Synthesis, characterization and photophysical properties of two heteroleptic 2-(4,6-difluorophenyl)pyridyl iridium(III) complexes with amidate ancillary ligands

Article abstract of DOI:10.1016/j.inoche.2011.05.035

Two novel iridium(III) complexes containing 2-(4,6-difluorophenyl)pyridyl cyclometalating ligand and amidate ancillary ligands were synthesized and characterized by NMR and X-ray crystallographic methods. Photophysical properties including UV-vis absorpti

Full text of DOI:10.1016/j.inoche.2011.05.035

Inorganic Chemistry Communications 14 (2011) 1414–1417
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization and photophysical properties of two heteroleptic
2
-(4,6-difluorophenyl)pyridyl iridium(III) complexes with amidate ancillary ligands
Songlin Zhang ⁎, Feng Wu, Wei Yang, Yuqiang Ding ⁎
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel iridium(III) complexes containing 2-(4,6-difluorophenyl)pyridyl cyclometalating ligand and
amidate ancillary ligands were synthesized and characterized by NMR and X-ray crystallographic methods.
Photophysical properties including UV–vis absorption and photoluminescence spectra were studied,
indicating that the electronic properties of the amidate ancillary ligand exhibit a significant effect on the
emitting excited states of these iridium(III) amidate complexes and thus lead to different emission colors.
Received 6 April 2011
Accepted 25 May 2011
Available online 1 June 2011
Keywords:
Iridium complex
Amidate
©
2011 Elsevier B.V. All rights reserved.
Photoluminescence
2-(4,6-difluorophenyl)pyridine
Heteroleptic complex
In recent years, iridium(III) complexes containing cyclometalating
ligands have been studied intensively as promising candidates for
phosphorescent emitters in OLEDs [1–8]. These complexes have been
shown to exhibit high quantum efficiencies due to strong spin-orbital
coupling of iridium center, which leads to mixing of singlet and triplet
excited states and subsequent harvesting of “spin-forbidden” triplet
states for photon emission [1–3]. Generally, there are two major kinds
sized and characterized, and have been demonstrated to exhibit high
photoluminescent efficiencies and quite wide color bands [24,25]. On
the other hand, iridium complexes containing 2-(4,6-difluorophenyl)
pyridyl (dfppy) cyclometalating ligand have been shown to exhibit
distinct photopysical properties from classical iridium phenylpyridyl
complexes due to the involvement of small, strongly electronegative
fluorine atoms that can effectively alter the energy level of the HOMO
and LUMO orbitals of the complexes [26–30]. In line with these
studies, we report herein the synthesis, characterization and photo-
physical properties of two unprecedented iridium(III) complexes
containing 2-(4,6-difluorophenyl)pyridine as cyclometalating ligand
and amides as the ancillary ligand (Scheme 1, complexes 2 and 3).
These complexes have shown good photoluminescence. The substit-
uent effect of amide on photophysical properties was recognized.
Cyclometalating ligand dfppy was synthesized by using Suzuki
coupling reaction (Scheme 1a). Dichloro-bridged complex 1
3
of iridium(III) emitters: homoleptic Ir(C^N) [9–12] and heteroleptic
(
C^N) Ir(LX) [13–19] complexes (C^N denotes a cyclometalating
2
ligand, such as 2-phenylpyridine etc.; LX denotes a monoanionic
ancillary ligand such as acetylacetonate and picolinate etc.). Homo-
leptic Ir(III) complexes, with three identical cyclometalating ligands,
have been shown to possess high photoluminescent efficiencies and
their internal efficiencies can reach 100% in principle. In heteroletpic
Ir(III) emitters, usually two main cyclometalating ligands, associated
with an ancillary ligand, cooperatively coordinate to an iridium
center. The efficiency and color of the Ir(III) emitters can be readily
tuned by the variation of electronic and/or steric properties of
cyclometalating ligands as well as ancillary ligands, resulting in a
broad range of readily tunable emission color bands.
2 2 3
[(dfppy) IrCl] was obtained by reaction of dfppy ligand with IrCl
according to a general procedure (Scheme 1b) [31]. Into a Schlenk
tube containing complex 1, amide ligand and sodium methanolate
2 2
were added CH Cl solvent under dinitrogen atmosphere (Scheme 1c).
The commonly used ancillary ligands include acetylacetonate,
picolinate, N-methylsalicyliminyl, triazolate and others [20–23]. As a
result of our continuing effort, we have shown that amides are able to
bind to iridium center via κ2 mode and thus form a four-membered
metallocycle. Consequently, a number of photoluminescent Ir(III)
complexes with several amidate ancillary ligands have been synthe-
The mixture was stirred at room temperature for 48 h. After work-up,
column chromatography, and recrystallization, yellow crystals 2 and 3
were obtained.
These two complexes have been fully characterized by H, 13C and
1
1
9
F NMR methods, and X-ray crystallography where possible.
Chemical shifts and splittings of signals in 1H, C and F NMR for
complexes 2 and 3 are consistent with the formation of the [dfppy] Ir
amidate) complexes. For instance, for complex 2 H chemical shifts in
13
19
2
1
(
⁎
Corresponding authors. Tel./fax: +86 510 85917763.
the aromatic range and splitting patterns are consistent with relevant
E-mail addresses: slzhang4@mail.ustc.edu.cn (S. Zhang), yding@jiangnan.edu.cn
(Y. Ding).
Ir(III) 2-(4,6-difluorophenyl)pyridyl complexes previously reported
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.035

S. Zhang et al. / Inorganic Chemistry Communications 14 (2011) 1414–1417
1415
Scheme 1. Synthesis of Ir(III) complexes with amidate ancillary ligands.
by Thompson, Park and Chi groups [26–30] and Ir(III) amidate
chemical shift at 2.11 (s, 3H) for acetamidate α-methyl hydrogens,
and this methyl carbon atom is found at 23.85 in 13C NMR spectrum of
complex 3 (for more information, please refer to the supplementary
material).
To unambiguously elucidate the structure, single crystal X-ray
diffraction analysis was done for complex 2, as shown by Fig. 1.
Complex 2 adopts an octahedral geometry with the two dfppy ligands
located in a cis C, C’ and trans N, N’ manner. This orientation has been
previously reported for analogous heteroleptic complexes containing
Ir-bis(ppy) and Ir-bis(dfppy) fragment. The other two coordination
positions have been occupied by amidate O and N atoms, forming a
four-membered metallocycle. The Ir1\C11, Ir1\C22, Ir1\N1 and
Ir1\N2 bond lengths of 1.989, 2.012, 2.045 and 2.044 Å are consistent
complexes reported by us [24,25]. The typical chemical shift at 179.19
13
in C NMR spectrum is characteristic of the amidate carbon atom in
19
the four-membered metallocycle. Furthermore, the F chemical shifts
at 111.14 and 110.78 are attributed to the two 6-F atoms of the two
4
,6-difluorophenylpyridyl ligands and the dd splitting at 109.11 is
attributed to the two 4-F atoms [26–30]. Due to the different chemical
environments for the two 4,6-difluorophenylpyridyl ligands (with
one C trans to amidate nitrogen atom and the other C’ trans to amidate
oxygen atom), there are slight differences for the chemical shifts of
the four fluorine atoms. Finally, X-ray crystallographic structure
presents the most convincing and intuitional evidence (vide infra).
For complex 3, additional evidence in the 1H NMR spectrum is the
Fig. 1. ORTEP diagram of complex 2 with thermal ellipsoids shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and
angles (°): Ir(1)\C(11) 1.989(6), Ir(1)\N(1) 2.045(4), Ir(1)\C(22) 2.012(5), Ir(1)\N(2) 2.044(4), Ir(1)\O(1) 2.207(4), Ir(1)\N(3) 2.154(5), C(23)\O(1) 1.294(6), N(3)\C(23)
1.308(7), C(22)\Ir(1)\N(2) 81.2(2), C(11)\Ir(1)\N(1) 80.8(2), N(3)\Ir(1)\O(1) 59.66(16), C(11)\Ir(1)\C(22) 90.7(2), N(1)\Ir(1)\N(2) 174.97(16).

1416
S. Zhang et al. / Inorganic Chemistry Communications 14 (2011) 1414–1417
Fig. 2. UV–vis absorption and normalized photoluminescent emission spectra of complexes 2 and 3 at a concentration of 10−5 mol/L in CH
2 2
Cl solution at 298 K.
with other complexes containing Ir-(dfppy) fragment [26–30,32]. The
Ir1\N3 and Ir1\O1 bond lengths are 2.154 and 2.207 Å, in agreement
with those of other Ir(III) amidate complexes reported by us
previously [24,25].
Complexes 2 and 3 show similar absorption spectra, as reflected by
the shape of Fig. 2a and b and absorption wavelengths in Table 1.
However, the emission spectra for complexes 2 and 3 are distinct from
each other. Complex 2 shows a significant maximum emission at
Additionally, there should be significant inter-ligand energy
transfer during emission process of complex 3, resulting in the
stabilization of the emitting states and subsequently a second longer
emission wavelength [29]. But this inter-ligand energy transfer
process should be unfavorable for complex 2. According to the
inter-ligand energy transfer theory, much more stable LX3 state is
predicted for complex 3 than for complex 2. This is reasonable
because there is better delocalization of electrons in benzamidate
ligand due to the incorporation of phenyl group in complex 2 and
3
94 nm (purple light emission, Fig. 2c) whereas complex 3 features
3
two significant emission bands at 448 and 478 nm (blue light
emission, Fig. 2d) [27,28]. It is noteworthy that the purple emission
of complex 2 is quite unprecedented for such heteroleptic Ir(III)
complexes in the literature. In addition, these results suggest that
excitation of complexes 2 and 3 should involve a common transition
subsequently it should be more energy-costly to form the LX state for
complex 2. This explains why inter-ligand energy transfer process is
unfavorable for complex 2.
In conclusion, we report here the synthesis and characterization of
2
two novel Ir(dfppy) complexes with different amidate ancillary
(
possibly a metal-to-ligand charge transfer) [27–29,33], but there is
ligands. These complexes show distinct emission colors, demonstrating
the crucial role of the electronic properties of ancillary ligands on
photoluminescent properties. The dichotomy of photophysical proper-
ties has been proposed to be caused by the more delocalized nature of
benzamidate ligand.
significant differences for the emitting excited states for complexes 2
and 3. In view of the two lower-energy emission bands for complex 3,
the energy levels of emitting states for complex 3 should be
substantially lower than that for complex 2. DFT calculation results
[34] indicate that the energy level of HOMO of the triplet complex 2 is
much higher than that for complex 3 (−0.110 vs. −0.154), therefore
validating the fact that shorter wavelength has been observed for
complex 2.
Acknowledgments
This study was supported by the National Natural Science
Foundation of China (no. 20971058) and the Fundamental Research
Funds for Central Universities (no. JUSRP 11105).
Table 1
Summary of photophysical properties of complexes 2 and 3.
Appendix A. Supplementary material
Complex
Absorption bands (nm)
Emission (maximum, nm)
2
3
229, 252, 380
229, 254, 383
394
478
Supplementary data to this article can be found online at doi:10.
016/j.inoche.2011.05.035.
1

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1417
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