Controlling Supramolecular Microstructure to Realize Highly Efficient

May 21, 2008 - Controlling Supramolecular Microstructure to Realize Highly Efficient Nondoped Deep Blue Organic Light-Emitting Devices: The Role of ...
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9066

J. Phys. Chem. C 2008, 112, 9066–9071

Controlling Supramolecular Microstructure to Realize Highly Efficient Nondoped Deep Blue Organic Light-Emitting Devices: The Role of Diphenyl Substituents in Distyrylbenzene Derivatives Zengqi Xie, Weijie Xie, Feng Li,* Linlin Liu, Huan Wang, and Yuguang Ma* State Key Lab of Supramolecular Structure and Materials, Jilin UniVersity, 2699 Qianjin AVenue, Changchun 130012, People’s Republic of China ReceiVed: February 4, 2008; ReVised Manuscript ReceiVed: March 15, 2008

trans-Distyrylbenzene (trans-DSB), the most typical model compound for poly(p-phenylenevinylene) (PPV) derivatives, has highly efficient and pure blue emission in dilute solution; while in solid or film state, its photoluminescence efficiency decreases dramatically with red-shifted emission, because of the formation of H-aggregate. Herein, we introduce two phenyl substituent groups to the central phenyl ring of trans-DSB, which restrains the formation of H-aggregate efficiently in the condensed state and then induces highly luminescent molecular aggregate modes (X-aggregate and J-aggregate). Most important, the introduction of the phenyl substituent groups makes the resulting compounds form high quality films easily under vacuum deposition conditions. High performance nondoped deep blue organic light-emitting device with CIE (0.15, 0.10), maximum luminous efficiency of 4.2 cd/A (corresponding external quantum efficiency of 3.9%), is achieved on the basis of this kind of material. Introduction After the discovery of electroluminescence (EL) phenomena of conjugated small molecules and polymers in thin film devices, poly(p-phenylenevinylene) (PPV) type luminescent oligomeric and polymeric materials have attracted wide interest, because of tunable emitting color, high luminescent efficiency, and convenient synthesis methods.1–4 Great progresses have been made to increase the solid-state luminescent efficiency by the use of substitutional side chains to suppress molecular aggregation as well as to improve the solubility in common solvents (e.g., MEH-PPV).5 Nevertheless, there still exists a significant tendency toward molecular aggregation because aggregation in some sense is inherent in film formation, which gives rise to strong effects on their light emission properties.6–12 Thus, it would be an ideal case if the emitters adopt a special molecular stacking that can restrain the luminescence quenching. trans-Distyrylbenzene (trans-DSB, Chart 1) is the most typical model molecule for PPV derivatives, and was extensively studied for a long period.13 It possesses very high photoluminescence (PL) efficiency and deep blue emission in dilute solution, while in solid state or thin film, the PL efficiency decreased dramatically with obvious red-shifted emission, which can be attributed to the formation of H-aggregate.14 According to the exciton interaction model described by Bohn and Kasha et al., some special dipole stacking system, such as X-aggregate (cross dipole stacking) and J-aggregate (head to tail dipole stacking), could suppress the fluorescence quenching and even enhance the fluorescence emission in the solid state.15,16 Then the construction of special dipole stacking by molecular design becomes a channel to realize high solid-state efficiency and furthermore to fabricate devices with high performance.17,18 Crystal structure analysis is a direct method to get the information about the interdipole interaction mode (molecular stacking mode) in crystal. Usually, the molecular stacking mode

CHART 1: Molecular Structures and Dipole Direction of trans-DSB, trans-DPDSB, and trans-DPDMSB, and the Schematics of H-Aggregate, X-Aggregate, and J-Aggregate

* Corresponding author. E-mail: [email protected] (F.L.). (Y.M.) and [email protected] (F.L.).

Figure 1. (a) Molecular X-stacking of trans-DPDSB in polymorphR. (b) Molecular J-stacking of trans-DPDSB in polymorph-β.

can be changed easily by adding substituent groups, and sometimes one compound tends to form different polymorphs

10.1021/jp801033j CCC: $40.75  2008 American Chemical Society Published on Web 05/21/2008

Diphenyl Substituents in Distyrylbenzene Derivatives

Figure 2. ORTEP drawing of trans-DPDMSB in the plate crystal grown in the mixture of ethanol and THF. The ellipsoid probability is 25%.

CHART 2: Schematic of the Selected Dihedral Angles

with a totally different crystal structure.19,20 For example, the most classical electroluminescent material tris(8-hydroxyquinoline) aluminum(III) (Alq3) tends to form different polymorphs in different conditions, and of course the optical properties are affected by the molecular stacking mode.21 The tendency to form different polymorphs of one material seems very important for high quality deposited film under vacuum condition, although the crystallization process in solvent is easy. In fact, devices with high performance and long lifetime have been prepared based on the high quality films of Alq3. The case in Alq3 system indicates that we could design luminescent molecules with easily changeable molecular conformation to form different polymorphs easily, and then to get high quality films for thin film devices. In this contribution, we will present the crystal structures, luminescent properties, and EL devices of a phenyl-substituent trans-DSB system, in which the phenyl groups connect to the central phenyl ring of trans-DSB (Chart 1). There are special considerations for the choice of phenyl-substituent groups: (1) the phenyl-substituent trans-DSB can represent a class of highly luminescent PPVs (DP-PPVs);22 (2) the phenyl groups on the side of trans-DSB will destroy the possibility of the parallel molecular (dipole) stacking (H-aggregate), and then to form highly luminescent molecular (dipole) stacking (X-aggregate or J-aggregate); and (3) the phenyl groups have large torsional (and changeable) angles relative to the trans-DSB backbone, which make the title molecules suitable to form high quality films for the application in OLEDs. In fact, the EL devices based on the phenyl-substituent trans-DSB possess very high performance with attractive deep blue emission that is still rare up to date. Herein is the result. Experimental Section The 1H NMR spectra were recorded on an AVANCZ 500 spectrometer at 298 K by utilizing deuterated dimethylsulfoxide (DMSO) as solvent and tetramethylsilane (TMS) as standard.

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9067 The compound was characterized by a Flash EA 1112, CHNS-O elemental analysis instrument. The time-of-flight mass spectra were recorded using a Kratos MALDI-TOF mass system. IR spectra were recorded on Perkin-Elmer spectrophotometer in the 4000-400 cm-1 region using a powdered sample on a KBr plate. The thermal analysis was determined using a NETZSCH (DSC-204) instrument differential scanning calorimeter at 10 °C/min with nitrogen flushing. AFM images were recorded under ambient conditions using a Digital Instrument multimode nanoscope IIIa operating in the tapping mode. Si cantilever tips (TESP) with a resonance frequency of approximately 300 kHz and a spring constant of about 40 N m1- were used. Electrochemical measurements were performed with a BAS 100 W Bioanalytical Systems, using a platinum disk (Φ ) 2.0 mm) as working electrode, platinum wire as auxiliary electrode, with porous ceramic wick, Ag/Ag+ as reference electrode, standardized for the redox couple ferricinium/ferrocene (E1/2 ) +0.40 V, ∆EP ) 76 mV). Cyclic voltammetric studies were carried out at a scan rate of 50 mV s-1 on 0.01 M solutions in DMF containing 0.1 M Bu4NBF4 as supporting electrolyte. All solutions were purged with nitrogen stream for 10 min before measurement. The procedure was performed at room temperature and a nitrogen atmosphere was maintained over the solution during measurements. UV-vis absorption and fluorescence spectra were recorded on UV-3100 and RF-5301PC spectrophotometers, respectively. The diffraction experiments were carried out on a Rigaku R-AXIS RAPID diffractometer equipped with a Mo KR and Control software using the RAPID AUTO at 293 ((2) °C. Empirical absorption corrections were applied automatically. The structures were solved with direct methods and refined with a full-matrix least-squares technique using the SHELXS v. 5.1 programs,23 respectively. The space groups were determined from the systematic absences and their correctness was confined by successful solution and refinement of structures. Anisotropic thermal parameters were refined for all of the non-hydrogen atoms. CCDC 634120 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Organic layers were deposited by high-vacuum (10-6 Torr) thermal evaporation onto a cleaned glass substrate precoated with transparent, conductive indium-tin oxide (ITO). The layer thickness of the deposited material was monitored in situ using an oscillating quartz thickness monitor. Finally, a LiF buffer layer and Al cathode were deposited at a background pressure of 10-6 Torr onto the organic films. The luminance, current-voltage characteristics, EL spectra, and CIE coordinates of devices were measured simultaneously by the programmable Keithley model 2400 and PR650 spectrometer. All of the measurements were carried out at room temperature under ambient conditions. The synthesis process and structure determination of transDPDSB and 2,5-diphenyl-1,4-dip-methylstyrylbenzene with two cis double bonds can be found in our previous publication.19,24 trans-DPDMSB was prepared by refluxing the p-xylene solution of the cis compound for 12 h using a little iodine as catalyzer. After the reaction was completed, most of the solvent was moved under reduced pressure and then the concentrated solution was dropped into stirred methanol. The pale yellow product was obtained by filtration (yield, 96%; mp 251.3 °C). 1H NMR (500 MHz, 25 °C, DMSO, TMS, ppm): δ ) 7.76 (s, 2H; benzo H), 7.54 (t, J (HH) ) 7.4 Hz, 4H; benzo H), 7.48 (d, J (HH) ) 6.9 Hz, 4H; benzo H), 7.47 (t, J (HH) ) 7.2 Hz, 2 H; benzo H), 7.27 (d, J (HH) ) 8.1 Hz, 4H; benzo H), 7.26

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TABLE 1: Selected Dihedral Angles in Three Different Molecular Conformations of trans-DPDSB and in trans-DPDMSBa dihedral angle C(1)#-C(2)-C(3)-C(4) C(11)-C(2)-C(3)-C(4) C(2)-C(3)-C(4)-C(5) C(3)-C(4)-C(5)-C(6) C(3)-C(4)-C(5)-C(10) C(3)-C(2)-C(11)-C(12) C(1)-C(11)-C(12)-C(17) C(2)-C(11)-C(12)-C(17) C(1)-C(11)-C(12)-C(13) C(2)-C(11)-C(12)-C(13)

trans-DPDSB(R)-1 # -x + 1, trans-DPDSB(R)-2 # -x + 1, trans-DPDSB(β) # -x + 1, trans-DPDMSB # -x, -y, -z -y + 2, -z + 1 -y + 1, -z + 1 -y + 1, z 18.7(6) -163.7(4) -178.2(4) -0.5(7) 178.2(4) 3.9(5) -129.5(4) 49.5(5) 48.4(5) -132.6(4)

19.5(6) -162.0(4) 177.7(4) 24.1(7) -155.6(4) 5.3(6) 55.7(5) -127.3(4) -119.4(4) 57.6(5)

16.7(2) -169.24(15) -174.70(13) 17.6(2) -162.02(15) 8.4(2) -123.23(17) 55.0(2) 53.3(2) -128.52(16)

31.7(5) -149.5(4) 175.4(3) 0.6(5) -175.8(4) 0 -135.2(4) 43.9(5) 43.4(5) -137.6(3)

a The carbon numbers are shown in Chart 2, and the dihedral angles are reported in degree. # represents the symmetry transformations used to generate equivalent atoms.

Figure 3. PL spectra of (a) trans-DSB, (b) trans-DPDSB, and (c) transDPDMSB in THF solutions and films.

(d, J (HH) ) 16 Hz, 2H; olefinic H), 7.13 (d, J (HH) ) 7.9 Hz, 4H; benzo H), 7.01 (d, J (HH) ) 16 Hz, 2H; olefinic H). FTIR (KBr pellet, cm-1) ν ) 3047(m), 3018(m), 2914(m), 2858(m), 1599(m), 1512(m), 1477(m), 1447(m), 1394(w), 1315(w), 1082(m), 1024(m), 968(s), 899(m), 864(m), 810(s), 766(s), 735(m), 708(vs). Anal. Calcd. for C36H30: C, 93.46; H, 6.54. Found: C, 93.35; H, 6.65. UV/vis (THF): λmax () ) 354.6 nm (51 100 mol-1 L3 cm-1), 278.0 nm (24 100 mol-1 L3 cm-1). MALDI-TOF-MS (M) 462.6 (100%). Result and Discussion Crystal Structures and Dipole Stacking Modes. The crystal structure of trans-DSB was reported by Wu et al., which has crystal system of orthorhombic with lattice parameters: a ) 5.873 Å, b ) 7.696 Å, c ) 34.866 Å.25 From the crystal data, we know that the most obvious feature is the trans-DSB molecules are packed in layer-by-layer mode and within one layer the adjacent molecules are parallel to each other (herringbone structure, see Chart 1), which is like most unsubstituent rigid molecules.26–28 Because the molecular transfer dipole of trans-DSB is along the long molecular direction, H-aggregate is formed in the parallel molecular stacking, which will decrease the luminescence efficiency dramatically compared with the dilute solutions.29,15,16

In order to restrain the formation of the parallel molecular stacking, two much larger phenyl substituent groups are introduced on the central phenyl ring of trans-DSB backbone to construct the new trans-DSB derivatives, trans-DPDSB and trans-DPDMSB (Chart 1). The single crystals of trans-DPDSB were grown from solutions carefully, and two polymorphs, which are different with herringbone molecular stacking, were formed as crystals prepared under different conditions. In polymorph-R (the crystal was grown in the mix solvents of chloroform and methanol (vol. 1:2), the adjacent molecules are packed in X-stacking with a cross angle of 70°, where the C-H · · · π hydrogen bonds act as the driving force (Figure 1a).24 While in polymorph-β (the crystal was grown in acetone), the adjacent molecules are packed in J-stacking with θ angle of 42° as shown in Figure 1b, where the arene-arene interactions act as the driving force.30 (Note that here both X-stacking and J-stacking are formed in the polymorphs, which make them highly luminescent in crystalline state as we reported before.24,30) Both C-H · · · π hydrogen bond and arene-arene interaction are very weak when compared with common hydrogen bond and π-π interaction. The tendency to form different polymorphs and the absence of strong intermolecular interaction make transDPDSB easier to form a high quality thin film as thermalevaporation in vacuum used for film growth during the device preparation process. The thermal-evaporation films of transDPDSB are almost amorphous (more details in next section), and even though few microcrystallines or aggregates exist in films, they may possess X-stacking and J-stacking, which is a benefit to high luminescence. Methyl substituent derivative trans-DPDMSB shows low crystalline trend as behaving to be difficult to form single crystal in many solvent systems, such as acetone, hexane, and mixtures of different solvents. Finally, we found that just in the mixture of ethanol and tetrahydrofuran (THF) two kinds of crystals with different morphology were formed (needle crystal and plate crystal). It is impossible to determine the structure of the needle crystal because the diffraction is very weak, which indicates the disorder of the molecules in this kind of crystal. As for the plate crystal, the crystal structure was determined, and the molecules are packed similar with polymorph-R of transDPDSB, with relatively smaller intermolecular cross angle (25°). In the crystal structure, no strong π-π interaction is formed. The molecular conformation of trans-DPDMSB is noncentrosymmetric in the crystal as shown in Figure 2, which is different from that of trans-DPDSB in both polymorphs (centrosymmetric molecular conformations). This means, in the plate crystal of trans-DPDMSB, the molecular conformation is not the most stable one, and there must be another polymorph

Diphenyl Substituents in Distyrylbenzene Derivatives

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9069

Figure 4. AFM height images (10 × 10µm) of (a) trans-DSB, (b) trans-DPDSB, and (c) trans-DPDMSB on the glass/ITO/NPB substrates.

Figure 5. (a) EL spectra of the device A and device B at the luminance of about 1000 cd/m2. (b) Current density-voltage characteristics of devices A and B. Inset shows the relative energy level of TCTA (NPB), trans-DPDMSB, and TPBI.

TABLE 2: Performances of the OLEDs Based on trans-DPDMSB device No.

turn-on voltage [V]

ηa [cd/A]

ηb [%]

Lcmax [cd/m2]

CIE coordinates (x,y)

A B

4.1 5.0

2.5 4.2

2.2 3.9

10170 3254

0.15,0.10 0.15,0.10

a Maximum luminous efficiency. b Maximum external quantum efficiency. c Maximum luminance before damaged.

with centrosymmetric molecular conformation. On the basis of the polymorphism in trans-DPDSB and trans-DPDMSB systems, we can conclude that the multiple molecular stacking modes can be formed easily under different conditions, which is helpful to form high quality films during the process of fabricating OLEDs. There are totally three different molecular conformations in the two polymorphs of trans-DPDSB, two in polymorph-R and one in polymorph-β. The selected dihedral angles of the three molecular conformations together with that of trans-DPDMSB are shown in Chart 2 and Table 1. As we can see, there are large dihedral angles dispersed from 40° to 60° between the central phenyl ring and the substituent phenyl groups (the angle of C11, C12, and their adjacent carbons), which make the multimolecular conformations and restrain the parallel molecular stacking in solid state. Also the dihedral angles between the double bonds and the adjacent phenyl groups can be changed easily under different conditions, for example, the angles of C3-C4-C5-C6 are -0.5(7) and 24.1(7)° in a different molecular configuration of trans-DPDSB in polymorph-R. The flexible molecular conformation of the phenyl substituent DSB derivatives is the essential reason for their polymorphism, and it is a benefit to prepare high quality films for EL devices with high performance. PL Properties and EL Devices. As shown in Figure 3a, the PL spectrum of trans-DSB film shows a very large red shift compared with the emission from the dilute solution (from deep blue to sky blue), attributed to the absence of 0 f 0 transition

from the H-aggregate formed in the film. While in the film of trans-DPDSB, the emission spectrum shows a distinct 0 f 0 transition band (Figure 3b), which comes from the X-aggregate in the film. The existence of a distinct 0 f 0 transition band makes the spectrum locate on the deep blue region as in dilute solution. As for the film of trans-DPDMSB, the 0 f 0 transition band locates at about 420 nm, although the intensity is not as strong as that of trans-DPDSB (Figure 3c). On the basis of the PL spectra in the thin films of trans-DPDSB and transDPDMSB, it is possible to fabricate pure blue EL devices using these materials as the emitting layer. Figure 4 shows the height images of trans-DSB, transDPDSB, and trans-DPDMSB on ITO/NPB substrates, prepared by vacuum deposition method. It is obvious that trans-DSB tends to form micrometer sized domains (microcrystal), while trans-DPDSB tends to form a relative plane film just with some particles here and there. The better film quality of trans-DPDSB is attributed to the flexible molecular conformations. As for trans-DPDMSB, a uniform film can be formed, and most importantly, the morphology of this film is very stable under atmosphere conditions, which means trans-DPDMSB is the best one for the preparation of thin film EL devices. Considering the nice film quality of trans-DPDMSB, we choose it as the blue-emitting layer to fabricate OLEDs with the device structure of ITO/NPB (50 nm)/trans-DPDMSB (20 nm)/TPBI (40nm)/LiF (0.5 nm)/Al (120 nm) (device A), where NPB (N,N-diphenyl-N,N-bis(1-naphthyl)-(1,1-biphenyl)-4,4-diamine) and TPBI (1,3,5-tri(phenyl-2- benzimidazolyl)-benzene) are used as hole-transporting layer (HTL) and electrontransporting layer (ETL) (TPBI also as hole-blocking layer), respectively. The LiF buffer layer is used to increase the electron injection from the aluminum cathode. Note that here transDPDMSB is a nondoped layer in the full fabricating process, which is more suitable for mass production compared with the complex doping process. As shown in Figure 3c and Figure 5a, the EL spectrum of the device is similar to the PL spectrum of trans-DPDMSB with Commission Internationale de l’Eclairage (CIE) of (0.15, 0.10), which is in the deep blue zone.31 The similar EL and PL spectra also confirm that the EL emission comes from the trans-DPDMSB layer. The turn-on voltage, maximum luminous efficiency, and maximum luminance before being damaged are 4.1 V, 2.5 cd/A, and 10170 cd/m2, respectively, for device A (Table 2). To further confine and enhance electron-hole recombination in the emissive layer (EML) and thus increase the device efficiency, we make device B with device structure of ITO/ PEDOT:PSS/TCTA (40 nm)/trans-DPDMSB (20 nm)/TPBI (40 nm)/LiF (0.5 nm)/Al (120 nm), where the PEDOT:PSS/TCTA (4,4′,4″-tri(N-carbazolyl)-triphenylamine) layers are used to replace the NPB layer. The EL spectrum and CIE coordinates of device B remain almost the same as those of device A (Figure 5a and Table 2). While the maximum luminous efficiency

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TABLE 3: Luminous Efficiency and Color Purity Data for the Present Blue OLED device based on trans-DPDMSB 4-N,N-diphenylamino-4′- [(4-N′,N′-diphenylamino) styryl]biphenyl 7,8,10-triphenylfluoranthene 9,10-bis(4-(1,2,2-triphenylvinyl) phenyl)anthracene 6,6′-bis(2-p-biphenyl)-4-phenylquinoline) 2,3-dicyano-5,6-di-(4-(2,3,4,5-tetraphenylphenyl)phenyl) pyrazine NTSC blue standards: a

doped or nondoped

maxumun efficiency cd/A, EQEa

CIE coordinates (x,y)

nondoped doped nondoped nondoped nondoped nondoped

4.2, 3.9% 5.0, 4.2% 3.02, 1.83% 3.93, --7.12, 6.56% 5.2, ---

0.15, 0.10 0.15, 0.14 0.18, 0.24 0.16, 0.14 0.15, 0.16 0.16, 0.28 0.14, 0.08

reference this work [31] [32] [33] [34] [35]

External quantum efficiency.

Conclusion

Figure 6. Luminance efficiency-voltage and luminance-voltage characteristics of device B.

increases dramatically to 4.2 cd/A of device B, corresponding to an external quantum efficiency of 3.9%. To the best of our knowledge, such efficiency with color purity is among the best fluorescence-based blue OLEDs reported recently, as summarized in Table 3. As can be seen, the CIE coordinate of our device is the closest to the National Television System Committee (NTSC) standards (0.14, 0.08). The energy gaps of TCTA, trans-DPDMSB, and TPBI are 3.3, 3.1, and 3.4 eV, respectively (energy data of TCTA and TPBI are from ref 36 and that of trans-DPDMSB is from the cyclic-volt-ampere measurement). The energy barriers of holes from transDPDMSB to TPBI and electrons from trans-DPDMSB to TCTA are 0.6 and 0.3 eV, as shown in the inset of Figure 5b. So the holes, electrons, and excitons can be confined effectively in the trans-DPDMSB layer resulting in the highly efficient emission from trans-DPDMSB in device B. From Table 2, the turn-on voltage of device B is a little larger than that of device A, and as shown in Figure 5b, the current density at the same voltage of device B is a little smaller than that of device A. Since the only difference between device B and device A is using PEDOT:PSS/TCTA to replace NPB as the HTL (the energy gap of TCTA is larger than that of NPB, as shown in the inset of Figure 5b), the hole current in device B is decreased, which induces the further carrier balance in device B and subsequently the higher luminous efficiency. Figure6showsluminanceefficiency-voltageandluminance-voltage characteristics of device B. It should be noted that the luminous efficiency of device B decreases rapidly with the increase of the voltage. The similar phenomenon also exists in the devices based on TCTA in our other work,37 which might be due to the impurity of the commercial TCTA we used. Generally, the lifetime is defined as the time that the luminance of the device decays to half of the initial luminance. We carry out a simple test about the lifetime of device B, which is about 10 h without being capsulated when the initial luminance is 100 cd/m2.

In summary, the phenyl substituent groups on trans-DSB derivatives are efficient to restrain the parallel molecular stacking. Thus, the molecules tend to form highly luminescent molecular stacking modes, X-stacking and J-stacking. The introduction of phenyl substituent groups makes the molecules have multiple conformations and form different polymorphs under different conditions, which make it easy to prepare high quality films for EL devices. Highly efficient nondoped organic light-emitting devices based on trans-DPDMSB were fabricated, which have deep blue emission with CIE coordinates of (0.15, 0.10). The maximum luminous efficiency can reach 4.2 cd/A (corresponding to an external quantum efficiency of 3.9%). To our knowledge, such efficiency and color purity are among the best fluorescence-based blue OLEDs reported. Acknowledgment. We thank the financial support from National Science Foundation of China (Grants 20573040, 20474024, 90501001, 60706016, and 50303007), Ministry of Science and Technology of China (Grant 2002CB6134003), 111 project (B06009), and PCSIRT. Supporting Information Available: Crystallographic information files (CIF) of trans-DPDMSB. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Bourroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burn, P. L.; Homes, A. B. Nature 1990, 347, 539. (3) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (4) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471. (5) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 110, 416. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (6) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B. R. J. Phys. Chem. A 1999, 103, 2394. (7) Yan, M.; Rothberg, L. J.; Kwock, E. W.; Miller, T. M. Phys. ReV. Lett. 1995, 75, 1992. (8) Chen, S. H.; Su, A. C.; Han, S. R.; Chen, S. A.; Lee, Y. Z. Macromolecules 2004, 37, 181. (9) Vanden Bout, D. A.; Yip, W. T.; Hu, D.; Fu, D. K.; Swager, T. M.; Barbara, P. F. Science 1997, 277, 1074. (10) Nguyen, T. Q.; Doan, V.; Schwartz, B. J. J. Chem. Phys. 1999, 110, 4068. (11) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (12) van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma, C. W. T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. AdV. Funct. Mater. 2004, 14, 425. (13) Colaneri, N. F.; Bradley, D. D. C.; Friend, R. H.; Burn, P. L.; Holmes, A. B.; Spangler, C. W. Phys. ReV. B 1990, 42, 11670. (14) He, F.; Xu, H.; Duan, Y.; Tian, L. L.; Huang, K. K.; Ma, Y. G.; Liu, S. Y.; Feng, S. H.; Shen, J. C. AdV. Mater. 2005, 17, 2710. (15) Bohn, P. W. Annu. ReV. Phys. Chem. 1993, 44, 37.

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