Highly efficient, near-infrared electrophosphorescence from a Pt-metalloporphyrin complex

Article abstract of DOI:10.1002/anie.200604240

(Graph Presented) Red-shifting beyond red: The nonplanar porphyrin complex [Pt(tpbp)] (tpbp = tetraphenyltetrabenzoporphyrin) has been used as a phosphorescent dopant in highly efficient electrophosphorescent devices that emit in the near-infrared region

Full text of DOI:10.1002/anie.200604240

Angewandte
Chemie
DOI: 10.1002/anie.200604240
Light-Emitting Diodes
Highly Efficient, Near-Infrared Electrophosphorescence from a
Pt–Metalloporphyrin Complex**
Carsten Borek, Kenneth Hanson, Peter I. Djurovich, Mark E. Thompson,* Kristen Aznavour,
Robert Bau, Yiru Sun, Stephen R. Forrest, Jason Brooks, Lech Michalski, and Julie Brown
Organic light-emitting diodes (OLEDs) have been the subject
of a significant research effort for the past two decades with a
focus on devices that emit almost exclusively in the visible
part of the electromagnetic spectrum.[1] Recently, there has
been a growing interest in OLEDs that emit in the near-
infrared (NIR) region (700–2500 nm).[2–4] Applications for
these NIR OLEDs are particularly interesting for night-
vision-readable displays[5] and sensors.[6] The efficiency of
OLEDs are markedly improved when fluorescent emissive
dopants are replaced with phosphorescent heavy-metal com-
plexes that can effectively harvest both the singlet and triplet
excitons formed in electroluminescence, with wavelengths (l)
ranging from the near-ultraviolet into the red (with peak
emission at lmax = 380–650 nm).[7–10] Herein, we report on an
efficient NIR OLED that utilizes a phosphorescent Pt–
metalloporphyrin dopant, with an external quantum effi-
ciency (EQE) greater than 6% at lmax = 765 nm and a full
width at half maximum of 31 nm (500 cmÀ1).
lanthanide ion.[11] Schanze et al. have reported an NIR OLED
utilizing Ln3+ in conjunction with a porphyrin/polystyrene
matrix, with EQE ranging from 8.0 ” 10À4 to 2.0 ” 10À4 % at
approximately 1 mAcmÀ2.[5] Similarly, a Nd(phenalenone)3-
based OLED had an EQE of 0.007% at lmax = 1065 nm.[4] The
second class of NIR OLEDs is transition-metal complexes,
similar to those used in the visible region. A recent report of
an electrophosphorescent device that used a cyclometalated
[(pyrenyl–quinolyl)2Ir(acac)] complex as the phosphor gave
lmax = 720 nm and an EQE of 0.1%.[6]
A family of complexes that have shown intense absorption
and emission in the red-to-NIR region of the spectrum are the
metalloporphyrins.[12,13] There are a number of reports of
OLEDs fabricated with [Pt(oep)], [Pt(tpp)] (oep =
2,3,7,8,12,13,17,18-ocatethylporphryin, tpp = 5,10,15,20-tetra-
phenylporphyrin), or analogues of these compounds as
phosphorescent emitters, with emission maxima between
630 and 650 nm,[7,14–22] however, there has been no apparent
effort to shift the Pt–porphyrin-based OLED emission into
the NIR region. Porphyrin chromophores with fused aromatic
moieties at the b-pyrrole positions, for example, tetrabenzo-
porphyrin (bp), exhibit a bathochromic shift (relative to
unsubstituted porphyrin) of the absorption and emission
energy, owing to the expansion of the p-electronic system of
the porphyrin core.[23] The addition of bulky groups to the
meso positions of the porphyrin macrocycles with b-substi-
tuted pyrroles leads to the formation of nonplanar porphyrins,
and further red-shifts the absorption spectra.[24] Coordination
of a heavy-metal atom increases the rate of the intersystem
crossing between singlet and triplet states of the metal-
loporphyrins, thereby enhancing the rate of radiative decay
from the triplet state. For these reasons, we focus here on PtII–
tetraphenyltetrabenzoporphyrin, [Pt(tpbp)] (Figure 1), as a
phosphorescent dopant.
Previously, two classes of phosphorescent complexes have
been employed as dopants in NIR-emitting OLEDs. The first
utilizes trivalent lanthanide cations (Ln3+) as the emitting
centers, for example, Er3+ or Nd3+, chelated with chromo-
phoric ligands to sensitize excitation-energy transfer to the
[*] C. Borek, K. Hanson, Dr. P. I. Djurovich, Prof. M. E. Thompson,
K. Aznavour, Prof. R. Bau
Department of Chemistry
University of Southern California
Los Angeles, CA 99089 (USA)
Fax: (+1)213-740-8594
E-mail: met@usc.edu
Y. Sun
Department of Electrical Engineering
Princeton University
Princeton, NJ 08544 (USA)
Analysis of the [Pt(tpbp)] complex by X-ray crystallog-
raphy reveals a nonplanar molecular structure with a saddle-
type distortion, similar to that found in other tpbp deriva-
tives.[25] Crystallographic data are available in the Supporting
Information and from the Cambridge Crystallographic Data
Centre (CCDC-627735).[26] An analysis of [Pt(tpbp)] using
normal-coordinate structural decomposition (NSD) soft-
ware[27,28] quantifies the various distortions that accompany
the macrocyclic deformation (Figure 1, top). The total out-of-
plane distortion (Doop = 2.83 Š) is almost exclusively de-
scribed by saddling (B2u = 2.83 Š), while doming (A2u =
0.045 Š), wave x and wave y deformations (similar to a
chair conformation in cyclohexane, in either the x or
y direction; Eg = 0.042 and 0.068 Š, respectively), propeller-
ing (A1u = 0.0003 Š), and ruffling (B1u = 0.006 Š) contribute
Prof. S. R. Forrest
Department of Electrical Engineering and Computer Science and
Department of Physics
University of Michigan
Ann Arbor, MI 48109 (USA)
Dr. J. Brooks, L. Michalski, Dr. J. Brown
Universal Display Corporation
Ewing, NJ 08618 (USA)
[**] The authors acknowledge financial support from the U.S. Dept. of
the Army, CECOM for the phase II SBIR program (contract
no. W15P7T-06-C-T201), and Universal Display Corporation.
Supporting information for this article, including the procedure
used to prepare and purify [Pt(tpbp)], details of the crystallographic
work, NSD analysis, and the methods used to prepare and test
OLEDs, is available on the WWW under http://www.angewand-
te.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 1109 –1112
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e430nm = 2.03 ” 105 mÀ1 cmÀ1, and the Q band with lmax
=
611 nm (e611nm = 1.35 ” 105 mÀ1 cmÀ1), which differ slightly
from the values for the analogous [Pd(tpbp)] complex.[30]
The phosphorescence spectrum of thoroughly degassed
[Pt(tpbp)] has lmax = 765 nm, with a transient lifetime of t =
53 ms at room temperature (298 K); these values shift to
lmax = 751 nm and t = 73 ms at 77 K. The blue shift is due to a
rigidochromic effect at low temperature. Assuming that the
radiative rate constant does not show a temperature depend-
ence between 298 K and 77 K, and the nonradiative decay
rate is negligible at 77 K, the ratio of the lifetimes gives the
photoluminescence (PL) efficiency. The photophysical prop-
erties of Pt–tetrabenzoporphrin [Pt(bp)] support these
assumptions.[31] The photoluminescent quantum yield (FPL
)
estimated by this method is 0.7. Radiative (kr = 1.3 ” 104 sÀ1)
and nonradiative (knr = 5.8 ” 103 sÀ1) decay rates were esti-
mated from the lifetime (t) data (kr = 1/t77K; knr = 1/t298KÀkr).
The nonradiative rate is similar to that reported for [Pd-
(tpbp)] (knr = 4.2 ” 103 sÀ1), but the stronger spin–orbit cou-
pling of Pt significantly increases the radiative rate (compare
[Pd(tpbp)]: kr = 8.8 ” 102 sÀ1).[30]
To minimize concentration quenching in [Pt(tpbp)]-based
OLEDs, [Pt(tpbp)] was doped into a tris(8-hydroxyquino-
line)aluminum (Alq3) host.[7] The dopant singlet and triplet
levels fall well below those of Alq3 (S1 = 500 nm, T1 =
590 nm).[32] The OLEDs prepared here incorporated a neat
Alq3 exciton-blocking layer (EBL) to prevent exciton
quenching at the cathode surface. Thus, the structure used
was: ITO/(400 Š) NPD/(400 Š) Alq3 + 6 wt% [Pt(tpbp)]/
(500 Š) Alq3 /(10 Š) LiF/(1100 Š) Al (ITO = indium tin
Figure 1. Crystal structure of [Pt(tpbp)]. Top: top view; bottom: edge
view (meso-phenyl substituents removed for clarity) with results of
NSD analysis.
only slightly to the nonplanarity. This distortion is greater
oxide,
NPD = N,N’-bis(1-naphthyl)-N,N’-diphenyl-1,1’-
than that found in [Zn(tpbp)] (Doop = 2.35 Š),[25] yet is less
biphenyl-4,4’-diamine), hereafter referred to as device 1.
This device has an EQE of 6.3% at 0.1 mAcmÀ2, which
gradually decreases as the current density is increased
(Figure 3). The intensity is roughly 0.1 mWcmÀ2 at 3 V and
750 mWcmÀ2 at 12 V. The electroluminescence (EL) spectrum
for device 1 displays strong NIR emission at 769 nm, with no
Alq3 signal (520 nm) at any bias level (Figure 3), thus
indicating complete energy transfer from host to guest.
Devices were also prepared to test the operational
stability of [Pt(tpbp)]-based OLEDs. For these tests, the
following structure (device 2) was used: ITO/(400 Š) NPD/
(300 Š) Alq3 + 6 wt% [Pt(tpbp)]/(400 Š) BAlq/(10 Š) LiF/
(1000 Š) Al, where BAlq is aluminum(III) bis(2-methyl-8-
quinolinato)4-phenylphenolate, and serves as the exciton-
blocking layer. The charge transport, blocking, and host
materials have previously been demonstrated to give high
efficiencies and long electrophosphorescent device life-
times.[33,34] At a low current density of 0.1 mAcmÀ2, device 2
has a maximum EQE of 3%, falling to 1.1% at 10 mAcmÀ2
(Figure 3). The electroluminscence spectrum is identical to
that for device 1.
than that of
a [Ni(tpbp(CO2Me)8)] derivative (Doop =
3.43 Š).[23]
Electrochemical analysis of [Pt(tpbp)], versus an internal
ferrocene reference, shows a reversible oxidation at 0.24 V
and quasireversible reduction at À1.85 V. The highest occu-
pied (HOMO) and the lowest unoccupied (LUMO) molec-
ular orbitals calculated from these data are 4.9 and 2.5 eV in
energy relative to vacuum, respectively.[29] The absorption
spectrum (Figure 2) displays strong transitions for the Soret
band at lmax = 430 nm, with an extinction coefficient of
The device was aged at a high constant current of
40 mAcmÀ2 corresponding to an initial intensity of
740 mWcmÀ2. The data in Figure 4 (see inset) suggest that
device 2 will maintain greater than 90% of its initial intensity
after 1000 h of operation. A 0.5-V rise in voltage occurring
during the first 50 h corresponds to the initial fast decay in
device intensity. After 100 h, the voltage stabilizes at 13.3 V.
Figure 2. Room-temperature absorption spectrum (solid line), and
normalized emission spectra at room temperature (closed circles) and
at 77 K (open squares) of [Pt(tpbp)] in 2-methyl-THF.
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lifetime for [Pt(oep)]-based OLEDs (> 106 h measured at low
luminance),[35] thus illustrating the high device stability of Pt–
porphyrin-based OLEDs.
In conclusion, we have demonstrated highly efficient NIR
electrophosphorescent devices utilizing [Pt(tpbp)] emitters
with a device peak EQE of 6.3% at 0.1 mAcmÀ2, and lifetime
of greater than 1000 h to 90% efficiency at 40 mAcmÀ2. The
very high efficiencies of the NIR devices make them suitable
for many night-vision display and sensing applications.
Achieving even longer-wavelength emission is possible by
extending the conjugation length of the ligand structure.
Received: October 17, 2006
Published online: January 9, 2007
Keywords: luminescence · near-infrared emission · OLEDs ·
.
phosphorescence · porphyrinoids
[1] Z. H. Kafafi, Organic Electroluminescence, CRC, Boca Raton,
2005.
[2] B. S. Harrison, Appl. Phys. Lett. 2001, 79, 3770.
[3] L. H. Slooff, A. Polman, F. Cacialli, Appl. Phys. Lett. 2001, 78,
2122.
[4] A. OꢀRiordan, E. OꢀConnor, S. Moynihan, P. Nockemann, P.
Fias, R. Van Deun, D. Cupertino, P. Mackie, G. Redmond, Thin
Solid Films 2006, 497, 299.
[5] K. S. Schanze, J. R. Reynolds, J. M. Boncella, B. S. Harrison, T. J.
Foley, M. Bouguettaya, T.-S. Kang, Synth. Met. 2003, 137, 1013.
[6] E. L. Williams, J. Li, G. E. Jabbour, Appl. Phys. Lett. 2006, 89,
083506.
[7] M. A. Baldo, D. F. OꢀBrien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151.
[8] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E.
Lee, C. Adachi, P. E. Burrows, S. R. Forrest, M. E. Thompson, J.
Am. Chem. Soc. 2001, 123, 4304.
Figure 3. Top: external quantum efficiency versus current density.
Device 1: ITO/(400 Š) NPD/(400 Š) Alq3 +6 wt% [Pt(tpbp)]/(500 Š)
Alq3 /(10 Š) LiF/(1100 Š) Al; device 2: ITO/(400 Š) NPD/(300 Š)
Alq3 +6 wt% [Pt(tpbp)]/(400 Š) BAlq/(10 Š) LiF/(1000 Š) Al. Inset:
Intensity and current density versus voltage for devices 1 and 2.
Bottom: normalized electroluminescence spectra for device 1 at 5 V
to10 V, in 1-V steps. Spectra offset for clarity.
[9] E. Holder, B. M. W. Langeveld, U. S. Schubert, Adv. Mater. 2005,
17, 1109.
[10] R. J. Holmes, S. R. Forrest, T. Sajoto, A. Tamayo, P. I. Djurovich,
M. E. Thompson, J. Brooks, Y. J. Tung, B. W. DꢀAndrade, M. S.
Weaver, R. C. Kwong, J. J. Brown, Appl. Phys. Lett. 2005, 87.
[11] J. Kido, Y. Okamoto, Chem. Rev. 2002, 102, 2357.
[12] K. Kalyanasundaram, Photochemistry of Polypyridine and
Porphyrin Complexes, Academic Press, London, 1992.
[13] T. V. Duncan, K. Susumu, L. E. Sinks, M. J. Therien, J. Am.
Chem. Soc. 2006, 128, 9000.
[14] D. F. OꢀBrien, M. A. Baldo, M. E. Thompson, S. R. Forrest,
Appl. Phys. Lett. 1999, 74, 442.
[15] M. Colle, C. Garditz, M. Braun, J. Appl. Phys. 2004, 96, 6133.
Figure 4. Intensity versus operating time, normalized to initial inten-
sity (closed squares) and voltage (open circles). The initial intensity
was 740 mWcmÀ2. The inset shows the time versus radiance data
plotted on a semilogarithmicscale with extrapolation to 1000 h.
[16] H. D. F. Burrows, M.
, J. S. de Melo, A. P. Monkman, S.
Navaratnam, J. Am. Chem. Soc. 2003, 125, 15310.
[17] Y. Wang, Appl. Phys. Lett. 2004, 85, 4848.
[18] V. A. Montes, C. Perez-Bolivar, N. Agarwal, J. Shinar, P.
Anzenbacher, J. Am. Chem. Soc. 2006, 128, 12436.
[19] M. Ikai, F. Ishikawa, N. Aratani, A. Osuka, S. Kawabata, T.
Kajioka, H. Takeuchi, H. Fujikawa, Y. Taga, Adv. Funct. Mater.
2006, 16, 515.
[20] L. Yanqin, R. Aurora, S. Marco, M. Marco, H. Cheng, W. Yue, L.
Kechang, C. Roberto, G. Giuseppe, Appl. Phys. Lett. 2006, 89,
061125.
The operating voltage is sensitive to variations in temper-
ature, which may account for some of the observed fluctua-
tion. These initial results demonstrate that [Pt(tpbp)] devices
are stable at high drive currents, although further study is
needed to determine the device lifetime at radiances suitable
for display use, assuming the viewer is using night-vision
goggles. The long device lifetimes estimated for [Pt(tpbp)]-
based OLEDs are consistent with the previously reported
[21] Q. Hou, Y. Zhang, F. Li, J. Peng, Y. Cao, Organometallics 2005,
24, 4509.
[22] J. Kalinowski, W. Stampor, J. Szmytkowski, M. Cocchi, D. Virgili,
V. Fattori, P. D. Marco, J. Chem. Phys. 2005, 122, 154710.
Angew. Chem. Int. Ed. 2007, 46, 1109 –1112
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Communications
[23] V. V. Rozhkov, M. Khajehpour, S. A. Vinogradov, Inorg. Chem.
[31] T. J. Aartsma, M. Gouterman, C. Jochum, A. L. Kwiram, B. V.
Pepich, L. D. Williams, J. Am. Chem. Soc. 1982, 104, 6278.
[32] W. Humbs, E. van Veldhoven, H. Zhang, M. Glasbeek, Chem.
Phys. Lett. 1999, 304, 10.
[33] R. C. Kwong, M. R. Nugent, L. Michalski, T. Ngo, K. Rajan, Y.-J.
Tung, M. S. Weaver, T. X. Zhou, M. Hack, M. E. Thompson,
S. R. Forrest, J. J. Brown, Appl. Phys. Lett. 2002, 81, 162.
[34] R. C. Kwong, M. S. Weaver, M.-H. M. Lu, Y.-T. Tung, A. B.
Chwang, T. X. Zhou, M. Hack, J. J. Brown, Org. Electron. 2003,
4, 155.
2003, 42, 4253.
[24] O. S. Finikova, S. E. Aleshchenkov, R. P. Brinas, A. V. Chepra-
kov, P. J. Carroll, S. A. Vinogradov, J. Org. Chem. 2005, 70, 4617.
[25] R.-J. Cheng, Y.-R. Chen, S. L. Wang, C. Y. Cheng, Polyhedron
1993, 12, 1353.
[26] http://www.ccdc.cam.ac.uk/data_request/cif.
[27] W. Jentzen, X.-Z. Song, J. A. Shelnutt, J. Phys. Chem. B 1997,
101, 1684.
[28] http://jasheln.unm.edu/jasheln/content/nsd/nsd_welcome.htm.
[29] B. W. DꢀAndrade, S. Datta, S. R. Forrest, P. Djurovich, E.
Polikarpov, M. E. Thompson, Org. Electron. 2005, 6, 11.
[30] J. E. Roger, K. A. Nguyen, D. C. Hufnagle, D. G. McLean, W.
Su, K. M. Gossett, A. R. Burke, S. A. Vinogradov, R. Patcher,
P. A. Fleitz, J. Phys. Chem. A 2003, 107, 11331.
[35] P. E. Burrows, S. R. Forrest, T. X. Zhou, L. Michalski, Appl.
Phys. Lett. 2000, 76, 2493.
1112
www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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