Blue-light-emitting complexes: Cationic (2-phenylpyridinato)iridium(III) complexes with strong-field ancillary ligands

Article abstract of DOI:10.1002/ejic.200600920

The emission λ_max are strikingly shorter (451, 482 nm) for the cationic dicarbonyl complex [Ir(ppy)₂(CO)₂] ⁺ (2a) than those (473, 495 nm) of the anionic dicyano complex [Ir(ppy)₂-(CN)₂]^-, where CO and CN^- are both sp carbon coordinating ligands and known to be strong-field ligands showing a high trans effect. Sky-blue-light emission (λ_max = 441-443 and 470-471 nm) could be obtained by modifying the ppy ligand to F ₂Meppy (2,4-difluoro-2-phenyl-m-methylpyridinato anion) to prepare [Ir(F₂Meppy)₂LL′]⁺ [LL′ = (CO) ₂ (2aF₂Me), (PPh₂Me)₂ (2bF ₂Me)]. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200600920

SHORT COMMUNICATION
DOI: 10.1002/ejic.200600920
Blue-Light-Emitting Complexes: Cationic (2-Phenylpyridinato)iridium(III)
Complexes with Strong-Field Ancillary Ligands
Chong Shik Chin,*[a] Min-Sik Eum,[a] Song yi Kim,[a] Choongil Kim,[a] and
Sung Kwon Kang[b]
Keywords: Iridium / Luminescence / Blue-light emitter / Strong-field ligand
The emission λmax are strikingly shorter (451, 482 nm) for the
cationic dicarbonyl complex [Ir(ppy)2(CO)2]+ (2a) than those
(473, 495 nm) of the anionic dicyano complex [Ir(ppy)2-
(CN)2]–, where CO and CN– are both sp carbon coordinating
ligands and known to be strong-field ligands showing a high
trans effect. Sky-blue-light emission (λmax = 441–443 and
470–471 nm) could be obtained by modifying the ppy ligand
to
F2Meppy
(2,4-difluoro-2-phenyl-m-methylpyridinato
anion) to prepare [Ir(F2Meppy)2LLЈ]+ [LLЈ = (CO)2 (2aF2Me),
(PPh2Me)2 (2bF2Me)].
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
plexes than that of the anionic complexes. A cationic com-
Introduction
+
∧
plex such as [Ir(C N)2(CO)2] (not reported previously)
would show a blueshifted emission λmax from that of the
Research of blue-light-emitting materials is currently un-
der thorough investigation because efficient blue-light emit-
ters are in need in the display industry.[1] Iridium complexes
of ppy (2-phenylpyridinato) have drawn much attention as
they emit phosphorescence in the visible region with high
quantum yields and the emission λmax can be tuned by
modification of the ligands.[1–4] Theoretical calculations for
[Ir(ppy)3],[2c] [Ir(ppy)2L2],[4b] and [Pt(ppy)(acac)][5] suggest
–
∧
anionic complex [Ir(C N)2(CN)2] because both CO and
CN– are well-known strong-field ligands that give large d-
orbital energy splitting between dπ-orbitals (eg) and dσ-or-
bitals (t2g). One advantageous application of cationic com-
plexes is their use for single-layer electroluminescent de-
vices.[6]
We now wish to report new cationic complexes
+
+
that the HOMO of these metal complexes include dπ-orbit-
als of the metal and orbitals of L2
∧
∧
∧
[Ir(C N)2L2] and [Ir(C N)2LLЈ] [C N = ppy, F2ppy,
F2Meppy; L, LЈ = strong-field ligands: CO, PPh2Me,
P(OPh)3] and their photoluminescent characteristics in the
blue color region.
[4b]
or acac.[5] Spectro-
∧
∧
scopic data analysis for Ir(C N)2LLЈ (C N = ppy and its
derivatives)[4b] revealed that ancillary ligands (L, LЈ) alter
1
the MLCT energy mainly by changing the HOMO energy
level. The HOMO energy level may then be lowered by
strong-field ancillary ligands, which causes a large d-orbital
energy splitting. It is also likely that strong-field ancillary
ligands L and LЈ lengthen the bond lengths of Ir–C (ppy)
trans to L and LЈ to lower the energy levels of the dπ-orbit-
als of the metal as strong-field ligands are known to show
a high trans effect.
+
∧
Cationic complexes [Ir(C N)2L2] (L = neutral ligands
such as CO) would be harder to oxidize than anionic com-
–
plexes [Ir(C N)2A2] (A = anionic ligands such as CN–)
∧
when L and A are the ligands having comparable ligand-
field strength. One could expect, therefore, a larger energy
gap between the HOMO and LUMO of the cationic com-
Results and Discussion
Cationic complexes [Ir(ppy)2L2]+ (2) and [Ir(ppy)2LLЈ]+
(3, 4) have been newly prepared (Scheme 1) and unambigu-
ously characterized in detail by H-, 13C-, 31P NMR, IR
1
[a] Chemistry Department, Sogang University,
Seoul 121-742, Korea
spectral and elemental analysis data and also X-ray diffrac-
tion data analysis for single crystals of [Ir(ppy)2-
(PPh3)(NCMe)]+ (4a). Assignment of the spectral signals is
straightforward by comparison with those of related com-
pounds previously reported (see Supporting Information).
Fax: +82-2-701-0967
E-mail: cschin@sogang.ac.kr
[b] Chemistry Department, Chungnam National University
Daejeon 305-764, Korea
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
372
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 372–375

Blue-Light-Emitting Complexes
SHORT COMMUNICATION
For example, the Ir(ppy)2 moiety is unambiguously iden-
tified by detailed 1H- and 13C-NMR spectroscopic data
while carbonyl (CO), cyano (CN), and MeCN complexes
are characterized by IR absorptions due to ν(CϵO),
ν(CϵN), and ν(NϵC).
Figure 1. ORTEP drawing of [Ir(ppy)2(NCMe)(PPh3)]+ (4a) with
50% thermal ellipsoids probability. Selected bond lengths [Å]: Ir–
N1 = 2.081(6); Ir–N13 = 2.046(6); Ir–N25 = 2.111(7); Ir–C12
=
2.052(7); Ir–C24 = 2.026(8), Ir–P28 = 2.4342(19). Selected bond
angles [°]: N1–Ir–N13 = 171.3(2); C12–Ir–P28 = 178.44(19); C24–Ir–
N25 = 171.3(3).
dissociation of the labile MeCN ligand from 2 is sup-
pressed, although no measurable emission is found in
CH2Cl2 for 2e as reported previously by Watts.[7]
Table 1. Photophysical data[a,b,c] for cationic complexes [Ir(ppy)2-
L2]+ (2), [Ir(ppy)2(CO)(L)]+ (3), and [Ir(ppy)2(NCMe)(L)]+ (4).[a]
Scheme 1. Synthetic routes for [Ir(ppy)2L2]+ (2), [Ir(ppy)2(CO)L]+
(3), and [Ir(ppy)2(NCMe)L]+ (4).
[c]
(L)(LЈ)
λmax
ΦPL
τ
[nm]
[µsec]
Attempts to prepare bis(PPh3) complex [Ir(ppy)2(PPh3)2]+
have been unsuccessful probably because PPh3 is too large
for the cis-Ir(ppy)2 moiety to have two PPh3 in the cis posi-
tions. Figure 1 shows that replacement of the MeCN ligand
of 4a with another PPh3 would make the metal center too
crowded to give the stable bis(PPh3) complex. A smaller
phosphane (PPh2Me) and a less congested phosphite
[P(OPh)3] fit into the cis positions of [Ir(ppy)2L2]+ to give
stable complexes, 2b and 2c.
2a
2b
2c
2d
2e
3a
3b
3c
3d
4a
4b
4c
5
(CO)(CO)
451, 482
460, 491
458, 486
480, 508
469, 500
454, 485
466, 487
455, 485
451, 482
464, 495
463, 493
457, 488
473, 495
0.073
0.32
0.092
0.73
0.021
0.054
0.41
0.089
0.19
0.015
0.29
0.10
0.15
0.019
0.10
0.80
0.25
1.1
0.027
0.029
0.34
2.6
(PPh2Me)(PPh2Me)
[P(OPh)3][P(OPh)3]
(py)(py)
(NCMe)(NCMe)
(CO)(NCMe)
(CO)(py)
(CO)(PPh3)
(CO)[P(OPh)3]
(PPh3)(NCMe)
(PPh2Me)(NCMe)
[P(OPh)3](NCMe)
(CN)(CN)
0.034
0.040
0.13
Dicarbonyl complex [Ir(ppy)2(CO)2]+ (2a) was the first
complex synthesized in this study because the ligand-field
strength of CO is as strong as that of CN– and photophysi-
cal data for the anionic complex [Ir(ppy)2(CN)2]– (5)[4a] are
available to compare with those of 2a.
0.79[b]
[a] Compounds 2a–d and 3b–d in CH2Cl2 and 2e, 3a, 4a–4c, and 5
in CH3CN at 298 K under an atmosphere of Ar. [b] Measured in
this study and in agreement with the reported value,[4a] within ex-
perimental errors. [c] Calculated employing fac-Ir(ppy)3 (ΦPL
=
Cationic complex 2a shows emission λmax at 451 and
482 nm which are much shorter than those (473, 495 nm)
of anionic complex 5 (see Table 1). Cationic complexes 2b
0.40[8]) as the standard.
The quantum yields of complexes 2a–d decrease signifi-
and 2c with other strong-field ligands PPh2Me and cantly with a shift of the emission λmax to shorter wave-
P(OPh)3 also emit blue color, whereas a longer emission lengths (see Table 1 and Figure 2). These results agree with
λmax (480, 508 nm) is measured for complex 2d containing the suggestion that the luminescence quantum yields de-
the medium-strength-field ligand pyridine (Figure 2). Com- cline with increasing emission energies.[4b] Among com-
plex 2e with the other N-based ligand MeCN shows the plexes prepared in this study, complex 2d shows the longest
emission at λmax at 469 and 500 nm in MeCN where the emission λmax and the highest quantum yield. Both emis-
Eur. J. Inorg. Chem. 2007, 372–375
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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373

C. S. Chin, M.-S. Eum, S. y. Kim, C. Kim, S. K. Kang
SHORT COMMUNICATION
shorter emission λmax than does the sp2 N-based ligand pyr-
idine as observed for 2d, 2e, 3a, and 3b (see Table 1). The
effect of the field strength of the ancillary ligand on the
emission energy is unambiguously confirmed again by re-
placing medium-field ligand pyridine with stronger-field li-
gand CO, which causes the emission λmax to be blueshifted
from 480 and 508 nm for [Ir(ppy)2(py)2]+ (2d) to 466 and
487 nm for [Ir(ppy)2(py)(CO)]+ (3b).
It is noticed that the quantum yield is relatively higher
for P(OPh)3 complex 3d (0.19) than it is for both MeCN
(3a) and PPh3 (3c) complexes although the emission λmax is
shorter for 3d than it is for 3a and 3c, which may be under-
stood by the fact that P(OPh)3 is more basic than PPh3 and
MeCN and less dissociation of the ligand from the metal is
expected for 3d than for 3a and 3c.
Unlike bis-MeCN complex 2e, mono-MeCN complexes
3a and 4a–c exhibit measurable emission in CH2Cl2 even in
the absence of excess MeCN. Dissociation of the labile
MeCN ligand, however, seems significant for most MeCN
containing complexes and much lower quantum yields and
redshifted emission λmax are observed in the absence of ex-
cess MeCN.
Cyclic voltammetric measurements for complexes pre-
pared in this study show, in general, poorly resolved revers-
ible waves for both oxidation and reduction. It is, however,
confirmed that cationic complex 2a clearly shows a more
positive oxidation potential at 1.15 V than anionic com-
plexes 5 (0.9 V)[4a] in CH2Cl2 (vs. Fc2+/Fc+) while 5 shows
a more negative reduction potential (–1.70 V)[4a] than 2a
(–1.20 V) (see Supporting Information for voltammog-
rams).
In order to see the effects of ppy ligand modification on
the emission of [Ir(ppy)2L2]+ (2), complexes of F2ppy and
F2Meppy (2aF2, 2aF2Me, 2bF2, 2bF2Me) have been pre-
pared by the same method used for the preparation of 2
(Scheme 1). The blueshifts of the emission λmax caused by
the modification of the ppy ligand are somewhat larger for
the bisphosphane complexes (2b) than for the dicarbonyl
complexes (2a) (see Table 2 and Figure 3). It should be
Figure 2. Normalized emission spectra of [Ir(ppy)2L2](OTf) (2,
1ϫ10–4 ) in CH2Cl2 (2a–d), and MeCN (2e) at room
temperature;·L = CO (2a), PPh2Me (2b), P(OPh)3 (2c), py (2d),
MeCN (2e).
sion λmax and quantum yield of cationic complex 2d (λmax
= 480, 508 nm;
= 0.73 in CH2Cl2) are close to those of
PL
anionic complex 5 (λmax = 473, 495 nm; ΦPL = 0.79 in
MeCN) although pyridine is a much weaker-field ligand
than CN–. The lifetimes of cationic complexes prepared in
this study are significantly shorter than that of anionic com-
plex 5 and shorter lifetimes are measured for the complexes
with relatively higher quantum yields (see Table 1).
The results observed for [Ir(ppy)2(L)2]+ (2) agree with the
anticipation that the emission λmax would be blueshifted by
the strong-field (high trans effect) ancillary ligands L such
as C- and P-based ligands, whereas a longer emission λmax
would be seen for medium-field ligands L such as N-based
ligands.
Complexes having two different strong-field ligands,
[Ir(ppy)2(CO)(PPh3)]+ (3c) and [Ir(ppy)2(CO){P(OPh)3}]+
(3d) also show shorter emission λmax, whereas those with
one MeCN or py (3b and 4a–c) show λmax at relatively
longer wavelengths (Table 1). It seems certain that among
the N-based ligands, the sp N-based ligand MeCN gives
Figure 3. Normalized emission spectra of [Ir(ppy)2(CO)2]+ (2a), [Ir(F2ppy)2(CO)2]+ (2aF2), [Ir(F2Meppy)2(CO)2]+ (2aF2Me), [Ir(ppy)2-
(PPh2Me)2]+ (2b), [Ir(F2ppy)2(PPh2Me)2]+ (2bF2) and [Ir(F2Meppy)2(PPh2Me)2]+ (2bF2Me) in CH2Cl2 at 25 °C. Complex 2aF2Me (left)
displays somewhat of a more bluish emission than does 2bF2Me (right).
374
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Blue-Light-Emitting Complexes
SHORT COMMUNICATION
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∧
Table 2. Emission spectral data for [Ir(C N)2L2]+ (2) in CH2Cl2 at
25 °C.
∧
Complex C N
L
λmax
ΦPL
τ
[nm]
[µsec]
2aF2
F2ppy
(CO)
445, 474
441, 470
0.079
0.097
0.71
0.17
0.15
1.41
1.16
2aF2Me F2Meppy (CO)
2bF2 F2ppy
2bF2Me F2Meppy (PPh2Me) 443, 471
(PPh2Me) 446, 475
0.80
In summary, the emission λmax of [Ir(ppy)2(L)2]+ and
[Ir(ppy)2(L)(LЈ)]+ significantly shifts to shorter wavelengths
with strong-ligand-field ancillary ligands (L and LЈ) such as
CO and phosphorus-based ligands [PPh3, PPh2Me,
P(OPh)3] compared with the emission λmax of less-strong-
field ligands such as nitrogen-based ligands (MeCN, py).
For ancillary ligands of comparable ligand-field strengths
such as CO and CN–, the cationic complex [Ir(ppy)2(CO)2]+
emits shorter wavelength than does the anionic one
[Ir(ppy)2(CN)2]–. Blue-light emission is obtained from cat-
ionic complexes 2aF2Me and 2bF2Me which contain
strong-field ancillary ligands, CO and PPh2Me, and a modi-
fied ppy ligand, F2Meppy.
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1
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Acknowledgments
The authors wish to thank the Korea Research Foundation for
their financial support of this study through the Basic Science Re-
search Institute Program (Grant No. KRF-2005-015-C00222). Our
appreciation also goes to Professor B. Lee, Phys. Dept., Sogang
Univ. for the loan of the apparatus used to perform the phospho-
rescence lifetime measurements and Professor W. Shin, Chem.
Dept., Sogang Univ. for the cyclic voltammetric measurements.
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Received: September 29, 2006
Published Online: December 12, 2006
Eur. J. Inorg. Chem. 2007, 372–375
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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