MM quadruple bonds supported by cyanoacrylate ligands. Extending photon harvesting into the near infrared and studies of the MLCT states

Article abstract of DOI:10.1039/c3sc50322d

The compounds trans-M₂(TⁱPB)₂(L) ₂ and trans-M₂(TⁱPB)₂(L′) ₂ have been prepared from the reactions between M₂(T ⁱPB)₄ (TⁱPB = 2,4,6-triisopropylbenzoate, M = Mo or W) and LH or L′H (～2 equiv.), respectively, where L = O ₂CC(CN)CH-C₆H₄-NPh₂ and L′ = O₂CC(CN)CH-C₄H₃S-C₆H ₄-NPh₂. These cyanoacrylate ligands promote intense M ₂δ to L or L′ π*-transitions that span the range 550-1100 nm. The two molybdenum complexes have been characterized by single crystal X-ray studies that reveal the extensive L-M₂-L or L′-M₂-L′ M₂δ-ligand π-conjugation. The new compounds have been characterized by electronic structure calculations employing density functional theory (DFT) and time-dependent-DFT, cyclic voltammetry, electronic absorption and steady state emission spectroscopy and femtosecond (fs) and nanosecond (ns) time resolved transient absorption (TA) and fs time-resolved infrared spectroscopy (TRIR). The latter allows the determination of the S₁ states as ¹MLCT that are delocalized over both L and L′. For molybdenum the T₁ states are ³MoMoδδ* whereas for tungsten they are ³MLCT.

Full text of DOI:10.1039/c3sc50322d

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MM quadruple bonds supported by cyanoacrylate
ligands. Extending photon harvesting into the near
infrared and studies of the MLCT states†
Cite this: Chem. Sci., 2013, 4, 2105
Samantha E. Brown-Xu, Malcolm H. Chisholm,* Christopher B. Durr,
Sharlene A. Lewis, Vesal Naseri and Thomas F. Spilker
i
i
0
2
The compounds trans-M
2
(T PB)
(L)
2 2
and trans-M
2
(T PB)
2
(L )
have been prepared from the reactions
i
i
0
between M
2
(T PB)
4
(T PB ¼ 2,4,6-triisopropylbenzoate, M ¼ Mo or W) and LH or L H (ꢀ2 equiv.),
0
respectively, where L ¼ O
2
CC(CN)]CH–C
H
6 4
–NPh
2
and L ¼ O
2 4 3 6 4 2
CC(CN)]CH–C H S–C H –NPh . These
0
cyanoacrylate ligands promote intense M
2
d to L or L p*-transitions that span the range 550–1100 nm.
The two molybdenum complexes have been characterized by single crystal X-ray studies that reveal the
0 0
extensive L–M –L or L –M –L M d–ligand p-conjugation. The new compounds have been characterized
2 2 2
by electronic structure calculations employing density functional theory (DFT) and time-dependent-DFT,
cyclic voltammetry, electronic absorption and steady state emission spectroscopy and femtosecond (fs)
Received 4th February 2013
Accepted 27th February 2013
and nanosecond (ns) time resolved transient absorption (TA) and fs time-resolved infrared spectroscopy
DOI: 10.1039/c3sc50322d
1
(
TRIR). The latter allows the determination of the S
1
states as MLCT that are delocalized over both L
0
3
3
www.rsc.org/chemicalscience
and L . For molybdenum the T
1
states are MoModd* whereas for tungsten they are MLCT.
Introduction
There is considerable current interest in the synthesis of molecular
systems that enhance spectral absorption across the solar spectrum
1
for photon harvesting and application to photovoltaics, PV. Both
organic and metalloorganic compounds are under consideration in
various forms of dye sensitized solar cell (DSSC) devices. Numerous
push–pull molecules with a donor–acceptor (D–A) or donor–p–
bridge acceptor (D–p–A) architecture are being intensely studied
for application in optoelectronic devices such as solar cells; since it
is possible to tune the photophysical and structural properties of
these systems through the nature and arrangement of D and A
2–6
Chart 1 Representative examples of organic and metalloorganic compounds
moieties. These push–pull molecules have been employed to
7
–10
7
used in solar cell devices.
enhance spectral expansion to cover more of the sun's radiation.
Representative examples of some of the organic and metal-
7–10
loorganic compounds are shown in Chart 1.
2
The Ru(II)bipyr- ligand charge transfer (MLCT) transitions as a result of M d to
idine-based dyes have been largely employed in Gr ¨a tzel type solar ligand p* interactions. The energy of the MLCT may be tuned by
7,11
cells while platinum based polymers and organic oligomers are the choice of metal (Mo or W) and by the type of ligand. With an
8–10
employed in thin lm polymer or bulk heterojunction PV cells.
interest in extending these transitions toward the near-infrared
We are currently evaluating the photophysical properties of and enhancing spectral overlap, we were attracted towards the
carboxylate supported MM quadruply bonded complexes and employment of triphenylamine–cyanoacrylic acid D–p–A
their oligomers with regard to photon harvesting and their ligands and report here on the attachment of these ligands LH
0
potential in PV devices. These complexes show intense metal to and L H shown as I and II respectively below.
The Ohio State University, 100 West 18th Avenue, Columbus, OH, 43210, USA. E-mail:
chisholm@chemistry.ohio-state.edu; Fax: +1 614 292 0368; Tel: +1 614 688 3525
†
Electronic supplementary information (ESI) available: Table S1, Fig. S1–S6.
CCDC 919555 and 919556. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3sc50322d
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LH displays an intense intraligand charge transfer (ILCT)
transition from triphenylamine to the cyanoacrylic acid moiety.
This transition occurs in the visible region of the spectrum,
because the introduction of the cyanoacrylic acid moiety
considerably lowers the energy of the p* molecular orbital
0
(
MO). L H also exhibits an intense ILCT, but it is red shied
relative to that of LH because the thienyl moiety extends the
6,12,13
conjugation of the system.
It is anticipated that the incor-
poration of these ligands into MM quadruply bonded systems
will provide two main advantages. First, the relatively low-lying
p* MO will signicantly red-shi the energy of the MLCT
transition. This may be particularly useful when M ¼ Mo as Mo- Fig. 1 ORTEP representation of compound MoL drawn at 50% probability.
Solvent molecules and hydrogens excluded for clarity. Gray ¼ carbon, blue ¼
nitrogen, scarlet ¼ oxygen, green ¼ molybdenum.
based MM complexes exhibit higher energy MLCT transitions
due to the lower energy of the Mo d MO. Second, the MM
2
complexes will exhibit two intense absorption bands in the
visible region due to the ILCT and MLCT transitions. This will
enhance spectral overlap of these compounds. The synthesis,
characterization and photophysical properties of the complexes
are reported here.
Results and discussion
Syntheses
i
The reaction between the homoleptic compounds M (T PB)
2
4
where M ¼ Mo or W and the respective carboxylic acids (ꢀ2
equiv.) 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid, LH
and 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid,
0
0
L H in toluene at room temperature lead to the formation of the Fig. 2 ORTEP representation of compound MoL drawn at 50% probability.
0
Solvent molecules and hydrogens excluded for clarity. Gray ¼ carbon, blue ¼
compounds ML and ML as microcrystalline precipitates which
nitrogen, scarlet ¼ oxygen, yellow ¼ sulfur, turquoise ¼ molybdenum.
were collected by centrifugation and washed with toluene and
i
hexanes to remove unreacted M
2
(T PB)
4
. The molybdenum
0
complexes hereaer abbreviated MoL and MoL and tungsten
complexes, WL and WL , are air-sensitive and highly colored:
MoL, red-brown; MoL , brown; WL, green and WL , yellow
green. Crystals of MoL suitable for X-ray crystallography were
obtained from a concentrated THF solution which was layered
0
0
both structures the nitrogen atoms of the respective L and L
ligands are planar indicative of the Npp-ring conjugation. The
(O C) units are comparable to those seen in
2 2 4
numerous carboxylate complexes incorporating the quadruply
bonded Mo2 unit. The structures are also similar to those we
have recently proposed for a vinylthienyl carboxylate complex.
0
0
central Mo
4
+
0
with hexanes while crystals of MoL were obtained by a vapor
1
4
diffusion of pentane into a THF solution. The compounds were
1
characterized by H NMR spectroscopy and gave molecular ions
by MALDI-TOF mass spectrometry. Characterization data are
given in the Experimental.
Electronic structure calculations
Electronic structure calculations employing density functional
theory and the commercially available Gaussian suite of
programs have been performed on the model compounds where
Solid-state and molecular structures
0
i
The molecular structures of MoL and MoL are shown in Fig. 1 formate has been substituted for T PB. This saves on computa-
and 2, respectively. In both structures we see the trans-dispo- tional time and resources and is a reasonable approximation
i
i
sition of the ligands. The aryl groups of the T PB ligands are given the lack of aryl group conjugation within the T PB ligands
ꢁ
twisted close to 90 from their attendant carboxylate planes as seen from the dihedral angles in the structures of MoL and
0
0
which effectively removes the aromatic ring from conjugation MoL . We refer to the model compounds as ML and ML .
0
with the M
2
d orbital. In contrast the ligands L and L are
The calculated frontier molecular orbitals for the molecules
0
arranged in a planar manner so as to maximize L–M
2
–L and L – MoL and WL and their respective energies are shown in Fig. 3.
d orbitals. Each In both molecules the HOMO is an M
0
M
2
–L conjugation of the ligand p and M
2
2
d with extensive mixing
molecule has a crystallographically imposed center of inversion with the ligand p-system. This mixing is greater for MoL relative
and in MoL the –C H -ligand attached to the cyanoacrylate is to WL. This difference arises from the relative energies of the
6
4
seen to extend the conjugation as in the –C H S-group in the respective M d orbitals. The HOMO-1 is an in-phase combina-
4
2
2
0
0
MoL complex. In MoL the –C
6 4
H moiety attached to the tion of the ligand-based p orbital that does not have the correct
ꢁ
diphenylamine group is twisted ꢀ25 from the C
4
2
S plane. In symmetry to interact with the M d orbital, while the HOMO-2 is
2
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Fig. 3 Frontier molecular orbitals and calculated energies of MoL and WL.
the out-of-phase ligand p combination which does mix with the ligand-based p* orbitals and as can be seen the LUMO+1 mixes
M
2
d as can be seen in Fig. 3.
with the M
2 2
d orbital. The M d* orbital has minimal mixing with
2
Again this mixing is greater for MoL than for WL. The Mo d
the carboxylate non-bonding p orbitals. The calculated HOMO–
(
ꢀ
HOMO) and the ligand-based lled p-orbitals differ only by LUMO gap is 2.35 eV for MoL and 2.08 eV for WL.
0
0.5 eV while the difference in the tungsten complex is ꢀ1.0 eV.
The calculated frontier orbitals for the complexes MoL and
0
Below the HOMO-2 (shown in Fig. 3) come the MM p orbitals WL and their respective energies are given in Fig. 4. The calcu-
0
(
6.7 eV for MoL and ꢀ6.0 eV for WL) that have minimal inter- lations were done in C symmetry and predict for MoL a near
1
actions with the ligands. The LUMO and LUMO+1 are for both planar geometry involving the thienyl and its neighboring phenyl
0
complexes the in-phase and out-of-phase combinations of the group. For WL this extended planarity is not so pronounced and
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0
0
Fig. 4 Frontier molecular orbitals and calculated energies for MoL and WL .
0
it can be seen in the GaussView isosurface contour plots of the structures of the M -containing complexes. For L , we observe a
2
0
0
respective HOMOs that the Mo d–ligand p mixing is greater than smaller HOMO–LUMO gap (2.3 eV for MoL and 1.89 eV for WL )
2
0
its tungsten counterpart. Again this reects the proximity of the and a greater mixing with the Mo
Mo d and ligand p-orbital energies. The HOMO-1 and HOMO-2
are the in-phase and out-of-phase ligand p-orbital combinations
2
d orbital for L relative to L .
2
Electronic absorption spectra
2
and again the HOMO-2 has some Mo d contribution. Below those
0
orbitals we see that for MoL there are two other ligand p The absorption spectra for the free ligand LH and the
combinations that lie higher than the Mo p-orbitals whereas for complexes MoL and WL recorded in THF at room temperature
2
0
WL the W p-orbitals lie directly below the HOMO-2. The LUMO are shown in Fig. 5a. Each compound shows an intense
2
and LUMO+1 are the in-phase and the out-of-phase ligand p* absorption at ꢀ410 nm which we assign to an intraligand pp*
combinations and the LUMO+2 is the M
ligands LH and L H are closely related, as are the electronic tions on the free carboxylic acid LH indicate that the HOMO is
2
d* orbital. Clearly the charge transfer transition (ILCT). Electronic structure calcula-
0
2
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Fig. 6 (a) Fluorescence (solid blue, lexc ¼ 600 nm) and phosphorescence
0
(
dashed blue, lexc ¼ 658 nm) spectra of MoL and emission spectra of MoL at
Fig. 5 (a) Absorption spectra of LH (purple), MoL, (blue) and WL (red). (b)
room temperature (solid brown-green, lexc ¼ 658 nm) and 77 K (dashed brown-
green, lexc ¼ 658 nm). (b) Emission spectra of WL at room temperature (red,
0
0
0
Absorption spectra of L H (sky-blue), MoL , (brown-green) and WL (yellow-
green). Taken in THF at room temperature.
l
exc ¼ 658 nm,) and 77 K (dashed red, lexc ¼ 658 nm) and WL0 at room
temperature (yellow-green, lexc ¼ 785 nm).
6 4 2
principally located on the C H NPh moiety while the LUMO is
polarized toward the cyanoacrylate portion of the molecule. studies on molybdenum complexes and the large Stokes shi
Calculated energies and GaussView isosurface contour plots of allow the band at 1100 nm to be assigned to phosphorescence
0
3
0
the frontier orbitals of LH and L H are given in the ESI (Fig. S1†). from the MoModd*. (Fig. 6a) MoL shows a broad emission
Occurring to lower energy in the metal containing complexes band that extends from the visible into the infrared. This band
are broad and intense absorptions associated with the M d to starts at ꢀ650 and shows peaks at ꢀ940, 1080 and 1270 nm.
2
ligand p* MLCT. The absorption spectrum of WL has a lmax
ꢀ
Upon cooling to 77 K the relative intensity of the band at 1080
9
00 nm which shows evidence of a vibronic feature with an increases and vibronic features are observed. The vibronic
ꢂ1
ꢂ1
energy separation of ꢀ1500 cm and a long tail toward higher spacing was determined to be ꢀ330 cm . This result indicates
energy. Similar spectral features were seen for the complex that the broad band extending from ꢀ650 to 1400 nm consists
i
14
trans-W
2
(T PB)
2
(O
2
CCH]CHC
4
H
0
3
S)
2
.
of two bands. The band with a peak at ꢀ940 is assigned to
1
0
0
The absorption spectra for L H, MoL and WL recorded in uorescence from the MLCT and the band that is centered at
3
THF at room temperature are shown in Fig. 5b and show 1080 nm is due to phosphorescence from the MoModd*.
parallel features to those described above. There is an ILCT
WL shows a broad emission extending from ꢀ850 to above
transition at ꢀ450 nm which is relatively unchanged in energy 1400 nm with a peak at ꢀ960 nm and evidence of vibronic
in the metal containing complexes (a slight bathochromic shi features. When cooled to 77 K the vibronic features are better
0
1
is seen for WL ). To lower energy we see the MLCT transitions resolved and the vibronic spacing is determined to be
0
0
0
ꢂ1
0
at 600 nm for MoL and ꢀ1000 nm (lmax) for WL . Again for WL
ꢀ1200 cm . WL also shows a broad emission extending from
we see evidence of vibronic features and an extensive tailing of ꢀ920 to above 1500 nm with a peak at ꢀ1100 nm and vibronic
1
ꢂ1
the MLCT into the visible region of the spectrum.
features with a spacing of ꢀ1200 cm . Based on the small Stokes
0
shi and vibronic spacing the emission from WL and WL are
1
Steady state emission
assigned as uorescence from the MLCT state. No phosphores-
0
cence was observed from WL and WL within the detection
The steady state emission spectra for all four compounds were
recorded at room temperature in THF. (Fig. 6) The emission
window. This is likely due to non-radiative decay. The triplet states
were however detected with time-resolved spectroscopy, vide infra.
0
spectra of MoL and WL were also measured at 77 K in 2-
methyltetrahydrofuran. Compound MoL shows dual emission
with peaks at ꢀ720 and 1100 nm upon excitation at 600 and
Electrochemistry
6
58 nm respectively. Based on its small Stokes shi the peak at In THF solutions, the four compounds under consideration all
1
7
20 nm is assigned to uorescence from the MLCT. Previous show reversible one electron oxidation waves by cyclic
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0
0
a
Table 1 Electrochemical data for compounds MoL, MoL , WL and WL
obtain information pertaining to their lifetimes and where
electron density resides in these states. Results from the TA data
detailed below are well complemented by fs-TRIR studies pre-
sented in the following section. All compounds were excited
into the MLCT band in THF solutions, and kinetic traces are
shown in ESI Fig. S3.†
ox
½
red
E
, V
½
E , V
MoL
WL
0.10
ꢂ1.97
ꢂ1.90
ꢂ1.77
ꢂ1.66
1
ꢂ0.48
0
MoL
0.14
0
WL
ꢂ0.48
1 n
In the fs-TA spectra of MoL (Fig. 7a), a prominent S / S
a
+/0
The redox potentials are referenced to the Cp
2
Fe couple.
absorption is observed at 470 nm; however, it is likely that this
band extends out to lower energies where it is obscured by the
MLCT ground state (GS) bleach (500–600 nm). Also present is a
1
bleach at 410 nm from the ILCT transition associated with the
voltammetry. This corresponds to removal of an electron from
acrylate ligand. In the S state, electron density is largely located
the M d orbital. The molybdenum complexes have oxidation
1
2
0
/+
on the cyanoacrylate portion of the ligand, preventing charge
transfer from the triphenylamine unit and leading to depletion
of the ILCT. The recovery of the bleach coincides with the decay
of the singlet absorption, giving a singlet lifetime of 7.1 ps. As
noted above, the triplet state is MoModd* in nature, which
potentials close to the Cp Fe
tungsten complexes fall close to ꢂ0.5 V vs. Cp Fe . Upon
couple while those of the
2
0
/+
2
reduction all complexes show evidence of a ligand-based one
electron reduction wave which is quasi-reversible and the
tungsten complexes show evidence of a second reduction wave
which is irreversible. The electrochemical data are summarized
in Table 1 and the cyclic voltammograms are shown in the ESI
should not greatly affect the ILCT transition. The 410 nm band
1
is no longer depleted in the T state, however the MLCT GS
1
bleach is apparent at ꢀ550 nm along with a T / T absorption
(
Fig. S2†). Of note is the fact that the rst reduction wave occurs
1
n
0
at 465 nm. The triplet state does not decay on the timescale of
this experiment and it has been further characterized by ns-TA
at lower negative potentials for WL and WL than their respec-
tive molybdenum complexes.
0
along with MoL .
0
The features of the spectra for MoL (Fig. 7b) are similar to
Time-resolved spectroscopic studies
those described above though the bands are shied to lower
0
Femtosecond transient absorption spectra. For all four energy in accordance with the stabilization of L p* vs. L p*. In
1
1 1
compounds, the properties of both S and T
excited states have the MLCT state, a bleach of the intraligand CT state occurs at
been examined by ultrafast spectroscopic techniques in order to 460 nm while S
/ S absorptions are observed at 565 and
n
1
0
0
Fig. 7 fs-TA spectra of (a) MoL, lex ¼ 675 nm; (b) MoL , lex ¼ 675 nm; (c) WL, lex ¼ 800 nm and (d) WL , lex ¼ 800 nm; recorded in THF at RT.
2
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10 nm. An isosbestic point (485 nm) is also observed during
Chemical Science
5
decay of the singlet state (s ¼ 9.0 ꢃ 0.2 ps), indicating direct
3
conversion to the MoModd* state. Following this process, a
T / T absorption at 490 nm and GS bleach to lower energy
1
n
persist throughout the duration of the experiment.
For WL, the initial spectra are dominated by broad singlet
state absorption from 450–650 nm with peaks at 470 and
5
S
55 nm. As for MoL, the ILCT bleach is present at 410 nm. The
/ S absorption also occurs at similar energies for the two
1
n
compounds since this should largely be a L p / L p* transi-
tion. The lowest readily accessible pump wavelength for the
tungsten compounds was 800 nm which injects excess energy
into the complex. Consequently, a noticeable cooling of the 555
nm peak at early times (s ꢀ 0.5 ps) is observed. The lifetime of
1
the MLCT state of WL is 16.3 ps, much longer than any of the
other compounds investigated even though the heavier element
tungsten is present. We have been interested in determining the
factors that contribute to excited state lifetimes with one
possible component being the separation of the electron and
hole. Supported by the fs-TRIR data, vide infra, the electron may
be separated further from the metal center in WL thereby
3
slowing its recombination. Aer decay to the MLCT state, a
lower intensity, broad absorption remains with lmax ¼ 545 nm.
Though the GS MLCT bleach is not visible in this region, the
ILCT bleach remains as this transition is still affected in the
3
MLCT state. A longer decay component (s < 3 ns) is observable
in the data which can be attributed to decay of the triplet state.
3
Ditungsten tetracarboxylate MLCT lifetimes are typically short
15
(
3–10 ns) in quadruply bonded metal carboxylates, and the low
Fig. 8 (a) ns-TA spectra of MoL recorded at 1 ms (closed) and 50.5 ms (open) (b)
energy of this state in WL enhances the rate of GS repopulation.
0
0
ns-TA spectra of MoL recorded at 0.04 ms (closed) and 1.4 ms (open).
Finally, WL (Fig. 7d) displays many of the same excited state
properties as those previously detailed. Following 800 nm
excitation, a broad S / S absorption peaks at 540 nm and
1 n
tails into the near IR. The position of this band is similar to that orbital. This observation may be related to the continued
0
of MoL , again indicating the transient absorption is mainly growth of the MLCT GS bleach at long times during the fs
ligand localized. The decay of this band coincides with recovery experiment.
0
of the ILCT state bleach (470 nm), giving rise to an isosbestic
The MoL spectra are qualitatively similar to those at long
times in the fs experiment and show an absorption at ꢀ510 nm
and a bleach at 620 nm. Kinetics obtained from these bands
1
3
point at 490 nm and a MLCT lifetime of 8.8 ps. The MLCT T
absorption appears broad and shied to lower energy (lmax
1
ꢀ
600 nm), however it decays almost completely during the give an average T
1
lifetime of 2.5 ms which is roughly an order of
timeframe of the experiment leading to an estimation of the magnitude shorter than lifetimes commonly seen for
3
15
1
triplet lifetime as 1 ns. The rapid decay is again due to the small
MoModd* states. The low energy of the MLCT state (S
ꢀ
0
–0
ꢂ1
3
energy gap between T and the ground state which enhances the 12 500 cm ) means it is possible for the MLCT state to be very
1
3
ꢂ1
3
rate of radiationless decay. The ILCT bleach is also present in near in energy to MoModd* (ꢀ9100 cm ). MLCT states are
3
the MLCT state and decays on the same timescale, producing a known to be shorter-lived and the proximity may enable more
second isosbestic point at 465 nm.
efficient decay back to the ground state.
Nanosecond transient absorption spectra. Though the
Time-resolved infrared spectra. Each of the compounds has
triplet states of the tungsten compounds are not long-lived been examined by fs-TRIR spectroscopy in THF solution with
1
enough to be studied by ns-TA, the technique can be used for excitation into the MLCT absorptions. Spectra were collected in
0
ꢂ1
MoL and MoL . The spectra of MoL (Fig. 8a) show bleaches at the region of the C^N vibration (2280–2080 cm ) as well as in
ꢂ1
3
80 and 560 nm and a transient absorption at ꢀ440 nm. The T
lower energy regions (1650–1350 cm ) where organic ligand
lifetime obtained from decay of this band (average of 54 ms) is in vibrations occur. Kinetic traces are given in ESI Fig. S5.† First,
agreement with the lifetime of T states in previously studied the MoL and WL compounds will be compared (Fig. 9). In MoL,
molybdenum complexes and is assigned to the MoModd* a weak transient signal is observed at 2168 cm which we
state. The kinetic trace is shown in Fig. S4.† Presently, it is assign to n(CN) of the acrylate ligand L. This is shied 52 cm
1
1
15
3
ꢂ1
ꢂ1
unclear why the ILCT state remains bleached in this triplet state to lower energy relative to the ground state. The absorption
where the excited electron now resides in a d-antibonding disappears on conversion to the triplet state, consistent with the
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clear what determines the relative intensities of the excited state
vibrations such as why n(CN) is so much more prominent for
WL relative to MoL. What can be seen is that n(CN) shis to
higher energy in the T state relative to the S state, which we
1
1
propose is associated with the charge on the ligand being
further removed from the CN group, namely toward the
aromatic centers. We note, for example, that the S
1
lifetime for
i
the compound trans-W
2
(T PB)
2
(O
2
CC^C-9-anthracene) is also
2
14
longer that its dimolybdenum counterpart. Evidence has been
given that the ligand centered electron is more spatially
removed from the hole on the W center and in the triplet state
2
16
lies predominantly on the anthracene moiety.
The lower energy portion of the spectrum is much less
intense in Fig. 9b and an expanded section of this spectrum is
shown in the ESI Fig. S6.† Again we see bleaches assignable to
ꢂ1
C]C and –CO
2
vibrations at 1570 and 1505 cm , as well as a
ꢂ1
ꢂ1
shi of these modes to lower energy (1550 cm , 1480 cm ) in
the MLCT state. We also note that the long-lived state does not
1
ꢂ1
3
show n (CO ) ꢀ1540 cm that is more characteristic of a dd*
as
2
17
state.
The TRIR spectra of the compounds containing ligand L
0
(
Fig. 10) present many features similar to those described
0
above. For MoL , we again observe a weak C^N stretch in the
1
ꢂ1
ꢂ1
MLCT state at 2170 cm (Dn(CN) ¼ ꢂ49 cm ). The lower
0
Fig. 9 fs-TRIR spectra of (a) MoL, lex ¼ 675 nm; and (b) WL, lex ¼ 800 nm; energy region displays ground state bleaches associated with L
ꢂ1
ꢂ1
recorded in THF at RT.
C]C and asymmetric –CO vibrations at 1585 and 1505 cm
,
2
1
along with prominent MLCT absorptions shied ꢀ30 cm to
lower energy from the GS frequencies. With a lifetime of 7 ps,
3
assignment of T as MoModd* which should not greatly affect the C^N band and intense ligand vibrations at 1550 and 1480
1
the cyano stretch. In the lower energy region, we observe several
new IR bands associated with both the S and T1 states.
1
ꢂ1
Bleaches at 1510 and 1585 cm can be assigned to the ground
2
state nas(CO ) and n(C]C) of L, respectively, as supported by
DFT calculations. We propose that the intense absorption in the
ꢂ1
S
1
state at 1480 cm is predominantly the nas(CO
2
) mode while
ꢂ1
the S
1
band at 1530 cm arises from n(C]C). Upon photoex-
citation the L p* orbital is populated leading to a reduction in
bond strength and a weaker stretching frequency for these
vibrations. It is not presently clear how to assign the absorp-
tions at 1590 in the S state, but it is likely associated with ring
1
vibrations of the TPA moiety. In the long lived spectra assigned
ꢂ1
to the T
1
state, the absorption at 1540 cm is due to nas(CO
2
) of
3
the MoModd* state for the reasons outlined in previous TRIR
studies of related carboxylates. Based on DFT calculations, the
15
ꢂ1
bands ꢀ1400 cm may be attributed to n (CO ) of L.
s
2
In contrast to its molybdenum counterpart, WL exhibits an
ꢂ1
intense IR absorption at 2184 cm assignable to n(CN) in the
MLCT state. The 41 cm shi to lower energy from the ground
1
ꢂ1
state is smaller than the shi observed with MoL, suggesting
this singlet state is spread over more of the ligand and is less
concentrated on the cyano portion. As noted in the fs-TA, this
increased delocalization may also contribute to the longer S
1
lifetime of WL. The band decays with the singlet state but leaves
ꢂ1
a weaker C^N vibration at 2190 cm which we can readily
3
assign to n(CN) in the MLCT state. This is notably weaker than
the n(CN) of the S
1
state, in part due to the lower population of
0
0
Fig. 10 fs-TRIR spectra of (a) MoL , lex ¼ 675 nm; and (b) WL , lex ¼ 800 nm;
the T
1
state following singlet decay. However, it is not entirely recorded in THF at RT.
2
112 | Chem. Sci., 2013, 4, 2105–2116
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ꢂ1
0
3
cm disappear, causing L nas(CO
2
) of the MoModd* state to (ref. 22) were prepared by reported procedures. NMR spectra
ꢂ1
become visible at 1540 cm
.
were recorded on a 400 MHz Bruker DPX Advance 400 spec-
0
1
In the spectra of the related compound WL , the intense trometer. All H NMR chemical shis are in ppm relative to the
n(CN) absorption of the MLCT state is observed at 2177 cm , a protio impurity in tetrahydrofuran (THF)-d at 3.58 ppm.
shi of 48 cm from the ground state at 2225 cm . As was
1
ꢂ1
8
ꢂ1
ꢂ1
seen in the TRIR of WL, this band moves slightly to higher Electronic absorption spectra
ꢂ1
energy (2182 cm ) in the longer-lived T
1
state. Ligand vibra-
UV-Vis-NIR electronic spectra in THF solutions were measured
at room temperature using a Perkin-Elmer Lambda 900 spec-
trometer using a 1 cm ꢄ 1 cm quartz cuvette sealed with a
Kontes top.
tions in the S
1
state initially appear similar to those observed for
MoL , but with time these decay and the region is relatively
silent in the MLCT state. This observation, along with the
weakness of the C^N stretch in the triplet state, can be
0
3
attributed to rapid decay of the T state in accordance with the
energy gap law, as supported by the TA data.
1
Electrochemical studies
ꢂ1
Cyclic voltammograms were collected at a scan rate of 50 mV s
One question frequently raised in studying MLCT states of
molecules of this type is whether the excited state electron
density is located on one of the ligands or delocalized over both.
using a Princeton Applied Research (PAR) 173A potentiostat-
galvanostat equipped with a PAR 176 current-to-voltage
converter. The measurements were performed under an argon
2
+
3
In the case of heavily studied [Ru(bpy) ] -type complexes the
n
18,19
atmosphere in a 0.1 M solution of Bu NPF in THF inside a
charge is localized on a single ligand,
while we have found
4
6
single-compartment voltammetry cell that was equipped with a
platinum wire auxiliary electrode, a platinum working electrode
examples of both limiting cases in previous investigations of
20
excited state mixed valence. In the present study we note that
only one n(CN) is observed in the excited MLCT states for all
four compounds, while we might expect to see two IR active CN
stretches if the environment of each ligand was notably
different. In addition, calculations on the anions of the model
and a pseudoreference electrode consisting of a silver wire in
n
0
.1 M Bu
4
NPF
6
–THF separated from the bulk solution by a
+
Vycor tip. The potentials are referenced to the Cp
couple.
2 2
Fe/Cp Fe
ꢂ
0
ꢂ
complexes [ML] and [ML ] predict n(CN) to shi to lower
ꢂ1
ꢂ1
0
Mass spectrometry
energy by ꢀ50 cm for L and 25 cm for L . In these calcula-
tions the charge is delocalized over both ligands and the elec-
tron formally occupies the LUMO of the MO conguration of
the neutral molecules. The observed values of n(CN) in the
excited state fall close to the calculations and are not vastly
Matrix assisted laser desorption ionization time-of-ight
MALDI-TOF) mass spectra were obtained on a Bruker Microex
(
mass spectrometer. Dithranol was used as the matrix.
2 2 2 2
larger, as was seen for the compounds trans-M (O CMe ) -
Electronic structure calculations
i
20
2 2
[(N Pr) CC^C-Ph] which have localized MLCT states. Thus
The model complexes were optimized in the gas-phase using
density functional theory (DFT) utilizing the Gaussian09 suite of
we propose that the MLCT states of the four compounds
reported here have the photo-excited electron delocalized over
23,24
25,26
programs.
The B3LYP functional
was used in conjunc-
0
both L and L .
tion with the SDD energy consistent pseudopotentials and the
SDD energy consistent basis set for molybdenum and tung-
Conclusion
27
sten and the 6-31G* basis set for H, C, N, O and S atoms.
Vibrational frequency analysis was used to conrm that the
optimized structures were minima on the potential energy
surface. GaussView isosurface contour plots are shown with an
isovalue of 0.02. Electronic absorption spectra were calculated
using the time dependent DFT (TDDFT) method.
The attachment of the push–pull aminocyanoacrylate ligands to
the MM quadruply bonded centers allows the MLCT absorption
spectra to be extended well into the near IR with intense ILCT
bands in the visible. These absorptions are intense and the
26
1
MLCT bands have strong tails to higher energy. The S states
have relatively long lifetimes, ꢀ10 ps, when compared with
1
Crystallographic information
most transition metal MLCT states that undergo inter-system
0
crossing within 100 fs to triplet states. These two features Single crystals of MoL and MoL were isolated as red-brown and
suggest that these compounds should nd applications as brown crystals respectively and handled under a pool of uo-
photon harvesters and this is the topic of on-going research.
rinated oil. Examination of the diffraction pattern was done on
a Nonius Kappa CCD diffractometer with Mo Ka radiation. All
work was done at 150 K using an Oxford Cryosystems Cryo-
stream Cooler. Data integration was done with Denzo, and
Experimental
General procedures
28
scaling and merging of the data was done with Scalepack. The
The reactions (with the exception of ligand syntheses) were structures were solved by the direct methods program in
2
conducted under inert atmosphere (argon or nitrogen) using SHELXS-97. Full-matrix least-squares renements based on F
29
standard glovebox and Schlenk techniques and solvents used in were performed in SHELXL-97, as incorporated in the WinGX
these reactions were dried and distilled by standard methods package. For each methyl group, the hydrogen atoms
30
i
i
and degassed before use. Mo
2
(T PB)
4
(ref. 21) and W
2
(T PB)
4
were added at calculated positions using a riding model with
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U(H) ¼ 1.5Ueq (bonded carbon atom). The rest of the hydrogen fundamental laser beam is split to pump an OPA equipped with
atoms were included in the model at calculated positions using either an SFG or SHG attachment to produce pump pulses
a riding model with U(H) ¼ 1.2Ueq (bonded atom). Neutral atom tunable throughout the visible spectrum and an OPA with a
scattering factors were used and include terms for anomalous difference frequency attachment to produce mid-IR pulses (2–
3
1
dispersion.
10 mm). The IR beam is split into a probe and a reference beam
i
0
One of the isopropyl moieties on the T PB moiety of MoL was by a Ge beamsplitter. These beams then pass through the
disordered over two positions and modeled accordingly. In both sample, where the pump beam is overlapped with the probe,
0
MoL and MoL there was evidence of severely disordered solvent and are directed into a Triax 320 spectrometer. The probe and
in the lattice that was difficult to model sufficiently. In the case reference are spectrally dispersed onto separate HgCdTe array
0
of MoL this electron density was quite diffuse and removed (32 element) detectors cooled by liquid nitrogen. The pump and
32
33
using the SQUEEZE command in PLATON which excluded 33 probe pulses are synchronized by passing the pump through a
ꢂ
3
˚
e from a void of 162 A which roughly corresponds to one THF chopper operating at 500 Hz, allowing for measurement of the
molecule. In the case of MoL the disordered THF molecule was probe signal under pump on/off conditions. Signal from the
able to be found in the difference map, but had notably high U reference beam is subtracted to obtain the overall signal.
values and required numerous restraints and constraints.
Modeling of this THF molecule in multiple locations was demountable cell with a 1.0 mm Teon spacer between a 4 mm
unsuccessful and aer several attempts renement remained and a 2 mm CaF window. Solutions were prepared in a glove
unstable. Ultimately the SQUEEZE command was employed and box with THF as the solvent such that the absorbance at the
Samples were sealed in a Perkin-Elmer rectangular semi-
2
ꢂ
3
˚
excluded 43 e (THF) from a void of 290 A .†
MLCT lmax was 1.0–2.0. The molybdenum samples were excited
at 680 nm and the tungsten samples at 800 nm. The excitation
power at the sample was kept at ꢀ1 mJ. The static sample cell
was manually translated with an XYZ stage between scans to
ensure photo-decomposition was not an issue. The spectra
shown consist of multiple experiments covering a wide range of
IR probe energies. Gaps present in the spectra are due to the
probe wavelengths not overlapping between experiments.
Femtosecond transient absorption measurements
Femtosecond transient absorption (fsTA) experiments were
carried out using laser and detection systems that have been
34
previously described. The molybdenum samples were excited
at 675 nm and the tungsten samples at 800 nm (with excitation
power of 1–2 mJ at the sample) by the output of an optical
parametric amplier (OPA) equipped with a harmonics attach-
ment (UV/Vis). During the measurements the samples were kept
in constant motion by manual movement of an XYZ stage in the
vertical and horizontal directions. To ensure that no photode-
composition occurred during data collection, absorption
spectra were recorded before and aer the TA measurements.
The measurements were repeated several times at each of the
pump–probe delay positions to conrm data reproducibility
throughout the experiment, and the resulting spectra were
Steady state emission spectra
The steady-state visible luminescence spectra between 290 and
840 nm were acquired on a SPEX Fluoromax-2 spectrouo-
rometer. The room temperature data were taken using THF as
the solvent while 2-methyltetrahydrofuran was used as the
solvent for measurements done at 77 K. The steady-state near-IR
luminescence spectra were measured on a home-built instru-
ment utilizing a germanium detector. Compounds MoL, WL
3
5
corrected for the chirp in the white-light continuum. Kinetics
were t in general to a sum of exponential decay components of
the form S(t) ¼ S A exp(ꢂt/s ) + C, with amplitudes A , lifetime,
0
0
and MoL were excited at 658 nm and compound WL was
excited at 785 nm (laser diode max power: 65 mW).
i
i
i
i
s , and offset, C, using Microcal Origin 6.0. Error bars for the
i
Syntheses
lifetime are reported as the standard error of the t.
2-Cyano-3-(4-(diphenylamino)phenyl)acrylic acid (LH). 4-
Diphenylaminobenzaldehyde was prepared by a Vilsmeier for-
mylation reaction between triphenylamine, dimethylforma-
mide (DMF) and POCl . This was followed by the Knoevenagel
3
Nanosecond transient absorption measurements
Nanosecond transient absorption (nsTA) experiments were
36
carried out systems that have been previously described.
condensation of the monoaldehyde with cyanoacetic acid in the
2
Measurements were carried out in 1 ꢄ 1 cm quartz cuvettes
equipped with Kontes stopcocks. Nanosecond transient
absorption spectra were measured on a home-built instrument
pumped by a frequency-tripled (355 nm) Spectra-Physics GCR-
12
presence of piperidine to yield LH.
5
-[4-(Diphenylamino)phenyl]thiophene-2-cyanoacrylic acid
0
(
L H). Triphenylamine was brominated with N-bromosuccini-
38
mide to yield 4-(diphenylamino)bromobenzene. A Stille
coupling between 2-(tributylstannyl)thiophene and 4-(dipheny-
lamino)bromobenzene yielded 2-[4-diphenylamino)phenyl]
150 Nd:YAG laser (fwhm 8 ns, 5 mJ per pulse). The signal from
the photomultiplier tube (Hamamatsu R928) was processed by a
Tektronics 400 MHz oscilloscope (TDS 380).
39
thiophene. This compound was formylated by means of the
Vilsmeier formylation reaction using an adapted procedure.
Briey, DMF (0.47 mL, 6.11 mmol) was cooled to 0 C in an ice
Time-resolved infrared measurements
ꢁ
The time-resolved infrared (TRIR) experiment utilizes
a
bath and then POCl (0.30 mL, 3.05 mmol) was added dropwise.
3
ꢁ
Ti:Sapphire oscillator and regenerative amplier combination The reaction temperature was increased to 30 C and 2-[4-
37
operating at 1 kHz that has been previously described. The (diphenylamino)phenyl]thiophene (0.500 g, 1.53 mmol) was
2
114 | Chem. Sci., 2013, 4, 2105–2116
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added in one portion. The reaction mixture was then heated to
Acknowledgements
ꢁ
45–50 C and stirred for 3 hours. The mixture was poured onto
ice and sodium hydroxide was added until a pH of 10–11 was We thank the National Science Foundation, Grant CHE 095719,
reached. It was then extracted with dichloromethane, dried over and the Ohio State University Materials Research Institute for
magnesium sulfate and concentrated under vacuum to yield a nancial support, and the Ohio Super Computing Center for
dark yellow residue. The residue was puried via column computing resources. Samantha Brown-Xu thanks the National
chromatography (silica gel, CH
-[4-(diphenylamino)phenyl]formyl-thiophene-2-yl as a dark
2 2
Cl
–hexanes, 4 : 1 v/v) to yield Science Foundation for a graduate fellowship.
5
12
yellow oil (yield: 40%). This was followed by a Knoevenagel
condensation of the monoaldehyde with cyanoacetic acid to
yield L H (yield: 74%).
Notes and references
0
13
i
Mo (T PB) [O CC(CN)]CH-C H -NPh ] (MoL). A solution
1 J. Roncali, Acc. Chem. Res., 2009, 42, 1719–1730.
2 N. Cai, Y. Wang, M. Xu, Y. Fan, R. Li, M. Zhang and
P. Wang, Adv. Funct. Mater., 2012, DOI: 10.1002/
adfm.201202562.
2
2
2
6
4
2 2
i
of Mo
added to a suspension of LH (0.098 g, 0.288 mmol) in toluene (ca.
mL). A brown reaction mixture was formed. The reaction was
2 4
(T PB) (0.185 g, 0.156 mmol) in toluene (ca. 5 mL) was
5
stirred for 3 days. The solid was collected by centrifugation and
washed twice with toluene (ca. 5 mL) and once with hexanes (ca.
3 W.-Y. Wong, X.-Z. Wang, Z. He, A. B. Djuri ˇs i ´c , C.-T. Yip,
K.-Y. Cheung, H. Wang, C. S. K. Mak and W.-K. Chan, Nat.
Mater., 2007, 6, 521–527.
10 mL) to yield a red-brown powder. Crystals were grown by
layering a saturated THF solution with hexanes. (Yield: 60%)
4 W.-Y. Wong and P. D. Harvey, Macromol. Rapid Commun.,
2010, 31, 671–713.
NMR (THF-d 400 MHz): 8.36 (s, 2H), 8.07 (d, 4H) 7.43 (m, 8H),
8
7
4
2
.29 (m, 8H), 7.27 (m, 4H), 7.14 (s, 4H), 7.10 (d, 4H), 3.24 (septet,
H), 2.97 (septet, 2H), 1.32 (d, 36H). UV-Vis (THF, 298 K) 569, 412,
5 W.-Y. Wong, Macromol. Chem. Phys., 2008, 209, 14–24.
6 A. Baheti, P. Tyagi, K. R. J. Thomas, Y.-C. Hsu and J. T. Lin,
J. Phys. Chem. C, 2009, 113, 8541–8547.
7 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595–6663.
+
91, 262 nm. MALDI-TOF: found: 1365, M . Calculated: 1371.
i
W
2
(T PB)
2
[O
(0.220 g, 0.162 mmol) in toluene (ca. 5 mL) was added
2
CC(CN)]CH-C
6
H
4
-NPh
2
]
2
(WL). A solution of
i
W
2
(T PB)
4
to a suspension of LH (0.098 g, 0.289 mmol) in toluene (ca. 5
mL). A green suspension was formed. The reaction was stirred
for 3 days. The solid was collected by centrifugation and washed
twice with toluene (ca. 5 mL) and once with hexanes (ca. 10 mL)
8 W.-Y. Wong and C.-L. Ho, Acc. Chem. Res., 2010, 43, 1246–
1256.
9 Y. Liu, X. Wan, B. Yin, J. Zhou, G. Long, S. Yin and Y. Chen,
J. Mater. Chem., 2010, 20, 2464–2468.
to yield a green powder. (Yield: 54%) NMR (THF-d
8
400 MHz): 10 Y.-H. Chen, L.-Y. Lin, C.-W. Lu, F. Lin, Z.-Y. Huang,
7
4
.90 (d, 4H), 7.62 (s, 2H), 7.41 (m, 8H), 7.27 (m, 8H), 7.21 (m,
H), 7.13 (m, 8H), 2.99 (m, 6H), 1.31 (d, 12H), 1.27 (d, 24H). UV-
H.-W. Lin, P.-H. Wang, Y.-H. Liu, K.-T. Wong, J. Wen,
D. J. Miller and S. B. Darling, J. Am. Chem. Soc., 2012, 134,
13616–13623.
Vis (THF, 298 K) 900, 422, 289, 262 nm. MALDI-TOF: found:
1
+
544, M . Calculated: 1540.
11 B. O'Regan and M. Gr ¨a tzel, Nature, 1991, 353, 737–740.
12 C. Teng, X. Yang, S. Li, M. Cheng, A. Hagfeldt, L. Wu and
L. Sun, Chem.–Eur. J., 2010, 16, 13127–13138.
i
0
Mo (T PB) [O CC(CN)]CH-C H S-C H -NPh ] (MoL ).
A
2
2
2
i
4
2
6
4
2 2
solution of Mo (T PB) (0.135 g, 0.114 mmol) in toluene (ca. 5 mL)
was added to a suspension of L H (0.090 g, 0.213 mmol) in 13 D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura,
2
4
0
toluene (ca. 5 mL). A dark green suspension was formed. The
P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt and L. Sun,
reaction was stirred for 3 days. The solid was collected by
J. Org. Chem., 2007, 72, 9550–9556.
centrifugation and washed twice with toluene (ca. 5 mL) and once 14 B. G. Alberding, M. H. Chisholm, B. J. Lear, V. Naseri and
with hexanes (ca. 10 mL) to yield a brown powder. Crystals were C. R. Reed, Dalton Trans., 2011, 40, 10658–10663.
grown through vapor diffusion of pentane into a THF solution. 15 M. H. Chisholm, T. L. Gustafson and C. Turro, Acc. Chem.
Yield: 45%) NMR (THF-d 400 MHz): 8.57 (s, 2H), 8.11 (d, 2H),
Res., 2012, 46, 529–538.
.74 (d, 4H), 7.71 (d, 2H), 7.38 (m, 8H), 7.14 (m, 16H), 7.02 (m, 4H) 16 B. G. Alberding, S. E. Brown-Xu, M. H. Chisholm,
(
8
7
3
4
.02 (m, 6H), 1.29 (d, 12H), 1.09 (d, 24H). UV-Vis (THF, 298 K) 609,
J. C. Gallucci, T. L. Gustafson, V. Naseri, C. R. Reed and
C. Turro, Dalton Trans., 2012, 41, 12270–12281.
17 B. G. Alberding, M. H. Chisholm and T. L. Gustafson, Inorg.
Chem., 2012, 51, 491–498.
+
57, 303 nm. MALDI-TOF: found: 1530, M . Calculated: 1533.3.
i
0
W
2
(T PB)
solution of W
2
[O
2
CC(CN)]CH-C
4
H
2
S-C
6
H
4
-NPh
2
]
2
(WL ).
A
i
2
(T PB)
4
(0.184 g, 0.136 mmol) in toluene (ca. 5 mL)
0
was added to a suspension of L H (0.074 g, 0.174 mmol) in 18 V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni,
toluene (ca. 5 mL). A yellow green suspension was formed. The
reaction was stirred for 3 days. The solid was collected by 19 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser
centrifugation and washed twice with toluene (ca. 5 mL) and once
and A. von Zelewsky, Coord. Chem. Rev., 1988, 84, 85–277.
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Chem. Rev., 1996, 96, 759–834.
3
2
6
3
4%) NMR (THF-d
H), 7.60 (d, 4H), 7.34 (m, 8H), 7.17 (m, 12H), 7.13 (m, 8H) 3.0 (m,
H), 1.30 (d, 12H), 1.27 (d, 24H). UV-Vis (THF, 298 K) 1004, 465, 21 F. A. Cotton, L. M. Daniels, E. A. Hillard and C. A. Murillo,
01 nm. MALDI-TOF: found: 1704.6, M . Calculated: 1705.
8
400 MHz): 7.72 (s, 2H) 7.66 (s, 2H), 7.64 (s,
T. L. Gustafson, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 8152–
8156.
+
Inorg. Chem., 2002, 41, 1639–1644.
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