Synthesis and characterization of amino-functional, blue light-emitting copolymers and their composites with CdTe nanocrystals

Article abstract of DOI:10.1016/j.polymer.2010.08.068

Random side-chain functionalized copolymers were synthesized, utilizing a facile Yamamoto protocol by applying 2,7-dibromo-9,9-bis(6-bromohexyl)-9H-fluorene, (E)-1,2-bis(4-bromophenyl)ethene, 2,7-dibromo-9,9-dioctyl-9H-fluorene as comonomers. The precurso

Full text of DOI:10.1016/j.polymer.2010.08.068

Polymer 51 (2010) 5669e5673
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Polymer Communication
Synthesis and characterization of amino-functional, blue light-emitting
copolymers and their composites with CdTe nanocrystals
Ioannis Kanelidis a, Victoria Elsner a, Monique Bötzer a, Maren Butz a, Vladimir Lesnyak b,
,
Alexander Eychmüller b, Elisabeth Holder a
*
a Functional Polymers Group and Institute of Polymer Technology, University of Wuppertal, Gaußstrasse 20, D-42097 Wuppertal, Germany
b Physical Chemistry/Electrochemistry, TU Dresden, Bergstrasse 66b, D-01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Random side-chain functionalized copolymers were synthesized, utilizing a facile Yamamoto protocol by
applying 2,7-dibromo-9,9-bis(6-bromohexyl)-9H-fluorene, (E)-1,2-bis(4-bromophenyl)ethene, 2,7-dibromo-
9,9-dioctyl-9H-fluorene as comonomers. The precursor copolymers were post-functionalized utilizing
di-n-propylamine and the resulting target copolymers were fully characterized. The optical classification
parameters have been determined in solutions and in thin films as well. The copolymers revealed blue light
emission, wide optical bandgaps Egopt of at least 2.84 eV and remarkable quantum yields up to 0.78 in
chloroform solutions. The amino-functional copolymers allowed tying semiconductor CdTe nanocrystals.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 28 May 2010
Received in revised form
10 August 2010
Accepted 12 August 2010
Available online 1 October 2010
Keywords:
Conjugated copolymers synthesis
Photoluminescence
Nanocrystal-copolymer composites
1. Introduction
In order to design and synthesize poly(arylene)s, to which PFs
belong to, several CeC coupling reactions are available to date.
A series of wide bandgaps, blue light-emitting copolymers, like
poly(para-phenylene)s (PPP)s [1], poly(carbazole)s (PC)s [2], or poly
(fluorene)s (PF)s [3e5], have been described over the last decades.
PFs are of special importance as light-emitting materials often used
in polymer light-emitting diodes (PLED)s [6e8]. PF is in particular
of significance due to its high quantum yield [3,7,8], and rather
large bandgap. Therefore, PF-based copolymers with emission
colors over the entire visible range were synthesized by intro-
ducing comonomers revealing a lower bandgap [7e11]. C9-alky-
lated PFs are well-known to undergo oxidation upon heating, in
turn leading to color instability and green electroluminescence
[12e15]. In order to avoid this undesired effect, copolymerization
with functional and bulky moieties that keep the bandgap wide and
to some extent suppress the disturbing keto-defect emission
[16e18] of the PFs, is demanded respectively. Furthermore, func-
tional polymers comprised of functional monomers that can be
connected to other functional, or light-emitting species such as
nanoparticles or semiconductor nanocrystals (NCs) [19], are of
increasing impact in modern light-emitting technology [20].
Important is the oxidative coupling [21], but also the reductive
coupling, represented by the straightforward Yamamoto coupling
[7,20,22]. Likewise, palladium-interceded cross-coupling reactions
like the Suzuki polycondensation [23] are useful to achieve PFs. For
the functionalization of the conjugated copolymer backbones, the
emphasis is on transformation reactions like the substitution of
bromo-functional side-chains [24,25]. In order to introduce func-
tional groups like amino-functionalities [26], post-functionaliza-
tion has proven to be very efficient [25].
In this work, we introduce the synthesis and characterization of
two novel amino-functionalized fluorene-based copolymers by
a post-treatment of bromo-side chain functionalized precursors.
The latter were designed with 2,7-dibromo-9,9-bis(6-bromohexyl)-
9H-fluorene, (E)-1,2-bis(4-bromophenyl)ethene and 2,7-dibromo-
9,9-dioctyl-9H-fluorene as building blocks following a simple
Yamamoto protocol. The resulting random copolymers were post-
functionalized in order to tether di-n-propylamine groups at the
side-chains of the conjugated copolymer backbone. The final
products revealed blue light emission with an emission maximum
at 434 nm and high relative quantum yields up to 0.78 when
compared to the reference fluorophore PF, revealing a quantum
yield of 0.45 [27]. Due to the amino-functionalities, the random
copolymers might function to tie to a variety of nanoparticles
* Corresponding author. Tel.: þ49 202 438 3879; fax: þ49 202 438 3880.
E-mail address: holder@uni-wuppertal.de (E. Holder).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.08.068

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I. Kanelidis et al. / Polymer 51 (2010) 5669e5673
[28,29]. Here, we show the interconnection of the functional
copolymers with CdTe semiconductor NCs indicated by enhanced
deactivation of the amino-functional copolymers emission upon
the addition of the CdTe NCs.
2. Experimental part
Materials and instrumentation as well as the synthesis and
experimental details of the involved monomers, polymers, nano-
crystals and composites are described in the Electronic
Supplementary Material (ESM).
2.1. Synthesis of the monomers
2,7-Dibromo-9,9-bis(6-bromohexyl)-9H-fluorene (3) was synthe-
sized by a base-mediated alkylation using a phase transfer catalyst
(PTC) [24]. 6,6-(2,7-Dibromo-9H-fluorene-9,9-diyl)bis(N,N-dipro-
pylhexan-1-amine) (4) was obtained bya substitutionreactioninhigh
yield (91%) [30], while (E)-1,2-bis(4-bromophenyl)ethene (5) was
synthesized using 4-bromobenzaldehyde as starting material in the
presenceof TiCl4 [31] accordingtoaMcMurryreaction, yielding48%of
the E-isomer after recrystallization from ethyl acetate (Scheme 1).
Subsequently, a series of side-chain functionalized copolymers were
synthesized and post-functionalized (Scheme 2).
2.2. Synthesis of the copolymers e general procedure
The polymerizations were performed according to a facile Ni(0)-
mediated Yamamoto CeC-coupling protocol [32] using 2,7-dibromo-
9,9-dioctyl-9H-fluorene (2), 2,7-dibromo-9,9-bis(6-bromohexyl)-9H-
fluorene (3) and (E)-1,2-bis(4-bromophenyl)ethene (5) as building
blocks. The nickel(0)-catalyzed polycondensations were conducted in
Scheme 2. Synthesis of functional copolymers 6a, 7a and 6b, 7b.
THF (6a, 7a) by means of Ni(COD)2 and 2,20-bipyridine as the organ-
ometallic counterpart (Scheme 2).
3. Results and discussion
A series of random copolymers 6a, 7a and Pref (Scheme 2) were
obtained by varying the proportions of the building blocks. For
copolymer 6a the molar feed ratio of dioctyl-fluorene moiety 2 to
bis(6-bromohexyl)-fluorene 3 to (E)-stilbene derivative 5 was
40:10:50% and for copolymer 7a the ratio of the comonomers 2:3:5
was 35:15:50%, correspondingly. A reference copolymer Pref
comprised of dioctyl-fluorene 2 and (E)-stilbene derivative 5 in
a feed ratio of 50:50% was additionally prepared.
In order to remove impurities and to obtain unimodal molecular
weight distributions, the copolymers 6a,7a and Pref were exhaustively
purified in a Soxhlet-apparatus by using ethanol as extraction medium.
After purification, the functional copolymers were obtained as yellow
solids in yields of about 30% [33] and were easily soluble in solvents of
Scheme 1. The synthetic route for monomers 3, 4, and 5. NMR labels at compounds 3
and 4.

I. Kanelidis et al. / Polymer 51 (2010) 5669e5673
5671
additionally proven by detection of nitrogen utilizing elemental
analysis. A nitrogencontentof8.3% and9% wasdetected for polymers
6b and 7b, respectively. However, the determined values remained
slightly below theoretical values of the initial monomer feed ratio
of 10% and 15% used for the preparation of precursor copolymers 6a
and 7a, correspondingly. The successful post-functionalization was
furthermore confirmed by the detection of the vibrational bands at
2050 cmꢁ1 and 2348 cmꢁ1 in the infrared spectra, assigned to the
CeNHþ stretching vibration.
Gel permeation chromatography analysis of all copolymers
resulted in unimodal molecular weight distributions. The number
average molecular weight, (Mn) of Pref is about 12000 g ꢂ molꢁ1
with a polydispersity index (PDI) of 2.20, while for the precursor
copolymers 6a and 7a it is in the order of 18000 g ꢂ molꢁ1 with PDI
values of 2.49 and 2.75, accordingly. The purification of the amino-
functional copolymers 6b and 7b by precipitation into acetone lead
to about by half reduced Mn values with corresponding PDI values
of 2.28 and 1.90. This finding is most likely due to an increased
solubility of the higher molecular weight fractions in polar solvents
like acetone [29]. Nonetheless, acetone seems to be the optimal
medium for amino-functionalized copolymers. Compared to more
polar solvents like methanol the yields of the amino-functional
copolymers 6b and 7b were drastically reduced even below 10%
(side-chain solubility). The same holds true for less polar solvents
than acetone (backbone solubility).
Fig. 1. 1H NMR spectra of bromo-functionalized compound 3 and amino-functional-
ized derivative 4. All spectra were recorded in CDCl3.
medium polarity like toluene, chloroform or dichloromethane.
Moderate yields are expected in systems with complicated backbone
composition and side-chain functionalities, which enhance the solu-
bility and the losses upon purification via extraction and/or several
times of precipitation [33,34]. The post-functionalization was con-
ducted in a THF/DMF 1:1 mixture at 85 ꢀC reaction temperature by
addition of di-n-propylamine at 0 ꢀC and precipitation of the polymers
using acetone.
Fig. 1 shows the 1H NMR spectra recorded in deuterated chloro-
form (CDCl3) monitoring the transformation of compound 3 into its
amino-functionalized counterpart 4. As shown in Fig. 1, the shifts of
the resonances of the C6 protons (for NMR labels of 3 and 4 see
Scheme 1), allow the observation of the applied synthetic steps. For
compound 3, the triplet resonance at 3.29 ppm, which is assigned to
the protons of the C6 carbon atom and its perfect integration indi-
cate the step-wise conversion of 2,7-dibromofluorene (1) into
compound 3. The five high-field shifted resonances observable at
0.56e1.95 ppm represent the remaining protons of the aliphatic
eCH2egroups of the functionalized hexyl side-chains. As expected,
integration of the intensity indicates the presence of two protons in
each of the eCH2e groups, accordingly. In case of monomer 4, the
attachment of the di-n-propylamine moiety shifted the respective
C6 protonresonances tohigher fieldvalues (3.29 /2.91 ppm, Fig.1).
These resonances are in parts overlapping with the C7 protons of the
di-n-propylamine group (Fig. 1). The 1H NMR spectra of copolymers
6a, 7a are indicative for polymer spectra due to the observed reso-
nance broadening and the resonances at 3.31 ppm are therefore
given evidence for the incorporation of 2,7-dibromo-9,9-bis(6-bro-
mohexyl)-9H-fluorene (3) in each of the copolymers’ backbone.
Random copolymers 6b, 7b were obtained after a substitution
reaction replacing the C6-bromo-atoms in the side-chains of 6a, 7a
using di-n-propylamine in THF. For copolymers 6b, 7b an analogue
signal for the nitrogen-neighbored C6 protons is not as pronounced
as for 6a, 7a, however, the success of the post-functionalization was
The optical properties of copolymers 6a,b, 7a,b and Pref were
additionally determined. Table 1 illustrates the optical properties in
solution and film of the synthesized copolymers, which all exhibit
blue light emission. The absorption and emission spectra of the
precursor 6a, b and post-functionalized copolymers 7a, b were
illustrated in Fig. S1 in the supporting information file.
The amino-functional copolymers 6b and 7b possessed remark-
able quantum yields in the range of 0.77 and 0.78, correspondingly.
These quantum yields are increased by a significant factor of 1.28 and
1.42 compared to the bromo-functional precursor copolymers 6a and
7a, accordingly. Moreover, the copolymers revealed wide bandgaps
of 2.84 eV in solution while in the films the optical bandgaps
were slightly reduced to 2.70 eV (Table 1). Therefore, the functional
copolymers might suit to host lower bandgap compounds such
as semiconductor NCs. Consequently, solid-state measurements of the
optical properties of copolymers and NC-polymer systems were con-
ducted. In Fig. 2a, the relative emission intensities of bare polymers
and the NC-polymer films are depicted. These fluorescence
measurements, using an excitation wavelength of 410 nm, showed
that all polymers efficiently emit blue light. Upon addition of the NCs,
a reduction of the polymers fluorescence signal occurred. The
sequence of the quenching effect (QE) assigned to the polymer back-
bone band follows the order QEPref < QE6b < QE7b. In particular, the
signal assigned to the nanocomposite comprised of Pref is quenched
bya factorof 8.4compared tothe bare polymer, whilethe intensities of
Table 1
Optical properties of the copolymers 6a, b, 7a, b and Pref in CHCl3 solutions and in thin films.
a,c
a,d
Abssola(lg
e
) [nm] (L ꢂ molꢁ1 ꢂ cmꢁ1
)
Absfilm [nm]
Emsola,b [nm]
Emfilmb [nm]
Egsol [eV]
Egfilmc [eV]
Vsol
6a
7a
6b
7b
382 (5.86)
388 (5.80)
388 (5.39)
388 (5.37)
391 (6.06)
400
397
397
398
401
432/455
435/458
434/456
434/458
434/461
451/477
451/480
451/479
449/478
451/477
2.88
2.85
2.84
2.84
2.89
2.42
2.41
2.70
2.69
2.71
0.60
0.55
0.77
0.78
0.59
Pref
a
In chloroform solution (10ꢁ7 mol/L).
lexc. 390 [nm].
Calculated from the absorption band edge.
b
c
d
Determined according to Demas and Crosby [38] using PF (Vsol 0.45) [27] as a reference. Note in literature, the V values for PF vary over a wide range [28,39] also
depending on the structural features of the investigated copolymer [26,27].

5672
I. Kanelidis et al. / Polymer 51 (2010) 5669e5673
electro-optical investigations will be carried out in our laboratories
taking the observed deactivation process of differently labeled NCs
into consideration.
4. Conclusion
We prepared two bromo-functionalized fluorene-based copoly-
mers using a facile Yamamoto protocol, which were further amino-
functionalized. The copolymers were comprised of varying contents
of 2,7-dibromo-9,9-bis(6-bromohexyl)-9H-fluorene, (E)-1,2-bis(4-
bromophenyl)ethene and 2,7-dibromo-9,9-dioctyl-9H-fluorene. The
amino-functional copolymers revealed wide bandgaps of 2.84 eV,
favorable quantum yields of up to 0.78 in solution and proved
themselves to interconnect to the surface of CdTe NCs. The investi-
gation of the deactivation and the intensity enhancement process of
the polymer and the CdTe nanocrystal emission illustrated a depen-
dence on the bromo-functions of the NC stabilizing ligands and the
nitrogen content of the copolymers. Thus, tethering CdTe NCs on an
amino-functional copolymer backbone, leading to an emission
intensity enhancement of the NCs emission, might be regarded as
a
nitrogen-content depending process and the luminescence
decrease of the copolymers is promising for sensing bromo-con-
taining aromatic compounds.
Acknowledgements
The Deutsche Forschungsgemeinschaft (DFG) is acknowledged
for financial support within the grant application HO3911/2-1:
“Hybrid polymer/nanocrystals structures: fabrication and studies of
energytransfer, chargegenerationandtransport”. Prof. UllrichScherf
is acknowledged for granting access to the tools of the Macromo-
lecular Chemistry, while Prof. Matthias Rehahn is acknowledged for
many helpful comments supporting our work.
Fig. 2. Emission (a) and normalized emission (b) spectra of copolymers Pref, 6b, 7b
(1 mg/mL in THF) and their composites with CdTe (lex.: 410 nm) drop casted from THF and
a DMF/THF (1/3) mixture, respectively. Pictures of 7b, CdTe and their composite (1/1)
(in that order left to right) with no and under illumination (lex.: 366 nm) (c).
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.polymer.2010.08.068.
the 6b and 7b composites were reduced by a magnitude of 12.5 and
24.7, correspondingly, regarding their neat copolymer bands. Addi-
tionally, the normalized emission spectra of the investigated nano-
composite films are illustrated in Fig. 2b. The emissions at 610 nm,
originating from the CdTe nanocrystals, exposed intensity enhance-
ment (IE) in the order: IE7bþCdTe > IEPrefþCdTe > IE6bþCdTe. The
improved fluorescence deactivation in the case of 7b may thus have
a correlation with the content of the amino-groups in the copolymer
side-chain, most likely influencing the surface states of the CdTe NCs
[35,36]. The pictures in Fig. 2c show the deactivation of 7b upon CdTe
addition that might most probably occur via collisional quenching, for
whichbromobenzeneisparticularlyknownasanefficient quencher of
many fluorophores [37]. Obviously, the NCs stabilizer HSeC6H4eBr
representing a substituted bromobenzene may act as a photo-
luminescence quencher by a direct contact with the polymer in solid
films. Additional luminescence drop can be provided by remaining
N,N-dimethylformamide used as a solvent for the CdTe NCs, which is
also known for its quenching ability. Therefore, this observed lumi-
nescence decrease is promising for sensing bromo-containing
aromatic compounds. In order to further enhance the luminescence
properties of the NCs-polymer composites, a robust chemical binding
between the CdTe NCs and the respective polymer molecules under
elimination of the bromo-substituent will be performed in the future
as well as the employment of different stabilizing molecules in the NCs
synthesis. More detailed measurements involving photo-physical and
References
[1] Grimsdale AC, Müllen K. Adv Polym Sci 2006;199:1e82.
[2] Morin J-F, Leclerc M, Adès D, Siove A. Macromol Rapid Commun 2005;26:
761e78.
[3] Neher D. Macromol. Rapid Commun 2001;22:1365e85.
[4] Leclerc M. J Polym Sci Part A Polym Chem 2001;39:2867e73.
[5] Chen P, Yang G, Liu T, Li T, Wang M, Huang W. Polym Int 2006;55:473e90.
[6] Müllen K, Scherf U, editors. Organic light-emitting devices. Synthesis, prop-
erties and applications. Weinheim: WILEY-VCH; 2006.
[7] Tsami A, Yang X-H, Galbrecht F, Farrell T, Li H, Adamczyk S, et al. J Polym Sci
Part A Polym Chem 2007;45:4773e85.
[8] Tsami A, Yang X-H, Farrell T, Neher D, Holder E. J Polym Sci Part A Polym Chem
2008;46:7794e808.
[9] Bernius MT, Inbasekaran M, O’Brien J, Wu W. Adv Mater 2000;12:1737e50.
[10] Luo J, Peng J, Cao Y, Hou Q. Appl Phys Lett 2005;87:261103/1e3.
[11] Taranekar P, Abdulbaki M, Krishnamoorti R, Phanichphant S, Waenkaew P,
Patton D, et al. Macromolecules 2006;39:3848e54.
[12] Lee J, Klaerner G, Miller RD. Synth Met 1999;101:126. 126.
[13] Teetsov J, Fox MA. J Mater Chem 1999;9:2117e22.
[14] Weinfurtner K-H, Fujikawa H, Tokito S, Saga Y. Appl Phys Lett 2000;76:
2502e4.
[15] Yu W-L, Pei J, Huang W, Heeger A. Adv Mater 2000;12:828e31.
[16] Chen Y, Araki Y, Doyle J, Strevens A, Ito O, Blaue W. J Chem Mater 2005;17:
1661e6.
[17] Marsitzky D, Vestberg R, Blainey P, Tang BT, Hawker CJ, Carter KR. J Am Chem
Soc 2001;123:6965e72.
[18] Chiu CH, Shu CF. Macromloecules 2002;35:9673e7.
[19] Holder E, Tessler N, Rogach A. J Mater Chem 2008;18:1064e78.
[20] Demir HV, Nizamoglu S, Ozel T, Mutlugun E, Huyal IO, Sari E, et al. New J Phys
2007;9:362e70.

I. Kanelidis et al. / Polymer 51 (2010) 5669e5673
5673
[21] Leclerc M, Martinez Diaz F, Wegner G. Makromol Chem Rapid Commun
[30] Stork M, Gaylord BS, Heeger AJ, Bazan GC. Adv Mater 2002;14:361e6.
[31] Jeong K, Kim S, Shin U. J Am Chem Soc 2005;127:17672e85.
[32] Kinder L, Kanicki J, Petroff P. Synth Met 2004;146:181e5.
[33] Asawapirom U, Scherf U. Macromol Rapid Commun 2001;22:746e9.
[34] Wang L-Y, Li K-C, Lin H-C. Polymer 2010;51:75e83.
[35] Agrawal M, Rubio-Retama J, Zafeiropoulos NE, Gaponik N, Gupta S, Cimrova V,
et al. Langmuir 2008;24:9820e4.
[36] Soreni-Harari M, Yaacobi-Gross N, Steiner D, Aharoni A, Banin U, Millo O, et al.
Nano Lett 2008;8:678e84.
1989;190:3105e16.
[22] Yamamoto T, Yamada W, Takagi M, Kizu K, Maruyama T, Ooba N, et al.
Macromolecules 1994;27:6620e6.
[23] Schlüter AD. J Polym Sci Part A Polym Chem 2001;39:1533e56.
[24] Woo EP, Inbasekaran M, Shiang WR, Roof GR. WO//05184, 1997.
[25] Huang F, Zhang Y, Liu MS. Adv Funct Mater 2009;19:2457e66.
[26] Huang F, Wu H, Wang D, Yang W, Cao Y. Chem Mater 2004;16:708e16.
[27] Liu J, Tu G, Zhou Q, Cheng Y, Geng Y, Wang L, et al. Mater Chem 2006;16:1431e8.
[28] Lutich AA, Jiang G, Susha AS, Rogach AL, Stefani FD, Feldmann J. Nano Lett
2009;9:2636e40.
[37] Lakowicz JR. Princples of fluoresence spectroscopy. 3rd ed. New York:
Springer Science& Business Media, LCC; 2006.
[29] Guo ZS, Zhao L, Pei J, Zhou ZL, Gibson G, Brug J, et al. Macromolecules
2010;43:1860e6.
[38] Demas JN, Crosby GA. J Phys Chem 1971;75:991e1024.
[39] Ranger M, Rondeau D, Leclerc M. Macromolecules 1997;30:7686e91.