Metal-metal quadruple bonds supported by 5-ethynylthiophene-2-carboxylato ligands: Preparation, molecular and electronic structures, photoexcited state dynamics, and application as molecular synthons

Article abstract of DOI:10.1021/ja400508u

From the reaction between M₂(TⁱPB)₄ and 2 equiv of 5-ethynylthiophene-2-carboxylic acid (H-ThCCH) in toluene, the complexes trans-M₂(TⁱPB)₂(ThCCH)₂, where M = Mo (I) or W (II) and TⁱPB = 2,4,6-triisopropyl benzoate, have been isolated and characterized by ¹H NMR, IR, MALDI-TOF MS, UV-vis, steady-state emission, transient absorption, and time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms the trans-substitution pattern and the extended conjugation of the ethynylthienyl ligands via interaction with the Mo₂δ orbital. The HOMO of both I and II is the M ₂δ orbital, and the intense color of the compounds (I is red and II is blue) is due to the M₂δ-to-ThCCH ¹MLCT transition. The S₁ states for I and II are ¹MLCT. The T₁ state is ³MLCT for II, but ³MoMoδδ* for I. The TRIR spectra of the ν(Ci - C) stretch in the MLCT states are consistent with the delocalization of the electron over both ThCCH ligands. Compound I is shown to be a synthon for the preparation of trans-Mo₂(TⁱPB) ₂(ThCCPh)₂ (III) and trans-Mo₂(T ⁱPB)₂(ThCCAuPPh₃)₂ (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray diffraction. The structure of III indicates the extended π-conjugation of the trans-ethynyl-thienyl units extends to the added phenyl rings.

Full text of DOI:10.1021/ja400508u

Article
pubs.acs.org/JACS
Metal−Metal Quadruple Bonds Supported by 5‑Ethynylthiophene-2-
carboxylato Ligands: Preparation, Molecular and Electronic
Structures, Photoexcited State Dynamics, and Application as
Molecular Synthons
Samantha E. Brown-Xu, Malcolm H. Chisholm,* Christopher B. Durr, and Thomas F. Spilker
Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
S
* Supporting Information
ABSTRACT: From the reaction between M2(TiPB)4 and 2 equiv of 5-ethynylthiophene-2-carboxylic acid (H-ThCCH) in
toluene, the complexes trans-M2(TiPB)2(ThCCH)2, where M = Mo (I) or W (II) and TiPB = 2,4,6-triisopropyl benzoate, have
1
been isolated and characterized by H NMR, IR, MALDI-TOF MS, UV−vis, steady-state emission, transient absorption, and
time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms
the trans-substitution pattern and the extended conjugation of the ethynylthienyl ligands via interaction with the Mo2δ orbital.
The HOMO of both I and II is the M2δ orbital, and the intense color of the compounds (I is red and II is blue) is due to the
M2δ-to-ThCCH 1MLCT transition. The S1 states for I and II are 1MLCT. The T1 state is 3MLCT for II, but 3MoMoδδ* for I.
The TRIR spectra of the ν(CC) stretch in the MLCT states are consistent with the delocalization of the electron over both
ThCCH ligands. Compound I is shown to be a synthon for the preparation of trans-Mo2(TiPB)2(ThCCPh)2 (III) and trans-
Mo2(TiPB)2(ThCCAuPPh3)2 (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray
diffraction. The structure of III indicates the extended π-conjugation of the trans-ethynyl-thienyl units extends to the added
phenyl rings.
prepare polymers/oligomers containing M2 units by the
reaction between the homoleptic carboxylates M2(O2CR)4
and conjugated dicarboxylic acids. For M = Mo and ter- and
pentathienyl (α,α′-linked) dicarboxylic acids, oligomers were
obtained suitable for spin coating and, when fabricated, showed
electroluminescence.24 However, this procedure gave only low-
molecular-weight materials having 6−8 M2 units and containing
molecular loops, triangles, and other species. Indeed, reactions
involving the smallest of dicarboxylic acids, namely oxalic acid,
gave almost exclusively triangles.25 We are therefore seeking
alternate strategies for the synthesis of oligomers and polymers
by employing ethynyl and vinyl end groups that may be used in
more well-developed carbon−carbon coupling reactions and
eliminate the reliance on metathetic reactions at the M2 center.
1. INTRODUCTION
Conjugated organic polymers and even small oligomers are
important in a range of rapidly emerging technologies including
light-emitting diodes, field effect transistors, photovoltaic
devices, and sensors.1−6 The introduction of metal ions into
these systems affords a variety of additional features, as is
evident from fundamental studies by Hagihara7 and the work of
Wong in the introduction of platinum8−14 and other metals15,16
into conjugated ethynylthiophenes. The metal can enhance
emission from triplet states by intersystem crossing and may
allow hole transport by dπ−pπ conjugation.17 We have for
some time had an interest in the introduction of metal−metal
(M2) quadruply bonded units into organic π-systems and in the
preparation of metalated polymers. The M2δ orbital can
interact directly with a π-conjugated organic molecule that is
attached to the M2 unit via a carboxylate or the related
amidinate ligand.18−23 We have previously attempted to
Received: January 16, 2013
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of I−IV
a
Ar = 2,4,6-triisopropylphenyl. Reagents: (a) toluene, (b) excess phenyl iodide, THF:Et3N, Pd(PPh3)2Cl2, CuI; (c) 2 equiv of Ph3PAuCl, Et3N,
THF:DCM.
We describe here the synthesis of thienylcarboxylates bearing
terminal ethynyl groups bound to the M2 center and show that
these are viable synthons for the synthesis of extended π-
systems and molecular triads. The two principal quadruply
bonded complexes are also shown to exhibit very interesting
photophysical properties.
2. RESULTS AND DISCUSSION
2.1. Preparation. The synthesis of the new compounds
reported in this work is outlined in Scheme 1.
The compounds trans-M2(TiPB)2(ThCCH)2, where M =
Mo (I) and W (II), TiPB = 2,4,6-triisopropyl benzoate, and
ThCCH = 5-ethynylthiophene-2-carboxylate, are soluble in
toluene, THF, and dichloromethane (DCM). They are air-
sensitive and intensely colored: I is red, and II is blue. They
1
give molecular ions in the MALDI-TOF MS, and their H
NMR and other characterization data are given in the
Supporting Information.
The compound trans-Mo2(TiPB)2(ThCCPh)2 (III) was
Figure 1. Thermal ellipsoid plot of I shown at 50% probability.
Hydrogens and solvent molecules have been omitted for clarity. Color
code: gray, carbon; blue, nitrogen; scarlet, oxygen; yellow, sulfur;
green, molybdenum.
prepared by Sonogashira coupling involving phenyl iodide.
Compound III is purple, in contrast to its precursor I. The
gold-containing complex, trans-Mo2(TiPB)2(ThCCAuPPh3)2
(IV), is red like its precursor I. Further characterization data
for III and IV are given in the Supporting Information.
2.2. Single-Crystal and Molecular Structures. The
molecular structures of the dimolybdenum-containing com-
plexes I, III, and IV are shown in Figures 1, 2, and 3,
respectively.
molecules, THF (I and III) and DMF (IV), along the Mo−Mo
axis.
Further crystallographic details can be found in the
Supporting Information.
Each structure has a molecular center of inversion, and the
phenyl rings of the TiPB ligands are essentially out of
conjugation with the carboxylate π-system. The dihedral planes
involving the CO2 and C6 rings are 86.7° (I), 85.2° (III), and
81.9° (IV). In contrast, the thienyl ring and its attendant
carboxylate are nearly planar, with dihedral angles of 3.8° (I),
7.7° (III), and 1.4° (IV). For compound III, the dihedral angle
between the phenyl and thienyl rings is 9.3°. The central
2.3. Electronic Structure Calculations. In order to aid in
the interpretation of the spectral data, we performed electronic
structure calculations on the model compounds I′, II′, and III′,
where formate is substituted for the TiPB ligands. This assists in
reducing the computational time and resources and has proven
useful for related compounds and interpretation of their
spectral data. The calculations employed Density Functional
Theory (DFT) and time-dependent DFT (TD-DFT). We also
performed calculations on the anions [I′]− and [II′]− to assist
in the determination of electron delocalization in the MLCT
states.
4+
Mo2(O2R)4 units are typical of Mo2 quadruply bonded
complexes, with Mo−Mo and Mo−O bond distances of ∼2.10
Å.26 All three compounds crystallized with donor solvent
B
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highest occupied molecular orbital (HOMO) is M2δ with a
significant degree of mixing with the ligands. The lowest
unoccupied molecular orbital (LUMO) in both cases is the
symmetrical combination of the ethynylthienyl carboxylate π*
orbital that cannot mix with the M2δ. For molybdenum, the
HOMO−1 and HOMO−2 are filled combinations of the
ethynylthienyl carboxylate, which lie higher in energy relative to
the M2π and M2σ orbitals. In the case of tungsten, the order of
the ligand and metal π orbitals is reversed as the metal-based
orbitals, δ and π for tungsten, lie roughly 0.5 eV higher relative
to their molybdenum counterparts. For molybdenum, the
LUMO+1 is the Mo2δ* orbital, and the LUMO+2 is the ligand-
based π* orbital of the ethynylthienyl carboxylate that mixes
with the M2δ. For tungsten, M2δ* is the LUMO+2, and it is
well separated from the ligand π* combinations, the LUMO
and LUMO+1. The HOMO−LUMO gap for tungsten is also
0.5 eV smaller than that of its molybdenum analogue.
Figure 2. Thermal ellipsoid plot of III shown at 50% probability.
Hydrogens and solvent molecules have been omitted for clarity. Color
code: gray, carbon; blue, nitrogen; scarlet, oxygen; yellow, sulfur;
green, molybdenum.
A similar comparison of the frontier orbitals of I′ and III′
along with the isosurface contour plots of III′ are shown in
Figure 5. The calculations for both I′ and III′ were done in C2h
Figure 3. Thermal ellipsoid plot of IV shown at 50% probability.
Hydrogens and solvent molecules have been omitted for clarity. Color
code: gray, carbon; blue, nitrogen; scarlet, oxygen; yellow, sulfur;
maroon, phosphorus; orange, gold; green, molybdenum.
Figure 4 displays the frontier molecular orbital energy level
diagram comparing I′ and II′ and isosurface contour plots for
II′, which are very similar to those for I′. For each molecule, the
Figure 5. Energy diagram for I′ and III′. Orbitals for III′ are shown.
symmetry, which emphasizes the maximum ligand π−M2δ
conjugation. For III′ we see a greater mixing of the ligand filled
π orbitals with the M2δ as a result of the higher energy of the
ligand filled π orbitals in III′ relative to I′. This has the effect of
raising the energy of the HOMO, while the LUMO in III′ is
lowered due to the extended π conjugation. The energy
separation between the LUMO and LUMO+2 orbitals,
representing the two ligand π* systems, is also greater for
III′, 0.33 versus 0.26 eV, as a result of the greater M2δ ligand π
orbital mixing.
2.4. Electronic Absorption Spectra. The UV−vis−NIR
spectra of compounds I and II in THF at room temperature are
compared in Figure 6. At high energy, ∼300 nm, we see the
ethynylthienyl carboxylate ligand-based π−π* transition that is
hardly influenced by the metal. In contrast, to lower energy we
see the intense 1MLCT (metal-to-ligand charge-transfer) bands
arising from the M2δ to ethynylthienyl carboxylate π*,
HOMO→LUMO, that track with the relative energy of the
M2δ orbital. We also see evidence of a vibronic progression in
the spectrum of II for which the (0,0) transition is the most
intense. The vibronic features appear better resolved at low
temperature in 2-MeTHF, and the spectrum at 125 K is shown
Figure 4. Energy diagram of compounds I′ and II′. Orbitals for II′
were generated with Gaussview 5.0.
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Figure 6. Electronic absorption spectra for I (red) and II (blue).
MLCT λmax = 499 (I) and 712 nm (II).
in the Supporting Information. This feature is common to
many tungsten complexes of this type and has been discussed
elsewhere.27−29 Also, for both compounds we see the M2δ-to-
CO2π* transition associated with the TiPB ligand: 402 nm for
M = W and 335 nm for M = Mo.
An analogous comparison of the absorption spectra of the
Mo2-containing compounds, I, III, and IV, is given in Figure 7.
Figure 8. Solvent-dependent absorption and emission spectra of I (a)
and II (b) in THF (red), DCM (black), and toluene (blue).
II in toluene was measured as a function of concentration and
displays no shift in the spectrum, ruling out any self-absorption
(SI Figure 1).
At room temperature, the absorption spectra correspond to a
Boltzmann distribution of rotamers along the ethynylthienyl
carboxylate, and both trans-ligands are relatively weakly
coupled. However, upon photoexcitation, the promoted
electron formally occupies the LUMO, which has a stronger
preference for extended planarity. Thus, photoexcitation may
well be expected to lead to an initially polar excited state that
relaxes to one that is nonpolar. The steady-state emission
spectra support this view that the S1 state has little if any dipole
moment. Alternately stated, S1 is a delocalized excited state with
the positive charge on the M2 center and the electron
delocalized over both ethynylthienyl carboxylate ligands. This
view is further supported by the TRIR spectra, vide infra.
We have not determined the quantum yield for the emission,
but it is akin to other dimetal tetracarboxylates for which the
efficiency was estimated to be less than 0.1%.31
Figure 7. Electronic absorption spectra of I, III, and IV in THF at
room temperature. MLCT λmax = 499 (I), 517 (III), and 489 nm (IV).
Here we see the similarity of compounds I and IV, which both
appear red. This is not too surprising since the [AuPPh3]+
cation is commonly referred to as a “large proton”.30 In
contrast, the 1MLCT of compound III is notably red-shifted to
lower energy compared to I as a result of the extended π
system. The intraligand π→π* transition in III (321 nm) also
occurs at lower energy than in I (290 nm).
Compound I also shows phosphorescence from its triplet
(T1) state at ∼1100 nm which is typical of Mo2-containing
2.5. Solvent Dependence and Steady-State Emission.
3
The absorption and steady-state emission spectra associated
carboxylates and corresponds to a MoMoδδ* state (SI Figure
1
with the MLCT states have been recorded in THF, toluene,
2).18,20−22,32 At 77 K, the emission intensity increases notably
on cooling, and a vibronic progression corresponding to the
Mo−Mo stretch (∼350 cm−1) is visible. Compound II has
singlet (S1) emission with λmax ≈ 870 nm, which is followed by
tailing to ∼1100 nm. There is a definite weak emission at 1260
nm which we suggest arises from weak phosphorescence from
and DCM at room temperature. These are shown for I and II
in Figure 8, where it can be seen that while there is a significant
solvent dependence in the absorption spectra, there is very little
in the emission from the S1 state between toluene and DCM,
but in the case of THF we do see a significant shift, which is not
surprising because THF can behave as a good ligand in axial
3
the MLCT state at room temperature (SI Figure 2). We have
5+
coordination to the M2 center in the MLCT state. This
seen this type of behavior previously for tungsten, and it leads
essentially stabilizes the MLCT state in THF. The emission of
to ∼3500 cm−1 in terms of the energy separation of the S1 and
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T1 MLCT states (SI Figure 2, inset).33 It should be noted that
the 3WW δδ* state is notably higher in energy than the 3MoMo
δδ* state because of the relative energies of the M2δ orbitals.
This is nicely seen in the absorption spectra of M2(TiPB)4
compounds, where the λmax occurs at longer wavelength for M2
= W2, whereas the phosphorescence is in the inverse order.32
2.6. Time-Resolved Spectroscopy. Compounds I and II
have been examined by femtosecond time-resolved infrared (fs-
TRIR) and nanosecond transient absorption (ns-TA) spectros-
copies. From these we can estimate the S1 lifetimes as 3.57 ps
for I and 4.4 ps for II (SI Figure 3), while the T1 lifetimes are
78.8 5.9 μs for I and 10−30 ns for II (SI Figure 4). Most
pertinent to the matter of charge location in the excited states
are the TRIR spectra associated with the ethynyl group. In the
ground state, this is a weak IR-active stretch seen at ν = 2099
cm−1 for both compounds. Upon photoexcitation, the vibration
has enhanced intensity and is shifted to lower energy due to
population of the π* orbital. For complex II (Figure 9), the IR
Table 1. Experimental and Computational ν(CC)
Frequencies (in cm−1) and Corresponding Shifts for I and II
I
II
I′
II′
S0
2099
2033
−66
2099
neutral
anion
2130
2073
−57
2128
2073
−55
S1 (T1)
Δν(CC)
2047 (2031)
−52 (−68)
Δν(CC)
favorably with those observed in the S1 states, supporting the
view of a delocalized excited state.36
Time-resolved spectroscopic studies of III and IV will be
reported later in conjunction with other compounds of similar
nature. Evaluating the shift of ν(CC) in comparison to the
parent complexes should provide a measure of the electronic
interaction of their appended groups in the excited states.
3. CONCLUDING REMARKS
The spectroscopic data for compounds I and II indicate that
the S1 states and the T1 state for II are fully delocalized. In
terms of excited-state mixed valency,37 this is Class III on the
Robin and Day scheme.38 There are, to our knowledge, no
detailed TRIR studies of transition metal alkynyl complexes in
their S1 states, despite the fact that some fluoresce and have
picosecond lifetimes. The T1 states that have been examined
have broken symmetry where the charge is centered upon one
3
ligand. The delocalized MLCT states of II thus appear very
different from all studies of trans-bisalkynyl Pt(II) complexes to
date, which show lowest energy electronic transitions that are at
least in part MLCT in character.39−47
In terms of the synthetic strategy for the synthesis of higher
order assemblies, the preparation of III and IV from I is very
encouraging. Although the C−C coupling reactions employed
are “standard” in organic synthesis, and have been successfully
employed by others in the synthesis of later transition metal-
containing organic conjugated oligomers and polymers,48−54
the application to the kinetically labile M24+ center is notable, as
many reactions may lead to undesired products due to
metathetic reactions of the carboxylates. For example, the use
of organomagnesium or organozinc reagents as in Kumada or
Negishi coupling reactions is not possible due to carboxylate
scrambling. Further work involving C−C coupling reactions of
this type involving vinyl groups is currently underway.
Figure 9. fs-TRIR of ν(CC) of II in THF at room temperature, λex
= 720 nm.
band assignable to ν(CC) is initially shifted by 52 cm−1 in
1
the MLCT state. This peak decays with time to leave an IR
3
band assignable to the MLCT state of II shifted 68 cm−1 to
lower energy. The appearance of a single IR-active mode is
consistent with a delocalized excited state and contrasts with
the spectra of compounds of the form trans-PtL2((CCAr)2
that show two IR stretches in their T1 states: one shifted to
lower energy and the other to higher energy relative to the
ground state.34,35 These have been ascribed to broken or C2v
symmetry in the excited state. The further shift of the ν(CC)
to lower energy in II with the change from S1 to T1 suggests
that there is slightly greater CC π* character in the triplet
state. The molybdenum complex I (SI Figure 5) shows similar
behavior, with Δν(CC) = −66 cm−1 in the S1 state.
Following decay to the T1 state, however, the ethynyl stretch
ASSOCIATED CONTENT
■
S
* Supporting Information
Materials and methods, synthesis and characterization of I−IV,
NIR emission spectra, kinetic traces of I and II from the fs-
TRIR and of I from the ns-TA, TRIR spectrum of I, and
crystallographic information (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org. CCDC
records 919547−919549 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
3
becomes IR silent, consistent with a MoMoδδ* state.
Since the IR bands associated with ligands in MLCT states
are very similar to those of the reduced form of the ligand, we
have calculated the values of ν(CC) for [I′]− and [II′]−. The
calculations pertain to a delocalized anion with the negative
charge distributed principally upon the two trans-ethynylthienyl
carboxylates. A comparison of the experimental values for
compounds I and II in their ground and photoexcited states is
given in Table 1, along with those calculated for the ground
states of I′ and II′ and their respective anions [I′]− and [II′]−.
The calculated shifts for the anions (∼55 cm−1) compare
AUTHOR INFORMATION
■
Corresponding Author
chisholm@chemistry.ohio-state.edu
Notes
The authors declare no competing financial interest.
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(25) Chisholm, M. H.; Durr, C. B.; Hadad, C. M.; Spilker, T. F. J.
Cluster Sci. 2012, 23, 767−780.
ACKNOWLEDGMENTS
■
The authors acknowledge Dr. Judith Gallucci for helpful
discussion and Prof. Claudia Turro for use of instrumentation.
This work was supported by NSF grant CHE-0957191 and the
OSU Institute for Materials Research. Computational resources
were provided by the Ohio Supercomputing Center. S.E.B.-X.
acknowledges support from the National Defense Science and
Engineering Graduate (NDSEG) Fellowship. During this work
we discovered that compound I has been prepared and
structurally characterized independently by Thomas P.
Robinson in the group of Prof. Paul R. Raithby in the
University of Bath, UK.
(26) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds
Between Metal Atoms, 3rd ed.; Springer Science and Business Media,
Inc.: New York, 2005.
(27) Bursten, B. E.; Chisholm, M. H.; Clark, R. J. H.; Firth, S.;
Hadad, C. M.; MacIntosh, A. M.; Wilson, P. J.; Woodward, P. M.;
Zaleski, J. M. J. Am. Chem. Soc. 2002, 124, 3050−3063.
(28) Alberding, B. G.; Chisholm, M. H.; Lear, B. J.; Naseri, V.; Reed,
C. R. Dalton Trans. 2011, 40, 10658−10663.
(29) Chisholm, M. H.; Lear, B. J. Chem. Soc. Rev. 2011, 40, 5254−
5265.
(30) Raubenheimer, H. G.; Schmidbaur, H. Organometallics 2012, 31,
2507−2522.
(31) Chisholm, M. H.; Gustafson, T. L.; Turro, C. Acc. Chem. Res.
2013, 46, 529−538.
(32) Alberding, B. G.; Chisholm, M. H.; Chou, Y.-H.; Gallucci, J. C.;
Ghosh, Y.; Gustafson, T. L.; Patmore, N. J.; Reed, C. R.; Turro, C.
Inorg. Chem. 2009, 48, 4394−4399.
(33) Chisholm, M. H.; Chou, P.-T.; Chou, Y.-H.; Ghosh, Y.;
Gustafson, T. L.; Ho, M.-L. Inorg. Chem. 2008, 47, 3415−3425.
(34) Emmert, L. A.; Choi, W.; Marshall, J. A.; Yang, J.; Meyer, L. A.;
Brozik, J. A. J. Phys. Chem. A 2003, 107, 11340−11346.
(35) Batista, E. R.; Martin, R. L. J. Phys. Chem. A 2005, 109, 9856−
9859.
(36) Alberding, B. G.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.;
Gustafson, T. L. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 8152−8156.
(37) Plummer, E. A.; Zink, J. I. Inorg. Chem. 2006, 45, 6556−6558.
(38) Robin, M. B.; Day, P. In Advances in Inorganic Chemistry and
REFERENCES
■
(1) Mishra, A.; Bauerle, P. Angew. Chem., Int. Ed. 2012, 51, 2020−
̈
2067.
(2) Osaka, I.; Shimawaki, M.; Mori, H.; Doi, I.; Miyazaki, E.;
Koganezawa, T.; Takimiya, K. J. Am. Chem. Soc. 2012, 134, 3498−
3507.
(3) Silvestri, F.; Marrocchi, A.; Seri, M.; Kim, C.; Marks, T. J.;
Facchetti, A.; Taticchi, A. J. Am. Chem. Soc. 2010, 132, 6108−6123.
(4) McCulloch, I.; Ashraf, R. S.; Biniek, L.; Bronstein, H.; Combe, C.;
Donaghey, J. E.; James, D. I.; Nielsen, C. B.; Schroeder, B. C.; Zhang,
W. Acc. Chem. Res. 2012, 45, 714−722.
(5) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant,
J. R.; Shakya Tuladhar, P.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk,
M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.;
McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272−3275.
(6) Tsao, H. N.; Cho, D.; Andreasen, J. W.; Rouhanipour, A.; Breiby,
́
Radiochemistry; Emeleus, H. J., Sharpe, A. G., Eds.; Academic Press:
San Diego, 1968; Vol. 10, pp 247−422.
(39) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus,
D. W.; Pisula, W.; Mullen, K. Adv. Mater. 2009, 21, 209−212.
̈
M.; Khan, M. S.; Raithby, P. R.; Kohler, A.; Friend, R. H. J. Am. Chem.
̈
(7) Sonogashira, K.; Takahashi, S.; Hagihara, N. Macromolecules
1977, 10, 879−880.
Soc. 2001, 123, 9412−9417.
(40) Silverman, E. E.; Cardolaccia, T.; Zhao, X.; Kim, K.-Y.; Haskins-
Glusac, K.; Schanze, K. S. Coord. Chem. Rev. 2005, 249, 1491−1500.
(41) Prusakova, V.; McCusker, C. E.; Castellano, F. N. Inorg. Chem.
2012, 51, 8589−8598.
̌ ́
(8) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Chan, K.-K.; Djurisic, A. B.;
Cheung, K.-Y.; Yip, C.-T.; Ng, A. M.-C.; Xi, Y. Y.; Mak, C. S. K.; Chan,
W.-K. J. Am. Chem. Soc. 2007, 129, 14372−14380.
̌ ́
(9) Zhan, H.; Wong, W.-Y.; Ng, A.; Djurisic, A. B.; Chan, W.-K. J.
(42) Beljonne, D.; Wittmann, H. F.; Kohler, A.; Graham, S.; Younus,
̈
Organomet. Chem. 2011, 696, 4112−4120.
́
M.; Lewis, J.; Raithby, P. R.; Khan, M. S.; Friend, R. H.; Bredas, J. L. J.
(10) Wong, W.-Y.; Ho, C.-L. Acc. Chem. Res. 2010, 43, 1246−1256.
(11) Wong, W.-Y. Macromol. Chem. Phys. 2008, 209, 14−24.
(12) Ho, C.-L.; Wong, W.-Y. Coord. Chem. Rev. 2011, 255, 2469−
2502.
Chem. Phys. 1996, 105, 3868−3877.
(43) Muro, M. L.; Diring, S.; Wang, X.; Ziessel, R.; Castellano, F. N.
Inorg. Chem. 2009, 48, 11533−11542.
(44) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S.
Inorg. Chem. 2001, 40, 4053−4062.
̌ ́
(13) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisic, A. B.; Yip, C.-T.;
Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat. Mater.
2007, 6, 521−527.
(45) Archer, S.; Weinstein, J. A. Coord. Chem. Rev. 2012, 256, 2530−
2561.
(14) Wong, W.-Y.; Ho, C.-L. Coord. Chem. Rev. 2006, 250, 2627−
2690.
(46) Glik, E. A.; Kinayyigit, S.; Ronayne, K. L.; Towrie, M.;
Sazanovich, I. V.; Weinstein, J. A.; Castellano, F. N. Inorg. Chem. 2008,
47, 6974−6983.
(47) Ramakrishna, G.; Goodson, T.; Rogers-Haley, J. E.; Cooper, T.
M.; McLean, D. G.; Urbas, A. J. Phys. Chem. C 2009, 113, 1060−1066.
(48) Chen, W.-Z.; Fanwick, P. E.; Ren, T. Organometallics 2007, 26,
4115−4117.
(49) Chen, W.-Z.; Ren, T. Inorg. Chem. 2006, 45, 9175−9177.
(50) Xu, G.-L.; Ren, T. Organometallics 2005, 24, 2564−2566.
(51) Zhang, L.; Huang, Z.; Chen, W.-Z.; Negishi, E.; Fanwick, P. E.;
Updegraff, J. B.; Protasiewicz, J. D.; Ren, T. Organometallics 2007, 26,
6526−6528.
(15) Li, P.; Ahrens, B.; Choi, K.-H.; Khan, M. S.; Raithby, P. R.;
Wilson, P. J.; Wong, W.-Y. CrystEngComm 2002, 4, 405−412.
(16) Wong, W.-Y.; Choi, K.-H.; Lu, G.-L.; Lin, Z. Organometallics
2002, 21, 4475−4481.
(17) Holliday, B. J.; Swager, T. M. Chem. Commun. 2005, 23−36.
(18) Burdzinski, G. T.; Chisholm, M. H.; Chou, P.-T.; Chou, Y.-H.;
Feil, F.; Gallucci, J. C.; Ghosh, Y.; Gustafson, T. L.; Ho, M.-L.; Liu, Y.;
Ramnauth, R.; Turro, C. Proc. Natl. Acad. Sci. U.S.A. 2008, 105,
15247−15252.
(19) Chisholm, M. H.; D’Acchioli, J. S.; Hadad, C. M. J. Cluster Sci.
2006, 18, 27−49.
(20) Alberding, B. G.; Chisholm, M. H.; Ghosh, Y.; Gustafson, T. L.;
Liu, Y.; Turro, C. Inorg. Chem. 2009, 48, 8536−8543.
(21) Brown-Xu, S. E.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.;
Gustafson, T. L.; Reed, C. R. Dalton Trans. 2012, 41, 2257−2263.
(22) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Gustafson, T. L.;
Naseri, V.; Spilker, T. F. J. Am. Chem. Soc. 2012, 134, 20820−20826.
(23) Chisholm, M. H.; Patmore, N. J. Acc. Chem. Res. 2007, 40, 19−
27.
(52) Iii, S. W. T.; Yagi, S.; Swager, T. M. J. Mater. Chem. 2005, 15,
2829−2835.
(53) Ren, T. Organometallics 2005, 24, 4854−4870.
(54) Ren, T. Chem. Rev. 2008, 108, 4185−4207.
(24) Chisholm, M. H.; Epstein, A. J.; Gallucci, J. C.; Feil, F.; Pirkle,
W. Angew. Chem., Int. Ed. 2005, 44, 6537−6540.
F
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