Synthesis, structure, and optoelectronic properties of phosphafluorene copolymers

Article abstract of DOI:10.1021/ol801068m

(Chemical Equation Presented) Copolymers of phosphafluorenes are obtained through Suzuki copolymerization. The phosphorus-containing copolymers show unique optical, electrochemical, and optoelectronic properties. Blue and white electroluminescence can be

Full text of DOI:10.1021/ol801068m

ORGANIC
LETTERS
2008
Vol. 10, No. 13
2913-2916
Synthesis, Structure, and Optoelectronic
Properties of Phosphafluorene
Copolymers
Run-Feng Chen, Rui Zhu, Qu-Li Fan, and Wei Huang*
Jiangsu Key Laboratory for Organic Electronics & Information Displays (KLOEID)
and Institute of AdVanced Materials (IAM), Nanjing UniVersity of Posts and
Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210046, China
iamwhuang@njupt.edu.cn; wei-huang@njupt.edu.cn
Received May 9, 2008
ABSTRACT
Copolymers of phosphafluorenes are obtained through Suzuki copolymerization. The phosphorus-containing copolymers show unique optical,
electrochemical, and optoelectronic properties. Blue and white electroluminescence can be observed, depending on the modifications of the
phosphorus atoms. It is the first time that conjugated polymers containing phosphafluorene have been prepared and used in PLEDs.
Phosphafluorenes are new building blocks for conjugated oligomers and polymers.
Phosphorus has wide applications1 in inorganic semiconduc-
tors, organic synthesis, catalysts, pesticides, and recently
electronic and optoelectronic devices, functioning not only
as ligands2 for heavy-metal and rare-earth complexes, but
also as materials3 for light-emitting diodes. The incorporation
of phosphorus moieties into conjugated molecules offers
considerable promise for the development of new functional
materials with novel properties because of the versatile
reactivity and electronic properties of phosphorus centers.4
The P-P skeleton is a powerful σ-scaffold to establish
through-bond electronic interaction between π-chromophores
with promising perspectives in optoelectronic applications.
The phosphorus analogues of PPV have interesting electronic
and spectroscopic properties.5 The phosphorus-containing
poly(N-arylaniline) copolymers are a new type of π-conju-
gated polymer with low oxidation potentials and electronic
delocalization through phosphorus along the polymer chain.6
Phosphole, the phosphorus analogue of pyrrole, displays a
high potential for use in π-conjugated organic materials
because of its favored exocyclic delocalization along the
polymer chain.7,8 The dithienophospholes have extraordinary
optoelectronic properties, allowing for fine-tuning the elec-
tronic structure by simple chemical modifications and for
its incorporation into the functional light-emitting diodes
based on organophophorus materials.9,10 These intriguing
features of phosphorus-containing materials strongly support
incorporating phosphorus atoms into conjugated molecules.
(5) Smith, R. C.; Protasiewicz, J. D. Eur. J. Inorg. Chem. 2004, 998.
(6) Jin, Z.; Lucht, B. L. J. Am. Chem. Soc. 2005, 127, 5586.
(7) Delaere, D.; Dransfeld, A.; Nguyen, M. T.; Vanquickenborne, L. G.
(1) See for example:(a) Ouali, A.; Laurent, R.; Caminade, A.-M.;
Majoral, J.-P.; Taillefer, M. J. Am. Chem. Soc. 2006, 128, 15990. (b) Xie,
J.-H.; Zhou, Q.-L. Acc. Chem. Res. 2008, 41 (5), 581–593. (c) Tang, W.;
Zhang, X. Chem. ReV. 2003, 103 (8), 3029–3070.
J. Org. Chem. 2000, 65, 2631
(8) Delaere, D.; Nguyen, M. T.; Vanquickenborne, L. G. J. Organomet.
Chem. 2002, 643, 194
(9) Dienes, Y.; Durben, S.; Karpati, T.; Neumann, T.; Englert, U.;
Nyulaszi, L.; Baumgartner, T. Chem. Eur. J. 2007, 13, 7487
(10) Baumgartner, T.; Neumann, T.; Wirges, B. Angew. Chem., Int. Ed.
2004, 43, 6197
.
.
(2) Xu, H.; Wang, L. H.; Zhu, X. H.; Yin, K.; Zhong, G. Y.; Hou, X. Y.;
Huang, W. J. Phys. Chem. B 2006, 110, 3023.
.
(3) Sun, Y. M.; Wang, C. S. Polymer 2001, 42, 1035.
(4) Hobbs, M. G.; Baumgartner, T. Eur. J. Inorg. Chem. 2007, 3611.
.
10.1021/ol801068m CCC: $40.75
Published on Web 06/07/2008
2008 American Chemical Society

Among π-conjugated polymers, polyfluorenes (PFs) have
many advantages11 for polymeric light-emitting diodes
(PLEDs). Polyfluorene-based Red, Green, and Blue materials
for PLEDs have been realized.12 However, PFs often show
a tailed emission band at long wavelengths13 in solid states
and devices, leading to color instability. The polysilafluo-
renes14,15 formed by replacing the vulnerable C-9 carbons
in PFs with silicon atom has strong blue electroluminesence
(EL) with high thermal and optical stabilities. The phospho-
rus analogue of fluorene and silafluorene (phosphafluorene)
has been used in asymmetric catalysis,16 and unsymmetrical
9-phosphafluorene oxides are of interest for P-chiral liquid
crystals.17 However, the applications of phosphafluorene in
luminescence devices have not been found in the literature
so far. The chemical and electronic configuration18 of
phosphafluorene is also very similar to that of carbazoles
(nitrogenafluorenes) which are currently the core-building
units for light-emitting materials and hole transporting
materials.19 It is interesting to investigate the optical and
electronic properties of phosphafluorene, in which the
phosphorus atom is in the same subgroup as nitrogen.18
In this Letter, we successfully synthesized copolymers of
phosphafluorene (dibenzophosphole) and fluorene via Suzuki
polymerization, and the optoelectronic properties of the
polymers were investigated. In contrast to the polysilafluo-
renes which emit only blue light as polyfluorene, the prepared
phosphafluorene-containing copolymers show not only strong
blue light emitting but also single layer white EL. It is
surprising that the oxidized phosphorus atom on phos-
phafluorene significantly changed the blue EL of polyfluorene
into white, although the actual incorporation ratio of phos-
phafluorene oxide is only 11% revealed by HNMR. The
extraordinary optical and electronic features of phosphafluo-
rene suggest its great potential in opto and electro devices
and applications.
Scheme 1. Synthesis of Phosphafluorene Copolymers
phafluorene, BuPFO is 3,6-dimethoxyl-9-butyl-9-phos-
phafluorene oxide, and the feed content is 10 mol %. The
number-average molecular weight (Mn), weight-average
molecular weight (Mw), and polydispersity index (PI ) Mw/
Mn) of the copolymers determined by gel permeation
chromatography (GPC) with polystyrene standards are listed
in Table 1. The lower molecular weight of PFO-BuPFO10
Suzuki coupling polycondensation between the comono-
mers of I/II, III, and IV20,21 shown in Scheme 1 afforded
the copolymers identified as PFO-PhPF10 and PFO-
BuPFO10, respectively, in which PFO is poly[2,7-(9,9-
dioctylfluorene)], PhPF is 3,6-dimethoxyl-9-phenyl-9-phos-
Table 1. Composition, Number-Average (Mn) and
Weight-Average (Mw) Molecular Weight, Polydispersity Index
(Mw/Mn), and Thermal Properties of Phosphafluorene
Copolymersa
(11) Burroughes, J. H.; Bradley, D. D.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347,
539.
compositionc/
polymerb
PFO
PFO-PhPF10
PFO-BuPFO10
%
Mn
Mw Mw/Mn Tg/°C Td/°C
(12) Wu, W. S.; Inbasekaran, M.; Hudack, M.; Welsh, D.; Yu, W. L.;
Cheng, Y.; Wang, C.; Kram, S.; Tacey, M.; Bernius, M.; Fletcher, R.;
Kiszka, K.; Munger, S.; O’Brien, J. Microelectron. J. 2004, 35, 343.
(13) Zeng, G.; Yu, W. L.; Chua, S. J.; Huang, W. Macromolecules 2002,
35, 6907.
14 000 20 000
15 400 35 800
9 500 10 200
1.4
2.3
1.1
78
81
52
426
403
403
7
11
a Measured by GPC, using polystyrene as standard. b The mol feed
(14) Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B. J. Am.
Chem. Soc. 2005, 127, 7662
(15) Chan, K. L.; Watkins, S. E.; Mak, C. S.; McKiernan, M. J.; Towns,
C. R.; Pascu, S. I.; Holmes, A. B. Chem. Commun. 2005, 5766
.
content of phosphafluorenes is 10%. c The mol content of the phosphafluo-
1
rene unit in polymers is calculated via H NMR.
.
(16) Diaz, A. A.; Young, J. D.; Khan, M. A.; Wehmschulte, R. J. Inorg.
Chem. 2006, 45, 5568.
(17) Duran, E.; Gordo, E.; Granell, J.; Velasco, D.; Lopez-Calahorra,
F. Tetrahedron Lett. 2001, 42, 7791.
is probably due to the harmful interactions between the Pd
catalyst and BuPFO, resulting in reduced catalyst ef-
ficiency.22 The large polydispersity of the molecular weight
of PFO-PhF10 may also be due to the interactions between
the Pd catalyst and PhPF, which has a similar structure of
(18) Chen, R. F.; Zheng, C.; Fan, Q. L.; Huang, W. J. Comput. Chem.
2007, 28, 2091.
(19) Levesque, I.; Bertrand, P. O.; Blouin, N.; Leclerc, M.; Zecchin,
S.; Zotti, G.; Ratcliffe, C. I.; Klug, D. D.; Gao, X.; Gao, F. M.; Tse, J. S.
Chem. Mater. 2007, 19, 2128.
(20) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30,
7686
.
(21) Chen, R.-F.; Fan, Q.-L.; Zheng, C.; Huang, W. Org. Lett. 2006, 8,
(22) Frahn, J.; Karakaya, B.; Schafer, A.; Schluter, A. D. Tetrahedron
1997, 53, 15459.
203
.
2914
Org. Lett., Vol. 10, No. 13, 2008

Table 2. The Optical and Electrochemical Properties of the Phosphafluorene Copolymers
λmax/solution (nm)
λmax/film (nm)
em.
a
polymer
abs.
em.
abs.
391
Eop (eV)
HOMO/LUMO (eV)
QEb (%)
PFO
PFO-PhPF10
PFO-BuPFO10
383
384
387
420, 438 (475)
418, 442 (472)
418, 442 (472)
438, 468 (495)
438, 462 (492)
441, 462, 492
2.84
2.81
2.74
-5.84/-2.16
-5.87/-2.19
-/-
59.1
53.1
52.4
393, 429
402, 430
a Optical bandgap (Eop) calculated from the onset of absorption spectra of the solid film. b Absolute PL quantum yield (QE) measured in the integrating
sphere.
triphenylphosphine. The Suzuki polymerization conditions
tion breakage of the PFO chain. The separate absorption and
emission of phosphafluorene units cannot be observed since
phosphafluorene has similar poly-p-phenylene backbone
chromophores as PFs.18
for phosphafluorenes need to be optimized and investigated
at the mechanism level.23
The chemical structure of the copolymers was verified by
1H NMR spectra. The NMR signal of methoxyl around 4.05
ppm was considered as a chemical structure probe of
phosphafluorene units.24 From this useful probe, the PhPF10
and BuPFO composition in the copolymer was calculated
to be 7 and 11 mol %, respectively, suggesting the co-
polymerization reactivity of the comonomers decreases in
the following order: BuPFO > fluorene > PhPF. The much
lower copolymerization ability of the dibromated PhPF than
that of BuPFO is probably due to the bulkier substituent of
phenyl and its lower solubility in the copolymerization
medium. The slightly higher actual content of BuPFO than
its feed content suggests that the copolymerization ability
of dibromated BuPFO is higher than that of dibromofluorene
although the spatial hindrance of methoxyl substituents
beside the polymerization sites may hinder the process of
Suzuki polymerization.24 The quite different copolymeriza-
tion behavior of the phosphafluorenes indicates the great
tuning ability by chemical modifications of the phosphorus
center.15
The copolymers have good solubility in chloroform,
tetrahydrofuran, and other common organic solvents and
uniform thin films can be cast from the solutions. The starting
temperatures of thermal decomposition (Td) of the copoly-
mers measured by thermogravimetric analysis (TGA) are
both higher than 403 °C in nitrogen. The glass transition
temperatures (Tg) of PFO-PhPF10 and PFO-BuPFO10 were
determined to be around 81 and 52 °C, respectively,
according to differential scanning calorimetry (DSC). The
low Tg of PFO-BuPFO10 may be due to the flexible butyl
substitutes and its lower molecular weight.
From Figure 1a, PFO-PhPF10 and PFO-BuPFO10 display
PFO-segment-dominated25 absorption and emission spectra
in THF (see Table 2) with slightly red-shifted main absorp-
tion peak at 384 and 387 nm, respectively, in comparison
with that of PFO, and with strong photoluminescence (PL)
emission peaks around 418, 441, and 472 nm, indicating that
the phosphafluorene incorporation does not result in conjuga-
Figure 1. Absorption (UV) and fluorescence (PL) spectra of PFO-
PhPF10 and PFO-BuPFO10 in dilute THF solution (a) and in thin
film (b).
The absorption spectra of PFO-PhPF10 and PFO-BuPFO10
in cast film (see Figure 1b) show peaks at 393 and 402 nm,
respectively, and shoulders around 430 nm. The red-shifted
absoption peaks in comparison with that of PFO (391 nm)26
suggest that conjugated length in the solid state increases
after phosphafluorene incorporation and BuPFO has greater
effects than PhPF. The additional absorption shoulder around
430 nm is related to the ꢀ-phase,27 indicating that phos-
phafluorene incorporation increases intrachain order. The
emission specta are remarkably similar to that of PFO (ꢀ-
phase) but with enhanced 0-1 transition (462 nm) especially
(23) Tyukalova, O. V.; Ratovskii, G. V.; Shmidt, F. K.; Russ, J. Gen.
Chem. 2001, 71, 888.
(26) O’Carroll, D.; Iacopino, D.; O’Riordan, A.; Lovera, P.; O’Connor,
E.; O’Brien, G. A.; Redmond, G. AdV. Mater. 2008, 20, 42.
(27) Ariu, M.; Sims, M.; Rahn, M. D.; Hill, J.; Fox, A. M.; Lidzey,
D. G.; Oda, M.; Cabanillas-Gonzalez, J.; Bradley, D. D. Phys. ReV. B 2003,
67, 195333.
(24) Chen, R.-F.; Fan, Q.-L.; Liu, S.-J.; Zhu, R.; Pu, K.-Y.; Huang, W.
Synth. Met. 2006, 156, 1161.
(25) Chen, X. W.; Tseng, H. E.; Liao, J. L.; Chen, S. A. J. Phys. Chem.
B 2005, 109, 17496.
Org. Lett., Vol. 10, No. 13, 2008
2915

for PFO-BuPFO10, showing that phosphafluorene units make
the polymer more ordered (ꢀ-phase) and hinder the exiton
migration to lowest energy gap segments (0-0 transition).26
The oxidation of the phosphorus center results in the red-
shift of the absorption and emission peaks, which is also
observed in phosphole and bisthienylphosphole systems.4 The
optical bandgap calculated from the onset of absorption
spectra of the polymeric solid film decreases with the
introduction of the phosphafluorene unit into PFO, suggesting
that the phosphafluorene incorporation leads to increased
conjugation length despite the spacial hindrance of the
methoxyl substituents of phosphafluorene units which will
lead to shorter conjugation length. The oxidation of the
phosphorus center results in a decreased energy bandgap.4
High photoluminescence efficiency was observed for PFO-
PhPF10 and PFO-BuPFO10 with the absolute quantum
efficiency (QE) of 53.1% and 52.4% respectively. The small
incorporation ratio of phosphafluorene into PFO significantly
changes the photophysical properties of PFO in the solid
state, while in the dilute THF solution the UV and PL spectra
remain identical with those of PFO. It seems that phos-
phafluorene units influence the photophysical properties
mainly through interchain interactions.
PFO-PhPF10 has reversible electrochemical redox behav-
ior as indicated by cyclic voltammetry (CV), while PFO-
BuPFO10s CV is irreversible. From the onset potential of
electrochemical reduction and oxidation, LUMO and HOMO
of PFO-PhPF10 were calculated to be -5.87 and -2.19 eV,
respectively. The incorporation of phosphafluorene into PFO
results in decreased HOMO, decreased LUMO, and un-
changed energy gap (∆E ), indicating improved electron
injection and transport ability of the copolymer. The co-
polymer even has slightly lower LUMO than that of
polysilafluorene (-2.18 eV),14 suggesting the much greater
impact of phosphorus than that of silicon and comparative
electron injection and transfer properties of PFO-PhPF10 in
comparison with those of polysilafluorenes.
Preliminary results for the EL emission spectra of PFO,
PFO-PhPF10, and PFO-BuPFO10 (see Figure 2) were
measured on a single layer device in the configuration ITO/
PEDOT/polymer/Ba/Al. PFO-PhPF10 showed a blue EL
(CIE ) 0.21, 0.24) with emission maxima at 424, 450, and
478 nm, while the EL of PFO-BuPFO10 is very different
and shows white-light emission (CIE ) 0.34, 0.36). The
maximum luminescent brightness of PFO-PhPF10 is 1423
cd/m2, which is much higher than the 142 cd/m2 of PFO-
BuPFO10 in the same device configuration. The EL spectrum
of PFO-PhPF10 is blue-shifted in comparison with that of
PFO with lower brightness. The interesting EL spectra of
phosphafluorene-incorporated copolymers show not only
their great potential as blue light-emitting and single-layer
white EL materials, but also the great property adjustability
of the small ratio incorporated phosphafluorene units through
the phosphorus atom modifications. The high impact of the
phosphafluroenes on their EL spectra needs to be further
investigated.
Figure 2. Electroluminescence (EL) spectra of the phosphafluorene
copolymers with the following device configuration: ITO/PEDOT/
polymer/Ba/Al.
In conclusion, we successfully synthesized new phosphorus-
containing copolymers based on phosphafluorene and fluo-
rene units. The influence of phosphafluorenes on polymer-
ization reaction, thermoproperties, and photophysical,
electrochemical, and electroluminesent properties was in-
vestigated. It is the first time that conjugated polymers
containing phosphafluorene have been prepared and used in
PLEDs. The PFO-PhPF10 and PFO-BuPFO10 show unique
optical, electrochemical, and optoelectronic properties. The
chemical modifications of the phosphorus atom of 11 mol
% phosphafluorene in the copolymer significantly changed
the properties of the polymer, and tuned the blue EL into a
white one. The successful synthesis and the studies of the
solution-processable phosphafluorene-fluorene conjugated
copolymers indicate that the 2,7-functionalized phosphafluo-
rene is a new building block for conjugated polymers with
strong blue light-emitting and single-layer white EL. The
further optimization of the materials is to prepare a higher
content of phosphafluorene units and their homopolymers
as well as oligomers. The development of new functional
devices based on the phosphafluorene π-conjugated materials
is currently underway in our laboratory.
Acknowledgment. This work was financially supported
by the National Natural Science Foundation of China under
Grants 20774043, 20574012, and 90406021, the Fok Ying
Tung Education Foundation under Grants 111051 as well
as the Nanjing University of Posts and Telecommunications
(NUPT) under Grant NY206073.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data, and NMR spectra for new
compounds and polymers. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL801068M
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Org. Lett., Vol. 10, No. 13, 2008