Synthesis and luminescence studies of hydrocarbon-branched tris-cyclometallated liridium (III) complexes

Article abstract of DOI:10.1080/15421401003609228

We synthesized the tris-cyclometallated iridium complexes containing the substituted styryl groups and their saturated analogs. As the styryl iridium complexes, Ir(F-ppy-4-CH=CHC₆H₄R)₃ (where R=Me, NMe₂, OMe) we

Full text of DOI:10.1080/15421401003609228

Mol. Cryst. Liq. Cryst., Vol. 520: pp. 60=[336]–67=[343], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003609228
Synthesis and Luminescence Studies of
Hydrocarbon-Branched Tris-Cyclometallated
Iridium (III) Complexes
HYUN-SHIN LEE AND YUNKYOUNG HA
Department of Information Display Engineering, Hongik University,
Seoul, Korea
We synthesized the tris-cyclometallated iridium complexes containing the substi-
tuted styryl groups and their saturated analogs. As the styryl iridium complexes,
=
=
Ir(F-ppy-4-CH CHC6H4R)3 (where R Me, NMe2, OMe) were prepared via
direct functionalization of the methyl groups in the ppy (2-phenylpyridine) ligand
at the iridium complexes. The corresponding saturated analogs, Ir(F-ppy-
=
4-CH2CH2C6H4R)3 (where R Me, NMe2), were synthesized during the two step
=
reactions of the IrCl3 ꢀ xH2O with F-ppy-4-CH CHC6H4R via in situ hydrogen-
ation. Their photophysical properties were investigated both in solution and in film.
The longer p-conjugation in the cyclometallating ligands leads to the bathochromic
shift in photoluminescence of their iridium complexes. Among R groups, the NMe2
end group had the strongest push-pull effect with the F group at the other end, and
led to the effective control of the ILCT transition.
Keywords Hydrocarbon-branched; iridium complex; OLED; phosphorescence
1. Introduction
Phosphorescent transition-metal complexes are being widely utilized in the
light-emitting diodes (OLEDs) [1–3]. Among the transition metal complexes, the
iridium complexes were known to have high efficiency, utilizing large population
of the emitting triplet state by strong spin-orbit coupling induced by the metal center
[4–9]. The cyclometallating ligands of the iridium complexes were reported to deter-
mine the major color of phosphorescence since the energy gaps between the HOMO
and LUMO levels of the complexes can be controlled by variation of these ligands
[10,11].
The oligomeric hydrocarbon compounds have been studied as macromolecular
phosphors for such a purpose [12]. There were also several reports regarding the
synthesis and photophysical properties of the iridium complexes containing
hydrocarbon-branched cyclometallating ligands [13,14]. Furthermore, the iridium
complexes with the hydrocarbon-branched ligands can make the solution-processed
fabrication of OLEDs possible. Direct functionalization of the ligands in the iridium
Address correspondence to Yunkyoung Ha, Department of Science, College of Engineer-
ing, Hongik University, 72-1 Sangsu-Dong, Mapo-Ku, Seoul 121-791, Korea. Fax: þ82-2-320-
1490; E-mail: ykha@hongik.ac.kr
60=[336]

Tris-Cyclometallated Iridium (III) Complexes
61=[337]
complexes was previously reported [13,14]. In these literatures, unsaturated
=
tris-cyclometallated iridium(III) styryl complexes, Ir(ppy-4-CH CHC6H4R)3 and
their saturated analogs, Ir(ppy-4-CH2CH2C6H4R)3, were investigated and reported
to exhibit green phosphorescence. The ‘push-pull’ effect between R and the pyridyl
groups in the C^N ligand was also discussed with the (C^N)2Ir(acac) systems where
C^N represents the para-substituted styryl-pyridyl ligands [13].
Herein, we designed the new hydrocarbon-branched iridium complexes contain-
ing the 40-F-ppy ligand backbone. We expected that the 40-F-ppy backbone can
induce emission of their complexes at shorter wavelengths than the unsubstituted
ppy ligand. The complexes of the saturated and unsaturated hydrocarbon-branches
were prepared via different synthetic routes. Their photophysical properties were
compared, emphasizing the emission wavelength control and the long range
push-pull effect between p-R groups and 40-F-ppy moiety in the complexes. As
R groups, ꢁMe, ꢁOMe and ꢁNMe2 groups were introduced in the styryl portion
of the complexes. Photoluminescence (PL) of the complexes was investigated in both
organic solutions and polymeric matrixes.
2. Experimental Section
All reactions were carried out under the dry argon atmosphere.
2.1. Synthesis of Ligands
Synthesis of 4-Me-40-F-ppy. 4-methyl-4-fluorophenylpyridine ligand was obtained
from the reaction of 2-chloro-4-methylpyridine with 4-fluorophenylboronic acid
by the modified Suzuki coupling [15]. (yield: 60%)
0
=
Synthesis
of
F-ppy-4-CH CHC6H4-R. 2-(4 -Fluorophenyl)-4-(4-methylstyryl)
pyridine and 2-(40-fluorophenyl)-4-p-R-pyridine ligand were obtained from the
4-Me-40-F-ppy with the corresponding aldehydes, p-tolylaldehyde and
4-(dimethylamino)benzaldehyde, respectively [13,14]. (yield: 83%)
2.2. Synthesis of Complexes
0
=
Synthesis of Ir(F-ppy-4-CH CHC6H4-R)3. Ir(4-Me-4 -F-ppy)3 was prepared
according to the literature preparation [16]. And then, these crudes were reacted
with the corresponding aldehydes, p-tolualdehyde, 4-(dimethylamino)benzaldehyde
and 4-methoxybenzaldehyde, respectively [13,14]. (yield: 68%)
=
Ir(F-ppy-4-CH CHC6H4Me)3 FAB-MS: calculated 1057.2, found 1057.
=
Ir(F-ppy-4-CH CHC6H4NMe2)3 FAB-MS: calculated 1144.4, found 1144.
=
Ir(F-ppy-4-CH CHC6H4OMe)3 FAB-MS: calculated 1105.2, found 1105.
Synthesis of Ir(F-ppy-4-CH2CH2C6H4-R)3. Ir(F-ppy-4-CH2CH2C6H4-R)3 were
=
obtained from the crude dimer (F-ppy-4-CH CHC6H4-R)2Ir(l-Cl)2Ir(F-ppy-
=
=
4-CH CHC6H4-R)2 with the corresponding styryl ligands, F-ppy-4-CH CHC6H4-
R (2.5 mmol) [17]. (yield: 44%)
Ir(F-ppy-4-CH2CH2C6H4CH3)3 FAB-MS: calculated 1063.3, found 1063.
Ir(F-ppy-4-CH2CH2C6H4NMe2)3 FAB-MS: calculated 1150.4, found 1150.

62=[338]
H.-S. Lee and Y. Ha
2.2. Optical Measurements
UV-Vis absorption spectra were measured on a Hewlett Packard 8425A spec-
trometer. PL spectra were measured on a Perkin Elmer LS 50B spectrometer.
UV-Vis and PL spectra of the iridium complexes were measured in a 10ꢁ5 M dilute
CH2Cl2 solution and in a PMMA film. The PMMA film was fabricated by the
spin-coating onto the glass substrate with 10 wt% Ir complexes of PMMA in
1,2-dichloroethane solution followed by solvent evaporation.
3. Results and Discussion
The intraligand charge transfer (ILCT) properties in the unsaturated styryl iridium
complexes can be enhanced with the electron-donating end group (ꢁMe, ꢁNMe2,
ꢁOMe) of the styryl moiety via the p linker. The effective charge transfer from
the styryl end group to the pyridine acceptor might lead to the bathochromic shift,
compared with the saturated complex analogs. Furthermore, addition of the
electron-withdrawing group such as F to the phenyl ring in the complex might effec-
tively induce the electron density from the pyridine ring to the phenyl ring. Thus, the
new tris-cyclometallated Iridium complexes containing the modified F-ppy-4-styryl
derivatives were designed and synthesized. To investigate the p-conjugation effects
of the hydrocarbon-branches at the ligand, the saturated hydrocarbon-branched
iridium complexes were also prepared for comparison. The discontinuity of the
p-conjugation of the ligand in the complex make ILCT prohibited, inducing the
hypsochromic shift of the emission by the complex.
The unsaturated and saturated hydrocarbon-branched iridium complexes were
=
obtained according to the literature [13]. The ligands (L), F-ppy-4-CH CHC6H4-R,
were synthesized via two steps. The precursor, the 4-Me-40-F-ppy, was prepared
according to the modified Suzuki coupling method, as illustrated in Figure 1(a).
=
The styryl susbstituted ppy ligands, F-ppy-4-CH CHC6H4-R, was obtained from
the reaction of the 4-Me-40-F-ppy with the corresponding aldehyde. The unsaturated
complexes were formed from the reaction of Ir(4-Me-40-F-ppy)3 with the correspond-
ing aldehyde at the room temperature via direct functionalization of the methyl group
in the ligands [13]. The saturated styryl complexes were prepared from the two-step
reactions of the iridium precursor with their corresponding free ligands at 200ꢂC
via in situ ligand hydrogenation [14]. The overall synthetic scheme is shown in of
Figure 1.
The UV-Vis absorption spectra of the complexes in CH2Cl2 are shown in
Figure 2. The ILCT absorption by the longer p-conjugation in the unsaturated styryl
complexes are observed between 300 and 350 nm. For comparison, the strong
absorption bands between 250 and 300 nm by the saturated complexes are assigned
1
to the spin-allowed p-pꢃ transition of the styryl ligands in the complexes, which
support the relatively high energy absorption due to discontinuity of p-conjugation.
The ILCT transition was not observed in saturated complexes due to discontinuity
of the p linker, as expected. It is also noted that ‘push-pull’ effects between NMe2
end group in the pyridine ring and F end group in the phenyl moiety become
conspicuous from the absorption peaks around 400 and 420 nm in Ir(F-ppy-
=
CH CHC6H4NMe2)3 and Ir(F-ppy-CH2CH2C6H4NMe2)3, respectively. The com-
plexes containing the Me and OMe end group did not have absorption peaks
around in this region. The weaker absorption bands at the longer wavelengths can

Tris-Cyclometallated Iridium (III) Complexes
63=[339]
0
=
Figure 1. (a) Synthesis of the 4-Me-4 -F-ppy, (b) Synthesis of the F-ppy-4-CH CHC6H4-R,
=
(c) Synthesis of the unsaturated styryl Ir complexes, Ir(F-ppy-4-CH CHC6H4-R)3, (d) Syn-
thesis of the saturated styryl Ir complexes, Ir(F-ppy-4-CH2CH2C6H4-R)3.
ꢃ
3
3
be attributed to the spin-forbidden MLCT and spin-orbit coupling enhanced p-p
transition. The formally spin-forbidden MLCT gains the intensity by mixing with
3
1
the higher-lying MLCT transition through the strong spin-orbit coupling on the
iridium center.
The photoluminescence (PL) spectra of the unsaturated iridium complexes in
10ꢁ5 M CH2Cl2 solution show the emission peaks at 468, 492 and 490 nm for
=
Ir(F-ppy-4-CH CHC6H4-R)3 where R is Me, NMe2 and OMe, respectively. These

64=[340]
H.-S. Lee and Y. Ha
=
Figure 2. UV-vis absorption spectra of Ir(F-ppy-4-CH CHC6H4-R)3 and Ir(F-ppy-
4-CH2CH2C6H4-R)3 in 10ꢁ5 M CH2Cl2 solution.
PL maxima are slightly shifted bathochromically with respect to those of the
corresponding free ligands shown in Figure 3 because the electron-withdrawing
effect of F-ppy backbone in the ligands increases upon their coordination to the irid-
ium center. On the other hand, the emission spectra of the saturated complexes,
=
Ir(F-ppy-CH2CH2C6H4R)3 where R Me and NMe2, exhibit the emission around
453 and 425 nm, respectively, shifted toward the shorter wavelengths due to disconti-
nuity of p linker. Similar spectroscopic phenomena were reported in the emission
Figure 3. PL spectra of the free stryl ligands, F-ppy-4-CH CHC6H4-R in 10ꢁ5 M CH2Cl2
solution.
=

Tris-Cyclometallated Iridium (III) Complexes
65=[341]
=
spectra of Ir(ppy-4-CH CHC6H4-OMe)3 (526 nm) and Ir(ppy-4-CH2CH2C6H4-
OMe)3 (490 nm) [13]. It is also noteworthy that involvement of the electron-
withdrawing group, F, to the phenyl ring of the complex in our study changes
the electron balance between the pyridine and phenyl ring, resulting in hypso-
=
chromic shift in PL, compared with the results of unsubstituted Ir(ppy-4-CH
CHC6H4-OMe)3.
Additionally, we fabricated the PMMA (poly(methylmethacrylate)) film
of the complexes studied herein, and investigated their PL characteristics. The
hydrocarbon-branched iridium complexes can be compatible with polymers and thus
may be applied as emitting materials for the polymer light-emitting device (PLED).
Moreover, sterically bulky hydrocarbon branches may help to prevent the T-T
annihilation of the phosphors. PMMA is chosen as a host because its non-emitting
property within the visible range could provide the PL of the iridium complexes only
[18]. In Figure 4, the overall PL spectra of the PMMA films for tris-cyclometallated
styryl complexes were bathocromically shifted relative to their solution PLs. Pre-
viously, Guerchais, et al. reported the PL of the tris-cylcometallated iridium styryl
complexes containing the ppy backones and compared them with those of their satu-
rated analogs [13]. In his study, the temperature effect on their PLs was also elabo-
rated. He claimed that presence of the extended p-conjugation and=or strong
intramolecular donor-acceptor interaction into the ligand of the complex induced
a strong red shift upon the temperature variation from 298 to 77 K. The emission
mechanism of the complexes in our study in the rigid PMMA film can be thought
similar to that occurred at 77 K. The rigidity of the PMMA film could make
the ILCT character of the styryl moiety dominant, and the bathochromic shift of
the film could be explained by this increased ILCT. Meanwhile, the motional relax-
ation of the styryl moiety in solution at the higher temperatures may make ILCT
character weaker and limit the contribution of the p-conjugation to the emission
of their complexes.
=
Figure 4. PL spectra of Ir(F-ppy-4-CH CHC6H4-R)3 and Ir(F-ppy-4-CH2CH2C6H4-R)3 on
the PMMA films.

66=[342]
H.-S. Lee and Y. Ha
=
The emission maxima of Ir(F-ppy-4-CH CHC6H4NMe2)3 are observed at 542
=
and 623 nm, and those of Ir(F-ppy-4-CH CHC6H4OMe)3 occur at 509 and
=
600 nm, while Ir(F-ppy-4-CH CHC6H4Me)3 show broad emission with the maxima
at 535 nm. These PLs were blue-shifted due to the F substitutent of the ppy ligand in
the complex, compared to those of the complexes containing the unsubstituted ppy
ligand. The saturated complex, Ir(4-F-ppy-CH2CH2C6H4NMe2)3 exhibits the emis-
sion maxima at 518 and 635 nm. Interestingly, the solid PLs of the styryl complexes
containing NMe2 group show similar emission patterns regardless of the presence of
the p linker. It might be ascribed to the strong push effect by NMe2 to the complex in
both cases.
4. Conclusion
The tris-cyclometallated iridium complexes containing the substituted styryl groups
and their saturated analogs were prepared and their luminescence properties of the
iridium complexes were investigated. In the UV-vis spectral patterns of the iridium
complexes, the absorption around 350 nm was assigned to the ILCT by the extended
p-conjugation of the styryl moiety. Furthermore, the strong push-pull effect between
the NMe2 and F end groups led to the effective control of the ILCT transition,
extending the absorption of the complexes up to 400 nm. The PLs in both the sol-
ution and the film of these unsaturated styryl complexes were bathochromically
shifted with respect to those of their saturated analogs, as expected from the elonga-
tion of p-conjugation. Involvement of the electron withdrawing group, F, to the
phenyl ring in the complex, led to the hypsochromic shift in PL, compared with
the complexes without F substituents. In the PMMA film, the emission spectra of
the branched iridium complexes were broad and red-shifted presumably due to
their rigid environment.
Acknowledgement
This work was supported by the Korea Research Foundation (KRF-2008-
531-C00036).
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