Self-encapsulation of poly-2,7-fluorenes in a dendrimer matrix

Article abstract of DOI:10.1021/ja010020g

The synthesis and characterization of complex dendritic, rigid rod poly-2,7-fluorene homopolymers and copolymers via a macromonomer approach is reported. Several 2,7-dibromofluorene monomers containing benzyl ether dendrons (generations 1, 2, and 3) in th

Full text of DOI:10.1021/ja010020g

VOLUME 123, NUMBER 29
J ULY 25, 2001
© Copyright 2001 by the
American Chemical Society
Self-Encapsulation of Poly-2,7-fluorenes in a Dendrimer Matrix
Dirk Marsitzky, Robert Vestberg, Paul Blainey, Beverly T. Tang, Craig J. Hawker, and
Kenneth R. Carter*
Contribution from the IBM Almaden Research Center, NSF Center for Polymeric Interfaces and
Macromolecular Assemblies, 650 Harry Road, San Jose, California 95120-6099
ReceiVed January 2, 2001
Abstract: The synthesis and characterization of complex dendritic, rigid rod poly-2,7-fluorene homopolymers
and copolymers via a macromonomer approach is reported. Several 2,7-dibromofluorene monomers containing
benzyl ether dendrons (generations 1, 2, and 3) in the 9,9′-position of the fluorene ring were prepared and
employed in condensation polymerizations to yield both homopolymers and copolymers with diethylhexy-
lfluorene. Fluorescence measurements of the materials reveal extensive conjugation along the polymer backbone.
The determination of the solid-state PL spectra and quantum efficiencies showed that there is an apparent
optimum size of the dendritic side groups with the [G-2]-derivatives showing high reactivity with associated
site isolation of the conjugated chain. AFM analysis and DSC results confirmed that the hybrid polymers and
copolymers did not show any sign of a microphase-separated morphology. First EL-results demonstrated that
the homopolymers have higher turn-on voltages then the corresponding copolymers.
Introduction
their tendency to aggregate and form excimers leading to blue-
green emission and fluorescence quenching.16,17 Concepts
Electroluminescent organic polymers have transitioned in the
recent years from being pure scientific curiosities to materials
of commercial interest.1 An important issue under investigation
is the development of efficient and stable blue-emitting materi-
als.2,3 In this area the stepladder polymer poly-2,7-(9,9-dialkyl)-
fluorene (2,7-PF) has attracted a great deal of attention because
of its chemical and thermal stability and the possibility to vary
the properties of the resulting polymer via the substitution
pattern of the 9-position of the fluorene unit.4-15 The problems
encountered with polymeric rodlike blue emitters in general are
(5) Kreyenschmidt, M.; Kla¨rner, G.; Fuhrer, T.; Ashenhurst, J.; Karg,
S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller, R. D. Macromolecules
1998, 31, 1099.
(6) Kla¨rner, G.; Davey, M. H.; Chen, W.-D.; Scott, J. C.; Miller, R. D.
AdV. Mater. 1998, 10, 993.
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M.; Woo, E. P.; Soliman, M. Acta Polym. 1998, 49, 439.
(8) Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Wu, W. W.; Woo,
E. P. AdV. Mater. 1999, 11, 241.
(9) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.;
Nothofer, H. G.; Scherf, U.; Yasuda, A. AdV. Mater. 1999, 11, 671.
(10) Kla¨rner, G.; Miller, R. D. Macromolecules 1998, 31, 2007.
(11) Lee, J.-I.; Kla¨rner, G.; Miller, R. D. Polym. Prepr. 1998, 1047.
(12) Kla¨rner, G.; Miller, R. D.; Hawker, C. J. Polym. Prepr. 1998, 1006.
(13) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306.
(14) Fukuda, M.; Sawada, K.; Yoshino, K. J. Polym. Sci., A: Polym.
Chem. 1993, 31, 2465.
(1) Part of this increased focus is indicated by the growing number of
companies performing research and attempting to commercialize products
in this area, e.g., IBM, Covion, Philips, Dow Chemicals, DuPont, Kodak,
Xerox, Pioneer, CDT, Uniax, etc.
(2) Kraft, A.; Grimsdale, A.; Holmes A. B. Angew. Chem., Int. Ed. 1998,
110, 402.
(15) Marsitzky, D., Klapper, M., Mu¨llen, K. Macromolecules 1999, 32,
8685.
(16) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765.
(17) Samuel, I. D. W.; Rumbles, G.; Collison, C. J. Phys. ReV. B., 1995,
52, R11573.
(3) Marsitzky, D.; Mu¨llen, K. In AdVances in Synthetic Metals: Twenty
Years of Progress in Science and Technology; Bernier P., Lefrant, S., Bidan,
G., Eds.; Elsevier: New York, 1999; pp 1-97.
(4) Pei, Q.; Yang Y. J. Am. Chem. Soc. 1996, 118, 7416.
10.1021/ja010020g CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/28/2001

6966 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Scheme 1. Synthesis of the Macroinitiators 5-7
Marsitzky et al.
applied to avoid this detrimental π-aggregation behavior include
the statistical copolymerization of 2,7-dibromofluorene with
suitable comonomers,6 the use of bulky side chains as solubi-
lizers, and the incorporation of dendritic endgroups.12 Dendritic
side-chain functionalization of several conjugated polymer
systems has been reported, namely poly(p-phenylenevinylene),18
poly(p-phenyleneethynylene),19 poly(thiophene),20 poly(tri-
acetylene)21 and poly(p-phenylene).22-24 The attachment of
polyether dendrons to lanthanide complexes has been shown
by Fre´chet et al. to improve the luminescence properties of these
complexes both in solution and solid state. The observed
enhancement of luminescence activity of the dendron-function-
alized La3+-complexes was attributed to both an “antenna
effect”, the transfer of energy from the ligand to the core, and
a “shell effect”, the site isolation of the La3+-cores leading to
decreased rate of self-quenching.25 While this manuscript was
in review, a report by Mu¨llen et al. appeared which described
the combination of a conjugated polymer with a first-generation
polyphenylene dendron. They particularly investigated the
shielding effect provided by different phenylene side groups to
avoid aggregation.26
polymer of commercial interest, 2,7-PF.27 Our motivation for
the synthesis of these novel hybrid materials was 3-fold: first,
the dendrons serve as excellent solubilizers due to their highly
branched structure, and second, they envelope and protect the
polymer rods from aggregation and degradation. A third aspect
includes the possibility to fine-tune the charge-transport proper-
ties via statistical or alternating copolymerization with other 2,7-
dibromofluorene derivatives. Here we report the results of the
homo- and copolymerization of the macromonomers 5, 6, and
7, the determination of the optical properties of the resulting
dendritic 2,7-PFs and the corresponding copolymers as well as
the AFM investigation and the first results of the electrolumi-
nescent properties evaluation of these novel materials.
Results and Discussion
In general, two different synthetic routes can be applied to
yield dendron-functionalized homopolymers: the first route
utilizes a polymer backbone with anchor groups to either
convergently or divergently attach a dense sequence of dendrons
(grafting-onto route). The second route utilizes monomers
already carrying dendrons which are subjected to polymerization
or polycondensation (macromonomer route). We selected the
macromonomer route to circumvent the intrinsic problems of
the grafting-onto procedure, for example achieving complete
coverage of the polymer backbone with dendrons and purifying
the product.
The macromonomers 5-7 were designed in a way to, (1)
ensure a certain shielding behavior of the polyfluorene backbone
by choosing sufficiently large dendrons while (2) not shielding
the reaction site of the transition-metal-mediated polyconden-
sation reactions. A third aspect of the monomer design involves
the potential dilution of the electroluminescent properties of 2,7-
We wished to explore the utility of incorporation of side-
chain dendrons in a thermo-oxidatively stable blue-emitting
(18) Bao, Z.; Amundson, K. R.; Lovinger, A. J. Macromolecules 1998,
31, 8647.
(19) Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658.
(20) Malenfant, P. R. L.; Fre´chet, J. M. Macromolecules 2000, 33, 3634.
(21) Schenning, A. P. H. J.; Martin, R. E.; Ito, M.; Diederich, F.; Boudon,
C.; Gisselbrecht, J.-P.; Gross, M. Chem. Commun. 1998, 1013.
(22) Schlu¨ter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864.
(23) Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W., Schlu¨ter,
A.-D. J. Am. Chem. Soc. 1997, 119, 3296.
(24) Bo, Z.; Zhang, C.; Severin, N.; Rabe, J. P.; Schlu¨ter A.-D.
Macromolecules 2000, 33, 2688.
(25) Kawa, M., Fre´chet, J. M. J., Chem. Mater. 1998, 10, 286.
(26) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mu¨llen,
K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123,
946.
(27) Woo, E. P.; Inbasekaran, M.; Shiang, W.; Roof, G. R. International
Patent, WO 97/05184, 1997.

Self-Encapsulation of 2,7-PF in a Dendrimer Matrix
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 6967
Figure 1. 13C NMR spectrum (125 MHz, CDCl3) and structure of 6.
Scheme 2. Synthesis of the Dendrimer-Polyfluorene Hybrid Materials 8-10, 12-14, and 16-18
PF by incorporating a large wt % of dendrimer. As one can
see, the correct choice of dendrimer size is dependent on the
subtle interplay of a number of different issues.
The macromonomers were then subjected to a Yamamoto-
type polymerization29 (Scheme 2a) utilizing bis(cyclooctadiene)-
nickel(0), 2,2′-bypyridine, and cyclooctadiene in a toluene/DMF
mixture followed by chain termination with bromobenzene. The
polymerization of macromonomer 5 proceeded smoothly and
yielded after workup the 2,7-PF 8 carrying [G-1]-dendrons as
side groups. The larger second-generation macromonomer 6 did
not polymerize using the same reaction parameters (concentra-
tion of monomer, amount of reactive coupling reagent, tem-
perature) as used in the literature for other 2,7-dibromofluorene
The monomers 5-7 were synthesized by selective nucleo-
philic substitution reactions (SN) of the 9,9′-position of 1 with
the respective dendrons 2-4 (see Scheme 1).27 The benzyl
bromide-functionalized dendrons 2-4 were synthesized accord-
ing to established literature procedures.28 The SN reaction
proceeded with high yield (84-91% after purification) and gave
1
the macromonomers 5-7 as confirmed by H- and 13C NMR
analysis (see Figure 1).
(29) Yamamoto, T.; Hayashi, Y.; Yamamoto, Y. Bull. Chem. Soc. Jpn.
1978, 51, 2091.
(28) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638.

6968 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Marsitzky et al.
Figure 2. MALDI-TOF spectrum of 9b.
derivatives.5 Since it is known from the palladium(0)-mediated
synthesis of dendron-functionalized poly-p-(phenylene)s (PPP)
that the concentration of the macromonomers plays an important
role in respect to the polymerization results, we increased the
concentration of 6.22-24 The resulting polymer 9a was obtained
as a white solid and showed good solubility in common organic
solvents such as toluene and THF (maximum solubilities range
from 15 to 20 mg/mL solvent).
The molecular weight of the material was determined using
gel permeation chromatography (GPC) calibrated with poly-
styrene standards to be Mn ) 6240 g/mol. The molecular weight
determination of conjugated, rigid rod-type polymers using
relative methods such as GPC calibrated with coil-like poly-
styrene standards, however, is known to only give an estimation
of the actual molecular weight.30 This is even more true for the
conjugated polymer-dendrimer hybrid materials studied here.
A further increase of the monomer concentration to the
saturation limit led to only a small incremental increase of the
molecular weight of 2,7-PF 9 (see Table 1). The polymerization
of 7 was also conducted using the highest possible macromono-
mer concentration; however, it yielded only a low-molecular
weight sample of 2,7-PF 10, reflecting the sterically crowded
reaction site of 10 carrying two bulky [G-3]-dendrons.
Table 1. Molecular Weight (Mn in g/mol, GPC, THF, Polystyrene
Calibration) and Polydispersity Index (PDI), Monomer
Concentration (cM in wt %), Decompostion Temperature (TD after
5% Weight Loss in °C) and Optical Properties (λmax, λem, and ΦF)
in Solution (THF, ΦF: Toluene) of the Dendronized 2,7-PFs 8-10
Mn/PDI
cM
TD
TG
λ
max/λem
ΦF
8
51 400/2.01
6200/1.44
9300/1.66
4400/1.11
5.3
7.7
9.3
9.5
320
340
360
320
94
50
50
10
388/414
370/408
380/411
340/391
0.78
0.97
1.0
9a
9b
10
0.41
agent (see Figure 2). The signal intensities in all cases were
quite low, allowing only a qualitative analysis of the molecular
weight. All derivatives 9-10 show up to four peaks per repeat
unit due to endgroup variations and fragmentation reactions
caused by absorption of laser energy by these high band-gap
materials.31,32 The G-1-functionalized 2,7-PF 8 could not be
analyzed, probably due to a combination of factors, its high
molecular weight (51 400 g/mol), and high polydispersity (D
) 2.01). Despite the oligomeric nature and the low polydis-
persities of the alternating coplymers 16-18 these dendron-
functionalized 2,7-PFs could not be analyzed by MALDI-TOF
analysis.
The statistical copolymerization of macromonomers 4-6 with
2,7-dibromo-9,9-(bis-2′-ethylhexyl)fluorene 11 (Scheme 2b) was
also performed via the nickel(0)-mediated Yamamoto-polym-
erization (feed ratio 1:1) with the aim of synthesizing high-
The molecular weight of the dendron-functionalized PFs
8-10 were also examined by MALDI-TOF mass spectrometry.
The oligomeric 2,7-PFs 9a, 9b, and 10 with low polydispersities
could be analyzed using
a 1,8,9-trihydroxyanthracene
(31) Ra¨der, H. J., Spickermann, J., Kreyenschmidt, M., Mu¨llen, K.
Macromol. Chem. Phys. 1996, 197, 3285.
(32) Remmers, M.; Mu¨ller, B.; Martin, K.; Ra¨der, H. J.; Ko¨hler, W.
Macromolecules 1999, 32, 1073.
() dithranol) matrix and silver-trifluoroacetate as cationization
(30) Kreyenschmidt, M.; Uckert, F.; Mu¨llen, K. Macromolecules 1995,
28, 4577.

Self-Encapsulation of 2,7-PF in a Dendrimer Matrix
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 6969
Table 2. Molecular Weight (Mn in g/mol, GPC, THF, Polystyrene
Calibration) and Polydispersity Index (PDI), Incorporation Ratio
1
(via H NMR Analysis), Decomposition Temperature TD (at 5%
Weight Loss in °C) and Optical Properties (λmax, λem, and ΦF) in
Solution (THF, ΦF: Toluene) of the Statistical Copolymers 12-14
Mn/PDI
m:n (NMR)
TD
TG
λ
max/λem
ΦF
12
13
14
62 800/2.22
26 200/2.33
68 300/2.21
1:1.5
1:0.9
1:0.07
350
290
320
73
48
51
384/411
383/411
385/411
0.69
0.92
0.76
Table 3. Molecular Weight (Mn in g/mol, GPC, THF, Polystyrene
Calibration) and PDI, Monomer Concentration (cM in wt %),
Decompostion Temperature (TD in °C), Glass Transition
Temperature TG (in °C) and Optical Properties (λmax, λem, and ΦF) in
Solution (THF, ΦF: Toluene) of the Dendronized 2,7-PFs 15-17
Mn/PDI
cM
TD
TG
λ
max/λem
ΦF
15
16
17
3900/1.46
16 300/2.46
11 900/1.65
22
26
20
340
370
360
78
50
49
372/411
388/419
374/410
0.53
1.0
0.84
Figure 3. UV/vis- and PL spectra (λexc. ) λmax) of the dendron-
functionalized 2,7-PFs 8-10 in solution (THF).
[G-3]-dendron. The [G-2]-macromonomer 6, however, gave the
highest degree of polymerization under the conditions employed
with respect to catalyst choice, solvents, and reaction temper-
atures and unoptimized Suzuki-reaction parameters (Dp ) 8).
The thermal properties of the conjugated materials 8-18 were
analyzed by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). All materials exhibit excellent
thermal stability up to 300 °C (see Tables 1-3). Only one Tg
was detected for each of the copolymers with no evidence of
two-phase behavior. The glass transitions of the dendron-
functionalized 2,7-PFs 8-18 decline with increasing size of the
attached dendrons and increasing amount of dendrimers (see
Tables 1-3).
PL- and EL-properties. All dendron-functionalized 2,7-PFs
8-10, 12-14, and 16-18 exhibit good solubility in common
organic solvents such as chloroform, THF, and toluene and show
bright blue fluorescence in solution. The absorption spectrum
of 8 resembles a superposition of the [G-1]-UV spectrum (λmax
) 288 nm) and a high-molecular weight 2,7-PF spectrum (λmax
) 388 nm)4 while the fluorescence spectrum of 8 (λexc. ) 388
nm) represents principally the optical properties of the 2,7-PF
backbone (λem ) 414 nm, see Figure 3). The absorption and
emission data of 9a, 9b, and 10 are in the same range as those
reported for the respective 2,7-PF oligomers10,15 and indicate
that the attached dendrons of generation 2 and 3 at the 9-position
of the fluorene monomer units do not inhibit conjugation along
the polymer main chain (see Table 1). The optical properties
of the statistical copolymers in solution (see Table 2) are
dominated by the conjugated backbone and are identical to high-
molecular weight polyfluorenes carrying alkyl chains as solu-
bilizers. The absorption and emission characteristics of the
lower-molecular weight alternating copolymers 16 and 18
resemble those reported for fluorene oligomers up to six repeat
units.10 The alternating 2,7-PF 17, however, has passed the
effective conjugation length of 11 repeat units (for fluorescence)
and emits blue light at the same wavelength as high-molecular
weight 2,7-PFs.
molecular weight polymers with excellent film-forming prop-
erties. The monomer concentrations of the copolymerization
reactions were comparable to the homopolymerization reactions.
All copolymerizations proceeded smoothly, and the resulting
copolymers were obtained after several washing steps and
reprecipitations as high-molecular weight materials (see Table
2) showing excellent solubility in a variety of organic, apolar
solvents. The incorporation ratio of the dendron-functionalized
fluorenes was checked by comparing the relative 1H NMR signal
intensities of the 2′-ethylhexyl resonances (δ ) 0.39-1.01) with
the R-benzyl ether signals ([G-1] δ ) 3.42; [G-2] δ ) 3.32;
[G-3] δ ) 3.19). The results (Table 2) show that copolymer 12
(Mn ) 62 800 g/mol) is slightly enriched with respect to the
[G-1]-functionalized fluorene (incorporation 1:1.5), copolymer
13 (Mn ) 46 600 g/mol) nearly displays the expected incorpora-
tion ratio (1:0.9), while the incorporation ratio of [G-3]-
functionalized fluorene 7 in 14 (Mn ) 68 300 g/mol) is quite
low (1:0.07). We assume, that the reactivity of 5 carrying the
less bulky and more polar [G-1]-benzyl ether dendrons in the
side chain is slightly higher than that of 11, leading to a slightly
preferred incorporation of 5 in the resulting copolymer. The
reactivity of the macromonomer 6, bearing dendrons of genera-
tion 2 in the side chains, on the other hand, is almost the same
as that of 2,7-dibromofluorene 11, resulting in a conformity of
incorporation and feed ratio. In contrast, the reaction site of
macromonomer 7 with the bulky [G-3]-dendrons as solubilizing
groups, however, is very crowded, leading to a low incorporation
ratio in the copolymer 14.
For the synthesis of alternating copolymers consisting of 9,9-
bis(ethylhexyl)fluorene and the corresponding dendronized
fluorenes, 5-7, we utilized the palladium(0)-mediated Suzuki
polycondensation33 in the two-phase system THF and 2 N
potassium carbonate solution at 90 °C (see Scheme 2c). The
synthesis of 15 was performed according to an established
literature procedure.34,35 All alternating copolymerizations were
again carried out with a minimum amount of solvent. The
molecular weight of the alternating copolymers as determined
by GPC are shown in Table 3. Surprisingly the alternating
copolymer 16 carrying the [G-1]-dendrons shows the same
degree of polymerization as 18, Dp ) 3, corresponding to six
coupled fluorene units which contain the sterically crowded
The PL quantum yields (ΦF) of the 2,7-PFs 8-10, 12-14,
and 16-18 in solution (toluene) were measured in comparison
to 9,10-diphenylanthracene (ΦF ) 1.0)36 and range from 0.41
to 1.0 (see Tables 1-3). These PL efficiency values are
comparable to many laser dyes in solution37 and are exception-
(33) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2557.
(34) Ranger, M.; Leclerc, M. J. Chem. Soc., Chem. Commun. 1997, 1597.
(35) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30,
7686.
(36) (a) Bowen, E. J.; Sahu, J. J. Phys. Chem. 1959, 63, 4. (b) Heinrich,
G.; Schoof, S.; Gusten, H. J. Photochem. 1974/75, 3, 315.
(37) Scha¨fer, F. P., Ed. Dye Lasers; Springer: Berlin, 1973.

6970 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Marsitzky et al.
Table 4. Thin-Film Absorption and Emission (λmax ) λem) Data of
the Dendronized 2,7-PFs after Thermal Treatment (30 min at 100
°C) as Well as PL-Quantum Efficiencies (Relative to
Poly-2,7-9,9-dioctylfluorene)
absorption (nm)
emission (nm)
PL-QE (φF)
8
388
372
382
345
388
388
388
380
388
380
421, 446, 522
416, 438
418, 438
419 (broad)
417, 440, 533
415, 438
416, 449
415, 438, 523
419, 440
0.25
0.66
0.98
0.15
0.29
0.46
0.35
0.12
0.73
0.56
9a
9b
10
12
13
14
16
17
18
414, 436
ally high for the derivatives 9a, 9b, 13, and 16 compared to the
ΦF values reported for 2,7-PF homopolymers (e.g., poly-2,7-
[9,9-(3,6-dioxaheptyl)-fluorene]: ΦF ) 0.77)4 as well as
copolymers (ΦF ) 0.36-0.79).5
Figure 4. Solid-state PL spectra of 8-10 (λexc. ) λmax). Films were
prepared by spin-coating (CHCl3, c ) 15 mg/mL, 2000 rpm) and drying
at 100 °C for 30 min.
To evaluate the potential of these materials as the emissive
layer in blue LEDs, the thin film photoluminescence was
examined (Figures 3-5). The UV- and fluorescence data of the
homopolymers 8-10 and the copolymers 14-16 and 16-18
are summarized together with the photoluminescence quantum
efficiencies in Table 4. The films were prepared by spin-coating
the polymers from p-xylene solutions (c ) 15 mg/mL) for 60
s at 1500 rpm on quartz substrates. All thin-films were thermally
annealed (30 min at 100 °C) well above their glass-transition
temperatures since it has been shown by real-time IR-analysis,
that the temperature in operating LED devices can reach up to
86 °C.38
An AFM study by Friend et al. revealed that the morphology
present in polyfluorene blends can greatly affect the electroop-
tical properties of the blend films.39 Our AFM investigation of
the 2,7-PFs 8-10, 12-14, and 16-18, however, did not reveal
any macroscopic order or nanophase separation in the dendron-
containing materials, and they were not significantly different
from other polyfluorene samples examined. AFM of polymer
films only probes the surface of the film so that nanoscopic
features embedded within the polymer film cannot be strictly
ruled out.
The PL spectrum of the [G-1]-2,7-PF 8 still exhibits a tail in
the long-wavelength region (540 nm) (probably caused by
excimer formation since the absorption spectrum does not
change) even though this tail is much less pronounced than in
films of thermally treated films of 2,7-PF derivatives carrying
straight alkyl chains as solubilizing groups. The [G-2]-2,7-PFs
9a and 9b, however, showed no additional band in the long-
wavelength region (see Figure 4). The [G-3]-functionalized 2,7-
PF 10 displayed only a weak and broad fluorescence spectrum
in the solid state which is probably due the poor optical quality
of the spin-coated films. The solid-state PL spectra of the
statistical copolymers 12-14 after thermal treatment are shown
in Figure 5. The copolymer 12 exhibits a broad additional band
at 542 nm attributed to excimer formation (no change of
absorption spectrum before and after annealing), an effect which
is less pronounced in the PL spectrum of 13. Only the copolymer
14 with the fluorene-G-3 incorporated does not show any sign
of aggregate and excimer formationsan astonishing result when
regarding the low incorporation of the [G-3]-macromonomer
in the copolymer 14. In the case of the alternating copolymers
Figure 5. Solid-state PL spectra of 12-14 (λexc. ) λmax). Films were
prepared by spin-coating from p-xylene solutions (c ) 15 mg/mL, 2000
rpm) and drying at 100 °C for 30 min.
16-18 only 16, containing the small [G-1]-dendron exhibits
the broad excimer-band at 540 nm, the alternating copolymers
17 and 18 emit both deep blue light even after thermal treatment
(see Figure 6).
The fluorescence quantum yields (φF) of these films (see
Table 4) were generated by comparing the fluorescence intensity
of polymers 8-10 with that of a sample of poly-2,7-(9,9-
dioctylfluorene) excited at 384 nm (φF ) 0.55).40 Even though
this relative method has to be considered an estimate (e.g.,
different absolute PL-QE values for poly-2,7-9,9-dioctylfluo-
rene are given in the literature41) it gives a first impression of
the PL performance of these materials compared to those for
2,7-PFs with straight alkyl chains. We observed that the quantum
yield of the [G-1]-2,7-PF 8 (φF ) 0.25) has dropped significantly
compared to that of the 2,7-PF standard. This low quantum yield
is probably due to the aggregated nature of the conjugated
backbone leading to excimer emission and fluorescence quench-
ing. The PL-QE of the [G-2]-2,7-PF 9a (φF ) 0.66), however,
is higher than that of the standard, while the higher molecular
weight derivative, 9b (φF ) 0.98), shows an exceptionally high
(40) The quantum yield of 55% was determined for a poly-2,7-(9,9-
dioctylfluorene) with an integrated sphere by: Grice, A. W.; Bradley, D.
D. C.; Bernius, M. T.; Inbasekaran, M.; Wu, W. W.; Woo, E. P. Appl.
Phys. Lett. 1998, 75, 629.
(38) Zhou, X.; He, J.; Liao, L. S.; Lu, M.; Ding, X. M.; Hou, X. Y.;
Zhang, X. M.; He, X. Q.; Lee, S. T. AdV. Mater. 2000, 12, 265.
(39) Halls, J. M.; Arias, A. C.; MacKenzie, J. D.; Wu, W.; Inbasekaran,
M.; Woo, E. P.; Friend, R. H., AdV. Mater. 2000, 12, 498.
(41) Tsetsov, J., Fox, M. A., J. Mater. Chem. 1999, 9, 2117.

Self-Encapsulation of 2,7-PF in a Dendrimer Matrix
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 6971
Conclusions
In summary, we report the synthesis and characterization of
complex dendritic, rigid-rod homopolymers and copolymers, in
which site isolation of the fluorescent poly-2,7-fluorene back-
bone is achieved by self-encapsulation in a dendritic matrix. A
macromonomer approach was followed, and several 2,7-
dibromo-fluorene monomers containing benzyl ether dendrons
(generation 1, 2, and 3) in the 9,9′-position of the fluorene ring
were prepared. These macromonomers were employed in
condensation polymerizations to yield bothshomopolymers (8-
10) and copolymers with diethylhexylfluorene (12-14, 16-
18). The solution- and solid-state fluorescence data revealed
extensive conjugation along the polymer backbone. Interestingly,
we discovered that there was an apparent optimum size of the
dendritic side groups, in respect to PL quantum efficiency, with
the [G-2]-derivatives showing high reactivity with associated
chain isolation. This leads to aggregation-free solid-state spectra
and high PL quantum efficiencies. AFM analysis and DSC
results confirmed that these hybrid copolymers-polymers did
not show any signs of a microphase-separated morphology.
Attempts to optimize the EL properties when tested in standard
device configurations are currently in progress.
Figure 6. Solid-state PL spectra of 15-17 (λexc. ) λmax). Films were
prepared by spin-coating from p-xylene (c ) 15 mg/mL, 2000 rpm)
and drying at 100 °C for 30 min.
quantum efficiency. The reason for the decreased PL efficiency
of the shorter derivative 9a compared to 9b might be the better
optical quality of the spin-coated film of 9b. The low φF of 10
(0.15) is probably due to the low degree of polymerization of
the [G-3]-functionalized 2,7-PF leading to crystallization phe-
nomena and fluorescence quenching. The PL-efficiencies of the
copolymers, 12-14 and 16-18, show trends similar to those
of the homopolymers 8-10. The materials that incorporate the
small [G-1]-dendron all exhibit relatively high aggregation and
excimer formation tendencies and thus possess relatively low
photoluminescence yields (12, φF ) 0.29; 16, φF ) 0.12). The
[G-2]-functionalized 2,7-PF copolymers, however, exhibit the
highest PL-efficiencies (13, φF ) 0.46; 17, φF ) 0.73) relative
to their [G-1]- and [G-3]-analogues. The PL-efficiency of the
[G-3]-functionalized statistical copolymer 14 (φF ) 0.35) is
lower than that of the standard 2,7-PF, while the PL-efficiency
of the alternating copolymer 18 (φF ) 0.56) is in the same range
as that of the 2,7-PF reference.
Light-emitting diode structures from the dendron-function-
alized 2,7-PF homopolymers 8, 9b as well as from the [G-2]-
functionalized statistical and alternating copolymers 13 and 17
were prepared in the device configuration indium-tin-oxide
(ITO)/PEDOT () poly-2,5-(3,4-ethylenedioxo)-thiophene)/2,7-
PF 8, 9b, 13, or 17 / Ca/Al. The fabrication process was
accomplished by spin-coating the first hole-transport layer
PEDOT onto the commercially available glass-ITO substrates.
Then the respective materials 8, 9b, 13, or 17 were deposited
by spin-coating from p-xylene solutions (c ) 10 mg/mL, film-
thickness ≈ 40 nm) followed by stepwise evaporation of the
calcium/aluminum cathode onto the EL-active layer. The diodes
with the [G-1]- and [G-2]-functionalized 2,7-PF homopolymers
8 and 9b started to emit blue light around 16 V. The LEDs
built from copolymers 13 and 17, however, showed considerably
lower turn-on voltages of 4.5 and 6 V, respectively. These diodes
were more stable than those made of the 2,7-PF derivatives 8
and 9b. However, all devices possessed short functional
lifetimes, which may be attributed to the thin EL-layers (ca.
35-40 nm compared to usual 80-120 nm in 2,7-PF LEDs42)
in combination with the unoptimized diode structure used. It
should be noted that we were unable to reproducibly obtain high-
quality thin films of the polymers when spun on the ITO/PEDOT
substrates, leading to shorts and diode failure.
Experimental Section
Materials. The compounds 2,7-dibromofluorene, benzyl(triethyl)-
ammoniumchloride, hexylbromide, 2,2′-bipyridine. and 1,5-cycloocta-
diene were purchased from Aldrich and used without further purifica-
tion. The Yamamoto coupling reagent bis(1,5-cyclooctadiene)nickel(0)
was purchased from Strem and handled under inert atmosphere. The
solvents were dried according to standard procedures. The 2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(2′-ethylhexyl)-
fluorene (15) were prepared according to already published reaction
protocol.13,34,35 All other materials were purchased from Aldrich and
used as received. All reactions were carried out under an argon
atmosphere.
Characterization. The 1H- and 13C NMR spectra were recorded on
a Bruker AF 250 MHz spectrometer. TGA measurements of the
dendron-functionalized 2,7-PFs were performed under a nitrogen
atmosphere at a heating rate of 10 °C/min using a Perkin-Elmer TGS-2
instrument. The polymer molecular weights were determined using a
Waters 150-C gel permeation chromatography instrument calibrated
with polystyrene standards. The optical properties were investigated
with a Hewlett-Packard 8452A diode array spectrophotometer and an
SA Instruments FL3-11 fluorimeter. The solution PL quantum yields
(ΦF) were determined by excitation of the respective polymer solutions
(toluene) at λmax and comparison with the solution emission of the 9,-
10-diphenylanthracene (ΦF ) 1.0).36 The solid-state fluorescence
efficiencies were measured by comparison of the fluorescence of the
respective dendron-functionalized 2,7-PF films on quartz substrates
(film thickness: 40-70 nm) with that of a standard poly-2,7-(9,9-
dioctylfluorene) sample (Mn ) 52 000 g/mol, PDI ) 2.4) spin-coated
from p-xylene solution onto quartz (film thickness: 50 nm).40 The
analysis of the film thickness was performed using an Alpha-Step 200
from Tencor Instruments. The MALDI-TOF measurements were carried
out with a Reflex I mass spectrometer (Bruker, Bremen, Germany),
equipped with a 337 nm nitrogen laser. The spectra were recorded in
the linear mode with the Bruker HIMAS detector at an acceleration
voltage of 30 kV. The mass spectrometer was calibrated externally with
polystyrene standards (PSS, Mainz, Germany). The matrix 1,8,9-
trihydroxyanthracene (dithranol) (Aldrich) and the analytes were
dissolved in THF and mixed at a mole ratio of approximately 1:500
(analyte:matrix).
Synthesis of the [G-1]-, [G-2]-, and [G-3]-dendrons 2-4. The
dendrons 2-4 were synthesized according to well-known literature
procedures by Hawker et al.28
General Procedure for the Synthesis of Macromonomers 5-7.
2,7-Dibromofluorene (1 g, 3.09 mmol), the phase-transfer catalyst
benzyl(triethyl)ammoniumchloride (0.22 g, 0.07 mmol) and the respec-
(42) Chen, J. P.; Kla¨rner, G.; Lee, J.-I.; Markiewicz, D.; Lee, V. Y.;
Miller, R. D.; Scott, J. C. Synth. Met. 1999, 107, 129.

6972 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Marsitzky et al.
tive dendron 2-4 (6.8 mmol) were dissolved in 90 mL of DMSO. The
solution was degassed three times and purged with argon. Then 5 mL
of a 50 wt % solution of NaOH were added via a syringe, and the
reaction mixture was stirred for 4 h. After the reaction was quenched
with the addition of 50 mL of water, 100 mL of dichloromethane were
added and the phases were separated. The aqueous phase was extracted
twice with dichloromethane, and the combined organic phases were
dried over MgSO4. After removal of the solvent the residue was
chromatographed using silica gel and a hexane/ethyl acetate solvent
mixture (2:1).
MHz, CDCl3) δ 0.48-1.05 (m, b 30 H), 1.82-2.01 (s, b, 4 H), 3.42
(s, b, 4 H), 4.41-4.65 (m, b, 8 H), 6.00-6.08 (m, b, 4 H), 6.55 (s, b,
2 H), 6.94-7.89 (m, b, 26 H); UV-vis (THF) λmax ) 384 nm.
Poly-9,9-bis[(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyloxy)-
methyl]-co-9,9-bis[2′-(ethyl)hexyl)]-2,7-fluorene 13: yield 200 mg
(76%); 1H NMR (250 MHz, CDCl3) δ 0.39-0.95 (m, b, 30 H), 1.89-
2.10 (s, b, 4 H), 3.32 (s, b, 4 H), 4.41-4.96 (m, b, 24 H), 5.95-6.07
(b, 6 H), 6.27-6.59 (m, b, 12 H), 7.11-7.55 (m, b, 46 H); UV-vis
(THF) λmax ) 383 nm.
Poly-9,9-bis[(3,5-bis(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyl-
9,9-Bis[(3,5-bis(benzyloxy)benzyloxy)methyl]-2,7-dibromo-
oxy)benzyloxy)-methyl]-co-9,9-bis[2′-(ethyl)hexyl)]-2,7-fluorene
1
fluorene 5: yield 2.62 g (91%); H NMR (250 MHz, CDCl3) δ 3.22
1
14: yield 175 mg; H NMR (250 MHz, CDCl3) δ 0.53-0.93 (m, b,
(s, 4 H), 4.72 (s, 8 H), 5.91 (t, J ) 2 Hz, 2 H), 6.29 (t, J ) 2 Hz, 4 H),
7.21-7.58 (m, 26 H); 13C NMR (68 MHz, CDCl3) δ 45.4, 57.2, 70.0,
101.5, 109.3, 120.6, 121.4, 127.3, 127.8, 128.1, 128.5, 130.5, 137.0,
138.2, 139.0, 150.5, 159.0.
30 H), 1.92-2.10 (s, b, 4 H), 3.19 (s, 4 H), 4.62 (s, b 8 H), 4.79-4.89
(m, b, 48 H), 6.08 (s, w, 4 H), 6.47-6.57 (m, b, 38 H), 7.19-7.27 (m,
b, 80 H), 7.42-7.61 (m, b, 6 H); UV-vis (THF) λmax ) 385 nm.
General Procedure for the Synthesis of Alternating 2,7-PF-
Copolymers 16-18. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-
2-yl)-9,9-bis(2′-ethylhexyl)fluorene (0.17 mmol), 0.17 mmol of the
respective macromonomers 5-7, and 5 mol % tetrakis(triphenylphos-
phine)palladium(0) were dissolved in an nitrogen atmosphere in 1.2
mL of degassed THF in a two-neck Schlenk-flask equipped with a reflux
condenser. K2CO3 (0.75 mL of a 2 M solution) was added via a syringe,
and the reaction mixture was degassed and purged with nitrogen three
times. The two-phase system was then heated under vigorous stirring
for 36 h at 95 °C. The workup included adding 10 mL of toluene and
water, separating the phases and washing the organic phase three times
with water. The organic phase was concentrated and precipitated in an
excess of a 10:1 mixture of methanol:acetone. The precipitate was
collected by filtration, intensively washed with acetone, and dried in a
vacuum to afford the alternating copolymers as pale yellow powders.
9,9-Bis[(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyloxy)-methyl]-
2,7-dibromo-fluorene 6: yield 4.67 g (85%); 1H NMR (250 MHz,
CDCl3) δ 3.17 (s, 4 H), 4.65 (s, 8 H), 4.99 (s, 16 H), 5.88 (t, J ) 2 Hz,
4 H), 6.26 (t, J ) 2 Hz, 2 H), 6.52 (t, J ) 2 Hz, 4 H), 6.58 (d, J ) 2
Hz, 8 H), 7.26-7.51 (m, 46 H); 13C NMR (68 MHz, CDCl3) δ 45.3,
57.2, 69.8, 70.1, 101.5, 101.6, 106.2, 109.3, 120.5, 121.4, 127.5, 128.0,
128.1, 128.6, 130.4, 136.9, 138.2, 139.0, 139.5, 150.5, 158.8, 160.1.
9,9-Bis[(3,5-bis(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyloxy)-
1
benzyloxy)-methyl]-2,7-dibromofluorene 7: yield 9.01 g (84%); H
NMR (250 MHz, CDCl3) δ 3.14 (s, 4 H), 4.63 (s, 8 H), 4.90 (s, 16 H),
4.97 (s, 32 H), 5.89 (d, J ) 4 Hz, 4 H), 6.29 (t, J ) 2 Hz, 2 H),
6.48-6.65 (m, 36 H), 7.26-7.50 (m, 86 H); 13C NMR (68 MHz,
CDCl3) δ 45.2, 57.2, 68.0, 70.0, 70.1, 101.5, 101.7, 106.3, 106.4, 109.3,
120.5, 121.5, 125.5, 127.5, 127.9, 128.0, 128.6, 130.4, 136.8, 138.3,
138.9, 139.3, 139.4, 150.4, 158.8, 160.0, 160.2.
Poly-9,9-bis[(3,5-bis(benzyloxy)benzyloxy)methyl]-alt-9,9-bis[2′-
General Procedure for the Synthesis of the Dendron-Function-
alized 2,7-PFs 8-10 and 12-14. A Schlenk tube containing 2 mL of
degassed toluene, 1 mL of degassed DMF, bis(1,5-cyclooctadiene)-
nickel(0) (100 mg, 3.64 mmol), 2,2′-bipyridine (60 mg, 3.84 mmol),
and 1,5-cyclooctadiene (0.04 mL, 3.84 mmol) was heated under argon
to 80 °C for 20 min. The respective monomers (0.12 mmol), dissolved
in 0.75 mL of degassed toluene, were added to the above solution via
a syringe, and the polymerization was maintained at 80 °C for 72 h.
Then 1.5 mL of bromobenzene was added to remove residual bromine
endgroups, and the reaction mixture was stirred for additional 3 h at
80 °C. After the reaction 50 mL of toluene was added, and the solution
was washed three times with EDTA (aq). The organic solution was
concentrated, and the polymer was precipitated from an equivolume
mixture of methanol and acetone. Finally the polymer was dried at 50
°C under vacuum.
1
(ethyl)hexyl)]-2,7-fluorene 16: yield 148 mg (75%); H NMR (250
MHz, CDCl3) δ 0.42-0.89 (m, 30 H), 1.83-1.99 (s, b, 4 H), 3.27-
3.35 (m, 4 H), 4.56-4.64 (m, 8 H), 5.95-6.04 (m, 4 H), 6.29 (s, b, 2
H), 7.08-7.32 (m, b, 20 H), 7.48-7.64 (m, b, 6 H); UV-vis (THF)
λmax ) 372 nm.
Poly-9,9-bis[(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyloxy)-
methyl]-alt-9,9-bis[2′-(ethyl)hexyl)]-2,7-fluorene 17: yield 283 mg
(82%); 1H NMR (250 MHz, CDCl3) δ 0.38-0.82 (m, b, 30 H), 1.82-
1.97 (s, 4 H), 3.36-3.50 (s, b, 4 H), 4.56 (s, b, 8 H), 4.84 (s, b, 16 H),
6.06 (s, 4 H), 6.29 (s, 2 H), 6.44-6.49 (m, b, 12 H), 7.18-7.54 (m, 46
H); UV-vis (THF) λmax ) 388 nm.
Poly-9,9-bis[(3,5-bis(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyl-
oxy)benzyloxy)-methyl]-alt-9,9-bis[2′-(ethyl)hexyl)]-2,7-fluorene 18:
yield 390 mg (90%); 1H NMR (250 MHz, CDCl3) δ 0.37-0.89 (m, b,
30 H), 1.82-2.01 (s, b, 4 H), 3.41 (b, 4 H), 4.50-4.95 (m, b, 56 H),
5.85 (b, 2 H), 6.02 (b, 4 H), 6.39-6.59 (m, b, 36 H), 7.12-7.32 (m,
b, 80 H), 7.37-7.68 (m, 6 H); UV-vis (THF) λmax ) 374 nm.
Poly-9,9-bis[(3,5-bis(benzyloxy)benzyloxy)methyl]-2,7-fluorene
1
8: yield 215 mg (52%); H NMR (250 MHz, CDCl3) δ 3.25 (s, b, 4
H), 4.57 (s, b, 8 H), 6.04-6.33 (b, 4 H), 7.13-7.32 (m, 2 H), 7.35-
7.51 (m, 26 H); UV-vis (THF) λmax ) 388 nm.
Poly-9,9-bis[(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyloxy)-
Acknowledgment. This research was supported in part by
NSF Materials Science and Engineering Research Center Grant
9808677 to the Center for Polymer Interfaces and Macro-
molecular Assemblies at Stanford University. We acknowledge
Dr. Hans-Joachim Ra¨der and Dipl.-Ing. Stefan Tu¨rk (Max-
Planck-Institute for Polymer Research) for measuring the
MALDI-TOF spectra and Luisa Bozano and Professor Sue
Carter for investigating the EL properties of the dendron-
functionalized homo-2,7-PFs.
1
methyl]-2,7-fluorene 9: yield 148 mg (54%); H NMR (250 MHz,
CDCl3) δ 3.24 (s, b, 4 H), 4.42 (s, b, 8 H), 4.60-4.77 (b, 16 H), 5.82-
5.90 (b, 2 H), 6.32-6.41 (b, 16 H), 7.14-7.50 (m, 46 H); 9a: UV-
vis (THF) λmax ) 370 nm; 9b: UV-vis (THF) λmax ) 380 nm.
Poly-9,9-bis[(3,5-bis(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyl-
oxy)benzyloxy)-methyl]-2,7-fluorene 10: yield 180 mg (45%); 1H
NMR (250 MHz, CDCl3) δ 3.14 (s, b, 4 H), 4.27-4.93 (m, 56 H),
5.79 (b, 4 H), 6.25-6.55 (m, 38 H), 7.15-7.41 (m, 86 H); UV-vis
(THF) λmax ) 340 nm.
Poly-9,9-bis[(3,5-bis(benzyloxy)benzyloxy)methyl]-co-9,9-bis[2′-
1
(ethyl)hexyl)]-2,7-fluorene 12: yield 250 mg (90%); H NMR (250
JA010020G