Synthesis, crystal structure and photophysical properties of a series of new neutral iridium(III) complexes with 2-phenylpyridine

Article abstract of DOI:10.1016/j.inoche.2009.06.017

A series of new neutral iridium(III) complexes containing strong-field ancillary ligands, [Ir(ppy)₂(PPh₃)L] (ppy = 2-phenylpyridine, PPh₃ = triphenylphosphine, L = NCS^-, 1; N₃^-, 2; NCO

Full text of DOI:10.1016/j.inoche.2009.06.017

Inorganic Chemistry Communications 12 (2009) 785–788
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, crystal structure and photophysical properties of a series of new
neutral iridium(III) complexes with 2-phenylpyridine
*
*
Xuan Shen , Hao Yang, Xiu-Hua Hu, Yan Xu, Feng-Ling Wang, Su Chen, Dun-Ru Zhu
College of Chemistry and Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new neutral iridium(III) complexes containing strong-field ancillary ligands, [Ir(ppy)2(PPh3)L]
(ppy = 2-phenylpyridine, PPh3 = triphenylphosphine, L = NCSꢀ, 1; N3ꢀ, 2; NCOꢀ, 3), have been synthesized
and fully characterized by 1H NMR, IR, ESI mass spectral and elemental analysis. The crystal structure of 1
has been determined by X-ray analysis. The photoluminescence (PL) spectra of 1–3 show emission max-
ima at 477, 489 and 485 nm, respectively, corresponding to blue light-emitting of 1 and blue-green light-
emitting of 2 and 3. PL quantum yields (PLQYs) of 1–3 are 0.39, 0.13 and 0.43, respectively.
Ó 2009 Elsevier B.V. All rights reserved.
Received 7 May 2009
Accepted 10 June 2009
Available online 14 June 2009
Keywords:
Neutral iridium(III) complexes
Strong-field ligands
Crystal structure
Photoluminescence
Blue–green light emitting
PL quantum yields
Phosphorescent materials play a major role in bringing organic
light-emitting diode (OLED) displays and lighting to a wide market
[1]. These materials are generally based on heavy metal complexes
which can emit from states with a large degree of metal-to-ligand
charge transfer (MLCT) character and have the advantage that both
singlet (1MLCT) and triplet (3MLCT) excitons can emit light [2].
Ir(III) complexes containing 2-phenylpyridine (ppy) and its deriva-
tives are known to exhibit high triplet quantum yields due to mix-
ing the singlet and the triplet excited states via spin–orbit
coupling, which enhances the triplet-state subsequently, leading
to high phosphorescence efficiencies. Furthermore, by modifying
the ancillary ligands, the HOMO energy level can be changed, lead-
ing to alter the MLCT energy. Different energy gaps between the
HOMO and LUMO of complexes generate light-emitting at different
wave lengths.
Up to now, cationic, anionic and neutral iridium(III) complexes
containing ppy and its derivatives (C^N) have all been reported.
Cationic complexes [Ir(C^N)2LL0]+ containing two same or different
neutral monodentate ancillary ligands L and L0 [3] and cationic
complexes [Ir(C^N)2(L2)]+ in which there is only one neutral biden-
tate ancillary ligand L2 [4] have been fully investigated. However,
anionic complexes [Ir(C^N)2L2]ꢀ with two same anionic monoden-
tate ancillary ligands have been less reported [5]. This may be due
to the larger energy gaps between the HOMO and LUMO of the cat-
ionic complexes than those of the anionic complexes. In addition,
cationic complexes would be harder to be oxidized than anionic
complexes when their ancillary ligands have comparable ligand
field strength. Because the design and selection of bidentate ancil-
lary ligands in ꢀ1 valence state are comparatively simple, a large
number of neutral complexes in the type of [Ir(C^N)2(L2)] have
been synthesized [6]. However, few neutral complexes in the type
of [Ir(C^N)2LL0] containing one neutral monodentate ancillary li-
gand and one ꢀ1 valence anionic monodentate ancillary ligand
have been reported recently [7]. Herein, as extension of the re-
search, we present a series of new neutral luminescent Ir(III) com-
plexes [Ir(ppy)2(PPh3)L] (PPh3 = triphenylphosphine, L = NCSꢀ, 1;
Nꢀ3 , 2; NCOꢀ, 3) which have been synthesized using [Ir(p-
py)2(PPh3)Cl] [7b] as starting reagent [8–10].
The crystal structure of 1 has been determined by X-ray analysis
[11]. An ORTEP view of 1 is shown in Fig. 1. In 1, the Ir(III) atom
is coordinated by one NCSꢀ anion, one PPh3 ligand and two
N,C-bidentate ppy ligands. In neutral [Ir(C^N)2LL0] type complexes,
only the structures of [Ir(ppy)2(PPh3)Cl] [7b] and [Ir(dfppy)2
(t-BuNC)(CN)] (dfppy = 2-(2,4-difluorophenyl)pyridine, t-BuNC =
t-butyl isocyanide), [Ir(dfppy)2(dmpNC)(CN)] (dmpNC = 2,6-
dimethylphenyl isocyanide) [7c] have been reported. Even
as above-mentioned three complexes, two ancillary ligands
(NCSꢀ and PPh3) in 1 are also in the cis positions. However, in
cationic [Ir(C^N)2LL0]+ type complex, [Ir(ppy)2(PPh3)2]+, two ancil-
lary ligands (PPh3) are in the trans positions [3c] obviously because
PPh3 group is too large for the cis-Ir(ppy)2 moiety to have two PPh3
in the cis positions. In 1, the pyridyl rings of two ppy ligands are in
the trans positions and the phenyl rings of two ppy ligands locate
in the cis positions with respect to Ir(III) center.
* Corresponding authors. Tel.: +86 25 8358 7717; fax: +86 25 8336 5813.
E-mail addresses: shenxuan@njut.edu.cn (X. Shen), zhudr@njut.edu.cn (D.-R.
Zhu).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.06.017

786
X. Shen et al. / Inorganic Chemistry Communications 12 (2009) 785–788
with the increase of another ancillary ligands’ sizes
(Clꢀ < NCSꢀ < P(OPh)3).
The absorption and photoluminescence (PL) data of complexes
1–3 are summarized in Table 1. Fig. 2 shows a comparison of
absorption spectra of 10ꢀ4 M solutions of 1–3 in dichloromethane
at 298 K. Because of the similar characteristics of NCSꢀ, N3ꢀ, NCOꢀ
ligands, the absorption spectra of the three complexes do not show
great difference. The high-energy bands at around 280 nm are as-
*
signed to the intraligand p–p transition of 2-phenylpyridine and
the low-energy bands at around 470 nm are categorized as me-
tal-to-ligand charge-transfer transitions. However, the low-energy
MLCT bands in 1 (465 nm) and 3 (468 nm) slightly blue shift com-
pared to that of 2 (474 nm), indicating the extent of the p-acceptor
strength of the NCSꢀ and NCOꢀ ligands compared to the Nꢀ3 ligand.
In the spectra, the 1MLCT and 3MLCT bands are not distinct, as
shown by the broad bands at around 384 nm. This is an indication
of the strong-field ligands character in the MLCT states of the
complexes.
Fig. 3 shows a comparison of PL spectra of 10ꢀ4 M solutions
of 1–3 in dichloromethane at 298 K. Compared with the absorp-
tion spectra of 1–3, PL spectra of 1–3 also show analogous
curves and emission peaks because of the similar ligands and
structures. The PL maxima of 1–3 are 477, 489 and 485 nm,
respectively, corresponding to blue luminescence of
1 and
Fig. 1. ORTEP drawing of 1. The thermal ellipsoids are drawn at the 50% probability
blue–green luminescence of 2 and 3. Judging from the PL max-
ima of 1–3, it can be concluded that the HOMO–LUMO energy
gaps for 1–3 increase in the order of 2 < 3 < 1. It can also be
clearly seen that the PL maxima are all blue shifted when com-
pared with [Ir(ppy)2Cl]2, which has an emission peak at around
512 nm [12]. The blue shift in the PL spectra of 1–3 relative to
[Ir(ppy)2Cl]2 is consistent with the fact that the HOMO–LUMO
energy gaps for 1–3 are larger than that of [Ir(ppy)2Cl]2. The
effective strategy to magnify the PL quantum yields (PLQYs) of
this class of Ir(III) complexes is to increase the HOMO–LUMO en-
ergy gaps by introducing the ligands such as NCSꢀ, N3ꢀ, and
NCOꢀ, which are known to have strong ligand field stabilization
energy. In such type of complexes, the charge transfer excited
states decay through radiative pathways. The PLQYs of 1–3 are
0.39, 0.13 and 0.43, respectively, which are comparable with
those of the related complexes.
level. The H atoms and the solvent (CHCl3) are omitted for clarity.
Two Ir–C(ppy) bond lengths are slightly different (2.024(7) and
2.031(8) Å) in [Ir(ppy)2(PPh3)Cl]. However, in 1, two Ir–C(ppy)
bond lengths are obviously different. The bond length between Ir
atom and C(ppy) atom which is in the cis position of NCSꢀ ligand
with respect to the metal center (2.044(4) Å) is longer than that be-
tween Ir atom and C(ppy) atom which is in the trans position of
NCS- ligand (2.005(5) Å). This may be due to greater trans effect
of NCSꢀ in 1 compared with that of Clꢀ in [Ir(ppy)2(PPh3)Cl]. More-
over, two Ir–N(ppy) bond lengths are slightly different (2.056(3)
and 2.069(3) Å) because two N(ppy) atoms are all in cis positions
of NCSꢀ ligand with respect to the metal center. The average bond
length of Ir–N(ppy) is 0.060 Å shorter than that of IrꢀN(NCSꢀ)
(2.122(4) Å). Ir–P bond length in 1 (2.4186(12) Å) is comparable
with that in Ir(ppy)2(PPh3)Cl (2.426(2) Å).
The bond angles about Ir atom in 1 do not indicate substantial
distortion from octahedral regularity. The N3–C39–S1 bond angle
of NCSꢀ ligand is nearly linear at 178.8(5)°, whereas the Ir1–N3–
C39 angle is slightly bent at 172.9(4)°. Two ppy groups are pushed
away from the PPh3 ligand and the N–Ir–N bond angle is
169.18(14)°, which is a little smaller than that in [Ir(ppy)2(PPh3)Cl]
(170.7(2)°) and is bigger than that in [Ir(ppy)2(PPh3){P(OPh)3}]+
(164.3(4)°) [3c]. In 1, [Ir(ppy)2(PPh3)Cl] and [Ir(ppy)2(PPh3){-
P(OPh)3}]+, two ancillary ligands are all in the cis positions and
one of ancillary ligands are same (PPh3). Different sizes of another
ancillary ligands result in different steric hindrances between
ancillary ligands and two ppy ligands. The N–Ir–N bond angles in
three complexes decrease in the order of [Ir(ppy)2(PPh3)-
Cl] > 1 > [Ir(ppy)2(PPh3){P(OPh)3}]+, which is strictly consistent
In summary, a series of new neutral luminescent iridium(III)
complexes [Ir(ppy)2(PPh3)L] (L = NCSꢀ, 1; N3ꢀ, 2; NCOꢀ, 3) have
been synthesized and unambiguously characterized in detail by
1H NMR, IR, ESI mass spectral and elemental analysis data and also
X-ray diffraction data analysis for single crystals of 1. The photo-
physical properties of 1–3 have also been investigated. The absorp-
tion spectra of 1–3 can be divided into two regions: the short
*
wavelength region at around 280 nm being due to the
p
–p
transi-
tions of the ppy ligands, while the longer wavelength absorptions
being due to the MLCT transitions. The PL spectra of 1–3 show
emission maxima at 477, 489 and 485 nm, respectively, corre-
sponding to blue luminescence of 1 and blue–green luminescence
of 2 and 3. The PLQYs of 1–3 are 0.39, 0.13 and 0.43, respectively.
These results suggest the potential applications of 1–3 as phospho-
rescent materials.
Table 1
Photophysical properties of complexes 1–3 in dichloromethane solution at 298 K.
Complex
abs. kmax (nm) (
e
, 103 Mꢀ1 cmꢀ1
)
PL kmax (nm)a
PLQYsb
1
2
3
465 (0.18), 382 (5.55), 346sh (7.77), 281 (28.08)
474sh (0.35), 386sh (2.83), 279 (18.90)
468 (0.29), 384 (5.50), 349sh (8.18), 283 (26.60)
477, 505
459, 489
485, 509
0.39
0.13
0.43
a
Excitation wavelength for the emission measurements was 360 nm.
PLQYs were measured relative to 1 lg/mL quinine sulfate solution in 1 N sulfuric acid. Corrections were made due to the change in solvent refractive indices.
b

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This work was sponsored by the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education Minis-
try (51101024) and the National Natural Science Foundation of
China (20771059).
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[8] 1: Ammonium thiocyanate (49 mg, 0.65 mmol) was added to a solution of
[Ir(ppy)2(PPh3)Cl] (100 mg, 0.13 mmol) in 60 mL mixture of chloroform and
methanol (1:1 v/v). The reaction mixture was refluxed with stirring for 24 h
under argon atmosphere. After being cooled to r.t., the solution was filtered
and insoluble crude 1 was obtained as a yellow solid after being washed with
hexane. Yellow block crystals of 1 containing one molecule of chloroform,
which were suitable for X-ray crystal structure analysis, were obtained by
recrystallization from chloroform and hexane (2:3 v/v) in a yield of 80 mg
(68%). Anal. Calc. for 1ꢁCHCl3 (C42H32Cl3IrN3PS, 940.34): C, 53.65; H, 3.43; N,
CCDC 725338 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.inoche.2009.
06.017.
4.47. Found: C, 53.56; H, 3.59; N, 4.51%. IR (KBr, cmꢀ1):
1582, 1479, 1436
m
(C@C, C@N) 1606,
m
(C„N) 2098. ESI-MS: m/z = 763 (M–NCSꢀ)+. 1H NMR
References
(300 MHz, CDCl3) (ppm): d 8.68 (d, J = 5.6 Hz, 1H), 8.59 (d, J = 5.7 Hz, 1H), 7.92
(d, J = 7.9 Hz, 1H), 7.75 (t, J = 7.2 Hz, 1H), 7.58 (d, J = 7.9 Hz, 1H), 7.51 (d,
J = 4.0 Hz, 2H), 7.41 (d, J = 7.4 Hz, 1H), 7.31–7.28 (m, 2H), 7.19–7.11 (m, 12H),
6.92–6.71 (m, 6H), 6.63 (t, J = 7.1 Hz, 1H), 5.96 (d, J = 7.6 Hz, 1H), 5.82 (t, J = 5.0,
1H).
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[9] 2: Sodium azide (42 mg, 0.65 mmol) was added to
a solution of
[Ir(ppy)2(PPh3)Cl] (100 mg, 0.13 mmol) in 30 mL chloroform. The reaction
mixture was refluxed with stirring for 24 h under argon atmosphere. After

788
X. Shen et al. / Inorganic Chemistry Communications 12 (2009) 785–788
being cooled to r.t., unreacted sodium azide was removed by filtration and
8.72 (t, J = 6.2 Hz, 2H), 7.91 (d, J = 7.3 Hz, 2H), 7.75–7.70 (m, 2H), 7.61–7.41
(m, 4H), 7.19–7.09 (m, 9H), 6.91–6.69 (m, 4H), 6.63 (t, J = 7.4 Hz, 1H), 6.55 (t,
J = 7.5 Hz, 1H), 6.07 (d, J = 7.7 Hz, 1H), 5.91 (d, J = 7.9 Hz, 1H), 5.86–5.80 (m,
2H).
evaporating the filtrate led to nigger-brown crude product. Brown micro-
crystals of 2 were obtained by recrystallization from methanol in a yield of
66 mg (65%). Anal. Calc. for 2 (C40H31IrN5P, 804.90): C, 59.69; H, 3.88; N, 8.70.
Found: C, 59.76; H, 3.82; N, 8.65%. IR (KBr, cmꢀ1):
m
(C@C, C@N) 1640, 1617,
(N„N) 2034. ESI-MS: m/z = 763 (M–Nꢀ3 )+. 1H NMR (500 MHz,
CDCl3) (ppm): 9.32 (d, J = 5.7 Hz, 1H), 8.87 (d, J = 5.3 Hz, 1H), 8.69 (d,
[11] Single-crystal X-ray diffraction data of 1 were collected on on a Bruker SMART
1476, 1431;
m
APEX II CCD area detector diffractometer equipped with a rotating anode
0
d
generator using monochromated Mo Ka radiation (k = 0.71073 ÅA) at 293(2) K.
J = 5.9 Hz, 1H), 8.62 (d, J = 5.7 Hz, 1H), 7.95 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 8.0 Hz,
1H), 7.73–7.66 (m, 2H), 7.58–7.41 (m, 4H), 7.19–7.09 (m, 9H), 6.92 (t,
J = 7.3 Hz, 1H), 6.84–6.71 (m, 3H), 6.64 (t, J = 7.4 Hz, 1H), 6.55 (t, J = 7.3 Hz, 1H),
6.17 (d, J = 7.6 Hz, 1H), 5.91 (d, J = 7.7 Hz, 1H), 5.87–5.84 (m, 2H).
Cell parameters were retrieved using SMART software and refined using
SAINTPLUS for all observed reflection. Data reduction and correction for decay
were performed using the SAINTPLUS software. Absorption corrections were
applied using SADABS. The structure of 1 was solved by the direct methods
using the SHELXL program of the SHELXTL-97 package and refined with
SHELXTL. All non-hydrogen atoms were refined anisotropic thermal
parameters. Hydrogen atoms were introduced in calculated positions and
included in the refinement, riding on their respective parent atoms.
Crystallographic data for 1ꢁCHCl3: C41H31IrN3PSꢁCHCl3, Mr = 940.29,
[10] 3: Sodium cyanate (42 mg, 0.65 mmol) was added to
a solution of
[Ir(ppy)2(PPh3)Cl] (100 mg, 0.13 mmol) in 30 mL chloroform. The reaction
mixture was refluxed with stirring for 24 h under argon atmosphere. After
being cooled to r.t., unreacted sodium cyanate was removed by filtration and
evaporating the filtrate led to crude product. Yellow micro-crystals of 3 were
obtained by recrystallization from chloroform and hexane (1:3 v/v) in a yield
of 65 mg (64%). Anal. Calc. for 3 (C41H31IrN3OP, 804.90): C, 61.18; H, 3.88; N,
0
0
monoclinic
P21/n
with
a = 13.5460(11) ÅA,
b = 15.3833(13) ÅA,
0
0
c = 18.4552(15) ÅA, b = 91.1470(10)°, V = 3845.0(5) ÅA3, Z = 4, Dc = 1.624 g cmꢀ3
,
5.22. Found: C, 61.09; H, 3.95; N, 5.23%. IR (KBr, cmꢀ1):
1581, 1478, 1436;
(300 MHz, CDCl3) (ppm): d 9.33 (d, J = 5.2 Hz, 1H), 8.87 (d, J = 5.1 Hz, 1H),
m
(C@C, C@N) 1606,
F(0 0 0) = 1856, GOF = 1.079, R1 = 0.0355, xR2 = 0.0656 for all data.
m
(C„N) 2225. ESI-MS: m/z = 763 (M–NCOꢀ)+. 1H NMR
[12] Y.-M. Wang, F. Teng, L.-G. Gan, H.-M. Liu, X.-H. Zhang, W.-F. Fu, Y.-S. Wang, X.-
R. Xu, J. Phys. Chem. C 112 (2008) 4743.