Charge-transfer and ligand-localized photophysics in luminescent cyclometalated pyrazolate-bridged dinuclear platinum(II) complexes

Article abstract of DOI:10.1021/om400276v

We present the synthesis, photophysical characterization, and electrochemistry of three series of cyclometalated binuclear platinum(II) complexes, each bridged by two 3,5-disubstituted pyrazolate ligands (μ-R ₂pz). These neutral compounds have the general formula [C NPt(μ-R₂pz)]₂, where C N is a cyclometalating ligand corresponding to 2-(2′-thienyl)pyridine (thpy), 1-phenylisoquinoline (piq), or 7,8-benzoquinoline (bzq) with R = H, Me, ⁱPr, Ph, corresponding to series I-III dimers, respectively. Systematic variation of the cyclometalating ligands in addition to the bridging pyrazolates renders colorful structures exhibiting a range of electrochemical and spectroscopic behavior with absorption and photoluminescence properties tuned over a wide portion of the visible spectrum. Steric bulk introduced into the 3,5-positions on the pz bridges readily modulates intramolecular d⁸-d⁸ metal-metal σ interactions strongly affecting the frontier orbitals' electronic structure, manifested by changes in absorption and emission energy, excited-state lifetime, and photoluminescence quantum yield. Cyclic voltammetry revealed the presence of two very closely spaced reversible CN ligand-based reductions ranging between -1.97 and -2.56 V vs Fc⁺/Fc, and the first metal-metal-centered oxidation wave was found to be reversible in dichloromethane and irreversible in coordinating THF in most instances. All the complexes of series I displayed triplet ligand-localized excited states at all temperatures, while an increase of steric bulk in the pz bridge in the two other molecular series resulted in a variation of photophysical behavior ranging from charge transfer to ligand localized, including admixture behavior.

Full text of DOI:10.1021/om400276v

Article
pubs.acs.org/Organometallics
Charge-Transfer and Ligand-Localized Photophysics in Luminescent
Cyclometalated Pyrazolate-Bridged Dinuclear Platinum(II)
Complexes
Arnab Chakraborty, Joseph C. Deaton, Alexandre Haefele, and Felix N. Castellano*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403,
United States
S
* Supporting Information
ABSTRACT: We present the synthesis, photophysical characterization,
and electrochemistry of three series of cyclometalated binuclear
platinum(II) complexes, each bridged by two 3,5-disubstituted
pyrazolate ligands (μ-R2pz). These neutral compounds have the general
formula [C∧NPt(μ-R2pz)]2, where C∧N is a cyclometalating ligand
corresponding to 2-(2′-thienyl)pyridine (thpy), 1-phenylisoquinoline
i
(piq), or 7,8-benzoquinoline (bzq) with R = H, Me, Pr, Ph,
corresponding to series I−III dimers, respectively. Systematic variation
of the cyclometalating ligands in addition to the bridging pyrazolates
renders colorful structures exhibiting a range of electrochemical and
spectroscopic behavior with absorption and photoluminescence proper-
ties tuned over a wide portion of the visible spectrum. Steric bulk
introduced into the 3,5-positions on the pz bridges readily modulates
intramolecular d8−d8 metal−metal σ interactions strongly affecting the frontier orbitals’ electronic structure, manifested by
changes in absorption and emission energy, excited-state lifetime, and photoluminescence quantum yield. Cyclic voltammetry
revealed the presence of two very closely spaced reversible C∧N ligand-based reductions ranging between −1.97 and −2.56 V vs
Fc+/Fc, and the first metal−metal-centered oxidation wave was found to be reversible in dichloromethane and irreversible in
coordinating THF in most instances. All the complexes of series I displayed triplet ligand-localized excited states at all
temperatures, while an increase of steric bulk in the pz bridge in the two other molecular series resulted in a variation of
photophysical behavior ranging from charge transfer to ligand localized, including admixture behavior.
state chemistry of the binuclear [Pt2(μ-P2O5H2)4]4− has been
INTRODUCTION
■
reported in the literature,49 while there are numerous related
Cyclometalated coordination compounds of transition metals,
chromophores that display changes in their photophysical
particularly square-planar complexes of platinum(II) and
properties with variation of ancillary and charge transfer
palladium(II), have generated research interest for their
ligands.33,50−55 In addition to Pt, there have been several
numerous potential applications in addition to fundamental
reported studies on binuclear complexes of Pd and Ir.56−59 In
contributions in molecular materials. Such molecules have been
2
used as photocatalysts,1−4 optoelectronic devices,5−10 cyanogen
select binuclear Pt(II) complexes, the dz orbital of each
halide detection,11 cation sensing,12,13 and pH sensing,14,15 as
platinum center is forced to σ overlap and this interaction gives
well as for DNA applications,16,17 as metallomesogens,18,19 and
as anticancer agents.20 All of these multidisciplinary applica-
tions consequently attracted substantial research interest
toward extensive photophysical investigations on mononuclear
cyclometalated Pt(II) complexes.21−39 The strong σ-donating
ability of the cyclometalating ligands results in large d-orbital
splitting effectively inactivating ligand field states, rendering
strongly emissive molecules. The origin of emission in such
mononuclear cyclometalated Pt(II) complexes generally results
from ligand-centered (LC), ligand-to-ligand charge-transfer
(LLCT), and/or metal-to-ligand charge transfer (MLCT)
excited states.
rise to filled dσ and dσ* orbitals. This electronic structure
effectively transforms high-energy absorbing mononuclear
structures into colorful dinuclear molecules exhibiting reversible
metal−metal-based oxidations with riveting photophysics.60−63
When the metal−metal coupling is sufficient, the photophysical
properties can be considered to result from metal−metal-to-
ligand charge transfer (MMLCT) excited states, involving
charge transfer between a filled Pt−Pt dσ* orbital and a ligand-
localized vacant π*orbital on the cyclometalating ligand (Figure
1).
Thompson and co-workers previously reported a series of
binuclear platinum(II) complexes with 2-(2,4-difluorophenyl)-
Binuclear transition-metal complexes based on Pt(II) have
also generated profound interest because of their intriguing
photophysical properties.6,40−48 In particular, the rich excited-
Received: April 3, 2013
© XXXX American Chemical Society
A
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properties. A range of photophysical behavior was observed,
including ligand-centered phosphorescence and charge-transfer
photoluminescence, as well as admixtures of these two
extremes.
RESULTS AND DISCUSSION
■
Syntheses and Structural Characterization. The inter-
mediate cyclometalated dichloro-bridged platinum(II) dimers
were prepared following a literature procedure with slight
modification.33 Instead of directly refluxing K2PtCl4 with the
desired cyclometalating ligand, the precursor Pt(DMSO)2Cl2
was substituted and refluxed in a mixture of 2-ethoxyethanol
and water (3:1) (Scheme 1). This reaction proceeds without
the formation of platinum black, which is often obtained when
K2PtCl4 is utilized. The subsequent pyrazolate-bridged dimers
were synthesized in a manner analogous to that previously
developed in our laboratory.67 In all instances, satisfactory mass
Figure 1. Simplified molecular orbital diagram illustrating the d8−d8
interaction in cofacial dinuclear Pt(II) complexes.
pyridyl as the C∧N ligand and showed that the photophysical
properties of such platinum dimers can be readily tuned by
introducing steric bulk in the bridging pyrazolate ligands.50 In
another report, the same group showed that such complexes
can be used as phosphorescent dopants in OLEDs.64 Recently
there has been increasing interest in these classes of complexes
in both ground- and excited-state properties.65,66 For example,
an investigation of the temperature-dependent spectral and
photophysical properties in [Pt(ppy)(μ-Ph2pz)]2 was recently
reported in addition to its solid-state thermochromic
behavior.67
In the present contribution we report the synthesis,
photophysical characterization, and electrochemistry of an
array of pyrazolate-bridged cyclometalated dinuclear platinum-
(II) complexes. All these complexes are emissive at both 77 K
and room temperature. Here we have investigated how the
systematic variation of cyclometalating ligand and the bridging
pyrazolates affects the photophysical properties of these
complexes. The bridging pyrazolates provide a framework to
initiate metal−metal interactions in the ground-state A-frame
geometry, where the extent of this interaction is controlled by
the bulkiness introduced in the 3- and 5-positions of the
bridging pyrazolates (Figure 2). Three series of platinum
dimers were synthesized by using 2-(2′-thienyl)pyridine (thpy;
series I), 1-phenylisoquinoline (piq; series II) and 7,8-
benzoquinoline (bzq; series III) as the cyclometalating ligands
and in each series methyl-, isopropyl-, and phenyl-disubstituted
pyrazolates were used as bridging ligands in order to evaluate
the effect of steric strain on the resultant photophysical
1
spectra, elemental analyses or HR-MS spectra, and H NMR
data were obtained for each newly synthesized Pt(II) dimer.
Almost all of these dimers were isolated as a mixture of cis and
trans isomers with respect to the relative orientation of the two
C∧N ligands, and the trans form was the major product with a
trans:cis ratio of ∼2:1. This is not surprising, as we have already
described an identical isomerism in the analogous 2-phenyl-
pyridine-containing molecules,67 which is easily confirmed
1
through H NMR.
1
The H NMR spectra of trans-[Pt(thpy)(μ-Me2pz)]2 (1b)
and cis/trans [Pt(thpy)(μ-H2pz)]2 (1a) are presented in Figure
1
3 and serve as representative examples for cis/trans H NMR
assignments in these classes of molecules. The 1H NMR
spectrum of compound 1b, which was obtained as the pure
trans isomer, shows only one signal for each of the
characteristic pairs of protons 1−3 (Figure 3A). Because of
the presence of a C2 symmetry axis in the trans isomers, these
protons are magnetically equivalent and have identical chemical
shifts. In contrast, this symmetry axis is not present in the cis
isomers, being replaced by a plane of symmetry passing through
the two pyrazolate moieties. This change in symmetry revealed
in the cis isomers implies that the two cyclometalating ligands
remain magnetically equivalent (but with different chemical
shifts with respect to the trans isomers) and the two pyrazolates
become chemically distinguishable. This cis/trans effect is most
visible on the signal corresponding to the pyrazolate proton 2
in the 1H NMR spectrum of the mixture of cis-/trans-1a (Figure
3B). Only one triplet, at 6.5 ppm, is observed for the trans
Figure 2. Structures of the three series of pyrazolate-bridged dinuclear Pt(II) complexes under investigation.
B
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Scheme 1. Synthetic Route to Producing Dinuclear Pt(II) Complexes of Series I−III
Figure 3. 1H NMR spectra of 1b (A) and 1a (B) focusing only on the aromatic region recorded in CD2Cl2. Labeled protons 1−3 are distinguishing
signals corresponding to one proton in the pyridyl, pyrazolyl, and thienyl moieties respectively, while the asterisks (*) correspond to peaks from the
residual 1,2-dichlorobenzene solvent. The structures of these associated isomers are presented for clarity.
isomer, whereas two triplets at 6.4 and 6.6 ppm are observed
for the cis isomer, illustrating the difference in magnetic
environment experienced by the two pyrazolate moieties.
Protons 3 and 1 also produce different signals for the cis and
trans isomera, the latter being used to determination the actual
cis/trans isomer ratio. Similar NMR patterns were observed for
all other Pt dimers investigated herein, when obtained as a
mixture of cis and trans isomers.
the redox potentials and electrochemical reversibility of the
newly synthesized Pt(II) dimers. In all instances these
measurements took place under a dry and inert atmosphere
provided by a glovebox. The coordinating solvents included
acetonitrile, 1/1 acetonitrile/toluene, and THF, selected to
provide appropriate solubility and electrochemical potential
measurement windows for both oxidations and reductions,
along with noncoordinating CH2Cl2, which was able to reveal
metal−metal-based reversible oxidations in several instances.
Peak potentials (Epa or Epc) were measured by DPV because
Electrochemistry. Cyclic voltammetry (CV) and differ-
ential pulse voltammetry (DPV) were both utilized to evaluate
C
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this method was necessary in instances where the redox couples
were irreversible. The redox potentials obtained for all of the
Pt(II) complexes under investigation are collected in Table 1.
positions of the pyrazolate bridges (Table 1). The increase in
steric bulk in the bridging ligands results in an increased Pt−Pt
σ interaction along their respective z axes, resulting in a
profound impact on the electrochemical oxidation of the
molecules. In the interest of harnessing potential excited-state
redox chemistry from such molecules, we also performed cyclic
voltammetry studies in noncoordinating CH2Cl2 to see if the
irreversible oxidative processes observed in THF could be
rendered completely reversible, potentially paving the way for
multielectron reversible oxidative chemistry as a result of having
two proximate Pt(II) centers. In several instances, particularly
in those molecules possessing the strongest Pt−Pt interactions,
the cyclic voltammograms in CH2Cl2 became completely
reversible, exhibiting the characteristics of a one-electron
oxidation. A representative example of such reversible electro-
chemical behavior is presented for 2c in Figure 4. We believe
a
Table 1. Electrochemical Data for the Platinum(II) Dimers
b
d
complex
E1/2(ox) (V)
E1/2(ox) (V)
E1/2(red) (V)
e
1a
1b
1c
1d
2a
2b
2c
3a
3b
3c
3d
c
0.59ir
0.51ir
0.48ir
0.45ir
0.48ir
0.43ir
0.36ir
0.53ir
0.48ir
0.44ir
0.38ir
−2.39r
e
0.50ir
0.41r
0.42r
0.42ir
0.35r
0.33r
c
−2.41r, −2.52
e
−2.41r, −2.55
e
−2.38r, −2.56
f
−2.00r, −2.10
f
−1.98r, −2.11
f
−1.97r, −2.08
e
−2.21ir
e
0.46r
0.37ir
0.38r
−2.24ir, −2.33
−2.27ir, −2.37
e
e
−2.21i.r, −2.37
a
All measurements were performed with 0.1 M TBAPF6 as the
supporting electrolyte, and potentials are given vs the Fc+/Fc couple. ir
= irreversible, and r = reversible. Measured in DCM. Could not be
b
c
d
measured due to rapid decomposition in DCM. Measured in THF.
e
f
Measured in 1/1 acetonitrile/toluene. Measured in acetonitrile.
All the platinum dimers in series I and II exhibited two very
closely spaced one-electron reductions, with the exception of
1a, where the first reduction is completely reversible. The
complexes of series III are unique in the fact that each molecule
exhibited electrochemically irreversible one-electron reductions.
This is not too surprising, given that similar observations have
been reported for mononuclear platinum(II) complexes bearing
7,8-benzoquinoline C∧N ligands. Among the several square-
planar cyclometalated platinum(II) complexes reported by
Thompson and co-workers, it was seen only in the case of
bzqPt(dpm) that the ligand-based reduction became irrever-
sible. It is believed that the rigid planarity along with the
increased conjugation exhibited by this cyclometalating ligand
weakens the metal−ligand interaction, thereby rendering each
of the series III complexes susceptible to irreversible
reduction.33,68 Collectively, the first one-electron reduction
potentials across all three series of molecules were in the range
of −1.97 to −2.56 V, depending upon the specific cyclo-
metalating ligands. These values were completely invariant
(within experimental error) within each series of molecules,
irrespective of the nature of the bridging pyrazolate ligand.
These observations leave little doubt that in every instance the
first reduction is localized on the cyclometalating ligand.
Oxidative cyclic voltammetry studies performed in THF
revealed irreversible processes in every single molecule
regardless of the C∧N ligand or bridging pyrazolate moiety.
Therefore, the associated ground-state oxidation potentials
acquired in THF were ascertained by DPV and are collected in
Table 1. The measured values here are in close agreement with
the reported oxidation potentials of closely related pyrazolate-
bridged binuclear platinum(II) complexes of type C∧NPt(μ-
pz)2PtC∧N, where C∧N = 2-(4′,6′-difluorophenyl)pyridinato-
Figure 4. Cyclic voltammogram for oxidation of 2c in argon-saturated
0.1 M TBAPF6 in dichloromethane. A platinum disk was used as the
working electrode, platinum wire as the counter electrode, and silver
wire as the reference electrode.
this experimental observation is completely consistent with a
ground-state electronic structure featuring frontier HOMO
orbitals dominated by a σ interaction occurring between the
two Pt(II) centers. In essence, the irreversible oxidation
processes normally exhibited in mononuclear Pt(II) complexes
can be made reversible by adopting a dinuclear A-frame
structure, in agreement with that previously suggested by
Thompson and co-workers in related structures. Considering
that the LUMO’s in each series are fixed as ascertained by the
reductive electrochemistry discussed above, the pyrazolate
bridge-sensitive HOMO in each molecule fine-tunes the
spectroscopic properties exhibited by these chromophores, as
discussed in detail below.
Electronic Spectroscopy. The UV−vis absorption spectra
all of 3 series of molecules measured in THF at room
temperature are presented in Figure 5, and all of the relevant
associated data are collected in Table 2. In all instances, the
higher energy features and more intense absorption bands
below 350 nm are assigned to π−π* ligand-centered (LC)
transitions, ascertained by direct comparison with each
respective uncoordinated ligand (see the Supporting Informa-
tion). On the basis of assignments in related molecules,42 the
lowest energy features between 350 and ∼500 nm in the
electronic spectra are attributed to charge-transfer (CT)
transitions terminating onto the π* LUMOs of each respective
C∧N ligand in the three series of complexes. In instances where
there is little to no steric bulk imposed by the pyrazolate bridge,
when R = H, CH3, the low-energy absorption bands can be
considered as metal-to-ligand charge transfer (MLCT) in
character, since the dinuclear chromophore essentially behaves
N,C2′.64 The irreversibility of the first oxidation process is not
surprising, given the fact that the square-planar platinum
centers are highly susceptible to nucleophilic attack by
coordinating solvents, resulting in permanent oxidative addition
products.33,64,69 The first oxidation process occurring in each
Pt(II) dimer series systematically becomes easier (>100 mV
variance) with increasing steric bulk accumulated on the 3,5-
D
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Figure 5. Absorption spectra of 1a−d (A), 2a−c (B), and 3a−d (C) measured in THF at room temperature.
a
Table 2. Photophysical Properties of the Pt(II) Dimers
g
h
i
λ(em)rt,77K
τrt, τ77K
d
τTA
(μs)
Φ
kr (104
knr (104
ΔEs
c
c
e
emf
complex
λabs/nm (ε, M‑1 cm‑1)
(nm)
(μs)
(%)
46.8
s‑1)
s‑1)
(cm‑1)
191
1a
300 (20483), 338 (17952), 365 (10341), 410 (5779), 430 (5197)
563, 556
18.12,
20.75
2.58
2.94
1b
301 (14382), 332 (10499), 351 (8942), 381 (7173), 412 (5400),
440 (3988)
565, 563
16.68, 18.4
54.0
3.24
2.76
189
1c
1d
2a
2b
281 (72592), 348 (12685), 383 (9908), 412 (8627), 444 (5370)
276 (52911), 410 (6931), 440 (4725)
565, 560
561, 560
604, 603
610, 603
16.3, 17.58
11.8, 16.97
0.46, 3.83
0.33, 4.00
57.4
61.5
1.9
3.52
5.53
4.13
3.94
2.61
3.47
213
300
184
96
310 (25600), 355 (13204), 378 (9363), 423 (13204), 470 (6087)
0.44
0.34
82
310 (35671), 355 (20303), 384 (14036), 425 (20442), 474
(10417)
1.3
250
2c
297 (45572), 355 (14699), 378 (10021), 417 (11237), 478
(6273)
713, 596
0.28, 5.83
0.26
5.6
20.0
337
3453
b
3a
314 (12159), 337 (9778), 374 (6381), 424 (2492)
314 (17851), 341 (12226), 395 (9252), 440 (3957)
310 (17445), 346 (11283), 403 (9037), 448 (3678)
346 (12768), 406 (8740), 448 (4626)
575, 486
596, 489
605, 490
631, 539
0.53, 166.7
0.24, 111.7
0.66, 83
0.52
0.22
0.62
1.51
6.0
2.8
11.3
11.6
13.6
12.7
177
405
138
54.4
3227
3588
4408
3b
3c
3d
9.0
1.49, 7.12
19.0
3366
a
b
Room-temperature and low-temperature measurements were performed in THF or MTHF respectively, unless otherwise noted. Room-
temperature measurements and absolute quantum yield determinations were performed in toluene because of decomposition in ether-based solvents.
c
d
e
Absorption and emission maxima, 2 nm. Intensity decays, 5%. TA decays were measured at multiple wavelengths and were finally averaged.
f
g
h
i
Absolute quantum yields, 5%. kr = Φ/τ. knr = (1 − Φ)/τ. ΔEs = E0−0(77 K) − E0−0(298 K).
Figure 6. (A) Photoluminescence spectra of the series I chromophores measured in degassed THF at room temperature. λex: 430 nm (1a); 440 nm
(1b); 443 nm (1c); 450 nm (1d). (B) Photoluminescence spectra of 1b measured as a function of temperature in MTHF. τex = 440 nm.
like two independent mononuclear complexes.50 However,
when larger substituents are incorporated onto the 3,5-
positions of the bridging pz ligands (when R = Pr, phenyl),
find that absorption for the most sterically hindered complex 2c
tailed beyond 500 nm. Similarly, as we moved across series III
from 3a to 3d, there was a small but progressive red shift in the
shoulder centered around ∼450 nm. These defining character-
istics can be readily inferred from the electronic spectra
provided in Figure 5.
Photoluminescence Spectroscopy. The photolumines-
cence spectra of all of the molecules of series I measured in
degassed THF at room temperature are presented in Figure 6A.
The photoluminescence profiles are almost completely
invariant with respect to one another within experimental
error, with the exception of slight intensity differences in the
intermediate vibronic structure and the somewhat bluer-
i
the resulting A-frame topology produces significant metal−
metal σ interactions markedly affecting the electronic structure
of the frontier HOMO levels, ascertained in part by the
electrochemical experiments described above. Therefore, in
these instances, the charge-transfer process is best described as
being metal−metal-to-ligand charge transfer (MMLCT) in
character; hence, the absorption bands red-shift in series II and
series III with increasing steric bulk in the 3,5-substituted μ-pz
ligands. This effect is observed for the lowest energy absorption
band/shoulder centered at ∼470 nm in series II, where we can
E
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Figure 7. (A) Photoluminescence spectra of series II at room temperature in THF. (B) Photoluminescence spectra of 2c measured as a function of
temperature in MTHF. τex = 478 nm. (C) Transient absorption difference spectra of the series II chromophores measured in degassed THF at room
temperature following 472 nm (2a), 475 nm (2b), or 478 nm (2c) pulsed laser excitation (3−4 mJ/pulse, 5−7 ns fwhm) measured at 20 ns delay
time. Each difference spectrum represents the average of 64 transients.
emitting 1d. This result immediately suggests that the nature of
the emissive excited state at room temperature is precisely the
same across this series. Similarly, the 77 K spectra measured in
MTHF glasses exhibit similar spectral characteristics in
comparison to those measured for the identical corresponding
chromophore at room temperature (see the Supporting
Information and Table 2). As a representative example, the
temperature-dependent photoluminescence spectra of 1b are
presented in Figure 6B, which emulates the same photo-
luminescence behavior observed in each chromophore in series
I. The vibronic progression of the emission spectrum sharpened
slightly at lower temperatures, but no significant hypsochromic
shift was observed, as the solution was frozen into the rigid
matrix at temperatures at and below the Tg of MTHF. Such
temperature-dependent emission characteristics are consistent
with an excited state that is relatively nonpolar and is quite
unlikely charge transfer in nature. The magnitude of the
thermally induced Stokes shift (ΔEs = E00(77 K) − E00(300
K)) has been previously utilized as a metric in elucidating the
nature of the lowest lying excited state in Pt(II) charge-transfer
chromophores as a function of temperature.52,53,70−76 Applying
this tool to the molecules of series I produced a single value of
∼190 cm−1 for 1a−c and 96 cm−1 for 1d, thereby yielding a
clear-cut assignment of thpy ligand-centered (LC) emission for
all of the complexes of this series based solely on the magnitude
of ΔEs. The excited-state lifetimes for these molecules at room
temperature and 77 K lie well beyond those typically measured
in triplet Pt(II) charge transfer excited states (Table 2), and the
values obtained in 1a−c closely paralleled one another (τrt =
16.3−18 μs), whereas that for 1d was somewhat shorter (τrt =
11.8 μs) than for the other three molecules at room
temperature. The latter may be incorporating a small fraction
of triplet charge-transfer character that effectively shortens its
excited-state lifetime in comparison to the other molecules of
series I. In all instances, the single-exponential room-temper-
ature luminescence intensity decays are entirely consistent with
3
a thpy LC excited state. Within the thpy dimer series, the
excited-state lifetime was also largely invariant to changes in
temperature (τ changed by <15% in the most extreme case),
indicating that the nature of the excited state remains constant
between room temperature and 77 K. As already implied by the
other photoluminescence data presented above, the lowest
excited state in series I at all temperatures between room
3
temperature and 77 K is attributed to a thpy LC excited state
and the nature of the bridging pz ligand has no profound effect
on the resultant photophysics in this series of chromophores.
Although not as structured as the photoluminescence
observed in 1a−d, the room-temperature emission spectra of
the piq-containing 2a,b in THF each exhibit vibronic side bands
in their profiles, whereas that of 2c is completely structureless
and is significantly red-shifted in comparison (Figure 7A). The
recorded ΔEs values for both 2a and 2b are quite small, 82 and
3
250 cm−1, respectively, reminiscent of LC excited states at all
temperatures between room temperature and 77 K, whereas the
corresponding value in the Ph2pz-containing 2c (ΔEs = 3453
cm−1) is more characteristic of the rigidochromic behavior of a
polar charge transfer excited state (Figure 7B). Moreover, the
low-temperature emission spectrum of 2c has the same
structured appearance as those of 2a,b. Interestingly, the
excited-state lifetimes in these series II chromophores measured
using transient photoluminescence spectroscopy range between
280 and 460 ns at room temperature (Table 2), each entirely
consistent with lifetimes exhibited in many classes of Pt(II)
charge transfer excited states,40,77 while much shorter than
those exhibited by the series I chromophores. The radiative
decay rate constants (kr) of 2a,b were similar to those of series
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I; however, that of 2c is approximately 5 times greater,
indicative of charge-transfer character. The corresponding
lifetimes determined at 77 K in MTHF glasses are also quite
consistent with those typically exhibited in related mononuclear
Pt(II) charge-transfer chromophores.33 The ambiguity regard-
ing the nature of the lowest energy excited state within this
series was addressed by studying time-resolved electronic
absorption spectroscopy. Nanosecond transient absorption
difference spectra for complexes 2a−c were acquired in THF
at room temperature (Figure 7C). The single-exponential
kinetics exhibited by the transient absorption decays for all of
the complexes of this series were identical across each
respective difference spectrum (see the Supporting Informa-
tion) and were in perfect agreement with the values determined
from time-resolved photoluminescence decay measurements
(Table 2). The resulting difference spectra for 2a,b were almost
identical, having a small bleach at ∼430 nm and excited state
absorptions centered near 400 and 525 nm. In contrast, 2c
displayed a distinct ground-state bleaching feature at 480 nm,
consistent with the depletion of the 1CT character upon
formation of the excited state, followed by a substantial red shift
in the primary transient absorption feature now present at 630
nm. The key spectroscopic feature in the difference spectrum in
2c is the prevalent ground-state bleach characteristic of that
expected from the formation of a CT excited state. Specifically,
the combination of 2c’s broad and structureless luminescence
spectrum that exhibits strong rigidochromism possessing a large
ΔES value, along with its characteristic transient difference
spectrum and excited state decay parameters, strongly suggest
Figure 8. Photoluminescence spectra of the series III chromophores
measured in degassed THF at room temperature. τex: 400 nm (3a);
440 nm (3b); 443 nm (3c); 450 nm (3d). 3a was recorded in toluene.
MTHF matrix, progressing from structureless above Tg to a
bathochromically shifted structured emission profile below Tg
(Figure S4, Supporting Information), except for the emission of
3d, which remains quite broad at all measured temperatures.
The recorded ΔEs values in series III (Table 2) are consistent
with a polar charge-transfer excited state at room temperature
and suggest a possible crossover from 3CT at room temperature
3
to LC at 77 K as ΔEs > 3200 cm−1. This potential change in
the nature of the excited state was further supported by time-
resolved photoluminescence decay measurements performed at
room temperature and 77 K (Figure 9B and Table 2). The
excited-state lifetimes of 3a−d at room temperature range
between 0.24 and 1.49 μs, each consistent with typical lifetimes
of Pt(II) charge-transfer excited states. The excited-state
lifetimes of 3a−c at 77 K are considerably longer than those
at room temperature, ranging from 83 to 166 μs, whereas that
of 3d only moderately increased to 7.1 μs. The latter
3
that this excited state is likely CT in nature. However, the
combined photophysical data acquired for 2a,b is more
illustrative of that expected from 3LC states. Both the
photoluminescence and transient absorption studies revealed
that increasing steric strain by the introduction of the 3,5-
phenyl groups in the bridging pz ligands of 2c substantially
impacted the photophysics of this chromophore with respect to
that of 2a,b. This dramatic change in photophysical behavior
can be accounted for as a consequence of an enhanced metal−
metal interaction in 2c, effectively altering the energetic
3
observation likely results from remnant CT character at 77
K that shortens the excited-state lifetime in comparison to the
other entries of series III. The radiative decay rate constants
(kr) of 3a−d at room temperature were similar to each other in
magnitude and were 3−4 times larger than those of series I,
thereby suggesting charge transfer character in the lowest lying
emissive state for each molecule at room temperature (Table
2). Transient absorption difference spectra were recorded for
all the complexes in this series, and the single-exponential
kinetics exhibited by the transient absorption features were
quantitatively identical with those measured in the correspond-
ing photoluminescence intensity decays. The difference spectra
acquired for 3b−d possess a significant ground-state bleach
(∼410 nm) in the region of the charge-transfer absorption band
(similar to the case for 2c) and an excited-state absorption
centered near ∼510 nm. The difference spectrum of 3a differs
from those of 3b−d, having excited-state absorptions centered
at 400, 450, and 500 nm. The photophysical behavior cited
above leads to the conclusion that the lowest energy emissive
3
3
ordering of LC and CT in this molecule. This enhancement
of the Pt−Pt interaction results in significant destabilization of
the frontier dσ* orbital, as represented in Figure 1. As a result,
the HOMO should now be considered as metal−metal in
character, whereas the LUMO remains fixed on the cyclo-
metalated ligand π* orbital. The lowest energy CT transition in
3
2c is therefore termed MMLCT and its CT excited state can
3
be characterized specifically as MMLCT. As the HOMO is
dσ* in nature, formation of the MMLCT excited state is
expected to result in a transient increase in the Pt−Pt bond
order by 0.5. This type of bond shortening process has been
experimentally observed previously by Gray and co-workers in
[Pt2(H2P2O5)4]4−.50,67,78−81
3
state at room temperature for 3b−d is indeed CT in nature.
Similar to the case for 2c, room-temperature emission
profiles of the 7,8-benzoquinoline-containing 3b−d in THF are
broad and structureless, suggesting a polar charge-transfer
excited state (Figure 8). The higher energy shoulders in 3a
likely arise from a nearby 3LC state in thermal equilibrium with
Additionally, the room-temperature photoluminescence data
revealed the broad unstructured emission shifted to pro-
gressively lower energy as the steric bulk in the bridging pz
ligand was increased along the series, consistent with that
observed previously.50,67 Thus, the same reasoning leads us to
the conclusion that the lowest energy excited state at room
3
the CT state.
3
The temperature dependence of the photoluminescence
spectra in series III was evaluated, and the data acquired for 3b
are given in Figure 9A and are intended to be representative of
the entire series. Every compound exhibits a dramatic change in
its respective emission profile at temperatures below Tg of the
temperature for 3b−d is best characterized as MMLCT in
nature. As 3a is expected to exhibit the largest Pt−Pt separation
in this series, the higher energy shoulder observed in its
emission profile probably arises due to the presence of a
3
3
thermal equilibrium between LC and MLCT states at room
G
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Figure 9. Temperature dependence of photoluminescence (A) and photoluminescence intensity decay (B) of 3b in MTHF.
Figure 10. Energy level diagram describing the photophysical processes of series I (A), series II (B), and series III (C) Pt(II) dimer chromophores at
room temperature. Solid lines represent radiative transitions, and dashed lines represent nonradiative transitions.
temperature. A synopsis compiling the qualitative Jablonski
diagrams for the three series of the Pt(II) dimers at room
temperature is presented in Figure 10.
increasing steric bulk in the pz bridge. However, cyclic
voltammetry performed in noncoordinating dichloromethane
resulted in reversible one-electron-based oxidations in most
instances, indicative of the coupled metal−metal nature of the
ground-state electronic structure. The photophysical properties
of these complexes were tuned by substitution in the bridging
pyrazolate ligands. Introduction of steric bulk in the pz bridges
resulted in a change of HOMO−LUMO energy gap among the
dimers of series III and 2c, which was manifested by the change
of emission energy, lifetime, and quantum yield; 2c had the
lowest energy gap and hence was the most red-shifted among
all the dimers investigated. The nature of cyclometalating
ligands also played an important part in controlling the
photophysical behavior. Time-resolved photoluminescence
CONCLUSIONS
■
In summary, three classes of pyrazolate-bridged platinum(II)
dimers were synthesized and thoroughly characterized. Most of
these complexes were isolated as a mixture of cis and trans
isomers, having the trans form as the major product.
Electrochemical studies revealed that most of these complexes
exhibited two reductions which were cyclometalating ligand-
centered in nature. In contrast, one-electron irreversible
oxidation of platinum was observed in coordinating tetrahy-
drofuran whose oxidation potential was found to decrease with
H
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[Pt(thpy)(μ-Me2pz)]2 (1b). Yield: 37% (87 mg). The solid was
recrystallized from THF/DCM and washed with pentane. The
compound was isolated exclusively as the trans isomer. 1H NMR
(500 MHz, CD2Cl2; ppm): 8.07 (dd, 2H), 7.65 (dt, 2H), 7.32 (d, 2H),
7.29 (dd, 2H), 6.80 (dt, 2H), 6.72 (d, 2H), 6.05 (s, 2H), 2.30 (s, 6H),
2.25 (s, 6H). MALDI-TOF MS: m/z 900. Anal. Calcd (found) for
C28H26N6S2Pt2·0.25CH2Cl2·0.25C4H8O: C, 37.37 (37.60); H, 3.06
(2.78); N, 8.94 (8.64).
studies at both room temperature and 77 K along with
nanosecond transient absorption spectroscopy aided in
distinguishing the nature of lowest energy excited states in
these chromophores. All the complexes of series I displayed
3LC-based excited states at all temperatures, while an increase
of steric bulk in the pz bridge in the two other molecular series
resulted in a variation of photophysical behavior ranging from
charge transfer to ligand localized, including admixture
behavior. The excited-state behavior of these chromophores
appear most suitable for a myriad of fundamental and applied
electron- and energy-transfer studies.
[Pt(thpy)(μ-iPr2pz)]2 (1c). Yield: 41% (163 mg). The solid was
recrystallized from DCM/hexane. The compound was isolated as a
mixture of cis and trans isomers. 1H NMR (500 MHz, THF-d8; ppm):
8.20 cis and 8.04 trans (dd, 2H total), 7.69 (dt, 2H), 7.28 (d, 2H), 7.26
(dd, 2H), 6.83 (dt, 2H), 6.64 trans and 6.56 cis (d, 2H total), 6.12 cis,
6.04 trans and 5.96 cis (t, 2H total), 3.30 (m, 4H), 1.32 (d, 12H), 1.23
(d, 12H). MALDI-TOF MS: m/z 1012. Anal. Calcd (found) for
C36H42N6S2Pt2: C, 42.68 (42.72); H, 4.18 (4.07); N, 8.30 (8.11).
[Pt(thpy)(μ-Ph2pz)]2 (1d). Yield: 52% (170 mg). The solid was
recrystallized from DCM/ethanol and washed with pentane. The
compound was isolated as a mixture of cis and trans isomers. 1H NMR
(500 MHz, THF-d8; ppm): 8.69 cis and 8.64 trans (dd, 4H total), 8.49
trans and 8.43 cis (dd, 4H total), 8.07 cis and 8.00 trans (dd, 2H total),
7.55 (m, 4H), 7.33−7.12 (m, 14H), 7.01 (d, 2H), 6.54 (dt, 2H), 6.41
trans and 6.38 cis (d, 2H total). MALDI-TOF MS: m/z 1149. Anal.
Calcd (found) for C48H34N6S2Pt2·C6H4Cl2: C, 50.04 (49.74), H, 2.96
(2.78), N, 6.48 (6.40).
EXPERIMENTAL SECTION
■
General Considerations. All reagents purchased from commercial
suppliers were used without any further purification. The compounds
7,8-benzoquinoline, 2-(2′-thienyl)pyridine, silver trifluoromethanesul-
fonate, potassium tetrachloroplatinate(II), 3,5-dimethylpyrazole, and
3,5-diphenylpyrazole were purchased from Aldrich Chemical Co..
Pyrazole was purchased from Alfa Aesar, and 1-phenylisoquinoline was
obtained from TCI. 3,5-Diisopropylpyrazole and Pt(DMSO)2Cl2 were
prepared by following established literature procedures.82,83 1H NMR
spectra were recorded with a Bruker Advance III 500 (500 MHz)
spectrometer. All chemical shifts were referenced to residual solvent
signals. MALDI-TOF mass spectra were measured by a Bruker-
Daltonics Omniflex spectrometer. Elemental analyses were performed
in house using a Perkin-Elmer 2400 CHN Elemental Analyzer. ESI-
HRMS measurements were performed at the Michigan State
University Mass Spectrometry Facility.
[Pt(piq)(μ-Me2pz)]2 (2a). Yield: 52% (180 mg). The solid was
recrystallized from DCM/ethanol and washed with pentane. The
1
compound was isolated as a cis/trans mixture. H NMR (500 MHz,
CD2Cl2; ppm): 8.84 (d, 2H), 8.35 cis and 8.22 trans (d, 2H total), 8.00
(dd, 2H), 7.79 (dd, 2H), 7.74 (dt, 2H), 7.65 (dt, 2H), 7.34 (dd, 2H),
7.15 (dd, 2H), 7.07 (dt, 4H), 6.12 cis, 6.09 trans and 6.07 cis (s, 2H
total), 2.30 (s, 6H), 2.23 (s, 6H). MALDI-TOF MS: m/z 988. HRMS-
ESI: found 989.2198 (MH+), calcd for C40H35N6Pt2 989.2219.
[Pt(piq)(μ-iPr2pz)]2 (2b). Yield: 33% (150 mg). The solid was
recrystallized from THF/ethanol and washed with pentane. The
compound was isolated only as the trans isomer. 1H NMR (500 MHz,
CD2Cl2; ppm): 8.84 (d, 2H), 8.16 (d, 2H), 7.98 (dd, 2H), 7.79 (dd,
2H), 7.74 (dt, 2H), 7.65 (dt, 2H), 7.31 (dd, 2H), 7.10 (dd, 2H), 7.05
(dt, 4H), 6.07 (s, 2H), 3.34 (m, 2H), 3.10 (m, 2H), 1.29 (d, 6H), 1.24
(d, 12H), 1.16 (d, 6H). MALDI-TOF MS: m/z 1100. HRMS-ESI:
found 1101.3519 (MH+), calcd for C48H51N6Pt2 1101.3471.
[Pt(piq)(μ-Ph2pz)]2 (2c). Yield: 61% (314 mg). The solid was
recrystallized from THF/ethanol and washed with pentane. The
compound was isolated as a mixture of cis and trans isomers. 1H NMR
(500 MHz, CD2Cl2; ppm): 8.76 cis and 8.73 trans (d, 2H total), 8.59
cis and 8.57 trans (dd, 4H total), 8.36 (dd, 4H), 8.24 (d, 2H), 7.89 cis
and 7.85 trans (d, 2H total), 7.66 (dd, 4H), 7.58 (dt, 2H), 7.34−7.12
(m, 14H), 7.06 (dd, 4H), 6.91 (dt, 2H), 6.84 (dt, 2H). MALDI-TOF
MS: 1236. HRMS-ESI: found 1237.2889 (MH+), calcd for
C60H43N6Pt2 1237.2845.
General Synthetic Procedure for the Preparation of Pt(II)
Dimers. All of the reactions were performed under an inert
atmosphere using standard Schlenk techniques. The intermediate
cyclometalated dichloride -bridged platinum(II) dimers were prepared
following a literature protocol with slight modification,33 which
involved refluxing Pt(DMSO)2Cl2 with 2.5 equiv of the respective
cyclometalating ligand in a mixture of 2-ethoxyethanol and water (3/
1) overnight. The reaction mixture was concentrated, and the product
was precipitated by the addition of a water/ethanol (1/1) mixture. The
solid obtained was filtered and dried under vacuum. In most cases,
these dichloro-bridged dimers were not purified or characterized and
were directly utilized for the synthesis of the title complexes.33,84−86
The pyrazolate-bridged dimers were synthesized by adapting a
literature procedure.63 Most of these complexes are new, although
1a and 3a have been previously reported by Che and co-workers.42
Briefly, each dichloro-bridged platinum(II) dimer was dissolved in 100
mL of 1,2-dichlorobenzene and 2 equiv of the respective pyrazolate
was then added, followed by the addition at least 2 equiv of silver
triflate and a few drops of tributylamine. The reaction mixture was
refluxed under argon for 30 min, and the AgCl precipitate was
removed by filtration through a pad of alumina. The filtrate was
concentrated, and the solid product was precipitated by addition of 1/
1 ethanol/hexane and finally collected by vacuum filtration. The
resultant dimer was purified by recrystallization from a suitable solvent
and finally washed with pentane and dried. In most instances, the
dimers were isolated as a mixture of cis and trans isomers (relative to
the position of each C∧N ligand in the dimer), with the trans form (the
two C∧N ligands have their C’s and N’s trans to each other) as the
predominant product: trans:cis ≈ 2:1. All attempts to separate these
two isomers were unsuccessful; however, the mixture of these two
isomers had no impact whatsoever on the photophysical properties.
[Pt(thpy)(μ-H2pz)]2 (1a). Yield: 44% (119 mg). The solid was
recrystallized from DCM/hexane and washed with pentane. The
compound was isolated as a mixture of cis and trans isomers. 1H NMR
(500 MHz, CD2Cl2; ppm): 8.31 cis and 8.15 trans (dd, 2H total), 7.78
(dd, 2H), 7.71 (dt, 2H), 7.62 (dd, 2H), 7.42 trans and 7.40 cis (d, 2H
total), 7.37 (dd, 2H), 6.90-trans and 6.79 cis (d, 2H total), 6.87 (dt,
2H), 6.56 cis, 6.45 trans and 6.34 cis (t, 2H total). MALDI-TOF MS:
m/z 844. Anal. Calcd (found) for C24H18N6S2Pt2·0.25C6H4Cl2: C,
34.75 (34.49); H, 2.17 (2.11); N, 9.53 (9.51).
[Pt(bzq)(μ-H2pz)]2 (3a). Yield: 53% (92 mg). The solid was
recrystallized from DCM/hexane and washed with pentane. The
1
compound was isolated as a cis/trans mixture. H NMR (500 MHz,
CD2Cl2; ppm): 8.80 cis and 8.67 trans (dd, 2H total, 8.31 (dd, 2H),
7.95 (dd, 2H), 7.79 (dd, 2H), 7.76 (d, 2H), 7.59 (dd, 2H), 7.56 (d,
2H), 7.48−7.33 (m, 6H), 6.66 cis, 6.57 trans, 6.48 cis (t, 2H total).
MALDI-TOF MS: m/z 880. Anal. Calcd (found) for C32H22N6Pt2: C,
43.64 (43.37); H, 2.52 (2.38); N, 9.54 (9.19).
[Pt(bzq)(μ-Me2pz)]2 (3b). Yield: 49% (85 mg). The solid was
recrystallized from DCM/hexane and washed with pentane. The
compound was isolated as a mixture of cis and trans isomers. 1H NMR
(500 MHz, THF-d8; ppm): 8.72 cis and 8.66 trans (dd 2H total), 8.31
(dd, 2H), 7.62 (d, 2H), 7.48 (d, 2H), 7.42−7.36 (m, 4H), 7.31−7.24
(m, 4H), 6.08 cis, 6.04 trans, 6.00 cis (s, 2H total), 2.37 (s, 6H), 2.33
(s, 6H). MALDI-TOF MS: m/z 937. Anal. Calcd (found) for
C36H30N6Pt2 C2H5OH: C, 46.44 (46.62); H, 3.69 (3.35); N, 8.55
(8.55).
[Pt(bzq)(μ-iPr2pz)] (3c). Yield: 40% (160 mg). The solid was
recrystallized from DCM/hexane and washed with pentane. The
compound was isolated as a mixture of cis and trans isomers. 1H NMR
I
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Article
(500 MHz, CD2Cl2; ppm): 8.62 cis and 8.56 trans (dd, 2H total), 8.21
(dd, 2H), 7.64 (d, 2H), 7.37 (d, 2H), 7.26 (dd, 2H), 7.29−7.18 (m,
4H), 7.17 (dd, 2H), 6.22 cis, 6.16 trans and 6.10 cis (s, 2H total), 3.37
(m, 4H), 1.36 (d, 12H), 1.23(d, 12H). MALDI-TOF MS: m/z 1048.
HRMS-ESI: found 1049.3181 (MH+), calcd for C44H47N6Pt2
1049.3158.
[Pt(bzq)(μ-Ph2pz)]2 (3d). Yield: 47% (411 mg). The solid was
recrystallized from THF/ethanol. The compound was isolated as a
mixture of cis and trans isomers. 1H NMR (500 MHz, THF-d8; ppm):
8.74 trans and 8.71 cis (dd, 4H total), 8.61 cis and 8.57 trans (dd, 4H
total), 8.53 trans and 8.48 cis (dd, 2H total), 8.04 (dd, 2H), 7.41 (d,
2H), 7.31 (s, 2H), 7.27 (d, 2H), 7.24−7.01 (m, 20H). MALDI-TOF
M S : m / z 1 1 8 4 . A n a l . C a l c d ( f o u n d ) f o r
C56H38N6Pt2·0.5C6H14·0.5C6H4Cl2: C, 57.21 (57.22); H, 3.64
(3.32); N, 6.46 (6.49).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by funding from the National Science
Foundation (No. CBET-0731153) and the State of Ohio
through the Ohio Research Scholars program. Dr. Catherine E.
McCusker, Dr. Mykhaylo Myahkostupov, Dr. Aaron A.
Rachford, and Dr. Valentina Prusakova are gratefully acknowl-
edged for their assistance and valuable discussions.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text and figures giving synthetic details, electrochemical data,
and static and dynamic photoluminescence spectra at various
temperatures. This material is available free of charge via the
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AUTHOR INFORMATION
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Corresponding Author
*E-mail for F.N.C.: castell@bgsu.edu.
1999, 18, 1801.
J
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dx.doi.org/10.1021/om400276v | Organometallics XXXX, XXX, XXX−XXX