Aromatic Oxadiazole-Based Conjugated Polymers withExcited-State Intramolecular Proton Transfer: TheirSynthesis and Sensing Ability for ExplosiveNitroaromatic Compounds

TAE HOON KIM, HYUNG JUN KIM, CHAN GYU KWAK, WON HO PARK, TAEK SEUNG LEE

Organic and Optoelectronic Materials Laboratory, Department of Organic Materials and Textile System,Chungnam National University, Daejeon 305-764, Korea

Received 7 September 2005; accepted 7 January 2006DOI: 10.1002/pola.21319Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Aromatic polyoxadiazole derivatives containing 9,90-dioctylfluorene weresuccessfully synthesized via the Suzuki coupling reaction. The oxadiazole moiety inthe polymer backbone was linked with the bis(hydroxyphenyl) group in its 2-positionto exhibit a large Stokes shift in the emission spectrum due to the excited-state intra-molecular proton transfer. To prepare the polymer via the Suzuki cross-coupling reac-tion, the hydroxyl group in the monomer was protected with the t-butoxycarbonylgroup before polymerization and removed after polymerization to a desirable extent.The polymer with the free hydroxyl group showed a considerable sensitivity for nitro-aromatic compounds, exhibiting fluorescence quenching in a chloroform solution. Theinteraction between the electron-donating OH group and electron-deficient nitroaro-matic compounds seemed to play a decisive role in fluorescence quenching. VVC 2006

Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2059–2068, 2006

Keywords: conjugated polymers; excited-state intramolecular proton transfer; poly-bis(hydroxyphenyl)oxadiazole; luminescence; sensors

INTRODUCTION

In recent years, organic fluorescent conjugatedpolymers have received much attention for poten-tial applications such as electroluminescent devi-ces, photoconductors, nonlinear optical materials,and sensors. Various kinds of conjugated poly-mers have been synthesized and reported in theliterature.1–4 Among them, conjugated polymerswith nitrogen-containing fused-ring systems areone of the most promising categories of fluores-cent polymers because of their high fluorescence,high electron mobility, and easy tailoring of thechemical structure.5–8

Aromatic oxadiazole-based compounds havehigh electron affinities, which facilitate both elec-tron injection and transport.9,10 Polymers contain-ing an oxadiazole moiety have been widely usedin electronic devices as electron-transporting ma-terials because the presence of the oxadiazole ringin the molecular backbone affects the electronicproperties of the resulting polymers.11–14

In addition to the excellent electron-transport-ing properties, oxadiazole-based materials with alarge Stokes shift of the fluorescence emissioncan be easily prepared with a structural modifica-tion. The incorporation of the ortho-hydroxy-phenyl group in the 2-position of oxadiazole pro-vides the excited-state intramolecular protontransfer (ESIPT) process, which equilibrates withthe tautomeric enol and keto forms, as shown inScheme 1. Such ESIPT can be induced by intra-

Correspondence to: T. S. Lee (E-mail: tslee@cnu.ac.kr)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 2059–2068 (2006)VVC 2006 Wiley Periodicals, Inc.

2059

molecular hydrogen bonding upon photoexcita-tion, and the presence of an adjacent free hy-droxyl group is essential to heterocyclic com-pounds such as benzazoles, quinolines, and oxa-diazoles.15–18 ESIPT provides two emission bandsfrom enol and keto forms; the keto form is respon-sible for the long wavelength emission, as shownin Scheme 1.

Recently, the synthesis of oxadiazole-basedpolymer derivatives with adjacent hydroxyphenylgroups and their application to fluorescent che-mosensors were reported during the preparationof this article on polymers with a similar struc-ture.19 Although it was reported that such poly-mers did not show ESIPT upon excitation, wefound completely different photophysical resultsin comparison with the results for a similar poly-mer prepared with a different synthetic method.

It has been reported that organic conjugatedpolymers can be used as a potential fluorescentchemosensors for explosive nitroaromatic com-pounds, providing portable detection devices.Conjugated polymers composed of polypentipty-cene and polyacetylene have been reported to behighly sensitive fluorescent chemosensors for thevapors of 2,4,6-trinitrotoluene and 2,4-dinitro-toluene (DNT) because of the molecular wireeffect.20–22 Besides, fluoroalkylated polysilanehas been used as a chemosensor for explosivenitroaromatic compounds because of electron-donating and -accepting interactions.23 Here weattempted to synthesize a bis(hydroxyphenyl)oxa-diazole polymer, which showed strong ESIPT,

and to investigate the chemosensing property to-ward electron-deficient explosive nitroaromaticcompounds. Although the chemical structure ofthe polymer is similar to one already reported,ESIPT in the resulting polymer was newly found,and its sensing ability for nitroaromatic com-pounds was investigated in terms of an electron-transfer interaction.

EXPERIMENTAL

Materials and Reagents

All the chemicals and reagents were purchasedfrom Aldrich and used as received unless other-wise specially noted. Tetrahydrofuran (THF),used as a Suzuki coupling reaction solvent, waspurified by a conventional distillation methodwith sodium/benzophenone.

Characterization

1H NMR spectra were obtained with a BrukerDRX-300 spectrometer with tetramethylsilane asan internal standard (Korea Basic Science Insti-tute). The elemental analysis was determinedwith a CE Instruments EA-1110 elemental ana-lyzer. Differential scanning calorimetry (DSC)was performed on a DuPont model 2100 thermalanalyzer equipped with a 2910 DSC instrumentat a heating rate of 10 8C/min under a nitrogenatmosphere. Thermogravimetric analysis (TGA)

Scheme 1. ESIPT process in bis(hydroxyphenyl)oxadiazole.

2060 KIM ET AL.

was conducted with a PerkinElmer TGA 7equipped with a TGA 7/3 instrument controller ata heating rate of 20 8C/min under nitrogen. Ultra-violet–visible (UV–vis) spectra were measuredwith a PerkinElmer Lambda 35 spectrometer.Steady-state fluorescence spectra were taken on aPerkinElmer LS 45 spectrofluorometer. The mo-lecular weight was determined by gel permeationchromatography (GPC) with THF as an eluentwith a polystyrene standard. The absolute quan-tum yield measurements were carried out with afluorescence photometer (QuantaMaster PhotonTechnology International) equipped with an inte-grating sphere. Time-resolved fluorescence wasmeasured with an FL 900 Mira model 900-Plaser. The time-correlated single-photon countingmethod with an argon-ion laser, a pumped tita-nium–sapphire laser (repetition rate ¼ 76 MHz,pulse range ¼ 3 ps, time-resolved range ¼ 10 psto 100 ls) with a pulse selector, and a second andthird harmonic generator was used.

2,5-Bis(5-bromo-2-hydroxyphenyl)-1,3,4-oxadiazole (1)

To 15 mL of poly(phosphoric acid) (PPA), 5.0 g(23.03 mmol) of 5-bromo-2-hydroxybenzoic acidand 2.54 g (23.03 mmol) of aminourea hydrochlor-ide were added. The mixture was stirred at 150–160 8C for 3 h and then poured into ice water. Thesolid was isolated by filtration, washed withwater, and then recrystallized from acetic acid togive white crystals (2.41 g, 51%).

1H NMR (CDCl3, d, ppm): 9.88 (s, 2H), 8.03 (d,2H), 7.6 (d, 2H), 7.1 (d, 2H). ELEM. ANAL. Calcd.for C14H8N2O3Br2: C, 40.80%; H, 1.96%; N,6.80%. Found: C, 40.33%; H, 1.83%; N, 6.60%.

2,5-Bis[5-bromo-2-(tert-butyloxycarbonyloxy)-phenyl]-1,3,4-oxadiazole (2)

1 (1.0 g, 2.42 mmol) and 1.27 g (5.82 mmol) ofdi-tert-butyldicarbonate were dissolved in THF(100 mL), and 0.05 equiv of 4-dimethylaminopyri-dine (DMAP) was added to the solution. The com-pletion of the reaction was judged by thin-layerchromatography (TLC), and the mixture waspoured into the water. After filtration, the iso-lated solid was washed with ethanol. The whitepowder was dried under reduced pressure to give2 (1.44 g, 97%).

1H NMR (acetone-d6, d, ppm): 8.4 (d, 2H), 7.9(d, 2H), 7.4 (d, 2H), 1.5 (s, 18H).

2,5-Bis[5-bromo-2-(tert-butyldimethylsilanyloxy)-phenyl]-1,3,4-oxadiazole (3)

Into a solution of 1 (1.5 g, 3.64 mmol) in 30 mL ofN,N-dimethylformamide (DMF) were slowly added1.65 g (10.93 mmol) of tert-butyldimethylsilyl chlo-ride (tBDMSCl) and 1.74 g (25.55 mmol) of imidaz-ole under nitrogen. After the appropriate reactiontime judged by TLC, the mixture was poured intowater and extracted with ethyl ether. The organiclayer was separated, dried over MgSO4, and fil-tered, and the solvent was evaporated. The productwas further purified by column chromatography(silica gel, n-hexane/ethyl acetate ¼ 1:1 v/v) andprecipitated in methanol to yield 3 (1.58 g, 67%).

1H NMR (CDCl3, d, ppm): 8.0 (d, 2H), 7.5 (d,2H), 6.8 (d, 2H), 0.9 (s, 12H), 0.2 (s, 18H).

Synthesis of Polymer 4

To a stirred mixture of 0.91 g (1.63 mmol) of 9,9-dioctylfluorene-2,7-bis(trimethylene borate), 1.0 g(1.63 mmol) of 2, and dry THF (10 mL) under nitro-gen were added 1 g of the phase-transfer catalyst(Aliquat 336), 0.11 g (0.01 mmol) of tetrakis(triphe-nylphosphine)palladium(0), and a 2 M aqueous so-dium carbonate solution (10 mL). The mixture washeated to gentle reflux with stirring in an oil bathunder nitrogen for 72 h. After the reaction, thereaction mixture was cooled and poured slowly intomethanol, and the precipitates were filtered. Thesolid was washed with deionized water and metha-nol several times. The polymer was dissolved inchloroform and precipitated in hexane. The precip-itate was collected by filtration and dried underreduced pressure (yield ¼ 0.93 g, 67%).

1H NMR (CDCl3, d, ppm): 7.4–8.5 (m, 12 H),2.1 (s, 4H), 1.5 (s, 18 H), 0.8–1.3 (m, 30H).

Chemical Deprotection for Polymer 5

t-Butoxycarbonyl (t-BOC)-protected polymer 4 (1 g)was dissolved in 30 mL of CHCl3 and treated with3 equiv of trifluoroacetic acid (TFA). The mixturewas stirred at room temperature for 3 h. After thereaction, the mixture was washed with an aqueousNaHCO3 solution. The organic layer was dried overMgSO4 and evaporated.

Thermal Deprotection for Polymer 6

t-BOC-protected polymer 4 was treated in anoven at 190 8C for 20 min to yield polymer 6 with100% free hydroxyl groups.

OXADIAZOLE-BASED CONJUGATED POLYMERS 2061

1H NMR (CDCl3, d, ppm): 10.1 (s, 2H), 7.4–8.5(m, 12H), 2.1 (s, 4H), 0.8–1.3 (m, 30H).

RESULTS AND DISCUSSION

Polymer Synthesis and Characterization

The synthetic procedures for preparing the mono-mers and polymers are illustrated in Scheme 2.

The condensation of 5-bromo-2-hydroxybenzoicacid in PPA afforded the monomer precursor 1 ina 51% yield. Monomers 2 and 3 were synthesizedby the reaction of 1 with di-tert-butylcarbonate ortBDMSCl, respectively, for Suzuki coupling poly-merization. We reported that the protection of thehydroxyl group was essential to complete thepolymerization because the presence of the freehydroxyl group would inhibit the Suzuki coupling

Scheme 2. Synthesis of the monomers and polymers.

2062 KIM ET AL.

type reaction.24 We used 2 or 3 for the polymeri-zation reaction with 9,9-dioctylfluorene-2,7-bis(trimethylene borate) and found that a polymerwas not formed when 3 was used because the pre-cipitation was observed as soon as monomer 3was added to the reaction mixture, presumablybecause of a facile removal of the tert-butyldime-thysilyl (tBDMS) group to produce the free hy-droxyl group in the presence of an aqueousNa2CO3 solution acting as a catalyst for basic hy-drolysis.

Polymer 4, composed of 9,9-dioctylfluorene andt-BOC-protected 2,5-bis(dihydroxyphenyl)oxadia-zole, was successfully synthesized in a 67% yield.The corresponding polymer 5, the deprotectionproduct of 4, was also prepared by the hydrolysisof the protecting t-BOC group under an acidiccondition. Both polymers were soluble in organicsolvents such as chloroform, THF, and DMF.Polymer 5 showed better solubility than 4.

The chemical structures of the polymers wereconfirmed by 1H NMR spectra. The 1H NMR spec-tra of polymers 4, 5, and 6 were in good agree-ment with the polymer structures. The degree ofdeprotection in polymer 5 was calculated fromthe ratio of the free hydroxyl resonance at10.2 ppm and tert-butyl group resonance in the t-BOC moiety at 1.52 ppm versus the reference re-sonance of the methylene group in 9,9-dioctyl-fluorene (2.1 ppm), as shown in Figure 1. Thedegree of deprotection, that is, the content of free

hydroxyl groups in polymer 5, was found to be50%. As shown in Figure 1(c), the proton signalsat 8.48 and 1.52 ppm disappeared after thermaldeprotection, and this implied complete removalof t-BOC groups from polymer 5 in comparisonwith Figure 1(b). In accordance with NMR calcu-lations, the free hydroxyl group content was con-firmed by TGAweight-loss calculations. The TGAthermogram in Figure 2 shows the thermally ini-tiated deprotection of polymer 4 started from150 8C with the release of isobutene and carbondioxide, corresponding to a 12% weight loss.25 At

Figure 1. 1H NMR spectra of polymers (a) 4, (b) 5, and (c) 6 in CDCl3.

Figure 2. TGA thermograms of polymers (—) 4, (- - -)5, and (� � �) 6 under a nitrogen atmosphere.

OXADIAZOLE-BASED CONJUGATED POLYMERS 2063

around 190 8C, t-BOC groups were cleaved andremoved, and this resulted in the formation of oxa-diazole polymer 6 with 100% free hydroxyl groups.Thus, we carried out the thermal deprotection of 4to obtain polymer 6 at 190 8C for 20 min.

A TGA thermogram of 6 revealed that theonset decomposition temperature of thermallydeprotected polymer 6 under nitrogen was mea-sured to be 438 8C, similar to the onset decomposi-tion temperatures of polymers 4 and 5. The DSCheating scan of polymer 6 exhibited a clear tran-sition at 135 8C without a melting transition. Theexact glass-transition temperature (Tg) of poly-mers 4 and 5 was impossible to measure becauseof the facile and concomitant conversion to 6 upontemperature elevation during the scanning. Thisindicates that the incorporation of the rigid 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole unit intothe polyfluorene backbone reduced the segmentalmotion and suppressed the dense packing andcrystallization of polymer chains (polyfluorenehomopolymer Tg ¼ 67 8C and melting temperature¼ 152 8C).26 The molecular weights of these poly-mers were determined with GPC against poly-styrene standards and are shown in Table 1. Inthe case of 4, 4 or 5 repeat units were connected inthe polymer backbone on the basis of the number-average molecular weight. It is presumed thatthe difference can be attributed not to a change inthe actual molecular weight but to an alteration inthe molecular expansion size of the polymer chains.

Optical Properties

Optical absorption spectra of monomer precursor1 and monomers 2 and 3 had similar features,with absorption maxima ranging from 270 to350 nm, except for t-BOC-protected monomer 2,which exhibited a single absorption at 278 nm inchloroform, as shown in Figure 3(b). The p–p*transition of the fluorene units contributed the350-nm absorption, and that of oxadiazole unitscontributed the 270-nm absorption.27 In contrast

to the similarities in the UV spectra, only 1 had along wavelength emission (ca. 530 nm) resultingfrom ESIPT, no matter what kinds of solventswere used [Fig. 3(a)]. The long wavelength emis-sions varied according to the solvent used, and ithas been reported that the emission wavelengthis dependent on the solvent polarity.24,28 Mono-mers 2 and 3 exhibited only short wavelengthemissions around 355–365 nm due to the blockingof ESIPT; it is presumed that the presence of thehydroxyl group is essential to facilitate ESIPT[Fig. 3(b)]. Regardless of the kinds of protectinggroups, such as t-BOC and tBDMS groups, ESIPTin monomers 2 and 3 was suppressed efficiently

Table 1. Molecular Weights of the Polymersa

Polymer Mn Mw Polydispersity

4 6,000 13,000 2.15 10,000 22,000 2.26 8,150 15,560 1.9

a The number-average (Mn) and weight-average (Mw) mo-lecular weights were determined by GPC with polystyrenestandards.

Figure 3. Absorption and fluorescence spectra of (a)1 in (� � �) CHCl3, (- - -) THF, and (—) DMF with aconcentration of 25 lM and (b) (—) 20 lM 2 and (- - -)50 lM 3 in CHCl3. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

2064 KIM ET AL.

by the presence of the protecting groups. Thisresult complied with the literature alreadyreported, which described the protection with themethoxy group.18

The optical absorption and fluorescence emis-sion spectra of the polymers in dilute chloroformsolutions are shown in Figure 4. All the polymersshowed a low energy absorption band at 330 nm.Similarly to monomer 1, a long wavelength emis-sion at 528 nm was observed in the cases of 5 and6 resulting from ESIPT. As shown in the emissionspectra of 2 and 3, polymer 4 did not exhibit aketo form emission (long wavelength emission)because of the effective blocking of proton trans-fer by the t-BOC group. However, the long wave-length emission at 528 nm from ESIPT in poly-mer 5 or 6 was not observed with the same chemi-cal structure that was reported earlier.19 In thatarticle, a clear explanation for the nonexistenceof ESIPT in such polymers is not given, eventhough their monomeric forms showed ESIPT.The quantum yields for 4 and 6 in chloroformsolutions were found to be 57 and 5%, respectively.Such a low value of the quantum yield of polymer6 may be a main cause for not recognizing theESIPT process in the earlier study. In this article,we are reporting ESIPT in bis(hydroxyphenyl)oxa-diazole polymer for the first time. Polymer 5showed a long wavelength emission similar to thatof 6, indicating that the degree of deprotection,that is, the hydroxyl group content, is not crucialto exhibit ESIPT. Besides the strong emission at528 nm, a significantly weak emission at a short

Figure 4. Absorption and fluorescence spectra of (—)4, (- - -) 5, and (� � �) 6 in CHCl3. The concentration ofthe chloroform solution was 10 lM for the UV spec-trum and 5 lM for the fluorescence spectrum. [Colorfigure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

Figure 5. Absorption and fluorescence spectroscopychanges of 5 (5.0 lM) upon the addition of PA, DNT,and DNB in CHCl3: (a) [PA] ¼ 0, 3.3 � 10�5, 6.6 � 10�5,and 1.0 � 10�4 M for UV spectra and 0, 5.0 � 10�5, 1.0� 10�4, and 4.0 � 10�4 M for fluorescence spectra, (b)[DNT] ¼ 0, 3.3 � 10�5, 5.0 � 10�5, and 1.0 � 10�4 M forUV spectra and 0, 1.6 � 10�4, 6.66 � 10�4, and 1.3� 10�3 M for fluorescence spectra, and (c) [DNB] ¼ 0,3.33 � 10�5, 6.66 � 10�4, and 1.0 � 10�3 M for UV spec-tra and 0, 6.66 � 10�4, 2.33 � 10�3, and 6.66 � 10�3 Mfor fluorescence spectra.

OXADIAZOLE-BASED CONJUGATED POLYMERS 2065

wavelength around 410 nm was also observed in 5and 6, which resulted from the enol form relaxa-tion, as shown in Scheme 1. Excitation spectra ofemissions of 410 and 528 nm (which are not shownhere) were exactly the same; it was presumed thatthe emission at 528 nm came from the molecularbackbone of 5 or 6 and not from other moleculessuch as oxidized impurities from the polyfluorenechain.

Sensing Nitroaromatic Compounds

With the aim of attaining the detection of nitro-aromatic compounds with the synthesized poly-mers, we titrated nitroaromatic compounds suchas picric acid (PA), DNT, and m-dinitrobenzene(DNB) into chloroform solutions of 5 and exam-ined the changes in the UV–vis absorption andfluorescence spectra. With this investigation inthe good solvent chloroform, we could evaluatethe optical responses from each nitroaromaticcompounds and the relative sensitivity of thepolymer. As soon as each of the electron-deficientnitroaromatic compounds was added to 5 in chlo-roform, dramatic fluorescence quenching wasobserved, as shown in Figure 5. Concomitantly,the absorption of 5 showed a hyperchromic shiftupon exposure to PA and DNB without a redshiftor blueshift, presumably as a result of an electrontransfer occurring within the electron-deficientnitroaromatic compounds and bis(hydroxyphenyl)oxadiazole moiety. When a solution of 5 in chloro-form was exposed to nitroaromatic compounds, ef-ficient fluorescence quenching occurred at a lowconcentration of each nitroaromatic compound.This can be explained by the Stern–Volmer rela-tionship:29

ðF0=FÞ ¼ 1þ Ksv½Q� ð1Þ

This provides a quantitative correlation betweenthe loss of fluorescence intensity (F0/F) and theconcentration of added quencher [Q]. KSV, theslope of the plot, is the Stern–Volmer constant.Figure 6 shows the Stern–Volmer plot of the expo-sure of 5 to nitroaromatic compounds. The fluo-rescence quenching of 5 was linear at a low con-centration of nitroaromatics but deviated fromlinearity at higher quencher concentrations. TheKSV values, based on the initial linear parts of the

Figure 6. Stern–Volmer plots for 5 (5.0 lM) uponthe addition of (a) PA, (b) DNT, and (c) DNB in CHCl3(excited at 330 nm). M ¼ molarity.

2066 KIM ET AL.

curves, are 7.43 � 104, 6.64 � 103, and 4.37 � 104

M�1 for PA, DNT, and DNB. The KSV results sug-gest that 5 is an efficient sensory material forelectron-deficient nitroaromatic compounds andshow that PA is the most efficient quencher, fol-lowed by DNB and DNT. All the nitroaromaticcompounds gave rise to nonlinear curves (positivecurvatures for PA and DNT and a negative onefor DNB). A deviation from a linear functionmeans that the quenching system is related tomore complex species. The fluorescence lifetimeis affected by dynamic quenching but not affectedby static quenching. As a result, monitoring life-time changes represents the conventional prac-tice for determining the dynamic quenching con-stant independently of the static quenching con-stant. In our experiment of fluorescence lifetimemeasurements, as the fluorescence lifetime didnot change according to the addition of the nitro-aromatic compound, static quenching was domi-nant in our quenching system.30

It has been reported that the hydrogen-bondingability of the receptor and analyte molecules is oneof the important factors affecting the sensingbehavior of materials.31,32 It is thought that theelectron-withdrawing property of nitroaromaticcompounds affects the acidity of the phenolic pro-ton in 5, leading to higher binding affinity. Thus,the presence of a hydroxyphenyl group adjacent tooxadiazole is essential for achieving proper sens-ing ability. It is presumed that the interactionbetween the electron-donating hydroxyl group andelectron-deficient nitroaromatic compound is re-sponsible for the electron transfer resulting in flu-orescence quenching. One of the crucial advan-tages of the chemical species with ESIPT is closelyrelated to the large Stokes shift, which could cir-cumvent the self-absorption of their own emissionbecause of the least overlapping of absorption andemission resulting in an enhancement of the emis-sion efficiency. Thus, this enhancement in emis-sion efficiency enables their use in aqueous media.

CONCLUSIONS

We synthesized oxadiazole-based conjugated pol-ymers with adjacent hydroxyphenyl groups in themain chain with various contents of free hydroxylgroups. The intramolecular hydrogen bond be-tween oxadiazole and hydroxyphenyl groupsoffered ESIPT, in conjunction with a large Stokesshift, which provided a useful sensing signalrequired for optical chemosensors. This interac-

tion could be altered upon exposure to nitroaro-matic compounds, which were the model com-pounds for explosives, showing UV–vis absorp-tion and fluorescence emission changes. KSV of 5for PA was found to be 7.43 � 104 M�1, which iscomparable to that of other sensing materialsreported so far.

Financial support from the Korea Research Founda-tion (R05-2004-000-11091-0) is gratefully acknowledged.

REFERENCES AND NOTES

1. Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Na-ture (London) 2000, 403, 750.

2. Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Bur-roughes, J. H.; Marks, R. N.; Taliani, C.; Bradlev,D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Log-dlund, M.; Salaneck, W. R. Nature (London) 1999,397, 121.

3. Wise, D. L.; Wnek, G. E.; Trantolo, D. J.; Cooper,T. M.; Gresser, J. D. Photonic Polymer Systems;Marcel Dekker:New York, 1998.

4. McQuade, D. T.; Pullen, A. E.; Swager, T. M.Chem Rev 2001, 101, 2537.

5. Zhang, X.; Jenekhe, S. A. Macromolecules 2000,33, 2069.

6. Zhu, Y.; Jenekhe, S. A. Polym Prepr 2004, 45(1),176.

7. Fang, Q.; Yamamoto, T. J Polym Sci Part A:Polym Chem 2004, 42, 2686.

8. Hou, S.; Ding, M.; Gao, L. Macromolecules 2003,36, 3826.

9. Schulz, B.; Bruma, M.; Brehmer, L. Adv Mater1997, 9, 601.

10. Wang, J. F.; Jabbour, G.; Mash, E. A.; Anderson,J.; Zhang, Y.; Lee, P. A. Adv Mater 1999, 11, 1266.

11. Yang, Y.; Pei, Q. J Appl Phys 1995, 77, 4807.12. Shin, D.-C.; Ahn, J.-H.; Kim, Y.-H.; Kwon, S.-K.

J Polym Sci Part A: Polym Chem 2000, 38, 3086.13. Xu, B.; Pan, Y.; Zhang, J.; Peng, Z. Synth Met

2000, 114, 337.14. Sung, H.-H.; Lin, H.-C. Macromolecules 2004, 37,

7945.15. Tanaka, K.; Kumagai, T.; Aoki, H.; Deguchi, M.;

Iwata, S. J Org Chem 2001, 66, 7328.16. Kocher, C.; Weder, C.; Smith, P. J Mater Chem

2003, 13, 9.17. Tong, H.; Zhou, G.; Wang, L.; Jing, X.; Wang, F.;

Zhang, J. Tetrahedron Lett 2003, 44, 131.18. Lee, J. K.; Kim, H.-J.; Kim, T. H.; Lee, C.-H.;

Park, W. H.; Kim, J.; Lee, T. S. Macromolecules2005, 38, 9427.

19. Zhou, G.; Cheng, Y.; Wang, L.; Jing, X.; Wang, F.Macromolecules 2005, 38, 2148.

20. Yang, J.-S.; Swager, T. M. J Am Chem Soc 1998,120, 11864.

OXADIAZOLE-BASED CONJUGATED POLYMERS 2067

21. Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C.J Am Chem Soc 2003, 125, 3821.

22. Liu, Y.; Mills, R. C.; Boncella, J. M.; Schanze, K.S. Langmuir 2001, 17, 7452.

23. Saxena, A.; Fujiki, M.; Rai, R.; Kwak, G. ChemMater 2005, 17, 2181.

24. Lee, J. K.; Lee, T. S. J Polym Sci Part A: PolymChem 2005, 43, 1397.

25. Frenett, M.; Coenjarts, C.; Scaiano, J. C. Macro-mol Rapid Commun 2004, 25, 1628.

26. Kulkarni, A. P.; Zhu, Y.; Jenekhe, S. A. Macromo-lecules 2005, 38, 1553.

27. Peng, Z.; Zhang, J. Chem Mater 1999, 11, 1138.28. Nguyen, T.; Doan, V.; Schwartz, B. J. J Chem

Phys 1999, 110, 4068.29. Lakowicz, J. R. Principles of Fluorescence

Spectroscopy, 2nd ed.; Plenum: New York,1999.

30. Zhao, D.; Swager, T. M. Macromolecules 2005, 38,9377.

31. Jursıkova, K.; Sessler, J. L. J Am Chem Soc 2000,122, 9350.

32. Mizuno, T.; Wei, W. H.; Eller, L. R.; Sessler, J. L.J Am Chem Soc 2002, 124, 1134.

2068 KIM ET AL.