Preparation and characterization of luminescent SCS and NCN pincer platinum complexes derived from 3,5-bis(anilinothiocarbonyl)toluene

Article abstract of DOI:10.1021/om060016j

Starting from 5-methyl-1,3-bis(anilinothiocarbonyl)-benzene (1), two types of luminescent pincer platinum complexes with κ³S,C,S and κ³N,C,N coordination modes were obtained. Reaction of 1 with K₂PtCl₄ afforded a pincer Pt(II) complex (3) with the κ³S,C,S coordination mode, whereas oxidative cyclization of 1 yielded 5-methyl-1,3-bis(benzothiazol-2-yl)-benzene (2), which provided a pincer Pt(II) complex (4) with the κ³N.C.N coordination mode. Molecular structures and photochemical properties of the complexes have been examined. The decay lifetimes of the emission from the complexes were in the range 10^-5-10^-6 s, which were indicative of phosphorescent emission.

Full text of DOI:10.1021/om060016j

4026
Organometallics 2006, 25, 4026-4029
Notes
Preparation and Characterization of Luminescent SCS and NCN
Pincer Platinum Complexes Derived from
3,5-Bis(anilinothiocarbonyl)toluene
Ken Okamoto, Takaki Kanbara,* Takakazu Yamamoto,* and Akihide Wada
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama, 226-8503 Japan
ReceiVed January 6, 2006
Scheme 1. New Pincer Pt(II) Complexes 3 and 4 Derived
from 1
Summary: Starting from 5-methyl-1,3-bis(anilinothiocarbonyl)-
benzene (1), two types of luminescent pincer platinum complexes
with κ3S,C,S and κ3N,C,N coordination modes were obtained.
Reaction of 1 with K2PtCl4 afforded a pincer Pt(II) complex
(3) with the κ3S,C,S coordination mode, whereas oxidatiVe
cyclization of 1 yielded 5-methyl-1,3-bis(benzothiazol-2-yl)-
benzene (2), which proVided a pincer Pt(II) complex (4) with
the κ3N,C,N coordination mode. Molecular structures and
photochemical properties of the complexes haVe been examined.
The decay lifetimes of the emission from the complexes were in
the range 10-5-10-6 s, which were indicatiVe of phosphorescent
emission.
Introduction
Square-planar cycloplatinated complexes are often light
emissive,1 and use of such light emissive metal complexes,
especially phosphorescent metal complexes, as emitters in light-
emitting diodes (LEDs) has attracted much attention.2
We previously reported that κ3S,C,S pincer Pt(II) complexes
containing tertiary thioamide-based pincer ligands exhibited
strong phosphorescent emission in the glassy frozen state and
in the solid state.1q We have extended the research and now
report that the secondary thioamide compound 1 is a good
starting material to afford the two new pincer Pt(II) complexes
3 and 4.
* Corresponding authors. E-mail: tkanbara@res.titech.ac.jp. Fax: +81-
45-924-5976.
(1) (a) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba,
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C.-M. J. Chem. Soc., Dalton Trans. 1996 1645. (f) Lu, W.; Mi, B.-X.; Chan,
M. C.-W.; Hui, Z.; Zhu, N.; Lee, S.-T.; Che, C.-M. Chem. Commun. 2002,
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W. B. Inorg. Chem. 2002, 41, 2275. (p) Kanbara, T.; Yamamoto, T. J.
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T.; Ogawa, H.; Inoue, T. J. Organomet. Chem. 2004, 689, 1860. (r) Jude,
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Secondary thioamides have been used in the field of medical
and organic chemistry,3,4 and it is known that oxidative
cyclization of secondary N-arylthioamides with K3[Fe(CN)6]
gives benzothiazole derivatives.4 Such a cyclization reaction can
proceed with bis(secondary thioamide)s, and 1 can be trans-
formed into 2; thus, from the ligands 1 and 2, the new complexes
3 and 4 are obtained. Consequently, expansion of the complex
synthesis exhibited in Scheme 1 by modifying the aromatic
groups in 1 is expected to give various Pt complexes of the
types 3 and 4.
Cyclometalated κ2C,N Ir(III) and Pt(II) complexes of
2-arylbenzothiazoles1a,2c and related κ3N,N,N type metal com-
plexes of neutral tridentate ligands such as 2,6-bis(benzothiazol-
(3) (a) Jagodzin˜ski, T. S. Chem. ReV. 2003, 103, 197. (b) Klingele, M.
H.; Brooker, S. Eur. J. Org. Chem. 2004, 3422. (c) Murai, T.; Aso, H.;
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E.; Langeveld, B. M. W.; Schubert, U. S. AdV. Mater. 2005, 17, 1109, and
references therein.
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Mathis, C. A.; Wang, Y.; Holt, D. P.; Huang, G.-F.; Debnath, M. L.; Klunk,
W. E. J. Med. Chem. 2003, 46, 2740.
10.1021/om060016j CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/29/2006

Notes
Organometallics, Vol. 25, No. 16, 2006 4027
Table 1. Selected Bond Lengths and Bond Angles for 1-4
Bond Lengths (Å)
1
3
2
4
Pt-C
Pt-Cl
C-S
1.957(7)
2.389(2)
1.700(7)
1.714(7)
1.31(1)
1.924(5)
2.4254(14)
1.712(5)
1.724(5)
1.349(7)
1.327(7)
1.671(2)
1.331(2)
1.728(8)
1.768(6)
1.311(9)
1.190(9)
C-S
C(S)-N
C(S)-N
1.32(1)
Bond Angles (deg)
3
4
Cl-Pt-C
S-Pt-S
N-Pt-N
178.76(17)
170.77(6)
177.72(16)
158.57(18)
lengths are somewhat shorter than those observed with 1,
indicating that the S-coordination to Pt decreases the double-
bond character of the CdS bond. Two DMSO molecules in
the crystal are hydrogen bonded to the N-H protons of 3
through the O of DMSO (N‚‚‚O ) 2.809 and 2.852 Å), in
agreement with known hydrogen-bonding donor ability of the
N-H hydrogen of the secondary thioamide.7
Figure 1. X-ray crystal structures of (a) 3 and (b) 4 with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms and
solvated molecules are omitted for simplicity.
As depicted in Figure 1b, 4 has a κ3N,C,N coordination mode.
The Pt-C bond length is shorter than those in 3 and related
pincer Pt(II) complexes.1e,8 The cycloplatination brings about a
close contact between the H(4′) hydrogen of the benzothiazole
ring and the Cl ligand in 4, as depicted in Scheme 1, and
distances of 2.66 and 2.70 Å are obtained for the distance
between H(4′) and Cl by X-ray crystallography. The distance
is smaller than the sum (2.95 Å) of the van der Waals radii of
H and Cl, indicating the presence of an intramolecular hydrogen
bonding interaction between H(4′) and Cl.9 The two benzothia-
zole rings are nearly parallel to the center benzene ring. The
2-yl)pyridine have been reported.5 However, a κ3N,C,N pincer
complex such as 4 constituted of the benzothiazole unit has not
yet been reported to our knowledge. On this basis, we here report
that 1 can give two luminescent pincer Pt(II) complexes, 3 and
4, which are composed of different donor sets with κ3S,C,S and
κ3N,C,N pincer coordination modes, respectively. Molecular
structures and photochemical properties of the complexes are
also described.
1
Results and Discussion
intramolecular hydrogen-bonding interaction agrees with a H
NMR shift of the H(4′) hydrogen peak of 2 to a lower magnetic
field by 1.77 ppm by the complexation with Pt, as shown in
Figure S1.
Preparation of 1 is described in the Experimental Section.
The oxidative cyclization of 1 using K3[Fe(CN)6] afforded 2.
Reactions of 1 and 2 with K2PtCl4 in acetic acid led to
regioselective ortho,ortho-cycloplatination at the C-2 position
of the centered benzene ring with Pt(II) and afforded the
corresponding pincer complexes 3 and 4 in 63 and 51% yields,
respectively.
3 and 4 were fairly stable to air and water and were thermally
stable up to 300 °C. 3 showed good solubility in polar organic
solvents such as MeOH, EtOH, DMF, and DMSO. 4 was less
soluble and showed only low solubility in CH2Cl2 and CHCl3.
The 1H NMR spectra of 3 and 4 shown in Figure S1 agree with
their structures. In the 13C NMR spectrum of 3, the ortho,ortho-
cycloplatinated carbon (Cipso) was shifted to a higher magnetic
field by 24 ppm according to the σ-bonding to Pt.
The ORTEP drawings of 3 and 4 are presented in Figure 1,
and selected bond lengths and bond angles of 1-4 are
summarized in Table 1. 3 and 4 have a distorted square-planar
geometry similar to each other. As shown in Figure 1a, 3 has
a κ3S,C,S pincer complex strucrture, similar to previously
reported thioamide-based pincer complexes.1q,6 The Pt-C and
Pt-Cl bond lengths are consistent with those of the reported
thioamide-based pincer Pt(II) complexes. The CdS bond lengths
are longer than those observed with 1, and the C(S)-N bond
The crystal-packing diagrams of 3 and 4 exhibit that the
complexes are stacked in a head-to-tail fashion with interplanar
distances of 3.56 and 3.40 Å for 3 and 4, respectively, as
depicted in Figure S4. The molecular planes are slipped and
the Pt‚‚‚Pt distances between Pt in the upper complex and Pt in
the lower complex are 6.00 and 5.51 Å for 3 and 4, respectively.
These data indicate that there is no significant d8-d8 interaction.
Electrochemical Response. Redox behavior of 3 and 4 has
been investigated by cyclic voltammetry, and the data are given
in Table 2. Electrochemical reduction of 3 occurs at -1.78 V
vs Cp2Fe+/Cp2Fe, which is considered to occur mainly at the
ligand. The reduction peak is located at a higher potential than
red
that of 1 (Ep ) -2.38 V), presumably due to bonding to
(6) (a) Takahashi, S.; Nonoyama, M.; Kita, M. Transition Met. Chem.
1995, 20, 528. (b) Nojima, Y.; Nonoyama, M.; Nakajima, K. Polyhedron
1996, 15, 3795. (c) Hossain, M. A.; Lucarini, S.; Powell, D.; Bowman-
James, K. Inorg. Chem. 2004, 43, 7275. (d) Akaiwa, M.; Kanbara, T.;
Fukumoto, H.; Yamamoto, T. J. Organomet. Chem. 2005, 690, 4192. (e)
Begum, R. A.; Powell, D.; Bowman-James, K. Inorg. Chem. 2006, 45, 964.
(7) (a) Lee, H.-J.; Choi, Y.-S.; Lee, K.-B.; Park, J.; Yoon, C.-J. J. Phys.
Chem. A. 2002, 106, 7010. (b) Inoue, Y.; Kanbara, T.; Yamamoto, T.
Tetrahedron Lett. 2003, 44, 5167.
(8) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750. (b) Albrecht, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc.
2001, 123, 7233. (c) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem.
Eur. J. 1998, 4, 759, and references therein.
(5) (a) Boc´a, M.; Boc´a, R.; Kickelbick, G.; Linert, W.; Svoboda, I.; Fuess,
H. Inorg. Chem. Acta 2002, 338, 36. (b) Ru¨ttimann, S.; Moreau, C. M.;
Williams, A. F. Polyhedron 1992, 11, 635. (c) Addison, A. W.; Burman,
S.; Wahlgren, C. G. J. Chem. Soc., Dalton Trans. 1987, 2621. (d)
Livingstone, S. E.; Nolan, J. D. J. Chem. Soc., Dalton Trans. 1972, 218.
(9) (a) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des.
2001, 1, 277. (b) Thallapally, P. K.; Nangia, A. CrystEngComm 2001, 27,
1.

4028 Organometallics, Vol. 25, No. 16, 2006
Notes
Table 2. Photochemical, Electrochemical, and Thermal Data
of 3 and 4
3
4
absorptiona (UV-vis)
max/nm
(ꢀ, M-1 cm-1
294 (25 300)
383 (5900)
425 (6400)
478 (11 100)
544 (1300)
631
321 (23 800)
388 (5900)
425(8300)
λ
)
449 (12 800)
emission (PL) λmax/nmb
550, 597, 649, 717sh
550, 597, 649, 718sh
702
0.33
9.4 (4.1)
-1.98
λ
λ
max/nmc
max/nmd
741
0.08
3.7
-1.78
0.64
331
φfe
τ/µsf
Epred/Vg
Epox/Vg
T
d10/°Ch
326
a In THF. The values in the parentheses are molar absorption coefficients.
b In a CH2Cl2-THF (3:2) matrix at 77 K. c In a CH2Cl2-THF (3:2) solution
at 298 K. d For a microcrystalline sample at 298 K. e Quantum yield in a
CH2Cl2-THF (3:2) matrix at 77 K. fac-Tris(2-phenylpyridine)iridium (φf
) 0.4 ( 0.1) was used as a reference.13 f Emission decay lifetime at 77 K
and that at 298 K in parentheses. g Electrochemical oxidation and reduction
peak current potentials vs Cp2Fe+/Cp2Fe. Measured in a THF solution of
[(n-Bu)4N][BF4] (0.10 M). h Temperature for 10% weight loss under N2.
Figure 3. Excitation and emission spectra of (a) 3 and (b) 4 in a
frozen matrix of CH2Cl2-THF (3:2) at 77 K.
overlapped absorption bands in the region 400-500 nm, and
the shape of the spectra in the region is similar to that of reported
cycloplatinated complexes.1j,s,11 Absorption bands of similar Pt
complexes in this region have been assigned to metal-to-ligand
charge-transfer (MLCT) transitions.1q,t The position of the UV-
vis peak of 3 roughly agrees with the difference between the
electrochemical oxidation and reduction potentials of 3 (0.64
- (-1.98) ) 2.62 V or 21 000 cm-1) and supports the
assignment of the UV-vis band to the MLCT band. The UV-
vis peak of 3 at about 470 nm exhibits solvatochromism, as
shown in Table S1; such solvatochromism is often observed
for charge-transfer bands and gives additional support for the
assignment to the MLCT band.12
3 and 4 were photoluminescent in a glassy frozen state and
in the solid; 4 was light emissive even in solutions at room
temperature. The photoluminescence data of 3 and 4 are
included in Table 2, and Figure 3 shows photoluminescence
spectra of 3 and 4. As seen from Figure 3a, 3 shows a broad
emission band at about 630 nm when excited with 455 nm light
in the glassy frozen state at 77 K. The emission peak is located
at a longer wavelength than the onset position of the UV-vis
absorption band at about 580 nm, and the emission gives a decay
lifetime (τ) of 3.7 µs in the glassy state. These data are indicative
of phosphorescent emission. The emission spectrum and lifetime
τ of 3 are similar to those of reported tertiary thioamide-based
pincer Pt(II) complexes,1q and the reported emission of the Pt-
(II) complexes has been assigned to the emission occurring from
a 3MLCT excited state. A microcrystalline sample of 3 exhibited
Figure 2. UV-vis spectra of the ligands and the complexes in
THF at room temperature: (a) 1 and 3; (b) 2 and 4.
electron-donating Pt. The oxidation of 3 with Epox of 0.64 V is
considered to take place mainly at Pt, because 1 is oxidized at
ox
a considerably higher positive potential (Ep ) 1.01 V). 4
exhibits a reduction peak at -1.98 V, and the reduction potential
red
is also at a higher potential than that of 2 (Ep ) -2.49 V).
However, oxidation of 2 and 4 was not observed up to 1.2 V.
Optical Data. Figure 2 shows absorption spectra of 1-4.
The strong absorption bands around 300 nm are attributed to a
π-π* transition in the ligands. 1 exhibits a tail absorption in
the range 380-500 nm, which is assigned to an n-π* transition
in the thioamide group.10,11 Absorption spectra of 3 and 4 show
(12) (a) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439. (b) Hissler,
M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa, D.; Lachicotte,
R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447. (c) Pomestchenko, I. E.;
Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2003,
42, 1394.
(10) (a) Petiau, M.; Fabion, J. THEOCHEM 2001, 538, 253. (b)
Olszewska, T.; Gdaniec, M.; Polon´ski, T. J. Org. Chem. 2004, 69, 1248.
(11) Dimitra, K.-D.; Paras, N. Y.; Mavroudis, A. D.; Mauro, C. J. Inorg.
Biochem. 2000, 78, 347.

Notes
Organometallics, Vol. 25, No. 16, 2006 4029
mixture was stirred for 24 h at room temperature. After water was
added, the resulting yellow precipitate was collected by filtration.
The precipitate was washed with chloroform and ether (543 mg,
30% yield).
FAB-mass: m/z 363 [M + H]+. 1H NMR (400 MHz in DMSO-
d6): δ 11.83 (s, 2H), 8.03 (s, 1H), 7.81-7.80 (m, 6H), 7.44 (t, J )
7.2 Hz, 4H), 7.28 (t, J ) 7.2 Hz, 2H), 2.46 (s, 3H). 13C NMR (100
MHz in DMSO-d6): δ 196.7, 142.2, 140.0, 137.3, 130.3, 128.5,
126.4, 124.2, 123.5, 21.0. Anal. Calcd for C21H18N2S2: C, 69.58;
H, 5.00; N, 7.73; S, 17.69. Found: C, 69.55; H, 4.85; N, 7.57; S,
17.42.
Synthesis of 5-Methyl-1,3-bis(2-benzothiazolyl)benzene (2).
To a suspension of 1 (181 mg, 0.5 mmol) in ethanol (1 mL) was
added a 30% solution of aqueous NaOH (1 mL). The solution was
added dropwise to freshly prepared aqueous potassium ferricyanide
(1.317 g, 4.0 mmol) at 80 °C, and the reaction mixture was stirred
at 80 °C for 30 min. After cooling to room temperature, the
precipitate was washed thoroughly with water to give a pale yellow
powder of 2 (116 mg, 65% yield).
FAB-mass: m/z 359 [M + H]+. 1H NMR (400 MHz in CDCl3):
δ 8.55 (s, 1H), 8.11 (d, J ) 7.6 Hz, 2H), 8.07 (s, 2H), 7.94 (d, J
) 7.2 Hz, 2H), 7.52 (s, 2H), 7.42 (t, J ) 7.6 Hz, 2H), 2.56 (s, 3H).
13C NMR (100 MHz in CDCl3): δ 167.1, 154.0, 139.8, 135.1,
134.4, 130.3, 126.4, 125.3, 123.3, 121.6, 21.6. Anal. Calcd for
C21H14N2S2: C, 70.36; H, 3.98; N, 7.81; S, 17.89. Found: C, 70.49;
H, 3.94; N, 7.46; S, 17.39.
a weak emission at about 740 nm at room temperature, which
was significantly red-shifted from that observed in the glassy
frozen state, suggesting stabilization of the excited state by
formation of excimer-like adduct(s) in the solid.14 The layer-
to-layer packed structure of 3 seems to be suited for the
formation of such an adduct in the solid.
As shown in Figure 3b, 4 in the glassy frozen state exhibits
a vibronic-structured emission band in the range 530-750 nm,
with a spacing of about 1400 ( 60 cm-1, which corresponds to
skeletal vibrational frequencies of the benzothiazole ligand.1a,i,j
Essentially the same emission spectrum was obtained at room
temperature, as depicted in Figure S5. The emission peak of 4
also deviates from the onset position of the UV-vis absorption
band. 4 gave long lifetimes of 9.4 and 4.1 µs in the glassy frozen
state and in a 3:2 mixture of CH2Cl2 and THF at room
temperature, respectively. The large Stokes shift and the
relatively long lifetime suggest that the emission of 4 is also a
phosphorescent emission. With reference to previous spectro-
scopic studies on related pincer Pt(II) complexes such as
[5-methyl-1,3-di(2-pyridyl)phenyl-C2,N,N′]chloroplatinum(II) (5),1j
the emission from 4 would be assigned to a ligand-centered
3(π*π) transition. The excitation and emission peaks of 4 were
observed in a lower energy region by about 1620 cm-1 than
those of 5, suggesting that the expansion of the aromatic system
in the ligand resulted in the reduction of the band gap. In the
solid state, the emission peak of 4 was red-shifted to about 700
nm, which was also related to the layer-to-layer stacking of the
complex in the solid.
Synthesis of [5-Methyl-1,3-bis(anilinothiocarbonyl)phenyl-
C2,S,S′]chloroplatinum(II) (3). A mixture of 1 (36 mg, 0.1 mmol)
and K2PtCl4 (41 mg, 0.1 mmol) in acetic acid (3 mL) was refluxed
for 72 h under N2. The resulting orange precipitate was filtered
and washed with methanol and water to give a bright reddish
powder of 3 (38 mg, 63% yield).
Conclusion and Scope
FAB-mass: m/z 556 [M - Cl]+. 1H NMR (400 MHz in DMSO-
d6): δ 12.09 (s, 2H), 8.04 (s, 2H), 7.46-7.63 (m, 10H), 2.34 (s,
3H). 13C NMR (100 MHz in DMSO-d6): δ 202.2, 161.2, 153.9,
143.2, 137.4, 129.2, 129.1, 128.2, 125.6, 20.5. Anal. Calcd for
C21H17ClN2PtS2: C, 42.60; H, 2.89; N, 4.73; Cl, 5.99; S, 10.83.
Found: C, 42.03; H, 2.89; N, 4.40; Cl, 5.96; S, 11.17.
Synthesis of [5-Methyl-1,3-bis(2-benzothiazolyl)phenyl-C2,N,N′]-
chloroplatinum(II) (4). Preparation of 4 was carried out in analogy
with that of 3 using 2 to afford a yellow powder of 4 (51% yield).
S-Coordinating and oxidative cyclization ability of 1 provided
two phosphorescent pincer Pt(II) complexes, 3 and 4, with
different κ3S,C,S and κ3N,C,N coordination systems. The
electrochemical, UV-vis, and photoluminescence data support
3
a notion that the emission from 3 originates from the MLCT
excited state. On the other hand, the emission from 4 is
3
considered to originate from (π*π) emission. By modifying
the structure of the starting material 1, various new Pt complexes
of the types 3 and 4 are considered to be obtained, and the
chemistry of the luminescent pincer complexes is expected to
be expanded.
1
FAB-mass: m/z 587 [M + H]+, 552 [M - Cl]+. H NMR (400
MHz in CDCl3): δ 9.88 (d, J ) 8.8 Hz, 2H), 7.83 (d, J ) 8.0 Hz,
2H), 7.66 (t, J ) 8.0 Hz, 2H), 7.47 (t, J ) 8.8 Hz, 2H), 7.39 (s,
2H), 2.36 (s, 3H). Anal. Calcd for C21H13ClN2PtS2: C, 42.90; H,
2.23; N, 4.76; Cl, 6.03; S, 10.91. Found: C, 42.91; H, 2.51; N,
4.42; Cl, 6.23; S, 10.23.
Experimental Section
Synthesis of 5-Methyl-1,3-bis(anilinothiocarbonyl)benzene (1).
To magnesium powder (255 mg, 10.5 mmol) was added dropwise
a THF (5 mL) solution of 3,5-dibromotoluene (1.25 g, 5 mmol).
The reaction mixture was refluxed under N2 for 12 h. The resulting
mixture was allowed to cool at 0 °C, and a THF (5 mL) solution
of phenylisothiocyanate (1.35 g, 10 mmol) was added. The reaction
Acknowledgment. The authors gratefully thank Dr. T.
Koizumi of our laboratory for his experimental support. This
work has been partly supported by a Grant-in-Aid from the
Ministry of Education, Science, Sports, and Culture of Japan.
Supporting Information Available: General procedures, X-ray
1
crystallographic data, H NMR spectra, and UV-vis and photo-
(13) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985,
107, 1431.
(14) Delahaye, S.; Loosli, C.; Liu, S.-X.; Decurtins, S.; Labat, G.; Neels,
A.; Loosli, A.; Ward, T. R.; Hauser, A. AdV. Funct. Mater. 2006, 16, 286,
and references therein.
luminescence data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM060016J