Structural and spectroscopic studies on Pt...Pt and π-π interactions in luminescent multinuclear cyclometalated platinum(II) homologues tethered by oligophosphine auxiliaries

Article abstract of DOI:10.1021/ja039727o

The synthesis and structural, spectroscopic, and electrochemical properties of a series of trinuclear tridentate cyclometalated platinum(II) complexes tethered by bis(diphenylphosphinomethyl)phenylphosphine (dpmp) have been studied and compared with their mono- and binuclear homologues and a propeller-like congener. The X-ray crystal structures of several derivatives show the presence of a variety of intramolecular Pt...Pt, π-π, and C-H...O(crown ether) and intermolecular π-π interactions. The trinuclear complexes display strong absorption in the 400-600 nm region and show intense red to near-infrared phosphorescence with microsecond lifetimes in fluid and glassy solutions and in the solid state. These emissions are generally assigned as ³MMLCT [dσ*→π*(C/N/N)] in nature. The close similarities between the emission energies in acetonitrile solution and in the solid state at 298 K indicate that comparable Pt...Pt and π-π configurations are maintained in both media, and hence a relationship between the photophysical behavior of these lumophores and their solid-state structural features is proposed. The tendencies of the absorption and emission energies to red-shift from mono- to linearly tethered bi- and trinuclear Pt(II) species are evident. A light-emitting electrochemical cell using a trinuclear Pt(II) derivative as emitter has been demonstrated.

Full text of DOI:10.1021/ja039727o

Published on Web 05/28/2004
Structural and Spectroscopic Studies on Pt‚‚‚Pt and π-π
Interactions in Luminescent Multinuclear Cyclometalated
Platinum(II) Homologues Tethered by Oligophosphine
Auxiliaries
Wei Lu, Michael C. W. Chan, Nianyong Zhu, Chi-Ming Che,* Chuannan Li, and
Zheng Hui
Contribution from the Department of Chemistry and HKU-CAS Joint Laboratory on
New Materials, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong SAR, China
Received November 21, 2003; E-mail: cmche@hku.hk
Abstract: The synthesis and structural, spectroscopic, and electrochemical properties of a series of trinuclear
tridentate cyclometalated platinum(II) complexes tethered by bis(diphenylphosphinomethyl)phenylphosphine
(dpmp) have been studied and compared with their mono- and binuclear homologues and a propeller-like
congener. The X-ray crystal structures of several derivatives show the presence of a variety of intramolecular
Pt‚‚‚Pt, π-π, and C-H‚‚‚O(crown ether) and intermolecular π-π interactions. The trinuclear complexes
display strong absorption in the 400-600 nm region and show intense red to near-infrared phosphorescence
with microsecond lifetimes in fluid and glassy solutions and in the solid state. These emissions are generally
assigned as 3MMLCT [dσ*fπ*(C∧N∧N)] in nature. The close similarities between the emission energies in
acetonitrile solution and in the solid state at 298 K indicate that comparable Pt‚‚‚Pt and π-π configurations
are maintained in both media, and hence a relationship between the photophysical behavior of these
lumophores and their solid-state structural features is proposed. The tendencies of the absorption and
emission energies to red-shift from mono- to linearly tethered bi- and trinuclear Pt(II) species are evident.
A light-emitting electrochemical cell using a trinuclear Pt(II) derivative as emitter has been demonstrated.
Introduction
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configuration, have been extensively studied for their long triplet
excited-state lifetimes, high emission quantum yields, and
planar d8 electronic configuration, have also been demonstrated
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J. AM. CHEM. SOC. 2004, 126, 7639-7651
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A R T I C L E S
Lu et al.
to be strongly emissive in fluid solution and the solid state, and
have recently received considerable attention in material
science.6-11 The open square-planar geometry of Pt(II) allows
axial substrate-binding, and this results in different photophysical
and -chemical properties when compared to d6 octahedral metal
lumophores which display sphere-like coordination environ-
ments. Diverse excited states that emit in the visible spectral
range have been reported for this class of complexes, and these
include metal-centered 3(dd), ligand-centered 3(ππ*), excimeric
σ-donor than terpyridine but a more capable π-acceptor than
C∧N∧C. Hence, the unique nature of the C∧N∧N auxiliary
apparently affords 3(dd) excited states (responsible for deactiva-
tion of Pt(II) photoluminescence) that are higher in energy
3
3
relative to the MLCT and/or (ππ*) states. However, strong
visible absorptions and near-infrared (NIR) emissions in fluid
solutions are difficult to attain in monomeric [(C∧N∧N)PtL]n+
(L ) halide, acetylide, phosphine, pyridine, etc.; n ) 0, 1)
complexes, which typically show absorption and emission bands
that are higher in energy compared to the well-studied
[Ru(aromatic R-diimine)3]2+ system. This clearly hinders the
utilization of cyclometalated Pt(II) complexes as photocatalysts
for solar-energy conversion and as luminescent probes in
biomedical applications. We envisioned that it would be feasible
for the absorption and emission energies of this class of lumo-
phores to be red-shifted through d8‚‚‚d8 and/or ligand-ligand
interactions between [(C∧N∧N)Pt]+ moieties.16 The aim of the
present study is to illustrate the intriguing structural and spectro-
scopic properties of multinuclear cyclometalated Pt(II) com-
plexes that are tethered in linear or propeller conformations by
oligophosphine ligands. Furthermore, the perturbation of low-
energy emissions by weak noncovalent interactions, such as
d8‚‚‚d8 (Pt‚‚‚Pt), π-π, and C-H‚‚‚O, will be highlighted.
3
3(ππ*), metal-to-ligand charge-transfer MLCT, and metal-
metal-to-ligand charge-transfer 3MMLCT (or [dσ*fπ*]). Pt(II)
complexes displaying emission in the yellow to red region or
showing intense absorption in the near-infrared region have
found applications in high-performance organic light-emitting12
and photovoltaic dye-sensitized13 devices. Furthermore, there
have been active studies in developing emissive Pt(II) complexes
as luminescent probes for biomolecular targets such as DNA
and proteins.14 Very recently, the realization of supramolecular
recognition derived from Pt‚‚‚Pt interactions in a predesigned
metal-based molecular receptor was described by Bosnich and
co-workers.15
The [(C∧N∧N)Pt]+ moiety (HC∧N∧N ) 6-aryl-2,2′-bipyri-
dine) and its derivatives11 were chosen in this study as the
building unit for assembly of oligonuclear Pt(II) complexes due
to their rich photoluminescent properties in fluid solutions. With
regards to the terpyridine9 and C∧N∧C (HC∧N∧CH ) 2,6-
diphenylpyridine)10 complexes, the C∧N∧N ligand is a superior
Results
Synthesis. The tridentate cyclometalating ligands used in this
study contain the 2,2′-bipyridine moiety plus a 6-aryl group
which acts as a proton-donor to facilitate the cyclometalating
process. Depending on the pendant functional groups, these
ligands can be synthesized by various routes (see Scheme S1
in Supporting Information). 4,4′-Bis(tert-butyl)-6-phenyl-2,2′-
bipyridine was prepared by reaction of phenyllithium and 4,4′-
bis(tert-butyl)-2,2′-bipyridine, a method adopted by Hamilton
and co-workers in the synthesis of 2-phenyl-1,10-phenanthro-
line.17 4-Hydroxycarbonyl-6-phenyl-2,2′-bipyridine and 6-(2-
thienyl)-2,2′-bipyridine were obtained by condensation between
acalkylpyridinium salts and R,â-unsaturated ketones in the
presence of ammonium acetate in methanol.18 Compounds 319
and 4 are useful precursors to 4-substituted (HC∧N∧N) ligands,
because the nBu3Sn and iodo groups can be readily substituted
through established palladium(II)/copper(I)-catalyzed cross-
coupling and amination protocols.20 Heating a mixture of 4 and
aza-15-crown-5 gave 5 bearing a crown ether moiety. This
method was previously employed by Ward and co-workers to
prepare 4-(aza-18-crown-6)-2,2′:6′,2′′-terpyridine.21 Reflux-
ing these ligands with K2PtCl4 in CH3CN/H2O or with PtCl2
in C2H5OC2H4OH/H2O gave the desired Cl-ligated precursors.
The tridentate ligand bis(diphenylphosphinomethyl)phenyl-
phosphine (dpmp) was synthesized according to the literature
method.22
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Trinuclear Cyclometalated Pt(II) Luminophores
A R T I C L E S
Chart 1
The trinuclear [{Rx(C∧N∧N)}3Pt3(µ3-triphosphine)]3+ (Rx
denotes substituent(s) on (C∧N∧N), Chart 1) derivatives were
prepared by reacting [Rx(C∧N∧N)PtCl] with the triphosphine
ligands 1,1,1-tris(diphenylphosphinomethyl)ethane (tppe) and
dpmp in a CH2Cl2/CH3OH mixture followed by counterion
metathesis. For comparison, their mono- and binuclear homo-
logues with triphenylphosphine (PPh3) and bis(diphenylphos-
phino)methane (dppm) ligands, respectively, have also been
prepared. Several attempts to synthesize the Pd3 counterparts
were undertaken but have not been successful; treatment of three
molar equivalents of [(C∧N∧N)PdCl] with dpmp resulted in the
isolation of the binuclear complex [(C∧N∧N)2Pd2(µ-dpmp)]2+
(11), which has been characterized by 1H (only one set of signals
for (C∧N∧N) ligands) and 31P (one signal for bound P and one
for free P [lower intensity]) NMR spectroscopy to contain
tethered [(C∧N∧N)Pd] moieties on the two terminal phosphorus
atoms with a free central phosphorus atom.
195Pt signals from mono- to bi- and trinuclear species in
the present study is evident; hence, the 195Pt NMR signals
appear at -4160, -4008, and -3843/-3969 (central/terminal)
ppm for 12(ClO4), 13(ClO4)2, and 14(ClO4)3 in the [4,4′-
tBu2(C∧N∧N)Pt] series, -4157, -3982, and -3820/-3962 ppm
for 18(ClO4), 19(ClO4)2, and 20(ClO4)3 in the [4-EtO2C-
(C∧N∧N)Pt] series, and -4240, -4092, and -3941/-4061 ppm
for 23(ClO4), 24(ClO4)2, and 25(ClO4)3 in the [(S∧N∧N)Pt]
series, respectively. It is interesting to note that for each higher
homologue in the series, the corresponding 195Pt NMR signal
systematically shifts downfield by 160 ( 10 ppm. This trend
to shift downfield for the 195Pt NMR signals is consistent with
that observed by Bosnich and co-workers in their studies on
the “supramolecular” recognition and formation of metal-based
host-guest complexes through Pt‚‚‚Pt/ligand-ligand inter-
actions.15a
Solid-State Pt‚‚‚Pt and π-π Interactions. The crystal
structures of 7(ClO4)3, 10(ClO4)3‚H2O, 10(ClO4)3‚2Et2O‚CH3-
CN, and 25(ClO4)3‚1.5Et2O‚CH3CN were reported in our
previous communication,16 and the remaining crystal data are
listed in the Supporting Information. The structure of 7(ClO4)3
shows three [(C∧N∧N)Pt]+ units arranged in C3V symmetry
through coordination of the P donors of the tripodal tppe ligand
to Pt(II). In contrast, the dpmp ligand can position multiple
[(C∧N∧N)Pt]+ moieties into linear face-to-face configurations.
Upon diffusion of diethyl ether vapor into an acetonitrile solution
of 10(ClO4)3, three crystal forms of yellow, orange, and red
coloration were obtained. For 10(ClO4)3‚H2O (red form), the
intramolecular metal‚‚‚metal contacts of 3.194 Å at Pt1‚‚‚Pt2
and 3.399 Å at Pt2‚‚‚Pt3, with the Pt1‚‚‚Pt2‚‚‚Pt3 angle being
169°, indicate weak d8‚‚‚d8 interactions across Pt1‚‚‚Pt2‚‚‚Pt3.
The yellow and orange forms quickly transform into the red
form upon removal from the mother liquor. Nevertheless, the
molecular structure of the orange form (10(ClO4)3‚2Et2O‚CH3-
CN) was determined by X-ray crystallography. The intramo-
lecular metal‚‚‚metal contacts are 3.364 Å (Pt1‚‚‚Pt2) and 3.617
The 31P NMR spectrum of 10(ClO4)3 contains only one signal
1
(with 195Pt satellites: JP-Pt ) 4070 Hz) at 26 °C in CD3CN,
but there are clearly at least two types of phosphorus nuclei in
different chemical environments in this molecule. Subsequent
variable-temperature 31P NMR measurements for 10(ClO4)3 in
CD3COCD3/CD3CN (1/1, v/v) revealed two groups of signals
(see Supporting Information for spectra). Upon decreasing the
temperature from +60 to -60 °C, the chemical shift for the
central P atom experiences a downfield shift from 11.3 to 13.7
ppm, while that for the peripheral P atoms (higher intensity)
shifts upfield from 12.6 to 11.7 ppm. No fluxional behavior
was detected for 10(ClO4)3; thus, it appears that the central
[(C∧N∧N)Pt]+ unit “swings” between the two terminal [(C∧N∧N)-
Pt]+ moieties at a sufficiently fast rate so that the environments
of the latter average out (or remain the same) even down to
-60 °C.
The 195Pt[31P] NMR spectrum of 10(ClO4)3 shows two
doublets at -3847 (central) and -3976 (terminal) ppm (1JP-Pt
) 4091 and 4114 Hz, respectively). A downfield shift for the
9
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A R T I C L E S
Lu et al.
CN, but the crystal packing of 20(ClO4)3‚2CH3CN is different.
The [4-EtO2C(C∧N∧N)Pt]+ moieties in 20(ClO4)3‚2CH3CN are
arranged into infinite chains through intramolecular Pt‚‚‚Pt and
intermolecular π-π interactions, as depicted in Figure 2
(bottom). The intermolecular π-π stacking occurs between the
[4-EtO2C(C∧N∧N)Pt]+ planes of Pt1 and Pt3, which contrasts
with that in crystal 10(ClO4)3‚2Et2O‚CH3CN where dimeric
π-π contacts occur between two [(C∧N∧N)Pt]+ moieties at Pt3.
As a consequence of the noncovalent interactions, the molecular
cations are packed in a coiled/helical conformation, where one
turn contains two cations and a screw-pitch of 20.8 Å is
observed.
In the crystal structure of 6 (see Supporting Information for
perspective view and packing diagram), the oxygen atoms on
the crown ether moiety are not involved in intermolecular
interactions. Neighboring (C∧N∧N) planes are stacked into
dimeric structures in a head-to-tail manner through π-π
interaction. The interlayer distance is ca. 3.6 Å, but the shortest
Pt‚‚‚Pt distance is 6.609 Å.
Perspective views of the cation in crystal 22(PF6)2 are shown
in Figure 3. Unlike previous dppm-tethered [(C∧N∧N)Pt]
complexes, the long Pt‚‚‚Pt separation of 3.827 Å in 22(PF6)2
is beyond the range expected for appreciable intermetal interac-
tion. The two [4-(aza-15-crown-5)(C∧N∧N)Pt]+ moieties are
staggered with a torsion angle of 43.2° about the Pt‚‚‚Pt axis.
Close scrutiny of this structure reveals short contacts between
H3 on the 4′-position of the diimine ligand and the oxygen atoms
(O2-O4) on the crown ether pendant of the adjacent [4-(aza-
15-crown-5)(C∧N∧N)Pt]+ fragment, and vice versa (e.g.
C3‚‚‚O3 3.439 and H3‚‚‚O3 2.598 Å). These oxygen atoms
protrude toward the closest hydrogen atom, while the nitrogen
atoms do not appear to be significantly involved in these weak
hydrogen-bonding interactions.
Figure 1. Perspective view of the cation of 14(ClO4)3‚3CH3CN.
Å (Pt2‚‚‚Pt3), and the Pt1‚‚‚Pt2‚‚‚Pt3 angle is 162°. Hence, a
weak d8‚‚‚d8 interaction exists between Pt1‚‚‚Pt2, but the value
of the Pt2‚‚‚Pt3 separation (3.617 Å) is beyond the range of
intermetal distances (3.09-3.50 Å) normally observed in
monomeric Pt(II) extended linear-chain structures.6d In addition,
it is apparent that two neighboring cations of 10 in the orange
form are engaged in intermolecular π-π interactions (ca. 3.5
Å between [(C∧N∧N)Pt]+ planes at Pt3). Furthermore, the color
of crystalline complex 10 depends on the counterions. For
10(BF4)3, only the orange form was found in the mother liquor,
and this slowly transformed into the red form upon collection
on a sinter-glass filter. For 10(PF6)3, only the red form was
observed in the mother liquor or under reduced pressure. After
prolonged standing, all orange/red forms of these solids slowly
turn brownish red in color, presumably due to lattice contraction
induced by loss of solvated molecules.
For complex 14, two tert-butyl groups are introduced at each
(C∧N∧N) ligand to increase the steric repulsion between
neighboring [(C∧N∧N)Pt] moieties. A perspective view of the
complex cation of 14(ClO4)3‚3CH3CN is shown in Figure 1.
The three [4,4′-tBu2(C∧N∧N)Pt]+ fragments are partially stag-
gered, with torsion angles of 29.6° and 9.3° about the Pt1-Pt2
and Pt2-Pt3 axes, respectively (defined by the angle between
the Pt1-Pt2-N2 and Pt1-Pt2-N4 planes and that between
the Pt2-Pt3-N2 and Pt2-Pt3-N6 planes, respectively). The
intramolecular metal‚‚‚metal contacts are 3.217 Å at Pt1‚‚‚Pt2
and 3.601 Å at Pt2‚‚‚Pt3, and the Pt1‚‚‚Pt2‚‚‚Pt3 angle is 157°,
resembling those values for the orange form of 10. Similarly,
we propose that the weak d8‚‚‚d8 interaction is present between
Pt1‚‚‚Pt2 but not Pt2‚‚‚Pt3. The salient difference between the
crystal structure of 14(ClO4)3‚3CH3CN and 10(ClO4)3‚2Et2O‚CH3-
CN (orange form) is that the latter undergoes dimeric π-π
interactions with an adjacent molecule, but the bulkiness of the
[4,4′-tBu2(C∧N∧N)Pt]+ moieties presumably disfavors this.
A perspective view of the complex cation of 20(ClO4)3‚2CH3-
CN is shown in Figure 2 (top). The three [4-EtO2C(C∧N∧N)-
Pt]+ groups are staggered, with torsion angles of 33.7° about
the Pt1-Pt2 axis and 8.9° about Pt2-Pt3, respectively. The
intramolecular metal‚‚‚metal contacts are 3.466 Å at Pt1‚‚‚Pt2
and 3.647 Å at Pt2‚‚‚Pt3, and the Pt1‚‚‚Pt2‚‚‚Pt3 angle is 162°;
hence, Pt1‚‚‚Pt2 but not Pt2‚‚‚Pt3 appears to undergo a weak
d8‚‚‚d8 interaction. These structural data are similar to those for
10(ClO4)3‚2Et2O‚CH3CN (orange form) and 14(ClO4)3‚3CH3-
Perspective views of cations in 24(ClO4)2‚2CH3CN and
25(ClO4)3‚1.5Et2O‚CH3CN are shown in Figure 4. In the
binuclear structure 24, the two [(S∧N∧N)Pt]+ fragments are
staggered with a torsion angle of 29.0° about the Pt1-Pt2 axis.
The intramolecular Pt‚‚‚Pt distance is 3.321 Å. In the trinuclear
structure 25, the three [(S∧N∧N)Pt]+ units are staggered with
torsion angles of 33.0° about the Pt1-Pt2 axis and 18.7° about
Pt2-Pt3, respectively. The intramolecular metal‚‚‚metal separa-
tions are 3.254 Å at Pt1‚‚‚Pt2 and 3.389 Å at Pt2‚‚‚Pt3, and the
Pt1‚‚‚Pt2‚‚‚Pt3 angle is 163°. Hence, weak d8‚‚‚d8 interactions
apparently exist between Pt1‚‚‚Pt2 in 24 and across Pt1‚‚‚
Pt2‚‚‚Pt3 in 25.
Electronic Spectroscopy. The UV-vis absorption data of
the complexes in the present study are summarized in Table 1.
A comparison between the electronic spectra of 7(ClO4)3,
10(ClO4)3, and 17(ClO4)3 is shown in Figure 5. The UV-vis
spectra of 10(ClO4)3 and 17(ClO4)3 in acetonitrile at 298 K
feature broad absorptions in the 400-600 nm range with average
ꢀ values of ∼5000 dm3 mol-1 cm-1, which are almost double
that (average ꢀ ≈ 2550 dm3 mol-1 cm-1) previously found for
the binuclear homologue [(C∧N∧N)2Pt2(µ-dppm)](ClO4)2
[9(ClO4)2]. The lowest-energy absorption peak maximum is
located at 517 and 522 nm for 10(ClO4)3 and 17(ClO4)3,
respectively. These transitions are not observed in the UV-vis
spectra of the propeller-shaped 7(ClO4)3 and the mono- [8(ClO4)
and 15(ClO4)] and binuclear [9(ClO4)2 and 16(ClO4)2] relatives.
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Trinuclear Cyclometalated Pt(II) Luminophores
A R T I C L E S
Figure 2. Top: perspective view of the cation of 20(ClO4)3‚2CH3CN. Bottom: view of four neighboring cations of 20(ClO4)3‚2CH3CN (bold lines highlight
the coiled nature of the structure; phenyl groups of dpmp ligands are omitted for clarity).
Figure 3. View along the Pt‚‚‚Pt axial (left) and side view (right, phenyl groups of dppm ligand are omitted for clarity) of the cation of 22(PF6)2 (C2 axis
exists for cation), with dashed lines indicating contacts shorter than the sum of van der Waals radii.
A comparison between the electronic spectra of 12(ClO4),
13(ClO4)2, and 14(ClO4)3 is shown in Figure 6. The UV-vis
spectra of 13(ClO4)2 and 14(ClO4)3 in acetonitrile at 298 K
feature broad absorptions in the 400-550 nm range, and the
lowest-energy absorption peak maximum is ca. 500 nm for both
13(ClO4)2 and 14(ClO4)3. Clearly, such broad absorptions are
9
J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7643

A R T I C L E S
Lu et al.
Figure 4. Perspective view of the cation of 24(ClO4)2‚2CH3CN (left) and 25(ClO4)3‚1.5Et2O‚CH3CN (right).
Table 1. Absorption Dataa
3
complex
λ
abs/nm (ꢀ/104 dm mol-1 cm-1
)
[(C∧N∧N)3Pt3(µ3-tppe)](ClO4)3, 7(ClO4)3
332 (3.17), 350 (2.95), 400 (sh, 0.14), 415 (sh, 0.12)
337 (2.41), 424 (broad, 0.52), 482 (broad, 0.46), 513 (broad, 0.41)
331 (1.24), 347 (1.18), 390 (broad sh, 0.05)
328 (1.41), 336 (1.40), 418 (broad, 0.44), 460 (broad sh, 0.31), 490 (broad sh, 0.18)
in CH3CN: 332 (2.56), 340 (sh, 2.47), 414 (broad, 0.56), 463 (broad sh, 0.41),
495 (0.25)
[(C∧N∧N)3Pt3(µ3-dpmp)](ClO4)3, 10(ClO4)3
[4,4′-tBu2(C∧N∧N)PtPPh3](ClO4), 12(ClO4)
[{4,4′-tBu2(C∧N∧N)}2Pt2(µ-dppm)](ClO4)2, 13(ClO4)2
[{4,4′-tBu2(C∧N∧N)}3Pt3(µ3-dpmp)](ClO4)3, 14(ClO4)3
in CH2Cl2: 336 (3.01), 346 (sh, 2.86), 417 (broad sh, 0.63), 463 (broad sh, 0.48),
495 (0.34)
in C2H5OH: 332 (2.54), 340 (sh, 2.47), 417 (0.56), 462 (broad sh, 0.41), 491
(broad sh, 0.26)
in C6H6: 336 (2.81), 344 (sh, 2.71), 424 (broad, 0.67), 464 (broad sh, 0.54), 493
(broad sh, 0.37)
344 (6.60), 420 (sh, 1.02), 494 (0.57), 522 (0.59)
342 (1.47), 364 (1.08), 442 (broad sh, 0.04)
336 (1.51), 351 (1.54), 438 (0.35), 490 (broad sh, 0.22), 525 (broad sh, 0.16)
342 (2.32), 356 (2.16), 441 (broad sh, 0.39), 492 (broad sh, 0.27), 539 (0.21)
317 (1.12), 321 (1.12), 374 (0.55)
320 (1.69), 368 (0.93), 439 (broad sh, 0.28)
342 (1.46), 460 (0.04)
345 (3.48), 437 (0.89), 495 (broad sh, 0.34)
346 (2.79), 468 (0.85), 528 (broad sh, 0.41)
[{4-(4-MeOC6H4)(C∧N∧N)}3Pt3(µ3-dpmp)](ClO4)3, 17(ClO4)3
[4-EtO2C(C∧N∧N)PtPPh3](ClO4), 18(ClO4)
[{4-EtO2C(C∧N∧N)}2Pt2(µ-dppm)](ClO4)2, 19(ClO4)2
[{4-EtO2C(C∧N∧N)}3Pt3(µ3-dpmp)](ClO4)3, 20(ClO4)3
[4-(aza-15-crown-5)(C∧N∧N)PtPPh3](PF6), 21(PF6)
[{4-(aza-15-crown-5)(C∧N∧N)}2Pt2(µ-dppm)](PF6)2, 22(PF6)2
[(S∧N∧N)PtPPh3]ClO4, 23(ClO4)
[(S∧N∧N)2Pt2(µ-dppm)](ClO4)2, 24(ClO4)2
[(S∧N∧N)3Pt3(µ3-dpmp)](ClO4)3, 25(ClO4)3
[(S∧N∧N)PdPPh3]ClO4, 26(ClO4)
311 (sh, 2.12), 319 (2.20), 421 (broad, 0.24)
312 (sh, 3.23), 425 (broad sh, 0.39)
[(S∧N∧N)2Pd2(µ-dppm)](ClO4)2, 27(ClO4)2
a Recorded in CH3CN solution at 298 K unless stated otherwise.
Figure 5. UV-vis absorption spectra of 7(ClO4)3, 10(ClO4)3, and 17(ClO4)3
in acetonitrile at 298 K. Inset: Plot of ∆ꢀ versus wavelength for 7(ClO4)3
and 10(ClO4)3 in acetonitrile at 298 K.
Figure 6. UV-vis absorption spectra of 12(ClO4), 13(ClO4)2, and
14(ClO4)3 in acetonitrile at 298 K. Inset: Plot of ∆ꢀ versus wavelength for
12(ClO4) (ꢀ values multiplied by 3) and 14(ClO4)3 in acetonitrile at 298 K.
not observed beyond 400 nm in the spectrum of the mononuclear
species 12(ClO4).
Comparisons between the electronic spectra of 18(ClO4),
19(ClO4)2, and 20(ClO4)3 and between 23(ClO4), 24(ClO4)2, and
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Trinuclear Cyclometalated Pt(II) Luminophores
A R T I C L E S
Table 2. Emission Data
solution,a 298 K
max/nm (τ/µs; φb)
glassy solution,c 77 K
max/nm (τ/µs)
solid state, 298 K
max/nm (τ/µs)
solid state, 77 K
λmax/nm (τ/µs)
complex
λ
λ
λ
[(C∧N∧N)3Pt3(µ3-tppe)](ClO4)3,
7(ClO4)3
520 (8.0; 0.12)
710 (0.1; 0.02)
500 (16.0), 535
544 (5.9)
533 (9.6), 571
[(C∧N∧N)3Pt3(µ3-dpmp)](ClO4)3,
10(ClO4)3
516 (16.7), 552; 677 (4.0)
orange form: 630 (1.5)
red form: 674 (0.9)
brown form: 709 (0.4)
532 (3.5)
red form: 680 (3.4)
brown form: 730 (2.3)
[4,4′-tBu2(C∧N∧N)PtPPh3](ClO4),
12(ClO4)
532 (1.0; 0.11)
625 (0.5; 0.05)
488 (23.5), 524
516 (4.2), 548
592 (3.5)
[{4,4′-tBu2(C∧N∧N)}2Pt2(µ-
dppm)](ClO4)2, 13(ClO4)2
[{4,4′-tBu2(C∧N∧N)}3Pt3(µ3-
dpmp)](ClO4)3, 14(ClO4)3
493, 530, 598 (6.5)
501, 538, 607 (7.1)
593 (2.0)
617 (2.1)
in CH3CN:
628 (1.7; 0.08)
in C6H6:
619 (3.9)
621 (2.7; 0.18)
in CH2Cl2:
631 (2.3; 0.15)
in C2H5OH:
649 (0.6; 0.05)
710 (1.0; 0.05)
[{4-(4-MeOC6H4)(C∧N∧N)}3Pt3(µ3-
dpmp)](ClO4)3, 17(ClO4)3
[4-EtO2C(C∧N∧N)PtPPh3](ClO4),
18(ClO4)
697 (3.6)
710 (1.1)
628 (3.5)
662 (1.8)
648 (0.9)
530 (2.7)
677 (1.6)
612 (5.2)
664 (3.9)
645 (3.4)
n.d.
562 (0.1; 0.01)
nonemissive
nonemissive
530 (2.3; 0.09)
527 (20.0), 560
550 (10.7), 662 (6.2)
562, broad
[{4-EtO2C(C∧N∧N)}2Pt2(µ-
dppm)](ClO4)2, 19(ClO4)2
[{4-EtO2C(C∧N∧N)}3Pt3(µ3-
dpmp)](ClO4)3, 20(ClO4)3
[4-(aza-15-crown-
5)(C∧N∧N)PtPPh3](PF6),
21(PF6)
n.d.
[{4-(aza-15-crown-
589 (0.7; 0.07)
475, 587 (4.7)
570 (0.4)
517, 578 (2.2)
5)(C∧N∧N)}2Pt2(µ-dppm)](PF6)2,
22(PF6)2
[(S∧N∧N)PtPPh3]ClO4,
23(ClO4)
603 (4.7; 0.05)
629 (3.1; 0.02)
657 (1.3; 0.01), broad
nonemissive
613 (21.4), 665
710 (e0.1), broad
627 (1.0)
627 (7.7)
642 (1.2)
[(S∧N∧N)2Pt2(µ-dppm)](ClO4)2,
24(ClO4)2
723 (e0.1)
658 (0.4)
685 (0.5)
[(S∧N∧N)3Pt3(µ3-dpmp)](ClO4)3,
25(ClO4)3
649 (1.3)
[(S∧N∧N)PdPPh3]ClO4,
26(ClO4)
547 (471), 592
558 (217), 602
nonemissive
nonemissive
nonemissive
nonemissive
[(S∧N∧N)2Pd2(µ-dppm)](ClO4)2,
27(ClO4)2
nonemissive
a Measured in CH3CN solution unless stated otherwise. b [Ru(bpy)3](ClO4)2 in CH3CN (φ ) 0.062) as reference. c Measured in MeOH/EtOH (1/4, v/v),
complex concentration ∼1 × 10-5 mol dm-3. n.d. ) not determined.
25(ClO4)3 are shown in the Supporting Information. The UV-
vis spectra for the bi- and trinuclear complexes in acetonitrile
at 298 K feature broad absorptions in the 400-550 nm range,
and the lowest-energy absorption peak maximum appears at ca.
529, 543, 437, and 468 nm for 19(ClO4)2, 20(ClO4)3, 24(ClO4)2,
and 25(ClO4)3, respectively. Such broad bands beyond 400 nm
are not observed in the spectra of the mononuclear homologues
18(ClO4) and 23(ClO4).
The UV-vis absorption spectra of 21(PF6) and 22(PF6)2
in acetonitrile are shown in Figure 7. The salient differ-
ence between these two spectra occurs in the low-energy region;
a broad band in the 425-500 nm range for the binuclear
complex 22(PF6)2 is absent for the monomeric species
21(PF6).
Figure 7. UV-vis absorption spectra of 21(PF6) and 22(PF6)2 in acetonitrile
at 298 K. Inset: Plot of ∆ꢀ versus wavelength for 21(PF6) (ꢀ values
multiplied by 2) and 22(PF6)2 in acetonitrile at 298 K.
Steady-State Emission Spectra. The emission data of the
complexes in the present study are summarized in Table 2. All
Pt(II) complexes are emissive in the solid state, and in glassy
and fluid solutions except for 19(ClO4)2 and 20(ClO4)3, which
are virtually nonemissive in CH3CN solution at 298 K. The
observed emission lifetimes (in the microsecond range) and
quantum yields in fluid solutions are comparable or superior to
the widely studied [Ru(bpy)3]2+ complex.
The green-yellow solid-state emission (Figure 8, λmax 544
nm) of the propeller-shaped derivative 7(ClO4)3 is blue-shifted
from the emission of the related monomeric [(C∧N∧N)PtPPh3]-
9
J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7645

A R T I C L E S
Lu et al.
Figure 9. Normalized emission spectra of 12(ClO4) (- ‚ -), 13(ClO4)2
(- - -), and 14(ClO4)3 (s) in the solid state (left) and in acetonitrile solution
(right) at 298 K (λex 350 nm).
Figure 8. Normalized emission spectra of 7(ClO4)3 (- - -) and 10(ClO4)3
(s) in the solid state (left) and in acetonitrile solution (right) at 298 K (λex
350 nm).
in the excitation spectrum of [(C∧N∧N)PtCtNCy](ClO4) (λem
1
710 nm) was recently assigned to the MMLCT transition.24
3
(ClO4) (λmax ≈ 560 nm).11b Because the IL emission of the
Pd(II) analogue [(C∧N∧N)PdPPh3](ClO4) occurs at λmax 480
nm,23 we assign the 544 nm emission of the 7(ClO4)3 solid to
an excited state with mixed 3MLCT and 3IL parentage. The red
emission (λmax 630 nm) from the orange form of 10(BF4)3 is
comparable with the emission energy of the binuclear homo-
logue [(C∧N∧N)2Pt2(µ-dppm)](ClO4)2, which was previously
The emission of 10(ClO4)3 in n-butyronitrile glass at 77 K is
dependent on the excitation wavelength (see Supporting Infor-
mation for spectra). Upon excitation at 360 nm, a structured
emission at λmax 510 nm with vibronic spacing of ∼1290 cm-1
(lifetime ≈ 15 µs), together with a broad structureless emission
at λmax 633 nm (lifetime 6.6 µs), are observed. These two
emission bands are tentatively assigned to the monomeric and
3
3
assigned to a (ππ*) excimeric emission with some MMLCT
character. This assignment coincides with the fact that only one
close Pt‚‚‚Pt contact (less than 3.5 Å) is revealed in the crystal
structure of 10 (orange form) and the packing is dominated by
intra- and intermolecular π-π interactions. The deep red
emissions from the red (λmax 674 nm) and brown red (Figure 8,
3
excimeric (ππ*) excited states, respectively. Upon excitation
at 440 and 510 nm, structureless emission at λmax 630 and 650
nm, respectively, is dominant. The site-selective emission of
10(ClO4)3 implies the existence of multiple emissive species in
the n-butyronitrile glassy medium, presumably due to varying
degrees of aggregation of [(C∧N∧N)Pt]+ lumophores. For
7(ClO4)3 and 17(ClO4)3, intense emission that is independent
of complex concentration and excitation wavelength is observed
at λmax 520 and 710 nm, respectively, in acetonitrile solution at
298 K.
λmax 709 nm) forms of 10(ClO4)3 plus 17(ClO4)3 (λmax 710 nm)
are seldom encountered in cyclometalated oligopyridyl Pt(II)
complexes. Recently, we detected the 3MMLCT emissions from
concentrated solutions of [(C∧N∧N)Pt(isocyanide or carbonyl)]+
salts at λmax ≈ 700 nm.24 The emission from the yellow
crystalline form of 10(ClO4)3 cannot be accurately recorded due
to the facile loss of solvated molecules. The emission energies
of 7(ClO4)3, 10(ClO4)3, and 17(ClO4)3 (Figure 8, λmax 520, 710,
and 710 nm, respectively) in acetonitrile at 298 K are similar
to those in the solid state, thus indicating that comparable
intramolecular configurations are maintained in both media. It
is noteworthy that the emission maxima in acetonitrile at 298
K for the mono- and binuclear homologues of 10(ClO4)3 and
17(ClO4)3 occur at 535 and 652 nm [for 8(ClO4) and 9(ClO4)2,
respectively] and 655 nm [for binuclear 16(ClO4)2],11e and thus
the tendency of the emission energy to red-shift from mono-
nuclear to linearly tethered bi- and trinuclear Pt(II) species is
demonstrated.
The emission energy of 10(ClO4)3 in acetonitrile at 298 K is
concentration-dependent (see Supporting Information for spec-
tra). With the increment of the complex concentration from 8.3
× 10-6 to 8.3 × 10-4 M, the emission maximum red-shifts from
670 to 710 nm. The excitation spectrum monitored at λem 710
nm shows a sharp peak at 560 nm, which is assigned to the
ground-state aggregation of [(C∧N∧N)Pt]+ units. Consistent with
this, the well-defined low-energy absorption at ca. λmax 500 nm
The emission spectra for 12(ClO4), 13(ClO4)2, and 14(ClO4)3
in the solid state and in acetonitrile at 298 K are shown in Figure
9. The solid-state emission maximum is red-shifted from mono-
(532 nm) to bi- (593 nm) and trinuclear (617 nm) complexes.
All three complexes are highly emissive in fluid solution with
emission maximum occurring at 532, 625, and 628 nm,
respectively. Interestingly, the emission energies of the mono-
nuclear species 12(ClO4) are identical in the solid state and
in diluted solution, while a minimal red-shift of emission energy
in CH3CN is detected when the number of tethered
[4,4′-tBu2(C∧N∧N)Pt] fragments is increased from two to three.
The emission spectra for 18(ClO4), 19(ClO4)2, and 20(ClO4)3
in the solid state and in acetonitrile (for 18(ClO4) only) at 298
K are shown in the Supporting Information. It is notable that
the emission maxima for the mono- (628 nm), bi- (662 nm),
and trinuclear (648 nm) derivatives are comparable. Only
18(ClO4) in this series is emissive in fluid solutions with
emission maximum at 562 nm, and this is red-shifted from that
of [(C∧N∧N)PtPPh3](ClO4) (535 nm) and [4,4′-tBu2(C∧N∧N)-
PtPPh3](ClO4) (532 nm). Hence, the electron-withdrawing
ethoxycarbonyl group on the 4-position of (C∧N∧N) may lead
3
3
to dd or MLCT excited states that are lower in energy than
3
the IL state (with a greater energy gap).
(23) Lai, S. W.; Cheung, T. C.; Chan, M. C. W.; Cheung, K. K.; Peng, S. M.;
Che, C. M. Inorg. Chem. 2000, 39, 255-262.
(24) Lai, S. W.; Lam, H. W.; Lu, W.; Cheung, K. K.; Che, C. M. Organome-
tallics 2002, 21, 226-234.
For 21(PF6) and 22(PF6)2, which bear a crown ether moiety
at the 4-position of the (C∧N∧N) ligand, the emission maximum
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7646 J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004

Trinuclear Cyclometalated Pt(II) Luminophores
A R T I C L E S
Table 3. Electrochemical Dataa of Selected Pt(II) Complexes
reduction anodic peak
complex
E
1/2 (V)b pa (V)b
E
[(C∧N∧N)2Pt2(µ-dppm)](ClO4)2, (9)c -1.23, -1.46
-
1.05
-
[(C∧N∧N)3Pt3(µ3-dpmp)]3+, (10)
[4,4′-tBu2(C∧N∧N)PtPPh3]+, (12)
[{4,4′-tBu2(C∧N∧N)}2Pt2(µ-
dppm)]2+, (13)
-1.30, -1.46, -1.59
-1.62
-1.49, -1.78
1.05
[{4,4′-tBu2(C∧N∧N)}3Pt3(µ3-
dpmp)]3+, (14)
-1.50, -1.77, -2.10
1.09
1.00
[{4-(4-MeOC6H4)(C∧N∧N)}3Pt3(µ3- -1.51, -1.64, -1.90
dpmp)]3+, (17)
[4-EtO2C(C∧N∧N)PtPPh3]+, (18)
[{4-EtO2C(C∧N∧N)}2Pt2(µ-
dppm)]2+, (19)
-1.26, -1.81
-1.07, -1.30, -2.04
1.36
1.10
[{4-EtO2C(C∧N∧N)}3Pt3(µ3-
dpmp)]3+, (20)
-1.03, -1.18, -1.42
1.13
[(S∧N∧N)PtPPh3]+, (23)
[(S∧N∧N)2Pt2(µ-dppm)]2+, (24)
[(S∧N∧N)3Pt3(µ3-dpmp)]3+, (25)
[(S∧N∧N)PdPPh3]+, (26)
[(S∧N∧N)2Pd2(µ-dppm)]2+, (27)
-1.47, -2.02
1.17
1.10
1.12
1.19
1.29
Figure 10. Normalized emission spectra of 21(PF6) (- - -) and 22(PF6)2
(s) in the solid state (left) and in acetonitrile solution (right) at 298 K (λex
350 nm).
-1.29, -1.54
-1.21, -1.41, -1.59
-1.60, -1.90
-1.46, -1.64, -1.90
(530 and 589 nm, respectively) recorded in acetonitrile solution
approaches that in the solid state (530 and 570 nm, respectively),
as shown in Figure 10. The emission energy of the monomeric
species 21(PF6) is comparable to that of the [(C∧N∧N)PtPPh3]-
(ClO4) and [4,4′-tBu2(C∧N∧N)PtPPh3](ClO4) (12) congeners.
Significantly, the fluid and solid-state emissions for the binuclear
species 22(PF6)2 are remarkably blue-shifted from those of the
[(C∧N∧N)2Pt2(µ-dppm)](ClO4)2 (∼650 nm) and [{4,4′-
tBu2(C∧N∧N)}2Pt2(µ-dppm)](ClO4)2 (13) (∼625 nm) analogues,
consistent with weaker Pt‚‚‚Pt/π-π interactions.
a Determined in CH3CN at 298 K with 0.1 M Bu4NPF6 as supporting
n
electrolyte; scanning rate: 50 mV s-1 b Values versus E1/2 (Cp2Fe+/0
. )
[0.04-0.05 V vs Ag/AgNO3 (0.1 M in CH3CN) reference electrode].
c Values taken from ref 11e.
The emission spectra for crystalline forms of 23(ClO4),
24(ClO4)2, and 25(ClO4)3 at 298 K are shown in the Supporting
Information. Clearly, the emission maximum red-shifts from
mono- (613 nm) to tri- (627 nm) and binuclear (710 nm)
derivatives. All three complexes are emissive in fluid solutions
with λmax occurring at 603, 629, and 657 nm for the mono-,
bi-, and trinuclear species, respectively. The emission maximum
of the mononuclear complex 23(ClO4) in this series is red-shifted
from that for [(C∧N∧N)PtPPh3](ClO4) (535 nm), but λmax for
the trinuclear species is blue-shifted from that of [(C∧N∧N)3Pt3-
(µ3-dpmp)](ClO4)3 (710 nm).
Figure 11. Cyclic voltammograms of 12(ClO4), 13(ClO4)2, and 14(ClO4)3
in acetonitrile at 298 K (with 0.1 M nBu4NPF6 as supporting electrolyte;
scanning rate: 50 mV s-1).
Spectroscopy of Palladium(II) Relatives. To gain further
insight into the thienyl-substituted series, the spectroscopic
properties of the Pd(II) congeners were investigated. Notably,
the UV-vis spectra of the mononuclear 26(ClO4) and binuclear
27(ClO4)2 in acetonitrile at 298 K feature comparable broad
absorptions at λmax 421 and 425 (sh) nm, respectively, and these
are distinctly blue-shifted from the lowest-energy absorptions
observed for the Pt2 and Pt3 analogues. The Pd(II) derivatives
are emissive in alcoholic glassy solutions at 77 K (see
Supporting Information for spectra) but not in the solid state or
fluid solution. Complexes 26(ClO4) and 27(ClO4)2 exhibit
vibronically structured emission with λmax (also λ0-0) at 547
and 558 nm, respectively, and the former is blue-shifted by ca.
1900 cm-1 from the value (613 nm) recorded for the Pt analogue
23(ClO4) under the same conditions. The dpmp-supported Pd2
derivative 11(ClO4)2 is nonemissive in the solid state or in fluid
solution.
18(ClO4), and 23(ClO4) in CH3CN at 298 K exhibit a quasi-
reversible reduction couple E1/2 at -1.60, -1.26, and -1.47 V
versus Cp2Fe+/0 respectively. Substitution at the 4-position of
the (C∧N∧N) ligand has an effect upon these E1/2 values; an
electron-withdrawing ethoxycarbonyl group in 18(ClO4) shifts
the first reduction wave to -1.26 V, while this reduction appears
at -1.60 V for 12(ClO4) which bears two tert-butyl groups on
the (C∧N∧N) ligand. The first reduction wave at -1.60 V for
the Pd(II) derivative 26(ClO4) occurs at a more cathodic
potential than that for the Pt(II) analogue 23(ClO4) (-1.47 V),
indicating the extent of orbital mixing between the metal and
cyclometalating ligand.
Upon going from mono- to bi- and trinuclear complexes, the
first reduction wave(s) based at the cyclometalating ligand(s)
is found to split into two and three couples, respectively, as
shown in Figure 11 for the [4,4′-tBu2(C∧N∧N)Pt] series (see
Supporting Information for other analogues). For example, the
reduction waves are observed at -1.62, -1.49/-1.78, and
-1.50/-1.77/-2.10 V for the mono-, bi-, and trinuclear species,
respectively, in the [4,4′-tBu2(C∧N∧N)Pt] series; -1.26, -1.07/-
1.30, and -1.03/-1.18/-1.42 V in the [4-EtO2C(C∧N∧N)Pt]
Cyclic Voltammetry. The electrochemical data of selected
Pt(II) complexes recorded in CH3CN at 298 K are listed in Table
3. There is an irreversible oxidation wave at 1.00-1.36 V versus
Cp2Fe+/0 for the Pt(II) complexes in this study. The cyclic
voltammograms of the mononuclear Pt(II) complexes 12(ClO4),
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J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7647

A R T I C L E S
Lu et al.
emission under a forward bias, with maximum luminance levels
of 10 cd m-2 at a current density of ca. 2.2 mA cm-2. No
emission was observed in the reverse bias, although the negative
current was recorded. The electroluminescence maximum of ca.
634 nm is close in energy to the fluid and solid-state photolu-
minescence of 14(ClO4)3, indicating the same origin for the
electro- and photoluminescence. The low turn-on voltage (<
3.0 V), relatively long response time (ca. 10 s at 6 V), and
measurable reverse current suggest an electrochemical mech-
anism instead of diode-like for this light-emitting device.25 The
poor performance is presumably due to an irreversible one-
electron oxidation process.
Discussion
Figure 12. Time-resolved difference absorption spectra of 14(ClO4)3 in
CH3CN at 298 K.
Weak Intra- and Intermolecular Interactions. The salient
features in the crystal structures of the tethered oligopyridine
cyclometalated Pt(II) complexes are the intramolecular Pt‚‚‚Pt
contacts and intra- and intermolecular π-π interactions. Table
4 has been compiled to facilitate comparison between the
literature-reported data and the present work. Three parameters,
Pt‚‚‚Pt distance, torsion angle between the Pt(II) coordination
planes, and Pt‚‚‚Pt‚‚‚Pt angle (for trinuclear species), have been
selected to characterize and illustrate these interactions. Gener-
ally, shorter Pt‚‚‚Pt distances are accompanied by a larger torsion
angle between the ligated Pt(II) planes. The optimized config-
uration for face-to-face π-stacking interactions between two
aromatic planes is staggered (i.e., torsion angle significantly
larger than 0°),26 whereas face-to-face π overlap in an eclipsed
fashion (i.e., torsion angle ca. 0°) introduces repulsive forces.
However, the stabilization effect of Pt‚‚‚Pt contacts in these
tethered complexes apparently overcomes this repulsion between
the ligated Pt(II) moieties, and all Pt3 complexes adopt a syn,syn-
configuration. The partially staggered geometry observed is in
essence the compromise between Pt‚‚‚Pt attraction(s) and
repulsion induced by unfavorable π-stacking interactions. A
consequence of this compromise is that the Pt‚‚‚Pt‚‚‚Pt chains
in the tethered Pt(II) complexes in this study deviate from
linearity, as shown by the intramolecular Pt‚‚‚Pt‚‚‚Pt angles of
162 ( 7°. The “ideal” combination of Pt‚‚‚Pt and π-π
interactions can apparently be achieved in oligomers of unteth-
ered Pt(II) complexes through self-assembly, as is reported for
the green form of [(tpy)Pt(CtC-CtCH)](CF3SO3),9f where
the Pt‚‚‚Pt distance is 3.388 Å, the torsion angle is 134.4°, and
the Pt‚‚‚Pt‚‚‚Pt angle is 179°. The intriguing coiled chains
observed in the crystal packing of the trinuclear complex
20(ClO4)3‚2CH3CN can be rationalized by a combination of
intramolecular Pt‚‚‚Pt and intermolecular π-π interactions. We
note that trinuclear Pd(II) counterparts have not been prepared
or isolated; Pd‚‚‚Pd interactions are considerably weaker and
presumably cannot compete with π-stacking interactions.
series; and -1.47, -1.29/-1.54, and -1.21/-1.41/-1.59 V in
the [(S∧N∧N)Pt] series. This indicates that the cyclometalating
ligands in the bi- and trinuclear complexes are inequivalent upon
sequential reduction.
Photophysical and Electrochemical Studies on [{4,4′-tBu2-
(C∧N∧N)}3Pt3(µ3-dpmp)](ClO4)3,14(ClO4)3.Complex14(ClO4)3,
which bears two tert-butyl substituents on each (C∧N∧N) ligand,
displays superior solubility in a variety of organic solvents and
was chosen as an illustrative example to test the effects of
solvent on the absorption and emission properties (see Sup-
porting Information for spectra). The effect of solvent polarity
on the absorption is minimal. The emission maximum (lifetime,
quantum yield) changes from 621 (2.7 µs, 0.18) to 628 (1.7 µs,
0.08), 631 (2.3 µs, 0.15), and 649 (0.6 µs, 0.05) nm when the
solvent is changed from benzene to CH3CN, CH2Cl2, and EtOH.
To characterize the photophysical nature of these Pt3 lumo-
phores in detail, the time-resolved difference absorption (TA)
spectra of 14(ClO4)3 in CH3CN were recorded (Figure 12). The
decay of the absorption signals (1.4-1.8 µs) matched the
emission lifetime (1.7 µs), which is in accordance with the
absorption originating from the emissive excited state. The
salient features in the TA spectra are intense luminescence
bleaching in the 500-775 nm range and strong absorption in
the 380-500 nm range (with λmax ca. 400 and 470 nm).
By employing the electrochemical data plus the spectroscopic
measurements, it is possible to estimate the excited-state
potential of 14(ClO4)3 by using the following equation: E(*143+
/
142+) ) E(143+/142+) + E0-0(143+). The cyclic voltammogram
of 14(ClO4)3 reveals the first quasi-reversible one-electron
reduction wave [E(143+/142+), (ipc/ipa) ≈ 1] at -1.50 V versus
Cp2Fe+/0 (-1.10 V vs NHE) in acetonitrile. The 0-0 transition
energy of 143+, E0-0(143+), can be estimated from the overlap
of the emission and excitation spectra, which occurs at 2.4 eV.
Thus, E(*143+/142+) is estimated to be ca. 0.9 V versus Cp2Fe+/0
(1.3 V vs NHE), which signifies that *143+ is a strong
photooxidant. This value is slightly smaller than that previously
calculated for E(*92+/9+) [1.61 V vs NHE].11e
When crown ether units are introduced into this system, the
observed trend for Pt‚‚‚Pt distances and torsion angles can be
broken. As listed in Table 4, the Pt‚‚‚Pt distance in 22 is 3.827
Å, which is considerably larger than that in [(C∧N∧N)2Pt2(µ-
dppm)](ClO4)2,11e even though the torsion angle is comparable
Complex 14(ClO4)3 was chosen to investigate the electrolu-
minescent properties of positively charged Pt(II)-based triplet
lumophores in light-emitting devices. Thus, 14(ClO4)3 was spin-
coated onto ITO glass to give a film with a thickness of ca. 0.1
µm, and an aluminum electrode was vacuum-deposited onto
this cast film. Preliminary results (see Supporting Information
for plots) show that the single-layer device displays orange-red
(25) (a) Lee, J. K.; Yoo, D. S.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett.
1996, 69, 1686-1688. (b) Buda, M.; Kalyuzhny, G.; Bard, A. J. J. Am.
Chem. Soc. 2002, 124, 6090-6098. (c) Welter, S.; Brunner, K.; Hofstraat,
J. W.; De Cola, L. Nature 2003, 421, 54-57.
(26) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
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Trinuclear Cyclometalated Pt(II) Luminophores
A R T I C L E S
Table 4. Selected Structural Parameters Concerning Pt‚‚‚Pt Interactions in Tethered Oligopyridine Cyclometalated Pt(II) Complexesa
Pt‚‚‚Pt
torsion
Pt‚‚‚Pt‚‚‚Pt
complex
distance/Å
angle/deg
angle/deg
ref
[(tpy)2Pt2(µ-can)]3+
[(tpy)2Pt2(µ-gua)]3+
[(tpy)2Pt2(µ-pz)]3+
[(tpy)2Pt2(µ-az)]3+
[(tpy)2Pt2(µ-dpf)]3+
[(tpy)2Pt2(µ-SNS)]2+
[(C∧N∧N)2Pt2(µ-pz)]+
2.998
3.090; 3.071
3.432
3.13
3.049
3.908
3.612
3.270
3.150
3.245
3.827
3.321
3.194; 3.399
3.364; 3.617
3.217; 3.601
3.466; 3.647
3.254; 3.389
3.262; 3.303
3.388
21
31
32
33
34
34
35
11e
11b
36
28.6; 22.1
14
[(C∧N∧N)2Pt2(µ-dppm)]2+ (9)
44.6
27.2
20.7
43.2
[{4-(4-ClC6H4)-(C∧N∧N)}2Pt2(µ-dppm)]2+
[{4-(4-MeC6H4)-(C∧N∧N)}2Pt2(µ-dppm)]2+
[{4-(aza-15-crown-5)-(C∧N∧N)}2Pt2(µ-dppm)]2+ (22)
[(S∧N∧N)2Pt2(µ-dppm)]2+ (24)
11e
this work
this work
this work
this work
this work
this work
this work
15a
29.0
[(C∧N∧N)3Pt3(µ3-dpmp)]3+ (10, red form)
[(C∧N∧N)3Pt3(µ3-dpmp)]3+ (10, orange form)
[{4,4′-tBu2(C∧N∧N)}3Pt3(µ3-dpmp)]3+ (14)
[{4-EtO2C(C∧N∧N)}3Pt3(µ3-dpmp)]3+ (20)
[(S∧N∧N)3Pt3(µ3-dpmp)]3+ (25)
26.3; 15.5
34.6; 4.7
29.6; 9.3
33.7; 8.9
33.0; 18.7
169
162
157
162
163
173
179
154
[Bosnich’s Pt2 receptor: Pt(salap)NH3]2+
[(tpy)Pt(CtC-CtCH)]+ (green form)
[(tpy)Pt(CtC-CtCH)]+ (red form)
134.4
49.7; 57.1
9f
9f
3.394; 3.648
a Hgua ) guanidine; Hpz ) pyrazole; Haz ) azaindole; Hdpf ) diphenylformamidine; Hcan ) arginine; H2SNS ) dithiouracil; H2salap )
2-salicylideneaminophenol.
(43.2° versus 44.6°). It is apparent that the C-H‚‚‚O interactions
between H3 on the 4′-position of one diimine ligand and the
oxygen atoms (O2-O4) on the adjacent [4-(aza-15-crown-5)-
(C∧N∧N)Pt]+ fragment are more important than the intramo-
lecular Pt‚‚‚Pt interaction in terms of their impact upon the
crystal packing. No Pt‚‚‚Pt interaction is expected in fluid
solutions, because the emission maximum (589 nm) recorded
in acetonitrile solution is comparable to that in the solid state
(570 nm). It is noteworthy that interactions of crown ethers with
alkaline and alkaline earth metal and ammonium ions are
ubiquitous but the binding of a C-H group to a crown ether
moiety is comparatively rare.
MMLCT Transition in Electronic Spectroscopy. The
MMLCT excited states in Pt(II) complexes have attracted
Figure 13. Plot of ∆ꢀ versus wavelength (s) for 10(ClO4)3 and 14(ClO4)3
in acetonitrile at 298 K, and excitation spectrum (- - -) of 10(ClO4)3 in
MeOH/EtOH (1/4, v/v) at 77 K (monitored at λem 677 nm, complex
concentration ∼1 × 10-5 mol dm-3).
considerable attention since the early works by the Yersin and
Miskowski groups.6 Previous studies by Che and co-workers
on binuclear derivatives bearing cyclometalated 6-phenyl-2,2′-
bipyridine, [(C∧N∧N)2Pt2(µ-L)]n+ (L ) pyrazole, dppm), re-
vealed interesting photophysical properties, including distinctive
low-energy emissions in fluid solutions.11e However, assignment
nm (ꢀ ≈ 4000 dm3 mol-1 cm-1); this is apparently derived from
ground-state Pt‚‚‚Pt interaction, and the inhomogeneous broad-
ening presumably arises from the changeability of the Pt‚‚‚Pt
separation in solution. The corrected absorption difference
spectrum (Figure 7, inset) between 21(PF6) and 22(PF6)2 in
acetonitrile at 298 K features a broad low-energy band with a
maximum at 430 nm (ꢀ ≈ 3000 dm3 mol-1 cm-1). As noted in
the crystal structures, there is no Pt‚‚‚Pt interaction in 22(PF6)2
due partly to the influence of the crown ether moieties, but
14(ClO4)3 may contain short Pt‚‚‚Pt contacts. Therefore, the low-
energy band at λmax 430 nm is tentatively assigned to the ground-
state aggregation of the lumophores in 22 through intramolecular
π-π interaction.
A stark observation is apparent when the absorption difference
spectrum between 10(ClO4)3 and 14(ClO4)3 in acetonitrile at
298 K (Figure 13) is scrutinized. This spectrum features a well-
structured low-energy band at 523 nm (peak maximum) with ꢀ
and vibronic progression of ∼3000 dm3 mol-1 cm-1 and 1080
cm-1, respectively (the vibronic progression may correspond
to the excited-state aromatic ring deformation). The energy and
vibronic progression coincide with the corresponding values in
3
of these low-energy emissions to MMLCT excited states is
complicated by the fact that the corresponding excimeric ππ*
emissions occur in a similar spectral region. We envisaged that
the tethered trinuclear Pt(II) complexes in the present study
would enable us to elucidate the impact of metal‚‚‚metal and
π-π interactions upon MMLCT excited states.
The emission maxima of the [4,4′-tBu2(C∧N∧N)Pt] and
[4-(aza-15-crown-5)(C∧N∧N)Pt] series of complexes in aceto-
nitrile solution at 298 K are virtually identical to their respective
values in the solid state; therefore, the extent of intramolecular
Pt‚‚‚Pt and π-π interactions revealed in their crystal structures
is likely to be maintained in fluid solutions, assuming that the
emissions are directly related to these weak noncovalent
interactions. The difference between the absorption spectra of
these complexes can shed light on the net effect of extending
from mononuclear to tethered bi- and trinuclear Pt(II) homo-
logues. The corrected absorption difference spectrum (Figure
6, inset) between 12(ClO4) and 14(ClO4)3 in acetonitrile at 298
K features a broad low-energy band with a maximum at 422
9
J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7649

A R T I C L E S
Lu et al.
the excitation spectrum of 10(ClO4)3 in MeOH/EtOH (1/4, v/v)
at 77 K (monitored at λem 677 nm), as shown in Figure 13.
Furthermore, the energy of this band is comparable to that of
the 500 nm band in the excitation spectrum of [(C∧N∧N)PtCt
NCy](ClO4) (concentration ∼7 × 10-3 mol dm-3) in acetonitrile
at 298 K when monitored at 710 nm, and the latter was ascribed
to a MMLCT transition.24 Hence, we propose that the absorption
band at λmax 523 nm, derived from the absorption difference
spectra, can be considered as a distinct singlet MMLCT
transition and corresponds to the low-energy 3MMLCT emission
at λmax ≈ 700 nm for 10(ClO4)3 in fluid solutions and the solid
state. In view of the fact that there are two Pt‚‚‚Pt contacts in
the crystal structure of 10(ClO4)3, but only one in 14(ClO4)3,
the 523 nm band in the absorption difference spectrum can be
tentatively attributed to the net effect of one Pt‚‚‚Pt interaction
(note that in this case the interligand effect has been off-set).
Significantly, Bosnich and co-workers recently proposed that
the presence of an absorption band beyond 500 nm (ꢀ ≈ 1500
dm3 mol-1 cm-1) can be similarly attributed to the association
of a Pt2 terpyridine molecular receptor with neutral and anionic
Pt-based guests through Pt‚‚‚Pt interactions.15a
Tuning of the Emission Energy. To obtain cyclometalated
Pt(II) complexes that display lower emission energies in fluid
solution, two strategies could be employed. The first is the
introduction of electron-withdrawing groups on the cyclometa-
lating moiety (or electron-donating substituents on the ancillary
ligands), assuming that the emissions are MLCT in nature and
the cyclometalated ligand is the electron acceptor upon photo-
excitation. However, this approach is limited by the energy gap
law, which states that low emission energy for a charge-transfer
excited state correlates to low quantum yield and short lifetime.
The second method, which can lead to high-efficiency low-
energy Pt(II) emitters, is to connect discrete lumophore units
into multinuclear species through Pt‚‚‚Pt and π-π interactions.
In solution, the Pt‚‚‚Pt/π-π contacts would be locked into place
by judiciously chosen oligophosphine ligands, which tether the
lumophore units in a face-to-face linear conformation to afford
highly emissive MMLCT excited states.
charge-transfer bands cover the visible spectrum. Hence, these
molecular materials may find applications in dye-sensitized
solar-energy cells and light-emitting electrochemical cells
(LECs), like the red LEC demonstrated in this work, although
the efficiency of the device clearly requires considerable
improvement. The NIR emissions from these lumophores are
ideal signal outputs for biomolecular probes and chemosensors.
The nature of the excited states, which are extremely sensitive
to the microenvironment, have been probed and subsequently
assigned. Evidence from the present study indicates that both
the ground and excited states of the higher homologues of this
class of luminescent Pt(II) complexes can readily engaged in
weak noncovalent interactions such as d8-d8 contacts, π-π
stacking, and C-H‚‚‚O hydrogen bonding.
Experimental Section
Materials and General Procedures. All starting materials were
purchased from commercial sources and used as received unless stated
otherwise. The solvents used for synthesis were of analytical grade.
The compounds 3-(2-pyridyl)-5-phenyl-1,2,4-triazine,27 4-hydroxycar-
bonyl-6-phenyl-2,2′-bipyridine,28 bis(diphenylphosphinomethyl)phenyl-
phosphine (dpmp),22 [(C∧N∧N)PtCl],29 [(C∧N∧N)PdCl],29 [4-(4-
MeOC6H4)(C∧N∧N)PtCl],24 [(S∧N∧N)PtCl],30 and [(S∧N∧N)PdCl]30
1
were prepared according to literature methods. H, 13C, 31P, and 195Pt
NMR spectra were recorded on a Bruker DRX 500, Avance 400, or
DRX 300 FT-NMR spectrometer with TMS (1H and 13C), H3PO4 (31P),
and H2PtCl6 (195Pt) as references. Mass spectra (FAB) were obtained
on a Finnigan MAT 95 mass spectrometer. Elemental analyses were
performed by Beijing Institute of Chemistry, Chinese Academy of
Sciences. Details of solvent treatment for photophysical studies and
cyclic voltammetric studies have been described earlier.11e UV-vis
spectra were recorded on a Perkin-Elmer Lambda 19 UV/vis spectro-
photometer. Solution samples for emission and time-resolved difference
absorption measurements were degassed by at least four freeze-pump-
thaw cycles. Emission spectra were obtained on a SPEX Fluorolog-2
model F111 fluorescence spectrophotometer. Emission lifetime mea-
surements were performed with a Quanta Ray DCR-3 pulsed Nd:YAG
laser system (pulse output 355 nm, 8 ns). Luminescent quantum yields
were referenced to [Ru(bpy)3](ClO4)2 in acetonitrile (φ ) 0.062)
(estimated error (15%). Time-resolved difference absorption experi-
ments were performed with the above-mentioned Nd:YAG laser system.
The monitoring light was from a 300 W continuous wave tungsten-
halogen or xenon lamp oriented orthogonally to the direction of the
laser pulse. Transient species were monitored by monochromatic light
from a tungsten or xenon lamp. The difference absorption signals at
individual wavelengths were fed to the Tektronix 2430 or TDS 350
oscilloscope. The entire optical difference spectrum was generated point
to point by monitoring at each wavelength. Details of cyclic voltam-
metric studies have been described earlier.12a
The tuning of the emission energies from mono- to bi- and
trinuclear cyclometalated complexes has been demonstrated; for
example, the emission maxima for the [(C∧N∧N)Pt] series in
CH3CN solution are 560, 650, and 710 nm, respectively.
However, for all of the linearly tethered trinuclear complexes,
the emission energies appear in the λmax 657-710 nm range
(in acetonitrile solutions at 298 K), and further red-shifted
emissions from these MMLCT excited states have not thus far
been achieved through ligand modification. There may be a
fundamental limitation for the tethering approach; that is, the
conflict between close intramolecular Pt‚‚‚Pt attraction and π-π
repulsion of adjacent [(C∧N∧N)Pt] moieties in an eclipsed or
staggered manner can prevent a more intimate approach between
Pt(II) centers. Nevertheless, further extension of the tethered
series of cyclometalated Pt(II) complexes to tetranuclear and
higher homologues can be envisaged.
General Procedure for Syntheses of Phosphine-Ligated Com-
plexes. (Caution! Perchlorate salts are potentially explosiVe and should
(27) Culbertson, B. M.; Parr, G. R. J. Heterocycl. Chem. 1967, 4, 422-424.
(28) Neve, F.; Crispini, A.; Di Pietro, C.; Campagna, S. Organometallics 2002,
21, 3511-3518.
(29) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J. Chem.
Soc., Chem. Commun. 1990, 513-515.
(30) Constable, E. C.; Henney, R. P. G.; Raithby, P. R.; Sousa, L. R. J. Chem.
Soc., Dalton Trans. 1992, 2251-2258.
(31) Ratilla, E. M. A.; Scott, B. K.; Moxness, M. S.; Kostic´, N. M. Inorg. Chem.
1990, 29, 918-926.
(32) Yip, H. K.; Che, C. M.; Zhou, Z. Y.; Mak, T. C. W. J. Chem. Soc., Chem.
Commun. 1992, 1369-1371.
Summary
(33) Bailey, J. A.; Gray, H. B. Acta Crystallogr. 1992, C48, 1420-1422.
(34) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Acta Crystallogr. 1993, C49,
793-796.
Control over the absorption and emission energies of multi-
nuclear cyclometalated Pt(II) complexes has been achieved by
modifying the shape and chelating directionality of the tethering
oligophosphine ligand. For “linear” Pt3 species, the allowed
(35) Tzeng, B. C.; Fu, W. F.; Che, C. M.; Chao, H. Y.; Cheung, K. K.; Peng,
S. M. J. Chem. Soc., Dalton Trans. 1999, 1017-1023.
(36) Wu, L. Z.; Cheung, T. C.; Che, C. M.; Cheung, K. K.; Lam, M. H. W.
Chem. Commun. 1998, 1127-1128.
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Trinuclear Cyclometalated Pt(II) Luminophores
A R T I C L E S
be handled with care and in small amounts.) A mixture of [Rx(C∧N∧N)-
MCl] (M ) Pt or Pd) and the phosphine ligand in CH2Cl2/CH3OH
(1/5, v/v) was stirred for 12 h at room temperature. The resulted clear
solution was filtered into a saturated CH3OH solution of LiClO4 (or
Et4NBF4, NH4PF6). After the volume of the mixture was reduced to
about 5 mL, Et2O (100 mL) was added to precipitate the product, which
was collected on sinter glass and washed with H2O/CH3OH and Et2O,
then recrystallized from CH3CN/Et2O unless otherwise stated.
formula unit, including one anion located in two sets of positions (each
with half occupancy). One of the anions was disordered in the mode
of sharing three F atoms for two parts, due to its closeness to the special
position; restraints were applied to the disordered anion assuming similar
P-F bond lengths. The end-side of the crown ether fragments showed
high displacement parameters, so restraints were also applied assuming
similar O-C, C-C, and 1,3-O‚‚‚C bond lengths or distances, respec-
tively. In the final stage of least-squares refinement, the F atoms of
the disordered anion and 8 non-H atoms of the crown ether end-side
were refined isotropically, and the remaining non-hydrogen atoms were
refined anisotropically. For 24(ClO4)2‚2CH3CN, one crystallographic
asymmetric unit consists of one and a half formula unit, including one
anion (in two positions) and one CH3CN solvent molecule. For
25(ClO4)3‚1.5Et2O‚CH3CN, one crystallographic asymmetric unit con-
sists of one formula unit, including three anion, one CH3CN, and one
and a half Et2O solvent molecules. In the final stage of least-squares
refinement, the O atoms of ClO4- and atoms of solvate molecules were
refined isotropically, and the other non-hydrogen atoms were refined
anisotropically.
The syntheses and characterization data of all new ligands and
complexes are provided in the Supporting Information.
X-ray Crystallography. Single crystals of 10(ClO4)3‚H2O (red
form: slow diffusion of benzene vapor into a CH3CN solution), 6,
9(ClO4)3, 10(ClO4)3‚2Et2O‚CH3CN (orange form), 14(ClO4)3‚3CH3CN,
20(ClO4)3‚2CH3CN, 22(PF6)2, 24(ClO4)2‚2CH3CN, and 25(ClO4)3‚
1.5Et2O‚CH3CN (slow diffusion of Et2O into CH3CN solutions) were
obtained. For 14(ClO4)3‚3CH3CN, one crystallographic asymmetric unit
consists of one formula unit, including three anions and three acetonitrile
solvent molecules. Three ClO4- anions were located at four positions.
The anion containing Cl3 was located at the special position, while
the anion containing Cl4 was considered to have half occupancy. One
of the tert-butyl group was disordered into two sets of positions, in the
mode of rotation along C-C bond. For convergence of least-squares
refinement, restraints were applied assuming similar C-C and O‚‚‚C
bond lengths, respectively. For 20(ClO4)3‚2CH3CN, one crystallographic
asymmetric unit consists of one formula unit, including three anions
and two acetonitrile solvent molecules. The CH3 group of the central
ethoxyl group is disordered into two positions, and restraints were
applied assuming similar C-C and O‚‚‚C bond lengths or distances
for the disordered C atoms, respectively. For 6, one crystallographic
asymmetric unit consists of one formula unit. In the final stage of least-
squares refinement, only Pt and Cl atoms were refined anisotropically,
and the other non-hydrogen atoms were refined isotropically. For
22(PF6)2, one crystallographic asymmetric unit consists of half of one
Acknowledgment. We are grateful for financial support from
The University of Hong Kong, the Research Grants Council of
Hong Kong SAR, China [HKU 7039/03P], and the Croucher
Foundation of Hong Kong. We thank the reviewers for helpful
comments.
Supporting Information Available: Crystal data (CIF);
synthetic and characterization data; supplementary UV-vis
absorption, emission spectra, and cyclic voltammograms; scheme
of ligand synthesis. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA039727O
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