Synthesis and characterization of poly(fluorene-coalt-phenylene) containing 1,3,4-oxadiazole dendritic pendants

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alt-phenylene) Containing 1,3,4-Oxadiazole

Dendritic Pendants

CHUNG-WEN WU, HSIAO-HSIEN SUNG, HONG-CHEU LIN

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China

Received 7 April 2006; accepted 3 August 2006 DOI: 10.1002/pola.21720

Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A series of poly(fluorene-co-alt-phenylene)s containing various generations of dendritic oxadiazole (OXD) pendent wedges were synthesized by the Suzuki polyconden-sation of OXD-functionalized 1,4-dibromophenylene with 9,9-dihexylfluorene-2,7-dibor-onic ester. The obtained polymers possessed excellent solubility in common solvents and good thermal stability. Photophysical studies showed that the dendronized polymers appended with higher generations of OXD dendrons exhibited enhanced photolumines-cence efficiencies and narrower values of the full width at half-maximum. This was attrib-uted to the shielding effect induced by the bulky dendritic OXD side chains, which pre-vented self-quenching and suppressed the formation of aggregates/excimers. The energy transfer from the OXD dendrons to the polymer backbones was very efficient when excita-tion of the peripheral OXD dendrons resulted mainly in the polymer backbone emission alone. In particular, the photoluminescence emission intensities by the sensitized excita-tions of OXD dendrons in solid films of the polymers were all stronger than those by the direct excitations of their polymer conjugated backbones.VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6765–6774, 2006

Keywords: dendrimers; dendritic pendant; dendronized polymers; energy transfer; oxadiazole (OXD); polyﬂuorene

INTRODUCTION

Organic light-emitting diodes (OLEDs) have the potential to achieve low-cost and full-color ﬂat-panel displays because of their high brightness, easy fabrication, and wide range of emission col-ors.1,2However, some important and fundamental challenges remain unsolved, including the maximi-zation of external quantum efﬁciencies (EQEs), the design and synthesis of new materials with purer

colors, and modes of addressing devices for a full-color display with optimized resolution. A major factor responsible for low device EQEs is that the charge (electron and hole) injection and transporta-tion in emissive materials are generally unbal-anced. In general, most electroluminescent poly-mers inject and transport holes more efficiently than electrons because of their inherent richness of p electrons. One approach for improving the electron-transport properties of polymers is to in-corporate electron-deficient segments into main chains3–5 or as pendants attached to the back-bones.6–10Oxadiazole (OXD) units are among the most widely investigated electron-transport struc-tures for OLEDs because of the high electron affin-ity of the OXD segments in OLED molecules. Sev-eral OXD-containing light-emitting polymers have also been prepared in recent years.11–13

This article includes Supplementary Material available from the authors upon request or via the Internet at www. interscience.wiley.com/jpages/0887-624X/suppmat

Correspondence to: H.-C. Lin (E-mail: [email protected]. tw)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 6765–6774 (2006)

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Dendronized polymers may exhibit superior properties in future applications in comparison with their nondendronized analogues for a num-ber of reasons. The dendritic side chains, acting as solubilizers, can enhance processability with-out the loss of mechanical or thermal stability provided by the rigid polymer backbones. The dense dendron decorations may provide efﬁcient shields, such as protection against chemical reac-tants and prevention of molecular aggregation. In addition, the dendrons may serve, depending on their chemical nature, as light-harvesting anten-nae, charge carriers, and so on.14

Polyfluorenes (PFs) have attracted a great deal of attention because of their excellent thermal and chemical stability as well as high photolumi-nescence (PL) quantum efficiencies.15–18 How-ever, the problems encountered with PFs in gen-eral are their tendency to aggregate and form excimers, which lead to blue-green emissions with fluorescence quenching. In previous reports, re-search regarding PFs appended with polypheny-lene dendrons19,20 or Frećhet-type dendrons21–23 demonstrated that the shielding effect provided by the dendritic side chains on the conjugated backbones of PFs suppresses the formation of aggregates/excimers. Recently, Bo et al.24 first reported the synthesis of dendronized PFs con-taining functional carbazole dendrons, with their focal benzyl groups directly bonded to the C-9 car-bon of the fluorene units. The copolymerization approach has been widely used in the preparation of conjugated polymers to achieve specific elec-tronic and physical properties. It has also been demonstrated that the copolymerization of fluo-renes with various aryl partners allows for the tunability of the electronic properties with en-hanced stability.25–27

In hopes of combining both excellent electron affinities and energy-transfer (light-antenna) prop-erties in polymers, a new family of polyfluorene-co-alt-phenylenes carrying peripheral OXD func-tional dendrons attached to the 2- and 5-positions of the phenylene rings is presented in this report. Thus, the dendritic wedges play the roles of effi-cient site isolation and excellent electron affinity. In particular, the enhanced PL emission proper-ties of dendronized polymers have been observed in contrast to those of analogous conjugated main chains without dendritic pendants, for which sim-ilar PFs (lacking alternative phenylene back-bones) containing different generations of poly (benzyl ether) dendritic wedges with OXD periph-eral functional groups have also been reported

recently.28Therefore, the light harvest of chromo-phores from the luminescence of the dendritic pendants by the light-antenna design is more efﬁ-cient than direct excitation at the maximum absorption of light-emitting segments by an external light source.

EXPERIMENTAL

Characterization

1

H NMR spectra were recorded on a Varian Unity 300-MHz spectrometer with CDCl3as the solvent.

Elemental analyses were performed on a Heraeus CHN-OS rapid elemental analyzer. Monomers M1–M3 (4–6) were characterized with 1H NMR, elemental analyses, and fast atom bombardment [FAB; or matrix-assisted laser desorption/ioniza-tion time-of-flight (MALDI-TOF)] mass spectrom-etry (MS). Transition temperatures were deter-mined with differential scanning calorimetry (DSC; Diamond model, PerkinElmer) at a heating and cooling rate of 10 8C/min. Thermogravimet-ric analysis (TGA) was conducted on a DuPont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer at a heating rate of 20 8C/min. Gel permeation chromatography (GPC) analysis was conducted on a Water 1515 separation module with polystyrene as a stand-ard and tetrahydrofuran (THF) as an eluant. Ultraviolet–visible (UV–vis) absorption spectra were recorded in dilute chloroform solutions (106 M) on an HP G1103A spectrophotometer, and fluorescence spectra were obtained on a Hita-chi F-4500 spectrophotometer. Polymer solid films were spin-coated on quartz substrates from THF solutions with a concentration of 10 mg/mL. Cy-clic voltammetry measurements of the polymer films were performed on a BAS 100 B/W electro-chemical analyzer in acetonitrile with 0.1 M tet-rabutylammonium hexafluorophosphate as the supporting electrolyte at a scanning rate of 100 mV/s. A polymer film on a glassy carbon disk elec-trode was used as the working elecelec-trode. A plati-num wire was used as the counter electrode, and a silver wire was used as the reference electrode. All preparations and measurements were carried out at room temperature under nitrogen. The potentials were measured against a Ag/Agþ(0.01 M AgNO3) reference electrode with ferrocene as

the internal standard. The onset potentials were determined by the intersection of two tangents drawn at the rising current and background cur-rent of the cyclic voltammogram.

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Materials

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylﬂuorene (M4) was synthesized according to literature procedures.29The synthe-sis and characterization of compounds 1–3 and their intermediates are described in the supple-mental material. The chemicals and solvents were reagent-grade and were purchased from Aldrich, Acros, TCI, and Lancaster Chemical Co. Dichloromethane and THF were distilled to remain anhydrous before use, and the other chemicals were used without further puriﬁcation.

General Synthetic Procedure for Dendronized Monomers M1–M3 (4–6)

A mixture of corresponding compound 1, 2, or 3 (2.1 equiv), 2,5-dibromobenzene-1,4-diol (1 equiv), K2CO3(2.5 equiv), and 18-crown-6 (0.2 equiv) in

dry THF was heated to reﬂuxing and stirred under nitrogen for 48 h. The mixture was evapo-rated to dryness under reduced pressure, and the residue was partitioned between water and CH2Cl2.

The aqueous layer was extracted with CH2Cl2,

and the organic layers were dried over MgSO4.

The crude product was puriﬁed as outlined in the following text.

M1 (4)

Monomer M1 (4) was puriﬁed by column chroma-tography with EA/CH2Cl2(1:10) to obtain a white

solid.

Yield: 75%. 1H NMR (ppm, CDCl3): 0.90–0.97

(m, 12H), 1.34–1.54 (m, 16H), 1.75–1.77 (m, 2H), 3.92 (d, 4H), 5.16 (s, 4H), 7.03 (d, 4H), 7.21 (s, 2H), 7.63 (d, 4H), 8.06 (d, 4H), 8.16 (d, 4H). ELEM.

ANAL. Calcd. for C52H56Br2N4O6: C, 62.91%; H,

5.69%; N, 5.64%. Found: C, 63.01%; H, 5.73%; N, 5.64%. MS (FAB): m/z [Mþ] 993.20, calcd. m/z 992.83.

M2 (5)

Monomer M2 (5) was puriﬁed by column chroma-tography with EA/CH2Cl2 (1:6) to obtain a white

solid.

(m, 24H), 1.35–1.55 (m, 32H), 1.72–1.78 (m, 4H), 3.92 (d, 8H), 5.01 (s, 4H), 5.14 (s, 8H), 6.58 (s, 2H), 6.70 (s, 4H), 7.01 (d, 8H), 7.09 (s, 2H), 7.57 (d, 8H), 8.05 (d, 8H), 8.13 (d, 8H). ELEM. ANAL. Calcd.

for C112H120Br2N8O14: C, 68.56%; H, 6.16%; N,

5.71%. Found: C, 68.91%; H, 6.27%; N, 5.80%. MS

(MALDI-TOF): m/z [Mþ] 1962.84, calcd. m/z 1962.00.

M3 (6)

Monomer M3 (6) was puriﬁed by column chroma-tography with EA/CH2Cl2(1:12 gradually

increas-ing to 1:8) to obtain a white solid.

(m, 48H), 1.25–1.54 (m, 64H), 1.70–1.76 (m, 8H), 3.89 (d, 16H), 4.96 (s, 4H), 4.99 (s, 8H), 5.08 (s, 16H), 6.50 (s, 2H), 6.54 (s, 4H), 6.65 (s, 4H), 6.66 (s, 8H), 6.99 (d, 16H), 7.06 (s, 2H), 7.53 (d, 16H), 8.02 (d, 16H), 8.08 (d, 16H). ELEM. ANAL. Calcd.

for C232H248Br2N16O30: C, 71.44%; H, 6.41%; N,

5.75%. Found: C, 71.34%; H, 6.50%; N, 5.76%. MS (MALDI-TOF): m/z [Mþ] 3900.28, calcd. m/z 3900.35.

General Synthetic Procedure for Dendronized Polymers P1–P3

A mixture of corresponding dendritic monomer M1, M2, or M3 (4, 5, or 6), M4, K2CO3, toluene,

and H2O was degassed, and Pd[P(p-tolyl)3]3 was

added under a nitrogen atmosphere. The reaction mixture was heated to refluxing and stirred under nitrogen for 48 h. End-group capping was performed through the heating of the solution under refluxing for 6 h sequentially with phenyl-boronic acid and iodobenzene. After cooling, the crude polymers were dissolved in THF and puri-fied by precipitation from methanol, and dendron-ized polymers P1–P3 were collected and dried in vacuo.

Dendronized Polymer P1

M4 (273.5 mg, 0.46 mmol), M1 (4; 463 mg, 0.46 mmol), K2CO3 (1.1 g), Pd[P(p-tolyl)3]3 (8.00 mg),

toluene (10 mL), and H2O (4 mL) were used in the

reaction mixture. Polymer P1 was obtained as a slightly yellow solid.

(m, 34H), 1.26–1.75 (m, 22H), 3.90 (d, 4H), 5.16 (s, 4H), 7.00 (d, 4H), 7.20 (s, 2H), 7.49 (d, 4H), 7.63 (broad, 4H), 7.83 (d, 2H), 8.03 (d, 4H), 8.09 (d, 4H). ELEM. ANAL. Calcd. for (C77H88N4O6)n: C,

79.35%; H, 7.61%; N, 4.81%. Found: C, 78.12%; H, 7.56%; N, 4.45%.

M4 (120 mg, 0.20 mmol), M2 (5) (400 mg, 0.20 mmol), K2CO3 (1.1 g), Pd[P(p-tolyl)3]3 (8.00 mg),

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Yield: 68%.1H NMR (ppm, CDCl3): 0.63–0.97 (m,

46H), 1.31–1.73 (m, 40H), 3.84–3.92 (broad, 8H), 4.89 (s, 8H), 5.10 (broad, 4H), 6.46 (s, 2H), 6.61 (s, 4H), 6.93–7.03 (broad, 10H), 7.42 (d, 8H), 7.74 (broad, 6H), 7.97–8.14 (broad, 16H). ELEM. ANAL.

Calcd. for (C137H152N8O14)n: C, 77.08%; H, 7.18%;

N, 5.25%. Found: C, 75.99%; H, 7.17%; N, 4.67%.

M4 (60 mg, 0.10 mmol), M3 (6) (400 mg, 0.10 mmol), K2CO3 (1.1 g), Pd[P(p-tolyl)3]3 (8.00 mg),

toluene (10 mL), and H2O (4 mL) were used in the

(m, 70H), 1.29–1.66 (m, 72H), 3.80–3.89 (broad, 16H), 4.88 (broad, 28H), 6.44–6.59 (broad, 18H),

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6.87–6.98 (broad, 18H), 7.39–7.57 (broad, 22H), 7.88–8.07 (broad, 32H). ELEM. ANAL. Calcd. for

(C257H280N16O30)n: C, 75.78%; H, 6.93%; N,

5.50%. Found: C, 74.75%; H, 6.91%; N, 5.05%.

RESULTS AND DISCUSSION

Synthesis and Characterization

The synthetic routes of monomers M1–M3 (4–6) and polymers P1–P3 are shown in Schemes 1 and 2. In the presence of K2CO3in THF at reﬂux,

2,5-dibromobenzene-1,4-diols were reacted with various generations of corresponding dendrons to obtain monomers M1–M3 (4–6), which were then copolymerized with M4 by Suzuki polycondensa-tions (SPCs). In the beginning, when Pd(PPh3)4

was used as a catalyst precursor, polymer P3 could not be obtained because of the shielding

effect of M3 on the reactive site, so Pd[P(p-tolyl)3]3 seems to be a superior choice in many

SPC cases.30 Freshly prepared Pd[P(p-tolyl)3]3

was used as the catalyst precursor31 with Ali-quate 336 as the phase-transfer reagent in a bi-phasic system (toluene/aqueous K2CO3) and gave

alternating copolymers P1–P3. All dendronized polymers could be fully dissolved in common or-ganic solvents, such as methylene chloride, chlo-roform, toluene, and THF. The molecular weights of the polymers determined by GPC with polysty-rene standards are summarized in Table 1. GPC analysis showed that the weight-average molecu-lar weights and polydispersity indices of the den-dronized polymers were in the range of 3.3–4.4 104

and 1.4–2.4, respectively. The data reveal that the degree of polymerization decreased as the size of the dendritic monomer increased. The same observation was also reported for the syn-thesis of similar dendronized polymers.21,23 The

Scheme 2. Synthetic routes of polymers P1–P3.

Table 1. Molecular Weights and Thermal Properties of Polymers P1–P3

Polymer Mna Mwa Polydispersity Indexa Tg (8C)b Td (8C)c P1 17,500 42,000 2.4 122 310 P2 24,100 44,300 1.8 94 341 P3 24,000 33,500 1.4 77 353

a_{Number-average molecular weight (M}

n) and weight-average molecular weight (Mw)

deter-mined by GPC in THF with polystyrene standards (polydispersity index¼ Mw/Mn). b_{Glass-transition temperature determined by DSC at a heating rate of 10}

8C min1_. c

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thermal properties of the dendronized polymers were investigated with TGA and DSC, and the results are presented in Table 1. All the polymers exhibited good thermal stability: less than 5% weight loss decomposition at about 310–353 8C under nitrogen but more than 50% weight loss decomposition at 450–460 8C (Fig. 1) were ob-served. The glass-transition temperatures of the dendronized polymers were in the range of 77– 1228C and declined with the increasing size of the attached dendrons. In comparison with a previous report, the glass-transition temperatures of dendro-nized polymers P1–P3 were higher than that of poly [(9,9-dihexylﬂuorene)-alt-co-(2,5-dihexyl-2,5-phenyl-ene)] (glass-transition temperature ¼ 50 8C) or poly[(9,9-dihexylﬂuorene)-alt-co-(2,5-dihexyloxy-1,4-phenylene)] (glass-transition temperature¼ 72 8C).32

Optical Properties

The spectroscopic properties of polymers P1–P3 were measured in both solutions (THF) and solid

films. The optical properties of polymers P1–P3 are summarized in Table 2. The dendronized poly-mers showed very similar absorption and emis-sion characteristics in THF. As shown in Figure 2, the polymers in THF solutions exhibited an absorption peak at about 300 nm whose intensity increased with the generation number; this was due to the absorption of the peripheral OXD moi-eties. This result further confirmed the accom-plishment of various generations of OXD den-drimers. The additional absorption peak at about 367 nm was assigned to the p–p* transition con-tributed from the conjugated backbones of the polymers, which was not affected by the genera-tion of dendronized polymers. Upon the excitagenera-tion of the backbones of polymers P1–P3 at 367 nm, the PL emission spectra displayed the maximum intensity at about 414 nm. The PL spectra in THF are almost identical for polymers P1–P3, as shown in Figure 3. In contrast to dilute solutions (in THF), the absorption spectra of the polymers in solid films are similar (a little redshifted) to those in solutions. The optical band gaps deter-mined from the absorption edges of the UV–vis spectra of polymers P1–P3 in solid films were found to be 3.03 eV. The PL emission spectra of polymers P1–P3 in solid films are only 4–7 nm bathochromically redshifted compared with those in solutions. In particular, because of the stronger aggregation in the solid films of the lowest gener-ation polymer, P1, having the smallest dendron size, shows a more obvious shoulder followed by a long, featureless tail (extending into the red region) than the higher generation polymers P2 and P3 (Fig. 4). It can be concluded that the higher generation polymers have more site-isola-tion or dilusite-isola-tion effects because of the larger size of the dendrons. A previous publication33 reported that the various lengths of alkoxy side chains in

Figure 1. TGA traces of polymers P1–P3.

Table 2. Absorption and PL Emission Spectral Data of Polymers P1–P3 in THF and Solid Films

Polymer kabs,sola (nm) kabs,filmb (nm) Band Gapc (eV) kPL,sold (nm) kPL,filme (nm) FwhmPL,filmf (nm) FPL,solg FPL,filmh P1 301,364 323,388 3.03 414 421 44 0.60 0.08 P2 300,367 322,380 3.03 414 421 45 0.87 0.18 P3 300,367 321,376 3.03 415 419 44 0.82 0.26 a_k

abs,solwere deﬁned from the peaks of absorption spectra in solution. b

kabs,filmwere defined from the peaks of absorption spectra in solid films.

c_{Band gaps were calculated from the onsets of UV-visible absorption spectra of P1–P3 in solid ﬁlms.} d

kPL,solwere deﬁned from the peaks of PL spectra in solution. e

kPL,filmwere defined from the peaks of PL spectra in solid films. f_Fwhm

PL,filmwere defined from full width at half maximum of PL spectra in solid film. g

Solution fluorescence quantum efficiency measured in THF, relative to 9,10-diphenylanthracene (FPL¼ 0.90). h_{PL quantum efficiency estimated relative to 9,10-diphenylanthracene in poly(methyl methacrylate) as a standard (}

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poly[(9,9-dihexylﬂuorene)-alt-co-1,4-phenylene)] (PDHFP) were attached to the phenylene rings of the backbones and that the full width at half-maxi-mum (fwhm) values of these polymers strongly de-pended on the lengths of the attached alkoxy side chains. The fwhm value decreased from 62 nm in PDHFP (without any side chain on the phenylene ring) to 46 nm in poly(9,9-dihexylﬂuorene)-(2,5-didecyloxy-1,4-phenylene) (PDHFDDOP) (longer side chains ofOC10on the phenylene ring). The

report explained that the smaller fwhm values of the polymers with longer side chains were due to the emission resulting from more isolated main-chain ﬂuorophores. Thus, the fwhm values of the emission curves in solid ﬁlms of dendronized poly-mers P1–P3 were also quite narrow (44–45 nm),

and purer blue-light emissions were yielded because of their outstanding site-isolation effect. In addition, compared with PDHFP, the vibronic structures in PL emissions of P2 and P3 were remarkably reduced, and no noticeable spectral shoulder above 500 nm was observed.

A comparison of the PL emission spectrum of model compound 1 and the absorption spectra of dendronized polymers P1–P3 in THF (Fig. 2) shows that the overlap of the OXD emission peak (363 nm) and P1–P3 backbone absorption peaks (ca. 367 nm) was extremely large. The large spec-tral overlap between the two interacting chromo-phores indicates that the probability of donor– acceptor energy transfer should be high. The energy-transfer efficiency of the dendritic wedge is estimated by the intensity ratio of the maxi-mum absorption of OXD dendrons in the absorp-tion spectrum to that in the fluorescence excita-tion spectrum.34According to this estimation, the energy-transfer efficiencies of various genera-tions of OXD dendrons were 59, 43, and 49% for P1, P2, and P3, respectively. It was consistently found that the energy-transfer efficiency decreased as the generation (i.e., the size) of the surface-functionalized poly(benzyl ether)-type dendritic wedge increased. Upon excitation at either OXD dendrons or polymer backbones of polymers P1– P3 in THF, the obtained PL emission spectra are identical, as shown in Figure 5(a). The almost complete disappearance of the OXD emission at 363 nm indicates that energy-transfer efficiency was extremely high in these molecules. A compar-ison of the PL emission intensity from the sensi-tized excitation (excited at the maximum absorp-tion of OXD dendrons) and that from the direct

Figure 2. Normalized UV–vis absorption spectra of polymers P1–P3 and PL emission spectrum of com-pound 1 in THF. The absorption spectra are normalized at the absorption peaks of the polymer backbones around 367 nm.

Figure 3. Normalized PL spectra of polymers P1–P3 in THF.

Figure 4. Normalized PL spectra of polymers P1–P3 in solid ﬁlms.

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excitation (excited at the maximum absorption of the polymer backbones) of dendronized polymers P2 and P3 shows more intense PL backbone

emissions by the sensitized excitation from the energy transfer of OXD dendrons than by the direct excitation from the absorption of chromo-phore backbones. This indicates that the overall fluorescence of dendronized polymers P1–P3 resulted not only from the contribution of the polymer backbones but also from that of the pe-ripherally dendritic OXD units. In addition, the OXD-functionalized dendritic wedges were very efficient light-harvesting moieties for funneling energy to the polymer backbones. As shown in Figure 5(b), the excitations of the peripheral OXD moieties of dendronized polymers P1–P3 in solid films resulted in fluorescence patterns similar to those excited at the maximum absorption of the polymer backbones alone. In particular, the PL emission intensities by the sensitized excitations of OXD dendrons in solid films of polymers P1– P3 were all stronger than those by the direct exci-tations of their polymer conjugated backbones. Actually, the PL emission intensity of polymer P3 in a solid film excited at peripheral OXD den-drons was about 86% higher than that excited at the backbones. In general, similar effects have been commonly described for dye-labeled den-drimers.35–38 However, the result demonstrates that dendronized polymers may show stronger emissions through efficient energy transfer from peripheral dendrons to emitting backbones.

As listed in Table 2, the PL quantum yields of polymers P1–P3 in THF solutions were meas-ured with 9,10-diphenylanthracene as a reference standard39 (cyclohexane, PL quantum yield ¼ 0.9), and the highest quantum yield reached 0.87. The PL quantum yields in solid films were meas-ured with the same standard in poly(methyl methacrylate).40 The shielding effect of the den-dritic side chains on the polymer backbones was also reflected in the emission efficiency of the

Figure 5. PL emission spectra of polymers P1–P3 (a) in solutions (THF) and (b) in solid ﬁlms, which were excited at the maximum absorption of the polymer backbones (solid symbols) and the polymer-periphery OXD dendrons (open symbols).

Table 3. HOMO and LUMO Energies and Electrochemical Properties of Polymers P1–P3 Polymer Ered/onset (V)a Ered/peak (V)b Eox/onset (V)a EHOMO (eV)c ELUMO (eV)c Eg (eV)d P1 2.07 2.57 0.93 5.73 2.73 3.00 P2 2.01 2.58 0.94 5.74 2.79 2.95 P3 2.03 2.54 0.93 5.73 2.77 2.96 a

Ered/onsetand Eox/onsetwere determined from the intersection of two tangents drawn at the ris-ing and background currents of the CV measurements.

b_Ered/peak_{was deﬁned from the ﬁrst peak in the cathodic sweep.}

c_EHOMO_{and E}LUMO_{of polymers were estimated with regard to the energy level of the ferrocene}

reference (4.8 eV below the vacuum level).

d

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dendronized polymers. Although this relative method could give only an estimation of the fluo-rescence quantum yields of the polymers, the data still indicated that the fluorescence quantum yields of the polymers depended on the sizes of the attached dendrons; that is, the larger the den-drons were, the higher the PL quantum yields were. Therefore, the second generation of dendron-ized polymer P3 in the solid film showed the high-est quantum yield, which was attributed to the minimization of the self-quenching by the attach-ment of bulky pendent dendrons onto the polymer backbones.

Electrochemical Properties

Cyclic voltammetry measurements were carried out to determine the energies of the highest occu-pied molecular orbital (HOMO) and lowest unoc-cupied molecular orbital (LUMO) of polymers P1–P3. The HOMO, LUMO, and electrochemical

properties of the polymers are summarized in Table 3. As shown in Figure 6, polymers P1–P3 showed almost identical behavior during anodic and cathodic scans because of the same backbone structure of poly(ﬂuorene-co-alt-phenylene) with different generations of OXD dendrons. In the cyclic voltammogram of polymer P1, the differ-ence between the anodic and cathodic onset potentials was 3.00 V, which implied that thep– p* band gap of the polymer was 3.00 eV. The data were similar to those obtained from the absorp-tion edge of the UV–vis spectrum. With respect to the energy level of the ferrocene reference (4.8 eV below the vacuum level),41the HOMO and LUMO energy levels of polymer P1 were estimated to be 5.73 and 2.73 eV, respectively.

CONCLUSIONS

A series of novel poly(fluorene-co-alt-phenylene)s containing different generations of dendronized side chains, including Frećhet-type poly(aryl ether) dendrons and functional peripheral OXD groups, were synthesized. The resulting polymers pos-sessed good thermal stability and excellent solu-bility in common organic solvents. The emission spectral quality (narrow fwhm and reduced tail) could be improved by the insertion of bulky OXD dendrons as side chains of the polymers because of less molecular close stacking. In addition, it was demonstrated that the peripheral OXD den-drons had specific light-antenna and enhanced backbone luminescence properties. The PL emis-sion intensities by the sensitized excitations of OXD dendrons in solid films of polymers P1–P3 were all stronger than those by the direct excita-tions of their polymer conjugated backbones.

The authors thank the National Science Council of Tai-wan (Republic of China) for its ﬁnancial support through NSC 93-2113-M-009-011. Yu-Chie Chen (ma-trix-assisted laser desorption/ionization time-of-ﬂight mass spectrometry) and Ching-Fong Shu (gel permea-tion chromatography and cyclic voltammetry measure-ments) at the Department of Applied Chemistry of National Chiao Tung University (Taiwan) are also acknowledged for their instrumental support.

REFERENCES AND NOTES

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Figure 6. Cyclic voltammetry of polymers P1–P3 during (a) the oxidation processes and (b) the reduction processes.

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