Synthesis and energy-transfer properties of hydrogen-bonded oligofluorenes

Article abstract of DOI:10.1021/ja052054k

A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki) couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via quadruple hydrogen bonds, were attached to both ends of the oligof

Full text of DOI:10.1021/ja052054k

Published on Web 07/29/2005
Synthesis and Energy-Transfer Properties of
Hydrogen-Bonded Oligofluorenes
Stephen P. Dudek, Maarten Pouderoijen, Robert Abbel,
Albertus P. H. J. Schenning,* and E. W. Meijer*
Contribution from the Laboratory of Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands
Received March 31, 2005; E-mail: a.p.h.j.schenning@tue.nl; e.w.meijer@tue.nl
Abstract: A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki)
couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via
quadruple hydrogen bonds, were attached to both ends of the oligofluorenes. The resulting bis-UPy-
terminated oligomers self-assemble into supramolecular chain polymers. For comparison, oligofluorenes
of the same oligomer lengths but without terminal hydrogen-bonding groups were synthesized. Chains of
hydrogen-bonded fluorenes can be simply endcapped by a variety of chain stoppers, molecules that have
one UPy group. In this manner, we have endcapped the hydrogen-bonded fluorene chains with either
oligo(p-phenylenevinylene) or perylene bisimide. Energy-transfer experiments in solution and the solid state
demonstrate that oligofluorenes can donate energy to a variety of energy acceptors, but that this energy
transfer occurs most effectively when the donor fluorene is hydrogen-bonded to the acceptor.
Introduction
hydrogen-bonding groups without having phase segregation
between the energy donor and acceptor material. To demonstrate
π-Conjugated polymers1 have been extensively investigated
as alternatives to inorganic semiconductor components in
devices such as field-effect transistors, solar cells, and light-
emitting diodes. Polymers, however, can suffer from inhomo-
geneities such as chain defects that may limit electronic
performance. Instead, many groups2,3 have synthesized and
studied defect-free π-conjugated oligomers to determine how
the electronic properties of a material change with chain
length.3,4 Yet, it can be difficult to synthesize oligomers that
are long enough to have mechanical properties useful for
incorporation into devices. Short oligomers often need to be
vacuum-deposited and do not readily form films.
this concept of modularity, we have chosen to work with
oligomers of fluorene (Chart 1). Chiefly explored for blue light-
emitting diodes, oligo- and polyfluorenes7 are thermally stable
and have high fluorescence quantum yields.8-13 2-Ureido-4[1H]-
pyrimidinone (UPy)14 groups were attached to the ends of a set
of oligofluorenes (Chart 1, UPy-OFn-UPy). The UPy group
dimerizes strongly via four hydrogen bonds with a dimerization
constant of 6 × 10-7 M in chloroform15 (Chart 1). The blue
emissive bis-UPy oligofluorenes can be endcapped with either
UPy-terminated oligo(p-phenylenevinylene) or perylene bis-
imide, green and red emissive dyes, respectively (Chart 1).
The well-defined electronic properties of the monodisperse
oligomer and the processability of a polymer can be combined
using supramolecular polymers composed of oligomers. Previ-
ously, we attached a conjugated oligo(p-phenylenevinylene) to
aliphatic molecules bis-terminated with self-complementary
hydrogen-bonding groups,5 resulting in supramolecular hydrogen-
bonded polymers useful in photovoltaic devices.6 One advantage
of supramolecular polymers is their modularity, permitting
tuning of the polymer’s electronic properties. For example, the
emission wavelength in supramolecular polymers can be
adjusted by mixing in other emissive units functionalized with
(7) (a) Fukuda, M.; Sawada, K.; Yoshino, K. J. Polym. Sci., Part A: Polym.
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Chem. Mater. 2002, 14, 1332-1339. (c) Geng, Y.; Culligan, S. W.;
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A. J. Nature 1992, 357, 477-479.
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(3) Meier, H.; Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379-384.
(4) Klaerner, G.; Miller, R. D. Macromolecules 1998, 31, 2007-2009.
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J. AM. CHEM. SOC. 2005, 127, 11763-11768
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A R T I C L E S
Dudek et al.
Chart 1. Chemical Structure of the Synthesized Oligofluorenes
(OFn), Bis-UPy-Terminated Oligofluorenes (UPy-OFn-UPy),
UPy-Terminated Oligo(p-phenylenevinylene) (UPy-OPV), and
UPy-Terminated Perylene Bismide (UPy-Pery)
coupling, after which it can be converted back to a halogen
with iodo monochloride. To expedite the synthesis of longer
oligomers, asymmetric fluorene dimers were synthesized. Fluo-
rene dimers 7 and 12 are generated along with symmetric
fluorene trimers from Suzuki couplings (Scheme 1); the
asymmetric dimers can be readily separated from the symmetric
trimers by silica gel column chromatography. Coupling of dimer
fragments instead of monomer fragments to both ends of the
fluorene core reduces the number of coupling steps required to
make longer oligomers.
Bis-UPy-oligofluorenes were synthesized by attachment of
activated ureidopyrimidinones to bis-NH2-terminated oligo-
fluorenes (Scheme 2). Since the palladium-catalyzed coupling
of fluorenes did not proceed in the presence of either the UPy
or NH2 functional group under standard Suzuki conditions, an
amine-protecting group was sought. Although there are several
ways to introduce amine functionality on fluorenes, nitration18
of a fluorene monomer followed by reduction of the nitro group
after the oligofluorene chain is constructed was chosen as the
most robust and simplest method.19 Nitration of oligofluorenes
longer than one unit generated products that were multiply
nitrated along the chain and not just at the desired terminal
positions. Therefore, first a fluorene monomer building block
was singly nitrated (14), next the nitro monomer was incorpo-
rated into the oligofluorene chain by Suzuki coupling, and finally
the bis-nitro-oligofluorene was reduced to the bis-amine coun-
terpart as a penultimate step. Reduction of the bis-nitro-fluorene
monomer and trimer proceeded smoothly by hydrogenation on
palladium/carbon, but this method was too sluggish for the
longer oligomers. For the pentamer and heptamer, SnCl2
reduction proved more effective. One main advantage of this
stepwise conversion of bis-nitro-terminated oligofluorenes to
bis-amines and then to bis-ureidopyrimidinones is the successive
change in polarity of the products after each transformation.
The increase in polarity easily permits separation by column
chromatography of the desired products from undesired non-
or singly functionalized oligomers.
Energy-transfer experiments demonstrate that these oligofluo-
renes can efficiently donate energy to both energy acceptors.
Previously, we reported energy donor-acceptor systems con-
nected by (uriedo)triazine hydrogen bonds.16,17 However, this
hydrogen-bonded system required additional π-stacking between
the donor and acceptor groups for room temperature stability.
The dimerization constant of the UPy groups is more than 100
times stronger than that for uriedotriazine systems, permitting
polymer assemblies based purely on hydrogen bonding.
The syntheses of the hydrogen-bonded oligophenylene-
vinylene (UPy-OPV, Chart 1) and perylene bisimide acceptors
(UPy-Pery, Chart 1) are based on previously reported synthetic
routes (Scheme 3).
The OFn and UPy-OFn-UPy and intermediate products were
Results and Discussion
1
characterized by standard techniques such as H NMR, 13C
Synthesis. The synthesis of simple, nonterminated oligo-
fluorenes has been reported by several research groups,9 most
notably Tsutsui,10 Vial,11 and Chen.12 A similar synthetic
strategy was adopted here to construct the fluorene backbone
of both the hydrogen- and UPy-terminated oligofluorenes (OFn,
UPy-OFn-Upy, Chart 1, Schemes 1 and 2). The fluorene chain
was elongated by stepwise Pd(0) (Suzuki) coupling of two
asymmetric fluorene monomers to both ends of a symmetric
bis-functionalized mono- or oligofluorene core. The trimethyl-
silyl (TMS) group serves as a halogen-protecting group: the
trimethylsilyl group can be first introduced through lithiation
of a bromofluorene followed by addition of trimethylsilyl
chloride; the TMS group is tolerant of the pursuant Suzuki
NMR, MALDI-TOF, IR, and elemental analysis (see Supporting
Information). The hydrogen-terminated oligofluorenes are floc-
culent white powders; the bis-UPy-terminated oligofluorenes
consist of pale brown thin films suggesting substantial inter-
molecular hydrogen bonding. Likewise, the OFn is sufficiently
soluble in common organic solvents such as THF or CHCl3,
while the UPy-OFn-UPy has low solubility in these solvents.
The solubility of UPy-OFn-UPy in chloroform increases with
chain length. On addition of a small amount of a competing
hydrogen bond source such as trifluoroacetic acid (TFA) or
(18) Halles, G.; Hepworth, J. D.; Waring, D. R. J. Chem. Soc. B 1970, 975-
979.
(19) Incorporation of amine functionality was also attempted via attachment of
phthalimide (Gabriel reaction) to bromofluorenes followed by hydrazine
cleavage of the phthalimide, but this route led to lower yields and incomplete
cleavage of the phthalimide. See: (a) Sato, M.; Ebine, S.; Akabori, S.
Synthesis 1981, 472-473. (b) Grisorio, R.; Mastrolilli, P.; Nobile, C. F.;
Romanazzi, G.; Suranna, G. P.; Meijer, E. W. Tetrahedron Lett. 2004, 45,
5367-5370.
(16) Hoeben, F. J. M.; Herz, L. M.; Daniel, C.; Jonkheijm, P.; Schenning, A. P.
H. J.; Silva, C.; Meskers, S. C. J.; Beljonnne, D.; Phillips, R. T.; Friend,
R. H.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 1976-1979.
(17) Schenning, A. P. H. J.; van Herrikhuijzen, J.; Jonkheijm, P.; Chen, Z.;
Wurthner, F.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 10252-10253.
9
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Properties of Hydrogen-Bonded Oligofluorenes
A R T I C L E S
Scheme 1. Synthesis of OFna
a Reaction conditions: (i, iii) C6H13Br, DMSO/aq NaOH, 96% (OF1), 46% (5); (ii, iv, vi, viii) nBuLi, 2-isopropoxy-pinacolatoborolane, THF, -78 °C,
98% (3), 77% (6), 72% (8), 45% (10); (v, ix, xi, xii) Pd(PPh3)4, Na2CO3, dioxane, 90 °C, 54% (OF3) and 39% (7), 73% (11) and 27% (12); 74% (OF5),
46% (OF7); (vii) nBuLi, TMSCl, Et2O, -78 °C, 81% (9); (x) ICl, CH2Cl2, 4 °C, 86%.
hexafluoro-2-propanol, the bis-UPy-terminated oligofluorenes
do dissolve readily in common organic solvents. The 1H NMR
spectra of the UPy-OFn-UPy show that the ureidopyrimidinone
protons are shifted strongly downfield to 12.3, 12.4, and 13.1
ppm, indicating hydrogen bonding.14 On addition of TFA, the
three downfield signals disappear (see Supporting Information).
No enol tautomeric proton signals are present in the NMR
spectrum, suggesting that the pyrimidinones are predominately
in the keto tautomer. When UPy-OFn-UPy solutions are serially
diluted, the 1H NMR peak positions of the pyrimidinone protons
do not shift and no cyclic structures are observed. Assuming
an association constant similar to earlier reported UPy systems,15
the virtual degree of polymerization is about 10 000 at a
concentration of 1 M.
Optical Properties. Figure 1a shows the UV-visible spectra
of the bis-UPy-terminated oligofluorene series as compared with
the spectra of the parent hydrogen-terminated oligofluorenes.
All samples were dissolved in chloroform at a constant
concentration of 6 × 10-7 M. Both sets of oligofluorenes, UPy-
and hydrogen-terminated, absorb strongly between 325 and 425
nm with vibronically unstructured bands. The hydrogen-
terminated monomer, OF1, has a much lower absorption due
to its short conjugation length. The peak position of maximum
absorption (λmax) shifts bathochromically with increasing oli-
gomer length; likewise, the extinction coefficient increases with
oligomer length. These trends are similar to those reported for
oligofluorenes4,9,10 and other sets of π-conjugated oligomers.3
The bis-UPy-terminated oligofluorenes absorb more strongly
than their hydrogen-terminated oligofluorene counterparts. This
slight increase in the extinction coefficient for the bis-UPy
oligofluorenes may be due to electron donation by the urea
groups, enlarging the effective conjugation length of the
oligofluorene to which it is attached. Table 1 summarizes
absorption and photoluminescence spectra for oligofluorenes in
CHCl3.
When excited at their respective maximum absorption
wavelengths, the oligofluorenes emit strongly. The fluorescence
spectra of these compounds show three primary vibronically
structured bands with vibronic spacing of about 1300 cm-1. As
with absorption, the fluorescence spectra exhibit a bathochromic
shift with increasing oligomer length (Figure 1b). The bis-UPy-
terminated oligo-9,9-dihexylfluorene series appears to have a
maximum fluorescence effective conjugation length of about
five units as the fluorescence peak maximum does not shift
appreciably further with increasing chain length. For compari-
son, Klaerner and Miller4 report a fluorescence effective
conjugation length of six fluorene units for oligo-9,9-dihexyl-
fluorene. The bis-UPy-terminated oligofluorenes fluoresce less
strongly than the hydrogen-terminated oligofluorenes of the
same length. The fluorescence quantum yields (ΦPL) of the
fluorenes were determined relative to that of quinine sulfate
and anthracene (Table 1).20 Except for the monomer, the
nonterminated oligofluorenes OFn have high quantum yields
(0.88-0.96) that are approximately the same. The trimer OF3
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A R T I C L E S
Dudek et al.
Scheme 2. Synthesis of UPy-OFn-UPya
a Reaction conditions: (i, ii) HNO3/HOAc, 53% (NO2-OF1-NO2), 98% (14); (iii) bis(pinacolato)diboron (Bpin), Pd(dppf)Cl2, NaOAc, DMF, 90 °C,
52%; (iv, v, vi, viii) Pd(PPh3)4, Na2CO3, dioxane, 90 °C; 97% (NO2-OF3-NO2), 68% (NO2-OF5-NO2), 68% (16), 83% (NO2-OF7-NO2); (vii) ICl, CH2Cl2,
4 °C, 100%; (ix) H2, Pd/C, MeOH/CH2Cl2, or SnCl2, EtOH/EtOAc, reflux 19-74%; (x) N-(6-tridecyl-4[1H]-pyrimidinon-2-yl)-1H-imidazole-1-carboxamide,
CHCl3, 64-100%.
Scheme 3. Synthesis of UPy-OPV and UPy-Perya
a (i) UPy-imidazolide, CHCl3, reflux, 85%; (ii) 1,4-diamino-2,5-di-t-butylbenzene, Zn(OAc)2, DMA, reflux, 80%; (iii) N-(6-tridecyl-4[1H]-pyrimidinon-
2-yl)-1H-imidazole-1-carboxamide, CHCl3, reflux, 67%.
has the highest ΦPL of the OFn series. The bis-UPy-terminated
oligofluorene series exhibits stronger variation of ΦPL with chain
length. The longer UPy-OFn-UPy has quantum yields similar
to that of the OFn of the same oligomer length, while the shorter
UPy-OFn-UPy has a quantum yield much lower than that of
the OFn.
The data above show that the bis-UPy-terminated oligomers
self-assemble into supramolecular chain polymers that are still
(20) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107-1114.
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Properties of Hydrogen-Bonded Oligofluorenes
A R T I C L E S
Table 1. Summary of Absorption (λmax) and Photoluminescence
(PLmax) Spectra for Oligofluorenes in CHCl3
1
λ
max (nm)
ꢀ
(M-1 cm-
)
PLmax (nm)
φPL
τ
±
(ns)
0.2
molecule
±
1
±
25%
±
2
±
0.05
OF1
OF3
OF5
OF7
UPy-OF1-UPy
UPy-OF3-UPy
UPy-OF5-UPy
UPy-OF7-UPy
304
355
369
375
333
364
372
377
9900
320
398
413
415
391
407
415
416
<0.01
0.96
0.89
0.88
<0.01
0.30
2.0
1.7
1.6
1.9
2.4
1.9
2.0
2.0
75000
140000
220000
7600
100000
160000
230000
0.65
0.86
Energy-Transfer Experiments
We have carried out energy-transfer experiments to adjust the
emission wavelength of our hydrogen-bonded systems. Optically,
oligofluorenes are blue emitters that can act as efficient energy donors
when coupled with appropriate energy acceptors such as UPy-terminated
green emissive oligo(p-phenylenevinylene) (UPy-OPV) and red emis-
sive perylene bisimide (UPy-Pery) (Chart 1). We have used a mono-
UPy-functionalized acceptor to endcap the hydrogen-bonded energy
donor polymer with energy acceptors and thereby adjust the degree of
polymerization. Moreover, because of the quadruple hydrogen bonds,
no phase separation takes place between the donor and acceptor.
Because energy transfer is strongly dependent on the distance (d)
between donor and acceptor (ket ≈ d-6), energy-transfer experiments
such as these are helpful to elucidate the connectivity of supramolecular
assemblies. In dilute solutions, efficient energy transfer between the
donor and acceptor should be observed if two species are hydrogen-
bonded. Energy-transfer experiments show that both UPy-OPV and
UPy-Pery quench the fluorescence of the UPy-OFn-UPy set. For
example, Figure 2a shows a series of photoluminescence spectra for a
titration of UPy-OF3-UPy with UPy-OPV (see Supporting Information
for an example titration with UPy-Pery). As expected, on addition of
UPy-OPV, there is concomitant decrease in the fluorescence of the
oligofluorene with an increase of fluorescence of the oligo(p-phen-
ylenevinylene). Figure 2b summarizes this titration in a Stern-Volmer
Figure 1. UV-visible (a) and fluorescence (b) spectra of oligofluorenes
UPy-OFn-UPy (s) and OFn (- - -) at 6 × 10-7 M in CHCl3. Numbers
indicate oligomer length (n).
plot (2). The calculated Stern-Volmer constant of 2.9 × 106 M-1
,
which is reflecting the dimerization constant of the UPy units, is of
the same order of magnitude of other Stern-Volmer constants
calculated for energy-transfer experiments between UPy hydrogen-
bonded donors and acceptors diads.21 Energy transfer between UPy-
luminescent in solution. The degree of polymerization is high,
indicating that these polymers can be processed by spin-coating.
Figure 2. (a) Fluorescence spectra from a titration of UPy-OF3-UPy with UPy-OPV. (b) Stern-Volmer plot of energy-transfer titrations at 6 × 10-7
M
donor concentration in CHCl3 between UPy-OF3-UPy (s) or OF3 (‚‚‚) and UPy-OPV (9) or UPy-Pery (2) and a depiction of the energy tranfer (ET)
concept in the hydrogen-bonded oligofluorenes.
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A R T I C L E S
Dudek et al.
Figure 3. (a) Normalized absorption (s) and fluorescence (- - -) spectra of thin films of UPy-OF3-UPy (black lines) and OF3 (gray lines) spin-coated
from chloroform solutions. (b) Normalized fluorescence spectra of thin films spin-coated from solutions of 95:5 fluorene-UPy-OPV.
OF3-UPy and UPy-Pery (9; Stern-Volmer constant ) 1.3 × 106 M-1
)
polymers. The synthesis described here to functionalize the ends
of oligofluorenes is general and should permit construction of
even longer UPy-oligofluorenes as well as the synthesis of other
supramolecular π-conjugated polymers such as those made from
oligothiophenes. The resulting UPy-terminated oligofluorenes
are modular and can be readily endcapped by simple addition
of mono-UPy-terminated groups. Supramolecular co-block
polymers should be possible by mixing of two bis-UPy-
terminated monomers. We have capped the UPy-terminated
oligofluorenes with energy acceptors UPy-perylenebisimides and
UPy-oligo(p-phenylenevinylene) to show that these oligofluo-
renes can act as energy donors and that hydrogen bonding
enhances energy transfer. Because of their advantageous
electronic and material properties, noncovalently connected
oligofluorenes are promising candidates for light-emitting
diodes.
is not as efficient as that between UPy-OF3-UPy and UPy-OPV as
expected because of less spectral overlap: UPy-OF3-UPy fluoresces
most strongly at 407 nm, whereas UPy-OPV absorbs at 436 nm and
UPy-Pery at 515 and 553 nm (Figure 2b). Energy transfer is less than
25% as efficient if either of the donor-acceptor components lacks a
hydrogen-bonding group. For example, the dotted Stern-Volmer curves
in Figure 2b show Stern-Volmer plots of energy-transfer titrations
between OF3 and either UPy-OPV (2, Stern-Volmer constant ) 5.5
× 105 M-1) or UPy-Pery (9, Stern-Volmer constant ) 3.1 × 105
M-1). At the concentration studied, hydrogen bonding seems essential
for efficient energy transfer.
The energy-transfer properties of the mixed systems were also studied
in thin films. As expected for supramolecular polymers, solutions of
bis-UPy-terminated oligofluorene could be easily spin-coated on glass
yielding smooth films. Figure 3a shows the normalized absorbance and
fluorescence spectra of thin films of OF3 and UPy-OF3-UPy deposited
from chloroform. The peak positions of maximum absorption are the
same in solution as in the solid state for both samples, but the emission
maxima shift bathochromically from solution to solid state (Figure 3).
When a small amount (5%) of UPy-OPV was mixed with either OF3
or UPy-OF3-UPy, the fluorescence of the UPy-OF3-UPy is completely
quenched, indicating efficient energy transfer to the oligo(phen-
ylenevinylene) in the solid state (Figure 3b), while in the case of OF3,
incomplete fluorescence quenching is observed (Figure 3b).
Acknowledgment. We thank Dr. X. Lou for MALDI-TOF
analyses, Rint Sijbesma and Rene Janssen for helpful discussions
on hydrogen-bond assemblies and energy transfer, and Dorothee
Wasserberg for determination of quantum yields. This work,
as part of the European Science Foundation EUROCORES
Programme SONS, was supported by funds form The Nether-
lands Organization for Scientific Research (NWO) and the EC
Sixth Framework Programme.
Conclusion and Outlook
In conclusion, we have developed a new class of supramo-
lecular π-conjugated polymers: oligomers of fluorene of four
different lengths that are noncovalently connected by quadruple
hydrogen bonds to form polymers. 1H NMR spectroscopy and
the resulting material properties indicate that bis-ureidopyrimi-
dinone-terminated oligofluorenes do indeed form supramolecular
Supporting Information Available: Synthesis and charac-
terization of all intermediates, hydrogen-terminated oligofluo-
renes (OFn), and UPy-terminated oligofluorenes (UPy-OFn-
UPy), UPy-OPV, and UPy-Pery, H NMR spectra of UPy-
OFn-UPy, and energy-transfer titration plots. This material is
1
available free of charge via the Internet at http://pubs.acs.org.
(21) Neuteboom, E. E.; Beckers, E. H. A.; Meskers, S. C. J.; Meijer, E. W.;
Janssen, R. A. J. Org. Biomol. Chem. 2003, 1, 198-203.
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