Luminescent poly(p-phenylenevinylene) with 4-methylcoumarin side groups: Synthesis, optical properties and photo-crosslinking behaviors

REACTIVE & FUNCTIONAL POLYMERS

Reactive & Functional Polymers 66 (2006) 1327–1335

www.elsevier.com/locate/react

Luminescent poly(p-phenylenevinylene) with 4-methylcoumarin side groups: Synthesis, optical properties and photo-crosslinking behaviors Cheng-Jang Tsai, Yun Chen

*

Department of Chemical Engineering, National Cheng Kung University, Da-Syue Road, Tainan 701, Taiwan, ROC Received 25 August 2005; received in revised form 3 March 2006; accepted 18 March 2006 Available online 5 May 2006

Abstract Poly(p-phenylenevinylene) (PPV) derivatives HOPPV and COUPPV, containing hexyloxy side groups and 7-oxy-4methylcoumarin groups via hexyloxy spacer, respectively, have been synthesized and characterized. Their optical properties were investigated with absorption and photoluminescence (PL) spectral methods. The emissions of COUPPV and model HOPPV films are similar and show peaks around 543–550 nm which is attributed to PPV backbone. Accordingly, efficient energy transfer from the 4-methylcoumarin chromophores to PPV backbone occurred readily in COUPPV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels have been estimated from their cyclic voltammograms. As compared with HOPPV (LUMO: 2.86 eV), the electron affinity of COUPPV (LUMO: 3.65 eV) was enhanced greatly by introducing 4-methylcoumarin groups. Double-layer EL devices (Al/PEDOT/COUPPV or HOPPV/ITO) revealed green electroluminescence. The photo-crosslinking behaviors of COUPPV film under ultraviolet irradiation were investigated by FT-IR and PL methods. Both C@C of PPV and 4-methylcoumarin dimerized to cyclobutane derivatives and lead to crosslinking of the polymers. However, COUPPV exhibited higher photo-crosslinking rate due to pendant 4-methylcoumarin chromophores. Patterned emission from PLED utilizing this effect was also demonstrated.  2006 Elsevier B.V. All rights reserved. Keywords: Luminescent; 4-Methylcoumarin; Optical property; Photo-crosslinking

1. Introduction Polymeric light emitting diodes (PLEDs) have been developed over 10 years since Holmes et al. first reported the electroluminescence (EL) of full conjugated poly(p-phenylenevinylene) (PPV) [1]. Among those, low efficiency of polymeric EL *

Corresponding author. Tel./fax: +886 62085843. E-mail address: [email protected] (Y. Chen).

devices is one of the major restraints. To date, many strategies such as fabrication of multi-layer structure, using low-work function metal as cathode, blending of luminescent polymers and so on, have been investigated for performance improvement. Amid those methods, the blending of conjugated polymers is a versatile and efficient method for enhancing efficiency of the device [2]. However, phase separation may occur due to limited solubility and lead to decreased efficiency and shortened life-

1381-5148/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.03.016

1328

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335

is attributed to a photo-induced [2 + 2] cycloaddition of the C@C double bonds, which is assumed to occur in PPV derivatives under UV illumination [9]. Coumarin is a compound existing in many plants and applied to the preparation of photosensitive polymers. Upon irradiation with ultraviolet light (k > 300 nm), it readily dimerizes to cyclobutane to form four types of dimers: anti head-tohead, anti head-to-tail, syn head-to-head, and syn head-to-tail coumarin dimers [10]. Accordingly, irradiation of COUPPV film by UV light renders dimerization of the pendant 4-methylcoumarin groups and results in gradual crosslinking. Therefore, the copolymers are negative-type photoresist materials [11]. This work presents the preparation and characterization of PPV derivatives COUPPV and model polymer HOPPV. We attempted to enhance the electron affinity and reactivity of COUPPV by introducing the 4-methylcoumarin side groups. The optical, EL, electrochemical, photo-crosslinking and patterned emission properties of COUPPV have been investigated in detail.

time. To circumvent the disadvantages of such blends, we now explore to chemically attach 4-methylcoumarin sensitizer moiety to PPV backbone through flexible spacer (Scheme 1). The resulting polymer is expected to give interesting results, such as enhancement of electron affinity without phase separation problems. Red, green and blue (RGB) pixels have to be realized for full color applications. The methods to achieve RGB displays can be categorized as external conversion layers [3], filters [4], and printing techniques such as ink-jet printing [5]. A technique relies on the lithographic patterning of functionalized conjugated polymers (crosslinking of oxetane groups by UV irradiation) has been reported [6]. Previous results with the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have shown that it can be efficiently photo-bleached by UV irradiation in the presence of gaseous reagents such as hydrazine and thiols [7]. These reagents lead to an efficient saturation of C@C double bonds, which is utilized for the set-up of color-patterned light emitting devices. However, both hydrazine and thiols are toxic and odorous reagents which cause problems during photochemical processing [7]. Buchgraber investigated UV-assisted bleaching of MEH-PPV with silane or thiol reagents and compared with those in air or in Argon (>99.9999%) [8]. UV irradiation of MEHPPV under inert conditions also led to decreased absorption at 503 nm although at a slower rate. This

2. Experimental 2.1. Materials The synthetic procedures of the monomers 1, 2 and model compound MC were analogous to those provided elsewhere [12,13]. The p-divinylbenzene (3)

R

R

O I

I

+

P(Ph-Me)3 , DMF

O

O

3

1 or 2

n

O

Pd(OAc)2, NEt3

HOPPV or COUPPV

R

R for 1, HOPPV

R= H R=

O

for 2, MC and COUPPV

R O

MC

O

Scheme 1.

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335

was separated from a mixture (Aldrich) of p-divinylbenzene and m-divinylbenzene according to the literature [14]. Triethylamine (NEt3) was distilled over KOH before use. Palladium (II) acetate [Pd(OAc)2; Acros], tri-(o-tolyl)phosphine [P(PhMe)3; Acros] and potassium carbonate (K2CO3; Showa) were used without further purification. N,N-Dimethylformamide (DMF; Tedia), chloroform (CHCl3; Tedia), acetone (CH3COCH3, Tedia), tertrahydrofuran (THF; Tedia) and other solvents were HPLC grade reagents and used without further purification. 2.2. Instrumentations All compounds were identified by 1H NMR, FTIR, and elemental analysis (EA). The 1H NMR spectra were recorded on a Bruker AMX400 MHz FT-NMR, and chemical shifts are reported in ppm using tetramethylsilane (TMS) as an internal standard. The FT-IR spectra were measured as KBr pellets on a Fourier transform infrared spectrometer, model Valor III from Jasco. The elemental analysis was carried out on a Heraus CHN-Rapid elemental analyzer. The thermogravimetric analysis (TGA) of the polymers was performed under nitrogen atmosphere at a heating rate of 20 C/min using a Perkin–Elmer TGA-7 thermal analyzer. Thermal properties of the polymers were measured using a differential scanning calorimeter (DSC), Perkin–Elmer DSC 7, under nitrogen atmosphere at a heating rate of 10 C/ min. Absorption spectra were measured with Jasco V-550 spectrophotometer and photoluminescence (PL) spectra were obtained using a Hitachi F-4500 spectrofluorometer. Cyclic voltammograms were recorded using a voltammetric analyzer, model CV-50W from BAS, under nitrogen atmosphere using ITO glass as working electrode, Ag/AgCl electrode as reference electrode and platinum wire electrode as auxiliary electrode supporting in 0.1 M (n-Bu)4NClO4 in acetonitrile. The energy levels were calculated using the ferrocene (FOC) value of 4.8 eV with respect to vacuum level, which is defined as zero [15]. 2.3. Fabrication and characterization of EL devices Double-layer device (Al/PEDOT/polymer/ITO) was fabricated by spin-coating (3000 rpm) onto ITO glass (25 X/square) of poly(3,4-ethylenedioxythiophene) (PEDOT) solution to obtain a film first,

1329

followed with drying in a vacuum oven at 150 C for 30 min. Then the emitting layer was spin-cast (1000 rpm) onto the ITO from COUPPV or HOPPV solutions (6–3 mg/1 ml in C2H2Cl4), followed with drying at 80 C under vacuum, to obtain a film of about 90–110 nm thickness (measured by Atomic Force Microscope, AFM). Finally, thin aluminum layer was deposited as the cathode by thermal evaporation under a vacuum about 105 Torr. The active area of each device is about 4 mm2. The luminance and electric characteristics of the devices were measured by Keithley power supply (model 2400) and luminance meter (BM8 from TOPCON), respectively. The electroluminescence spectra were measured by Hitachi F-4500 spectrofluorometer. All the fabrications and characterizations of the devices were under ambient environment. 2.4. Irradiation equipment Thin polymer films were prepared by casting their solution on quartz glass plate or potassium bromide pellet. The films were allowed to dry at room temperature overnight under a dark and inert environment. UV exposure was carried out by putting the polymer film under a UVP Mineralight lamp (model UVGL-58) in a MBRAUN glove box (model Unilab). The main emission peak of the UV lamp was at 366 nm and the power at the reaction site was 3.01 mW/cm2, which was measured by a UV meter (model UVX-36 from UVP). For patterning of EL device, it was settled in the glove box filled with nitrogen (H2O < 1 ppm, O2 < 1 ppm) and the light was irradiated from ITO-glass side using a mask made of aluminum foil. 2.5. Polymer synthesis A typical polymerization procedure is described as follows: triethylamine (0.35 ml, 2.5 mmol) was added to a solution of p-divinylbenzene (3; 130 mg, 1 mmol), monomer 1 or 2 (1 mmol), Pd(OAc)2 (9.0 mg, 0.04 mmol), and tri-o-tolylphosphine (60.9 mg, 0.2 mmol) in 5 ml of DMF. The mixture was allowed to react at 100 C for 15 h under a nitrogen atmosphere and then poured into 200 ml of methanol. The precipitated polymer was collected by filtration and further purified by redissolving in a minimum amount of hot chloroform and re-precipitating in acetone. The resulting polymer was further extracted with methanol for 3 days

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335

3. Results and discussion 3.1. Polymerization A typical Heck reaction condition was employed to the polymerization of aromatic diiodide (1 or 2) and p-divinylbenzene (3) (Scheme 1) in the preparation of HOPPV or COUPPV. The molecular weight and thermal properties of COUPPV and HOPPV are summarized in Table 1. The weightaverage molecular weights (Mw) of COUPPV and HOPPV, determined by gel permeation chromatography using polystyrene as standard, were 10,012 and 4696, respectively, with polydispersity indexes of 2.03 and 1.56. The polymers are readily soluble in common organic solvents such as chloroform, Table 1 Polymerization yield, molecular weight and Tds of HOPPV and COUPPV Polymers

Yield (%)

Mna

Mwa

PDIa

Td (C)b

HOPPV COUPPV

42.4 67.4

3001 4922

4696 10,012

1.56 2.03

327 418

a

Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3. b Decomposition temperature at 5% weight loss was measured by TGA at a heating rate of 20 C/min under nitrogen.

tetrahydrofuran (THF), 1,1,2,2-tetrachloroethane (C2H2Cl4), and N,N-dimethylformamide (DMF). The decomposition temperatures (Td) at 5% weight loss are 418 and 327 C for COUPPV andHOPPV, respectively, indicating that the former with 4-methylcoumarin groups is thermally more stable than HOPPV. No melting temperature and obvious Tg (glass transition temperature) can be observed below 250 C on their differential scanning calorimetric (DSC) measurements. 3.2. Optical properties Fig. 1 shows the absorption and photoluminescence (PL) spectra of model compound (MC) and model polymer (HOPPV), which simulate the two emitting fluorophores in COUPPV (as shown in Scheme 1). HOPPV is the model polymer corresponding to emitting backbone of COUPPV, whereas MC is corresponding to 7-oxy-4-methylcoumarin chromophores in side chain [16]. Emitting HOPPV shows maximal PL peak at 511, but electron-transporting models MC exhibits a band peaking at 380 nm. The absorption and PL spectrum of COUPPV in chloroform solutions are also shown in Fig. 1 and the related optical data are listed in Table 2. The absorption spectra of COUPPV locate around 330 and 460 nm which are basically the overlap of those of MC and HOPPV. However, the PL spectra of COUPPV and HOPPV (under excitation with 320 nm light) are very similar in shape and exhibit maximal PL peak at 511 nm, although COUPPV contains extra electron-transporting pendant 7-oxy-4-methylcoumarin groups.

PL Intensity (a.u.)

using a Soxhlet extractor and was then dried under a vacuum at 45 C for 2 days. COUPPV (67.4% yield): FT-IR (KBr pellet): m 2943–2859 (–CH2–), 1718 (lactone), 1153 (–CH2– O–Ar), and 970 cm1 (trans –CH@CH–). 1H NMR (CDCl3): d 7.48–7.42 (d, 7H, Cou H-5, –CH@CH– and H-Ar); 7.12 (s, 4H, H-ortho and –CH@CH–); 6.79–6.74 (d, 2H, Cou H-6, H-8 and –CH@CH–); 6.07 (s, 2H, Cou H-3); 4.10–4.00 (t, 8H, –CH2O– cou and –CH2OAr); 2.31 (s, 6H, –CH3); 1.88 (broad, 8H, ArOCH2CH2– and –CH2CH2O–); 1.57–1.25 (d, 8H, –CH2–). Anal. Calc. for C48H48O8: C, 76.57; H, 6.43. Found: C, 77.13; H, 6.90%. HOPPV (42.4 % yield): FT-IR (KBr pellet): m 2930–2859 (–CH2–), 1206 (–CH2–O–Ar), and 961 cm1 (trans –CH2@CH2–). 1H NMR (CDCl3): d 7.54–7.39 (m, 5H, –CH@CH– and H-Ar); 7.17– 7.04 (m, 5H, H-ortho and –CH@CH–Ar– CH@CH–); 4.07–3.96 (t, 4H, –CH2OAr); 1.89 (broad, 4H, –CH2CH2O–); 1.56 (broad, 4H, –CH2 CH3); 1.39 (broad, 8H, –CH2CH2 CH2 CH2O–); 0.94–0.85 (broad, 6H, –CH3). Anal. Calc. for C28H36O2: C, 83.12; H, 8.97. Found: C, 81.97; H, 8.10%.

Absorbance (a.u.)

1330

300

400

500

600

Wavelength (nm)

Fig. 1. Absorption and photoluminescence spectra (excitation at 320 nm) of COUPPV (—), MC (- - - -) and HOPPV (  ) in 1 · 105 M CHCl3 solutions.

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335 Table 2 Optical properties of the polymers in solution and film states Polymers

COUPPV HOPPV

UV kmax (nm)a

PL kmax (nm)a

Soln.

Film

Soln.

Film

330, 456 454

335, 460 460

511 511

543 550

UPL b 0.61 0.56

a UV-visible absorption and photoluminescence spectra of polymers in CHCl3 solution. b These values were measured by using quinine sulfate (dissolved in 1 N H2SO4(aq) with a concentration of 105 M, assuming UPL of 0.55) as a standard at 24–25 C. The excitation wavelength is 320 nm for these solutions, which had absorbance of less than 0.05.

The PL bands of pendant 4-methylcoumarin groups, i.e., 380 nm of MC, disappear completely. Therefore, the PL spectrum of COUPPV is mainly dominated by the fluorophores with longer emissive wavelength (emitting backbone), indicating that efficient excitation energy transfer from electrontransporting 7-oxy-4-methylcoumarin to hole-transporting PPV backbone has occurred in COUPPV under photo-excitation. The emission spectra of MC are covered significantly with the absorption of HOPPV (Fig. 1). This suggests that excitation energy might transfer from MC (energy donor) to HOPPV (energy acceptor) if the distance between the donor and acceptor is sufficiently short. As shown in Fig. 2, the PL spectra of MC and HOPPV mixtures (excited with 320 nm, which is the absorption maximum of MC) exhibit emission peaks around 380 and 511 nm, respectively. The relative intensity of HOPPV (at ca. 511 nm), as compared to that of MC (at ca. 380 nm), enhances significantly with increasing concentration (from 5 · 105 M to

1331

5 · 104 M). The transformation of PL spectra with concentration would be caused by increased reabsorption and excitation energy transfer due to reduced intermolecular distance that is mainly controlled by the concentration in solution. Accordingly, COUPPV show only PL peaks of hole-transporting PPV main chain around 511 nm (Fig. 2). This is attributable to efficient intrachain energy transfer because the energy donor (7-oxy-4-methylcoumarin side chain) and the energy acceptor (PPV backbone) are connected only via a flexible spacer. The PL spectra of COUPPV and HOPPV in thin films both show about 32 and 39 nm red-shifts, respectively, and are characteristically broader as compared with those in solution (Table 2 and Fig. 5). These red-shifts are due to intermolecular p–p interactions between the chromophores in solid state. The relative quantum yields (UPL) of COUPPV and HOPPV in solution are 0.61 and 0.56, respectively, suggesting that efficient energy transfer from electron-transporting 7-oxy-4-methylcoumarin to emitting PPV backbone in COUPPV enhance the quantum yields slightly. 3.3. Electrochemical property The cyclic voltammograms (CV) of COUPPV and HOPPV are shown in Fig. 3 and their electrochemical data are summarized in Table 3. The onset oxidation potentials (versus FOC) of COUPPV and HOPPV are the same (0.29 V) because they contain the same hole-transporting PPV backbone. However, their onset reduction potentials are very differ800

600

Current (10-6 A)

PL Intensity (a. u.)

400

200

0

-200

-400 350

400

450

500

550

600

Wavelength (nm)

Fig. 2. Photoluminescence spectra (excitation at 320 nm) of COUPPV (—: 1 · 105 M), and mixture of HOPPV and MC (- - - -: 1 · 104 M,   : 1 · 105 M) in CHCl3 solutions.

2

1

0

-1

-2

Potent ial (V)

Fig. 3. Cyclic voltammograms of COUPPV (—) and HOPPV (- - - - -) films coated on ITO glass electrode immersed in CH3CN solution of Bu4NClO4 (0.1 M); scan rate: 50 mV/s.

1332

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335

Table 3 Electrochemical data of HOPPV and COUPPV Polymers

Eonset(ox) vs. Ag/Ag+ (V)

Eonset(red) vs. Ag/Ag+ (V)

Eonset(ox) vs. EFOC (V)a

Eonset(red) vs. EFOC (V)a

EHOMO (eV)b

ELUMO (eV)c

Eelg (eV)d

HOPPV COUPPV

0.74 0.74

1.49 0.70

0.29 0.29

1.94 1.15

5.09 5.09

2.86 3.65

2.23 1.44

4-methylcoumarin moieties can be observed at blue–violet region even at high electric field (Fig. 4). The EL spectral maximum of COUPPV and HOPPV devices are 547 and 544 nm, respectively, whose features are similar to those observed in PL spectra of the corresponding polymer films. Fig. 5 shows the current density (J)–electric field (F)–luminance (L) diagram of the devices. The turn-on electric fields for current density are 4.1 · 105 and 6.2 · 105 V/cm with luminance maxima of 103 and 74 cd/m2 for COUPPV and HOPPV, respectively. The overall luminance of the devices is rather low probably due to the use

(a)

PL intensity (a.u.)

(b)

PL intensity (a.u.)

ent and locate at 1.15 and 1.94 V, respectively. The COUPPV reduces much more easily than HOPPV due to the isolated electron-transporting 7-oxy-4-methylcoumarin side groups. The estimated HOMO and LUMO energy levels of COUPPV are 5.09 and 3.65 eV, whereas those of HOPPV are 5.09 and 2.86 eV, respectively. Obviously, the 7oxy-4-methylcoumarin groups enhance electron affinity of COUPPV significantly by reducing its LUMO level. The optical band gap (Eopt g ) of MC and model polymer (HOPPV), calculated from their edge absorption, are 3.30 and 2.33 eV, respectively [12,17]. The LUMO levels of hole-transporting backbone can be estimated from the HOMO levels of COUPPV by adding the Eopt of corresponding g model polymer (HOPPV). So the LUMO level of hole-transporting PPV backbone in COUPPV is estimated to be 2.76 eV by adding the Eopt of the g corresponding HOPPV (2.33 eV) with its HOMO level (5.09 eV). The HOMO levels of electrontransporting 7-oxy-4-methy-coumarin groups can be estimated similarly. The electrochemically-determined band of COUPPV is the overlapped portions of corresponding models MC and HOPPV. Therefore, electron affinity of COUPPV can be promoted significantly caused by electron-withdrawing 7-oxy4-methy-coumarin groups. In general, decrease in LUMO levels enhances electron affinity which leads to reduced electron-injection barriers. Therefore, the electron- and hole-injection would be more balanced in COUPPV when employed as emitting layer in EL devices.

EL Intensity (a. u.)

c d

Eonset(ox or red) vs. EFOC = Eonset(ox or red) vs. Ag/Ag+(V)  EFOC, where EFOC = 0.45 V vs. Ag/Ag+. EHOMO = e(Eonset(ox), vs. EFOC + 4.8 V). ELUMO = e(Eonset(red), vs. EFOC + 4.8 V). Band gaps obtained from electrochemical data: Eelg ¼ ELUMO  EHOMO .

EL Intensity (a. u.)

a b

3.4. Double-layer LED device Double-layer light emitting diodes [ITO/ PEDOT/Polymers/Al] were fabricated for optoelectronic investigation. The electroluminescence (EL) spectrum of COUPPV shows only the emission of PPV backbone and no emission of the side 7-oxy-

400

450

500

550

600

Wavelength (nm)

Fig. 4. Electroluminescenct spectra (solid line) and photoluminescent spectra (excitation at 320 nm) (broken line) of (a) HOPPV and (b) COUPPV.

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335 120

2.5

100

2.0

80 1.5 60 1.0 40 0.5

Luminescence ( cd/m2 )

Current Density (mA/cm2 )

1333

20

0.0

0 0

2

4

6

8

Electric Field (105 V/cm)

Fig. 5. Luminance–electric field (hollow) and current density–electric field (full) characteristics of double layer devices of Al/PEDOT/ COUPPV/ITO (circle) and Al/ PEDOT/HOPPV/ITO (triangle).

(a)

PL Intensity (a.u.)

Absorbance (a.u.)

of high work function aluminum (4.3 eV) as cathode. The COUPPV film shows lower turn-on electric fields (for current density) and greater luminance maxima, which can be attributed to enhanced electron injection due to electron-withdrawing 7-oxy-4-methylcoumarin groups and occurrence of energy transfer from 7-oxy-4-methylcoumarin to emitting PPV backbone. 3.5. Photo-crosslinking behaviors of HOPPV and COUPPV

PL Intensity (a.u.)

(b)

Absorbance (a.u.)

As shown in the Fig. 6(a), the UV and PL spectra of HOPPV film show the peaks at 460 and 550 nm which can be attributed to the K band absorption and the emission of PPV main chain, respectively. Upon irradiating with 366 nm UV light under nitrogen, both the absorption and emission intensity decrease with time or irradiation energy (Fig. 7). This suggests gradual dimerization of C@C double bond of PPV backbone to cyclobutane derivatives and lead to the interruption of conjugation [8]. The dimerization of C@C double bonds of PPV main chain can also be confirmed by FT-IR spectra as shown in Fig. 8(a) for HOPPV. Upon irradiation of 366 nm light for 2 h (irradiation energy: 21.67 J/cm2) the peak at 970 cm1, which is assigned to the stretching of trans –CH@CH– in the main chain, decreases in intensity gradually. As shown in Fig. 6(b), the UV and PL spectra of COUPPV film exhibit peaks at 460 and 543 nm, which can also be attributed to the K band absorption and the emission of PPV conjugated backbone, respectively. Additional peak at 330 nm can be ascribed to the absorption of pendant 7-oxy-4-methylcoumarin groups. Upon irradiation

300

400

500

600

Wavelength (nm)

Fig. 6. Photoluminescence (excitation at 320 nm) and UV–Vis absorption spectra of HOPPV (a) and COUPPV (b) in film state before (—) and after irradiation for 10 min (  : 1.81 J/cm2), 50 min (– .. – .. –: 9.03 J/cm2) and 2 h (–h–h–: 21.67 J/cm2) under nitrogen.

with 366 nm light, both emission and absorption peaks decrease with irradiation time and energy (Fig. 7). The intensity decrease in absorption at 460 nm is faster than that at 330 nm. Interestingly, the UV and PL of COUPPV decay significantly

1.0

1.0

0.9

0.8

0.8

0.6

0.7

0.4

0.6

0.2

0.5

Normalized PL intensity (a.u.)

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335

N o r m a l i z e d a b s.

1334

0.0 0

5

10

15

20

2

Irradiation energy (J/cm )

Fig. 7. Photo-bleaching of HOPPV (triangle) and COUPPV (circle) under nitrogen: normalized absorbance (full) and normalized photoluminescence intensity (hollow, excitation at 320 nm) at 460 and 550 nm as a function of the UV irradiation energy, respectively.

Normalized Absorbance

(a)

-CH2-

-CH=CH-

(b) Normalized Absorbance

-CH=CH-

-CH2-

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Fig. 8. FTIR spectra of (a) HOPPV and (b) COUPPV in film state, before (upper) and after (lower) irradiation for 2 h (irradiation energy: 21.67 J/cm2) under nitrogen.

faster than those of HOPPV. This is probably due to the additional dimerization of 7-oxy-4-methylcoumarin chromophores to cyclobutane derivatives [10]. However, the role of 7-oxy-4-methylcoumarin as sensitizer can not be neglected since it possibly

enhances photo-crosslinking of C@C double bond of PPV via energy transfer. The reaction of COUPPV can also be confirmed by FT-IR spectra as shown in Fig. 8(b). The peak at 970 cm1, which is assigned to stretching of trans –CH@CH–, disappears almost completely after irradiating 366 nm light for 2 h under nitrogen. The peak at 2920 and 2856 cm1 due to –CH2– stretching increases slightly after the irradiation, suggesting dimerization of C@C double bond of PPV and coumarin to form cyclobutane derivatives. The photosensitivity of HOPPV and COUPPV films has been evaluated by their characteristic curves, which were established by tracing the residual weight of the films after irradiation and development, and expressed as W/W0 versus irradiation energy plot (W0 is weight before exposure, whereas W is the residual weight after exposure and developed in chloroform). The W/W0 ratio depicts the fraction of crosslink (gel fraction) after irradiation. As shown in Fig. 9, both COUPPV and HOPPV crosslink steadily under irradiation with 366 nm light and COUPPV reveals much faster rate than HOPPV. From the above results, clearly COUPPV is a more sensitive negative-type photoresist and can be employed as patterned emission materials [6,11]. Therefore, by using a mask during photo-irradiation, patterned emission was realized as shown in Fig. 10. It should be noted that the patterning method of PLED [ITO/PEDOT/ COUPPV/Al] proposed here was carried out after device fabrication, i.e. after evaporation of the metal electrode, in contrast to other methods such as ink-jet printing of polymer solutions or UV irradiation in the presence of toxic and odorous reagents such as hydrazine and thiols.

C.-J. Tsai, Y. Chen / Reactive & Functional Polymers 66 (2006) 1327–1335 100

W/W0 (%)

80

60

40

20

0 0

5

10

15

20

Irradiation energy (J/cm2)

Fig. 9. Characteristic curves of HOPPV (.) and COUPPV (d). W is the weight of the film after irradiation with 366 nm light under nitrogen followed by developing in chloroform, where W0 is the weight before irradiation.

1335

HOPPV. The emission in COUPPV is mainly attributed to PPV main chain due to efficient energy transfer from electron-transporting 7-oxy-4-methylcoumarin chromophores. Furthermore, electron affinity of COUPPV is enhanced by 7-oxy-4-methylcoumarin chromophores. Double-layer EL devices of both polymers emit green light. COUPPV device showed lower turn-on electric fields and greater maxima luminance, which can be attributed to enhanced electron injection due to electron-withdrawing 7-oxy-4-methylcoumarin groups. The photo-crosslinking rate of COUPPV film under ultraviolet irradiation was higher than HOPPV due to additional 4-methylcoumarin chromophores in pendant groups. Patterned EL emission device using COUPPV as emitting layer has been successfully fabricated. References

Fig. 10. Patterned (8) emission from two-layer EL device (Al/ PEDOT/COUPPV/ITO).

4. Conclusions We have successfully prepared two PPV derivatives HOPPV (as model polymer) and COUPPV by typical Heck reaction. Corresponding model compound MC of pendant 7-oxy-4-methylcoumarin have also been prepared for comparison. The COUPPV shows better thermal stability than

[1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [2] I.N. Kang, D.H. Hwang, H.K. Shim, T. Zyung, J.J. Kim, Macromolecules 29 (1996) 165. [3] S. Tasch, C. Brandstaetter, F. Meghdadi, G. Leising, G. Froyer, L. Athouel, Adv. Mater. 9 (1997) 33. [4] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [5] T.R. Hebner, C.C. Wu, D. Marcy, M.H. Lu, J.C. Sturm, Appl. Phys. Lett. 72 (1998) 519. [6] C.D. Mu¨ller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature (London) 421 (2003) 829. [7] A. Pogantsch, G. Trattnig, E. Zojer, H. Tillmann, H.H. Ho¨rhold, U. Scherf, G. Langer, W. Kern, Synth. Met. 138 (2003) 85. [8] C. Buchgraber, J. Spanring, A. Pogantsch, M. Turner, W. Kern, Synth. Met. 147 (2004) 91. [9] H.F.M. Schoo, Optimisation of conjugated polymers and performance in organic devices, in: Lecture Presented at MRS Spring Meeting, San Francisco, USA, April 21–25, 2003. [10] C.H. Krauch, S. Farid, G.O. Schenk, Chem. Ber. 99 (1966) 625. [11] Y. Chen, J.L. Geh, Polymer 37 (1996) 4473. [12] Y. Chen, S.W. Hwang, T.H. Yu, Polymer 44 (2003) 3827. [13] A.R.A. Palmans, P. Smith, C. Weder, Macromolecules 32 (1999) 4677. [14] B.T. Storey, J. Polym. Sci. A 3 (1965) 265. [15] Y. Liu, M.S. Liu, A.K.Y. Jen, Acta Polym. 50 (1999) 105. [16] S.W. Hwang, Y. Chen, Macromolecules 35 (2002) 5438. [17] S.W. Hwang, S.H. Chen, Y. Chen, J. Polym. Sci. A 40 (2002) 2215.