Highly luminescent palladium(II) complexes with sub-millisecond blue to green phosphorescent excited states. Photocatalysis and highly efficient PSF-OLEDs

Article abstract of DOI:10.1039/c6sc00462h

Palladium(ii) complexes supported by tetradentate [N^C^C^N] and [O^N^C^N] ligand systems display sky blue to red phosphorescence with emission quantum yields and emission lifetimes up to 0.64 and 272 μs, respectively. Femtosecond time-resolved fluorescenc

Full text of DOI:10.1039/c6sc00462h

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Highly luminescent palladium(II) complexes with
sub-millisecond blue to green phosphorescent
excited states. Photocatalysis and highly efficient
PSF-OLEDs†
Cite this: DOI: 10.1039/c6sc00462h
Pui-Keong Chow,a Gang Cheng,ab Glenna So Ming Tong,a Chensheng Ma,c
Wai-Ming Kwok,d Wai-Hung Ang,a Clive Yik-Sham Chung,a Chen Yang,a Feng Wanga
ab
*
and Chi-Ming Che
Palladium(II) complexes supported by tetradentate [N^C^C^N] and [O^N^C^N] ligand systems display sky
blue to red phosphorescence with emission quantum yields and emission lifetimes up to 0.64 and 272 ms,
respectively. Femtosecond time-resolved fluorescence (fs-TRF) measurements on these Pd(II) complexes
reveal a fast intersystem crossing from singlet to triplet manifolds with time constants of 0.6–21 ps.
DFT/TDDFT calculations revealed that, as a result of the spiro-fluorene and bridging tertiary amine units
of the ligands, the T1 excited state is more ligand-localized and has smaller structural distortion, leading
to slower non-radiative decay as well as radiative decay of T1 / S0 transition and thereby highly
emissive, long-lived triplet excited states. The Pd(II) complexes have been found to be efficient catalysts
for visible light-driven, reductive C–C bond formation from unactivated alkyl bromides with conversions
and yields of up to 90% and 83%, respectively. These complexes have also been employed as
photosensitizers for [2 + 2] cycloaddition of styrenes, with conversions and yields comparable to those
of the reported Ir(III) complexes. Both green and sky blue organic-light emitting devices (OLEDs) have
been generated with these Pd(II) complexes as guest emitters. Maximum external quantum efficiencies
(EQE) of up to 16.5% have been achieved in the sky blue OLEDs. The long emission lifetimes render the
Pd(II) complexes good sensitizers for phosphor-sensitized fluorescent OLEDs (PSF-OLEDs). By utilizing
these phosphorescent Pd(II) complexes as sensitizers, highly efficient green and yellow PSF-OLEDs
having high EQE (up to 14.3%), high colour purity and long operation lifetimes, with 90% of initial
luminance (LT90) for more than 80 000 h, have been realized.
Received 29th January 2016
Accepted 23rd May 2016
DOI: 10.1039/c6sc00462h
www.rsc.org/chemicalscience
bio-molecule sensing,2 and sensitizers for energy conversion
processes such as triplet–triplet annihilation (TTA) energy
transfer.3 Most phosphorescent complexes of Ru(II), Pt(II) and
Introduction
Phosphorescent transition metal complexes having long-lived
(100 ms range), high-energy triplet excited states (Et ꢀ 2.4–2.7 eV)
accompanied by high emission quantum yields are highly
desirable since they can be promising photo-catalysts in
photochemical reactions,1 probes for bio-imaging and
Ir(III) have emission lifetimes within 0.1–10 ms owing to their
emissive excited states having 3MLCT (MLCT ¼ metal-to-ligand
charge transfer) character; the large spin–orbit coupling (SOC)
constants of the heavy metal ion facilitates radiative decay (kr)
from the lowest energy triplet excited state (T1) to the ground
state (S0).4 Introducing naphthalenediimide, coumarin, Bodipy,
anthracene or pyrene to the ligand system, or as ancilliary
ligand of these metal complexes, was found to result in low-
energy phosphorescence in the red to near-infrared spectral
region (580–732 nm) with long emission lifetimes in the tens to
hundreds of microseconds.5–7 This has been explained by the
localization of emitting triplet excited states on the ligands
[3pp* or 3LLCT (LLCT ¼ ligand-to-ligand charge transfer)].
Thus, the SOC effect is diminished and kr is subsequently
decreased. Our recent works showed that neutral complexes of
Pd(II) and Au(III) exhibit green phosphorescence with long
aState Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional
Materials, HKU-CAS Joint Laboratory on New Materials and Department of
Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
E-mail: cmche@hku.hk
bHKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, China
cSchool of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen
518060, China
dDepartment of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong SAR, China
† Electronic supplementary information (ESI) available. CCDC 1048773 and
1449190. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6sc00462h
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¨
emission lifetimes in the 100 ms range and high emission singlet manifold of a uorescent dye via a Forster resonance
quantum yields of up to 22% and 61%, respectively.8,9 The energy transfer mechanism (FRET).15a,b This energy conversion has
applications of these luminescent Pd(II) and Au(III) complexes been shown to be an effective means for a uorescent OLED to
in photocatalytic C–H functionalization, energy conversion acquire a higher EL efficiency than its statistical limit of 25%.15
by TTA, and OLEDs have also been examined. More recently, Luminescent Ir(III) complexes have previously been used as
tris-cyclometalated Pt(IV) complexes, fac-[Pt(C^N)3]OTf phosphor sensitizers in most reported PSF-OLEDs, but the energy
(H–C^N ¼ 2-phenylpyridine/1-phenylpyrazole), which emanate transfer from the Ir(III)-phosphor to organic uorophores is usually
ligand-centred phosphorescence at 402–453 nm with emission incomplete, and hence a signicant residual phosphorescence is
lifetimes of up to 319 ms, have been reported.10
present. Incomplete energy transfer arises because the phospho-
Few reports describe Pd(II) complexes that display phospho- rescence lifetimes of most reported luminescent Ir(III) complexes
rescence in the blue to green spectral region (lem ꢀ 450–540 nm) at are relatively short (usually below 10 ms).16 Since the radiative decay
room temperature. A common perception is that the low lying of the triplet excited state and energy transfer of phosphor to the
d–d excited states of Pd(II) complexes usually occur at an energy organic uorophore are two competitive processes, by using
lower than that of the intraligand and/or MLCT excited state, a phosphor with a long emission lifetime (say in the 100 ms range),
thereby effectively quenching the emission of the complexes.9,11 complete energy transfer from the phosphor to the organic
Deliberate ligand design, such as introducing strong s-donor uorophore becomes more likely. This is crucial to attain high
atoms to the ligand system to destabilize the d–d state, or efficiency PSF-OLEDs with good colour purity.
employing a multidentate ligand system to minimize excited
In the present work, we describe a panel of new luminescent
state structural distortion, has been found to be effective in Pd(II) complexes (Scheme 1) which display phosphorescence in the
harvesting intense phosphorescence for most platinum group blue to red spectral region (lem ¼ 466–599 nm) and with long
metal (PGM) complexes.12,13 Our recent work shows that Pd(II) lifetimes of up to 272 ms (in CH2Cl2) as well as high Fem of up to
complexes supported by the tetradentate [O^N^C^N] ligands that 64% (in PMMA) at room temperature. High EQEs of 14.5% and
have deprotonated carbon donor atom(s) exhibit intense green 16.5% have been achieved for the green OLED with the Pd-G-1
phosphorescence at room temperature with emission maxima emitter and the sky blue OLED with the Pd-B-1 emitter, respec-
(lem) at 498–540 nm, high emission quantum yields (Fem up to tively. By using these Pd(II) complexes as sensitizers in the energy
0.22) and long emission lifetimes (sobs ¼ 62–122 ms).9 Detailed down conversion process, highly efficient PSF-OLEDs having EQEs
analysis by DFT/TDDFT calculations together with fs-time-resolved of up to 14.3%, high colour purity, slow efficiency roll-offs, and
uorescence (TRF) and fs/ns-transient absorption (TA) measure- long device operation lifetimes (LT90) of more than 80 000 h have
ments suggest that structural distortions of the emissive triplet been produced. Using these Pd(II) complexes as photocatalysts, the
excited states of this class of complexes are small; this has been visible light-driven reductive C–C bond formation of alkyl
plausibly attributed to the rigid metal chelating ring system. The bromides has been investigated. Conversions and product yields of
destabilization of the d–d excited state by the strong C donor atom up to 90% and 83%, respectively, have been achieved. The [2 + 2]
also leads to less effective quenching of the emissions. The long cycloaddition of styrenes, induced by energy transfer from the
phosphorescence lifetimes of these Pd(II) complexes, being in the excited state of the Pd(II) complexes, has been found to have
100 ms range, are attributed to the emission derived from substrate conversions and product yields of up to 100% and 97%,
intraligand excited states with little metal parentage.
respectively.
Long-lived excited states are desirable for photochemical
reactions as they have sufficient time for bimolecular
energy/electron transfer processes to take place. While the
visible light-induced reductive C–C bond formation from alkyl
halides and [2 + 2] styrene cycloaddition by energy transfer
mechanism have been widely studied using fac-Ir(ppy)3,
[Ir{dF(CF3)ppy}2(dtbbpy)]PF6 or Ru(bpy)3Cl2 as a sensitizer,1a,b,14
the utilization of other luminescent transition metal complexes
for these photo-catalytic reactions has been less explored. This
is especially true for Pd(II) complexes, whose photochemistry is
under-developed. While the severe efficiency roll-off caused by
long emission lifetimes (up to 100 ms) would disfavour the
utility of luminescent Pd(II) complexes as phosphorescent
dopant materials for OLEDs, the long emission lifetimes would
be advantageous in phosphor-sensitized uorescent OLEDs
(PSF-OLEDs). The PSF-OLED works by using a phosphorescent
sensitizer to excite a uorescent dye.15a,b In brief, as the
phosphor contains a heavy metal ion, all the high energy singlet
excited states of the phosphor would rapidly decay to the lowest
energy triplet excited state due to the spin–orbit coupling effect.
Results
Design and synthesis of ligands and complexes
In previous study, we reported a series of Pd(II) complexes
supported by [O^N^C^N] ligands that show an intense green
This triplet excited state undergoes energy transfer to the radiative Scheme 1 Chemical structures of the Pd(II) complexes.
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phosphorescence (Fem up to 22%) with a long sobs (up to 122 ms) ꢀ10 ppm assignable to protons on the spiro-uorene unit. The
at room temperature. These [Pd(O^N^C^N)] complexes were 1H NMR spectra of Pd-B-1 at a temperature of 273–323 K are
1
found to sensitize triplet–triplet annihilation (TTA) of depicted in Fig. S1 of the ESI.† All the H signals retain their
9,10-diphenylanthracene (DPA) (Fdelayed
of up to chemical shis except that the protons on the spiro-uorene
uorescence
21%), catalyse photo-induced aerobic oxidation of secondary become broader at temperatures >313 K. Thermal stability of
amines with up to 1650 turnovers in 2 h, and serve as Pd-B-1, Pd-B-2, Pd-B-4, Pd-G-1, Pd-G-2 and Pd-N-1 has been
a phosphorescent dopant in OLEDs with an EQE of up to 7.4%.9 investigated by thermogravimetric analysis (TGA). Except for
As the ligand structure has
a profound effect on the Pd-B-4, all the complexes show a decomposition temperature
photophysical and photochemical properties of the metal (Td) at 315 to 392 ꢁC (Fig. S2 in the ESI†). High purity samples of
complex, a new panel of tetradentate ligands have been Pd-B-1, Pd-B-2, Pd-G-1 and Pd-G-2 were obtained by sublimation
ꢃ5
ꢁ
prepared and used for preparing highly luminescent Pd(II) at 285 to 300 C under 4 ꢂ 10 Torr. The structures of Pd-B-1
complexes. Pd-G-1 was designed by modication of the reported and Pd-B-3 were characterized by X-ray crystallography.
[Pd(O^N^C^N)] complexes by replacing the n-butyl with methyl
groups.9 This modication is envisaged to lead to shorter
intermolecular contact. Pd-N-1 features a bridging tertiary
amine between the phenyl and terminal pyridine groups of the
[O^N^C^N] ligand resulting in a 6-5-6-membered metallocycle.
Complexes Pd-B-1, Pd-B-3 and Pd-B-4 have a [N^C^C^N] ligand
scaffold containing a spiro-uorene group installed between
a phenyl ring and a pyridine ring.
X-ray crystal structure of Pd-B-1 and Pd-B-3
Crystals of Pd-B-1 and Pd-B-3 were obtained by slow diffusion of
Et2O into CHCl3 solutions of the metal complexes. Their X-ray
crystal structures (Fig. 1 and S3†) reveal a distorted coordination
geometry from planarity with
Cphenyl-Pd-Npyridine angles of
165.96(7)–173.74(8)ꢁ. The Pd–N distances of 2.0968(19)–2.1441(18)
˚
The tetradentate ligands of complexes Pd-N-1, Pd-B-1 and
Pd-B-3 were prepared by the procedures depicted in Scheme 2
(the synthetic procedure for the ligand of Pd-B-4 can be found in
the ESI†). The Pd(II) complexes were synthesized by reacting the
corresponding ligands with Pd(OAc)2 in reuxing CH3CO2H,
and were puried by ash column chromatography on a SiO2
column using a hexane–ethyl acetate mixture as eluent. The 1H
NMR data of all reaction intermediates, ligands and metal
complexes are given in the ESI.† Assignments of the 1H signals
are based on 2D COSY and NOESY NMR spectra. Complexes
Pd-B-1, Pd-B-3 and Pd-B-4 show a downeld 1H signal at
A are longer than those of the reported [Pd(O^N^C^N)] complexes
9
˚
(1.982(1)–2.061(3) A). The spiro-uorene moiety forces the pyri-
dine ring nearby to twist at an angle of ꢀ52.2ꢁ (Pd-B-1) or ꢀ54.4ꢁ
(Pd-B-3) to the normal plane of the Pd1–C11–C6–C5–N1 (Pd-B-1) or
Pd1–C6–C7–C5–N1 (Pd-B-3) ring, resulting in a highly distorted,
rigid ligand scaffold (inset in Fig. 1 and S3†). It is noted that the
hydrogen atoms residing on C25 (Pd-B-1) and C42 (Pd-B-3) point
toward the Pd1–C11–C6–C5–N1 (Pd-B-1) and Pd1–C6–C7–C5–N1
(Pd-B-3) chelating rings, respectively, with a distance separation of
˚
2.543–2.556 A (Fig. 1 and S3†). There is no close Pd/Pd contact in
the crystal structures.
Scheme 2 General synthetic scheme for the ligands of Pd-B-1, Pd-B-3 and Pd-N-1.
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Fig. 1 X-ray crystal structure of Pd-B-1. Thermal ellipsoids are drawn
at the 35% probability level (note: the distance between the hydrogen
atom residing on C25 and the normal plane of the Pd1–C11–C6–C5–
N1 chelating ring is 2.556 A˚); the inset depicts the angle between the
pyridine ring and the aforementioned chelating ring.
Fig.
2
Cyclic voltammograms of Pd-B-1 and Pd-N-1 in DMF
(0.1 mol dmꢃ3 nBu4NPF6 as supporting electrolyte) at 298 K. Scan rate:
100 mV sꢃ1
.
Electrochemical properties of Pd(II) complexes
oxygen atom (Pd-B-2). However, the LUMO is mainly localized on
the C^N fragment (the part without the linker unit, see MO
surfaces in Fig. S5†) such that Pd-B-1 and Pd-B-2 have similar
Er1e/2d; for Pd-B-4, due to the extended p-conjugation of the
1-isoquinoline pendant moiety of the C^N unit, the LUMO is
stabilized resulting in less cathodic Er1e/2d compared to Pd-B-1.
When the anodic scan was performed prior to the cathodic
scan, the cyclic voltammograms of Pd-B-1 and Pd-B-2 showed
cathodic waves at around +0.03 V, ꢃ1.22 V (Pd-B-1) and ꢃ0.09 V
(Pd-B-2), respectively (Fig. S6†). As these cathodic waves were
not observed in the initial cathodic scan, they are attributable to
the decomposition of the electrochemically oxidized species of
the palladium(II) complexes.
The electrochemical data of the Pd(II) complexes in
DMF (0.1 mol dmꢃ3 nBu4NPF6 as the supporting electrolyte) are
summarized in Table 1. Cyclic voltammograms of Pd-N-1,
Pd-B-1, Pd-B-2, Pd-B-4, Pd-G-1, and Pd-G-2 in DMF show
a quasi-reversible reduction couple at E1/2 of ꢃ2.16 to ꢃ2.54 V
and an irreversible oxidation wave at Epa of +0.53 to +0.81 V
versus Cp2Fe+/0 (Fig. 2 and S4†). Pd-N-1 shows a similar Epa value
(ꢀ+0.53 V) but a more cathodic Er1e/2d (E1re/2d with a cathodic shi of
260 mV) compared to those of Pd-G-1 and Pd-G-2. The less
red
1/2
cathodic E
of Pd-G-1 and Pd-G-2 is ascribed to the two
electron-withdrawing F substituents on the [O^N^C^N] ligand
of Pd-G-1 and Pd-G-2 which cause lowering of the LUMO of
these two complexes when compared to Pd-N-1. On the other
hand, complexes Pd-N-1, Pd-G-1, and Pd-G-2 have similar
Epa values attributable to their HOMOs being mainly localized
on the phenoxide ion of the [O^N^C^N] ligand (MO surfaces
was shown in Fig. S5 in the ESI†). For the Pd(II) complexes with
UV/Vis absorption and steady-state emission measurements
All the complexes show intense absorption bands at 250–370 nm
with molar absorptivities (3) of the order of 104 dm3 molꢃ1 cmꢃ1
,
[N^C^C^N] ligands, Pd-B-1, Pd-B-2 and Pd-B-4, they display
and less intense absorption bands at 370–450 nm with 3 values
of the order of 103 dm3 molꢃ1 cmꢃ1 (Fig. 3). Compared to
Pd-B-1, Pd-B-2, and Pd-B-3, the low-energy absorption bands of
Pd-B-4 are red-shied. Such red shis can be attributed to the
lower LUMO level of Pd-B-4 as revealed by the electrochemical
data and DFT calculations on these complexes. Complexes
Pd-G-1, Pd-G-2 and Pd-N-1 show more intense absorption bands
at l > 400 nm. The intense absorptions at 250–320 nm (Pd-B-1)
and 250–350 nm (Pd-G-1 and Pd-G-2) are attributed to 1IL
(intraligand) p / p* transitions as the free ligands show
similar absorption bands (Fig. S7 and S8 in the ESI†) and
insignicant changes in absorption maxima with solvent
polarity are observed (Fig. S9 and S10 in the ESI†). On the other
hand, the less intense lower-energy absorption bands display
negative solvatochromic shis (lmax of the lowest energy
absorption band of Pd-B-1 is blue-shied from 421 nm in hexane
to 383 nm in CH3OH; lmax of Pd-G-2 is blue-shied from 446 nm in
red
similar Epa values but different E values. This is attributed to
1/2
their HOMOs being mainly localized on the C-deprotonated
phenyl group of the [N^C^C^N] ligand with some electron density
delocalized to the bridging uorene unit (Pd-B-1 and Pd-B-4) or
Table 1 Electrochemical data of selected complexes
a
reda
1/2
Epa (V)
E
(V)
HOMOb (eV)
LUMOb (eV)
Pd-N-1
Pd-B-1
Pd-B-2
Pd-B-4
Pd-G-1
Pd-G-2
0.53
0.81
0.73
0.81
0.53
0.59
ꢃ2.47
ꢃ2.54
ꢃ2.44
ꢃ2.16
ꢃ2.24
ꢃ2.21
ꢃ5.33
ꢃ5.61
ꢃ5.53
ꢃ5.61
ꢃ5.33
ꢃ5.39
ꢃ2.33
ꢃ2.26
ꢃ2.36
ꢃ2.64
ꢃ2.56
ꢃ2.59
a
Determined in DMF at 298 K with 0.1 mol dmꢃ3 nBu4NPF6 as the
supporting electrolyte. Scan rate: 100 mV sꢃ1. Values are versus hexane to 391 nm in CH3OH), indicating that these absorptions
Cp2Fe+/0. Cp2Fe+/0 occurs at 0.06–0.08 V versus Ag/AgNO3 (0.1 mol
are charge transfer in nature (Fig. S9 and S10 in the ESI†).17
dmꢃ3 in CH3CN) reference electrode. Estimated from Epa/E1/2 using
b
a Cp2Fe+/0 value of 4.8 eV below the vacuum level.
All the Pd(II) complexes are strongly emissive with peak
maxima (lem) at 466–599 nm, lifetimes (sobs) in the range of
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modication. For example, Pd-B-3 which has two uorine
substituents on the cyclometalated ligand shows
a blue-shied (ꢀ10 nm) emission band whereas Pd-B-4, with
a 1-isoquinoline ring instead of the pyridine ring in Pd-B-1,
shows a remarkable red-shied emission band (lmax at 599 nm)
compared to that of Pd-B-1. However, the Fem values of Pd-B-3
and Pd-B-4 are lower than that of Pd-B-1 (0.47 [Pd-B-1] versus
0.20 [Pd-B-3] versus 0.07 [Pd-B-4]). Although the emission
spectra of Pd-B-1 and Pd-B-2 are similar, the emission intensity
ratio of the v00 ¼ 1 to v00 ¼ 0 transition is higher for Pd-B-2.
Together with the faster knr (knr ¼ 5.00 ꢂ 103 sꢃ1 [Pd-B-1]; 1.27 ꢂ
104 sꢃ1 [Pd-B-2]), the emissive excited state of Pd-B-2 has
a greater structural distortion and hence a lower Fem for Pd-B-2
(Fem ¼ 0.47 [Pd-B-1] versus 0.39 [Pd-B-2]) is found. The effect of
temperature on the emission of Pd-B-1 in toluene has been
investigated (Fig. S13 in the ESI†). Upon increasing the
temperature from 275 K to 353 K, only a small change in the
emission intensity was observed (a decrease of 6.2% at 353 K).
Variable-temperature emission lifetime of Pd-B-1 in the solid
state and in PMMA. The emission spectrum of Pd-B-1 in the
solid state resembles that in CH2Cl2 solution (Fig. 4). At 77 K,
the emission spectra of Pd-B-1 in the solid state and in the
glassy solution (1 ꢂ 10ꢃ5 mol dmꢃ3 in 2-methyltetrahydrofuran)
are similar, showing slightly blue-shied, well-resolved emis-
sion bands relative to the solid state emission at room
temperature (Fig. 4). No emission from excimer/ground-state
aggregates is observed. The emission lifetimes of Pd-B-1 in the
solid state and in PMMA (5% dopant concentration) at
Fig. 3 UV/Vis absorption and emission spectra of Pd-B-1, Pd-B-2, Pd-
B-3, Pd-B-4, Pd-G-1 and Pd-N-1 in degassed CH2Cl2 solution (4 ꢂ
10ꢃ5 mol dmꢃ3) at room temperature.
48–272 ms, and quantum yields (Fem) of up to 0.47 in
degassed CH2Cl2 solutions and 0.64 in thin lm samples at
room temperature (Fig. 3 and Table 2). Increasing solvent
polarity results in a blue shi in the emission maximum of
Pd-B-1 (lmax ¼ 468 [toluene]; 464 nm [DMF]) and Pd-G-2
(lmax ¼ 506 nm [hexane]; 501 nm [CH3OH]) (Fig. S11 and S12
in the ESI†). This nding, together with the long sobs in the 10
to 100 ms range, suggests that the emission of these Pd(II)
complexes is mainly derived from 3IL p/ p* excited states.17
The emission energy of Pd-B-1 can be tuned by ligand
Table 2 Photo-physical characteristics of the Pd(II) complexes
Emission
UV/Vis absorption labs (nm)
(3 (104 dm3 molꢃ1 cmꢃ1))a
lmax (nm) (sobs (ms))a
Fem
kr; knr (sꢃ1
)
f
Complex
Pd-B-1
259 (3.39), 282 (2.83), 298
(sh, 2.21), 317 (1.73), 339
(0.98), 387 (sh, 0.17)
249 (2.61), 294 (1.12), 323
(1.21), 338 (sh, 1.02), 387 (sh,
0.16)
257 (3.49), 281 (sh, 2.73), 298
(sh, 2.01), 318 (1.61), 335 (sh,
0.97), 380 (sh, 0.16)
271 (3.57), 280 (sh, 3.38), 297
(sh, 2.27), 3.16 (1.70), 356
(1.33), 372 (sh, 1.12), 404 (sh,
0.18)
253 (4.75), 284 (sh, 2.10), 323
(sh, 1.00), 338 (1.48), 357 (sh,
1.00), 403 (br, 0.71)
254 (3.48), 301 (sh, 1.16), 329
(sh, 0.88), 347 (1.10), 400 (br,
0.57), 422 (sh, 0.52)
466, 495 (106)
0.47b; 0.64c
4.43 ꢂ 103; 5.00 ꢂ 103
8.13 ꢂ 103; 1.27 ꢂ 104
2.90 ꢂ 103; 1.16 ꢂ 104
1.59 ꢂ 103; 2.11 ꢂ 104
Pd-B-2
Pd-B-3
Pd-B-4
466, 497 (48)
456, 483 (69)
599 (44)
0.39b; 0.54c
0.20b; 0.09c
0.07d; 0.04c
Pd-G-1
Pd-N-1
517 (55.4)
515 (272)
0.22d; 0.27e
0.18d; 0.01c
3.97 ꢂ 103; 1.41 ꢂ 104
6.62 ꢂ 102; 3.01 ꢂ 103
a
b
Determined in degassed CH2Cl2 (2 ꢂ 10ꢃ5 mol dmꢃ3). Emission quantum yield was measured by using the optical dilute method with BPEA
c
(9,10-bis(phenylethynyl)anthracene) in degassed benzene as the reference (Fem ¼ 0.85). Emission quantum yield was measured in PMMA thin
d
lm with a dopant concentration of 10 wt%. Emission quantum yield was measured by the optical dilution method with [Ru(bpy)3](PF6)2 (bpy
e
¼ 2,20-bipyridine) in degassed CH3CN as the reference (Fem ¼ 0.062). Emission quantum yield was measured in mCP thin lm with a doping
f
concentration of 10 wt%. Radiative decay rate constant estimated from the equation kr ¼ Fem/s; nonradiative decay rate constant estimated
from the equation knr ¼ (1 ꢃ Fem)/s.
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Fig.
4 Emission spectra of Pd-B-1 in the solid state at room
temperature (black line) and 77 K (red line), and glassy solution (2-
methyltetrahydrofuran at 77 K, green line) (lex ¼ 350 nm).
Fig. 5 Electron density difference maps for the T1 / S0 transition of
Pd-B-1 (top left), Pd-B-2 (top right), and Pd-N-1 (bottom). Green:
increase in electron density; magenta: decrease in electron density.
temperatures from 50 to 300 K are depicted in Fig. S14 in the
ESI.† The emission lifetime of the complex in the solid state and
in PMMA decreases from 55 ms to 2 ms and from 245 ms to 80 ms,
respectively. As there is no abrupt change in the emission
lifetime as the temperature is increased from 77 to 300 K
(Fig. S14 in the ESI†), the emissive excited state of Pd-B-1 is
unlikely to undergo reverse intersystem crossing (RISC) from T1
to S1. Hence, thermally activated delayed uorescence (TADF) is
not expected.4,18 Instead, the decrease in emission lifetime with
increasing temperature is ascribed to an increase in the acces-
sibility of vibrational states and/or cross-over to non-radiative
excited states at higher temperatures. The much longer
emission lifetimes of Pd-B-1 in PMMA relative to those in the
solid state at various temperatures can be rationalized by the
facile TTA in the solid state in which closer intermolecular
interactions are allowed. This facile TTA also contributes to the
more signicant decrease in emission lifetime of Pd-B-1 in the
solid state as compared to that in PMMA.
3
transitions that have signicant pp*(ppy) character, there is
also a non-negligible contribution from HOMO / LUMO
transition (13%) with the HOMO localized on both phenyl
moieties of the tetradentate ligand (see Fig. S16 in the ESI† for
the MO surfaces). This means that the T1 excited state of Pd-B-1
involves more delocalized orbitals than that of Pd-B-2. Hence,
the T1 excited state of Pd-B-1 experiences a smaller structural
distortion than that of Pd-B-2, which is supported by the smaller
Huang–Rhys factor (S) of the aromatic C]C/C]N stretching
mode calculated for Pd-B-1 (S ¼ 0.98) compared to Pd-B-2
(S ¼ 1.02; see ESI†). Therefore, using an O atom instead of
a spiro-uorene as the linker would result in more localized
orbitals and hence, a larger excited state structural distortion.
Thus, a faster non-radiative decay rate of the T1 / S0 transition
is observed for Pd-B-2. In addition, with an O atom as the linker,
the metal character in both the H ꢃ 1 and LUMO are higher (see
Fig. S15 and S16 in the ESI† for the MO characters). The higher
metal character in the frontier orbitals gives rise to larger
spin–orbit coupling (SOC), leading to an increase in both kr and
knr. As a result, Pd-B-2 shows faster kr and knr values than those
of Pd-B-1.
Pd-N-1 has the longest emission lifetime among the
complexes studied in this work. Based on the photophysical
data, the radiative and non-radiative decay rates of Pd-N-1 at
room temperature are 6.62 ꢂ 102 sꢃ1 and 3.01 ꢂ 103 sꢃ1
respectively. Comparing these values with those of Pd-B-1, both
complexes have similar knr but Pd-N-1 has a signicantly
smaller kr. Examination of the T1 excited state of Pd-N-1 by
DFT/TDDFT calculations revealed that the triplet excited state is
derived from HOMO / LUMO (58%), H ꢃ 1 / LUMO (21%),
and H ꢃ 2 / LUMO (10%) transitions. The electron density
difference map of the T1 excited state and MO surfaces are
depicted in Fig. 5 and S17 in the ESI,† respectively. The orbitals
are delocalized over the tetradentate ligand, thus the T1 / S0
transition is conceived to be accompanied with a small
structural distortion, as in the case of Pd-B-1 (Huang–Rhys
DFT/TDDFT calculations on the Pd(II) complexes. Based on
the photophysical data, Pd-B-1 shows a much longer sobs than
Pd-B-2. This can be explained by the slower radiative decay
and non-radiative decay rates of Pd-B-1 compared to those of
Pd-B-2 (Pd-B-1: kr ¼ 4.43 ꢂ 103 sꢃ1 and knr ¼ 5.00 ꢂ 103 sꢃ1
;
Pd-B-2: kr ¼ 8.13 ꢂ 103 sꢃ1 and knr ¼ 1.27 ꢂ 104 sꢃ1). To shed
light on the effect of the linker between the phenyl ring and the
terminal pyridine ring (a spiro-uorene [Pd-B-1] versus an O
atom [Pd-B-2]) on the photophysics of the Pd(II) complexes, we
performed DFT/TDDFT calculations on Pd-B-1 and Pd-B-2.
Geometry optimizations using the DFT method revealed that
the lowest triplet excited state (T1) is mainly of 3pp*(ppy)
character (ppy ¼ phenylpyridine; see Fig. 5 for the electron
density difference map of the T1 excited state of these two
complexes). For Pd-B-2, the major contribution (80%) is H ꢃ 1
/ LUMO transition where both H ꢃ 1 and LUMO are respec-
tively p and p* orbitals of the ppy moiety of the tetradentate
ligand (with less than 15% Pd character; see Fig. S15 in the ESI†
for the MO surfaces and their compositions). For Pd-B-1,
although the major contributions to the T1 excited state are
derived from H ꢃ 2 / LUMO (43%) and H ꢃ 1 / LUMO (31%)
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factor for the C]C/C]N stretch is ꢀ0.89 for Pd-N-1). In
addition, the metal contribution is found to be smaller in the
case of Pd-N-1 than in Pd-B-1 (e.g., for the dominant contribu-
tion to the T1 excited state of Pd-N-1 [HOMO / LUMO] and
Pd-B-1 [Hꢃ2 / LUMO]; HOMO of Pd-N-1 has only 3% Pd
character but H ꢃ 2 of Pd-B-1 has 7% Pd character). With this
lower metal parentage in the T1 excited state and hence
a smaller SOC, the kr of Pd-N-1 should be slower than that of
Pd-B-1, such that the former has the longest emission lifetime
of 272 ms.
Excited state properties of Pd(II) complexes
Time-resolved uorescence measurements. Femtosecond
time-resolved uorescence (fs-TRF) measurements of Pd-N-1,
Pd-G-1, Pd-B-1, and Pd-B-2 in CH2Cl2 solutions with excitation
at 300 nm, have been conducted. The fs-TRF spectra and related
kinetic decay of the uorescence of the four complexes are
shown in Fig. S18 and S19 in the ESI.† The fs-TRF spectra of
Pd-N-1 and Pd-G-1 are similar (Fig. S18a and b†), featuring
intensity decay accompanied with dynamic Stokes shi (DSS) of
lmax from 475 nm to 495 nm (Pd-N-1) and from 480 nm to
510 nm (Pd-G-1) in several picoseconds. The TRF of both
complexes were observed to vanish completely by about 100 ps
aer photo-excitation. Analysis of the decay of TRF intensity
(Fig. S19c and d†) revealed bi-exponential dynamics with time
constants (s1 and s2) of 0.9 ps and 21 ps for Pd-N-1 and 0.9 ps
and 13 ps for Pd-G-1. The short-lived species in both cases
(s1 ¼ 0.9 ps) are probably ascribable to the DSS arisen from the
vibrational and structural relaxation while s2 (21 ps and 13 ps
for Pd-N-1 and Pd-G-1, respectively) can be attributed to the
process of intersystem crossing (ISC). On the other hand, fs-TRF
of Pd-B-1 and Pd-B-2 (Fig. S19 in the ESI†) followed
bi-exponential decay with s1 of 0.3 (Pd-B-1)/0.4 ps (Pd-B-2) and s2
of 1.0 (Pd-B-1)/0.6 ps (Pd-B-2), that is, the decays of fs-TRF of
both Pd-B-1 and Pd-B-2 are faster than those of Pd-N-1 and
Pd-G-1. The short- and long-lived species are similarly accoun-
ted for by DSS (lmax of both the complexes are shied from
430 nm to 445 nm) and ISC of the complexes, respectively.
Nano-second, time-resolved absorption and emission measure-
ments. Excited state dynamics of Pd-B-1 on a nanosecond to
microsecond time scale have been investigated by nanosecond
time-resolved emission (ns-TRE) and transient absorption
(ns-TA) measurements with the details depicted in Fig. S20 of
the ESI.† The emission spectra with gate delays of 0–100 ns/
0–1000 ns/0–160 ms at time intervals of 20 ns/100 ns/20 ms show
similar emission proles with no new emission band (Fig. 6).
Fig. 6 ns-TRE of Pd-B-1 in CH2Cl2 (5 ꢂ 10ꢃ5 mol dmꢃ3) with gate
delays of 0–100 ns (top left), 0–1 ms (top right) and 0–160 ms (bottom)
after laser excitation (355 nm).
new absorption band could be found in the absorption
difference spectrum for the sample saturated with O2 (Fig. S21
in the ESI†). This suggests that the excited state of Pd-B-1 is
quenched by O2 via an energy transfer pathway.
The excited state potentials, E(M*/Mꢃ1) and E(M+/M*), of
Pd-B-1, Pd-B-2, Pd-G-1 and Pd-N-1 are estimated from the
electrochemical and spectroscopic data using eqn (1) and (2)
E(M*/Mꢃ1) ¼ E + E0–0
red
(1)
(2)
1/2
E(M+/M*) ¼ Epa ꢃ E0–0
Er1e/2d and Epa are obtained from the cyclic voltammograms of
the complexes (Table 1). E0–0 is estimated from the triplet
emission band. The E(M*/Mꢃ1
)
and E(M+/M*) of these
complexes are in the range of +0.01 to +0.37 V and ꢃ1.90 to
ꢃ2.08 V, respectively (Table 3).
The photo-induced electron transfer reactions between
Pd-B-1 and electron donor/electron acceptor were studied by
nanosecond time resolved absorption spectroscopy. The time
resolved absorption difference spectra of a CH3CN solution of
For the ns-TA spectra measured at similar time intervals, the Table 3 Electrochemical data of selected complexes
decay kinetics at 409 nm has sobs of 60 ms which is similar to
that of the ns-TRE at 466 nm, suggesting that the emission of
Pd-B-1 originates from the triplet excited state.
E(M*/Mꢃ1
(V)
)
E(M+/M*)
(V)
reda
1/2
Epaa (V)
E
(V)
E0–0b (eV)
The long-lived excited state of Pd-B-1 is found to be easily
quenched by O2. In the presence of air, the time resolved
absorption of Pd-B-1 in CH3CN follows a faster decay than that
in the degassed solution. Kinetic study of the TA at 398 nm
reveals a sobs of 122 ns in aerated conditions, which is about
260-fold faster than that of the degassed sample (Fig. S21 in the
ESI†). It is noteworthy that the absorption proles for the
degassed and non-degassed samples are almost the same as no
Pd-N-1
Pd-B-1
Pd-B-2
Pd-G-1
0.53
0.81
0.73
0.53
ꢃ2.47
ꢃ2.54
ꢃ2.44
ꢃ2.24
2.48
2.71
2.70
2.61
0.01
0.17
0.26
0.37
ꢃ1.95
ꢃ1.90
ꢃ1.97
ꢃ2.08
a
Determined in DMF at 298 K with 0.1 mol dmꢃ3 nBu4NPF6 as the
supporting electrolyte. Scan rate: 100 mV sꢃ1. Values are versus
Cp2Fe+/0. Cp2Fe+/0 occur at 0.06–0.08 V versus Ag/AgNO3 (0.1 mol dmꢃ3
b
in CH3CN) reference electrode. Estimated from lmax of the triplet
emission band at 77 K.
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Pd-B-1 (5 ꢂ 10ꢃ5 mol dmꢃ3) and tetramethylethylenediamine follows a reductive quenching pathway, by which the Pd(II)
(TMEDA; 0.69 mol dmꢃ3) showed strong absorptions at 390 and complex in the excited state is reductively quenched by TMEDA
543 nm at delay time > 5 ms aer 355 nm laser excitation (Fig. 7 [driving force (DE) ¼ +0.15 V] but not by iPr2NEt (DE ¼ ꢃ0.16 V).1a,b
and S22 in the ESI†). Kinetic analysis of the decay at 398 nm
The reaction mechanism for the photo-reductive C–C bond
revealed one short-lived component and one long-lived formation catalysed by Pd-N-1 is different from that of Pd-B-1.
component with time constants of 3.1 and 36 ms, respectively. This is because the reduction of Pd-N-1 in the excited state by
The short-lived species is assigned to the triplet excited state of iPr2NEt, with a DE value of ꢃ0.32 V, is not thermodynamically
Pd-B-1 (Fig. S22 in the ESI†). Since the electron transfer reaction favourable. On the other hand, no new absorption band
between Pd-B-1 in the excited state and TMEDA is thermody- attributable to [Pd-N-1]+ could be observed in the time-resolved
namically favoured with a driving force (DE) of +0.15 eV, the absorption difference spectra of the solutions containing
i
long-lived species is tentatively assigned to the one-electron Pd-N-1 and Pr2NEt, or Pd-N-1 and substrate A1, aer laser
reduced species of Pd-B-1, [Pd-B-1]ꢃ. The emission of Pd-B-1 is excitation (lex ¼ 355 nm; Fig. S25 in the ESI†). Also, the emissive
quenched by MV2+ with a quenching rate constant (kq) of 8.66 ꢂ excited state of Pd-N-1 is not quenched by substrate A1 or
109 dm3 molꢃ1 sꢃ1 (Fig. S23 in the ESI†). The time resolved iPr2NEt. Nevertheless, as no product A2 was observed in the
absorption difference spectra of a CH3CN solution of Pd-B-1 reaction mixture with addition of 1 equivalent of 2,2,6,6-tetra-
(5 ꢂ 10ꢃ5 mol dmꢃ3) and MV2+ (2.5 ꢂ 10ꢃ4 mol dmꢃ3) recorded methyl-1-piperidinyloxy (TEMPO), which is a well-known radical
with delay time of 0–2 ms aer 355 nm laser excitation showed scavenger, the involvement of radical intermediate(s) in
the strong absorptions at 398 and 605 nm (Fig. S24 in the ESI†). photo-catalysis is suggested.
Kinetic analysis of the decay at 398 nm revealed a long-lived
Visible light photo-catalysis of [2 + 2] styrene cycloadditions.
species with a sobs of ꢀ80 ms. These ndings altogether reveal Complexes Pd-B-1 and Pd-N-1, having high energy (up to 2.71 eV)
that the electron transfer reaction between Pd-B-1 in the excited long-lived triplet excited states with lifetimes up to 272 ms, were
state and MV2+ has a DE of +1.01 eV, leading to the formation of found to catalyse the formation of [2 + 2] cycloadducts E2–H2
MV+ cation radical.19
from E1–H1 with substrate conversions and yields of up to 100%
and 97% under irradiation with a xenon lamp (l > 350 nm)/blue
Photochemical properties of the Pd(II) complexes
Photo-reductive C–C bond formation. The Pd(II) complexes LED (lmax at 462 nm)/23 W compact uorescent lamp (CFL). No
have long-lived excited states with sobs of up to 272 ms, which is product was detected in the control experiment, that is, in the
useful for photo-catalysis. In this work, the photo-induced absence of Pd(II) complexes (entry 5 in Table 5).
reductive cyclization of alkyl bromides using Pd-B-1, Pd-B-2,
The results obtained with the two Pd(II) complexes are
Pd-G-1 or Pd-N-1 as photo-redox catalyst has been examined. In comparable to those of parallel experiments using fac-Ir(ppy)3
the presence of diisopropylethylamine (iPr2NEt) as a sacricial as the catalyst (Table 5) and those reported by Yoon using
electron donor in CH3CN and using a blue LED as the light Ir(dF(CF3)ppy)2(dtbbpy)(PF6)2 under similar reaction con-
source (lem ¼ 420–520 nm, lmax ¼ 462 nm), cyclized products ditions.14b,c The reactions presumably proceed through the
were found in the reactions of Pd-N-1 with conversions and Dexter energy transfer pathway as electron transfer reactions
yields of up to 90% and 83%, respectively (Table 4). For Pd-B-1 between the substrate E1 and Pd-B-1 or Pd-N-1 are not
or Pd-B-2, no cyclized product was found with excitation at thermodynamically favourable (DE ¼ ꢃ0.94 to ꢃ1.10 eV;
lex > 370 nm using a xenon lamp as light source. The low Pd-B-1/Pd-N-1 in the excited state can act as oxidant or reduc-
absorptivities of Pd-B-1 and Pd-B-2 in the visible spectral region tant, Table S5 in the ESI†). No cyclised product was observed
may contribute to this nding. Nevertheless, the cyclized when Pd-B-4 was used as the sensitizer (entry 3 in Table 5) as
product A2 with 15% yield was found when TMEDA was used this complex has a lower triplet energy (2.07 eV), thereby
instead of iPr2NEt in the reaction of Pd-B-1 (entry 3 in Table 4). lending support to an energy transfer pathway. It is noted
This suggests that the photo-cyclization catalysed by Pd-B-1 that the kq between the triplet excited state of Pd-B-1 and E1
(kq ¼ 2.51 ꢂ 109 dm3 molꢃ1 sꢃ1) is one order of magnitude
larger than that between Pd-N-1 and E1 (2.62 ꢂ 108 dm3 molꢃ1 sꢃ1
)
(Fig. S26 in the ESI†).
Application of Pd(II) complexes in blue and green OLEDs as
well as in green and yellow PSF-OLEDs
Based on their electrochemical data and photo-physical
properties (Tables 1 and 2), different device structures were
used to fabricate green (Pd-G-1 and Pd-G-2) and blue (Pd-B-1
and Pd-B-2) OLEDs from these Pd(II) complexes. For the devices
with Pd-G-1 and Pd-G-2, an architecture of ITO/MoO3 (2 nm)/
TAPC (40 nm)/mCP(10 nm)/mCP: emitter(s) (20 nm)/TmPyPb
(40 nm)/LiF (1.2 nm)Al (150 nm) was constructed. 4,40-Cyclo-
hexylidenebis[N,N-bis(4-methylphenyl)benzamine] (TAPC) and 3,30-
[50-[3-(3-pyridinyl)phenyl][1,10:30,100-terphenyl]-3,300-diyl]bispyridine
Fig. 7 Left: Time resolved absorption difference spectra of Pd-B-1 (5
ꢂ 10ꢃ5 mol dmꢃ3) in degassed CH3CN monitored at 0 s, 5 ms and 100
ms; Right: time resolved absorption difference spectra of Pd-B-1 (5 ꢂ
10ꢃ5 mol dmꢃ3) and TMEDA (0.69 mol dmꢃ3) in CH3CN monitored at
0 s, 5 ms and 100 ms.
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Table 4 Photo-induced C–C bond formation of alkyl bromide
Reaction time
(h)
Entry
Cat.
Substrate
Product
Conv.; yield f(%)
1a,c
2a,d
3d,e
Pd-N-1
Pd-B-1
Pd-B-1
10
10
10
90; 83
0; 0
30; 15
4b,c
Pd-N-1
Pd-N-1
Pd-N-1
10
10
20
84; 63
88; 66
90; 68
5b,c
6a,c
a
i
b
i
Procedure a: Pd(II) complex (1 mol%) Pr2NEt (2 equiv.), CH3CN. Procedure b: Pd(II) complex (1 mol%) Pr2NEt (5 equiv.), HCO2H (2 equiv.),
c
d
CH3CN. The reaction mixture was irradiated with a blue LED (lmax ¼ 462 nm) The reaction mixture was irradiated with a xenon lamp (lex
>
e
i
f
1
370 nm). The procedure is the same as a except TMEDA was used instead of Pr2NEt. Determined by H NMR spectroscopy using 4,40-
dimethyl-2,20-bipyridine as an internal standard. Ts ¼ Tosylate.
(TmPyPB) were the hole-transporting layer (HTL) and electron- PSF-OLEDs in which 9,10-bis[N,N-di-(p-tolyl)-amino]anthracene
transporting layer (ETL), respectively. In the emissive layer (EML), (TTPA) was used as the uorescent emitter.20b
1,3-bis(carbazol-9-yl)benzene (mCP) was used as the host, in which
Normalized EL spectra and EQE-luminance characteristics
Pd-G-1 or Pd-G-2 was dispersed as the phosphorescent emitter. For of all phosphorescent OLEDs based on Pd(II) complexes at
yellow PSF-OLEDs, Pd-G-1 or Pd-G-2 was used as the phosphor optimized dopant concentrations are depicted in Fig. 8. The
sensitizer while 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12- dependence of EL spectra and EQE on the doping concentra-
diphenyltetracene (TBRb) was the yellow uorescent emitter.20a An tions of these Pd(II) complexes are depicted in Fig. S27–S30
additional mCP layer was inserted between HTL and EML as an (ESI†) and the ndings are summarized in Table 6. For the
exciton-blocking layer (EBL) to conne triplet excitons inside the green light-emitting Pd-G-1 and Pd-G-2, their electrolumines-
EML. In the cases of Pd-B-1 and Pd-B-2, a more complicated device cent (EL) spectra are almost identical to their solution
structure of ITO/MoO3 (2 nm)/TAPC (40 nm)/TCTA (10 nm)/CzSi photoluminescence (PL) spectra (Fig. 3). With increasing
(3 nm)/CzSi: emitter(s) (20 nm)/TSPO1 (10 nm)/TmPyPb (40 nm)/LiF complex concentration, the EL spectra of both Pd-G-1 and
(1.2 nm)/Al (150 nm) was used. Considering the high triplet Pd-G-2 showed a red shi in the emission maximum, attribut-
energies of Pd-B-1 and Pd-B-2, EML and EBL having higher triplet able to intermolecular interactions of the complex (details are
energies
(Et)
were
used;
9-(4-tert-butylphenyl)-3,6-bis given in Fig. S27a and S28a of ESI†). However, no emission from
(triphenylsilyl)-9H-carbazole (CzSi; Et ¼ 3.02 eV) as the host in the aggregated states, such as excimer, was observed even at the
EML, bilayer tris(4-carbazoyl-9-ylphenyl)amine (TCTA; Et ¼ 2.8 eV)/ high dopant concentration of 20 wt%. As depicted in Fig. 8b,
CzSi (Et ¼ 3.02 eV) as a hole-transporting EBL between the HTL and a maximum EQE of 14.45% was achieved for the OLED with
EML,21b
and
diphenyl-4-triphenylsilylphenyl-phosphineoxide 4 wt% Pd-G-1. This is nearly 2-fold higher than that of other
(TSPO1; Et ¼ 3.36 eV) as an electron-transporting EBL between the reported Pd(II)-OLEDs.9 For the Pd-G-2 device, a maximum EQE
EML and ETL.16e Pd-B-1 or Pd-B-2 was used as the phosphorescent of 10.65% was achieved at a higher dopant concentration of
emitter in blue phosphorescent OLEDs or as the sensitizer in green 10 wt%. In addition, with increasing dopant concentration, the
EQE of the Pd-G-2 device decreases slowly (Fig. S28b in the
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Table 5 Photo-catalysis of [2 + 2] styrene cycloadditiona
Reaction time
(h)
Entry
Cat.
Substrate
Product
Conv.; yield (%) (d. r.)e
3
4
4
4
4
4
4
4
4
100; 88 (4 : 1)b
27; 14 (4 : 1)d
100; 97 (4 : 1)c
79; 73 (4 : 1)d
0; 0b
1
2
Pd-B-1
Pd-N-1
3
4
Pd-B-4
0; 0c
fac-Ir(ppy)3
100; 77 (4 : 1)c
76; 62 (4 : 1)d
0; 0b
5
6
7
—
Pd-B-1
Pd-N-1
8.5
24
90; 74(>10 : 1)b
100; 82 (7 : 1)c
8
9
10
Pd-B-1
Pd-N-1
fac-Ir(ppy)3
8.5
24
24
100; 94 (6 : 1)b
100; 97 (5 : 1)c
100; 61 (6 : 1)c
11
12
13
Pd-B-1
Pd-N-1
fac-Ir(ppy)3
8.5
16
16
100; 85 (7 : 1)b
>99; 80 (7 : 1)c
>99; 79 (6 : 1)c
a
Reaction condition: metal complex (1 mol%), substrate (0.01 mol dmꢃ3) in CH3CN. b Xenon lamp (lex > 350 nm) as the light source. c blue LED
(lem ¼ 462 nm) as the light source. d 23 W compact uorescent lamp (CFL) as the light source. e Product yield determined by 1H NMR spectroscopy
using 4,40-dimethyl-2,20-bipyridine as an internal standard.
device red-shied from 501 nm to 534 nm when the dopant
concentration was increased from 2 to 10 wt% (Fig. S29a and
S30a of ESI†). The dopant concentration-dependent EL
behaviour of Pd-B-1 and Pd-B-2 devices is attributable to
different structures of these two Pd(II) complexes; the ligand
of Pd-B-1 is more sterically bulky (Fig. 1) and hence weak
intermolecular interactions among the complex molecules are
anticipated. Thus it is not surprising to nd that the EL
spectrum of the Pd-B-1 device was quite insensitive to dopant
concentration. A maximum EQE of 16.48% was achieved at
a low luminance of ꢀ1 cd mꢃ2 for the OLED with 2 wt% Pd-B-1.
This value is comparable to those of the most efficient blue
Pt-OLEDs.21 For the Pd-B-2 device, the low EQE of 5.60% can be
accounted for by the lower PL quantum yield of the Pd(II)
complex (Table 2).
For the Pd-OLEDs studied in this work, the efficiency roll-off
with increasing luminance was quite signicant (Fig. 8b). The
EQEs of devices with Pd-G-1 (4 wt%), Pd-G-2 (10 wt%), Pd-B-1
(2 wt%) and Pd-B-2 (2 wt%) at 500 cd mꢃ2 were 5.72%, 2.11%,
4.00% and 1.67%, corresponding to roll-offs of 60.4%, 80.2%,
75.7% and 70.2%, respectively. The rapid efficiency roll-off
could be due to saturation of triplet excited states of the Pd(II)
complexes, which have very long emission lifetimes.4,9 Never-
theless, these long emission lifetimes render the Pd(II)
Fig. 8 (a) Normalized EL spectra and (b) EQE-luminance character-
istics of OLEDs with Pd-G-1, Pd-G-2, Pd-B-1 and Pd-B-2 with opti-
mized dopant concentrations.
ESI†), suggesting that triplet–triplet annihilation of Pd-G-2 is
less efficient. Normalized EL spectra and EQE-luminance curves
of OLEDs with Pd-B-1 and Pd-B-2 respectively are depicted in
Fig. 8. The EL spectra of Pd-B-1 and Pd-B-2 devices have similar
proles at a low dopant concentration of 2 wt% and sky blue
emission with Commission Internationale De L'Eclairage (CIE)
coordinates of (0.24, 0.46) and (0.24, 0.45), respectively. To our
knowledge, these are the rst examples of sky blue Pd-OLEDs.
The EL performance of Pd-B-1 and Pd-B-2 devices varied with
dopant concentration. The EL spectrum for the Pd-B-1 device
remained unchanged while the EL maximum of the Pd-B-2
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Table 6 Key performance data of OLEDs with Pd-G-1, Pd-G-2, Pd-B-1 and Pd-B-2
EQE (%)
Max.
at 500 cd
mꢃ2
Complex (wt%)
CEa (cd Aꢃ1
)
PEb (lm Wꢃ1
)
CIEc (x, y)
Pd-G-1 (4%)
Pd-G-1 (10%)
Pd-G-1 (20%)
Pd-G-2 (4%)
Pd-G-2 (10%)
Pd-G-2 (20%)
Pd-B-1 (2%)
Pd-B-1 (6%)
Pd-B-1 (10%)
Pd-B-2 (2%)
Pd-B-2 (6%)
Pd-B-2 (10%)
46.11
40.79
20.07
30.76
33.90
24.68
42.62
28.71
17.36
14.75
16.25
13.12
48.29
42.17
20.97
32.22
35.50
22.81
39.38
25.06
15.15
13.63
15.60
12.03
14.45
12.24
6.54
10.20
10.65
7.29
16.48
10.47
6.31
5.72
7.24
3.11
0.92
2.11
2.10
4.00
3.21
2.16
1.67
1.12
0.87
0.27, 0.56
0.30, 0.58
0.34, 0.58
0.26, 0.51
0.27, 0.55
0.30, 0.58
0.24, 0.46
0.24, 0.46
0.24, 0.47
0.24, 0.45
0.26, 0.49
0.29, 0.52
5.60
5.64
4.32
a
Max. current efficiency. b Max. power efficiency. c CIE coordinates at 500 cd mꢃ2
.
complexes useful sensitizers for PSF-OLEDs. Both yellow and tetradentate ligand of Pd-G-2 that may hamper energy transfer
green PSF-OLEDs using the Pd(II) complexes as sensitizer have to TBRb. Meanwhile, the Pd-G-1-PSF OLED device showed
been investigated and the results are summarized in Table 7. a signicant improvement of efficiency roll-off; this can be
The EL spectra and EQE-luminance performances of the yellow attributed to the expansion of the carrier recombination site
OLEDs using TBRb as uorescent emitter with and without and/or the reduction of exciton/polaron quenching.22 For the
Pd-G-1 or Pd-G-2 sensitizer are depicted in Fig. 9a and b, green PSF-OLEDs using 1 wt% uorescent TTPA as the emitter,
respectively. Although the doping concentration of the Pd(II) phosphorescent Pd-B-1 or Pd-B-2 was employed as the sensitizer
complex was 10-fold higher than that of TBRb, the yellow (Fig. 9c). Like those of the yellow PSF-OLEDs depicted in Fig. 9a,
light-emitting devices with and without the Pd(II) complexes as phosphorescence from the Pd(II) complex in the green Pd-B-1 or
sensitizer showed identical EL spectra, revealing complete Pd-B-2 PSF-OLED device was almost completely quenched and
quenching of the phosphorescence of these Pd(II) complexes a strong emission from TTPA was observed. It is noteworthy that
¨
via Forster resonance energy transfer (FRET) to TBRb. without the phosphorescent Pd(II) sensitizer, a low EQE of
Consequently, a maximum EQE of 14.32% for the Pd-G-1-PSF 3.14% was recorded in the TTPA-only device (Fig. 9d). On the
OLED device was more than 3-fold higher than that of the one other hand, although the efficiency of the Pd-B-2-based OLED
without Pd(II) sensitizer. A relatively lower EQE of 7.0% for the was lower than that of the Pd-B-1 one (Fig. 8b), the EQEs of both
Pd-G-2-PSF OLED device could be attributed to the bulky PSF-OLEDs with Pd-B-1 or Pd-B-2 as the sensitizer were similar;
10.41% for the former and 9.73% for the latter. These values are
more than 3-fold higher than the EQE of the TTPA-only device.
Compared to the reported PSF-OLEDs using phosphorescent
Ir(III) complexes as sensitizer, improved colour purity and
higher efficiency have been realized in the PSF-OLEDs with the
phosphorescent Pd(II) sensitizers herein described.15
Preliminary studies on the operational stability of OLEDs
with Pd-G-1 as the emitter and PSF-OLEDs with Pd-G-1 as the
phosphorescent sensitizer were undertaken, using the device
structure of ITO/MoO3 (5 nm)/NPB (70 nm)/mCBP: dopant(s)
(30 nm)/BAlq (10 nm)/Alq (30 nm)/LiF (1.2 nm)/Al (150 nm).23
In these devices, N,N0-di(1-naphthyl)-N,N0-diphenyl-(1,10-
biphenyl)-4,40-diamine (NPB) was used as the hole-transporting
layer, with 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) as the host
material, bis(2-methyl-8-quinolinolato-N1,O8)-(1,10-biphenyl-4-
olato)aluminum (BAlq) as the hole-blocking layer, and tris-(8-
hydroxyquinoline)aluminum (Alq) as the electron-transporting
layer. Pd-G-1 (10 wt%) and Pd-G-1 (10 wt%):TBRb (1 wt%) were
used as dopant(s) for Pd-OLED and PSF-OLED, respectively. The
Fig. 9 (a) Normalized EL spectra and (b) EQE-luminance character-
dependence of relative luminance on operation time of both
devices is depicted in Fig. S31 (in the ESI†). The Pd-OLED and
istics of yellow PSF-OLEDs based on the emission of TBRb. (c)
Normalized EL spectra and (d) EQE-luminance characteristics of green
PSF-OLEDs based on the emission of TTPA.
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the Pd(II)-PSF-OLED were operated at constant current densities of the excited state. In literature, palladium(II) complexes of
of 10 and 20 mA cmꢃ2, respectively. For the Pd-OLED, its benzoporphyrin and naphthoporphyrin (Fig. 10) were reported
lifetime at 90% initial luminance (LT90) was found to be 135 h. to show phosphorescence in the red to near-infrared region
With the formula LT90(L1) ¼ LT90(L0) ꢂ (L0/L1)1.7
,
24 where L1 and L0 (673–882 nm) with Fem and sobs of up to 23% and 520 ms,
respectively. These Pd(II) porphyrin complexes have been widely
respectively represent the objective and experimental (930 cd mꢃ2
,
here) initial luminance, LT90 at an objective luminance of used for TTA and oxygen sensing.26 The signicant energy
100 cd mꢃ2 was estimated to be 5980 h. For the yellow PSF-OLED, difference between the d–d and low energy emissive 3pp*
a LT90 of 182 h was found at L0 ¼ 3810 cd mꢃ2, corresponding to excited states of the porphyrin ligand is the main reason
more than 80 000 h at the objective luminance of 100 cd mꢃ2. The accounting for the observed intense phosphorescence from
longer device lifetime of the yellow PSF-OLED can be attributed to Pd(II) porphyrins. In previous work, we showed that Pd(II)
the slower efficiency roll-off of this device as well as the stability of complexes containing C-deprotonated R–C^N^N–R0 and
the uorescent emitter TBRb.
pentauorophenylacetylide ligands display orange to red
phosphorescence (570–600 nm) in CH2Cl2 solutions at high
concentrations (Fig. 10).11h This nding was attributed to the
formation of intermolecular aggregates where the energy level
of the emissive triplet excited state would be lowered upon
aggregation, thereby reducing the quenching via thermal
population of the d–d state. Nevertheless, high energy emissive
excited states in the blue to green spectral region, which are
desirable in the context of photo-catalysis and energy down
conversion processes, are sparsely found in Pd(II) complexes. To
overcome this challenge, we designed luminescent Pd(II)
complexes with ligands having a rigid scaffold and strong donor
atoms for minimizing structural distortion in the T1 excited
state and for destabilizing the d–d excited state to a thermally
inaccessible energy level. Based on this design strategy, we
Discussion
All the Pd(II) complexes were synthesized by reacting Pd(OAc)2
with the corresponding ligand in glacial acetic acid. H NMR
spectra of Pd-B-1 at 273 K to 323 K show that the H signals of
1
1
spiro-uorene unit become broader at temperatures >313 K
whereas all other 1H signals retain their chemical shis
and shapes. This nding is suggestive of swinging of the
spiro-uorene moiety at higher temperatures, resulting in
indistinguishable chemical environments experienced by the
protons. The minimal changes found for the other proton
signals of Pd-B-1 at elevated temperatures are attributable to the
rigid ligand scaffold of this complex. It is noted that complexes
Pd-B-1, Pd-B-3 and Pd-B-4 display a 1H signal at ꢀ10 ppm
corresponding to the aromatic C–H proton of the spiro-uorene.
As aromatic C–H protons are usually found at 6–9 ppm, the
downeld 1H signal at ꢀ10 ppm could be attributed to the
C–H/p/C–H/Pd interactions, which has also been revealed
in the X-ray crystal structures of Pd-B-1 and Pd-B-3. Due to the
C–H/p/C–H/Pd interactions, the C–H(H25) protons of the
spiro-uorene unit were observed to point into the metal chelating
previously reported
a series of Pd(II) complexes with the
tetradentate [O^N^C^N] ligands showing green phosphorescence
at 498–540 nm with Fem and sobs of up to 22% and 122 ms,
25
˚
ring with distances of 2.543–2.556 A.
Palladium(II) complexes are seldom reported to display
intense phosphorescence in the blue to green spectral region. It
is generally conceived that the d–d excited states are close in
energy to, or lower in energy than, the emissive excited state(s)
of most reported luminescent Pd(II) complexes containing
non-porphyrin ligands.11 Population of the d–d state leads to
severe structural distortion and hence facile non-radiative decay Fig. 10 Selected examples of phosphorescent Pd(II) complexes.
Table 7 Key performance data of PSF-OLEDs
EQE (%)
at 1000 cd
EML
CEa (cd Aꢃ1
)
PEb (lm Wꢃ1
)
Max.
mꢃ2
CIEc (x, y)
TBRb (1%)
14.88
48.70
22.44
11.80
38.85
36.82
13.75
45.00
20.14
11.60
38.14
36.15
4.38
14.32
7.00
3.14
10.41
9.73
2.21
0.47, 0.51
0.47, 0.52
0.47, 0.51
0.32, 0.60
0.33, 0.60
0.33, 0.60
Pd-G-1 (10%): TBRb (1%)
Pd-G-2 (10%): TBRb (1%)
TTPA
Pd-B-1 (10%), TTPA (1%)
Pd-B-2 (10%), TTPA (1%)
10.60
3.10
1.45
6.55
5.24
a
Max. current efficiency. b Max. power efficiency. c CIE coordinates at 1000 cd mꢃ2
.
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respectively (Fig. 10).9 These were the rst reported Pd(II) minimize structural distortion in the excited states of the
complexes which display long-lived and high-energy triplet excited complexes, as revealed by the slight variations in the integrated
states. Subsequently, Li and co-workers reported two Pd(II) area of the emission spectra of the complexes with increasing
complexes, namely PdN3N and PdN3O (Fig. 10), which temperature. Together with the destabilization of the
display intense green phosphorescence in CH2Cl2 (lem ꢀ 530 nm; non-radiative d–d excited state by the strong C donor atoms of
Fem
(MADF).23b
¼
0.72–0.76) and metal-assisted delayed uorescence the tetradentate ligands, the Pd(II) complexes herein show
a much smaller knr than many other reported Pd(II) complexes.
The Pd(II) complexes described herein are supported by the Moreover, the emission properties of this class of Pd(II)
tetradentate ligand systems [N^C^C^N] and [O^N^C^N]; they complexes can be readily manipulated by modifying the
display intense phosphorescence with lem ¼ 466–599 nm, long chemical structure of the ligand system. For example, Pd-B-4,
emission lifetime (sobs up to 272 ms), and Fem up to 0.47 in which is derived from Pd-B-1 with the replacement of a pyridine
CH2Cl2 and 0.64 in PMMA. The long emission lifetime is ring by a 1-isoquinoline ring, shows a remarkable red shi in
attributed to little metal character and small structural the emission maximum relative to the latter (466 nm [Pd-B-1]
distortion in the T1 excited state, rendering a slower decay from versus 599 nm [Pd-B-4]). From the DFT/TDDFT analysis, the
the T1 to S0, as is also revealed by the DFT/TDDFT calculations. LUMO of Pd-B-1 is mainly localized on the 2-phenylpyridine
In addition, the Pd(II) complexes described in this work feature moiety of the ligand (Fig. 3). Therefore, the red shi in the
fast ISC from singlet to triplet manifolds with time constants of emission maximum of Pd-B-4 can be attributed to the lower
0.6 to 21 ps. These time constants are about one to two orders of LUMO level of the 1-phenylisoquinoline moiety than that of the
magnitude larger than those of the Pt(II) analogues which is 2-phenylpyridine moiety.
attributed to the larger spin–orbit coupling constant of Pt(II) ion
There has been a growing interest in applying organome-
than Pd(II) ion. Notably, the temporal evolution of fs-TRF of tallic photo-catalysts in organic synthesis owing to the mild
Pd-B-1 and Pd-B-2 and time constants of the uorescence decay reaction conditions, such as excitation wavelength in the visible
(s1 ¼ 0.3–0.4 ps for DSS and s2 ¼ 0.6–1 ps for ISC) are different region, which can be better tolerated by organic molecules
from those of Pd-N-1 and Pd-G-1 (s1 ¼ 0.9 ps for DSS and containing functional groups that are sensitive to UVC radiation
s2 ¼ 12–21 ps for ISC), and other Pd(II) complexes containing (100–290 nm).14b,c Employing transition metal complexes other
tetradentate [O^N^C^N] ligands.9 Thus the ISC process of the than that of Ir(III) and Ru(II) for photo-reductive C–C bond
Pd(II) complexes can be signicantly tuned by the structure of formation is very rare. Notable examples include [Au2(m-dppm)2]
the tetradentate ligand scaffold.
Cl2 and [Pt(R–C^N^N–R0)(C^CArF5)] for reductive C–C bond
The fast ISC revealed from fs-TRF data and long emission formation of unactivated alkyl/aryl bromide, and [Pt(C^N)acac] for
lifetime as well as the unchanged emission prole observed in triuoromethylation of alkenes and heteroarenes.13h,27 Although
ns-TRE measurements with microseconds delay time lend the use of an organic sensitizer for photocatalytic [2 + 2] cycload-
support to the emissions from the current Pd(II) complexes to dition induced by visible light has been reported,28 Ir(III) sensitizers
have triplet parentage. For Pd-B-1 there is no drastic change in such as [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 are advantageous as they
the emission lifetime and kr with increasing temperature from have a high triplet energy (ꢀ2.6 eV) and intense absorptions in the
77 to 300 K; thus, this complex does not show TADF visible region. Nonetheless, there have been no examples of
properties.18
transition metal complexes that show both high-energy emission
Pd-B-1 exhibits blue phosphorescence accompanied with (Et ꢀ 2.5 eV) and long emission lifetimes on the order of 100 ms as
a high emission quantum yield, properties that are unprece- well as with intense absorptions in the visible region. Complexes
dented for Pd(II) complexes. Thus, the [N^C^C^N] ligand can that possess all these properties are good candidates for
raise the energy of Pd(II) d–d excited states well above that of photochemical reactions driven by visible light. In this work,
a blue–green emission. DFT/TDDFT calculations reveal that the Pd-N-1, having Et ꢀ 2.48 eV and sobs ꢀ 272 ms, and fairly strong
spiro-uorene of Pd-B-1 plays a determinant role in the highly absorptions (3 ꢀ6 ꢂ 103 dm3 molꢃ1 cmꢃ1) at labs of 400–450 nm,
emissive and long-lived character of the excited state of this was found to be an efficient photo-catalyst for both C–C bond
Pd(II) complex. With the spiro-uorene linkage, the T1 excited formation of unactivated alkyl bromide and [2 + 2] cycloaddition of
state of Pd-B-1 shows little metal character, while the O-linkage styrene using blue LED/23 W CFL as the light source.29 These
in Pd-B-2 favours more metal character in the T1 excited state. are the rst examples of applying Pd(II) complexes in these
This leads to a faster radiative decay process and hence photo-catalytic reactions.
a larger kr is found for Pd-B-2 (kr ¼ 8.13 ꢂ 103 sꢃ1) than for
Fluorescence-based OLEDs (u-OLEDs) are attractive owing
Pd-B-1 (kr ¼ 4.43 ꢂ 103 sꢃ1). However, the higher metal to their high colour purity and long operational lifetime.22
parentage in the T1 excited state of Pd-B-2 also results in a faster However, the EQE of u-OLEDs is limited to 5%, assuming the
SOC for the T1 / S0 non-radiative decay process than that of out-coupling efficiency of OLEDs to be 20%.22 Recently, there
Pd-B-1. Therefore, Pd-B-2 shows a larger knr (1.27 ꢂ 104 sꢃ1
)
has been a surge of interest to use thermally activated delayed
than Pd-B-1 (5.00 ꢂ 103 sꢃ1), and this can account for the lower uorescence (TADF) compounds as dopant; TADF-assisted
emission quantum yield of Pd-B-2 than that of Pd-B-1. Never- uorescence OLEDs (TAF-OLEDs) and triplet harvesting in
theless, the Pd(II) complexes described herein display more u-OLEDs with EQEs of 13.4–18% have been reported.22 In this
intense emission than many other reported Pd(II) complexes.9,11 work, Pd(II) complexes having long-lived excited states have
The rigid organic framework of tetradentate ligands helps been used to fabricate green and yellow PSF-OLEDs with devices
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having high colour purity, EQE of up to 14.32% and maximum
LT90 > 80 000 h realized. The gentle efficiency roll-off of these
Pd(II)-based PSF-OLED devices is attributed to the short
emission lifetime of the uorescent emitter as well as rapid
energy transfer from the Pd(II) complex to the uorescent
dopant. The cascade of energy transfer in these PSF-OLEDs is
rather simple, involving a direct energy transfer from triplet
excited state of the phosphorescent Pd(II) complex to singlet
K.-H. Leung, D. S.-H. Chan and C.-H. Leung, Angew. Chem.,
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W.-Y. Wong and C.-L. Ho, J. Mater. Chem., 2009, 19, 4457;
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¨
excited state of the uorescent dopant via Forster resonance
energy transfer.15a Although the energy loss via the Dexter
mechanism cannot be excluded as in the case of
TAF-OLEDs,18,22,30 we envision that the device performance can
be further improved by ne-tuning the device structure. Since
the triplet energy of the Pd(II) complexes can be tuned, as shown
in this work, utilizing strongly phosphorescent Pd(II) complexes
in PSF-OLEDs provides a simple alternative for TAF-OLEDs.
Conclusion
We have developed a panel of highly phosphorescent Pd(II)
complexes which display sky blue to red phosphorescence with
emission lifetimes of up to 272 ms. With these Pd(II) complexes
as emitters, green and sky blue phosphorescent OLEDs have
been fabricated. High EQEs of 14.45% and 16.48% have been
achieved for the green and blue Pd-OLEDs by using Pd-G-1 and
Pd-B-1 as emitters, respectively. By using these Pd(II) complexes
as sensitizers, green and yellow PSF-OLEDs with high EQEs of
up to 14.32%, high colour purity and long operational lifetimes
LT90 of more than 80 000 h have been realized. The visible
light-catalysed reductive C–C bond formation of alkyl bromide
using the Pd(II) complexes as the catalysts has been investigated
with conversions and yields of up to 90% and 83%, respectively.
The [2 + 2] cycloaddition of styrenes catalysed by energy transfer
from Pd(II) complexes in the excited state has been explored,
with conversions and yields comparable to those reported for
Ir(III) complexes.
5 Selected
examples
on
Ru(II)
complexes:
(a)
N. D. McClenaghan, Y. Leydet, B. Maubert, M. T. Indelli
and S. Campagna, Coord. Chem. Rev., 2005, 249, 1336; (b)
R. Lincoln, L. Kohler, S. Monro, H. Yin, M. Stephenson,
R. Zong, A. Chouai, C. Dorsey, R. Hennigar, R. P. Thummel
and S. A. McFarland, J. Am. Chem. Soc., 2013, 135, 17161;
(c) G. Ragazzon, P. Verwilst, S. A. Denisov, A. Credi,
G. Jonusauskas and N. D. McClenaghan, Chem. Commun.,
2013, 49, 9110.
6 Selected examples on Pt(II) complexes: (a) Y. Liu, W. Wu,
J. Zhao, X. Zhang and H. Guo, Dalton Trans., 2011, 40,
9085; (b) W. Wu, L. Liu, X. Cui, C. Zhang and J. Zhao,
Dalton Trans., 2013, 42, 14374.
7 Selected examples on Ir(III) complexes: (a) J. Sun, W. Wu,
H. Guo and J. Zhao, Eur. J. Inorg. Chem., 2011, 3165; (b)
L. Ma, S. Guo, J. Sun, C. Zhang, J. Zhao and H. Guo, Dalton
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W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma, W.-M. Kwok,
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2013, 52, 6648; (c) G. Cheng, K. T. Chan, W.-P. To and
C.-M. Che, Adv. Mater., 2014, 26, 2540.
Acknowledgements
This work was supported by the Theme-Based Research Scheme
(T23-713/11), the National Key Basic Research Program of China
(2013CB834802), the Hong Kong Research Grants Council
(HKU 700812P, HKU17300614), the Hong Kong UGC Special
Equipment Grant (SEG HKU09), and the Guangdong Special
Project of the Introduction of Innovative R&D Teams, China.
9 P. K. Chow, C. Ma, W.-P. To, G. S. M. Tong, S.-L. Lai,
S. C. F. Kui, W.-M. Kwok and C.-M. Che, Angew. Chem., Int.
Ed., 2013, 52, 11775.
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from oxyen-perturbed absorption spectrum has some errors
(e.g. ꢄ 1.3 kcal for the case of indene), together with the
possibility of the existence of relaxed triplet state of the
substrate at a lower energy level (ꢀ8 kcal less) than
X.-H. Zhang and C.-S. Lee, Adv. Mater., 2015, 27, 2025; (b)
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