Oligothiophenes incorporating metal-metal quadruple bonds

Article abstract of DOI:10.1002/anie.200500138

Illuminating quadruple bonds: The polymeric materials [M₂(O ₂CC₆H₂-2,4,6-Me₃)₂(O ₂C-(Thp)_n·CO₂)]_n (M = Mo or W (green); Thp = thiophene (S atoms

Full text of DOI:10.1002/anie.200500138

Angewandte
Chemie
on the use of the Group 8 metals (the metals of the iron,
cobalt, and nickel triads), in which the metal ions are d6 or d8.
We are interested in incorporating metal–metal quadruple
bonds with a s2p4d2 configuration into organic conjugated
polymers because the dinuclear center is redox-active and its
ease of oxidation can be tuned over a considerable range by
the selection of the metal (M = Cr, Mo, W) and the ancillary
ligands,[5] even to the point that the M2 center is more easily
oxidized than cesium.[6] The M2 center may also be brought
into conjugation with the p system of the organic polymer
through the agency of a carboxylate link.[7] The carboxylate
p system couples with both the p system of the polymer
backbone and the M2 d orbitals. We recently illustrated this by
the preparation of some discrete model complexes of the type
[M2(O2CR)4], in which R = mono-, bi-, and terthienyl (a,a’)
groups.[8]
Herein, we describe our synthesis of oligomeric/polymeric
thienyl-based materials incorporating metal–metal quadruple
bonds. The general reaction is shown in Equation (1),
22 o
C
½M ðO CRÞ Š þ HOOC-ðThpÞ -COOH
ƒƒƒ!
2
2
4
n
toluene
ð1Þ
½M2ðO2CRÞ2ðO2-CðThpÞn-CO2ފm þ 2 RCOOH
although, as noted below, there are important restrictions
(Thp = thiophene = 2,5-C4H2S). Full experimental details are
given in the Supporting Information.
Metal–Metal Bonds
DOI: 10.1002/anie.200500138
To prepare soluble oligomers/polymers of the desired
empirical formula, we found it necessary to introduce chain-
branching alkyl substituents on both the spectator carboxyl-
ate and the thienyl dicarboxylate. Thus far, we have enjoyed
most success with 2,4,6-triisopropylbenzoate (2,4,6-
iPr3C6H2CO2 = TiPB) as the ancillary ligand. This group has
the added advantage that the trans-disubstituted product
appears to be the preferred thermodynamic product. This is
also evident from the preparations of the discrete compounds
for molybdenum [Eq. (2)] and for tungsten [Eq. (3)]. For
Oligothiophenes Incorporating Metal–Metal
Quadruple Bonds**
Malcolm H. Chisholm,* Arthur J. Epstein,*
Judith C. Gallucci, Florian Feil, and Wesley Pirkle
There is considerable interest at this time in inorganic–
organic hybrid materials, and metal-containing polymers
constitute an important section within this general field. The
work of Manners and co-workers[1] with metallocene-derived
polymers, Friend, Raithby, and co-workers[2] with metallated
polyynes and poly(phenylenvinylene)s, Wolf and co-work-
ers[3] with metallated polythiophenes, and Thompson, Forrest,
and co-workers[4] with iridium and platinum complex doped
conjugated polymers, represent four prime areas of recent
research interest. The majority of efforts thus far have focused
22 o
C
½Mo ðTiPBÞ Š þ 2 HOOC-ðThpÞ -R
ƒƒƒ!
2
4
n
trans-½Mo2ðTiPBÞ2ðO2C-ðThpÞn-RÞ2Štoþluen2e TiPB-H
ð2Þ
ð3Þ
22 o
C
½W ðTiPBÞ ðtBuCO Þ Š þ 2 HOOC-ðThpÞ -R
ƒƒƒ!
2
2
2
2
n
toluene
trans-½W2ðTiPBÞ2ðO2C-ðThpÞn-RÞ2Š þ 2 tBuCOOH
molybdenum, we used the corresponding homoleptic carbox-
ylate [Mo2(TiPB)4][9] and for tungsten, the mixed compound
[W2(TiPB)2(tBuCO2)2] as starting materials. Even with the
use of only 1 equivalent of the thienyl carboxylic acid, the
bis,bis-substituted product [M2(TiPB)4ꢀx(O2C-(Thp)n-R)x]
(Thp = 2,4-C4H2S; n = 1,2: R = H; n = 3: R = Me) is formed
in preference to other combinations.
[*] Prof. Dr. M. H. Chisholm, J. C. Gallucci, Dr. F. Feil
Department of Chemistry, The Ohio State University
Columbus, Ohio 43210 (USA)
Fax: (+1)614-292-0368
E-mail: chisholm@chemistry.ohio-state.edu
Only one isomer is observed in solution for these M2
complexes and, as is shown in Figure 1, steric factors favor the
trans substitution about the M2 paddle-wheel core. The
twisting of the phenyl rings out of the plane of their attendant
carboxylate groups removes M2(d)-to-C6(p) conjugation. In
the UV/Vis spectrum, the M2(d)-to-(aryl carboxylate) tran-
sition is very weak in comparison with the M2(d)-to-(thienyl
carboxylate) transition. As is shown in Figure 2, the M2(d)-to-
thienyl(p*) transition moves to lower energy as n increases
Prof. Dr. A. J. Epstein, W. Pirkle
Department of Physics, The Ohio State University
Columbus, OH 43210 (USA)
[**] We thank the National Science Foundation for financial support, Dr.
Kari Green-Church for recording MALDI mass spectra, and Prof.
Claudia Turro, Dr. Ramkrishna Ramnauth, and Dr. Yao Liu for
measuring the emission spectra.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 6537 –6540
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acid TiPB-H and any unreacted monomeric M2 tetracarbox-
ylates. The resulting dried red (M = Mo) or bluish-green (M =
W) powders are somewhat soluble in THFand swell to form
gels by absorbing approximately 20 times their mass of THF.
1
The H NMR spectra of the samples in [D8]THFonly show
evidence for one type of TiPB and O2C-(Thp)n-CO2 groups.
However, the MALDI MS data show molecular ions that
+
correspond to [M2(TiPB)2(O2C-(Thp)n-CO2)]2 loops, [M2-
+
(TiPB)2(O2C-(Thp)n-CO2)]3 triangles, and even higher
oligomers. These can only arise from a cis-substitution pattern
at the M24+ center. We propose that this substitution pattern is
less common than that of trans substitution, but that it is this
4+
property, together with some tersubstitution at the Mo2
center, that facilitates the gel-forming capability of the
entangled chains. The system can be described as a dynamic
equilibrium of chains, cycles, and cross-linked oligomers/
polymers[12] as a result of facile carboxylate-exchange reac-
tions, which are well-documented to be catalyzed by both acid
and base (H+, carboxylate).[13] A reasonable estimate of the
molecular weight is over 10000 Daltons, or more than
10 repeating units.
The electronic absorption spectra of these oligomers
closely resemble those of the model compounds (Figure 2) in
that they have well-defined, although broad, absorptions
assignable to M2(d)-to-thienyl(bridge)(p*) and thienyl(p)-to-
thienyl(p*) (bridge-to-bridge) transitions. The emission spec-
tra show only the thienyl(p*)-to-M2(d) transition, as shown in
Figure 1. Molecular structure of the centrosymmetric [Mo2(TiPB)2(O2C-
Thp-H)2] molecule found in the solid state with two axially coordinating
DMSO molecules and one noncoordinating DMSO molecule. The
ꢀ
ꢀ
ꢀ
Mo Mo, Mo O(TiPB carboxylate), and Mo O(thiophene carboxylate)
interatomic distances are 2.113, 2.111 (av), and 2.110 Š (av), respec-
tively. The dihedral angle between the thienyl ring and its carboxylate
group is 0.358 (av).
from 1 to 2 to 3 and is markedly red-shifted and more intense
on going from M = Mo to W. The latter is a result of the higher
orbital energy of the W2 d electrons and the greater degree of
W2(d)-to-thienyl(p*) overlap.
In reactions of Equation (1), we employed the n-hexyl-
substituted thienyl dicarboxylic acids (DHTT)H2 (3’,4’-
dihexyl-2,2’:5’,2’’-terthiophene-5,5’’-dicarboxylic acid)[10] and
(DHQT)H2
(3’’,4’’-dihexyl-2,2’:5’,2’’:5’’,2’’’:5’’’,2’’’’-quinque-
thiophene-5,5’’’’-dicarboxylic acid)[11] shown below. Further-
more, an excess of [Mo2(TiPB)4] or [W2(TiPB)4] was used to
ensure the total conversion of the dicarboxylic acids. The
polymeric materials [M2(TiPB)2(O2C-(Thp)n-CO2)] are
formed as flocculent precipitates that are difficult to filter.
They are best isolated by centrifugation followed by repeated
washings, first with toluene and then hexanes, to remove the
Figure 2. a) Electronic absorption spectra of [Mo2(TiPB)2(O2C-(Thp)-
H)2] (yellow), [Mo2(TiPB)2(O2C-(Thp)2-H)2] (red), and [Mo2(TiPB)2(O2C-
(Thp)3-Me)2] (blue) in THF at room temperature. b) Electronic absorp-
tion spectra of [W2(TiPB)2(O2C-(Thp)-H)2] (blue), [W2(TiPB)2(O2C-
(Thp)2-H)2] (green), and [W2(TiPB)2(O2C-(Thp)3-Me)2] (yellow) in THF
at room temperature.
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Angewandte
Chemie
Figure 3 for the terthienyl-bridged polymer. These photo-
physical properties can be described in terms of the Jablonski
diagram shown in Figure 4.
Figure 3. Absorption spectrum (red) of “Mo2(TiPB)2(DHTT)” in THF at
room temperature, and photoluminescence spectrum (green) with
excitation at 531 nm. Higher-energy excitation (360 nm) results in a
similar spectral profile. The excitation spectrum is shown in blue.
Figure 5. a) Structure of an Al-“Mo2(TiPB)2(DHTT)”-ITO device; b) cur-
*
rent as a function of voltage for the same device ( : IV curve from 14
!
to ꢀ14 V; : IV curve from 16 to ꢀ16 V).
films of the molybdenum terthienyl-bridged material display
diode properties, with current being drawn nearly equally in
both the forward and reverse bias, reminiscent of interface-
controlled symmetrically configured ac-light-emitting
(SCALE) devices.[14] Whereas the terthienylmolybdenum-
containing films do not show electroluminescence with
aluminum, the pentathienyl analogue (derived from
(DHQT)H2) does, and both the ter- and pentathienyl-derived
thin films show electroluminescence when lower-work-func-
tion calcium was used as the cathode (Figure 6). The emission
from these films is red-shifted relative to that of the parent
Figure 4. Proposed Jablonski scheme of the absorptions and emission
of [M2(TiPB)2(O2C-(Thp)n-CO2)] oligomers. Absorptions to S1 and S2
are observed; emission occurs from S1 while a nonemissive T1 state is
also observed. IC=internal conversion; ISC=intersystem crossing.
The gels dry to form thin films, and THFsolutions can be
used to deposit thin films of these metallated oligomers by
spin-coating in a dry-box under argon. Deposition on an
indium–tin oxide (ITO)-coated glass plate followed by vapor
deposition of aluminum or calcium was carried out to make
conductivity measurements. As shown in Figure 5, the thin
Figure 6. Absorption spectra of an “Mo2(TiPB)2(DHTT)” thin film
(red) and an “Mo2(TiPB)2(DHQT)” thin film (purple), and electroemis-
sion of a Ca-“Mo2(TiPB)2(DHTT)”-ITO device (blue) and a Ca-“Mo2-
(TiPB)2(DHQT)”-ITO device (green).
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www.angewandte.org
6539

Communications
acids (DHTT)H2 and (DHQT)H2.[15] Furthermore, the elec-
troluminescence spectra display an unusual blue shift relative
to the photoluminescence spectra, which suggests the impor-
tant role of charge trapping in these materials.
In conclusion, we believe that the preliminary findings
reported herein indicate that polythiophenes incorporating
metal–metal quadruple bonds should have interesting and
tunable electronic properties. The M2 d electrons lie within
the band-gap of the polymer, but are strongly coupled by
M2(d)-to-CO2(p) conjugation. We anticipate that the electro-
luminescence for tungsten can be moved into the near-IR
region. Further work in this area is in progress.
[13] M. H. Chisholm, A. M. Macintosh, J. Chem. Soc. Dalton Trans.
1999, 1205.
[14] Y. Z. Wang, D. D. Gebler, L. B. Lin, J. W. Blatchford, S. W.
Jessen, H. L. Wang, A. J. Epstein, Appl. Phys. Lett. 1996, 68, 894.
[15] N. DiCesare, M. Belletete, A. Donat-Bouillud, M. Leclerc, G.
Durocher, J. Lumin. 1999, 81, 111.
Received: January 13, 2005
Revised: May 30, 2005
Published online: September 15, 2005
Keywords: electroluminescence · metal–metal interactions ·
.
molybdenum · sulfur heterocycles · tungsten
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