Accelerated luminophore discovery through combinatorial synthesis

Article abstract of DOI:10.1021/ja047156+

A method for accelerating the discovery of ionic luminophores using combinatorial techniques is reported. The photophysical properties of the resulting transition-metal-based chromophores were compared against a series of analogous, traditionally prepared species. The strong overlap between these two sets confirms the identity of the parallel synthesis products and supports the truthfulness of the combinatorial results. Further support for the combinatorial method comes from the adherence of these complexes to the energy gap law. The relationship between the structure of a complex and its photophysical properties was also considered, and static DFT calculations were used to assess whether it is feasible to predict the luminescent behavior of novel materials.

Full text of DOI:10.1021/ja047156+

Published on Web 10/08/2004
Accelerated Luminophore Discovery through Combinatorial
Synthesis
Michael S. Lowry, William R. Hudson, Robert A. Pascal, Jr., and Stefan Bernhard*
Contribution from the Department of Chemistry, Princeton UniVersity, Frick Laboratory,
Princeton, New Jersey 08544
Received May 14, 2004; E-mail: sbernhar@princeton.edu
Abstract: A method for accelerating the discovery of ionic luminophores using combinatorial techniques
is reported. The photophysical properties of the resulting transition-metal-based chromophores were
compared against a series of analogous, traditionally prepared species. The strong overlap between these
two sets confirms the identity of the parallel synthesis products and supports the truthfulness of the
combinatorial results. Further support for the combinatorial method comes from the adherence of these
complexes to the energy gap law. The relationship between the structure of a complex and its photophysical
properties was also considered, and static DFT calculations were used to assess whether it is feasible to
predict the luminescent behavior of novel materials.
Introduction
There are three electrochemical stages associated with light
emission in a diode-type device: (1) charge injection, (2) charge
Ionic transition-metal complexes have emerged as promising
candidates for applications in solid-state electroluminescent
devices. These materials serve as multifunctional chromophores,
into which electrons and holes are injected and then migrate
and recombine to exhibit light emission.1-6 In particular,
transition-metal complexes are appealing luminescent materials
because their properties (and net charge) can be tuned through
ligation, thereby enabling facile, synthetic control over device
performance. Consequently, the design of novel complexes for
OLED applications has become a chief synthetic goal in
materials research.7-9
transport, and (3) electron-hole recombination.10,11 Although
OLEDs operate through electron-induced emission, solution-
based photoluminescence studies provide a simple means of
modeling and examining the radiative pathways in new materials
and, thus, can be used to optimize their photophysical properties.
The majority of ionic chromophores that have been studied
for single-layer devices involve osmium(II),12 ruthenium(II),1-6,13
and rhenium(I).14,15 These materials emit primarily in the orange-
red portion of the spectrum (600-650 nm) and have shown
little sign of improvement with respect to color-tunability.
Recently, however, Slinker et al. observed efficient yellow
electroluminescence from a single-layer of a mixed-ligand
iridium(III) complex, [Ir(ppy)2(dtbbpy)](PF6) (where ppy )
2-phenylpyridine; dtbbpy ) 4,4′-di-tert-butyl-2,2′-dipyridyl),
marking the highest emission energy from a single-layer device,
to date.16
The usefulness and applicability of new luminescent materials
can be assessed according to their stability, external quantum
yield, excited-state lifetime, and emission energy. It is highly
desirable to tailor transition-metal complexes that will emit
across the visible spectrum and to produce molecules in which
light emission predominates over nonradiative decay. These
properties can be evaluated using both photo- and electrolumi-
nescent methods.
Because of its success, it is important to consider Slinker et
al.’s complex more intimately. The dtbbpy-complex has a higher
emission energy, a longer lifetime, and better photo- and
electroluminescent quantum yields than does its more thoroughly
investigated parent complex, [Ir(ppy)2(bpy)](PF6).16-19 Although
a systematic relationship between photo- and electrolumines-
(1) Slinker, J.; Bernards, D.; Houston, P. L.; Abrun˜a, H. D.; Bernhard, S.;
Malliaras, G. G. Chem. Commun. 2003, 2392-2399.
(2) Gao, F. G.; Bard, A. J. J. Am. Chem. Soc. 2000, 122, 7426-7427.
(3) Rudmann, H.; Shimada, S.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124,
4918-4921.
(4) Buda, M.; Kalyuzhny, G.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 6090-
6098.
(10) Forrest, S.; Burrows, P.; Thompson, M. IEEE Spectrum 2000, 29-34.
(11) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-915.
(12) Bernhard, S.; Gao, X.; Malliaras, G. G.; Abrun˜a, H. D. AdV. Mater. 2002,
14, 433-436.
(13) Reveco, P.; Schmehl, R. H.; Cherry, W. R.; Fronczek, F. R.; Selbin, J.
Inorg. Chem. 1985, 24, 4078-4082.
(14) Gong, X.; Ng, P. K.; Chan, W. K. AdV. Mater. 1998, 10, 1337-1340.
(15) Spellane, P.; Watts, R. J.; Vogler, A. Inorg. Chem. 1993, 32, 5633-5636.
(16) Slinker, J. D.; Gorodetsky, A. A.; Lowry, M. S.; Wang, J.; Parker, S.;
Rohl, R.; Bernhard, S.; Malliaras, G. G. J. Am. Chem. Soc. 2004, 126,
2763-2767.
(17) Wilde, A. P.; Watts, R. J. J. Phys. Chem. 1991, 95, 622-629.
(18) Colombo, M. G.; Gu¨del, H. U. Inorg. Chem. 1993, 32, 3081-3087.
(19) Colombo, M. G.; Hauser, A.; Gu¨del, H. U. Inorg. Chem. 1993, 32, 3088-
3092.
(5) Bernhard, S.; Barron, J. A.; Houston, P. L.; Abrun˜a, H. D.; Ruglovsky, J.
L.; Gao, X.; Malliaras, G. G. J. Am. Chem. Soc. 2002, 124, 13624-13628.
(6) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.;
Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem.
2001, 40, 1704-1711.
(7) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.;
Adachi, C.; Burrows, P.; Forrest, S. R.; Thompson, M. E. J. Am. Chem.
Soc. 2001, 123, 4304-4312.
(8) Pohl, R.; Montes, V. A.; Shinar, J.; Pavel Anzenbacher, J. J. Org. Chem.
2004, 69, 1723-1725.
(9) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.;
Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino,
M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971-12979.
9
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J. AM. CHEM. SOC. 2004, 126, 14129-14135
14129

A R T I C L E S
Lowry et al.
Figure 1. Ten cyclometalating (C∧N) and 10 neutral (N∧N) ligands.
cence in solid-state devices has yet to be revealed, improved
photoluminescence appears to be a good predictor of improved
luminescence in electronic devices. It is currently believed that
such a relationship will persist for similar ligand systems.
Moreover, the source of these differences in luminescent
properties - a function of their chemical structure - is not yet
well understood and needs to be addressed. It is important,
therefore, to synthesize a large series of structurally similar,
noncongruent complexes to assess the impact of chemical
modifications on the photophysical properties of multifunctional
chromophores.
As the effect of ligation on luminescent behavior becomes
better understood, it will become possible to predict the
properties of light emission for a novel complex from its
structural design. Yet, before any intelligent predictions can be
made, the number of luminescent compounds whose photo-
physical properties are well-established must be dramatically
increased. Expansion of the known library of luminescent
molecules is limited by the time and effort required to
synthesize, purify, and characterize each individual complex by
traditional synthetic methods. The introduction of combinatorial
chemistry to this class of research is extremely appealing
because it can be used to expedite the production of a
molecularly diverse library of compounds.
In the present investigation, a library of 100 complexes has
been prepared by coordinating one neutral, diimine and two
equivalent cyclometalating (C∧N) ligands to an iridium(III)
metal center, [Ir(C∧N)2(N∧N)]Cl. By simultaneously and
independently varying these two ligands groups, it is possible
to monitor photophysical changes as a function of ligand
structure in the coordination sphere. The compounds in this
library were synthesized using 10 cyclometalating and 10
diimine-type ligands (Figure 1) [where C∧N ) ppy, benzo[h]-
quinoline (bhq), 2-thienylpyridine (thpy), 4,5′-di-tert-butyl-2-
phenylpyridine (dtbppy), and six 5-methyl-2-(4-“R”-phenyl)-
pyridines (R-mppy); and N∧N ) bpy, dtbbpy, 4,4′-di-methyl-
2,2′-dipyridyl (4,4′-dmbpy), 5,5′-di-methyl-2,2′-dipyridyl (5,5′-
dmbpy), 1,10-phenanthroline (phen), 4-methyl-1,10-phenanthroline
(4-Me-phen), 5-methyl-1,10-phenanthroline (5-Me-phen), 3,4,7,8-
tetra-methyl-1,10-phenanthroline (Me4-phen), bis-(diphenylphos-
phino)ethylene (dppe), and dipyridophenazine (dppz)]. A parallel
synthetic approach was employed, and photophysical properties
were measured directly from the reaction mixtures. The ef-
fectiveness of this methodology was assessed by comparing the
resulting photophysical data to the data that were obtained for
a small subset of analogous complexes prepared by traditional
methods. Efforts to relate the structure of a given complex to
its luminescent behavior using static DFT calculations were also
considered and are presented below.
Experimental Section
One hundred iridium(III) and 10 ruthenium(II) complexes were
synthesized using parallel synthetic techniques, and 11 control com-
plexes were prepared using traditional methods. A generic reaction
scheme for the synthesis of mixed-ligand iridium(III) complexes is
presented in Figure 2. All solvents and reagents for these procedures
were purchased from Aldrich and used without further purification.
Synthesis of Tetrakis-(C∧N)-µ-(dichloro)-diiridium(III). Sto-
ichiometric quantities of the appropriate cyclometalating ligand (ligand
synthesis is described in the Supporting Information) were added to a
mixture of iridium chloride trihydrate hydrochloride in a methoxyetha-
nol:water (5:1) solution. The reaction vessel was heated under reflux
(120 °C) with constant stirring for 12 h. The resulting precipitate was
collected by suction filtration, washed with water and diethyl ether,
and dried to yield the product, [(C∧N)2Ir-µ-Cl]2 (yield: 72-85%).
Synthesis of Control Complexes (“Traditional Synthesis”):
[(N∧N)-Bis-(C∧N)-iridium(III)] Hexafluorophosphate. The dichloro-
bridged dimer (0.1 mmol) was permitted to react with the appropriate
neutral ligand (0.22 mmol) in ethylene glycol (5.0 mL) under reflux
(150 °C) with constant stirring for 15 h. Upon cooling to room
temperature, the mixture was transferred to a separatory funnel with
water (3 × 30 mL) and washed with diethyl ether (3 × 50 mL) (except
in the case of C∧N ) dtbppy where hexanes (3 × 50 mL) were used
to wash the mixture). The aqueous layer was heated to 70 °C for 5
min using a heat gun to remove residual organic solvents. The vessel
was then placed on ice, and 10 mL of aqueous ammonium hexafluo-
9
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Accelerated Luminophore Discovery
A R T I C L E S
Figure 2. Synthetic pathway for the preparation of ionic iridium(III) chromophores. The boxed portion of the figure represents the step that is explored
through parallel synthesis.
rophosphate solution (1.0 g in 10 mL of deionized water) was slowly
added to the reaction mixture, yielding a colored suspension. The
precipitate was collected by suction filtration and allowed to air-dry
overnight. This product was recrystallized by acetonitrile:diethyl ether
vapor diffusion (except for C∧N ) dtbppy, where hexane:acetone vapor
diffusion was used instead) and dried in vacuo (100 °C) for 24 h to
yield the pure product, [Ir(C∧N)2(N∧N)](PF6) (yield: 68-80%).
reaction mixtures were subsequently diluted with 10.0 mL of acetonitrile
and transferred to a 1.0 cm quartz cuvette for the collection of absorption
data.
Traditional Synthesis. Solutions of each monomeric complex [25
µM] were prepared in acetonitrile. Aliquots of each solution were
transferred to a 1.0 cm capped quartz cuvette and degassed with nitrogen
for 10 min before measuring their emission, absorption, and excited-
state lifetime.
Structural Identification. Structures for the 11 traditional synthesis
Quantum Yield. The emission quantum yield (Φem) was calculated
for each complex according to the equation:20
products were confirmed using NMR and MS data. A Varian Inova-
1
500 spectrometer was used to collect all H and 13C NMR spectra. A
Hewlett-Packard 5898B (Electrospray) MS Engine was used to measure
the mass spectra. These results are summarized in the Supporting
Information.
Φs ) Φr • (Is/Ir) • (Ar/As)
(1)
Parallel Synthesis Protocol: [(N∧N)-Bis-(C∧N)-iridium(III)] Chlo-
ride. Homogeneous stock solutions of “n” iridium dichloro-bridged
dimers [4 mM] were prepared in ethylene glycol, and stock solutions
of “m” neutral ligands [8 mM; N∧N] were prepared in acetonitrile. In
each of the (n × m) reaction vessels (medium test tubes), 250 µL of
the appropriate dimer solution (0.001 mmol) was reacted with 270 µL
of neutral ligand solution (0.0022 mmol) in ethylene glycol (750 µL)
at 150 °C for 15 h to give the product, [Ir(C∧N)2(N∧N)]Cl. Upon
cooling to room temperature, all (n × m) products were available for
screening and spectroscopic characterization.
where Φs is the quantum yield of the sample, As and Ar are the
absorbance of the sample and the reference at the excitation wavelength,
and Is and Ir represent the points of maximum intensity in the corrected
emission spectra. Φr is the quantum yield for the reference complex;
[Ru(bpy)3](PF6)2 (Φ ) 6.20%) is the reference for the control products,
[Ir(ppy)2(bpy)]Cl (Φ ) 6.22%) is the reference for the parallel synthesis
products, and [Ru(bpy)3]Cl2 (Φ ) 6.20%) is the reference for the
ruthenium combinatorial products.1-6
Computational Studies. Hybrid density functional calculations
(B3LYP/LANL2DZ) were performed by using Gaussian 98.21 The built-
in default thresholds for gradient convergence were employed, but the
threshold for wave function convergence was slightly relaxed [SCF)-
(CONVER)7)]. All geometry optimizations were carried out under
the constraint of C2 symmetry.
Parallel Synthesis of [Ruthenium(II) tris-(N∧N)] Dichloride
Complexes. Homogeneous stock solutions of ruthenium(III) chloride
[1 mM, ethylene glycol] and the 10 neutral ligands [8 mM, acetonitrile]
were prepared. The metal solution (1.0 mL, 0.001 mmol) reacted with
the appropriate ligand solution (337 µL, 0.0033 mmol) at 150 °C for
15 h to give the product, [Ru(N∧N)3]Cl2. Upon cooling to room
temperature, all 10 products were available for screening and spectro-
scopic characterization.
Results and Discussion
Ligand Synthesis. Five novel cyclometalating ligands (C∧N
) dtbppy, F-mppy, Cl-mppy, Br-mppy, Ph-mppy) were syn-
thesized. Each of these five new systems was prepared by
following a modified literature procedure, utilizing either the
Kro¨hnke pyridine synthesis22 (“R”-mppy) or the Suzuki coupling
reaction (dtbppy).23,24 Details for the synthesis and characteriza-
tion of these systems are described in the Supporting Informa-
tion.
Screening and Photophysical Characterization. UV spectra were
recorded at room temperature in a 1.0 cm quartz cuvette using a
Hewlett-Packard 8453 spectrometer equipped with a diode-array
detector.
Emission spectra were recorded using a Jobin-Yvon Fluorolog-3
spectrometer equipped with double monochromators and a Hamamatsu-
928 photomultiplier tube (PMT) as the detector. Front face detection
was used for the combinatorial experiments, and right angle detection
was used for the control experiments. All complexes were excited at
400 nm (except for members of the dppe-series, which were excited at
360 nm). All emission spectra were adjusted according to the calibrated
correction factors of the instrument.
(20) Parker, C. A. Measurement of Fluorescence Efficiency; Elsevier Publishing
Co.: New York, 1968; pp 261-269.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98; Gaussian, Inc.: Pittsburgh, PA, 1998.
Excited-state lifetimes were measured using the emission mono-
chromator and PMT detector of the Jobin-Yvon Fluorolog-3 spectrom-
eter. The samples were excited at 337 nm with an N2 laser (Laser
Science, Inc. VSL-337LRF, 10 ns pulse), and the emission decay was
recorded using a Tektronix TDS 3032B digital phosphor oscilloscope.
Sample Preparation: Parallel Synthesis. All 110 samples were
diluted with 5.0 mL of acetonitrile, the reaction vessels were sealed
with rubber septa, and the samples were degassed with nitrogen for 10
min prior to emission and excited-state lifetime analyses. These
parameters were measured directly from the reaction vessel. The
(22) Gianini, M.; Forster, A.; Haag, P.; von Zelewsky, A.; Stoeckli-Evans, H.
Inorg. Chem. 1996, 35, 4889-4895.
(23) Ali, N. M.; McKillop, A.; Mitchell, M. B.; Tebelo, R. A.; Wallbank, P. J.
Tetrahedron 1992, 48, 8117-8126.
(24) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 207-210.
9
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A R T I C L E S
Lowry et al.
the measured emission energy, quantum yield, and excited-state
lifetime are approximately the same regardless of which method
was used to prepare the luminophoress and that parallel synthesis
is viable for the production of transition-metal complexes.
The library was subsequently expanded to include 76 new
mixed-ligand iridium(III) complexes and 10 tris-diimine ruthe-
nium(II) complexes; six additional control species were also
prepared. To confirm that there were no systematic errors
between the initial subset and the expanded library, an internal
standard, [Ir(ppy)2(dtbbpy)]Cl, was synthesized and character-
ized both times. The emission maxima (λ ) 570 nm, 570 nm),
corrected quantum yields (Φem ) 17.51%, 17.89%), and excited-
state lifetimes (τ ) 617 ns, 621 ns) are extremely close and are
consistent with the values reported by Slinker et al. for the pure,
PF6-salt of this complex.16
Figure 3. An application of the energy gap law to the compounds that
were prepared in the initial subset and in the full expanded library. The
matching linear trends for the subset and the expanded library as well as
their close correlation to the 11 control points suggest that the complexes
have been accurately prepared and consistently measured.
To construct an energy gap plot for the expanded library,
the thpy-series and the dppz-series had to be excluded. As was
the case for the initial subset, the thpy-complexes were
eliminated because they constantly fluoresced at a single energy
across multiple N∧N ligands. Similarly, the members of the
strongly absorbing dppz-series could not be included because
their excited states were too short-lived to be measured by our
instrument. A plot featuring the remaining 81 iridium(III)
complexes shows that the library conforms to the energy gap
law and that the new data are consistent with the first subset. A
matching linear relationship was also observed for the control
complexes (Figure 3).
Evaluating the Combinatorial Approach. A subset of 24
complexes from the 100-member iridium(III) library was
initially prepared to examine the effectiveness of the combina-
torial techniques. This subset was prepared using four cyclo-
metalating ligands (ppy, dtbppy, bhq, and thpy) and six neutral
ligands (bpy, dtbbpy, 5,5′-dmbpy, phen, 4-Me-phen, 5-Me-
phen); each of the resulting 24 iridium(III) complexes was
characterized for its emission energy, quantum yield, and
excited-state lifetime. The results of these analyses were
considered with respect to their adherence to the energy gap
law and their closeness to the values obtained from five
analogous control complexes.
An energy gap plot (Figure 3) was prepared for these 24
complexes (and later extended to include the rest of the library)
by plotting the natural log of the nonradiative decay rate constant
(knr) as a function of emission energy. The value of knr can be
calculated according to the equation:25
The normalized absorption and emission spectra of the
complexes in the expanded library closely match those of the
controls. These strong spectral similarities (e.g., band-shapes
and emission maxima) suggest that the appropriate complexes
were prepared during the parallel synthesis. Examples of
overlaid spectra are presented in the Supporting Information.
A summary of the photophysical properties for the control
complexes with their corresponding combinatorial products is
presented in Table 1. It is worth noting that the use of different
counterions (Cl-1 vs PF6-1) and slightly different solvent
systems between the combinatorial and traditional methods did
not significantly impact these results.
knr ) 1/τ - kr ) [(1 - Φem)/τ]
(2)
where the radiative rate constant (kr) is equal to the ratio of the
quantum yield over the emission lifetime (τ, in s). Because all
three relevant photophysical parameters are taken into account,
conformity to the energy gap law (a linear plot with negative
slope) suggests that the data have been consistently and
accurately measured. Perhaps equally important, it is easy to
identify which reaction mixtures and measurements are incon-
sistent by locating outliers in the plot.25
Along these lines, the thpy-series had to be excluded from
the initial subset because it did not follow the same linear trend
as the other 18 data points. It is currently hypothesized that the
reaction of [(thpy)2Ir-µ-Cl]2 with a neutral ligand does not yield
the desired product under the given combinatorial conditions.
The relatively constant emission energy of these complexes (592
nm) across different N∧N ligands as well as the small Stokes
shift exhibited by the thpy-series suggests that a fluorescent
byproduct of the reaction is being probed rather than the desired
target complex.
Photophysical Observations. As can be seen in Table 1, a
broad range of values were revealed for the emission energy,
quantum yield, and excited-state lifetime of the complexes in
the expanded library. Three-dimensional plots of these properties
for the complete library are shown in Figure 4. Due to the order
of magnitude deviations between the highest and lowest lifetime,
these values were plotted on a logarithmic scale. The results
for all 110 complexes are tabulated in the Supporting Informa-
tion.
Perhaps the most intriguing observation was the color
versatility expressed by the complexes in this library (Figure
4). The emission maxima spanned from blue for the dppe-series
(450 nm) to red-orange (615 nm) for the ruthenium(II)-series
with corrected photoluminescent yields ranging from 0.02% to
32.01% and lifetimes that span from nano- to microseconds.
Such properties will be extremely useful for introducing the
visible spectrum to single-layer, ionic OLED displays. A
collection of novel luminophores were targeted from this library
for their interesting photophysical properties and have been
tested in optoelectronic devices. These results will be reported
in a future publication.26
The remaining 18 complexes obey the energy gap law,
demonstrating a linear relationship that perfectly corresponds
with the five controls. This matching relationship implies that
(25) Kalyanasundaram, K.; Photochemistry of Polypyridine and Porphyrin
Complexes; Academic Press: New York, 1992; pp 172-173.
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Accelerated Luminophore Discovery
A R T I C L E S
Table 1. A Comparison of the Photophysical Properties of the
rapidly become commonplace for accelerated luminophore
discovery. The techniques that have been presented above will
be of particular importance for continuing research with ionic,
mixed-ligand iridium(III) systems and will also have extensions
with different luminophore complexes. For instance, the neutral,
heteroleptic iridium(III) complexes6,7 investigated by Thompson
et al. can be prepared by following a similar synthetic
pathway: A dichloro-bridged dimer is reacted with an ancillary
ligand to yield the desired product.
Products from the Expanded Library (Cl-1) and Their Analogous
-1
a
Controls (PF6
)
1
-1
sample complex
Cl-
PF6
[Ir(ppy)2(bpy)]+1
λ ) 583 nm
Φ ) 6.22 ( 0.28%
τ ) 329 ( 13 ns
Φ ) 7.07 ( 0.39%
τ ) 269 ( 11 ns
[Ir(ppy)2(dtbbpy)]+1
λ ) 570 nm
Φ ) 17.89 ( 0.82% Φ ) 17.16 ( 1.10%
τ ) 495 ( 20 ns
Φ ) 4.66 ( 0.21%
τ ) 424 ( 17 ns
τ ) 621 ( 25 ns
Φ ) 4.46 ( 0.20%
τ ) 258 ( 11 ns
[Ir(dtbppy)2(bpy)]+1
λ ) 597 nm
[Ir(dtbppy)2(dtbbpy)]+1
λ ) 583 nm
Φ ) 11.84 ( 0.54% Φ ) 9.47 ( 0.47%
τ ) 389 ( 16 ns τ ) 444 ( 18 ns
Φ ) 20.12 ( 0.92% Φ ) 20.53 ( 0.94%
τ ) 877 ( 36 ns τ ) 1166 ( 48 ns
Φ ) 12.49 ( 0.78% Φ ) 12.11 ( 0.55%
τ ) 685 ( 28 ns τ ) 877 ( 36 ns
Φ ) 27.96 ( 1.28% Φ ) 37.68 ( 3.17%
τ ) 1604 ( 66 ns τ ) 1866 ( 77 ns
Φ ) 26.99 ( 2.05% Φ ) 26.27 ( 1.24%
τ ) 1080 ( 44 ns τ ) 1221 ( 50 ns
The ruthenium(II) complexes that were prepared in the current
library demonstrate the ability to apply this technique to other
metals. The tris-N∧N-series was selected for this study because
of its agreement with the reaction conditions (ethylene glycol,
150 °C, 15 h), but the synthetic protocol could have easily been
modified to accommodate a diverse range of solvents and
reaction conditions. Other suitable candidates for this approach
include rhenium(I),14,15,27,28 platinum(II),22,29,30 osmium(II),12 and
rhodium(III).31-33
[Ir(ppy)2(5,5′-dmbpy)]+1
λ ) 558 nm
[Ir(ppy)2(phen)]+1
λ ) 577 nm
[Ir(ppy)2(Me4-phen)]+1
λ ) 534 nm
[Ir(F-mppy)2(dtbbpy)]+1
λ ) 543 nm
[Ir(F-mppy)2(Me4-phen)]+1 Φ ) 28.15 ( 1.29% Φ ) 44.99 ( 4.70%
λ ) 506 nm
τ ) 3026 ( 124 ns
Φ ) 29.10 ( 1.33% Φ ) 30.12 ( 2.44%
τ ) 1378 ( 56 ns
Φ ) 7.11 ( 0.41%
τ ) 980 ( 40 ns
τ ) 5884 ( 241 ns
[Ir(Br-mppy)2(dtbbpy)]+1
λ ) 537 nm
3
3
Excited-State Characteristics. MLCT and π-π* ligand-
centered transitions are commonly observed in a vast majority
of the presently investigated mixed-ligand transition-metal
complexes. While it has been shown that there is a strong degree
of mixing between these two emissive states in the [Ir(ppy)2-
(bpy)](PF6) model,19 excited-state character has not been
extensively studied with other ligand combinations. In [Ir(ppy)2-
τ ) 1256 ( 52 ns
Φ ) 6.29 ( 0.30%
τ ) 990 ( 42 ns
[Ru(dtbbpy)3]+2
λ ) 614 nm
a These values are remarkably similar, and they support the notion that
combinatorial results can be used as accurate predictors of the emission
energy, quantum yield, and excited-state lifetime of pure, traditionally
synthesized materials.
3
The development of such a structurally diverse library would
have taken considerably longer using traditional methods, and,
as such, we believe that the use of combinatorial chemistry will
(bpy)](PF6), the MLCT transition arises from the promotion
of an electron from a filled t2g-orbital on the metal to a vacant
π*-orbital on the neutral, aromatic bpy-ligand. The ligand-
Figure 4. Three-dimensional plots for the (A) emission energy, (B) quantum yield, and (C) log(excited-state lifetime) for 100 iridium(III) complexes with
the form [Ir(C∧N)2(N∧N)]Cl. C∧N ) ppy (1), dtbppy (2), bhq (3), thpy (4), F-mppy (5), Cl-mppy (6), Br-mppy (7), MeO-mppy (8), Ph-mppy (9), H-mppy
(10). N∧N ) dppz (a), dppe (b), Me4-phen (c), 5-Me-phen (d), 4-Me-phen (e), phen (f), 5,5′-dmbpy (g), 4,4′-dmbpy (h), dtbbpy (i), bpy (j). As can be seen
in panel (D), these complexes exhibit remarkable luminescent color versatility, spanning from blue to orange-red emission.
9
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A R T I C L E S
Lowry et al.
centered 3π-π* transition typically involves the movement of
electrons between filled and vacant π-orbitals on the cyclom-
etalating ppy-ligand.18,19,34
To understand the photoluminescent properties of mixed
3
3
systems, it is important to observe pure MLCT and π-π*
systems first. Thus, the ruthenium(II)- (containing only N∧N
ligands) and dppe-complexes (containing no aromatic diimine
ligands) were incorporated into the current library, respectively.
All of the remaining complexes in the library have mixed excited
states due to the presence of both cyclometalating and aromatic,
diimine ligands. Trisubstituted ruthenium(II) was used in place
of trisubstituted iridium(III) because the ruthenium complexes
exhibit only 3MLCT transitions, whereas their iridium analogues
exhibit mixed-states with strong 3π-π* character.32 Moreover,
the synthesis of the tris-diimine ruthenium species was also more
compliant with the given reaction conditions.33
Figure 5. The evolution of a mixed excited state from 3π-π* (dppe) and
3MLCT (ruthenium complex). The series shown represents the normalized
emission spectra for a series of [Ir(Br-mppy)2(N∧N)]Cl complexes, where
N∧N ) dppe, Me4-phen, 5,5′-dmbpy, and bpy. [Ru(bpy)3]Cl2 is included
From observation of the luminescence spectra of pure 3MLCT
3
and π-π* species, certain defining features become evident.
3
due to its strict MLCT character. Changes in the character of the excited
3
For instance, the MLCT emission bands tend to be broader
state can be monitored by the shape and position of the emission band,
and lower in energy than the vibrationally structured, blue-
shifted 3π-π* bands. These spectral differences make it possible
to assess the contribution of each transition-type to light
emission in mixed-state systems. Although a quantitative method
for describing the degree of mixing has yet to be determined,
the current library shows us that this value is strongly ligand-
dependent. The convolution of a mixed state from pure 3π-π*
where blue-shifted dual bands are indicative of more 3π-π* character and
3
a broad, red-shifted band suggests more MLCT character.
3
and MLCT excited states is depicted in Figure 5.
By comparing the photophysical properties of the pure
systems with those from the mixed-state systems, it further
becomes clear that a simple correlation between the two does
not exist. For instance, Figure 6 demonstrates the relationship
between the emission energy of the pure transitions and their
closest comparator from the mixed-state systems. These com-
parators were defined as the systems with the strongest spectral
similarities to ruthenium(II) and dppe across all N∧N or C∧N
Figure 6. A comparison of the energy values obtained for the pure 3MLCT
Ru(II)-series (orange [) with its nearest mixed-state analogue (MeO-mppy),
as well as for the pure 3π-π* dppe-series (blue 2) with its nearest mixed-
state analogue (Me4-phen). As can clearly be seen in the plot, a simple
correlation between the pure and the mixed state species does not exist.
3
variables (i.e., MeO-mppy was used for the MLCT and Me4-
library of such complexes is necessary, and no accurate
predictions of structure-property relationships can currently be
made.
phen was used for the 3π-π*). The nonlinear relationship
between these data suggests that it may not be feasible to predict
the luminescent properties of a mixed-state system through
simple extrapolation from the pure excited-state materials and
that a more convoluted interplay between these states exists.
The electronic structure and orbital geometries of the ligands,
as well as the manner in which those ligands interact with each
other, dictate the emissive properties of the complex. Due to
the fact that it is not a priori clear how these traits will be
manifest in a given system, the study of a molecularly diverse
Computational Prediction. To more efficiently plan future
libraries, the measured emission maxima of selected complexes
were correlated with energy differences obtained from static
DFT calculations (B3LYP/LANL2DZ). The consistency of the
spectroscopic measurements in the library facilitates such
comparisons. Three different strategies were used on seven
complexes with phosphorescence maxima ranging from 2.07
eV (600 nm) to 2.53 eV (487 nm). The fastest calculations
involved the optimization of the singlet geometry of the
complexes and the correlation of the HOMO-LUMO gap to
the luminescence maxima. The assumption that the singlet-
triplet splitting energy and the nuclear rearrangements are similar
in this family of complexes was confirmed by the overall linear
relationship depicted in Figure 7. The only exception is the [Ir-
(F-mppy)2(dppe)]+ complex, which exhibits a pure π-π*
transition and is, therefore, likely to have a larger singlet-triplet
splitting energy than the complexes with mixed excited states.
In a second method, the triplet geometry was optimized, and
the difference in the total energy of the triplet and singlet states
at this geometry was correlated with the observed emission
maxima. The third approach was based on the work by
(26) Slinker, J. D.; Lowry, M.; Parker, S.; Bernhard, S.; Malliaras, G. G.,
manuscript in preparation.
(27) Li, F.; Zhang, M.; Cheng, G.; Feng, J.; Zhao, Y.; Ma, Y.; Liu, S.; Shen,
J. Appl. Phys. Lett. 2004, 84, 148-150.
(28) Ranjan, S.; Lin, S.; Hwang, K.; Chi, Y.; Ching, W.; Liu, C. Inorg. Chem.
2003, 42, 1248-1255.
(29) Brooks, J.; Yelizaveta, B.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau,
R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055-3066.
(30) DePriest, J.; Zheng, G. Y.; Goswami, N.; Eichhorn, D. M.; Woods, C.;
Rillema, D. P. Inorg. Chem. 2000, 39, 1955-1963.
(31) Frei, G.; Zilian, A.; Raselli, A.; Gu¨del, H. U.; Burgi, H.-B. Inorg. Chem.
1992, 31, 4766-4773.
(32) Ghizdavu, L.; von Zelewsky, A.; Stoeckli-Evans, H. Eur. J. Inorg. Chem.
2001, 993-1003.
(33) Ghizdavu, L.; Kolp, B.; von Zelewsky, A.; Stoeckli-Evans, H. Eur. J.
Inorg. Chem. 1999, 1271-1279.
(34) Wilde, A. P.; King, K. A.; Watts, R. J. J. Phys. Chem. 1991, 95, 629-
634.
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14134 J. AM. CHEM. SOC. VOL. 126, NO. 43, 2004

Accelerated Luminophore Discovery
A R T I C L E S
coherence between the experimental data and the controls
confirms that the appropriate complexes were prepared and that
the photophysical properties of pure complexes can be predicted
from their crude analogues. Thus, combinatorial methods will
be extraordinarily useful for accelerating the discovery of
luminophore complexes. Similarly, the use of combinatorial
techniques will be economically shrewd and environmentally
friendly because it will reduce the expenditure of resources while
also dramatically decreasing the amount of waste generated
during multiple syntheses.
From a materials science perspective, the combinatorial
approach has proven to be a valuable tool for evaluating the
relationship between ligand structure and emissive behavior.
This technique is not limited to the arena of OLED discovery,
however, and it may have extensions in a multitude of other
scientific avenues. For instance, TiO2-based photovoltaic cells,37-39
photoinduced water-splitting,40,41 biomolecular probes,42,43 and
chiral luminophores44-46 all present intriguing opportunities for
further exploration.
Figure 7. Correlation of the experimental luminescence maxima with the
results of DFT calculations. Red b: The HOMO-LUMO gap at optimized
singlet geometry. Green 9: The difference of the absolute energy of the
triplet configuration and the singlet configuration at the optimized triplet
geometry. Blue 2: The orbital energy difference of the HSOMO (triplet)
and the HOMO (singlet) calculated at the triplet geometry. The data points
represent the following: 1 [Ir(MeO-mppy)2(bpy)]+, 2 [Ir(mppy)2(bpy)]+,
3 [Ir(mppy)2(4,4′-dmbpy)]+, 4 [Ir(F-mppy)2(bpy)]+, 5 [Ir(F-mppy)2(5,5′-
dmbpy)]+, 6 [Ir(F-mppy)2(Me4-phen)]+, and 7 [Ir(F-mppy)2(dppe)]+.
Acknowledgment. This work was supported in part by a
Camille and Henry Dreyfus Foundation New Faculty Award,
which is gratefully acknowledged. This work was partially
supported by the NSF through grant DMR-0213706 to the
MRSEC-Princeton Center for Complex Materials.
Thompson et al. in which the difference of the HOMO energy
of the singlet and the highest singly occupied molecular orbital
(HSOMO) energy of the triplet was used to predict the
maxima.29,35 Again, both orbital energies were calculated at the
optimized nuclear geometry of the triplet state. Data from the
triplet state geometry optimization result in more consistent
predictions, independent of the nature of the considered excited
state.
Overall, static DFT calculations offer an acceptable prediction
of the phosphorescence energy. However, no information
regarding the excited-state lifetimes and phosphorescence
quantum efficiency can be obtained from such DFT calculations.
To this end, an adaptive substituent reordering algorithm is
currently being explored that would interpolate the emission
energy, quantum yield, and excited-state lifetime of new
materials based on structure-property relationships.36
Supporting Information Available: Procedures for the syn-
thesis of five novel cyclometallating ligands, seven dichloro-
bridged diiridium(III) dimers, and nine iridium(III) hexafluo-
rophosphate monomers. 1H NMR chemical shifts, coupling
constants, and peak identifications for all of the traditionally
prepared species. 13C NMR and MS data for the traditionally
prepared monomeric complexes. Normalized emission spectra
for select library luminophores and their traditional analogues.
Tables of emission energy, photoluminescent quantum yield,
and excited-state lifetime for (n × m) complexes in the
combinatorial library and 10 ruthenium(II) species. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA047156+
(37) Hou, Y.; Xie, P.; Zhang, B.; Cao, Y.; Xiao, X.; Wang, W. Inorg. Chem.
1999, 38, 6320-6322.
(38) Anandan, S.; Latha, S.; Maruthamuthu, P. J. Photochem. Photobiol., A
2002, 150, 167-175.
Conclusion
(39) Mosurkal, R.; Kim, Y.; Kumar, J.; Li, L.; Walker, J.; Samuelson, L. A. J.
Macromol. Sci., Pure Appl. Chem. 2003, A40, 1317-1325.
(40) Harriman, A.; West, M. A. Photogeneration of Hydrogen; Academic
Press: New York, 1982.
The luminescent properties that were measured for this library
of novel luminophores are astonishingly accurate considering
that the products were analyzed directly from their reaction
vessels without further workup or purification. The strong
(41) Koelle, U. New J. Chem. 1992, 16, 157-169.
(42) Lo, K. K.; Chung, C.; Lee, T. K.; Lui, L.; Tsang, K. H.; Zhu, N. Inorg.
Chem. 2003, 42, 6886-6897.
(43) Lo, K. K.; Chung, C.; Zhu, N. Chem.-Eur. J. 2003, 9, 475-483.
(44) Ghizdavu, L.; Lentzen, O.; Schumm, S.; Brodkorb, A.; Moucheron, O.;
Kirsh-De Mesmaeker, A. Inorg. Chem. 2003, 42, 1935-1944.
(45) Ziegler, M.; von Zelewsky, A. Coord. Chem. ReV. 1998, 177, 257-300.
(46) Mu¨rner, H.; Belser, P.; von Zelewsky, A. J. Am. Chem. Soc. 1996, 118,
7989-7994.
(35) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.;
Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377-
7387.
(36) Liang, F.; Feng, X.; Lowry, M.; Bernhard, S.; Rabitz, H., manuscript in
preparation.
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