Controlling emission energy, self-quenching, and excimer formation in highly luminescent N∧C∧N-coordinated platinum(II) complexes

Article abstract of DOI:10.1021/ic051049e

A series of cyclometalated platinum(II) complexes have been prepared, [PtLηCl], containing N∧C∧N-coordinating, terdentate ligands based on 1,3-dipyridylbenzene (HL¹), incorporating aryl substituents at the central 5 position of the ligand. All of the new complexes are intensely luminescent in a degassed solution at 298 K (φ = -0.46-0.65 in CH ₂Cl₂) with lifetimes in the microsecond range (7.9-20.5 μs). The introduction of the aryl substituents leads to a red shift in the lowest-energy, intense charge-transfer absorption band compared to [PtL ¹Cl] (401 nm in CH₂Cl₂), in the order H < mesityl < 2-pyridyl < 4-tolyl < 4-biphenylyl < 2-thienyl < 4-(dimethylamino)phenyl (431 nm in CH₂Cl₂), which correlates with the decreasing order of oxidation potentials. A similar order is also observed in the emission maxima, ranging from 491 nm for [PtL ¹Cl] to 588 nm for the 4-(dimethylamino)phenyl-substituted complex. The emission spectra of all of the complexes, except for the amino-substituted compound, are highly structured in a dilute solution in CH₂Cl ₂, and the emission is assigned to excited states of primarily ³LC (ligand-centered) character. At higher concentrations, self-quenching accompanied by structureless excimer emission centered at 700 nm is observed, but the aryl groups attenuate the self-quenching compared to the parent compound [PtL¹Cl], particularly for the most sterically hindered mesityl complex. The introduction of the strongly electron-donating 4-dimethylamino substituent leads to a switch in the nature of the lowest-energy excited state from ³LC to one of primarily intraligand charge-transfer (ILCT) character in CH₂Cl₂: this complex displays a structureless and much broader emission band than the other compounds and a high degree of positive solvatochromism. No excimer emission is observed in CH₂Cl₂, and self-quenching is an order of magnitude lower than that for the other complexes. However, in nonpolar solvents such as CCl₄, the ILCT state is destabilized, such that the ³LC remains the lowest-energy excited state. Reversible switching between the ILCT and ³LC states can also be achieved in a CH₂Cl₂ solution by protonation of the amine, with an accompanying large change in the emission maxima of >100 nm. The X-ray structures of the biphenylyl- and methyl-substituted complexes are reported, together with those of the 2-pyridyl- and mesityl-substituted ligands and the key synthetic intermediate 1-bromo-3,5-di(2-pyridyl)benzene.

Full text of DOI:10.1021/ic051049e

Inorg. Chem. 2005, 44, 9690−9703
Controlling Emission Energy, Self-Quenching, and Excimer Formation in
Highly Luminescent N N-Coordinated Platinum(II) Complexes
∧C∧
Sarah J. Farley, David L. Rochester, Amber L. Thompson, Judith A. K. Howard, and
J. A. Gareth Williams*
Department of Chemistry, UniVersity of Durham, South Road, Durham DH1 3LE, U.K.
Received June 25, 2005
A series of cyclometalated platinum(II) complexes have been prepared, [PtLnCl], containing N
∧
C N-coordinating,
∧
terdentate ligands based on 1,3-dipyridylbenzene (HL1), incorporating aryl substituents at the central 5 position of
the ligand. All of the new complexes are intensely luminescent in a degassed solution at 298 K (φ ) 0.46
−0.65
in CH2Cl2) with lifetimes in the microsecond range (7.9 20.5 s). The introduction of the aryl substituents leads to
−
µ
a red shift in the lowest-energy, intense charge-transfer absorption band compared to [PtL1Cl] (401 nm in CH2Cl2),
in the order H < mesityl < 2-pyridyl < 4-tolyl < 4-biphenylyl < 2-thienyl < 4-(dimethylamino)phenyl (431 nm in
CH2Cl2), which correlates with the decreasing order of oxidation potentials. A similar order is also observed in the
emission maxima, ranging from 491 nm for [PtL1Cl] to 588 nm for the 4-(dimethylamino)phenyl-substituted complex.
The emission spectra of all of the complexes, except for the amino-substituted compound, are highly structured in
a dilute solution in CH2Cl2, and the emission is assigned to excited states of primarily 3LC (ligand-centered) character.
At higher concentrations, self-quenching accompanied by structureless excimer emission centered at 700 nm is
observed, but the aryl groups attenuate the self-quenching compared to the parent compound [PtL1Cl], particularly
for the most sterically hindered mesityl complex. The introduction of the strongly electron-donating 4-dimethylamino
3
substituent leads to a switch in the nature of the lowest-energy excited state from LC to one of primarily intraligand
charge-transfer (ILCT) character in CH2Cl2: this complex displays a structureless and much broader emission
band than the other compounds and a high degree of positive solvatochromism. No excimer emission is observed
in CH2Cl2, and self-quenching is an order of magnitude lower than that for the other complexes. However, in
3
nonpolar solvents such as CCl4, the ILCT state is destabilized, such that the LC remains the lowest-energy excited
state. Reversible switching between the ILCT and 3LC states can also be achieved in a CH2Cl2 solution by protonation
of the amine, with an accompanying large change in the emission maxima of >100 nm. The X-ray structures of the
biphenylyl- and methyl-substituted complexes are reported, together with those of the 2-pyridyl- and mesityl-substituted
ligands and the key synthetic intermediate 1-bromo-3,5-di(2-pyridyl)benzene.
Introduction
excited states of different orbital origin according to the
concentration and proximity of the molecules,5,6 and to
chemical and photochemical reactivity.7,8 Potential applica-
In the field of inorganic photochemistry, the coordinative
unsaturation of square-planar platinum(II) complexes gives
rise to the possibility of ground- and excited-state interactions
that are not possible in octahedral and tetrahedral metal
complexes. Such axial interactions can lead, inter alia, to
self-quenching and cross-quenching, including excimer and
exciplex formation,1-4 to aggregation and switching between
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* To whom correspondence should be addressed. E-mail: j.a.g.williams@
durham.ac.uk.
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9690 Inorganic Chemistry, Vol. 44, No. 26, 2005
10.1021/ic051049e CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/12/2005

N∧C∧N-Coordinated Platinum(II) Complexes
tions arising from such interactions include chemical sensing
and vapochromism7 and the development of molecular probes
for biological macromolecules.9 In addition, the triplet nature
of the luminescence of platinum(II) metal complexes,
promoted by the high spin-orbit coupling constant of the
metal ion, renders the most emissive complexes of interest
for use in optoelectronics, for example, as triplet-harvesting
agents in organic light-emitting devices (OLEDs).10
pyridyl ligand can be replaced by a cyclometalating analogue
because the very strong ligand field induced by the cyclo-
metalated carbon again serves to raise the energy of the
deactivating d-d states. This strategy has led to a variety of
emissive complexes, including several containing N∧C-
coordinated ligands such as 2-phenylpyridine.8c,10a,d,18,19
A
rich chemistry of platinum(II) with cyclometalating terdentate
ligands has also been developed.20-24 The platinum(II)
complexes of N∧N∧C-binding ligands20 [e.g., 6-phenyl-2,2′-
bipyridine (phbpy) and analogues] display 3MLCT emission,
which has been explored in a variety of applications,
particularly by Che and co-workers.6,10c,21 Bis-cyclometalated,
C∧N∧C-coordinated platinum(II) complexes (e.g., with 2,6-
Many simple platinum(II) complexes, such as [Pt-
(bpy)2Cl2], are either nonemissive or only very weakly
luminescent at room temperature, owing to the presence of
low-lying metal-centered (d-d) excited states, which are
severely distorted compared to the ground state and hence
subject to efficient nonradiative decay.11 Three distinct
strategies have emerged to promote luminescence from
pyridyl-based platinum(II) complexes. Che et al. have shown
that the introduction of aryl substituents at the 4′ position of
the terpyridine ligand in [Pt(tpy)Cl]+ leads to emission in a
fluid solution at room temperature, with lifetimes on the
microsecond time scale.12 The influence of the substituents
is most pronounced for the most electron-rich groups.
McMillin and co-workers have interpreted a similar effect
of polycyclic aromatic substituents, such as 9-phenanthrenyl
and 1-pyrenyl, in terms of a switch from the usual metal-
to-ligand charge-transfer state (3MLCT) to a lower-energy
state with significant aryl-to-tpy intraligand charge transfer
(1,3ILCT).13 The ILCT state is not subject to efficient
nonradiative decay via the higher-lying d-d state, and a
luminescence lifetime as long as 64 µs has been observed
for the 1-pyrenyl-substituted complex.13b,c The other two
approaches seek to increase the ligand field strength at the
metal in order to raise the energy of the deactivating d-d
state and hence reduce its influence. This may be achieved
through the replacement of the chloride ligands in, for
example, [Pt(tpy)Cl]+ or [Pt(bpy)Cl2], by a strong-field ligand
such as an acetylide -CtCR, leading to complexes that are
emissive at room temperature.14-17 Alternatively, the poly-
3
diphenylpyridine) display emission from a LC (ligand-
centered) state.22
In the case of N∧C∧N-coordinated platinum(II) com-
plexes, two distinct types of behavior have emerged. Connick
and co-workers have observed low-energy, metal-centered
emission at 77 K from complexes with cyclometalating
“pincer” ligands, in which the lateral coordinating nitrogen
atoms are aliphatic amines [e.g., the 2,6-bis(piperidylmethyl)-
phenyl anion].23 On the other hand, we recently discovered
that the N∧C∧N coordination of platinum(II) by 1,3-
dipyridylbenzene (HL1) leads to a highly luminescent
complex, [PtL1Cl] (Chart 1), which emits from a 3LC state.24
Two simple derivatives incorporating alkyl or ester substit-
uents at the 5 position of the central benzene ring were also
prepared (Chart 1), displaying small red and blue shifts of
the emission, respectively, when compared to the parent
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Inorganic Chemistry, Vol. 44, No. 26, 2005 9691

Farley et al.
Scheme 1
Chart 1
Results and Discussion
Synthesis and Characterization of Ligands. The 5-aryl-
substituted 1,3-dipyridylbenzene ligands were prepared from
1,3,5-tribromobenzene (tbb) using different sequences of
cross-coupling reactions, as shown in Scheme 1. The C3-
symmetric ligand 1,3,5-tris(2-pyridyl)benzene, HL4, was
prepared readily in one step by palladium-catalyzed Stille
cross-coupling of tbb with an excess of 2-(tri-n-butylstannyl)-
pyridine in toluene (Scheme 1, route A). All of the other
ligands have 2-fold symmetry, and their syntheses require
the selective substitution of two of the three bromine atoms
of tbb with pyridyl moieties and the third bromine atom with
the appropriate aryl group. Previous syntheses of 5-aryl-1,3-
dipyridylbenzenes have proceeded via the intermediate
1-bromo-3,5-di(2-pyridyl)benzene (dpyb-Br), obtained by
cross-coupling of tbb with 2 equiv of (trialkylstannyl)-
pyridine25a or with a related organozinc species prepared in
situ (Negishi-type coupling);25b,c the remaining bromine atom
is then available for further cross-coupling to introduce the
requisite aryl group. This route was employed here to prepare
the mesityl- and biphenylyl-substituted ligands HL5 and HL6
by reaction of dpyb-Br with mesitylboronic acid and 4-bi-
phenylylboronic acid, respectively, in a Suzuki-type cross-
coupling (Scheme 1, route B). Previous work on the synthesis
complex. The luminescence quantum yields of 0.58-0.68
and lifetimes of 7.0-8.0 µs (in degassed CH2Cl2) are an order
of magnitude greater than those for analogous Pt(N∧N∧C)-
Cl complexes, attributed to the very short Pt-C bond in the
N∧C∧N-coordinated complexes.24 This serves to further
raise the energy of the d-d excited state, effectively severing
completely this pathway of nonradiative deactivation.
In this paper, we describe the synthesis and luminescence
properties of a new series of N∧C∧N-coordinated complexes
incorporating pendent aryl groups at the 5 position and
demonstrate that the introduction of such substituents not
only enables the excited-state energy to be varied over a wide
range of at least 5000 cm-1, while maintaining the excep-
tionally high quantum yields, but also allows the rate of self-
quenching and extent of excimer formation to be controlled.
The unique luminescence properties of a p-(dimethylamino)-
phenyl-substituted complex, which are profoundly modified
upon protonation of the amine, are also described.
(25) (a) Beley, M.; Chodorowski, S.; Collin, J.-P.; Sauvage, J.-P. Tetra-
hedron Lett. 1993, 34, 2933. (b) Jouaiti, A.; Geoffroy, M.; Collin,
J.-P. Inorg. Chim. Acta 1996, 245, 69. (c) Chavarot, M.; Pikramenou,
Z. Tetrahedron Lett. 1999, 40, 6865.
9692 Inorganic Chemistry, Vol. 44, No. 26, 2005

N∧C∧N-Coordinated Platinum(II) Complexes
of dpyb-Br by Stille coupling reported yields of 35%.25a It
was noted during the present work that this compound is
susceptible to competitive hydro-debromination under the
Stille conditions. Thus, the proportion of the desired product
in the reaction mixture initially increases as the Stille
coupling proceeds but then begins to fall off as debromination
occurs to give HL1. The optimal reaction time was found to
be about 20-24 h, leading to yields of 50-55%. Significant
quantities of HL1 side product were isolated when longer
reaction times were employed.
An alternative approach of monosubstituting the tbb
starting material with the requisite aryl group by Suzuki
coupling, prior to reaction with the stannylpyridine, was also
investigated and shown to be a viable alternative. Ligands
HL7, HL8, and HL9 were prepared in this way (Scheme 1,
route C): palladium-catalyzed Suzuki cross-coupling of tbb
with the appropriate arylboronic acid, and chromatographic
purification to remove disubstituted material and unreacted
tbb, was followed by Stille coupling with 2-(tri-n-butyl-
stannyl)pyridine.
The ligands were characterized by two-dimensional 1H and
13C NMR spectroscopy, mass spectrometry, and elemental
1
analysis. Complete assignment of H NMR spectra was
possible using 1H-1H NOESY spectroscopy, which crucially
allows resonances on the pendent ring to be assigned on the
basis of the through-space coupling of Hb to H4′ (for ligands
HL6-HL9; the atom-numbering system is shown in Scheme
1).
(a) Crystal Structures of dpyb-Br, HL4, and HL5. Single
crystals of these compounds suitable for X-ray diffraction
analysis were obtained by the slow evaporation of a
dichloromethane solution containing the ligand. The crystal
structure of dpyb-Br (Figure 1a) shows a head-to-tail
disposition of the two pyridyl rings. This configuration may
be adopted to minimize unfavorable electrostatic interactions
between the lone pairs and resembles the transoid arrange-
ment of the pyridine units about the interannular bonds in
2,2′:6′,2′′-terpyridines.26,27 However, both nitrogen atoms
participate in weak intermolecular C-H‚‚‚N interactions,
which may also influence the configuration [C‚‚‚N ) 3.407-
(3) and 3.495(3) Å for C11-H11‚‚‚N1 and C14-H14‚‚‚N2,
respectively]. The three aromatic rings are not quite coplanar,
with angles of 8.34(11)° and 26.35(8)° between the planes
of the lateral pyridyl rings and that of the central benzene
ring. There are no other examples of uncoordinated 1,3-di-
(2-pyridyl)benzenes in the Cambridge Crystallographic Da-
tabase to enable structural comparisons to be performed.
However, the structure of 1,3-di(quinolin-2-yl)benzene dis-
plays a similar head-to-tail arrangement of the quinolyl
groups, albeit with larger angles of 41.1 and 57.5° between
the planes of the quinolyl rings and the central aryl ring.28
Figure 1. Molecular structures of the key synthetic intermediate dpyb-Br
(a) and of ligands HL4 (b) and HL5 (c) at 120 K, with displacement ellipsoids
at the 50% level.
X-ray diffraction analyses of single crystals of HL4 and HL5
reveal similar features (parts b and c of Figure 1, respec-
tively). In the case of HL4, there are three pyridyl groups,
and all three adopt a head-to-tail arrangement (i.e., with a
pseudo C3 axis through the center of the benzene ring). The
pyridyl rings are not quite coplanar with the central benzene
ring, with the angles being 15.99(7), 24.58(6), and
18.91(6)°. As for dpyb-Br, there are also very weak
C-H‚‚‚N interactions [the strongest is C22-H22‚‚‚N3 )
3.516(2) Å]. In the case of HL5, the mesityl group is close
(26) For example, see: (a) Constable, E. C.; Lewis, J.; Liptrot, M. C.;
Raithby, P. R. Inorg. Chim. Acta 1990, 178, 47. (b) Constable, E. C.;
Khan, F. A.; Raithby, P. R.; Marquez, V. E. Acta Crystallogr., Sect.
C 1992, C48, 932.
(27) Leslie, W.; Batsanov, A. S.; Howard, J. A. K.; Williams, J. A. G.
Dalton Trans. 2004, 623.
(28) Jones, P. G.; Dubenitschek, P.; Dyker, G.; Nouroozian, M. Z.
Kristallogr. 1997, 212, 87 (CSD No. 402930).
Inorganic Chemistry, Vol. 44, No. 26, 2005 9693

Farley et al.
to perpendicular to the central ring with an angle of
80.53(4)°, as would be expected because of the increased
steric bulk of the substituent. However, the pyridyl rings are
also more inclined than those in dpyb-Br and HL4, with
angles of 34.64(6) and 30.06(6)° between the lateral pyridyl
rings and the central benzene ring.
Synthesis and Characterization of Complexes. With the
exception of [PtL9Cl], all of the complexes were prepared
by the reaction of the corresponding ligand with K2PtCl4 in
acetic acid at reflux under nitrogen, as originally reported
by Ca´rdenas et al. for the parent complex [PtL1Cl].29 The
resultant complexes have relatively low solubility and
precipitate from the acetic acid solution. A simple purification
procedure (see the Experimental Section) allows the com-
pounds to be isolated in yields typically of around 50-60%.
In the case of the pyridyl-appended complex [PtL4Cl], a
second potential N∧C-coordinating binding site for platinum
is present, although no evidence for the competitive forma-
tion of a dimetallic complex was observed, even in the
presence of excess K2PtCl4. For the dimethylamino-
substituted complex [PtL9Cl], an alternative solvent system
of acetonitrile/water was employed {3:1 (v/v), as used
previously for [PtL2Cl]24} to avoid problems associated with
the protonation of the pendent amino group of the product
that would occur in an acetic acid solution.
Apart from the obvious loss of the H2′ resonance (see
Scheme 1 for the numbering system), the main systematic
changes in the 1H NMR spectra of the ligands upon
coordination to platinum(II) in an N∧C∧N manner were
found to be (i) a shift to high frequency of the H6 resonance
(∆δ 0.5-0.6 ppm), accompanied by the appearance of well-
1
defined 195Pt (I ) /2) satellites about this signal (3J of ca.
40 Hz), (ii) a shift to low frequency of the H4′ resonance of
the central ring (∆δ 0.6-0.75 ppm), in some instances
accompanied by resolvable 195Pt satellites (4J of ca. 6 Hz),
and (iii) a small shift of the pyridyl H4 resonance to higher
frequency (∆δ 0.1-0.2 ppm) and a comparable shift of H3
to lower frequency, the summative effect of which is to lead
to a reversal of the ordering of these two signals in the spectra
of the complexes compared to those of their respective
ligands. The chemical shifts of resonances in the pendent
aryl groups were not substantially changed upon coordina-
Figure 2. Molecular structures of [PtL6Cl] (a) and [PtL3Cl] (b) at 120 K,
with displacement ellipsoids at the 50% level. Selected bond lengths (Å)
and angles (deg): for [PtL6Cl], Pt1-C21 1.912(3), Pt1-N1 2.045(2), Pt1-
Cl1 2.422(1), N1-Pt1-N1′ 161.13(11); for [PtL3Cl], Pt1-C131 1.908(6)/
1.920(6), Pt1-N11 2.044(5)/2.031(5), Pt1-N12 2.033(6)/2.025(5), Pt1-
Cl1 2.406(2)/2.416(2), N11-Pt1-N12 161.4(2)/161.6(2).
strain that such ligands suffer when bound terdentately, a
feature shared with other transition-metal complexes of
terpyridines {e.g., the corresponding angle in [Pt(4-Ph-tpy)-
Cl]+ is 162.8(4)°}.30 This strain is also manifested in the
rather compressed angle of 112.6(2)° between the central
benzene ring and the pyridyl substituents. In all of these
features, the structure closely resembles those of [PtL1Cl]
and [PtL2Cl] reported previously.24,29 It is significant that
the Pt-C bond length [1.912(3) Å] is considerably shorter
than that found in complexes incorporating 6-phenyl-2,2′-
bipyridine and its derivatives (typically around 2.04 Å), and
the Pt-N bond lengths are also somewhat shorter than that
for the bond to the lateral nitrogen in the latter class of
complexes [2.045(2) Å compared with values of around 2.14
Å31]. The angle between the plane of the cyclometalated ring
1
tion, although H-1H NOESY was again useful for unam-
biguous assignments.
(a) Crystal Structures of [PtL6Cl] and [PtL3Cl]. Single
crystals of the biphenylyl-substituted complex [PtL6Cl] were
obtained from a chloroform solution, and the structure was
determined using X-ray diffraction. In the crystal structure,
the [PtL6Cl] molecule lies on a crystallographic 2-fold axis
that passes through the metal ion, the chloride ligand, and
the cyclometalated carbon (Figure 2a and Table S1 of the
Supporting Information). The platinum ion, the three coor-
dinating rings, and the chloride ligand are planar, but the
N-Pt-N axis is distorted away from linearity, with an angle
of 161.1(1)°. This reflects the distortion and chelate ring
(29) Ca´rdenas, D. J.; Echavarren, A. M.; Ram´ırez de Arellano, M. C.
(30) Buchner, R.; Cunningham, C. T.; Field, J. S.; Haines, R. J.; McMillin,
Organometallics 1999, 18, 3337.
D. R.; Summerton, G. C. J. Chem. Soc., Dalton Trans. 1999, 711.
9694 Inorganic Chemistry, Vol. 44, No. 26, 2005

N∧C∧N-Coordinated Platinum(II) Complexes
are almost all the same (Table S1 in the Supporting
Information).
Electrochemistry. Within the range -2.4 to +1.0 V (vs
Fc/Fc+), all of the complexes show one irreversible reduction
process [in 9:1 (v/v) CH3CN/CH2Cl2], at a similar value of
around -2.0 V in each case and comparable to that observed
for the parent complex (Table 1). The presence of the aryl
substituents has only a small and erratic effect on the
reduction potential, probably because the lowest unoccupied
molecular orbital in these complexes is dominated by the
pyridyl rings, with little contribution expected from the
formally anionic cyclometalating ring or its pendent aryl
substituent.
Complexes [PtL4Cl]-[PtL7Cl] display one irreversible
oxidation wave within this region, while [PtL8Cl] and [PtL9-
Cl] also display a second oxidation process. The oxidation
potential shows rather more variation with the substituent
than the reduction potential (Table 1), suggesting that the
central phenyl ring makes a more substantial contribution
to the highest occupied molecular orbital (HOMO). The
observed order, mesityl > 2-pyridyl > 4-tolyl > 4-biphenylyl
> methyl > 2-thienyl > 4-(dimethylamino)phenyl, can be
rationalized in terms of the electron-withdrawing or -donating
ability of the substituents. Thus, in terms of the σ-bonding
framework, simple aryl substituents are electron-withdrawing
relative to methyl, accounting for the higher Eox values
exhibited by the first four complexes listed compared to R
) methyl.33 However, this effect can be offset by donation
through the π-bonding framework, which will be most
significant for the most conjugated substituent, biphenylyl
(hence, a lower Eox), and least significant for R ) mesityl,
where the steric encumbrance associated with the o-methyl
groups will inhibit the attainment of the coplanar conforma-
tion required for efficient conjugation (hence, the highest
Eox for this complex). Thiophene is an electron-rich hetero-
cycle, while the dimethylamino substituent is strongly
electron-donating through the π framework, accounting for
the particularly low Eox values observed for [PtL8Cl] and
[PtL9Cl], respectively. The second oxidation process ob-
served for [PtL9Cl] is reversible, E1/2 ) 0.29 V. Because
the free ligand HL9 also displays a reversible oxidation at
only a slightly smaller potential (E1/2 ) 0.24 V), we attribute
the second oxidation process in the complex to oxidation of
the (dimethylamino)phenyl substituent and the first to the
Pt(N∧C∧N)Cl core of the complex, as for the other
compounds.
Figure 3. Crystal packing in [PtL6Cl] showing the stacking parallel (a)
and perpendicular (b) to the Pt(N∧C∧N)Cl plane.
and that of the first ring of the biphenylyl pendent is
29.63(16)°, while the angle between the constituent rings of
the biphenylyl unit is 26.99(19)° in the opposing direction,
such that the cyclometalated ring and the terminal ring are
almost coplanar [the angle between their planes is only 2.6-
(2)°]. Twisting about the interannular C-C bonds has
similarly been observed by Alcock et al. for a series of 4′-
biphenylyl-substituted 2,2′:6′,2′′-terpyridyl (biptpy) com-
plexes containing transition-metal ions, M2+, with values
reported of around 30°.32 The packing of [PtL6Cl] can be
compared to that seen in the [M(biptpy)2]2+ metal complexes,
where four different packing motifs were identified.32 In the
case of [PtL6Cl], the molecules stack flat in an offset head-
to-tail arrangement, similar to the type B stacking reported
by Alcock for [Cd(biptpy)2](PF6)2. However, when compared
with [Cd(biptpy)2](PF6)2 (where one of the pyridine rings
sits above the first of the pendent biphenyl rings), the
molecules are laterally further apart, so that the pyridine rings
are positioned partially over the cyclometalated ring of the
next molecule (Figure 3).
The crystal structure of the methyl derivative [PtL3Cl] has
also been determined during this work (Figure 2b and Table
S1 in the Supporting Information) and displays bond lengths
and angles very similar to those found in the Pt(NCN)Cl
core of [PtL6Cl] discussed above and in the other crystal-
lographically characterized complexes, [PtL1Cl] and [PtL2-
Cl].24,29 In this case, two different environments in the crystal
were observed, but corresponding bond lengths and angles
Photophysical Properties. Absorption and emission data
for the complexes prepared in this study are listed in Table
1, together with corresponding values for the three previously
studied complexes for comparison.24
Absorption. All of the new complexes display very intense
1
absorption bands below 330 nm, because of π-π* transi-
(31) A search in the CCD reveals a mean value of 2.04 Å for the Pt-C
bond length in six N∧N∧C complexes, 2.14 Å for the lateral Pt-N
bond, and 1.97 Å for the bond to the central N.
(32) Alcock, N. W.; Barker, P. R.; Haider, J. M.; Hannon, M. J.; Painting,
C. L.; Pikramenou, Z.; Plummer, E. A.; Rissanen, K.; Saarenketo, P.
J. Chem. Soc., Dalton Trans. 2000, 1447.
(33) The opposing effects of σ withdrawal and π donation on the
electrochemistry of 4′-aryl-substituted [Pt(N∧N∧N)Cl] complexes are
described in ref 13c. A detailed discussion of similar observations in
[Ru(N∧N)3]2+ complexes is provided by: Damrauer, N. H.; Boussie,
T. R.; Devenney, M.; McCusker, J. K. J. Am. Chem. Soc. 1997, 119,
8253.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9695

Farley et al.
Figure 4. (a) UV-visible absorption spectra of the platinum(II) complexes
in a dichloromethane solution at 295 K, normalized at 380 nm for
comparison (extinction coefficients are given in Table 1). (b) Expansion of
the low-energy region, showing the weak bands attributed to direct
3
population of LC excited states.
tions localized on the ligands (ꢀ > 20 000 L mol-1 cm-1),
together with a set of intense bands in the region 350-430
nm (ꢀ ) 5000-10 000 L mol-1 cm-1; Figure 4a). The latter
comprise at least three bands, of which the position of one,
at 380 ((1) nm, is the same in each complex and hence
must arise from a transition that is independent of the 5′
substituent. On the other hand, the component band at lowest
energy is increasingly red-shifted from 401 to 431 nm in
the order mesityl < 2-pyridyl < 4-tolyl ) 4-biphenylyl <
2-thienyl < 4-dimethylamino (values in CH2Cl2). This order
correlates with the decreasing order of the oxidation poten-
tials of the complexes discussed above. In fact, the absorption
energy (Eabs) increases linearly with an increase in the
oxidation potential, at a rate of 2900 cm-1 V-1 (see Figure
S1 in the Supporting Information for a plot of Eabs vs Eox),
confirming the influence of the substituent on the HOMO
energy. For each complex, this band displays significant
negative solvatochromism, typically increasing in energy by
around 1500 cm-1 on going from the least polar solvent
investigated, CCl4, to the most polar solvent, CH3CN.
Absorption data for all of the complexes in a range of
solvents are provided in Table S3 in the Supporting Informa-
tion. Such behavior is typical of transitions with an ap-
preciable degree of charge-transfer character and, normally,
to an excited state of lower dipole moment than the ground
state.34
A much weaker band (ꢀ ≈ 200 L mol-1 cm-1) is
discernible at lower energy for all of the complexes (Figure
9696 Inorganic Chemistry, Vol. 44, No. 26, 2005

N∧C∧N-Coordinated Platinum(II) Complexes
compared to that formed initially upon excitation into the
strong, solvatochromic absorption band. We have previously
observed a similar ordering (H ≈ mesityl < ω-pyridyl <
3
4-tolyl < 4-biphenylyl) in the LC emission of a series of
4′-substituted bis(terpyridyl)iridium(III) complexes.27
As described in the previous section, the lowest-energy
band in the absorption spectra, attributed to direct excitation
3
to the LC state, occurs at almost the same wavelength for
all of the aryl-substituted complexes (494 ( 2 nm). Hence,
given the trend in the emission energy, there is a clear,
3
progressive increase in the Stokes shift between the LC
absorption band and the emission band (0-0 transition) in
the same order: H < mesityl < 2-pyridyl < 4-tolyl <
4-biphenylyl < 2-thienyl. Presumably, this arises from the
need for torsional rearrangement to occur following light
absorption for maximal conjugation of the aryl substituent
with the N∧C∧N core. An approximately coplanar confor-
mation of the pendent and cyclometalating rings will be
required to maximize the conjugation in the excited state,
whereas in the initially formed conformation of the excited
state, the mean dihedral angle is expected to be larger, similar
to that in the ground state (e.g., around 30° according from
the X-ray structure of [PtL6Cl]). To verify this hypothesis,
measurements in a frozen glass at 77 K have been performed,
where torsional rearrangement is inhibited. The spectrum of
the mesityl-substituted complex at 77 K [2:2:1 (v/v) ether/
isopentane/ethanol] is unchanged compared to the room-
temperature spectrum, whereas the emission maxima of the
other complexes are blue-shifted by 5-10 nm, consistent
with the requirement for a change in the conformation for
maximal conjugation to be attained.
Figure 5. Emission spectra of the complexes in a dilute solution (10-5
M) in dichloromethane at 295 K (λex ) 400 nm; band-passes of 1.0 nm).
4b), except for the thienyl and amino compounds, where it
may be obscured by the tail of the charge-transfer band. The
wavelength of this band is very similar in each case (492-
496 nm), comparable to that for the methyl-substituted
compound (495 nm), and displays a much weaker degree of
solvatochromism than the intense charge-transfer band. This
band is attributed to the direct population of states of
3
primarily LC character, facilitated by the high spin-orbit
coupling associated with the platinum(II) ion.24,35
Emission. All of the complexes studied are intensely
luminescent in solution at room temperature (Figure 5 and
Table 1). The luminescence quantum yields in a degassed
dichloromethane solution are on the order of 0.5-0.6 in each
case, values which are particularly high among the numerous
luminescent platinum(II) complexes reported to date. The
emission spectra of [PtL1Cl]-[PtL8Cl] are highly structured,
typical of luminescence from states of primarily 3LC
character. The component band of highest intensity is the
one of highest energy (i.e., the 0-0 transition), indicating a
minimal difference in geometry between the ground and
excited electronic states; the Stokes shifts are small; and the
spectra show only very weak negative solvatochromism (e.g.,
shifting by only 200-250 cm-1 on going from CH3CN to
CCl4; see Table S3 in the Supporting Information for data).
These are all further features that are typically more
characteristic of ligand-centered, as opposed to charge-
transfer, states. The amino-substituted complex [PtL9Cl] is
anomalous in this respect, displaying a broad structureless
emission band and strong positive solvatochromism. Its
behavior will be discussed separately.
The emission maxima are increasingly red-shifted by the
substituent in the order mesityl < 2-pyridyl < 4-tolyl <
4-biphenylyl < 2-thienyl. Again, the emission energy, Eem,
correlates roughly linearly with the oxidation potential
(Figure S2 in the Supporting Information), in line with the
influence of the aryl substituents on the energy of the
HOMO. However, at 4900 cm-1 V-1, the dependence is
markedly steeper than that observed for the absorption band,
consistent with a different nature to the emissive state
(a) Concentration Dependence, Self-Quenching, and
Excimer Formation. For the complexes [PtL1Cl]-[PtL8-
Cl], increasing the concentration leads to the appearance of
a new, broad, structureless emission band centered at 690-
700 nm, which becomes progressively more intense at the
expense of the shorter wavelength bands.36 This is charac-
teristic of excimer formation and emission.37 The temporal
decay of the emission registered in the region of monomer
luminescence (480-550 nm), where no contribution from
the excimer is anticipated, remains monoexponential over
the whole range of concentrations studied (7.5 × 10-6-1.5
× 10-4 M). In contrast, the emission kinetic trace registered
at 680 nm shows a monoexponential growth of the emission
intensity (kobs ≈ 106 s-1) as the excimer responsible for the
emission at this wavelength is formed from the monomer,
followed by a monoexponential decay with the same rate
constant as that of the monomer. The observed emission
(36) No significant differences were observed in the position or spectral
profile of the excimer formed by the different aryl-substituted
complexes, being effectively identical with that reported previously
for [PtL1-3Cl].23
(37) Many square-planar platinum(II) complexes are subject to self-
quenching, owing to attractive intermolecular interactions between the
excited state and its ground-state analogue. Only in some cases,
however, is excimer emission detectable, for example, in [Pt(diimine)-
(CN)2] complexes2 and, though to a lesser extent, certain [Pt(phen)-
(acetylide)2] systems.3 Cyclometalated complexes (e.g., those of phbpy)
usually display self-quenching that is not accompanied by detectable
excimer emission.6
(34) A detailed discussion of this effect in platinum(II) diimine complexes
is provided by: Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc.
1996, 118, 1949. See also ref 23.
(35) Zheng, G. Y.; Rillema, D. P.; DePriest, J.; Woods, C. Inorg. Chem.
1998, 37, 3588.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9697

Farley et al.
decay rate constants (kobs) increase linearly with the con-
centration, fitting well to an expression of the form
primarily responsible here, where the aryl groups would be
disposed on opposite sides of the dimer.
Efficient quenching of the emission of each complex by
dissolved molecular oxygen is observed, with quenching rate
constants close to 109 M-1 s-1 (Table 1).
kobs ) k0 + kQ[Pt]
(1)
where k0 is the emission rate constant at infinite dilution and
kQ is an apparent rate constant of self-quenching.2,3,38 This
latter parameter has been shown to be determined by the
product of the equilibrium constant for excimer formation
(KE) and the sum of the rate constants of the radiative and
nonradiative decay pathways of the excimer.2,38 Thus, kQ
provides an indication of the susceptibility of the complex
to self-quenching through excimer formation.
Values of k0 and kQ have been determined for each
complex in this way from measurements of kobs over a range
of concentrations. The lifetimes of emission at infinite
dilution τ0 listed in Table 1 are the values of 1/k0. The
lifetimes are all similar to one another and to that of the
parent complex (around 8 µs), with the exception of the
thienyl-substituted complex, which has a significantly longer
lifetime of 21 µs (values in degassed dichloromethane at
room temperature). The lifetimes at 77 K [2:2:1 (v/v) ether/
isopentane/ethanol] were the same, within the experimental
uncertainty on the measurement, as those determined at room
temperature, a remarkable testament to the success of the
N∧C∧N coordination in eliminating all pathways of potential
thermally activated nonradiative decay.
The complexes reveal considerable variation in the value
of kQ (Table 1). The introduction of aryl substituents into
the 5′ position of the ligand might be expected to reduce the
tendency to form excimers compared to the parent complex,
owing to steric inhibition by the pendent aryl group prevent-
ing the necessary face-to-face approach of a complex in its
excited state to a ground-state molecule. This prediction was
borne out by most of the new complexes, and there is
evidence for a trend in kQ with respect to the increasing steric
demand of the substituent. Thus, kQ is reduced from 3.3 ×
109 M-1 s-1 in the methyl-substituted complex to 2.5 × 109
M-1 s-1 in the tolyl-substituted analogue and further reduced
in the most sterically encumbered mesityl system to 1.0 ×
109 M-1 s-1. The values for the biphenylyl and thienyl
complexes lie between those of the tolyl and mesityl
compounds. On the other hand, the pyridyl substituent in
[PtL4Cl] might reasonably be expected to have a steric
influence comparable to that of a tolyl group, yet kQ for this
complex is actually higher than that for the simple, methyl-
substituted complex. Clearly, steric effects are only part of
the picture, and the electronic influence of the substituent
must also play a role in determining the affinity of the
excited-state molecule for a ground-state molecule. An
interesting question pertaining to excimer formation in
platinum diimine complexes is the nature of the attractive
interactions, e.g., Pt-Pt or ligand-ligand or combinations
of these.2 That excimer formation in the present set of aryl
complexes occurs at all, and is relatively significant even in
the presence of sterically demanding substituents in the
ligands, may suggest that a head-to-tail interaction is
(b) Emission Properties of the Amino-Substituted
Complex [PtL9Cl]. This complex, which carries a 4-(di-
methylamino)phenyl substituent at the 4 position of the
N∧C∧N core, exhibits very different luminescence properties
from all of the other complexes in a dichloromethane solution
(Figure 5). (i) The emission band at room temperature is
broad (half-height width of 120 nm) and unstructured. (ii)
The emission is substantially red-shifted compared to the
other compounds (λmax ) 588 nm in CH2Cl2): the position
of this complex in the plot of Eem vs Eox is anomalous (see
Figure S2 in the Supporting Information). (iii) The complex
is subject to very little self-quenching: the apparent rate
constant of self-quenching kQ (≈0.12 × 109 M-1 s-1) is
approximately 20 times smaller than that for the tolyl-
substituted complex. (iv) The emission in this complex is
substantially more sensitive to oxygen than in all of the other
complexes; for example, the emission quantum yield is
reduced by a factor of 65 in an aerated versus degassed
solution in CH2Cl2 (Table 1).
These differences suggest a switch in the nature of the
lowest excited state from 3LC in [PtL1-8Cl] to one of
primarily ILCT character in the amino complex [PtL9Cl]. It
is likely that the introduction of the strongly electron-donating
Me2N- substituent raises the energy of the pendent aryl
group to such an extent that there is a high-energy molecular
orbital localized predominantly on this group, and the lowest-
energy excited state corresponds to an ILCT (πMe N-Ar
π*NCN) transition rather than the locally excited states (πNCN
-
-
2
π*NCN) of the other complexes. The electrochemical data for
this complex lend support to this hypothesis: the Me2N-
C6H5- group displays an oxidation potential only slightly
higher than that of the Pt(N∧C∧N)Cl core, as discussed
earlier. A related effect has been observed by McMillin and
co-workers in a series of [Pt(X-tpy)Cl]+ complexes, where
X-tpy is a 4′-substituted terpyridine ligand: the introduction
of increasingly electron-donating substituents, such as -NMe2
or pyrene, leads to an increase in the contribution of ILCT
character in the emissive state.13 Similarly, we recently
observed a switch in the nature of the lowest-energy excited
3
state from LC to ILCT in bis(terpyridyl)iridium(III) com-
plexes, upon the introduction of a -C6H4-NMe2 substituent
into the terpyridine.39 Notably, the luminescence quantum
yield of [PtL9Cl] is again very high (φ ) 0.46), despite the
lower energy of the excited state.
Further evidence in support of the charge-transfer assign-
ment in a dichloromethane solution is provided by the very
pronounced positive solvatochromism in the emission spec-
trum of this complex (Figure 6). For example, the emission
energy is almost 4000 cm-1 lower in CH3CN than in CCl4.
In fact, the spectrum in the least polar solvent investigated,
(39) Leslie, W.; Poole, R. A.; Murray, P. R.; Yellowlees, L. J.; Beeby, A.;
(38) Horvath, A.; Stevenson, K. L. Coord. Chem. ReV. 1996, 153, 57.
9698 Inorganic Chemistry, Vol. 44, No. 26, 2005
Williams, J. A. G. Polyhedron 2004, 23, 2769.

N∧C∧N-Coordinated Platinum(II) Complexes
Figure 7. Emission and excitation spectra of [PtL9Cl] in CH2Cl2 (6.0 ×
10-5 M) at 295 K (red solid and dotted lines, respectively), and the
corresponding blue-shifted spectra (in blue), after the addition of trifluo-
roacetic acid (10-2 M). λex ) 408 nm in both cases (isosbestic point); λem
) 588 and 488 nm, respectively; band-passes ) 2 nm. Note the appearance
of an excimer band for the protonated complex.
Figure 6. Emission spectra of [PtL9Cl] at 295 K in several solvents of
differing polarity, showing the strong positive solvatochromism. Note the
change to a structured, high-energy emission spectrum in CCl4, resembling
that displayed by the other complexes, and the presence of excimer emission
in toluene at the concentration employed (10-4 M).
CCl4, resembles the spectra of the other complexes, display-
ing structured emission and with an emission maximum
similar to that of the pyridyl-substituted complex. Thus, we
excimer formation at elevated concentrations become sig-
nificant, and the limiting lifetime in a dilute solution is
shortened by a factor of 2. These changes in emission are
accompanied by a blue shift of the intense charge-transfer
band in absorption (Figure 7). All of these effects are
consistent with a switch in the nature of the lowest-energy
3
conclude that the LC and ILCT states are close in energy
in [PtL9Cl]. In the least polar solvents, the former is the lower
in energy and structured emission is observed. In more polar
solvents, the ILCT energy is reduced, owing to the high
degree of charge separation, to such an extent that in CH2-
Cl2 and CH3CN it is lower in energy than the 3LC. The trend
is thus reminiscent of the classical behavior of organic
molecules such as 4-(dimethylamino)benzonitrile that may
display either localized or twisted intramolecular charge-
transfer emission, according to the environment.40
3
excited state from the ILCT character of [PtL9Cl] to the -
LC character observed for all of the other complexes, which
can be attributed directly to the removal of the influence of
the electron-donating amino group upon protonation. Indeed,
the spectral parameters for [PtL9HCl]+ are very similar to
those of the mesityl-substituted complex [PtL5Cl], in line
with the presence of a pendent aryl group that now has little
influence. The luminescence quantum yield remains high
(φ ) 0.40). The lower value of kQ for [PtL9HCl]+ than, for
example, the tolyl-substituted complex [PtL7Cl], can be
attributed to the net positive charge of the former, which
should disfavor the close approach of the excited- and
ground-state molecules. On the other hand, the fact that
excimer formation remains quite significant, despite the
positive charge, again supports the notion that a head-to-tail
disposition of the two complexes in the excited-state dimer
is more likely.
The ILCT character of the lowest excited state in CH2Cl2
might also account for the greatly attenuated self-quenching
and absence of excimer formation for [PtL9Cl] in this solvent
because the excited state will have lower affinity for a
ground-state molecule, which plays the role of a Lewis base
in the process of excimer formation. A somewhat analogous
effect has been reported by McMillin and co-workers in their
study of 4′-aryl-substituted terpyridylplatinum complexes,
where the increasing ILCT character correlated well with a
systematic moderation of exciplex quenching by, for ex-
ample, butyronitrile.13 If such an explanation indeed applies
in the present instance, then excimer formation might be
predicted in CCl4, in which solvent the emissive excited state
This protonation-induced switching between two different
types of excited states with significantly different energies,
both of which are highly emissiVe, is an unusual feature.
Several platinum group metal complexes incorporating amino
or pyridyl functionality in one or more of the ligands display
luminescence, which can be modulated through protonation
of the nitrogen atom.41 However, in most cases, one or the
other of the two forms is quenched, and the effect is usually
a change in the intensity of the emission, as opposed to the
emission energy. The effect of protonation is fully reversible
upon the addition of a stoichiometric amount of base required
for neutralization, and the complex [PtL9Cl] is stable in the
presence of larger amounts of base (e.g., no significant
decomposition was observed in the presence of 0.2 M Et3N
3
has LC character, as discussed above. Unfortunately, the
complex has poor solubility in this solvent, and concentra-
tions sufficient for excimer emission to be observed are not
achievable. However, a saturated solution in toluene does
show clear evidence of excimer emission (Figure 6). Toluene
is probably sufficiently nonpolar to ensure that the 3LC state
remains lower in energy.
(c) Effect of Protonation of the Pendent Amine. Pro-
tonation of [PtL9Cl] in CH2Cl2 by the addition of trifluoro-
acetic acid results in a dramatic set of changes in the
absorption and luminescence properties (Figure 7 and Table
1). The emission maximum is blue-shifted by 3700 cm-1,
the spectrum becomes more structured, self-quenching and
(41) For a recent example involving platinum complexes, see: Yang, Q.-
Z.; Tong, Q.-X.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H.
Eur. J. Inorg. Chem. 2004, 1948 and ref 14b.
(40) Rettig, W. Angew. Chem., Int. Ed. 1986, 25, 971.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9699

Farley et al.
ionization, as indicated. All solvents used in preparative work were
at least Analar grade, and water was purified using the Purite
system. Solvents used for optical spectroscopy were high-
performance liquid chromatography grade.
over a period of several hours). In principle, the switching
effect observed here could be utilized as a component of a
sensory system for ratiometric sensing, given that both the
emission and excitation spectra are substantially shifted.
Ligand Prepared Using Route A: 1,3,5-Tris(2-pyridyl)-
benzene (HL4). Toluene (15 mL) was added to a mixture of tbb
(0.87 g, 2.76 mmol), 2-(tri-n-butylstannyl)pyridine (4.34 g of 81%
purity, equivalent to 9.55 mmol, remainder mostly tetra-n-butyltin),
lithium chloride (1.21 g, 38.6 mmol), and bis(triphenylphosphine)-
palladium(II) chloride (154 mg, 0.22 mmol) contained in an oven-
dried Schlenk tube. The mixture was degassed via five freeze-
pump-thaw cycles and then heated at reflux under an atmosphere
of dinitrogen for 20 h. A saturated aqueous solution of potassium
fluoride (10 mL) was then added to the solution, after cooling to
ambient temperature, and stirred for 1 h. The insoluble residue
formed was removed by filtration and washed with toluene. The
toluene was removed from the combined filtrate and washings under
reduced pressure, giving a crude product, which was taken up into
dichloromethane (150 mL) and washed with aqueous NaHCO3 (5%
by mass, 2 × 100 mL). The organic phase was dried over anhydrous
potassium carbonate and evaporated to dryness. The brown residue
was purified by chromatography on silica gel, gradient elution from
100% hexane to 100% diethyl ether, giving HL4 as an off-white
solid (470 mg, 55%). 1H NMR (400 MHz, CDCl3): δ 8.75 (3H, d,
Conclusion
In summary, this study demonstrates the versatility of
terdentate, N∧C∧N-coordinating ligands, based on the 1,3-
di(2-pyridyl)benzene structure, for obtaining highly lumi-
nescent, cyclometalated platinum(II) complexes. In particular,
the introduction of aryl substituents into the 5 position of
the benzene ring of the ligand has several important
implications. First, it allows the emission wavelength to be
tuned over a wide range across the visible region, from 481
nm ([PtL2Cl]) to 640 nm ([PtL9Cl] in CH3CN) (an energy
range of 5000 cm-1) while retaining the outstandingly high
luminescence quantum yields of 50-60%. Second, the
presence of aryl substituents in the complexes diminishes
their propensity for self-quenching at elevated concentrations.
Finally, the introduction of the strongly electron-donating
4-(dimethylamino)phenyl group leads to a switch in the
nature of the lowest excited state to one of primarily ILCT
character in polar solvents, while in nonpolar solvents, the
3LC remains lower in energy. Moreover, protonation of the
3
4
3J ) 4.5 Hz, H6), 8.74 (3H, s, H2′), 7.99 (3H, dd, J ) 7.5 Hz, J
3
4
) 1.0 Hz, H3), 7.81 (3H, td, J ) 7.5 Hz, J ) 2.0 Hz, H4), 7.29
(3H, ddd, 3J ) 7.5 and 4.5 Hz, 4J ) 1.0 Hz, H5). 13C NMR
(CDCl3): δ 157.1 (quat), 149.7 (C2′), 140.5 (quat), 137.1 (C4), 126.3
3
amine also leads to reversible switching to the LC state,
manifested as a dramatic change in emission maxima from
588 to 482 nm.
1
(C6), 122.6 (C5), 121.2 (C3). MS (ES+): m/z 310 [M + H+]. H
NMR data are consistent with values previously reported in CD2-
Cl2 for this compound, obtained as a side product from the reaction
of 2-ethynylpyridine with CpCo(PPh3)2.43
Highly emissive, charge-neutral platinum group metal
complexes may be suitable for applications as triplet-
harvesting agents in OLEDs, and indeed a very recent study
has shown excellent results for [PtL3Cl] in a polymer-based
device.42 Thus, the availability of a family of complexes with
tunable emission over a wide range offers considerable scope
in the further development of this field. The effect of the
aryl substituents in reducing self-quenching is also an
attractive feature for practical applications such as OLED
technology, where local concentrations may be quite high,
and self-quenching is a wasteful energy sink that diminishes
device efficiencies. Finally, the sensitive solvent dependence
of the emission of [PtL9Cl], and the protonation-induced
switching between excited states in this complex, could form
the basis of novel sensory systems for a variety of species
through incorporation of the amine into an appropriate
receptor.
Ligands Prepared Using Route B. (a) Bromo-Substituted
Intermediate: dpyb-Br. The method employed was similar to that
described above for HL4, but in this case, only 2.2 equiv of the
pyridylstannane was used. Thus, a mixture of tbb (1.43 g, 4.55
mmol) and 2-(tri-n-butylstannyl)pyridine (4.10 g of 90% purity,
equivalent to 10.03 mmol) was stirred in toluene (15 mL) at reflux
under nitrogen, in the presence of lithium chloride (1.56 g, 36.7
mmol) and bis(triphenylphosphine)palladium(II) chloride (191 mg,
0.27 mmol), for 20 h. Workup as above led to a pale-brown residue,
which was purified by chromatography on silica gel, gradient elution
from hexane to 50% hexane/50% diethyl ether, giving dpyb-Br as
1
a colorless solid (740 mg, 52%). H NMR (400 MHz, CDCl3): δ
8.72 (2H, dd, 3J ) 5.0 Hz, 4J ) 1.5 Hz, H6), 8.55 (1H, t, 4J ) 1.5
Hz, H4′), 8.22 (2H, d, 4J ) 1.5 Hz, H2′), 7.80 (4H, m, H3, H4), 7.28
(2H, ddd, 3J ) 6.5 and 5.0 Hz, 4J ) 1.5 Hz, H5). 13C NMR (CDCl3):
δ 155.8 (quat), 149.9 (quat), 141.7 (quat), 137.1 (C4), 130.5 (C2′),
124.2 (C6), 123.7 (C4′), 123.0 (C5), 120.9 (C3). MS (ES+): m/z
311.0205 [M + H+]; calcd for C16H12N2Br, 311.0184. A small
amount of 1,3-dibromo-5-(2-pyridyl)benzene was also isolated at
lower polarity (104 mg, 10%).
Experimental Section
Synthetic Details. 1H and 13C NMR spectra, including NOESY
and COSY, were recorded on a Varian 400- or 500-MHz instrument
and referenced to residual protiosolvent resonances. Coupling
constants are in Hertz. Electrospray ionization (EI) mass spectra
of ligands were acquired on a time-of-flight Micromass LCT
spectrometer; high-resolution spectra for accurate mass determina-
tions were also carried out on this instrument. The mass spectra of
the complexes were recorded at the EPSRC National Mass
Spectrometry Service Centre, Swansea,U.K., using electrospray, fast
atom bombardment, or liquid secondary ion mass spectrometry
(LSIMS)
(b) 5-Mesityl-1,3-di(2-pyridyl)benzene (HL5). Dimethoxy-
ethane (10 mL) was added to a mixture of dpyb-Br (208 mg, 0.67
mmol), (2,4,6-trimethylphenyl)boronic acid (121 mg, 0.74 mmol),
and Ba(OH)2‚8H2O (632 mg, 2.00 mmol) in an oven-dried Schlenk
tube. The mixture was degassed via three freeze-pump-thaw
cycles and placed under nitrogen. Tetrakis(triphenylphosphine)-
palladium(0) (23 mg, 0.02 mmol) was added under a flow of
nitrogen and the mixture heated at reflux (85 °C) under a nitrogen
(42) Sotoyama, W.; Satoh, T.; Sawatari, N.; Inoue, H. Appl. Phys. Lett.
2005, 86, 153505.
(43) Wakatsuki, Y.; Yoshimura, H.; Yamazaki, H. J. Organomet. Chem.
1989, 366, 215.
9700 Inorganic Chemistry, Vol. 44, No. 26, 2005

N∧C∧N-Coordinated Platinum(II) Complexes
atmosphere for 24 h. The solvent was then removed under reduced
pressure and the crude residue taken up into dichloromethane (50
mL) and NaHCO3 (5% by mass, 50 mL). The organic layer was
separated, washed with NaHCO3 (2 × 50 mL), and dried over
anhydrous K2CO3. The solvent was removed under reduced
pressure, and the pale-brown residue was recrystallized from ethanol
to give off-white crystals of the desired product (27 mg, 12%). 1H
eluted products of higher polarity was 1-bromo-3,5-di(4-tolyl)-
benzene (Rf ) 0.5), also a colorless solid (430 mg, 19%). 1H NMR
(400 MHz, CDCl3): δ 7.67 (3H, m, H2, H4, H6), 7.51 (2H, d, 3J )
3
8.0 Hz, Hb), 7.27 (4H, d, J ) 8.0 Hz, Ha), 2.41 (6H, s, CH3).
(b) 5-(2-Thienyl)-1,3-dibromobenzene. This compound was
prepared by a procedure similar to that above, starting from tbb
(2.24 g, 7.10 mmol) and 2-thiophenylboronic acid (1.00 g, 7.81
mmol) in dimethoxyethane (25 mL). After refluxing for 18 h, a
similar workup followed by purification by column chromatography
on silica gel, eluting with hexane, gave the required product (Rf )
3
NMR (300 MHz, CDCl3): δ 8.74 (2H, d, J ) 6.0 Hz, H6), 8.69
(1H, t, 4J ) 1.5 Hz, H2′), 7.85 (4H, m, H3, H4′), 7.77 (2H, td, 3J )
7.5 Hz, 4J ) 1.8 Hz, H4), 7.26 (2H, m, H5), 6.97 (2H, s, Ha′), 2.35
(3H, s, p-CH3), 2.09 (6H, s, o-CH3). 13C NMR (CDCl3): δ 157.4
(quat), 149.9 (quat), 142.4 (quat), 140.3 (quat), 138.8 (quat), 137.0
(C4), 136.9 (quat), 136.2 (quat), 128.7 (C5), 128.3 (C4′), 124.2 (C2′),
122.7 (C5), 121.0 (C3), 21.2 (p-CH3), 21.1 (o-CH3). MS (ES+): m/z
351.1852 [M + H+]; calcd for C25H23N2, 351.1856. Anal. Calcd
for C25H22N2: C, 85.68; H, 6.33; N, 7.99. Found: C, 85.58; H,
6.30; N, 7.90.
1
0.7 in hexane) as a colorless solid (870 mg, 39%). H NMR (400
MHz, CDCl3): δ 7.66 (2H, d, 4J ) 2.0 Hz, H4), 7.56 (1H, t, 4J )
3
4
2.0 Hz, H2), 7.34 (1H, dd, J ) 5.0 Hz, J ) 1.0 Hz, Ha), 7.31
(1H, dd, 3J ) 3.5 Hz, 4J ) 1.0 Hz, Hc), 7.09 (1H, dd,3J ) 5.0 and
3.5 Hz, Hb). 13C NMR (CDCl3): δ 140.9 (quat), 137.7 (quat), 133.0
(C2), 128.3 (Cb), 127.5 (C4, C6), 126.4 (Ca), 124.6 (Cc), 123.3 (quat).
MS (EI): m/z 318 (M+).
(c) 5-(4-Biphenylyl)-1,3-di(2-pyridyl)benzene (HL6). 4-Bi-
phenylylboronic acid (96 mg, 0.49 mmol) and dpyb-Br (134 mg,
0.43 mmol) were suspended in dimethoxyethane (3 mL), and an
aqueous solution of sodium carbonate (137 mg, 1.29 mmol in 0.5
mL water) was added. The mixture was degassed via four freeze-
pump-thaw cycles and placed under dinitrogen, and tetrakis-
(triphenylphosphine)palladium(0) was added (30 mg, 0.03 mmol).
The mixture was stirred at room temperature for 1 h before being
heated at 80 °C for 67 h. The solvent was then removed and the
residue taken up into dichloromethane (50 mL). After washing with
water (4 × 50 mL), the organic phase was dried over anhydrous
potassium carbonate and the solvent removed under reduced
pressure. The beige residue was purified by chromatography on
silica gel, gradient elution from hexane to 50% hexane/50% diethyl
(c) 5-[4-(Dimethylamino)phenyl]-1,3-dibromobenzene. This
compound was prepared similarly, starting from tbb (2.00 g, 6.35
mmol) and [4-(dimethylamino)phenyl]boronic acid (1.15 g, 6.99
mmol) in dimethoxyethane (25 mL). After refluxing for 18 h, the
mixture was worked up as above but using potassium carbonate as
the drying agent in place of magnesium sulfate. The crude product
was purified by column chromatography on silica gel, gradient
elution from hexane to 75% hexane/25% diethyl ether (v/v), leading
to the required product (Rf ) 0.6 in 75% hexane/25% ether) as a
pale-yellow solid (650 mg, 29%). 1H NMR (400 MHz, CDCl3): δ
7.61 (2H, d, 4J ) 2.0 Hz, H4), 7.52 (1H, t, 4J ) 2.0 Hz, H2), 7.43
(2H, d, 3J ) 8.0 Hz, Hb), 6.78 (2H, d, 3J ) 8.0 Hz, Ha), 3.01 (6H,
s, CH3). 13C NMR (CDCl3): δ 150.5 (quat), 144.8 (quat), 131.0
(C2), 127.8, 127.7 (C4, Cb), 125.9 (quat), 123.1 (quat), 112.5 (Ca),
40.4 (CH3). MS (EI): m/z 355 (M+). Among side products eluted
at higher polarities (Rf ) 0.2) was 1-bromo-3,5-bis[4-(dimethyl-
amino)phenyl]benzene. 1H NMR (400 MHz, CDCl3): δ 7.63 (1H,
t, 4J ) 1.5 Hz, H4), 7.57 (2H, d, 4J ) 1.5 Hz, H2), 7.51 (2H, d, 3J
1
ether, leading to a colorless solid (80 mg, 48%). H NMR (500
3
4
MHz, CDCl3): δ 8.78 (2H, dd, J ) 5.0 Hz, J ) 1.0 Hz, H6),
8.65 (1H, t, 4J ) 1.5 Hz, H2′), 8.40 (2H, d, 4J ) 1.5 Hz, H4′), 8.01
(2H, d, J ) 8.0 Hz, H3), 7.89 (4H, m, Ha, H4), 7.73 (2H, d, J )
3
3
8.0 Hz, Hb), 7.67 (2H, dd, J ) 8.0 Hz, J ) 1.0 Hz, Hb′), 7.48
(2H, t, 3J ) 8.0 Hz, Ha′), 7.36 (3H, m, Hc′, H5). 13C NMR
(CDCl3): δ 156.7, 142.3, 140.8, 140.7, 139.6, 138.0, 129.0 (Ca′),
128.0 (Ca), 127.7 (Cb), 127.6 (Cc′), 127.2 (Cb′), 126.9 (C4′), 125.0,
122.9 (C5), 121.6 (C3). MS (ES+): m/z 385 [M + H+]; 385.1694
[M + H+]; calcd for C28H21N2, 385.1699, 407.1514 [M + Na+];
calcd for C28H20N223Na, 407.1519. Anal. Calcd for C28H20N2: C,
87.47; H, 5.24; N, 7.28. Found: C, 86.77; H, 5.22; N, 7.14.
3
4
3
) 8.5 Hz, Hb), 6.80 (2H, d, J ) 8.5 Hz, Ha), 3.01 (6H, s, CH3).
13C NMR (CDCl3): δ 150.2 (quat), 143.5 (quat), 133.2 (quat), 127.8
(Cb), 126.7 (C4), 123.1 (C2), 122.9 (quat), 112.6 (Ca), 40.5 (CH3).
Anal. Calcd for C22H23N2Br: C, 66.84; H, 5.86; N, 7.09. Found:
C, 66.76; H, 5.85; N, 7.07.
Ligands Prepared Using Route C. (a) 5-(4-Tolyl)-1,3-di(2-
pyridyl)benzene (HL7). 1,3-Dibromo-5-(4′-tolyl)benzene (0.60 g,
1.84 mmol), 2-(tri-n-butylstannyl)pyridine (1.88 g of 91% purity,
equivalent to 4.78 mmol), bis(triphenylphosphine)palladium(II)
chloride (0.10 g, 0.15 mmol), and lithium chloride (0.78 g, 18.4
mmol) were placed in an oven-dried Schlenk tube, and toluene (10
mL) was added. The mixture was degassed via five freeze-pump-
thaw cycles and then heated at reflux under a nitrogen atmosphere
for 36 h. After cooling to room temperature, a saturated aqueous
solution of potassium fluoride (10 mL) was added and the mixture
stirred for 1.5 h. The highly insoluble residue that formed was
filtered off under vacuum and washed with more toluene. The
solvent was then removed under reduced pressure and the residue
taken up into a mixture of dichloromethane (150 mL) and NaHCO3
(5% by mass, 100 mL). The layers were separated, and the aqueous
layer was then further extracted with dichloromethane (2 × 50 mL).
The combined organic layers were washed twice with NaHCO3 (2
× 100 mL), then dried over anhydrous potassium carbonate, and
evaporated to dryness. The product was purified by column
chromatography on silica gel, gradient elution from hexane to 60%
hexane/40% diethyl ether (v/v), affording the required product (Rf
) 0.3 in 50% hexane/50% ether) as a colorless solid (350 mg, 59%).
Dibrominated Precursors for Ligands Prepared Using Route
C. (a) 5-(4-Tolyl)-1,3-dibromobenzene. tbb (2.11 g, 6.69 mmol),
p-tolylboronic acid (1.00 g, 7.36 mmol), and dimethoxyethane (25
mL) were placed in an oven-dried Schlenk tube, and sodium
carbonate (2.13 g, 20.1 mmol), dissolved in the minimum volume
of water, was added. The solution was degassed via three freeze-
pump-thaw cycles and placed under nitrogen. Tetrakis(tri-
phenylphosphine)palladium(0) (0.23 g, 0.20 mmol) was added under
a flow of nitrogen and the mixture heated at reflux for 18 h. The
residue obtained upon removal of solvent was taken up into a
mixture of dichloromethane (50 mL) and water (50 mL), and the
organic layer was separated and washed with more water (3 × 50
mL). After drying of the organic phase over anhydrous magnesium
sulfate, the solvent was removed under reduced pressure. The pale-
brown residue was purified by column chromatography on silica
gel, eluting with hexane, to afford the required product (Rf ) 0.8
1
on TLC in hexane) as a colorless solid (650 mg, 29%). H NMR
(400 MHz, CDCl3): δ 7.56 (2H, d, 4J ) 2.0 Hz, H4), 7.53 (1H, t,
3
3
4J ) 2.0 Hz, H2), 7.35 (2H, dd, J ) 8.0 Hz, Hb), 7.18 (2H, d, J
) 8.0 Hz, Ha), 2.32 (3H, s, CH3). MS (EI): m/z 326 (M+). Among
1H NMR (300 MHz, CDCl3): δ 8.74 (2H, d, J ) 4.5 Hz, H6),
3
Inorganic Chemistry, Vol. 44, No. 26, 2005 9701

Farley et al.
8.57 (1H, t, 4J ) 1.5 Hz, H2′), 8.29 (2H, d, 4J ) 1.5 Hz, H4′), 7.89
(2H, d, 3J ) 7.5 Hz, H3 ), 7.79 (2H, td, 3J ) 7.5 Hz, 4J ) 2.0 Hz,
H4), 7.67 (2H, d, 3J ) 8.0 Hz, Hb), 7.27 (4H, m, H5, Ha), 2.41 (3H,
s, CH3). 13C NMR (CDCl3): δ 157.2 (quat), 149.6 (C6), 142.2
(quat), 140.3 (quat), 137.9 (quat), 137.3 (quat), 136.7 (C4), 129.4
(C5), 127.2 (Cb), 126.1 (C4′), 124.2 (C2′), 122.3 (Ca), 120.8 (C3),
21.2 (CH3). MS (EI): m/z 322 (M+). Mp: 187 °C.
under reduced pressure; for [PtL8Cl], further extraction using
acetone was necessary. In the case of [PtL6Cl], the product was
found to be partially protonated at the pendent pyridyl ring; the
nonprotonated compound was obtained by washing the dichloro-
methane solution of the complex with triethylamine (0.05 mL)
followed by water (2 × 15 mL).
(i) [PtL4Cl] 2-pyridyl: yellow solid, yield 57% after deproto-
nation as described above. Data for the protonated species [PtL4-
HCl]+ initially formed. 1H NMR (500 MHz, CDCl3): δ 9.15 (2H,
(b) 5-(2-Thienyl)-1,3-di(2-pyridyl)benzene (HL8). This com-
pound was prepared similarly, starting from 1,3-dibromo-5-(2-
thienyl)benzene (0.75 g, 2.36 mmol), 2-(tri-n-butylstannyl)pyridine
(2.66 g of 85% purity, equivalent to 6.14 mmol), bis(triphenylphos-
phine)palladium(II) chloride (0.13 g, 0.19 mmol), and lithium
chloride (1.00 g, 23.6 mmol) in toluene (10 mL). After refluxing
for 48 h, the mixture was worked up as above and purified by
column chromatography on silica gel, gradient elution from hexane
to 25% hexane/75% diethyl ether (v/v), to give the required product
(Rf ) 0.4 in 75% hexane/25% ether) as a colorless solid (530 mg,
71%). Recrystallization from ethanol enabled the remaining trace
3
4
3
dd, J ) 6.0 Hz, J ) 1.0 Hz, J(195Pt) ) 36.2 Hz, H6), 8.72 (1H,
3
4
dd, J ) 5.0 Hz, J ) 1.0 Hz, H6-pendent py), 8.51 (2H, s, H4′),
8.32 (2H, d, 3J ) 7.5 Hz, H3), 8.26 (2H, td, 3J ) 7.5 Hz, 4J ) 1.5
Hz, H4), 8.23 (1H, d, 3J ) 8.0 Hz, H3-pendent py), 8.02 (1H, td, 3J
) 8.0 Hz, J ) 1.5 Hz, H4-pendent py), 7.61 (2H, ddd, J ) 7.0
and 6.0 Hz, 4J ) 1.5 Hz, H5), 7.47 (1H, ddd, 3J ) 7.5 and 5.0 Hz,
4J ) 1.0 Hz, H5-pendent py). Data for the nonprotonated final
product [PtL4Cl]. 1H NMR (500 MHz, CDCl3): δ 9.31 (2H, dd, 3J
) 5.5 Hz, 4J ) 1.0 Hz, 3J(195Pt) ) 41.4 Hz, H6), 8.69 (1H, d, 3J )
4
3
5.0 Hz, H6-pendent py), 8.06 (2H, s, H4′), 7.94 (2H, td, J ) 7.5
3
1
impurities to be removed. H NMR (500 MHz, CDCl3): δ 8.62
4
Hz, J ) 1.5 Hz, H4-pendent py), 7.80 (4H, m, overlapping H3,
(2H, d, 3J ) 5.0 Hz, H6), 8.40 (1H, t, 4J ) 1.5 Hz, H2′), 8.20 (2H,
t, 4J ) 1.5 Hz, H4′), 7.76 (2H, d, 3J ) 7.5 Hz, H3), 7.64 (2H, td, 3J
H3-pendent py, H4), 7.27 (3H, m, overlapping H5, H5-pendent py).
13C NMR (CDCl3): δ 167.2, 163.2, 157.7, 152.3, 149.8, 141.4,
139.3, 137.2, 134.8, 123.4 (C5-pendent py), 123.1 (C4′), 122.1 (C5),
120.2, 119.7. MS (EI+): m/z 539.051 [M+]; calcd for [C21H14N3-
ClPt], 539.052. Anal. Calcd for C21H14N3ClPt‚H2O: C, 45.29; H,
2.90; N, 7.55. Found: C, 45.04; H, 2.88; N, 7.15.
4
3
4
) 7.5 Hz, J ) 2.0 Hz, H4), 7.40 (1H, dd, J ) 3.5 Hz, J ) 1.0
Hz, Hc), 7.24 (1H, dd, J ) 5.0 Hz, J ) 1.0 Hz, Ha), 7.18 (2H,
ddd, 3J ) 7.5 and 5.0 Hz, 4J ) 1.5 Hz, H5), 7.02 (1H, dd, 3J ) 5.0
and 3.5 Hz, Hb). 13C NMR (CDCl3): δ 156.8 (quat), 149.6 (C6),
144.0 (quat), 140.5 (quat), 136.8 (C4), 135.4 (quat), 128.0 (Cb),
125.0, 124.9 (Ca, C4′), 124.5 (C2′), 123.8 (Cc), 122.4 (C5), 120.8
(C3). MS (EI): m/z 314 (M+).
3
4
(ii) [PtL5Cl] Mesityl. Complexation was carried out on a smaller
scale (0.07 mmol) in this case, leading to the product as an orange
1
solid in 20% yield. H NMR (200 MHz, CDCl3): δ 9.12 (2H, d,
(c) 5-[4-(Dimethylamino)phenyl]-1,3-di(2-pyridyl)benzene
(HL9). This compound was prepared in a similar way, starting from
1,3-dibromo-5-[4-(dimethylamino)phenyl]benzene (0.60 g, 1.69
mmol), 2-(tri-n-butylstannyl)pyridine (2.07 g of 78% purity,
equivalent to 4.39 mmol), bis(triphenylphosphine)palladium(II)
chloride (0.09 g, 0.14 mmol), and lithium chloride (0.72 g, 17
mmol) in toluene (10 mL). After refluxing for 24 h, the mixture
was worked up as described above, and the product was purified
by column chromatography on silica gel, gradient elution from
hexane to 30% hexane/70% diethyl ether, to give the required
product (Rf ) 0.3 in 75% hexane/25% ether) as a colorless solid
(400 mg, 67%). 1H NMR (400 MHz, CDCl3): δ 8.76 (2H, d, 3J )
3
3
3J ) 6.0 Hz, J(195Pt) ) 20.0 Hz, H6), 7.87 (2H, td, J ) 7.5 Hz,
4J ) 2.0 Hz, H4), 7.58 (2H, d, J ) 7.5 Hz, H3), 7.26 (2H, dd, J
) 7.5 Hz, 6.0, H5), 7.18 (2H, s, H4′), 6.92 (2H, s, Ha), 2.01 (9H, s,
CH3). Anal. Calcd for C25H21N2ClPt: C, 51.77; H, 3.65; N, 4.83.
Found: C, 51.04; H, 3.85; N, 4.49.
3
3
(iii) [PtL6Cl] biphenylyl: yellow solid in 66% yield. 1H NMR
3
4
(500 MHz, CDCl3): δ 9.35 (2H, dd, J ) 6.0 Hz, J ) 2.0 Hz,
3J(195Pt) ) 39.0 Hz, H6), 7.96 (2H, td, J ) 8.0 Hz, J ) 1.5 Hz,
3
4
H4), 7.76 (2H, d, J ) 8.0 Hz, H3), 7.71 (4H, m, Hb′, H4′), 7.67
3
(4H, m, H2, Ha), 7.49 (2H, t, J ) 7.5 Hz, Ha′), 7.39 (1H, tt, J )
3
3
7.5 Hz, J ) 1.8 Hz, Hc′), 7.29 (2H, ddd, J ) 7.3 and 5.9 Hz, J
) 1.5 Hz, H5). 13C NMR (CDCl3): δ 167.3, 152.5, 141.5, 140.7,
140.2, 139.3 (C4), 129.1 (Ca′), 127.9 (Cc′), 127.6, 127.4 (Cb or Cb′),
127.2 (Cb or Cb′), 123.5 (C5), 123.3 (Ca), 199.5 (C3). MS (EI): m/z
614.089 [M+]; calcd for [C28H19N2ClPt], 619.088; 613.09 [M -
Cl+].
4
3
4
4
4
5.0 Hz, H6), 8.52 (1H, t, J ) 1.5 Hz, H2′), 8.28 (2H, d, J ) 1.5
Hz, H4′), 7.93 (2H, d, 3J ) 8.0 Hz, H3), 7.82 (2H, td, 3J ) 8.0 Hz,
4J ) 2.0 Hz, H4), 7.71 (2H, d, 3J ) 8.5 Hz, Hb), 7.30 (2H, ddd, 3J
) 7.5 and 5.0 Hz, 4J ) 1.5 Hz, H5), 6.88 (2H, d, 3J ) 8.5 Hz, Ha),
3.03 (6H, s, CH3). 13C NMR (CDCl3): δ 157.4 (quat), 150.0 (quat),
149.4 (C6), 142.3 (quat), 140.1 (quat), 136.9 (C4), 128.0 (Cb), 125.6
(C4′), 123.4 (C2′), 122.3 (C5), 121.0 (C3), 112.9 (Ca), 40.7 (CH3).
MS (EI): m/z 351 (M+). HRMS (ES+): 352.1809 (M + H+), calcd
for C24H22N3 (M + H+): 352.1808. Anal. Calcd for C24H21N3: C,
82.02; H, 6.02; N, 11.96. Found: C, 81.77; H, 5.99; N, 11.95.
Platinum(II) Complexes. (a) General Procedure for the
Preparation of Platinum Complexes [PtL4Cl]-[PtL8Cl]. Acetic
acid (5 mL) was added to a mixture of the ligand HLn (typically
0.15 mmol) and K2PtCl4 (1 equiv) in an oven-dried Schlenk tube.
The mixture was degassed via three freeze-pump-thaw cycles
and then heated at reflux under a nitrogen atmosphere for 3 days,
generating a yellow or yellow-orange precipitate. Upon cooling to
room temperature, the precipitate was separated off from the yellow
solution, washed sequentially with methanol, water, ethanol, and
diethyl ether (typically 3 × 5 mL of each), and dried under vacuum.
Further purification was required for [PtL5Cl]-[PtL8Cl] to separate
traces of protonated ligand, involving the extraction of the solid
into dichloromethane (5 × 5 mL) and subsequent removal of solvent
(iv) [PtL7Cl] tolyl: orange solid in 56% yield. H NMR (400
MHz, CDCl3): δ 9.31 (2H, dd, 3J ) 5.5 Hz, 4J ) 1.0 Hz, 3J(195Pt)
) 37.6 Hz, H6), 7.96 (2H, td, 3J ) 8.0 Hz, 4J ) 1.2 Hz, H4), 7.72
1
(2H, d, J ) 8.0 Hz, H3), 7.58 (2H, s, H4′), 7.51 (2H, d, J ) 8.4
Hz, Hb), 7.28-7.24 (4H, m, H5, Ha), 2.42 (6H, s, CH3). 13C NMR
(CDCl3): δ 168.1 (quat), 152.0 (C6), 142.3 (quat), 140.1 (C4), 138.3
(quat), 136.4 (quat), 136.1 (quat), 129.4 (C5), 127.1 (Cb), 123.2
(Ca), 123.0 (C4′), 119.0 (C3), 20.4 (CH3). MS (LSIMS): m/z 516
(M+ - Cl). Anal. Calcd for C23H17N2PtCl: C, 50.05; H, 3.10; N,
5.08. Found: C, 49.52; H, 3.13; N, 4.80.
3
3
(v) [PtL8Cl] thienyl: orange solid in 23% yield. 1H NMR (400
MHz, CDCl3): δ 9.14 (2H, d, J ) 5.0 Hz, J(195Pt) ) 37.6 Hz,
3
H6), 7.84 (2H, t, J ) 8.0 Hz, H4), 7.60 (2H, d, J ) 8.0 Hz, H3),
7.43 (2H, s, H4′), 7.20 (2H, m, Ha, Hc), 7.13 (2H, dd, 3J ) 8.0 and
5.0 Hz, H5), 7.00 (1H, m, Hb). 13C NMR (CDCl3): δ 150 (C6-py),
144 (quat), 142 (quat), 138 (C4 -py), 130 (quat), 129 (C4-thio), 125
(C5-thio), 124 (C5-py), 123 (C3-thio), 122 (C4 and C6), 120
(C3-py). MS (LSIMS): m/z 508 (M+ - Cl).
3
3
9702 Inorganic Chemistry, Vol. 44, No. 26, 2005

N∧C∧N-Coordinated Platinum(II) Complexes
(vi) [PtL9Cl] Dimethylamino. This complex was prepared using
a different solvent system, as used previously for [PtL2Cl].24 HL9
(50 mg, 0.14 mmol) was dissolved in acetonitrile (4 mL) in a
Schlenk tube, and K2PtCl4 (59 mg, 0.14 mmol) was dissolved in
water (2 mL) in a separate Schlenk tube. Both solutions were
degassed via three freeze-pump-thaw cycles and placed under
nitrogen, and the K2PtCl4 solution was then transferred to the ligand
solution using a cannula. The mixture was heated at reflux under
nitrogen for 3 days. Upon cooling, the orange precipitate that had
formed was separated by centrifugation, washed with water (4 ×
5 mL), ethanol (4 × 5 mL), and diethyl ether (4 × 5 mL), and
10-6-1.5 × 10-4 M. Plots of the observed decay rate constants
versus concentration were linear, and the quoted lifetime at infinite
dilution was obtained from the intercept, as described in the Results
and Discussion section. The estimated uncertainty in the quoted
lifetimes is (10% or better. Bimolecular rate constants for
quenching by molecular oxygen were determined from the lifetimes
in degassed and air-equilibrated solutions, taking the concentration
of oxygen in CH2Cl2 at 0.21 atm of O2 to be 2.2 mmol dm-3 45
.
Crystallography. The single-crystal X-ray diffraction experi-
ments were carried out at 120 K, using graphite-monochromated
Mo KR radiation (λ ) 0.710 73 Å) on Bruker SMART area detector
diffractometers, equipped with Cryostream N2 open-flow cooling
devices.46 A series of narrow ω scans (0.3°) were performed at
several æ settings in such a way as to cover a sphere of data to a
maximum resolution between 0.70 and 0.77 Å. Cell parameters
were determined and refined using the SMART software,47 and raw
frame data were integrated using the SAINT program.48 All three
structures were solved by direct methods and refined by full-matrix
least squares on F 2 using SHELXTL software.49 Reflection intensi-
ties were corrected by numerical integration using SHELXTL
software, based on crystal measurements and indexing of the faces.49
All non-hydrogen atoms were refined with anisotropic displacement
parameters, and the hydrogen atoms were positioned geometrically
and refined using a riding model. For the structure of HL5, the
terminal methyl group was modeled as rotationally disordered with
occupancies 60% and 40% for the two components. Crystal data
and structure refinement parameters are given in Table S2 of the
Supporting Information, and further data are in the accompanying
CIF file.
1
finally dried under vacuum (48 mg, 58%). H NMR (400 MHz,
3
4
3
CDCl3): δ 9.31 (2H, dd, J ) 5.5 Hz, J ) 1.5 Hz, J(195Pt) )
37.6 Hz, H6), 7.92 (2H, td, J ) 8.0 Hz, J ) 1.6 Hz, H4), 7.71
3
4
(2H, dd, J ) 8.0 Hz, J ) 1.0 Hz, H3), 7.55 (2H, s, H4′), 7.51
3
4
(2H, d, 3J ) 7.0 Hz, Hb), 7.24 (2H, dd, 3J ) 7.0 Hz, 4J ) 1.6 Hz,
H5), 6.83 (2H, d, J ) 8.4 Hz, Ha), 3.02 (6H, s, CH3). 13C NMR
3
(CDCl3): δ 208.1 (C2), 168.4 (quat), 152.0 (C6), 142.1 (quat), 140
(C4), 128 (Cb), 123 (C5), 122.5 (Ca), 120.3 (C3), 113.0 (C3′), 40.1
(CH3). MS (LSIMS): m/z 581.1 (M+), 545.1 (M - Cl+). Anal.
Calcd for C24H20N3PtCl‚1/2H2O: C, 48.86; H, 3.59; N, 7.12.
Found: C, 48.92; H, 3.33; N, 6.73.
Electrochemical and Photophysical Measurements. Cyclic
voltammetry was carried out using a Princeton Applied Research
potentiostat 263, using 1 mM solutions of the complexes in 90%
CH3CN/10% CH2Cl2, in the presence of [Bu4N][BF4] (0.1 M) as
the supporting electrolyte. Solutions were purged with nitrogen gas
prior to each scan. A three-electrode system was employed using
platinum working, reference, and counter electrodes. The voltam-
mograms were referenced to a ferrocence-ferrocenium couple as
the standard (E0 ) 0.42 vs SCE).
Absorption spectra were measured on a Biotek Instruments XL
spectrometer, using quartz cuvettes of 1-cm path length. Steady-
state luminescence spectra were measured using a Jobin Yvon
FluoroMax-2 spectrofluorimeter, fitted with a red-sensitive Hamamat-
su R928 photomultiplier tube; the spectra shown are corrected for
the wavelength dependence of the detector, and the quoted emission
maxima refer to the values after correction. Samples for emission
measurements were contained within quartz cuvettes of 1-cm path
length modified with the appropriate glassware to allow connection
to a high-vacuum line. Degassing was achieved via a minimum of
three freeze-pump-thaw cycles while connected to the vacuum
manifold; the final vapor pressure at 77 K was <10-2 mbar, as
monitored using a Pirani gauge. Luminescence quantum yields were
determined by the method of continuous dilution, using quinine
sulfate in 1 M H2SO4 (φ ) 0.54644) as the standard; the estimated
uncertainty in φ is (20% or better.
Acknowledgment. We thank the EPSRC for a student-
ship (D.L.R.), Frontier Scientific Ltd. for key starting
materials, and the EPSRC National Mass Spectrometry
Service Centre for several mass spectra. We are indebted to
Dr. A. Beeby for use of the Nd:YAG laser and Dr. R. Kataky
and N. Barooah for assistance with electrochemistry. We are
grateful to Dr. J. Weinstein for helpful comments and for
stimulating discussions on the photophysics of platinum(II)
complexes.
Supporting Information Available: Bond lengths and angles
for [PL6Cl] and [PL3Cl], crystal data and structure refinement
parameters, solvatochromic data for [PL4Cl]-[PL9Cl], correlations
of absorption and emission energies vs oxidation potential, and
further data in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC051049E
Samples for time-resolved measurements were excited at 355
nm using the third harmonic of a Q-switched Nd:YAG laser. The
luminescence was detected with a Hamamatsu R928 photomultiplier
tube and recorded using a digital storage oscilloscope, before
transfer to a PC for analysis. For each complex, a series of at least
six solutions were examined over the concentration range 7.5 ×
(45) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker: New York, 1993.
(46) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105.
(47) SMART-NT, Data Collection Software, version 5.0; Bruker Analytical
X-ray Instruments Inc.: Madison, WI, 1999.
(48) SAINT-NT, Data Reduction Software, version 5.0; Bruker Analytical
X-ray Instruments Inc.: Madison, WI, 1999.
(49) SHELXTL, version 5.1; Bruker Analytical X-ray Instruments Inc.:
Madison, WI, 1999.
(44) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9703