Near-Infrared Photo- and Electroluminescence of Alkoxy-Substituted

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Near-Infrared Photo- and Electroluminescence of Alkoxy-Substituted Poly(p-phenylene) and Nonconjugated Polymer/Lanthanide Tetraphenylporphyrin Blends Benjamin S. Harrison, Timothy J. Foley, Alison S. Knefely,† Jeremiah K. Mwaura, Garry B. Cunningham, Tae-Sik Kang, Mohamed Bouguettaya, James M. Boncella,*,† John R. Reynolds,* and Kirk S. Schanze* Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611-7200 Received January 9, 2004. Revised Manuscript Received May 13, 2004

The photoluminescent and electroluminescent properties of near-infrared (near-IR) emitting lanthanide monoporphyrinate complexes, Ln(TPP)L (L ) hydridotris(1-pyrazolyl)borate (Tp) or (cyclopentadienyl)tris(diethylphosphinito)cobaltate(I) L(OEt)) blended into conjugated and nonconjugated polymer hosts were characterized. A blue-emitting alkoxysubstituted poly(p-phenylene) (PPP-OR11) was used as the conjugated polymer host and nonconjugated hosts included polystyrene, poly(methyl methacrylate), poly(n-butyl methacrylate), and poly(bisphenol A-carbonate). Complete quenching of the PPP-OR11 host fluorescence (i.e., > 95%) is observed at 5 mol % of Ln(TPP)Tp, and host quenching is accompanied by sensitization of near-IR emission from the lanthanide complex. The photoluminescence results suggest that energy transfer occurs from PPP-OR11 to Ln(TPP)L, presumably via the Fo¨rster mechanism. Near-IR light emitting diodes (PLEDs) consisting of Yb(TPP)Tp blended into PPP-OR11 and the nonconjugated polymer hosts were characterized. PLEDs fabricated with PPP-OR11 exhibited turn-on voltages of approximately 4 V, whereas nonconjugated polymer devices had higher turn-on voltages (ca. 8 V), independent of the polymer used. Comparable external electroluminescence (EL) efficiencies on the order of 10-4 were observed from both the conjugated and nonconjugated polymer host devices. Taken together, the available evidence suggests that the dominant mechanism operating in the EL devices involves the Ln(TPP)L complex as the charge-transport material, the center for electron-hole recombination, and the emitter.

Introduction Since the first demonstration of electroluminescence (EL) from poly(phenylene vinylene) (PPV),1 extensive research on π-conjugated polymer-based light-emitting diodes (PLEDs) has been carried out.2-5 Research and development in this area has developed to a stage where PLEDs are nearing commercial application in monochrome and full-color display applications. To prevent aggregation quenching of emitting chromophores, a strategy that involves blending of nonemitting polymers with active polymers or with small molecule dyes has * Authors to whom correspondence may be addressed. E-mail: [email protected], [email protected], [email protected]. † Current Address: Nuclear Materials Technology Division, Los Alamos National Laboratory, PO Box 1663, Mail Stop J-582, Los Alamos, NM 87545. (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539. (2) 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.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (3) Cao, Y.; Parker, I. D.; Yu, G.; Zhang, C.; Heeger, A. J. Nature 1999, 397, 414-417. (4) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982-1984. (5) Visser, R. J. Philips J. Res. 1998, 51, 467-477.

been successfully employed to fabricate LEDs with improved device performance.6 In most applications where conjugated polymers are blended with small molecule dopants, the role of the polymer is to transport charge and/or transfer energy to the dopant molecules, which emit. Hence, energy transfer-sensitization of dopants in conjugated polymer blends, investigated using both photoluminescence and electroluminescence, has been of recent interest. In particular is work done in the visible region using conjugated polymer hosts doped with red-emitting organic dyes.7-10 This concept has been extended to polymer blends that contain visible light emitting metal-organic complexes, for example, (6) He, G.; Liu, J.; Li, Y.; Yang, Y. Appl. Phys. Lett. 2002, 80, 18911893. (7) Lyons, B. P.; Wong, K. M.; Monkman, A. P. J. Chem. Phys. 2003, 118, 4707-4711. (8) Virgili, T.; Lidzey, D. G.; Bradley, D. D. Adv. Mater. 2000, 12, 58-62. (9) Brunner, K.; van Haare, J. A. E. H.; Langeveld-Voss, B. M. W.; Schoo, H. F. M.; Hofstraat, J. W.; van Dijken, A. J. Phys. Chem. B. 2002, 106, 6834-6841. (10) Morgado, J.; Cacialli, F.; Iqbal, R.; Moratti, S. C.; Holmes, A. B.; Yahioglu, G.; Milgrom, L. R.; Friend, R. H. J. Mater. Chem. 2001, 11, 278-283.

10.1021/cm049937f CCC: $27.50 © 2004 American Chemical Society Published on Web 07/02/2004

PL and EL of Lanthanide Monoporphyrinate Polymer Blends

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Figure 1. Chemical structures of the Ln complexes and polymeric compounds used in this work: Ln(TPP)Tp, Ln(TPP)L(OEt) (Ln is Yb, Er, Nd, or Ho), PPP-OR11, polystyrene (PS), poly(bisphenenol-A-carbonate) (PC), poly(methyl methacrylate) (PMMA), and poly(n-butyl methacrylate) (PBMA).

iridium(III) and platinum(II) complexes with an emphasis on wavelength shifting, color purity, and enhancing efficiency in electroluminescent devices.11-13 In addition, sensitization has been shifted into the nearIR region by using dye dopants such as porphyrins and phthalocyanines that emit at wavelengths greater than 800 nm.14-16 It has long been known that lanthanide emission can be sensitized via the “antenna effect” by using complexes with ligands that absorb in the UV-visible region resulting in narrow bandwidth emission from the metal ion. Recently, it has been shown that this effect can be extended to conjugated polymer systems by blending the lanthanide complex into a luminescent host.17 Thus, the conjugated polymer host acts to sensitize emission from the lanthanide complex in the visible and/or the nearIR, and this concept has been demonstrated in electroluminescent devices by our group and others. Specifically, we have shown that when poly[2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is used as a host with Ln(TPP)acac, (Ln ) lanthanide, TPP ) 5,10,15,20-tetraphenylporphyrinate, acac ) acetoacetonate) as a dopant, the visible PL and EL of the host is quenched, and it is replaced by the near-IR emission characteristic of the lanthanide complex.18 Similar effects have been seen with blends consisting of an alkoxy-substituted poly(p-phenylene) host and complexes of the type Ln(DBM)3phen (DBM ) dibenzoyl(11) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 1999, 71, 2095-2106. (12) Kwong, R. C.; Sibley, S.; Dubovoy, T.; Baldo, M.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 1999, 11, 3709-3713. (13) Noh, Y.-Y.; Lee, C.-L.; Kim, J.-J.; Yase, K. J. Chem. Phys. 2003, 118, 2853-2864. (14) Flora, W. H.; Hall, H. K.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 1142-1150. (15) Suzuki, H. Appl. Phys. Lett. 2000, 76, 1543-1545. (16) Ostrowski, J. C.; Susumu, K.; Robinson, M. R.; Therien, M. J.; Bazan, G. C. Adv. Mater. 2003, 15, 1296-1300. (17) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. Adv. Mater. 1999, 11, 1349-1354. (18) Harrison, B. S.; Foley, T. J.; Bouguettaya, M.; Boncella, J. M.; Reynolds, J. R.; Schanze, K. S.; Shim, J.; Holloway, P. H.; Padmanaban, G.; Ramakrishnan, S. Appl. Phys. Lett. 2001, 79, 3770-3772.

methane and DNM ) dinaphthoylmethane).19 Concurrently, other groups have reported visible and near-IR electroluminescence from polymer/lanthanide complex blends.20,21 There are also examples of other routes to near-IR electroluminescence which include the use of vapor deposited films of small molecule emitters and near-IR emitting semiconducting nanoparticles.22-27 In this paper, we report a complete study of the photoluminescent and electroluminescent properties of blends of Ln(TPP)L complexes (L ) hydridotris(1pyrazolyl)borate (Tp) or (cyclopentadienyl)tris(diethylphosphinito)cobaltate(I) L(OEt)) and a blue-emitting poly(p-phenylene) host (PPP-OR11) whose structures are illustrated in Figure 1. The Ln(TPP)L complexes have been shown to be relatively efficient near-IR emitters, and energy transfer from the PPP-OR11 polymer host to the Ln(TPP)L dopant is efficient. This system was designed based on the concept that the PPPOR11 host would act as a charge transport material and excitons produced on the host by electron/hole recombination would undergo energy transfer to the Ln(TPP)L dopant resulting in near-IR EL. The results obtained in the present study, which probes the PL and EL properties of the blends, suggest that, while the PPPOR11 luminescence is effectively quenched by the (19) Kang, T.-S.; Harrison, B. S.; Bouguettaya, M.; Foley, T. J.; Boncella, J. M.; Schanze, K. S.; Reynolds, J. R. Adv. Funct. Mater. 2003, 13, 205-210. (20) Slooff, L. H.; Polman, A.; Cacialli, F.; Friend, R. H.; Hebbink, G. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Appl. Phys. Lett. 2001, 78, 2122-2124. (21) Sun, R. G.; Wang, Y. Z.; Zhang, Q. B.; Zhang, H. J.; Epstein, A. J. J. Appl. Phys. 2000, 87, 7589-7591. (22) Kawamura, Y.; Wada, Y.; Yanagida, S. Jpn. J. Appl. Phys. 1 2001, 40, 350-356. (23) Curry, R. J.; Gillian, W. P. Synth. Met. 2000, 111-112, 3538. (24) Curry, R. J.; Gillian, W. P. Appl. Phys. Lett. 1999, 75, 13801382. (25) Tessler, N.; Medvedev, V.; Kazaes, M.; Kan, S.; Banin, U. Science 2002, 295, 1506-1508. (26) Bakueva, L.; Musikhin, S.; Hines, M. A.; Chang, T. W. F.; Tzolov, M.; Scholes, G. D.; Sargent, E. H. Appl. Phys. Lett. 2003, 82, 2895-2897. (27) Steckel, J. S.; Coe-Sullivan, S.; Bulovic, V.; Bawendi, M. G. Adv. Mater. 2003, 15, 1862-1866.

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Ln(TPP)L dopant, contrary to our initial expectations, charge transport in the blends is dominated by the Ln(TPP)L complexes. By taking advantage of this effect, we recently communicated the fabrication of electroluminescent devices in which the active material consists of near-IR emitting Ln(TPP)L complexes blended with polystyrene, a nonelectroactive and nonconjugated polymer.28 The EL efficiencies of these devices were found to be comparable in magnitude to the efficiencies obtained with the conjugated polymer host, a finding which is consistent with the TPP ligand acting as the primary charge-carrying moiety. This discovery opens up many opportunities for the use of inexpensive nonelectroactive polymers in fabricating PLEDs with TPP-based dopants. This concept is elaborated here through fabrication of Ln(TPP)L-based near-IR electroluminescent devices using several commercially available nonconjugated polymers with varying physical properties as polymer hosts. Experimental Section Materials. Yb(TPP)Tp and Yb(TPP)L(OEt) and PPP-OR11 were synthesized according to procedures reported elsewhere.29-31 Polystyrene (Mw ≈ 125 000-250 000 g‚mol-1) was purchased from Polyscience, Inc., poly(methyl methacrylate) (Mw ≈ 120 000 g‚mol-1) and poly(bisphenol A-carbonate) (Mw ≈ 65 000 g‚mol-1) were purchased from Aldrich, and poly(nbutyl methacrylate) (Mw ≈ 100 000 g‚mol-1) was purchased from Scientific Polymer Products, Inc. UV Absorption and Photoluminescence. Absorption spectra were obtained on a double-beam Cary-100 UV-visible spectrometer. Visible and near-IR photoluminescence spectra were recorded using a front-face geometry on a Fluorolog-2 fluorescence spectrophotometer (Jobin Yvon Inc.). For measurements in the visible region (300-800 nm) a thermoelectrically cooled Hamamatsu R928 PMT detector was used, and in the near-IR (800-1600 nm) a liquid N2 cooled InGaAs photodiode was used. Photoluminescence quantum efficiency measurements of PPP-OR11 were made in CH2Cl2 relative to perylene (Φ ) 0.93, ethanol), and the PL quantum efficiency of the material as a spin-coated film was determined as described in the literature.32 Device Fabrication. Electroluminescent devices were prepared by masking, then etching, ITO coated glass (Delta Technologies, Rs ) 8-12 Ω 0-1) by exposure to aqua regia vapors. Following etching, the ITO glass was cleaned by sonication for 20 min in each of the following solvents: aqueous SDS (Fisher, Versa-Clean solution), Milli-Q water, acetone, and isopropyl alcohol. After the ITO glass was rinsed, it was air-dried. A hole transport layer of PEDOT-PSS (Bayer Baytron P VP Al 4083) was spin-coated onto the ITO surface at 4000 rpm and subsequently dried in a vacuum oven at 150 °C for 4 h. The Ln(TPP)Tp or Ln(TPP)L(OEt) complex was first dissolved in CHCl3, and the solution was added to another solution containing 2 mg‚mL-1 of PPP-OR11 in CHCl3 to give the desired lanthanide complex-to-polymer ratio. For EL devices with nonconjugated polymers, polystyrene, poly(methyl methacrylate), poly(butyl methacrylate), and poly(bisphenol (28) Kang, T.-S.; Harrison, B. S.; Foley, T. J.; Knefely, A. S.; Boncella, J. M.; Reynolds, J. R.; Schanze, K. S. Adv. Mater. 2003, 15, 1093-1097. (29) Foley, T. J.; Abboud, K. A.; Boncella, J. M. Inorg. Chem. 2002, 41, 1704-1706. (30) Foley, T. J.; Harrison, B. S.; Knefely, A. S.; Abboud, K. A.; Reynolds, J. R.; Schanze, K. S.; Boncella, J. M. Inorg. Chem. 2003, 42, 5023-5032. (31) Balanda, P. B. Ph.D. Thesis, University of Florida, Gainesville, FL, 1997; p 243. (32) Greenham, N. C.; Samuel, I. D. W.; Hayes, G. R.; Phillips, R. T.; Kessener, Y. A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Chem. Phys. Lett. 1995, 241, 89-96.

Harrison et al. A-carbonate), 5 mg‚mL-1 solutions in 1,2-dichloroethane were used. The resulting solutions were spin coated onto the ITO substrate at 800 rpm for 30 s (500 µL of solution was used for each substrate). The resulting films were dried under vacuum (10-6 Torr) for 12 h at room temperature, and their thickness was measured with a Dektak 3030 (Veeco Instruments Inc.) profilometer. Calcium and aluminum layers were sequentially deposited by thermal evaporation at 4 × 10-6 Torr without breaking the vacuum between metal depositions. The thicknesses of the Ca and Al layers were determined to be 50 and 2000 Å, respectively, using a calibrated oscillating quartz crystal thickness monitor. After metal deposition, the devices were encapsulated with epoxy (Loctite quick set epoxy) under an argon atmosphere to minimize exposure to oxygen and moisture. All device measurements were made at room temperature. EL Device Characterization. Power for the electroluminescence (EL) measurements was supplied using a Keithley 2400 voltage-current source. EL spectra were collected on an ISA-SPEX Triax 180 spectrograph fitted with a liquid N2 cooled CCD detector (Hamamatsu back-illuminated CCD, 1024 × 64 pixels, 400-1100 nm). A secondary standard tungsten lamp was used to calibrate the CCD detector in irradiance units (µW‚cm-2‚nm-1). Measurements were made normal to the surface of the devices, and in the computation of the EL quantum efficiencies it was assumed that the spatial distribution of the emission was Lambertian. External device quantum efficiencies were calculated as described in the literature.33,34 Near-IR images of the EL devices were recorded using a Sony DCR-TVR730 video recorder in night-shot mode. An RG830 Shott glass filter was placed between the EL device and the camera to block all visible light emitted by the device.

Results and Discussion Optical Properties of the Blend Components. The PPP-OR11 conjugated polymer host and the Ln(TPP)L and complexes used in the near-IR emitting devices described here were selected to optimize energy transfer for efficient near-IR emission. The optical properties of the lanthanide porphyrin complexes in solution have previously been reported.30 These complexes display absorption bands typical of “regular” metalloporphyrins35 (Soret ∼425 nm, Q-bands ∼550 and 590 nm), and the identity of the coordinated lanthanide ion has little effect on the position of the absorption bands. Photoluminescence from Ln-based F-states is observed with efficiencies ranging from 3% for Yb to 0.09% for Er.30 The conjugated polymer PPP-OR11 was synthesized by a Suzuki step growth polymerization31 and the sample used in this work had Mn ≈ 20 000 g‚mol-1 and Mw ≈ 37 000 g‚mol-1 corresponding to a degree of polymerization, Xn of ∼ 42. In solution, the polymer’s absorption is centered at 348 nm with an intense PL emission at 406 nm (ΦPL ) 0.82). The optical properties of the polymer as a spin coated film are similar, with λmax,abs ) 367 nm and λmax,em ) 415 nm (ΦPL ) 0.8 ( 0.1). Photoluminescence of Ln(TPP)Tp/PPP-OR11 Blends. Energy Transfer From the Host to the Dopant. There is excellent spectral overlap between the blue fluorescence of PPP-OR11 and the Soret absorption band of the lanthanide tetraphenylporphyrin complexes, (33) He, Y.; Hattori, R.; Kanicki, J. Rev. Sci. Instrum. 2000, 71, 2104-2107. (34) Greenham, N. C.; Friend, R. H.; Bradley, D. D. Adv. Mater. 1994, 491-494. (35) Gouterman, M. In The Porphyrins, Vol. 3; Dolphin, D., Ed.; Academic Press: New York, 1978; pp 1-165.


Figure 2. (a) Quenching of the fluorescence from spin-cast films of PPP-OR11 by added Yb(TPP)Tp. In order of decreasing fluorescence intensity: concentration of Yb(TPP)Tp is 0, 1, 2, 5, 10, 15, and 20 mol % per polymer repeat unit. The inset shows the fraction of PPP-OR11 fluorescence quenched (plotted as 1 - I/I0, see text) by Ln(TPP)Tp where Ln is Ho3+ (b), Er3+ (1), Tm3+ (9), and Yb3+ ((). (b) Near-infrared emission of Yb(TPP)Tp in PPP-OR11 from the same series of blend films upon excitation at 367 nm. The inset shows the near-infrared emission intensity to be linear with Yb(TPP)Tp concentration up to 20 mol % Yb(TPP)Tp.

Ln(TPP)L. Calculations using Fo¨rster energy transfer theory36,37 show that the Fo¨rster radius, Ro, which is defined as the donor-acceptor distance that gives rise to a 50% probability of dipole-dipole energy transfer, is 4.6 nm in the PPP-OR11/Ln(TPP)L blends. This fact suggests that Fo¨rster-type energy transfer from the singlet excited state of PPP-OR11 to Ln(TPP)L complexes in blends should be quite efficient. To test this concept, thin films were prepared by first adding a measured amount of the complex Ln(TPP)Tp into a solution of PPP-OR11 in chloroform, followed by spincasting the solution onto a glass substrate. Photoluminescence from these films produced by excitation at 367 nm (the polymer’s π,π* absorption) is shown in Figure 2a. Note that the polymer’s emission is quenched as the concentration of the Ln(TPP)Tp complex in the blend increases. The inset in Figure 2a shows the dependence of the polymer’s emission intensity on the amount of Ln(TPP)Tp in the blend. (This plot shows “fraction quenched,” given by 1 - I/Io, where Io is the fluorescence intensity from a pure PPP-OR11 film and I is the (36) Fo¨rster, T. Ann. Physik 1948, 2, 55-75. (37) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum: New York, 1999.

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intensity from the PPP-OR11/Ln(TPP)Tp blend.) The PL quenching experiments establish that, independent of which lanthanide ion is coordinated to the porphyrin, complete quenching of the PPP-OR11 host fluorescence (i.e., > 95%) is observed at 5 mol % of the Ln(TPP)Tp in the blend. The efficient quenching of the polymer fluorescence is believed to arise from the high degree of spectral overlap between the PPP-OR11 fluorescence and the absorption of the TPP ligand. Several recent investigations have explored the efficiency of energy transfer from conjugated polymer hosts to fluorescent dopants.8-10 In one study that is directly relevant to the present work, the efficiency of energy transfer from the blue-emitting host polymer poly(9,9-dioctylfluorene) to tetraphenylporphyrin (PFO and TPP, respectively) was examined.8 Calculations using Fo¨rster theory indicate that for the PFO/TPP blend system Ro ) 4.8 nm, which is in excellent agreement with the Ro value calculated for the PPPOR11/Ln(TPP)L system. The good agreement between the Fo¨rster radii in the PFO/TPP and PPP-OR11/ Ln(TPP)L systems arises because the fluorescence and absorption properties of the two polymer/dopant systems are very similar. However, despite the fact that the Fo¨rster radii are similar in the two systems, energy transfer is considerably more efficient at lower dopant concentrations in the PFO/TPP system. Specifically, in this system energy transfer is 100% complete for blends that contain 0.5 mol % TPP in PFO.8 By contrast, as seen in Figure 2a, blends that contain 1 mol % Ln(TPP)Tp in PPP-OR11 still exhibit significant emission from the host, indicating that energy transfer is still relatively inefficient at this concentration of Ln(TPP)Tp in the blend. We believe that the origin of the reduced quenching efficiency seen in the PPP-OR11/Ln(TPP)Tp blends may arise because the Ln(TPP)Tp is not uniformly dispersed within the host polymer matrix. Fluorescence microscopy was used to explore the morphology of the PPP-OR11/Ln(TPP)Tp blends, and these studies demonstrated that phase segregation is not evident on length scales of µm. However, this does not rule out the possibility that there is phase segregation or aggregation of the Ln(TPP)Tp in the blends on the nanoscale. To confirm that quenching of the PL from the PPPOR11 host arises due to energy transfer to the Ln(TPP)Tp dopant, photoluminescence experiments were carried out to detect near-IR emission from the lanthanide complex. These experiments were predicated on the idea that if energy transfer is the mechanism for quenching of the polymer’s fluorescence, then quenching will be accompanied by sensitization of the near-IR emission from the Ln(TPP)Tp complex (the energy acceptor). As expected on the basis of the energy transfer model, near-IR emission from Yb(TPP)Tp is observed (see Figure 2b) when the complex is blended into a spin-coated film of PPP-OR11 at concentrations as low as 2 mol %. The central transition at 977 nm corresponding to the 2F5/2 f 2F7/2 transition of Yb3+ is clearly observed in the spectrum, along with additional high and low energy bands that result from ligand field splitting of the 2F5/2 f 2F7/2 states. As shown in the inset of Figure 2b, the intensity of the near-IR emission from Yb(TPP)Tp increases linearly with complex loading

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Harrison et al. Scheme 1

Figure 3. Absorption and near-IR emission excitation spectra of spin-coated films consisting of Yb(TPP)Tp blended into PPPOR11 at 1, 2, 5, 10, 15, and 20 mol % concentration. The arrows indicate the trends in absorbance (and excitation intensity) with increasing Yb(TPP)Tp concentration. (a) Absorbance of spin-cast films. (b) Excitation spectra obtained while monitoring the 977 nm emission; spectra are normalized at 370 nm.

(over the range 0-20%). Within this range of complex loading concentration quenching is not observed. The lack of concentration quenching observed in the Yb(TPP)Tp system may be due to the excellent “steric shield” provided by the bulky TPP and Tp ligands. As shown in Figure 3a, the absorption spectra of thin films of the PPP-OR11/Yb(TPP)Tp blends feature the polymer absorption band at λmax ) 367 nm, along with absorptions corresponding to the porphyrin Soret transition at λmax ) 427 nm and the Q-bands at λmax ) 554 and 591 nm. Near-IR emission excitation spectra for the blends measured while monitoring the Yb3+ emission at λem ) 977 nm confirm that the PPP-OR11 host sensitizes emission from the lanthanide complex. As shown in Figure 3b, the near-IR emission excitation spectra are essentially identical to the absorption spectra. Specifically, the excitation spectra feature two excitation bands at 420 and 550 nm corresponding to the Soret and Q-band absorptions of the TPP chromophore, respectively. In addition, an excitation band is observed at λ ) 360 nm which corresponds to the π,π* absorption of PPP-OR11. The presence of this band clearly indicates that energy transfer from the polymer to Yb(TPP)Tp complex occurs, leading to sensitization of the near-IR emission from the lanthanide ion. On the basis of the absorption and PL observations, we conclude that energy transfer leading to near-IR light emission occurs as depicted in Scheme 1. Specifically, light is absorbed by PPP-OR11 resulting in the production of a singlet exciton on the host. Energy

transfer from PPP-OR11 to the Ln(TPP)L complex then occurs, presumably via the Fo¨rster mechanism. This process leads to production of the 1π,π* state of TPP chromophore. Rapid intersystem crossing then ensues to produce the 3π,π* state on the TPP chromophore. Finally, energy transfer occurs from the TPP ligand to the coordinated lanthanide ion, producing the f-centered excited state, which subsequently relaxes by radiative and nonradiative decay processes (radiative decay occurs with emission of a near-IR wavelength photon). Although the model developed above to explain energy transfer from PPP-OR11 to Yb(TPP)Tp emphasizes Fo¨rster transfer as being the dominant mechanism, on the basis of the available experimental evidence we cannot rule out the Dexter exchange mechanism38 as being a contributing pathway for energy transfer. However, in another investigation we have explored the relationship between the efficiency of energy transfer and spectral overlap between PPP-OR11 fluorescence (the energy donor) and the absorption of a series of Ln-complexes (the energy acceptors).39 This work supports the notion that the Fo¨rster mechanism is the dominant pathway for energy transfer from the host polymer to the Ln complex in the blends.39 For example, PPP-OR11 quenching experiments using the acceptor Yb(dibenzoylmethane)3(1,10-phenanthroline), a complex (38) Dexter, D. L. J. Chem. Phys. 1953, 21, 836-850. (39) Harrison, B. S. Ph.D. Thesis, University of Florida, Gainesville, FL, 2003.


Figure 4. Electroluminescence spectrum of PPP-OR11 at 9 V. The inset shows the current (b) and irradiance (4) as a function of voltage for the ITO/PEDOT-PSS/PPP-OR11/Ca/Al device

having poor spectral overlap with the fluorescence of PPP-OR11,19,39 showed that quenching of the polymer’s fluorescence was incomplete even in blends that contain more than 20 mol % of the Yb complex. This result contrasts with the findings of the present study, where nearly complete quenching of the PPP-OR11 fluorescence is observed at 5 mol % of the Ln(TPP)Tp in the blend. Electroluminescence of PPP-OR11. Before electroluminescence studies were carried out on the PPPOR11/Ln(TPP)L blends, the EL properties of the pure polymer were probed. Polymer light emitting diodes (PLEDs) were prepared with the following architecture: ITO/PEDOT-PSS/PPP-OR11/Ca/Al 40:50:5:200 nm. As shown in Figure 4, the device exhibits blue emission with λmax ) 420 nm, and the EL spectrum is similar to the PL spectrum of the solid film, indicating that the electroluminescence results from a singlet π,π* exciton which has the same structure as that produced by photoexcitation. Although the EL spectrum is qualitatively similar to the PL spectrum, close comparison of the two spectra reveals that in the EL spectrum the emission intensity is enhanced on the red side of the band. This result suggests that a substantial fraction of the electroluminescence emanates from interchain aggregate “trap” sites. As shown in the insert to Figure 4, the PPP-OR11 device turns on at 4 V, and the EL intensity increases with voltage, peaking at 9 V, followed by a decrease in intensity at higher voltages, possibly due to device breakdown. At 6 V the irradiance emitted from the device is 4 µW‚cm-2, and by 9 V the irradiance reaches 70 µW‚cm-2 (115 cd‚m-2 at 420 nm) at a current density of 140 mA‚cm-2. The external electron-to-photon quantum efficiency of the PPP-OR11 EL device as a function of current density is shown in Figure 5 (triangles). It is apparent that the efficiency increases up to a current density of 30 mA‚cm-2, where it peaks at 2.6 × 10-3, after which the efficiency is relatively constant up to about 100 mA‚cm-2. The external quantum efficiency observed for the PPP-OR11 based devices is comparable to that reported for single-layer EL devices that contain other alkoxy-substituted PPPs.40 (40) Huang, J.; Zhang, H.; Tian, W.; Ma, H.; Shen, J.; Shiyong, L. Synth. Met. 1997, 87, 105-108.

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Figure 5. External electroluminescence efficiencies of devices that contain PPP-OR11 only (4, visible EL), or a blend of 15 mol % Yb(TPP)Tp in PPP-OR11 (b, near-IR EL) as the active material.

Figure 6. Electroluminescence spectra of (a) 5, (b) 10, and (c) 15 mol % Yb(TPP)Tp in PPP-OR11 at 8 V.

Electroluminescence of PPP-OR11/Yb(TPP)Tp Blends. Light emitting diodes containing blends of PPPOR11 and Yb(TPP)Tp or Yb(TPP)L(OEt) were prepared to characterize the EL device properties, including detailed studies of efficiency as a function of blend composition. Very similar results were obtained using the two complexes, therefore in this manuscript we discuss only results obtained with blends containing Yb(TPP)Tp. (Results for the Yb(TPP)L(OEt) blend devices are included as Supporting Information.) The configuration of all EL devices tested was ITO/PEDOTPSS /PPP-OR11 /Ln(TPP)Tp/ Ca/ A1 40:50:5:200. EL spectra of devices containing 5, 10, and 15 mol % Yb(TPP)Tp blended into PPP-OR11 are shown in Figure 6. It is clear from the spectra that the near-IR emission at 977 nm arising from Yb(TPP)Tp dominates at all complex concentrations, and that the visible emission from the PPP-OR11 host is entirely suppressed. As in the PL studies, the 977 nm emission originates from the 2F5/2 f 2F7/2 transition of Yb3+. As the Yb(TPP)Tp concentration in the blend increases, the spectral shape of the Yb3+ based emission begins to show a defined band at 920 nm, which is due to the crystal field splitting of the Yb3+ F states.41,42 The EL in the visible (41) Asano-Someda, M.; Kaizu, Y. J. Photochem. Photobiol. A 2001, 139, 161-165.

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Figure 7. Current and irradiance profiles of devices consisting of Yb(TPP)Tp in PPP-OR11. (a) Current density of devices that contain 5 (0), 10 (3), and 15 (O) mol % of Yb(TPP)Tp in PPPOR11; ()) current density of device that contains only PPPOR11. (b) Near-IR irradiance of 5 (9), 10 (3), and 15 (b) mol % of Yb(TPP)Tp in PPP-OR11.

region from all of the blends is weaker than in the nearIR by a factor of 102-103. The very weak band that can be seen at 600 nm in the EL spectrum of the 15 mol % blend is likely TPP-based fluorescence from the Yb(TPP)Tp complex. This assignment is based on previous photoluminescence studies which showed that weak porphyrin-based fluorescence is observed from Ybporphyrin complexes.43 An additional set of weak EL bands is observed from the blends between 620 and 800 nm. This weak emission is believed to arise due to a combination of fluorescence from a free base porphyrin impurity, and porphyrin-based phosphorescence from Yb(TPP)Tp.43 The current-voltage (i-V) profiles and irradiances of the PPP-OR11/Yb(TPP)Tp based devices are shown in Figure 7 along with the current-voltage profile of a (visible light emitting) device that contains pure PPPOR11. All of the blend devices turn on between 4 and 5 V, which is similar to the behavior of devices that contain only PPP-OR11. It is also evident from the i-V plots that devices which contain larger amounts of Yb(TPP)Tp exhibit lower resistivity, as reflected by the fact that at any given voltage the current density increases with increasing concentration of Yb(TPP)Tp. This observation strongly suggests that Yb(TPP)Tp (42) Crosby, G. A.; Kasha, M. Spectrochim. Acta 1958, 10, 377382. (43) Gouterman, M.; Schumaker, C. D.; Srivastava, T. S.; Yonetani, T. Chem. Phys. Lett. 1976, 40, 456-461.

Harrison et al.

plays an important role in charge transport through the devices (vide infra). Concomitant with the increase in current density, the near-IR irradiance of the devices increases with Yb(TPP)Tp concentration. For example, at 9 V the 5 mol % devices emit with a near-IR irradiance of 1.6 µW‚cm-2 while passing an average current of 100 mA‚cm-2. At 10 mol %, the irradiance and current increase to 5.0 µW‚cm-2 and 160 mA‚cm-2 at 9 V, respectively. Finally, at 15 mol %, the emission increases to 10.0 µW‚cm-2 with a current of 315 mA‚cm-2 at 9 V. The peak external quantum efficiency (Φext, max ) for near-IR emission from the 5, 10, and 15 mol % PPPOR11/Yb(TPP)Tp blend devices is approximately 10-4 (Φext,max values: 5 mol %, 5.0 × 10-5 @ 50 mA‚cm-2; 10 mol %, 1.8 × 10-4 @ 10 mA‚cm-2; 15 mol % 1.3 × 10-4 @ 30 mA‚cm-2). The overall dependence of Φext on the current density for the 15 mol % blend device shown in Figure 5 is typical of that observed for the other blends. The efficiency rises with current density and reaches a plateau (Φext ≈ 10-4) at a current density ≈ 30 mA‚cm-2. The efficiency falls gradually at high current density (>50 mA‚cm-2). The decrease in efficiency may occur because of saturation in the devices due to the long lifetime of the lanthanide excited state (∼50 µs).30 Note that Φext for near-IR emission from the 15 mol % PPPOR11/Yb(TPP)Tp blend device is lower than Φext for the (visible-emitting) device that contains pure PPP-OR11. The decreased efficiency for the near-IR emitting devices is believed to be due to the low intrinsic PL quantum yield for the Yb(TPP)Tp emitter (see below). The Φext,max values are related to the maximum internal electron-to-photon quantum efficiency (Φint,max) by the expression, Φint,max ) 2n2 Φext,max, where n is the refractive index of the host polymer (estimated to be 1.6 for PPP-OR11).34 Using this expression, we find that, on average the Φint,max values for near-IR emission from the PPP-OR11/Yb(TPP)Tp blend devices are in the range 3-9 × 10-4. Clearly these values are low compared to devices that feature phosphorescent emitters such as Pt(OEP) or Ir(ppy)2(acac) (where OEP ) octaethylporphyrin and ppy ) 2-phenylpyridine).44,45 This is not surprising considering the PL efficiency (ΦPL) for Yb(TPP)Tp is ∼10-2 which ultimately limits the maximum EL efficiency that can be attained. The internal quantum efficiency of an EL device is related to both the efficiency of electron-hole recombination (ηe/h) and the intrinsic photoluminescence quantum efficiency (ΦPL) of the emitter as shown in eq 1

Φint ) ηe/h ΦPL

(1)

Given that the Φint,max values for the PPP-OR11/ Yb(TPP)Tp blend devices are in the range 10-4 to 10-3, while the ΦPL values for the Yb(TPP)Tp emitter are in the 10-2 range, application of eq 1 leads to the conclusion that ηe/h for the blend devices is low, and it is in the range between 10-2 and 10-1. We believe that the low ηe/h arises due to charge carrier transport imbalance. Specifically, it is likely that hole injection is more efficient and hole transport is more rapid compared to (44) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048-5051. (45) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154.


electron injection and transport. This situation leads to an intrinsic imbalance in carrier injection and transport (i.e., holes dominate) in the devices. Although it is evident that hole transport is the dominant mechanism for conduction in the PPP-OR11/ Yb(TPP)Tp blend devices, an interesting question remains as to what material acts as the dominant hole carrier (i.e., PPP-OR11 or Yb(TPP)Tp). At the onset of this investigation it was anticipated that sensitization of near-IR emission in the EL devices would occur via a mechanism similar to the one operating for PPP-OR11 sensitized near-IR PL from Ln(TPP)L. In particular, as illustrated in Scheme 1, holes injected from the ITO/ PEDOT electrode and electrons injected via the Ca electrode would be transported and recombine on the PPP-OR11 host, producing the polymer-based exciton. From this point, the cascade of events leading to nearIR emission would be the same as that proposed for PL sensitization of near-IR emission (vide supra). However, this model is inconsistent with the fact that the electrical and optical properties of the PPP-OR11/Yb(TPP)Tp blend devices provide clear evidence that the porphyrin is the dominant charge transport material. Specifically, it is quite evident from the i-V curves shown in Figure 7 that the conductivity of the PPP-OR11/Yb(TPP)Tp blends increases substantially as the fraction of Yb(TPP)Tp in the blend increases. Moreover, although the data are not presented herein, it was also found that EL devices that contain less than 5 mol % of the lanthanide complex in the blend display significant (i.e., unquenched) visible emission from the PPP-OR11 host. Taken together, these observations indicate that the Ln(TPP)L complexes act as efficient hole traps in the blends. Moreover, when the concentration of the porphyrin complex becomes sufficiently large in the blends, then this material can act alone as the charge transporter, and electron-hole recombination also occurs exclusively on the complexes as suggested by Scheme 2.46 More insight into the electronic properties of the PPPOR11/Yb(TPP)Tp blends comes from consideration of an approximate energy band diagram for the material. On the basis of the first oxidation and reduction potentials (measured in solution),47 the HOMO and LUMO levels for Yb(TPP)Tp are estimated to lie at approximately 5.3 and 3.1 eV, respectively (relative to the vacuum level). On the basis of electrochemical data obtained on a structurally related polymer, and the band gap (3.1 eV), the HOMO and LUMO levels of PPP-OR11 are estimated to lie at approximately 5.3 and 2.2 eV, respectively.48-50 Using these energy values along with the work functions of the ITO and Ca/Al electrodes, an energy band diagram for the PPP-OR11/Yb(TPP)Tp blend EL devices is constructed (Scheme 1). As can be (46) Lane, P. A.; Palilis, L. C.; O’Brien, D. F.; Giebeler, C.; Cadby, A. J.; Lidzey, D. G.; Campbell, A. J.; Blau, W.; Bradley, D. D. C. Phys. Rev. B 2001, 63, 235206. (47) The first oxidation and reduction potentials are observed at E1/2 (0.58 V) and E1/2 (-1.67 V) vs Fc/Fc+ in dichloroethane and tetrahydrofuran, respectively. (48) Child, A. D.; Reynolds, J. R. Macromolecules 1994, 27, 19751977. (49) Yang, Y.; Pei, Q.; Heeger, A. J. Synth. Met. 1996, 78, 263267. (50) Birgerson, J.; Fahlman, M.; Broms, P.; Salaneck, W. R. Synth. Met. 1996, 80.

Chem. Mater., Vol. 16, No. 15, 2004 2945 Scheme 2

seen from this diagram, hole injection is expected to be equally facile for both PPP-OR11 and Yb(TPP)Tp. By contrast, electron injection is expected to be more facile for the Yb(TPP)Tp. Although it is not possible to draw any firm conclusions from the energy level diagram, the fact that the two materials are expected to be approximately equivalent with respect to the energetics of hole injection, we conclude that the carrier imbalance which gives rise to the low observed EL efficiency likely results from the fact that the hole mobility is larger than the electron mobility in the blends. This conclusion is supported by recent unpublished investigations in which we have demonstrated that addition of Al(isoquinolate)3, a well-known hole blocking material, to the PPP-OR11/ Yb(TPP)Tp blends results in a decrease in the current density with a concomitant increase in EL brightness, leading to a 10-fold increase in the quantum efficiency for near-IR electroluminescence. Tuning the Wavelength of Near-IR Emission by Varying the Ln Ion. It is well-established that different Ln3+ ions feature emission from F-based states at various wavelengths in the near-IR region. We recently characterized the photoluminescence of the series of lanthanide porphyrin complexes, Ln(TPP)L (L ) Tp and L(OEt)), and found that PL could be observed from solutions of the complexes where Ln ) Yb, Er, Nd.30 In the present investigation, we sought to establish that it is possible to fabricate near-IR emitting EL devices which emit at the characteristic wavelengths of the different Ln ions by using blends of PPP-OR11 and the Ln(TPP)L complexes as the active materials. To accomplish this objective, a series of EL devices were prepared which contained blends of PPP-OR11 with Ln(TPP)Tp complexes (15 mol %) with Ln ) Yb, Nd, Ho, and Er. In each case, the devices exhibited near-IR EL emission at wavelengths characteristic of the Ln ion in the porphyrin complex (see Figure 8 for EL spectra). Specif-

2946


Harrison et al.

Figure 8. Near-IR electroluminescence spectra of Ln(TPP)Tp in PPP-OR11 (15 mol %) where Ln ) Yb, Nd, Ho, or Er.

ically, for Ln ) Yb, emission is apparent at 977 nm arising from the 2F5/2 f 2F7/2 transition; for Ln ) Nd, bands are observed at 880 nm (4F3/2 f 4I9/2), 1060 nm (4F3/2 f 4I11/2), and 1300 nm (4F3/2 f 4I13/2); for Ln ) Er, a single band at 1560 nm is observed which arises from the 4I13/2 f 4I15/2 transition; and for Ho a single weak emission is noted at 1200 nm from the 5I6 f 5I8 transition. The EL efficiencies exhibited by this series of devices qualitatively mirror the PL efficiencies determined for the respective lanthanide complexes, with Yb > Nd . Er > Ho.30 Nonconjugated Polymer Hosts. On the basis of the work outlined above which strongly implies that the conjugated polymer host plays a minor role in carrier transport and exciton generation in the PPP-OR11/ Ln(TPP)L EL devices, we have utilized nonconjugated polymers as host materials. In a preliminary communication we showed that EL devices fabricated using polystyrene/Yb(TPP)L blends exhibit electrical characteristics and quantum efficiencies similar to those based on PPP-OR11/Yb(TPP) L blends.28 In the course of the present investigation we have expanded this concept by fabricating Yb(TPP)Tp-based near-IR EL devices using three other nonconjugated polymer hosts having varying physical properties. The polymers used as hosts in this expanded study include poly(methyl methacrylate) (PMMA, glass transition temperature (Tg) ) 105 °C), poly(n-butyl methacrylate) (PBMA, Tg ) 15 °C), and poly(bisphenol A-carbonate) (PC, Tg ) 150 °C) (see Figure 1 for repeat unit structures of these polymers). The electrical and optical characteristics of EL devices with loadings of 25, 33, 50, and 67 wt % of Yb(TPP)Tp in this set of nonconjugated polymer hosts were characterized. In all cases, consistent with the behavior of the polystyrene/Yb(TPP)Tp blend devices which were reported previously,28 near-IR light output increases with the Ln-complex loading. Therefore, we present herein only the data for devices with the highest complex loading (67 wt %) for a comparative study among the different polymers tested. The current density-voltage (J-V) and light outputvoltage characteristics of devices with blends of Yb(TPP)Tp and PMMA, PBMA, or PC are compared in Figure 9. In each case the devices turn-on at approximately 8 V and the current density and near-IR

Figure 9. (a) Current density and (b) near-IR irradiance characteristics of blend devices with 67 wt % of Yb(TPP)Tp in PMMA (9), PBMA (b), and PC(2) as a function of applied voltage.

light output increases sharply above 10 V. As can be seen from Figure 9b, the near-IR emission increases with applied voltage up to 15 V. Interestingly, the turnon voltage of these devices is slightly higher than that in devices that contain PPP-OR11/Yb(TPP)Tp, which turn on at ∼4 V. This difference in turn-on voltage suggests that at low applied voltage the PPP-OR11 host may play a role in carrier transport in the conjugated polymer based devices. The higher turn-on and drive voltage may arise from the fact that the nonelectroactive polymers add a resistive barrier to the devices due to interfacial effects. Thickness differences in the active layer (∼50 nm in PPP-OR11/Yb(TPP)Tp compared to ∼100 nm in Yb(TPP)Tp/nonconjugated poymer films) are also expected to increase the turn-on voltage for the devices that contain the nonconjugated polymers. The devices which contain PMMA, PBMA, or PC as the host also operate at higher applied voltage without failure (i.e., up to 15 V, see Figure 9), compared to 10-12 V for PPP-OR11 based devices. However, the external quantum efficiencies (shown in Figure 10) for the nonconjugated polymer host devices are comparable to those with PPP-OR11 as host, with values ranging between 1 and 3 × 10-4. This suggests that a similar charge transport and recombination mechanism is taking place in devices that contain PPP-OR11 or nonconjugated polymer hosts. Another point of note is that in the nonconjugated polymer host devices Φext decreases at higher current density. This effect was also observed for the PPP-OR11/ Yb(TPP)Tp blend EL devices (vide supra) and we attribute it to saturation effects which arise due to the long lifetime of the lanthanide-ion based excited states.51,52 In view of the fact that all of the nonconju(51) Capecchi, S.; Renault, O.; Moon, D. G.; Halim, M.; Etchells, M.; Dobson, P. J.; Salata, O. V.; Chritou, V. Adv. Mater. 2000, 12, 1591-1594.


Figure 10. External quantum efficiency as a function of current density for blend devices with 67 wt % of Yb(TPP)Tp in PMMA (9), PBMA (b), and PC(2) as a function of applied voltage.

Chem. Mater., Vol. 16, No. 15, 2004 2947

almost as well as those having host polymers with Tg above ambient. Finally, to demonstrate the feasibility of imaging the near-IR output from a Yb(TPP)Tp based PLED, we fabricated a device on a 2.5 × 2.5 cm substrate in which a recognizable pattern was etched in the ITO conductive layer. The device contained a polystyrene/Yb(TPP)Tp blend (66 wt % Ln complex), it was operated at 9 V, and it was imaged using a commercial video camera operating in night-shot mode (SONY Videocam, silicon CCD) through an 850-nm long-pass filter. Figure 11 shows an image captured using the video camera; this experiment clearly demonstrates that the light output from the device can be imaged without specialized equipment. (Under the conditions of this imaging experiment the output of the device was ∼1 µw‚cm-2). Summary and Conclusions

Figure 11. Near-IR image of a Yb(TPP)Tp device in polystyrene captured using a commercial video camera capable of recording images in the near-infrared. An RG850 Schott glass filter was placed between the device and the camera to filter all visible light from reaching the camera.

gatedpolymers used in this series of investigations are wide-band gap insulators, it is quite clear that the Yb(TPP)Tp complex acts as both the charge transport material (presumably via a hopping mechanism)53,54 as well as the near-IR emitting species. Among the set of nonconjugated polymer hosts characterized in the present investigation PMMA was found to result in the best device characteristics, as reflected by the near-IR irradiance and EL efficiencies (see Figures 9 and 10). This is possibly due to the fact that Yb(TPP)Tp is more soluble in PMMA allowing for better quality spin-cast films with fewer pinhole defects. Nevertheless, it is interesting that it is possible to fabricate devices with host polymers having Tg as low as 15 °C (PBMA), and that these devices performed (52) Kido, J.; Okamoto, Y. Chem. Rev. 2002, 102, 2357-2368. (53) Lawrence, M. F.; Huang, Z.; Langford, C. H.; Ordonez, I. J. Phys. Chem. 1993, 97, 944-951. (54) Savenije, T. J.; Moons, E.; Boschloo, G. K.; Goossens, A.; Schaafsma, T. J. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 9685-9692.

The PL and EL properties of the blue-emitting polymer PPP-OR11, blends of Ln(TPP)L complexes with PPP-OR11, and blends of Ln(TPP)L complexes with several nonconjugated polymers have been characterized. The results of the combined PL and EL study indicate that, although Fo¨rster energy transfer is a reasonable mechanism to explain the sensitized nearIR photoluminescence in (PPP-OR11)/Yb(TPP)L blends, a different mechanism leading to near-IR emission operates in EL devices. In the EL devices that contain PPP-OR11/Ln(TPP)L blends, the results suggest that hole transport is dominated by the Ln(TPP)L material, and carrier transport is believed to occur via a site-tosite hopping mechanism. Excitations are likely created by direct electron-hole recombination on the Ln(TPP)L complexes (rather than by electron-hole recombination on the host polymer, followed by energy transfer). The ability to fabricate EL devices using blends of Yb(TPP)Tp with various nonconjugated polymer hosts that display emission EL efficiencies comparable to those obtained with the PPP-OR11 conjugated polymer host supports the proposed charge-hopping mechanism that is postulated to be operating in the PPP-OR11/Ln(TPP)L blend devices. Moreover, the ability to use nonconjugated polymers as hosts for Ln(TPP)L based LEDs opens up many opportunities for the use of inexpensive nonelectroactive polymers in fabricating PLEDs with tetraphenylporphyrin based emitters. Work in progress seeks to improve the near-IR EL efficiency of the devices by incorporating hole-blocking and/or electron transporting materials into the active medium. In addition, ongoing studies seek to shift the EL emission into the mid-infrared region. Results of this ongoing work will be reported soon. Acknowledgment. This work was supported by the Army Research Office and the Defense Advanced Research Projects Agency (DAAD19-00-1-0002). Supporting Information Available: Device characterization data for near-IR EL devices that contain Yb(TPP)L(OEt)/PPP-OR11 blends as the active material (2 figures; pdf). This information is available free via the Internet at http:// pubs.acs.org. CM049937F

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