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`2
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`

`

`Synthesis of a novel cationic water-soluble efficient blue photoluminescent
`conjugated polymer
`
`Bin Liu,a Wang-Lin Yu,h Yee-Hing Lai*a and Wei Huang*h
`
`a Department of Chemistry, National University of Singapore, Singapore 119260, Republic of Singapore
`b Institute of Materials Research and Engineering (/MRE), National University of Singapore, 3 Research Link,
`Singapore I 17602, Republic of Singapore. E-mail: wei-huang@imre.org.sg
`
`Received (in Cambridge, UK) 24th January 2000, Accepted 23rd February 2000
`Published on the Web 17th March 2000
`
`3
`C'")
`0
`3
`3
`
`A novel cationic conjugated polymer, poly[(9,9-dihexyl-
`2,7-fluorene)-alt-co-(2,S-bis(3-[(N,N-dimethyl)-N-ethylam(cid:173)
`monium]-1-oxapropyl}-1,4-phenylene)] dibromide, which is
`water-soluble and emits bright blue fluorescence both in
`solutions and as films, is synthesized through Suzuki
`coupling reaction and a post-polymerization treatment.
`
`Water-solubility of conjugated polymers may offer many new
`application opportunities. Potential applications of water(cid:173)
`soluble conjugated polymers include the construction of active
`layers in organic light-emitting diodes through a layer-by-layer
`self-assembly approach, 1 as buffer layer and emissive layer
`materials in inkjet printing fabricated organic LEDs,2 and as
`highly sensitive fluorescent sensory materials in living bodies.3
`Such applications generally favor high molecular weights and
`high photoluminescence (PL) efficiencies and require different
`ionic types. Water-solubility of semiconducting conjugated
`polymers was first demonstrated in 3-substituted polythio(cid:173)
`phenes4·S and was then extended to poly(para-phenylene
`vinylene) (PPV)-based6 and poly(para-phenylene) (PPP)-based
`polymers.7·8 The water-solubility
`in such polymers was
`achieved by functionalizing the substituted side chains with
`terminal carboxylate or sulfonate groups. These polymers are,
`therefore, anionic polyelectrolytes. Until the most recent report
`on the synthesis of ammonium-functionalized PPPs from
`Reynolds' group,9 there are no cationic water-soluble conju(cid:173)
`gated polymers available. Fluorene-based conjugated polymers
`have received considerable attention in the past few years for
`the high efficiencies both in PL and in electroluminescence
`(EL).IO Moreover, we recently demonstrated that conjugated
`polymers based on alternating fluorene and phenylene back(cid:173)
`bones are promising efficient and stable blue luminescent
`materials. 11 This work presents
`the successful effort
`in
`developing a cationic water-soluble conjugated polymer based
`on the alternating fluorene and phenylene backbone, which
`represents the first example of fluorene-based water-soluble
`conjugated polymer and exhibits efficient blue light emission.
`The chemical structure of the new water-soluble conj ugated
`poly[(9,9-d ihexyl-2,7 -fluorene)-a/t-co-(2,5-bis { 3-
`polymer,
`[(N,N-dimethyl)-N-ethylammonium]-1-oxapropyl}-1 ,4-
`phenylene)] dibromide (PFP-NMe2EtBr) and the synthetic
`route are depicted in Scheme I. Monomer I, 9,9-dihexyl(cid:173)
`fluorene-2,7-bis(trimethylene boronate), was synthesized from
`2,7-dibromofluorene as the starting material. 11 Monomer II,
`2,5-bis[3-(N,N-dimethylamino )-1-oxapropyl)-1 ,4-dibromoben(cid:173)
`zene], was prepared from 2,5-dibromohydroquinone by reac(cid:173)
`tion with 2-ch loroethyldimethylam ine in refluxing acetone in
`the presence of an excess of anhydrous potassium carbonate.9
`The polymerization was carried out in a mixture of toluene and
`aqueous potassium carbonate solution (2M) containing I mol %
`Pd(PPh3)4 under vigorous stirring at 85-90 °C for three days.
`The neutral polymer, polymer I, was obtained as a fibrous white
`solid with a yield of ca. 70% after purification and drying.
`Conversion of the neutral polymer to the final water-soluble
`polymer was achieved by treatment with bromoethane in
`dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) (I :4).
`4J
`DOl: 10.1039/b000740o
`
`s
`1
`0
`
`Monorrer l
`
`OH
`
`I
`N-
`
`0
`
`-"~
`
`HO
`
`0
`
`0
`
`~ PJ
`Co, -o.;p-~
`f"
`~ a,
`--:;
`°
`H13C 6 c 6H13
`J
`Br-o-Br _i_ Br-o-Br
`S Mono~r u
`-N
`fN Br
`N-
`I
`I
`f
`~ -iii-~
`So
`c , 11, ,+So
`
`\ j
`+\
`
`0
`
`H13c 6 CoJ-113
`
`H13c,
`
`-~
`I
`
`Polymer I
`
`Br N
`\
`'
`
`Polyme r II
`
`Scheme 1 Chemical structures and synthetic route towards the polymers.
`Reagents and conditions: i, 2-chlorotrimethylamine hydrochloride, anhy(cid:173)
`drous potassium carbonate, acetone, reflux , 3 days; ii , toluene/aqueous
`potassium carbonate solution (2 M), Pd(PPh 3) 4 , 85- 90 °C, 3 days; iii,
`bromoethane, DMSO-THF (I :4), SO °C, 3 days.
`
`The structures of both the neutral and the final water-soluble
`polymers were confirmed by NMR and elemental analysis.t
`The characterization of molecular weight is often a problem
`for water-soluble conjugated polymers. The post-polymeriza(cid:173)
`tion approach for the realization of water-solubility allows us to
`characterize the molecular weight at the stage of the neutral
`polymer. The neutral polymer, polymer I, can be readily
`dissolved in CHC13, THF, toluene and aqueous acid, but is
`insoluble in DMSO, methanol and water. Gel-permeation
`chromatography (GPC) measurement using THF as eluent and
`polystyrenes as the standards indicated the weight average
`molecular weight to be 47 000, with a polydispersity of 1.61.
`Another advantage of the post-polymerization approach is that
`the quaternization degree can be controlled and thus the water(cid:173)
`solubi lity of the resultant polymer is tunable. The tunable
`solubility is useful for the application of such materials as buffer
`layers in inkjet printing fabrication of LEDs. 2 The degree of
`quaternization cou ld be determined by 1 H NMR spectra. As
`shown in Fig. I, the neutral polymer exhibits three peaks in the
`region 87.8-7.6 arising from the aromatic protons in fluorene
`and one peak at 8 7.15 due to the protons in the phenylene ring.
`The well resolved peaks at 8 4.09, 2.67 and 2.30 correspond to
`the oxygen (-OCHr) and
`methylene groups adjacent to
`nitrogen (-CH 2N-) atoms and
`the methylamino groups
`(-NCH3), respectively. After the treatment with bromoethane,
`the peaks in the aromatic region remain almost unchanged,
`whereas all the signals corresponding to -OCH2- , -CH2N-, and
`-NCH3 split into two peaks, which arise from the quaternarized
`(lower field) and un-quaternized components, respectively. The
`
`Chern. Commun., 2000, 551-552
`
`551
`
`This journal is © The Royal Society of Chemistry 2000
`
`3
`
`

`

`Fig. 2. The absorption and emission peaks appear at 359 and 416
`nm, respectively.
`From the application point of view, one of the most attractive
`properties of the polymers is the relatively high PL quantum
`yield ( <Pp1). Both the neutral polymer and the quaternized
`polymers display strong blue fluorescence either in solutions or
`as films upon exposure to UV light. The <Pn of the neutral
`polymer (polymer I) is as high as 97 % as measured from its
`dilute solution in chloroform.:j: For polymer II (with a degree of
`quaternization of 80%), the <Pp1 was measured to be 86% from
`its dilute solution in methanol. When the measurement was
`conducted in aqueous solution, the corresponding value of ct>P1
`is 25 %. The decrease of PL efficiency may be attributed to the
`aggregation of the polymer in aqueous solution. This was
`supported by a further reduced PL efficiency measured in the
`solid state (films on quartz plate cast from methanol solution),
`which
`is 4% compared with 9, 10-diphenylanthracene as
`standard (dispersed in PMMA films with a concentration lower
`than 1 X 10- 3 M, assuming a PL efficiency of 83%). 12
`In summary, we have synthesized a new cationic water(cid:173)
`soluble conjugated polymer based on the alternating fluorene
`and phenylene backbone structure through a facile post(cid:173)
`polymerization approach , which permits a full structural
`characterization of the polymer and control of the degree of
`cation formation. The polymer emits intense blue fluorescence
`both in solutions and in film states. The good water-solubility
`and high fluorescence quantum yield make it attractive for
`applications
`in fabricating organic LED devices and as
`fluorescent bio-sensory material s.
`The work was partially supported by the National University
`of Singapore through a research grant (RP97061 0).
`
`Notes and references
`t NMR and elemental analyses data for polymers I and II : polymer l : 8 (300
`MHz, CDC13) 7.79 (br, 2H , Ar-H), 7.66-7 .60 (br, 4H , Ar-H), 7. 15 (s, 2H,
`Ar-H), 4.09 (br, 4H, -OCH2), 2.67 (br, 4H, - CH 2N), 2.27 (s, 12H, NCH 3),
`2.05 (br, 4H, fluorene 9-H), 1.12---0.78 (br, 22 H, -CH2 , -CH3). Calc. for
`C39H540 2N2 : C, 80.41; H , 9.27; N, 4.81 ; Br, 0 (terminal group). Found : C,
`79.60; H, 8.99; N , 4.90; Br, 0%.
`Polyme r II: 8(300 MHz, CD30D) 7.93 (br, 2H, Ar-H), 7.70--7.64 (br, 4H,
`Ar-H), 7.24 (br, 2H, Ar-H), 4.56 (br, 3.2H, - OCH 2), 4.38 (br, 0.8H,
`- OCH 2), 3.77 (br, 3.2H, -CH2N), 3.52 (br, 0.8H, - CH2N), 3.42 (br, 3.2H,
`NCH2CH3), 3.06 (br, I2H, NCH3), 2.87 (br, 4.8H , NC H2CH3), 2. 16 (br, 4H,
`flu orene 9- H), 1.28---0.80 (br, 22H, -CH2 , -CH3). Calc. for C39Hs40 2
`N2-4H 20 · 1.6C2H5Br (the amounts of H20 and C2H5Br were based on TGA
`analysis and 1H NMR): C , 61.10; H, 8.45; Br, 15.46; N, 3.38. Found: C,
`60.63; H, 8.29; Br, 16.04; N , 3.52%.
`:j: The quantum yields were measured using a Perkin Elmer LS SOB
`luminescence spectrometer with dilute solutions (A < 0.2) at room
`temperature.I J Quinine sulfate solution (ca. 1.0 X 10- s M) in 0.10 M
`H2S04 (quantum yie ld, 55%) was used as a standard.
`
`M. Ferreira and M. F. Rubner, Macromolecules, 1995, 28, 710 I; A. C.
`Fou, 0 . Onitsuka, M. Ferreira and M. F. Rubner, J. Appl. Phys. , 1996,
`79, 7501; J. W. Baur, S. Kim, P. B. Balanda, J. R. Reynolds and M. F.
`Rubner, Adv. Mater., 1998, 10, 1452.
`2 J. Bharathan and Y. Yang, Appl. Phys. Lett., 1998, 72, 2660; S. C.
`Chang, J. Bharathan andY. Yang, Appl. Phys. Leu. , 1998, 73, 256 1.
`3 K Fiiid and M. Leclere, Chern. Commun., 1996, 2761 ; J . Am. Chern .
`Soc., 1998, 120, 5274.
`4 A . 0. Patil, Y. lkenoue, F. Wudl and A . J. Heeger, J. Am. Chern. Soc.,
`1987, 109, 1858.
`5 P. Pickup, J. Electroanal. Chern ., 1987, 225, 273 .
`6 S. Shi and F. Wudl , Macromolecules, 1990, 23, 2 11 9.
`7 T. I. Wallow and B. M. Novak , J. Am. Chern. Soc., 199 1, 113, 74 11.
`8 A. D. Child and J. R. Reynolds, Macromolecules, 1994, 27, 1975.
`9 P. B. Balanda, M. B. Ramey and J. R. Reynolds, Macromolecules, 1999,
`32, 3970.
`10 A. W. Grice, D. D. C. Bradley, M. T. Bemius, M. Inbasekaran , W. W.
`Wu and E. P. Woo, Appl. Phys. Leu ., 1998, 73, 629.
`II W . L. Yu, J. Pe i, Y. Cao, W. Huang and A. J. Heeger, Chern. Commun. ,
`1999, 1837.
`12 J. N. Demas and G . A . Crospy, J . Phys. Chern., 197 1, 75, 99 1.
`13 H. S. Joshi , R. Jamshidi andY. Tor, Angew. Chern., Int. Ed., 1999, 38,
`2722.
`
`Communication b000740o
`
`CO,OD
`
`C!l)OD
`
`(b)
`
`9.0
`
`8.0
`
`7.0
`
`6.0
`
`s.o
`
`4.0
`
`3.0
`
`2.0
`
`1.0
`
`0.0
`
`Fig.l 1H NMR spectra of polymers I (a) and II (b).
`
`relative integrals of each pair of the split peaks can thus be used
`to estimate the degree of quaternization. The highest degree of
`quaternization obtained in our experiments is ca. 80%. With
`this degree of quaternization, the resulting polymer shows
`solubility characteristics opposite to that of polymer I, being
`completely sol uble
`in DMSO, methanol, and water but
`insoluble in CHC1 3 and THF.
`Polymer II also possesses good thermal stabi lity. The onset
`degradation temperature of this polymer is 300 °C in nitrogen,
`whereas it starts to decompose above 230 °C in air, with a small
`amount of water loss at lower temperatures. In air, no residue
`remained after heating to 800 °C.
`The UV-VIS absorption spectra of polymer I in chloroform
`solution and as a film (on quartz plate, spin-cast from
`chloroform solution) are almost
`identical with
`the same
`maximum absorption at 366 nm. The PL spectrum of the
`polymer sol uti on peaks at 414 nm, whereas the polymer film
`exhibits an emission maximum at 424 nm with a vibronic
`shoulder around 444 nm. The emission spectral feature of the
`polymer in the film state is very similar to that of the polymer
`having the same backbone structure and substitution on fluorene
`unit but without the terminal amino group in the phenylene side
`chains. 11 This implies that the terminal amino groups are
`unlikely to affect the conformation of the backbone in the film
`state. For the quaternized sample with the highest degree of
`quaternization (ca. 80%), the electronic spectra are remarkably
`dependent on the solvent, showing a bathochromic shift with a
`decrease in solvent polarity. As displayed in Fig. 2, the polymer
`shows absorption maxima at 343, 354 and 367 nm in water,
`methanol and DMSO, respectively. The corresponding PL
`maxima appear at 409, 409 and 419 nm, respectively. Uniform
`and transparent films of the polymer on quartz plates were
`prepared by spin-casting its aqueous solution. The UV-VIS
`absorption and PL spectra of the polymer film are also shown in
`
`1000
`2 ·c:
`iii z::
`
`800
`
`600
`
`:::J
`..ci
`
`400
`
`1.00
`Vi
`:g 0.80
`:::J
`..ci
`~0.60
`Ql
`()
`
`<(
`
`552
`
`Chern. Commun., 2000, 551- 552
`
`·u;
`~ 0.40
`c
`Ql
`..0
`0 .2 0.20
`~
`200 _J a..
`0.00 -1----...---.---..:s~~~~iliiilo;:-==l-
`0
`250 300 350 400 450 500 550 600
`Wavelength/nm
`Fig. 2 UV- VIS absorption and photoluminescence spectra of polymer II in
`solutions and as films. (a) UV in aqueous solution, (b) UV in MeOH, (c) UV
`in film , (d) UV in DMSO, (e) PL in MeOH , (f) PL in aqueous solution,
`(g) PL as film and (h) PL in DMSO.
`
`4
`
`