Synthesis and characterization of a multicomponent rhenium(I) complex for application as an OLED dopant

Article abstract of DOI:10.1002/anie.200504532

(Figure Presented) Three's not a crowd: A modular approach was used to prepare 1, an electroactive molecular material that integrates distinct electron-transporting, hole-transporting, and emissive units (see picture). The trifunctional molecule holds pro

Full text of DOI:10.1002/anie.200504532

Communications
Electrooptics
complex, which acts as the charge-recombination and emis-
sion center.[5,6] Maximization of the brightness of a device is a
key consideration in the production of OLEDs, which is why
phosphorescent metal complexes are favored over fluorescent
organic materials in many situations.[7] The dopant used also
greatly affects the wavelength and broadness of the emission,
as well as the efficiency of the device, so that an OLED may
be tuned by appropriate selection of the metal center and
ligand(s).
Herein, we describe our endeavors to design an electro-
active material composed of simple modular units, each of
which performs a specific function, namely electron transfer,
hole transfer, and emission.Furthermore, we chose units that
DOI: 10.1002/anie.200504532
Synthesis and Characterization of a
Multicomponent Rhenium(i) Complex for
Application as an OLED Dopant**
Natasha J. Lundin, Allan G. Blackman,
Keith C. Gordon,* and David L. Officer
Dedicated to Professor John J. McGarvey
may be easily derivatized, thereby giving great flexibility to
[8]
Current studies in the field of organic light-emitting diodes
(OLEDs) are aimed at improving display technology through
production of devices with increased efficiency, flexibility, and
long lifetimes.[1] An OLED converts electrical energy into
light by allowing holes and electrons, which are transported
from opposing electrodes, to combine to form an exciton, or
localized excited state, in an emissive material.Subsequent
this design strategy.Wong et al.
and Gong et al.[9] have
synthesized integrated molecules containing all three func-
tionalities and shown that they are effective as a single charge-
transport and emitting layer in OLEDs.Trifunctional poly-
mers have also been prepared.[10] We designed and synthe-
sized a trifunctional molecule that comprises the key units for
an OLED material: an emissive chromophore based on a ReI
polypyridyl complex containing dipyrido[3,2-a:2’,3’-c]phena-
zine (dppz), an electron-transporting 1,3,4-oxadiazole group,
relaxation of the exciton results in the emission of light.[1]
A
successful OLED requires facile and balanced charge trans-
port as well as a high conversion efficiency of excitons to
light.[2] As electrons and holes are transported through the
lowest unoccupied molecular orbitals (LUMOs) and highest
occupied molecular orbitals (HOMOs), respectively, in
OLED components, the relative band gap and HOMO and
LUMO energies of these materials is of crucial importance to
the efficiency and brightness of an OLED.[3] Balanced charge
injection is required so that charge recombination does not
occur close to the electrode/polymer interface, because a
metal electrode can deactivate excitons, lowering the effi-
ciency of electroluminescence of the device.[2]
I
and a hole-transporting terthiophene unit.Re polypyridyl
complexes are known to be highly emissive[11,12] and have
been used as emitting dopants in a variety of OLED
structures.[13–15] 1,3,4-Oxadiazole heterocycles are widely
used as electron-transporting groups owing to their electron
deficiency and good thermal stability.[2] The terthiophene
subunit can act as a hole-transport group[16] and can also be
easily functionalized and polymerized.[17] These units were
linked to form a trifunctional molecule as outlined in
Scheme 1.
Ligand 2-(11-dipyrido[3,2-a:2’,3’-c]phenazine)-5-phenyl-
To fulfill these requirements, OLEDs often contain a
number of layers that individually perform the roles of charge
transport and light emission.[4] Each layer requires materials
with the correct HOMO and LUMO energies to either
promote or disrupt the flow of charge in the device.In many
devices the emissive layer consists of a polymer such as
poly(N-vinylcarbazole) (PVK) doped with a transition-metal
1,3,4-oxadiazole (2), which was prepared from dppz-
+
CO2H,[18] was treated with [Re(CO)5Cl] to give fac-3.Ag
-
induced solvolysis of 3 in MeCN[19] and reaction of the
resulting complex 4 with (E)-1-((2,2’:5’,2’’-terthiophen)-3’-yl)-
2-(4’-pyridyl)ethane (5) gave the multifunctional complex 6 as
the hexafluorophosphate salt (Scheme 1).Elemental analysis
suggested the presence of a single molecule of solvent (ethyl
1
acetate), and both H and 13C NMR spectra were consistent
with the desired product.Despite numerous attempts using
different solvents and counterions, we were unable to grow
crystals of 6 of suitable quality for X-ray analysis.
[*] N. J. Lundin, Dr. A. G. Blackman, Prof. K. C. Gordon
Department of Chemistry
The MacDiarmid Institute for Advanced Materials and Nano-
technology
University of Otago
P.O. Box 56, Dunedin (New Zealand)
Fax: (+64)3-479-7906
E-mail: kgordon@chemistry.otago.ac.nz
Spectroscopic data for dppz, 2, 3, 5, and 6 are presented in
Table 1 and Figure 1.The ligand dppz is commonly thought to
contain partitioned orbitals, as it displays limited communi-
cation between the phenazine and bipyridyl sections of the
ligand.[20,21] If ligand 2 behaves in a similar manner, it is
expected that introduction of an electron-withdrawing oxa-
diazole moiety, or coordination of a metal center, should have
little effect on the properties of the ligand.However, both
ligand substitution and metal coordination resulted in a shift
in the absorption maxima.The emission maximum of 2 is at
the same wavelength as that of dppz but exhibits a higher
quantum yield of emission.Complexes 3 and 6 both display
broader emission bands than 2, showing the influence of the
rhenium(i) center.The absorption spectra of 3 and 6 show tails
Prof. D. L. Officer
Department of Chemistry
Nanomaterials Research Centre
Massey University
P.O. Box 11222, Palmerston North (New Zealand)
[**] We acknowledge financial support from the MacDiarmid Institute
for Advanced Materials and Nanotechnology. OLED=organic light-
emitting diode.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Figure 1. Normalized absorption (A) and photoluminescence (PL) data
of dppz (c), 2 (a), 3 (g), 5 (b), and 6 (d). Data were
collected at room temperature from solutions (ꢀ10À5 m) in CH2Cl2
degassed with N2.
of 3 than of 5, which suggests that the HOMO and
LUMO of 6 are more similar to those of 3 than of 5.[24]
Electrochemical data for dppz, 2, 3, 5, and 6 are
presented in Table 2 and Figure 2.HOMO and LUMO
Scheme 1. Synthesis of the multicomponent molecule 6. dppz=dipyrido-
[3,2-a:2’,3’-c]phenazine; DMF=N,N-dimethylformamide; Tf=trifluoro-
methanesulfonyl.
Table 2: Electrochemical data for dppz, 2, 3, 5, and 6.
Compound
Reduction
potential
[mV][a]
Oxidation
potential
[mV][a]
E
(HOMO)
E(LUMO)
G
Table 1: Spectroscopic data for dppz, 2, 3, 5, and 6.
Compound lmax
[nm][a]
e
lPL Quantum Stokes shift
[nm][b] yield[c] [cmÀ1 [d]
A
]
G
]
dppz
À1600[26]
À851 (irr)
À746
n.d.
À6.22[27]
À5.87
À5.79
À5.35
À5.38
À2.96[27]
À3.31
À3.36
À3.13
À3.45
dppz
380
18000
1300
29000
21000
3400
427
427
497
487
474
0.009
0.040
0.001
0.130
0.003
2900
1600
2600
8700
3400
2
3
5
6
1690 (irr)
1620 (irr)
1110
2
3
5
6
400
388
342
409
À1030 (irr)
À648
1170 (irr)
[a] Potentials were measured with reference to decamethylferrocenium/
decamethylferrocene. Determined from solutions in dichloromethane
(1.0mm), containing tetrabutylammonium perchlorate (0.10m) as the
supporting electrolyte. Samples were purged with N2 for 5 min before
use. Scan rate: 100 MvsÀ1. n.d.=not determined; irr indicates an
irreversible process. [b] Calculated using the method of Li et al.[25]
[a] Data were collected at room temperature from solutions in CH2Cl2
(concentrations in the 10À5 m range) degassed with N2. [b] Maximum
wavelength of photoluminescence upon excitation at l=380 nm.
[c] Absolute values, calculated by the method of Wang et al.[22] [d] Calcu-
lated from the difference between the lowest energy absorption
maximum and the emission wavelength maximum.
energies were calculated from the electrochemical data by
known methods.[25] The introduction of an electron-with-
drawing oxadiazole group had the predicted effect, with
compounds 2 , ,3 and 6 being reduced at less-negative
potential than dppz.Complex 6 was reduced at less-negative
potential than its components, and its LUMO is correspond-
ingly lower in energy.This observation correlates with the
complex being easier to reduce than dppz, 2, 3, or 5 and thus a
suitable candidate for electron transport.The presence of a
terthiophene moiety in 5 and 6 was reflected in the oxidation
potentials of the compounds: Compounds 5 and 6 were
oxidized at less-positive potentials than dppz, 2, or 3,
indicating a greater propensity for 5 and 6 to lose an electron.
Resultantly, the HOMO energies of 5 and 6 are greater than
at lower energy, indicative of a metal–ligand state that is not
present in the free ligand.
The Stokes shifts of dppz, 2, 3, 5, and 6 were calculated in
wavenumbers from the difference between the lowest energy
absorption maximum and the emission maximum of each
compound.[23] The oxadiazole moiety decreases the size of the
Stokes shift of 2 relative to that of dppz.However, coordi-
nation of the rhenium(i) center increases the Stokes shift, as
seen by a comparison of the data for 3 and 6 with those of 2
(see Table 1).The large Stokes shift as observed for 5 may be
caused by significant structural differences between the
ground state and excited state, as has been predicted for 5
and related molecules.[16] The Stokes shift of 6 is closer to that
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2583

Communications
aimed at derivatizing the modular components and making
devices containing 6 and its derivatives.
Received: January 21, 2006
Keywords: electron transport · heterocycles ·
.
molecular electronics · rhenium
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allowing the electronic properties of the multifunctional
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