Inter-ligand energy transfer and related emission change in the cyclometalated heteroleptic iridium complex: Facile and efficient color tuning over the whole visible range by the ancillary ligand structure

Article abstract of DOI:10.1021/ja052880t

We report a novel color tuning methodology in the electrophosphorescent iridium complex by substituting one cyclometalating ligand to an ancillary ligand as an emitting center. Highly efficient exothermic inter-ligand energy transfer (ILET) from the MLCT<

Full text of DOI:10.1021/ja052880t

Published on Web 08/18/2005
Inter-Ligand Energy Transfer and Related Emission Change in the
Cyclometalated Heteroleptic Iridium Complex: Facile and Efficient Color
Tuning over the Whole Visible Range by the Ancillary Ligand Structure
Youngmin You and Soo Young Park*
School of Materials Science and Engineering, Seoul National UniVersity, San 56-1, Shillim-Dong, Kwanak-Gu,
Seoul 151-744, Korea
Received May 2, 2005; E-mail: parksy@snu.ac.kr
Scheme 1. Synthetic Procedure of Heteroleptic Iridium
Complexes and Their Structures
Initiated by the pioneering work of the Thompson and Forrest
group,1 enormous advances in cyclometalated iridium complexes
have been made in pursuit of a highly efficient electrophospho-
rescent organic light-emitting diode (OLED). Many different classes
of homoleptic (Ir(III)(C∧N)3) and heteroleptic (Ir(III)(C∧N)2 (LX))
iridium complexes have been developed, where (C∧N) is a
monoanionic cyclometalating ligand (e.g., 2-phenylpyridine, 2-phe-
nylbenzothiazole) and (LX) is an ancillary ligand (e.g., acetyl
acetonate, picolinate). It has been well demonstrated that structural
changes in the skeletal as well as the substituent groups of the
cyclometalating ligand (C∧N) afford significant color tuning of
electrophosphorescence. Therefore, the design and synthesis of
novel cyclometalating ligands have been enthusiastically investi-
gated as the main strategy of phosphorescence color tuning.2,3
Unfortunately, however, the synthesis of an electrophosphores-
cent iridium complex from the newly synthesized cyclometalating
ligand (C∧N) is not always feasible because the chloride-bridged
iridium dimer intermediate, (C∧N)2Ir(µ-Cl)2Ir(C∧N)2, is often
difficult to prepare for steric and electronic reasons. Therefore, at
least, in principle, emission color tuning by the ancillary ligand
structure in the heteroleptic iridium complex of the already known
cyclometalating ligand (C∧N) is considered a much easier, efficient,
and competitive approach. In this regard, heteroleptic iridium
complexes incorporating ancillary ligands, such as acetylacetonate,
picolinate, N-methylsalicylimine, triazolate, and tetrazolate deriva-
tives, have been investigated by many groups. However, discourag-
ingly, it was found that the emission color was largely governed
by the nature of the cyclometalating ligand, while the ancillary
ligand operated insignificant control.3,4
Recently, observation and calculational prediction of some color
tuning (over the 532-603 nm range) in heteroleptic iridium
complexes with 1-isoquinolinate and 3-isoquinolinate as emitting
ancillary ligands were reported.5 In addition to the small range of
color tuning and low phosphorescence quantum yield (<0.14),
however, no viable mechanism explaining the photophysical
processes based on inter-ligand energy transfer (ILET)6 was
suggested.
In this report, we demonstrate the full potential of a color tuning
methodology with an ancillary ligand and suggest its most probable
mechanism in heteroleptic iridium complexes. We employ the
common cyclometalating ligand 2-(2,4-difluorophenyl)pyridine
(dfppy) and show the blue to red range emission control in a series
of heteroleptic iridium complexes by using novel ancillary ligands.
Briefly, excitation from the iridium-centered HOMO to the cyclo-
metalating dfppy-centered LUMO (so-called metal to ligand charge
transfer (MLCT) transition) is followed by efficient inter-ligand
energy transfer (ILET) to the “emitting ancillary ligand”, providing
a novel channel of phosphorescence color tuning.
Scheme 1 illustrates the chemical structure and synthesis of
dfppy-based heteroleptic iridium complexes ((dfppy)2Ir(LX)) in-
vestigated in this work. Ancillary ligands with different band gap
energy were designed by changing the fused ring conjugation of
pyridine- and pyrazine-based ligand structures. A carboxylate group
was included as the metal chelating unit in six ancillary ligands.
Synthesis of two amide-containing ligands was accomplished in
order to assess the possible perturbation of HOMO levels in iridium
complexes.3e
Chloride-bridged dimer (dfppy)2Ir(µ-Cl)2Ir(dfppy)2 was synthe-
sized according to the procedure described in the literature.3a
A
sodium carbonate-mediated ligand exchange reaction with an
ancillary ligand was carried out to give a heteroleptic iridium
complex in a quantitative yield (see Supporting Information).
Photophysical characteristics of seven newly synthesized iridium
complexes (see Scheme 1 for designations) together with the
already-known complex FIrpic4c are summarized in Table 1.
Photographs showing the spectral tuning of photoluminescence (PL)
and related spectral features are included in Figure 1.7 It is clearly
noted that the emission color is broadly controlled from sky blue
(468 nm) to red (666 nm), depending on the ancillary ligand
structure, although their absorption spectra were almost the same.
This observation implies the presence of a specific energy transfer
process, most likely ILET, from the common excited states in a
series of these heteroleptic iridium complexes. All the high
phosphorescence quantum efficiencies (0.27-0.39) from six car-
boxylate-containing heteroleptic complexes compare well to that
of Ir(ppy)3 (0.40), suggesting a highly efficient ILET process. In
contrast with our expectations, however, insignificant HOMO level
modulation (see Table 1) and much lower phosphorescence
efficiencies (0.03 and 0.04 for FIrprza and FIrpca, respectively)
were obtained for amide-type ancillary ligands.8
9
12438
J. AM. CHEM. SOC. 2005, 127, 12438-12439
10.1021/ja052880t CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Photophysical Properties of Iridium Complexes
Emission
solution
λ
max (nm)
film
HOMO (eV)
Φp
∆E
LUMO (eV)a
FIrpic
FIrmpic
FIrqnd
FIrpca
FIriq
FIrprz
FIrprza
FIrqnx
468, 494
478, 504
573
574
581
587
621
666
474, 500
479, 501
551
484, 504
539, 565
554, 564
554, 569
615
5.8
5.9
5.9
5.8
5.8
5.9
5.8
5.8
0.42
0.31
0.35
0.04
0.27
0.31
0.03
0.39
0.000
0.000
-0.048
-0.181
-0.196
-0.466
-0.694
-0.801
Figure 2. (a) A suggested mechanism of color tuning invoked by the inter-
ligand energy transfer (ILET) to ancillary ligand. HOMO (left) and LUMO
(right) of (b) FIrmpic and (c) FIriq calculated with BLYP functional and
DNP basis set under effective core potential.
a Calculated LUMO energy difference relative to FIrmpic.
energies of Type II ancillary ligands are all lower than that of
FIrmpic (Type I) and are well correlated with the changes in
emission wavelength. Furthermore, DFT calculations show that the
LUMO of FIrmpic (Type I) locates largely on the dfppy ligand,
while that of FIriq (Type II) locates on the isoquinolinate ligand
exclusively (see Figure 2). Therefore, tunable emission via the
ILET-to-LX3 scheme and possible solvatochromism of Type II
complexes are consistently explained.
In conclusion, we have successfully demonstrated and elucidated
the broad range color tuning of heteroleptic iridium complexes via
relative energy level (LX3) control of the ancillary ligand.
Figure 1. (a) Emission spectra (black for 1.0 × 10-5 M in CH2Cl2 solution
and gray for PMMA film) of FIrmpic (Type I), (b) emission spectra (black
for 1.0 × 10-5 M in MeCN solution and gray for PMMA film) of FIrqnx
(Type II), and (c) a plot of absorption and emission maxima of FIrmpic
(Type I, blue) and FIrqnx (Type II, red) as a function of solvent polarity
parameter; n and ꢀ are the refractive index and dielectric constant of solvent,
respectively.
Acknowledgment. This work was supported by the Ministry
of Science and Technology of Korea through National Research
Laboratory (NRL) program awarded to Prof. Soo Young Park and
in part by Dongwoo FineChem Co., Ltd.
As seen in Figure 1, two different types of phosphorescence
emission and related iridium complexes were identified. Type I
complexes (FIrmpic, FIrpic) exhibited a blue emission with vibronic
structures and showed an invariant spectral emission both in solution
and in solid state, as reported earlier for FIrpic. On the other hand,
Type II complexes (FIrqnd, FIrpca, FIriq, FIrprz, FIrprza, and
FIrqnx) exhibited a broad spectral emission at longer wavelength
and a characteristically hypsochromic spectral shift in the solid state
compared to the solution state. Furthermore, it is worth noting that
the Type II complexes show distinct positive solvatochromism in
the phosphorescence emission (see Figure 1c) in contrast that of
Type I complexes, indicating that the emitting states of Type II
complexes comprise an intramolecular charge transfer (ICT) state.9
On the basis of these experimental observations, the most
probable mechanism of phosphorescence emission from the hetero-
leptic iridium complex is shown in Figure 2a. After the MLCT1
excitation from iridium to dfppy in the singlet manifold, highly
efficient inter-system crossing to MLCT3 occurs due to the strong
spin-orbit coupling. This dfppy-centered MLCT3 state has different
fates (Types I and II), depending on the triplet energy level of the
ancillary ligand (LX3) compared to that of MLCT3, as depicted in
Figure 2. Type I emission is attributed to the phosphorescent decay
from the MLCT3 state of dfppy, which must occur when the level
of LX3 locates higher than that of MLCT3. On the other hand, ILET
to the LX3 state of the ancillary ligand followed by phosphorescent
decay (Type II emission) occurs when the level of LX3 is lower
than that of MLCT3. This ILET-mediated phosphorescence emission
is strongly supported by the relative LUMO energies calculated
by the DFT method (see Table 1). It is clearly noted that the LX3
Supporting Information Available: A summary of detailed
synthetic procedure, spectral assignment, full absorption and photo-
luminescence spectra, and characterization methods. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154.
(2) 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.
(3) (a) 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. (b) Anthopoulos, T. D.; Frampton, M. J.; Namdas,
E. B.; Burn, P. L.; Samuel, I. D. W. AdV. Mater. 2004, 16, 557-560. (c)
Yeh, S.-J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho, M.-H.;
Hsu, S.-F.; Chen, C. H. AdV. Mater. 2005, 17, 285-289. (d) Li, J.;
Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson,
M. E. Polyhedron 2004, 23, 419-428. (e) Nazeeruddin, M. K.; Humphry-
Baker, R.; Berner, D.; Rivier, S.; Zuppiroli, L.; Graetzel, M. J. Am. Chem.
Soc. 2003, 125, 8790-8797.
(4) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-
E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am.
Chem. Soc. 2001, 123, 4304-4312. (b) Coppo, P.; Plummer, E. A.; De
Cola, D. Chem. Commun. 2004, 1774-1775. (c) Adachi, C.; Kwong, R.
C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.;
Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082-2084.
(5) Kwon, T.-H.; Cho, H. S.; Kim, M. K.; Kim, J.-H.; Kim, J.-J.; Lee, K. H.;
Park, S. J.; Shin, I.-S.; Kim, H.; Shin, D. M.; Chung, Y. K.; Hong, J.-I.
Organometallics 2005, 24, 1578-1585.
(6) Schoonover, J. R.; Dattaelbaum, D. M.; Malko, A.; Klimov, V. I.; Meyer,
T. J.; Stryers-Barnett, D. J.; Gannon, E. Z.; Granger, C.; Aldridge, W. S.,
III; Papanikolas, J. M. J. Phys. Chem. A 2005, 109, 2472-2475.
(7) For the spectra of other iridium complexes, see Supporting Information.
(8) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc.
1984, 106, 6647-6653.
(9) Seo, J.; Kim, S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 11154-11155.
JA052880T
9
J. AM. CHEM. SOC. VOL. 127, NO. 36, 2005 12439