Highly luminescent mixed-metal Pt(II)/Ir(III) complexes: Bis-cyclometalation of 4,6-diphenylpyrimidine as a versatile route to rigid multimetallic assemblies

Article abstract of DOI:10.1021/ic200706e

The proligand 4,6-di-(4-tert-butylphenyl)pyrimidine LH₂ can undergo cycloplatination with K₂PtCl₄ at one of the two aryl rings to give, after treatment with sodium acetylacetonate, a mononuclear complex Pt(N^{^}C-LH)(acac) (denoted Pt). If an excess of K ₂PtCl₄ is used, a dinuclear complex of the form [Pt(acac)]₂{μ-(N^{^}C-L-N^{^}C)} (Pt₂) is obtained instead, where the pyrimidine ring acts as a bridging unit. Alternatively, the mononuclear complex can undergo cyclometalation with a different metal ion. Thus, reaction of Pt with IrCl₃?3H ₂O (2:1 ratio) leads, after treatment with sodium acetylacetonate, to an unprecedented mixed-metal complex of the form Ir{μ-(N^{^}C-L- N^{^}C)Pt(acac)}₂(acac) (Pt₂Ir). The mononuclear iridium complex Ir(N^{^}C-LH)₂(acac) (Ir) has also been prepared for comparison. The UV-visible absorption and photoluminesence properties of the four complexes and of the proligand have been investigated. The complexes are all highly luminescent, with quantum yields of around 0.5 in solution at room temperature. The introduction of the additional metal centers is found to lead to a substantial red-shift in absorption and emission, with λ_max in the order Pt < Pt₂ < Ir < Pt ₂Ir. The trend is interpreted with the aid of electrochemical data and density functional theory calculations, which suggest that the red-shift is due primarily to a progressive stabilization of the lowest unoccupied molecular orbital (LUMO). The radiative decay constant is also increased. This versatile design strategy may offer a new approach for tuning and optimizing the luminescence properties of d-block metal complexes for contemporary applications.

Full text of DOI:10.1021/ic200706e

ARTICLE
pubs.acs.org/IC
Highly Luminescent Mixed-Metal Pt(II)/Ir(III) Complexes:
Bis-Cyclometalation of 4,6-Diphenylpyrimidine As a Versatile
Route to Rigid Multimetallic Assemblies
Valery N. Kozhevnikov,*,† Marcus C. Durrant,† and J. A. Gareth Williams*,‡
†School of Life Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, U.K.
‡Department of Chemistry, Durham University, Durham, DH1 3LE, U.K.
S Supporting Information
b
ABSTRACT: The proligand 4,6-di-(4-tert-butylphenyl)pyrimidine LH2 can undergo
cycloplatination with K2PtCl4 at one of the two aryl rings to give, after treatment with
sodium acetylacetonate, a mononuclear complex Pt(N∧C-LH)(acac) (denoted Pt). If
an excess of K2PtCl4 is used, a dinuclear complex of the form [Pt(acac)]2{μ-(N∧C-L-
N∧C)} (Pt2) is obtained instead, where the pyrimidine ring acts as a bridging unit.
Alternatively, the mononuclear complex can undergo cyclometalation with a different
metal ion. Thus, reaction of Pt with IrCl3 3H2O (2:1 ratio) leads, after treatment with
3
sodium acetylacetonate, to an unprecedented mixed-metal complex of the form
Ir{μ-(N∧C-L-N∧C)Pt(acac)}2(acac) (Pt2Ir). The mononuclear iridium complex
Ir(N∧C-LH)2(acac) (Ir) has also been prepared for comparison. The UVꢀvisible absorption and photoluminesence properties
of the four complexes and of the proligand have been investigated. The complexes are all highly luminescent, with quantum yields of
around 0.5 in solution at room temperature. The introduction of the additional metal centers is found to lead to a substantial red-
shift in absorption and emission, with λmax in the order Pt < Pt2 < Ir < Pt2Ir. The trend is interpreted with the aid of electrochemical
data and density functional theory calculations, which suggest that the red-shift is due primarily to a progressive stabilization of the
lowest unoccupied molecular orbital (LUMO). The radiative decay constant is also increased. This versatile design strategy may
offer a new approach for tuning and optimizing the luminescence properties of d-block metal complexes for contemporary
applications.
’ INTRODUCTION
of the same metal. In the case of Pt(II), for example, studies have
generally been focused on the use of flexible or rigid linkers that
bring together two square-planar, cyclometalated platinum units
in a face-to-face manner, promoting Pt---Pt or ligand---ligand
interactions and induction of lower-energy emission.11 Mean-
while most of the examples involving iridium are represented by
systems in which individual mononuclear metal complexes are
connected either through an ancillary ligand or by a bridging
ligand that acts as a partially insulating spacer. Examples include
compounds that make use of para-phenylene-bridged bis-
bipyridines to link two Ir(ppy)2 units12 or one such unit to a
ruthenium(II) center;10 related assemblies employing tridentate
cyclometallating ligands in conjunction with bridging terpyridines;13
and recent workwith bridging acetylacetonate derivatives.14 In such
compounds, the metal centers retain properties that are similar to
those of the isolated units, with the bridging ligand playing a
relatively minor role.
Cyclometalated iridium(III) and platinum(II) complexes of
2-phenylpyridines and related ligands are often exceptionally
good luminophores.1ꢀ3 Current interest from academia and
industry is driven, in particular, by their suitability for applica-
tions as phosphors in organic light-emitting diodes (OLEDs).4
The success of such complexes in this field is mainly due to the
spinꢀorbit coupling (SOC) effect induced by cyclometalated
third row transition metal centers, which promotes efficient
phosphorescence from the triplet state, allowing the harvesting
of both singlet and triplet type excitons.5 Meanwhile, other
applications have emerged in parallel. For example, such com-
plexes are attractive as the light-emitting units of sensory
molecular systems whose emission is modulated upon binding
of a target analyte,6 or as intrinsically emissive probes that may
localize in selected organelles within living cells.7 They are also
under investigation as potential photocatalysts for “water split-
ting” to generate hydrogen,8 and as sensitizers of energy- and
electron-transfer reactions, also relevant to the conversion of
solar energy to electrical or chemical energy.2,9,10
Very few rigid systems in which two cyclometalated metal
centers are connected by coordination to a common heterocyclic
ring have been reported, and even fewer where the photophysical
properties have been investigated. Hexa-azatriphenylene (HAT)
The vast majority of cyclometalated Pt(II) and Ir(III) com-
pounds studied to date incorporate a single metal center.
Examples of luminescent multinuclear systems with cyclometal-
lating ligands are less common, and mostly comprise two atoms
Received: April 6, 2011
Published: May 31, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Proligand LH2 by Suzuki Cross-Coupling
and 3,5-dipyridyl-1,2,4-triazolate were used in two early studies
linking Ir(ppy)2 and Ru(bpy)2 units,15,16 but the pyrazine-type
bridges introduced severe nonradiative decay.17 Steel and co-
workers described the bis-cyclopalladation of bis-aryl-pyrimi-
dines, -pyrazines, and -pyridazines,18 and, more recently, Rourke
and co-workers prepared a set of homometallic dinuclear Pd(II)
and Pt(II) complexes with 3,6-diphenylpyridazines, though
excited-state properties were considered in neither case.19 Iva-
nova and colleagues observed luminescence at 77 K from
charged, homometallic dinuclear Pd(II) and Pt(II) complexes
of 4,6-diphenylpyrimidine.20 Otherwise, such chemistry has not
been extended to the design of luminescent Pt(II) or Ir(III)
complexes. Indeed, there are apparently no mixed-metal poly-
metallic, cyclometalated complexes having both Pt(II) and Ir-
(III) cyclometalated metal centers present in a single integrated
luminophore, despite the potentially interesting properties that
might be anticipated.
In this contribution, we describe the synthesis of a rigid, poly-
nuclear cyclometalated complex incorporating one Ir(III) and two
Pt(II) centers, using doubly metalated 4,6-di-(4-tert-butylphenyl)-
pyrimidine as a bridging ligand. The corresponding monometallic
complexes and a related dinuclear platinum(II) complex have also
been prepared. All of these complexes are intensely luminescent,
and the trends in excited state energies are interpreted with the aid
of computational and electrochemical studies.
Pt in place of LH2 as the N∧C ligand led to the trinuclear,
heterometallic complex Ir{μ-(N∧C-L-N∧C)Pt(acac)}2(acac)
(Pt2Ir) in 42% yield. All four complexes were very soluble in
dichloromethane owing to the tert-butyl groups, and their
purification was easily accomplished by using standard column
chromatography. The products were characterized by NMR
spectroscopy, mass spectrometry, and elemental analysis. Proton
NMR spectra are well-resolved and straightforward to interpret
because of the presence of a number of characteristic signals.
Even in the most complex structure of Pt2Ir, all 15 signals in the
spectrum can be assigned to the structure (see Supporting
Information ꢀ Section 2).
2. Electrochemistry. The ground-state oxidation and reduction
processes exhibited by the four complexes were probed using cyclic
voltammetry in dichloromethane solution over the range ꢀ2.5 to
1.5 V (potentials versus the ferrocene/ferrocenium couple). Data are
listed in Table 1. The mononuclear platinum complex Pt displays
one quasi-reversible oxidation process within the potential window
investigated, at 0.17 V versus Fc|Fcþ (Supporting Information ꢀ
Section 3). This behavior contrasts with that typically observed for
analogous complexes with simple arylpyridine ligands, where the
oxidation processes are generally chemically irreversible and appear
at more positive potentials; for example, for Pt(N∧C-ppy)(dpm),
Epox = þ1.0 V in DMF versus Fc|Fcþ.22 The irreversibility in such
cases is generally attributed to the susceptibility of the electrogener-
ated Pt(III) species to nucleophilic attack by the solvent. Thus, the
chemical reversibility in the present instance, as well as the substan-
tially lower potential of the oxidation, suggests that the resulting
charge may be more delocalized over the pyridimine-based cyclo-
metallating ligand than in the pyridyl analogues. Meanwhile,
the complex Pt undergoes one quasi-reversible reduction process
atꢀ2.17 V. The potential of this process is of a similar magnitude to
that displayed by a number of Pt(N∧C)(O∧O) complexes, where-
in reduction is assumed to be largely ligand-localized.22
The dinuclear complex Pt2 displays two quasi-reversible
oxidation processes at 0.27 and 0.56 V (Figure 1). These may
reasonably be attributed to oxidation associated with each of the
two metalated units of the ligand, the oxidation of the monoca-
tion being expected to be somewhat disfavored compared to the
charge-neutral complex. On the other hand, only one reduction
process is observed. This would be consistent with the reduction
being based on the single heterocyclic ring of the ligand, as in
complexes of other aryl-heterocycle ligands such as phenylpyr-
idine, where reduction is thought to occur primarily at the pyridyl
ring. The anodic shift of this reduction process compared to
Pt (the potentials are ꢀ1.93 and ꢀ2.16 V for Pt2 and Pt
respectively) indicates that the introduction of the second
platinum(II) center stabilizes the system with respect to reduction.
The mononuclear iridium(III) complex Ir displays one rever-
sible oxidation at 0.00 V (the same potential as Fc|Fcþ)
(Supporting Information ꢀ Section 3), but no reduction wave
’ RESULTS
1. Synthesis of Complexes. The synthetic strategy used to
prepare the target complexes is depicted in Schemes 1 and 2. The
key proligand 4,6-di-(4-tert-butylphenyl)pyrimidine, LH2, was
synthesized by a Suzuki cross-coupling reaction of 4,6-dichlor-
opyrimidine with 2.6 equiv of 4-tert-butylbenzeneboronic acid
(Scheme 1). The tert-butyl functionality was introduced to
increase the solubility of the final metal complexes, and also to
inhibit aggregation of the Pt(II) compounds, a phenomenon
commonly encountered with square-planar d8 metal complexes.21
The mono- and dinuclear Pt(II) complexes, Pt(N∧C-LH)-
(acac) and [Pt(acac)]2{μ-(N∧C-L-N∧C)} (which will be hence-
forth be referred to respectively as Pt and Pt2) were obtained
from the dichloro-bridged intermediates by heating a mixture of
the proligand with, respectively, 1 or 2 equiv of K2PtCl4 in acetic
acid (Scheme 2). The highly insoluble precipitates were collected
by filtration and treated with hot dimethylsulfoxide followed by
sodium acetylacetonate to give complexes Pt and Pt2 in yields of
52 and 30%, respectively. The corresponding mononuclear
iridium complex Ir(N∧C-LH)2(acac) (referred to as Ir) was
prepared from LH2 upon reaction with IrCl3 3H2O (2.2: 1
3
molar ratio), followed by treatment with excess sodium acetyla-
cetonate, without isolation of the intermediate dichloro-bridged
dimer. An analogous reaction of IrCl3 3H2O with the complex
3
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Scheme 2. Preparation of the Mononuclear Complexes (Ir and Pt), the Homometallic Dinuclear Compound (Pt2), and the
Trinuclear Complex (Pt2Ir), from Proligand LH2
is observed within the accessible solvent window. This pattern of
behavior is consistent with that normally exhibited by Ir(N∧C)2-
(acac) complexes.23 As in the case of platinum, the oxidation
process is seen to be facilitated compared to complexes of
with each of the three arylated metals and, based on the values for
the mononuclear complexes, it seems likely that the Ir center
would undergo oxidation first. It is noticeable, however, that even
the first oxidation process for this compound is substantially
shifted anodically compared to that of Ir, indicating that the
presence of the two platinum(II) cations makes the system less
readily oxidized. Two reversible reduction waves are observed, at
potentials similar to those found for Pt and Pt2, an observation
again supportive of the notion that reduction is based on the
pyrimidine rings.
ox
arylpyridines; for example, for Ir(ppy)2(acac), E1/2 = 0.45 V
versus Fc|Fcþ. Again, this implies that the pyrimidine unit is able
to stabilize the resulting cationic species.
In the trinuclear, mixed-metal complex Pt2Ir, three close-lying
oxidation processes can be discerned (Figure 2), though they are
not clearly resolved by cyclic voltammetry. Square-wave voltam-
metry allows them to be discriminated more readily. These
processes can reasonably be attributed to oxidations associated
3. Absorption Spectroscopy. The UVꢀvisible absorption
spectra of the four complexes in dichloromethane solution at
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Table 1. Ground-State Oxidation and Reduction Potentials and UV-visible Absorption Data for the Four Complexes and
Proligand LH2.a
complex
absorption λmax / nm (ε/Mꢀ1 cmꢀ1
)
E1/2ox / V (ΔE/mV)b
E1/2red / V (ΔE/mV)b
proligand LH2
304 (24500), 289 (24200), 259 (25000)
399 (11900), 338sh (18500), 308 (38100), 255 (26000)
Pt
0.17 (200)
0.14 (160)
0.43 (220)
0.00 (180)
0.44 d
0.77d
1.02d
ꢀ2.16 (240)
ꢀ1.93 (180)
Pt2
530 (1430), 474 (24500), 417 (16600), 396 (12200), 333 (47200), 256 (36900)
c
Ir
513 (6210), 497sh (6050), 421 (12600), 376 (20200), 308 (63500), 278 (50800)
568 (13100), 536 (11400), 470 (26300), 443 (24400), 410 (24800), 388 (24500),
330 (69700), 278 (41900), 256 (51300)
Pt2Ir
ꢀ2.10 (120)
ꢀ2.33 (140)
a In CH2Cl2 at 298 ( 3 K. b Potentials obtained by cyclic voltammetry, using Bu4NPF6 (0.1 M) as the supporting electrolyte. Potentials are given relative
to the Fc|Fcþ couple measured under the same conditions. c No reduction wave was observed for this complex within the accessible solvent window.
d The three oxidations observed for this complex are overlapping in the CV. The quoted values are therefore those obtained by square-wave
potentiometry.
Figure 2. Cyclic voltammograms showing the first two reductions and
Figure 1. Cyclic voltammograms showing the first reduction and first
the first three oxidation processes of Pt2Ir in CH2Cl2 at room tempera-
two oxidation processes of Pt2 in CH2Cl2 at room temperature.
ture (solid line). The oxidation processes are more clearly resolved using
square-wave voltammetry (dashed line).
ambient temperature, together with the spectrum of proligand
LH2 under the same conditions, are shown in Figure 3. The main
the iridium(III) systems, as in the current instance (Eox = 0.00
absorbance maxima and their extinction coefficients are com-
piled in Table 1. The mononuclear platinum complex Pt displays
intense bands in the far-UV region around 250ꢀ260 nm, similar
to those observed for the proligand, and attributed to ligand-
based πꢀπ* transitions. As typically observed for cyclometalated
Pt complexes,24 including those such as Pt(ppy)(acac),22 there is
an additional set of bands extending into the visible region
beyond 400 nm. On the basis of many previous studies,22,24
and on time-dependent density functional theory (TD-DFT)
studies in the present case (vide infra), at least some of these
bands can be attributed to charge-transfer transitions from the
arylꢀmetal unit to the heterocyclic ring. The mononuclear
iridium complex Ir displays a set of bands that extend further
into the visible region, beyond 500 nm. This contrasting behavior
of iridium(III) compared to platinum(II) with a given cyclome-
tallating ligand mirrors the trend observed for complexes with
N∧C-coordinating aryl-pyridine ligands, where bis-cyclometa-
lated Ir complexes invariably absorb out to longer wavelengths
than their monocyclometalated Pt counterparts.25 It may be
tempting to interpret this in part to the more facile oxidation of
compared to 0.17 V for the mononuclear Ir and Pt complexes
respectively), although such a simplistic argument overlooks the
fact that the ligand-based reduction in the Ir complexes is
frequently cathodically shifted to such an extent that it lies
beyond the normally accessible solvent window. Moreover,
transitions involving direct absorption to triplet states may also
contribute to the low-energy region of the spectrum, and the
spinꢀorbit coupling pathways that facilitate such processes are
typically more efficient for octahedral complexes of d6 metal ions
than for square planar complexes of d8 metal ions with compar-
able spin-orbit coupling constants,5b,26 leading to more signifi-
cant triplet contributions to the absorption spectrum of iridium
complexes.
The spectrum of the dinuclear platinum complex Pt2 is quite
different from its mononuclear counterpart. It displays a very
strong absorption band (ε ∼ 25000 Mꢀ1cmꢀ1) at substantially
longer wavelength (474 nm) than the longest wavelength band of
Pt. There is also an additional, weaker low-energy band apparent
at 530 nm, which may tentatively be attributed to direct excita-
tion to the triplet state (vide infra). Clearly, the dinuclear system
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no detectable emission bands corresponding to the mononuclear
unit. Likewise, the mixed-metal trimer Pt2Ir shows no emission
bands corresponding to the Pt unit. Excitation spectra match
closely the absorption spectra. As in absorption, the multinuclear
systems are thus again seen to behave as phosphors that are
intrinsically different from their constituent units.
At room temperature, vibrational structure is just discernible
in the mononuclear platinum complex, while it is more clear-cut
in the dinuclear compound. At 77 K (Figure 4b), the structure
becomes well-resolved in both cases, showing a vibrational
Figure 3. UVꢀvisible absorption spectra of the metal complexes in
CH2Cl2 solution at 298 K, together with the spectrum of the proligand
LH2 under the same conditions. Legend: Pt = blue line; Pt2 = green; Ir =
orange; Pt2Ir = red; LH2 = black.
does not behave simply as two interconnected, discrete mono-
nuclear platinum units, since in that case, the band positions
would be expected to be similar to those in Pt, albeit with higher ε
values. The introduction of the second platinum ion has a more
profound effect on the excited state properties.
The mixed-metal trinuclear complex Pt2Ir is striking in that it
is vivid red in color, in contrast to the yellow or orange colors of
the other complexes. Its absorption spectrum reveals strong
bands that extend out to 600 nm, substantially lower in energy
than those displayed by either of the constituent mononuclear
units Pt and Ir. A series of bands in the 370ꢀ500 nm region are
around twice as intense as those of Ir in the same region.
4. Photoluminescence Properties. All four complexes are
highly luminescent, both in the solid state and in solution at room
temperature. Key photoluminescence parameters are summar-
ized in Table 2. The emission spectra at ambient temperature in
solution and at 77 K are shown in Figure 4, together with the
spectrum of LH2 at 77 K for comparison. There is no detectable
emission from the proligand at room temperature, but at 77 K
phosphorescence is observed. From the position of its 0,0 band,
the energy of the triplet state in the proligand is estimated to be
23500 cmꢀ1. The introduction of the metal ions is seen to induce
a stabilization of the triplet state, as always observed upon
cyclometalation. The trend in emission λmax, Pt < Pt2 < Ir <
Pt2Ir, mirrors that observed for the lowest-energy absorption
bands. In particular, the dinuclear platinum complex Pt2 shows
Figure 4. (a) Top: Emission spectra of the four metal complexes in
CH2Cl2 solution at 298 K. (b) Bottom: Emission spectra of the
complexes and of proligand LH2 at 77 K in diethyl ether/isopentane/
ethanol (2:2:1 v/v). Color scheme as in Figure 3.
Table 2. Luminescence Data for the Metal Complexes at 298 K in CH2Cl2 and at 77 K (Including Proligand LH2) in EPAa
emission 77 K
compound
Pt
emission λmax/nm
Φlum
τ/nsb
kQO2/108 Mꢀ1 sꢀ1c
kr/105 sꢀ1d
∑knr/105 sꢀ1d
λmax/nm
τ/μs
5.6
513, 536sh
550, 591, 647
585
0.31
0.54
0.39
0.41
2100 (340)
1700 (650)
1000 (190)
580 (230)
11
4.3
19
12
1.5
3.2
3.9
7.1
3.3
2.7
6.1
10
494, 531, 569, 616
540, 561, 575sh, 585, 631
554, 595
Pt2
5.8
Ir
16
Pt2Ir
626
e
596, 643
12
1.8 ꢁ 106
LH2 proligand
425, 455, 482, 516
a EPA = diethyl ether/isopentane/ethanol (2:2:1 v/v). b Luminescence lifetime in degassed solution; corresponding values in air-equilibrated solution
are given in parentheses. c Bimolecular rate constants for quenching by dissolved molecular oxygen, assuming [O2] = 2.2 mmol dmꢀ3 in CH2Cl2 at 1 atm
pressure of air. d Radiative (kr) and nonradiative (∑knr) decay constants estimated from the measured quantum yields and lifetimes, assuming that the
emissive state is formed with unitary efficiency upon excitation. e No emission detectable at room temperature.
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progression of ∼1400 cmꢀ1 similar to that of the proligand HL2.
The 0,0 component band displays the highest intensity in both
cases, indicative of a rigid structure with little distortion upon
formation of the excited state compared to the ground state. The
spectra of the iridium-containing complexes Ir and Pt2Ir are
unstructured at room temperature, with a shoulder to the low-
energy side visible at 77 K. Such behavior is quite typical of metal
complexes with a high degree of metal-to-ligand charge-transfer
(MLCT) character {for example, the archetypal cases of
[Ru(bpy)3]2þ and Ir(ppy)3},27 while more structured spectra
are generally associated with an increased contribution of ligand-
centered character to the excited state.
The quantum yields are high in each case (0.31ꢀ0.54,
Table 2). It is particularly noticeable that a high value is retained
even for the lowest-energy, red-emitting complex Pt2Ir, whereas
the efficiency of many metal-based phosphors tends to fall off
toward the red, owing to increased nonradiative decay processes
through coupling to low-energy vibrations.28,29 The lumines-
cence lifetimes are of the order of a microsecond, a little less for
Pt2Ir, and a little more for the platinum complexes. Assuming
that the emitting state is formed with unitary efficiency upon
photoexcitation, the radiative (kr) and nonradiative (∑knr) decay
constants can be estimated from the measured lifetimes and
quantum yields (Table 2). Two trends emerge from a considera-
tion of the kr values. First, the values for the iridium-containing
complexes are larger than for the platinum-only complexes. This
is quite typical for iridium(III) versus platinum(II) complexes of
the same ligands,25 and is a further reflection of the more efficient
spinꢀorbit coupling pathways associated with an octahedral
metal center compared to a square-planar one with similar ζ.
The presence of three degenerate filled d orbitals in the former
case facilitates the mixing of the triplet state with higher-lying
1MLCT states.26
Second, comparing Pt2 with Pt, it can be seen that kr increases
upon introduction of the second metal center, despite the
emission energy decreasing. Other things being equal, a decrease
in the radiative rate would be expected as the energy decreases,
according to the Einstein coefficient which depends on ν3.
Likewise, the presence of the platinum centers in Pt2Ir is seen
to enhance kr compared to that of Ir, despite the emission energy
of the former being lower. The kr value roughly doubles on going
from Pt to Pt2 and likewise from Ir to Pt2Ir. Apparently,
therefore, the T1fS0 process is promoted by the presence of
the additional metal center(s). It might be intuitively reasonable
to attribute this effect to the spinꢀorbit coupling of the extra
metal ions facilitating intersystem crossing. However, it can be
seen from the absorption spectra (Figure 3) that the S0fS1
process also increases in intensity from Pt to Pt2 and from Ir to
Pt2Ir (by a similar 2-fold factor to the increase in triplet kr in each
case), suggesting that the oscillator strength also increases for the
spin-allowed singlet transitions. These conclusions are supported
by the results of calculations discussed in the next section.
5. TD-DFT Calculations. To facilitate the interpretation of the
electrochemical and photophysical results, TD-DFT calculations
have been carried out on the molecules, both in the gas phase and
in dichloromethane solution using the polarized continuum
solvent model.30 We have also included the hypothetical complex
Pt(acac){μ-(N∧C-L-N∧C)}Ir(acac)(N∧C-LH) (PtIr) for com-
parison. The B3LYP functional was used, with the LanL2DZ
basis set for Pt and Ir and 6-31þG(d) basis set for all other atoms,
as implemented in Gaussian09W.31 Full geometry optimizations
were carried out, but the larger complexes Ir, PtIr, and Pt2Ir were
truncated by removal of the tert-butyl groups to keep the job
times manageable. The optimized geometry of proligand LH2
shows twists of 14° and 16° in the dihedral angle between the
central pyrimidine and the noncoordinated flanking aryl rings,
reflecting the anticipated opposing balance between conjugation
and steric effects. In the complexes, the dihedral angles for the
equivalent noncoordinated aryl rings range from 6° (Ir) to 16°
(PtIr), while the coordinated aryl rings are essentially coplanar
with the pyrimidine ring in each case (dihedral angles <1.5°).
The calculated PtꢀC and PtꢀN bond lengths for the four Pt
moieties range from 1.980 to 1.983 Å and from 2.020 to 2.036 Å,
respectively, while the IrꢀC and IrꢀN bond lengths for the three
Ir units are in the range 2.001ꢀ2005 Å and 2.053ꢀ2.065 Å
respectively. Each Pt center has one longer and one shorter
PtꢀO bond, in the ranges of 2.147ꢀ2.153 Å and 2.036ꢀ2.044 Å,
respectively, reflecting the subtly greater CdO double bond
character trans to the metalated carbon (higher trans influence
than N) and, conversely, CꢀOꢀ trans to the pyrimidine N atom.
This is also reflected in slightly different CꢀO and CꢀC
distances in each acac ligand coordinated to Pt. In contrast, the
acac ligands on Ir, all of which are trans to carbon, show
equivalent CꢀO and CꢀC bond lengths; and the IrꢀO dis-
tances, which span the range of 2.058ꢀ2.196 Å, are equivalent in
each of the three Ir complexes. All of the calculated bond lengths
around Pt and Ir are comparable to those for related complexes of
arylpyridines determined by X-ray diffraction.22,32,24h
The calculations reveal that the lowest-energy singlet transitions
(S0fS1) are composed predominantly of HOMOfLUMO
excitations in each case (Tables S1 and S2 in Supporting Informa-
tion ꢀ Section 4). The frontier orbitals of the Pt complexes are
shown in Figure 5. In the mononuclear Pt complex, the highest
occupied molecular orbital (HOMO) is seen to be based primarily
on the metalated aryl ring and the metal, whereas the LUMO is
mostly localized on the pyrimidine ring. Such a trend has been
commonly encountered in complexes of arylpyridines, and is
consistent with the usual dPt/πN∧Cfπ*N∧C assignment of the
emissive excited state.24 In the dinuclear complex Pt2, the pattern
for the HOMO is similar, being based on each metalated aryl ring,
with little conjugation apparent between the two halves of the
molecule. In contrast, the LUMO is delocalized over both units
and the central pyrimidine. We can thus expect a stabilization of
the LUMO upon introduction of the second platinum center, with
little effect on the HOMO, and indeed the calculated energies of
the orbitals reveal this to be the case; the HOMO energies for Pt
and Pt2 in vacuo are ꢀ5.72 and ꢀ5.73 eV, respectively, while the
LUMO energies are ꢀ2.28 and ꢀ2.51 eV, respectively.
This conclusion is consistent with the electrochemical results,
where the reduction potential was anodically shifted in Pt2 com-
pared to Pt, while there was little effect on the oxidation potential. As
a result of the stabilization of the LUMO, the HOMOfLUMO gap
is decreased from 3.52 eV for Pt to 3.24 eV for Pt2 (calculated values
in CH2Cl2; the corresponding value calculated for the proligand is
4.49 eV). This trend is in line with the observed shift of the lowest
energy absorption band to longer wavelengths on going from LH2
to Pt to Pt2. The absorption spectra simulated on the basis of the
TD-DFT results are shown in Figure 6 and, indeed, they reveal a
good match to the observed spectra of Figure 3, in terms not only of
the positions of the bands, but also their relative intensities. In
addition, a calculation including the triplet excited states for Pt2 in
dichloromethane placed the formally spin-forbidden S0fT1 transi-
tion at 537 nm (predominantly HOMOfLUMO), in good
agreement with the observed weak band at 530 nm.
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ARTICLE
Figure 5. Frontier orbitals of the mononuclear and dinuclear platinum complexes and of the mixed-metal trinuclear complex, obtained by TD-DFT,
plotted at an isovalue of 0.04.
Figure 6). This is consistent with the experimental results of
absorption spectroscopy discussed in Section 4, a trend that was
also apparent in the radiative rate constants of emission from the
corresponding triplet states.
’ CONCLUDING DISCUSSION
In this work, we have prepared a binuclear platinum complex
and a trinuclear heterometallic complex incorporating iridium-
(III) and platinum(II) ions, in which the metals are cyclometa-
lated to a common ligand leading to an integrated luminophore.
The corresponding mononuclear complexes have also been
prepared, allowing the influence of the extra metal ions to be
probed. The combination of electrochemistry, TD-DFT, absorp-
tion and emission spectroscopy reveals that the multimetallic
systems behave as chromophores that are intrinsically different
from their mononuclear counterparts, owing primarily to the
conjugation of the LUMO across the entire aromatic ligand, with
the pyrimidine acting as a bridge of communication. Thus, unlike
many other multimetallic systems in supramolecular photochem-
istry, the metallic units do not retain their separate identities, and
Figure 6. Simulated UVꢀvisible absorption spectra of the compounds
by TD-DFT calculations with CH2Cl2 implicit solvent model. Colors as
in Figure 3, namely, black, LH2; blue, Pt; green, Pt2; orange, Ir; red,
it is not appropriate to consider them as individual units between
which energy transfer occurs. Rather, the introduction of addi-
tional metal centers is seen to allow additional control over the
excited state properties of the important class of phosphorescent
compounds based on cyclometalated metal complexes. In parti-
cular, in the present instance, it can be seen that the absorption
and emission of an iridium complex can be signficantly shifted
further into the red region through introduction of the platinum
ions. The ability to shift absorption to the red is desirable, for
example, in the absorbing units of dye-sensitized solar cells.
Moreover, the augmentation of the radiative rate constants in
response to the added metal ions, as well as the rigidity of the
systems which keeps nonradiative decay to a minimum, ensures
Pt2Ir; in addition, the spectrum of the hypothetical complex PtIr is
included in gray.
The calculated spectrum of the hypothetical complex PtIr is
very similar to that of Pt2Ir, and the HOMOfLUMO gaps for
these two complexes are identical (2.99 eV in dichloromethane,
compared to 3.18 eV for Ir), confirming that the key determinant
of the energy of the lowest-energy band is the presence of the two
different metals.33 It may also be noted that the calculations
reveal a significant increase in the oscillator strength of the
lowest-energy transitions on going from Pt to Pt2 and from Ir
to Pt2Ir (Supporting Information, Tables S1 and S2 and
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that high emission quantum yields are retained in the red region,
a desirable property in, for example, solar concentrators and red
phosphors for OLEDs alike. We expect that these first examples,
which make use of the versatile aryl pyrimidine unit, will lead to
new opportunities for controlling excited states for such con-
temporary applications.
washed with water and acetone (5 mL). The product was then purified
by filtering through a short plug of silica gel, which was eluted with a
mixture of CH2Cl2 and ethyl acetate (10/1). The solvent was evaporated
to dryness; the residue was treated with acetone and filtered, to give the
product as bright orange crystals (90 mg, 51%). 1H (400 Hz, CDCl3) δ =
9.87 (br s, 1H), 7.63 (d, 2H, J = 2.0), 7.44 (br s, 1H), 7.41 (d, 2H, J = 8.2),
7.14 (dd, 2H, J = 8.2, 2.0), 5.44 (s, 2H), 2.01 (s, 6H), 1.97 (s, 6H), 1.36
(s, 18H). Elemental analysis calcd for C34H40N2O4Pt2: C 43.87, H 4.33,
N 3.01%; found C 44.00, H 4.25, N 3.20%. MS (MALDI, DCTB/
DCM): m/z = 928.2 [M]þ.
’ EXPERIMENTAL SECTION
1H and 13C NMR spectra were recorded on Bruker Avance-400 or
JEOL Delta-270 spectrometers operating at the frequencies indicated.
Chemical shifts (δ) are in ppm, referenced to residual protio-solvent
resonances, and coupling constants are in hertz. Mass spectra were
recorded at the EPSRC National Mass Spectrometry Service Centre
using MALDI ionization on an Applied Biosystems Voyager instrument,
using DCTB as the matrix. Elemental analyses were carried out using a
Carlo Erba 1108 Elemental Analyzer controlled with CE Eager 200
software, run in accordance with the manufacturer’s instructions and
weighed using a certified Mettler MX5Microbalance.
Mononuclear Iridium Complex Ir. A mixture of LH2 (205 mg,
0.6 mmol), IrCl3 3H2O (95 mg, 0.27 mmol) and ethoxyethanol was
3
heated at 110 °C for 14 h. Sodium acetylacetonate (165 mg, 1.35 mmol)
was added, and the mixture was stirred at 90 °C for 5 h. The solvent was
removed by rotary evaporation, and the residue purified by column
chromatography (silica gel, CH2Cl2) to give the product as an orange
solid (374 mg, 64%). 1H (400 Hz, CDCl3) δ = 9.17 (d, 1H, J = 1.2), 8.20
(AA0XX0, 4H, J = 8.6), 8.11 (d, 2H, J = 1.2), 7.67 (d, 2H, J = 8.2), 7.63
(AA0XX0, 4H, J= 8.6), 6.95(dd, 2H, J = 8.2, 1.9), 6.46(d, 2H, J= 1.9), 5.24
(s, 1H), 1.83 (s, 6H), 1,43 (s, 18H), 1.08 (s, 18H). Elemental analysis
calcd for C53H61IrN4O2: C 65.07, H 6.28, N 5.73%; found C 64.87, H
6.77, N 5.81%. MS (MALDI, DCTB/DCM): m/z = 976.4 [M]þ.
Mixed-Metal Trinuclear Complex Pt2Ir. A mixture of Pt
4,6-Di-(4-tert-butylphenyl)pyrimidine LH2. A mixture of
4-tert-butylphenylboronic acid (2.0 g, 11.2 mmol), 4,6-dichloropyrimi-
dine (643 mg, 4.3 mmol), 2 M aqueous solution of K2CO3 (13 mL,
26 mmol), and 1,4-dioxane was deaerated by bubbling argon through the
mixture for 10 min. A catalytic amount of Pd(PPh3)4 was added, and
argon was bubbled through the mixture for an additional 10 min. The
reaction mixture was then stirred at 95 °C for 24 h. After completion of
the reaction, toluene (20 mL) was added, and the mixture was washed
with water. The organic layer was dried over MgSO4 and evaporated to
dryness. The product was then purified by column chromatography
(silica gel, ethyl acetate/hexane, 1/4) followed by recrystallization from
(340 mg, 0.53 mmol), IrCl3 3H2O (85 mg, 0.24 mmol) and ethox-
3
yethanol was heated at 110 °C for 12 h. Sodium acetylacetonate
(150 mg, 1.23 mmol) was added, and the mixture was stirred at 80 °C
for 4 h. The solvent was removed by rotary evaporation, and the residue
was purified by column chromatography (silica gel, CH2Cl2, followed by
a second column on silica gel, CH2Cl2/hexane, 1/1). Fractions with the
product were combined and evaporated to dryness. The residue was
treated with methanol and filtered to give the product as bright red solid
(163 mg, 42%). 1H (270 Hz, CDCl3) δ = 9.35 (d, 2H, J = 0.9), 7.76 (br.s,
2H), 7.68 (d, 2H, J = 1.9), 7.57 (d, 2H, J = 8.5), 7.54 (d, 2H, J = 8.5), 7.19
(dd, 2H, J = 8.2, 2.0), 6.89 (dd, 2H, J = 8.3, 1.9), 6.48 (d, 2H, J = 1.9),
5.40 (s, 2H), 5.20 (s, 1H), 1.96 (s, 6H), 1.84 (s, 6H), 1.80 (s, 6H), 1.34
(s, 18H), 1.01 (s, 18H). Elemental analysis calcd for C63H73IrN4O6Pt2:
C 48.36, H 4.70, N 3.58%; found C 48.41, H 4.98, N 3.69%. MS
(MALDI, DCTB/DCM): m/z = 1560.4 [M]þ.
1
methanol, giving the product as a colorless solid (990 mg, 67%). H
(270 MHz, CDCl3): δ = 9.27 (d, 1H, J = 1.2), 8.07 (AA0XX0, 4H, J =
8.7), 8.06 (br s, 1H), 7.54 (AA0XX0, 4H, J = 8.7), 1.41 (s, 18H). 13C (68
MHz, CDCl3): δ = 164.5, 159.1, 154.4, 134.3, 127.0, 126.0, 112.3, 34.9,
31.2. Elemental analysis calcd for C24H28N2: C 83.68, H 8.19, N 8.13%;
found C 83.75, H 8.06, N 8.12%.
Mononuclear Platinum Complex Pt. A mixture of LH2
(450 mg, 1.3 mmol) and potassium tetrachloroplatinate (542 mg, 1.3
mmol) in acetic acid (125 mL) was heated under reflux for 14 h. The
solvent was removed by rotary evaporation, and the residue was
dissolved in dimethylsulfoxide (DMSO, 20 mL), heated under reflux
for 1 min, and allowed to cool to room temperature. Water (70 mL) was
added, and the solid that precipitated was filtered off, washed with water,
and dissolved in acetone (70 mL). Sodium acetylacetonate (1.6 g,
13 mmol) was added, and the mixture was heated under reflux for
5 h. Water (70 mL) was added, and the precipitated solid was filtered off,
washed with water, and dried. Purification by column chromatography
(silica gel, CH2Cl2) led to the product as a yellow solid (429 mg, 52%).
1H (400 Hz, CDCl3) δ = 9.59 (d, 1H, J = 1.2), 8.12 (AA0XX0, 2H, J =
8.7), 7.84 (d, 1H, J = 1.2), 7.73 (d, 1H, J = 2.0), 7.58 (AA0XX0, 2H, J =
8.7), 7.57 (d, 1H, J = 2.0), 7.23 (dd, 1H, J = 8.2, 2.0), 5.52 (s, 1H), 2.06 (s,
3H), 2.04 (s, 3H), 1.41 (s, 9H), 1.40 (s, 9H). Elemental analysis calcd for
C29H34N2O2Pt: C 54.62, H 5.37, N 4.39%; found C 54.63, H 5.52, N
4.47%. MS (MALDI, DCTB/DCM): m/z = 636.1 [M]þ.
Dinuclear Platinum Complex Pt2. A mixture of LH2 (345 mg, 1
mmol) and potassium tetrachloroplatinate (830 mg, 2 mmol) in acetic
acid (125 mL) was heated under reflux for 3 days. The solvent was
removed by rotary evaporation, the residue dissolved in DMSO
(20 mL), heated to reflux for 3 min, and then the solvent was evaporated
under reduced pressure. The residue was treated with methanol to give a
solid which was filtered off and washed with methanol (470 mg, 58%). A
mixture of the above solid (150 mg, 0.19 mmol), sodium acetylacetonate
(230 mg, 1.9 mmol), and acetone (75 mL) was heated under reflux for
1 h. Water (75 mL) was added, and the precipitated solid was filtered off,
Density Functional Theory Calculations. All DFT calculations
were carried out using Gaussian 09W.31 The B3LYP hybrid functional
was used, together with the LanL2DZ basis set for Pt and Ir, and
6-31þG(d) for all other atom types. Geometries were fully optimized in
vacuo. UVꢀvis spectra were obtained by single point TD-DFT calcula-
tions at the optimized geometries, using the same functional and basis
sets, either in vacuo or with a PCM solvent correction30 for dichloro-
methane. The molecular orbitals and spectra were visualized using
GaussView 5.0,34 with Gaussian line shapes and an arbitrary half-width
at half-height of 1000 cmꢀ1 for the latter.
Electrochemistry. Cyclic voltammetry was carried out using a
μAutolab Type III potentiostat with computer control and data storage
via GPES Manager software. Solutions of concentration 1 mM in
CH2Cl2 at 298 ( 3 K were used, containing [Bu4N][PF6] as the
supporting electrolyte. A three-electrode assembly was employed, con-
sisting of a glassy carbon working electrode, and platinum wire counter
and reference electrodes. Solutions were purged for at least 5 min with
solvent-saturated nitrogen gas with stirring, prior to measurements
being taken under a nitrogen atmosphere without stirring. The voltam-
mograms were referenced to the ferrocence-ferrocenium couple mea-
sured under the same conditions.
Photophysical Measurements. Absorption spectra were mea-
sured on a Biotek Instruments XS 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 Hamamatsu R928 photomultiplier tube; the spectra shown are
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ARTICLE
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 appropriate glassware to allow connection to a
high-vacuum line. Degassing was achieved via a minimum of three
freezeꢀpumpꢀthaw cycles while connected to the vacuum manifold;
final vapor pressure at 77 K was <5 ꢁ 10ꢀ2 mbar, as monitored using a
Pirani gauge. Luminescence quantum yields were determined using
[Ru(bpy)3]Cl2 in degassed aqueous solution as the standard (φ =
0.04235); estimated uncertainty in Φ is (20% or better.
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The luminescence lifetimes of the complexes were measured by time-
correlated single-photon counting, following excitation at 374.0 nm with
an EPL-375 pulsed-diode laser. The emitted light was detected at 90°
using a Peltier-cooled R928 PMT after passage through a monochro-
mator. The estimated uncertainty in the quoted lifetimes is (10% or
better. Lifetimes at 77 K in excess of 10 μs were measured by multi-
channel scaling following excitation with a μs-pulsed xenon lamp; an
excitation wavelength of 374 nm (band-pass 5 nm) was selected with a
monochromator. Bimolecular rate constants for quenching by molecular
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b
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Corresponding Author
*E-mail: valery.kozhevnikov@northumbria.ac.uk, j.a.g.williams@
durham.ac.uk.
’ ACKNOWLEDGMENT
We thank the School of Life Sciences, Northumbria University
for funding, the EPSRC National Mass Spectrometry Service
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