Competition between the Formation of Excimers andExcitons during the Photoluminescence of Light-EmittingPolymer Blends

J. S. LEE,1 C. H. KIM,1 J.-W. YU,2 J. K. KIM,2 D. Y. KIM,2 N. W. SONG,3 C. Y. KIM4

1Polymer Department, Inha University, Inchon, Korea

2Polymer Materials Laboratory, Korea Institute of Science and Technology, Seoul, Korea

3Korea Research Institute of Standards and Science, Daejeon, Korea

4Electronic Materials Laboratory, Samsung Advanced Institute of Technology, Suwon, Korea

Received 27 January 2003; accepted 22 July 2003

ABSTRACT: Divinylenediethylhexyloxyphenyl and either triphenyltriazine (PTPV) ordiphenoxyphenyltriazine (POTPV) were alternately connected to inhibit the dimeremission of the divinylenediethylhexyloxyphenyl unit and to enhance the electronmobility of light-emitting polymers. A photoluminescence (PL) spectrum of a PTPV filmshowed characteristics of excimer emissions, whereas POTPV generated a singletexciton PL emission. A blend of POTPV with poly(vinyl carbazole) (PVK) enhanced thePL intensity of POTPV upon the photoexcitation of PVK, and this indicated efficientenergy transfer between the two polymers. Contrary to expectations based on the goodoverlap of spectra for the PL emission of the donor and the light absorption of theacceptor, the energy transfer from PVK to PTPV was incomplete upon the photoexci-tation of the blend at the ultraviolet–visible absorption maximum of PVK. However, theexcimer PL emission of PTPV was completely suppressed. The lifetime ratio of theexcitations of the donor and the acceptor was proven to be one of the critical factors inthe energy transfer, in addition to the Forster energy-transfer principle. The excimeremission was suppressed by the photoexcitation of the chromophores below a certaincritical energy level. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42:557–565, 2004Keywords: photoluminescence; lifetime; energy transfer; light-emitting diodes(LED); conjugated polymers; excimers; excitons

INTRODUCTION

There has been significant progress in the fabri-cation of polymer light-emitting diodes (PLEDs)since their discovery in 1990.1 It is expected thatmobile phones equipped with PLEDs will be onthe market this year and that PLEDs with bigger

screens will be introduced in 3 years.2,3 PLEDshave some advantages over organic light-emittingdiodes (OLEDs) in the fabrication of display de-vices. The processes include spin casting,4,5 inkjetprinting,6 and flexible substrates,7 which maymake the mass production of these devices possi-ble with low production costs.

Despite all the progress in PLEDs in recentyears, the power efficiency and color purity of thedevices need to be improved to compete againstliquid-crystal displays, plasma display panels,and OLEDs. Most light-emitting polymers with

Correspondence to: C. Y. Kim (E-mail: cykimem@empal.com)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 557–565 (2004)© 2003 Wiley Periodicals, Inc.

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aromatic structures such as phenylene vinyleneand fluorene units show hole-transporting char-acteristic.8 It is known that hole mobility isgenerally 100 times faster than electron mobil-ity in light-emitting polymers.9 It is importantto have balanced injections of the charges intothe light-emitting polymers for high power effi-ciency.8,10

Light-emitting polymers consisting of aromat-ics, heterocyclics, or vinyl units in the polymerbackbones are rigid, and the polymer moleculestend to be extended. The flat chromophores inlight-emitting polymer molecules are stacked eas-ily with one another. It is common for a photolu-minescence (PL) spectrum of a light-emittingpolymer to show a PL emission maximum with avibronic feature and secondary emissions. Thecolor purity is reduced when the full width athalf-maximum of the PL spectrum is large or thePL spectrum has a secondary emission. Excimersor exciplexes generate the secondary emission,11

but there is a possibility of exciton emission of achromophore of a longer conjugation rather thanthe secondary emission.12

The modification of a polymer molecule by theattachment of pendant groups onto the mainchain reduces the excimer emission of poly(phe-nylene vinylene)s.13,14 The secondary PL emis-sion by excimers is reduced in a polymer solutionand in a polymer blend. Moreover, in polymerblends, the fluorophores of an acceptor with anarrower band gap are used as light emitters,whereas energy is absorbed by chromophores of adonor with a wider band gap. There is an energytransfer from the donor to the acceptor when theForster critical energy-transfer conditions aremet.14–16 However, there is another importantfactor controlling the energy transfer between adonor and an acceptor.17

We examined two poly(phenylene vinylene)(PPV) derivatives with triazine, an electron-transporting unit, in the polymer main chains todetermine if the intensity of the excimer PL emis-sion of poly(phenylene vinylene) would be reducedby the insertion of the unit and the copolymerwould keep the reduction characteristic of theunit. We also determined if the secondary emis-sion would be reduced on dilution via blendingwith poly(vinyl carbazole) (PVK), a hole-trans-porting polymer. A suppression mechanism of thesecondary emission of PPV derivatives was sug-gested during the photoexcitation of a donor in adonor–acceptor system, which failed to follow theForster energy-transfer mechanism. There was astrong possibility that the lifetime ratio between

excitations of chromophores of wide and narrowband gaps controlled the efficiency of the energytransfer and the degree of the suppression of theexcimer emission.

EXPERIMENTAL

Schematics of the synthetic routes of the alternat-ing copolymers are shown in Scheme 1. Both poly-mers were synthesized by the Heck reaction.18

Synthesis of 2,4-Dichloro-6-phenyl-1,3,5-s-triazine(S1)

Cyanuric acid (18.44 g, 0.1 mol) was dissolved in200 mL of tetrahydrofuran (THF), and the solu-tion was flushed with nitrogen at 5 °C. A phenylmagnesium bromide solution (100 mL, 1 mmol) inTHF was added dropwise through a double-tipneedle. The rate was 24 drops per minute withstirring at 5 °C. The reactants were kept for 12 h.The product was filtered and then refined by col-umn chromatography with n-hexane as a diluent.The product was recrystallized in n-hexane toyield needlelike, transparent crystals. The yieldwas 17.87 g (79%).

mp: 120–121 °C. 1H NMR (CDCl3, ppm, �): 7.5[m, triazineOPh (m), 2H], 7.62 [m, triazineOPh(p), H], 8.45 [m, triazineOPh (o), 2H]. 13C NMR(CDCl3, �): 128.9, 129.8, 132.5, 134.6, 171.9,174.6. Mass (m/z): 225. Fourier transform infra-red (FTIR; KBr disc, cm�1): 452, 651, 770, 1110,1257, 1496, 1597, 2500.

Synthesis of 2,4-Bis(4-vinylphenyl)-6-phenyl-1,3,5-s-triazine (M1)

4-Chlorostyrene (12.6 g, 90.88 mmol) and magne-sium (93.32 g, 136 mmol) in THF were stirred for12 h to yield a Grignard reagent. The Grignardreagent solution was added to a solution of S1 (5 g,22.72 mmol) dissolved in 70 mL of THF dropwisethrough a double-tip needle with stirring. Afterthe addition, the reactants were stirred for 30 h at50 °C. The product was refined with silica gel.After flushing through column chromatographywith methylene chloride, the product was recrys-tallized with methylene chloride and n-hexane toyield white crystals.

Yield: 2.02 g (25%). 1H NMR (CDCl3, �): 5.37 (d,OPhOCHACHaHb, 2H), 5.91 (d, OPhOCHACHaHb), 6.81 (dd, OPhOCHACHaHb, 2H), 7.60[m, triazineOPh (m,p), triazineOPhOCHACH2(m), 7H], 8.73 [m, triazineOPh (o), 6H]. 13C NMR

558 LEE ET AL.

(CDCl3, �): 115.8, 126.4, 128.5, 128.9, 129.2, 132.4,135.6, 136.4, 141.5, 171.1, 171.5. ELEM. ANAL. Calcd.for C25H19N3: C, 83.08%; H, 5.30%; N, 8.08%.Found: C, 83.31%; H, 6.02%; N, 11.77%. Mass (m/z):361. FTIR (KBr disc, cm�1): 3088, 2981, 1587, 1517,1370, 1182.

Synthesis of 2,4-Bis(4-iodophenoxy)-6-phenyl-1,3,5-s-triazine (M2)

Sodium hydroxide (2.97 g, 74.4. mmol) and tetra-methyl ammonium chloride (1.14 g, 74.4 mmol)were dissolved in 50 mL of distilled water, and then4-iodophenol (16.4 g, 74.4 mmol) was added to thesolution. A solution of S1 (5.6 g, 24.8 mmol) in 80 mLof chloroform was poured into the aforementioned

solution, and the mixture was stirred for about 6 hat 70 °C. The product was extracted with an aque-ous solution of NaOH three times and then withwater three times; this was followed by drying inanhydrous magnesium sulfate. The liquid was fil-tered, and the solvent was stripped off in vacuo. Theproduct was recrystallized with methylene chlorideand n-hexane. The yield was 10.3 g (70%).

mp: 157–158 °C. 1H NMR (CDCl3, �): 6.98 (d,OOOPh, 2H), 7.42 [t, triazineOPh (m), 2H], 7.55[t, triazineOPh (p), 1H], 8.26 (d, IOPh, 2H), 8.26[d, triazineOPh (o), 2H]. 13C NMR (CDCl3, �):90.1, 123.7, 128.6, 129.1, 133.4, 134.0, 138.5,151.5, 172. 4, 176.0. ELEM. ANAL. Calcd. forC21H13N3O2I2: C, 42.52%; H, 2.21%; N, 7.08%.Found: C, 42.50%; H, 2.20%; N, 7.05. Mass (m/z):

Scheme 1. Schematics of the synthesis routes of PTPV and POTPV.

PHOTOLUMINESCENCE OF POLYMER BLENDS 559

593. FTIR (KBr disc, cm�1): 3059, 3029, 1640,1580, 1500, 1358, 1306, 1271.

Synthesis of 1,4-Bis[(2-ethylhexyl)oxy]benzene (S2)

Potassium hydroxide (2 g, 428 mmol) and 120 mLof dimethyl sulfoxide were mixed and stirred for60 min at room temperature. Hydroquinone (5.51g, 50 mmol) and 2-ethylhexyl bromide (38 g, 197mmol) were added to the mixture. The reactantwas kept at room temperature for 2 h and thenpoured into ice water. An aqueous solution layerwas extracted with n-hexane and dried with an-hydrous magnesium sulfate. The solvent wasstripped off in vacuo, and a colorless liquid wasobtained. The yield was 12.5 g (75%).

1H NMR (CDCl3, �): 6.8 (s, Ph, 4H), 3.76 (d,OOCH2O, 4H), 1.67 (m, OCHO, 2H), 1.43, 1.30(m, OCH2O, 8H), 0.91 (m, OCH3, 12H). ELEM.ANAL. Calcd. for C22H38O2: C, 78.99%; H, 11.45%.Found: C, 79.03%; H, 11.53%. Mass (m/z): 335.

Synthesis of 1,4-Bis[(2-ethylhexyl)oxy]-2,5-diiodobenzene (M3)

Distilled water (20 mL), 4.8 mL of 98% sulfuricacid, and 270 mL of acetic acid were mixed; 14.9 gof S2 (44.5 mmol), 12.6 g of iodine (49.6 mmol),and 3.83 g of potassium iodate (17.9 mmol) wereadded to the aqueous solution and refluxed for16 h. The mixture was then cooled to room tem-perature, and a 20% aqueous solution of sodiumsulfate was added to the mixture until the browncolor of iodine disappeared. The mixture wasadded to ice water, and then the organic layer wasseparated from the aqueous solution. The aque-ous solution was extracted with n-hexane threetimes and dried with anhydrous magnesium sul-fate. The solvent was stripped off by evaporation,and a pale yellowish liquid was collected by col-umn chromatography with n-hexane. The yieldwas 20.1 g (75%).

1H NMR (CDCl3, �): 7.14 (s, Ph, 2H), 3.79 (d,OOCH2O, 2H), 1.70 (m, OCHO, 2H), 1.50 (m,OCH2O, 8H), 1.30 (m, OCH2O, 8H), 0.91 (m,OCH3, 12H). 13C NMR (CDCl3, �): 153.09, 122.57,86.27, 72.56, 39.67, 30.73, 29.27, 24.16, 23.25,14.33, 11.43. ELEM. ANAL. Calcd. for C22H36I2O2:C, 45.07%; H, 6.19%. Found: C, 45.44%; H, 6.33%.Mass (m/z): 586.

Synthesis of 1,4-Bis[(2-ethylhexyl)oxy]-2,5-divinylbenzene (M4)

M3 (3.8 g, 6.48 mmol) with tributyl(vinyl)tin (4.11g, 12.96 mmol) and tetrakis(triphenyl phosphine)

palladium (0.3 g, 0.26 mmol) was dissolved in 50mL of DMF and kept at 80 °C for 10 h. Thereactant was cooled to room temperature and fil-tered for the separation of the black precipitate.The filtrate was washed with water and diethylether sequentially, and this was followed by dry-ing with anhydrous magnesium sulfate. The prod-uct was flushed in column chromatography withn-hexane to obtain a greenish liquid. The yieldwas 12 g (48%).

1H NMR (CDCl3, �): 7.05 (t, CHaHbACHO,2H), 6.97 (s, Ph, 2H), 5.71 (dd, CHaCHbACHO,2H), 5.23 (dd,CHaCHbACHO, 2H), 3.84 (d,PhOOCH2O,4H), 1.70 (m, PhOOCH2OCHO,2H), 1.39 (m, OCH2O, 16H), 0.88 (m, OCH3,12H). 13C NMR (CDCl3, �): 150.63, 131.45,127.03, 113.83, 109.98, 71.32, 39.6, 30.63, 29.1,26.61, 24.0, 23.03, 16.37, 14.07, 13.56, 11.19.Mass (m/z): 386.

Synthesis of Poly{(2,4,6-triphenyl-1,3,5-triazine)-4�,4�-diylenevinylene-alt-[1,4-(bis-2-ethylhexyl)oxy]phenylene vinylene} (PTPV)

With a Heck reaction,18 M1 (0.8 g, 2.2 mmol) andM3 (1.3 g,, 2.2 mmol) with palladium acetate (0.02g, 0.09 mmol), tri-o-tolylphosphine (0.14 g, 0.44mmol), and 0.5 mL of triethylamine were dis-solved in 40 mL of NMP and stirred at 150 °C for12 h. The reactant was precipitated in methanoland dissolved again in THF, and this was fol-lowed by precipitation by acetone. The precipita-tion was repeated twice with methanol and ace-tone, respectively. The yellowish solid was driedin vacuo at 40 °C for 24 h. The yield was 1.4 g(67%).

1H NMR (THF-d8, �): 0.81–1.98 [m, OOOCH2CH(CH2)4(CH3)2, 30H], 4.17 (s, OOOCH2O,2H), 7.46–7.54 (br, PhOCHACHOPhO, 4H),7.70–7.84 [br, Ph-triazineO (m,p), triazineOPhOCHACHO (m),OCHACHOPhO, 9H], 8.89[br, triazineOPh (o), 6H]. 13C NMR (CDCl3, �):1.9, 12.3, 15.1, 24.7, 30.8, 41.6, 72.8, 111.6, 126.8,127.9, 128.5, 129.4, 130.0, 130.3, 130.8, 133.9,136.6, 138.0, 144.1, 153.1, 172.7, 173.0. ELEM.ANAL. Calcd. for C47H55N3O2: C, 81.35%; H,7.99%; N, 6.06%. Found: C, 80.00%; H, 8.10%; N,6.06%. FTIR (KBr, cm�1): 2958, 1601, 1511, 1365,1180, 1025, 813.

Synthesis of Poly{(2,4-diphenyloxy-6-phenyl-1,3,5-triazine)-4�,4�-diylenevinylene-alt-[1,4-bis(-2-ethylhexyl)oxy]phenylene vinylene} (POTPV)

With a Heck reaction,18 M2 (0.61g, 1.0 mmol) andM4 (0.38 g, 1.0 mmol) with palladium acetate

560 LEE ET AL.

(0.0093 g, 0.04 mmol), tri-o-tolylphosphine (0.063g, 0.2 mmol), and 0.4 mL of triethylamine weredissolved in 7 mL of NMP and stirred at 130 °Cfor 20 h. The product was precipitated with meth-anol three times and then dried in vacuo for 24 h.The yield was 0.2 g (20%).

1H NMR (CDCl3, �): 0.707–1.794 [m,OOOCH2CH(CH2)4(CH3)2, 30H], 3.48–3.89 (m,OOOCH2, 4H), 5.29 (m, OPhOCHACHaCHb,1H), 5.74 (m, OPhOCHACHaCHb, 1H), 6.84–7.57 [m, triazineOPh (m,p), OtriazineOOOPh,OCHACHOPh, OCHACHO, 17H], 8.27–8.39[m, triazineOPh (o), 2H]. 13C NMR (CDCl3, ppm,�): 172.88, 151.31, 134.55, 133.13, 130.59, 129.23,128.51, 128.04, 127.34, 124.02, 121.74, 120.96,116.18, 115.73, 110.55, 71.94, 70.88, 39.82, 39.58,30.93, 30.3, 29.22, 24.23, 23.56, 23.03, 13.98,11.28, 11.03. ELEM. ANAL. Calcd. for C47H55N3O4:C, 77.76%; H, 7.64%; N, 5.79%. Found: C, 73.45%;H, 7.29%; N, 6.58%. FTIR (KBr, cm�1): 2958,2859, 1665, 1593, 1514, 1420, 1359, 1167, 1029.

Instrumentation

A melting-point apparatus from Laboratory De-vices was used for the measurements. Bruker AC250 (250 MHz) and (56 MHz) spectrometers wereemployed for 1H NMR and 13C NMR, respectively.IR analyses were performed on a PerkinElmer IRSpectrum Explorer with KBr pellets. Elementalanalyses were performed on a VG Trio 2000 withan EI� method. The molecular weight determina-tion was carried out with a Waters GPC 150 at-tached to a Waters 410 differential refractometerand a 486 ultraviolet (UV) detector. Polystyrenestandards were used for calibration. Cyclic volta-mmograms were obtained with a potentiostat(EG&G PAR 273) with a data acquisition module.A PL TGA 1000 and a PerkinElmer DSC 7 wereemployed for thermal characterizations. Polymerblend films were prepared by the spin casting ofblend solutions in chlorobenzene after filtrationthrough a 0.2-�m-hole membrane filter. The sur-face morphology of the blend was studied with aPSI Autoprobe CP atomic force microscopy (AFM)instrument. Ultraviolet–visible (UV–vis) absorp-tion (AB) spectra were recorded with an HP8452A diode array spectrophotometer, and the PLspectra were recorded with an ISS PL-1 fluorom-eter. Time-correlated single-photon counting (TC-SPC) based on a femtosecond titanium–sapphirelaser (Tsunami Spectra Physics) and a Jobin–Yvon HR320 monochromator was employed forthe time-resolved PL measurements. A detailed

description of the instrumental setup for TCSPCwas reported elsewhere.19

RESULTS AND DISCUSSION

The number-average molecular weights (Mn’s) ofPOTPV and PTPV are shown in Table 1. The lowMn of PTPV might have been due to the poorsolubility and early precipitation in the reactantsolvent before sufficient propagation during poly-merization. No end capping of the polymers wasattempted because the polymers were used not asefficient light-emitting materials but as energyacceptors in the study of the energy-transfermechanism. The glass-transition temperature in-creased with an increase in Mn, but the decompo-sition temperature of the polymers depended lesson Mn. Cyclic voltammograms of both POTPV andPTPV showed only the oxidation potential, de-spite the triazine units in the polymer backbones.The triazine unit lost the ability to accept elec-trons from the cathode during the reduction cycle,and this indicated that electrons were delocalizedintramolecularly between the triazine unit andthe phenoxy or dialkoxyphenylenedivinylphe-nylene unit of the polymers, respectively, andthat the chromophore favored electrooxidationover electroreduction.

UV–vis AB and PL spectra of a POTPV filmand a PTPV film and a solution of PTPV in chlo-robenzene are shown in Figures 1 and 2, respec-tively. The AB spectra show the AB maxima at380 and 440 nm for POTPV and PTPV, respec-tively. The PL spectrum of POTPV on photoexci-tation at 380 nm shows the emission maximum at470 nm with a vibronic shoulder at 485 nm and aweak tail with a peak at 520 nm. No shift in thePL maximum was observed from the solution inchlorobenzene, and this revealed that the photo-excited chromophores formed no dimer conforma-tion with adjacent chromophores, which might bedue to the flexible ether linkage between the tri-azine and dialkoxyphenylenedivinylphenyleneunits. The high intensity of the vibronic peakagainst the PL maximum reveals strong self-ab-sorption of the PL emission because the AB spec-

Table 1. Physical Properties of Polymers

Mn Polydispersity Tg (°C) Td (°C)

PTPV 1.3 � 104 2.2 279 355POTPV 3.2 � 103 1.3 90 398

PHOTOLUMINESCENCE OF POLYMER BLENDS 561

trum overlaps the PL spectrum substantially inthe wavelength range of 435–470 nm. The sec-ondary peak at 520 nm might be assigned to ex-tended chromophores12 because the shoulder stillappears in the PL spectrum of the polymer solu-tion, in which the polymer molecules were dilutedenough not to form aggregates.

The PTPV film on photoexcitation at 440 nmshowed a featureless PL emission spectrum witha peak maximum at 610 nm [Fig. 2(c)]. The fea-tureless PL spectrum of the PTPV film was alsoobserved on photoexcitation at 340 and 365 nmwhen the polymer showed low extinction coeffi-cients. The featureless PL spectrum must be aresult of radiative excimer emission. Coplanarityseems to have developed between the dialkoxy-phenylene–divinylphenylene and triazine unitsin PTPV, whereas coplanarity was not formed inPOTPV because of the ether linkage enhancingthe molecular motion in the polymer backbone.The AB maximum of a PTPV solution of 10�5

mol/L in chlorobenzene moved little from that ofthe film. However, the PL spectrum of the solu-

tion in chlorobenzene on photoexcitation at 440nm exhibits PL maxima at 505 and 540 nm [Fig.2(b)]. The two peaks in the PL spectrum of thesolution might indicate the reduction of the chainstiffness in the diluted solution.

The values of the highest occupied molecularorbital and the lowest unoccupied molecular or-bital (LUMO) of POTPV and PTPV were deducedfrom the results of cyclic voltammetry and AB, aspresented in Table 2. It is apparent that the etherlinkage broke the coplanarity between the tri-azine unit and dialkoxyphenylenedivinylphe-nylene unit. The band gap of POTPV was widerand the LUMO was lower than for PTPV. It wasexpected that the electron injection would be eas-ier for PTPV than for POTPV.

The photoexcitation of PVK at 340 nm gener-ated a substantial number of excimers andshowed a strong PL emission.20 A polymer blendof PTPV and PVK with a blend ratio of 1:9 wasprepared to dilute PTPV in PVK and to suppressexcimer formation. The blend showed a signifi-cant spectrum overlap between PL of PVK andAB of either POTPV or PTPV, suggesting thatPVK and the polymers had a relationship of adonor and an acceptor, fulfilling the Forster energy-transfer mechanism, as shown in Figure 3. How-ever, AB of POTPV overlapped the higher energyside of the PL emission spectrum of PVK, whereasthat of PTPV overlapped the lower side of the PLspectrum. The energy-transfer efficiency may be

Figure 1. Spectra of UV–vis absorption (a) of aPOTPV film and photoluminescence on photoexcitationat 380 nm (b).

Figure 2. UV–vis absorption spectrum (a) of a PTPVfilm and PL spectra of the solution in chlorobenzene(1.25 � 10�5 mol/L) (b) and the film (c) of the copolymeron photoexcitation at 440 nm.

Table 2. HOMO, LUMO, and Band Gaps ofPolymers

HOMO (eV) LUMO (eV) Band-gap (eV)

PTPV �5.4 �3.1 2.3POTPV �5.3 �2.6 2.7

Figure 3. Overlapping of PL emission spectrum ofPVK (a) and UV–vis absorption spectra of POTPV (b)and PTPV (c).

562 LEE ET AL.

correlated not only to the degree of overlap butalso to the character of the overlap.

The photoexcitation of a PVK/POTPV (9:1 w/w)blend film at 380 nm (i.e., the direct photoexcita-tion of POTPV) displayed the same PL emissionspectrum as POTPV itself. However, the PL in-tensity of the blend film increased almost fivetimes on photoexcitation at 340 nm, indirect pho-toexcitation at the AB maximum of PVK, asshown in Figure 4. There is no trace of the PLspectrum of PVK, and this indicates a completeenergy transfer from PVK to POTPV on the pho-toexcitation of PVK. The PL spectrum of the blendfilm on photoexcitation at 340 nm shows a shoul-der at 520 nm. The shoulder was also observed onphotoexcitation of a POTPV film and a solution inchlorobenzene at 340 nm. This was, therefore, notan excimer emission but a radiative singlet exci-ton emission. This means that there was a chro-mophore in POTPV generating PL emission withthe PL maximum at 520 nm. It is suggested thatan excited chromophore might form a new onewith the next chromophore unit in the polymerchain and might display a redshifted radiativesinglet decay emission.

The direct photoexcitation of PTPV in theblend at 440 nm, the AB maximum of the poly-mer, generated a featureless PL spectrum, asshown in Figure 5(b). No effect of dilution by PVKwas observed, and this indicated the existence ofPTPV aggregations. When the blend of PPTV andPVK was photoexcited at the AB maximum ofPVK of 340 nm, two sets of PL emissions wereobtained, as shown in Figure 5(a): one with thePL maximum at 420 nm and the other with themaximum at 505 nm. The PL maximum at 420nm was assigned to the excimer emission peak ofPVK because the photoexcitation of PVK at 420nm gave the same PL spectrum shown in Figure

5(c). The latter part of the spectrum with the PLmaximum at 505 nm accompanies a strong shoul-der with the peak at 540 nm and has all thecharacteristics of the PL spectrum of the PTPVsolution in chlorobenzene. The vibronic progres-sion with the peak at 540 nm appears clearly asthe second peak, probably due to the rigid chainin the solid state.21 The characteristics of theexcimer PL emission of the PTPV film completelydisappeared, and the intensity of the excimeremission at 610 nm was completely suppressed.

AFM showed domains formed by the spin cast-ing of the polymer solution in chlorobenzene dueto phase separation, as shown in Figure 6. Theblend must have had aggregations of either thedonor or one of the acceptors, and each domainwas richer in one polymer component over theother. It was difficult to prevent phase separationin these polymer blends because the low entropy

Figure 4. PL spectra of a PVK/POTPV (9:1 w/w)blend film photoexcited at 340 nm (a) and 380 nm (b),respectively.

Figure 5. PL spectra of a PVK/PTPV (9:1 w/w) blendfilm photoexcited at 340 nm (a) and 440 nm (c), respec-tively, and of a PVK film photoexcited at 340 nm (b).

Figure 6. AFM image of a PVK/PTPV (9:1 w/w) blendfilm.

PHOTOLUMINESCENCE OF POLYMER BLENDS 563

of mixing prevented the homogeneous dispersionof one polymer in the other.22 It was not surpris-ing to observe that the PL spectrum of the blendfilm of PVK/PTPV on photoexcitation at 440 nm,the AB maximum of PTPV, was the same as thatof a PTPV-only film photoexcited at the samewavelength, as shown in Figure 5(b), becausePTPV formed its own rich domains in the blendfilm. The PL spectrum displays a featureless ex-cimer characteristic of the emission maximum at610 nm, showing no dilution effect by PVK.

The indirect photoexcitation of PTPV throughPVK generated no excimer emission from thePTPV-rich domains, although there was no com-plete energy transfer from PVK to PTPV. It isapparent that a few of the PVK excitations trans-ferred energy to PTPV, but the excited PTPVchromophores failed to form the excimers inPTPV. There must be a critical energy level forthe formation of excimers in PTPV. A suppressionmechanism of the excimer emission of a polymeris suggested in Figure 7. When PTPV was photo-excited directly, the chromophores received en-ergy with a Gaussian distribution. The excitedchromophores with energy greater than the crit-

ical level (Scr) formed the PTPV excimers. Whenthe excitations of PVK relaxed by vibration, theenergy level of the excitations reached the inter-molecular internal conversion energy level (Str),and the excitations transferred the energy fromPVK to PTPV. However, the transferred energylevel was not high enough in the PVK–PTPV sys-tem to form excimers in PTPV.

The lifetime of PVK excimers was determinedwith TCSPC. Figure 8 shows that the PL inten-sity of PVK at 410 nm in the blend film on pho-toexcitation at 340 nm decreased exponentially.The lifetimes of excimers in PVK were deduced byan exponential curve-fitting method, as shown inTable 3. The PVK excimer in the PVK/POTPVblend had a shorter lifetime than that of the PVK/PTPV blend. It was, therefore, expected that theenergy transfer from the excited chromophore of adonor to the fluorophore of an acceptor was moreefficient when the lifetime of the fluorophore inthe acceptor was shorter than the excited speciesof the donor. The indirect photoexcitation of thePVK/POTPV blend enhanced the PL intensitysignificantly, but the PVK/PTPV blend showedpoor results.

It is generally accepted that a good overlapbetween the PL emission spectrum of a donor andthe AB spectrum of an acceptor predicts a fasterenergy transfer, but the Forster equation gives nohint on the energy-transfer efficiency based onthe donor–acceptor lifetime ratio. It is apparentthat a donor with a longer lifetime in the excitedstate compared with that of the acceptor willshow high efficiency of the energy transfer to theacceptor.

Figure 7. A schematic representation of the path-ways of energy transfer and excimer formation isshown. Thin arrows represent excitation, solid arrowsrepresent light emission, and broken lines representenergy transfer. S0 is the ground state; S1(d) and S1(a)are the lowest excited states of the donor and acceptor,respectively; Sv is the energy transfer level; Scr is thecritical energy level for the formation of excimers; Sex isthe energy level of excimers; h� is the light emission.

Figure 8. TCSPC profiles of POTPV (a) and PTPV (b)films.

564 LEE ET AL.

CONCLUSIONS

Two copolymers with a triphenyltriazine unit, anelectron-transporting group, attached to a poly-mer backbone of a dialkoxyphenylenedivinylphe-nylene unit failed to be reduced electrochemically.However, LUMOs of the copolymers increased,and this made electron injection easier from acathode. POTPV generated no excimer PL emis-sion on photoexcitation, but the PL intensity wasgreatly increased by the indirect photoexcitationof a blend film of PVK/POTPV with a blend ratioof 9:1. PTPV generated only an excimer PL emis-sion on photoexcitation. The polymer blend withPVK with a blend ratio of 1:9 still generated ex-cimers on photoexcitation at the AB maximum ofPTPV, and this indicated that the blend had do-mains richer in one component on phase separa-tion. However, no excimer emission of PTPV wasobserved on the photoexcitation of PVK. The en-ergy transfer was incomplete, showing both theexcimer PL emission of PVK and the singlet ex-citon emission of PTPV on the photoexcitation ofPVK in the PVK–PTPV blend. This means thatthe excimer emission of PTPV was suppressed. Itis suggested that there is a critical energy levelfor the formation of excimers in a light-emittingpolymer. No excimer emission was detected in aPVK/PTPV blend on photoexcitation at the ABmaximum of PVK because the energy level atwhich the energy transfer occurred between thetwo polymers was below the critical energy levelfor forming excimers. The degree of energy trans-fer between PVK and POTPV or PTPV dependedon the lifetime ratio of PVK excimers and accep-tor fluorophores. A PTPV singlet exciton had along lifetime and showed poor energy transferfrom PVK. There was inefficient energy transferbetween the two polymers not because of the poorspectrum overlap of the emission of the donor andthe AB of the acceptor but because of the longerlifetime of the excited fluorophore of the acceptor.

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Table 3. PL Lifetimes of the Polymers Photoexcitedat the Respective AB Maxima

Lifetime in Solid (ps)

PTPV 230POTPV 55

PHOTOLUMINESCENCE OF POLYMER BLENDS 565