Photoinduced electron transfer in rhenium(I)-oligotriarylamine molecules

Article abstract of DOI:10.1021/ic501620g

Two molecular triads with an oligotriarylamine multielectron donor were synthesized and investigated with a view to obtaining charge-separated states in which the oligotriarylamine is oxidized 2-fold. Such photoinduced accumulation of multiple redox equivalents is of interest for artificial photosynthesis. The first triad was comprised of the oligotriarylamine and two rhenium(I) tricarbonyl diimine photosensitizers each of which can potentially accept one electron. In the second triad the oligotriarylamine was connected to anthraquinone, in principle an acceptor of two electrons, via a rhenium(I) tricarbonyl diimine unit. With nanosecond transient absorption spectroscopy (using an ordinary pump-probe technique) no evidence for the generation of 2-fold oxidized oligotriarylamine or 2-fold reduced anthraquinone was found. The key factors limiting the photochemistry of the new triads to simple charge separation of one electron and one hole are discussed, and the insights gained from this study are useful for further research in the area of charge accumulation in purely molecular (nanoparticle-free) systems. An important problem of the rhenium-based systems considered here is the short wavelength required for photoexcitation. In the second triad, photogenerated anthraquinone monoanion is protonated by organic acids, and the resulting semiquinone species leads to an increase in lifetime of the charge-separated state by about an order of magnitude. This shows that the proton-coupled electron transfer chemistry of quinones could be bene ficial for photoinduced charge accumulation. (Chemical Equation Presented).

Full text of DOI:10.1021/ic501620g

Article
pubs.acs.org/IC
Photoinduced Electron Transfer in Rhenium(I)−Oligotriarylamine
Molecules
Annabell G. Bonn, Markus Neuburger, and Oliver S. Wenger*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
S
* Supporting Information
ABSTRACT: Two molecular triads with an oligotriarylamine
multielectron donor were synthesized and investigated with a
view to obtaining charge-separated states in which the
oligotriarylamine is oxidized 2-fold. Such photoinduced
accumulation of multiple redox equivalents is of interest for
artificial photosynthesis. The first triad was comprised of the
oligotriarylamine and two rhenium(I) tricarbonyl diimine
photosensitizers each of which can potentially accept one
electron. In the second triad the oligotriarylamine was
connected to anthraquinone, in principle an acceptor of two
electrons, via a rhenium(I) tricarbonyl diimine unit. With
nanosecond transient absorption spectroscopy (using an
ordinary pump−probe technique) no evidence for the
generation of 2-fold oxidized oligotriarylamine or 2-fold reduced anthraquinone was found. The key factors limiting the
photochemistry of the new triads to simple charge separation of one electron and one hole are discussed, and the insights gained
from this study are useful for further research in the area of charge accumulation in purely molecular (nanoparticle-free) systems.
An important problem of the rhenium-based systems considered here is the short wavelength required for photoexcitation. In the
second triad, photogenerated anthraquinone monoanion is protonated by organic acids, and the resulting semiquinone species
leads to an increase in lifetime of the charge-separated state by about an order of magnitude. This shows that the proton-coupled
electron transfer chemistry of quinones could be beneficial for photoinduced charge accumulation.
signatures, which should be distinguishable from one another
by transient absorption spectroscopy.7 Indeed, a recent study of
OTA−Ru(2,2′-bipyridine)32+−TiO2 systems was able to
provide evidence for the 2-fold oxidation of an OTA molecule
after a two-photon excitation process.5c In our work, we
synthesized a molecule in which an OTA unit is covalently
connected to two rhenium(I) tricarbonyl diimine photo-
sensitizers (Re2−OTA, Scheme 1). We aimed to explore
whether through simultaneous excitation of the two photo-
sensitizers it would be possible to access a (presumably short-
lived) charge-separated state in which the OTA unit is oxidized
twice, while each of the rhenium complexes would be in its one-
electron reduced form. A reference molecule with a simple
triarylamine (TAA) one-electron donor was also investigated
(Re2−TAA, Scheme 1).
Quinones are two-electron acceptors, and the uptake of the
second electron occurs with particular ease in the presence of
hydrogen-bond donors or Brønsted acids.8 9,10-anthraquinone
(AQ) produces relatively clear spectroscopic signatures in its
reduced forms, which are readily detectable by transient
absorption spectroscopy.8b,9 Therefore, we decided to synthe-
size a triad comprised of an OTA donor, a single rhenium(I)
INTRODUCTION
■
Donor−photosensitizer−acceptor molecules have been very
frequently employed for driving-force or distance-dependence
studies of (photoinduced) electron transfer.1 Many important
insights regarding the so-called Marcus inverted region and the
mechanisms of long-range electron transfer were gained by
such investigations.2 In numerous cases relatively long-lived
electron−hole pairs were observed after excitation with visible
light, and the primary charge-separation events in bacterial
photosynthetic reaction centers were successfully mimicked.3
However, as long as single electrons are separated from single
holes, it is difficult to perform much useful secondary chemistry
with photogenerated charge-separated states, because many of
the most interesting fuel-forming reactions require multiple
redox equivalents.4 Light-driven separation of multiple
electrons from multiple holes remains a significant challenge.
Nanoparticles can readily accommodate multiple charges,5 but
in purely molecular systems the accumulation of several
electrons or holes is more difficult, and there are relatively
few experimental studies that have focused on this particular
aspect of photoinduced electron transfer in donor−sensitizer−
acceptor molecules.6
Oligotriarylamines (OTAs) can release multiple electrons at
decent electrochemical potentials, and their one- and two-
electron oxidized forms often have characteristic spectroscopic
Received: July 8, 2014
© XXXX American Chemical Society
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Scheme 1. Chemical Structures of the Key Compounds
Investigated in This Study
Scheme 2. Synthetic Strategies Leading to the Amino-
Decorated Pyridine Ligands Required for the Re2−OTA and
a
Re2−TAA Molecules
a
(a) 4-(N,N-Dimethylamino)pyridine, di-tert-butyl dicarbonate, THF,
reflux, 3 h. (b) BuONa, Pd(dba)2, (HPtBu3)BF4, toluene, reflux. (c)
t
photosensitizer, and an AQ acceptor (OTA−Re−AQ, Scheme
1). The purpose was to explore whether through 2-fold
(consecutive) photoexcitation of the same rhenium(I)
sensitizer a charge-separated state containing OTA2+ and
AQ2− (or protonated forms thereof) could be accessible.
Rhenium(I) tricarbonyl diimine sensitizers are popular
choices for studies of photoinduced electron (and energy)
transfer,10 and we used them because their molecular structure
offered some advantages for the synthesis of our target
molecules. The p-xylene linkers ensure a suitable balance
between conformational dynamics facilitating electron transfer
and good solubility.11
CF3COOH, acetone, 20 °C; (d) Pd(PPh3)4, THF/H2O, Na2CO3,
reflux, 24 h.
of isolated pure Re2−OTA and Re2−TAA were 37% and 50%,
respectively.
The synthetic pathway leading to the AQ−Re−OTA triad is
shown in Scheme 3. 2-Bromo-9,10-anthraquinone (11) was
Scheme 3. Synthetic Strategy Leading to the AQ−Re−OTA
a
Triad
We were not able to detect any 2-fold oxidized OTA or 2-
fold reduced AQ in either of the systems from Scheme 1 but
gained some valuable insights, which we consider useful for
future research in the area of generating two-electron oxidized
or two-electron reduced photoproducts in purely molecular
systems. In addition, our study provides insight into the
influence of acids of different strengths on the accessible
photoproducts (and their lifetimes) in systems with quinone
acceptors. The proton-coupled electron transfer (PCET)
chemistry of quinones could indeed prove to be useful for
the accumulation of multiple electrons on such units.
RESULTS AND DISCUSSION
■
Synthesis and Crystallographic Studies. The triaryl-
amine- and oligotriarylamine-decorated ligands of the Re2−
TAA and Re2−OTA complexes were synthesized following the
strategy outlined in Scheme 2. The starting point was
commercial bis(4-bromophenyl)amine (1), which was pro-
tected with a Boc group (2) prior to performing Buchwald−
Hartwig coupling with dianisylamine (3).7c The coupling
product (4) was deprotected,12 and the resulting secondary
amine (5) was reacted with compound 8, which in turn was
synthesized from 1,3,5-tribromobenzene (6) and pyridine-4-
boronic acid hydrate (7). Compounds 5 and 8 were isolated in
pure forms with yields of 84% and 36%, respectively, and the
final coupling reaction to afford ligand 9 proceeded with a yield
of 71%. Ligand 10 was obtained in 83% yield via Pd-catalyzed
N−C coupling between dianisylamine (3) and compound 8.
Coordination of ligands 9 and 10 to the final rhenium(I)
complexes was accomplished using the [Re(bpy)(CO)3(OTf)]
precursor (bpy = 2,2′-bipyridine, OTf = triflate).13 The yields
a
(a) Pd(PPh3)4, toluene/EtOH/H2O, Na2CO3, reflux. (b) ICl,
CH2Cl2/CH3CN, 0 °C. (c) Pd(PPh3)4, m-xylene, reflux, 70 h. (d)
Re(CO)5Cl, toluene, reflux. (e) CF3SO3H, CH2Cl2, Et2O, 20 °C. (f)
tBuOK, Pd(dba)2, (HPtBu3)BF4, toluene, 100 °C. (g) CH3OH,
CHCl3, reflux, 43 h.
coupled to 4-trimethylsilyl-2,5-dimethylphenylboronic acid
(12)14 using the Pd(PPh3)4 catalyst. The coupling product
(13) was deprotected with ICl, and the resulting iodo
compound (14) was coupled to 5-(tri(n-butyl)stannyl)-2,2′-
bipyridine (15)15 to afford the final anthraquinone-equipped
bpy ligand (16) in 73% global yield. For this part of the
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Figure 1. Crystallographic structure of oligotriarylamine 5. Anisotropic displacement parameters are drawn at the 50% probability level.
synthesis we followed our own previously published strategy.16
Coordination to rhenium(I) occurred by reacting ligand 16
with pentacarbonylchlororhenium(I), followed by exchanging
the chloro ligand of complex 17 by a very weakly coordinating
triflate (18). The OTA-decorated pyridine ligand (22) was
obtained by reacting pyridine-4-boronic acid hydrate (7) with
1-bromo-4-trimethylsilyl-2,5-dimethylbenzene (19), followed
by deprotection of the trimethylsilyl group of the coupling
product (20) with ICl. The resulting iodo compound (21) was
coupled to oligotriarylamine 5 to afford ligand 22, which was
reacted with complex 18 to give AQ−Re−OTA in 47% yield.
All key compounds were characterized by 1H NMR
spectroscopy, high-resolution ESI mass spectrometry, and
CHN elemental analysis. Their NMR and mass spectra are
shown in the Supporting Information, and detailed synthetic
procedures and product characterization data for all new
compounds appearing in Schemes 1−3 can be found in the
Experimental Section.
(CO)3(py)]+ (py = pyridine) reference complex (ref, black
trace) the molecules from Scheme 1 have a significantly higher
molar extinction coefficient, which is presumably an effect of
increased π-conjugation.11b,17 The luminescence emitted by
Re2−OTA, Re2−TAA, and AQ−Re−OTA in CH3CN
following excitation at 350 nm is significantly weaker than
that of [Re(bpy)(CO)3(py)]+ (Figure 2b), and this is due to
excited-state quenching by intramolecular electron transfer as
demonstrated below.
Cyclic voltammograms of [Re(bpy)(CO)3(py)]+, Re2−TAA,
Re2−OTA, and AQ−Re−OTA measured in CH3CN with 0.1
M TBAPF6 are shown in Figure 3. Some of the voltammograms
Single crystals of oligotriarylamine 5 were obtained by slow
evaporation of an acetone solution. The result of an X-ray
diffraction study is shown in Figure 1, and crystallographic
details are in the Supporting Information. The key observation
in Figure 1 is the propeller-shaped structure of all triarylamino
units, which is a common feature of this class of compounds
and which is responsible for their relatively low basicity, an
aspect that will become important in the optical spectroscopic
studies presented below.
Optical Spectroscopy and Electrochemistry. In Figure
2a the optical absorption spectra of the three compounds from
Scheme 1 in CH3CN are shown. Compared to the [Re(bpy)-
Figure 3. Cyclic voltammograms of the [Re(bpy)(CO)3(py)]+
reference complex (ref) and the three compounds from Scheme 1
in dry CH3CN with 0.1 M TBAPF6. The voltage sweep rate was 0.1 V/
s; the waves at −0.51 V are due to decamethylferrocene, which was
added in small quantities for internal voltage calibration. (a, b, and d)
Oxidative (solid lines) and reductive (dashed lines) sweeps were
performed separately because this gave higher quality results than
scans over the entire −2.7 to 1.7 V vs Fc+/Fc range.
were recorded in two separate voltage sweeps, one extending
from −0.8 V vs Fc+/Fc to positive potentials (solid lines) and
another one between −0.4 V vs Fc+/Fc and more negative
potentials (dashed lines) because this gave higher quality
results. The waves at −0.51 V are due to decamethylferrocene,
which was added in small quantities for internal voltage
calibration.18
Figure 2. (a) Optical absorption spectra of the three compounds from
Scheme 1 and [Re(bpy)(CO)3(py)]+ (ref) in CH3CN. (b)
Normalized luminescence spectra of the same four compounds in
deoxygenated CH3CN obtained after excitation at 350 nm. The
relative intensities of the individual luminescence spectra were
corrected for differences in absorbance at the excitation wavelength.
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Table 1. Reduction Potentials (in volts vs Fc+/Fc) for the Various Electrochemically Active Components of Re2−TAA, Re2−
OTA, AQ−Re−OTA, and [Re(bpy)(CO)3(py)]OTf (ref) in CH3CN as Determined from the Cyclic Voltammetry Data in
a
Figure 3
compound
amine+/0
amine2+/+
amine3+/2+
Re2+/+
bpy0/−
AQ0/−
AQ−/2−
Re2−TAA
Re2−OTA
AQ−Re−OTA
ref
0.46
−0.07
−0.05
1.23
0.88
0.95
1.37
−1.76
−1.88
−1.8
0.25
0.21
0.65
0.74
−1.29
−1.8
−1.77
a
Peak-to-peak separations for the various reversible and quasi-reversible oxidation and reduction processes are given in the Supporting Information
(Table S1).
Irreversible rhenium-based oxidations are detected at peak
potentials ranging from 1.37 to 0.88 V vs Fc+/Fc (Table 1), in
line with prior reports.10a,19 Evidently, the attachment of
electron-rich amines (TAA, OTA) to the rhenium complexes
makes them easier to oxidize. In Re2−TAA (Figure 3, green
trace) oxidation of the TAA group occurs at 0.46 V vs Fc+/Fc
as commonly observed,20 and this is a reversible process as long
as the voltage sweep does not extend over the rhenium
oxidation. In Re2−OTA and AQ−Re−OTA two consecutive
and reversible one-electron oxidation processes associated with
the OTA unit are detected at ca. −0.05 V and near 0.2 V vs
Fc+/Fc (Table 1). A third OTA-based one-electron oxidation
process near 0.7 V vs Fc+/Fc is irreversible. Thus, OTA is a far
stronger donor than TAA, and OTA may in fact donate two
electrons at less positive potentials than what is required for
one-electron oxidation of TAA. Reductions based on the bpy-
ligands occur near −1.8 V vs Fc+/Fc in all four compounds, in
line with prior studies.9d,19b,c In AQ−Re−OTA this overlaps
with the second one-electron reduction of the AQ moiety; its
first one-electron reduction occurs at −1.29 V vs Fc+/Fc (Table
1).8b
In Figure 4a the spectral changes associated with the addition
of increasing amounts of Cu(ClO4)2 to Re2−TAA in CH3CN
are shown. Cu(ClO4)2 can be used to oxidize TAA by one
electron.20a Below 320 nm the absorbance decreases upon
converting Re2−TAA to Re2−TAA+, whereas between 330 and
420 nm it increases. The most prominent feature, however, is a
new absorption band centered ∼770 nm, which is typical for
triarylamine monocations.9b,20a,21 In Figure 4b the transient
absorption spectrum detected after excitation of a 35 μM
solution of Re2−TAA in CH3CN at 355 nm is shown. Laser
pulses of ∼10 ns duration were employed, and detection
occurred by time-averaging over the first 200 ns immediately
after the pulses. Bands at 360 and 770 nm are observed,
compatible with the formation of TAA+ as the comparison with
Figure 4a shows readily. Reduction of the bpy ligand of one of
the rhenium complexes is expected to result in a bleach below
300 nm,22 but this cannot be detected because the optical
density of the solutions useable for transient absorption
spectroscopy is too high in the relevant spectral range.
Nevertheless, the formation of a charge-separated state
comprised of TAA+ and a reduced rhenium complex after
photoexcitation of Re2−TAA is undisputable. On the basis of
the redox potentials from Table 1 and assuming a triplet metal-
to-ligand charge transfer (3MLCT) energy of ∼2.8 eV for the
[Re(bpy)(CO)3(py)]+ unit, electron transfer from TAA to the
photosensitizer is associated with a reaction free energy
(ΔGET0) of approximately −0.6 eV. In freeze−pump−thaw
deoxygenated CH3CN at 25 °C this charge-separated state has
a lifetime of 39 ns (Supporting Information, Figure S1a),
Figure 4. (a) Changes in optical absorption spectra of Re2−TAA in
CH3CN upon addition of increasing amounts of Cu(ClO4)2. (b)
Transient difference spectrum recorded on a 35 μM solution of Re2−
TAA in deoxygenated CH3CN following excitation at 355 nm with
laser pulses of ∼10 ns duration. The spectrum was measured by time-
averaging over a 200 ns period following immediately after excitation.
(c) Changes in optical absorption spectra of Re2−OTA in CH3CN
upon addition of increasing amounts of Cu(ClO4)2. (d) Transient
difference spectrum recorded on a 10 μM solution of Re2−OTA in
deoxygenated CH3CN under identical conditions as those in (b).
similar to what was reported for other rhenium-based dyads
with various redox partners under comparable conditions.23
Analogous experiments were performed with Re2−OTA, and
the outcome of the chemical oxidation with Cu(ClO4)2 in
CH3CN is shown in Figure 4c. Cu(ClO4)2 is able to oxidize
OTA to OTA+ and OTA2+.20a The one-electron oxidized form
of OTA exhibits absorption maxima at 425, 780, and 1320 nm
and thus resembles TAA+. By contrast, OTA2+ displays
absorptions maximizing at 600 and 1140 nm and hence is
spectroscopically clearly distinct from OTA+, particularly in the
near-infrared (NIR) spectral range. In Figure 4d the transient
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absorption spectrum of a 10 μM solution of Re2−OTA in
CH3CN recorded under analogous conditions as described
above for Re2−TAA is shown. This spectrum shows all the
features expected for OTA+, and we conclude that the main
photoproduct is a charge-separated state comprised of OTA+
of the transient absorption spectra and the fact that OTA2+
photoproducts might be significantly shorter-lived than the
main (OTA+) photoproduct.
Aside from the multitude of open decay channels for two-
electron photoproducts, the two-photon excitation process
itself is already challenging even when sufficiently high photon
fluxes can be used. One possible pitfall is quenching of the
3MLCT-excited photosensitizer by energy transfer once OTA+
is present because the latter has low-energy absorptions.
However, luminescence lifetime measurements performed on
Re2−TAA+ and Re2−OTA+ in deoxygenated CH3CN (pro-
duced by chemical oxidation with Cu(ClO4)2) demonstrate
that the 3MLCT lifetime in both cases is on the order of 20 ns,
similar to what is measured for the Re2−TAA and Re2−OTA
compounds before amine oxidation (data not shown).
However, even with energy transfer quenching being relatively
inefficient, there is still the possibility that once the photo-
products identified on the basis of Figure 4 have been formed
via absorption of one photon, the absorption of a second
photon by the yet unreacted rhenium center will actually
0
and a reduced rhenium complex (ΔGET ≈ −1.0 eV), in
complete analogy to the Re2−TAA system. In deoxygenated
CH3CN this state has a lifetime of 36 ns (Supporting
Information, Figure S1b), very similar to what was found for
Re2−TAA.
The observation that OTA+ dominates the transient
absorption spectrum in Figure 4d is no surprise. If OTA2+ is
generated at all, the population of a charge-separated state
comprised of two one-electron reduced rhenium complexes and
an OTA2+ unit is likely to be small compared to the population
of the main photoproduct detected in Figure 4d. The reasons
for this are simple: Generation of OTA2+ requires two-photon
excitation, which is inherently less efficient than a one-photon
process. In addition, more decay pathways are open for the
more energy-storing state comprised of OTA2+ and two
reduced rhenium complexes than for the simple charge-
separated state that dominates the spectrum in Figure 4d.
Two-photon processes usually exhibit a quadratic dependence
on the laser excitation power.24 Hence, we reasoned that when
performing variations in the excitation density we might be able
to detect spectral changes that are indicative of OTA2+
formation. In Figure 5a the transient absorption spectra
3
induce (reverse) electron transfer from the MLCT-excited
photosensitizer to TAA+ or OTA+.
Compared to the Re2−OTA dyad, the AQ−Re−OTA triad
has the advantage that electrons and holes can be separated
from each other over a greater distance, potentially leading to
longer lifetimes, which in turn will simplify photoproduct
detection.9b,23e Its main disadvantage is that it contains only
one photosensitizer. Hence, after its initial excitation, formation
of a first charge-separated state comprised of OTA+ and AQ−
coupled to relaxation of the sensitizer to the ground state would
have to occur within less than 10 ns, such that the second
excitation of the same rhenium complex can still occur within
the duration of the same laser pulse.25 On the basis of our own
recent studies of triarylamine−ruthenium/osmium−anthraqui-
none triads this is not an unreasonable expectation because in
these triads the charge-separated state containing TAA+ and
AQ− was formed within ∼200 ps.9b,d This double-excitation
process would resemble the sequence of ground-state
absorption and excited-state absorption in upconversion
materials, in which the same chromophore is excited twice
within the same ∼10 ns laser pulse.26 For photoinduced two-
electron transfer double-excitation with two separate laser
pulses has been previously achieved in a system containing
Figure 5. (a) Transient absorption spectra recorded on a 10 μM
solution of Re2−OTA in deoxygenated CH3CN after excitation at 355
nm. Detection occurred by time-averaging over 200 ns immediately
after excitation with laser pulses of ∼10 ns duration. The black and
blue spectra were recorded using different laser powers. The blue trace
was multiplied by a factor of 3.45 (= 3.8/1.1). The red trace is the
difference between the black and blue spectra. (b) Analogous sets of
data for a 10 μM solution of AQ−Re−OTA in deoxygenated CH3CN,
using a multiplication factor of 3.09 (= 3.4/1.1) for the blue trace.
5b
2+
Ru(bpy)3 and TiO2 nanoparticles.
Chemical oxidation of AQ−Re−OTA in CH3CN by
Cu(ClO4)2 leads to similar results as for Re2−OTA (Figure
6a). One- and two-electron oxidation products are clearly
distinguishable from each other; nearly the same spectral
features as in Figure 4c are detected for OTA+ and OTA2+. The
transient absorption spectrum averaged over a 200 ns time
window after excitation of a 10 μM solution of AQ−Re−OTA
in pure CH3CN at 355 nm (Figure 6b, black trace) exhibits
absorption maxima at 1280, 563, and 428 nm along with a
bleach at 330 nm. The bands at 1280 and 428 nm are
compatible with formation of OTA+, and based on prior studies
the absorption at 563 nm and the bleach at 330 nm can be
attributed unambiguously to AQ−.8b,9 An increase in laser
excitation power from 1.1 to 3.4 mJ per pulse did not result in
any changes in the transient absorption spectrum, which could
reasonably be interpreted in terms of a manifestation of OTA2+
or AQ2− (Figure 5b).
recorded with laser excitation powers of 3.8 (black) and 1.1
(blue) mJ per pulse are shown. The blue trace was multiplied
by a factor of 3.45 (the quotient of 3.8 mJ and 1.1 mJ). The red
trace is the result of a subtraction of the blue trace from the
black spectrum. As seen above (Figure 4c), OTA2+ has a
significantly higher molar extinction coefficient at 600 nm than
that of OTA+; hence, with increasing laser power one could
have expected increasing transient absorption at 600 nm.
However, this effect cannot be detected in the red difference
spectrum of Figure 5a, and we must conclude that there is no
evidence for the formation of OTA2+ under these conditions.
Higher excitation powers lead to sample decomposition, and
detection in the NIR spectral range is hampered by the
comparatively poor sensitivity of the NIR detector. A further
complication might come from the necessity for time-averaging
The transient absorption signals for the AQ−−Re−OTA+
state appear immediately after excitation with pulses of ∼10 ns
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relatively stable hydroquinones can be formed.8,27 Therefore,
we explored the photochemistry of the AQ−Re−OTA triad in
CH3CN in presence of 0.2 M benzoic acid and 0.2 M
chloroacetic acid; the respective transient absorption spectra are
shown as orange and blue traces in Figure 6b. Even in the
presence of such high concentrations of organic acids, spectral
features clearly attributable to OTA+ can be detected near 410
and 1280 nm, indicating that OTA is not protonated and can
still undergo electron transfer under these conditions. Triaryl-
amines are poorly basic compared to other tertiary amines due
to their propeller-like structure, and evidently this is also the
case for OTA (Figure 1). Significant differences between the
three transient absorption spectra from Figure 6b occur in the
spectral range between 625 and 475 nm: the band detected at
563 nm in pure CH3CN (black trace), attributed above to AQ−,
shifts to 510 nm in presence of 0.2 M benzoic acid (orange)
and finally disappears (or is hidden below the band at 410 nm)
in presence of 0.2 M chloroacetic acid (blue). The bleach at
330 nm persists. All transient absorption signals in CH3CN
with 0.2 M benzoic acid decay in a biexponential manner with
average lifetimes of 100 and 2250 ns, respectively (Figure 7b,
Table 2). In CH3CN with 0.2 M chloroacetic acid the decays
are single-exponential with an average lifetime of 3680 ns
(Figure 7c, Table 2).
Figure 6. (a) Changes in optical absorption spectra of AQ−Re−OTA
in CH3CN upon addition of increasing amounts of Cu(ClO4)2. (b)
Transient difference spectra recorded on a 10 μM solution of AQ−
Re−OTA in deoxygenated CH3CN following excitation at 355 nm
with laser pulses of ∼10 ns duration. The spectra were recorded by
time-averaging over a 200 ns period following immediately after
excitation.
duration (Figure 7a), indicating their instantaneous formation.
Consequently, the photosensitizer returns to its electronic
Table 2. Lifetimes of the Detectable Photoproducts in
Deoxygenated Solvents
τ [ns]
CH3CN 0.2 M
C6H5COOH
CH3CN 0.2 M
ClCH2COOH
compound
CH3CN
Re2−TAA
39
Re2−OTA
36
AQ−Re−OTA
205
100/2250
3680
The combined observations of anthraquinone-related
spectral band-shifts (Figure 6b) and an increase in photo-
product lifetime upon addition of organic acids (Figure 7b,c)
are compatible with the formation of the protonated (semi-
quinone) form of anthraquinone (AQH).8b,28 The occurrence
of biexponential decays in the presence of benzoic acid (Figure
7b) with one decay component resembling that detected in
pure CH3CN (205 vs 100 ns, Table 2) and a second
component similar to that observed in the presence of 0.2 M
chloroacetic acid (2250 vs 3680 ns, Table 2) suggests that the
presence of 0.2 M benzoic acid leads to a mixture of AQ− and
AQH photoproducts. The pKa of AQH in CH3CN is not
known,8b but given the acidity constants reported for benzoic
acid and chloroacetic acid in CH3CN (21.5 vs 18.8),29 it is clear
that the latter will protonate AQ− more readily than benzoic
acid. Thus, in the presence of acid, proton-coupled electron
transfer (PCET) occurs with our AQ−Re−OTA triad, similar
to the electrochemically induced PCET chemistry reported
earlier for quinone-based systems.8a Our findings are in line
with recent studies in which hydrogen-bond donating solvents
(e. g., hexafluoroisopropanol, trifluoroethanol) were found to
increase the lifetimes of charge-separated states with quinone
acceptors,16,28,30 and they are similar to what was observed
earlier for the primary charge separation events of bacterical
photosynthesis.31
Figure 7. (a) Decays of the transient absorption signals at 790 (red),
565 (green), 428 (blue), and 390 nm (black) of a 10 μM solution of
AQ−Re−OTA in deoxygenated CH3CN following excitation at 355
nm with laser pulses of ∼10 ns duration. (b) Transient absorption
decays recorded at 790 (red), 670 (green), and 420 nm (black) in an
analogous experiment in the presence of 0.2 M benzoic acid. (c)
Transient absorption decays recorded at 1240 (red), 790 (green), 675
(blue), and 410 nm (black) in an analogous experiment in presence of
0.2 M chloroacetic acid.
ground state very rapidly and re-excitation with a second
photon within the duration of one pulse would indeed appear
to be possible, as anticipated above. Irrespective of detection
wavelength, the transient absorption signals associated with the
AQ−−Re−OTA+ state exhibit a lifetime of 205 ns in
deoxygenated CH3CN.
Two-electron reduction of quinones occurs with particular
ease in protic solvents or, better, in presence of acids because
F
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and tetrahydrofuran (THF) were dried in a solvent purification system
from Innovative Technology. Dry toluene was bought from Sigma-
Aldrich (crown-capped, over molecular sieve). Silicycle silica gel (40−
63 mm) was used for column chromatography. Thin-layer
chromatography was performed on silica gel plates (60 F254) from
Merck.
SUMMARY AND CONCLUSIONS
■
The lack of evidence for two-electron photoredox products
(OTA2+, AQ2−) in Re2−OTA and AQ−Re−OTA can have
several origins. A key factor is the relatively short excitation
wavelength (355 nm), which is required to excite these
rhenium(I) tricarbonyl diimine based systems. This is
detrimental in several respects: First of all, light of this
wavelength does not lead to selective excitation of the
photosensitizer but (based on the UV−vis spectra in Figure
2) occurs at least partly into absorptions localized on OTA and
AQ. Second, the photoredox product OTA+ has a large molar
extinction coefficient at 355 nm (Figure 4c, Figure 6a);
consequently, once a first photoinduced electron transfer step
has occurred, excitation of OTA+ might interfere with the
desired processes that would lead to OTA2+. Third, the
photodamage threshold of the investigated molecules for 355
nm excitation is relatively low, and the phototriggered loss of
CO ligands at the rhenium photosensitizer is a well-known
phenomenon.32 We consider the use of a photosensitizer that
permits excitation at longer wavelengths of pivotal importance
for our future endeavors of detecting two-electron photoredox
products in purely molecular (nanoparticle-free) systems.
Obviously this remains a significant challenge because
numerous decay channels are open for two-photon excited
intermediates and two-electron photoredox products,4b,5b,c,6a,33
and it is difficult to design a system in which undesired (energy-
wasting) electron and energy transfer processes can be
efficiently suppressed. Pump−pump−probe experiments that
make use of two temporally delayed excitation pulses (possibly
even of different wavelengths) would certainly represent a
major advantage, particularly when performed with high
temporal resolution (<10 ns) to detect very short-lived
photoproducts.5c,6e In our specific cases explored in this
study, the applicable photon flux was likely too low. We
estimate that for the useable laser excitation powers and
concentrations, the photon flux was on the order of 1−2
photons per molecule per pulse. With sufficiently high photon
fluxes a possible additional problem is two-photon ionization of
the rhenium(I) photosensitizer (producing solvated electrons),
similar to what has been previously observed for Ru(bpy)23+.34
In principle, the consecutive 2-fold excitation of the
photosensitizer in the AQ−Re−OTA triad should be possible
within the duration of a 10 ns laser pulse because the AQ−−
Re−OTA+ state is formed very rapidly (Figure 7).25 In closely
related ruthenium- and osmium-based triads full charge-
separation with an electron on the acceptor and a hole on
the donor took as little as ∼200 ps.9d,35 The use of one single
photosensitizer for accessing two-electron photoredox prod-
ucts, albeit certainly challenging, should therefore in principle
be possible, at least when using relatively long (∼10 ns) and
sufficiently strong excitation pulses.
Compound 2. For the synthesis of this molecule we followed a
previously published procedure.7c Commercially available bis(4-
bromophenyl)amine 1 (2.00 g, 6.12 mmol), 4-(N,N-dimethylamino)-
pyridine (149.5 mg, 1.22 mmol), and di-tert-butyl dicarbonate (2.11
mg, 9.17 mmol) were dissolved in dry THF (15 mL) under N2. The
yellow solution was heated to reflux for 3 h. After it was cooled to
room temperature and the subsequent removal of the solvent on a
rotary evaporator, the crude orange solid was filtered over SiO2 with a
2:1 (v:v) pentane/dichloromethane eluent mixture, which afforded the
1
pure product as a white solid (2.60 g, 6.09 mmol, ∼100%). H NMR
(400 MHz, CDCl3) δ: 7.45−7.41 (m, 4H), 7.08−7.04 (m, 4 H), 1.44
(s, 9 H).
Compound 4. This synthesis was performed by adapting a
previously published protocol.7c Compound 2 (1.00 g, 2.34 mmol),
t
commercially available dianisylamine 3, (1.34 g, 5.86 mmol), BuONa
(4.5 g, 46.8 mmol), Pd(dba)2 (67.2 mg, 0.12 mmol), and (HPtBu3)BF4
(34.2 mg, 0.12 mmol) were dissolved in dry and deoxygenated toluene
(30 mL) under nitrogen. The reaction mixture was heated to 120 °C
for 19 h. Water (100 mL) was added to the cooled mixture, and the
aqueous phase was extracted with dichloromethane (3 × 50 mL). The
combined organic phases were dried over anhydrous MgSO4, and
subsequently the solvent was evaporated. Column chromatography on
silica gel with dichloromethane as the eluent gave the product as a
1
beige solid (1.42 g, 1.96 mmol, 84%). H NMR (400 MHz, acetone-
d6) δ: 7.09−7.05 (m, 4 H), 7.04−7.00 (m, 8 H), 6.91−6.87 (m, 8 H),
6.82−6.78 (m, 4 H), 3.78 (s, 12 H), 1.41 (s, 9 H).
Compound 5. This reaction step was performed following a
published procedure.12 Compound 4 (2.00 g, 2.76 mmol) was
dissolved in acetone (40 mL) under nitrogen. Trifluoroacetic acid
(TFA, 10 mL, 0.13 mol) was added drop by drop, and the solution was
stirred at room temperature overnight. Then the reaction mixture was
evaporated to dryness. Column chromatography on basic alumina with
ethyl acetate as the eluent yielded the product as a beige solid (1.70 g,
2.73 mmol, 99%). Single crystals for X-ray diffraction were obtained
from acetone solution by slow evaporation. 1H NMR (400 MHz,
acetone-d6 with a drop of TFA) δ: 7.40−7.37 (m, 4 H), 7.18−7.14 (m,
8 H), 6.98−6.94 (m, 8 H), 6.87−6.83 (m, 4 H), 3.80 (s, 12 H).
Compound 8. 1,3,5-Tribromobenzene 6 (0.50 g, 1.59 mmol),
commercial pyridine-4-boronic acid hydrate 7 (586.3 mg, 4.77 mmol),
and Na2CO3 (3.00 g, 28.3 mmol) were suspended in a mixture
comprised of water (10 mL) and THF (20 mL). After deoxygenating
the mixture by bubbling N2 for 30 min, Pd(PPh3)4 (184 mg, 0.16
mmol) was added. Then the reaction mixture was deoxygenated
further, and finally it was heated to 90 °C for 24 h. After it was cooled
to room temperature, water was added (50 mL), and the mixture was
extracted with dichloromethane (3 × 50 mL). The combined organic
phases were dried over anhydrous Na2SO4, and the solvents were
removed on a rotary evaporator. Purification by column chromatog-
raphy on silica gel using dichloromethane with 2% of triethylamine
yielded the desired product as a light green solid (0.18 g, 0.58 mmol,
36%). 1H NMR (400 MHz, CDCl3) δ: 8.75−8.69 (m, 4 H), 7.83 (d, 2
H, J = 1.6 Hz), 7.77 (t, 1 H, J = 1.6 Hz), 7.54−7.49 (m, 4 H).
Ligand 9. Compound 8 (180.5 mg, 0.58 mmol), compound 5 (0.30
g, 0.48 mmol), tBuONa (1.11 g, 11.6 mmol), Pd(dba)2 (13.8 mg, 0.02
mmol), and (HPtBu3)BF4 (6.90 mg, 0.02 mmol) were dissolved in dry
and deoxygenated toluene (25 mL) under N2. The reaction mixture
was heated to 125 °C for 19 h. After it cooled to room temperature,
water (100 mL) was added, and the mixture was extracted with
dichloromethane (3 × 50 mL). After drying over anhydrous MgSO4
the organic solvents were evaporated, and the crude product was
purified by column chromatography on silica gel using dichloro-
methane with 2% triethylamine as the eluent. Subsequent recrystalliza-
tion from hexane gave the product as an olive-green solid (292 mg,
Our study of the AQ−Re−OTA triad shows that
anthraquinone monoanion (AQ−) and the protonated semi-
quinone form (AQH) are distinguishable by UV−vis transient
absorption spectroscopy. This might be useful for studies of
photoinduced PCET with quinones.36 Thus, the PCET
chemistry of quinones can potentially be exploited for
accumulation of multiple electrons on such units. We are
currently exploring this possibility.
EXPERIMENTAL SECTION
Synthesis and Product Characterization. Commercially avail-
able chemicals were used as received. Dichloromethane, diethyl ether,
■
1
0.34 mmol, 71%). H NMR (400 MHz, CDCl3) δ: 8.66 (d, 4 H, J =
G
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4.7 Hz), 7.44 (d, 4 H, J = 6.2 Hz), 7.29 (s, 3 H), 7.08−7.04 (m, 8 H),
7.02−6.99 (m, 4 H), 6.91−6.87 (m, 4 H), 6.85−6.81 (m, 8 H), 3.79
(s, 12 H).
(m, 2 H), 7.79−7.76 (dd, 1 H, J = 8.0, 1.9 Hz), 7.40 (s, 1 H), 7.11 (s, 1
H), 2.49 (s, 3 H), 2.30 (s, 3 H), 0.38 (s, 9 H).
Compound 14. Compound 13 (1.97 g, 5.12 mmol) was dissolved in
dichloromethane (10 mL) under N2 and cooled to 0 °C. A solution of
ICl (563 μL, 10.8 mmol) in acetonitrile (40 mL) was added very
slowly, and the resulting suspension was stirred at 0 °C for 15 min.
Then the ice bath was removed, and the reaction mixture was stirred at
room temperature overnight. Following addition of aqueous Na2S2O3
(5%, 200 mL), the mixture was extracted with dichloromethane (3 ×
50 mL). The combined organic phases were washed with water (50
mL) and then dried over anhydrous Na2SO4. After evaporation to
Re2−OTA. Ligand 9 (0.10 g, 0.12 mmol) and [Re(bpy)(CO)3OTf]
(148.2 mg, 0.26 mmol)13 were dissolved in a mixture of methanol (10
mL) and chloroform (3 mL). After deoxygenating for 25 min, the
reaction mixture was heated to 85 °C for 1.5 d. Then the solvents were
evaporated, and the crude product was purified by column
chromatography on silica gel. The eluent was a mixture of pure
acetone, deionized water, and saturated aqueous KNO3 solution in the
ratio of 200:9:1 (v/v/v). An orange solid was obtained, and this was
dissolved in water (50 mL) and extracted with dichloromethane (3 ×
50 mL). The combined organic phases were dried over anhydrous
Na2SO4 prior to evaporating the solvent. The solid residue was
recrystallized from hexane, yielding the desired complex in the form of
its nitrate salt as a yellow-brownish solid (83 mg, 0.04 mmol, 37%). 1H
NMR (400 MHz, CD3CN) δ: 9.23 (ddd, 4 H, J = 5.5, 1.6, 0.7 Hz),
8.40 (dt, 4 H, J = 8.3, 1.1 Hz), 8.27 (td, 4 H, J = 7.9, 1.6 Hz), 8.23−
8.19 (m, 4 H), 7.80 (ddd, 4 H, J = 7.7, 5.5, 1.3 Hz), 7.41−7.36 (m, 4
H), 7.20 (s, 1 H), 7.12 (s, 2 H), 6.88 (m, 24 H), 3.78 (s, 12 H). ESI-
MS (m/z) calculated for C82H63N9O10Re2: 853.6902; found: 853.6915.
Elemental analysis calculated for C82H63N11O16Re2·3H2O (%): C,
52.25; H, 3.69; N, 8.17; found: C, 52.14; H, 3.79; N, 8.09.
1
dryness a yellow solid was obtained (2.2 g, 5.12 mmol, ∼100%). H
NMR (400 MHz, CDCl3) δ: 8.37−8.31 (m, 3 H), 8.25 (d, 1 H, J = 1.8
Hz), 7.85−7.80 (m, 2 H), 7.79 (s, 1 H), 7.74−7.72 (dd, 1 H, J = 8.0,
1.8 Hz), 7.14 (s, 1 H), 2.45 (s, 3 H), 2.23 (s, 3 H).
Ligand 16. Compound 14 (820 mg, 1.87 mmol) and 5-(tri(n-
butyl)stannyl)-2,2′-bipyridine 15 (1.00 g, 2.25 mmol)11d,15 were
dissolved in m-xylene (80 mL). After bubbling N2 gas for 30 min,
Pd(PPh3)4 (108 mg, 0.09 mmol) was added, and the reaction mixture
was further deoxygenated prior to heating to 150 °C for 70 h. Water
(150 mL) was added to the cooled black suspension. This mixture was
extracted with dichloromethane (3 × 50 mL). The combined organic
phases were dried over anhydrous MgSO4 prior to solvent
evaporation. The crude product was purified by column chromatog-
raphy on silica gel using first pure dichloromethane, then dichloro-
methane with 1% triethylamine as the eluent. Subsequent recrystalliza-
Ligand 10. Compound 8 (66 mg, 0.21 mmol), dianisylamine (58.3
t
mg, 0.25 mmol), BuONa (404 mg, 4.2 mmol), Pd(dba)2 (6 mg, 0.01
mmol), and (HPtBu3)BF4 (3 mg, 0.01 mmol) were dissolved in dry
and deoxygenated toluene (5 mL) under N2. This mixture was heated
to 100 °C for 22.5 h. After cooling to room temperature, water (50
mL) was added, and the mixture was extracted with dichloromethane
(3 × 50 mL). The combined organic phases were dried over
anhydrous Na2SO4 and then evaporated. Column chromatography on
silica gel occurred with dichloromethane containing 1% of triethyl-
amine. This procedure afforded the product as a beige solid (80 mg,
0.17 mmol, 83%). 1H NMR (400 MHz, CDCl3) δ: 8.62 (m, 4 H), 7.41
(m, 4 H), 7.29 (t, 1 H, J = 1.6 Hz), 7.20 (d, 2 H, J = 1.6 Hz), 7.16−
7.12 (m, 4 H), 6.90−6.86 (m, 4 H), 3.82 (s, 6 H).
1
tion from hexane gave a yellow solid (640 mg, 1.37 mmol, 73%). H
NMR (400 MHz, CDCl3) δ: 8.73 (ddd, 2 H, J = 5.5, 2.0, 0.9 Hz), 8.50
(dd, 1 H, J = 8.1, 0.8 Hz), 8.46 (dt, 1 H, J = 8.0, 1.1 Hz), 8.40 (d, 1 H,
J = 8.0 Hz), 8.38−8.32 (m, 3 H), 7.89−7.80 (m, 6 H), 7.34 (ddd, 1 H,
J = 7.5, 4.8, 1.8 Hz), 7.27 (s, 1 H), 2.36 (m, 6 H).
Complex 17. Ligand 16 (0.15 g, 0.32 mmol) and Re(CO)5Cl (116
mg, 0.32 mmol) were suspended in toluene (25 mL) and heated to
reflux overnight. After it was cooled to room temperature, the yellow
precipitate was filtered, washed with diethyl ether, and finally dried in
1
vacuum (225 mg, 0.29 mmol, 91%). H NMR (400 MHz, CDCl3) δ:
9.13−9.07 (m, 2 H), 8.42 (d, 1 H, J = 8.0 Hz), 8.38−8.32 (m, 3 H),
8.26 (dd, 2 H, J = 9.7, 8.0 Hz), 8.11 (ddd, 2 H, J = 8.6, 6.9, 1.9 Hz),
7.87−7.80 (m, 3 H), 7.57 (ddd, 1 H, J = 7.6, 5.5, 1.2 Hz), 7.30 (d, 2 H,
J = 6.6 Hz), 2.39 (m, 6 H).
Re2−TAA. Ligand 10 (0.10 g, 0.22 mmol) and [Re(bpy)-
(CO)3OTf] (276 mg, 0.48 mmol)13 were dissolved in methanol (10
mL). Prior to heating to 85 °C overnight, the reaction mixture was
deoxygenated by bubbling N2 gas during 45 min. After evaporating to
dryness, the crude product was purified by column chromatography
using the same conditions as described above for Re2−OTA. An
orange fraction was collected and dissolved in water (50 mL). This
solution was extracted with dichloromethane (3 × 50 mL), and the
combined organic phases were dried over Na2SO4 and evaporated.
Recrystallization from hexane gave the desired complex in the form of
its nitrate salts as an orange solid (161 mg, 0.11 mmol, 50%). 1H NMR
(400 MHz, CD3CN) δ: 9.21 (ddd, 4 H, J = 5.5, 1.6, 0.8 Hz), 8.37 (dt,
4 H, J = 8.3, 1.0 Hz), 8.25 (td, 4 H, J = 7.9, 1.5 Hz), 8.20−8.15 (m, 4
H), 7.77 (ddd, 4 H, J = 7.6, 5.5, 1.3 Hz), 7.37−7.32 (m, 4 H), 7.19 (t,
1 H, J = 1.6 Hz), 7.07−7.02 (m, 4 H), 6.97 (d, 2 H, J = 1.6 Hz), 6.89−
6.84 (m, 4 H), 3.75 (s, 6 H). ESI-MS (m/z) calculated for
C56H41N7O8Re2: 656.6061; found: 656.6075. Elemental analysis
calculated for C56H41N9O14Re2·2H2O (%): C, 45.68; H, 3.08; N,
8.56; found: C, 45.62; H, 3.21; N, 8.63.
Compound 13. 2-Bromoanthraquinone 11 (3.32 g, 11.6 mmol), 4-
trimethylsilyl-2,5-dimethylphenylboronic acid 12 (3.08 g, 13.9
mmol),11d and Na2CO3 (3.67 g, 34.7 mmol) were suspended in a
mixture of water (12 mL), ethanol (10 mL), and toluene (60 mL).
After bubbling N2 gas during 15 min, Pd(PPh3)4 (1.33 g, 1.16 mmol)
was added, and the reaction mixture was further deoxygenated prior to
refluxing for 1.5 d. After it was cooled to room temperature, deionized
water (100 mL) was added, and the mixture was extracted with
dichloromethane (3 × 50 mL). The combined organic phases were
dried over anhydrous Na2SO4 and then evaporated to dryness.
Column chromatography on silica gel occurred with a pentane−
dichloromethane 1:1 (v/v) mixture. This procedure afforded the
product as a yellow solid (4.44 g, 11.6 mmol, ∼100%). 1H NMR (400
MHz, CDCl3) δ: 8.38−8.32 (m, 3 H), 8.30 (d, 1 H, J = 1.8 Hz), 7.82
Complex 18. Complex 17 (201 mg, 0.26 mmol) was suspended in
dry dichloromethane (15 mL). Trifluoromethanesulfonic acid (1.50
mL, 17 mmol) was added dropwise at room temperature, and the
resulting homogeneous solution was stirred for 2 h at this temperature.
Then diethyl ether was added slowly, and the product precipitated as a
yellow solid. After storing the mixture in the refrigerator overnight, the
yellow solid was filtered, washed with diethyl ether, and finally dried in
1
vacuum (190 mg, 0.21 mmol, 83%). H NMR (400 MHz, CDCl3) δ:
9.16−9.10 (m, 2 H), 8.42 (dd, 1 H, J = 8.0, 2.0 Hz), 8.39−8.32 (m, 3
H), 8.29 (m, 2 H), 8.24−8.15 (m, 2 H), 7.87−7.79 (m, 3 H), 7.66
(ddd, 1 H, J = 7.6, 5.4, 1.3 Hz), 7.30 (m, 2 H), 2.42−2.35 (m, 6 H).
Compound 20. Pyridine-4-boronic acid hydrate 7 (1.00 g, 5.14
mmol), 1-bromo-4-trimethylsilyl-2,5-dimethylbenzene 19 (1.37 g, 6.17
mmol),11c and Na2CO3 were suspended in a mixture of water, ethanol,
and toluene (8:6:20; v/v/v) under N2. After bubbling N2 gas through
the suspension for 20 min, Pd(PPh3)4 (60 mg, 0.05 mmol) was added,
and the suspension was further deoxygenated by bubbling N2 for 10
min prior to refluxing for 1.5 d. Water (100 mL) was added to the
cooled reaction mixture, and then the mixture was extracted with
dichloromethane (3 × 50 mL). After drying over anhydrous MgSO4
the combined organic phases were evaporated, and the crude product
was purified by column chromatography on silica gel. The eluent was a
mixture of pentane and dichloromethane (2:1, v:v) containing 1%
triethylamine. This procedure afforded the desired product as a pale
1
yellow solid (1.30 g, 5.09 mmol, 99%). H NMR (400 MHz, CDCl3)
δ: 8.65−8.63 (m, 2 H), 7.37 (s, 1 H), 7.27−7.26 (m, 2 H), 7.02 (s, 1
H), 2.46 (s, 3 H), 2.26 (s, 3 H), 0.36 (s, 9 H).
Compound 21. Compound 20 (1.30 g, 5.09 mmol) was dissolved in
dichloromethane (5 mL) under N2. After this solution cooled to 0 °C,
a solution of ICl (520 μL, 10.2 mmol) in acetonitrile (20 mL) was
H
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added drop by drop. The reaction mixture was stirred at room
temperature overnight. Then saturated aqueous Na2S2O3 (200 mL)
was added, and the product was extracted with dichloromethane (3 ×
50 mL). The combined organic phases were dried over Na2SO4, and
then the solvents were removed on a rotary evaporator. Purification
occurred by passing the crude product through some silica gel and by
washing with an eluent mixture comprised of pentane and dichloro-
methane (2:1, v/v) with 1% triethylamine. This gave the pure product
as a beige solid (1.52 g, 4.92 mmol, 97%). 1H NMR (400 MHz,
CDCl3) δ: 8.68−8.61 (m, 2 H), 7.76 (s, 1 H), 7.25−7.18 (m, 2 H),
7.06 (s, 1 H), 2.43 (s, 3 H), 2.19 (s, 3 H).
on a Cary 5000 UV−vis−NIR spectrophotometer from Varian. For
steady-state luminescence spectroscopy a Fluorolog-322 instrument
from Horiba Jobin-Yvon was used. Cyclic voltammetry experiments
were performed with a Versastat3−200 potentiostat from Princeton
Applied Research using a conventional three-electrode setup. A glassy
carbon disk served as a working electrode, and two silver wires were
used as counter and quasi-reference electrodes, respectively. Dry,
argon-saturated acetonitrile with 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as supporting electrolyte was used in all
cases. Transient absorption spectroscopy was measured on an LP920-
KS spectrophotometer from Edinburgh Instruments, equipped with an
iCCD camera from Andor and an R928 photomultiplier or an NIR
301/2 (InGaAs) detector (900−1650 nm, ∼100 ns response time).
The frequency-tripled output of a Quantel Brilliant b laser was used for
excitation. The duration of the laser excitation pulses was
approximately 10 ns, the repetition rate was 10 Hz. Transient
absorption spectra were time-averaged over a duration of 200 ns
directly after excitation. Quartz cuvettes from Starna were employed
for all optical spectroscopic experiments.
Ligand 22. Compound 21 (177 mg, 0.57 mmol), amine 5 (429 mg,
t
0.69 mmol), BuOK (1.28 g, 11.4 mmol), Pd(dba)2 (33 mg, 0.06
mmol), and (HPtBu3)BF4 (16.5 mg, 0.06 mmol) were dissolved in dry
and deoxygenated toluene (15 mL) under N2. This reaction mixture
was heated to 100 °C for 2.5 d. Deionized water (150 mL) was then
added to the cooled mixture, and the product was extracted with
dichloromethane (3 × 50 mL). After it was dried over anhydrous
Na2SO4 and subsequent solvent evaporation, the crude product was
purified by column chromatography on silica gel. At first, the eluent
was pure dichloromethane, and later dichloromethane with 1%
triethylamine was used. This procedure afforded the product as a
brownish solid (0.43 g, 0.53 mmol, 94%). 1H NMR (400 MHz,
CDCl3) δ: 8.64−8.61 (m, 2 H), 7.29−7.26 (m, 2 H), 7.06−6.99 (m,
10 H), 6.87−6.77 (m, 16 H), 3.78 (s, 12 H), 2.20 (s, 3 H), 2.05 (s, 3
H).
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data for compound 5 in CIF format.
NMR and high-resolution ESI mass spectra, additional transient
absorption data. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystallographic data (exclud-
ing structure factors) for the structure in this paper were
deposited with the Cambridge Crystallographic Data Center;
the deposition number is 1011917. Copies of the data can be
obtained, free of charge, on application to the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: +44−1223−
336033 or e-mail: deposit@ccdc.cam.ac.uk].
AQ−Re−OTA. Complex 18 (81 mg, 0.08 mmol) and ligand 22 (74
mg, 0.10 mmol) were suspended in a mixture of methanol (5 mL) and
chloroform (5 mL). Prior to refluxing for 43 h the reaction mixture
was deoxygenated by bubbling N2 gas for 20 min. Then the solvents
were evaporated, and the solid residue was subjected to column
chromatography on silica gel. At first the eluent was dichloromethane
with 1% methanol, followed by dichloromethane with 3% methanol.
The product was further purified by dissolving in a minimal amount of
dichloromethane and then precipitated by diethyl ether. This
procedure afforded the pure product as a dark orange solid (64 mg,
AUTHOR INFORMATION
Corresponding Author
*E-mail: oliver.wenger@unibas.ch.
■
1
0.04 mmol, 47%). H NMR (400 MHz, CD3CN) δ: 9.27−9.23 (m, 1
H), 9.19 (d, 1 H, J = 1.7 Hz), 8.44 (dd, 2 H, J = 8.2, 4.2 Hz), 8.35−
8.32 (m, 1 H), 8.31−8.24 (m, 7 H), 7.88 (dt, 3 H, J = 6.8, 1.5 Hz),
7.83−7.77 (m, 1 H), 7.44 (s, 1 H), 7.37 (s, 1 H), 7.24 (s, 2 H), 6.86
(sbroad, 26 H), 3.73 (s, 12 H), 2.35 (m, 12 H). ESI-MS (m/z)
calculated for C88H70N6O9Re: 1541.4675; found: 1541.4774. Ele-
mental analysis calculated for C89H70F3N6O12ReS·2Et2O (%): C,
63.29; H, 4.57; N, 4.76; found: C, 63.01; H, 4.68; N, 5.11.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
X-ray Crystallography. Crystal data for 5: formula C40H37N3O4,
M = 623.75, F(000) = 1320, green block, size 0.080 × 0.190 × 0.330
mm3, orthorhombic, space group Pccn, Z = 4, a = 10.2046(8) Å, b =
15.4020(12) Å, c = 20.5009(16) Å, α = 90°, β = 90°, γ = 90°, V =
3222.2(4) Å3, Dcalc = 1.286 Mg·m−3. The crystal was measured on a
Bruker Kappa Apex2 diffractometer at 123 K using graphite-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Swiss NSF (Grant No.
200021_146231/1). Support from COST action CM1202 is
acknowledged.
monochromated Cu Kα radiation with λ = 1.541 78 Å, Θmax
=
68.340°. Minimal/maximal transmission 0.88/0.95, μ = 0.665 mm−1.
The Apex2 suite has been used for data collection and integration.37
From a total of 40 707 reflections, 2937 were independent (merging at
r = 0.055). From these, 2910 were considered as observed (I >
2.0σ(I)) and were used to refine 215 parameters. The structure was
solved by charge flipping using the program Superflip.38 Least-squares
refinement against F was carried out on all non-hydrogen atoms using
the program CRYSTALS.39 R = 0.0349 (observed data), wR = 0.0384
(all data), GOF = 1.0472. Minimal/maximal residual electron density
= −0.15/0.22 e Å−3. Chebychev polynomial weights were used to
complete the refinement.40 Plots were produced using Mercury.41
Additional crystallographic data are available in the Supporting
Information.
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