Chapter 23
Silylene-Tethered Divinylarene Copolymers: A New Class of Electroluminescent Polymer 1
1
2
Tien-Yau L u h , Ruey-Min Chen , Zhenbo Deng , and Shuit-Tong Lee 1
2
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong
Semiconducting Polymers Downloaded from pubs.acs.org by YORK UNIV on 12/09/18. For personal use only.
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A new class o f electroactive copolymers o f silylene-spaced conjugated segments is conveniently synthesized by hydrosilylation bis-vinylic silyl hydrides and bisalkynes. Flexible silylenedivinylbenzene copolymer exhibits strong intrachain aggregation at both ground and excited states leading to longer wavelength emission in the blue light region. More rigid polymers, on the other hand, shows compatible fluorescence spectra as those o f the corresponding monomeric model compounds. Copolymers containing triphenylenevinylene-vinylene chromophore 8c can serve as an emitting dopant for the fabrication o f a blue-green organic light emitting diode ( L E D ) . The peak o f the electroluminescence ( E L ) position o f the L E D device can be blue-shifted with increasing applied voltage. The present observation o f the voltage-dependent E L emission suggests a new avenue for controlling the color o f L E D s .
Organic electroluminescent devices have become a real possibility for applications as flat panel displays, since the first report o f Tang and VanSlyke o f an efficient double-layered device based on small organic molecules (/). A number o f organic materials has now been found with improved emission spectral range, long-term stability and conversion efficiency (2). Conjugated polymers have been demonstrated to exhibit diverse electroactive properties. Poly(phenylene-vinylene) (PPV) was the first kind o f such polymers to serve as an emission material in the light emitting diode ( L E D ) (3) Since then, numerous conjugated polymers, copolymers, dye-doped polymers and metal complex polymers have been studied for L E D applications (2-7). Various model systems suggest that the photophysical properties o f certain conjugated polymers can be represented by those o f a short fragment o f the corresponding chromophores (S). The wavelength and quantum efficiency o f the emitted light o f a conjugated moiety in an electroluminescence experiment may be determined by the conjugation length. Accordingly, introduction o f spacers between well-defined chromophores in the polymeric chain can occasionally increase the processibility and, at the same time, the photophysical properties can be predicted
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©1999 American Chemical Society
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(9,10). In particular, such short conjugated segments could furnish desired H O M O L U M O energy gap such that efficient multicolor display applications in the L E D device could be achieved. The use o f aliphatic spacers has been well documented. Since the first discovery of the randomly segemented PPV-based conjugated polymer 1, which exhibits the blue shifted light output relative to P P V ( / / ) , a more rational design by employing ether linkage to insulate the conjugated segments (e.g. 2 and 3) has been reported. These latter copolymers exhibit blue light emission {12). Copolymers having aliphatic spacer 4 and 5 have also been prepared and their electroluminescent properties behave similarly (75). A n alternative approach to tackle this problem is by attaching the chromophore(s) as pendant to a polymeric backbone (14).
5 In this paper we summarize our recent investigations on the silylene-spaced conjugated copolymers. Silylene Spacer The presence o f a silyl substituents on the conjugated system may change the band gap. Depending on the position o f the silyl group, the band gap can be either increased (75) or reduced (16). There has been an increasing use o f a tetrahedral silylene moiety as a bridge connecting chromophores in polymers (16-20). Wittig reaction o f a dicarbonyl compound with preformed aryl-silane linkage provides a useful entry for the silylene-bridged copolymer 6 (17,20). Copolymers containing oligothiophenes and silanylene spacers 7 exhibit emission ranging from blue to red according to the number o f thiophene and silane units (18). More recently, we have employed the strategy using hydrosilylation o f alkynes for the preparation o f copolymers 8 for optoelectronic interests (19). The conjugated
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moiety in 9 are obtained conveniently by employing coupling reactions (eq 1) or Heck reaction (eq 2)1 The key idea o f this hydrosilylation approach relies on our convenient procedure (21) for the synthesis o f vinylic silyl hydrides 10 from the corresponding dithioacetals 9 (eq 3). Representative examples are summarized in Table 1. The corresponding monomers 11 are prepared in a similar manner for comparison.
pc H 4
9
BuBu
0
si-siV1
1
/n
C H 0 4
9
Bu Srr^N ^CH(OEt) 3
s
78«-
2
!3
Ρ
(1)
9b
(i)l-C H -l, cat.PdCI (PPh ) , (ii) 10%HCI, (iii) 1,2-ethanedithiol, BF OEt 6
4
2
3 2
3
2
OC4H9
(2) C H 0 4
9
9c
(i) l-[2,5-(BuO) C H2]-l, cat.Pd(OAqJi P(o-tolyl) , NEt , DMF 2
-S
e
3
3
.S HMe Sr
0•s>* Ar0-
SiMe H
2
9
2
10
m
(3) 7
7
Me Me
Me Me 8
a: A r =
c: A r =
C H 0 4
9
(i) ('PrOJMe^iCHzMgCI, NiCI (PPh ) (ii) LiAIH (iii) H G E C C H C ^ C H , RhCI(PPh ) 2
3
2i
4
6
4
3 3
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Table 1 Synthesis of Polymers 8 %yield (10)
%yield(8)
a
44
85
10667(3.5)
b
62
80
6167(2.0)
c
49
81
9759(1.6)
~9
\ /
Mn (PDI)
\ /
11
Photoluminescence The fluorescence spectra for polymers 8 were examined and are compared with those of the corresponding monomers 11 (Figures 1-3). A s shown in Figure 1, 8a in solution exhibited dual fluorescence spectra. The profiles remains essentially unchanged with the solvent and with the concentration ( 1 0 to Ι Ο M ) . The higher energy emission at ca 340 and 360 mn for 8a is compatible with those for the monomer 11a. The relative intensity of the emission in the blue light region (ca 420 nm) increases with the molecular weight o f 8a, such interaction becomes more important as the polymer becomes larger (79). Intramolecular exeiplex formation between the chromophores in 8a has been proposed. Hartree Fock (3-21G*) calculations on divinyl and distyrylsilanes suggested that the molecules are quite flexible (79). Accordingly, the opportunity for one chromophore unit in 8a located proximal to the other i n space would increase with the molecular weight. Related -5
5
- 7
y
Figure 1 Emission spectra o f a: l i a (1 x 10' M in CHC1 ), b: 8a (2 x 1 0 M in CHC1 ), and c: thin film o f 8a. 3
3
378
,
400
ι
,
_ΖΓ=
500
600
Emission(nm) 5
1 0
Figure 2 Emission spectra o f a: l l b ( l χ ΙΟ" M i n CHC1 ); b: 8b (8 χ Ι Ο M in CHC1 ),. 3
3
500
600
700
Emission(nm) 6
6
Figure 3 Emission spectra o f 8c (3 χ ΙΟ" M in CHC1 ), l i e (3 χ ΙΟ" M in C H C 1 , 3
photophysical behavior has also been observed in block copolymers obtained by ring opening metathesis polymerization o f [2,2]paracyclophanene and norbornene (13b). A s shown i n Figure 1, the vibronic fine structures (Δν = 1507 c m ) for 8a were also observed i n this blue light region. This observation indicates that intrachain aggregation may also occur in the ground state. Intermolecular aggregation is known for polymers in solid film and therefore exhibits characteristic emission at longer wavelength (22). Indeed, the fluorescence o f the thin film o f 8a appears at -1
379
much longer wavelength than that o f 8a in solution (Figure 1 ). These results further suggest that intramolecular interaction between lumiphores in 8a is responsible for the emission i n solution at ca 420 nm. Because of such kind o f interaction, divinylbenzene chromophore in these silylene-spaced copolymers will emit in the blue light region. In contrast to the photophysical properties o f 8a, there is not much difference in the emission spectra between polymers 8b, c and the corresponding monomers l i b , c (Figures 2 and 3). This implies that the above-mentioned intrachain interaction may not occur i n these polymers. Relatively speaking the conjugated moieties in 8b and c are more rigid and therefore the steric requirement might prohibit the chromophores in these polymers in close proximity. The fluorescence quantum yields o f polymers 8 and the corresponding monomers i n CHCI3 solution are summarized in Table 2. Polymers 8b and c were chosen for the electroluminescence ( E L ) investigation because o f its high efficiency in photoluminescence (PL). Table 2 Excitation warelength (λθχ) andfluorescencequantum yield (Φ) of 8 and 11 Substrate 8a 8b 8c 11a 11b 11c
λβχ (nm)
Φ
300 375 407 300
0.026 0.327 0.562 0.006
375 407
0.217 0.451
Electroluminescence A blue-green emitting thin film electroluminescent device using 8c was investigated. Universal device was employed for the L E D studies (Figure 4). It is known that polymer blends o f a hole-transporting polymer polyvinylcarbazole ( P V K ) with an emitting polymer such as polyphenylenephenylenevinylene ( P P P V ) w i l l increase quantum efficiencies (23). In the present study, the hole-transport/ emitting layer containing a mixture o f P V K doped with 8c (1:0.2) was spin-coated onto the ITO glass. The thickness o f the film measured with a profilometer (Alpha Step 500) was controlled by the speed o f spin-coater and the concentration o f the solution. The electron-transport layer, tris(quinoline-8-hydroxylate)-aluminum (Alq), and the cathode, M g : A g (10:1) were vacuum evaporated. Figure 5 shows the E L spectra o f
Figure 4 Structure o f L E D device
380
the device driven at 18 and 28 volt and the P L spectrum o f the thin film of 8c. The intensity o f the E L peak varies with the applied voltage although the half-width remains at about 100 nm, and the position o f the peak is at 516 nm (green) at the low voltage (18 V ) but shifts to 496 nm (blue-green) at the high voltage (28 V ) .
500
r
300
400
500
600
700
800
Wavelength (nm) Figure 5 The P L spectrum o f 8c and the E L spectra o f the device The brightness-current-voltage (B-I-V) curves o f the device are shown in Figure 6. The threshold voltage was observed to be 12 V in the device. The E L brightness reached 800 cd/m under 28 V and about 89 m A / c m . The E L peak emission o f the device blue-shifted with increasing applied voltage. Figure 7 shows the peak position o f E L versus the applied voltage. The wavelength of the peak position decreased by 24 nm (from 520 to 495 nm) as the applied voltage increased from 12 to 28 V . A s shown in Figure 5, the E L emission o f the L E D is similar to the P L spectrum of the thin film o f 8c, particularly when the device is driven at 18 V , indicating that the E L emission originates mainly from 8c. This observation together with the absence o f the blue emission from P V K suggested that there was an energy transfer from P V K to 8c or the energy o f the excitons was too low for P V K to emit. Under lower applied voltage, the E L is perhaps attributable to the emission from the A l q layer because the E L peak o f the device is at 520 nm, which is close to the A l q emission at 524 nm. This suggests that at low electric field, the electrons are slowly moving so that electron-hole recombination occurs predominantly in the A l q layer. With increasing applied voltage, the electron mobility increases and the electronhole recombination zone moves increasingly towards the P V K / 8 c layer. A s a result, with increasing voltage the E L emission has an increasing contribution from 8c and the E L peak shifts more to the blue. A t still higher field, the recombination occurs near the I T O electrode from which reflection may become important and the effect of microcavity may set in, resulting in a further shortening o f the wavelength to less than the P L peak o f 8c. 2
2
381
H
800
600 ,
H
400
H
200
E
Ο
10
15
20
Voltage (V)
Figure 6 The brightness-current-voltage characteristics o f the device
520 k — — β ^ ·
·
496nm- - - · - ·
15
20
Applied voltage (V)
Figure 7 The change o f E L peak position versus applied voltage (dot: experimental Value; dashed line: fitted curve o f the data).
382
As shown in Figure 2, 8b emits in the blue region. A similar device as shown in Figure 4 was made but without the alq electron transporting layer. This device gave blue light emission when the threshold voltage 15 V was applied. Conclusion In summary, we have demonstrated a new class of electroactive copolymers of silylene-spaced conjugated segments 8 which can be conveniently synthesized from the readily accessible starting materials. The photophysical studies indicate that 8a exhibit strong intrachain aggregation at both ground and excited states leading to longer wavelength emission in the blue light region. Polymer 8c can serve as as an emitting dopant for the fabrication of a blue-green organic L E D . The peak E L position of the L E D device can be blue-shifted with increasing applied voltage. The present observation of the voltage-dependent EL emission suggests a new avenue for controlling the color of LEDs. Acknowledgment. This work was supported by the National Science Council of the Republic of China (to T Y L and RMC) and by the Strategic Research Grant (Project No. 7000771 ) of City University of Hong Kong (to Z B D and STL).
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