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J. Phys. Chem. C 2007, 111, 6544-6549
Intermolecular Hydrogen Bonds Induce Highly Emissive Excimers: Enhancement of Solid-State Luminescence Yang Liu,† Xutang Tao,*,† Fuzhi Wang,‡ Jinghua Shi,† Jianliang Sun,† Wentao Yu,† Yan Ren,† Dechun Zou,‡ and Minhua Jiang† State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan, 250100, People’s Republic of China, and Department of Polymer Science and Engineering, College of Chemistry, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: January 13, 2007; In Final Form: March 3, 2007
Aggregation induced emission (AIE) is an amazing property for light emitting materials and has attracted much attention. Here, we report a new kind of AIE materials: fluorenone derivates 2,7-dip-tolyl-fluorenone (DTFO) and 2,7-bis(4-(tert-butylthio)phenyl)-fluorenone (DSFO). Strong light emissions with a large Stokes shift and long lifetime in the solid state originate from the formation of excimers. The crystal structure of DSFO shows that every two molecules are bound together even in the ground state by intermolecular hydrogen bonds and form a particular dimer. When this dimer is excited, it turns into an excimer without arrangement adjustment and likewise without repulsive interactions when the excimer decays back to the dimer; so, the nonradiative decay pathways that exist in common excimers are greatly reduced and thus induce a strongly enhanced luminescence in the solid state. OLED devices employing DTFO as light emitting layers are fabricated and evaluated.
Introduction Organic light emitting materials have attracted much attention in recent years. They were investigated and are used in the fields of light emitting diodes (LEDs),1 organic lasers,2 photovoltaic cells,3 optical sensers,4 etc. Most of these applications require the light emitting materials to be used in the solid state (e.g., thin films or crystals), while common organic materials are usually weakly luminescent when concentrated to the solid states5 (commonly called a concentration quenching effect). This is believed to be a result of organic molecules in the solid state aggregating to form H-aggregations or other less emissive species (e.g., excimers).6 Chemical, physical, and engineering approaches have been used by many groups to reduce the effect of aggregation on fluorescence quenching. In 2001, an intriguing phenomenon of aggregation induced emission (AIE) in siloles was observed. 1,1-Substituted 2,3,4,5-tetraphenylsilole derivatives were found to be faintly emissive in ethanol or chloroform solutions, but their aggregates or solid states were strongly luminescent.6 Also, several other compounds bearing this character have been reported (e.g., 1-cyano-trans-1,2-bis(4methylbiphenyl)ethylene (CN-MBE),7 cis,cis-1,2,3,4-tetraphenylbutadiene (TPBD),8 and cis-2,5-diphenyl-1,4-distyrylbenzene (DPDSB).9 Most of these enhanced solid-state emissions were believed to be caused by restricted intramolecular rotations (e.g., in the typical siloses, AIE is believed to be caused by restricted intramolecular rotations of the peripheral aromatic rings upon the axes of the single bonds linked to the central silole cores).10 The nonplanar structure reduces the intermolecular interaction and the likelihood of excimer formation and increases PL efficiency in the solid state.11 Another viewpoint is that the unique fluorescence change is related to the effects of intramo* Corresponding author. E-mail:
[email protected]. † Shandong University. ‡ Peking University.
lecular planarization or a specific aggregation (H- or Jaggregation) in the solid state.7 Highly emissive solids are needed for light emitting applications, and so research on such new materials and mechanisms would greatly help us to design more materials. Fluorene containing compounds have been widely studied for photonic applications, while their oxidation products, fluorenone derivates, are less studied. Here, we report a series of simple fluorenone derivates as a new class of materials with enhanced solid-state luminescence. Distinguishingly, their enhanced fluorescence is found not from the single molecules but from the excimers. Excimers, the abbreviation of the excited-state dimer, usually form when two aromatic systems are within sufficient proximity (∼3 Å)12 and are characterized by broad emissive peaks, long emissive lifetimes, and red-shift emissions.13 It is well-known that the formation of excimers will drastically decrease the PL quantum yields of the materials because of the greater number of nonradiative decay pathways for depopulation of the excited state.14 The formation of excimers is found in many light emitting materials. Generally, it is an unwanted effect that often reduces the luminescent efficiency and decreases the emission color purity.5,15 Much effort has been performed to prevent excimer formation.16 Whereas the excimer does not always play a negative role in highly luminescent materials, for example, white light emissions have been realized by using the excimer (exciplex) emissions,17 and highly emissive conjugated polymer excimers have also been reported in the literature.18 Here, we report the excimer induced luminescence enhancement in two fluorenone compounds: DTFO and DSFO. Results and Discussion Synthesis. Scheme 1 shows the synthesis routes and molecular structures of the two fluorenone derivates. The key synthesis step of the two compounds is a Suzuki coupling reaction,19 and
10.1021/jp070288f CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007
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SCHEME 1: General Synthetic Routes to DTFO and DSFO
the yields of both title compounds are above 90%; it is very easy to obtain the pure products by recrystallization. The solutions (in CH2Cl2, CHCl3, ethanol, etc.) of the two compounds are not luminescent, while the separated solids of them are strongly luminescent under the UV lamp (365 nm) (see Figure 1). Furthermore, the drops of these compounds’ solutions on the TLC plate do not luminesce even after solvent evaporation, which is entirely different from the other AIE materials.6-8 We can regard the dark spots on the TLC as the solid-state solutions, and this suggests that the luminescence of the compounds is a collective behavior of the compound molecules and that single molecules (molecules that do not interact from each other), even in the solid state, are not luminescent. Photoluminescence and Absorption Properties of DTFO and DSFO with AIE Characteristics. To validate our speculation, we did a solvent-non-solvent photoluminescence test, which is commonly used in studying the AIE properties.6-8 DTFO and DSFO were separately dispersed in ethanol (solvent)water (non-solvent) mixture systems, with the concentration being kept at 5 µM. Figure 2 shows the photoluminescence spectra of DTFO and DSFO in such systems. V(x) stands for the water fraction. As we can see from Figure 2, when the water fractions are not more than 30%, the solutions are almost not luminescent; when the water fractions increase to above 50%, the systems become turbid, and the fluorescence becomes stronger and stronger. The insets of Figure 2 give the profiles of PL peak intensity versus solvent composition. The PL intensity at V90 (the water fraction is 90%) is about 25 times higher than that at V10 (the water fraction is 10%). This indicates that the fluorescence comes from the aggregation state of the compounds, their solutions, vagariously, have nearly no luminescence. Figure 3 shows the UV-vis absorption spectra of DTFO and DSFO in water-ethanol mixtures. Contrary to the fluorescence’s changing trends, the absorption intensities gradually become weaker from V10 to V90. Moreover, when the water fractions increase to above 50% (V50), the absorption spectra profiles show a red-shift, and a long absorption tail, which are characteristic of the nanoparticles’ absorption. The formation of more solid particles decreases the solutions’ concentration, and the absorption intensity depends on the compound that dissolved in the solvent, while the solid-phase particles that separated from the solution contribute the most to the fluorescence. This is also why we provide the profiles of the PL peak intensity versus solvent composition but not the fluorescence quantum yield in our experiments. In our opinion, since the absorbance and fluorescence come from contributions of different parts, the quantum yield calculated by the reference method will have no meaning. Excimer Emission and Mechanism of Excimer Formation of Fluorenone Derivates in Solutions. Excimer formation usually induces weak fluorescence in solutions.14,20 As shown
Figure 1. Fluorescence emissions of DSFO (a) and DTFO (b) and solids vs solutions (1 × 10-4 M in CHCl3) under 365 nm illumination.
in Figure 2, the emissions of DTFO and DSFO essentially coincide with the rules of excimer emission. When the water fraction is low, shorter wavelength emissions can be seen around 400 nm (DTFO) and 450 nm (DSFO), and they are the single molecules’ emissions. However, the emission peak at 550 nm shows three characters: (i) a large Stokes shift (∼220 nm) relative to that of the main absorbance; (ii) a featureless emission band (absence of vibronic structure); and (iii) a relatively long fluorescence lifetime (6.87 ns in solution and 6.21 ns in thin films of DTFO). All of these results indicate that this emission peak originates from excimer emissions. However, on the TLC plate, the DTFO and DSFO molecules are molecularly isolated, and the two molecules cannot connect to each other to form a fluorescent excimer, so that only a dark spot remains on the TLC plate. This is the reason as to why the two compounds show no luminescence on the TLC plate. To gain insight as to why these fluorenone derivates are prone to form excimers and contribute to the mechanism of excimer formation, we obtained their HOMOs and LUMOs based on B3LYP/6-31G calculations. As shown in Figure 4, panels b and d are the HOMOs and panels a and c are the LUMOs of DTFO and DSFO, respectively. Notably, the electrons of the DTFO (DSFO)’s LUMO are confined mostly in the fluorenone core, and only negligible ones are on the phenyl rings linked to the 2,7--positions. The configurations of the HOMOs and LUMOs are quite similar to that of the fluorenone containing fluorene oligomers reported by Zojer et al.21 They pointed out that these molecules usually possess a CT π-π* state high above the n-π* state, and according to Kasha’s rule, only in the case where the energy of the CT π-π* state comes to lie close to or even below that of the n-π* state will light emissions be observed. This is the reason as to why the single molecules’ emissions are very weak (in solutions) or even unseen (in the solid state). On the basis of these theoretical revelations, we speculate that, as a result of the large electron distribution on the oxygen atom of the LUMO, the oxygen atom at this time (the excited state) must have a considerable electronegativity, and this will facilitate it to combine one or two comparatively electropositive hydrogen atoms of another ground-state molecule lying in proper distance simply by Coulomb interactions; then, through the O‚‚‚H bonds, the admixture of the wavefunctions of the intramolecular singlet excited state and the intermolecular charge transfer state can be realized, and an excimer forms. Furthermore, the forbidden relaxation of the singlet excited state gives enough time for the excimer formation. When relaxing back to the ground state, the two molecules repulse to separate into two individuals once more, as illustrated in Figure 4. Thus, the localized orbitals (to the central O atom) of the excited state give the most contribution to the formation of the excimer. Mechanism of Enhanced Emissions in Solid State. Figure 7 gives the PL spectra of the DTFO film; just like in the waterethanol mixture, it shows a strong solid-state luminescence.
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Figure 2. PL spectra changes of DTFO (a) and DSFO (b) depending on the water fractions. V(x) stands for the water fraction. Insets give the profile of PL peak intensity vs solvent composition of the water-ethanol mixture.
Figure 3. UV-vis absorption spectra changes of DTFO (a) and DSFO (b) depending on the water fractions in ethanol. V(x) stands for the water fraction.
Figure 4. HOMO and LUMO of DTFO and DSFO. (a and c) LUMOs and (b and d) HOMOs. Right: Scheme of proposed mechanism of excimer formation.
Excimers, as we know them, often reduce luminescence efficiency and decrease the emission color purity, so why here do they play a positive role in enhancing the solid-state emission? None of these proposed mechanisms of AIE, such as the restricted intramolecular rotations or the special J- or H-aggregates, are suitable to explain the AIE phenomena in these two compounds as their emissions are from excimers. The
actual clues should be found in the aggregated mode of the solid state. Fortunately, the crystal structure of DSFO was resolved. The DSFO molecules were found to adopt a coplanar conformation, with a dihedral angle between the fluorenone core and the 2,7-linked phenyl rings of 179.6°, while the optimized geometry shows a twist configuration with a dihedral angle of 144.4° for the isolated molecules. As expected, novel intermolecular
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J. Phys. Chem. C, Vol. 111, No. 17, 2007 6547
Figure 5. Packing of the molecular pairs of DSFO. (a) View along the vertical direction of the molecular planes. (b) View along the parallel direction of the molecular planes. (c) O···H hydrogen bonds formed between two DSFO molecules.
Figure 6. PL lifetime spectra of DTFO in toluene (5 × 10-6 M) (a) and thin film (b).
interactions were noticed. As shown in Figure 5, in the crystal, there are a number of molecular pairs, and they each pack in a parallel but staggered style. The oxygen atom of one DSFO molecule forms two hydrogen bonds with two hydrogen atoms of the other molecule with bond lengths of 2.539 and 2.547 Å. The hydrogen bonds join every two adjacent molecules together. Regarding our proposed mechanism of excimer formation of the fluorenone derivates in the previous paragraph, here, every molecular pair joined by hydrogen bonds builds a special dimer. When one of the two molecules is photo- or electro-excited, the dimer will instantly become an excimer by delocalizing the excitation through the O‚‚‚H bond channels. The process of this special excimer’s formation and decay is different from the traditional excimer’s formation and decay. Commonly, the two molecules that form an excimer should first undergo suitable arrangement adjustments to combine together and then a repulsive process to decay to the ground state, these actions consume much excited state energy and subsequently effectively quench the fluorescence.13 However, for DTFO and DSFO, every two molecules are combined together even before being excited, so there is no need to adopt those energy-consuming actions in the process of excimer formation and decay. The PL lifetime detection can also testify this point. Figure 6 gives the PL lifetime spectra of DTFO in a toluene solution and thin film. In toluene, DTFO shows a weak but detectable yellow light (λem ) 550 nm) when being excited at 340 nm, with a PL lifetime of 6.87 ns; nevertheless, under the same conditions, the PL lifetime of the DTFO thin film is 6.21 ns. As we
know, for general chromophores, their PL lifetime in the solid state is obviously longer than that in solutions.22 The abnormality is believed to be a contribution of the special dimer (excimer). Thanks to the concise process of excimer formation and decay, the excimer is more ready to form and irradiate to the ground state and thus acquires a shorter PL lifetime in the solid state. Electroluminescence Properties. Employing DTFO as light emitting layers, OLED devices are fabricated. Two device architectures are used: (i) ITO/NPB (30 nm)/DTFO (40 nm)/ BCP (10 nm)/AlQ (20 nm)/Mg/Ag (150 nm, 10:1)/Ag (10 nm) (device A) and (ii) ITO/NPB (40 nm)/DTFO (40 nm)/TPBi (20 nm)/Mg/Ag (150 nm, 10:1)/Ag (10 nm) (device B), where NPB acts as a hole transporting layer, BCP as a hole blocking layer, and AlQ and TPBi act as electron transporting layers. Figure 7 shows the PL spectra of the DTFO thin film and the EL spectra of the two devices. Noticeably, the shorter wavelength emission from the single species is not observed in both PL and EL spectra; this indicates that the excimer formation in the solid state has a very high efficiency. The performances of the two devices are quite similar. They have comparatively low turnon voltages of 3.8 V (device A) and 4.2 V (device B), and their maximum brightness are 5347.83 cd/m2 (device A) and 5428.04 cd/m2 (device B). The main EL performances of the two devices are listed in Table 1. In summary, the devices employing DTFO as light emitting layers show a single color emission from the excimers, with considerable performances without further optimized device configurations.
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Figure 7. PL spectra of DTFO film and EL spectra of device A and device B using DTFO as light emitting layers.
TABLE 1: Performances of OLED Devices
devices
power emission turn-on maximum luminance wavelengtha voltageb brightness efficiency efficiency (lm/W) (cd/m2) (cd/A) (nm) (V)
device A 535 (558) device B 567 (535)
3.8 4.2
5347.83c 5428.04f
0.649d 0.644g
0.35e 0.346h
a In parentheses is the wavelength of shoulder peak emission. b At a brightness of 1 cd/m2. c At a bias of 13.25 V. d At a bias of 7.25 V, current density of 69.781 mA/cm2, and brightness of 453.043 cd/m2. e At a bias of 3.5 V, current density of 0.107 mA/cm2, and brightness of 0.416 cd/m2. f At a bias of 13.75 V. g At a bias of 7.5 V, current density of 38.058 mA/ cm2, and brightness of 24.923 cd/m2. h At a bias of 3.75 V, current density of 0.068 mA/cm2, and brightness of 0.282 cd/m2.
Conclusion We found and reported two fluorenone compounds with AIE. The mechanism of AIE in these two compounds is found to be due to the formation of excimers through hydrogen bonds, which is entirely different from those of other reported AIE materials. In virtue of quantum chemical calculations, we proposed a mechanism of excimer formation in solutions of the fluorenone derivates. In the solid state, the crystal structure of DSFO revealed that every two molecules are bound together even in the ground state by intermolecular hydrogen bonds, forming a particular dimer. When the dimer is excited, it turns into an excimer without arrangement adjustment, and when the excimer decays back to the dimer, there is no repulsive interaction either. Therefore, the nonradiative decay pathways are greatly reduced and thus induce a strongly enhanced luminescence in the solid state. Employing DTFO as light emitting layers, pure excimer emission OLED devices are fabricated, and relatively good device performances are proven. The novel intermolecular interaction may be used in other areas, such as sensors and charge transport, and work on these aspects are underway. Experimental Procedures Materials and Methods. All reagents and starting materials are commercially available and used as received. Pd(PPh3)4 was purchased from Acros. The melting point was measured on a diamond differential scanning calorimeter under nitrogen atmosphere. 1H NMR and 13C NMR spectra were recorded at 25 °C using a Bruker Avance 400 spectrometer. Microanalyses (C, H, and N) were performed using a German Vario EL III elemental analyzer. Electrospray mass spectrum (ES-MS) was
recorded on a Finnigan LCQ mass spectrograph, and the concentration of the samples was about 1.0 mmol/mL. The diluted solution was electrosprayed at a flow rate of 5 × 10-6 L/min with a needle voltage of 4.5 kV. Absorption measurements were carried out on a TU-1800 spectrophotometer using a quartz cuvette having a 1 cm path length. Photoluminescence (PL) measurements were recorded using a Hitachi F-4500 fluorescence spectrophotometer with a 150 W Xe lamp. The fluorescence lifetime measurement was performed on the Edinburgh FLS920 spectrofluorimeter with a hydrogen flash lamp (pulse duration 1 ns) as the excitation source on the same spectrofluorometer. Solvents were purified and dried according to standard procedures. OLED devices were fabricated by thermal vacuum deposition under 1.333 × 10-4 Pa. The active area of the device was about 4 mm2. The thickness of the films was measured by a Dektak surface profilometer. Synthesis of 2,7-dip-Tolyl-fluorenone (DTFO) (Scheme 1). Under an argon atmosphere, to a stirred mixture of 0.606 g (1.8 mmol) of 2,7-dibromo-fluorenone, 0.61 g (4.49 mmol) of 4methylphenylboronic acid in 6 mL THF, and 4 mL of 2 M Na2CO3, 0.02 g of Pd(PPh3)4 was added. The mixture was heated to 80 °C for 8 h. After cooling, the organic phase was extracted with 50 mL of CH2Cl2, washed with water, dried with Na2SO4, filtrated, and recrystallized from CH2Cl2/ethanol (10:1) and gave 0.6 g of yellow-orange crystalline solids. (Yield 92%) mp 248 °C. 1H NMR (CDCl3, 400 MHz, ppm) δ: 2.413 (s, 6H), 7.276 (d, J ) 8.0 Hz, 4H), 7.534 (d, J ) 8.1 Hz, 4H), 7.584 (d, J ) 7.8 Hz, 2H), 7.722 (dd, J ) 7.7 Hz, J ) 1.7 Hz, 2H), 7.912 (d, J ) 1.4 Hz, 2H). 13C NMR (CDCl3, 100.57 MHz, ppm) δ: 20.659, 120.167, 122.322, 126.131, 129.183, 132.532, 134.740, 136.488, 137.345, 141.609, 142.390, 193.413. MS (EI) (Calcd for C27H20O, 360.15) found, 361.6 (M+). Anal. Calcd for C27H20O: C, 89.97; H, 5.59. Found: C, 89.44; H, 5.414. Synthesis of 2,7-Bis(4-(tert-butylthio)phenyl)-fluorenone (DSFO). Prepared according to the same procedure of DTFO from 0.606 g (1.8 mmol) of 2,7-dibromo-fluorenone, 0.94 g (4.48 mmol) of 4-(tert-butylthio)phenylboronic acid, and 0.02 g of Pd(PPh3)4. Recrystallization from CH2Cl2/ethanol (10:1) gave 0.85 g of orange crystalline solids. (Yield 93%) mp 264 °C. 1H NMR (CDCl3, 400 MHz, ppm) δ: 1.334 (s, 18H), 7.592 (d, J ) 6.3 Hz, 4H), 7.613 (d, J ) 6.2 Hz, 4H), 7.641 (s, 2H), 7.764 (dd, J ) 7.8 Hz, J ) 1.7 Hz, 2H), 7.943 (d, J ) 1.5 Hz, 2H). 13C NMR (CDCl3, 100.57 MHz, ppm) δ: 30.546, 45.766, 120.411, 122.536, 126.262, 132.268, 132.800, 134.811, 137.437, 139.545, 141.016, 142.786. Anal. Calcd for C33H32OS2: C, 77.91; H, 6.34. Found: C, 77.78; H, 6.247. Crystal data for DSFO (C33H32OS2): Mr ) 508.71, monoclinic, space group C2/ c, a ) 44.525(2) Å, R ) 90°, b ) 6.2063(3) Å, β ) 92.087(3)°, c ) 19.6664(9) Å, γ ) 90°, V ) 5430.9(4) Å3, Z ) 8, Fcalcd ) 1.244 Mg/m3, T ) 293 (2) K, crystal size 0.48 mm × 0.46 mm × 0.03 mm, R1 ) 0.0883 (wR2 ) 0.1547) [I > 2σ(I)]. Acknowledgment. The authors are grateful to the State National Natural Science Foundation of China (Grants 50325311 and 50590403) and the 863,973 program of the People’s Republic of China (Grants 50125310, 90101013, and 2004CB619002) for financial support. Supporting Information Available: CIF files of DSFO. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 57, 531.
Intermolecular H-Bonds Induce Emissive Excimers (2) Schneider, D.; Rabe, T.; Riedl, T.; Dobbertin, T.; Kro¨ger, M.; Becker, E.; Johannes, H.-H.; Kowalsky, W.; Weimann, T.; Wang, J.; Hinze, P.; Gerhard, A.; Sto¨ssel, P.; Vestweber, H. AdV. Mater. 2005, 17, 31. (3) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (4) Zeng, L.; Miller, E. W.; Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10. (5) Friend, R. H.; Gymer, R. W.; Holms, 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. (6) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740. (7) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (8) Chen, J.; Xu, B.; Ouyang, X.; Tang, B. Z.; Cao, Y. J. Phys. Chem. A 2004, 108, 7522. (9) Xie, Z.; Yang, B.; Cheng, G.; Liu, L.; He, F.; Shen, F.; Ma, Y.; Liu, S. Chem. Mater. 2005, 17, 1287. (10) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535. (11) Yu, G.; Yin, S.; Liu, Y.; Chen, J.; Xu, X.; Sun, X.; Ma, D.; Zhan, X.; Peng, Q.; Shuai, Z.; Tang, B.; Zhu, D.; Fang, W.; Luo, Y. J. Am. Chem. Soc. 2005, 127, 6335.
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6549 (12) Conwell, E. M. Synth. Met. 1997, 85, 995. (13) (a) Sims, M.; Bradley, D. D. C.; Ariu, M.; Koeberg, M.; Asimakis, A.; Grell, M.; Lidzey, D. G. AdV. Funct. Mater. 2004, 14, 765. (b) Birks, J. B. Photophysics of Aromatic Molecules; John Wiley and Sons: London, 1970. (14) Kulkarni, A. P.; Kong, X.; Jenekhe, S. A. J. Phys. Chem. B 2004, 108, 8689. (15) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (16) (a) Schweikart, K.-H.; Hohloch, M.; Steinhuber, E.; Hanack, M.; Lu¨er, L.; Gierschner, J.; Egelhaaf, H.-J.; Oelkrug, D. Synth. Met. 2001, 121, 1641. (b) Liu, X.-M.; He, C.; Hao, X.-T.; Tan, L.-W.; Li, Y.; Ong, K. S. Macromolecules 2004, 37, 5965. (17) D’Andrade, B. W.; Forrest, S. R. AdV. Mater. 2004, 16, 1585. (18) Kim, Y.; Bouffard, J.; Kooi, S. E.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 13726. (19) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (20) Jaramillo-Isaza, F.; Turner, M. L. J. Mater. Chem. 2006, 16, 83. (21) Zojer, E.; Pogantsch, A.; Hennebicq, E.; Beljonne, D.; Bre´das, J.-L.; Freitas, P. S.; Scherf, U.; List, E. J. W. J. Chem. Phys. 2002, 117, 6794. (22) Ren, Y.; Lam, J. W. Y.; Dong, Y.; Tang, B. Z.; Wong, K. S. J. Phys. Chem. B 2005, 109, 1135.
