Living Long and Prosperous: Productive Intraligand Charge-Transfer States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced Light Absorption

Article abstract of DOI:10.1021/acs.inorgchem.0c01939

The ground- and excited-state properties of six rhenium(I) κ²N-tricarbonyl complexes with 4′-(4-substituted-phenyl)terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly 4-fold variation of the triplet metal-to-ligand charge-transfer (³MLCT) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe₂group, we observed in the κ²N complex the appearance of a very strong absorption band, red-shifted by ca. 100 nm with respect to the other complexes. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 vs 1.5 ns), and a character change from ³MLCT to intraligand charge transfer (³ILCT), despite the remote location of the substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as photosensitizers for hydrogen production, with the κ²N-NMe₂complex resulting in a stable and efficient photocatalytic system reaching TON_Revalues of over 2100, representing the first application of the ³ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.

Full text of DOI:10.1021/acs.inorgchem.0c01939

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Article
Living Long and Prosperous: Productive Intraligand Charge-Transfer
States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced
Light Absorption
́
́
́
Ricardo Fernandez-Teran* and Laurent Severy
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ABSTRACT: The ground- and excited-state properties of six
rhenium(I) κ2N-tricarbonyl complexes with 4′-(4-substituted-phenyl)-
terpyridine ligands bearing substituents of different electron-donating
abilities were evaluated. Significant modulation of the electrochemical
potentials and a nearly 4-fold variation of the triplet metal-to-ligand
charge-transfer (3MLCT) lifetimes were observed upon going from CN
to OMe. With the more electron-donating NMe2 group, we observed
in the κ2N complex the appearance of a very strong absorption band,
red-shifted by ca. 100 nm with respect to the other complexes. This
was accompanied by a dramatic enhancement of the excited-state
3
lifetime (380 vs 1.5 ns), and a character change from MLCT to
intraligand charge transfer (3ILCT), despite the remote location of the
substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy,
IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as
photosensitizers for hydrogen production, with the κ2N-NMe2 complex resulting in a stable and efficient photocatalytic system
3
reaching TONRe values of over 2100, representing the first application of the ILCT state of a rhenium(I) carbonyl complex in a
stable photocatalytic system.
efforts in this direction have focused on the introduction of
electron-withdrawing groups or the increase of conjugation of
the diimine ligand framework (Figure 1A); in these cases, the
absorption is red-shifted because of stabilization of the ligand-
centered π* orbitals [typically the lowest unoccupied
molecular orbital (LUMO)].22
While the effects of substitution in the vicinity of the metal
center have been previously described for rhenium(I)
tricarbonyl complexes,23−26 further understanding of the
effects of substitution far away from the metal center (Figure
1B) is needed and constitutes a promising alternative to fine-
tuning the properties of these complexes.
The photophysics and photochemistry of remote substituent
effects in rhenium(I) complexes have been explored in the past
by many groups. Among them, studies by Schanze and co-
workers focused on remote substitution of the axial ligand in
[Re(bpy)(CO)3(L)]+-type complexes, where the L ligand was
decorated with a dimethylamino group27 or a cation-
INTRODUCTION
■
Rhenium(I) tricarbonyl complexes bearing polypyridine
ligands are very popular and well-studied molecules, especially
concerning their photochemistry and photophysics. These
complexes have found many applications, especially in solar
energy conversion, where their use for photoelectrochemical
CO2 reduction1−6 and as photosensitizers for hydrogen
production are the most prominent ones.7−18 The fac-
{Re(CO)3}+ moiety makes these complexes excellent IR
absorbers. The high sensitivity of the CO stretching modes
to the electron density around the metal center, their very high
IR absorption coefficients, and the overall stability of this
organometallic core allowed rhenium(I) tricarbonyl complexes
to become one of the most studied systems by transient IR
(TRIR) spectroscopy and can be considered the closest
analogues of [Ru(bpy)3]2+ (bpy = bipyridine) in terms of their
widespread use as photosensitizers.19,20
Rhenium(I) tricarbonyldiimine complexes typically exhibit a
purely metal-to-ligand charge-transfer (MLCT) or mixed
ligand-to-ligand charge-transfer (LLCT) absorption band
centered around 380 nm and the corresponding ligand-
centered excitation bands at higher energies.21 Consequently,
these dyes absorb almost exclusively ultraviolet or blue light,
which has prompted the design, synthesis, and study of
complexes with enhanced absorption at lower energies. Most
Received: June 30, 2020
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Despite the abundant literature on terpyridine-based
rhenium(I) tricarbonyl complexes, an in-depth systematic
exploration of tuning of the ground- and excited-state
properties is missing. Moreover, the effect of substituents in
the character of the lowest excited states has only been
discussed in brief. In this regard, a recent report by Shillito and
co-workers showed how switching in the character of the
lowest excited states (from MLCT to ILCT) frustrates tuning
of the properties in rhenium(I) and platinum(II) complexes
bearing triphenylamine-substituted 1,10-phenanthroline li-
gands.49
Figure 1. Different strategies for red shifting the absorption of
rhenium(I) carbonyl dyes: (A) by substitution of the bpy ligands
(stabilizing the ligand π* orbitals); (B) through remote substitution,
affecting the HOMO and allowing for excited-state character
switching.
In the present work, we focus on the effect of remote
substitution across the 4′-(4-R1-phenyl)-2,2′:6′,2″-terpyridine
framework on the photophysical and photochemical properties
of rhenium(I) tricarbonyl complexes. We show that, in a
complementary manner, as has been described before, the
introduction of electron-donating substituents changes the
localization of the highest occupied molecular orbital
(HOMO; instead of tuning the LUMO, as shown in ref 23),
leading to a change in the character of the lowest excited state
from MLCT to ILCT.
We report on the synthesis, spectroscopic, crystallographic,
and electrochemical characterization of a series of rhenium(I)
κ2N-tricarbonyl (2a−2f) complexes with substituted 4′-(4-R1-
phenyl)-2,2′:6′,2″-terpyridine ligands (La−Lf). Studying a
series of complexes bearing substituents of different electron-
donating abilities (from CN to NMe2; Scheme 1) allowed us
to correlate the observed properties with the electron density
on the terpyridine ligand. The ground- and excited-state
properties of these complexes were studied by UV pump−IR
probe (in the femtosecond to microsecond range), electro-
chemistry, Fourier transform infrared (FT-IR) spectroelec-
trochemistry (IR-SEC) and photoluminescence measurements.
In addition, through a combined experimental and computa-
tional approach, we could establish the key differences in the
character of the lowest-lying excited states. Finally, we
evaluated the potential utility of these complexes for
photochemical energy conversion with proof-of-principle
measurements of photocatalytic hydrogen evolution. To the
best of our knowledge, this demonstrates for the first time the
application of productive 3ILCT states of rhenium(I) carbonyl
complexes in photocatalysis.
responsive crown macrocycle.28 This strategy was later
combined with a dipyrido[3,2-α:2′,3′-c]phenazine ligand,
replacing the bpy ligand,29 and a photoisomerizable stilbene-
like moiety, as shown by Perutz and co-workers.30 The ultrafast
dynamics of intramolecular electron transfer in an azacrown-
ether-decorated complex were reported by Perutz and co-
workers,31 who used transient UV−vis absorption. Similarly,
porphyrin-appended rhenium(I) carbonyl complexes have
been studied by ultrafast TRIR and transient UV−vis
absorption spectroscopy,32,33 and it was shown that photo-
excitation of these complexes may lead to ligand dissociation.34
In addition, the electron-transfer dynamics of porphyrin-
appended rhenium(I) carbonyl complexes have been studied
in NiO surfaces.35
Most of the examples mentioned up to now focus on
substitution of the axial ligand. In our approach, we use
2,2′:6′,2″-terpyridine (terpy) to operate on the α-diimine
ligand instead. The terpy framework represents an ideal
structural backbone for modification by substitution. In
particular, phenyl-substituted terpyridine ligands have shown
remarkable photophysical and photochemical properties by
themselves,36 as well as their complexes with ruthenium-
(II),37,38 platinum(II),39 zinc(II),40 iridium(III),41 and other
transition metals. In most of these cases, the terpyridine ligand
acts as a tridentate ligand, adopting a meridional configuration
around the metal center. In contrast, fac-rhenium(I)
tricarbonyl complexes with terpyridine ligands contain a
bidentate (κ2N) terpyridine ligand in a bpy-like coordination
fashion.20,42−44
The photophysical properties of a few rhenium-based
terpyridine complexes,45,46 as well as those with structurally
related bidentate N^N ligands, have been previously
studied.47,48 Some of these evidenced long-lived intraligand
charge-transfer (ILCT) states.48
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The 4′-(4-R1-phenyl)-
2,2′:6′,2″-terpyridine ligands (La−Lf) were readily obtained in
one step from 2-acetylpyridine and the corresponding 4-R1-
Scheme 1. Synthetic Route and Structures of the Compounds Studied in the Present Work
B
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̈
benzaldehyde using a modified Krohnke methodology, as
reported previously.50,51 This ligand framework and synthetic
methodology enable easy modification at the 4′ position of the
terpyridine ligand. The κ2N-tricarbonyl complexes (2a−2f)
were obtained in excellent yield from ReI(CO)5Cl as air-stable
yellow solids (2f as a yellow-green solid) by refluxing in a
methanol/toluene mixture [see the Supporting Information
(SI) for details]. Single-crystal X-ray diffraction analysis was
performed for the κ2N-tricarbonyl complexes 2a, 2b, and 2e.
Figure 2 shows the displacement ellipsoid plot of the structure
of 2a as a representative example.
Figure 3. (A) UV−vis and (B) FT-IR spectra of 2d−2f in DMF, as
representative examples. The dashed line in part A indicates the
excitation wavelength used in the transient absorption experiments
(420 nm).
Figure 2. Displacement ellipsoid representation at the 50%
probability level of the crystal structure of complex 2a. Hydrogen
atoms and solvent molecules are omitted for clarity.
a very intense absorption band in 2f, bearing the more
electron-donating R1 = NMe2 moiety. This additional low-
energy band (Figure 3, red) has a significantly higher
extinction coefficient than typical MLCT or even ligand-
centered π−π* transitions, providing a first hint into its
different character.
In the FT-IR absorption spectra of 2a−2f in DMF, we
observe the typical features of the fac-{Re(CO)3}+ moiety:
three strong νCO bands at ca. 2019, 1915, and 1892 cm−1,
corresponding to modes of A′(1), A″, and A′(2) symmetries,
respectively. The νCO frequencies shift only 2−4 cm−1 (Table
S2), contrasting with the higher shifts observed upon direct
substitution at the bpy ligand (on the order of 5−20 cm−1),23
and can be explained by the remote character of substitution in
the complexes described herein.
Character of the Lowest Excited States. We now turn
to (TD-)DFT calculations to understand the effects of the
substituents in the electronic structures of these complexes. We
observe an increase in the relative energies of the HOMOs and
LUMOs with more donating groups, with the energies of the
latter having a more pronounced dependence (Figure S4).
While the location of the LUMO remains largely unaltered
throughout the series, we observe that, as the electron-
donating character of the substituent is increased, the HOMO
gradually shifts from the {Re(CO)3}+ moiety to the 4′-phenyl
moiety of the terpy ligand (Figure 4), until it is localized
entirely on the latter, for R1 = NMe2.
These observations are supported by the charge-density-
difference (CDD) isosurfaces calculated for the S1 ← S0
transition, as shown in Figure 5 for the unsubstituted and
OMe- and NMe2-substituted complexes (2d−2f, respectively).
The CDD isosurfaces were calculated using MultiWfn, version
3.5.54
The structures of the κ2N-tricarbonyl complexes show a bpy-
like coordination of the terpyridine ligand to the fac-
{Re(CO)3}+ core, with the two rings being twisted by about
10°. The third pyridine ring is rotated by ca. 45−50°, and the
4′-phenyl ring is rotated by 15−25° with respect to the plane
of the central pyridine ring. A brief discussion of the crystal
structures and a table of crystallographic parameters are given
1
in Table S1. In the solution H NMR spectra of complexes
2a−2f, we observe two sets of signals for the peripheral
pyridine rings, supporting the κ2N coordination with a
dangling pyridine observed in the solid state.
Steady-State Spectroscopic Properties. The UV−vis
and FT-IR spectra of 2d−2f in N,N-dimethylformamide
(DMF; as representative examples) are shown in Figure 3
(other spectra are provided in Figures S1 and S2 and Table
S2). In the UV−vis absorption spectra of the κ2N-tricarbonyl
complexes (2a−2f), we observe a band centered around 380
nm with an extinction coefficient ε ≈ 4000 cm−1 M−1, typical
of an MLCT band in rhenium(I) complexes.21 The higher-
energy bands are attributed to ligand-centered π−π*
transitions. Time-dependent density functional theory (TD-
DFT) calculations performed in Gaussian 09, revision D.01,52
nicely reproduce all of these features (Figure S3).
The MLCT absorption band shows a hypsochromic shift of
ca. 720 cm−1 as the electron-donating character of the
substituent increases from CN to OMe (values provided in
Table S2). These shifts are much smaller than those obtained
upon direct substitution at the 4 and 4′ positions of the bpy
ligand (ca. 6250 cm−1 between NO2 and NH2)23,53 and
suggest a small, albeit observable, electronic effect of
substitution in the 4′-phenyl ring of the terpy ligands upon
going from CN to OMe. While the spectra of the complexes
with electron-withdrawing substituents do not show significant
changes, complexes with more electron-donating groups show
distinct features in their UV−vis spectra. For 2e (R1 = OMe),
we observe a new shoulder at lower energy, which evolves into
For complex 2d (and those with more electron-withdrawing
substituents), it is clear that this transition has the expected
MLCT character. Surprisingly, despite the change in the
localization of the HOMO observed for complex 2e, the S1 ←
S0 transition conserves an MLCT character (further supported
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Figure 6. FT-IR spectrum (top) and magic-angle UV pump−IR
probe transient spectra (bottom) of 2d (5 mM in DMF) at different
pump−probe delays.
Figure 4. Calculated HOMOs and LUMOs of 2d−2f. Orbitals shown
at |ϕ| = 0.025 au.
Figure 7. FT-IR spectrum (top) and UV pump−IR probe transient
spectra (bottom) of 2f (5 mM in nitrogen-purged DMF) at different
delays (picosecond to microsecond time scales). The IR-SEC
difference spectrum at −1.8 V versus Fc+/Fc (first reduction) is
overlaid with the pump−probe spectra (purple dotted line, ΔAbs ×
0.4).
Figure 5. CDD isosurfaces of the S1 ← S0 transition of 2d−2f, shown
at |Δρ| = 0.002 au.
by TRIR data and discussed next). In contrast, the S1 ← S0
transition in the NMe2-substituted complex has strong ILCT
characterwith the NMe2 group acting as the donor and the
metal-coordinated κ2N-terpy as the acceptor.
Excited-State Dynamics. To understand the excited-state
dynamics of these complexes, we performed transient
absorption experiments with UV pump and ultrashort
broadband mid-IR pulses derived from a home-built optical
parametric amplifier as the probe (complete experimental
details are given in section 1.3 of the SI).55−57
The red shifts of the ESA bands suggest an increase of the
electron density around the rhenium(I) center upon photo-
excitation, closely resembling the difference spectra of the
singly reduced species (Figure 7, purple dotted line) observed
by IR-SEC studies. These features are very well reproduced by
DFT calculations of the lowest triplet excited state and the
singly reduced NMe2-substituted complex (Figure S5). After
the initial blue shift of the bands in a 5 ps time scale, the
observed signal remained constant for more than 4 ns (Figure
S6). To observe the decay of these features (and, hence, extract
the actual excited-state lifetimes), we performed a second set of
TRIR measurements comprising time scales from 10 ps to over
10 μs, using a setup based on two synchronized amplifiers.59
From these measurements, we obtained a lifetime of 380 20
ns in a nitrogen-purged DMF solution for 2f. The significantly
longer lifetime of this complex (ca. 260 times longer than that
of 2d) can be explained by the different character of the lowest
triplet excited states.
TRIR spectra of complexes 2a−2e in air-saturated DMF,
recorded upon excitation with 420 nm ultrashort pulses, show
3
the typical features of the MLCT excited state of rhenium(I)
tricarbonyl dyes within 500 fs of excitation. The TRIR spectra
of 2d as a representative example is shown in Figure 6. These
bands show a time-dependent blue shift, on a time scale of ca.
5 ps, attributed to a convolution of ultrafast intersystem
crossing (ISC; within ∼100 fs) and vibrational relaxation on
the excited state, in agreement with previous reports.10,48,58
For complex 2f, the situation is different: TRIR spectroscopy
3
yielded definitive experimental evidence for a ILCT state in
These observations contrast with the TRIR data of
complexes 2a−2e (Figure 6). In those cases, excitation of
the MLCT band decreases the electron density around the
metal center (as evidenced by blue-shifted ESA bands) and the
transient signals decay significantly faster.
On the picosecond-to-microsecond setup and under the
same measurement conditions, we obtained a lifetime of 2.3
this complex. Immediately after excitation, we observed in the
TRIR spectra of 2f in DMF (Figure 7) two sets of excited-
state-absorption (ESA) bands in the νCO region: a single,
clearly resolved band around 1994 cm−1 and a set of two
unresolved bands in the 1860 cm−1 region that partially overlap
with the ground-state bleaches.
D
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0.2 ns for 2e (R1 = OMe) in nitrogen-purged DMF, which
agrees quantitatively with the lifetime obtained in the ultrafast
experiments in air-saturated DMF solutions. This illustrates the
negligible effects of oxygen on the lifetimes of all other
complexes, showing also the consistency of the kinetics
obtained from the different setups.
The photophysical properties of the ILCT states of
rhenium(I) tricarbonyls have been previously studied in
complexes bearing 3-substituted 1-(2-pyridyl)imidazo[1,5-α]-
pyridine ligands.48 The ILCT character of these complexes was
largely independent from the substituents (ranging from NO2
to NMe2), in sharp contrast with our observation for the
rhenium(I) complexes with 4′-(4-R1-phenyl)-2,2′:6′,2″-terpyr-
idine ligands. In our case, only the NMe2-substituted complex
(2f) shows a significant ILCT, displaying similar characteristic
TRIR signatures, but a significantly longer excited-state
lifetime.
observed for the singly reduced 2f−, obtained by spectroelec-
trochemistry (see below). Overall, these numbers illustrate the
small but significant difference between a formally reduced
complex and an apparent reduced environment around the fac-
{Re(CO)3}+ moiety arising from intramolecular charge
transfer in the excited state.
Photochemical and Redox Properties. To better
understand the excited-state properties and to estimate the
applicability of these complexes as photosensitizers, we
evaluated the excited-state redox potentials, which were
estimated according to eqs 1 and 2:
Ered°′(3[ReI]*) = Ered°′ + ΔGST
(1)
Eox°′(3[ReI]*) = Eox°′ − ΔGST
(2)
where Ered°′ and Eox°′ are the ground-state reduction and
oxidation potentials, respectively, and ΔGST is the singlet−
triplet energy gap.
The ground-state redox properties of these complexes were
studied by cyclic voltammetry (CV) in 0.1 M [Bu4N][PF6] in
DMF at room temperature. The cyclic voltammograms of
selected complexes are shown in Figure 9. All complexes show
A complementary approach, based on tuning of the axial
3
ligand, has also shown that ILCT lifetimes can be modulated
from 27 ns to 6 μs when the axial ligand is changed from
pyridine to bromide. This opens additional pathways for future
modifications of the complexes studied in this work and
highlights the interplay between close-lying MLCT and long-
lived ILCT states. In their work, Gordon and co-workers show
that the change in the emission lifetimes does not arise from
the switching of molecular orbitals (as in our case, where the
HOMO shifts with the substituent), close-lying deactivating
states, contributions from a halide-to-ligand charge-transfer
state, or changes in spin−orbit coupling.60
3
In our case, we observe a systematic change in the MLCT
lifetimes and reduction potentials with different substituents
on the terpy ligand (Figures 8 and S7). The lifetimes increase
Figure 9. Cyclic voltammograms of selected complexes in 0.1 M
[Bu4N][PF6] in DMF. Scan rate: 100 mV s−1.
a partially irreversible two-electron reduction (Ered°′), taking
place at ca. −1.7 V versus Fc+/Fc. The oxidation was outside of
the electrochemical window of our experiment (Eox°′ > 0.5 V
vs Fc+/Fc).
From this, we observe that Ered°′ becomes more negative
with more electron-donating substituents (Table 1), showing
an excellent linear correlation with the Hammett σp substituent
constants (Figure 8). This demonstrates that tunability in a
range of 200 mV can be achieved without significantly
altering the optical properties of these complexes (for 2a−2e),
3
Figure 8. Linear correlations between the MLCT lifetimes (from
TRIR, in blue), the observed ground-state reduction potentials (in
red), and the Hammett σp substituent constants for complexes 2a−2e.
Complex 2f deviates significantly from the linearity of the lifetime plot
and is excluded for clarity.
Table 1. Ground- and Excited-State Redox Properties of the
Studied Complexes in DMF
from 0.58 to 2.3 ns between the CN- and OMe-substituted
complexes, while the reduction potentials become more
negative, shifting from −1.6 to −1.8 V versus ferrocenium/
ferrocene (Fc+/Fc; discussed next). The fitted lifetimes of all
complexes can be found in Table S2.
a
b
c
d
exp
ST
theo
ST
R1
Ered°′ (V) ΔG (eV) ΔG (eV) (Ered°′)* (V)
2a
2b
2c
2d
2e
2f
CN
CF3
Br
H
OMe
NMe2
−1.61
−1.66
−1.70
−1.73
−1.77
−1.81
2.42
2.38
0.80
0.79
0.76
0.75
0.73
0.56
2.46
2.46
2.48
2.50
2.37
2.44
2.46
2.49
2.47
2.06
The frequency shift of −17 cm−1 observed for the a′(1)
band in the TRIR spectra of 2f in DMF is similar to the ca.
−20 cm−1 shift reported for complexes with 3-substituted 1-(2-
pyridyl)imidazo[1,5-α]pyridine ligands (possessing also an
ILCT excited state)48 and the ca. −24 cm−1 shift observed
upon electron transfer from the porphyrin to the rhenium
moiety in the porphyrin-appended rhenium(I) tricarbonyl
complexes studied by Perutz and co-workers.32 The latter,
however, is in better agreement with the −24 cm−1 shift
a
b
Electrochemical potential in volts versus Fc+/Fc in DMF. From a
linear fit of the high-energy side of the 77 K emission spectrum in 2-
c
MeTHF. Energy difference between the optimized ground-state and
d
lowest triplet excited-state structures. Excited-state potentials
calculated from eq 1.
E
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a desirable feature for fine-tuning of the energetics of ground-
and excited-state electron-transfer processes involving these
complexes.
At the same time, we note that the long-lived ILCT state
achieved in the NMe2-substituted complex still has comparable
excited-state reduction potentials (only ca. 200 mV below
those of the rest of the complexes in the series), while
absorbing light with comparable extinction coefficients over
100 nm further to the red.
The emission spectra of all complexes were measured in
DMF at room temperature and at 77 K in a 2-
methyltetrahydrofuran (2-MeTHF) glass (Figure S8). The
ΔGST values were obtained from a linear fit of the high-energy
side of the low-temperature emission spectra, as described
previously (Figure S9).22 These values also show a very good
correlation with the Hammett σp substituent constants (except
for 2f) and illustrate that more electron-donating substituents
can increase ΔGST up to a certain limit, before any further
increase in the electron density results in a decrease of ΔGST
instead.
At room temperature, all complexes exhibit a moderately
intense, broad, and unstructured emission (very strong in the
case of the NMe2 complex, 2f). The positions of the emission
maxima correlate well with the Hammett σp substituent
constants, showing a blue shift with more electron-donating
groups. At cryogenic temperatures (77 K), the emission
spectra of complexes 2a−2e show significant hypsochromic
shifts, characteristic of emission from a 3MLCT state. Complex
2f, in contrast, deviates significantly from this behavior. The
emission of 2f shows instead a bathochromic shift upon
cooling and the appearance of a vibronic structure, suggestive
of emission from a ligand-localized triplet state.61 These
observations suggest that the character of the lowest triplet
excited state of the NMe2-substituted complex changes upon
cooling. At room temperature, it has significant ILCT
character, also supported by the results discussed in the
previous section.
The two-electron reduction process could be resolved in the
IR-SEC experiments into two overlapping and consecutive
single-electron reductions, as evidenced by significant
incremental red shifts of the νCO bands (Figure 10).
Further evidence for the two-electron nature of this
reduction comes from the ca. 96 mV peak-to-peak separation
(at a scan rate of 100 mV s−1) and the small shoulder observed
at less negative potentials (Figure 9). In all cases, an excellent
agreement was found between the calculated and experimental
difference spectra of the corresponding redox states. The
partial irreversibility of the second reduction process is
attributed in both cases to the loss of chloride ligands (also
reproduced in the DFT calculations, where optimization of the
doubly reduced species led to chlorine dissociation), agreeing
with previous reports of related complexes.62
Photocatalytic Hydrogen Evolution Experiments.
Considering the excited-state redox potentials and lifetimes
exhibited by these complexes, we performed a series of
homogeneous proof-of-principle photocatalysis experiments in
DMF. To focus only on the hydrogen evolution half-reaction,
triethanolamine (TEOA, 1 M) was used as a sacrificial electron
donor and triflic acid (TfOH, 0.1 M) as the proton source. For
these test experiments, we chose [Co(dmgH)2]2+ (0.5 mM) as
a catalyst, which was prepared in situ from Co(BF4)2·6H2O
and excess dimethylglyoxime (dmgH2). As representative
examples, we tested complexes 2d and 2f, as well as
[Re(bpy)(CO)3(NCS)] (RePS) under the same conditions
(with [Re] = 0.5 mM and λexc = 390 nm). The latter complex
was used to benchmark the performance of our complexes in
comparison to a well-studied system.63,64
Under these conditions, we observed significant and
prolonged hydrogen evolution only for 2d (Figure S10),
yielding a final turnover number (TON) for the photo-
sensitizer of 580 40 over 7 days. We observed a bimodal
decay of the turnover frequency (TOF) and a significant
decrease in the rate after 4.5 days. These results suggest that
the integrity of the catalyst represents the limiting factor for the
long-term stability of this photocatalytic system, as discussed in
previous reports of related systems.65 We did not observe
hydrogen evolution using complex 2f at these high
concentrations, which we attribute to accumulation of the
reduced photosensitizer and subsequent degradation.64
Considering this last result and the very long lifetime of
complex 2f and its intense absorption extending until ca. 500
nm, we performed additional photocatalytic experiments
(Figure 11) with λexc = 453 nm and higher dmgH2
concentrations (3.5 mM), while using a 10-fold lower
photosensitizer concentration ([2f] = 50 μM). All other
parameters were kept constant.
The DFT-calculated ΔGST values for the κ2N complexes
were in excellent agreement with those found in the
experiment, with a mean deviation of 0.02 eV for all
complexes (except 2f, which deviated by −0.3 eV). The
ground- and excited-state redox properties are summarized in
Table 1.
To identify the spectral signatures of the redox species
observed in the CV experiments, we performed IR-SEC studies
in 0.1 M [Bu4N][PF6] in DMF. The IR-SEC spectra of 2d are
shown in Figure 10 as a representative example.
The very high TONs (2130
120) and hydrogen
production rates (up to ca. 40 nmol s−1) obtained under
these conditions compare very favorably with those obtained
for a closely related system using RePS as the photo-
sensitizer.63,64 While strict comparisons of the TOF and
TON values are inadequate, it is worth mentioning that, under
these conditions, complex 2f is a remarkably stable photo-
sensitizer. The results shown in this work constitute, to the
best of our knowledge, the first example of a system, where the
Figure 10. Experimental (solid lines) and calculated (dashed lines)
difference IR spectra of the redox states of 2d. Experimental spectra
measured in 0.1 M [Bu4N][PF6] in DMF. Potentials versus Fc+/Fc.
Theoretical frequencies were scaled by 0.97 to better match the
experimentally obtained values.
3
long-lived ILCT state of a rhenium(I) complex is used for
photocatalytic hydrogen production, being involved in multi-
electron transfer steps in an efficient manner. We believe that
the long-term stability of this system might be improved by
F
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(dmgH)2]2+. This rate is also significantly slower than the
1.23 × 109 M−1 s−1 quenching rate between photoexcited
RePS* and [Co(dmgH)2]2+, thus excluding this mechanism.64
The absence of new transient features and long-lived signals
in the TRIR spectra of 2f with [Co(dmgH)2]2+ (independent
of the presence of TEOA) may suggest that either (i) a fast
back electron transfer from the cobalt complex in the absence
of TEOA or (ii) a fast electron transfer from TEOA (present in
large excess) to 2f+ takes place after the initial quenching event
in the three-component system.
Combining the insights gained from these experiments, we
believe that the formation of an exciplex between excited 2f*
and TEOA can simultaneously explain the changes in the
emission spectra and the unchanged TRIR data. The formation
of a metal complex−molecule exciplex has been observed
before,66 with the case of [Re(4,7-diphenyl-1,10-
phenanthroline)(CO)3Cl] and N,N-dimethylaniline (DMA)
in decalin being particularly relevant in the context of our
work. In this system, even very low concentrations (∼10−3 M)
of DMA were enough to observe a new red-shifted emission
band.67 While our specific scenario is differentnamely, a
more polar solvent and a different quenchercomplex 2f has
very large charge-transfer character upon photoexcitation (as
shown in Figure 5), leading to a large change in the dipole
moment, which would render the formation of a charge-
transfer exciplex with TEOA feasible.
Figure 11. Hydrogen evolution from 2f as a photosensitizer.
Conditions: [2f] = 50 μM, [Co] = 0.5 mM, [dmgH2] = 3.5 mM, 1
M TEOA, 0.1 M TfOH in DMF, and λexc = 453 nm.
fine-tuning of the conditions of the photocatalytic experiments,
which are outside the scope of the present work.
To gain a deeper understanding of the initial photochemical
steps in the photocatalytic cycle and, in particular, to evaluate
the potential differences between the ILCT and MLCT
excited-state reactivities, we performed emission quenching, as
well as TRIR experiments in the picosecond-to-microsecond
time scales of complexes 2d and 2f in the presence of TEOA.
For 2d, we observed a very small change in the excited-state
kinetics and emission intensities, leading to a diffusion-limited
quenching rate kQ ≈ 4 × 108 M−1 s−1. We find this number of
qualitative character, given the very short lifetime of 2d, which
limits the possibility of studying the intermolecular electron-
transfer steps in this complex.
Additional mechanistic studies of these complexes with the
purpose of unravelling the nature of this process and its solvent
dependence will reveal further differences in the reactivity and
3
3
photophysical properties of the ILCT versus MLCT excited
states, easily switched in these complexes by remote
substitution in the ligand framework.
Complex 2f, in contrast, presented an unexpected behavior.
Upon the addition of TEOA, we observed the appearance of a
strongly emissive blue-shifted band whose intensity increased
with increasing concentrations of TEOA. The absorption
spectra remained unchanged. At the same time, the spectral
features and kinetics of 2f (1 mM in degassed DMF) remained
largely unchanged in a concentration range from 1 mM to 1 M
TEOA (containing 10 mol % TfOH), hinting at a different
order of events in the photocatalytic cycle of 2f. TRIR spectra
recorded in the absence of TfOH also did not show any
difference with respect to those of pure 2f in DMF.
CONCLUDING REMARKS
■
The ground and excited states of a series of rhenium(I)
tricarbonyl complexes with substituted 4′-(4-R1-phenyl)-
2,2′:6′,2″-terpyridine ligands were analyzed by steady-state
and time-resolved spectroscopic methods. We found excellent
correlations between the evaluated spectroscopic and electro-
chemical properties of these complexes and the Hammett σp
substituent constants, showing their tunability even by
changing the substituent in a remote position of the ligand
framework. CV and IR-SEC revealed an irreversible stepwise
two-electron reduction, which, in turn, allowed us to assign the
identities and spectral signatures of the one- and two-electron-
reduced species.
3
In the “conventional” photocycle, the MLCT excited state
of a rhenium(I) complex (like RePS) is reductively quenched
in a bimolecular fashion by TEOA, and it is this reduced
species that transfers the electrons to the proton reduction
catalyst.14,64 If the 3ILCT excited state of 2f is not quenched by
TEOAand given the fact that the system does evolve
significant amounts of hydrogenwe hypothesize that this
complex transfers the electron directly to the cobalt catalyst
(considering its very long lifetime). After this electron transfer,
the oxidized 2f+ is regenerated by TEOA at longer time scales.
To prove this hypothesis, we performed TRIR experiments
of 2f with different concentrations of [Co(dmgH)2]2+ ranging
from 10 to 75 mM (the cobalt complex was prepared as a stock
solution from anhydrous CoBr2 and 3 equiv of dmgH2 in
DMF). From Stern−Volmer analysis of the TRIR kinetic data
(Figure S11), we obtained a second-order quenching rate
The NMe2-substituted complex (2f) showed notably
exceptional lifetime and spectroscopic properties compared
to other complexes of the same series. In this complex, the
lowest singlet and triplet excited states both have ILCT
character, as demonstrated by TRIR and emission spectra, and
are supported by detailed (TD-)DFT calculations. The
particularly long triplet lifetime of ca. 380 ns obtained for
the NMe2-substituted complex (vs ca. 1.5 ns for the
unsubstituted complex) suggests that very strongly donating
groups can lead to distinct photophysical and photochemical
properties and opens new avenues to exploring further
substituent-related modifications to the terpyridine ligand
framework. We conclude that judicious ligand-based sub-
stitutions can still be exploited to access long-lived, strongly
emissive yet sufficiently reducing excited states of complexes
with significantly red-shifted absorption, which would perform
as better photosensitizers by using a wider portion of the solar
constant of kCo = (2.0
0.2) × 108 M−1 s−1 for electron
transfer between 2f and [Co(dmgH)2]2+. This value of kCo
compares very well with the reported values of 1.3 × 108 and
2.5 × 108 M−1 s−1 for electron transfer between reduced
RePS− or [Re(bpy)(CO)3Br]−, respectively, and [Co-
G
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Article
spectrum. In this regard, our approach to enhancing light
absorption through remote substitution is complementary to
those that have been explored so far and benefits from the
simplicity and scalability of the synthesis of the substituted
terpyridine ligands.
Energy Grant PYAPP2 160586) and the University Research
Priority Program for Solar Light into Chemical Energy
Conversion (LightChEC) of the University of Zurich.
Notes
The authors declare no competing financial interest.
In proof-of-principle homogeneous photocatalytic experi-
ments, we demonstrated stable hydrogen evolution over a
period of 7 days with complex 2d and a fast and sustained
hydrogen evolution from complex 2f in a photocatalytic system
with 10-fold-smaller photosensitizer concentrations, reaching
TONs of over 2100 using standard [Co(dmgH)2]2+ as a
catalyst. Using TRIR spectroscopy, we tentatively assign direct
electron transfer from 2f* to the proton reduction catalyst,
instead of reductive quenching by the sacrificial donor
(TEOA). Further mechanistic studies are needed to explain
this in detail, focusing on the differences in the reactivity
ACKNOWLEDGMENTS
■
We thank Prof. Dr. Peter Hamm and Prof. Dr. S. David Tilley
for their valuable input and encouragement. We thank Prof.
Dr. Roger Alberto and his group for allowing us to use the
photochemical hydrogen evolution detection system and
laboratory infrastructure. Dr. Benjamin Probst-Rud is gratefully
̈
acknowledged for his valuable support in the photocatalytic
hydrogen evolution experiments and insightful discussions.
Prof. Dr. Bernhard Spingler is acknowledged for helpful
crystallographic discussions. We also thank Dr. Jan Helbing
3
3
between complexes operating from a ILCT versus MLCT
excited state. The application of 2f for photocatalytic hydrogen
evolution constitutes, to the best of our knowledge, the first
̈
̌
́
and Gokcȩ n Tek for insightful discussions and Olga Bozovic
for proofreading the manuscript and providing helpful
comments.
3
example of the use of long-lived ILCT states in rhenium(I)
tricarbonyl complexes for photocatalytic hydrogen production
and shows the potential of the terpyridine framework for
further modifications and mechanistic studies of the rich
photochemistry of these complexes.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01939.
Full experimental details (including synthetic proce-
dures, characterization details, crystallographic discus-
sions, details about the transient absorption and
electrochemistry experiments), UV−vis absorption,
emission, NMR and FT-IR spectra of all complexes, IR
spectroelectrochemical data, details of the computational
calculations, and additional tables and figures (PDF)
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CCDC 2012296−2012298 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
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AUTHOR INFORMATION
Corresponding Author
■
(8) Patrocinio, A. O. T.; Frin, K. P.; Murakami Iha, N. Y. Solid state
molecular device based on a rhenium(I) polypyridyl complex
immobilized on TiO2 films. Inorg. Chem. 2013, 52, 5889.
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properties of rhenium (I) tricarbonyl photosensitizer: [Re(4,4′-
(COOEt)2-2,2′-bpy)(CO)3py]PF6. Spectrochim. Acta, Part A 2009,
71, 2016.
́
́
Ricardo Fernandez-Teran − Department of Chemistry,
University of Zurich, Zurich CH-8006, Switzerland;
orcid.org/0000-0002-4665-3520; Phone: +41 44 635 44
73; Email: Ricardo.Fernandez@chem.uzh.ch; Fax: +41 44
635 68 38
(10) el Nahhas, A.; Consani, C.; Blanco-Rodríguez, A. M.; Lancaster,
Author
́
̌
K. M.; Braem, O.; Cannizzo, A.; Towrie, M.; Clark, I. P.; Zalis, S.;
́
Laurent Severy − Department of Chemistry, University of
Zurich, Zurich CH-8006, Switzerland; orcid.org/0000-
0002-6546-379X
̌
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D.; Batista, V. S.; Lian, T. Covalent attachment of a rhenium bipyridyl
CO2 reduction catalyst to rutile TiO2. J. Am. Chem. Soc. 2011, 133,
6922.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01939
Funding
(12) Liu, J.; Jiang, W. Photoinduced hydrogen evolution in
supramolecular devices with a rhenium photosensitizer linked to
FeFe-hydrogenase model complexes. Dalt. Trans. 2012, 41, 9700.
This research was funded through the Swiss National Science
Foundation (Sinergia Project CRSII2 160801/1 and AP
H
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