Strongly phosphorescent palladium(II) complexes of tetradentate ligands with mixed oxygen, carbon, and nitrogen donor atoms: Photophysics, photochemistry, and applications

Article abstract of DOI:10.1002/anie.201305590

A healthy glow: A series of palladium(II) complexes with tetradentate O N C N ligands (see structure) exhibit intense green phosphorescence and long-lived excited states with emission quantum yields and lifetimes up to 22 % and 122-μs, respectively. The c

Full text of DOI:10.1002/anie.201305590

Angewandte
Chemie
DOI: 10.1002/anie.201305590
Functional Materials
Strongly Phosphorescent Palladium(II) Complexes of Tetradentate
Ligands with Mixed Oxygen, Carbon, and Nitrogen Donor Atoms:
Photophysics, Photochemistry, and Applications**
Pui Keong Chow, Chensheng Ma, Wai-Pong To, Glenna So Ming Tong, Shiu-Lun Lai,
Steven C. F. Kui, Wai-Ming Kwok, and Chi-Ming Che*
Transition-metal complexes that show strong phosphores-
cence in solution at room temperature usually contain a RuII,
OsII, IrIII, or PtII ion, but luminescent AuIII complexes[1] have
also been studied increasingly. In contrast to the extensive
literature on luminescent PtII complexes,[2] examples of
luminescent PdII complexes that contain non-porphyrin
ligands and show decent emission quantum yields (Fem) are
sparse.[3] The crux of this long-standing situation lies in the
significantly lower ligand-field splitting of the PdII ion.[4]
Accordingly, the metal-centered d–d states of PdII complexes
are always thermally accessible, which enables facile popula-
tion of the antibonding 4dx2ꢀy2 orbital and hence leads to
effective nonradiative decay through severe excited-state
structural distortion.
(lmax = 668–797 nm; Fem up to 0.45) and PdII–Schiff base
complexes (lmax = 762–792 nm; Fem up to 0.023),[5] the emit-
ting excited states of both of which are lower-lying than the d–
d states. Thus, we asked the question: is it possible to prepare
strongly luminescent PdII complexes with d–d excited states
3
that are thermally inaccessible from the emissive IL (intra-
ligand) and/or 3MLCT (metal-to-ligand charge transfer)
excited states and which are efficient blue- or green-light
emitters?
Herein we report a new class of PdII complexes containing
tetradentate O^N^C^N ligands (Scheme 1). These com-
plexes emit green light (at ca. 530 nm) and exhibit high
Fem values of up to 0.22 and long emission lifetimes (to) of up
A strategy for suppression of the d–d deactivation is to use
strong-field ligands so as to raise the d–d states to a higher
energy. Although many bidentate and tridentate chelating
ligands, such as C-deprotonated C^N and C^N^N (HC^N =
2-phenylpyridine;
HC^N^N = 6-phenyl-2,2’-bipyridine)
ligands, give luminescent PtII complexes, they are not effective
for the design of luminescent PdII complexes. Most of the
reported luminescent PdII complexes containing non-porphy-
rin ligands are only weakly emissive (Fem < ca. 10ꢀ2) at low
temperature (77 K)[4] and non-emissive (Fem < ca. 10ꢀ4) in
solution at room temperature.[3] Notable examples of phos-
phorescent PdII complexes include PdII–porphyrin complexes
[*] P. K. Chow, Dr. C. Ma, Dr. W.-P. To, Dr. G. S. M. Tong, Dr. S.-L. Lai,
Dr. S. C. F. Kui, Prof. Dr. C.-M. Che
State 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)
Scheme 1. Structures of the PdII and PtII complexes 1–9.
E-mail: cmche@hku.hk
Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (China)
to about 120 ms in solution at room temperature. The series of
ligands were designed by the incorporation of a s-donating
phenolate (phol) group tethered to one pyridyl ring (PyA) of
the N^C^N moiety either through a single covalent bond
(complexes 1–3, type I) or through a methylene bridge
(complexes 4–8, type II). These O^N^C^N ligands are
strong s donors, strongly destabilize the antibonding PdII
4dx2ꢀy2 orbital, and can impart robustness and thermal stability
to the corresponding PdII complexes for practical applications
in materials. The structural rigidity of the O^N^C^N ligand
scaffold facilitates emission at relatively high energy and
prevents nonradiative decay resulting from ligand distortion.
Dr. W.-M. Kwok
Department of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong (China)
[**] This research was supported by the Area of Excellence Scheme AoE/
P-03/08, the National Key Basic Research Program of China (No.
2013CB834802), and the CAS-Croucher Foundation Funding
Scheme for Joint Laboratories
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305590.
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PtII complexes of O^N^C^N ligands have been reported to
display high emission quantum yields.[6]
103 dm3 molꢀ1 cmꢀ1). The absorption bands at 250–320 nm
and approximately 350 nm resemble those of the correspond-
ing free ligands. The absorptions at approximately 370–
450 nm show a moderate negative solvatochromic shift (e.g.
for 1, from ca. 411 nm in acetonitrile to ca. 431 nm in hexane;
see Figure S11 in the Supporting Information) and are
attributed, according to density functional theory (DFT)
and time-dependent DFT (TDDFT) calculations, to an
admixture of LLCT (ligand-to-ligand charge transfer;
pphol!p*N^C^N) and MLCT (dp!p*O^N^C^N) transitions. The
inclusion of dp!p* MLCT is in line with the red shift of the
lowest-energy absorption band from 5 (410 nm) to its PtII
congener 9 (430 nm) in dichloromethane, an observation
expected on the basis of the lower oxidation potential of PtII
relative to that of the PdII ion.[4a]
Details of the synthesis and characterization of the PdII
complexes 1–8 and a type II PtII analogue 9 are given in the
Supporting Information. The X-ray crystal structures of 1, 4,
and 7 were determined (see the Supporting Information for
details). In both 1 and 4, the Pd atom adopts a distorted
square planar geometry with N-Pd-N angles of 160.1(5)–
162.0(9)8. The Pd–C (C donor in the O^N^C^N ligand) and
Pd–N distances are 1.909(2)–1.984(4) and 1.982(1)–
2.061(3) ꢀ, respectively, and are thus similar to those
reported for PdII and PtII complexes with a N^C^N ligand.[7]
The Pd–O distances are 2.048(3)–2.116(5) ꢀ, slightly longer
than those in the complex [Pd(N^N)(OPh)2] (N^N = bipyr-
idine, 1.983–1.996 ꢀ).[8]
The steady-state absorption and emission spectra of 1 and
5 in dichloromethane are depicted in Figure 1 with key
spectral parameters listed in Table 1. The spectral data
obtained in other solvents are listed in Table S3 in the
Supporting Informaton. All of the complexes exhibit intense
Type I complexes are weakly emissive (Fem ꢁ 0.002) with
rather short emission lifetimes (t ꢁ 0.2–0.4 ms) and structure-
less emission bands (lmax ꢁ 535–543 nm) that display a sizeable
red shift in energy as the solvent polarity increases (e.g. from
506 nm in hexane to 549 nm in acetonitrile in the case of 1; see
Figure S12). Type II complexes are strongly emissive (Fem
ꢁ 0.14–0.22 for 4–8; Fem ꢁ 0.79 for the PtII congener 9) with
notably longer emission lifetimes (t ꢁ 83–122 ms for 4–8; t
ꢁ 10 ms for 9). Complexes 4–6 and 9 show structured emission
profiles with vibrational progressions of
absorption
bands
at
250–320 nm
(e = 4–7 ꢁ
104 dm3 molꢀ1 cmꢀ1), moderately intense absorption bands
with lmax ꢁ 350 nm (e = 1–2 ꢁ 104 dm3 molꢀ1 cmꢀ1), and rela-
tively weak absorptions at 370–450 nm (e = 3–5 ꢁ
about 1200–1600 cmꢀ1 attributed to the
=
=
ligand C C and/or C N stretching vibra-
tions. The emissions of all type II com-
plexes remain little changed upon variation
of the solvent polarity (e.g. from 516 nm in
hexane to 526 nm in acetonitrile in the case
of 4; see Figure S12). As revealed by the
DFT/TDDFT calculations, the emissions of
the type I and type II PdII complexes arise
from the lowest-energy triplet excited
3
states (T1) of LLCT(pphol!p*N^C^N) and
N^C^N-localized 3IL(p!p*) in parent-
age, respectively, both with less than 10%
3MLCT(dp!p*) character. The self-
quenching rate constants of these emis-
sions are on the order of 107 dm3 molꢀ1 sꢀ1
for 4, 5, and 9, and thus notably lower than
Figure 1. Absorption (solid lines) and emission spectra (dashed lines) of 1 (black lines) and
5 (gray lines) in dichloromethane at 298 K.
the values of approximately 109 dm3 molꢀ1 sꢀ1 reported for the
related complex [PtII(N^C^N)Cl]. Presumably the long alkyl
chains of O^N^C^N ligands disfavor intermolecular close
contact of the PdII and PtII complexes.
Table 1: Emission data of complexes 1–9 in dichloromethane.
[b]
Complex lmax [nm] to [ms][a] Fem
kr [103 sꢀ1 [c]
]
knr [103 sꢀ1 [d]
]
1
2
3
4
5
6
7
8
9
540
543
535
527
498
527
517
536
537
0.4
0.2
<0.2
105
121
122
62
0.0019 48
2500
5000
–
8.1
6.6
6.8
13
0.002
0.0016
0.15
10
–
[e]
[e]
DFT and TDDFT calculations performed on the type I
complex 1 and type II complexes 5 and 9 at their respective
optimized ground-state (S0) and T1 geometries showed that as
a result of the distinct electronic character of the T1 state in
the two types of complexes (3LLCT(p!p*) for type I versus
3IL(p!p*) for type II; Figure 2), the T1 state of 1 and that of
5 or 9 (3IL) feature different geometry with respect to that of
the corresponding S0 state. The structures of 5 and 9 remain
coplanar at both optimized S0 and T1 states. However, in the
case of 1, there is significant structural distortion upon the
transition from S0 to T1; the dihedral angle (dC1-C6-C7-N)
between the phenolate ring and the pyridine ring of the
O^N^C^N ligand changes from approximately 378 at the
1.4
1.7
1.4
3.5
1.7
77
0.20
0.17
0.22
83
10
0.14
10
20
0.79
[a] Emission lifetime at lmax. [b] Emission quantum yield measured at
1ꢀ10ꢀ5 moldmꢀ3 at room temperature with [Ru(bpy)3](PF6)2 (bpy=2,2’-
bipyridine) in degassed acetonitrile as a standard (Fem =0.062).
[c] Radiative-decay rate constant estimated from the equation kr =F/t.
[d] Nonradiative-decay rate constant estimated from the equation
knr =(1ꢀF)/t. [e] The value was not estimated, as the short emission
lifetime of 3 (<0.2 ms) is beyond the detection limit of our instrument.
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approximately 0.7 and 10.3 ps (Figure 3c). The time constant
of approximately 0.7 ps is close to the time constant (ca.
0.8 ps) that describes the DSS (inset in Figure 3c) and can be
understood to arise mainly from structural and vibrational
relaxation of the initial singlet excited state upon photo-
population to evolve to its energy minimum.
The TA recorded for 5 at 0.1 ps after fs laser excitation
shows a broad spectrum with the evolution of the band at lmax
ꢁ 450 nm according to a time constant of approximately
10.7 ps (Figure 3d) into a different profile featuring lmax
ꢁ 435/510 nm and an isosbestic point at approximately
480 nm (Figure 3b). This spectral evolution implies a precur-
sor–successor relationship between the TA at approximately
450 nm and that at about 435/510 nm. The TA at 435/510 nm
is long-lived and persists with no change in intensity over
a longer timescale from 40 ps to 6 ns (inset in Figure 3d; see
also Figure S15). The dynamics of TA with 10.7 ps time
constant matches well with the TRF time constant of
approximately 10.3 ps. The long-lived species, which has
a much lower radiative-decay rate than that of the singlet
excitation (as suggested by the lack of a corresponding long-
lived component in the fs-TRF) was ascribed to the T1 state
(3IL(p!p*)), an assignment which was further verified by
a nanosecond time-resolved emission measurement (see
Figure S16). We therefore assign the approximately 10 ps
process to singlet-to-triplet ISC with an ISC time constant
(tisc) of about 10 ps and an ISC rate constant (kisc) of ca.
1011 sꢀ1. Parallel TR studies revealed that 1 and 9 feature ISC
time constants of about 33 and 0.12 ps, respectively. In
Figure 2. a,b) Calculated electron difference density maps of the
lowest-energy triplet excited state (black: decrease in electron density;
gray: increase in electron density) of 1 (a) and 5 (b). c,d) Structures of
the T1 state of 1 (c) and 5 (d).
optimized S0 state to approximately 268 at the optimized T1
state. For both type I and type II complexes, there is no low-
lying d–d* excited state in either the singlet or the triplet
manifold in the excitation-energy range used in the spectro-
scopic study.
Femtosecond (fs) broadband time-resolved fluorescence
(TRF) and transient absorption (TA) experiments on 1, 5, and
9 revealed similar patterns
of excited-state cascades but
with different rates of sin-
glet-to-triplet intersystem
crossing (ISC; kisc) and dif-
ferent rates of decay of T1 to
S0 through radiative (kr) and
nonradiative processes (knr).
Typical data acquired for 5
are displayed in Figure 3
(for the corresponding
results for 1 and 9, see
Figures S13 and S14).
Upon excitation at
380 nm, 5 exhibited instan-
taneous TRF (0.1 ps) at lmax
ꢁ 460 nm from an electroni-
cally excited singlet state,
which decayed with an
accompanying
dynamic
Stokes shift (DSS; to lmax
ꢁ 510 nm by 3 ps) and dis-
appeared by about 50 ps
after excitation (Figure 3a).
Analysis of the kinetic
decay of the TRF intensity
(see the Supporting Infor-
mation for details) revealed
Figure 3. a,b) Temporal evolution of fs-TRF (a) and fs-TA (b) at the denoted time intervals (in picoseconds)
after the excitation of 5 at 380 nm in dichloromethane. The arrows show the direction of spectral evolution.
*
c,d) Normalized experimental ( , &) and fitted (lines) time profiles for TRF decay (c) and TA (d) at the
denoted wavelengths in the TRF and TA spectra. The inset in (c) shows the time dependence of the TRF
lmax value in the spectra in (a); the inset in (d) displays TA time profiles at time intervals from 0.1 to 6000 ps
after excitation.
biexponential
dynamics
with time constants of
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comparison to the value for 5 (tisc ꢁ 10 ps), the about 100-fold
faster ISC in the case of 9 is not surprising if the heavy-atom
effect that promotes spin–orbit coupling (SOC constant x =
1412/4000 cmꢀ1 for PdII/PtII) is taken into account; the slower
ISC in 1 shows that a subtle change in ligand structure can
notably alter the ISC rate. The ISC rates of the three
complexes are all very high and greatly exceed the radiative
rates of the singlet emission (ca. 108 sꢀ1). These rates suggest
a close-to-unitary ISC quantum efficiency (Fisc ꢁ 1) and
account for the apparent lack of fluorescence emission in
the corresponding steady-state spectra (Figure 1). They also
indicate that the phosphorescence quantum yields of the PdII
complexes are governed solely by the dynamic competition of
the radiative (kr) versus the nonradiative decay (knr) of the
emitting triplet state. The values of kr and knr derived from the
measured Fem and t0 values are included in Table 1.
The root cause for the high Fem and long t0 values of 5 and
all the other type II PdII complexes (4, 6–8) is their modest
kr value (ca. 1.4–3.5 ꢁ 103 sꢀ1) coupled with a similarly low
knr value (ca. 6.4–10 ꢁ 103 sꢀ1); the much lower/shorter Fem/t0
values exhibited by type I complexes (1–3) are primarily
a result of their about 500-fold higher knr value (ca. 2.5 ꢁ
106 sꢀ1) combined with a slightly higher kr value. The
modest radiative rate kr shown by type II PdII complexes is
an intrinsic result of the small metal contribution (9%
Figure 4. Top: Time-resolved emission spectra of a mixture of 7
(2ꢀ10ꢀ5 moldmꢀ3) and DPA (5ꢀ10ꢀ5 moldmꢀ3) in degassed dichloro-
methane as recorded from 1 to 175 ms after laser pulse excitation
(355 nm). The arrow indicates the change in emission intensity.
Bottom: Mechanism of TTA-based delayed fluorescence.
3) Another application of these complexes is in light-
3
[10]
ꢀ
according to DFT) in the IL(p!p*) T1 excitation of these
induced oxidative C H functionalization reactions. In this
complexes; the favorable low rate knr in this system is in line
with the similar coplanar structures of the T1 to S0 states. Thus,
providing that type I and II PdII complexes are both free from
d–d* deactivation (as suggested by DFT), the much faster rate
knr of type I complexes is caused by ligand deformation in the
nonplanar T1 state and the 3LLCT T1 parentage, both of which
study, 4–8 were found to be capable of catalyzing the
oxidation of secondary amines by oxygen under light
irradiation (Scheme 2; see also Table S5). Complex 8 dis-
3
are not present in the IL(p!p*) T1 excited states of the
strictly coplanar type II complexes.
The high emission quantum yields and long lifetimes of
the triplet excited states suggest a range of promising
applications, four of which are highlighted below.
Scheme 2. Photochemical reactions catalyzed by the PdII complexes.
Bn=benzyl.
1) The type II PdII complexes are good luminescent
oxygen sensors. For example, we observed a greater than
300-fold decrease in emission intensity (at lmax ꢁ 530 nm)
upon the exposure of a solution of 5 in dichloromethane to
air; this decrease in emission intensity corresponds to a rate
constant (kq) of emission quenching by oxygen of about 2.5 ꢁ
109 sꢀ1.
played the highest activity and furnished the products with up
to about 1650 turnovers in 2 h (see Table S5, entry 6). Singlet
oxygen sensitized by the triplet-excited PdII complex is
proposed to be the active oxidant.[11] This finding reveals
that PdII complexes containing non-porphyrin ligands may
also possess rich photochemical properties analogous to those
displayed by their PtII counterparts.
4) The application of 5 (decomposition temperature:
4188C) in organic light-emitting diodes (OLEDs) was also
examined. Complex 5 was vacuum-deposited as a dopant into
the emissive material layer of an OLED with the config-
uration ITO/NPB (40 nm)/x% 5: mCP (30 nm)/BAlq
(40 nm)/LiF (0.5 nm)/Al (80 nm; see the Supporting Infor-
mation for details of OLED fabrication; ITO = indium tin
2) In deoxygenated solutions of the PdII complexes 4–8
with 9,10-diphenylanthracene (DPA) in dichloromethane,
excitation with a 355 nm laser light (Nd:YAG laser) gave
rise to both prompt and delayed fluorescence of DPA (5 ꢁ
10ꢀ5 moldmꢀ3) together with minor phosphorescence of the
PdII complex (2 ꢁ 10ꢀ5 moldmꢀ3).[9] Upon excitation, the PdII
complex in its long-lived triplet excited state reacts with DPA
by energy transfer to produce triplet-excited DPA (3DPA*),
which undergoes a bimolecular triplet–triplet annihilation
(TTA) reaction to give a singlet-excited DPA molecule
(1DPA*) and a ground-state DPA molecule. The relaxation
of 1DPA* to its ground state gives rise to the observed delayed
fluorescence (Figure 4) with delayed fluorescence quantum
yields estimated to be 14–21% (see Table S4).
oxide,
NPB = N,N’-di(1-naphthyl)-N,N’-diphenyl-1,1’-
biphenyl-4,4’-diamine, mCP = 1,3-bis(N-carbazolyl)benzene,
BAlq = bis(2-methyl-8-quinolato)-(4-phenylphenolate)alu-
minum). Figure 5a shows the normalized electrolumines-
cence (EL) spectra of the OLEDs at a luminance of
100 cdmꢀ2 with various dopant concentrations of 5. The EL
4
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Keywords: organic light-emitting diodes · palladium ·
.
phosphorescence · photochemistry · tetradentate ligands
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Figure 5. a) EL spectra of the devices with different concentrations of 5
doped into the mCP layer at a luminance of 100 cdmꢀ2. b) Plots of the
external quantum efficiency (EQE) of the devices with 5 at different
dopant concentrations versus current density.
spectrum of the device with a 6% dopant concentration
showed emission maxima at 500, 535, and 589 nm, the CIE
coordinates (0.27, 0.44), a maximum current density of
20.0 cdAꢀ1, and
a
maximum power efficiency of
13.6 LmWꢀ1 (see Figure S17). Plots of the external quantum
efficiency (EQE) of devices with 5 at different dopant
concentrations versus current density are depicted in Fig-
ure 5b. With this simple device structure, a maximum EQE of
7.4% was reached, which is comparable to the EQE of
recently reported OLEDs based on PtII complexes.[12]
In conclusion, a series of PdII complexes supported by
tetradentate O^N^C^N ligands exhibited unprecedentedly
high emission quantum yields in the 510–550 nm spectral
region in solution at room temperature. These complexes
showed good performance in photocatalysis and displayed
promise for effective applications in materials. The discovery
of this class of luminescent PdII complexes, which show strong
green phosphorescence, indicates the usefulness of tetraden-
tate ligand systems with a rigid scaffold and strong donor
atoms in the development of highly robust and strongly
phosphorescent metal-based emitters.
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Received: June 28, 2013
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Functional Materials
A healthy glow: A series of palladium(II)
complexes with tetradentate O^N^C^N
ligands (see structure) exhibit intense
green phosphorescence and long-lived
excited states with emission quantum
yields and lifetimes up to 22% and
122 ms, respectively. The complexes show
potential as novel functional materials,
with promising results in a variety of
applications, such as oxygen sensing,
photocatalysis, and organic light-emitting
diodes.
P. K. Chow, C. Ma, W.-P. To,
G. S. M. Tong, S.-L. Lai, S. C. F. Kui,
W.-M. Kwok, C.-M. Che*
&&&& — &&&&
Strongly Phosphorescent Palladium(II)
Complexes of Tetradentate Ligands with
Mixed Oxygen, Carbon, and Nitrogen
Donor Atoms: Photophysics,
Photochemistry, and Applications
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!