Bespoke photoreductants: Tungsten arylisocyanides

Article abstract of DOI:10.1021/ja510973h

Modular syntheses of oligoarylisocyanide ligands that are derivatives of 2,6-diisopropylphenyl isocyanide (CNdipp) have been developed; tungsten complexes incorporating these oligoarylisocyanide ligands exhibit intense metal-to-ligand charge-transfer visible absorptions that are red-shifted and more intense than those of the parent W(CNdipp)₆ complex. Additionally, these W(CNAr)₆ complexes have enhanced excited-state properties, including longer lifetimes and very high quantum yields. The decay kinetics of electronically excited W(CNAr)₆ complexes (W(CNAr)₆) show solvent dependences; faster decay is observed in higher dielectric solvents. W(CNAr)₆ lifetimes are temperature dependent, suggestive of a strong coupling nonradiative decay mechanism that promotes repopulation of the ground state. Notably, W(CNAr)₆ complexes are exceptionally strong reductants: [W(CNAr)₆]⁺/W(CNAr)₆ potentials are more negative than -2.7 V vs [Cp₂Fe]⁺/Cp₂Fe. (Figure Presented).

Full text of DOI:10.1021/ja510973h

Article
pubs.acs.org/JACS
Bespoke Photoreductants: Tungsten Arylisocyanides
Wesley Sattler, Lawrence M. Henling, Jay R. Winkler, and Harry B. Gray*
Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Modular syntheses of oligoarylisocyanide ligands that
are derivatives of 2,6-diisopropylphenyl isocyanide (CNdipp) have
been developed; tungsten complexes incorporating these oligoar-
ylisocyanide ligands exhibit intense metal-to-ligand charge-transfer
visible absorptions that are red-shifted and more intense than those
of the parent W(CNdipp)6 complex. Additionally, these W(CNAr)6
complexes have enhanced excited-state properties, including longer
lifetimes and very high quantum yields. The decay kinetics of
electronically excited W(CNAr)6 complexes (*W(CNAr)6) show
solvent dependences; faster decay is observed in higher dielectric
solvents. *W(CNAr)6 lifetimes are temperature dependent,
suggestive of a strong coupling nonradiative decay mechanism
that promotes repopulation of the ground state. Notably, *W(CNAr)6 complexes are exceptionally strong reductants:
[W(CNAr)6]+/*W(CNAr)6 potentials are more negative than −2.7 V vs [Cp2Fe]+/Cp2Fe.
ylisocyanide tungsten complexes that possess even more
remarkable spectroscopic, photophysical, and photochemical
properties.
INTRODUCTION
■
The optical properties of luminescent molecules determine
their suitability for applications in solar energy concentration,1
lighting devices,2 and biological imaging.3,4 Luminescence is an
indicator of long-lived (>ns) electronic excited states that can
initiate chemical reactions unavailable to ground-state mole-
cules.5−7 This photosensitization capacity of luminophores is
finding renewed purpose in solar fuels research,8,9 as well as
inorganic and organic synthesis.10,11 In this regard, powerfully
reducing, organic-solvent soluble, photosensitizers with long
excited-state lifetimes are in particular demand.12 To deliver
photosensitizers custom-tailored for targeted applications, we
must acquire a deeper understanding of the photophysical
properties of luminescent molecules.
In our search for tunable new photosensitizers, we found that
M(CNAr)6 complexes of group 6 metals (M = Cr, Mo, and W)
are promising candidates, as they absorb strongly in the visible
region (400−550 nm) and luminesce with excited-state
lifetimes (τ) up to ∼80 ns, increasing when moving down
the periodic table.13,14 Notably, a 2,6-diisopropylphenyl
isocyanide (CNdipp)15 complex, W(CNdipp)6, was shown to
be a very strong photoreductant (τ ∼ 75 ns and E°(W+/*W0) =
−2.8 V vs Cp2Fe+/0 in THF; * denotes lowest energy excited
state).16 Rapid photoinduced electron transfer (ET) from
W(CNdipp)6 to anthracene (E° = −2.50 V in glyme),17,18
benzophenone (E° = −2.30 V in THF)17 and cobalticenium
ion (E° = −1.35 V in THF)17 was observed in THF solutions.
Anthracene was an especially interesting case, where both ET
and excitation energy transfer (EET) occurred, with the ET/
EET ratio tunable by variation of electrolyte ([nBu4N][PF6])
concentration.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. Increasing
the size of the ortho substituents (Me to Pr) on the
i
arylisocyanide ligands resulted in longer excited-state lifetimes
and greater photostabilities for W(CNAr)6 complexes.14,16 We
have now extended our studies to include functionalization at
the para position of CNdipp. The following methodology
allowed for four new oligoarylisocyanides, derivatives of
CNdipp, to be synthesized in two steps from a single synthetic
intermediate, N-formyl-4-bromo-2,6-diisopropylaniline.19 Suzu-
ki coupling of an arylboronic acid with N-formyl-4-bromo-2,6-
diisopropylaniline, followed by dehydration with OPCl3
(Scheme 1) afforded the oligoarylisocyanides (Figure 1).
OMe3
CNdippPh, CNdippPhOMe , and CNdippPh
are biaryliso-
2
cyanides, whereas CNdippPhPh is a terarylisocyanide.
In prior work,16 we obtained W(CNAr)6 complexes via
reduction of WCl6 with sodium amalgam, Na(Hg), in the
presence of free arylisocyanide (similar to the procedure used
by Yamamoto and co-workers).20 Product yields, however,
were low (28% for W(CNdipp)6), and purifications were
tedious due to tacky, viscous reaction mixtures. We developed
21
an improved synthesis from WCl4(THF)2 (Scheme 2) that
results in higher product yields and easier purification.
Crystalline samples of red W(CNAr)6 complexes, W-
(CNdipp)6, W(CNdippPh)6, W(CNdippPhPh)6, W-
Encouraged by these findings, we have extended our work to
include the synthesis and study of new homoleptic oligoar-
Received: October 24, 2014
© XXXX American Chemical Society
A
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Scheme 1
Table 1. Average Bond Lengths (Å), Bond Angles (deg), and
Dihedral Angles (deg) between Aryl Planes of W(CNAr)6
Complexes
a
W(CNAr)6
d(W−C)
d(CN)
∠(C−N−C)
φAr−Ar
b
W(CNdipp)6
2.062
2.045
2.055
2.056
2.046
1.176
1.176
1.171
1.175
1.168
164.4
171.2
166.3
169.4
171.7
n.a.
c
W(CNdippPh)6
W(CNdippPhPh)6
W(CNdippPhOMe
W(CNdippPhOMe
35.0
d
33.9
2
)
)
34.3
50.9
6
6
3
a
Determined by calculating the best least-squares plane through the
b
aryl carbons using the MPLN command in SHELXTL. Data taken
c
from ref 16. Average of two molecules in the asymmetric unit.
d
Average dihedral angle between the two aryl planes proximal to the
tungsten center.
in Table 1. Notably, in all of the W(CNAr)6 solid-state
structures, the orientation of the π-systems for trans
arylisocyanide ligands are approximately coplanar (Figure 2).
This orientation appears to be enforced by the bulky ortho
isopropyl groups, as the trans arylisocyanide π-systems in
W(CNXy)6 (Xy = 2,6-dimethylphenyl) are approximately
orthogonal.22−24 The absorption spectra of the oligoarylisocya-
nide tungsten complexes are similar to that of W(CNdipp)6
(vide infra), not W(CNXy)6. It is noteworthy that W(CNPh)6
has an absorption spectrum similar to that of W(CNXy)6.14 We
suggest that the lowest energy absorption is a property of the
fully conjugated orientation (coplanar trans arylisocyanide π-
systems), which is the lowest energy conformation with ortho
Figure 1. Arylisocyanides.
Scheme 2
isopropyl groups.16 Finally, the average dihedral angles (φAr−Ar
)
of the biaryl rings moieties are ∼35° for W(CNdippPh)6,
W(CNdippPhPh)6, and W(CNdippPhOMe )6 and slightly larger
2
(∼51°) for W(CNdippPhOMe )6. We attribute the larger
3
dihedral angle in W(CNdippPhOMe )6 to steric repulsion from
two meta methoxy groups oriented toward the metal center,
3
OMe3
(CNdippPhOMe )6, and W(CNdippPh
)6, were prepared in
2
whereas only one methoxy group points toward the tungsten
yields ranging from 65 to 87%.
center in W(CNdippPhOMe )6.
2
The molecular structures of the new W(CNAr)6 complexes
were determined by single-crystal X-ray diffraction (W-
The isocyanide stretching frequencies and 13C NMR
chemical shifts for the W(CNAr)6 complexes are very similar.
The CN vibrations for the complexes are broad and intense,
ranging from 1938−1949 cm−1 as thin films drop cast from
C6H6 solutions. In C6D6 solutions, the isocyanide 13C NMR
chemical shifts are all nearly identical (177.3−177.7 ppm). The
free oligoarylisocyanides also exhibit virtually identical iso-
cyanide stretching frequencies (ν(CN), 2111−2115 cm−1 as
thin films drop cast from C6H6 solutions and 2118−2119 cm−1
in CH2Cl2 solutions) and 13C NMR chemical shifts (δ =
169.1−169.3 ppm in CDCl3 solutions).
(CNdippPhOMe )6 shown in Figure 2). Examination of the
2
W−C bond lengths and C−N−C bond angles indicates that
the para substituents impart little structural change proximal to
the tungsten center; selected metrical parameters are provided
Absorption and Luminescence Spectra. Absorption and
luminescence spectra of W(CNAr)6 complexes in THF
solutions are shown in Figure 3 (spectra recorded in toluene
and MeCN solutions can be found in the SI). All of the
W(CNAr)6 complexes absorb extremely strongly in the visible
region, owing to highly allowed metal-to-ligand charge-transfer
(MLCT) transitions. Interestingly, the lowest energy MLCT
absorption maxima for oligoarylisocyanide complexes are not
only red-shifted compared to W(CNdipp)6, as would be
expected due to the increased conjugation, but also are more
intense. W(CNdipp)6 has a maximum extinction coefficient of
∼9.5 × 104 M−1 cm−1 at 462−464 nm, while values for the
biarylisocyanide complexes are roughly 1.3 × 105 M−1 cm−1 at
495−496 nm and ∼1.6 × 105 M−1 cm−1 at 506 nm for
W(CNdippPhPh)6. For comparison, the MLCT band in the
Figure 2. Molecular structure of W(CNdippPhOMe )6; 45° perspective
view; hydrogen atoms removed for clarity.
2
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observed when moving from W(CNdipp)6 to W(CNdippPh)6,
OMe3
W(CNdippPhOMe )6 and W(CNdippPh
)6, and further to
2
W(CNdippPhPh)6. W(CNdippPhOMe )6 is slightly blue-shifted
3
from W(CNdippPh)6 and W(CNdippPhOMe )6, which may be
2
attributed to the electronic influence of (i) the methoxy group
at the para position and/or (ii) the greater torsion angle
between the biaryl planes in W(CNdippPhOMe )6.
3
The luminescence bands red-shift and broaden with
increasing solvent polarity (full-width at half-maximum values
(fwhm) listed in Table 2), which implies that the excited states
are able to relax more due to solvent molecule reorientation in
higher dielectric media, consistent with a lower-symmetry
charge-transfer excited state that has an increased dipole
moment compared to the ground state. *W(CNdippPhOMe
)
2
6
and *W(CNdippPhOMe )6 are distorted significantly in MeCN,
apparent by the ∼70% increase in luminescence fwhm
compared with toluene (Table 2). For comparison, [Ru-
(bpy)3]2+ has a fwhm of ∼2750−3030 cm−1 depending on the
solvent employed.27
3
Figure 3. Absorption (solid lines) and emission (dotted lines) spectra
of W(CNdipp)6 (cyan/circle), W(CNdippPh)6 (red/square), W-
(CNdippPhPh)6 (blue/diamond), W(CNdippPhOMe
) (black/trian-
2
6
gle), and W(CNdippPhOMe )6 (maroon/hexagram) in THF solutions.
3
spectrum of [Ru(bpy)3]2+ is much weaker (∼1.45 × 104 M−1
cm−1 at 452 nm).25
It is also interesting to note the difference in energy of the
luminescence maximum (vmax) when changing solvent. W-
̃
(CNdipp)6 has only a 60 cm−1 decrease in vmax when changing
̃
Such intense absorptions in the visible region are remarkable;
we suggest that the intensities are due to the high degree of
delocalization and spatial overlap between the ground- and
excited-state wavefunctions (i.e., there is considerable charge-
transfer character in both ground and excited states).26 One
may view these MLCT transitions as excitation of an electron
from an orbital that has significant tungsten 5dπ and CN π*
character (t2g in idealized Oh symmetry) to one with mainly CN
π* character mixed with tungsten 6p- character (t1u and t2u,
Oh). Although there is substantial tungsten 5d delocalization in
the ground state, the net dipole is small due to the symmetry of
the molecules, as the absorption bands are insensitive to
changes in solvent static dielectric constants. Notably, each
W(CNAr)6 complex exhibits a shoulder with significant
extinction on the low-energy side of the most intense visible
MLCT band, which we assign to a series of singlet to triplet
transitions.
from toluene to THF solution, whereas W(CNdippPh)6,
W(CNdippPhOMe )6, and W(CNdippPh
) decrease by
6
OMe3
2
220−290 cm−1, and W(CNdippPhPh)6 decreases by 650
cm−1. This trend implies that W(CNAr)6 complexes containing
additional aryl groups have greater excited-state dipoles, which
are stabilized to a greater degree in more polar solvents.
Photoluminescence quantum yield (ϕPL, Table 3) measure-
ments were performed under optically dilute conditions by
comparison to [Ru(bpy)3][PF6]2 in MeCN (ϕPL = 0.062).27,28
The oligoarylisocyanide complexes all have ϕPL values of ∼0.4
in toluene solution, in striking contrast to W(CNdipp)6, which
has ϕPL ∼ 0.03. Switching from toluene to THF solutions
lowers ϕPL for W(CNdipp)6, W(CNdippPh)6, W-
OMe3
(CNdippPhOMe )6, and W(CNdippPh
) by ∼50%; and
2
6
even more dramatically for W(CNdippPhPh)6, where ϕPL drops
by a factor of 6 (Table 3). Finally, W(CNdippPhOMe )6 and
2
W(CNdippPhOMe )6 in MeCN solutions have reduced ϕPL
3
The most remarkable property of the oligoarylisocyanide
complexes is their brilliant luminescence in solution. Under
ambient light, toluene solutions of W(CNdippPh)6, W-
values, 0.01 and 0.02, respectively. These luminescence data
lead to the following conclusions about the photophysics of
W(CNAr)6 complexes: the excited states of the oligoaryliso-
cyanide complexes are more luminescent than complexes with
monoarylisocyanide ligands; addition of methoxy groups to the
biaryl ring has little effect; solvent plays an important role in the
excited-state decay dynamics; and greater excited-state dipoles
relaxed by solvent polarization (increased fwhm values) in
more polar solvents decrease ϕPL values.
OMe3
(CNdippPhPh)6, W(CNdippPhOMe )6, and W(CNdippPh
)
2
6
glow brightly, emitting yellow to red light easily observed by
eye. The spectacular photoluminescence observed from the
oligoarylisocyanide complexes prompted us to examine their
photophysics in more detail.
Luminescence spectra obtained from dilute THF solutions of
the W(CNAr)6 complexes are shown in Figure 3. Emission
spectra taken in both toluene and THF show similar energy
trends as the corresponding absorptions: red-shifting is
Excited-State Decay Kinetics. *W(CNAr)6 lifetimes (τ)
were determined in a variety of solvents (Table 3, 2-MeTHF in
a
Table 2. W(CNAr)6 Emission Maxima (λmax in nm and v in cm−1) and FWHM values (cm−1)
̃
max
toluene
THF
MeCN
W(CNAr)6
λmax (v
)
fwhm
λmax (v
)
fwhm
λmax (v )
max
fwhm
b
̃
̃
̃
max
max
b
W(CNdipp)6
575 (17300)
617 (16200)
629 (15800)
618 (16100)
612 (16300)
1610
1880
1970
1890
1850
577 (17300)
626 (15900)
656 (15200)
627 (15800)
623 (16000)
1750
2250
2670
2280
2180
b
b
b
b
W(CNdippPh)6
W(CNdippPhPh)6
W(CNdippPhOMe
W(CNdippPhOMe
2
)
)
670 (14700)
650 (15200)
3190
3050
6
6
3
a
Prior to determining emission maxima and fwhm values, the numbers of photons at a given wavelength (λ) were corrected to the wavenumber (v)
̃
scale by using the relationship, I(v) = I(λ) × λ2.29 Not soluble in MeCN.
b
̃
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a
Table 3. W(CNAr)6 Excited-State Decay Parameters in Toluene, THF, and MeCN Solutions
toluene
THF
kr (s−1
W(CNAr)6
τ
ϕPL
kr (s−1
)
knr (s−1
)
τ
ϕPL
)
knr (s−1
)
W(CNdipp)6
122 ns
1.73 μs
1.53 μs
1.65 μs
1.83 μs
0.03
0.41
0.44
0.42
0.41
2.3 × 105
2.4 × 105
2.9 × 105
2.6 × 105
2.2 × 105
8.0 × 106
3.4 × 105
3.7 × 105
3.5 × 105
3.2 × 105
75 ns
1.32 μs
350 ns
1.20 μs
1.56 μs
0.01
0.21
0.07
0.21
0.25
1.6 × 105
1.6 × 105
1.9 × 105
1.8 × 105
1.6 × 105
1.3 × 107
6.0 × 105
2.7 × 106
6.6 × 105
4.8 × 105
W(CNdippPh)6
W(CNdippPhPh)6
W(CNdippPhOMe
W(CNdippPhOMe
2
)
)
6
6
3
a
Values for W(CNdippPhOMe )6 in MeCN are τ = 94 ns, ϕPL = 0.01, kr = 9.6 × 104 s−1, knr = 1.1 × 107 s−1; values for W(CNdippPhOMe )6 in MeCN
2
3
are τ = 210 ns, ϕPL = 0.02, kr = 9.8 × 104 s−1, knr = 4.7 × 106 s−1.
SI). The oligoarylisocyanide complexes have longer-lived
excited states than W(CNdipp)6 (Figure 4 shows luminescence
this large decrease that leads to the bright luminescence. In
toluene solutions, *W(CNdippPh)6, *W(CNdippPhPh)6, *W-
(CNdippPhOMe )6, and *W(CNdippPh
) have virtually
6
OMe3
2
identical decay kinetics. Interestingly, in THF solution,
*W(CNdippPhPh)6 shows faster nonradiative decay; the
increase in knr by about an order of magnitude when changing
from toluene to THF is consistent with the larger distortion
(larger fwhm) in THF solution (Table 2). Increased distortions
in the excited state usually lead to faster nonradiative decay.31
While this does appear to be the case for each complex when
moving to more polar solvents, it is not the general case for the
W(CNAr)6 complexes. Specifically, W(CNdipp)6 has the
narrowest luminescence profile, implying minor distortions in
the excited state; this situation would be expected to produce
slow nonradiative decay. The opposite situation occurs,
however, where the oligoarylisocyanide complexes exhibit
slow nonradiative decay and broader luminescence bands
than W(CNdipp)6. W(CNdippPhPh)6 has the fastest radiative
decay in both toluene and THF, a finding consistent with the
overall greater absorptivity of W(CNdippPhPh)6 compared to
the other W(CNAr)6 complexes.32
Figure 4. Time-resolved luminescence traces (λex = 488 nm, 8 ns
pulse) of W(CNdipp)6 (cyan/circle), W(CNdippPh)6 (red/square),
W(CNdippPhPh)6 (blue/diamond), W(CNdippPhOMe )6 (black/trian-
2
gle), and W(CNdippPhOMe )6 (maroon/hexagram) in THF solutions.
3
Steady-state and time-resolved luminescence experiments on
W(CNAr)6 complexes also were performed at 77 K in toluene
and 2-methyltetrahydrofuran (2-MeTHF) glasses. In each case
there is sharpening on the high-energy side of the luminescence
profile, consistent with hot bands losing intensity at 77 K (SI).
Emission observed at λmax for each complex. The time at which the
intensities cross e−1 (horizontal dashed line) give the time constants.
decays in THF solutions), and τ values are smaller in more
polar solvents. Specifically, in going from toluene to THF
solutions, *W(CNAr)6 lifetimes decrease, most dramatically for
W(CNdippPhPh)6 (almost 80%). In comparison, τ values for
E
00 values estimated from these spectra are given in Table 4. As
*W(CNdipp)6, *W(CNdippPh)6, *W(CNdippPhOMe )6, and
2
Table 4. Ground-State and Excited-State Reduction
Potentials (V vs Cp2Fe+/0
*W(CNdippPhOMe
)
decrease by ∼15−40%. *W-
3
)
6
(CNdippPhOMe
) ) in MeCN sol-
and *W(CNdippPhOMe
2
3
6
6
a
b
W(CNAr)6
E°(W+/W0)
E00
E°(W+/*W0)
utions show a pronounced change, where lifetimes drop by
close to 90% compared to values in toluene solutions.
The collected data can be analyzed to determine the radiative
and nonradiative decay rate constants (kr and knr) as shown in
eq 1:30
W(CNdipp)6
−0.72
−0.68
−0.67
−0.65
−0.65
2.28
2.12
2.08
2.14
2.15
−3.00
−2.80
−2.75
−2.79
W(CNdippPh)6
W(CNdippPhPh)6
W(CNdippPhOMe
W(CNdippPhOMe
2
)
)
6
3
−2.80
6
1
a
b
Room temperature, 0.5 M [nBu4N][PF6]/CH2Cl2. 77 K in 2-
τ =
ϕPL = τkr
kr + knr
(1)
MeTHF.
Radiative decay rate constants for *W(CNAr)6 in toluene,
THF, and MeCN solutions (Table 3) are fairly constant in each
solvent, ranging from 2.2 to 2.9 × 105 and 1.6 to 1.9 × 105 s−1
in toluene and THF solutions, respectively. In MeCN,
expected, E00 values decrease in the order W(CNdipp)6 ≫
OMe2
W(CNdippPhOMe )6 > W(CNdippPh)6, W(CNdippPh
)6 >
3
W(CNdippPhPh)6. Interestingly, *W(CNAr)6 are significantly
longer lived at 77 K but do not follow single exponential decay
kinetics as expected for a single emitting excited state (SI).
While a single lifetime value does not exist for each W(CNAr)6
complex at 77 K, mean lifetimes can be obtained by integration
of the normalized luminescence traces, giving values which
range from 5.1−6.1 μs in toluene and 8.2−10.9 μs in 2-MeTHF
(see SI, Table S2). At this point, a tentative explanation is that
W(CNAr)6 complexes in frozen glasses have multiple static
OMe3
*W(CNdippPhOMe )6 and *W(CNdippPh
)6 have slightly
2
smaller kr values. Conversely, knr varies greatly, spanning almost
2 orders of magnitude. Thus, nonradiative decay is key in
determining W(CNAr)6 excited-state dynamics.
Further analysis of these data lead to the following
conclusions. First, and most notable, in a given solvent there
is a large decrease (over 1 order of magnitude) in knr for the
oligoarylisocyanide complexes relative to W(CNdipp)6; it is
D
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Journal of the American Chemical Society
Article
confirmations, giving rise to a distribution of excited-state
lifetimes. Regardless, these data imply that the faster room
temperature process is a thermally activated process (vide
infra).
favorable ground-state electrochemical properties: cyclic
voltammograms of W(CNAr)6 complexes at a platinum
working electrode in 0.5 M CH2Cl2 solution with [nBu4N]-
[PF6] as the supporting electrolyte (performed under dim
lighting conditions inside a nitrogen filled glovebox) exhibit
reversible waves at E° ∼ −0.7 V assigned to W+/W0 couples
(Table 4). The neutral oligoarylisocyanide complexes are all
slightly harder to oxidize (∼50 mV shift in E°) than
W(CNdipp)6. Similar to W(CNdipp)6 in THF solutions,16
irreversible oxidation events were observed in CH2Cl2 solutions
when scanning positive of the W+/W0 couple for all of the
W(CNAr)6 complexes. No evidence for a W0/W− couple was
found within the solvent window (−2.3 V).
Excited-State Decay Pathways. As radiative decay rate
constants are generally temperature independent,32 it follows
that increases in lifetimes when going from room temperature
to 77 K are due to slower nonradiative decay processes.
Temperature dependence in nonradiative decay kinetics implies
strong-coupling behavior (considerable displacement along the
horizontal axis of one electronic state’s potential energy surface
with respect to another),33 where a thermally activated pathway
is largely responsible for decay to the ground state. Although
additional in-depth variable temperature spectroscopic studies
will provide more insight into the decay process (e.g., apparent
activation energy, important vibrational frequencies), our
finding that the excited-state decay kinetics for W(CNAr)6
complexes are approximately the same in a specific glass at 77 K
suggests that a strongly coupled pathway is active for room
temperature decay.34 This type of behavior observed for
[Ru(bpy)3]2+ has been attributed to thermal activation to a
ligand field (LF) excited state.35,36
While the ground-state W+/W0 potentials span only ∼50 mV
for the W(CNAr)6 complexes, the excited-state energies range
over ∼200 meV (Table 4). The combination of these
electrochemical and photophysical data gives estimated
excited-state reduction potentials, E°(W+/*W0), ranging from
−2.7 to −3.0 V, with W(CNdipp)6 being the strongest
photoreductant. The W+/W0 couple for W(CNdipp)6 is
∼200 mV more negative in CH2Cl2 than in THF.16
It is highly unlikely that LF states of W(CNAr)6 complexes
are accessible for thermal activation from the T1u MLCT
Scheme 3
3
excited state. Estimates of the harmonic potential surfaces of
1
1
3
5
the T1g, T2g, T2g, and T2g LF states of W(CNAr)6 from
W(CO)6 spectroscopic data37 (the ligand field strength of
ArNC is comparable to that of CO)38 clearly show EFC(5T2g ←
1A1g) > 50,000 cm−1. Calculations of minima on the W(CO)6
potential surfaces place the lowest LF state (3T1g) well above
(E0 ≫ 25,000 cm−1) the ground state (see SI).39,40 As
luminescence proceeds from an MLCT excited state with E0 ∼
18,300 cm−1, a lower estimate (assuming same distortion
coordinate) of the activation energy required to populate any
LF state would be >8000 cm−1. Our finding that LF states of
W(CNAr)6 complexes are not accessible thermally is consistent
with the observation that photodissociation does not occur.
As mentioned above, the 77 K excited-state lifetimes of the
W(CNAr)6 complexes are ∼50% longer in 2-MeTHF than in
toluene; if we assume that the lifetimes at 77 K are
approximately equal to kr−1 (valid if the quantum yields are
close to unity, which appears to be the case for the W(CNAr)6
complexes), then kr will be ∼50% larger in toluene than in 2-
MeTHF. Interestingly, comparison to room temperature data
in toluene and THF shows a similar trend (kr ∼ 2.2−2.9 × 105
s−1 in toluene and 1.6−1.9 × 105 s−1 in THF). We conclude
that the transition between the ground state and the emitting
state has greater oscillator strength in toluene than in THF/2-
MeTHF.
The reactions of *W(CNdipp)6 and *W(CNdippPhOMe
)
6
2
with benzophenone and acetophenone41 are outlined in
Scheme 3: *W(CNAr)6, generated with a 488 nm laser pulse,
rapidly reduces benzophenone or acetophenone (second-order
rate constant, kq, Table 5) to give [W(CNAr)6]+ and a ketyl
Table 5. ET Quenching Rate Constants (kq in M−1 s−1) and
Reagent (Q) Concentrations Needed to Quench 50% of
*W(CNAr)6
a
benzophenone
acetophenone
W(CNAr)6
kq
[Q]50%
kq
[Q]50%
W(CNdipp)6
W(CNdippPhOMe
1.0 × 1010
2.7 × 109
1.3 mM
0.3 mM
2.2 × 108
2.1 × 106
0.06 M
0.40 M
2
)
6
a
Data for W(CNdipp)6 and benzophenone taken from ref 16.
It is apparent that employing oligoarylisocyanides rather than
monoarylisocyanides as ligands in W(CNAr)6 complexes has a
major impact on nonradiative decay behavior. For each
individual complex, ϕPL, emission fwhm, τ, and knr are all
directly related, implying that more pronounced excited-state
distortions in higher dielectric solvents promote nonradiative
decay. Addition of methoxy substituents on the distal aryl ring
)6) imparts higher
solubility in polar solvents but has little effect on photophysics
in these relatively nonpolar solvents.
Photoredox Reactions. We have demonstrated that
W(CNdipp)6 can effect visible light driven reductions of
challenging substrates.16 The oligoarylisocyanide complexes
have considerably longer lived excited states, along with
radical anion.42 Then the ET quenching products thermally
revert to ground-state reactants. As expected, ET from
*W(CNdippPhOMe
) is slower than from *W(CNdipp)6,
6
2
especially in the case of acetophenone, where the specific
quenching rate is attenuated by approximately 2 orders of
magnitude, likely attributable to a drop in driving force (Table
4) as well as an effect of the greater size (i.e., greater ET
OMe3
(W(CNdippPhOMe )6 and W(CNdippPh
2
distance) in the reaction with *W(CNdippPhOMe )6.
2
Although reactions with *W(CNdippPhOMe )6 are slower, its
2
longer lifetime may render it a more versatile photoredox
reagent. The concentrations needed to quench equivalent
amounts of excited photosensitizers depend both on τ and kq;
E
DOI: 10.1021/ja510973h
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
[nBu4N][PF6] as the supporting electrolyte. Sample concentrations
were approximately 1 mM.
concentrations needed to quench 50% of W(CNdipp)6 and
W(CNdippPhOMe )6 are listed in Table 5.
2
Photochemical Methods. UV−vis absorption measurements
were carried out using a Cary 50 UV−vis spectrophotometer with 1
cm path length quartz cuvettes. Steady-state and time-resolved
spectroscopic measurements were carried out in the Beckman Institute
Laser Resource Center (California Institute of Technology). Emission
and excitation spectra were recorded on a Jobin Yvon Spec Fluorolog-
3-11. Sample excitation was achieved via a xenon arc lamp with
wavelength selection provided by a monochromator. Right angle
luminescence was sorted using a monochromator and detected with a
Hamamatsu photomultiplier tube (PMT) with photon counting
(PMT model R928P for all spectra with the exception of
Of note, the best known inorganic photosensitizer,
ruthenium tris-bipyridine ([Ru(bpy)3]2+ bpy = 2,2′-bipyridine),
which has been investigated for over 40 years, has been
employed extensively in photoredox catalysis.10,11,25 The use of
[Ru(bpy)3]2+ in organic synthesis is limited by its reducing
potential (E°(Ru3+/*Ru2+) = −1.2 V vs Cp2Fe+/0 in acetonitrile
(MeCN) solutions).25 MacMillan and co-workers recently
demonstrated direct β-functionalization of cyclic ketones with
aryl ketones using photoredox and organocatalysis.12 While
these investigators were able directly to reduce diaryl ketones
such as benzophenone using fac-Ir(ppy)3 (ppy = (2-pyridinyl-
κN)phenyl-κ2C; E°(Ir4+/*Ir3+) = −2.1 V vs Cp2Fe+/0 in MeCN
solution),43,44 reduction of aryl alkyl ketones (e.g., acetophe-
none) was not observed. Electronically excited W(CNAr)6
complexes are capable of reducing such refractory substrates,
which should prove to be useful in driving catalytic photoredox
reactions.
OMe3
W(CNdippPhOMe )6 and W(CNdippPh
model R2658P was used).
)6 in MeCN, where PMT
2
For time-resolved measurements, laser excitation was provided by 8
ns pulses from a Q-switched Nd:YAG laser (Spectra-Physics Quanta-
Ray PRO-Series) operating at 10 Hz. The third harmonic was used to
pump an optical parametric oscillator (OPO, Spectra-Physics Quanta-
Ray MOPO-700) tunable in the visible region to provide laser pulses
at 488 nm. Probe light for transient absorption kinetics measurements
was provided by a 75-W arc lamp (PTI Model A 1010) that could be
operated in continuous wave or pulsed modes. After passing through
the sample collinearly with the laser beam, scattered excitation light
was rejected by suitable long pass and short pass filters, and probe
wavelengths were selected for detection by a double monochromator
(Instruments SA DH-10) with 1 mm slits. Transmitted light was
detected with a photomultiplier tube (PMT, Hamamatsu R928). The
PMT current was amplified and recorded with a GageScope transient
digitizer. The data were converted to units of ΔOD (ΔOD =
−log10(I/I0), where I is the time-resolved probe-light intensity with
laser excitation, and I0 is the intensity without excitation). Data were
averaged over approximately 100 shots. All instruments and electronics
in these systems were controlled by software written in LabVIEW
(National Instruments). Data manipulation was performed with either
MATLAB R2012a or MATLAB R2013a (Mathworks, Inc.).
CONCLUSIONS AND OUTLOOK
■
Development of a modular synthetic method for oligoaryliso-
cyanides has opened the way for the synthesis of new
W(CNAr)6 complexes. These complexes, which have extremely
rich photophysical and photochemical properties, are sure to
find application. Additional experimental and theoretical efforts
could shed new light on the electronic structures of their
ground and excited states, leading to further photosensitizer
customization.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations and spectroscopic
and electrochemical measurments were performed using a combina-
tion of glovebox, high vacuum, and Schlenk techniques under a
nitrogen or argon atmosphere.45 Solvents were purified and degassed
by standard procedures. NMR spectra were acquired at room
temperature unless otherwise noted through the use of Varian
Probe light for transient absorption spectra was provided by flash
lamps with either nanosecond or microsecond durations. Probe light
was transported via optical fiber and split by a partial reflector.
Approximately 70% of the probe light passed through the sample, the
remainder directed around the sample as a reference beam. Sample
excitation (λex = 488 nm) by the laser beam was collinear with the
probe light. Sample and reference beams were coupled by optical fibers
to a spectrograph and detected using two photodiode arrays (Ocean
Optics S1024DW Deep Well Spectrometer), with scattered excitation
light rejected by a 488 nm narrow notch filter. The timing
synchronization of the laser fire, flashlamp fire, and photodiode array
readout was controlled by a series of timing circuits triggered by either
a Q-switch advance logic pulse for nanosecond or a laser lamp sync
pulse for microsecond, lamp measurements. The photodiode readout
was interfaced with a PC via a National Instruments multifunction
input/output card. Measurements were made with and without
excitation, corrected for dark readout, and corrected for fluorescence
when necessary. Difference spectra were averaged over approximately
160 shots. All instruments and electronics in these systems were
controlled by software written in LabVIEW (National Instruments).
X-ray Structure Determinations. X-ray diffraction data were
collected on either a Bruker Kappa Apex II four circle diffractometer
1
spectrometers. H NMR chemical shifts are reported in ppm relative
to SiMe4 (δ = 0) and were referenced internally with respect to the
protio solvent impurity (δ 7.16 for C6D5H, 7.26 for CHCl3).46
13C{1H} NMR spectra are reported in ppm relative to SiMe4 (δ = 0)
and were referenced internally with respect to the solvent (δ 128.06
for C6D6 and δ 77.16 for CDCl3).45 Coupling constants are given in
hertz. Infrared spectra were recorded either in solution on a Nicolet
Avatar 370 DTGS spectrometer or as a thin film from evaporating a
benzene solution on the surface of a Bruker ALPHA ATR-IR
spectrometer probe (Platinum Sampling Module, diamond, OPUS
software package) at 2 cm−1 resolution and are reported in cm−1.
Samples for transient absorption and room temperature luminescence
measurements were prepared in dry, degassed solvents inside a
nitrogen-filled glovebox, placed into the cell of a high-vacuum 1 cm
path length fused quartz cuvette (Starna Cells), and isolated from
atmosphere by a high-vacuum Teflon valve (Kontes). All chemicals
were obtained from Aldrich. Acetic formic anhydride (HC(O)OC-
(O)Me),47 2,6-diisopropylphenylisocyanide (dippNC),16,48 4-bromo-
21
2,6-diisopropylaniline,49 and WCl4(THF)2 were prepared as
described previously.
Electrochemistry. Electrochemical measurments were made with
a Gamry Reference 600 potentiostat/galvanostat using a standard
three-electrode configuration. A platinum wire was used as the
working electrode. A platinum wire in a fritted (Vycor) glass tube
served as the counter electrode. Ag+/Ag was used as a quasi-reference
electrode, and the ferricenium/ferrocene couple (Cp2Fe+/Cp2Fe)
served as an internal reference. Measurements were performed at
room temperature in either THF solutions with 0.1 M [nBu4N][PF6]
as the supporting electrolyte or CH2Cl2 solutions with 0.5 M
OMe3
(W(CNdippPh)6, W(CNdippPhOMe )6, and W(CNdippPh
)6) or a
2
Bruker SMART 1000 three circle diffractometer (W(CNdippPhPh)6).
Crystal data, data collection, and refinement parameters are
summarized in Table S1. The structures were solved using direct
methods and standard difference map techniques and were refined by
full-matrix least-squares procedures on F2 with SHELXTL (Version
2014/2).50
F
DOI: 10.1021/ja510973h
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(13) Mann, K. R.; Cimolino, M.; Geoffroy, G. L.; Hammond, G. S.;
Orio, A. A.; Albertin, G.; Gray, H. B. Inorg. Chim. Acta 1976, 16, 97−
101.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and crystallographic data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
(14) Mann, K. R.; Gray, H. B.; Hammond, G. S. J. Am. Chem. Soc.
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(15) The abbreviation dipp for 2,6-diisopropylphenyl will be used in
this article, whereas the abbreviation Iph was used in references 14 and
16.
AUTHOR INFORMATION
Corresponding Author
*hbgray@caltech.edu
Notes
■
(16) Sattler, W.; Ener, M. E.; Blakemore, J. D.; Rachford, A. A.;
Labeaume, P. J.; Thackeray, J. W.; Cameron, J. F.; Winkler, J. R.; Gray,
H. B. J. Am. Chem. Soc. 2013, 135, 10614−10617.
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88, 5112−5117.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(19) Diemer, V.; Chaumeil, H.; Defoin, A.; Fort, A.; Boeglin, A.;
■
́
Carre, C. Eur. J. Org. Chem. 2006, 2727−2738.
We thank Michael Takase and David VanderVelde for
assistance with X-ray and NMR experiments. Discussions
with Aaron Rachford, Paul LaBeaume, Jim Thackeray, and Jim
Cameron in the early stages of this work were very helpful. The
Bruker KAPPA APEX II X-ray diffractometer was purchased via
an NSF CRIF:MU award to the California Institute of
Technology (CHE-0639094). Our work is supported by the
National Science Foundation Center for Chemical Innovation
in Solar Fuels (CHE-1305124) and a CCI postdoctoral
fellowship to W.S.
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