Synthesis and electronic spectroscopy of luminescent cyclometalated platinum-anthracenyl complexes

Article abstract of DOI:10.1021/om700706r

Dicyclometalated complexes syn- and anti-[Pt₂(L) ₂(PAnP-H₂)](OTf)₂ (Pt₂) are synthesized from reactions of 9,10-bis(diphenylphosphino)anthracene (PAnP) and Pt(L)(OTf)₂ (L = diphosphines, OTf = CF₃SO₃). Because of metal perturbation on the electronic structure of the anthracenyl ring, the ¹B_2u^- ← ¹Ag transition, which is forbidden in anthracene, is observed in the electronic spectra of Pt₂. The complexes display fluorescence arising from the ligand-centered Si excited state. No significant heavy atom effect is observed in the Pt₂ complexes. To understand the effect of the number of Pt ions on the extent of perturbation, a mononuclear analogue, [Pt(dppe)(PAn-H)] ClO₄ (PAn = 9-diphenylphosphinoanthracene), is prepared and its spectroscopy studied.

Full text of DOI:10.1021/om700706r

Organometallics 2007, 26, 6533–6543
6533
Articles
Synthesis and Electronic Spectroscopy of Luminescent Cyclometalated
Platinum-Anthracenyl Complexes
Jian Hu,‡ Ronger Lin,‡ John H. K. Yip,*,‡ Kwok-Yin Wong,† Dik-Lung Ma,† and
Jagadese J. Vittal‡
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore, 117543, and
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic UniVersity, Hung
Hom, Kowloon, Hong Kong SAR, People’s Republic of China
ReceiVed July 16, 2007
Dicyclometalated complexes syn- and anti-[Pt2(L)2(PAnP-H2)](OTf)2 (Pt2) are synthesized from reactions
of 9,10-bis(diphenylphosphino)anthracene (PAnP) and Pt(L)(OTf)2 (L ) diphosphines, OTf ) CF3SO3).
Because of metal perturbation on the electronic structure of the anthracenyl ring, the 1B2u- r 1Ag transition,
which is forbidden in anthracene, is observed in the electronic spectra of Pt2. The complexes display
fluorescence arising from the ligand-centered S1 excited state. No significant heavy atom effect is observed
in the Pt2 complexes. To understand the effect of the number of Pt ions on the extent of perturbation, a
mononuclear analogue, [Pt(dppe)(PAn-H)]ClO4 (PAn ) 9-diphenylphosphinoanthracene), is prepared
and its spectroscopy studied.
OLED.9–12,17d,20a The most extensively studied systems are the
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Introduction
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‡ National University of Singapore.
† The Hong Kong Polytechnic University.
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10.1021/om700706r CCC: $37.00
2007 American Chemical Society
Publication on Web 11/27/2007

6534 Organometallics, Vol. 26, No. 26, 2007
Hu et al.
derivatives.5-10,17d,18c,19c Among them, cyclometalated Pt com-
plexes have been commanding a lot of attention mainly because
of their relatively long-lived emissive excited states and tunable
electronic and molecular properties.5,7,9,12,17 Because of their
close intraligand π-π* and metal-to-ligand charge-transfer
(MLCT) excited states, the excited-state properties of the
complexes are rather sensitive to intermolecular metal-metal,
π-π interactions and medium effects.
While there are a lot of studies on Pt complexes of
polypyridine-type ligands, complexes of alternant aromatic
hydrocarbons (AAH) such as anthracene and pyrene have not
received much attention.12b,17b,21 Given the fact that many AAH
are well-known organic chromophores, it is envisioned that
metalated AAH could be developed into a class of new
luminescent organometallic materials with different photophysi-
cal properties and chemical reactivity. It is reasonable to expect
the photophysics of metalated AAH will be very different from
other cyclometalates. For instance, a salient feature of AAH is
that due to the large exchange interaction K, the energy
separation between the singlet and triplet excited states (∼2K)
is large. Accordingly, the intersystem crossing in metalated AAH
is dependent on the nature of the metal attached to the rings
and its perturbation on the electronic structures of the AAH.
Recently, applications of metal-π-conjugated systems such as
metal-acetylides as optoelectronic materials have been actively
explored. Metal-AAH complexes could be another direction
in the endeavor. The design of metal-AAH-based materials
requires an understanding of how metals interact with the
extended π-conjugated systems of the organic molecules, which
can be garnered by studying the electronic spectroscopy of the
complexes.
Our laboratory has recently reported the syntheses of a
diphosphine 9,10-bis(diphenylphosphino)anthracene (PAnP)
(Scheme 1).22a A special feature of the ligand is its chro-
mophoric anthracenyl backbone, which can impart luminescence
to its complexes.22 To further explore the coordination chemistry
of PAnP, we studied its reactions with [PtII(L)(OTf)2] (L )
bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphos-
phino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp))
(Scheme 1). Our results show that reactions produce dicyclo-
metalated platinum complexes syn- and anti-[Pt2(L)2(PAnP-
H2)](OTf)2 (Pt2). The spectroscopy and photophysics of Pt2
provide insights into the interactions between metal and alternant
aromatic hydrocarbons. To understand the effect of the number
of Pt ions on the extent of perturbation, a mononuclear analogue,
[Pt(dppe)(PAn-H)]ClO4 (4) (PAn ) 9-diphenylphosphinoan-
thracene), has been prepared and its spectroscopy studied.
Reported in this paper is our effort to understand the electronic
structures of the cyclometalated platinum-anthracenyl complexes.
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Experimental Section
General Methods. All syntheses were carried out in a N2
atmosphere. All the solvents used for synthesis and spectroscopic
measurements were purified according to the literature procedures.
PtCl2 was obtained from Oxkem; bis(diphenylphosphino)methane
(dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphe-
nylphosphino)propane (dppp), n-butyllithium, 9-bromoanthracene,
and 9,10-dibromoanthracene were purchased from Aldrich and used
without further purification. 9,10-Bis(diphenylphosphino)anthracene
(PAnP)22a and 9-(diphenylphosphino)anthracene (PAn)23 were
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24
prepared according to the reported methods. Pt(L)(OTf)2 were
prepared in situ by reacting Pt(L)Cl2 with 2 molar equiv of AgOTF
in CH3CN/CH2Cl2 based on modified literature methods.
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temperature emission spectra of the complexes were recorded on a
Shimadzu UV-1601 UV–visible spectrophotometer and a Perkin-
Elmer LS-50B luminescence spectrometer, respectively. The emis-
sion quantum yields were measured with anthracene as standard.
Solutions used for emission spectra and lifetime measurements were
degassed by four freeze–pump–thaw cycles. NMR experiments were
performed on a Bruker ACF 300, AMX500, or DRX500 spec-
trometer. All chemical shifts (δ) are reported in ppm and coupling
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Cyclometalated Platinum-Anthracenyl Complexes
Organometallics, Vol. 26, No. 26, 2007 6535
Scheme 1
constants (J) in Hz. 1H NMR chemical shifts are relative to
tetramethylsilane; the resonance of residual protons of solvents was
used as an internal standard. 31P{1H} NMR chemical shifts were
relative to 85% H3PO4 in D2O. Electrospray ionization mass spectra
(ESI-MS) were measured on a Finnigan MAT 731 LCQ spectrom-
eter. Elemental analyses of the complexes were carried out at the
Elemental Analysis Laboratory in the National University of
Singapore.
Emission Lifetime Measurements. The 77 K frozen glass
(MeOH/EtOH ) 1:4) emission spectra were recorded on a SPEX
Fluorolog-2 model fluorescence spectrophotometer. Emission life-
time measurements were performed with a Quanta Ray DCR-3
pulsed Nd:YAG laser system (pulse output 355 nm, 8 ns). Error
limits were estimated as follows: λ ((1 nm); τ ((10%); ꢀ ((10%).
The nanosecond emission lifetime measurements were performed
by time-correlated single-photon counting techniques, using a
Fluorocube fluorescence lifetime system equipped with a NanoLED
source (HORIBA Jobin Yvon IBH). The decays were analyzed by
means of DataStation software v2.3.
Molecular Orbital Calculations. The electronic structure of
PAnP with all its phenyl rings replaced with H atoms was calculated
using the SPARTAN semiempirical program SGI/R10K release
5.1.3 with geometry optimization. The model RHF/PM3 was used
in the calculations.
X-ray Crystallography. The diffraction experiments were
carried out on a Bruker AXS SMART CCD three-circle diffrac-
tometer with a sealed tube at 23 °C using graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å). The software used were
SMART25a for collecting frames of data, indexing reflections, and
determination of lattice parameters; SAINT25a for integration of
intensity of reflections and scaling; SADABS25b for empirical
absorption correction; and SHELXTL25c for space group determi-
nation, structure solution, and least-squares refinements on |F|2. The
crystals were mounted at the end of glass fibers (1a, 2a, 2b, 3b,
and 4) or sealed in a capillary tube (3a) and used for the diffraction
experiments. Anisotropic thermal parameters were refined for the
rest of the non-hydrogen atoms. The hydrogen atoms were placed
in their ideal positions. A brief summary of crystal data and
experimental details is given in Table 1.
Syntheses of syn-/anti-[Pt2(dppm)2(PAnP-H2)](OTf)2 (1a,b).
PAnP (33 mg, 0.060 mmol) was added to 40 mL of a CH3CN/
CH2Cl2 (1:1, v/v) solution of 2 molar equiv of Pt(dppm)(OTf)2 (93
mg, 0.121 mmol), which was prepared in situ. The solution was
stirred at room temperature overnight and then filtered. The filtrate
was concentrated by rotaevaporation, and yellow products were
precipitated by adding excess diethyl ether to the solution. Single
crystals of 1a suitable for X-ray diffraction studies were obtained
from slow diffusion of Et2O into a CH2Cl2 solution of the crude
products. Attempts failed to purify 1b from the mixture products.
Total yield: 102 mg, 85%. Anal. Calcd (%) for 1a (C90H70F6
1
O6P6Pt2S2): C, 54.00; H, 3.52. Found (%): C, 53.96; H, 3.66. H
NMR (300 MHz, δ/ppm) for 1a (in CD2Cl2): 7.85–7.82 (m, 2H,
H5,8 of anthracenyl ring (An)), 7.74–7.67 (m, 8H, Ph), 7.53–7.25
(m, 38H, Ph and H2,3 of An), 7.21–7.18 (m, 2H, H6,7, An), 7.16–7.03
(m, 16H, Ph), 4.83–4.61 (m, 4H, CH2). 31P{1H} NMR for 1a and
1b: see Table 2. ESI-MS (m/z): 1a and 1b 851.9 [M - 2OTf]2+
.
Syntheses of syn-/anti-[Pt2(dppe)2(PAnP-H2)](OTf)2 (2a,b).
PAnP (33 mg, 0.060 mmol) was added to 40 mL of a CH3CN/
CH2Cl2 (1:1, v/v) solution of 2 molar equiv of Pt(dppe)(OTf)2 (95
mg, 0.121 mmol), which was prepared in situ. The mixture was
stirred at room temperature overnight and then filtered. The filtrate
(25) (a) SMART & SAINT Software Reference Manuals, version 4.0;
Siemens Energy & Automation, Inc., Analytical Instrumentation: Madison,
WI, 1996. (b) Sheldrick, G. M. SADABS: A Software for Empirical
Absorption Correction; University of Gottingen: Gottingen, Germany, 1996.
(c) SHELXTL Reference Manual, version 5.03; Siemens Energy & Automa-
tion, Inc., Analytical Instrumentation: Madison, WI, 1996.

6536 Organometallics, Vol. 26, No. 26, 2007
Hu et al.

Cyclometalated Platinum-Anthracenyl Complexes
Organometallics, Vol. 26, No. 26, 2007 6537
a
Table 2. Summary of the 31P{1H} NMR Data of Pt2
chemical shifts δ (ppm)
coupling constants (Hz)
complex
P1
P2
P3
1JP1-Pt
1JP2-Pt
1JP3-Pt
2JP1-P2
2JP1-P3
JP2-P3
1a
1b
2a
2b
3a
3b
45.86
41.04
56.40
56.25
48.23
44.82
-23.26
-23.75
47.89
43.65
9.65
-31.09
-31.67
43.80
43.80
-5.09
-4.93
2828
2812
2741
2717
2804
2783
2444
2421
2791
2778
2707
2697
1461
1464
1765
1765
1738
1741
361
361
347
347
343
347
11
11
4
6
15
15
39
39
10
13
34
34
10.66
a 121.5 MHz, in CD2Cl2 for 1a and 1b, in acetone-d6 for 2a and 2b, in CD3CN for 3a and 3b, T ) 300 K.
was concentrated by rotaevaporation, and yellow products were
precipitated by adding excess diethyl ether to the solution. Slow
diffusion of Et2O into a CH2Cl2 solution of the crude products gave
a mixture of crystals of 2a and 2b, which could be separated, as
the crystals of 2a lost solvent easily and became opaque in the air
in a few minutes while the crystals of 2b remained transparent.
The separated crystals of 2a and 2b were purified by repeated
recrystallization from CH2Cl2/Et2O. Total yield: 115 mg, 95%. Anal.
Calcd (%) for 2a · CH2Cl2 (C92H74F6O6P6Pt2S2 · CH2Cl2): C, 52.82;
H, 3.62. Found (%): C, 53.22; H, 3.59. Calcd (%) for 2b · 0.5CH2Cl2
(C92H74F6O6P6Pt2S2 · 0.5CH2Cl2): C, 53.62; H, 3.65. Found (%): C,
H10, An), 8.10 (d, 1H, H5, An), 7.96–7.92 (m, 5H, H2 of An and
H2,2′ and H6,6′ of Ph2P1), 7.85 (dd, 1H, H4, An), 7.67–7.64 (m, 2H,
H4,4′, Ph2P1), 7.60–7.57 (m, 4H, H3,3′ and H5,5′ of Ph2P1), 7.52 (d,
1H, H8, An), 7.42–7.33 (m, 9H, H6 of An and Ph of dppe),
7.25–7.18 (m, 13H, H7 of An and Ph of dppe), 6.96–6.93 (m, 1H,
H3, An), 2.42–2.28 (m, 4H, CH2). 31P{1H} NMR (121.5 MHz,
CD2Cl2, δ/ppm): 54.82 (dd, P1), 44.41 (dd, P2), 42.16 (dd, P3);
1
1
1J(P1-Pt) ) 2746 Hz, J(P2-Pt) ) 2796 Hz, J(P3-Pt) ) 1751
2
2
Hz; J(P1-P2) ) 347 Hz, J(P1-P3) ) 8 Hz, J(P2-P3) ) 15 Hz.
ESI-MS (m/z): 954.4 [M - ClO4]+.
1
53.76; H, 3.86. H NMR (300 MHz, δ/ppm) for 2a (in CD3OD):
Results and Discussion
7.85–7.82 (m, 8H, Ph), 7.74–7.72 (m, 2H, H5,8, An), 7.62–7.22 (m,
54H, Ph and H2,3 of An), 7.04–7.01 (m, 2H, H6,7, An), 2.54–2.34
(m, 8H, CH2); 2b (in CD2Cl2): 7.88–7.16 (unresolved multiplets,
64H, Ph and H2,4,6,8 of An), 6.66–6.61 (m, 2H, H3,7 of An),
2.36–2.28 (m, 8H, PCH2). 31P{1H} NMR: see Table 2. ESI-MS
Synthesis. Reacting PAnP with Pt(L)(OTf)2 (L ) dppm,
dppe, and dppp) produced doubly cyclometalated complexes
[Pt2(L)2(PAnP-H2)](OTf)2 (Pt2) in high yields. The products
obtained display cluster peaks attributable to the doubly charged
[(PtL)2(PAnP-H2)]2+ (L ) dppm, dppe, or dppp) and the singly
charged [(PtL)2(PAnP-H2)OTf]+ in their ESI-MS spectra. A pair
of geometrical isomers, anti- and syn-[Pt2(L)2(PAnP-H2)](OTf)2,
are generated in each reaction (Scheme 1). The ratios of the
two isomers were obtained from the 31P{1H} spectra of the crude
products from the reactions in CH3CN/CH2Cl2 (1:1, v/v). The
double cyclometalation was regioselective, as the isomer ratios
(syn/anti) were 77:37, 90:10, and 77:33 for L ) dppm, dppe,
and dppp, respectively. Pure syn-isomers 1a, 2a, and 3a and
anti-isomers 2b and 3b were isolated by fractional recrystalli-
zation. The 31P{1H} NMR spectra of the Pt2 complexes are
similar; each spectrum consists of three mutually coupled
double-doublets, each flanked with two Pt satellites. The signals
(m/z): 2a and 2b 865.8 [M - 2OTf]2+
.
Syntheses of syn-/anti-[Pt2(dppp)2(PAnP-H2)](OTf)2 (3a,b). In
a typical reaction, PAnP (33 mg, 0.060 mmol) was added to 40
mL of a CH3CN/CH2Cl2 (1:1, v/v) solution of 2 molar equiv of
Pt(dppp)(OTf)2 (97 mg, 0.121 mmol), which was prepared in situ.
The solution was stirred at room temperature overnight and then
filtered. The filtrate was concentrated by rotaevaporation, and yellow
products were precipitated by adding excess diethyl ether to the
solution. The two isomers were separated by recrystallization. Slow
diffusion of Et2O into a CH2Cl2 solution of the crude products in
a test tube gave crystals of 3a and noncrystalline films of 3b
covering the test tube wall. The crystals of 3a were separated, and
the remaining was recrystallized. This process was repeated several
times to obtain purified 3a and 3b. Single crystals of 3b suitable
for X-ray study were obtained by evaporating a saturated CH2Cl2/
Et2O solution of purified 3b. Total yield: 110 mg, 90%. Anal. Calcd
(%) for 3a (C94H78F6O6P6Pt2S2): C, 54.87; H, 3.82. Found (%): C,
54.99; H, 3.79. Calcd (%) for 3b · 2H2O (C94H78F6O6P6Pt2S2 · 2H2O):
2
were assigned on the basis of the coupling constants JP-P and
1JPt-P and chemical shifts, which are listed in Table 2. Inspecting
the coupling constants of the signals shows that two of the P
2
atoms (P1, P2) in each complex show large JP-P of 343–361
1
C, 53.92; H, 3.95. Found (%): C, 53.93; H, 3.83. H NMR (300
1
Hz and JPt-P of 2421–2828 Hz, which are diagnostic for two
MHz, δ/ppm) for 3a (in CD3CN): 7.72–7.60 (m, 12H, Ph),
7.58–7.55 (m, 2H, H5, 8, An), 7.45–7.40 (m, 32H, Ph), 7.25–7.05
(m, 18H, Ph and H2,3 of An), 7.01–6.95 (m, 2H, H6,7, An), 2.48–2.40
(m, 8H, PCH2), 1.90–1.73 (m, 4H, CCH2C); 3b (in CD3CN):
7.82–7.76 (m, 8H, Ph), 7.69–7.63 (m, 2H, H2,6, An), 7.52–7.24 (m,
38H, Ph and H4,8 of An), 7.15–7.08 (m, 16H, Ph), 6.43–6.38 (m,
2H, H3,7, An), 2.45–2.38 (m, 8H, PCH2), 1.87–1.75 (m, 4H,
CCH2C). 31P{1H} NMR: see Table 2. ESI-MS (m/z): 3a and 3b,
trans-oriented P atoms (see Scheme 1 for labeling of the P
atoms).26 The signals are further split by the nuclear spin of
2
the third P atom (P3). The small JP-P (4–39 Hz) implies that
the third P atom is cis to the two trans-oriented P atoms. It is
noted that the 1JPt-P constant (1461–1765 Hz) of P3 is relatively
small, suggesting that it is trans to a strong σ-donor.26 The P3
is therefore assigned to a P atom in L that is trans to the
metalated carbon atom. As the P2 and P3 have similar chemical
shifts, they should belong to L, and the P1 is the P atom of
PAnP.
Reacting Pt(dppe)(ClO4)2 with 1 molar equiv of 9-(diphe-
nylphosphino)anthracene (PAn) gives the mononuclear cyclo-
metalated complex [Pt(dppe)(PAn-H)](ClO4) (4). The 31P{1H}
NMR spectrum of the complex shows a similar pattern to those
of corresponding dimetalated 2a and 2b.
879.8 [M - 2OTf]2+
.
Synthesis of [Pt(dppe)(PAn-H)]ClO4 (4). To 20 mL of a
dichloromethane solution of Pt(dppe)Cl2 (140 mg, 0.211 mmol)
was added 10 mL of an acetonitrile solution of AgClO4 (87 mg,
0.421 mmol). The solution was stirred in the dark for 2 h and
filtered. To the filtrate was added PAn (76 mg, 0.211 mmol). The
mixture was stirred at room temperature for 2 h and then filtered.
Addition of excess diethyl ether to the concentrated filtrate gave a
pale yellow precipitate. The product was purified by recrystallization
from diethyl ether/dichloromethane. Yield: 200 mg, 90%. Anal.
Calcd (%) for C52H42ClO4P3Pt: C, 59.24; H, 4.01. Found (%): C,
59.43; H, 3.28. 1H NMR (500 MHz, CD2Cl2, δ/ppm): 8.65 (d, 1H,
(26) (a) Allen, F. H.; Pidcock, A. J. Chem. Soc. A 1968, 2700. (b)
Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973, 10,
335.

6538 Organometallics, Vol. 26, No. 26, 2007
Hu et al.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1a · Et2O
Pt(1)-P(1) 2.2701(17)
Pt(1)-P(2) 2.3163(17)
Pt(1)-P(3) 2.3252(18)
Pt(1)-C(1) 2.043(6)
P(1)-Pt(1)-P(2)173.83(7)
P(1)-Pt(1)-P(3)106.12(6)
P(2)-Pt(1)-P(3)70.74(6)
P(1)-Pt(1)-C(1)83.78(17)
P(2)-Pt(1)-C(1)99.68(17)
P(3)-Pt(1)-C(1)169.69(18)
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
3a · 4.5CH2Cl2
Pt(1)-P(1) 2.2801(16)
Pt(1)-P(2) 2.3279(18)
Pt(1)-P(3) 2.3437(16)
Pt(1)-C(1) 2.078(6)
P(1)-Pt(1)-P(2)173.21(5)
P(1)-Pt(1)-P(3)98.93(5)
P(2)-Pt(1)-P(3)87.49(5)
P(1)-Pt(1)-C(1)80.83(14)
P(2)-Pt(1)-C(1)92.80(14)
P(3)-Pt(1)-C(1)178.96(18)
Figure 1. ORTEP diagram of 1a · Et2O (50% thermal ellipsoids).
H atoms and anions are omitted for clarity.
Table 5. Selected Bond Lengths (Å) and Angles (deg) for
3b · 4CH2Cl2
Pt(1)-P(1) 2.2817(13)
Pt(1)-P(2) 2.3257(13)
Pt(1)-P(3) 2.3309(13)
Pt(1)-C(1) 2.064(5)
P(1)-Pt(1)-P(2)171.79(5)
P(1)-Pt(1)-P(3)98.75(5)
P(2)-Pt(1)-P(3)86.88(5)
P(1)-Pt(1)-C(1)80.86(15)
P(2)-Pt(1)-C(1)93.75(15)
P(3)-Pt(1)-C(1)177.70(16)
C2V and C2h symmetry, respectively. The Pt-P (2.2701(17)-
2.3437(16) Å) and Pt-C (2.043(6)-2.106(1) Å) bond lengths
in the complexes are similar and typical for cyclometalated
Pt-phosphine complexes.9c,17a The anthracenyl rings are es-
sentially planar. The metal centers show a distorted square-
planar geometry and form two five-membered chelate rings with
the two P atoms of PAnP and carbon atoms of the anthracenyl
ring. In addition, the Pt atoms are chelated by dppm, dppe, and
dppp in forming four- (1a), five- (2a and 2b), and six-membered
(3a and 3b) rings, respectively. Expectedly, the bite angles
P2-Pt1-P3 follow the order 1a (70.74(6)°) < 2a (83.83(13)°)
and 2b (82.99(6)°) < 3a (87.49(5)°) and 3b (86.88(5)°). As a
consequence, the P2-Pt1-C1 angle in 3a (92.80(14)°) is the
smallest among all the complexes (cf. 1a, 99.68(17)°).
Figure 2. ORTEP diagram of 3a (50% thermal ellipsoids). H atoms,
solvent molecules, and anions are omitted for clarity.
As a result, the phenyl rings in 3a are highly congested. The
six-membered Pt(dppp) rings in 3a adopt a twisted chair
conformation, and the two phenyl rings on each P atom of dppp
are oriented either axially or equatorially. The dihedral angle
between the axial phenyl rings on P2 and the anthracenyl ring
is 75.8°, whereas the corresponding angle for the equatorial
phenyl rings on P2 are much smaller, being 49.3°. Because of
the small dihedral and P2-Pt1-C1 angles, the equatorial phenyl
rings in 3a are close to the anthracenyl ring and are extended
toward the opposite P2A atoms. To avoid severe steric repulsion,
the axial phenyl rings of P2 are directed facing the equatorial
phenyl rings of the P2A atom and vice versa.
Figure 3. ORTEP diagram of 3b (50% thermal ellipsoids). H atoms,
solvent molecules, and anions are omitted for clarity.
Structures of the Cyclometalated Complexes. The X-ray
crystal structures of syn-[Pt2(dppm)2(PAnP-H2)](OTf)2 · Et2O
(1a · Et2O)(Figure1),syn-[Pt2(dppe)2(PAnP-H2)](OTf)2 · 1.25CH2
Cl2 · 0.5Et2O (2a · 1.25CH2Cl2 · 0.5Et2O), anti-[Pt2(dppe)2(PAnP-
H2)](OTf)2 · 2CH2Cl2 (2b · 2CH2Cl2) (the ORTEP diagrams of
2a and 2b are included in the Supporting Information), syn-
[Pt2(dppp)2(PAnP-H2)](OTf)2 · 4.5CH2Cl2 (3a · 4.5CH2Cl2) (Fig-
ure 2), and anti-[Pt2(dppp)2(PAnP-H2)](OTf)2 · 4CH2Cl2 (3b · 4
CH2Cl2) (Figure 3) were determined. Selected bond distances
and angles are listed in Tables 3, 4, and 5. The structures of
1a, 2a, and 3a are similar, showing two syn-oriented Pt centers
attached to the C1 and C4 positions of the anthracenyl ring. On
the other hand, the C1 and C5 positions are metalated in the
syn-isomers 2b and 3b. The immediate coordination spheres of
the syn- and anti-isomers are slightly deviated from idealized
Complex 4 (Figure 4, Table 6 for selected bond distances
and angles) contains one Pt ion attached to the anthracenyl ring
at the C1 position. The coordination geometry and Pt-C and
Pt-P bond lengths are similar to those of 2a and 2b.
Electronic Spectroscopy. The UV–vis absorption spectra of
4 and PAn (Figure 5) display moderately intense vibronic bands
around 320–440 nm (the spectral data of Pt2 and 4 are
summarized in Table 7). The shapes of the bands are similar,
and the vibronic spacing in the band of 4 (1336 and 1350 cm-1
)
is close to those of PAn (1336, 1329 cm-1). The band of 4 is
red-shifted from that of PAn by ∼1200 cm-1. In addition, the
compounds display very intense absorption at 267 nm (4, εmax
) 9.25 × 104 M-1 cm-1) and 256 nm (PAn, εmax ) 1.02 ×
105 M-1 cm-1).

Cyclometalated Platinum-Anthracenyl Complexes
Organometallics, Vol. 26, No. 26, 2007 6539
Table 6. Selected Bond Lengths (Å) and Angles (deg) for 4 · CH2Cl2
Pt(1)-P(1) 2.2922(11)
Pt(1)-P(2) 2.3088(11)
Pt(1)-P(3) 2.3222(11)
Pt(1)-C(1) 2.077(4)
P(1)-Pt(1)-P(2)178.79(4)
P(1)-Pt(1)-P(3)97.58(4)
P(2)-Pt(1)-P(3)83.18(4)
P(1)-Pt(1)-C(1)82.77(12)
P(2)-Pt(1)-C(1)96.53(12)
P(3)-Pt(1)-C(1)176.53(10)
The spectra of the binuclear complexes (Figures 6 and 7, see
Table 7 for spectroscopic data) are different from that of 4. The
spectra of the syn-isomers 1a, 2a, and 3a display two intense,
overlapping absorption bands labeled A and B at 320–520 nm.
The higher energy and more intense absorption band B is
maximized around ∼350–420 nm (εmax ≈ (1.5-2) × 104 M-1
cm-1) and the lower energy band A at ∼430–500 nm (εmax ≈ 1
× 104 M-1 cm-1). Poorly resolved vibronic peaks could be
discerned. The anti-isomers 2b and 3b also display two intense
bands A and B in the same spectral range (300–500 nm). Both
bands A and B show distinct vibronic peaks with spacing of ∼1300
cm-1, with the former being more intense than the latter. Compar-
ing the spectra of the complexes shows that bands A of 2b and 3b
are slightly higher in energy than those of 2a and 3a.
The band A of Pt2 is red-shifted from the lowest energy 1(π*
r π) transition of the parent ligand PAnP by ∼2000 cm-1. An
intense high-energy band at 266–285 nm (εmax ) (6.26–8.76)
× 104 M-1 cm-1) is observed in the spectra of the binuclear
complexes and is slightly lower than the corresponding absorp-
tion of PAnP (εmax ) 261 nm, εmax ) 7.92 × 104 M-1 cm-1).
All the absorption bands are insensitive to solvent polarity, as
they are red-shifted by <100 cm-1 as the solvent is changed
from CH2Cl2 to DMSO.
Figure 5. UV–vis absorption spectra of PAn and 4 in CH2Cl2.
Shown in the inset are the bands at 240-320 nm.
statesthe so-called “plus” and “minus” states.27a,b,28 The very
1
+
intense band at 250 nm corresponds to the B2u
r
1Ag
transition. On the other hand, the 1B2u- r 1Ag transition, which
is slightly higher in energy than the 1B1u r 1Ag transition (1B1u
is lower than B2u by ∼1500 cm-1),29 is forbidden (ε ≈ 102
M-1 cm-1) and hidden under the latter.27a,b,28 Given the similar
spectra of 4 and anthracene, the 320–440 and 265 nm bands
1
-
1
1
+
are assigned to B1u
r
1Ag and B2u
r
1Ag transitions,
1
-
1
respectively. The B2u r Ag transition is not observed and
1
1
could lie under the B1u r Ag transition.
Notably, the LUMO r HOMO transition of 4 is red-shifted
from that of PAn by ∼1200 cm-1. It is due to perturbation of
the Pt ion on the π-system of the ligand (Vide infra). Nonethe-
less, the “anthracene-like” spectrum of 4 indicates that the
perturbation of a single Pt ion on the electronic structure of the
anthracenyl ring is rather small.
Spectral Assignments. The low-energy bands of 4 and PAn
have similar vibronic spacing and band shapes, suggesting that
both arise from an intraligand π* r π transition. In fact, they
resemble the spectrum of anthracene,27 which displays a
moderately intense vibronic band at 320–390 nm and a very
intense band at 255 nm. The absorption features of anthracene
arise from four electronic transitions, as shown by the unper-
turbed π-system in Scheme 2. The vibronic band arises from
On the other hand, the interactions between the two Pt ions
and the π-system are apparently stronger in Pt2. Unlike 4 and
anthracene, the spectra of Pt2 show two bands A and B in the
visible region. Given its energy and intensity, band A is assigned
to the LUMO r HOMO transition. The transition is red-shifted
from those of 4 and the parental ligand PAnP by ∼1200 and
2400 cm-1, respectively. The LUMO r HOMO transition of
the anti-isomers shows distinct vibronic peaks. Notably, the 0–0
transitions (407 nm for 2b and 409 nm for 3b) are slightly more
intense than the 0–1 transitions (381 nm for 2b and 385 nm for
3b). On the contrary, the 0–1 transitions are more intense than
the 0–0 transitions for 4 and anthracene, suggesting that the
anthracenyl framework is less distorted in the excited state of
the anti-isomers than in that of 4 and anthracene. The very
intense absorption around 260 nm corresponds to the 1B2u+ r
1Ag transition of anthracene. The bands B of 2b and 3b have
similar vibronic spacing of ∼1300 cm-1, suggesting that the
absorption is ligand-centered.
the LUMO r HOMO (1B1u
r
1Ag) transition (from here
onward, HOMO and LUMO refer to the highest occupied and
lowest unoccupied π- and π*-orbitals of the anthracenyl ring,
respectively). In a first-order approximation, the LUMO r
HOMO-1 and LUMO+1 r HOMO transitions, which have
the same energy, give rise to two degenerate 1B2u excited states.
Strong first-order configuration interactions between the states
1
+
1
-
give rise to a high-energy B2u state and a low energy B2u
Spectroscopic studies showed that the π-system of anthracene
could be significantly perturbed by substituents, especially the
(27) (a) Kessinger, M.; Michl, J. Excited States and Photochemistry of
Organic Molecules; VCH: New York, 1995. (b) Suzuki, H. Electronic
Absorption Spectra, and Geometry of Organic Molecules; an Application
of Molecular Orbital Theory; Academic Press: New York, 1967. (c) Turro,
N. J. Modern Molecular Photochemistry; Benjamin: Menlo Park, 1978.
(28) (a) Pariser, R. J. Chem. Phys. 1956, 24, 250. (b) Ham, N. S.;
Ruedenberg, K. J. Chem. Phys. 1956, 25, 13.
Figure 4. ORTEP diagram of 4 (50% thermal ellipsoids). H atoms,
solvent, and the anion are omitted for clarity.
(29) Kawashima, Y.; Hashimoto, T.; Nakano, H.; Hirao, K. Theor. Chem.
Acc. 1999, 102, 49.

6540 Organometallics, Vol. 26, No. 26, 2007
Hu et al.
Table 7. Spectroscopic and Photophysical Data of Pt2 and 4
Emission
maxima
at 77
band C/nm
solution
emission
maxima/nm
fluorescence
quantum
yield Φf
band A/nm
band B/nm
(ε, 104 M-1
kR/
kNR/
complex
(ε, 104 M-1 cm-1
)
(ε, 104 M-1 cm-1
)
cm-1
)
K/nm
τ/nsa
× 107 s-1 × 108 s-1
1a
434 (1.08), 460 (1.16)
346 (0.55), 365 (0.73),
382 (1.25), 405 (1.57)
380 (1.43), 394 (1.75)
347 (0.62), 364 (0.74),
381 (1.16), 407 (0.98)
394 (1.26)
266 (8.76)
510
503
1.7
0.055
3.24
5.56
2a
2b
443 (1.13), 468 (1.08)
430 (1.89), 457 (2.13)
273 (7.97) (s)
285 (8.65)
504
466
490
459
1.9
1.8
0.059
0.021
3.11
1.17
4.95
5.44
3a
3b
443 (1.00), 463 (1.01)
434 (1.72), 460 (1.86)
273 (6.26) (s)
272 (7.60) (s)
523
477
506
464
0.6
0.4
0.009
0.019
1.50
4.75
16.52
24.53
367 (0.66), 385 (0.86),
409 (0.89)
b
4
355 (0.49), 373 (0.78),
395 (1.06), 417 (0.95)
267 (9.25)
454
447
0.0016
a Measured in CH2Cl2 at 298 K. b The lifetime of the emissions is too short to be measured accurately.
affect the Coulomb integral Rt of the 2pz-orbital of the
substituted carbon atom i.31 The effect propagates along the
π-system and changes the energy of the molecular orbitals.
According to first-order perturbation theory, the change in the
energy of a molecular orbital R (δER) is related to the change
in Coulomb integral (δR) by the equation δEk ) i c2 δR
∑
ik
i
where cik is the linear-combination-of-atomic-orbital (LCAO)
coefficient of the 2pz-orbital of the substituted carbon atom i in
molecular orbital k. It is reasonable to expect the potential field
of the Pt ions would stabilize the ligand orbitals; that is, δR is
negative. Semiempirical calculation on a model for PAnP shows
that the LCAO coefficients for the 2pz-orbitals of the C1/4/5/8 in
the HOMO (0.29359) and LUMO (-0.27443) are close (see
SF6 for the molecular orbitals of the model), as one would
expect for alternant hydrocarbons. Accordingly, in a first-order
approximation, the two orbitals should be stabilized almost
Figure 6. UV–vis absorption spectra of the syn-isomers and PAnP
in CH2Cl2. Shown in the inset are the bands C.
equally by the potential field of the Pt ions; that is, δEHOMO
δELOMO. It suggests that the inductive effect alone cannot cause
≈
the 2400 cm-1 shift of the LUMO r HOMO transition.
Orbital interactions between the Pt ions and the anthracenyl
ring mainly involve the metal dπ-orbitals and the HOMO and
LUMO of the anthracenyl ring. For the anti-isomers in the
1
2
√
√
idealized C2h symmetry, the 1⁄ 2 (dxz + dxz ) (au) and 1⁄ 2
(dxz1-dxz2) (bg) orbitals can interact with the LUMO (au) and
HOMO (bg), respectively. Because of the stabilization of the
dπ-orbitals by the π-accepting phosphines, it is reasonable that
the metal orbitals are lower in energy than the HOMO, and
accordingly the interactions would destabilize the HOMO and
LUMO. The change in the energy of the orbitals, δE, is inversely
proportional to the energy difference between the metal and
ligand orbitals.32 As the metal orbitals are closer in energy to
the HOMO than to the LUMO, the former should be more
1
destabilized than the latter. Similarly, the 1⁄ 2 (6pz + 6pz2)
√
(au) and 1⁄ 2 (6pz - 6pz2) (bg) orbitals of the Pt ions can
1
√
overlap with the ligand orbitals. As the HOMO and LUMO are
lower in energy than the 6pz-orbitals, both of them would be
stabilized by the interactions, with the latter being more
stabilized than the former, as it is closer in energy to the 6pz-
orbitals. Collectively, the π-interactions reduce the HOMO-
LUMO energy gap. It accounts for the red-shift of the LUMO
r HOMO observed in the cyclometalated complexes. Since
the inductive effect and π-interactions are additive, the π-system
of Pt2 is more perturbed than that of 4. Both inductive effects
and π-interactions do not alter the energy of the HOMO-1 and
LUMO+1, as the orbitals have nodes at the substituted positions.
Figure 7. UV–vis absorption spectra of the anti-isomers in CH2Cl2.
Shown in the inset are the bands C.
ones that exert a strong mesomeric effect.30 The perturbation
would lower the symmetry of the π-system, leading to a
bathochromic shift of the LUMO r HOMO transition and, more
important, observation of the otherwise forbidden 1B2u- r 1Ag
transition. The Pt ions in the dicyclometalated complexes could
influence the π-system by their inductive effect and via
π-interactions. Longuet-Higgins and Coulson pointed out that
substitution of a H atom in an alternant aromatic ring would
(30) (a) Baba, H.; Suzuki, S. J. Chem. Phys. 1961, 35, 1501. (b) Tichý,
M.; Zahradník, R. J. Phys. Chem. 1969, 73, 534. (c) Rotkiewicz, K.;
Grabowski, Z. R. Trans. Faraday Soc. 1969, 65, 3263. (d) Steiner, R. P.;
Michl, J. J. Am. Chem. Soc. 1978, 100, 6861.
(31) Longuet-Higgins, H. C.; Coulson, C. A. J. Chem. Soc. 1949, 971.
(32) Creutz, C.; Newton, M. D.; Sutin, N. J. Photochem. Photobio. A
1994, 82, 47.

Cyclometalated Platinum-Anthracenyl Complexes
Organometallics, Vol. 26, No. 26, 2007 6541
Scheme 2. Left: HOMOs and LUMOs of anthracene. The red, blue, green, and black arrows represent the LUMO r HOMO,
LUMO r HOMO-1, LUMO+1 r HOMO, and LUMO+1 r HOMO-1 transitions, respectively. Right: Ground state and
four singlet excited states arising from the four electronic transitions (ref 27a)
Scheme 3
Scheme 3 shows the perturbed electronic structures (left) and
singlet excited states (right) of Pt2.
extinction coefficients of the π*(polypyridyl) r 5d(Pt) charge-
transfer transitions are ∼103 M-1 cm-1 4b,9c,12d,17a,34
.
Possibly
Electronic transitions from the HOMOs to the LUMOs in
Pt2 give rise to four singlet excited states. The major difference
between Pt2 and the unperturbed systems is that the degeneracy
of the LUMO r HOMO and LUMO+1 r HOMO-1 transi-
tions is removed by the metal–ligand interactions. In other
the MLCT transitions in the present cyclometalated complexes
are masked by the far more intense intraligand transition. It
should be mentioned that the anthracenyl ring is also perturbed
by the two PPh2 groups at the C9 and C10 positions. However
the effect is small, as the LUMO r HOMO transition in PAnP
is only red-shifted from that of anthracene by ∼1000 cm–1.
Accordingly, the strong perturbation observed in the spectra of
Pt2 should be mainly due to the Pt ions.
Emission Spectroscopy. All of the cyclometalated complexes
are emissive in degassed CH2Cl2 solution at room temperature
(Figure 8a, the photophysical data are summarized in Table 7).
The emission energies follow the order 4 > 2b, 3b > 1a, 2a,
and 3a and are parallel with the energy of band A of the
complexes. The Pt2 complexes (λmax ) 466–523 nm, Φ )
0.009–0.059) are more emissive than 4 (λmax ) 454 nm, Φ )
0.0016). The emission bands of 2b, 3b, and 4 show a vibronic
shoulder at a frequency of ∼1200 cm-1. On the other hand,
the emissions of the three syn-isomers (λem ) 502–522 nm)
are structureless. The emission lifetime (τ) of Pt2 is in the range
of a nanosecond (0.4–1.9 ns). On the other hand, the emission
lifetime of 4 is too short to be determined accurately. All
excitation spectra resemble the corresponding absorption spectra,
confirming that the emissions originate from the complexes.
All of the emissions become more intense and are slightly
blue-shifted in the frozen glass solutions of the complexes at
1
words, the two B2u states are no longer degenerate. Conse-
quently, configuration interactions between them become weaker
and the splitting of the “plus” and “minus” states should be
smaller than the one in the unperturbed systems. The bands B
+
and C of Pt2 should correspond to the 1B2u- r 1Ag and 1B2u
r Ag transitions in the unperturbed systems, respectively.33
1
1
For the unperturbed systems, the B1u
r
1Ag (LUMO r
1
-
1
HOMO) and B2u r Ag transitions are close in energy and
the latter is hidden under the intense former band. However,
the energy gap between the two transitions is widened in the
Pt2 because of the reduced configuration interactions and the
red-shift of the LUMO r HOMO transition. Taking the first
peaks of bands A and B as the 0–0 transitions, the energy
difference between the bands is estimated to be ∼4200 cm-1
,
which is larger than the corresponding separation of ∼1500
cm-1 in anthracene. While the B2u
r
1Ag transition is
1
-
forbidden in the unperturbed systems, it is allowed in Pt2
1
because the two B2u states are no longer degenerate. This
explains the high intensity of the band B.
No distinct MLCT absorption band can be discerned in the
spectra. Studies on Pt-polypyridyl complexes showed that the
(34) (a) Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray, H. B.
Inorg. Chem. 1996, 35, 6261. (b) Lai, S.-W.; Chan, M. C. W.; Cheung,
K.-K.; Che, C.-M. Inorg. Chem. 1999, 38, 4262. (c) Du, P.; Schneider, J.;
Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2006, 128, 7726. (d) Yam,
V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002, 124, 6506.
(33) The 1B1u r 1Ag, 1B2u- r 1Ag, and 1B2u+ r 1Ag transitions in the
1
1
1
-
1
1
+
unperturbed systems correspond to B1 r A1, A1 r A1, and A1
r
-
1A1 transitions in the C2V syn-isomers and 1Bu
r r
1Ag, 1Bu 1Ag, and
+
1Bu
r
1Ag transitions in the C2h anti-isomers, respectively.

6542 Organometallics, Vol. 26, No. 26, 2007
Hu et al.
Figure 8. (a) Emission spectra of degassed CH2Cl2 of the Pt complexes at 298 K. (b) Emission spectra of glassy solutions (MeOH/EtOH
) 1:4) of 2a, 2b, and 4 at 77 K.
77 K. The vibronic structures of the emissions of 2b, 4 (Figure
8b), and 3b (see Supporting Information) become more distinct.
The vibronic spacing between the first two peaks for 2b, 3b,
and 4 is 1420, 1300, and 1230 cm-1, respectively. The three
syn-isomers 1a, 3a (see Supporting Information), and 2a (Figure
8b) show similar emission (λmax ) 490–506 nm) with a vibronic
states, the cyclometalated complexes have MLCT and ligand
field excited states. However, the π-accepting phosphines should
push these excited states up; accordingly they are expected to
be higher in energy than S1 and therefore do not contribute
significantly to the nonradiative decay of the emissive state.
In principle, the Pt and P atoms in the complexes could
increase the rate of spin-forbidden transitions such as the S1 f
T1 and S1 f T2 intersystem crossing via their “heavy atom
effect”.27 As these radiationless pathways compete with radiation
decay, a direct consequence of the heavy atom effect is reduction
of quantum yield of fluorescence Φf.27,35 It is therefore
reasonable to assume the effect is at least partially responsible
for the exceptionally low Φf (0.0016) of 4 (cf. anthracene Φf
) 0.27).35 Having two Pt ions and two P atoms, the Pt2
complexes are expected to exhibit a stronger heavy atom effect
than 4. It is therefore surprising that the Φf values of the Pt2
(0.009–0.059) are higher than that of 4. A similar anomaly has
been observed: the Φf of 9,10-dibromoanthracene is found
higher than that of 9-bromoanthracene.37 The apparent absence
of heavy atom effect in 9,10-bromoanthracene is due to a change
in the order of the S1 and T2 energy. While the energy of S1
(ES1) is higher than the energy of T2 (ET2) in 9-bromoanthracene,
the order is reversed in 9,10-dibromoanthracene, i.e., ET2 > ES1.
shoulder separated from the peak by ∼1300 cm-1
.
Photophysics. The emissions of Pt2 have short lifetimes (τ
) 0.4-1.9 ns) and small Stokes shifts, indicating that the
emissions are fluorescent. In addition, rates of radiative decay
kR ((1.17–4.75) × 107 s-1, Table 7) for the complexes are similar
to those determined for fluorescence of anthracenes (e.g., kR
for anthracene ) 6.33 × 107 s-1).35 All these indicate that the
emissions arise from the lowest energy singlet excited states S1
(corresponds to the 1B1u) to the ground state S0 (1Ag). Huang–
Rhys factors, which are approximated by the ratio of the 0-0
and 0-1 transitions,36 are 1.2 for 4 and 0.5–0.83 for Pt2. The
lower Huang–Rhys factor for Pt2 implies the anthracenyl rings
in the complexes are less distorted in the S1 excited state than
that of 4. A possible reason is that the Pt2 complexes are more
rigid than 4. In addition, the small excited-state distortion of
the Pt2 could be partly due to stronger MLCT character in the
S1 state of the complexes.
Previous studies on the photophysics of anthracenes identified
three nonradiative decay pathways for S1, namely, internal
conversion from S1 to S0 (S1 f S0) and intersystem crossing
from S1 to two triplet states T1 and T2 (S1 f T1 and S1 f
T2).35,37 Because of the large energy gap between S1 and S0
and the rigid molecular scaffolds, the rate of the internal
conversion kS1-S0 is slow (kS1-S0 for anthracene is ∼106 s-1).
The T1 state, which arises from the LUMO r HOMO transition,
is much lower energy than the S1 because of the large exchange
integral. For instance, the S1-T1 gap is 1.2 × 104 cm-1 for
anthracene.35 This leads to an unfavorable Franck–Condon factor
for the S1 f T1 intersystem crossing and hence low intersystem
crossing rate kS1-T1. Lastly, there is an upper triplet state T2 (3B1g
or 3B1u) that is very close to S1 in energy. Because of the small
energy gap, the rate of the S1 f T2 intersystem crossing (kST)
is very high (∼108-109 s-1), and hence it is the major
nonradiative decay pathway for the emissive S1 state.37 Depend-
ing on the substituents, T2 could be higher or lower in energy
than the S1 state. In addition to the three ligand-centered excited
The kS1-T2 is dependent on the S1-T2 gap, i.e., kS1-T2
)
is
koS1-T2 + A exp E - ET2 kT (A is a constant and ko
(
)
/
S1
S1-T2
the intrinsic intersystem crossing rate).38 Accordingly, the heavy
atom effect, which would increase the kS1-T2o, is attenuated
when ET2 > ES1 (Scheme 4).
To determine the S1-T2 gap in Pt2 requires variable-
temperature quantum yield measurements. Unfortunately the
experimental setup is not available to us. Nonetheless, it is
possible to gauge the heavy atom effect in the complexes by
looking at the kS1-T2. As mentioned, there are three major
nonradiative decay pathways for the S1, and hence kNR ≈ kS0-S1
+ kS1-T1 + kS1-T2. Accordingly, the kNR can be taken as the
upper limit of the kS1-T2 for the Pt2 complexes. The upper limits
of kS1-T2 of 1a, b and 2a,b are similar (4.95–5.56 × 108 s-1).
More important, they are of the same order of magnitude as
the kS1-T2 of 1.8 × 108 s-1 estimated for anthracene. The similar
kS1-T2 suggests the heavy atom effect in the dinuclear complexes
is subdued, possibly by a widened T2-S1 gap, as in the case of
9,10-dibromoanthracene. The estimated kNR for 3a (16.52 ×
108 s-1) and 3b (24.53 × 108 s-1) are higher than those for
(35) Birks, J. B. Photophysics of Aromatic Molecules”; Wiley: London,
1970.
(36) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30, 4446.
(37) (a) Hiaryama, S.; Lampert, R. A.; Philips, D. J. Chem. Soc., Faraday
Trans. 2 1985, 81, 371. (b) Robinson, G. W.; Frosh, R. P. J. Chem. Phys.
1962, 37, 1962.
(38) (a) Tanaka, M.; Tanaka, I.; Tai, S.; Hamanoue, K.; Sumitani, M.;
Yoshihara, K. J. Phys. Chem. 1983, 87, 813. (b) Wu, K.-C.; Ware, W. R.
J. Am. Chem. Soc. 1979, 101, 5906. (c) Dreeskamp, H.; Pabst, J. Chem.
Phys. Lett. 1979, 61, 262.

Cyclometalated Platinum-Anthracenyl Complexes
Organometallics, Vol. 26, No. 26, 2007 6543
Scheme 4. Radiative (f) and nonradiative (dashed arrow)
Conclusion
pathways for the S1 excited state
Our spectroscopic study shows that the two Pt ions in the
doubly cyclometalated complexes can alter the electronic
structure of the anthracenyl ring significantly. The spectra of
the complexes display two intense absorption bands in the visible
region that correspond to the transitions in the unperturbed
systems. The observation of the otherwise forbidden transition
is due to the removal of the degeneracy of the excited states by
metal–ligand interactions. All of the cyclometalated complexes
display intraligand fluorescence from their S1 excited states.
There is an apparent absence of heavy atom effects on the S1-T2
intersystem crossing in the dicyclometalated complexes, which
is possibly due to a widened gap between the S1 and T2 states.
Acknowledgment. We thank Dr. Koh Lip Lin and Ms.
Tan Geok Kheng for solving the X-ray crystal structures of
the compounds, and National University of Singapore for
financial support. K.-Y.W. acknowledges support from the
Hong Kong Polytechnic University.
1a,b and 2a,b. This is unlikely due to an increase in kS1-T2 in
3a and 3b, as their electronic structures should be similar to
other Pt2 complexes. Possibly the high kNR is due to an increase
in kS1-S0, which could be due to motion of the flexible backbone
(the so-called loose bolt effect)39 and/or additional C-H bond
stretching of dppp in 3a and 3b.40
Supporting Information Available: CIF files for 1a, 2a, 2b,
3a, 3b, and 4. X-ray crystal structures and selected bond lengths
and angles for 2a and 2b. Emission spectra of 1a, 3a, and 3b in
glassy solutions at 77 K. Semiempirical molecular orbital diagram
of a model compound for PAnP. This material is available free of
charge via the Internet at http://pubs.acs.org.
(39) Guesten, H.; Mintas, M. Klasinc, L. J. Am. Chem. Soc. 1980, 102,
7936.
(40) (a) Kropp, J. L.; Windsor, M. W. J. Chem. Phys. 1965, 39, 2769.
(b) King, C.; Auerbach, R. A.; Fronczek, F. R.; Roundhill, D. M. J. Am.
Chem. Soc. 1986, 108, 5626.
OM700706R