Luminescence study of thioamide-based pincer palladium complexes in poly(vinylpyrrolidone) matrix

Article abstract of DOI:10.1246/cl.2010.385

Pincer palladium complexes bearing secondary thioamide units exhibit strong phosphorescence at room temperature upon incorporation into poly(vinylpyrrolidone) matrix; a hybrid film shows reversible switching of luminescence upon exposure to volatile organic compounds.

Full text of DOI:10.1246/cl.2010.385

doi:10.1246/cl.2010.385
Published on the web March 13, 2010
385
Luminescence Study of Thioamide-based Pincer Palladium Complexes
in Poly(vinylpyrrolidone) Matrix
Yasuyuki Ogawa,1 Ayako Taketoshi,1 Junpei Kuwabara,1 Ken Okamoto,1 Takashi Fukuda,2 and Takaki Kanbara*1
1Tsukuba Research Center for Interdisciplinary Materials Science (TIMS),
Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-8573
2National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565
(Received January 25, 2010; CL-100076; E-mail: kanbara@ims.tsukuba.ac.jp)
Pincer palladium complexes bearing secondary thioamide
units exhibit strong phosphorescence at room temperature upon
incorporation into poly(vinylpyrrolidone) matrix; a hybrid film
shows reversible switching of luminescence upon exposure to
volatile organic compounds.
polymer hybrid films were prepared by dissolving these
components with various weight ratios into DMF followed by
casting the solution under vacuum.
The complex 1 exhibits phosphorescent emission in a glassy
frozen matrix of CH2Cl2-THF at 77 K (-em = 505 nm, ¯em
=
0.24, ¸ = 71.8 ¯s), whereas the solution shows no emission
at room temperature due to molecular distortion which results
in nonradiative decay. The complex also shows only weak
emission in the solid state (-em = 682 nm, ¯em < 0.001),
presumably due to the formation of excimers. This situation is
quite different when 1 is incorporated into polymer matrices.
In preliminary studies the preparation of hybrid films of 1
and various polymers was surveyed, and we found that the
incorporation of 1 into PVP induced markedly enhanced
emission (-em = 532 nm) at room temperature relative to those
of isolated 1, as shown in Figure 1. Poly(2-ethyl-2-oxazoline)
and vinylacetate/vinylpyrrolidone copolymer also produced
hybrid films with good light emission. In terms of polymer
matrices, other polymers were less effective; only weak emission
was observed at room temperature. The luminescence intensity
of the 1/PVP hybrid film increased with increasing content of 1
and reached a maximum at about 10 wt % (the effect of content
of 1 is shown in Figure S1).6 The quantum yield (¯em) and
emission decay lifetime (¸) of the resulting film (9 wt %) were
0.08 and 16 ¯s, respectively, which are indicative of the
phosphorescent emission of 1 even in air at room temperature.
The yellowish film with a thickness of 1 ¯m showed good
transparency (>96%) at - > 470 nm. XRD analysis of the film
only gave broadened amorphous patterns, and no submicron
particles were observed by SEM, indicating that 1 is highly
Square-planar cycloplatinated complexes are often light-
emissive, and the use of such light-emissive metal complexes,
particularly phosphorescent metal complexes, as emitting
materials in light-emitting diodes and luminescent sensors has
been intensively investigated.1-3 Luminescent cyclopalladated
complexes are also potential luminescent materials; however,
there have been few luminescence studies on cyclopalladated
complexes at room temperature.4 We previously reported that
palladium complexes with thioamide-based pincer ligands
exhibit strong photoluminescence in glassy frozen matrices.4b
However, the pincer palladium complexes exhibit hardly any
light emission in solution at room temperature. To explore the
practical applications of luminescent pincer palladium com-
plexes, enhancement of the room-temperature luminescence
from the complex is desired.
Polymer matrices have often been regarded as ambient
temperature alternatives to low-temperature organic glasses as
media for isolating dispersed species for phosphorescence
measurements.5 Meanwhile, we recently found that photochem-
ical properties of pincer platinum complexes bearing secondary
thioamide groups were modulated upon subjection to chemical
stimuli.2g These observations motivated us to utilize an
appropriate polymer matrix to an alternative medium for pincer
palladium complexes; this methodology was expected to result
in light-emissive hybrid films at room temperature. We here
report phosphorescence and vapor-induced switching of lumi-
nescence of hybrid films consisting of a pincer palladium
complex bearing secondary thioamide units 1 and poly(vinyl-
pyrrolidone) (PVP) (Scheme 1).
1/Poly(vinylpyrrolidone)
1/Vinyl acetate
/Vinylpyrrolidone Copolymer
1/Poly(2-ethyl-2-oxazoline)
1/Poly(vinyl acetate)
1/Polystyrene
The pincer palladium complexes 1-3 were prepared from
thioamide ligands and K2PdCl4 in good yields.4b,6 All polymer
matrices were purchased and used as received. The complex/
-R =
H
1:
2:
N
Bn
n
H
N
R
R
N
O
iPr
450
500
550
600
650
S
Pd
Cl
S
Wavelength / nm
PVP
3:
N
Figure 1. Emission spectra of 1 (9 wt %) in various polymer
matrices at room temperature (-ex = 374 nm).
Scheme 1. Structures of pincer Pd complexes and PVP.
Chem. Lett. 2010, 39, 385-387
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

386
exposure to
CHCl3 vapor
Figure 2. Packing diagram of 1¢NMP.
miscible in PVP. The complex 2 also formed a similar light-
emissive hybrid film with PVP. In contrast to 1 and 2, the related
palladium complex 3 bearing a tertiary thioamide group
exhibited only weak luminescence in the PVP film. These
observations suggest that the presence of the N-H unit of the
complexes is essential to the high light-emissive hybrid films at
room temperature.
leaving
in air
0
10
20
t / min
30
40
Figure 3. Changes of emission intensity at 532 nm (-ex
374 nm) for 1/PVP film upon the cycle of exposure to
chloroform vapor and restoration in air.
=
Since PVP has a high Tg (175 °C), the high light emission at
room temperature of 1 in PVP is believed to originate from the
restricted distortion of the structure at the excited state, which
suppresses the nonradiative decay channel of 1 in PVP. A similar
explanation has been reported for the stable phosphorescence
of cyclometallated Ir complexes in PMMA.5e Meanwhile, our
observations suggest that the presence of the secondary
thioamide is associated with the emissive properties of 1 in
PVP. This is reminiscent of the related pincer platinum
complexes;2g the intermolecular hydrogen-bonding interaction
of the secondary thioamide framework causes the photolumi-
nescent properties of the platinum complex to be enhanced. In
the present study, the formation of hydrogen bonds between the
N-H hydrogen of 1 and the carbonyl group of PVP is thought to
increase the rigidity of the ancillary ligand, rendering the
relaxation of the excited state of 1 to the ground state via
nonradiative decay channels less efficient.2g,3c,3d,7 The results of
IR spectroscopy suggest the hydrogen-bonding interaction
between 1 and PVP; the ¯(N-H) band of 1 was observed at
was left in air. We observed similar behaviors for nitromethane,
methanol, THF, and DMF. These organic compounds serve as
good solvents for PVP; it is reasonable to consider that the
swelling of the polymer matrix due to the absorption of volatile
molecules disturbs the intermolecular interaction of 1 and PVP.
As described above, the hybrid films consisting of pincer
palladium complexes bearing secondary thioamide units and
PVP exhibit high light emission at room temperature. Although
fundamental studies on the photochemical properties of the
hybrid films are still crucial for investigating their new functions
and applications in inorganic and polymer chemistry, the present
methodology makes it possible to easily prepare light-emissive
films at room temperature and thus expand their range of
applications.
The authors are grateful to Prof. Y. Ootuka, Prof. T.
Nabeshima, Dr. C. Ikeda, and Mr. Y. Shoji of our Research
Center for measurement of SEM and PL quantum yield, and
Prof. T. Koizumi of Tokyo Institute of Technology for his
technical support. The authors thank the Chemical Analysis
Center of University of Tsukuba for measurements of X-ray
crystallography. This work was partly supported by Grants-in-
Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
¹1
3220 cm in polystyrene, whereas the ¯(N-H) band shifted to
¹1
3204 cm in PVP. The hydrogen-bonding interaction of 1 with
the pyrrolidone unit was also elucidated by X-ray crystallog-
raphy.8 Figure 2 shows the crystal structure of 1¢NMP. In the
crystal lattice, neither ³-stacking interaction of the centered
benzene unit nor intermolecular d8-d8 interaction is observed
with the Pd£Pd separation distance of 8.415 ¡. The distance
between N atom in the thioamide group and O atom in the NMP
molecule (2.768 ¡) is in the range of normal hydrogen bonding.9
Actually, the 1¢NMP crystals exhibit light emission at room
temperature (-em = 517 nm).
Vapochromic and vapoluminescent materials are attracting
increasing attention because of their potential application as
practical sensors.10 The luminescence of the 1/PVP hybrid film
was also sensitive to volatile organic compounds. When
chloroform vapor came in contact with the film, the emission
intensity immediately decreased approximately tenfold, as
shown in Figure 3. The spectral change was a reversible
process; the emission intensity was recovered when the film
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© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/