Synthesis and characterization of luminescent bis-cyclometalated platinumIV complexes

Article abstract of DOI:10.1021/ic100761f

Presented is the synthesis and characterization of a series of luminescent heteroleptic bis-cyclometalated platinum^IV complexes. An oxidation-facilitated cyclometalation is employed to convert platinum ^II pendant species into bis-cyc

Full text of DOI:10.1021/ic100761f

Inorg. Chem. 2010, 49, 11297–11308 11297
DOI: 10.1021/ic100761f
Synthesis and Characterization of Luminescent Bis-Cyclometalated PlatinumIV
Complexes
Dustin M. Jenkins† and Stefan Bernhard*,‡
†Department of Chemistry, Princeton University, Princeton, New Jersey 08540, United States, and
‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
Received April 20, 2010
Presented is the synthesis and characterization of a series of luminescent heteroleptic bis-cyclometalated platinumIV
complexes. An oxidation-facilitated cyclometalation is employed to convert platinumII pendant species into bis-
cyclometalated platinumIV dichlorides, which are transformed into the tris-chelated diimine complexes through ligand
substitution. The structure-property relationship is probed by judiciously varying substituents on both the C∧N and the
N∧N ligands resulting in a family of complexes exhibiting blue emission, long excited-state lifetimes, and highly efficient
oxygen quenching. Excited-state properties are corroborated by static and time-dependent density-functional theory
calculations of both the singlet and the triplet state.
Introduction
of applications such as optoelectronics,11-17 sensor tech-
nology,18-23 and photocatalysis.11,12,24-30
Since early investigations of exclusively orange-emitting
[Ru(bpy)3]2þ complexes, synthetic control over the excited
state, and thus the luminescent and electrochemical proper-
ties, has been attained. Third row metals such as OsII, IrIII,
and PtII, and stronger field ligands, like the cyclometalating
Ionic transition metal complexes have gained much recog-
nition because of their unique photophysical properties and
reversible electrochemical characteristics.1-5 Square planar
and octahedral architectures utilizing second or third row
metals and polypyridyl chelates have by far received the most
attention and remain at the forefront of current research. The
hallmarks of these complexes are long-lived triplet excited
states, redox stability, and broad control of emissive and
electrochemical attributes through judicious structural
modification.6-10 Such qualities lend them to a wide variety
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*To whom correspondence should be addressed. E-mail: bern@cmu.edu.
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r
2010 American Chemical Society
Published on Web 11/24/2010
pubs.acs.org/IC

11298 Inorganic Chemistry, Vol. 49, No. 24, 2010
Jenkins and Bernhard
2-phenylpyridine, have been incorporated to increase ligand
field splitting and eliminate thermal population of the non-
emissive MC state which precludes color tunability in
heteroleptic bis-cyclometalated PtIV complexes of the archi-
tecture [Pt(ppy)2(bpy)]2þ. Numerous complexes of PtIV have
been explored as anticancer agents40-43 and in various
catalytic reactions,44,45 but compounds with an octahedral,
triply chelated structure have, until now, been unknown. In
this work, green and blue emission is readily demonstrated in
such complexes through ligand modification, and full spec-
troscopic, photophysical, and electrochemical characteriza-
tion is provided alongside a computational treatment of both
ground- and excited-states using static and time-dependent
density-functional methods. Additionally, particular com-
plexes are shown to exhibit very long lifetimes and high
sensitivity to oxygen quenching.
3
[Ru(bpy)3]2þ and its derivatives.1,6,8 For instance, cationic
bis-cyclometalated IrIII complexes, such as [Ir(ppy)2(bpy)]þ,
demonstrate tunability across the visible spectrum and
through a wide range of electrochemical potentials,2,3,5,8,9
and have become key materials in a variety of fields.8,9,17 The
advantages of these complexes in technological applications
are numerous. Single-layer OLEDs incorporating ionic tran-
sition metal complexes31,32 demonstrate superior exciton
harvesting and, as a result, up to 100% theoretical
efficiency,33-35 turn-on voltages as low as 2.0 V, and can be
driven by an alternating current (AC) power supply.3 Device
architectures can be as simple as a luminophore sandwiched
between two metal electrodes, fabricated by spin-casting and
evaporation onto a glass substrate. Devices exhibiting ex-
ternal quantum efficiencies (EQEs) of up to 16% and power
efficiencies of nearly 37 lm/W have already been realized.36
Luminescence-based oxygen sensors utilizing ionic transi-
tion metal complexes demonstrate high accuracy owing to
the long triplet lifetimes and significant oxygen sensitivity of
such complexes. Simple devices consisting of an excitation
light source, photocell detector, and sensor have been shown
to be a promising replacement for the Clark cell since they do
not destroy the analyte being measured. The sensor itself
usually consists of the luminophore dispersed in an oxygen-
permeable polymer or self-assembled monolayer (SAM)
membrane and cast onto a surface.22,37-39
Experimental Section
Materials. Potassium tetrachloroplatinateII and 4,40-di-
methoxy-2,20-bipyridine were purchased from Aldrich and used
as received. 2-Phenylpyridine, ammonium hexafluoropho-
sphate, thalliumI nitrate, and silverI trifluoromethanesulfo-
nate were purchased from Alfa Aesar and used as received.
2,20-Bipyridine was purchased from Acros and used as received.
General Methods. Syntheses of the ligands 5-methyl-2-(4-
fluorophenyl)pyridine, 5-methyl-2-(4-methoxyphenyl)pyridine,
5-methyl-2-(biphen-4-yl)pyridine, and 5,50-difluoro-2,20-bipyr-
idine, and iodobenzene dichloride are described in the Supporting
Information.
Synthesis of PtII Pendant Complexes. The parent PtII pendant
complexes were synthesized using a procedure similar to that
employed by Niedermair et al.46 The 2-phenylpyridine was
combined with potassium tetrachloroplatinate in a mixture of
1:1 tert-butanol and water at 80 °C. The precipitate was filtered
and washed in methanol for good yields (60-89%). For spectro-
scopic analyses, fine crystals were obtained using the vapor
diffusion of diethyl ether into a solution of the complex in
dichloromethane.
Moreover, water-reduction systems exploiting ionic tran-
sition metal complexes as photosensitizers have received
much attention in the past decade as concern grows regarding
energy security and the need to utilize alternative fuels such as
hydrogen. Most of these systems make use of a noble metal
catalyst (such as colloidal platinum) and an amine-based
sacrificial reductant in a mixed aqueous/organic solution,
illuminated at an appropriate wavelength. Systems exhibiting
turnover numbers as high as 5,000 have been reported.27
The use of such luminescent metal complexes in the afore-
mentioned applications staunchly supports the continuing
interest in these materials. Specifically, the search for blue
emitters amenable to application in single-layer OLEDs is
ongoing,3,16,35,39 and the need for highly sensitive oxygen
sensors with faster response/recovery times persists. To
address some of these issues we have synthesized the first
Pt(ppy)(ppyH)Cl (1a). 2-Phenylpyridine (0.157 g, 1.011 mmol)
was combined with potassium tetrachloroplatinate (0.200 g,
0.482 mmol) in a 1:1 mixture of tert-butanol and water (10 mL)
at 80 °C for 12 h. The reaction was cooled and the yellow
precipitate was filtered and washed in methanol (30 mL) to
obtain the pure product (1a). Yield: 0.155 g (59.5%). Product
confirmed by 1H NMR as previously reported.46
MS (ESI) m/z calculated for ([M - Cl-]þ) 504.1039, found
504.0768.
Anal. Calcd for C22H17ClN2Pt: C, 48.90; H, 3.17; N, 5.19.
Found: C, 49.01; H, 3.24; N, 5.14.
Pt(F-mppy)(F-mppyH)Cl (1b). Reaction time: 24 h. Yield:
77.4%.
1H NMR (500 MHz, Acetone d6): δ 9.45 (“t”, 1H, JPt-H
=
19.2), 9.04 (“t”, 1H, JPt-H=21.7), 8.23 (td, 1H, J=6.8, 3.0), 8.06
(d, 1H, J=4.2), 7.84 (d, 1 H, J=4.3), 7.78 (d, 1H, J=4.3), 7.68
(d, 1H, J=4.3), 7.53 (td, 1H, J=6.9, 2.7), 7.12 (t, 2H, J=8.8),
6.72 (td, 1H, J=9.0, 2.2), 5.76 (“tdd”, 1H, JPt-H=26.5, J=10.0,
2.4), 2.52 (s, 3H), 2.40 (s, 3H).
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Article
13C NMR (500 MHz, Acetone d6): δ 164.91 (s), 164.68 (d, J=
Inorganic Chemistry, Vol. 49, No. 24, 2010 11299
13C NMR (500 MHz, DMSO d6): δ 160.70 (s), 159.63 (s),
147.65 (s), 142.89 (s), 141.99 (d, J = 6.4), 137.37 (d, J = 2.0),
135.19 (s), 127.77 (d, J=9.1), 121.48 (“t”, JPt-C=16.4), 113.29
(d, J=21.7), 112.99 (d, J=23.1), 18.15 (s).
29.3), 162.94 (s), 158.78 (s), 157.36 (s), 155.11 (d, J = 48.0),
151.19 (s), 145.46 (s), 140.63 (d, J=50.8), 139.80 (s), 136.41 (d,
J=3.2), 135.64 (s), 135.06 (s), 132.83 (d, J=8.6), 131.93 (d, J=
8.6), 127.96 (s), 126.20 (s), 125.83 (d, J=9.4), 119.07 (s), 116.05
(d, J=22.1), 115.36 (d, J=22.1), 30.63 (s), 18.13 (s), 17.89 (s),
17.85 (s).
19F NMR (300 MHz, DMSO d6): δ -106.86 (dd, J =
14.0, 8.2).
Pt(MeO-mppy)2Cl2 (2c). Yield: 15.6%.
19F NMR (300 MHz, Acetone d6): δ -111.49 (s), -113.06 (d,
J=169.6).
1H NMR (500 MHz, DMSO d6): δ 9.42 (t, 2H, J=13.9), 8.25
(d, 2H, J=8.3), 8.16 (dd, 2H, J=8.6, 1.2), 7.88 (d, 2H, J=8.7),
6.76 (dd, 2H, J=8.7, 2.2), 5.30 (“td”, 2H, JPt-H=18.5, J=2.3),
3.56 (s), 3.37 (s).
MS (ESI) m/z calculated for ([M - Cl-]þ) 568.1164, found
568.0874.
Pt(MeO-mppy)(MeO-mppyH)Cl (1c). Reaction time: 24 h.
Yield: 81.4%.
13C NMR (500 MHz, DMSO d6): δ 160.76 (t, J=27.0), 160.14
(t, 2H, J=26.5), 147.26 (s), 142.33 (s), 142.04 (s), 133.33 (“d”,
J=13.9), 127.04 (“t”, JPt-C=18.0), 120.36 (“t”, JPt-C=17.4),
112.81 (“t”, JPt-C=17.1), 109.71 (s), 55.03 (s), 17.97 (s).
Pt(Ph-mppy)2Cl2 (2d). Yield: 34.8%.
1H NMR (500 MHz, Acetone d6): δ 9.44 (“t”, 1H, JPt-H
=
17.35), 9.03 (“t”, 1H, JPt-H=20.3), 8.23 (d, 2H, J=4.8), 7.98 (d,
1H, J=4.1), 7.76 (d, 1H, J=4.4), 7.71 (d, 1H, J=4.4), 7.58 (d,
1H, J=4.2), 7.38 (d, 1H, J=4.7), 6.89 (d, 2H, J=4.6), 6.53 (dd,
1H, J=4.4, 2.3), 5.59 (“td”, 1H, JPt-H=26.7, J=2.4), 3.75 (s,
3H), 3.57 (s, 3H), 2.49 (s, 3H), 2.37 (s, 3H).
1H NMR (300 MHz, DMSO d6): δ 9.60 (“t”, 2H, JPt-H
=
14.5), 8.43 (m, 2H), 8.28 (d, 2H, J=8.7), 8.00 (d, 2H, J=8.2),
7.45 (dd, 2H, J=8.4, 1.2), 7.35 (m, 6H), 7.28 (m, 4H), 6.10 (“td”,
2H, JPt-H=17.4, J=1.5), 2.58 (s, 6H).
13C NMR (500 MHz, Acetone d6): δ 165.59 (s), 161.27 (s),
161.11 (s), 159.69 (s), 154. 92 (s), 150.90 (s), 144.46 (s), 140.35 (s),
139.92 (s), 138.07 (s), 134.41 (s), 133.45 (s), 131.95 (s), 131.40 (s),
127.56 (s), 125.25 (s), 118.19 (s), 116.47 (s), 113.85 (s), 108.97 (s),
55.50 (s), 55.04 (s), 18.07 (s), 17.78 (s).
13C NMR (300 MHz, DMSO d6): δ 161.00 (“t”, JPt-C=24.4),
148.49 (s), 143.04 (s), 142.81 (s), 141.83(s), 140.39 (“t”, JPt-C
=
14.0), 135.49 (“t”, JPt-C=14.4), 129.57 (s), 128.61 (s), 126.86 (s),
126.54 (“t”, JPt-C=15.4), 124.91 (s), 124.44 (“t”, JPt-C=17.4),
121.90 (“t”, JPt-C=15.4), 18.62 (s).
MS (ESI) m/z calculated for ([M - Cl-]þ) 592.1564, found
592.1384.
Synthesis of [Pt(ppy)2(bpy)](PF6)2 Derivatives. All [Pt(ppy)2-
(bpy)](PF6)2 derivatives were synthesized using similar condi-
tions. The PtIV dichloride was heated with an excess of the N∧N
ligand in a non-reducing solvent system (acetonitrile/water, tert-
butanol/water, or water) and chloride departure was assisted
using Agþ or Tlþ ion, when necessary, in a manner similar to
that used by McDaniel et al.48 The mixture was diluted with
water and ammonium hexafluorophosphate was added to pre-
cipitate a solid which was then filtered. The solid was dried in
vacuo at 100 °C for 24 h and the purified product obtained as
crystals using acetonitrile/ethyl ether vapor diffusion. All reac-
tions were performed in 40 mL sealed glass vials equipped with
septa.
Pt(Ph-mppy)(Ph-mppyH)Cl (1d). Reaction time: 24 h. Yield:
88.5%.
1H NMR (300 MHz, CDCl3): δ 9.48 (s), 9.16 (s), 8.34 (d, J=
8.3), 8.23 (d, J=8.3), 8.08 (d, J=8.3), 8.01 (m), 7.72 (m), 7.66
(m), 7.63 (m), 7.61-7.28 (m), 7.22 (J=1.8), 7.19 (J=1.8), 7.04
(J=8.0), 6.40 (J=1.7), 4.61 (s), 2.48 (s), 2.42 (J=2.7), 2.36 (s).
MS (ESI) m/z calculated for ([M - Cl-]þ) 684.1978, found
684.1851.
Synthesis of Pt(ppy)2Cl2 Derivatives. The precursor bis-cyclo-
metalated PtIV dichloride complexes were synthesized using a
method similar to that reported by Mamtora et al.47 The parent
PtII pendant complex was reacted with iodobenzene dichloride
in dichloromethane at room temperature. The precipitate was
filtered and washed in dichloromethane for modest to good
yields (16-72%). The bis-cyclometalated products can also be
obtained by oxidation of (1a)-(1d) with hydrogen peroxide as
reported by Newman et al.47
[Pt(ppy)2(bpy)](PF6)2 (3a). A 40 mL reaction vial, equipped
with a septum, was charged with Pt(ppy)2Cl2 (0.050 g, 87.1 μmol),
2,20-bipyridine (0.081 g, 519.3 μmol), and a magnetic stirrer in
10 mL of 3:1 acetonitrile/water. The reaction was stirred for 48 h
at 110 °C and then cooled. The reaction mixture was diluted to
20 mL with water, ammonium hexafluorophosphate (0.425 g,
2.61 mmol) was added, and the mixture diluted to 50 mL with
water to precipitate a solid. The solid was filtered and dried
overnight in a vacuum oven at 100 °C. Recrystallization by
diffusion of ethyl ether into acetonitrile containing the com-
plex was performed to arrive at the pure compound (3a)
(0.041 g, 50%).
Pt(ppy)2Cl2 (2a). Pt(ppy)(ppyH)Cl (0.222 g, 0.411 mmol) was
reacted with iodobenzene dichloride (0.113 g, 0.411 mmol) in
20 mL of dichloromethane for 24 h. A white solid precipitated
and was collected by filtration and washed in dichloromethane
(20 mL). Yield: 0.170 g (72.1%).
1H NMR (500 MHz, DMSO d6): δ 9.72 (“td”, 2H, JPt-H
=
14.3, J=6.0), 8.49 (d, 2H, J=7.9), 8.39 (ddd, 2H, J=9.0, 7.8,
1.1), 7.97 (d, 2H, J=7.9), 7.82 (td, 2H, J=6.7, 1.2), 7.16 (t, 2H,
J=7.4), 6.98 (td, 2H, J=7.6, 1.1), 5.94 (“td”, 2H, J Pt-H= 16.2,
J=7.9).
1H NMR (500 MHz, Acetone d6): δ 9.12 (d, 2H, J=8.6), 8.65
(td, 4H, J=7.9, 1.4), 8.47 (td, 2H, J=7.8, 1.3), 8.32 (q, 2H, J=
6.1), 8.24 (m, 4H), 8.03 (ddd, 2H, J=8.2, 5.5, 1.1), 7.57 (m, 4H),
7.33 (td, 2H, J=7.6, 1.4), 6.43 (“td”, 2H, JPt-H=13.8, J=8.2)
13C NMR (500 MHz, Acetone d6): δ 164.10 (s), 155.26 (s),
149.90 (s), 149.24 (s), 144.79 (s), 144.46 (s), 142.33 (s), 137.54 (s),
134.40 (“t”, JPt-C=18.6), 131.56 (“t”, JPt-C=8.2), 129.52 (“t”,
13C NMR (500 MHz, DMSO d6): 163.20 (“t”, JPt-C=28.1),
148.83 (s), 142.80 (s), 141.24 (s), 140.60 (“t”, JPt-C = 14.6),
131.60 (“t”, JPt-C=21.1), 126.46 (“t”, JPt-C=15.8), 125.82 (m),
124.99 (“t”, JPt-C=13.8), 121.69 (“t”, JPt-C=15.7).
Anal. Calcd for C22H16Cl2N2Pt: C, 46.01; H, 2.81; N, 4.88.
Found: C, 45.89; H, 2.89; N, 4.77.
JPt-C =14.7), 128.32 (“t”, JPt-C =13.9), 127.95 (“t”, JPt-C =
13.9), 127.85 (s), 124.23 (“t”, JPt-C=16.8).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 329.5825,
found 329.5368.
Pt(F-mppy)2Cl2 (2b). Yield: 54.7%.
1H NMR (500 MHz, DMSO d6): δ 9.45 (“t”, 2H, JPt-H
=
Anal. Calcd for C32H24F12N4P2Pt: C, 40.48; H, 2.55; N, 5.90.
Found: C, 40.62; H, 2.78; N, 6.02.
14.0), 8.38 (d, 2H, J=8.6), 8.26 (dd, 2H, J=8.3, 1.1), 8.07 (dd,
2H, J = 8.7, 5.8), 7.07 (td, 2H, J = 8.9, 2.4), 5.65 (“tdd”, 2H,
J
Pt-H=18.6, J=8.5, 2.4), 3.37 (s).
[Pt(ppy)2(dFbpy)](PF6)2 (3b). A 40 mL reaction vial, equipped
with a septum, was charged with Pt(ppy)2Cl2 (0.088 g, 153.2
μmol), 5,50-difluoro-2,20-bipyridine (0.119 g, 619.8 μmol), silverI
(47) (a) Mamtora, J.; Crosby, S. H.; Newman, C. P.; Clarkson, G. J.;
Rourke, J. P. Organometallics 2008, 27, 5559. (b) Newman, C. P.; Casey-Green,
K.; Clarkson, G. J.; Cave, G. W. V.; Errington, W.; Rourke, J. P. Dalton Trans.
2007, 3170.
(48) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J. Am.
Chem. Soc. 2008, 130, 210.

11300 Inorganic Chemistry, Vol. 49, No. 24, 2010
Jenkins and Bernhard
trifluoromethanesulfonate (0.083 g, 323.3 μmol), and a mag-
netic stirrer in 8 mL of 3:1 tert-butanol/water. The reaction was
stirred for 24 h at 60 °C and then cooled. The reaction mixture
was filtered, diluted to 20 mL with water, ammonium hexafluo-
rophosphate (0.667 g, 4.1 mmol) was added, and the mixture
diluted to 50 mL with water to precipitate a solid. The solid was
filtered and dried overnight in a vacuum oven at 100 °C. Re-
crystallization by diffusion of ethyl ether into acetonitrile con-
taining the complex was performed to arrive at the pure com-
pound (3b) (0.102 g, 68%).
8.31 (m, 4H), 8.01 (m, 4H), 7.37 (td, 2H, J = 8.8, 2.4), 6.14
(“tdd”, 2H, JPt-H=16.3, J=8.2, 2.5), 2.22 (s, 6H).
13C NMR (500 MHz, Acetone d6): δ 160.14 (s), 155.34 (s),
150.25 (s), 148. 32 (s), 145.70 (t, J=3.4), 144.48 (s), 139.09 (s),
138.99 (m), 138.02 (d, J=7.7), 131.50 (“t”, JPt-C=8.3), 130.17
(d, J=10.0), 127.97 (“t”, JPt-C=5.0), 123.77 (“t”, JPt-C=16.9),
116.96 (d, J=23), 116.58 (d, J=22.8), 114.02 (s), 18.40 (s).
19F NMR (300 MHz, Acetone d6): δ -105.12 (m).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 361.5886,
found 361.5665.
1H NMR (500 MHz, Acetone d6): δ 9.18 (dd, 2H, J=5.1, 4.2),
8.65 (m, 2H), 8.52 (ddd, 2H, J=9.6, 8.3, 2.7), 8.48 (td, 2H, J=
7.9, 1.2), 8.32 (“td”, 2H, J Pt-H=14.0, J=6.1), 8.23 (dd, 2H, J=
7.7, 1), 8.18 (dd, 2H, J=7.0), 7.57 (q, 4H, J=7.4), 7.33 (td, 2H,
J=7.7, 1.3), 6.41 (“td”, 2H, J Pt-H= 14.1, J=8.0).
13C NMR (500 MHz, Acetone d6): δ 163.16 (d, J = 262),
163.96 (s), 151.27 (d, J=3.3), 149.53 (s), 144.83 (s), 142.49 (s),
139.54 (d, J=32.8), 136.53 (s), 134.41 (“t”, J Pt-C=19.3), 131.50
(d, J=19.1), 130.00 (m), 129.77 (s), 129.67 (“t”, J Pt-C= 13.9),
128.46 (“t”, J Pt-C= 13.8), 127.92 (“t”, J Pt-C= 13.8), 124.36
(“t”, J Pt-C= 16.4).
Anal. Calcd for C34H26F14N4P2Pt 2.5 H2O: C, 38.57; H,
2.95; N, 5.29. Found: C, 38.53; H, 2.76; N, 5.30.
3
[Pt(F-mppy)2(dFbpy)](PF6)2 (3e). A 40 mL reaction vial,
equipped with a septum, was charged with Pt(F-mppy)2Cl2
(0.080 g, 125.4 μmol), 5,50-difluoro-2,20-bipyridine (0.144 g,
750.0 μmol), thalliumI nitrate (0.180 g, 675.7 μmol), and a
magnetic stirrer in 15 mL of water. The reaction was stirred
for 96 h at 110 °C and then cooled. The reaction mixture was
filtered, and ammonium hexafluorophosphate (0.300 g, 1.8 mmol)
was added to precipitate a solid. The solid was filtered and
dried overnight in a vacuum oven at 100 °C. Recrystallization
by diffusion of ethyl ether into acetonitrile containing the
complex was performed to arrive at the pure compound (3e)
(0.024 g, 18%).
19F NMR (300 MHz, Acetone d6): δ -115.17 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 347.5730,
found 347.5255.
Anal. Calcd for C32H22F14N4P2Pt H2O: C, 38.30; H, 2.41; N,
1H NMR (500 MHz, Acetone d6): δ 9.14 (dd, 2H, J=9.4, 3.7),
8.53 (m, 4H), 8.30 (m, 6H), 8.09 (“t”, 2H, JPt-H = 14.1), 7.38
(td, 2H, J=8.9, 2.0), 6.11 (“tdd”, 2H, JPt-H=17.0, 8.5, 2.2), 2.23
(s, 6H).
3
5.58. Found: C, 38.28; H, 2.38; N, 5.52.
[Pt(ppy)2(dMeObpy)](PF6)2 (3c). A 40 mL reaction vial,
equipped with a septum, was charged with Pt(ppy)2Cl2 (0.048 g,
83.6 μmol), 4,40-dimethoxy-2,20-bipyridine (0.108 g, 499.5 μmol),
and a magnetic stirrer in 6 mL of 3:1 acetonitrile/water. The
reaction was stirred for 48 h at 110 °C and then cooled. The
reaction mixture was diluted to 11 mL with water and washed
in ethyl ether (3ꢀ10 mL). Ammonium hexafluorophosphate
(0.380 g, 2.3 mmol) was added, and the mixture diluted to
50 mL with water to precipitate a solid. The solid was filtered
and dried overnight in a vacuum oven at 100 °C. Recrystalli-
zation by diffusion of ethyl ether into acetonitrile containing
the complex was performed to arrive at the pure compound
(3c) (0.058 g, 71%).
13NMR (500 MHz, Acetone d6): δ 164.96 (s), 163.01 (d, J=
261), 162.92 (s), 160.36 (s), 151.35 (d, J=3), 148.50 (s), 145.73
(s), 139.99 (d, J=33), 139.13 (s), 136.84 (d, J=7), 131.54 (d, J=
19), 130.28 (d, J=9.0), 130.07 (d, J=8.6), 123.94 (“t”, JPt-C
17.4), 117.12 (d, J=23.0), 116.83 (d, J=22.5), 18.38 (s).
=
19F NMR (300 MHz, Acetone d6): δ -104.94 (dd, J=17.6,
5.8), -115.18 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 379.5793,
found 379.5661.
Anal. Calcd for C34H24F16N4P2Pt H2O: C, 38.25; H, 2.45; N,
3
5.25. Found: C, 38.50; H, 2.55; N, 5.25.
1H NMR (500 MHz, Acetone d6): δ 8.65 (m, 2H), 8.59 (d, 2H,
J=2.6), 8.48 (ddd, 2H, J=9.3, 8.0, 1.1), 8.23 (m, 4H), 8.0 (d, 2H,
J=6.5), 7.61 (ddd, 2H, J=8.5, 5.9, 1.3), 7.53 (td, 2H, J=7.7),
7.46 (dd, 2H, J=6.7, 2.6), 7.3 (td, 2H, J=7.7, 1.3), 6.40 (“td”,
2H, JPt-H=13.5, J=7.8), 4.15 (s, 6H).
[Pt(F-mppy)2(dMeObpy)](PF6)2 (3f). A 40 mL reaction vial,
equipped with a septum, was charged with Pt(F-mppy)2Cl2
(0.070 g, 109.7 μmol), 4,40-dimethoxy-2,20-bipyridine (0.047 g,
217.4 μmol), thalliumI nitrate (0.073 g, 274.0 μmol), and a mag-
netic stirrer in 8 mL of 4:6 acetonitrile/water. The reaction was
stirred for 96 h at 115 °C and then cooled. The reaction mixture
was diluted to 16 mL with water, filtered, and washed in ethyl
ether (3 ꢀ 20 mL). Ammonium hexafluorophosphate (0.540 g,
3.3 mmol) was added, and the mixture diluted to 50 mL with
water to precipitate a solid. The solid was filtered and dried
overnight in a vacuum oven at 100 °C. Recrystallization by dif-
fusion of ethyl ether into acetonitrile containing the complex was
performed to arrive at the pure compound (3f) (0.045 g, 38%).
1H NMR (500 MHz, Acetone d6): δ 8.57 (d, 2H, J=2.2), 8.51
(d, 2H, J=8.3), 8.30 (m, 4H), 8.10 (“q”, 2H, JPt-H=6.6), 8.02
(“t”, 2H, JPt-H=14.3), 7.45 (dd, 2H, J=6.8, 2.4), 7.35 (td, 2H,
J =8.7, 2.0), 6.12 (“tdd”, 2H, JPt-H =15.7, 8.7, 2.0), 4.16 (s),
2.26 (s).
13C NMR (500 MHz, Acetone d6): δ 171.37 (s), 164.22 (s),
156.81 (s), 150.56 (“t”, JPt-C=9.0), 148.93 (s), 144.70 (s), 142.31
(s), 137.91 (s), 134.31 (“t”, JPt-C = 18.4), 129.49 (“t”, JPt-C
14.4), 129.31 (s), 128.21 (“t”, JPt-C=13.1), 127.88 (“t”, JPt-C
=
=
14.4), 124.12 (“t”, JPt-C = 16.4), 116.42 (“t”, JPt-C = 8.15),
114.37 (“t”, JPt-C=5), 58.04 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 359.5930,
found 359.5447.
Anal. Calcd for C34H28F12N4O2P2Pt 3H2O: C, 38.39; H,
3
3.22; N, 5.27. Found: C, 38.39; H, 3.17; N, 5.24.
[Pt(F-mppy)2(bpy)](PF6)2 (3d). A 40 mL reaction vial, equip-
ped with a septum, was charged with Pt(F-mppy)2Cl2 (0.080 g,
125.4 μmol), 2,20-bipyridine (0.039 g, 250.0 μmol), silverItrifluo-
romethanesulfonate (0.069 g, 268.8 μmol) and a magnetic stirrer
in 6 mL acetonitrile. The reaction was stirred for 24 h at 70 °C
and then cooled. The reaction mixture was filtered, diluted to
18 mL with water and washed in ethyl ether (3ꢀ20 mL). Am-
monium hexafluorophosphate (0.610 g, 3.7 mmol) was added,
and the mixture diluted to 50 mL with water to precipitate a
solid. The solid was filtered and dried overnight in a vacuum
oven at 100 °C. Recrystallization by diffusion of ethyl ether into
acetonitrile containing the complex was performed to arrive at
the pure compound (3d) (0.070 g, 63%).
13C NMR (500 MHz, Acetone d6): δ 171.34 (s), 165.09 (s),
163.05 (s), 160.58 (s), 156.87 (s), 150.97 (“t”, JPt-C = 10.7),
147.94 (s), 145.62 (s), 138.42 (s), 130.07 (d, J=9.3), 123.69 (“t”,
JPt-C=16.5), 116.90 (d, J=23.0), 116.46 (s), 116.26 (m), 114.52
(“t”, JPt-C=5), 58.11 (s), 18.45 (s).
19F NMR (300 MHz, Acetone d6): δ -105.29 (m).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 391.5992,
found 391.5755.
Anal. Calcd for C36H30F14N4O2P2Pt: C, 40.27; H, 2.82; N,
5.22. Found: C, 40.18; H, 2.90; N, 5.22.
1H NMR (500 MHz, Acetone d6): δ 9.06 (d, 2H, J=8.3), 8.66
(td, 2H, J=7.5, 1.7), 8.51(m, 2H), 8.39 (ddd, 2H, J=5.7, 6.6, 1.1),
[Pt(MeO-mppy)2(bpy)](PF6)2 (3g). A 40 mL reaction vial,
equipped with a septum, was charged with Pt(MeO-mppy)2Cl2

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11301
(0.043 g, 65.0 μmol), 2,20-bipyridine (0.060 g, 384.6 μmol), and a
magnetic stirrer in 10 mL of 3:1 acetonitrile/water. The reaction
was stirred for 48 h at 110 °C and then cooled. The reaction
mixture was diluted to 20 mL with water, and ammonium hex-
afluorophosphate (0.450 g, 2.8 mmol) was added. The mixture
was diluted to 50 mL with water to precipitate a solid. The solid
was filtered and dried overnight in a vacuum oven at 100 °C.
Recrystallization by diffusion of ethyl ether into acetonitrile
containing the complex was performed to arrive at the pure
compound (3g) (0.025 g, 37%).
13C NMR (500 MHz, Acetone d6): δ 171.20 (s), 162.96 (“t,”
Pt-C=24.0), 161.80 (s), 156.88 (s), 150.79 (“t”, JPt-C =10.0),
J
147.33 (s), 145.15 (s), 138.88 (s), 137.26 (s), 134.50 (s), 129.46
(“t”, JPt-C=16.0), 122.55 (s), 116.18 (“t”, JPt-C=9.0), 115.84
(“t”, JPt-C=16.0), 114.32 (“t”, JPt-C=5.0), 113.16 (s), 58.03 (s),
55.99 (s), 18.28 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 403.6192,
found 403.6086.
Anal. Calcd for C38H36F12N4O4P2Pt H2O: C, 39.63; H, 3.68;
3
N, 5.12. Found: C, 39.06; H, 3.65; N, 5.26.
1H NMR (500 MHz, Acetone d6): δ 9.05 (d, 2H, J=8), 8.65
(td, 2H, J=8.4, 1.3), 8.39 (m, 2H), 8.35 (d, 2H, J=6), 8.22 (d,
2H, J=8.5), 8.15 (d, 2H, J=8.8), 8.02 (dd, 2H, J=6.7), 7.92
(“t”, 2H, JPt-H =14.4), 7.13 (dd, 2H, J=8.7, 2.3), 5.77 (“td”,
2H, JPt-H=16.1, 2.3), 3.75 (s, 6H), 2.19 (s, 6H).
[Pt(Ph-mppy)2(bpy)](PF6)2 (3j). A 40 mL reaction vial, equip-
ped with a septum, was charged with Pt(Ph-mppy)2Cl2 (0.050 g,
66.3 μmol), 2,20-bipyridine (0.062 g, 394.7 μmol), and a magnetic
stirrer in 10 mL of 3:1 acetonitrile/water. The reaction was
stirred for 48 h at 110 °C and then cooled. The reaction mixture
was diluted to 20 mL with water and ammonium hexafluorop-
hosphate (0.425 g, 2.6 mmol) was added. The mixture diluted to
50 mL with water to precipitate a solid. The solid was filtered
and dried overnight in a vacuum oven at 100 °C. Recrystalliza-
tion by diffusion of ethyl ether into acetonitrile containing the
complex was performed to arrive at the pure compound (3j)
(0.016 g, 21%).
13C NMR (500 MHz, Acetone d6): δ 162.98 (s), 161.68 (s),
155.35 (s), 150.04 (“t”, JPt-C=8.0), 147.69 (s), 145.23 (s), 144.25
(s), 138.52 (s), 137.32 (s), 131.36 (“t”, JPt-C=7.5), 129.55 (“t”,
JPt-C=16.0), 127.79 (“t”, JPt-C=4.6), 122.61 (“t”, JPt-C=18),
115.90 (“t”, JPt-C=15), 113.25 (s), 56.02 (s), 18.23 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 373.6087,
found 373.6040.
1H NMR (300 MHz, Acetonitrile d3): δ 8.77 (d, 2H, J=8.2),
8.47 (td, 2H, J=8.0, 1.5), 8.26 (m, 2H), 8.14 (dd, 2H, J=0.9,
0.8), 8.12 (m, 2H), 8.03 (d, 2H, J=8.2), 7.82 (d, 1H, J=1.3), 7.80
(m, 2H), 7.76 (d, 1H, J=1.7), 7.53 (“tt”, 2H, J Pt-H=14.5, J=
0.9), 7.43 (m, 10H), 6.36 (“td”, 2H, J Pt-H=14.5, J=1.7), 2.18
(s, 6H).
Anal. Calcd for C36H32F12N4O2P2Pt H2O: C, 40.96; H, 3.25;
N, 5.31. Found: C, 40.98; H, 3.29; N, 5.44.
3
[Pt(MeO-mppy)2(dFbpy)](PF6)2 (3h). A 40 mL reaction vial,
equipped with a septum, was charged with Pt(MeO-mppy)2-
Cl2 (0.060 g, 90.6 μmol), 5,50-difluoro-2,20-bipyridine (0.104 g,
541.7 μmol), and a magnetic stirrer in 10 mL of 3:1 acetonitrile/
water. The reaction was stirred for 48 h at 110 °C and then
cooled. The reaction mixture was diluted to 20 mL with water,
washed in ethyl ether (3 ꢀ 10 mL) and ammonium hexafluor-
ophosphate (0.450 g, 2.8 mmol) was added. The mixture diluted
to 50 mL with water to precipitate a solid. The solid was filtered
and dried overnight in a vacuum oven at 100 °C. Recrystalliza-
tion by diffusion of ethyl ether into acetonitrile containing the
complex was performed to arrive at the pure compound (3h)
(0.045 g, 47%).
13C NMR (300 MHz, Acetonitrile d3): δ 161.95 (s), 155.10 (s),
150.41 (“t”, J Pt-C=8.1), 148.30 (s), 145.94 (s), 145.26 (s),144.05
(s), 141.29 (s), 139.86 (s), 138.93 (s), 137.36 (s), 131.21 (“t”, J Pt-C=
8.3), 130.14 (s), 129.68 (s), 128.41 (“t”, J Pt-C=14.2), 127.96 (m),
127.35 (“t”, J Pt-C= 15.8), 123.76 (s), 17.59 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 419.6294,
found 419.6114.
Anal. Calcd for C46H36F12N4P2Pt 2H2O: C, 47.39; H, 3.46;
3
N, 4.81. Found: C, 47.57; H, 3.37; N, 4.98.
1H NMR (500 MHz, Acetone d6): δ 9.13 (dd, 2H, J=9.0, 4.0),
8.53 (dd, 2H, J=9, 3), 8.38 (m, 2H), 8.22 (m, 2H), 8.14 (d, 2H,
J=8.9), 8.01 (“t”, 2H, JPt-H=14.0), 7.14 (dd, 2H, J=9.0, 2.0),
5.74 (“td”, 2H, JPt-H=16.0, 2.0), 3.75 (s, 6H), 2.21 (s, 6H).
13C NMR (500 MHz, Acetone d6): δ 162.99 (d, J = 261),
162.91 (s), 161.53 (s), 151.35 (d, J = 3), 147.97 (s), 145.25 (s),
139.65 (d, J=33.0), 137.46 (s), 137.38 (s), 134.62 (s), 131.32 (d,
J = 19.0), 129.92 (m), 129.65 (“t”, JPt-C = 16.0), 122.75 (“t”,
[Pt(Ph-mppy)2(dMeObpy)](PF6)2 (3k). A 40 mL reaction vial,
equipped with a septum, was charged with Pt(Ph-mppy)2Cl2
(0.100 g, 132.6 μmol), 4,40-dimethoxy-2,20-bipyridine (0.175 g,
809.4 μmol), and a magnetic stirrer in 14 mL of 3:1 acetonitrile/
water. The reaction was stirred for 48 h at 110 °C and then
cooled. The reaction mixture was diluted to 20 mL with water,
washed with ethyl ether (3ꢀ20 mL), and ammonium hexafluoro-
phosphate (0.680 g, 4.2 mmol) was added. The mixture diluted
to 50 mL with water to precipitate a solid, which was filtered and
dried overnight in a vacuum oven at 100 °C. Recrystallization by
diffusion of ethyl ether into acetonitrile containing the complex
was performed to arrive at the pure compound (3k) (0.026 g,
17%).
JPt-C = 18.0), 116.10 (“t”, JPt-C = 15.0), 113.56 (s), 56.05 (s),
18.21 (s).
19F NMR (300 MHz, Acetone d6): δ -115.62 (s).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 391.5992,
found 391.6077.
1H NMR (300 MHz, Acetone d6): δ 8.64 (d, 2H, J=2.6), 8.58
(m, 2H), 8.34 (d, 2H, J=8.6), 8.17 (m, 6H), 7.79 (dd, 2H, J=8.3,
1.5), 7.42 (m, 12H), 6.55 (d, 2H, J=1.5), 4.18 (s, 6H), 2.30 (s, 6H).
13C NMR (300 MHz, Acetone d6): δ 170.38 (s), 160.62 (s),
156.18 (s), 149.94 (t, J=9.3), 147.34 (s), 144.94 (s), 144.41 (s),
140.44 (s), 138.99 (s), 137.90 (s), 137.12 (s), 129.04 (s), 128.53 (s),
127.32 (t, J=13.7), 126.89 (s), 126.82 (s), 126.38 (t, J=15.4),
122.68 (t, J=17.3), 115.50 (t, J=8.5), 113.45 (t, J=5.0), 57.17
(s), 17.54 (s).
Anal. Calcd for C36H30F14N4O2P2Pt H2O: C, 39.61; H, 2.95;
N, 5.13. Found: C, 39.79; H, 3.18; N, 5.12.
3
[Pt(MeO-mppy)2(dMeObpy)](PF6)2 (3i). A 40 mL reaction
vial, equipped with a septum, was charged with Pt(MeO-
mppy)2Cl2 (0.060 g, 90.6 μmol), 4,40-dimethoxy-2,20-bipyridine
(0.118 g, 545.8 μmol), and a magnetic stirrer in 10 mL of 3:1
acetonitrile/water. The reaction was stirred for 48 h at 110 °C
and then cooled. The reaction mixture was diluted to 20 mL with
water, and ammonium hexafluorophosphate (0.450 g, 2.8
mmol) was added. The mixture diluted to 50 mL with water to
precipitate a solid. The solid was filtered and dried overnight in a
vacuum oven at 100 °C. Recrystallization by diffusion of ethyl
ether into acetonitrile containing the complex was performed to
arrive at the pure compound (3i) (0.023 g, 23%).
MS (ESI) m/z calculated for ([M - 2(PF6)-]2þ) 449.6400,
found 449.6415.
Anal. Calcd for C48H40F12N4O2P2Pt 3H2O: C, 46.35; H,
3
3.73; N, 4.50. Found: C, 45.95; H, 3.51; N, 4.64.
1
Spectroscopic and Spectrometric Determination. H and 13C
1H NMR (500 MHz, Acetone d6): δ 8.56 (d, 2H, J=2.5), 8.39
(m, 2H), 8.23 (d, 2H, J=8.4), 8.13 (d, 2H, J=9.0), 8.05 (“q”, 2H,
JPt-H=6.5), 7.93 (“t”, 2H, JPt-H=14.0), 7.46 (dd, 2H, J=6.6,
2.6), 7.11 (dd, 2H, J=8.8, 2.2), 5.74 (“td”, 2H, JPt-H=15.9, 2.4),
4.16 (s, 6H), 3.73 (s, 6H), 2.24 (s, 3H).
NMR spectra were recorded on either a Bruker Avance-300 or a
Bruker BioSpin Avance-500 MHz Spectrometer at room tem-
perature. 19F NMR spectra were recorded on a Varian-Mer-
cury-VX 300 MHz Spectrometer. Mass Spectra (MS) were
obtained using a Micromass Q-TOF II hybrid MS engine.

11302 Inorganic Chemistry, Vol. 49, No. 24, 2010
Jenkins and Bernhard
a
Scheme 1. Generalized Procedure for the Synthesis of Heteroleptic PtIV Complexes [Pt(C∧N)2(N∧N)](PF6)2
a Modification of substituents on the ligands produced a family of 11 derivatives (3a)-(3k).
Table 1. Initial Set of Cyclometalating and Diimine Ligands
Photophysical Characterization. Photoluminescence mea-
surements were performed at room temperature using 15 μM
solutions in acetonitrile using a capped quartz cuvette (1.0 cm).
These solutions were degassed using bubbling Argon for 10 min
prior to each measurement. UV-vis absorption spectra were
measured using either a Hewlett-Packard 8453 diode-array
spectrophotometer or a Varian Cary 50 UV-vis spectropho-
tometer. Photoluminescence emission spectra were measured on
a Jobin-Yvon Fluorolog-3 spectrophotometer equipped with
dual monochromators and a Hamamatsu-928 photomultiplier
tube (PMT) at right angle geometry. The emission spectra
were recorded at an excitation wavelength of 317 nm and were cor-
rected according to the calibration factors of the instrument.
Photoluminescent quantum yields (Φem) were measured against
a rutheniumII tris(2,20-bipyridine) hexafluorophosphate stan-
dard using the equation Φs=Φr (Is/Ir)(Ar/As)(δs/δr), where Φs is
the quantum yield for the sample, Is and Ir represent the points
of maximum intensity in the corrected emission spectra of the
sample and reference, As and Ar are the absorbance of the
sample and the reference at the excitation wavelength, and δs
and δr are the correction factors for sample and the reference for the
heightened sensitivity of the detector to longer wavelengths.
Φr is the quantum yield of the reference which is 6.2%.49
Excited-state lifetimes (τ) were measured with the emission
monochromator and PMT detector of the Jobin-Yvon spectro-
meter. The samples were excited at 337 nm using a Stanford
Research Systems NL 100 N2 laser (3.5 ns pulse). The emis-
sion decay was recorded using a Tektronix TDS 3032B digital
phosphor oscilloscope.
Electrochemical Analyses. Electrochemical potentials were
determined using a CH Instruments Model 600C Series Electro-
chemical Analyzer/Workstation with a potential sweep rate of
100 mV/s. Employed were a 1 mm2 platinum working electrode,
a platinum coil counter electrode, and a silver wire pseudo-
reference electrode. Ferrocene (Aldrich) was used as an internal
standard referenced against SCE as 0.371 V. Solutions of the
complexes were prepared at ∼400 μM and tetra-n-butylammo-
nium hexafluorophosphate (Fluka, electrochemical grade)
served as the supporting electrolyte at a concentration of 0.1
M. The solutions were degassed for 5 min with bubbling N2
prior to each measurement.
Computational Studies. Hybrid density functional theory
(DFT) calculations (B3LYP/LANL2DZ) were performed using
the Gaussian 03 suite.50 Default thresholds for gradient con-
vergence were used; however, the threshold for wave function
(50) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:
(49) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583.
Wallingford, CT, 2004.

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11303
convergence was relaxed to account for the presence of a heavy
metal (CONVER = 7). All geometric optimizations were per-
formed without point group constraints. For each metal com-
plex, time-dependent DFT (TD-DFT) calculations were
performed at the optimized ground-state geometry calculating
the energy, oscillator strength, and rotory strength for each of
the 200 lowest singlet excitations. Solvent effects (in acetonitrile)
were taken into account by using the conductor polarizable
continuum model (C-PCM, acetonitrile) in Gaussian 03. The
lowest 10 triplet excitations were also obtained by using re-
stricted B3LYP calculations at the optimized ground-state
geometry. Calculation of UV-vis spectra was accomplished
using GaussSum 2.0. Electronic transitions were expanded as
Gaussian curves, with a fwhm (full width at half-maximum) for
each peak set to 0.31 eV.
Results and Discussion
Synthetic Methods. At the onset, nine novel PtIV com-
plexes were synthesized. The procedure used to obtain the
complexes includes three steps, as seen below in Scheme 1.
The first two steps include adaptations from the work of
Neidermair et al.46 and Mamtora et al.47 utilizing PtII
pendant complexes to obtain the bis-cyclometalated PtIV
dichloride intermediates. Oxidation is accomplished by
the reaction of the PtII pendant complex with a hyperva-
lent iodide reagent in dichloromethane or via hydrogen
peroxide in acetone. The final step is a modification of the
procedure used by McDaniel48 et al. to substitute the N∧N
ligand for the chlorides and arrive at the target complexes.
A combination of six different C∧N and N∧N ligands
was chosen initially (Table 1). The choice of ligands was
influenced by the desire to methodically investigate the
effects of structural modification on the excited-state pro-
perties of the molecule and elucidate the degree of che-
mical tunability that could be attained. Electron-releasing
groups on the ligand have been shown to significantly de-
stabilize orbital energies, whereas electron-withdrawing
groups demonstrate the opposite effect.2 Therefore in
addition to the parent ligands, 2-phenylpyridine (ppyH)
and 2,20-bipyridine (bpy), the methoxylated and fluor-
inated derivatives 5-methyl-2-(4-methoxyphenyl)-pyridine
(MeO-mppyH), 4,40-dimethoxy-2,20-bipyridine (dMeObpy),
5-methyl-2-(4-fluorophenyl)-pyridine (F-mppyH), and
5,50-difluoro-2,20-bipyridine (dFbpy) were employed to
study structure-property relations.
Figure 1. Excitation (black line), UV-absorption (solid line), and emis-
sion (dashed line) profiles of the [Pt(ppy)2(N∧N)](PF6)2 (top) and [Pt(F-
mppy)2(N∧N)](PF6)2 series(middle), and UV-absorptionofthe[Pt(MeO-
mppy)2(N∧N)](PF6)2 series (bottom). Two primary transitions are seen
via UV-vis spectroscopy. The structured luminescence profiles and inde-
pendence of emission maxima from the N∧N ligand indicate a 3LC
emissive state on the C∧N ligand. All spectra were measured in a 1 cm
quartz cuvette using 15 μM solutions of the complex in acetonitrile,
bubbling with N2 for 10 min prior to the measurement. Emission profiles
were obtained with an excitation wavelength of 317 nm.
Figure 1). Solvatochromic studies of [Pt(ppy)2(bpy)](PF6)2
and [Pt(ppy)2(dMeObpy)](PF6)2 showed no shift of emis-
sion maxima nor any change in the shape of the emission
profiles in a range of solvents including tetrahydrofuran,
methanol, N,N0-dimethylformamide, and chloroform in
addition to acetonitrile. Therefore it can be concluded
that the emissive state is primarily ligand centered (LC)
arising from a π-π* transition on the C∧N ligand.
Excited-state lifetimes of the complexes were found to
be on the order of microseconds, ranging from 1.08 to
14.72 μs, and the quantum yields fell between 0.05% to
1.09% (Table 2). Longer excited-state lifetimes and high-
er quantum yields were observed for the ppy series than
the F-mppy series, and a strong dependence on the N∧N
ligand was seen for both quantities in the order of dMeObpy
> bpy > dFbpy. Furthermore, under an oxygen-saturated
atmosphere, the lifetime of [Pt(ppy)2(bpy)](PF6)2 decreased
from 8.69 to 1.64 μs indicating a triplet emissive state and
thus phosphorescence. Oxygen quenching studies showed
that the six complexes exhibit similar sensitivity to oxygen;
Upon characterizing the nine complexes mentioned
above, the library was expanded by synthesizing two
more complexes utilizing the C∧N ligand 5-methyl-
2-(biphen-4-yl)-pyridine (Ph-mppyH) and the N∧N ligands
bpy and dMeObpy.
Photophysical Characterization. Several interesting
trends were observed in the photoluminescent character-
ization of the initial nine complex library. The complexes
containing the ppy and F-mppy ligands exhibited blue,
structured, steady-state luminescence, while the MeO-
mppy derivatives were non-emissive in acetonitrile at
room temperature (Figure 1). Emission maxima were sig-
nificantly blue-shifted compared to the IrIII analogues
(482 nm for [Pt(ppy)2(bpy)](PF6)2 versus 582 nm for
[Ir(ppy)2(bpy)](PF6)7) and changed only marginally (∼3nm)
upon variation of the N∧N ligand, showing a sole depen-
dence on the C∧N ligand. The absorption and excitation
spectra of [Pt(ppy)2(bpy)](PF6)2 were identical indicating
that emission was not due to an impurity (Black line,

11304 Inorganic Chemistry, Vol. 49, No. 24, 2010
Jenkins and Bernhard
Table 2. Photophysical Data for the Initial Library of Bis-Cyclometalated PtIV Complexes and Other Reference Complexes
complex
absorptiona λmax(nm) emissionb λmax(nm) knr(103s-1
)
Tb(μs)
Φc (%)
KSV[O2]b(mbar-1
)
(3a) [Pt(ppy)2(bpy)](PF6)2
(3b) [Pt(ppy)2(dFbpy)](PF6)
316 (3.08), 332 (sh)
323 (2.39), 332 (sh)
305 (2.25), 332 (sh)
316 (2.91), 336 (sh)
324 (2.70), 336 (sh)
305 (2.37), 336 (sh)
315 (3.27), 350 (2.08)
323 (2.61), 349 (1.91)
452, 482, 511 (sh)
452, 484, 511(sh)
451, 482, 511 (sh)
457, 488, 520 (sh)
460, 491, 520 (sh)
457, 489, 520 (sh)
11.7
16.8
7.35
39.3
91.7
13.1
8.69 ( 0.06
6.22 ( 0.05
0.29 ( 0.01
0.27 ( 0.01
0.024 ( 0.001
0.020 ( 0.001
0.037 ( 0.002
0.014 ( 0.001
0.008 ( 0.001
0.026 ( 0.001
(3c) [Pt(ppy)2(dMeObpy)](PF6)2
(3d) [Pt(F-mppy)2(bpy)](PF6)2
(3e) [Pt(F-mppy)2(dFbpy)](PF6)2
(3f) [Pt(F-mppy)2(dMeObpy)](PF6)2
(3 g) [Pt(MeO-mppy)2(bpy)](PF6)2
(3 h) [Pt(MeO-mppy)2(dFbpy)](PF6)2
14.72 ( 0.05 1.09 ( 0.05
2.71 ( 0.07
1.08 ( 0.07
8.28 ( 0.05
0.09 ( 0.01
0.05 ( 0.01
0.48 ( 0.02
(3i)[Pt(MeO-mppy)2(dMeObpy)](PF6)2 304 (2.25), 350 (1.72)
[Ru(bpy)3](PF6)2
[Ir(ppy)2(bpy)](PF6)
PtOEP
608d
585d
646e
1000d
2850d
12.5e
0.94d
0.33d
50e
6.20d
6.22d
37.5e
0.024 ( 0.001
0.026 ( 0.001
1.253 ( 0.055
a Absorption data were measured in acetonitrile solution at room temperature. Molar extinction coefficients (ε) in 104 M-1 cm-1 are listed in
parentheses. b Emission spectra and excited-state lifetimes were measured in acetonitrile solution at room temperature after bubbling with argon for
10 min.; lifetimes were measured with a N2 laser (3.5 ns pulse). c Quantum yields were calculated using [Ru(bpy)3](PF6)2 as the reference standard.49
d Ref 7. e Ref 51.
Figure 3. Modest solvatochromic effect is observed for the absorption
of [Pt(F-mppy)2(dMeObpy)](PF6)2. In non-polar solvents, a large degree
Figure 2. Stern-Volmer plot of [Pt(ppy)2(N∧N)](PF6)2 derivatives.
Measurements were taken in a 1 cm quartz cuvette using 15 μM solutions
of the complex in acetonitrile, bubbling with Argon/Oxygen mixtures for
10min prior tothe measurement with anexcitationwavelength of317 nm.
of separation is seen between the 1LLCT and 1LC states; however, as
solvent polarity is increased the 1LLCT band is lowered in energy and the
separation decreases.
Stern-Volmer constants were on the order of 0.01 to 0.04
mbar-1. A Stern-Volmer plot of the [Pt(ppy)2(N∧N)](PF6)2
series is shown in Figure 2.
the N∧N ligand or a mixed ligand-metal to ligand charge
transfer depending upon the degree to which the metal
orbitals are involved. The unstructured band is assigned
to a ligand-centered (LC) transition on the C∧N ligand.
Electrochemical Properties. Characterization of the
PtIV complexes by cyclic voltammetry showed contrast-
ing and unique electrochemical behavior compared to
other analogous heteroleptic cyclometalated transition
metal complexes,4,5,12,52 most of which exhibit one-elec-
tron, metal-centered oxidation and one-electron, ligand-
centered reduction. Despite the difference in ligand struc-
ture, all PtIV complexes studied showed comparable redox
behavior (Table 3). While oxidation to PtV was not obser-
vable in the electrochemical window of the solvent, each
complex exhibited an irreversible reduction wave around
-1.0 V versus SCE that remained irreversible at scan rates
as high as 10 V/s for complex (3a), which was followed by
a quasi-reversible reduction near -1.96 V versus SCE.
To understand this redox behavior it is important to
note that species of PtIII are notoriously unstable because
of the occupation of a high energy antibonding eg*-type
orbital. Additionally, neutral bis-cyclometalled PtII
complexes such as Pt(ppy)2 also exhibit quasi-reversible
reduction at ∼ -1.96 V versus SCE.4,53 Therefore the
Characterization of the nine complexes by absorption
spectrophotometry showed two primary transitions: a
sharper, structured band at higher energies and a broad,
unstructured band at lower energies (Figure 1). The un-
structured band was partially or mostly obscured by the
structured band in the ppy and F-mppy complexes, but
was distinctly separate in the spectra of the MeO-mppy
complexes. Across a C∧N series, the structured band was
shifted to higher energies as the donor strength of the
N∧N ligand increased, while the unstructured band was
unaffected by the change. Additionally, when the C∧N
ligand was varied across a N∧N series, the position of the
unstructured transition was shifted, while no effect upon
the structured transition was observed.
The nature of these absorption transitions was further
probed in solvents of differing polarity. For [Pt(ppy)2-
(bpy)](PF6)2 and [Pt(F-mppy)2(dMeObpy)](PF6)2, as sol-
vent polarity increased, the structured transition was red-
shifted with regards to the unstructured transition, and
the separation between the two bands therefore decrea-
sed. Figure 3 shows the solvatochromic effect upon the
absorption profile of [Pt(F-mppy)2(dMeObpy)](PF6)2.
On the basis of these data, it is reasonable to assign the
structured band to a charge-separated state, either a ligand
to ligand charge transfer (LLCT) from the C∧N ligand to
(51) (a) Bansal, A. K.; Holzer, W.; Penzkofer, A.; Tsuboi, T. Chem. Phys.
2006, 330, 118. (b) Dienel, T.; Prohel, H.; Fritz, T.; Leo, K. J. Lumin. 2004,
110, 253.

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11305
Table 3. Electrochemical Data for the Initial Library of Bis-Cyclometalated PtIV Complexesa
complex
EpcPtIV/PtII (V vs SCE)
E1/2PtII(ppy)2 (V vs SCE)
ΔEp (mV)
(3a) [Pt(ppy)2(bpy)](PF6)2
(3b) [Pt(ppy)2(dFbpy)](PF6)2
-1.043
-0.986
-1.122
-0.982
-0.933
-1.063
-1.081
-1.017
-1.155
-1.965
-1.964
-1.975
-1.970
-1.973
-1.964
-2.082
-2.100
n/a
84
75
119
99
(3c) [Pt(ppy)2(dMeObpy)](PF6)2
(3d) [Pt(F-mppy)2(bpy)](PF6)2
(3e) [Pt(F-mppy)2(dFbpy)](PF6)2
(3f) [Pt(F-mppy)2(dMeObpy)](PF6)2
(3 g) [Pt(MeO-mppy)2(bpy)](PF6)2
(3 h) [Pt(MeO-mppy)2(dFbpy)](PF6)2
(3i) [Pt(MeO-mppy)2(dMeObpy)](PF6)2
79
102
145
133
n/a
a Measurements were taken of ∼400 μM solutions of the complex in acetonitrile after bubbling with N2 for five minutes. Tetrabutylammonium
hexafluorophosphate (TBAH) was used as the supporting electrolyte at a concentration of 0.1M. The experimental apparatus included a 1 mm2 platinum
working electrode, a platinum coil counter electrode, and a silver wire pseudo-reference electrode; ferrocene was used as an internal standard referenced
against SCE at 0.371 V.
electrochemical data are most likely explained by the
reduction of the PtIV to PtII (either by way of dispropor-
tionation of the transient PtIII species or two subsequent
one-electron transfers to PtIV) observed at ∼ -1 V versus
SCE. This process triggers the fast loss of the N∧N ligand
to form the bis-cyclometaled PtII complex, the reduction
of which is seen at ∼ -1.96 V versus SCE (Figure 4). The
formation of a PtIII complex is not altogether impossible,
as a few stable examples, most notably dimers, are recorded
in the literature.54,55 However, the evidence argues more
strongly for the formation of the PtII complex. This is in
agreement with the previously studied redox behavior of
octahedral PtIV amine-halide compounds whereby two
coordination sites are vacated and a square planar geo-
metry is adopted to accommodate the resulting d8 PtII
center. This process is commonly seen in the activation of
PtIV anticancer agents.56-58 Furthermore, Sargeson et al.
have reported on [Pt(tame)2]4þ, a complex with a related
structure, that undergoes the same conformational change
upon reduction.59
It should also be noted that the potential required for
the reduction of the PtIV center within a particular C∧N
series increases as a function of the donor ability of the
Figure 4. Cyclic voltammogram (top) of [Pt(ppy)2(bpy)](PF6)2 and the
corresponding electrochemical reactions (bottom). (A) Reduction of
N∧N ligand in the order of dFbpy < bpy < dMeObpy.
[Pt(ppy)2(bpy)](PF6)2 accompanied by loss of the N∧N ligand, (B)
[Pt(ppy)2]0/[Pt(ppy)2]- redox couple.
This variation corresponds to an increasing bond strength
between the N∧N ligand and Pt and correlates with the
raised energy of the eg* orbital. A similar trend was also
observed for luminescence intensity and quantum yield
similar complexes containing a heavy metal and organic
ligands was previously explored and proven to be a suit-
able method of computation with regards to the inclusion
of the metal center. Coughlin et al. compared time-depen-
dent computations using the LANL2DZ and 6-31G(d)
basis sets for a Zn hemicage complex and found that, con-
trary to initial supposition, the larger basis set did not
yield better results.60 Indeed, LANL2DZ modeled the
π-π* transition energies more accurately and was thus
utilized for this study. Harmonic frequency calculations
of [Pt(ppy)2(bpy)]2þ confirmed that the optimized struc-
ture was a minimum (see Supporting Information).
Orbital population analysis utilizing the singlet and
triplet geometries indicated that luminescence should
arise from a Ligand to Ligand Charge Transfer (LLCT)
state, in which the highest occupied molecular orbital
(HOMO) resides primarily on the C∧N ligand and the
lowest unoccupied molecular orbital (LUMO) involves
3
indicating that the non-emissive MC state is rendered
less accessible for thermal population.1
Computational Investigation. Geometry optimizations
for both the singlet and the triplet states were performed
on the nine PtIV compounds from the initial library using
DFT calculations. The use of B3LYP/LANL2DZ for
(52) Costa, R. D.; Viruela, P. M.; Bolink, H. J.; Orti, E. J. Mol. Struct.
THEOCHEM. 2009, 912, 21.
(53) Chassot, L.; Muller, E.; Von Zelewsky, A. Inorg. Chem. 1984, 23,
4249.
(54) Ghavale, N.; Wadawale, A.; Dey, S.; Jain, V. K. J. Organomet.
Chem. 2010, 695, 1237.
(55) Yamaguchi, T.; Kubota, O.; Ito, T. Chem. Lett. 2004, 33(2), 190.
(56) Choi, S.; Filotto, C.; Bisanzo, M.; Delaney, S.; Lagasee, D.; Whitworth,
J. L.; Jusko, A.; Li, C.; Wood, N. A.; Willingham, J.; Schwenker, A.;
Spaulding, K. Inorg. Chem. 1998, 37, 2500.
(57) Choi, H.-K.; Terzis, A.; Stevens, R. C.; Bau, R.; Haugwitz, R.;
Narayanan, V. L.; Wolpert-DeFilippes, M. Biochem. Biophys. Res. Commun.
1988, 156(3), 1120.
(58) Choi, S.; Mahalingaiah, S.; Delaney, S.; Neale, N. R.; Masood, S.
Inorg. Chem. 1999, 38, 1800.
(59) Brown, K. N.; Hockless, D. C. R.; Sargeson, A. M. J. Chem. Soc.,
Dalton Trans. 1999, 2171.
(60) Coughlin, F. J.; Oyler, K. D.; Pascal, R. A., Jr.; Bernhard, S. Inorg.
Chem. 2008, 47, 974.

11306 Inorganic Chemistry, Vol. 49, No. 24, 2010
Jenkins and Bernhard
Figure 6. Experimental and calculated absorption spectra of [Pt(ppy)2-
(bpy)]2þ. Calculated oscillator strengths are shown as green vertical lines;
thetransitiontothe excited statesS1 (the HOMO-LUMOtransition)and
S2 cannot be seen because of low oscillator strength. The notable excited
states S3 and S15, described in the text, are labeled above.
Figure 5. Energy level diagramshowing the first10excited states in both
the singlet and the triplet manifolds for [Pt(ppy)2(bpy)]2þ. The lowest
singly occupied molecular orbital (LSOMO) and highest singly occupied
molecular orbital (HSOMO) differ from the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO),
respectively, because of intersystem crossing to the 3LC state in lieu of the
3LLCT.
almost exclusively the N∧N ligand, both having slight
metal character (Figure 5).
Clearly, since the experimental data revealed that the
N∧N ligand does not affect the emission energy of the
complexes, its frontier orbitals do not strongly contri-
bute to the emissive state. Therefore TD-DFT calcula-
tions were undertaken to model UV-vis absorption
and investigate the discrepancy between the experimen-
tally observed photophysical properties and the initial
computations.
Figure 7. (3j) [Pt(Ph-mppy)2(bpy)](PF6)2 and (3k) [Pt(Ph-mppy)2-
(dMeObpy)](PF6)2.
The absorption spectrum obtainedfrom time-dependent
computations in the singlet state carried out on [Pt(ppy)2-
(bpy)]2þ closely resembled the experimental trace (Figure 6).
1
A LLCT state, involving the HOMO and LUMO, was
found again to be the lowest energy excited state (S1) as
shown by static calculations. However, the oscillator
strength for this transition was low ( f=0.0014). The first
transition with appreciably high oscillator strength in-
1
volved S3 (f = 0.1091) which was predominately a LC
π f π* transition on the C∧N ligand, and corresponded
to the lower energy shoulder of the primary absorption
band. The main transition of this band corresponding to
S15 (f = 0.3574) was predicted to be a charge transfer
mainly from the C∧N ligand to the N∧N ligand with a
slight degree of metal character.
In contrast, time-dependent triplet calculations showed
the lowest triplet state (T1) to be 3LC in character invol-
ving the C∧N ligand and having a large singlet-triplet
stabilization energy of 0.58 eV. The triplet state asso-
ciated with S1 was the fourth lowest in energy (T4) and
was 0.57 eV higher in energy than T1. Thus, the computa-
tional data corroborate the experimental results. So in
describing the excited state dynamics of this family of
complexes it can be concluded that after absorption takes
place into a mixed 1LC-1LLCT state, intersystem crossing
occurs into the 3LC state from which phosphorescence is
observed.
Figure 8. Relative intensities of emission of [Pt(ppy)2(bpy)](PF6)2,
[Pt(Ph-mppy)2(bpy)](PF6)2, and [Pt(Ph-mppy)2(dMeObpy)](PF6)2.
C∧N ligand 5-methyl-2-(biphen-4-yl)pyridine (Ph-mppyH)
for its extended π-system in hopes of increasing quantum
yield and influencing the emission color . Derivatives in-
corporating bpy and dMeObpy were synthesized as be-
fore (Figure 7) and photophysical characterization yielded
results that highlight the complexity of the interaction
between excited states in these luminophores.
Luminescence profiles of the Ph-mppy derivatives were
similar to the six previously described emissive PtIV com-
plexes exhibiting a structured emission arising from a
π fπ*transitionontheC∧N ligand. Quantum yields, 1.7%
and 3.5% for the bpy and dMeObpy complexes respec-
tively, out-shone the luminophores of the initial library
(∼1%) and are comparable to [Ru(bpy)3]2þ derivatives
and [Ir(ppy)2(bpy)]þ. The emission maxima were strongly
red-shifted to 551 nm (Figure 8), while lifetimes increased
more than 11-fold to 163 and 260 μs for the bpy and
dMeObpy complexes, respectively (Table 4), which is in
stark contrast to the Energy Gap Law. A plot of the
natural log of the non-radiative rate constant (knr) versus
Expansion of the Library. Additional synthetic work
was undertaken to further examine the excited-state
characteristics and extent of possible influence upon
quantum yield in this new family of PtIV complexes.
The library was expanded to include complexes of the

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11307
Table 4. Photophysical Data for the Expanded Library
complex
absorptiona λmax(nm) emissionbλmax(nm) knr(103s-1
)
Tb(μs)
Φc (%)
KSV[O2]b(mbar-1
)
(3j) [Pt(Ph-mppy)2(bpy)](PF6)2 316 (2.85), 348 (2.66)
(3k) [Pt(Ph-mppy)2(dMeObpy)](PF6)2 290 (3.29), 348 (2.73)
515, 551, 593 (sh)
513, 551, 593 (sh)
0.603
0.371
163.10 ( 0.03 1.84 ( 0.09
259.65 ( 0.02 3.50 ( 0.18
1.84 ( 0.09
2.12 ( 0.13
a Absorption data were measured in acetonitrile solution at room temperature. Molar extinction coefficients (ε) in 104 M-1 cm-1 are listed in
parentheses. b Emission spectra and excited-state lifetimes were measured in acetonitrile solution at room temperature after bubbling with argon for 10
min; lifetimes were measured with a N2 laser (3.5 ns pulse). c Quantum yields were calculated using [Ru(bpy)3](PF6)2 as the reference standard.49
Table 5. Electrochemical Data for the Heteroleptic PtIV Bis-Cyclometalated
Ph-mppy complexesa
E
pcPtIV/PtII
E
1/2PtII(ppy)2 ΔEp
(V vs SCE) (V vs SCE) (mV)
complex
(3j) [Pt(phmppy)2(bpy)](PF6)2
(3k) [Pt(phmppy)2(dmeobpy)](PF6)2
-1.027
-1.050
-1.921
-1.918
67
83
a Measurements were taken of 400 μM solutions of the complex in
acetonitrile after bubbling with N2 for 5 min. Tetrabutylammonium
Hexafluorophosphate (TBAH) was used as the supporting electrolyte at
a concentration of 0.1 M. The experimental apparatus included a 1 mm2
platinum working electrode, a platinum coil counter electrode, and a
silver wire pseudo-reference electrode; ferrocene was used as an internal
standard referenced against SCE at 0.371 V.
Figure 9. Energy-gap plotofthe eight emissive PtIV complexes. The blue
diamonds represent the complexes from the original library. The red
trianglesrepresent the Ph-mppy complexes, the change inthe natureofthe
excited state of which is evident from their non-adherence to the Energy-
Gap Law.
Figure 11. Stern-Volmer plot of [Pt(Ph-mppy)2(dMeObpy)](PF6)2,
PtOEP, and [Ru(bpy)3](PF6)2. [Pt(Ph-mppy)2(dMeObpy)](PF6)2 is shown
to demonstrate superior quenching. Measurements were taken in a 1 cm
quartz cuvette using 15 μM solutions of the complex in acetonitrile,
bubbling with Argon/Oxygen mixtures for 10 min prior to the measure-
ment with an excitation wavelength of 317 nm.
Further elucidation by the study of structure/property
relationships and additional computations is ongoing.
Electrochemical characterization provided more confir-
mation to the trends observed previously (Table 5), while
solvatochromic studies of [Pt(Ph-mppy)2(dMeObpy)](PF6)2
once again showed no shift of emission maxima in the same
range of solvents that were used for earlier studies.
Figure 10. Orbitals depicting the predicted emissive 3ILCT state of
[Pt(Ph-mppy)2(dMeObpy)]2þ. Left: lowest singly occupied molecular orbital
(LSOMO); Right: highest singly occupied molecular orbital (HSOMO).
emission energy for all eight emissive PtIV complexes
documenting this change is shown in Figure 9. The six
original luminophores adhere to the expected trend of
the Energy Gap Law, while the two Ph-mppy complexes
do not.
It was discovered that [Pt(Ph-mppy)2(bpy)](PF6)2 (3j)
and [Pt(Ph-mppy)2(dMeObpy)](PF6)2 (3k) were highly
sensitive to oxygen quenching; at oxygen concentrations
of 5% only minimal luminescence can be observed for
either complex. Therefore, their rates of quenching were
compared to that of [Ru(bpy)3](PF6)2 and platinumII 2,3,7,-
8,12,13,17,18-octaethyl-21H,23H-porphyrin (PtOEP), two
standards in the field of luminescent sensors18,51 (Figure 11).
[Ru(bpy)3](PF6)2 and PtOEP demonstrate long phosphor-
escence lifetimes (0.938 and 50 μs respectively) and sensitivity
Initial DFT calculations performed on [Pt(Ph-mppy)2-
(dMeObpy)](PF6)2 also predict a different emissive state
than that of [Pt(ppy)2(bpy)](PF6)2. Static and time-de-
pendent results of the singlet state showed similar frontier
orbitals to those of the latter. However, time-dependent
calculations of the triplet state show the lowest energy
excited state not to be a simple 3LC state, but an intrali-
gand charge transfer state (3ILCT) where the LSOMO
resides on the two phenyl rings and the HSOMO resides
on the pyridine and cyclometalated phenyl rings (Figure 10).
This change in the nature of the excited state is a possible
and likely explanation for why the Ph-mppy complexes
exhibit such different luminescent properties and do not
adhere to the Energy Gap Law as do the other complexes.
to oxygen quenching (KSV=0.024 mbar-1 and 1.254 mbar-1
,
respectively). It was found that (3j) and (3k) demonstrate
superior quenching to that of either [Ru(bpy)3](PF6)2 or
PtOEP with KSV values of 1.796 mbar-1 and 2.126 mbar-1
respectively. This is due primarily to the longer lifetimes
and favorable energy-transfer energetics of these complexes,

11308 Inorganic Chemistry, Vol. 49, No. 24, 2010
Jenkins and Bernhard
3
both of which are essential for application in sensor
technology.
of the LLCT state, the lowest-lying excited state initially
expected to be the source of phosphorescence.
The initial library of complexes showed a long-livedexcited
state, upward of 15 μs and quantum yields reaching 1%. The
elucidated structure-property relationships were used to
synthetically change the emission properties of two addi-
tional complexes. A shift of emission energies of 70 nm and
an 11-fold increase in quantum yield compared to [Pt(ppy)2-
(bpy)](PF6)2 was attained in the Ph-mppy derivatives. More-
over, the long-lived triplet states (lifetimes as high as 260 μs)
of these complexes demonstrate a high sensitivity to oxygen
quenching with KSV values up to 2.12 mbar-1. The discov-
ered structure-property relationship is now being judi-
ciously explored to tailor materials for optical oxygen sen-
sors, LECs, and other luminescence-based applications.
Conclusions
Presented is a library of the first heteroleptic bis-cyclome-
talated PtIV complexes of the architecture [Pt(C∧N)2(N∧N)]-
(PF6)2 which have been fully characterized with photophysi-
cal, electrochemical, and spectroscopic methods. Because of
the large ligand field splitting of the PtIV ion, the photo-
luminescence of the complexes is highly blue-shifted exhibit-
ing emission maxima as low as 482 nm. The most prominent
feature in the cyclic voltammogram is an irreversible reduc-
tion from PtIV to PtII, leading to ligand dissociation, followed
by a one-electron reduction of the resulting bis-cyclometa-
lated PtII species. The LC character of the excited state,
3
Supporting Information Available: Further details about the
synthetic procedures and characterizations, H NMR spectra
observed in photophysical measurements, was elucidated by
static and time-dependent density-functional theory calcula-
tions which showed that the singlet-triplet stabilization
1
for complexes (3a)-(3k), information about the DFT calcula-
tions, and complete citation of reference 50. This material is
available free of charge via the Internet at http://pubs.acs.org.
3
energy for the LC state was significantly greater than that