophene (OT) arms up to the third generation have been synthesized and investigated. The synthetic methods employed include a combination of palladium-catalyzed Stille cross-coupling reactions for oligothiophenes, Sonogashira cross-coupling reactions for building blocks, and carbodiimide-mediated esterification for building up the various dendrimers. The optical and electrochemical properties of this series of oligothiophenes-based dendrimers are shown to be strongly influenced by their morphologies as demonstrated by their pronounced solvatochromic and thermochromic responses under different environmental conditions. Introducing rigid oligothiophene arms to shape non-persistent ester-linked dendrimers causes higher generation dendrimers (G2 and G3) to exhibit solvatochromism and thermochromism, while their oligomeric counterpart (3b) and lower generation (G1) analogue do not. Spectroscopic changes due to both intramolecular and intermolecular aggregations are observed.

Full text of DOI:10.1021/jo0619581

Synthesis, Optical, and Electrochemical Properties of a New Family
of Dendritic Oligothiophenes
Yong Zhang,‡ Chunchang Zhao,† Jinhua Yang,† Mbiya Kapiamba,† Olesya Haze,†
Lewis J. Rothberg,*,†,‡ and Man-Kit Ng*,†
Department of Chemistry, and Department of Chemical Engineering, UniVersity of Rochester,
Rochester, New York 14627
rothberg@chem.rochester.edu; ng@chem.rochester.edu
ReceiVed September 21, 2006
A new class of semi-flexible dendrimers with oligothiophene (OT) arms up to the third generation have
been synthesized and investigated. The synthetic methods employed include a combination of palladium-
catalyzed Stille cross-coupling reactions for oligothiophenes, Sonogashira cross-coupling reactions for
building blocks, and carbodiimide-mediated esterification for building up the various dendrimers. The
optical and electrochemical properties of this series of oligothiophenes-based dendrimers are shown to
be strongly influenced by their morphologies as demonstrated by their pronounced solvatochromic and
thermochromic responses under different environmental conditions. Introducing rigid oligothiophene arms
to shape non-persistent ester-linked dendrimers causes higher generation dendrimers (G2 and G3) to
exhibit solvatochromism and thermochromism, while their oligomeric counterpart (3b) and lower
generation (G1) analogue do not. Spectroscopic changes due to both intramolecular and intermolecular
aggregations are observed.
Introduction
polyamide synthesis.2 These dendritic macromolecules have
been widely investigated for catalytic and therapeutic applica-
tions because of their solubility, flexibility, and biocompatibil-
ity.3 On the other end of the spectrum, Moore and co-workers
designed a series of phenylacetylene-based dendrimers and
developed the concept of “shape persistent” dendrimers associ-
ated with rigid, conjugated backbones.4a,b These molecules
combined many useful properties of dendrimers such as
uniformity and processability with those of semiconducting
conjugated materials such as charge transport and strong
luminescence efficiency.4
Dendrimers have a regular, highly branched, monodispersed,
and treelike globular architecture that differentiates them from
linear, branched, or cross-linked polymers and endows them
with distinct chemical, physical, and functional properties.1
Dendrimers can be characterized by whether the linking unit
renders them flexible (“shape non-persistent”) or rigid (“shape
persistent”). Most of the Fre´chet-type, ester- and amide-based
dendrimers are essentially flexible due to the presence of
benzylic ether, ester, or amide linkages that are conveniently
accessible by classical synthetic methodologies, including but
not limited to the Williamson ether synthesis, polyester, and
One of most investigated families of conjugated molecules
is the oligo- and polythiophenes, which have attracted intensein-
terest for applications in many promising organic electronic
† Department of Chemistry.
‡ Department of Chemical Engineering.
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10.1021/jo0619581 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/14/2006
J. Org. Chem. 2006, 71, 9475-9483
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Zhang et al.
SCHEME 1. Synthetic Routes for Dimeric and Tetrameric Oligothiophenes 2a-d and 3a-ca
a Reagents and conditions: (i) C6H13MgBr, 0.12 mol % Ni(dppp)Cl2, Et2O, reflux; (ii) NBS, CHCl3/HOAc, rt; (iii) (1) LDA, THF, 0 °C; (2) dry ice
(CO2), -78 °C; (iv) MeOH, cat. H2SO4, reflux; (v) 0.5 mol % Pd(PPh3)2Cl2, 1 mol % PPh3, DMF, 110 °C; (vi) (1) n-BuLi, THF, 0 °C; (2) Bu3SnCl; (vii)
NIS, CHCl3/HOAc, rt; (viii) TBAF, THF, rt.
devices,5 such as organic light-emitting diodes (OLEDs),6 field-
effect transistors (OFETs),7 photovoltaic devices (OPVs),8
photoconductive devices,9 and sensors.10 The optical and
electronic properties of oligothiophenes and polythiophenes are
very sensitive to conformation and morphology, and these
properties can be modified chemically11 through judicious
structural variation. Until recently, relatively few types of
branched or dendritic thiophene systems have been synthesized
and investigated. Advincula and co-workers have synthesized
rigid thiophene-based dendrimers that exhibited unique su-
pramolecular assembling properties of potential interest in
molecular electronics.12 Because of their rigid structures, the
optical and semiconducting properties of the conjugated den-
drimers studied to date are primarily sensitive to intermolecular
packing and are not subject to intramolecular interactions.13
In the present paper, we report on the synthesis and
characterization of a series of dendrimers with oligothiophene
arms connected by free rotating ester linkage cores. To improve
the solubility and processability of our dendritic oligothiophene
systems, a hexyl substituent is introduced on every other
thiophene unit in a pseudo “head-to-tail” coupled manner
(analogous to those in highly regioregular head-to-tail coupled
oligo- and poly-3-alkylthiophenes) with the aim of creating a
perfectly regioregular microstructure in each arm. In addition,
unlike the fully rigid thiophene-based dendrimers12 discussed
above, these macromolecules have labile morphology resulting
in properties highly sensitive to solvent composition (solvation)
and temperature changes. The oligothiophene arms can function
both as the rigid scaffold and as the internal “reporters” of the
dendrimer morphology. We also show how intramolecular
interaction reversibly affects their optical and electrochemical
behavior (solvatochromic and thermochromic properties) in
dilute solutions.
Results and Discussion
Synthesis and Characterization. The synthetic approach
used for various regioregular oligothiophenes (2a-d, 3a-c) is
outlined in Scheme 1. Palladium-catalyzed Stille reaction14 was
employed for the cross-coupling of R-bromo- or R-iodo-
thiophene with an appropriate thiophene organostannane to
generate dimers and tetramers with moderate yields of around
50-70% after purification by silica gel flash chromatography.
In the present work, using anhydrous DMF instead of toluene
or THF as a solvent for the cross-coupling reactions gave a better
yield and fewer side products. The tert-butyldimethylsilyl (TBS)
protecting group installed on the 5-position of several oligoth-
iophene intermediates (2a, 2b, and 3a), which served to achieve
regioregularity in a pseudo “head-to-tail” coupled fashion, was
found to be much more stable than a trimethylsilyl (TMS)
protecting group. No desilylation was observed during the
purification steps in the synthesis of these TBS-substituted
intermediates. Tetrabutylammonium fluoride (TBAF) was used
for the clean removal of the TBS protecting group, providing
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9476 J. Org. Chem., Vol. 71, No. 25, 2006

Properties of a New Family of Dendritic Oligothiophenes
SCHEME 2. Synthetic Routes for G1 and Dendrimer Building Block 6a
a Reagents and conditions: (i) AlCl3, benzene, reflux; (ii) (triisopropylsilyl)acetylene, 3 mol % Pd(PPh3CN)2Cl2, 6 mol % [(t-Bu)3PH]BF4, 2 mol % CuI,
2.5 equiv of HN(i-Pr)2, dioxane, rt; (iii) TBAF, THF, rt; (iv) 2 mol % Pd(PPh3)2Cl2, 1 mol % CuI, Et3N, reflux; (v) DHP, PPTS, CH2Cl2, 40 °C; (vi) NaOH,
EtOH/THF/H2O, reflux.
quaterthiophene ester 3b in 77% yield. It is worth mentioning
that the introduction of methyl ester functionality at the terminal
R-position facilitated chromatographic purification by increasing
polarity and provided a useful scaffold from which to construct
the dendrimers. Side products resulting from the homo-coupling
of iodo-thiophene (around 20%) were always observed under
these typical reaction conditions, and they could be readily
isolated by silica gel chromatography. Nevertheless, this iterative
synthetic strategy14b could be repeated to provide hexamer,
octamer, and other regioregular head-to-tail coupled oligoth-
iophenes with longer conjugation length.
Scheme 2 describes the methods to synthesize G1 and
building block 6. We chose 3,5- dibromophenol instead of 3,5-
diiodophenol as the precursor of the core 3,5-diethynylphenol
(5b) because the former was easier to synthesize, and we
prepared it according to a literature procedure.15a However, the
palladium-mediated Sonogashira cross-coupling of 3,5-dibro-
mophenol with (triisopropylsilyl)acetylene to obtain 5a proved
challenging. Typical workhorse palladium catalysts/ligands
combination such as Pd(PPh3)2Cl2/PPh3, Pd(PPh3)4, or Pd2(dba)3/
PPh3 all proved unsuccessful as they only generated monosub-
stituted instead of disubstituted product because of the reactivity
difference between bromo- and iodo-substrates. Gratifyingly,
the use of Fu’s recipe consisting of Pd(PhCN)2Cl2 and tri-tert-
butylphosphonium tetrafluoroborate as reported in the literature15b
eventually provided the desired cross-coupled product and led
to core 5b (following a TBAF-induced desilylation) in an overall
59% yield over two steps. G1 was also produced by palladium-
catalyzed Sonogashira cross-coupling of iodo-substituted qua-
terthiophene 3c with diethynyl core 5b. Starting from G1,
building block 6 was generated by hydrolyzing the tetrahydro-
pyranyl (THP)-protected G1 in good yield (82%). Higher
generation dendrimers were prepared using an esterification
reaction between phenol functionality at the focal point of Gn
and the dicarboxylic acid functional groups on the periphery of
building block 6.
The convergent assembly of dendrimers is outlined in Scheme
3. There were two main reasons for choosing esterification as
the key reaction for building dendrimers. One is the poly(ester)
dendrimer synthesis, which has been well developed, and
efficient condensation reagents are available. Instead of using
a rapid orthogonal coupling strategy,1d we followed the tradi-
tional methodology of repeating coupling and activation steps
enabling us to work under mild conditions, while retaining easy
purification and good yield. Condensation reactions16 between
oligothiophene carboxylic acid and phenol were used to furnish
up to the third generation ester-linked thiophene dendrimers
under neutral conditions. Furthermore, ester linkage provided
the whole molecule with a mobile structure to adopt different
conformations in response to environmental changes.
Dendrimers G1-G3 are soluble in common organic solvents
such as chloroform, tetrahydrofuran, chlorobenzene, and 1,2-
dichlorobenzene. The singlet peak signal at 3.89 ppm in the 1H
NMR spectra of various ester-substituted intermediates and
dendrimers can be assigned to the peripheral methyl ester
protons, while the region of 2.7-2.8 ppm is assignable to the
methylene protons on the hexyl side chain directly linked to
thiophene rings for all intermediates and dendrimers. In G1,
these two sets of signals have a ratio of 6:8, in G2 12:24, and
in G3 24:56, consistent with the number of peripheral ester CH3
protons and alkyl CH2 protons, confirming the successful growth
and structural uniformity of various dendrimers synthesized. In
13C NMR spectra, the diagnostic signals in the range of 80-95
ppm correspond to absorptions by alkyne carbons. In G1, there
are only two types of alkyne carbons with an integrated ratio
of 2:2, while in G2 and G3, there are essentially four types of
alkynyl carbons, and the ratios are 2:4:4:2 and 2:12:12:2,
respectively, proportional to the number of expected alkynyl
carbons in these molecules.
(13) For an interesting work on photochromic dendrimers containing an
azobenzene linkage in the internal dendrimer structure that is capable of
undergoing structural reorganization, see: Liao, L.-X.; Stellacci, F.;
McGrath, D. V. J. Am. Chem. Soc. 2004, 126, 2181-2185.
(14) (a) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. ReV. 2002, 102, 1359-1469. (b) Ng, M.-K.; Wang, L.; Yu, L. Chem.
Mater. 2000, 12, 2988-2995. (c) Zhao, C.; Zhang, Y.; Wang, C.; Rothberg,
L. J.; Ng, M.-K. Org. Lett. 2006, 8, 1585-1588.
(15) (a) Lin, C.; Tour, J. J. Org. Chem. 2002, 67, 7761-7768. (b)
Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295-4298.
Photophysical Properties. Figure 1 shows the absorption
spectra of dendrimers G1, G2, G3, and quaterthiophene 3b in
a dilute chloroform solution (good solvent) with the same
(16) Messmore, B. W.; Hulvat, J. F.; Sone, E. D.; Stupp, S. I. J. Am.
Chem. Soc. 2004, 126, 14452-14458.
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Zhang et al.
SCHEME 3. Convergent Synthetic Routes for Dendrimers G2 and G3 with Quaterthiophene Armsa
a Reagents and conditions: (i) DIPC, DPTS, CH2Cl2, reflux; (ii) 3 M HCl, THF, 40 °C; (iii) DIPC, DPTS, CHCl3, reflux.
concentration (4 × 10-6 M). Quaterthiophene 3b has an
absorption maximum at 395 nm with a molar extinction
coefficient ꢀ of 4.7 × 104 M-1 cm-1. It is noticed that G1’s
absorption shows a moderate red shift (λmax ) 415 nm) relative
to quaterthiophene 3b due to a slight elongation in effective
conjugation length upon coupling to a meta-linked phenyl bis-
(acetylenic) unit. Given the “meta”-like relationship between
the two quaterthiophene arms in G1, the effect of electronic
delocalization due to cross-conjugation between the individual
oligothiophene moieties is not expected to be significant. G1
has approximately doubled quaterthiophene 3b’s molar extinc-
tion coefficient because it has two quaterthiophene moieties per
molecule. On the contrary, both of the higher generation
dendrimers G2 and G3 exhibit λmax values at around 420 nm,
only barely red-shifted by another 5 nm with respect to that of
G1. Given the fact that the absorbance of higher generation
G3 is much greater than 1.0 at c ) 4 × 10-6 M, additional
spectroscopic measurements were performed on both G2 and
G3 at different concentration regimes. It was unambiguously
confirmed that λmax and molar absorption coefficient values of
9478 J. Org. Chem., Vol. 71, No. 25, 2006

Properties of a New Family of Dendritic Oligothiophenes
TABLE 1. Photophysical Properties of Quaterthiophene 3b and Dendrimers G1, G2, G3
abs
em
abs
b
λmax
nm
λmax
nm
ꢀ
ratio to ꢀ
of 3b
λonset
Eg
compound
×105 M-1 cm-1
Φfa
nm
eV
3b
G1
G2
G3
395
415
420
420
523
0.47
1.08
2.78
6.78
1
13%
24%
22%
14%
465
486
491
494
2.67
2.55
2.53
2.51
521, 547
545
2.3
5.9
14.4
552
a Optical absorption and emission studies were performed in chloroform, and quantum yields were determined relative to Rhodanmin 6G in methanol (Φf
) 0.94) as a standard.18a Fluorescence quantum yield measurements were performed at concentrations where A e 0.2 for all compounds. Refractive index
correction has been performed due to changing solvents. b Optical bandgaps Eg are estimated from onset absorption, which is 10% of the maximum absorption.18b
FIGURE 1. UV-vis absorption spectra of dendrimers G1, G2, G3,
FIGURE 2. Optical absorption and emission spectra of G2 in different
and quaterthiophene 3b in chloroform (c ) 4 × 10-6 M). Cartoon
structures of dendrimers are also illustrated. The ellipses represent
quaterthiophene arms, and short lines stand for the phenyl acetylene
cores.
mixtures of chloroform and methanol (c ) 1 × 10-5 M).
3b and all dendrimers are virtually independent of concentration
changes (from 10-5 to 10-7 M) typically employed in spectro-
scopic studies (Supporting Information Figure S1). Overall, the
molar extinction coefficients of various generations, within
experimental error, are proportional to the number of quater-
thiophene chromophores per dendrimer. On the basis of these
optical absorption data, one can safely assume that the quater-
thiophene moieties within each generation behave rather inde-
pendently in dilute solutions as has been reported for other
multichromophoric dendrimers in the recent literature.17
The basic photophysical data for 3b and the dendrimers are
summarized in the Table 1. The HOMO-LUMO gaps are
estimated from the absorption onset where we arbitrarily choose
an energy corresponding to where their extinction coefficients
are at 10% of their maximum values. The quantum yields for
fluorescence are determined to be 24%, 22%, and 14% for G1-
G3 in dilute chloroform solution, measured relative to Rhodamine
6G in methanol (Φf ) 0.94) as a standard.18 To minimize
extensive fluorescence quenching due to intermolecular ag-
gregations, all fluorescence quantum yield studies were carried
out in fairly dilute solutions where absorbance values are less
than 0.2, which typically correspond to c ) 10-6-10-7 M. The
gradual reduction in quantum yields for dilute solutions of G2
and G3 suggests increasingly important intramolecular fluores-
FIGURE 3. Optical absorption and emission spectra of G3 in different
mixtures of chloroform and methanol (c ) 2 × 10-6 M).
cence quenching behavior is taking place as more quater-
thiophene chromophore moieties are able to associate within
the flexible dendrimer in higher generation. The ability of the
chromophore units to aggregate internally, by virtue of the
presence of various ester linkages, makes these dendrimers
particularly sensitive to environmental conditions including
solvent composition and temperature changes as discussed
below.
Solvatochromism and Thermochromism. Notably, higher
generation dendrimers G2 and G3 were found to show rather
strong solvatochromic and thermochromic effects, in sharp
contrast to G1 and their linear oligomers. Figures 2 and 3 display
the steady-state UV-vis absorption and emission spectra of G2
and G3 in a mixture of chloroform and methanol with different
(17) (a) Chen, J.; Li, S.; Zhang, L.; Liu, B.; Han, Y.; Yang, G.; Li, Y.
J. Am. Chem. Soc. 2005, 127, 2165-2171. (b) Thomas, K. R. J.; Thompson,
A. L.; Sivakumar, A. V.; Bardeen, C. J.; Thayumanavan, S. J. Am. Chem.
Soc. 2005, 127, 373-383.
(18) (a) Jakubiak, R.; Bao, Z.; Rothberg, L. J. Synth. Met. 2000, 114,
61. (b) Tian, H.; Wang, J.; Shi, J.; Yan, D.; Wang, L.; Geng, Y.; Wang, F.
J. Mater. Chem. 2005, 15, 3026-3033.
J. Org. Chem, Vol. 71, No. 25, 2006 9479

Zhang et al.
FIGURE 4. Optical absorption and emission spectra of G1 in different
mixtures of chloroform and methanol (c ) 1 × 10-5 M).
FIGURE 6. Temperature-dependent absorption and emission spectra
of G3 in a 70:30 mixture of chloroform and methanol (c ) 2 × 10-6
M).
TABLE 2. Electrochemical Properties of Quaterthiophene 3b and
Dendrimers G1, G2, G3
Epa
Epc
E°′
E1°′
E2°′
compound
(eV)a
(eV)a
(eV)a
(eV)b
(eV)b
3b
1.04
0.99
0.87
0.84
0.87
0.88
0.74
0.68
0.96
0.94
0.81
0.76
G1
G2
G3
1.09c
1.14
1.16
1.46
1.46
a Measured in a 4:1 v/v mixture of CHCl3/CH3CN at room temperature.
Anodic (Epa), cathodic (Epc), and half-wave potentials (E°′ ) (Epa + Epc)/
2) are vs SCE (calibrated by Fc+/Fc couple). b Dendrimer thin films coated
on Pt wire working electrode and measured in CH3CN at room temperature.
Half-wave potentials (E°′ ) (Epa + Epc)/2) are vs SCE (calibrated by Fc+/
Fc couple). c Only Epa could be observed for the film of G1.
methanol leads to a morphological change of the flexible
dendrimer19 to a less mobile conformation where the thiophene
units can interact among themselves more effectively. This type
of intramolecular interaction is most pronounced in the higher
generations (G2 and G3), resulting in an improvement in
effective conjugation length, which concomitantly red shifts their
absorption and emission spectra (Figures 2 and 3). Intermo-
lecular π-π stacking interactions and interchain excitations
cause a concomitant reduction in fluorescence quantum yield,
as has been demonstrated in analogous conjugated systems.20,21
Lower generation G1 and quaterthiophene 3b lack the internal
ester core linkages and are therefore unable to reorganize
intramolecularly to support strong interactions between quater-
thiophene moieties and therefore do not exhibit any observable
solvatochromic behavior under the same conditions.
FIGURE 5. Temperature-dependent absorption and emission spectra
of G2 in a 70:30 mixture of chloroform and methanol (c ) 1 × 10-5
M).
composition. Methanol is a poor solvent for the rigid oligoth-
iophenes, and increasing its proportion in the solvent mixture
results in an apparent red spectral shift to longer wavelength
accompanied by a concomitant color change from yellow to
red. The absorption maximum bathochromically shifts by 22
nm for G2 and 27 nm for G3 upon going from 100% chloroform
to a mixture consisting of 60% chloroform and 40% methanol.
No aggregation or precipitation was observed with increasing
methanol content, and no further shifting of λmax values was
observed after methanol is increased up to 40%. The fluores-
cence red-shifting is somewhat in line with those observed for
the absorption, and the spectral change is accompanied by a
continuous and dramatic reduction in the fluorescence quantum
yield with increasing methanol content. In sharp contrast to both
G2 and G3, lower generation G1 and linear quaterthiophene
3b do not exhibit any observable solvatochromism under the
same conditions. Figure 4 shows UV-vis absorption and
emission spectra of G1 in a similar mixture of chloroform and
methanol. The fact that the spectral changes for both G2 and
G3 occur in extremely dilute solutions (∼10-5-10-7 M) and
do not occur in G1 or 3b leads us to conclude that the spectral
changes correspond to intramolecular interaction rather than
intermolecular aggregation typical of conjugated polymers.
Presumably, increasing the proportion of poor solvents such as
To lend further support to our argument, reversible thermo-
chromic behaviors were also observed in solutions of both G2
and G3, changing their color from red to yellow with increasing
temperature and reverting back to red when the temperature was
lowered in a reversible manner. Figures 5 and 6 display the
(19) Likos, C. N.; Ballauff, M. Top. Curr. Chem. 2005, 245, 239-252.
(20) (a) Le´vesque, I.; Leclerc, M. Synth. Met. 1997, 84, 203-204. (b)
DiCe´sare, N.; Belleteˆte, M.; Raymond, F.; Leclerc, M.; Durocher, G. J.
Phys. Chem. A 1997, 101, 776-782. (c) DiCe´sare, N.; Belleteˆte, M.; Leclerc,
M.; Durocher, G. Synth. Met. 1998, 98, 31-40.
(21) (a) Collison, C. J.; Rothberg, L. J.; Treemaneekarn, V.; Li, Y.
Macromolecules 2001, 34, 2346-2352. (b) Schenning, A. P. H. J.;
Kilbinger, A. F. M.; Biscarini, F.; Cavallini, M.; Cooper, H. J.; Derrick, P.
J.; Feast, W. J.; Lazzaroni, R.; Lecle`re, Ph.; McDonell, L. A.; Meijer, E.
W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002, 124, 1269-1275.
9480 J. Org. Chem., Vol. 71, No. 25, 2006

Properties of a New Family of Dendritic Oligothiophenes
FIGURE 7. (Right) Cyclic voltammograms of G1, G2, and G3 recorded in a 4:1 v/v mixture of CHCl3/CH3CN containing TBAPF6 (0.02 M) as
electrolyte at room temperature. The scan rate was 100 mV/s. Potentials are reported versus SCE and calibrated by Fc/Fc+ internal standard. Curves
have been offset vertically for clarity. (Left) Cyclic voltammograms of G1, G2, and G3 thin films deposited on Pt wire (1 mm diameter) electrodes
recorded in CH3CN containing TBAPF6 (0.1 M) as electrolyte at room temperature. The scan rate was 100 mV/s. Potentials are reported versus
SCE and calibrated by Fc/Fc+ internal standard.
temperature dependence of the absorption and emission spectra
of G3 and G2 in a 70/30 mixture of CHCl3/MeOH. As indicated
above, a solution of G3 in this particular mixed solvent system
exhibits an absorption maximum (λmax) value of 440 nm at near
room temperature (20 °C). Upon heating to 50 °C, the λmax value
was gradually blue-shifted by 20 nm, and, on the other hand,
while being cooled to 10 °C, the λmax was slightly red-shifted
to 442 nm. It is also clear that at the same time the emission
intensity of G3 in solution is dramatically and continuously
reduced along with decreasing temperature (Figure 6). This
process is fully reversible and can be repeated many times in
solution. Although essentially the same trend was observed for
G2 solution, no such thermochromic behavior could be observed
for G1 with any solvent composition within this temperature
range (0-50 °C). The thin-film absorption spectra of these
dendritic molecules have also been measured (Supporting
Information Figure S2), and all dendrimers show essentially the
same λmax at 437 nm in the solid state, which are bathochro-
mically shifted from their respective λmax in chloroform. The
overall effect of going from solution to solid state results in a
presumably higher degree of aggregation of the oligothiophene
units, analogous to what was observed in bad solvent systems
as well as when the solutions were cooled below room
temperature.
Electrochemical Properties. The redox behaviors of den-
drimers G1-G3 in solution and as thin films were investigated
by cyclic voltammetry. All potentials have been calibrated with
ferrocene/ferrocenium (Fc/Fc+) couple as the internal standard
and are reported versus saturated calomel electrode (SCE). The
electrochemical data are summarized in Table 2. These dendritic
materials demonstrate dramatic differences in both solid and
solution states because of their different structures and mor-
phologies. As shown in Figure 7, at room temperature in a 4:1
v/v mixture of CHCl3/CH3CN (c ) 10-4 M), the quater-
thiophene 3b and dendrimers G1-G3 showed quasi-reversible
redox behavior with decreasing anodic half-wave potentials Eï′
at 0.96, 0.94, 0.81, and 0.76, respectively. These results are fully
consistent with our optical absorption data, indicating a slight,
if not significant, improvement in effective conjugation length
as the generation increases.
Unlike the solution cases, the oligothiophene dendrimer thin
films cast on Pt electrodes behave electrochemically differently
in the -200 to 1500 mV range (vs SCE). While the film of G1
shows only one anodic wave, those of G2 and G3 can undergo
two successive oxidation processes due to the presence of a
larger number of oligothiophene units. One also notices that
there are significant increases of anodic potentials Epa values
from G1 to G2 (or G3), reflecting the fact that the compact
conformations of higher generation dendrimers (G2 and G3)
are able to inhibit electrolyte diffusion from solution to the
electrode surface. This observation indicates that the film
morphology indeed plays an important role in their electro-
chemical behavior.
In summary, we have designed and synthesized a new type
of dendrimer with oligothiophene arms up to the third genera-
tion. The synthetic strategy is highly repetitive and includes
Stille cross-coupling reactions for oligothiophenes, Sonogashira
cross-coupling reactions for building blocks, and esterification
reactions for the dendrimer assembly. The optical and electro-
chemical properties of these series of dendrimers are found to
be strongly influenced by their morphologies. The systematic
incorporation of rigid oligothiophene arms to shape non-
consistent ester-linked dendrimers causes higher generation ones
(G2 and G3) to exhibit thermochromism and solvatochromism
even though their oligomeric counterpart (3b) and lower
generation (G1) analogue do not. Their interesting solvatochro-
mic and thermochromic behaviors are comparable to those of
typical oligo- and polythiophenes. These structurally well-
J. Org. Chem, Vol. 71, No. 25, 2006 9481

Zhang et al.
hydroxide (0.029 g, 0.74 mmol) were added to a solution of the
resulting G1 in THF (10 mL), and the mixture was refluxed for 2
h. The solvent was removed under vacuum, and the residue was
acidified with acetic acid and extracted with ethyl acetate. The
organic phase was washed with DI water, dried over anhydrous
sodium sulfate, and solvent was removed in vacuo. The residue
was further dried in a vacuum oven to afford building block 6 (0.32
defined molecules are currently studied at the molecular level
by immobilization of the various dendritic molecules as mono-
layers on glass surface, to gain a better understanding of the
mechanism responsible for the spectral changes in different
morphological states.
1
Experimental Section
g, 82% over two steps) as a red powder. H NMR (d6-DMSO, δ
ppm): 0.86 (12H, br), 1.21-1.42 (24H, br), 1.52-1.70 (12H, br),
1.80 (2H, m), 2.75 (8H, t), 3.68 (2H, t), 5.61 (1H, t), 7.23 (4H, m),
7.28 (2H, d), 7.33 (2H, s), 7.35 (1H, s), 7.37 (2H, d), 7.45 (2H, d),
7.56 (2H, s). 13C NMR (d6-DMSO, δ ppm): 13.4, 18.0, 21.57,
21.59, 24.3, 28.0, 28.1, 28.3, 28.6, 29.2, 29.3, 29.4, 30.5, 30.6, 61.3,
82.7, 93.0, 95.9, 119.4, 121.1, 123.2, 124.7, 125.9, 126.7, 127.3,
127.8, 132.6, 128.3, 133.4, 134.0, 135.3, 135.6, 136.9, 139.8, 141.1,
156.4, 162.1.
Synthesis of Oligothiophenes. Scheme 1 shows the synthetic
routes for the oligothiophene monomer, dimers, and tetramers.
Compounds 1a, 1b, 2b, 2d, 3a, 2-(tert-butyldimethylsilyl)-5-
(tributylstannyl)thiophene, and 2-(tributylstannyl)thiophene were
prepared according to literature procedures.14 Typical synthetic
procedures for the Stille cross-coupling reaction used to create
dimers and tetramers are provided in the Supporting Information.
Desilylated quaterthiophene 3b (X ) H) was purified by flash
chromatography over silica gel to afford a bright orange-yellow
Synthesis of Dendrimers G2 and G3. Scheme 3 shows the
synthetic approach to making various generation dendrimers. A
synthetic procedure for G2 derived from the literature16 is described
as follows: To a mixture of building block 6 (0.144 g, 0.11 mmol),
G1 (0.317 g, 0.25 mmol), and DPTS (0.312 g, 1 mmol) in
dichloromethane (40 mL) was added 1,3-diisopropylcarbodiimide
(0.126 g, 1 mmol), and the reaction mixture was heated at reflux
for 24 h. Upon cooling to room temperature, the precipitate was
filtered and washed with CH2Cl2 to afford a red powder (0.30 g,
0.08 mmol). The crude solid was redissolved in THF (10 mL) and
treated with a few drops of HCl (3 M), and stirred for 5 h. The
solvent was removed under vacuum, and the residue was further
purified by flash chromatography on silica gel (eluting with
chloroform/diethyl ether from 200:1 ratio to 100:1) to provide G2
(0.292 g, 0.079 mmol, 72% over two steps) as a red solid. 1H NMR
(CDCl3, δ ppm): 0.91 (36H, br), 1.33-1.44 (72H, br), 1.65-1.75
(24H, br), 2.76 (20H, t), 2.85 (4H, t), 3.88 (12H, s), 6.98 (2H, s),
7.05 (12H, m), 7.11 (4H, d), 7.13 (4H, d), 7.15 (2H, d), 7.18 (2H,
d), 7.12-7.25 (6H, m), 7.28 (1H, s), 7.40 (4H, s), 7.58 (2H, s),
7.63 (4H, s), 7.82 (2H, s). 13C NMR (CDCl3, δ ppm): 14.0, 22.6,
22.7, 29.1, 29.2, 29.3, 29.4, 29.5, 29.7, 30.3, 30.5, 31.6, 31.7, 52.1,
83.0, 84.3, 92.8, 93.7, 119.5, 122.2, 122.7, 124.1, 124.3, 124.6,
125.5, 126.1, 127.0, 127.1, 127.6, 127.9, 128.5, 128.8, 129.6, 129.9,
130.1, 131.4, 132.7, 132.9, 133.8, 134.1, 134.7, 134.9, 136.2, 137.5,
1
solid. H NMR (CDCl3, δ ppm): 0.89 (6H, t), 1.31-1.39 (12H,
br), 1.69 (4H, m), 2.76 (4H, m), 3.89 (3H, s), 7.04 (1H, s), 7.08
(1H, t), 7.12 (2H, s), 7.15 (1H, d), 7.33 (1H, d), 7.63 (1H, s). 13C
NMR (CDCl3, δ ppm): 14.0, 22.5, 22.6, 29.1, 29.2, 29.3, 29.4,
30.2, 30.5, 31.5, 31.6, 52.1, 123.8, 124.0, 125.4, 125.9, 126.7, 127.4,
127.5, 129.9, 130.2, 133.7, 134.2, 135.7, 136.1, 137.7, 138.2, 139.9,
140.4, 162.5. HRMS (ESI) m/z calcd for C30H37O2S4 (M + H+),
557.1671; found, 557.1655.
Synthesis of Dendrimer G1 and Building Block 6. Scheme 2
shows the synthetic routes for the core G1 and building block 6.
Compounds 4, 5a, and 5b were prepared according to literature
procedures.15
G1. A mixture of iodo-quaterthiophene 3c (4.14 g, 6.06 mmol),
compound 5b (0.358 g, 2.52 mmol), copper(I) iodide (0.0048 g,
0.0252 mmol, 1 mol %), Pd(PPh3)2Cl2 (0.035 g, 0.0504 mmol, 2
mol %), and triethylamine (80 mL) was stirred at 70 °C under
nitrogen for 20 h. After the reaction was completed, the mixture
was slowly poured into HCl (80 mL, 3 M), cooled in an ice-water
bath, and then extracted with dichloromethane. The organic phase
was washed with deionized water and brine, dried over anhydrous
sodium sulfate, and concentrated under vacuum. The residue was
further purified by flash chromatography on silica gel (eluting with
CH2Cl2/hexane from 1:1 ratio to pure CH2Cl2) to provide G1 as
an orange-yellow powder in 55% yield. 1H NMR (CDCl3, δ ppm):
0.91 (12H, br), 1.30-1.42 (24H, br), 1.65-1.68 (8H, br), 2.77 (8H,
m), 3.89 (6H, s), 5.03 (1H, s), 6.98 (2H, s), 7.05 (4H, m), 7.12
137.7, 138.0, 138.1, 138.5, 139.6, 140.1, 140.4, 141.1, 141.2, 150.4,
+
156.8, 159.8, 162.5. MALDI-TOF calcd for C208H213O13S24
,
3690.48; found, 3690.22. Anal. Calcd for C208H214O13S24: C, 67.68;
H, 5.84. Found: C, 67.48; H, 5.68.
(2H, d), 7.13 (2H, d), 7.23 (2H, d), 7.30 (1H, s), 7.63 (2H, s). 13
C
A similar procedure was used to prepare G3, also isolated as a
red solid, which was further purified by flash chromatography on
silica gel (eluting with chlorobenzene/THF from 200:1 ratio to 50:
1), yield 70% for two steps. 1H NMR (CDCl3, δ ppm): 0.90-0.92
(84H, br), 1.33-1.44 (168H, br), 1.66-1.76 (56H, br), 2.79 (44H,
t), 2.85 (12H, t), 3.89 (24H, s), 6.99 (4H, s), 7.05-7.07 (28H, br),
7.11 (16H, d), 7.17 (12H, dd), 7.23-7.25 (14H, dd), 7.28 (2H, s),
7.40 (10H, s), 7.57 (5H, s), 7.62 (8H, s), 7.82 (6H, s). 13C NMR
(CDCl3, δ ppm): 14.0, 22.5, 22.6, 29.1, 29.2, 29.4, 29.5, 29.6, 30.3,
30.4, 31.6, 31.7, 52.0, 83.2, 84.3, 92.8, 93.5, 118.6, 122.5, 122.6,
122.9, 124.2, 124.3, 124.4, 124.5, 124.7, 125.7, 127.0, 127.2, 127.7,
128.1, 128.9, 129.7, 129.9, 130.0, 130.5, 131.5, 132.7, 132.9, 133.9,
134.3, 134.9, 135.0, 135.1, 136.1, 137.6, 137.7, 137.8, 138.1, 138.2,
138.6, 139.6, 140.3, 140.6, 141.3, 141.4, 150.6, 155.9, 159.8, 162.5.
MALDI-TOF for (C484H493O29S56 + Na)+ calcd, 8582.15; found,
8582.49. Anal. Calcd for C484H494O29S56: C, 67.83; H, 5.81.
Found: C, 67.61; H, 5.98.
NMR (CDCl3, δ ppm): 14.0, 22.5, 22.6, 29.1, 29.2, 29.4, 29.6,
30.3, 30.4, 31.6, 31.7, 52.1, 83.3, 93.3, 118.4, 122.7, 124.2, 124.6,
125.7, 127.1, 127.2, 127.7, 129.7, 130.4, 132.7, 134.3, 135.0, 136.1,
137.7, 137.9, 138.1, 140.3, 141.3, 155.5, 162.6. HRMS (MALDI-
TOF) calcd for C70H73O5S8+, 1250.8746; found, 1251.0809.
Building Block 6. A mixture of G1 (0.35 g, 0.28 mmol), 3,4-
dihydro-2H-pyran (0.071 g, 0.85 mmol), and pyridinium p-
toluenesulfonate (0.0014 g, 0.0056 mmol, 2 mol %) in dichlo-
romethane (20 mL) was stirred at 40 °C under nitrogen for 12 h.
The resulting mixture was washed with deionized water, extracted
with dichloromethane, dried over anhydrous sodium sulfate, and
concentrated under vacuum. The residue was further purified by
flash chromatography on silica gel (eluting with CH2Cl2/hexane
from 1:1 ratio to 3:1) to provide THP-protected G1 as orange-red
1
flakes (0.36 g, 98%). H NMR (CDCl3, δ ppm): 0.91 (12H, br),
1.30-1.42 (24H, br), 1.62-1.71 (10H, br), 1.89 (2H, m), 1.98 (2H,
m), 2.75-2.79 (8H, m), 3.68 (2H, t), 3.89 (6H, s), 5.48 (1H, t),
7.05 (4H, m), 7.13 (4H, dd), 7.20 (2H, d), 7.23 (2H, d), 7.33 (1H,
s), 7.63 (2H, s). 13C NMR (CDCl3, δ ppm): 14.0, 18.5, 22.5, 22.6,
25.1, 28.9, 29.0, 29.2, 29.3, 29.6, 30.2, 30.4, 31.6, 31.7, 52.1, 61.9,
83.0, 93.6, 96.4, 119.5, 122.6, 124.1, 125.5, 127.0, 127.5, 127.6,
129.6, 130.1, 132.6, 134.1, 134.8, 136.1, 137.66, 137.69, 138.0,
140.1, 141.1, 156.8, 162.5. For the subsequent basic hydrolysis
reaction, ethanol (10 mL), deionized water (5 mL), and sodium
Acknowledgment. We gratefully acknowledge financial
support by the University of Rochester, the American Chemical
Society PRF Type G (#43328-G7), and the New York State
OfficeofScience,Technology,andAcademicResearch(#C050045).
Mass spectrometry was provided by the Washington University
Mass Spectrometry Resource with support from the NIH
National Center for Research Resources (Grant No. P41RR0954).
9482 J. Org. Chem., Vol. 71, No. 25, 2006

Properties of a New Family of Dendritic Oligothiophenes
Supporting Information Available: Detailed experimental
procedures for the synthesis and characterization of dendrimers and
intermediates. This material is available free of charge via the
Internet at http://pubs.acs.org.
M.-K.N. thanks the NYSTAR James D. Watson Investigator
Program (2005). L.J.R. wishes to thank the National Science
Foundation (award number DMR0513416). O.H. is the recipient
of a summer fellowship through the NSF-REU program (2004).
We thank Prof. J. P. Dinnocenzo for helpful discussions.
JO0619581
J. Org. Chem, Vol. 71, No. 25, 2006 9483