Synthesis of monodisperse platinum acetylide oligomers end-capped with naphthalene diimide units

Article abstract of DOI:10.1021/om900195p

We report the synthesis and structural characterization of a series of monodisperse platinum acetylide oligomers with the general structure NDI-[Ph-C=C-Pt(PBu₃)₂-C=C-]_n.,Ph-NDI, where n = 2, 3, 6, or 10, Ph = 1, 4-phenylen

Full text of DOI:10.1021/om900195p

4210 Organometallics 2009, 28, 4210–4216
DOI: 10.1021/om900195p
Synthesis of Monodisperse Platinum Acetylide Oligomers End-Capped
with Naphthalene Diimide Units
Julia M. Keller and Kirk S. Schanze*
Department of Chemistry, University of Florida, PO Box 117200, Gainesville, Florida 32611
Received March 13, 2009
We report the synthesis and structural characterization of a series of monodisperse platinum
acetylide oligomers with the general structure NDI-[Ph-CtC-Pt(PBu3)2-CtC-]n-Ph-NDI,
where n=2, 3, 6, or 10, Ph=1,4-phenylene, NDI is a substituted 1,4,5,8-naphthalene diimide, and the
geometry at the Pt centers is trans. The oligomers were synthesized via an iterative-convergent
approach utilizing organometallic synthons that feature orthogonally protected terminal acetylene
units. The 31P NMR spectra of the oligomers are especially revealing as to their structure, due to a
difference in chemical shift for the internal and terminal Pt(PBu3)2 units. The oligomers were also
characterized by electrochemistry, UV-visible absorption, and photoluminescence spectroscopy.
The emission spectroscopy reveals that the triplet exciton is efficiently quenched in the NDI end-capped
oligomers, and the quenching is thought to arise due to photoinduced charge separation.
Introduction
metals influence the properties of the conjugated systems
and by their possible application in areas such as optoelec-
tronic devices and nonlinear absorption.10-15
Conjugated oligomers and polymers have been the focus
of a large body of research directed toward the development
of new advanced materials for optical and electronic appli-
cations, as well as for addressing fundamental scientific
questions.1-5 While most work in this field has focused on
organic conjugated materials, there has recently been in-
creasing interest in the study of conjugated materials with
heavy metal centers incorporated into the conjugated
network.6-9 Interest in these materials is driven both by
the need to understand fundamental aspects of how the
We have been investigating the properties of platinum (Pt)
acetylide oligomers and polymers of the type [-Pt(PR3)2-
CtC-Ar-CtC-], where Ar is an organic arene and the
stereochemistry at the square-planar Pt(II) center is trans.16-19
These materials provide considerable insight into the influ-
ence of the platinum center on a π-conjugated electronic
system. In addition, platinum acetylides are useful for appli-
cation in organic solar cells and as nonlinear absorption
materials.11-15,20 The most significant effect of the platinum
in these conjugated systems is that it introduces strong spin-
orbit coupling, effectively mixing the singlet and triplet
excited states, resulting in efficient production of long-lived
triplet excitons.21-23
*To whom correspondence should be addressed. Tel: 352 392 9133.
Fax: 352 392 2395. E-mail: kschanze@chem.ufl.edu.
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pubs.acs.org/Organometallics
Published on Web 06/19/2009
2009 American Chemical Society

Article
In several recent publications we described the results of
Organometallics, Vol. 28, No. 14, 2009 4211
Chart 1
spectroscopic studies that have characterized properties of
triplet excitons in platinum acetylide oligomers and poly-
mers.16-18 In addition, we have also carried out pulse radi-
olysis24 investigations to characterize the spectroscopy and
spatial extent of negative ion-radicals (polarons) produced
by reduction of the platinum acetylide chains.25 Taken
together, the spectroscopic and pulse radiolysis studies
reveal that triplet excitons and negative polarons are spa-
tially confined on the platinum acetylide chain, being delo-
calized over a segment corresponding to approximately a
single repeat unit, e.g., [-Pt-CtC-Ph-CtC-Pt-], where
Ph=1,4-phenylene.18,25 As a continuation of that work, we
are interested in developing long platinum acetylide oligo-
mers that are end-capped with molecular units that can act as
“traps” for triplet excitons or negative polarons produced on
the chains. Time-resolved spectroscopic studies of such end-
capped oligomers carried out using laser flash photolysis and
pulse radiolysis will provide considerable insight concerning
the dynamics of triplet exciton and negative polaron diffu-
sion along the π-conjugated chain.
In this paper we describe synthetic work carried out
toward the objective of synthesizing a series of monodisperse
platinum acetylide oligomers that are end-capped with
naphthalene diimide units, PtnNDI2, Chart 1. The objective
of the synthetic efforts was to prepare relatively long oligo-
mers, in order to enhance the opportunity to measure the
dynamics of exciton and polaron hopping along the chains.26-30
The synthetic method that was used to prepare these oligo-
mers is based on an iterative-convergent approach, which
was implemented via application of an organometallic syn-
thon that features orthogonally protected terminal acetyl-
enes. While there has been considerable research carried out
concerning the synthesis of monodisperse, organic π-con-
jugated oligomers,2,31-37 fewer studies have described the
preparation of oligomers that contain metal centers in the
π-conjugated sequence.9,16,38-51 This work is significant, as
the protecting group approach that is developed is general
and could be used toward the synthesis of a variety of
platinum acetylide based oligomers and dendrimers.
Results and Discussion
Molecular and Synthetic Design. The structures of the
PtnNDI2 oligomers (n=2, 3, 6, and 10) synthesized in the
course of this work are shown in Chart 1. Each of these
oligomers features a platinum acetylide chain with the repeat
unit structure [-Pt(PBu3)2-CtC-Ph-CtC-] (Ph =1,4-
phenylene) end-capped with 1,4,5,8-naphthalenediimide
(NDI) units. The NDI unit is used as the end-cap, as this
moiety is reduced at a relatively low potential, making it a
good electron acceptor for trapping negative polarons or
triplet excitons produced on the chains by pulse radiolysis or
optical excitation, respectively.52,53 The previously synthe-
sized dimeric complex Pt-216,25 was used as a reference
compound for photophysical characterization of the series.
Preparation of the longer oligomers (i.e., n = 6 and 10)
presented an interesting synthetic challenge. Consideration
of the problem suggested three basic synthetic strategies that
could be used. The simplest approach would be a fractiona-
tion method.2 Here, a short-lived AA/BB type polymeriza-
tion reaction of Pt(PBu3)2Cl2 and 1,4-diethynylbenzene
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4212 Organometallics, Vol. 28, No. 14, 2009
Keller and Schanze
Scheme 1a
a (a) Tetrabutylammonium fluoride (TBAF), THF, rt, 1 h; (b) MnO2, KOH, ether, rt, 5 h; (c) cis-Pt(PBu3)2Cl2, Et2NH, reflux, 12 h; (d) cis-Pt-
(PBu3)2Cl2, cat. CuI, THF/Et2NH, rt, 2 h; (e) 2b, cat. CuI, Et2NH, rt, 12 h.
Scheme 2a
a (a) cis-Pt(PBu3)2Cl2, iPr2NH/MeCN, reflux, 12 h; (b) cat. CuI, THF/Et2NH, rt, 3 h; (c) 5, cat. CuI, THF/Et2NH, rt, 3 h.
could be carried out, followed by quenching the reaction
early with the appropriate end-cap. The pitfall in this ap-
proach would be that the reaction would be uncontrolled,
resulting in a complex mixture of oligomers with different
chain lengths that would need to be separated. A stepwise,
totally directed approach2 would involve a completely linear
synthesis with group protection/deprotection needed at each
step in the sequence. While product isolation via this route
would be much less difficult, the synthesis would require
many steps, compromising the overall yields of the target
oligomers.38
Although we have previously developed an iterative meth-
od for preparation of monodisperse platinum acetylide
oligomers,16-18,25 because of the need for longer chain
lengths in the present study we decided that an iterative-
convergent approach is best suited for the task.2,54,55 This
route involves the use of orthogonal protecting groups to
construct a protected and unsymmetric platinum acetylide
oligomer segment as a primary synthon (structure 7,
Scheme 1). Building out from the center of the oligomer
ultimately reduced the number of overall synthetic steps
involved in assembly of the longer oligomers.
the other is protected with the triisopropylsilyl (TIPS) unit.
Hagihara coupling56,57 of this compound with cis-Pt-
(PBu3)2Cl2 in refluxing diethylamine afforded 3 in a 58%
overall yield from 1. Complex 3 was then subjected to
Hagihara coupling with compound 2b, which contains a free
ethynyl group and one protected with the HOM group. This
reaction affords the orthogonally protected complex 6a in
60% yield. This organometallic intermediate is a key build-
ing block toward the synthesis of the longer oligomers. First,
either of the acetylene protecting groups (HOM or TIPS) can
be selectively removed. Moreover, the complex is very
soluble in nonpolar solvents, and it can be easily separated
by silica chromatography from less polar impurities because the
polar HOM unit causes the complex to adsorb more strongly to
the support, increasing its retention time on a column. Finally,
removal of the HOM protecting group from 6a produces 6b,
which was coupled with cis-Pt(PBu3)2Cl2 to afford the TIPS-
protected synthon 7 in an 89% yield.
Synthesis of the shorter oligomers 11 and 12 was relatively
straightforward and consequently did not require synthon 7
(Scheme 2). The end-capped dimer Pt2NDI2 (11 in Scheme 2)
was prepared by the copper(I)-catalyzed Hagihara coupling
of known compounds 858 and 10.25 The end-capped trimer
Pt3NDI2 (12 in Scheme 2) was prepared by coupling known
complex 516 with the end-cap synthon 9. The latter was
prepared by Hagihara coupling of cis-Pt(PBu3)2Cl2 with 8.
Synthesis. The route used to access the primary synthon 7
is outlined in Scheme 1. First, the monomeric complex 3 was
prepared in a sequence beginning with selective deprotection
of the hydroxymethyl (HOM) group of known compound
1
benzene (2a) in which one of the ethynyl groups is free and
16,32 to afford an unsymmetrically protected 1,4-diethynyl-
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Article
Organometallics, Vol. 28, No. 14, 2009 4213
Scheme 3a
a (a) 7, THF/Et2NH, cat. CuI, rt, 4 h; (b) TBAF, THF, rt, 1 h; (c) 9, THF/Et2NH, cat. CuI, rt, 3 h.
As outlined in Scheme 3, synthons 7 and 9 were used to
allow rapid construction of the longer end-capped oligomers
Pt6NDI2 and Pt10NDI2 (15 and 17 in Scheme 3). Both end-
capped oligomers were synthesized via tetramer 14a, which
was produced in high yield by coupling 2 equiv of synthon 7
with diethynylbenzene. Removal of the TIPS protecting
groups in 14a using TBAF followed by coupling of the
resulting deprotected tetramer 14b with NDI end-cap 9
afforded Pt6NDI2. Finally, the end-capped decamer
Pt10NDI2 was synthesized in a sequence beginning with
copper(I)-catalyzed coupling of 14b with synthon 7 to yield
octamer 16a. The TIPS protecting groups were then removed
from 16a with TBAF, and the resulting deprotected octamer
(16b) was coupled with NDI end-cap 9 to afford the decamer
Pt10NDI2.
the central PtP2 unit. On the basis of the ratios, we conclude
that the resonance for the terminal PtP2 units is downfield by
ca. 0.2 ppm relative to that for the central PtP2. The down-
field shift for the terminal PtP2 units may arise due to the
electron-withdrawing effect of the NDI unit, which is a
relatively strong π-acceptor. Interestingly, for the hexamer,
Pt6NDI2 (Figure 1c), the center resonance also appears as
two closely spaced singlets, but in this case the downfield:
upfield ratio is 1:2, which is opposite from that for the trimer.
This spectrum is entirely consistent with the structure of the
hexamer, which features two terminal PtP2 units and four
central PtP2 units: on the basis of the structure we would
predict the ratio of the terminal and central resonance peaks
to be 1:2. Finally, a similar pattern is observed in the
spectrum of the decamer, Pt10NDI2, where the downfield
peak due to the terminal PtP2 units is considerably weaker
compared to the upfield signal due to the central PtPt2 units.
While we are unable to report the numerical integration
values for the two signals, inspection of the expanded
spectrum in Figure 1d indicates that the downfield:upfield
peak ratio is in qualitative agreement with the 1:4 ratio
expected on the basis of the oligomer’s structure.
31P NMR Characterization. The structures of the oligo-
mers were confirmed by 1H NMR, 31P NMR, and elemental
analysis (see Experimental Section for details). The 31P
NMR spectra of the oligomers are especially revealing as
to their structures (Figure 1). For each oligomer, the 31P
spectra appear as a pattern of three resonances centered at
δ ≈ 4 ppm; a weak “doublet” with J ≈ 2350 Hz that arises
from 195Pt-31P coupling, and a central resonance that is due
to PtP2 units with NMR-silent Pt isotopes (for the trimer,
hexamer, and decamer each of the three individual resonance
peaks is split as discussed below). The magnitude of the
195Pt-31P coupling constant confirms the trans stereochem-
istry of the PtP2 units.59
Now, to discuss the pattern of splitting that results from
the oligomer structures, we focus attention on the central
resonance (δ ≈ 4 ppm), which is shown expanded in the insets
in Figure 1. First, as shown in Figure 1a, for Pt2NDI2 the
central resonance is a singlet, consistent with the fact that the
two PtP2 centers are equivalent in the dimer. By contrast, for
the trimer, Pt3NDI2 (Figure 1b), the center resonance
appears as two closely spaced singlets with the downfield:
upfield ratio of 2:1. The splitting results from the fact that
there are two chemically inequivalent sets of phosphines in
the oligomer, those on the terminal PtP2 units and those on
Electrochemistry. The objective of the synthesis of the NDI
end-capped platinum acetylide oligomers is to use them to
investigate charge and exciton transport in π-conjugated
organometallic systems. In order to understand the proper-
ties of the excited and charged states of these systems, it is
essential to have information concerning the accessible redox
states. Thus, to obtain information regarding the redox
states, cyclic voltammetry was carried out on the simplest
member of the series, Pt2NDI2. As shown in Figure 2, the
cyclic voltammagram of Pt2NDI2 features two quasi-rever-
sible cathodic waves with E1/2=-0.65 V (ΔEp=92 mV) and
-1.05 V (ΔΕp=46 mV), along with a single quasi-reversible
anodic wave at E1/2=0.89 V (ΔΕp=126 mV) (potentials vs
SCE). The cathodic waves are due to sequential one-electron
reduction of the two NDI end-groups. Note that each
cathodic wave likely corresponds to simultaneous one-elec-
tron reduction of both of the NDI units, for example for the
first wave at E1/2=-0.65 V: NDI-Pt2-NDI þ 2e- f (NDI-•
-
Pt2-NDI-• 2-
) . This assignment is supported by a number of
previous studies of the NDI unit (including a study of a
(59) Mather, G. G.; Pidcock, A.; Rapsey, G. J. N. J. Chem. Soc.,
Dalton Trans. 1973, 2095–2099.

4214 Organometallics, Vol. 28, No. 14, 2009
Keller and Schanze
Figure 2. Cyclic voltammagram of Pt2NDI2 in methylene
chloride with 0.1 M tetrabutylammonium hexafluorophosphate
(TBAH) as electrolyte, vs SCE.
dominated by π,π* transitions localized on the NDI end-
group chromophore. The absorption of the NDI chromo-
phore features a pronounced vibronic progression with
maxima at 360 and 380 nm,52 and these bands are clearly
resolved in the Pt2NDI2 spectrum. As the length of the
platinum acetylide chain increases, the near-UV absorption
is increasingly dominated by the long-axis polarized π,π*
transition localized on the organometallic conjugated seg-
ment. For the decamer, the near-UV absorption consists of
ca. 85% absorption on the platinum acetylide chain, with
15% or less of the oscillator strength on the NDI end-groups.
Note that there is a series of weaker, transitions at higher
energy that become more prominent in the longer oligomers;
these transitions have been previously assigned as arising
from short-axis polarized π,π* transitions on the platinum
acetylide chain.16,60
Considerable insight into the excited-state dynamics of the
NDI end-capped oligomers comes from photoluminescence
spectroscopy. The right panel of Figure 3 compares the
emission of the PtnNDI2 series with the emission of the
model complex, Pt-2. The spectra were obtained for degassed
solutions, and the intensities are normalized to reflect the
relative emission quantum yields. First, for Pt-2 the emission
is dominated by a structured band with λmax=519 nm with a
vibronic shoulder at lower energy. This emission is observed
for all platinum acetylide oligomers with the repeat unit
structure [-Pt(PBu3)2-CtC-Ph-CtC-] and its phos-
phorescence arising from the triplet excited state (exciton).
In previous studies we have shown that the phosphorescence
energy is invariant with oligomer chain length, a fact that
suggests that for longer oligomers the triplet exciton is
spatially confined to a relatively short chain segment.16,18
The remarkable feature is that the phosphorescence is hardly
observed for any of the PtnNDI2 oligomers, a result that
clearly indicates that there is a nonradiative pathway avail-
able that very effectively quenches the triplet exciton before it
is able to phosphorescence. Emission quantum yield studies
show that the quenching of the platinum acetylide phosphor-
escence is nearly quantitative, with the quantum yields for
the NDI end-capped oligomers in the range ΦP ≈ 0.1-0.2%
compared to the yield of Pt-2, which has ΦP ≈ 5%.16
Figure 1. 31P NMR spectra for the PtnNDI2 series: (a) n=2;
(b) n=3; (c) n=6; (d) n=10.
platinum acetylide complex that contains NDI units), which
demonstrate that the NDI electrophore is reduced in two
sequential one-electron processes at potentials in the range
-0.6 and -0.8 V vs SCE.52,53,58 The anodic wave observed
for Pt2NDI2 is assigned to one-electron oxidation of an
electrophore consisting of the platinum acetylide unit, [-Pt-
(PBu3)2-CtC-Ph-CtC-Pt(PBu3)2-]. This assignment is
supported by our recent work on the electrochemical proper-
ties of the model complex Pt-2 and a series of platinum
acetylide oligomers with a repeat unit structure [-Pt-
(PBu3)2-CtC-Ph-CtC-]n.25 This study showed that
Pt-2 exhibits a one-electron oxidation at E1/2=0.89 V, while
the longer oligomers display similar waves at slightly less
anodic potentials.
Absorption and Photoluminescence Spectroscopy. The
UV-visible absorption spectra of the series of PtnNDI2
oligomers were recorded in THF, and the spectra are illu-
strated in the left panel of Figure 3. Each of the complexes
absorbs strongly in the near-UV region, with the molar
absorptivity increasing with chain length to a maximum of
nearly 800 000 M-1 cm-1 for the decamer. The near-UV
absorption arises from π,π* transitions localized on the
platinum acetylide chain and on the NDI end-groups (both
chromophores absorb in the 340-370 nm region). Close
inspection of the spectra shows that the shape of the near-UV
absorption varies with chain length. In particular, for the
dimer and trimer, there are two well-defined absorption
band maxima at ca. 360 and 375 nm; however, for the
hexamer and decamer the band sharpens and only a single
band maximum is seen at 370 nm. This variation arises
because the relative contribution of the platinum acetylide
chain and NDI chromophores varies across the series. In
particular, for the shorter oligomers, the absorption is
(60) Wilson, J. S.; Wilson, R. J.; Friend, R. H.; Kohler, A.; Al-Suti,
M. K.; Al-Mandhary, M. R. A.; Khan, M. S. Phys. Rev. B 2003, 67.

Article
Organometallics, Vol. 28, No. 14, 2009 4215
Figure 3. UV-visible absorption (left) and steady-state emission (right) spectra of PtnNDI2 series in degassed THF at 298 K. The
legend is shown in the left panel.
The electrochemical studies on Pt2NDI2 provide insight con-
cerning the mechanism for the triplet exciton quenching. In
particular, on the basis of the cyclic voltammetry studies we
conclude that a charge-separated state in which one NDI
acceptor is reduced and the platinum acetylide chain is oxidized,
e.g., NDI-[-(Pt(PBu3)2-CtC-Ph-CtC-)þ•]-NDI-•, lies
at ca. 1.55 eV relative to the ground state. (The charge transfer
state energy is estimated by taking the difference in the oxidation
and reduction potentials of Pt2NDI2.)61 This charge transfer
state lies well below the energy of the triplet exciton state, which
lies at ca. 2.4 eV, as estimated from the band maximum of the
phosphorescence. Thus, we conclude that the triplet quenching
arises from rapid intramolecular photoinduced electron transfer,
Precedence for the use of the HOM and TIPS as orthogo-
nal protecting groups in synthesis of oligomers comes from
the work of Godt et al.;32 this group demonstrated the
strategy in the synthesis of phenylene ethynylene oligomers
with nine repeat units. There have been many reports invol-
ving the synthesis of monodisperse Pt-acetylide materials,
some including use of a silyl-protected Pt-acetylides.41,42,44,49,62-73
Takahashi et al.38 used a TIPS-protected terminal acetylene
organometallic intermediate in a platinum acetylide dendri-
mer synthesis that was carried out through four generations.
However, prior to the present report we are unaware of the
incorporation of both of these orthogonal acetylene protect-
ing groups in organometallic intermediates toward the
synthesis of organometallic oligomers.
3
In ongoing physical experiments we are exploring the
dynamics of triplet exciton and negative polaron diffusion
along the NDI end-capped platinum acetylide oligomers
using time-resolved laser photolysis and pulse radiolysis
methods. Preliminary analysis of the results of these studies
indicate that transport along the platinum acetylide chains
occurs via a hopping mechanism, consistent with diffusion of
spatially confined exciton and polaron states. The results of
this work will be reported in a forthcoming paper.
ꢀ
NDI-½ - ðPtðPBu3Þ2 -CC-Ph-CC-Þ ꢁ-NDI f
NDI-½ -ðPtðPBu3Þ2-CC-Ph-CC- -þ•ÞꢁNDI-•
which is much faster compared to radiative decay of the triplet
exciton. In a forthcoming article we will provide details of
picosecond transient absorption spectroscopy experiments that
confirm that this process occurs and that also provide significant
insight concerning the dynamics of triplet exciton diffusion
within the longer platinum acetylide oligomers.
Experimental Section
Summary and Conclusions
Materials and Instruments. All solvents and chemicals used
for synthesis of the platinum acetylide oligomers were reagent
grade and used without purification unless noted. Silica gel
We have successfully developed an iterative-convergent
synthetic method to prepare a series of monodisperse plati-
num acetylide oligomers that contain as many as 10 repeat
units. The oligomers are end-functionalized with naphtha-
lene diimide electron acceptors for use in studies of exciton
and electron transport. The iterative-convergent synthetic
approach uses organometallic synthons that feature either
(or both) HOM or TIPS as protecting groups for the termi-
nal acetylene moieties. These protecting groups are orthogonal:
they are removed under different conditions in high synthetic
yields, affording monofunctional synthons that can be used in
subsequent iterative react-deprotect sequences to construct
the longer oligomers. In addition to serving as a protecting
group, the polar nature of the HOM unit also has the benefit
of facilitating the chromatographic separations. In particular,
oligomers that are end-functionalized with the HOM group
elute more slowly on silica gel columns compared to the
(less polar) reaction byproduct and oligomers that contain only
the TIPS protecting groups.
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4216 Organometallics, Vol. 28, No. 14, 2009
Keller and Schanze
˚
(Silicycle Inc., 230-400 mesh, 40-63 μm, 60 A) was used for all
dried over NaSO4(s), and the solvent was removed to give a dark
green solid, which was purified by flash chromatography (silica
gel, 1:1:0 to 1:1:(1%) hexane/CH2Cl2/(acetone)). Fraction 2
moved as a dark orange band down the column; solvent was
removed to afford a green solid product. Yield: 290 mg (49%).
Decomposition temp: 216-217 °C. 1H NMR (300 MHz,
CDCl3): δ 0.8-1.1 (br m, 62H), 1.1-1.8 (br m, 96H), 2.08 (br
m, 36H), 4.20 (t, 4H), 7.1 (br m, 12H), 7.41 (d, 4H), 8.80 (s, 8H).
flash chromatography. All silyl-acetylene starting materials
were purchased from GFS Chemical; potassium tetrachloropla-
tinate and palladium-bis(triphenylphosphyl) dichloride were
purchased from Strem Chemicals; all other chemicals were
purchased from Sigma-Aldrich. 1H and 31P NMR spectra were
obtained on a Varian 300 MHz spectrometer using deuterated
chloroform (CDCl3) as the solvent and tetramethylsilane (TMS)
as the internal reference. Elemental analysis was performed at
Robertson Microlit Laboratories for all of the final oligomers.
Decomposition temperatures were also estimated for all oligo-
mers synthesized using a standard Melt-temp apparatus. All
newly synthesized intermediates were characterized by TOF-
MS and elemental analysis.
Cyclic voltammetry experiments were performed in dry
methylene chloride (CH2Cl2) solution containing 0.1 M tetra-
n-butylammonimum hexafluorophosphate (TBAH) as the sup-
porting electrolyte. The three-electrode setup consisted of a
platinum microdisc (2 mm2) working electrode, a platinum wire
auxiliary electrode, and a silver wire reference electrode. Solu-
tions were degassed with argon flow prior to measurements,
maintaining positive argon pressure during the measurements.
The concentrations of oligomers in the solutions were 1 mM. A
scan rate of 100 mV/s was used. All potentials were internally
calibrated against the ferrocene/ferricinium couple (E=0.43 V
vs SCE in CH2Cl2) and potentials plotted vs SCE.
31P NMR (CDCl3): δ 4.2 (JPt-P = 2355.49 Hz), 4.0 (JPt-P
=
2363.43 Hz). Anal. Calcd for C152H220N4O8P6Pt3: C, 60.81; H,
7.39; N, 1.87. Found: C, 60.62; H, 7.40; N, 1.78.
Pt6NDI2 (15). To a 50 mL round-bottom flask with a stir
bar were added 9 (163 mg, 146 μmol), 14b (200 mg, 66 μmol),
THF (5 mL), and HNEt2 (5 mL). The solution was degassed
with argon for 5 min, followed by the addition of CuI
(2 mg). Within 2 min the reaction turned a dark orange color
and became somewhat opaque with salt formation. After
3 h, the reaction was quenched with deionized H2O, diluted
with CH2Cl2, and washed twice with brine. The organic
layer was dried over NaSO4(s) and then the solvent removed
to give a crude green-brown solid. The product was purified
by flash chromatography (silica gel, 2:1 CHCl3/hexane), eluting
the product as a red band down the column. Solvent
was removed from this fraction to afford a dark green
solid product, although some residual hexane proved diffi-
cult to remove. Yield: 170 mg (50.1%). Decomposition temp
> 400 °C. 1H NMR (300 MHz, CDCl3): δ 0.90 (m, 114H) 1.2-
1.7 (br m, 158H), 2.15 (br m, 72H), 4.20 (t, 4H), 7.0-7.2
(m, 24H), 7.42 (d, 4H), 8.8 (s, 8H). 31P NMR (CDCl3): δ 4.04
(JPt-P = 2365.3 Hz), 4.28 (JPt-P = 2356.1 Hz). Anal. Calcd
for C254H394N4O8P12Pt6: C, 58.96; H, 7.68; N, 1.08. Found:
C, 58.78; H, 8.00; N, 0.98.
Steady-state absorption spectra were recorded on a Varian Cary
100 dual-beam spectrophotometer. Corrected steady-state emis-
sion measurements were conducted on a SPEX F-112 fluorescence
spectrometer. Samples were degassed by argon purging for 30 min,
and concentrations were adjusted to produce “optically dilute”
solutions (i.e., Amax< 0.20) in THF. Quantum yields were calcu-
lated using Ru(byp)3Cl2 as a known reference.74
Pt10NDI2 (17). To a 25 mL round-bottom flask with a stir
bar were added 16b (400 mg, 65.9 μmol), 9 (161 mg, 145.2 μmol),
HNEt2 (8 mL), and THF (8 mL). The solution was degassed
under argon for 10 min, followed by the addition of CuI
(∼1 mg). The reaction was stirred at room temperature under
argon for 4 h. After this time, the solution was diluted
with CH2Cl2 and washed with both deionized water and brine.
The organic layer was dried over NaSO4(s), and then the
solvent was evaporated to give a dark green crude solid. The
product mixture was purified by flash chromatography
(silica gel, 2:1:(1%) to 3:1:(1%) to 5:1:(1%) CH2Cl2/hexane/
(MeOH)). Fractions 1 and 2 (both yellow color) were degraded
Synthesis. Pt2NDI2 (11). To a 50 mL flask with stir bar were
added 8 (60 mg, 0.13 mmol), 10 (80 mg, 0.06 mmol), THF (3.5
mL), and diethylamine (2 mL). The solution was stirred and
degassed with argon for 30 min, followed by the addition of CuI
(3 mg) under argon flow. The reaction was stirred at room
temperature for 1.5 h. The orange-brown reaction mixture was
diluted with CH2Cl2, then washed with deionized water and
brine, and dried over NaSO4(s). Evaporation of the organic
solvent gave a crude dark green solid. The product was purified
by flash chromatography (silica gel, 1:1 hexane/CH2Cl2, then
5:2:1 CH2Cl2/CHCl3/hexane) to elute the product as the second
fraction. Solvent was evaporated to give the product as a dark
green solid. Residual acetone proved difficult to remove. Yield=
starting materials. Fraction
3 (dark orange band) was
1
eluted, samples were collected, and solvent was evaporated to
afford a dark green solid as product. Yield: 240 mg (45.0%).
90 mg (69.4%). Decomposition temperature: 197-199 °C. H
NMR (300 MHz, CDCl3): δ 0.90 (m, 42H), 1.20-1.80 (br m,
72H), 2.15 (br m, 24H), 4.20 (t, 4H), 7.15 (d, 8H), 7.42 (d, 4H),
8.80 (s, 8H). 31P NMR (CDCl3): δ 4.27 (JPt-P=2354.33 Hz). MS
(MALDI-TOF): calcd for C118H126N4O8P4Pt2 [Mþ] 2278.07,
found [Mþ] 2278.06. Anal. Calcd for C118H126N4O8P4Pt2:
C, 62.20; H, 7.17; N, 2.46. Found: C, 62.40; H, 7.22; N, 2.27.
Pt3NDI2 (12). To a 50 mL round-bottom flask equipped with
a stir bar were added 5 (76 mg, 89.9 μmol), 9 (220 mg, 0.20
mmol), HNEt2 (5 mL), and THF (3 mL). The solution was
degassed under argon for 10 min, followed by the addition of
CuI (∼1 mg) to the solution. The reaction was stirred under
argon for 2 h. In this time, the mixture turned from a light orange
color to a dark orange-brown color. TLC (silica, 1:1 hexane/
CH2Cl2) revealed a complete consumption of the starting
materials. The solution was diluted with CH2Cl2 and washed
with both deionized water and brine. The organic layer was
1
Decomposition temp > 400 °C. H NMR (300 MHz, CDCl3):
δ 0.95 (m, 186H), 1.1-1.9 (br m, 264H), 2.15 (br s, 120H), 4.20
(br t, 4H), 7.13 (br m, 40H), 7.45 (br d, 4H), 8.80 (s, 8H).
31P NMR (CDCl3): δ 4.0 (JPt-P = 2365.26 Hz), 4.2 (JPt-P
=
2365 Hz). Anal. Calcd for C390H626N4O8P20Pt10: C, 58.05; H,
7.82; N, 0.69. Found: C, 57.98; H 7.68; N, 0.72.
Acknowledgment. We thank the National Science
Foundation for support of this research (Grant No.
CHE-0515066).
Supporting Information Available: The synthesis and char-
acterization of intermediate compounds 1-10, 13, 14, 16a, and
16b, as well as a CV scan of a Pt2NDI2/ferrocene mixture, is
provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
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