3580 Chem. Mater. 2010, 22, 3580–3582 DOI:10.1021/cm100407n
Blue-Violet Electroluminescence from a Highly Fluorescent Purine Yixing Yang,† Pamela Cohn,‡ Aubrey L. Dyer,‡ Sang-Hyun Eom,† John R. Reynolds,*,‡ Ronald K. Castellano,*,‡ and Jiangeng Xue*,† †
Department of Materials Science and Engineering and ‡ Department of Chemistry and the Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, 32611 Received February 25, 2010 Revised Manuscript Received May 20, 2010
Organic electroluminescent devices are considered ideal components of compact light sources because of their high efficiencies and the ability to rationally tune emission wavelength through molecular and supramolecular structural modifications.1 Although research efforts remain strong to develop organic materials that emit visible light, recent interest has turned to organic systems that emit at shorter wavelengths, from the blue, through the violet, to the ultraviolet regions. Ultraviolet (UV) to blue organic light-emitting diodes (OLEDs) have found, or are sought for, applications in biological and chemical sensing,2,3 high-density information storage devices,4 and full-color light-emitting displays.5,6 Despite a number of examples of blue to violet emissive materials based on organic fluorescent emitters in OLEDs, spanning materials from small molecules7-11 to polymers,12-15 only a handful7,9,13 realize external quantum efficiencies (EQE or ηEQE, defined as the ratio of the number of emitted *Corresponding author. E-mail:
[email protected] (J.R.R.);
[email protected] (R.K.C.);
[email protected] (J.X.).
(1) Li, Z.; Meng, H., Organic Light-Emitting Materials and Devices; CRC Press: Boca Raton, FL, 2006. (2) Shinar, J.; Shinar, R. J. Phys. D :Appl. Phys. 2008, 41, 133001. (3) Landgraf, S. J. Biochem. Biophys. Methods 2004, 61, 125. (4) van Santen, H.; Neijzen, J. H. M. Jpn. J. Appl. Phys., Part 1 2003, 42, 1110. (5) Fukuda, Y.; Watanabe, T.; Wakimoto, T.; Miyaguchi, S.; Tsuchida, M. Synth. Met. 2000, 111-112, 1. (6) Haldi, A.; Kim, J. B.; Domercq, B.; Kulkarni, A. P.; Barlow, S.; Gifford, A. P.; Jenekhe, S. A.; Marder, S. R.; Kippelen, B. J. Disp. Technol. 2009, 5, 120. (7) Okumoto, K.; Shirota, Y. Appl. Phys. Lett. 2001, 79, 1231. (8) Wee, K.-R.; Ahn, H.-C.; Son, H.-J.; Han, W.-S.; Kim, J.-E.; Cho, D. W.; Kang, S. O. J. Org. Chem. 2009, 74, 8472. (9) Chao, T.-C.; Lin, Y.-T.; Yang, C.-Y.; Hung, T. S.; Chou, H.-C.; Wu, C.-C.; Wong, K.-T. Adv. Mater. 2005, 17, 992. (10) Chen, C.-T.; Wei, Y.; Lin, J.-S.; Moturu, M. V. R. K.; Chao, W.-S.; Tao, Y.-T.; Chien, C.-H. J. Am. Chem. Soc. 2006, 128, 10992. (11) Hwu, J. R.; Hsu, Y. C.; Josephrajan, T.; Tsay, S.-C. J. Mater. Chem. 2009, 19, 3084. (12) Hoshino, S.; Ebata, K.; Furukawa, K. J. Appl. Phys. 2000, 87, 1968. (13) Yang, Y.; Pei, Q.; Heeger, A. J. J. Appl. Phys. 1996, 79, 934. (14) Huang, J.; Zhang, H.; Tian, W.; Hou, J.; Ma, Y.; Shen, J.; Liu, S. Synth. Met. 1997, 87, 105. (15) Kido, J.; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett. 1993, 63, 2627. (16) Tonzola, C. J.; Kulkarni, A. P.; Gifford, A. P.; Kaminsky, W.; Jenekhe, S. A. Adv. Funct. Mater. 2007, 17, 863.
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photons to the number of injected electrons) greater than 1%. Recently, there have been several reports16-19 of high-efficiency fluorescent blue OLEDs with a peak emission wavelength in the range of 440-480 nm that possess maximum ηEQE values of 3-6%, although the shorter wavelength emission generally leads to lower device efficiencies. Highly sought are organic materials that exploit the donor-acceptor concept8,10 to achieve emissive color tunability and provide high efficiency and brightness in the UV to violet region. The organic framework considered here is that of a purine, a heterocycle central to DNA/RNA structure that can be made highly fluorescent upon the judicious placement of electron donor and acceptor groups on its rings.20,21 Recently prepared derivatives have displayed fluorescence quantum yields over 80% and emission wavelengths (λem) that can be tuned from 350-450 nm in solution,20,21 suitable starting parameters for blueviolet OLED construction. The interest in using purines for electronic device development is further derived from their rich chemical structure that underlies molecular recognition and charge transport22 processes in natural systems. The molecules’ extended π-surface tends to facilitate aromatic stacking and can allow access to different hydrogen-bonding patterns along their heteroatom-lined edges.23 These structural features have already been shown to be important to the ordering of simple nucleobases in the solid state,24 where they are relevant to device performance.25,26 Hardly considered to date have been emissive purine analogs in solid-state devices, such as OLEDs. As a first example, efficient violet-emitting OLEDs based on one type of donor-acceptor purine, methyl-9benzyl-2-N,N0 -dimethylamino-9H-purine-8-carboxylate 1 (Figure 1, inset), are demonstrated in this paper. A maximum external quantum efficiency (EQE) of 3.1% is achieved in devices with the purine emitter dispersed in a (17) Lin, S.-L.; Chan, L.-H.; Lee, R.-H.; Yen, M.-Y.; Kuo, W.-J.; Chen, C.-T.; Jeng, R.-J. Adv. Mater. 2008, 20, 3947. (18) Matsumoto, N.; Miyazaki, T.; Nishiyama, M.; Adachi, C. J. Phys. Chem. C 2009, 113, 6261. (19) Zhang, T.; Wang, J.; Li, T.; Liu, M.; Xie, W.; Liu, S.; Liu, D.; Wu, C.-L.; Chen, C.-T. J. Phys. Chem. C 2010, 114, 4186. (20) Butler, R. S.; Myers, A. K.; Bellarmine, P.; Abboud, K. A.; Castellano, R. K. J. Mater. Chem. 2007, 17, 1863. (21) Butler, R. S.; Cohn, P.; Tenzel, P.; Abboud, K. A.; Castellano, R. K. J. Am. Chem. Soc. 2009, 131, 623. (22) Vura-Weis, J.; Wasielewski, M. R.; Thazhathveetil, A. K.; Lewis, F. D. J. Am. Chem. Soc. 2009, 131, 9722. (23) Sivakova, S.; Rowan, S. J. Chem. Soc. Rev. 2005, 34, 9. (24) Lena, S.; Brancolini, G.; Gottarelli, G.; Mariani, P.; Masiero, S.; Venturini, A.; Palermo, V.; Pandoli, O.; Pieraccini, S.; Samori, P.; Spada, G. P. Chem.;Eur. J. 2007, 13, 3757. (25) Rinaldi, R.; Maruccio, G.; Biasco, A.; Arima, V.; Cingolani, R.; Giorgi, T.; Masiero, S.; Spada, G. P.; Gottarelli, G. Nanotechnology 2002, 13, 398. (26) D’Amico, S.; Maruccio, G.; Visconti, P.; D’Amone, E.; Cingolani, R.; Rinaldi, R.; Masiero, S.; Spada, G. P.; Gottarelli, G. Microelectron. J. 2003, 34, 961.
Published on Web 05/25/2010
r 2010 American Chemical Society
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Figure 1. Photoluminescence spectra of purine 1 in CH2Cl2 solution (dashed-dotted line; λex = 320 nm) and in solid-state films (5 wt % in mCP) when excited at λex = 292 nm (solid line) and 362 nm (dashed line). Inset: The molecular structure of purine 1.. 0
N,N -dicarbazolyl-3,5-benzene (mCP) host matrix as the emissive layer (EML). The electroluminescence (EL) of the OLED has a peak emission wavelength of λ = 430 nm. The synthesis of purine 1 has been reported previously.21 Shown in Figure 1 are the photoluminescence (PL) spectra of 1 in solution and solid-state films (as a dispersion in mCP). The emission spectrum of 1 in CH2Cl2 (λex=320 nm) shows a single peak at 432 nm.21 Solid-state films were prepared by vacuum thermal evaporation and consisted of a 20 nm thick mCP layer doped with 5% (by weight) purine 1. Independent PL spectra for the films were obtained using excitation wavelengths that correspond to the absorption maxima of mCP (λex=292 nm) and purine 1 (λex = 362 nm). The PL spectra in solution and the solid state show identical emission maxima (λem = 432 nm) suggesting that the purine molecules are well-dispersed in the mCP host matrix and do not exhibit aggregation-induced red-shifted emission. Additionally, the solid-state film shows the same PL emission (λem = 432 nm) whether excited at the absorption maximum of mCP (λex =292 nm) or the absorption maximum of purine 1 (λex = 362 nm, a wavelength at which mCP has negligible absorption). This result indicates that F€ orster energy transfer27 of excitons occurs from the mCP host to the purine dopant molecules. Most encouraging is that while the PL quantum yield for 1 in organic solution (81% in CH2Cl2) is high relative to violet emitters reported previously,7,9,11 it is not the highest reported for donor-acceptor purines.21 Likewise, it is known that small changes to the donor and acceptor groups of 1 can tune its emission across the blue-UV region,21 potentially suitable for a variety of OLED applications. The energies of the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) for purine 1 were determined experimentally using cyclic voltammetry (CV) and differential pulse voltammetry (DPV); the results are shown in the Supporting Information (Figure S1). This knowledge has guided the choice of charge transport and injection layers for proper energy level alignment with the active material to maximize OLED (27) Pope, M.; Swenberg, C. E., Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999.
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Figure 2. Schematic energy level diagram of the OLEDs using purine 1 as the emitter. The energies (in eV) for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of the organic materials and the Fermi levels of the electrodes are referred from the vacuum level.29,30 The dashed lines in the mCP layer correspond to the LUMO and HOMO levels of purine 1.
efficiency. The E1/2 for oxidation and reduction (vs Fc/ Fcþ) were measured as 0.97 and -2.24 V, respectively (see Figure S1 in the Supporting Information). The HOMO and LUMO energy values were then calculated from the electrochemical data as 6.07 and 2.86 eV, respectively, considering that Fc/Fcþ = 5.1 eV relative to vacuum.28 The corresponding HOMO-LUMO gap energy (3.2 eV) lies between those reported previously21 from optical absorption data (3.0 eV) and electronic structure calculations (3.88 eV at the B3LYP/6-311þþG** level). The schematic energy level diagram of the multilayer OLED structure based on 1 is shown in Figure 2. The devices were fabricated using vacuum thermal evaporation29 on glass substrates that were commercially precoated with an indium tin oxide (ITO) anode (sheet resistance ∼20 Ω/0). Successively deposited onto the ITO were a 40 nm thick hole transporting layer (HTL) of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), a 20 nm thick EML consisting of the mCP host doped with purine, a 40 nm thick electron transporting layer (ETL) of 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7), and a 1 nm thick LiF layer followed by a 50 nm thick Al cathode. The electrode work functions and the HOMO/LUMO energies for TAPC, mCP, and OXD-7 in Figure 2 have been taken from the literature.29,30 Current density-voltage (J-V ) characteristics of an OLED based on purine 1 (at an optimized doping concentration of 4% by weight) are shown in Figure 3. Lower doping concentrations resulted in incomplete F€ orster energy transfer from mCP to the purine, whereas higher doping concentrations led to aggregate quenching.31 The radiant emittance, R, in the forward-viewing direction of the devices is also shown in Figure 3. Here, the actual optical power density of the OLED emission is reported instead of the more commonly used luminance, as the latter incorporates the human eye sensitivity and is intended for (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (29) Zheng, Y.; Eom, S.-H.; Chopra, N.; Lee, J.; So, F.; Xue, J. Appl. Phys. Lett. 2008, 92, 223301. (30) Ichikawa, M.; Kobayashi, K.; Koyama, T.; Taniguchi, Y. Thin Solid Films 2007, 515, 3932. (31) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610.
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Figure 3. Current density, J, and the radiant emittance, R, as functions of the voltage, V, for OLEDs based on purine 1.
display and lighting applications. Moreover, the luminance value of the OLED is dominated by the longer wavelength (blue and green) components in the emission spectrum, and does not clearly present the contributions from the shorter wavelength (UV to violet) emissions, the focus of this work. Nevertheless, a radiant emittance of 1 mW/cm2 for this OLED is equivalent to a luminance of approximately 130 cd/m2. The turn-on voltage of this device, which is defined as the operating voltage that yields detectable light emission (in this case it means R > 10-5 mW/cm2), is approximately VT = 2.9 ( 0.1 V. Note that the photon energy corresponding to the peak emission (λem = 432 nm) is 2.9 eV. This suggests that there is negligible energy loss to induce the EL from purine in this OLED device architecture. The light output increases drastically with the operating voltage at V > VT, and a radiant emittance in the range of 10 to 18 m W/cm2 is achieved at V > 15 V. Other ETL materials with higher electron mobility, such as 4,7-diphenyl-1,10-phenanthroline (BPhen) and tris(2,4,6-trimethyl-3-(pyridine3-yl)phenyl)borane (3TPYMB),32,33 were tested in place of OXD-7 (TAPC was kept as the HTL). Both the current density and radiant emittance at a given voltage increased by approximately 1 order of magnitude because of the significant enhancement of electron transport; however, the charge balance in the device34 was distorted and the external quantum efficiency decreased correspondingly. As shown in the inset of Figure 4, the EL spectrum of the device measured at J = 1 mA/cm2 reveals peak emission at λ = 430 nm with a full width at half-maximum of 60 nm, nearly identical to the PL emission obtained in solid-state films. The Commission Internationale de L’Eclairage (CIE) coordinates of this OLED are (0.15, 0.06). Figure 4 shows the dependency of ηEQE on the current density. The device has a maximum EQE of (3.1 ( 0.3)%, a value that approaches the highest efficiency OLEDs with peak emission approaching 440 nm;16-18 this is noteworthy given that the current device has yet to be fully optimized. The (32) Eom, S.-H.; Zheng, Y.; Wrzesniewski, E.; Lee, J.; Chopra, N.; So, F.; Xue, J. Org. Electron. 2009, 10, 686. (33) Tanaka, D.; Agata, Y.; Takeda, T.; Watanabe, S.; Kido, J. Jpn. J. Appl. Phys., Part 2 2007, 46, L117. (34) Chopra, N.; Lee, J.; Zheng, Y.; Eom, S.-H.; Xue, J.; So, F. ACS Appl. Mater. Interfaces 2009, 1, 1169.
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Figure 4. External quantum efficiency, ηEQE, of the purine-based OLED as a function of the current density, J. Inset: Normalized EL spectrum for the device at J = 1 mA/cm2.
power efficiency, defined as the ratio of the output optical power to the input electrical power, for the device reaches a maximum of (23 ( 3) mW/W at low current densities (1 10-3 < J
