Luminescence and photocatalytic properties of a platinum(II)-quaterpyridine complex incorporated in Nafion membrane

Article abstract of DOI:10.1039/b105601h

The non-emissive platinum(II) - quaterpyridine complex shows strong photoluminescence at room temperature upon incorporation into Nafion membrane; this complex is stabilized toward photochemical decomposition in Nafion even in the presence of oxygen, and can be used as a sensitizer to generate singlet oxygen to oxidize alkenes.

Full text of DOI:10.1039/b105601h

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Luminescence and photocatalytic properties of a
platinum(II)–quaterpyridine complex incorporated in Nafion
membrane
Xiao-Hong Li,a Li–Zhu Wu,*a Li–Ping Zhang,a Chen-Ho Tunga and Chi-Ming Cheb
a Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100101,
China. E-mail: g217@ipc.ac.cn
b Department of Chemistry and the HKU-CAS Joint Laboratory on New Materials, The University of
Hong Kong, Pokfulam Road, Hong Kong
Received (in Cambridge, UK) 26th June 2001, Accepted 19th September 2001
First published as an Advance Article on the web 11th October 2001
The non-emissive platinum(II)–quaterpyridine complex
shows strong photoluminescence at room temperature upon
incorporation into Nafion membrane; this complex is
stabilized toward photochemical decomposition in Nafion
even in the presence of oxygen, and can be used as a
sensitizer to generate singlet oxygen to oxidize alkenes.
acetonitrile solution. Furthermore, weak absorption bands from
375 to 450 nm were observed at high complex loading level and
attributed to the absorption of oligomerized species based on the
fact that excitation with light in this wavelength region
exclusively leads to excimeric emission (see below).
At room temperature, a degassed acetonitrile solution of
[Pt(QP)](CF3SO3)2 shows no emission upon excitation at 358
nm due to molecular distortion which results in nonradiative
decay. This situation is quite different when the complex is
incorporated into Nafion membrane. In water-swollen Nafion
membrane and in a nitrogen atmosphere, this complex shows a
very weak emission. However, after removal of the absorbed
water by evaporation under vacuum, the dry complex–Nafion
sample exhibits a strong luminescence in a nitrogen atmos-
phere. Possibly, the rigid matrix in dry Nafion causes the
deactivation process via molecular distortion to be blocked. Fig.
1 shows the emission spectra of dry Nafion samples at room
temperature. At low complex loading (ca. 131027 mol/g
Nafion) the Nafion sample shows a structured emission with
lmax at 476, 513 and 544 nm. and lifetime of ca. 3.42 ms. The
long emission lifetime suggests that the transition involved is a
spin-forbidden process. The vibrational progression is ca. 1330
cm21, which corresponds to the CNC and CNN stretching
modes, the dominant high-frequency acceptor modes of LC
levels involving polypyridine-type ligands.5 Thus, this emission
is assigned to be associated to the 3IL (p, p*) state of the
coordinated quaterpyridine ligand. At high loading a broad
emission band centered at 600 nm (t = 2.58 ms) prevails in
addition to the structured 3IL emission (Fig. 1). The broad
Luminescent square planar d8 platinum(II)–polypyridine com-
plexes may oligomerize in solution leading to supramolecular
architectures whose photophysical properties can be used to
elucidate ligand–ligand (p–p) and metal–metal interactions.1–4
However, in many cases they are weak emitters or even non-
emissive in fluid solution at room temperature.1–5 Thus, many
researchers have diverted their attention to exploit new
complexes which show strong luminescence at room tem-
perature.4–6 Here we report an approach to enhance the room-
temperature luminescence from Pt(II)–polypyridine complexes.
Our approach involves the use of Nafion membrane as the rigid
matrix. Upon incorporation into the membrane, the non-
emissive
platinum(II)–quaterpyridine
complex
[Pt(QP)](CF3SO3)2 (QP = 2,2A+6A,2B+6B,2BA-quaterpyridine)
exhibits strong photoluminescence at room temperature. Fur-
thermore, this complex in Nafion is stable and can be used as a
photosensitizer to generate singlet oxygen.
Nafion represents a family of polymers which consists of a
perfluorinated backbone and short pendant chains terminated by
sulfonic groups. When swollen in water, the structure of Nafion
is believed to resemble that of an inverse micelle.7 The water-
swollen Nafion can incorporate high concentrations of organic
and inorganic compounds, thus raising the possibility of
obtaining high local concentrations of substrates. The complex
[Pt(QP)](CF3SO3)2 was successfully incorporated into water-
swollen Nafion (in sodium form, Nafion-Na+) by immersing the
membrane samples in an aqueous suspension of the platinum
complex. The concentration of the platinum complex in the
membrane was measured by its absorption spectrum, assuming
that the absorption spectra obey Beer’s law in the concentration
range studied. Although this complex has a low solubility in
water, its concentration in the water-swollen Nafion-Na+ can be
rather high (ca. 2 3 1025 mol/g Nafion). It is likely that the
molecules of this complex are primarily solubilized in the
hydrophobic perfluorocarbon backbone region close to the
fluorocarbon/water interface.
The absorption spectrum of [Pt(QP)](CF3SO3)2 in acetoni-
trile has been described previously by one of the authors.8 This
complex shows intense absorption bands in the high-energy
1
region (l < 325 nm) which are assigned to intraligand IL(p,
p*) transitions, and moderately intense low-energy bands
ranging from 320 to 370 nm which can be assigned to spin-
allowed metal-to-ligand charge-transfer transitions (1MLCT)
involving a dp platinum orbital as the donor orbital and p*
quaterpyridine orbital as the acceptor orbital. Upon incorpora-
tion into Nafion membrane, all of the absorption bands are
slightly blue-shifted (ca. 2 nm) compared with those in
Fig. 1 The normalized emission spectra of [Pt(QP)](CF3SO3)2 incorporated
in Nafion at different concentrations and excitation wavelength: (a) in
acetonitrile, 131025 M, lex = 358 nm; (b) in Nafion, 131027 mol/g
Nafion, lex = 358 nm; (c) in Nafion, 1.4 3 1025 mol/g Nafion, lex = 358
nm; (d) in Nafion, 1.4 3 1025 mol/g Nafion, lex = 410 nm.
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Chem. Commun., 2001, 2280–2281
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b105601h

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Table 1 Photooxidation of alkenes sensitized by Nafion-incorporated
[Pt(QP)](CF3SO3)2 (7.02 3 1026 mol/g Nafion) in acetonitrile solution.
Photoirradiation
time/h
Substrate
Product
Yield (%)c
100
a
5
2
PhCHO
PhCO2Me
a
100
b
5
18+82
a The concentration is 1022 M. b The concentration is 1021 M. c Yields
were calculated based on the consumption of the substrates.
3
emission overlaps on the IL emission when the complex is
excited with light of wavelength < 375 nm. However, the broad
emission becomes dominant at an excitation wavelength > 380
nm (Fig. 1). Since this broad emission occurs only at high
loading, it is attributed to excimeric species originating from p–
p interactions between the partially stacked quaterpyridine
ligands in the oligomers. In other words, the broad emission
originates from excitation of the oligomerized species, differ-
ently from the 3IL emission which originates from excitation of
the ‘isolated’ monomers. The oligomerization of square planar
d8 metal complexes has been well precedented.1–6
Fig. 2 EPR spectrum of nitroxide radical generated by irradiation of the
Nafion-incorporated complex immersed in an oxygen-saturated acetonitrile
solution of TMP: (a) in the dark; (b) after the sample was irradiated for 400
s.
saturated acetonitrile, and immersed the complex-incorporated
Nafion membrane in the solution. Fig. 2 shows the EPR
spectrum obtained after 400 s irradiation of the solution and
clearly evidenced the nitroxide free radical. This obsevation
shows that singlet oxygen was generated in the Nafion
membrane and diffused into the solution to react with TMP to
give the nitroxide radical.
Financial support from the Ministry of Science and Technol-
ogy of China (grant No. G2000078104 and No. G2000077502)
and the National Nature Science Foundation of China are
gratefully acknowledged. We also acknowledge support from
the University of Hong Kong and the Hong Kong Research
Grants Council [HKU7298/99P].
The rigid matrix of Nafion stabilizes the [Pt(QP)](CF3SO3)2
complex towards photochemical decomposition. At room
temperature in aerated acetonitrile solution, this complex is
photochemically unstable. For example, photoirradiation of the
solution within 2 h resulted in a significant decrease in
absorption. However, photolysis of the complex in dry Nafion
membrane immersed in acetonitrile led to no change in the
absorbance of [Pt(QP)](CF3SO3)2 over a period of 10 h. The
matrix-stabilized complex was used as a photosensitizer to
generate singlet oxygen and to oxidize alkenes such as trans-
stilbene, trans-1,2-dimethoxystilbene and cyclohexene. The
photosensitized oxidations were carried out in oxygen-saturated
solution of the substrate in acetonitrile in which
[Pt(QP)](CF3SO3)2 incorporated Nafion membranes were sus-
pended. Photoirradiation of the complex yields the photooxida-
tion products as shown in Table 1. All the products were
identified by GC–MS. Significantly, the platinum complex–
Nafion sample was easily separated from the solution, and could
be used for many cycles without loss of activity. We propose
that this photooxidation of alkenes proceeds via singlet oxygen
mechanism. Acetonitrile can moderately swell the complex–
incorporated Nafion membrane and thus enables oxygen to
diffuse into the membrane from the solvent. Interaction between
Notes and references
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3
oxygen and the IL excited state of the complex results in
energy transfer to generate singlet oxygen, which diffuses back
to the solution to oxidize the alkenes. This proposal is supported
by EPR spectroscopy. It has been established that 2,2,6,6-tetra-
methylpiperidine (TMP) reacts with singlet oxygen to give the
stable free radical nitroxide, which can be readily detected by
EPR spectroscopy.9 Thus, we dissolved TMP in oxygen-
8 C. W. Chan and C. M. Che, Inorg. Chem., 1992, 31, 4874.
9 H. R. Li, L. Z. Wu and C. H. Tung, J. Am. Chem. Soc., 2000, 122, 2446
and references therein..
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