Highly phosphorescent organopalladium(ii) complexes with metal-metal-to-ligand charge-transfer excited states in fluid solutions

Article abstract of DOI:10.1039/c9dt02525a

Dinuclear pincer-type cyclometalated Pd(ii) complexes with foldable diacetylide ligands show crystallographically determined intramolecular Pd?Pd contacts of 3.203-3.380 ?. In deoxygenated fluid solutions, these Pd(ii) complexes are highly phosphorescent

Full text of DOI:10.1039/c9dt02525a

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DOI: 10.1039/C9DT02525A
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Highly phosphorescent organopalladium(II) complexes with
metal-metal-to-ligand charge-transfer excited states in fluid
solutions
Jinqiang Lin, Chao Zou, Xiaobao Zhang, Qin Gao, Sa Suo, Qihang Zhuo, Xiaoyong Chang, Mo Xie†
and Wei Lu*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Dinuclear pincer-type cyclometalated Pd(II) complexes with donating ligand that can raise the d-d state energies. However,
foldable diacetylide ligands show crystallographically determined mononuclear pincer-type cyclometalated Pd(II) σ-ethynyl
9
intramolecular PdPd contacts of 3.2033.380 Å. In deoxygenated complexes are non-emissive in fluid solutions.
fluid solutions, these Pd(II) complexes are highly phosphorescent in
It has also been investigated that dimeric or oligomeric Pt(II)
the red region with emission quantum yields up to 48%, which has complexes containing large π-conjugated ligands, if spatially
been ascribed to be metal-metal-to-ligand charge-transfer (MMLCT) arranged in a face-to-face manner allowing dispersion PtPt
excited states in nature.
interactions, usually exhibit low-lying dσ*-π* electronic
transitions and much red-shifted emissions from the triplet
metal-metal-to-ligand charge-transfer (MMLCT) excited
Square-planar d8 Pd(II) coordination complexes highly
luminescent in fluid solutions are scarce in the literature, and
1
10
states. However, this approach cannot be directly adopted to
hence Pd(II) photophysics, photochemistry and photo-
functionalized materials are far from developed, which is in
install PdPd interactions and hence to access MMLCT excited
states of Pd(II) complexes in solutions, because of the much
2
sharp contrast to the well-documented luminescent Pt(II) and
11
shorter van der Waals radius of palladium (1.63 Å) than that
3
recently flourishing Au(III) congeners. The significantly lower
of platinum (1.75 Å). The optimized separation for π-π stacking
ligand-field splitting of the Pd(II) ion, when compared with that
of Pt(II) 5d-orbitals, leads to effective nonradiative decay
through excited-state structural distortion by population of the
12
is about 3.5 Å, which matches perfectly to the intermetal
PtPt contacts but is too large for and disfavours the intermetal
PdPd contacts. Although a number of bridged dinuclear Pd(II)
antibonding 4d
x
2
-y
2
orbital. Coordination of a structurally rigid
complexes
showing
crystallographically
determined
4
tetradentate dianionic ligand, such as porphyrins and
phenylpyridines5 to the Pd(II) ion usually minimizes the
excited-state distortion and renders the resulted complexes
phosphorescent or delayed fluorescent from ligand-centered
intramolecular PdPd contacts shorter than the sum of the van
,6
13-16
der Waals radii (3.26 Å) have been studied,
none of them
were found emissive in diluted fluid solutions with appreciable
quantum yields.
(LC) or ILCT excited states with lifetimes of hundreds of
microseconds. Nevertheless, the ultralong lifetimes of these
known Pd(II) emitters are indicating that the lowest-energy
charge-transfer excited states involve mainly the tetradentate
ligands but not the metal orbitals of Pd(II).
R2
R2
R2
R1
Rigid
tridentate
cyclometalated
ligands
R1
N
N
R2
N
N
R1
N
Pd
Pd
Pd
N
Molecular designs for phosphorescent Pt(II) and Au(III)
complexes are usually not effective for their Pd(II) analogues.
For example, pincer-type cyclometalated Pt(II) and Au(III)
R1
This
work
Strong
-donating
acetylides
Close
Pd Pd
interactions
.
..

7
8
complexes containing σ-ethynyl ligands have been known as
phosphorescent emitters, because ethynyl is a strongly σ-
X
X
1
2
3
4
5
X = CH, R1 = R2 = H
X = CH, R1 = H, R2 = Bu
X = CH, R1 = F, R2 = H
X = N, R1 = R2 = H
t
em up to 48%
Non-emissive
Department of Chemistry, Southern University of Science and Technology,
Shenzhen, Guangdong 518055, China. Email: luw@sustech.edu.cn
Scheme 1Chemical structures of the pincer-type cyclometalated Pd(II) arylacetylide
complexes. Structural features of the phosphorescent dinuclear Pd(II) are highlighted to
the right.
†
Current address: College of Chemistry and Materials Science, Jinan University,
Guangzhou, Guangdong 510632, China.
Electronic Supplementary Information (ESI) available: Experiment details, additional
spectra and tables. CCDC 19098601909863. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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In the present work, we report that dinuclear pincer-type tert-butyl groups on the (C^N^N) ligands. Crystal 5VDieMw AFrt(icFleigO.n1lince)
cyclometalated Pd(II) complexes, bridged by triangular is isostructural to 2DMSO and contaDiOnsI: 10o.n10ly39/tCh9eDTf0o2l5d2e5dA
diarylacetylide ligands (Scheme 1), are unprecedented highly conformation of P-helicity with an intramolecular PdPd
phosphorescent from MMLCT excited states in diluted contact of 3.2400(5) Å.
solutions. This molecular design combines three structural
features: rigid cyclometalated ligand, strong σ-donating Table 1. Photoluminescence data of complexes 15.
acetylide ligands, and close intramolecular PdPd contacts. In
Medium
T/K

em,max/nm (em/s)
φ
em/%
k
r
/s1
k
nr/s1
2
98
560 (3.1)
3.0
the current study, [R-(C^N^N)PdCl] (R-HC^N^N = R-substituted
Crystalline
7
298
77
7
515 (35.3)
Non-emissive
476 (52.0)
1
1
7
6
-phenyl-2,2’-bipyridine) complexes were chosen as the
CH
2
Cl
2
MeTHF
precursors and the two diarylacetylides derived from 1,2-bis(2-
298
640 (2.1)
650 (14.4)
653 (1.2)
477 (49.1); 648 (11.8)
604 (2.4)
628 (13.0)
655 (2.0)
630 (2.5)
475 (60.7); 609 (10.6)
622 (1.5)
650 (12.7)
635 (2.0)
501 (1085.4); 630 (14.5)
622 (1.9)
630 (12.8)
56
20
79
Crystalline
7
7
1
8
2
ethynylphenyl)ethyne
and
1,2-bis(4-ethynylpyridin-3-
1.7105
6.7105
CH
2
Cl
2
298
77
MeTHF
yl)ethyne were judiciously chosen as the bridging ligands for the
dinuclear complexes 2–5, given that the free-rotations around
the three ethynylene moieties could allow favourable geometry
for the two cyclometalated Pd(II) termini interacting to each
other through π-π stacking and PdPd interactions. The
monomeric complex 1 was also prepared for comparison. These
2
7
98
7
Crystalline
5
5
5
5
3
Toluene
298
298
77
48
46
2.410
1.810
2.610
2.210
CH
2
Cl
2
MeTHF
298
19
30
27
30
Crystalline
7
7
4
5
CH
2
Cl
2
298
77
1.5105
3.5105
MeTHF
2
7
98
7
Crystalline
Pd--ethynyl complexes were prepared under the Sonogashira
CH
2
Cl
2
298
77
640 (1.9)
475 (68.5); 633 (14.6)
1.610
5
3.710
5
i
coupling reaction conditions (CuI, Pr
2
NH and CH
2
Cl
2
) and
MeTHF
characterized with a variety of spectroscopic methods including
IR, multi-nuclear NMR, and high-resolution MS.
As shown in Fig. 2a, both 1 and 2 modestly absorb in the 350–
50 nm region with molar extinction coefficient (ε) of around
5000 and 8000 dm mol cm , respectively. Complex 2 shows
4
3
–1
–1
a)
b)
c)
additional weak absorption bands beyond 450 nm and tailing to
3
–1
–1
5
50 nm with ε lower than 1500 dm mol cm , which have been
depicted in the inset of Fig. 2a as the mathematic difference Δε
between 2 and 1 (ε values multiplied by 2). Complex 2 exhibits
a structureless red emission with peak maximum, lifetime and
quantum yield of 653 nm, 1.2 μs and 20%, respectively, in a
Pd
N
C
deoxygenated diluted CH
constant of 1.7×10 s is two order higher than that calculated
for the ligand-based triplet ILCT excited states,
2
Cl
2
solution. The calculated radiation
5
–1
4
-6
P
P
and is
P
comparable with those previously reported for triplet MLCT and
MMLCT excited states that have significant parentage of metal
PdPd 3.2028(7) Å
PdPd 3.3796(6) Å
PdPd 3.2400(5) Å
2
,3,10b-c
orbitals.
The excitation spectrum upon monitored at em
Fig. 1
ORTEP plots of the complex molecules in single crystals 2DMSO (a), 3 (b), = 653 nm well-matches the absorption spectrum of 2. The
5
DMF (c). The solvate molecules and hydrogen atoms are omitted for clarity. Note that
emission energy of 2 in diluted solution is identical to the solid-
state emission at 77 K (Fig. 2c, 650 nm), and hence we inferred
that the folded conformation with the short intramolecular
PdPd contact could account for the emission observed in
diluted solutions. In sharp contrast, the mononuclear analogue
crystal 3 contain both folded conformations with M- and P-helicity, respectively, but only
the P-helicity of 3 is drawn and labelled here.
The structures of 13, and 5 were further confirmed by X-ray
crystallography (Table S1). For 1 (Fig. S32), head-to-head
dimeric π-π stacking with a interplanar distance of ca. 3.4 Å and
a shortest intermetal distance of 6.347 Å exclude any PdPd
interactions. The dinuclear complex 2 crystallized into an
1
is non-emissive in fluid solutions at ambient temperature but
in solid-state shows a vibronically structured emission with 0-0
peak at 515 nm (Fig. 2c) with a much longer lifetime of 35.5 μs,
indicating a ligand-based parentage of the low-energy excited
state. In diluted glassy MeTHF solutions at 77 K, 1 shows a
vibronically structured emission with 0-0 peak at 476 nm (52.0
μs) (Fig. 2d), while complex 2 shows dual emission bands at 477
1 1 1
orthorhombic P2 2 2 chiral space group with solvated DMSO
molecules. The folded conformation shows a helical sense by
distorting the diarylacetylide from coplanarity and by stacking
two (C^N^N)Pd planes in a staggered manner with an
interplanar distance of ca. 3.3 Å and a close intramolecular
PdPd contact of 3.2028(7) Å (Fig. 1a). It has been noted that
only the folded conformation with P-helicity was found in
crystal 2DMSO. Complex 3 crystallized into a triclinic P-1 space
group containing both folded conformations with M- and P-
helicity about a crystallographically imposed symmetrical
center. The slightly longer intramolecular PdPd contact of
(structured, 49.1 μs) and 648 (structureless, 11.8 μs) nm,
implying the co-existence of unfolded and folded
conformations of 2 in frozen state. Excitation spectrum
monitored at em = 648 nm for 2 gave a distinct low-energy band
at 480 nm, which is absent from the excitations monitored at

em = 543 nm for 2 and at em = 542 nm for 1 but is coincident
with the low-energy absorbance beyond 450 nm observed for 2
in CH Cl solution at 298 K. We tentatively ascribed the 653 nm
2
2
3
.3796(6) Å in 3 (Fig. 1b) is possibly due to the bulkiness of the
red emissions and the 480 nm transitions in the absorption and
2
| J. Name., 2012, 00, 1-3
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a) 0.20 Time Decay / s
b)
5 A
0
0
0
.12
.08
.04
507 nm,  = 2.1 s
710 nm,  = 2.5 s
excitation spectra of 2 to be MMLCT in nature with significant
metal character.
0
1
2
3
4
5
6
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1
DOI: 10.1039/C9DT02525A
0.15
2
0.00
0
0
0
.10
.05
.00
0
2
4
6
8
10
Time Decay / s
8
7
6
5
4
3
2
1
0
1
0
1
2
3
b)
3
5
a)
2, Emission
(ex = 400 nm)
4
41 nm
2, Excitation
-
-
-
(em = 653 nm)
2
1x2
300
400
500
600
700
800
-2.5 -2.0 -1.5 -1.0 -0.5 0.0
0.5
1.0
0/+
 / nm
Potential vs. Fc / V
300
x5
400
/ nm
500
600

2
1
Intensity / 5
300 400
Fig. 3
2 2
(a) Nanosecond transient absorption spectra of 3 in a deoxygenated CH Cl
4
3
solution at 298 K (concentration ~110 mol dm ). Inset is the decay curves of the 507
and 710 nm transient absorption bands. (d) Cyclic voltammograms of complexes 13,
3
5
00
400
500
600
500
 / nm
600
700
800

/ nm
2
2
2
1
1
, Emission (ex = 400 nm)
, Excitation (em = 648 nm)
, Excitation (em = 543 nm)
, Emission (ex = 380 nm)
, Excitation (em = 542 nm)
3
c)
2
1
d)
and 5 in deoxygenated CH
2
Cl
2
solutions at 298 K (supporting electrolyte: 0.1 mol dm
15 nm
650 nm
n
; scan rate: 50 mVs1).
4 6
Bu NPF
The electrochemical properties of 13, and 5 were studied by
cyclic voltammetry and the data are listed in Table S2. As
depicted in Fig. 3b, 13, and 5 shows one quasi-reversible
reduction couple with Ered at –1.90, –1.95, –2.04, and –1.92 V
5
00
600
700
800
300
400
500
 / nm
600
700
800

/ nm
+/0
versus Fc and an irreversible oxidation wave with Epa at 0.33,
.16, 0.29 and 0.37 V versus Fc+/0, respectively. The dinuclear 2
0
Fig. 2
2 2
(a) UV-vis absorption spectra of 1 and 2 in CH Cl at 298 K. Inset: the
mathematic difference  between 2 and 1 ( values multiplied by 2). (b) Emission and is more difficult to be reduced but easier to be oxidized than the

5
3
2 2
excitation spectra of 2 in deoxygenated CH Cl at 298 K (concentration ~110 mol dm ).
mononuclear 1. When compared with 2, the electron-donating
tert-butyl groups on the (C^N^N) ligand of 3 negatively shift the
reduction couple but positively shift the oxidation wave.
Electron-deficient dipyridylacetylide bridging ligand results in a
more positive oxidation wave. It could be reasonable to assign
the HOMO of the dinuclear complexes to be localized on the
diarylacetylide locked PdPd moieties and the LUMO, based on
the (C^N^N) ligands.
Inset: a cuvette containing the solution under a portable 365 UV lamp. (c) Solid-state
emission spectra of 1 and 2 at 77 K. (d) Emission and excitation spectra of 1 and 2 in
glassy 2-methyltetrahydrofuran (MeTHF) at 77 K (concentration ~110 mol dm ).

5
3
Emission of 2 is the reddest with peak maximum at 653 nm
but not the brightest among the dinuclear complexes studied
herein (Table 1). In deoxygenated CH
5
2 2
Cl solutions at 298 K, 3–
show intense structureless red emission with peak
The excited-state redox potentials (E*(M*/M–) and E*(M+/M*)) of
the dinuclear Pd(II) complexes could be derived by combining
the 0-0 transition energy (E0-0) and the quasi-reversible
reduction couple (Ered) and anodic oxidation wave (Epa),
respectively, from the electrochemical data. The E0-0 energy
could be estimated from the onset of the emission bands of the
complexes to be around 2.25 eV (550 nm). Accordingly, the
E*(M*/M–) and E*(M+/M*) potentials are estimated to be 0.30/–2.09
V, 0.21/–1.96 V, and 0.33/–1.88 V for 2, 3 and 5, respectively,
revealing that these dinuclear Pd(II) complexes in the excited
maximum/lifetime/quantum yield of 630 nm/2.5 μs/46%, 635
nm/2.0 μs/30%, and 640 nm/1.9 μs/30%, respectively (Table 1
and Fig. S34). Electron-donating tert-butyl groups on bipyridyl
and electron-withdrawing fluoro-groups on phenyl of the
(C^N^N) ligand or replacing phenyls of the diarylacetylide ligand
with electron-deficient pyridyls slightly blue-shift the emission
maximum and enhance the radiative constants, which is largely
in line with the energy-gap law. For 3, upon increase the solvent
polarity, the low-energy absorption bands in the 450–550 nm
region slightly blue-shifts (Fig. S35), and the emission maximum
accordingly blue-shifts from 652 nm for toluene to 622 nm for
DMF (Fig. S35), revealing a less polar excited state than its
ground state.
2
1
states are modest oxidants but strong reductants.
We performed time-dependent density functional theory
TD-DFT) calculations on complexes 1 and 2 in CH Cl . The
(
2
2
calculated UV-vis absorption spectrum (Fig. S51S52) is in line
with the respective experimental spectrum and the involved
frontier orbitals are shown in Fig. 4a. For 1, the calculated low-
energy transition at 477 nm (oscillator strength, f = 0.039) is
attributed to the HOMOLUMO ligand-to-ligand charge-
transfer (LLCT) transition from phenylacetylide to (C^N^N) with
2 2
Complex 3 in a deoxygenated CH Cl solution at 298 K shows
two broad intense transient absorption difference bands at 507
and 710 nm (Fig. 3a) with lifetimes of 2.1 and 2.5 μs,
respectively. The decay kinetics resemble that of the steady-
state emissions (630 nm, 2.5 μs), indicating the same excited
state. Given that the 505 and 710 nm transient absorption band
little contribution from the Pd d
dipenylacetylide is distorted from coplanarity and the
overlapping of the d orbitals due to the close PdPd contact
raises the d* orbital of 2 for 0.72 eV from d orbital of 1 (Fig.
a). Consequently, the lower-energy HOMOLUMO transition
z
2
orbital. For 2, the
1
9
resemble that of the radical anion of the diimine ligand and
3
+
20
Pd ions, respectively, we tentatively assigned the observed
excited state to be MMLCT in nature with one-electron reduced
z
2
z
2

II
III
(
C^N^N)• ligand and partially oxidized (Pd Pd ) moiety. No
4
triplet absorption was detectable for the mononuclear complex
after the nanosecond laser pulse excitation (Fig. S38).
corresponding to the diphenylacetylide to (C^N^N) charge
transfer of 2 is with a negligible f of 0.005 and the HOMO-
1
2
LUMO transition gains an f of 0.067 at 478 nm which is
dominated by a *(d
z
2
) orbital (PdPd interaction) to * orbital
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charge-transfer (MMLCT). Due to the promotion of one electron Conflicts of interest
View Article Online
DOI: 10.1039/C9DT02525A
from the antibonding *(d
from 3.34 Å for the optimized S
Å for the T state, and 2.85 Å for the T
z
2
), the PdPd contact is shortened
state to 2.86 Å for S state, 3.04
state (Fig. 4b). The
There are no conflicts to declare.
0
1
1
2
Notes and references
molecular rigidity is well-maintained in the excited state (see
structural parameter in Table S4). The energy loss due to
internal conversion is relatively small during the structure
relaxation of the excited state and hence the strongest emission
is possibly corresponding to the strongest absorption. As
depicted in Fig. S53, the spin density located on the Pd atom in
the T2 state is larger than that in the T1 state, which implies that
the probability of spin-orbital coupling (SOC) occurring in the T2
state is greater than in the T1 state. The emission from T2 to S0
will possess larger radiation constant. Therefore, the triplet T
emission corresponding to the LUMOH-1 of the folded
conformation of 2 at 603 nm gained significance, which is
essentially *(d )-π* MMLCT in nature (Fig. 4c). We note that
the calculated MMLCT absorption at 478 nm and triplet
1
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7
z
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
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Complex 1
Complex 2
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T
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LUMO *
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Emission
Pd···Pd 2.86 Å
Pd···Pd 2.85 Å
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S
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T
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CH Cl
optimized S
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(a) Energy diagram for the calculated frontier orbitals of complexes 1 and 2 in
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2
2
0
, S
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, T
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-S triplet emission.
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1
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In summary, we revealed dinuclear organopalladium(II)
complexes with unprecedentedly high phosphorescent
quantum yield in fluid solutions. The foldable geometries of the
bridging diarylacetylide ligand and the close intramolecular
PdPd contacts are salient features of these dinuclear Pd(II)
complexes. The compelling evidences have suggested the
MMLCT parentage of the emissive excited states. We envisage
that these complexes could serve as prototypes for novel class
of phosphorescent metal complexes.
1
1
4
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2
This work was supported by National Natural Science
Foundation of China (21571096 and 21873044) and Science and
Technology Innovation Commission of Shenzhen Municipality
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322.
(JCYJ 20160301114634613 and JCYJ20170817104715174).
4
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Dalton Transactions
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DOI: 10.1039/C9DT02525A
TOC Graphic
Pd
Pd
Pd
Non-emissive
Highly emissive
Making Pd(II) brightly shining: Mononuclear analogue is non-emissive, but folded dinuclear
palladium(II) diacetylide complexes phosphoresce from MMLCT excited states with quantum
yields up to 48%.