Sexithiophenes mediated by MM quadruple bonds: MM = Mo2, MoW, and W2

Article abstract of DOI:10.1021/ic901303a

The reactions between MM(TiPB)₄, where TiPB = 2,4,6-triisopropylbenzoate and MM = MoW and W₂, and the (2,2':5',2"-terthiophene)-5-carboxylic acid, TThH (2 equiv) leads to the formation of new compounds transMM(TiPB)₂(TTh)<

Full text of DOI:10.1021/ic901303a

8536 Inorg. Chem. 2009, 48, 8536–8543
DOI: 10.1021/ic901303a
Sexithiophenes Mediated by MM Quadruple Bonds: MM = Mo2, MoW, and W2
Brian G. Alberding, Malcolm H. Chisholm,* Yagnaseni Ghosh, Terry L. Gustafson,* Yao Liu, and Claudia Turro*
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210-1185
Received July 6, 2009
The reactions between MM(TiPB)4, where TiPB = 2,4,6-triisopropylbenzoate and MM = MoW and W2, and the
(2,20:50,200-terthiophene)-5-carboxylic acid, TThH (2 equiv) leads to the formation of new compounds trans-
MM(TiPB)2(TTh)2, II and III, respectively, as well as to the previously reported compound I, when MM = Mo2. The
1
compounds have been characterized by elemental analysis, H NMR spectroscopy, electronic absorption, and
emission spectroscopies together with cyclic voltammetry and differential pulse voltammetry. Calculations on the
model compounds I0, II0, and III0, where formate ligands substitute for TiPB, have been carried out employing density
1
functional theory (DFT) and time-dependent DFT. These complexes display intense MLCT absorptions (MMδ to
thienyl carboxylate) and have oxidations and reductions that are metal (MMδ) and thienyl ligand based, respectively.
All compounds show emission in the near-IR region. At low temperature the NIR emission from I and II shows clear
evidence of vibronic features due to υ(MM) ∼ 350-390 cm-1, and all compounds show evidence of a vibronic feature
due to υ(CO2) ∼ 1200 cm-1. Transient absorption spectroscopy reveals relatively short-lived S1 states, τ ∼ 10 ps,
and longer lived T1 states: τ ∼ 72 μs for I, ∼ 160 ns for II, and ∼ 90 ns for III. The chemistry described here reveals the
remarkable influence of MMδ to TTh π electronic coupling on the optoelectronic properties of the thienyl chains.
Introduction
solubility,9-11 and the introduction of electron withdrawing
groups can alter the band gap and conductivity.12,13 So, for
example, the introduction of fluoro or fluoroacyl groups can
convert a hole transporter to an electron conductor as
demonstrated by Marks et al.14-17 We are interested in
the influence of incorporating MM quadruple bonded units
(MM=Mo2, MoW, and W2) into oligothiophenes by way of
carboxylate linkers. The latter allow for both covalent
attachments by use of M-O σ bonds and electronic coupling
by way of MM δ to thienyl π conjugation.
In general, the introduction of metal ions into conjugated
organic π-systems leads to new classes of hybrid organic-
inorganic materials.18,19 The metal atoms may be attached as
side-chain appendages or incorporated in the backbone
of the polymer.20 By far the most studied of these systems
are those incorporating the group 8 transition metals and the
Organic conjugated polymers have attracted considerable
attention over the past two decades and now find numerous
applications in the rapidly emerging field of plastic electronic
devices.1-3 Prime among these materials are oligo- or poly-
thiophenes,4-7 many of which are sold commercially, for
example, P3HT (poly-3-hexyl thiophene) and PEDOT (poly
dioxoethylene thiophene). The materials are semiconductors
and when doped show good electrical conductivity via hole
transport.8An attractive feature of these polymers is that
chemical modification can be employed so as to enhance their
processability and physical properties. For example, the
introduction of alkyl groups at 3 and 4 positions enhances
*To whom correspondence should be addressed. E-mail: chisholm@
chemistry.ohio-state.edu (M.H.C.), gustafson@chemistry.ohio-state.edu
(T.L.G.), turro@chemistry.ohio-state.edu (C.T.).
(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347,
539.
(2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
1998, 37, 403.
(11) Hotta, S.; Soga, M.; Sonoda, N. Synth. Met. 1988, 26, 267.
(12) Li, L.; Counts, K. E.; Kurosawa, S.; Teja, A. S.; Collard, D. M. Adv.
Mater. 2004, 16, 180.
(13) Pomerantz, M.; Yang, H.; Cheng, Y. Macromolecules 1995, 28, 5706.
(14) Facchetti, A.; Letizia, J.; Yoon, M.-H.; Mushrush, M.; Katz, H. E.;
Marks, T. J. Chem. Mater. 2004, 16, 4715.
(3) Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A.; Elsenbaumer,
R. L.; Reynolds, J. R., Eds.; M. Dekker: New York, 1997; p 1112 ff.
(4) Roncali, J. Chem. Rev. 1997, 97, 173.
(15) Facchetti, A.; Mushrush, M.; Katz, H. E.; Marks, T. J. Adv. Mater.
2003, 15, 33.
(5) McCullough, R. D. Adv. Mater. 1998, 10, 93.
(6) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100,
2537.
(7) Reddinger, J. L.; Reynolds, J. R. Adv. Polym. Sci. 1999, 145, 57.
(8) Martin, R. E.; Geneste, F.; Holmes, A. B. C. R. Phys. 2000, 1, 447.
(9) Elsenbaumer, R. L.; Jen, K. Y.; Oboodi, R. Synth. Met. 1986, 15, 169.
(10) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F.
Macromolecules 1987, 20, 212.
(16) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Katz, H. E.; Marks, T. J.
Angew. Chem., Int. Ed. 2003, 42, 3900.
(17) Letizia, J. A.; Facchetti, A.; Stern, C. L.; Ratner, M. A.; Marks, T. J.
J. Am. Chem. Soc. 2005, 127, 13476.
(18) Holliday, B. J.; Swager, T. M. Chem. Commun. 2005, 23.
(19) Pickup, P. G. J. Mater. Chem. 1999, 9, 1641.
(20) Wolf, M. O. Adv. Mater. 2001, 13, 545.
r
pubs.acs.org/IC
Published on Web 08/12/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8537
Scheme 1. Synthesis of MM(TiPB)2(TTh)2 Complexes; MM = Mo2, MoW, and W2
coinage metals as exemplified by the work of Wolf,21-23
Raithby24,25 and Wong.26,27 These assemblies have attracted
great attention because of their luminescent properties and
their potential applications in various opto-electronic devices
and fluorescent detectors. The emission from these com-
plexes is from LLCT or MLCT. To our knowledge triplet
Results and Discussion
Synthesis. The general method for the preparation of II
and III, as well as the previously described compound, I,29
is outlined in Scheme 1. Reactions carried out in toluene
proceed with a marked color change and the precipitation
of the products.
3
emission from a metal centered state, MC, has not been
All three compounds are air-sensitive and appreciably
soluble in THF and other donor solvents but less soluble
in simple hydrocarbon solvents such as hexane and
toluene. The compounds show molecular ions (Iþ, IIþ,
IIIþ) by matrix assisted laser desorption/ionization time-
of-flight (MALDI-TOF) and they gave satisfactory ele-
mental analyses.
observed in this application. We recently reported the synth-
esis and electronic and photophysical properties of thienyl
ligated quadruply bonded dimolybdenum compounds, in-
cluding the compound, Mo2(TiPB)2(TTh)2 (I), where, TiPB=
2,4,6-triisopropylbenzoate and TTh = (2,20:50,200-terthio-
phene)-5-carboxylate, that showed dual emission, namely,
a 1MLCT fluorescence and 3MC phosphorescence.29 In this
paper we describe the synthesis and properties of a series of
compounds, MM(TiPB)2(TTh)2, of the structural type shown
in Scheme 1 where the dinuclear MM center is MoW and W2
and compare them with the related compound I, where MM is
Mo2. These compounds show remarkable properties that arise
from the electronic coupling of the MMδ and thienyl carbox-
ylate π-systems and in many ways are quite different from
those of earlier studies of metalated oligothiophenes. They are
model compounds for oligothiophenes incorporating these
MM units which are currently under study in this laboratory.
1
The H NMR data are consistent with the trans-sub-
stitution pattern at the MM center and 1H NMR data are
given in the Experimental Section. Previous studies have
also confirmed the trans geometry in the compounds
28
29
Mo2(TiPB)2(Th)2 and Mo2(TiPB)2(BTh)2 where Th
is thiophene-5-carboxylic acid and BTh is 2,20-bithio-
phene-5-carboxylic acid. In these structures the thienyl
rings are also essentially coplanar with the carboxylate
CO2 moieties as was also seen in the compound trans-
W2(TiPB)2(Azu)2,30 where Azu = 2-carboxy-6-ethoxy-
azulene. As we show later in the calculations, vide infra,
this planar trans-geometry favors and maximizes thienyl
carboxylate-MMδ-thienyl carboxylate interactions.
Electrochemical Studies. The compounds I, II, and III
show reversible one electron oxidation waves by cyclic
voltammetry and differential-pulse voltammetry that are
readily assignable to the reversible oxidation of the MM
center. The oxidation potentials reveal the ease of re-
moval of an electron from the MMδ orbital, and the
potentials are essentially identical to those of the parent
MM(TiPB)4 compounds.31 There is roughly 0.5 V differ-
ence between the Mo24þ/5þ and the W24þ/5þcouple.
(21) Wolf, M. O.; Patrick, B. O.; Stott, T. L. Can. J. Chem. 2007, 85, 383.
(22) Moorlag, C.; Sih, B. C.; Stott, T. L.; Wolf, M. O. J. Mater. Chem.
2005, 15, 2433.
(23) Moorlag, C.; Clot, O.; Wolf, M. O. Polym. Prepr.;Am. Chem. Soc.,
Div. Polym. Chem. 2004, 45, 437.
(24) Li, P.; Ahrens, B.; Feeder, N.; Raithby, P. R.; Teat, S. J.; Khan, M. S.
Dalton Trans. 2005, 874.
(25) Chawdhury, N.; Kohler, A.; Friend, R. H.; Wong, W. Y.; Lewis, J.;
Younus, M.; Raithby, P. R.; Corcoran, T. C.; Al-Mandhary, M. R. A.;
Khan, M. S. J. Chem. Phys. 1999, 110, 4963.
(26) 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.
(27) Wong, W.-Y.; Zhou, G.-J.; He, Z.; Cheung, K.-Y.; Ng, A. M.-C.;
Djurisic, A. B.; Chan, W.-K. Macromol. Chem. Phys. 2008, 209, 1319.
(28) Chisholm, M. H.; Epstein, A. J.; Gallucci, J. C.; Feil, F.; Pirkle, W.
Angew. Chem., Int. Ed. 2005, 44, 6537.
(29) 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. 2008, 105, 15247.
(30) Barybin, M. V.; Chisholm, M. H.; Patmore, N. J.; Robinson, R. E.;
Singh, N. Chem. Commun. 2007, 3652.
(31) 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.

8538 Inorganic Chemistry, Vol. 48, No. 17, 2009
Alberding et al.
Scheme 2. Orbital Interaction between M2 δ and -CO2 π* Orbitals in
a M2(TiPB)2(O2C-L)2 Type Compounda
a L = conjugated organic ligand.
The compounds also show reduction waves that are not
reversible but can be readily assigned to being associated
with the complexes, that is, not solvent based reductions.
What is most interesting is that the first reduction wave
shows clear evidence of having two components in going
from I to II to III. Given the higher energy of the MMδ
orbital with increasing W content in the MM core this is
consistent with the formation of ligand centered radicals,
I -, II-, and III- that may be classified as Class II or III on
the Robin and Day Scheme.32 This arises because of the
coupling of the out-of-phase π*-thienyl carboxylate orbi-
tals with the MMδ as shown in Scheme 2.
Figure 1. Differential pulse voltammograms for the compounds MM-
(TiPB)2(TTh)2 where, MM=Mo2 (I) in red; MM=MoW (II) in blue and
MM=W2 (III) in green, showing the metal based oxidation of the MMδ
orbital and the thienyl carboxylate reductions. The potentials are refer-
enced to the Cp2Fe0/þ couple.
The differential pulse voltammograms of the three
compounds are compared in Figure 1, and details are
reported in the Experimental Section.
Electronic Absorption Spectra. The absorption spectra
of compounds I, II, and III in tetrahydrofuran (THF) are
shown in Figure 2. The intense broad absorptions bands
1
in the visible region of the spectrum arise from MLCT
(MMδ to thienyl carboxylate π*). With the change from
MM=Mo2 to MoW to W2, we observe the systematic red
shift which correlates with the equivalent rise in energy of
the MMδ orbitals as noted above in the electrochemical
studies. At higher energy and λmax ∼ 380 nm, there is for
each compound an intense absorption that we assign to a
thienylcarboxylate 1LC transition. This absorption is
similar to that of the free acid, TTh-H (see the Supporting
Information, Figure S1).Whereas the molar absorptivity
of 1LC transition remains virtually constant, we note that
Figure 2. Absorption spectra for the compounds MM(TiPB)2(TTh)2
where, MM=Mo2 (I) in red; MM=MoW (II) in blue and MM=W2 (III)
in green in THF at room temperature.
MM = MoW and W2, we note that the emission is
significantly weaker for the MoW containing compound.
We should also note that compound I shows emission
at ∼700 nm at room temperature which we have pre-
viously assigned to fluorescence from the 1MLCT state.29
No such emission in the visible region is observed for
compounds II and III. It should be mentioned here that
the emission from the pure TTh-H acid occurs at a much
higher energy (see Supporting Information, Figure S1),
and it can therefore be assumed that the NIR emission
from these compounds arises not from any free ligand
impurity.
The low temperature emission spectra obtained from
frozen glasses in 2-MeTHF at 77 K are shown in Figure 3,
Bottom. Compound I shows clear evidence of vibronic
features associated with υ(MoMo) ∼ 390 cm-1. The
compound II, which shows weaker emission, does show
vibronic features at 77 K, and an estimate of vibrational
spacing is ∼330 cm-1 which may be assigned to υ(MoW).
The emission spectrum of the W2-containing complex
at 77 K does not show vibronic features due to υ(WW) but
rather an ill-resolved vibronic feature with a spacing of
∼1200 cm-1. A closer inspection of the spectra obtained
for I and II also reveals evidence of vibronic features to
lower energy with υ ∼ 1200 cm-1. This we suggest arises
1
the MLCT for MM=W2 is significantly more intense
than for MM=Mo2. A similar trend has been seen before
in related compounds and correlates with the general
“rule of thumb” that the intensity of an MLCT transition
is inversely proportional to the energy separation of the
two states and correlates with orbital overlap. However,
as can be seen in Figure 2, the relative intensity of the
1MLCT for the MoW containing complex does not
appear to follow this trend, though a quantitative mea-
sure of this is not easy to judge since the separation of the
1LC and 1MLCT absorptions are much greater for com-
pound II relative to compound I. Moreover in compound
1
I, there is likely a contribution from the tail of the LC
absorption to the intensity of the 1MLCT absorption.
Luminescence Studies. The compounds I, II, and III all
show room temperature emission in the near-infrared
(NIR) (950-1300 nm) as shown in Figure 3 Top. With
excitation at 532 nm for MM = Mo2 and at 658 nm for
(32) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8539
the 400-550 nm range, which is consistent with the
presence of the reduced or quinoid form of the TTh-H
ligand.34 Furthermore, this band is also comparable to
that observed for the 3MLCT state observed in the ns TA
experiment, vide infra. The lifetimes of the initial excited
states obtained by fs TA were found to be ∼4 ps for II and
1
∼ 20 ps for III which can be assigned as MLCT. This
1MLCT excited state decays by fluorescence in the NIR
region. Furthermore the NIR emission of III has a life-
time <10 ns, which is consistent with the fs TA measure-
ments. Regrettably for II low luminescence quantum
efficiency made determination of its emission lifetime
impossible. It should also be noted in Figure 4 that the
bleach signals of II and III are still evident at 12.5 ps and
100 ps, respectively, indicating that the complexes have
not yet returned to the ground state.
To investigate longer-lived excited states, ns TA mea-
surements were also carried out on these compounds.
Upon excitation of II and III at 532 nm, it was found that
the TA signals decayed monoexponentially with lifetimes
of τ=165 ns for MM=MoW and τ ∼ 90 ns for MM=W2.
Previous studies found a state with τ = 72 μs for MM=
Mo2 upon excitation at 532 nm.29 Upon examination of
the ns TA spectra for the compounds II and III shown in
Figure 5, similarities are again observed between the two.
Both complexes show bleaching in >400 nm region (1LC)
and positive, intense absorption features are observed at
lower energies, from 400 to 800 nm range. The spectral
differences between the compounds are the contribution
from the bleaching for II at ∼630 nm and for III at >700
nm. Sucha similarity inthe TA signals ofIIand III point to
their origin from very similar excited states. The broad,
intense absorption throughout the visible region is
consistent with that arising from a reduced TTh-H ligand
(TTh-), and therefore the states giving rise to the signal are
assigned to the 3MLCT excited states for these two com-
pounds. The spectral properties resemble that published
for a related terthiophene complex of Ru(II), with strong
3MLCT absorption from 400 to 800 nm.35 It can be
reasonably assumed that these 3MLCT excited states are
very low lying in energy, by virtue of the highly delocalized
TTh ligands making their deactivation primarily non-
radiative. This hypothesis is consistent with the lack of
low energy triplet emission for compounds II and III. In
contrast, for compound I, the longer lived triplet excited
Figure 3. NIR emission spectra for MM(TiPB)2(TTh)2 compounds,
where MM=Mo2 (red),=MoW (blue), and=W2 (green). Top: Room
temperature in THF, Bottom: 77 K in 2-MeTHF.
from a υ(thienyl carboxylate) mode. A similar ligand-
based progression is observed for Ru(bpy)32þ at 77 K.33
Whereas, the NIR emission of compound I does not
show solvatochromism, we do observe solvatochromism
for the NIR emission of compounds II and III, on
changing the solvent from THF to DMSO (see Support-
ing Information, Figure S2). Consequently, we suggest
that on the basis of the small Stokes shift from the
1MLCT absorption band and the solvatochromism,
the NIR emission of II and III corresponds to emission
1
from the MLCT excited state, while for compound I
the NIR emission arises from the 3MMδδ* state.
Further corroboration of our assignments comes from
the nanosecond (ns) and femtosecond (fs) transient absorp-
tion (TA) measurements which are discussed in the follow-
ing section.
3
state is assigned to be a MMδδ* state, which decays by
phosphorescence in the NIR region at ∼1100 nm.
As previously observed and shown in the Supporting
Information, Figure S3,29 the ns TA of I exhibits bleach-
ing at <380 nm and at ∼520 nm, consistent with its
ground state absorptions. Positive absorption is observed
with a maximum at ∼420 nm; however, no transient
signal is apparent from 600 to 800 nm. The strong
Transient Absorption Spectroscopy. Femtosecond TA
spectra were obtained upon excitation of the compounds
II and III at 675 nm. The TA spectra and the correspond-
ing lifetimes obtained by kinetic traces are shown in
Figure 4. An examination of TA spectra indicates the
similarity observed between the two compounds. Both
show an initial intense absorption in the low energy
region of the spectra (500-630 nm) as well as higher
energy bleach (380-400 nm) corresponding to the ground
state 1LC absorption. The initial intense absorption
decays and directly gives rise to the board absorption in
3
absorption of the MLCT of II and III are at λ > 720
nm and at 600-700 nm, respectively. The strong TA
signal throughout the visible and NIR is typical of MLCT
excited states involving reduced organic ligands. The
absence of this broad absorption in the Mo2-complex, I,
(34) Pappenfus, T. M.; Raff, J. D.; Hukkanen, E. J.; Burney, J. R.;
Casado, J.; Drew, S. M.; Miller, L. L.; Mann, K. R. J. Org. Chem. 2002, 67,
6015.
(35) Barbieri, A.; Ventura, B.; Flamigni, L.; Barigelletti, F.; Fuhrmann,
G.; Baeuerle, P.; Goeb, S.; Ziessel, R. Inorg. Chem. 2005, 44, 8033.
(33) Thompson, D. W.; Schoonover, J. R.; Graff, D. K.; Fleming, C. N.;
Meyer, T. J. J. Photochem. Photobiol., A 2000, 137, 131.

8540 Inorganic Chemistry, Vol. 48, No. 17, 2009
Alberding et al.
Figure 4. Femtosecond TA spectra for compounds MoW(TiPB)2(TTh)2 (II) and W2(TiPB)2(TTh)2 (III) in THF excited at 675 nm. Right: Representative
kinetics for the singlet state decay.
is consistent with the assignment of the long-lived tran-
sient as 3MMδδ*.
where MM=Mo2 (I0), MoW (II0), and W2 (III0) by employ-
ing density functional theory (DFT)43-46 as implemented in
Gaussian 03 suite of programs,47 and the frontier orbital
energy level diagrams for these three molecules are shown
in Figure 6. Gaussview48 plots for the ditungsten compound
are shown in Figure 7. In all cases the highest occupied
molecular orbital (HOMO) was the MMδ orbital with very
little mixing with the terthienyl carboxylate ligand. In going
from Mo2 to MoW to W2 there is a steady increase in orbi-
tal energy of the MMδ as expected based on the redox
chemistry.
Overall, it is seen, that when the dinuclear core has
MM = MoW and W2, the emissive properties and life-
times of the excited states are very different compared to
that for MM = Mo2, even when the same ligand, TTh, is
used. This is in sharp contrast to our observations on the
emissive properties of the compounds MM(TiPB)4,
where it was found that the singlet and triplet excited
states for all three compounds were the same, namely,
1MLCT and 3MMδδ* states, respectively.31
Thus we see that with increasing π-delocalization in the
ligand, for example, TTh, the nature of the triplet excited
state changes from MM based in Mo2 to M-L based in
MoW and W2. This is because with the heavier elements,
there is greater overlap between the MM δ orbitals and
TTh π* orbitals, causing greater stabilization of the
3MLCT states. The 3MMδδ* state in compounds II and
III exists but at a higher energy. The lowering of the
energy of the 3MLCT states in II and III is so pronounced
that the deactivation occurs via a non-radiative route,
which can be explained by the energy gap law.36-42
Electronic Structure Calculations. Calculations were car-
ried out on the model compounds trans-MM(O2CH)2(TTh)2
(43) Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 164.
(44) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 137, 1697.
(45) Parr, R. G.; Yang, W. Density-functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989; p 333.
(46) Salahub, D. R.; Zerner, M. C., Eds.; ACS Symposium Series 394: The
Challenge of d and f Electrons: Theory and Computation. Developed from a
Symposium at the 3rd Chemical Congress of North America Toronto, Can.,
June 5-11, 1988. 1989; p 405.
(47) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant,
J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.;
Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian,
H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain,
M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski,
J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian,Inc.: Wallingford, CT, 2004.
(48) Dennington, R. K. T., II; Millam, J.; Eppinnett, K.; Hovell, W. L.;
Gilliland, R., 2003.
(36) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Rev. 1977, 77, 793.
(37) Radiationless Processes in Molecules and Condensed Phases; Fong,
F. K., Ed.; Springer Verlag: Berlin, 1976; Topics in Applied Physics, Vol. 15,
p 360 ff.
(38) Freed, K. F. Acc. Chem. Res. 1978, 11, 74.
(39) Heller, E. J.; Brown, R. C. J. Chem. Phys. 1983, 79, 3336.
(40) Henry, B. R.; Siebrand, W. Org. Mol. Photophys. 1973, 1, 153.
(41) Excited States; Lim, E. C., Ed., Academic Press: New York, 1979; Vol. 4,
p 400 ff.
(42) Radiationless Transitions; Lin, S. H., Ed.; Academic Press: New York,
1980; p 425 ff.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8541
have thienyl ligand and metal based oxidations,49-52
and their electronic spectra are dominated by π to π*
transitions25,49,50,53 which are notably higher in energy than
1
the MLCT absorptions seen for I, II, and III. Spin orbit
coupling from the second and third row metal mixes with
these 1LC states so that the emission is seen from both S1 and
T1 states. The d6 ferrocenyl and Ru(II) alkynyl linked thienyl
derivatives or bipyridyl substituted thienyl derivatives have
metal based and thienyl oxidations. The carboxylate linker to
the MM4þ unit mediates the rate of intersystem crossing such
1
that the MLCT is detectable by TA spectroscopy, and for
M = Mo, both fluorescence and phosphorescence is seen.
However, the emissive metal based T1 state for molybdenum,
3MoMoδδ*, is most unusual for a transition metal ion since
metal based states (ligand field dd states) are dark and decay
rapidly.54 Thus the emissive 3MoMoδδ* state at ∼1100 nm is
more reminiscent of a lanthanide ion.55 With increasing
tungsten content the compounds II and III show only
1
emission from the S1, MLCT state and the triplet MLCT
state can only be detected by TA spectroscopy. These states
3
have notably shorter lifetimes than the MMδδ* state and
their photophysical properties are collectively summarized in
the Jablonski diagram shown in Figure 8.
Finally, the ability to tune the MMδ orbital energy within
the series MM=Mo2, MoW, and W2 allows the electronic
absorptions to traverse the entire region of the spectrum from
the UV to the NIR, and this is a property that we seek to
employ in photon harvesting devices based on oligothio-
phenes incorporating MM quadruple bonds.
Figure 5. Comparison of ns TA spectra of MoW(TiPB)2TTh2 (II) and
W2(TiPB)2TTh2 (III) in THF collected immediately after laser pulse
(lex = 532 nm, fwhm ∼8 ns).
Experimental Section
In all cases the lowest unoccupied molecular orbital
(LUMO) and LUMO þ1 are the in-phase and out-of-
phase π* combinations of the terthienylcarboxylate and
the energy separation between these orbitals increases
along the series Mo2 < MoW < W2. The energy separa-
tion is due to the mixing with the MMδ (MMδ to ligand
π* back-bonding), and this is greatest for W2 because of
both overlap and energy considerations.
Below the HOMO two thienyl ligand π-based orbitals
are calculated, one of which, the HOMO-2, mixes with
the M2δ orbital. Here the energy separation between the
HOMO-1 and HOMO-2 decreases in the order Mo2 >
MoW > W2 because of the relative energies of the MMδ
orbitals.
Above the LUMO and LUMO þ1 come the M2δ*
metal based orbitals, which have essentially no mixing
with any of the ligand orbitals. Below the HOMO -2 are
the metal-based MM π orbitals, followed by four sets of
ligand filled π orbitals and then the MMσ orbital.
As is evident from the energy level diagram shown in
Figure 7, the HOMO-LUMO gap steadily decreases from
Mo2>MoW > W2 and is thus consistent with the observed
trends in the 1MLCT absorptions shown in Figure 2.
Measurements. NMR spectra were recorded on a 400 MHz
Bruker DPX Advance400 spectrometer. All 1H NMR chemical
shifts are in ppm relative to the protio impurity in THF-d8 at
3.58 ppm.
Electronic spectra at room temperature were recorded using a
Perkin-Elmer Lambda 900 spectrometer in THF solution. A
10.00 mm IR quartz cell was employed.
The cyclic voltammogram (scan rate of 100 mV s-1) and
differential pulse voltammogram of all the complexes were
collected, using a Princeton Applied Research (PAR) 173A
potentiostat-galvanostat equipped with a PAR 176 current-to
voltage converter. Electrochemical measurements were per-
formed under an inert atmosphere in a 0.5 M solution of
nBu4NPF6 in THF inside a single compartment voltammetric
cell equipped with a platinum working electrode, a platinum
wire auxiliary electrode, and a pseudoreference electrode con-
sisting of a silver wire in 0.5 M nBu4NPF6/THF separated from
the bulk solution by a Vycorþtip. The potential values are
referenced to the FeCp2/FeCp2 couple, obtained by addition
of a small amount of FeCp2 to the solution.
Nanosecond TA measurements were carried out in 1 ꢀ 1 cm
square quartz cuvettes equipped with Kontes stopcocks. Nano-
second TA spectra were measured on a home-built instrument
pumped by a frequency doubled (532 nm) or tripled (355 nm)
Spectra-Physics GCR-150 Nd:YAG laser (fwhm ∼ 8 ns,
∼5 mJ per pulse). The signal from the photomultiplier tube
Concluding Remarks
(51) Moorlag, C.; Wolf, M. O.; Bohne, C.; Patrick, B. O. J. Am. Chem.
The present series of compounds show significantly differ-
ent electro-optical properties when compared to other
metalated oligothiophenes. The d8 and d10 metal complexes
Soc. 2005, 127, 6382.
(52) Trouillet, L.; De Nicola, A.; Guillerez, S. Chem. Mater. 2000, 12,
1611.
(53) Thomas, K. R. J.; Lin, J. T.; Lin, K.-J. Organometallics 1999, 18,
5285.
(49) Zhu, Y.; Wolf, M. O. Chem. Mater. 1999, 11, 2995.
(50) Zhu, Y.; Millet, D. B.; Wolf, M. O.; Rettig, S. J. Organometallics
1999, 18, 1930.
(54) Bargawi, K. R.; Llobet, A.; Meyer, T. J. J. Am. Chem. Soc. 1988, 110,
7751.
(55) Bunzli Jean-Claude, G.; Piguet, C. Chem Soc Rev 2005, 34, 1048.

8542 Inorganic Chemistry, Vol. 48, No. 17, 2009
Alberding et al.
Figure 6. Frontier molecular orbital energy level diagrams for the compounds MM(O2CH)2(TTh)2, where MM = Mo2 (I0) in red, MoW (II0) in blue, and
W2 (III0) in green.
Figure 8. Jablonski diagram depicting the primary excitation and
deactivation pathways and the excited state lifetimes for MM(TiPB)2-
(TTh)2 compounds, where MM = Mo2, MoW, and W2.
Figure 7. Gaussview plots of the frontier orbitals for the W2(O2CH)2-
(TTh)2 (II0) compound, where the HOMO is 186 and the LUMO 187,
drawn at isosurface value of 0.2.
power: 65 mW) and a RG830 long pass filter was placed between
the sample and the detector.
In the femtosecond TA experiments, samples of Mo2TTh
were excited at 365 and 514 nm (second harmonics) or 800 nm of
a femtosecond Ti-Sapphire oscillator (82 MHz, Spectra Physics)
and monitored with a supercontinuum probe pulse in the
(Hamamatsu R928) was processed by a Tektronics 400 MHz
oscilloscope (TDS 380).56
The steady-state NIR-luminescence measurements at 77 K
spectral range of 500-800 nm. The recorded spectra were
time-corrected for the chirp of the supercontinuum. All transi-
ent signals were linearly dependent on the excitation power. The
time resolution of the system is 300 fs, as determined by the two-
photon absorption of methanol in the sample cell. For
MoWTTh and W2TTh, the experiments were carried out
were carried out in J. Young NMR tubes. The spectra were
measured on a home-built instrument utilizing a germanium
detector. The sample was excited at 658 nm (laser diode max
(56) Byrnes, M. J.; Chisholm, M. H.; Gallucci, J. A.; Liu, Y.; Ramnauth,
R.; Turro, C. J. Am. Chem. Soc. 2005, 127, 17343.

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8543
using a different laser and detection system, which have been
previously described.57 The samples were excited at 675 nm
(with excitation power ∼1-2 μJ at the sample) and were
prepared with absorbance ∼0.4-0.8 at the excitation wave-
length. During the measurements, the samples were kept in
constant motion by manual movement of an XYZ stage in the
vertical and horizontal directions. In order ensure that no
photodecomposition occurred during data collection, absorp-
tion spectra were recorded before and after the TA measure-
ments. The measurements were repeated five times at each of the
pump-probe delay positions to confirm data reproducibility
throughout the experiment and the resulting spectra were
corrected for the chirp in the white-light super continuum.
The kinetics were fit to a single exponential decay of the form,
S(t)=A exp(-t/τ) þ C, with amplitude, A, lifetime, τ, and offset,
C, using SigmaPlot 10.0. Error bars for the lifetimes are reported
as the standard error of the exponential fit.
Microanalysis was performed by H. Kolbe Microanalytisches
Laboratorium, Germany. MALDI-TOF was performed on a
Bruker Reflex III (Bruker, Bremen, Germany) mass spectro-
meter operated in a linear, positive ion mode with an N2 laser.
Dithranol was used as the matrix and prepared as a saturated
solution in THF. Allotments of matrix and sample were thor-
oughly mixed together; 0.5 mL of this was spotted on the target
plate and allowed to dry.
(m, 4H), 2.88 (m, 2H), 1.24 (d, 12H, JHH = 7 Hz), 1.12 (d, 24H,
J
HH = 7 Hz) ppm. MALDI-TOF: Calculated monoisotopic mw
for C58H60O8S6MoW: 1358.1. Found: 1359 (Mþ). UV-vis-
NIR (in THF, 293 K, values of ε are given in parentheses): 632 br
(∼37000), 379 (∼81000) nm.
W2(TiPB)2(TTh)2. W2(TiPB)4 (0.099 g, 0.073 mmol) was
dissolved in 30 mL of dry toluene, and this yellow solution
was canulated to a Schlenck flask containing TTh-H (0.041 g,
0.142 mmol). The suspension was stirred at room temperature
for 4 days, at the end of which a green precipitate had formed.
This was centrifuged and washed with toluene (2ꢀ10 mL) before
being dried in vacuo to give 85 mg (81% yield) of a green solid.
Microanalysis found: C 48.08, H 4.21 C58H60O8S6W2 requires:
C 48.20, H 4.18. NMR (THF-d8): δH (400 MHz) 7.50 (d, 2H,
J
HH=4 Hz), 7.45 (d, 2H, JHH=4 Hz), 7.39 (dd, 2H, JHH=5 Hz),
7.30 (dd, 2H, JHH=4 Hz), 7.28 (d, 2H, JHH=4 Hz), 7.26 (d, 2H,
HH=4 Hz), 7.08 (dd, 2H, JHH=3.8 Hz), 7.07 (s, 2H), 2.97 (m,
4H), 2.90 (m, 2H), 1.27 (d, 12H, JHH = 7 Hz), 1.13 (d, 24H,
J
J
HH=7 Hz) ppm. MALDI-TOF: Calculated monoisotopic mw
for C58H60O8S6W2: 1442.2. Found: 1445 (Mþ). UV-vis-NIR
(in THF, 293 K, values of ε are given in parentheses): 769
(∼71000), 715 (∼62000), 384 (∼89000) nm.
Theoretical Approaches
Electronic structure calculations on the model compounds
I0, II0, and III0 were performed DFT with the aid of the
Gaussian03 suite of programs. The B3LYPexchange correla-
tion functional60,61 was used along with the 6-31G* basis set
for C, H, and O, 6-31þG (2d) basis set for S, and the SDD
energy consistent pseudopotentials for molybdenum and
tungsten. Geometry optimizations were performed in appro-
priate symmetry and were confirmed as local minima on the
potential energy surfaces using frequency analysis. Orbital
analyses were preformed using Gaussview.
Synthesis. All reactions were carried out under 1 atmosphere
of oxygen-free UHP-grade argon using standard Schlenck
techniques or under a dry and oxygen-free nitrogen atmosphere
using standard glovebox techniques. All solvents were dried and
degassed by standard methods and distilled prior to use. The
preparation of the starting materials MM(TiPB)4 has been
previously reported for MM = Mo2,58 MoW, and W2.31 The
synthesis of the terthienyl carboxylic acid (TTh-H) was accom-
plished by following reported procedures,59 and the synthesis of
the compound Mo2(TiPB)2(TTh)2 has also been reported.29
MoW(TiPB)2(TTh)2. MoW(TiPB)4 (0.183 g, 0.144 mmol)
was dissolved in 30 mL of dry toluene, and this yellow solution
was canulated to a Schlenck flask containing TTh-H (0.08 g,
0.27 mmol). The suspension was stirred at room temperature for
4 days, at the end of which a blue precipitate had formed. This
was centrifuged and washed with toluene (2 ꢀ 10 mL) before
being dried in vacuo to give 160 mg (82% yield) of a blue solid.
Microanalysis found: C 51.18, H 4.41 C58H60O8S6MoW re-
quires: C 51.33, H 4.46. NMR (THF-d8): δH (400 MHz) 7.69 (d,
2H, JHH=4 Hz), 7.39 (d, 2H, JHH = 4 Hz), 7.37 (d, 2H, JHH = 5
Hz), 7.33 (d, 2H, JHH = 4 Hz), 7.28 (d, 2H, JHH = 4 Hz), 7.22 (d,
2H, JHH = 4 Hz), 7.05 (dd, 2H, JHH=3.5 Hz), 7.02 (s, 2H), 3.03
Acknowledgment. We would like to thank the National
Science Foundation and the Institute of Materials at Ohio State
University for funding. We would also like to thank the Ohio
Supercomputing Center for computing resources. We also
thank Prof. Felix N. Castellano and Dr. Aaron Rachford at
Bowling Green State University for assistance with emissive
lifetime determinations of compound III.
Supporting Information Available: Listing of absorption,
emission, and excitation spectra for the ligand TTh-H, solvato-
chromic studies on compound III, and ns TA spectrum of
compound I. This material is available free of charge via the
Internet at http://pubs.acs.org.
(57) Burdzinski, G.; Hackett, J. C.; Wang, J.; Gustafson, T. L.; Hadad, C.
M.; Platz, M. S. J. Am. Chem. Soc. 2006, 128, 13402.
(58) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. Inorg.
Chem. 2002, 41, 1639.
(60) Becke, A. D. Phys. Rev. A: Gen. Phys. 1988, 38, 3098.
(61) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200.
(59) Wang, S.; Brisse, F. Macromolecules 1998, 31, 2265.