Dinuclear iridium(III) complexes linked by a bis(β-diketonato) bridging ligand: Energy convergence versus aggregation-induced emission

Article abstract of DOI:10.1002/ejic.201000275

Novel iridium(III)/iridium(III) and iridium(III)/platinum(II) dinuclear complexes, [{Ir(ppyFF)₂}₂(μ₂-L)] (4) and [{Ir(ppyFF)₂}-(μ₂-L){Pt(ppy)}] (5) [ppyFF = 2-(2,4-difluorophenyl)pyridine, ppy = 2-phenylpyridine, L = 1,3-bis(3-phenyl-3- oxopropanoyl) benzene], linked by an L bridging ligand were prepared, and their photophysical properties were investigated in solution and in the solid state. The photophysical properties of mononuclear iridium(III) and platinum(II) complexes, [Ir(ppyFF)₂(dbm)] (1) and [Pt(ppy)(dbm)] (2) bearing a dibenzoylmethane(dbm) ligand were also compared. Whereas the UV/Vis absorption spectra of 4 and 5 show independent light absorption at each metal-centered moiety, the photoluminescence spectra of 4 and 5 display almost identical features, but very weak emissions in solution at both room temperature and 77 K. The weak emission in solution is found to mainly originate from a ³LX state of the L bridging ligand, which reflects the occurrence of efficient energy convergence from the triplet states of the Pt(ppy) and Ir(ppyFF) moieties to the ³LX state of L. By contrast, intense orange-red emission, that is, aggregation-induced emission, is produced in the solid state of 4 and 5. Inspection of the crystal-packing structures of 5 reveals that strong intermolecular π-π interactions between the adjacent pyridine rings of ppyFF ligands in the Ir-centered moieties are responsible for the emissive metal-to-ligand-ligand charge-transfer [³M(LL)CT] state of the solid-state dinuclear systems. The electrochemical properties of 4 and 5 further indicate that the first two reductions occur at the dbm moieties of the L bridging ligand linked to each metal center, which is consistent with the fact that the lowest-energy excited state of the L bridging ligand dominates the excited-state properties of 4 and 5 in solution.

Full text of DOI:10.1002/ejic.201000275

FULL PAPER
DOI: 10.1002/ejic.201000275
Dinuclear Iridium(III) Complexes Linked by a Bis(β-diketonato) Bridging
Ligand: Energy Convergence versus Aggregation-Induced Emission
Chang Hwan Shin,[a] Jung Oh Huh,[a] Sun Jong Baek,[a] Sang Kyu Kim,[a]
Min Hyung Lee,*[b] and Youngkyu Do*[a]
Keywords: Aggregation / Dimetallic complexes / Phosphorescence / Iridium / Platinum
Novel iridium(III)/iridium(III) and iridium(III)/platinum(II) di-
nuclear complexes, [{Ir(ppyFF)2}2(µ2-L)] (4) and [{Ir(ppyFF)2}-
(µ2-L){Pt(ppy)}] (5) [ppyFF = 2-(2,4-difluorophenyl)pyridine,
which reflects the occurrence of efficient energy conver-
gence from the triplet states of the Pt(ppy) and Ir(ppyFF) moi-
eties to the 3LX state of L. By contrast, intense orange-red
emission, that is, aggregation-induced emission, is produced
ppy
= 2-phenylpyridine, L = 1,3-bis(3-phenyl-3-oxopro-
panoyl)benzene], linked by an L bridging ligand were pre- in the solid state of 4 and 5. Inspection of the crystal-packing
pared, and their photophysical properties were investigated
in solution and in the solid state. The photophysical proper-
ties of mononuclear iridium(III) and platinum(II) complexes,
[Ir(ppyFF)2(dbm)] (1) and [Pt(ppy)(dbm)] (2) bearing a diben-
zoylmethane (dbm) ligand were also compared. Whereas the
UV/Vis absorption spectra of 4 and 5 show independent light
absorption at each metal-centered moiety, the photolumines-
cence spectra of 4 and 5 display almost identical features,
but very weak emissions in solution at both room tempera-
ture and 77 K. The weak emission in solution is found to
mainly originate from a 3LX state of the L bridging ligand,
structures of 5 reveals that strong intermolecular π–π interac-
tions between the adjacent pyridine rings of ppyFF ligands
in the Ir-centered moieties are responsible for the emissive
metal-to-ligand–ligand charge-transfer [3M(LL)CT] state of
the solid-state dinuclear systems. The electrochemical prop-
erties of 4 and 5 further indicate that the first two reductions
occur at the dbm moieties of the L bridging ligand linked to
each metal center, which is consistent with the fact that the
lowest-energy excited state of the L bridging ligand domi-
nates the excited-state properties of 4 and 5 in solution.
complexes may be of particular interest owing to the excel-
lent emitting properties of molecular complexes[3] such as
the high quantum efficiency of their phosphorescent emis-
sion.[4]
Introduction
Luminescent solid materials have recently attracted great
attention in the areas of optoelectronic device applications
such as organic light-emitting diodes and sensors.[1] Al-
though the majority of organic dyes and luminescent metal
complexes are highly emissive in their molecular state, ag-
gregation into the solid state often has detrimental effects
on their light-emitting properties like emission quenching,
which may render them less suitable for many optical appli-
cations. Thus, it would be highly desirable to construct an
emissive solid state or to enhance the light emission of mo-
lecular species by means of aggregation-induced emission
(AIE).[2] Although a number of AIEs of organic solid mate-
rials that constitute fluorescent emission have been re-
ported, the solid-state emission based on heavy-metal ion
It has been well established in the square-planar plati-
num(II) complexes that PtII–PtII interactions and/or π–π
stacking of the ligands gives rise to the appearance of AIE
or in some cases to the multiple AIEs induced by poly-
morphism.[5] In contrast, the phosphorescent AIEs of
iridium(III) systems have been relatively less investigated
owing to the octahedral structure of the iridium(III) com-
plex.[6–11] Regarding the AIE phenomena, Zhao and Huang
et al. demonstrated that the phosphorescent AIEs of hetero-
leptic iridium(III) complexes such as [Ir(ppy)2(dbm)] (ppy
= 2-phenylpyridine, dbm = dibenzoylmethane)[11] can be in-
duced by the intermolecular excimer state, that is, the
metal-to-ligand–ligand charge-transfer [3M(LL)CT] state
(Scheme 1).[8,9] Park and You et al. also suggested that the
restricted intramolecular relaxation in the solid state is re-
sponsible for the AIE of the heteroleptic iridium(III) com-
plexes such as [Ir(ppyFF)2(pip)] [pip = 2-(phenylimino-
methyl)phenol] bearing an imine ancillary ligand[10] al-
though the proposed mechanism has been recently refuted
by Chou and Chi et al.[12] and Huang et al.[8] Very recently,
we have also reported that the dual phosphorescent emis-
[a] Department of Chemistry, KAIST,
Daejeon 305-701, Republic of Korea
Fax: +82-42-350-2810
E-mail: ykdo@kaist.ac.kr
[b] Department of Chemistry and Energy Harvest-Storage
Research Center,
University of Ulsan, Ulsan 680-749, Republic of Korea
Fax: +82-52-259-2348
E-mail: lmh74@ulsan.ac.kr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000275.
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Eur. J. Inorg. Chem. 2010, 3642–3651

Dinuclear IrIII Complexes Linked by a Bis(β-diketonato) Bridging Ligand
Scheme 1.
sion from a solid-state iridium(III) complex could be and AIE of dinuclear complexes. In this regard, we designed
achievable through the crystal-packing polymorphism in- novel IrIII/IrIII homodinuclear and IrIII/PtII heterodinuclear
duced by the modification of π–π interactions between the complexes that are linked by means of a bridging ligand
cyclometalated ligands of [Ir(ppyFF)2(dbm)] (1) [ppyFF = comprising two β-diketonato binding sites. As noted in pre-
2-(2,4-difluorophenyl)pyridine].[7] These examples of the vious reports, a β-diketonato ligand such as dbm can act as
3
iridium(III) systems reveal that the very weakly emissive or an energy-converging unit through its low-energy LX ex-
nonemissive 3LX excited state of the ancillary ligand can be cited state in the mononuclear systems. By the same anal-
switched into emissive states if a proper manipulation of ogy, we chose 1,3-bis(3-phenyl-3-oxopropanoyl)benzene
the crystal-packing structure is allowed in the solid state.
In a continuous effort to develop novel AIE, we turned connected by a 1,3-phenylene spacer as the bridging ligand.
our attention to the dinuclear iridium(III) systems bearing In the present study, the synthesis and characterization
(H2L),[18,19] which possesses two β-diketonato binding sites
photoactive cyclometalated ligands, as the multinuclearity of neutral IrIII/IrIII homodinuclear and IrIII/PtII heterodinu-
could lead to facile modulation of π–π stacking and/or clear complexes linked by a bis(β-diketonato) bridging li-
metal–metal interactions in the solid state. Furthermore, gand (L) are described to investigate the electronic energy
whereas the earlier investigations of phosphorescence have convergence from the metal centers to the bridging ligand
been extensively directed toward mononuclear IrIII com- in the molecular state, as well as the solid-state AIE.
plexes,[13–15] there have been few reports on the photophysi-
cal properties of dinuclear IrIII complexes.[16] Since the so-
lid-state AIE derived from dinuclear complexes could also
Results and Discussion
Synthesis and Crystal Structure
be useful for the development of color-tunable emitters for
full-color and white-light display applications,[17] it would
The mononuclear platinum complex [Pt(ppy)(dbm)] (2)
be intriguing to investigate the photophysical properties was obtained in good yield (68%) according to the analo-
Scheme 2. Reaction conditions: (i) 2-ethoxyethanol, Na2CO3, 25 °C, 24 h, 45%; (ii) 2-ethoxyethanol, Na2CO3, 130 °C, 24 h, 70%; (iii) 2-
ethoxyethanol, Na2CO3, 25 °C, 24 h, 40%.
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C. H. Shin, J. O. Huh, S. J. Baek, S. K. Kim, M. H. Lee, Y. Do
FULL PAPER
gous method applied to the synthesis of 1. The synthesis of
mono- and diiridium complexes of [Ir(ppyFF)2(HL)] (3)
and [{Ir(ppyFF)2}2(µ2-L)] (4) was achieved through the re-
action of the H2L ligand and a dimeric precursor,
[{Ir(ppyFF)2(µ-Cl)}2], by varying the reaction temperature
and the stoichiometry of the reactants according to the pro-
cedures outlined in Scheme 2. It is noteworthy that the reac-
tion temperature plays an important role in the syntheses
of 3 and 4. When [{Ir(ppyFF)2(µ-Cl)}2] was treated with
H2L (1–3 equiv.) at room temperature, monosubstituted β-
diketonato complex 3 was the sole product. In contrast, the
same reaction at 130 °C gave disubstituted 4 as the major
product. This result may indicate that 4 is a thermodynamic
product of the reaction between [{Ir(ppyFF)2(µ-Cl)}2] and
H2L. Indeed, 3 was slowly changed to 4 upon heating of
the solution. The reaction between 3 and the [{Pt(ppy)(µ-
Cl)}2] precursor at room temperature afforded an orange
solid of heterodinuclear complex [{Ir(ppyFF)2}(µ2-
L){Pt(ppy)}] (5) linked by an L bridge in 40% yield. All the
Figure 1. Molecular structure of 5 (50% thermal ellipsoid). Hydro-
gen atoms and solvent molecules (THF) are omitted for clarity.
Selected bond lengths [Å] and angles [°]: Ir–C41 1.992(7), Ir–C52
1.994(6), Ir–N1 2.043(5), Ir–N2 2.045(5), Ir–O4 2.120(5), Ir–O3
2.145(4), Pt–N3 1.911(8), Pt–C16 1.982(6), Pt–O2 2.034(5), Pt–O1
2.039(4); C41–Ir–N1 82.1(2), C52–Ir–N2 81.2(3), O4–Ir–O3
87.95(17), N3–Pt–C16 81.6(3), C16–Pt–O2 94.9(2), N3–Pt–O1
92.1(2), O2–Pt–O1 91.42(18).
1
complexes were characterized by H and 13C NMR spec-
ing from 350 to 400 nm are caused by π–π* transitions
(1LX) on the β-diketonato bridging ligands (L and dbm).[19]
Whereas the lower energy absorption in 400–500 nm is
weak for 2, the Ir-centered species 1, 4, and 5 exhibit more
intense bands with two shoulders in this region. The weak
shoulder at 437 nm is assignable to the spin-allowed MLCT
transition (1MLCT), and the other one at 465 nm, which
tails to 500 nm, can be assigned to the spin-forbidden
troscopy, MALDI-TOF mass spectrometry, and elemental
analysis. In particular, an X-ray diffraction study was per-
formed on single crystals of 5.
The molecular structure of 5 and selected interatomic
distances and angles are shown in Figure 1. The structure
of 5 clearly shows the heterodinuclear connectivity between
the Ir- and Pt-centered moieties by means of the L bridging
ligand. The Ir-centered moiety adopts a Λ-configuration
and bears two ppyFF ligands with a trans disposition of
pyridine rings. In the Pt-centered moiety, the N3–Pt–C16
angle of 81.6(3)° and the O2–Pt–O1 angle of 91.42(18)°
indicate a distorted square-planar geometry around the Pt
center similar to that of the reported cyclometalated Pt
complexes.[20] When compared to the dihedral angle be-
tween the phenyl rings in dbm of 1 (20.25°),[7] the dbm frag-
ment at the Ir center of 5 forms a much larger angle of
53.97° probably due to the steric hindrance exerted by the
adjacent ppy ligand of the Pt-centered moiety. In addition,
the planes defined by O3–Ir–O4 and O1–Pt–O2 are linked
to the central phenylene plane of the L bridging ligand by
a relatively small dihedral angle of 18.74 and 10.68°, respec-
tively, which indicates the presence of electronic conjugation
between the planes. This feature may suggest that efficient
electronic interactions can occur between the metal-cen-
tered moieties and the L bridging ligand through a π-orbital
overlap.[21] It seems that Ir and Pt atoms do not interact
with each other as judged by the large Ir···Pt separation of
9.926 Å.
1
MLCT transition (3MLCT). These features of MLCT and
3MLCT transitions are very similar to those of the reported
phosphorescent Ir complexes.[15,23] In particular, the ab-
sorption spectrum of 4 shows an identical absorption band
with an intensity twice that of the spectrum of 1. Heterodi-
nuclear 5 also exhibits an absorption feature corresponding
to the sum of the independent spectra of 1 and 2. These
results indicate that the electronic transition at each metal-
centered moiety in 4 and 5 is not affected by the L bridging
ligand.
Photophysical Properties and Solid-State Emission
Figure 2. Absorption spectra of 1, 2, 4, and 5 in CHCl3 at room
The absorption spectra of the complexes were recorded
in degassed solutions of chloroform at room temperature
(Figure 2 and Table 1). The intense high-energy bands be-
temperature. Inset: enlarged absorption spectra.
The room-temperature photoluminescence (PL) spectra
low the 300 nm region can be assigned to singlet spin-al- of solutions of 1, 2, 4, and 5 in degassed chloroform are
lowed π–π* ligand-centered (1LC) transitions of ppy and shown in Figure 3, and the photophysical data are summa-
ppyFF ligands.[15,20,22] The broad bands in the region rang- rized in Table 1. The selective excitation at a wavelength of
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Dinuclear IrIII Complexes Linked by a Bis(β-diketonato) Bridging Ligand
Table 1. Photophysical data.
Absorption[a]
Emission at 293 K[b]
Solution[a]
λmax [nm] τ [ns]
Emission at 77 K[b]
λabs [nm] (ε [10–4

–1 cm–1])
Solid
λmax [nm] τ [ns] λmax [nm]
[c]
Φem
τ [µs]
1
2
4
5
254 (4.88), 290 (3.51), 330 (1.88),
388 (1.08), 437 (0.34), 465 (0.15)
261 (3.72), 280 (3.12),
611
540
615
615
23
180
22
3.0ϫ10–2
2.3ϫ10–1
3.0ϫ10–2
2.3ϫ10–2
605[d]
548, 570
597
n.d.[e]
n.d.
67
555
1.3
495, 527,
555 (sh)
570
4.9
1.3
1.4
314 (1.88), 365 (1.56)
253 (10.0), 290 (6.75), 328 (4.07),
388 (2.17), 435 (0.76), 465 (0.31)
259 (8.58), 281 (6.81), 327 (3.83),
369 (2.72), 388 (2.43), 437 (0.58), 465 (0.21)
22
597
56
570
[a] In degassed CHCl3. [b] λex = 388 nm. [c] [Ir(ppy)3] (Φem = 0.40) was used as a standard. [d] For the orange-red crystals of 1. [e] Not
determined.
388 nm provides the most feasible quantitative explanation more, a comparison of the emission intensities for 1 and 5
of the electronic energy relaxation since the Ir- and Pt-cen- at the same molar concentration reveals that the 615 nm-
tered moieties are estimated to absorb nearly an equal band intensity of 5 is increased 1.5-fold. This result may
quantity of light at that wavelength, as judged from the sim- suggest that the internal conversion of the Tn state, which
ilar absorption coefficients of mononuclear complexes 1 is contributed mainly by the Pt(ppy) moiety, into an energy
and 2 (ε1 = 10804 –1 cm–1; ε2 = 11319 –1 cm–1). As re- state of 615 nm, that is, the T1 state, dominated by the 3LX
ported earlier, the very weak emission band at 611 nm in 1 state of the L bridging ligand, effectively takes place in 5.
could be assignable to the major contribution from the 3LX
state of the dbm ligand.[7,9] Moreover, although weak in
intensity, the observation of the emission band of 1 (Φem
=
0.03) with a relatively short emission lifetime (τ = 23 ns for
1) may further indicate the possible involvement of the
3MLCT state, thus leading to the proper description of the
lowest-energy excited state of 1 in solution as the mixed
3MLCT–3LX state. The absence of a 3MLCT band is likely
due to the large π–π* energy gap of the ppyFF ligand,
which causes the LUMO to be located mostly at the dbm
3
ligand, thus allowing the LX state of the dbm ligand to
dominate the lowest-lying excited state. Chou and Chi et
al. very recently demonstrated that all higher-energy excited
states of relevant heteroleptic IrIII complexes are strongly
coupled with each other.[12] In contrast, 2 exhibits an in-
tense emission band at 540 nm with high quantum effi-
ciency (Φem = 0.23), which indicates that the emission from
an excited state of the Pt(ppy) moiety is not quenched de-
Figure 3. Emission spectra of 1, 2, 4, and 5 in CHCl3 (1.0ϫ10–5 ;
λex = 388 nm) at room temperature.
To clarify the electronic energy convergence, low-tem-
spite the presence of the dbm moiety in 2 (vide infra). The perature PL experiments were performed in chloroform ri-
emission spectrum of homodinuclear complex 4 exhibits a gid matrices at 77 K. The emission spectra of 2, 4, and 5
very weak emission band at 615 nm with low quantum effi- are displayed in Figure 4, and that of 1 is shown for com-
ciency (Φem = 0.03) indicative of inefficient phosphores- parison. The mononuclear Pt complex 2 shows a structured
cence. This feature is in fact very similar to that observed band at 495 nm with a long emission lifetime of 4.9 µs, thus
in the mononuclear complex 1 (λem = 611 nm, Φem
=
confirming that the emission is originated from a mixed
0.03),[7] which suggests that the observed weak emission in MLCT–3LC(ppy) excited state[20] (Figure 4a). In addition,
4 is dominated by the L-based triplet excited state (3LX). the weak shoulder around 555 nm that was not observed in
Interestingly, heterodinuclear 5 also exhibits a weak emis- the room-temperature spectrum could be attributed to a
sion band at 615 nm with low quantum efficiency (Φem
=
dbm-based excited state (3LX) as similarly found in 1 (λem
0.023) as observed in 4. The characteristic emission band = 555 nm at 77 K).[7,9]
around 540 nm that appeared in the mononuclear Pt com-
This result indicates that the lowest-energy excited state
plex 2 is not observed in the emission spectrum of 5, of 2 could be described as a mixed MLCT–3LC(ppy)–
pointing to the quenching of the Pt(ppy)-based emission. 3LX(dbm) state presumably due to a small difference in en-
One order of magnitude decrease in quantum efficiency ergy between these states. In contrast, the emission band of
(Φem = 0.23 for 2 vs. 0.023 for 5) and a much reduced emis- the Pt(ppy) moiety in heterodinuclear 5 is completely
sion lifetime of 5 relative to that of 2 (τ = 180 ns for 2 vs. quenched at 77 K, only giving rise to a 570 nm band. This
22 ns for 5) are in agreement with this observation. Further- feature is actually similar to that observed at room tempera-
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C. H. Shin, J. O. Huh, S. J. Baek, S. K. Kim, M. H. Lee, Y. Do
FULL PAPER
3LX state of the L bridging ligand, the energy level of which
would be lower than that of dbm probably owing to a
longer conjugation length. Consequently, as depicted in the
schematic energy-level diagram of 5 (Figure 5), all these re-
sults together with the electrochemical data (vide infra) may
suggest that in the given molecular dinuclear IrIII systems
linked by the bis(β-diketonato) L bridging ligand, energy
convergence effectively takes place from the Tn states con-
tributed by metal-centered moieties to the T1 state domi-
nated by the L bridging ligand, which thus plays the role of
an energy “collector” or a “converging unit”.
Figure 4. Emission spectra (λex = 388 nm) of 1, 2, 4, and 5 in (a) a
CHCl3 rigid matrix at 77 K and (b) the solid state. The solid-state
spectrum of 1 was taken from an orange polymorph reported in
the literature.[10]
ture. Since the emission spectra of an equimolar mixture of
1 and 2 contain all of the emission bands derived from the
Pt complex 2 (Figure S1 in the Supporting Information),
this finding signifies the occurrence of an efficient Tn Ǟ T1
internal conversion from the Pt(ppy) moiety to the L bridg-
ing ligand in 5. Moreover, the shapes of emission bands and
the maximum emission wavelength of the dinuclear com-
plexes 4 and 5 are virtually identical, and the maximum
peak positions are also redshifted by 15 nm in comparison
with mononuclear 1. This feature further indicates that the
Figure 5. Schematic energy-level diagram and proposed emission
processes of 5 in solution. IC: internal conversion; ISC: intersystem
lowest-excited state of 4 and 5 is mainly involved with the crossing; VR: vibrational relaxation.
Figure 6. Crystal-packing structures of 5. (a) Overall packing structure. (b) π–π interaction between Ir-centered moieties. (c) Extended
π–π stacking structure between Pt-centered moieties. (d) Top views showing two kinds of π–π interaction between Pt-centered moieties:
left, Pt(A)L···Pt(B)L interactions; right, Pt(B)(ppy)···Pt(C)(ppy) interactions.
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Dinuclear IrIII Complexes Linked by a Bis(β-diketonato) Bridging Ligand
Unlike the very weak luminescence of 4 and 5 in solu- dation feature with an onset potential of 0.48 V. In the case
tion, the intense orange-red emission (i.e., AIE) centered at of heterodinuclear 5, two successive oxidations are ob-
597 nm is observed in the solid state (Figure 4b and Fig- served. Whereas the irreversible first oxidation ascribable to
ure S2 in the Supporting Information). This feature is in the Pt-centered moiety in 5 occurs at a potential identical
accord with those observed in solid-state iridium complexes to that of 2, the reversible second oxidation potential of the
such as [Ir(ppy)2(dbm)][9] and 1.[7] To elucidate the origin of Ir-centered moiety (0.78 V) is slightly higher than that of 1.
the AIE, the crystal-packing structures of heterodinuclear 5 This result indicates a stabilization of the filled d orbitals
were examined. The packing structures illustrated in Fig- of the Ir center in 5 due to the electron-withdrawing effect
ure 6 clearly show that each metal-centered moiety forms π– caused by the first oxidation of the linked Pt moiety as simi-
π interactions in the crystal (Figure 6a). One is the effective larly found in an RuII/OsII system.[24] In contrast, 4 un-
overlap between the ppyFF ligands of the Ir-centered moie- dergoes a single two-electron reversible oxidation process
ties in adjacent molecules (Figure 6b), and the other is π–π consistent with identical environments of both Ir-centered
stacking of the Pt-centered moieties (Figure 6c). In the Ir- moieties and thereby simultaneous oxidation. Hence, the in-
centered moieties, the interplanar separation between the crease of the Ir-centered oxidation potential in 4 (0.76 V)
adjacent pyridine rings of ppyFF ligands is estimated to relative to the potential of 1 is less apparent than that in 5.
be approximately 3.40 Å, comparable to those observed in
mononuclear [Ir(ppy)2(dbm)] (3.37 Å)[9] and 1 (3.40 Å),[7]
which reflects the presence of sufficiently strong π–π inter-
Table 2. Electrochemical data.[a]
Complexes
Oxidation
Reduction
actions. The Pt-centered moieties, on the other hand, pack
as an extended π–π stacking structure consisting of two
kinds of alternating π–π interactions, that is, Pt(A)L···
Pt(B)L and Pt(B)(ppy)···Pt(C)(ppy) interactions (Figure 6c
and d). This feature is quite different from the isolated
ppyFF···ppyFF interactions observed in the Ir-centered
moieties. Along with the apparent incorporation of Pt cen-
ters into the stacking structures, this could be ascribed to
the planar geometry of the Pt moiety. Whereas a direct
Pt···Pt interaction appears absent as judged by the long sep-
arations [6.575 Å in Pt(A)L···Pt(B)L and 4.299 Å in Pt(B)-
(ppy)···Pt(C)(ppy)], the short interplanar separations of ap-
proximately 3.36 and 3.42 Å, respectively, indicate the pres-
ence of strong π–π interactions between the Pt-centered
moieties.[20] Therefore, it can be suggested that in the solid
state of 5, the strong π–π interactions between the adjacent
ppyFF ligands induce the ppyFF-centered triplet excited
2nd
1st
1st
2nd
3rd
E
[V]
E
[V]
E
[V]
E
red
[V]
E
[V]
ox
ox
red
red
(IrIII/IrIV
)
(PtII/PtIII
)
1
2
4
5
0.73
–
–
–2.16
–2.00
–2.06
–1.94
–2.77[b]
–2.58
–2.27
–2.23
–
–
0.48[b]
–
0.76[c]
0.78
–2.68[b]
–2.64[b]
0.48[b]
[a] Conditions: 10–3  in CH3CN, 0.1  [nBu4N][PF6], scan rate
50 mV s–1, redox potential reported versus E (ferrocene/ferrocen-
½
ium). [b] Irreversible redox. [c] Single two-electron wave.
3
state, that is, the M(LL)CT state, which is lower in energy
than 3LX of the L bridging ligand. As a result, the
3M(LL)CT state dominates the lowest-energy excited state
leading to the intense emission. Furthermore, the essentially
identical emission features observed for both 4 and 5 in
terms of emission wavelength (λem = 597 nm) and lifetime
(τ = 67 ns for 4 and 56 ns for 5) imply the quenching of the
Pt(ppy)-based emission in the solid state of 5 (Figure 4b).
We attribute this to the extended π–π stacking structure
comprised of alternating Pt(A)L···Pt(B)L and Pt(B)-
(ppy)···Pt(C)(ppy) interactions. Although the Pt(B)(ppy)···
Pt(C)(ppy) interactions may lead to an emissive 3M(LL)CT
excited state, the energy of such a state appears to be ef-
Figure 7. Cyclic voltammograms of 1, 2, 4, and 5 showing (a) oxi-
dation and (b) reduction.
However, mononuclear 1 undergoes a two-step reduction
ficiently converged into the weakly emissive triplet state process with first and second reduction potentials of –2.16
(3LLX) formed by adjacent Pt(A)L···Pt(B)L interactions.
and –2.77 V, respectively. Since the second reduction poten-
tial falls in the typical range observed for Ir–ppy deriva-
tives[13] and it is known that the LUMO of the [Ir(ppy)2-
(dbm)] complex is distributed mainly on the dbm ligand,[9]
the first and second reductions of 1 can be assigned to the
Electrochemical Properties
The electrochemical properties of the complexes were ex- dbm and ppyFF ligand-based reduction. In the case of 2,
amined by cyclic voltammetry (Table 2 and Figure 7). The two-step reduction processes take place at a less negative
mononuclear Ir complex 1 shows reversible oxidation at potential (–2.00 and –2.58 V) than those observed for 1,
0.73 V, whereas Pt complex 2 exhibits an irreversible oxi- but the reduction features are quite similar to those of 1.
Eur. J. Inorg. Chem. 2010, 3642–3651
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3647

C. H. Shin, J. O. Huh, S. J. Baek, S. K. Kim, M. H. Lee, Y. Do
FULL PAPER
Scheme 3. First and second reduction processes in 4 and 5.
Moreover, [(ppy)Pt(OʝO)][20] (OʝOH = acetylacetone, dipi- lowest-energy excited state of dinuclear complexes receives
valoylmethane) and [Pt(ppy)2][25] complexes are reported to its major contribution from the 3LX state of the L bridging
have a ppy ligand-based reduction at around –2.4 to –2.5 V. ligand, and hence the absorbed energy was efficiently con-
Although the second reduction potential of 2 is slightly verged into the energy acceptor, the L bridging ligand be-
more negative than the reported values, presumably due to tween IrIII/IrIII- and IrIII/PtII-based luminophores. In con-
the already reduced state of 2, these results indicate that the trast to the weak luminescence in solution, intense emission
first and second reductions of 2 are the dbm and ppy li- (i.e., aggregation-induced emission) was produced in the so-
gand-based reductions, respectively. The dinuclear com- lid state. It is suggested from the crystal-packing structures
plexes 4 and 5 undergo a three-step reduction process with that strong π–π intermolecular interactions between adja-
a comparable reduction potential at each reduction step. cent pyridine rings of ppyFF ligands in the Ir-centered moi-
The first and third reductions could be assignable to the eties are responsible for the emissive 3M(LL)CT state of the
reduction at the dbm moiety in the L bridging ligand and solid-state dinuclear systems. The electrochemical proper-
at the ppyFF or ppy ligand in the metal-centered moieties, ties of 4 and 5 further indicate that the first two reductions
respectively. The second reduction, which exhibits a revers- occur at the dbm moieties of the L bridging ligand linked
ible feature, has its potential close to the first reduction po- to each metal center, which supports evidence that the L
tentials of 1 and 2, which suggests that the second reduction bridging ligand dominates the lowest excited state of 4 and
also occurs at the dbm moiety in the L bridging ligand. 5 in solution.
Furthermore, the first reduction potentials of 4 and 5 show
an anodic shift, to a similar extent, with respect to those of
1 and 2, respectively, implying that the LUMO localized on Experimental Section
the dbm moiety of the L bridging ligand has a lower energy
General: All manipulations were performed under nitrogen by
than that of the monomeric dbm complexes. This feature is
using standard Schlenk and glove-box techniques. Anhydrous-
in accord with the extended conjugation in the L bridging
grade solvents (Aldrich) were purified by passing them through an
activated alumina (Acros, 50–200 micron) column. All reagents
ligand. In addition to the anodic shift of the first reduction
potential of 5 (–1.94 V) with respect to that of 4 (–2.06 V),
were used without any further purification after being purchased
as similarly shown in 1 and 2, these findings indicate that from Aldrich (dimethyl isophthalate, acetophenone, 2-phenylpyr-
idine, 2-bromopyridine, 2,4-difluorophenylboronic acid, dibenzo-
ylmethane, 2-ethoxyethanol), Fluka (anhydrous Na2CO3), and
Strem [iridium(III) chloride hydrate, potassium tetrachloro-
platinate]. The compounds 2-(2,4-difluorophenyl)pyridine,[13] 1,3-
bis(3-oxo-3-phenylpropanoyl)benzene (H2L),[19] [{Ir(ppyFF)2(µ-
Cl)}2],[13] [{Pt(ppy)(µ-Cl)}2],[20] and [Ir(ppyFF)2(dbm)] (1)[7] were
prepared according to modified literature procedures. Deuterated
solvents (Cambridge Isotope Laboratories) were dried with acti-
vated molecular sieves (5 Å). 1H and 13C NMR spectra of the com-
pounds were recorded with a Bruker Spectrospin 400 spectrometer
at ambient temperature. All chemical shifts are reported in δ with
reference to the residual peaks of CDCl3 for proton (δ =7.24 ppm)
and carbon (δ =77.0 ppm) chemical shifts. Elemental analyses (EA)
were carried out with an EA1110-FISONS (CE Instruments) in-
strument by the Environmental Analysis Laboratory at KAIST.
MALDI-TOF MS were measured with a Voyager DE-STR 4700
the initial two reduction processes in 4 and 5 involve the
first reduction at the dbm moiety of the L bridging ligand
linked to the Ir and Pt centers, respectively, and is followed
by the second reduction at the dbm moiety linked to the
remaining Ir centers in the first reduced state of 4 and 5
(Scheme 3). These reduction behaviors are also in good
agreement with the fact that the L bridging ligand domi-
nates the lowest-energy excited state of 4 and 5 in solution.
Conclusion
Novel IrIII/IrIII homodinuclear and IrIII/PtII heterodinu-
clear complexes linked by a bis(β-diketonato) bridging li-
gand (L) were synthesized and characterized. According to
the photophysical results in solution, it was found that the proteomics analyzer at the Korea Basic Science Center (KBSC).
3648 Eur. J. Inorg. Chem. 2010, 3642–3651
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Dinuclear IrIII Complexes Linked by a Bis(β-diketonato) Bridging Ligand
135.23, 130.81, 129.15, 128.76, 128.37, 127.28, 126.97, 126.71,
UV/Vis and PL spectra were obtained with a Jasco V-530 spectro-
photometer and a Spex Fluorog-3 luminescence spectrometer,
respectively. Solution PL experiments were performed by using
HPLC-grade chloroform. All solutions in chloroform were de-
gassed by several freeze-pump-thaw cycles using a diffusion pump.
The emission quantum yields of the complexes were calculated by
using degassed fac-[Ir(ppy)3] in toluene (Φ = 0.40) as a reference.
Low-temperature measurements were recorded in 5 mm diameter
quartz tubes that were placed in a quartz-walled Dewar flask filled
with liquid nitrogen (77 K). Emission lifetimes were determined by
using a third-harmonic generator of an Nd:YAG laser (Spectra-
Physics LAB170, 10 Hz). A laser beam collected through a fused
silica window was applied to excite the sample, and the delay time
(310 µs) between the flash lamp and Q-switch was used to reduce
the laser output power. The final power of the laser beam to the
pump samples was about 100 µJpulse–1. A monochromator (Dong-
woo Optron, DM150i) and a general photomultiplier tube (PMT;
Hamamatsu, R928) were employed to select the wavelength of the
emission and to detect the strength of the emission. The signals
from the PMT were spread on an oscilloscope (Lecroy waverunner
104Xi, 1 GHz bandwidth) and recorded. Cyclic voltammetry was
performed by using an AUTOLAB/PGSTAT12 model system with
a three-electrode cell configuration consisting of platinum working
and counter electrodes and an Ag/AgNO3 (0.1  in acetonitrile)
reference electrode at room temperature. Anhydrous acetonitrile
was used as the solvent, and 0.1  tetrabutylammonium hexafluo-
rophosphate was used as the supporting electrolyte. The redox po-
tentials were recorded at a 50 mVs–1 scan rate and reported against
the ferrocene/ferrocenium (Cp2Fe/Cp2Fe+; Cp = cyclopentadienyl)
redox couple that was used as an internal standard.
126.27, 125.40, 122.82, 122.56, 122.35, 121.88, 121.68, 115.39,
115.21, 97.61, 97.27, 96.91, 95.14, 93.13 ppm. MALDI-TOF MS:
m/z = 941.99 [M+]. C46H29F4IrN2O4 (941.94): calcd. C 58.65, H
3.10, N 2.97; found C 59.49, H 3.09, N 2.91.
Synthesis of [{Ir(ppyFF)2}2(µ2-L)] (4): [{Ir(ppyFF)2(µ-Cl)}2] (0.61 g,
0.5 mmol), H2L (0.19 g, 0.5 mmol), and Na2CO3 (0.53 g, 5.0 mmol)
were mixed in 2-ethoxyethanol (30 mL), and the mixture was
heated at 130 °C for 24 h. Workup followed by flash column
chromatography (eluent: dichloromethane) yielded 4 as an orange
solid (0.53 g, 70%). 1H NMR (400.13 MHz, CDCl3): δ = 8.46–8.51
(m, 4 H), 8.23 (t, JH,H = 6.8 Hz, 4 H), 8.15 (d, JH,H = 19 Hz, 1 H),
7.81–7.86 (m, 2 H), 7.67–7.74 (m, 8 H), 7.44 (t, JH,H = 7.0 Hz, 2
H), 7.30–7.35 (t, JH,H = 7.7 Hz, 4 H), 7.26 (dd, JH,H = 10.1, 1.8 Hz,
1 H), 7.07 (t, JH,H = 6.2 Hz, 2 H), 7.00–7.04 (m, 2 H), 6.51 (s, 2
H), 6.32–6.42 (m, 4 H), 5.70–5.79 (m, 4 H) ppm. 13C NMR
(100.62 MHz, CDCl3): δ = 179.50, 178.77, 178.67, 165.28, 164.66,
164.35, 164.19, 162.55, 160.98, 160.80, 159.11, 158.94, 151.59,
151.19, 147.89, 141.06, 140.50, 138.26, 137.85, 130.61, 129.15,
128.75, 128.51, 128.33, 127.15, 126.93, 126.38, 122.51, 121.64,
121.00, 115.31, 115.09, 97.62, 97.27, 96.90, 95.13 ppm. MALDI-
TOF MS: m/z = 1514.15 [M+]. C68H40F8Ir2N4O4 (1513.49): calcd.
C 53.96, H 2.66, N 3.70; found C 53.57, H 2.57, N 3.73.
Synthesis of [{Ir(ppyFF)2}(µ2-L){Pt(ppy)}] (5): Compound 3 (0.43 g,
0.46 mmol), [{Pt(ppy)(µ-Cl)}2] (0.18 g, 0.23 mmol), and Na2CO3
(0.49 g, 4.6 mmol) were mixed in 2-ethoxyethanol (30 mL) and
stirred at room temperature for 24 h. Workup followed by flash
column chromatography (eluent: ethyl acetate/hexane, 1:4) yielded
5 as an orange solid (0.23 g, 40%). 1H NMR (400.13 MHz,
CDCl3): δ = 9.07 (d, JH,H = 5.6 Hz, 0.5 H), 9.02 (d, JH,H = 5.5 Hz,
0.5 H), 8.53–8.59 (m, 2.5 H), 8.43 (s, 0.5 H), 8.26 (d, JH,H = 8.4 Hz,
Synthesis of [Pt(ppy)(dbm)] (2): [{Pt(ppy)(µ-Cl)}2] (0.45 g,
0.58 mmol), dibenzoylmethane (0.39 g, 1.74 mmol), and Na2CO3
(0.62 g, 5.8 mmol) were mixed in 2-ethoxyethanol (30 mL) at room
temperature. The mixture was stirred and heated at reflux over-
night. After cooling to room temperature, water (30 mL) was
added, and the mixture was then filtered to give a dark crude prod-
uct, which was washed with water (30 mL) and diethyl ether
(30 mL). The pure complex was obtained by flash chromatography
on a silica column by using dichloromethane as the eluent to yield
2 H), 8.09–8.11 (m, 1 H), 7.96–8.00 (m, 3 H), 7.86 (d, JH,H
=
7.6 Hz, 1 H), 7.81 (d, JH,H = 7.6 Hz, 1 H), 7.72–7.76 (m, 4 H), 7.56
(t, JH,H = 7.7 Hz, 2 H), 7.40–7.48 (m, 5 H), 7.32 (t, JH,H = 7.6 Hz,
2 H), 7.24 (t, J = 7.5 Hz, 0.5 H), 7.07–7.12 (m, 4 H), 6.90 (t, JH,H
= 7.5 Hz, 0.5 H), 6.76 (s, 0.5 H), 6.67 (t, JH,H = 7.4 Hz, 1.5 H),
6.36–6.43 (m,
2 H), 5.78–5.83 (m, 2
H) ppm. 13C NMR
(100.62 MHz, CDCl3): δ = 180.06, 179.63, 179.01, 178.58, 177.92,
168.35, 165.24, 163.85, 162.05, 161.31, 159.47, 151.46, 151.25,
148.03, 147.21, 147.00, 144.77, 141.06, 140.62, 140.41, 140.04,
139.19, 138.78, 138.22, 138.05, 131.00, 130.84, 130.67, 130.57,
129.43, 128.96, 128.75, 128.62, 128.46, 127.03, 126.89, 125.10,
123.66, 123.12, 122.67, 122.48, 121.79, 121.36, 118.45, 115.32,
115.15, 97.52, 97.20, 97.02, 95.19, 95.07 ppm. MALDI-TOF MS:
m/z = 1289.78 [M+]. C57H36F4IrN3O4Pt (1290.20): calcd. C 53.06,
H 2.81, N 3.26; found C 53.86, H 2.73, N 3.28.
1
2 (0.45 g) as a yellow solid in 68% yield. H NMR (400.13 MHz,
CDCl3): δ = 9.11 (d, JH,H = 5.7 Hz, 1 H), 8.02–8.08 (m, 4 H), 7.75–
7.82 (m, 2 H), 7.63 (d, JH,H = 8.0 Hz, 1 H), 7.53–7.56 (m, 2 H),
7.45–7.50 (m, 5 H), 7.25 (td, JH,H = 7.3, 0.9 Hz, 1 H), 7.10–7.16
(m, 2 H), 6.75 (s, 1 H) ppm. 13C NMR (100.62 MHz, CDCl3): δ =
179.92, 178.94, 168.49, 147.28, 144.77, 140.27, 139.30, 138.88,
138.24, 130.90, 130.87, 130.74, 129.36, 128.63, 128.57, 127.11,
126.98, 123.74, 123.08, 121.35, 118.47, 97.55 ppm. MALDI-TOF
MS: m/z = 571.80 [M+]. C26H19NO2Pt (572.51): calcd. C 54.55, H
3.35, N 2.45; found C 54.49, H 3.19, N 2.44.
X-ray Crystallography: Single crystals of 5 suitable for X-ray dif-
fraction studies were obtained by slow cooling of solutions of 5 in
THF/hexane. Crystals were coated with Paratone oil, and diffrac-
Synthesis of [Ir(ppyFF)2(HL)] (3): [{Ir(ppyFF)2(µ-Cl)}2] (1.22 g,
1.0 mmol), H2L (1.11 g, 3.0 mmol), and Na2CO3 (1.06 g, 10 mmol) tion data were measured at 100 K with synchrotron radiation (λ =
were mixed in 2-ethoxyethanol (30 mL) and stirred at room tem-
perature for 24 h. Addition of water (30 mL) followed by filtration
and flash column chromatography (eluent; ethyl acetate/hexane,
1:4) yielded 3 as an orange solid (0.85 g, 45%). 1H NMR
(400.13 MHz, CDCl3): δ = 16.83 (br., 1 H), 8.51 (d, JH,H = 6.2 Hz,
0.74999 Å) by a 4AMXW ADSC Quantum-210 detector equipped
with a silicon double-crystal monochromator at the Pohang Accel-
erator Laboratory in Korea. The reflection data were collected as
π-scan frames with a width of 1° per frame and an exposure time
of 1 s per frame. HKL2000 (version 0.98.698)[26] was used for data
2 H), 8.3 (t, JH,H = 1.7 Hz, 1 H), 8.26 (d, JH,H = 8.3 Hz, 2 H), 8.07 collection, cell refinement, reduction, and absorption correction.
(d, JH,H = 8.1 Hz, 1 H), 7.90–7.93 (m, 3 H), 7.75–7.81 (m, 4 H),
7.4–7.60 (m, 5 H), 7.33 (t, JH,H = 7.2 Hz, 2 H), 7.08–7.13 (m, 2 H),
The structure was solved by direct methods and refined by full-
matrix least-squares methods using the SHELXTL program pack-
6.73 (s, 1 H), 6.63 (s, 1 H), 6.35–6.43 (m, 2 H), 5.75–5.80 (m, 2 H) age with anisotropic thermal parameters for all non-hydrogen
ppm. 13C NMR (100.62 MHz, CDCl3): δ = 186.07, 185.58, 185.34,
184.79, 180.01, 177.93, 165.19, 164.29, 162.47, 160.90, 159.05,
151.37, 151.16, 147.99, 141.17, 140.48, 138.28, 137.90, 135.76,
atoms. Hydrogen atoms were placed in their geometrically calcu-
lated positions and refined riding on the corresponding carbon
atoms with isotropic thermal parameters.
Eur. J. Inorg. Chem. 2010, 3642–3651
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3649

C. H. Shin, J. O. Huh, S. J. Baek, S. K. Kim, M. H. Lee, Y. Do
FULL PAPER
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Acknowledgments
We gratefully acknowledge the Korea Research Foundation (no.
KRF-2008-313-C00454 for Y. D.) and the Priority Research Cen-
ters Program of the National Research Foundation of Korea (no.
2009-0093818 for M. H. L.) for financial support and the Pohang
Accelerator Laboratory (PAL) for beam-line use (grant 2008-2041-
05).
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Received: March 10, 2010
Published Online: July 1, 2010
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