Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and ditungsten (M≡M): A comparison of their chemical and photophysical properties

Article abstract of DOI:10.1039/c1dt11889g

From the reactions between M₂(TⁱPB)₄, where TⁱPB = 2,4,6-triisopropylbenzoate and two equivalents each of 2-furan carboxylic acid, FuCO₂H, and 2-selenophene carboxylic acid, SpCO₂H in toluene, the new compounds trans-M₂(T ⁱPB)₂(O₂CFu)₂ (1a M = Mo, 2a M = W) and trans-M₂(TⁱPB)₂(O₂CSp) ₂ (1b M = Mo, 2b M = W) were formed. These new compounds have been characterized by ¹H NMR, steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo₂(T ⁱPB)₂(O₂CSp)₂ (1b) has been characterized by single crystal X-ray crystallography. These data are compared with those previously reported for related 2-thiophene carboxylate derivatives: M₂(TⁱPB)₂(O₂CTh)₂. The physico-chemical data correlate well with electronic structure calculations performed on model compounds. All compounds have detectible S₁ photoexcited states with lifetimes that vary from ～5 ps to < 1 ps. The molybdenum compounds have T₁ states with microsecond lifetimes that are assigned as MMδδ* whereas the T₁ states for tungsten are ³MLCT with lifetimes on the order of nanoseconds. In all cases, shorter lifetimes were seen in complexes containing heavier atoms. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1dt11889g

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PAPER
Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and
ditungsten (M M): a comparison of their chemical and photophysical
properties†
Samantha E. Brown-Xu, Malcolm H. Chisholm,* Judith C. Gallucci, Yagnaseni Ghosh, Terry L. Gustafson and
Carly R. Reed
Received 6th October 2011, Accepted 14th November 2011
DOI: 10.1039/c1dt11889g
From the reactions between M2(TiPB)4, where TiPB = 2,4,6-triisopropylbenzoate and two equivalents
each of 2-furan carboxylic acid, FuCO2H, and 2-selenophene carboxylic acid, SpCO2H in toluene, the
new compounds trans-M2(TiPB)2(O2CFu)2 (1a M = Mo, 2a M = W) and trans-M2(TiPB)2(O2CSp)2 (1b
M = Mo, 2b M = W) were formed. These new compounds have been characterized by 1H NMR,
steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse
voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo2(TiPB)2(O2CSp)2
(1b) has been characterized by single crystal X-ray crystallography. These data are compared with those
previously reported for related 2-thiophene carboxylate derivatives: M2(TiPB)2(O2CTh)2. The
physico–chemical data correlate well with electronic structure calculations performed on model
compounds. All compounds have detectible S1 photoexcited states with lifetimes that vary from ~5 ps to
< 1 ps. The molybdenum compounds have T1 states with microsecond lifetimes that are assigned as
MMdd* whereas the T1 states for tungsten are 3MLCT with lifetimes on the order of nanoseconds. In
all cases, shorter lifetimes were seen in complexes containing heavier atoms.
These types of substitutions have recently been studied in
Introduction
other materials such as polymers and porphyrin derivatives.
In previous studies in this laboratory, the properties of thienyl
Polymers that incorporate thiophene units are the most widely
carboxylates attached to MM quadruple bonds, as both monomer
used semiconducting polymers and are utilized in devices such as
models and oligomers, have been examined.1–6 These have shown
interesting photophysical properties and have potential applica-
tions for light emitting diodes (LEDs)7,8 and solar cells.3,9 For
compounds of the form trans-M2(TiPB)2L2, where L = O2CTh,
O2CBTh and O2CTTh (Th = 2-thienyl, BTh = 2,2¢-bithienyl
and TTh = 2,2¢,2¢¢-terthienyl), electronic transitions that span
the visible-NIR region of the spectrum (400–850 nm), that can
be tuned by choice of ligand, and relatively long lived singlet
and triplet photoexcited states assignable to MLCT, MLCT or
3MMdd* states have been observed.3–6 Since thiophene materials
have been so well studied,10–12 we became interested in what
influence the sulfur heteroatom has on chemical and photophysical
properties. Substitution of sulfur by another group VI element that
is either smaller or larger, oxygen or selenium, respectively, should
provide answers as to the effect of the heteroatom.
organic field-effect transistors (OFETs)13,14 and bulk heterojunc-
tion solar sells (BHJs),10–12,15 though the search for new materials
is on-going. It was believed that the polarizability of sulfur
contributed to the favorable charge mobility in OFETs, however,
furan-based polymers displayed similar field-effect mobilities as
their thiophene analogs, showing that the heteroatom has only
a slight effect on the material’s properties.13 In other studies,
the replacement of thiophene units by selenophene improved
the hole mobility of the polymer and produced an ambipolar
organic semiconductor.14,16 Generally, polyselenophene materials
are known to have lower optical band gaps than polythiophenes.15
This improves the overlap between the polymer absorption and
the solar emission spectrum, making polyselenophenes more
attractive as solar cell components, and work is currently being
done to improve efficiencies of these BHJ devices.15,17 Substitution
of selenium for sulfur was also seen to have an impact on excited
state lifetimes in a series of heteroatom-substituted expanded
porphyrins. When selenium was incorporated, both the singlet and
triplet lifetimes were decreased by 50–67%. This was attributed
to an increase in the rate of intersystem crossing due to the
heavy atom effect, as well as faster triplet decay to the ground
state according to the energy gap law since selenium stabilizes
the LUMO more than sulfur.18 Here, we report similar findings
1
3
Department of Chemistry, The Ohio State University, 100 W 18th Ave.,
Columbus, Ohio, 43210, USA. E-mail: Chisholm@chemistry.ohio-state.edu
† Electronic supplementary information (ESI) available: CIF for 1b,
selected calculated orbital energies, NIR emission spectra for 1a–2a, fs
TA spectra for 1b and 2b, and ns TA spectra for all compounds. CCDC
reference number 849730. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11889g
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concerning the use of furan and selenophene ligands in place of
thiophene in trans-M2(TiPB)2(L)2 complexes.
Pro 6.0, where Ai is the amplitude, t is the lifetime, and C is an
offset. Error bars for the lifetimes are reported as standard errors
of the exponential fits.
Nanosecond TA was performed on samples in 1 ¥ 1 cm square
quartz cuvettes with Kontes stopcocks. Measurements were made
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 pulse-1). Signal from a Hamamatsu R928
photomultiplier tube was processed with a Tektronics 400 MHz
oscilloscope (TDS 380).25
Experimental
Materials and methods
NMR spectra were recorded on a 400 MHz Bruker DPX
Advance400 spectrometer. All 1H NMR ppm chemical shifts are
relative to THF-d8 at 1.73 ppm.
Electronic structure calculations on the model compounds were
performed by DFT with the aid of the Gaussian03 program
suite.19,20 The B3LYP21,22 exchange correlation functional was used
with a 6-31G* basis set for non-metal atoms and SDD energy
consistent pseudopotentials for Mo and W. Optimizations of
the geometries were performed in appropriate symmetries. The
calculated energies were confirmed as local minima by frequency
analysis. Analysis of the orbitals from time-dependent calculations
was performed using Gaussview.23
MALDI-TOF was performed on a Bruker Reflex III mass
spectrometer operated in a linear, positive ion mode with an
N2 laser. The matrix was prepared as a concentrated solution of
dithranol in THF. A small amount of the samples were dissolved
in the matrix solution, then spotted onto a MALDI plate with a
pipet and allowed to dry.
All reactions and subsequent manipulations of the compounds
were performed under one atmosphere of oxygen-free UHP-
grade argon using standard Schlenk techniques or in a glove box
with a dry and oxygen-free nitrogen atmosphere using standard
techniques. Prior to use, all solvents were dried appropriately,
distilled, degassed, and stored in flasks equipped with Kontes taps
Steady-state photophysical measurements were carried out with
10.0 ¥ 2.0-mm2 quartz cuvettes equipped with Kontes stopcocks.
Electronic absorption spectra at room temperature were recorded
using a Perkin-Elmer Lambda 900 spectrometer in THF solution.
Fluorescence measurements were made on a SPEX Fluoromax-2
spectrofluorimeter in the UV-visible region. Emission measure-
ments in the near-infrared region were performed in J. Young
NMR tubes at 77 K in 2-methyl THF. Spectra were recorded
on a home-built instrument equipped with a germanium detector.
Molybdenum complexes were excited at 405 nm with a RG830 long
pass filter between sample and detector, while tungsten complexes
were excited at 532 nm and a RG630 long pass filter was used.
The cyclic voltammogram and differential pulse voltammo-
grams of the complexes were collected at a scan rate of 100 mV s-1
and 20 mV s-1, respectively. A Princeton Applied Research (PAR)
173A potentiostat-galvanostat equipped with a PAR 176 current-
to-voltage converter was employed. Electrochemical measure-
ments were wade under an inert N2 atmosphere in a 0.5 M solution
˚
over activated 4 A molecular sieves under an argon atmosphere.
All chemicals were used as received. The compounds 1a–2b were
prepared in a similar manner to M2(TiPB)2(O2CTh)2; M = Mo,
W, 5,6 and heteroleptic starting materials, M2(TiPB)4; M = Mo, W,
were first prepared according to established procedures.26 NMR,
MALDI-TOF, elemental analysis, and UV-vis data for 1a–2b are
provided below.
Synthesis of 2-selenophene carboxylic acid (SpCO2H). The
ligand 2-selenophene carboxylic acid was synthesized by the
carboxylation of selenophene with nBuLi and CO2 according to a
previously published procedure.27
Synthesis of Mo2(TiPB)2(O2CFu)2 (1a). Mo2(TiPB)4 (0.49 g,
0.41 mmol) and FuCO2H (0.09 g, 0.84 mmol) were combined
in toluene (20 mL) and the suspension was stirred at room
temperature for 3 days. The yellow precipitate was isolated from
solution through centrifugation and washed once with each
of toluene and hexanes (15 mL). The solid was dried, then
recrystallized through slow diffusion of hexanes in THF, giving
250 mg (66% yield) of a yellow crystalline solid. Microanalysis
found: C 50.54, H 5.27 C42H52Mo2O10 requires: C 50.67, H 5.51.
NMR (THF-d8): dH (400 MHz) 7.74 (d, 2H JHH = 1 Hz), 7.27 (d,
2H, JHH = 3.3 Hz), 7.03 (s, 4H), 6.66 (d, 2H, JHH = 3.5 Hz), 3.10
(m, 4H), 2.89 (m, 2H) 1.25 (d, 12H, JHH = 7 Hz), 1.11 (d, 24H,
JHH = 6.8 Hz) ppm. MALDI-TOF: calculated monoisotopic MW
for C42H52Mo2O10: 908.8. Found: 908.3 (M+). UV-vis (in THF,
293 K, e (M-1 cm-1) values given in parentheses): 420 (~13000),
345 (~10000) nm.
n
of Bu4NPF6 in THF inside a single compartment voltammetric
cell utilizing a platinum working electrode, a platinum wire
auxiliary electrode, and a pseudoreference electrode made up of a
silver wire in 0.5 M nBu4NPF6/THF solution separated by a Vycor
tip. For tungsten compounds, potential values were referenced to
+
the FeCp2/FeCp2 couple, obtained by adding a small amount
of FeCp2 to the solution. For molybdenum, the oxidation poten-
tial overlapped too closely with FeCp2 so decamethylferrocene
(DMFc) was used instead. The DMFc oxidation potential was
+
referenced back to FeCp2/FeCp2 .
Femtosecond transient absorption (TA) experiments were per-
formed using laser and detection systems described previously.24
Samples were prepared with absorbance ~0.3–0.8 at the excitation
wavelengths, which were 400 nm for 1a, 514 nm for 1b and 2a, and
600 nm for 2b. Excitation power at the sample was ~1–2 mJ. The
samples were kept in constant motion during the experiment by
manual movement of an XYZ stage to prevent photodecompo-
sition. The TA measurements at a series of pump–probe delay
times were performed three times to ensure reproducibility of
the results. Spectra collected underwent wavelength calibration
Synthesis of Mo2(TiPB)2(O2CSp)2 (1b). Mo2(TiPB)4 (0.40 g,
0.33 mmol) and SpCO2H (0.117 g, 0.67 mmol) were combined
in toluene (20 mL) and the suspension was stirred at room
temperature for 3 days. The orange precipitate was isolated
from solution through centrifugation and washed once with each
of toluene and hexanes (15 mL). The solid was dried, then
recrystallized through slow diffusion of hexanes in THF, giving
230 mg (67% yield) of orange blocky crystals. NMR (THF-d8):
and GVD corrections. In general, kinetics were fit to a sum of
ꢀ
exponential decay terms, S(t) = iAiexp(-1/ti) + C using Igor
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Table 1 Summary of crystallographic data
dH (400 MHz) 8.37 (d, 2H, JHH = 5.6 Hz), 8.10 (d, 2H, JHH
3.6 Hz), 7.46 (dd, 2H, JHH = 4 Hz), 7.02 (s, 4H), 3.09 (m, 4H),
2.88 (m, 2H), 1.22 (d, 12H, JHH = 7 Hz), 1.10 (d, 24H, JHH
=
Compound
1b
=
6.8 Hz) ppm. MALDI-TOF: calculated monoisotopic MW for
C42H52Mo2O8Se2: 1034.5. Found: 1034.4. UV-vis (in THF, 293 K):
465, 440 (shoulder), 340, 285 (shoulder), 260 nm.
Formula
Mr
C58H84Mo2O12Se2
1323.05
Tetragonal
I41/a
28.6622(2)
14.9089(1)
12248.03(15)
8
1.435
150(2)
5440
1.654
Crystal system
Space group
˚
Synthesis of W2(TiPB)2(O2CFu)2 (2a). W2(TiPB)4 (0.31 g,
0.23 mmol) and FuCO2H (0.05 g, 0.46 mmol) were combined
in toluene (20 mL) and the suspension was stirred at room
temperature for 3 days. The purple-red precipitate was isolated
from solution through centrifugation and washed once with each
of toluene and hexanes (15 mL). The solid was dried, then
recrystallized through slow diffusion of hexanes in THF, giving
230 mg (47% yield) of dark red needle-like crystals. NMR (THF-
d8): dH (400 MHz) 7.55 (d, 2H, JHH = 1 Hz), 7.02 (s, 4H), 6.96
(d, 2H, JHH = 3.5 Hz), 6.65 (d, 2H, JHH = 3.4 Hz) 2.90 (m, 6H),
1.23 (d, 12H, JHH = 7 Hz), 1.05 (d, 24H, JHH = 6.9 Hz) ppm.
MALDI-TOF: calculated monoisotopic MW for C42H52W2O10:
1084. Found: 1085.2 (M+). UV-vis (in THF, 293 K, e (M-1 cm-1)
values given in parentheses): 574 (~17000), 538 (~16000), 404
(~11000), 250 (~25000) nm.
a/A
˚
c/A
3
˚
V/A
Z
Dc/g cm-3
T/K
F(000)
m(Mo-Ka)/mm-1
Refl. collected
122942
Refl. indep. (Rint
)
7018 (0.051)
0.0332 (0.0505)
0.0796 (0.0861)
1.042
R1 [I > 2s(I)] (all)
wR2 [I > 2s(I)] (all)
Goodness-of-fit on F2
-3
˚
Drmax, min/e A
0.588 and -0.461
(bonded carbon atom). The rest of the hydrogen atoms were
included in the model at calculated positions using a riding model
with U(H) = 1.2Ueq (bonded atom). Neutral atom scattering
factors were used and include terms for anomalous dispersion.31
Synthesis of W2(TiPB)2(O2CSp)2 (2b). W2(TiPB)4 (0.3 g,
0.22 mmol) and SpCO2H (0.073 g, 0.42 mmol) were combined
in toluene (20 mL) and the suspension was stirred at room
temperature for 3 days. The black precipitate was isolated from
solution through centrifugation and washed once with each
of toluene and hexanes (15 mL). The solid was dried, then
recrystallized through slow diffusion of hexanes in THF, giving
170 mg (64% yield) of black block-shaped crystals. Microanalysis
found: C 41.38, H 4.61 C42H52W2O8Se2 requires: C 41.67, H 4.33.
NMR (THF-d8): dH (400 MHz) 8.09 (d, 2H, JHH = 5.5 Hz), 7.72 (d,
2H, JHH = 3.6 Hz), 7.47 (dd, 2H, JHH = 3.8 Hz), 7.01 (s, 4H), 2.90 (m,
6H), 1.23 (d, 12H, JHH = 7 Hz), 1.05 (d, 24H, JHH = 6.8 Hz) ppm.
MALDI-TOF: calculated monoisotopic MW for C42H52W2O8Se2:
1210.5. Found: 1211.7. UV-vis (in THF, 293 K, e (M-1 cm-1) values
given in parentheses): 655 (~15000), 611 (~13000), 407 (~8500) 264
(~22000) nm.
Results and discussion
Syntheses
The new compounds were prepared in a similar manner to
that of the former 2-thienylcaboxylates,5,6 namely in toluene the
compounds M2(TiPB)4 were allowed to react with two equivalents
of each of 2-furan carboxylic acid (FuCO2H) and 2-selenophene
carboxylic acid (SpCO2H). These reactions proceeded with a
rapid color change with the formation of the trans-M2(TiPB)2L2
compounds where L = O2CFu (1a M = Mo, 2a M = W) and O2CSp
(1b M = Mo, 2b M = W) (Fig. 1). The trans-geometry about the M2
unit is favored by the presence of the bulky TiPB ligands, which
twist such that the C6 ring is out of the plane of their carboxylate
CO2 plane.7 The new compounds have been characterized by H
1
Crystal structure determination and refinement†. A summary
of crystallographic data for compound 1b is presented in Table 1.
The data collection crystal was an orange chunk that was cut from
an intergrown mass of crystals. Examination of the diffraction
pattern on a Nonius Kappa CCD diffractometer indicated a
tetragonal crystal system. All work was done at 150 K using
an Oxford Cryosystems Cryostream Cooler. The data collection
strategy was set up to measure a quadrant of reciprocal space
with a redundancy factor of 4.5, which means that 90% of these
reflections were measured at least 4.5 times. Phi and omega scans
with a frame width of 1.0◦ were used.
Data integration was done with Denzo, and scaling and merging
of the data was done with Scalepack.28 Merging the data and
averaging the symmetry equivalent reflections for the 4/m Laue
group resulted in an R2 value of 0.051. The structure was solved by
the direct methods program in SHELXS-97.29 Full-matrix least-
squares refinements based on F2 were performed in SHELXL-97,29
as incorporated in the WinGX package.30
NMR, elemental analysis, and MALDI-MS and these data are
reported in the experimental section.
Single crystal and molecular structure of
trans-Mo2(TiPB)2(O2CSp)2
An ORTEP drawing of the molecular structure of 1b is shown
in Fig. 2. In the solid state there is a center of inversion in the
Mo2 unit and two molecules of THF are weakly ligated along
˚
the Mo–Mo axis with Mo–O equal to 2.58 A, which is notably
much longer than the Mo–O distances to the carboxylates (Mo–
˚
O ~2.10 A). Pertinent to the discussion which follows is the
observation that the C4Se rings lie essentially coplanar with their
O2C units with the dihedral angle less than 1◦ and, as in related
structures, this allows for trans Lp–M2d–Lp conjugation. The
molecular structure is essentially similar to that of the 2-thiophene
carboxylate derivative3 previously reported and we assume that
the 2-furan carboxylate is likewise related. A full listing of the
crystallographic data is given in the electronic supplementary
information (ESI).†
For each methyl group, the hydrogen atoms were added at
calculated positions using a riding model with U(H) = 1.5Ueq
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implemented by the Gaussian20 suite of programs. To reduce
computational resources we have substituted formate for the bulky
TiPB ligands as in our previous calculations of the related thienyl
carboxylate derivatives. The complexes were optimized in C2h
geometry. In all cases the HOMO was the M2d orbital and this
was notably higher in energy by ca. 0.5 eV for M = W relative
to M = Mo. For the molybdenum complexes, the LUMO was
M2d* in character and the LUMO+1 and LUMO+2 were in-
and out-of-phase ligand p* combinations. For tungsten, the W2d*
orbital was the LUMO+2 and the in-phase and out-of-phase
Lp* combinations were the LUMO and LUMO+1, respectively.
Higher in energy was the lowest CO2p* combination of the formate
ligands: LUMO+6 for M = Mo and LUMO+4 for M = W. T i m e
dependent DFT calculations indicated that the lowest energy
electronic transition with a high oscillator strength was the M2d to
in-phase ligand p* while that involving the M2d and CO2p* of the
formate was at higher energy and with a lower oscillator strength.
In all cases, the M2d to d* has a small oscillator strength and this
transition is expected to be masked by the fully allowed intense
M2d to Lp* transition.
The frontier MO energy level diagrams for compounds 1a–2b
are shown in Fig. 3. Here it is seen that for the series of C4H4E
1
rings, the MLCT energy follows the order E = O > Se for both
metals, and both the transitions in tungsten are lower in energy
than in molybdenum. Fig. 4 shows the calculated iso surfaces of the
HOMO, M2-d*, and in-phase and out-of-phase L-p* orbitals for
1a, while the orbitals for the other complexes are almost identical.
The calculated energies of these orbitals are given in Table S1,
ESI.† The energy difference between the in- and out-of-phase
Lp* orbitals can correlate with the coupling through the metal
center, with larger differences indicative of stronger coupling. This
DE increases in the order 1b < 1a < 2b < 2a, suggesting that a
tungsten metal center and furan ligands better promote electronic
communication through the metal center.
Fig. 1 General structure of the complexes under investigation where M =
Mo or W, E = O or Se, and R = triisopropyl benzene.
Fig. 2 ORTEP diagram of compound 1b with 50% probability displace-
˚
ment ellipsoids for the non-hydrogen atoms. Selected bond distances (A)
and angles: Mo–Mo 2.1024(4), Mo–O(1) 2.1173(16), Mo–O(5) 2.581(2),
Se–C(18) 1.861(2), O(2)–Mo–O(4) 89.61(6), O(2)–Mo–Mo 92.83(4),
C(21)–Se–C(18) 86.69(12), O(1)–C(1)–O(4) 122.6(2).
Electronic structure calculations
Fig. 3 Frontier orbital energy level diagram for model compounds of
1a–2b showing the M2-d, M2-d*, and in-phase and out-of-phase ligand
p* orbital energies. The lowest energy metal-to-ligand charge transfer
(MLCT) transition is depicted.
To aid in an interpretation of the electronic spectra and electro-
chemical data, we have employed the use of electronic structure
calculations based on density functional theory (DFT)32,19 as
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Steady state emission spectra
All of the compounds show weak fluorescence in THF at room
temperature (Fig. 6) which follows the trend that might be
anticipated based on their absorption spectra. With the exception
of 2b, the compounds also show phosphorescence (ESI, Fig. S1†)
in the near infrared (NIR). Our instrumentation changes at around
850 nm, which impedes the certainty with which we can report the
luminescence from 800–900 nm. This not withstanding, the energy
of the phosphorescence is seen to be constant for M = Mo and
the appearance of vibronic features with D ~400 cm-1 is indicative
of emission from the 3MMdd* states.5 For tungsten, we assign the
phosphorescence as arising from the 3MLCT state since a 3WWdd*
emission should occur ~820 nm and display prominent vibronic
features.5,26
Fig. 4 Orbital surfaces as represented by GaussView28 for the M2-d,
M2-d*, and in-phase and out-of-phase ligand p* orbitals of 1a (iso value =
0.02).
Fig. 6 Fluorescence spectra of compounds 1a (orange), 1b (red), 2a
(purple), and 2b (blue).
For the selenophene derivative, the 3MLCT state is likely to be
so low in energy that it relaxes by a non-radiative process.
A summary of the emissive data for the four compounds is
given in Table 2 where it can be seen that the Stokes’ shift for
the fluorescence is ~5000 cm-1 for the complexes where M = Mo
but only ~2000 cm-1 for M = W. We propose that this is due
to the greater conformational reorganization of the molybdenum
complexes in their 1MLCT states. The back bonding, M2d to Lp*,
follows the order M = W > Mo and thus in solution the Boltzmann
distribution of rotamers will favor the more planar structure when
M = W relative to M = Mo at room temperature. The photoexcited
state has a planar quinoidal geometry, therefore a smaller Stokes’
shift will be seen for the tungsten complexes.
Fig. 5 UV-visible absorption spectra for compounds 1a (orange), 1b (red),
2a (purple), and 2b (blue) in THF at room temperature.
Electronic absorption spectra
The colors of the compounds range from yellow (1a) to dark green
(2b), and their absorption spectra in THF are compared in Fig. 5.
As predicted by the time dependent DFT calculations on the model
1
compounds, the lowest energy MCLT absorptions are lower in
energy for M = W relative to M = Mo. The transitions are generally
more intense for the tungsten compounds, as well. Within each
metal-containing group, the energy of this transition decreases as
the atomic weight of the ligand heteroatom decreases such that O
> S > Se, when compared with previously published results for
M2(TiPB)2(O2CTh)2, M = Mo, W.3,5 Within each metal containing
series we observe a higher energy transition: lmax ~330 nm for
M = Mo and ~400 nm for M = W which is relatively invariant
with respect to the nature of the C4H4E ring. This we assign to
the M2d to TiPB carboxylate p* transition. Absorption at higher
energies is due to ligand p to p* transitions. Also notable from an
inspection of the spectra shown in Fig. 5 is the appearance of a
clear vibronic progression when M = W. This is also typical for
complexes of the type trans-W2(TiPB)2L2 where L = p-accepting
carboxylate ligands5,6,33 and is a sign of greater coupling in 2a and
2b.
Electrochemical studies
We have studied the electrochemical properties of the four com-
pounds by cyclic voltammetry and differential pulse voltammetry
in THF solutions (Table 3). All compounds show a reversible
oxidation wave corresponding to the reversible removal of an
electron from the M2d orbital.
This oxidation is little influenced by the nature of the heteroatom
in the C4E ring but as expected is dependent on the metal, with the
oxidation being close to that of the ferrocene/ferrocenium couple,
Fc+/0, for M = Mo and around -0.5 V for M = W. The compounds
also show evidence of reduction within the solvent window but the
reduction waves are not reversible. In terms of trends, the onset of
this reduction is seen to be heteroatom dependent with the ease of
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Table 2 Absorption and emission data and the Stokes’ shift for compounds 1a–2b
Compound
lmax Absorption/nm
lmax Fluorescence/nm
Stokes’ shift/cm-1
lmax Phosphorescence/nm
1a
1b
2a
2b
420
460
555
635
540a
610c
625e
730g
5300
5300
2000
2100
1080b
1080d
~800f
—
a
lex = 420 nm. b lex = 405 nm. c lex = 500 nm. d lex = 405 nm. e lex = 560 nm. f lex = 532 nm. g lex = 660 nm.
Table 3 Oxidation and reduction potentials for compounds 1a–2b in
THF, versus Cp2Fe0/+ couple
Compound
Ox. potential/V
Red. potential/V
1a
1b
2a
2b
0.048
0.034
-0.56
-0.57
-3.1
-2.7, -3.0
-2.9
-2.6, -2.9
reduction following the order E = Se > S > O5,6 in accordance with
the absorption spectra, with two reductions being visible for 1b and
2b. Also, for related pairs of compounds this reduction occurs at a
slightly lower potential for M = W. We therefore propose that this
is a ligand-based reduction, even in the case of 1a and 1b where
the calculated LUMO is metal-based for the neutral complexes,
with the formation of what is effectively an organic radical anion.
In principle, this could be localized on one arm or delocalized
over both trans-ligands. These would correspond to mixed valence
ligands of Class I/II and Class III, respectively, on the Robin and
Day Scheme of mixed valence ions.34,35 Their kinetic lability in this
instance prevents their detailed study.
Transient absorption spectroscopy
The new compounds have been studied by fs and ns transient
absorption spectroscopy in THF solution. Both singlet and triplet
states have been monitored. Representative fs TA spectra for
compounds 1a and 2a are shown in Fig. 7. The 1a spectrum reveals
the bleach and partial recovery of the ground state absorption
at ~400 nm and the appearance of a new absorption band at
~500 nm. The kinetic trace for the disappearance of the absorption
yields a lifetime for the singlet state of ~5 ps. For 2a, absorbance
bands appear to both higher and lower energies than the bleach
at ~550 nm and do not disappear within 2400 ps, indicating they
belong to a triplet state. Measurement of the kinetics at 630 nm
gives a singlet state lifetime of ~2.5 ps. TA spectra for 1b and 2b
are shown in ESI Fig. S2.†
Fig. 7 Transient absorption spectra of 1a (top) with lex = 400 nm and 2a
(bottom) with lex = 514 nm in THF at room temperature.
Table 4 Lifetimes of singlet and triplet excited states determined from fs
and ns TA spectroscopic measurements
Compound
t
1MLCT/ps
t
3MLCT/ns
t3MMdd*/ms
1a
5.29 0.15a
1.47 0.02c
4.1e
62.4 1.6b
66.7 1.2d
77e
1b
Mo2(TiPB)2(O2CTh)2
2a
2b
2.45 0.30f
0.86 0.10g
0.67e
3–10
3–10
3–10e
W2(TiPB)2(O2CTh)2
a
b
Estimations of the lifetimes of the triplet states were determined
by ns TA spectroscopy where in most instances the loss of the
bleach was monitored with time (ESI Fig. S3†). Triplet lifetimes
for the molybdenum complexes span a relatively narrow range: 70
10 ms. This is rather typical for the lifetimes of 3MoModd* states
in related molybdenum carboxylates.4,5,33 The tungsten triplet
lex = 400 nm, kinetics monitored at 450 nm. lex = 355 nm, kinetics
monitored at 420 nm. c lex = 514 nm, kinetics monitored at 390 nm. d lex
=
=
532 nm, kinetics monitored at 460 nm. e Values from reference 5. lex
f
514 nm, kinetics monitored at 635 nm. g lex = 600 nm, kinetics monitored
at 415 nm.
3
lifetimes are notably much shorter than the WWdd* state seen
For molybdenum, we see that the 1MLCT state has a lifetime that
varies from just over 5 ps to 1.5 ps with the decrease in lifetime
following the atomic number of the heteroatom. For tungsten, the
S1 lifetimes are in all cases somewhat shorter, but the trend in
heteroatom E = O, S, Se is less apparent as both E = S and Se have
lifetimes less than a picosecond.
for W2(TiPB)426 and are consistent with 3MLCT states. This, along
with the phosphorescence data of 2a, is very strong evidence that
the T1 state for the two tungsten complexes is 3MLCT. The lifetime
data for the four compounds, along with previously reported
results for M2(TiPB)2(O2CTh)2, M = Mo, W,5 are listed in Table 4.
2262 | Dalton Trans., 2012, 41, 2257–2263
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1
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3
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Acknowledgements
This material is based upon work supported by the National
Science Foundation under Grant No. 0957191. We also thank
The Ohio State University Institute for Materials Research
and the Ohio State Third Frontier Photovoltaic Initiative for
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