Enhancement of the photoluminescence of a thioamide-based pincer palladium complex in the crystalline state

Article abstract of DOI:10.1016/j.jorganchem.2010.12.037

A thioamide-based pincer Pd complex, [2,6-bis(benzylaminothiocarbonyl- κS)phenyl-κC¹]chloropalladium(II), was crystallized from different solvents. The structure of the complex in the solid state depends on hydrogen bonding interactions of cocrystallized solvent molecules with the metal complex in the crystal. The arrangement affects the intensity of the photoluminescence from the crystals. Strong emission was observed from the crystal having a densely packed arrangement of the complex, whereas the solutions and powders of the complex did not exhibit emission.

Full text of DOI:10.1016/j.jorganchem.2010.12.037

Journal of Organometallic Chemistry 696 (2011) 1289e1293
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Enhancement of the photoluminescence of a thioamide-based pincer palladium
complex in the crystalline state
*
Junpei Kuwabara, Yasuyuki Ogawa, Ayako Taketoshi, Takaki Kanbara
Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba 305-8573, Japan
a r t i c l e i n f o
a b s t r a c t
A thioamide-based pincer Pd complex, [2,6-bis(benzylaminothiocarbonyl-kS)phenyl-k
C1]chloropalla-
Article history:
Received 30 September 2010
Received in revised form
28 December 2010
dium(II), was crystallized from different solvents. The structure of the complex in the solid state depends
on hydrogen bonding interactions of cocrystallized solvent molecules with the metal complex in the
crystal. The arrangement affects the intensity of the photoluminescence from the crystals. Strong
emission was observed from the crystal having a densely packed arrangement of the complex, whereas
the solutions and powders of the complex did not exhibit emission.
Accepted 28 December 2010
Keywords:
Ó 2011 Elsevier B.V. All rights reserved.
Pincer complex
Photoluminescence
Hydrogen bonding
Thioamide
1. Introduction
effect of hydrogen bonding on emission, because the hydrogen
bonding and the molecular arrangement of the complex can be
Transition-metal complexes with cyclometalated ligands have
attracted great interest because of their emission properties. There
are many reports of cyclometalated Pt and Ir complexes with effi-
cient emission and their applications for organic light-emitting
diodes (OLEDs) [1]. A cyclometalated pincer ligand having a tri-
dentate coordination ability usually affords better thermal stability
to complexes than a bidentate ligand. We recently reported pincer
complexes with thioamide groups and their emitting properties
[2]. The Pt complex having the thioamide-based pincer ligand
exhibits stronger emission than the corresponding Pd complex,
which is usual phenomena owing to facile a nonradiative decay
pathway via molecular distortion in the case of a Pd complex [1c,3].
The fabrication of hybrid films of the pincer Pd complex bearing
secondary thioamide units and polyvinylpyrrolidone leads to
improvement of its emission compared to the original emission of
the Pd complex in the solid state at room temperature [4]. Since
polyvinylpyrrolidone is much more effective than polystyrene, one
can assume that reason for the enhancement of emission is
hydrogen bonding between the NeH moiety in the secondary
thioamide group and a pyrrolidone unit in the polymer. Hydrogen-
bonding interactions between the Clꢀ and the Pt complex having
the same ligand increase the photoluminescence in solution [2d].
The solid state of complexes can be an ideal way to evaluate the
determined by single-crystal X-ray diffraction analysis. Emission
from the solid state is usually weak owing to concentration-
dependent quenching processes [5]. However, some compounds
exhibit strong emissions in the solid state compared to the solution
state which is known as aggregation-induced emission [6e8]. Here
we report the relationship between photoluminescence of the
thioamide-based pincer Pd complex and its molecular arrange-
ment containing hydrogen bonds in the crystalline state. Some of
crystals exhibit stronger photoluminescence than that in dilute
solutions at room temperature.
2. Results and discussion
2.1. Molecular arrangement of Pd complex 1 in different crystals
The thioamide-based pincer Pd complex, [2,6-bis(benzylami-
nothiocarbonyl-
according to the literature methods [4]. The complex 1 has two
kS)phenyl-k
C1]chloropalladium(II), 1 was prepared
H
N
H
N
S
Pd
Cl
S
* Corresponding author. Tel.: þ8129 853 5066; fax: þ8129 853 4490.
E-mail address: kanbara@ims.tsukuba.ac.jp (T. Kanbara).
1
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.12.037

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J. Kuwabara et al. / Journal of Organometallic Chemistry 696 (2011) 1289e1293
Table 1
secondary thioamide groups acting as hydrogen-bonding donors
and a Cl ligand acting as a hydrogen-bonding acceptor. The complex
is only soluble in polar organic solvents such as dimethylformamide
(DMF), N-methylpyrrolidone (NMP), dimethylacetamide (DMAc),
and dimethylsulfoxide (DMSO) presumably due to a formation of
a hydrogen-bonding network. Since the polar solvents interact with
the secondary thioamide moieties, the interaction may change the
hydrogen-bonding network. To evaluate the supramolecular inter-
actions, complex 1 was crystallized from four kinds of solvents such
as DMF, NMP [4], DMAc, and DMSO acting as a hydrogen-bonding
acceptors. The crystal structures of the different solvates reveal the
molecular arrangement and hydrogen-bonding interactions of these
groups with the solvent molecules (Fig. 1). The crystallization of
complex 1 from a solution in DMF yielded two types of crystals.
Fig. 1a shows one of the crystal structures containing a DMF
molecule. One of the thioamide groups has a hydrogen bond to the
oxygen atom in the DMF molecule. The other thioamide group has
hydrogen-bonding interaction to the Cl ligand in an adjacent
complex. Similar hydrogen-bonding networks of the pincer
complexes have been reported in literature [9]. The NeH/Cl
interaction forms a linear arrangement of 1, hence termed 1$DMFL.
The distance between hydrogen-bonding donors and acceptors are
within the usual range (Table 1) [10]. The other crystal of 1 obtained
from a solution in DMF also has one DMF molecule interacting with
a thioamide group (Fig. 1b). The difference between two crystals is
Selected bond distance (Å), angle (deg) and distance of hydrogen bonds (Å).
1$DMFL
1$DMFZ
1$NMPa
1$DMAc
1$2DMSO
MeC1
MeS1
MeS2
MeCl1
S1eC7
S2eC15
N1eC7
N2eC15
1.962(2)
1.960(3)
1.970(14)
2.287(3)
2.281(3)
2.405(3)
1.682(13)
1.688(15)
1.301(15)
1.338(18)
177.8(3)
1.958(3)
2.2926(9)
2.2901(10)
2.3989(8)
1.713(3)
1.707(3)
1.317(5)
1.312(5)
174.22(9)
1.953(4)
2.2958(8)
2.2967(6) 2.2896(9)
2.2991(6) 2.2889(11)
2.4070(6) 2.3969(8)
1.717(2)
1.712(2)
1.311(3)
1.314(3)
2.3616(13)
1.704(3)
1.720(3)
1.707(3)
1.321(5)
1.313(5)
1.318(4)
Cl1eMeC1 175.74(7) 176.16(10)
S1eMeS2
H1/O1
180
171.13(3)
2.13
170.96(2) 170.92(3)
2.00
2.55
171.46(13) 170.82(2)
1.98
2.31
1.96
2.33
1.99
2.32
H2/Cl1
a
From Ref. [4].
the molecular arrangement formed by hydrogen-bonding network.
Fig. 1b shows that the Pd complexes are connected by NeH/Cl
hydrogen bonds in a zigzag manner, hence termed 1$DMFZ. The
angle of Pd1eCl1eH2 is 91.5ꢁ. Since the shortest PdePd distances in
crystal lattice of 1$DMFL and 1$DMFZ are 7.932 Å and 7.492 Å,
respectively, there are no d8ed8 metalemetal interactions which
induces aggregation-induced emission as found in some other cases
[7]. The crystal with an NMP molecule, 1$NMP, has a linear molec-
ular arrangement similar to 1$DMFL (Fig. 1c). On the other hand,
Fig. 1. Packing diagrams for (a) 1$DMFL, (b) 1$DMFZ, (c) 1$NMP, (d) 1$DMAc, and (e) 1$2DMSO. Hydrogen atoms except for NeH are omitted for clarity. Labeling and coloring
schemes are as follows: Pd (magenta); S (yellow); N (purple); Cl (lime green); C (gray); O (red); H (white) (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article).

J. Kuwabara et al. / Journal of Organometallic Chemistry 696 (2011) 1289e1293
1291
a crystal of 1 obtained with a DMAc molecule, 1$DMAc, has a zigzag
arrangement which is quite similar to 1$DMFZ (Fig. 1d). Crystalli-
zation from DMSO gave a crystal having two DMSO molecules per
one Pd complex, which is different from the above-mentioned
crystals possessing one solvent molecule. The crystal structure of
1$2DMSO shows that both of the thioamide groups interact with
DMSO molecules and not with the Cl ligand in the adjacent complex
(Fig. 1e). The molecule has a crystallographic mirror plane con-
taining Pd and Cl atoms. The PdePd distance in the crystal lattice is
5.651 Å which is too long for a d8ed8 metalemetal interaction.
However, the two Pd complexes are located close to each other in
comparison with the arrangement in the other crystals. The distance
between the Pd center and the nitrogen atom in the closest complex
is 3.702 Å.
Cl ligands in a late-transition-metal complex are usually strong
hydrogen-bonding acceptors [11]. Since the NeH groups are
hydrogen-bonded to DMSO molecules in 1$2DMSO, there are no
interactions between the Cl ligand and the NeH groups. However,
the Cl ligand does interact with the CeH group at the para position
of the cyclometalating benzene ring (Fig. 2). This interaction forms
a linear chain-like arrangement along the b axis in an orthorhombic
lattice. Therefore, the angle of CeH/Cl is 180ꢁ which is ideal for
a
hydrogen-bonding interaction [10]. The distance between
CeH/ClePd is 2.571 Å which is shorter than the sum of the van der
Waals radii for H and Cl (2.95 Å). Since the examples of a hydrogen
bond of a CeH group are very rare compared to classical hydrogen
bond of NeH and OeH groups [12], the clear evidence for the
hydrogen bond of CeH/ClePd is notable.
Fig. 3. Photographs of (a) 1$DMFL, (b) 1$DMFZ, (c) 1$NMP, (d) 1$DMAc, and (e)
1$2DMSO under UV light at the wavelength of 254 nm.
Consequently, an aggregation-induced emission was observed in
particular crystals such as 1$DMFL, 1$NMP, and 1$2DMSO.
2.2. Photoluminescence of Pd complex 1 in the crystalline state
Fig. 4 shows the emission spectra of 1$DMFL, 1$NMP, and
1$2DMSO as well as spectrum of 1 in a frozen matrix of CH2Cl2/THF
at 77 K. The emission from crystals appears at longer wavelength
than that of solutions. The spectra of 1$DMFL and 1$NMP are very
similar to each other. However, the spectrum of 1$2DMSO is
different from those of 1$DMFL and 1$NMP. The absorption spec-
trum of complex 1 in DMF exhibits the maximum absorption at
374 nm and shoulder peak at around 430 nm (See Supporting
information, Fig. S-1). The spectrum in DMSO exhibits a similar
absorption curve. On the bases of a DFT calculation of a thioamide-
based pincer Pd complex, the absorptions in the long-wavelength
region can be assigned as metal-to-ligand charge transfers (MLCT)
Pd complex 1 exhibits green emission in CH2Cl2/THF (3:2, v/v)
glass matrix at 77 K, whose quantum yield is 65%. On the other
hand, no emission was observed from the solution of 1 at room
temperature. The photoluminescence of the five kinds of crystals
were elucidated by the visual observation as a preliminary evalu-
ation. Fig. 3 shows the photographs of the crystals under UV light at
the wavelength of 254 nm at room temperature. Although the
solution of complex 1 does not exhibit emission, the crystals of
1$DMFL, 1$NMP, and 1$2DMSO exhibits greenish yellow emission
upon excitation, with quantum yields of 1.43, 2.11, and 4.88%,
respectively. Since the quantum yields of 1 in the powder state is
less than 0.1% at room temperature, the quantum yields of 1 are
improved by the crystallization process. On the other hand, 1$DMFZ
and 1$DMAc exhibits negligible emission as shown in Fig. 3b and d,
respectively. The quantum yields of the crystals are less than 0.1%.
Fig. 2. Space filling model of 1$2DMSO indicating hydrogen bonding between
CeH/ClePd. The DMSO molecules are omitted for clarity. Labeling and coloring
schemes are as follows: Pd (magenta); S (yellow); N (purple); Cl (lime green); C (gray);
H (white) (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article).
Fig. 4. Emission spectra of 1$DMFL, 1$NMP, 1$2DMSO (lex ¼ 374 nm) and 1 in a frozen
matrix of CH2Cl2/THF (3:2) at 77 K (lex ¼ 384 nm).

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J. Kuwabara et al. / Journal of Organometallic Chemistry 696 (2011) 1289e1293
Table 2
Crystal data and details of structure refinement.
1$DMFL
1$DMFZ
1$DMAc
1$2DMSO
Chemical formula
Formula weight
Crystal system
Space group
a, Å
C
22H19N2S2PdCl$C3H7NO
C22H19N2S2PdCl$C3H7NO
590.47
C22H19N2S2PdCl$C4H9NO
604.50
Monoclinic
P21 (No 4)
9.7192
11.9849(6)
11.9672(5)
110.4477(12)
1306.15(9)
2
C22H19N2S2PdCl$C4H12O2S2
673.64
Orthorhombic
Pbcn (No 60)
28.6445(12)
590.47
Monoclinic
P21/n (No 14)
9.1001(3)
18.7316(6)
14.6434(5)
96.9912(10)
2477.55(14)
4
Monoclinic
P21 (No 4)
9.3827(3)
11.3525(5)
12.4367(4)
108.8136(10)
1253.94(8)
2
b, Å
c, Å
10.6281(5)
9.3560(4)
b
, deg
V, Å3
2848.3(2)
4
10.659
1376
Z
m
, cmꢀ1
10.493
1200
1.583
10.366
600
1.564
9.971
616
1.537
F(000)
Dcalcd, g cmꢀ3
Crystal size, mm
Exposure rate sec./ꢁ
No. of data
1.571
0.20 ꢂ 0.15 ꢂ 0.10
0.20 ꢂ 0.20 ꢂ 0.10
0.50 ꢂ 0.30 ꢂ 0.10
0.40 ꢂ 0.15 ꢂ 0.15
60.0
60.0
60.0
60.0
22,834
5595
311
12,280
5627
281
12,569
5770
316
25,820
3253
170
No. of unique data
No. of variables
R (I > 2
s
(I))
0.0339
0.0437
0.0798
1.044
0.0299
0.0351
0.0731
1.109
0.0345
0.0355
0.0891
1.071
0.0421
0.0594
0.1055
1.038
R (All reflections)
RW (All reflections)
GOF
[2e]. The excitation spectra of 1$DMFL 1$2DMSO, and 1 in a frozen
matrix of CH2Cl2/THF at 77 K were measured for a comparison of
three different conditions (Fig. S-2). The excitation spectra show
two maximum peaks at around 375 nm and 430 nm. These peaks
correspond to the maximum absorption and a shoulder peak in the
absorption spectrum in the solution state at room temperature.
These results indicate no special photoexcitation such as metal-
metal-to-ligand charge transfer in the solid state and glass matrix
state. The origin of the emission is likely to be MLCT both in the
solid state and in the frozen matrix based on the DFT calculation
and the spectroscopic data.
3. Conclusions
Five kinds of crystal structures of complex 1 with a different
hydrogen bonding interactions were revealed by single-crystal X-
ray diffraction analyses. The molecular arrangements of the crystals
are divided into three groups which are linear, zigzag, and stacking
arrangements. The zigzag arrangement is unfavorable for emission
because of loose packing of the Pd complex in the crystal. The
crystals having high density exhibit aggregation-induced emission.
Thiswork reveals that a moleculararrangement, which is induced by
hydrogen bonding, can be important for emission in the solid state at
room temperature. This result will provide a guide for maximizing
the potential for emission by complexes in the solid state.
2.3. Relationship of crystal structure and photoluminescence of Pd
complex 1
4. Experimental
The emissive crystals, 1$DMFL and 1$NMP, have similar linear
molecular arrangements in the solid state (Fig. 1a and c). The non-
emissive crystals, 1$DMFZ and 1$DMAc, have the common zigzag
arrangement (Fig. 1b and d). In addition, the crystal of 1$2DMSO
having unique arrangement exhibits emission at different wave-
length from other crystals (Fig. 1e). These phenomena strongly
indicate that the molecular arrangement in the crystals affects the
intensity and wavelength of emission. In general, the origin of
aggregation-induced emission of a metal complex is an intermo-
lecular metalemetal interaction [7]. However, no intermolecular
interactions between Pd centers were observed in crystallo-
graphic results and spectroscopic measurements reported herein.
From the many of crystallographic data, we focus on the density of
the crystals (Table 2). The density of the crystal bearing the linear
molecular arrangement is 1.583 g cmꢀ3 (1$DMFL) and 1.591 g
cmꢀ3 (1$NMP) which are significantly higher than that of the
zigzag arrangement, 1.564 g cmꢀ3 (1$DMFZ) and 1.537 g cmꢀ3
(1$DMAc). The high density probably prevents the distortion of
the complex at the exited state resulting in suppression of non-
radiative decay pathway via molecular distortion. Therefore,
densely packed crystals with hydrogen bonds exhibit emission at
room temperature. In terms of 1$2DMSO, the density of the crystal
is 1.571 g cmꢀ3 which is relatively high compared with the crystals
with no emission. In addition, partial stacking of the Pd complex is
observed in the crystal lattice. These may be the reason for rela-
tively strong emission and different emission wavelength from
1$DMFL and 1$NMP.
4.1. General
The IR spectra were measured with JASCO FT/IR-300 spectro-
photometer. The UV/Vis absorption spectra were measured with
JASCO V-630 spectrophotometer. To measure the PL characteristics
of the complexes in the solution and the glass-matrix state, the
dissolved oxygen in the solution was removed by bubbling with N2
for 20 min. The emission spectra at room temperature were
measured with a JASCO FP-6200 spectrophotometer. The emission
spectra at 77 K were measured with a Hitachi F-4010 spectropho-
tometer using deaerated CH2Cl2/THF (3:2, v/v) as glass matrix. The
quantum yields at room temperature were measured by an abso-
lute PL quantum yield measurement system (C9920-02, Hama-
matsu photonics k.k.). The quantum yields at 77 K were measured
by the same equipment using deaerated CH2Cl2/THF (3:2, v/v) as
glass matrix. The Pd complex 1 was prepared according to the
literature method [4].
4.2. X-ray crystal structure determination
Single crystals were obtained from the concentrated solution of
complex 1 in the corresponding solvent. The crystal structure of
1$NMP has been reported in the previous literature [4]. Crystals
were mounted on a glass capillary tube. Intensity data were
collected on a Rigaku R-AXIS Rapid diffractometer with Mo K
radiation. Crystallographic data and details of refinement of the
a

J. Kuwabara et al. / Journal of Organometallic Chemistry 696 (2011) 1289e1293
1293
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complexes are summarized in Table 2. A full matrix least-squares
refinement was used for non-hydrogen atoms except for disordered
atoms with anisotropic thermal parameters method by SHELXL-97
program. Hydrogen atoms except for NeH group in 1$DMFL,
1$DMAc and 1$2DMSO were placed at the calculated positions and
were included in the structure calculation without further refine-
ment of the parameters.
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Acknowledgements
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The authors thank to the Chemical Analysis Center of University
of Tsukuba for the measurements of X-ray analysis. Prof. T. Nabe-
shima and Dr. M. Yamamura are grateful for the support of quantum
yield measurements. J.K. acknowledges the Kurata Memorial Hitach
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Appendix A. Supplementary data
CCDC 794884, 794885, 750102, 794883 and 794887 contain the
supplementary crystallographic data for 1$DMFL, 1$DMFZ, 1$NMP,
1$DMAc, and 1$2DMSO, respectively. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif.
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Appendix B. Supporting information
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646e649;
Supplementary data related to this article can be found online at
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