Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis

Article abstract of DOI:10.1002/anie.201907247

Understanding photodriven multielectron reaction pathways requires the identification and spectroscopic characterization of intermediates and their excited-state dynamics, which is very challenging due to their short lifetimes. To the best of our knowledg

Full text of DOI:10.1002/anie.201907247

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Accepted Article
Title: Unravelling the Light-Activated Reaction Mechanism in a
Catalytically Competent Key Intermediate of a Multifunctional
Molecular Catalyst for Artificial Photosynthesis
Authors: Linda Zedler, Alexander Klaus Mengele, Karl-Michael Ziems,
Ying Zhang, Maria Wächtler, Stefanie Gräfe, Torbjörn
Pascher, Stephan Kupfer, Sven Rau, and Benjamin Dietzek
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907247
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Angewandte Chemie International Edition
10.1002/anie.201907247
RESEARCH ARTICLE
Unravelling the Light-Activated Reaction Mechanism in a
Catalytically Competent Key Intermediate of a Multifunctional
Molecular Catalyst for Artificial Photosynthesis
Linda Zedler, Alexander Klaus Mengele, Karl Michael Ziems, Ying Zhang, Maria Wächtler, [a,b]
[
a]
[c]
[b]
[a,b]
[b]
[d]
[c]
[b]
[a,b]
Stefanie Gräfe, Torbjörn Pascher, Sven Rau,* Stephan Kupfer* and Benjamin Dietzek*
Abstract: Understanding photodriven multielectron reaction
pathways requires the identification and spectroscopic
approaches for heterogeneous and homogeneous systems have
been established and studied spectroscopically to elucidate the
reaction mechanism and to explore the factors determining
catalytic activity.[2] However, intramolecular photocatalystic
reactions often involve complex multi-step reactions with short-
lived and highly reactive intermediates. As water splitting is an
inherent multielectron process, elucidation of the properties of the
redox- activated intermediates is of paramount importance to
understanding overall catalytic activity. The identification of those
intermediates and their excited-state processes within the
electron transfer cascades are of fundamental importance to
understand the catalytic mechanism and to identify competing
deactivation pathways. Therefore, detailed knowledge with
respect to the structure, the photophysical properties and
photoinduced dynamics of short-lived photoexcited intermediates
is of utmost importance to develop highly active and stable
photocatalysts for e.g. hydrogen production.
characterization of intermediates and their excited-state dynamics,
which is very challenging due to their short lifetimes. To the best of
our knowledge, the manuscript reports for the first time on in-situ
spectroelectrochemistry as an alternative approach to study the
excited-state properties of reactive intermediates of photocatalytic
cycles. UV-vis, resonance Raman and transient absorption
spectroscopy have been employed to characterize the catalytically
II
I
competent
intermediate
[(tbbpy)
2
Ru (tpphz)Rh Cp*]
(Ru(tpphz)RhCp*),
of
a
[
(tbbpy) Ru(tpphz)Rh(Cp*)Cl]Cl(PF )
2
6 2
photocatalyst for hydrogenation of nicotinamide (NAD-analogue) and
proton reduction, generated by electrochemical and chemical
reduction. Electronic transitions shifting electron density from the
activated catalytic center to the bridging tpphz ligand significantly
reduce the catalytic activity upon VIS irradiation.
Therefore, the presented work investigates these intermediates
resulting from photoexcitation and subsequent electron transfer
by spectroelectrochemical methods (UV-vis absorption and
resonance Raman (rR) spectroscopy) in combination with
quantum chemical simulations.[3] RR is specifically suitable to
identify the initially photoexcitated state as exclusively vibrations
are enhanced, which are connected to structural changes coupled
to the electronic transition.[4] Time-resolved transient absorption
Introduction
Solar-driven water-splitting to release molecular hydrogen as a
carbon neutral energy source is an attractive solutions to satisfy
the rising global energy demand.[1] Despite considerable progress,
artificial photosynthesis does not reach its full potential in
sustainably converting solar energy into chemical energy.
Attempts to develop visible light-driven hydrogen-evolution
catalysts generally include a photosensitizer for light harvesting,
an inter- or intramolecular electron relay to achieve charge
separation and a catalytic center for hydrogen generation. Many
(
TA) spectroelectrochemistry (SEC) enables the investigation of
the photoinduced electron transfer in intermediates of the catalytic
cycles on femto- to nanoseconds timescales.
Here, we combine rR- and TA-SEC with the photocatalytic
characterization of a key intermediate in the photocatalytic cycle
of [(tbbpy)
(Figure 1A, left structure).
2
Ru(tpphz)Rh(Cp*)Cl]Cl(PF
6
)
2
(Ru(tpphz)RhCp*)
[
a]
b]
Dr. L. Zedler,[+] Dr. Y. Zhang, Dr. M. Wächtler, Prof. Dr. B. Dietzek
Department Functional Interfaces
Leibniz Institute of Photonic Technology Jena (IPHT)
Albert-Einstein-Straße 9, 07745 Jena, Germany
E-mail: benjamin.dietzek@leibniz-ipht.de
[5]
The catalyst features a Ru
II
chromophore, a tetrapyridophenazine (tpphz) bridging ligand and
III
a Rh (Cp*)Cl catalytic center and – in the presence of sacrificial
electron donors – photocatalytically reduces nicotine amides as
[
K. M. Ziems, Dr. Y. Zhang, Dr. M. Wächtler, Prof. Dr. S. Gräfe, Dr.
S. Kupfer, Prof. Dr. B. Dietzek
Institute of Physical Chemistry and Abbe Center of Photonics
Friedrich Schiller University Jena
biologically usable NAD-like (nicotinamide adenine dinucleotide)
cofactors (Figure 1A).[6] The preorganized structure of the
chromophore and the catalyst prevents formation of bioinactive
NAD(P) dimers.[ In addition, catalytic hydrogen evolution was
6-7]
Helmholtzweg 4, 07743 Jena, Germany
E-mail: benjamin.dietzek@uni-jena.de
observed more than six times longer (650 hours) than for
structurally similar tpphz-bridged photocatalysts.[2c, 8] However,
E-mail: stephan.kupfer@uni-jena.de
[
+]
[
c]
A. K. Mengele, Prof. Dr. S. Rau
Department of Inorganic Chemistry I
University of Ulm
using Ru(tpphz)RhCp* as
a
photocatalyst for hydrogen
+
production, the catalytic activity was much lower than for NAD -
reduction. Thus, the study presented here focuses on the
mechanism of the light-driven reduction of NAD-like cofactors.
The reductively fully activated intermediate in the respective
photocatalytic cycle containing two reduction equivalents, i.e.
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: sven.rau@uni-ulm.de
T. Pascher
Pascher Instruments AB
Stora Råby Byaväg 24, S-224 80 Lund, Sweden
These authors contributed equally to the work.
[
d]
+]
II
I
[
2
[(tbbpy) Ru (tpphz)Rh Cp*] (Figure 1, middle), is likely formed in
presence of an excess of sacrificial electron donor upon
irradiation with visible light. Here we exploit electrochemical
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201907247
RESEARCH ARTICLE
reduction and chemical reduction with CoCp
2
to form this species.
The band at 440 nm is due to metal-to-ligand charge transfer
II
It should be noted that finding a suitable reductant to access the
catalytically competent intermediate presents a delicate task as
the respective redox potentials have to be balanced to selectively
reduce the Rh ion only, a task however easily accomplished by
electrochemical reduction. The catalytic competence of the fully
reduced Ru(tpphz)RhCp* towards the formation of reduced
nicotine amides is shown (Figure 1).[6] To the best of our
knowledge, we present for the first time the results of early-time
photodynamics of electrochemically generated intermediates of a
fully competent photocatalyst, i.e., the doubly reduced
Ru(tpphz)RhCp*, under non-catalytic conditions to gain
important mechanistic insights into electron transfer cascades
occurring during the catalytic cycle. These joint catalytic-
spectroscopic mechanistic insights provide a detailed picture of
structure-function-activity correlations in this class of
photocatalysts for artificial photosynthesis.
(MLCT) transitions from the Ru center to both the tpphz and the
tbbpy ligands (Figure 2B).[ Bands between 350 and 400 nm
stem from tpphz-centered π-π* transitions, whereas the sharp
band peaking at 286 nm is due to intra-ligand π-π* transitions
located at the Cp* and the tbbpy moieties. Upon chemical
12]
2
reduction of Ru(tpphz)RhCp* by CoCp , a broad absorption at
650 nm arises, while the spectral changes in the other spectral
regions remain minute (Figure 2A). These reduction-induced
changes are also apparent upon electrochemical reduction
(Figure 2B), which can be performed at specific potentials
enabling a stepwise reduction of Ru(tpphz)RhCp*. Based on
literature no alterations in the UV-vis absorption is expected upon
III
I [5]
reduction of Rh to Rh . Hence, it is concluded that the electronic
II
configuration of the Ru -ion photocenter and, thus, Ru-MLCT
transitions are not affected by the reduced catalytic center.
However, UV-vis SEC experiments show that already the first
I
reduction (yielding a Rh center, vide supra) gives rise to a new
band at 650 nm (red line, Figure 2B). A similar absorption feature
2
was previously observed for singly reduced Ru(tpphz)PtCl and
Results and Discussion
assigned to an intra-ligand π-π* transition of the reduced tpphz-
ligand by rR-SEC and TDDFT simulations (red line, Figure 2C).[
This startling coincidence can be rationalized considering that
photoexcitation of the reduced Ru(tpphz)RhCp* may occur at the
3b]
Figure 1 schematically indicates key reaction steps underlying the
light-driven multielectron proton-coupled hydrogenation of N-
+
benzylnicotinamide (BNA ) by Ru(tpphz)RhCp*. To unravel
I
I
Rh center, i.e., a MLCT from the Rh to the tpphz ligand is excited.
Such transition is observed in the same range as intra-ligand π-
π* transitions of the reduced tpphz-ligand as evident from studies
mechanistic details of the reaction, specifically the photoinduced
reactivity of the key intermediate (shown in the red box of Figure
1), the photocatalytic reaction was first monitored by UV-vis
I
[14]
on the model complex (phen)Rh Cp*.
provided by means of quantum chemical calculations, predicting
bright MLCT transitions into S at 606 nm (Table S4 and
Further evidence is
spectroscopy (Figure 1B). In a second step, the catalytically active
species (Figure 1A, middle) was generated by chemical reduction
with CoCp and tested for its reactivity (Figure 1C, 1D, S18). Here,
2
7
Figure S8) and T11 at 652 nm (Table S5 and Figure S10) for the
doubly reduced Ru(tpphz)RhCp* within singlet and triplet
multiplicity, respectively. UV-vis SEC during the third reduction of
Ru(tpphz)RhCp* reveals a strong absorption increase between
irradiation of the chemically doubly reduced Ru(tpphz)RhCp* in
presence of BNA significantly hampered the catalytic turnover as
+
indicated by comparing the much stronger absorbance increase
at 355 nm (ascribed to the formation of the reduced product
BNAH) in the dark compared to the same sample under
continuous irradiation with visible light. Finally, the intermediate
was prepared in an electrochemical approach and characterized
by UV-vis, rR-SEC and TA-SEC and quantum chemical
simulations. Electrochemical characterization reveals that the first
and second reduction in Ru(tpphz)RhCp* is located at the Rh ion:
The irreversible reduction at -0.6 V vs. a silver pseudo reference
2
I
5
00 and 800 nm, related to MLCT transitions from Rh to the
I
tbbpy ligands (D23 and D24) as well as from Rh to the tpphz ligand
(D25), and to the intra-ligand π-π* transition (D28) of the now
reduced tpphz ligand (blue line, Figure 2B and C, see supporting
information for details).
III/I
electrode is due to a two-electron process in the Rh couple
(
Figure 2B, inset).[9] This process can be explained by two
possible pathways: Either a one-electron reduction is followed by
II
[9b]
fast disproportion of the Rh intermediate or potential inversion
II
I
III
II
of the redox potential of the Rh /Rh and the Rh /Rh couples
allows for an ECE (electrochemical reaction, chemical reaction,
electrochemical reaction) process at the applied potential.[10] The
reduction induces a loss of the chloro ligand altering the original
III
piano stool geometry of the Rh (Cp*)Cl center in
Ru(tpphz)RhCp*, i.e., the plane of the cyclopentadienyl ligand
orients perpendicular to the tpphz ligand (Figure 1A, middle).[11] In
analogy with previous studies, the reduction at -0.9 V is assigned
to the first reduction of the tpphz ligand (Figure 2B).[
Ru(tpphz)RhCp* exhibits similar absorption properties as the
5]
structurally
Ru(tpphz)PtX
related
compounds
Ru(tpphz)PdCl
2
and
(X = Cl, I).[3b, 12] The different catalytic centers do
2
not affect the steady-state absorption properties significantly.[
5, 13]
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RESEARCH ARTICLE
-
I
Figure 1. A: Molecular structure of the hetero-binuclear photocatalyst (left). Cleavage of the chloro ligand is observed during 2e -reduction, resulting in a Rh
II
I
+
intermediate Ru (tpphz)Rh Cp* (framed in red). The general mechanism of the light-driven catalytic hydrogenation of N-benzylnicotinamide (BNA to BNAH) is
displayed including various possibilities to generate the fully charged Ru (tpphz)Rh Cp* (blue: photocatalytic conditions; green: chemical reduction with CoCp
II
I
2
; red:
electrochemical reduction). B: UV-Vis spectroscopic monitoring of the light-driven (λexc = 463 nm) catalytic hydrogenation of a nicotinamide using 20 μM
Ru(tpphz)RhCp* in ACN:H
2
O = 1:1 with 0.1 M TEA and 0.08 M NaH
2
PO
4
. Inset in B: Calculated catalysis parameters TON and TOF based on the spectroscopic
+
changes at 355 nm induced by the formation of BNAH. C and D: UV-Vis spectroscopic changes during addition of 200 μM of subtrate BNA to a solution containing
2
chemically (by CoCp ) reduced Ru(tpphz)RhCp* and during irradiation with a 463 nm LED-stick. Insets in C and D: Relative Absorbance changes at 665 nm and
355 nm with time.
To reveal structural changes of Ru(tpphz)RhCp* upon reduction,
rR-SEC was performed at 405, 473 and 643 nm (Figure 2D-2F).
dominated by features, which are neither visible upon excitation
at 405 nor 473 nm (Figure 2F). Particularly, no bands associated
with the tbbpy ligands are observed, pointing to the fact that the
reduction-induced transition in the red part of the UV-vis
[
3b,
3
c, 4, 12, 15]
At 405 and 473 nm MLCT transitions are probed, which
II
involve both the tbbpy and the tpphz ligands around the Ru
center.[
4,
16]
II
In line with previous studies on closely related
absorption spectrum does not involve excitation of the Ru -center
I
complexes, the contribution of the tpphz ligand to the rR spectra
is higher upon 473 nm excitation as compared to excitation at 405
and the tbbpy ligands. Excitation of a Rh → Cp* transition is
unlikely due to the large electron density of the ligand.[9c,
14]
[
12-13, 17]
nm (Figure 2D and 2E).
However, the 643-nm rR-SEC spectrum compares to the
III
I
Rh /Rh reduction causes only minor changes in the rR spectra of
Ru(tpphz)RhCp* excited at 405 nm (Figure 2D): The tbbpy-
assigned bands at 1320, 1480 and 1540 cm-1 roughly keep their
intensity upon reduction, while tpphz-associated modes decrease
notably. Also the rR spectra recorded at 473 nm (Figure 2E) show
an intensity increase of tbbpy-related bands, while the intensity of
tpphz-associated bands diminishes. Hence, photoexcitation of the
reduced Ru(tpphz)RhCp* at 405 and 473 nm leads to a tbbpy-
MLCT being populated. Reduction of the Rh-center appears to
impede an MLCT transition to the tpphz ligand. This finding
agrees with the quantum chemical calculations and a previous
report[18] showing the excess charge at the Rh-center to be
partially delocalized on the phenanthroline fragment of the tpphz
ligand (see Figure S15). This charge distribution apparently
inhibits (additional) charge transfer from the RuII towards the
tpphz ligand (Figure S8).
spectrum of reduced Ru(tpphz)PtCl
2
(Figure 2F, inset).
Ru(tpphz)PtCl is structurally similar to Ru(tpphz)RhCp*, but, the
2
first reduction is localized on the tpphz ligand instead of the
catalytic metal-center.[3b] When comparing the 643 nm rR
spectrum of the reduced Ru(tpphz)RhCp* with the respective
II
•-
II
2
spectrum of the reduced Ru (tpphz )Pt Cl excited at 514 nm, it
becomes evident that the bands observed for the reduced
Ru(tpphz)RhCp* are associated with the tpphz ligand. Hence,
I
either a Rh → tpphz MLCT or a tpphz-intra-ligand (IL) is excited
at 643 nm. The rR-SEC studies do not allow for discrimination
between these two scenarios. TDDFT simulations on the doubly
reduced Ru(tpphz)RhCp*
multiplicity - show bright MLCT transitions from the Rh center to
the tpphz ligand, partially mixed with a local excitation of the tpphz
-
both in singlet and triplet
I
7
ligand, see S (606 nm) and T11 (at 652 nm) in Figure 2C.
Furthermore, a bright IL transition centered on the tpphz ligand
(T21), typical for the reduced tpphz, is predicted by TDDFT at 548
nm for the doubly reduced triplet species.
rR-SEC at 643 nm interrogates the reduction-induced absorption
band at 650 nm, which is characteristic for the doubly reduced
Ru(tpphz)RhCp* (Figure 2B). The rR spectrum at 643 nm is
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Figure 2. UV-Vis spectrum of the chemically reduced Ru(tpphz)RhCp* (green trace) using CoCp
2
as reducing agent and the spectrum recorded after reaeration
(grey trace) of the solution which regenerates the parent complex (A). Experimental in situ (B) and calculated (C) UV-vis spectroelectrochemical results of reduction
4
states of Ru(tpphz)RhCp*. Inset in B: CV of Ru(tpphz)RhCp* in ACN containing 0.1 M TBABF electrolyte. The applied potentials for acquisition of the UV-vis
spectra are coloured (scan rate 100 mV/s, glassy carbon working, Pt-counter, and Ag/AgCl-pseudo-reference electrodes). RR excitation and transient absorption
pump wavelengths are displayed as continuous and dashed vertical lines in the spectra, respectively. Experimental rR spectra of non-reduced (black), electro-
chemically doubly reduced (red) and chemically reduced (green) Ru(tpphz)RhCp*, excited at 405 (D), 473 (E), 643 and 647 nm (F). Modes assigned to the tbbpy
ligands are marked with an asterisk. Inset: RR spectrum of reduced Ru(tpphz)PtCl
with tpphz vibrations.
2
, excited at 514 nm for comparison. Grey shaded Raman bands are associated
Thus, rR-SEC shows that reduction of the RhIII center in
investigations also proved the similarity of the electrochemically
and photochemically (Figure 1, 2A and S17) generated doubly
reduced Ru(tpphz)RhCp*.
Ru(tpphz)RhCp* alters the electronic transitions available at the
II
III
I
Ru photocenter, i.e., the additional charge density upon Rh /Rh
reduction becomes partially delocalized over the tpphz ligand and
prohibits further charge density shift on the bridging ligand by
Evaluating the Catalytic Competences of
II
I
excitation of a RuII → tpphz MLCT. For Ru(tpphz)PtCl
, the
(tbbpy) Ru (tpphz)Rh Cp*
2
2
transfer of the second electron, which is indispensable for
hydrogen evolution at the Pt center, onto the tpphz bridging ligand
is impeded by the additional charge localized on the phenazine
II
I
The reactivity of Ru (tpphz)Rh Cp* towards the hydrogenation of
+
N-benzylnicotinamide (BNA to BNAH). can be monitored by the
phz) moiety.[3b] Contrary to that, the initial localization of the
increased absorbance of the product around 355 nm (Figure
B).[6] Here, we generated the “fully charged” Ru (tpphz)Rh Cp*
(
II
I
1
excited state on the tbbpy ligands in the doubly reduced
+
Ru(tpphz)RhCp* should not adversely affect the turnover at the
by chemical reduction with CoCp
2
. Upon reaction with BNA in the
I
RhCp* moiety as no third electron is required on the Rh center to
dark, formation of BNAH is apparent by an increased absorbance
promote typical metal-mediated catalysis.[19]
around 355 nm, the loss of absorbance of the Rh -tpphz MLCT
I
I
III
Resonance Raman experiments upon excitation at 647 nm on the
chemically reduced Ru(tpphz)RhCp* (see Figure 2F) yield
identical band pattern as the corresponding experiment on the
electrochemically reduced catalysts. Together with the striking
similarities of the respectively generated UV-vis spectra
band around 650 nm proved the re-oxidation of Rh to Rh (Figure
1
C). Performing the same reaction under irradiation with an LED
2
at λ = 463±12 nm (45 mW/cm ) showed only insignificant changes
of the absorption both at 650 and 340 nm (Figure 1D), i.e. no signs
for catalytic hydrogenation are observed.
(
Figure 2A and B), this provides clear evidence that both
approaches yield the same product, i.e.
Ru (tpphz)Rh Cp*]. Furthermore, UV-vis spectroscopic
II
I
[
(tbbpy)
2
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RESEARCH ARTICLE
Thus, we conclude that irradiation of the sample obviously impairs
the catalysts competence and limits its overall efficiency. To
identify and trace the deactivation pathway of the fully charged
and competent catalytic species during irradiation, transient
absorption experiments will be discussed in the following.
Femtosecond Time-Resolved Transient Absorption-
Spectroelectrochemistry
Time-resolved transient absorption experiments investigating the
light-induced electron transfer dynamics on timescales of sub-ps
to ns are performed as TA-SEC measurements using pump
wavelengths of 403, 492 and 600 nm. At 403 and 492 nm, both
the parent and the doubly reduced Ru(tpphz)RhCp* complex
absorb (see Figure 2B). The parent complex shows transient
absorption features reminiscent of those observed for related
[
(tbbpy)
2
Ru(tpphz)MX
2
]2+ (MX
2
= PdCl
2
, PtCl
2
, PtI
2
) upon MLCT
[
2d, 13, 17]
1
excitation:
An instantaneous bleach of the MLCT band
II
reflects the shift of electron density from the Ru to both tbbpy and
tpphz ligands (Figure 3A). In the latter case, the phenanthroline
(
phen) moiety serves as primary electron density acceptor. The 1
1
3
ps process contains contributions from MLCT → MLCT inter-
system crossing (ISC), vibrational relaxation and an inter-ligand
charge transfer (ILCT) from the tbbpy moiety to the
phenanthroline sphere of the tpphz ligand (Figure 2D and S16).
(
Due to the limited temporal resolution and the spectral
congestion of the processes, the individual contributions cannot
be resolved.) The phenanthroline-centered state relaxes into a
3
phenazine-centered MLCT state (τ
2
= 11 ps), which decays with
a lifetime of 450 ps (τ
3
) (Figure 3D and S16). The assignment of
3
the phenazine-centered MLCT state is based on the broad
absorption of the phenazine radical anion at about 590 nm, which
builds up within 20 ps concomitantly with a slight blue shift of the
absorption maximum (Figure 3A).
Further evidence for the origin of the excited-state absorption
(
ESA) at 590 nm is provided by TDDFT addressing the absorption
3
by spin-allowed triplet-triplet excitations, which reveal a bright IL
transition at 609 nm centered on the phenazine moiety, see T27 in
Figure 3B. This absorption subsequently loses intensity - in
particular - the blue part of the band (Figure 3A and 3C).
Figure 3. Transient absorption data including experimental (A) and calculated
(
B) transient absorption spectra at selected delay times, transient kinetics at key
wavelengths (C) and spectral changes associated with each kinetic process
DAS) (D) for non-reduced Ru(tpphz)RhCp* pumped at 492 nm. For
As a result, the 450 ps component in the decay-associated
spectra (DAS) shows no intensity towards the near IR, while the
infinite spectrum appreciably absorbs at 750 nm (Figure 3D). This
feature indicates electron transfer from the formally reduced
(
comparison, the inverted, i.e., negative, steady state absorption of the complex
Ru(tpphz)RhCp* is also plotted in grey and scaled to the largest ground state
bleach signal at 360 nm (A). Inset in B: Charge density differences for bright
-
III
spin-allowed triplet-triplet excitations into intra-ligand State T27 (with Cl ) and into
phenazine species towards the Rh catalytic center: TDDFT
-
metal-to-ligand charge transfer state T12 (without Cl ); charge transfer takes
III
II
reveals that the [(tbbpy)
2
Ru (tpphz)Rh Cp*] species shows
place from red to blue.
II
appreciable absorption at 738 nm due to a bright Rh → tpphz
3MLCT (T12), while [(tbbpy)
absorbs at around 600 nm based on the IL excitation into T27
III
•-
III
2
Ru tpphz Rh Cp*Cl] strongly
3
The excellent match between the experimentally obtained DAS
and the calculated spectra indicates not only electron transfer
(
Figure 3B). These results explain the spectral shape of the visible
excited-state absorption reflected in the DAS of τ
3
- and the long-
II
from the photoactive [(tbbpy)
2
Ru (tpphz)]-fragment to the
lived component (Figure 3D).
-
catalytically active Rh-center, it also proofs Cl -dissociation on the
50 ps time-scale upon metal-centered one-electron reduction.[
Notably, structural reorganization at the reduced Rh -center upon
Cl -dissociation leads to a linear tpphz-Rh-Cp* geometry, in which
the Rh -center is shielded by steric and electronic factors from
geminate recombination with the solvated Cl , hence leading to
20]
4
II
-
II
-
the long-lived species and in consequence to the long-lived red-
absorbing component in the transient absorption (blue infinite
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
component in the DAS, Figure 3D, S16). In a recent report, this
slow decay of the Rh species has been shown to obey second-
order kinetics, which was interpreted as a disproportionation
Interestingly, the inter-ligand charge transfer process is
comparably slow compared to inter-ligand hopping processes
observed in Ru -complexes and the parent non-reduced
II
II
I
III
[21]
reaction yielding Rh and Rh in the µs to ms time range.
After two-electron reduction of Ru(tpphz)RhCp*, accompanied
Ru(tpphz)RhCp* (11 ps), which is attributed to the fact that
II
electron density in the Rh -center (partially) extends towards the
-
III
by Cl -dissociation from the pentamethylcyclopentadienyl-Rh
tpphz-bridging ligand, as indicated by the respective spin density
in Figure S15. Hence, the additional negative charge on the
phenanthroline fraction of the tpphz-ligand coordinating the Rh-
ion slows the kinetics of inter-ligand electron transfer towards the
center,[
11,
22]
II
I
2
the photodynamics of [(tbbpy) Ru (tpphz)Rh Cp*],
i.e., the photoinduced processes in a “fully charged” molecular
photocatalyst, are observed (Figure 4). To the best of our
knowledge, the data discussed in the following present the first
ultrafast transient absorption study on an isolated, molecular
photocatalyst in its fully active electronic configuration.
3
bridging ligand. With the characteristic time constant of τ = 600
ps, the excess charge density on the tpphz-ligand thermalizes into
the lowest orbital available, i.e., charge density shifts from the
phenanthroline to the phenazine moiety of the bridging ligand.
This is manifested in the comparably low negative ΔOD amplitude
at around 630 nm, which stems from the superposition of the Rh-
center ground state bleach and the excited-state absorption of the
reduced tpphz-ligand, more specifically the reduced central
phenazine unit (DAS, Figure 4A).[23] The resultant
As discussed in the context of the rR data, excitation of
II
I
[
(tbbpy)
2
Ru (tpphz)Rh Cp*] at 403 nm populates an MLCT state,
in which electron density is shifted to the tbbpy ligands, i.e.,
•¯
III
I
[
(tbbpy)(tbbpy )Ru (tpphz)Rh Cp*] is formed (Scheme 1). The
ESA at 360 nm reflects intra-ligand absorption of the tbbpy radical
anion (Figure 4A). Nonetheless, photoexcitation leads to an
instantaneous (within the experimental time-resolution) formation
of a rather strong bleach signal at 650 nm, which is assigned to a
II
•¯
II
2
[(tbbpy) Ru (phen-phz -phen)Rh Cp*] bearing the electron in the
central phenazine sphere forms the long-lived species, which
decays back to the ground-state with a time-constant beyond the
experimentally accessible time range (Scheme 1).
I
Rh → tpphz MLCT transition based on TDDFT and UV-vis SEC
results (Figures 2B, 2C and 4A). This feature thus indicates an
interaction of the two metal centers and hole transfer from the
The slow ns-decay component observed in the TA-SEC data
upon 403-nm excitation corresponds to the intramolecular
electron transfer of the second electron onto a mono-reduced RhII
catalytic center. This electron transfer step is apparently much
slower than the corresponding first electron transfer yielding the
RhIII → RhII reduction (450 ps, see Figure 4A). As the
electrochemical analysis showed potential inversion of the
Rh /Rh and Rh /Rh reduction, the slowed electron transfer must
be attributed to the change in the relative orientation between the
tpphz bridging ligand and the Cp* ligand, which results in a kinetic
barrier for the electron transfer process.
III
I
photoexcited Ru -center to the Rh -center. This process occurs
rapidly, i.e., sub-500 fs, and leads to the formation of
•¯
II
II
[
(tbbpy)(tbbpy )Ru (tpphz)Rh Cp*]. This state is initially
vibrationally hot and cools down with the time constant τ = 1.8 ps.
1
Cooling causes a slight shift of both the ESA maxima at 470 nm
and the ΔOD = 0 crossing at 725 nm (Figure 4A). The relaxed
•
¯
II
II
III
II
II
I
[
7
(tbbpy)(tbbpy )Ru (tpphz)Rh Cp*] reveals a strong ESA above
II
20 nm, which the calculations assign to a tpphz → Rh -LMCT
transition. In addition the tbbpy absorption below 440 nm and the
bleach of the Rh → tpphz MLCT transition at around 630 nm is
apparent (Figure 4A). The strong bleach of the Ru -center at
•
¯
I
II
As concluded from the TA-measurements and the quantum
chemical calculations, a perpendicular arrangement between
tpphz bridging and Cp* ligand also exists at the
about 480 nm is not visible in DAS (τ
1
2
) and DAS (τ ) but only
appears on a 100 ps timescale. On this timescale, associated with
•
-
II
II
I
τ
2
= 43 ps, inter-ligand tbbpy → tpphz electron transfer takes
2
[(tbbpy) Ru (tpphz)Rh Cp*] state (Figure S16). Similar to the Rh
II
•¯
II
place, i.e., [(tbbpy)
2
Ru (tpphz )Rh Cp*] is formed (Scheme 1).
species, electron density redistribution from the strong donor
II
We hypothesize that the partial Ru-center bleach in
ligand Cp* over the connecting Rh core to the phenanthroline
•¯
II
II
•-
[
(tbbpy)(tbbpy )Ru (tpphz)Rh Cp*] is superimposed by a tbbpy
sphere of the tpphz ligand might establish an electronic barrier for
the electron localized at the phenazine sphere which drastically
reduces the transfer kinetics of the second electron onto the
catalytic unit. 600 nm excitation of the “fully charged”
Ru(tpphz)RhCp* photocatalysts allows for cross-validation of the
model put forward before.
→
tpphz excited-state absorption. This renders the net optical
density changes in the spectral region below 500 nm (slightly)
•
-
positive. Upon inter-ligand tbbpy → tpphz electron transfer, more
prominent GSB features below 500 nm are observed, while the
bleach of the Rh-center at 650 nm decreases (Figure 4A).
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Figure 4. Transient absorption data including transient absorption spectra at selected delay times, transient kinetics at key wavelengths and spectral changes
associated with each kinetic process (DAS), for doubly reduced Ru(tpphz)RhCp* pumped at 403 (A), 600 (B) and 492 nm (C). For comparison, the inverted, ie.,
negative, steady state absorption spectra of doubly reduced Ru(tpphz)RhCp* are plotted in addition in grey and scaled to the maximum ground state bleach
signal within the individual graphs.
Excitation within the reduction-induced absorption band (see
Figure 2B) leads to a charge-density shift associated with a RhI
ground-state bleach in the same spectral region (Figures 4A,
4B).[23]
→
tpphz MLCT transition. TDDFT calculations show that the
Photoexcitation of doubly reduced Ru(tpphz)RhCp* at 492 nm
leads to similar transient absorption features obtained with
403 nm excitation (Figure 4C). In agreement with rR-SEC at
excess charge density is localized on the phenanthroline part of
the bridging ligand that coordinates the Rh -ion. Hence, 600 nm
excitation of the doubly reduced Ru(tpphz)RhCp* catalyst initially
leads to a vibrationally hot [(tbbpy)
I
II
473 nm, a Ru → tbbpy MLCT state is initially populated (Figures
II
•¯
II
I
2
Ru (phen-phz-phen )Rh Cp*]
2E and 4C). Additionally, a Rh → tpphz MLCT transition is excited,
state (Scheme 1). The subsequent dynamics are characterized
by a 4.7 and a 485 ps process (DAS, Figure 4B), which lead to
since both absorption features are spectrally broad and
overlapping. Consequently, two overlapping decay processes
from both MLCT transitions are observed in parallel. Therefore,
the TA spectra obtained under 403 and 492 nm excitation are
qualitatively similar, but interpretation is significantly complicated.
Therefore, the data was not analyzed in detail, because too many
decay constants would be required rendering an in-depth kinetic
analysis unreliable.
the population of
experimentally accessible delay-time window of 1.8 ns. The
fastest component, τ = 4.7 ps, is spectrally characterized by a
a molecular species that outlives the
1
blue-shift of the ∆OD = 0 crossing at around 680 nm and the
I
disappearance of a (negative) shoulder in the red-edge of the Rh -
ground state bleach feature (Figure 4B). Hence, it is associated
II
•¯
II
with cooling of the [(tbbpy)
2
Ru (phen-phz-phen )Rh Cp*] state,
The combined catalytic, photophysical, electrochemical and
theoretical data paint a very similar picture, i.e., photoexcitation of
however, it should be noted that the excess electron density on
the tpphz-ligand is still considered to be localized on the
phenanthroline moiety of the ligand. Only during the process
I
the “fully charged” Ru(tpphz)RhCp* complex containing a Rh ion
II
leads to the photochemical formation of a Rh intermediate. The
associated with τ
central phenazine part of the tpphz ligand, i.e., forming the
2
= 485 ps, the charge density relaxes to the
same light required for photochemical generation of the
catalytically competent Rh state thus leads to its deactivation by
I
II
•¯
II
species [(tbbpy)
2
Ru (phen-phz -phen)Rh Cp*] (Scheme 1). This
MLCT formation. This is directly reflected by the absence of
II
I
species is found to be long-lived on the time-scale of the
femtosecond transient absorption experiment.
catalytic activity of the intermediate Ru (tpphz)Rh Cp* under
irradiation conditions. The photoinduced discharging produces a
relatively long-lived RhII state describing for the first time the
catalytic inactivity of a mono-reduced (N,N)RhCp* complex, i.e.
II
It should be pointed out that the formation of [(tbbpy)
2
Ru (phen-
•
¯
II
phz -phen)Rh Cp*] occurs with a charge density shift from the
I
II
Rh-fragment upon 600 nm excitation with a time-constant of 485
the Rh state, towards nicotinamide reduction. This is particularly
II
ps as well as from the Ru -fragment upon 403 nm excitation with
important as the electrochemical properties of virtually all
(N,N)RhCp* complexes only permit experimental access to the
a time-constant of 600 ps (Figures 4A and 4B). The similarity of
characteristic timescales reflects the apparent energetic
symmetry of the tpphz ligand even though it coordinates two
different metal ions. In either of these cases the phenanthroline
I
directly generated Rh state during metal-centered reduction.
Based on our mechanistic conclusions, we suggest to control the
overall catalytic process by sequential photochemical RhI
I
→
phenazine charge density shift is spectrally manifested in an
generation and thermal consumption of Rh coupled to substrate
apparent decrease of ground-state bleach in the red part of the
conversion. Utilizing this approach, BNAH formation was
successfully implemented (see Figure S19).
•¯
∆OD < 0 band. This is due to the fact that the phz -fragment
strongly absorbs at around 550 nm, i.e., partially overriding the
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Scheme 1. Schematic representation of the proposed photophysical pathways for the doubly reduced photocatalyst Ru(tpphz)RhCp* upon photoexcitation at 403
and 600 nm. At 403 nm a MLCT from Ru to the tbbpy ligand is excited, and then an electron hole is transferred on a sub-500 fs timescale, reducing RuIII and
II
I
oxidizing the Rh center. This process is followed by an inter-ligand transition to tpphz, finally decaying by an intra-ligand charge transfer from the phenanthroline to
I
the phenazine fragment of the bridging ligand, followed by a LMCT to the ground state. In contrast, at 600 nm excitation, a MLCT from Rh to tpphz is excited, which
decays via an intra-ligand charge transfer to the ground state. Here, the Ru-center and the tbbpy ligands are not involved in the photodynamical processes.
I
[19b, 25]
III
[19a, 26]
I
Conclusion
either a Rh (Cp*H)
or a Rh (Cp*)H
unit, formed upon
fast[26] oxidative addition of a proton to the Rh Cp* moiety. As this
hydride transferring agent does not exhibit a red absorbance,[
it is possible in these systems to escape the potential visible light-
driven inactivation of catalysis by working in sufficient acidic
media during photocatalysis.
With respect to future applications of molecular catalysts using
the whole solar spectrum for light-driven catalysis, the detected
MLCT transitions from the reductively fully activated catalytic
center back to the bridging ligand represent a putatively activity
limiting step. It is clear that a more careful design of the ligand
sphere supporting the catalytic center is crucial for supporting
sufficiently high activity. The presented results clearly show that
the design of photocatalytically active systems will benefit from
close interactions between spectroscopy, theory and synthesis,
leading to spectroscopy-aided design of next generation catalysts.
19b]
The elucidation of multi-electron reaction pathways, e.g. in
hydrogen evolving photocatalysis, requires the identification and
dynamic monitoring of intermediates, which can be mimicked by
sequential electroreduction. To the best of our knowledge, we
presented for the first time the results of in situ early-time
photodynamics of an isolated, molecular photocatalyst in its
active electronic configuration. Pulsed laser excitation of doubly
reduced Ru(tpphz)RhCp* at either 403 or 600 nm leads to a
charge density shift towards the central phenazine part of the
tpphz bridging ligand, irrespective of the fact that completely
II
different states are initially populated. At 403 nm, a Ru → tbbpy
MLCT transition is excited, while at 600 nm, a Rh → tpphz MLCT
state is populated as confirmed by rR-SEC and TDDFT. The long-
2
lived radical anion [(tbbpy) Ru (tpphz )Rh Cp*] in the described
I
II
•¯
II
relaxation cascade is possibly a potent precursor for an additional
chemical deactivation pathway of the catalyst.[
24]
The
investigations towards the effect of blue light irradiation of the
+
catalytic reaction mixture containing BNA clearly showed, that
once the doubly reduced Ru(tpphz)RhCp* state is formed,
+
photoexcitation impedes catalytic hydrogenation of BNA to
BNAH. This is in full agreement with the results of the detailed
spectroscopic and theoretical investigation which point to the
II
formation of an inactive Rh intermediate upon photoexcitation.
The excitation-induced charge redistribution in the doubly
I
II
reduced Ru(tpphz)RhCp* can be formally viewed as a Rh → Rh
discharging process leading to inactivation of the catalytic center.
However, the actual “H” transferring species in RhCp* catalysts is
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
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Entry for the Table of Contents (Please choose one layout)
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RESEARCH ARTICLE
[
a]
For
the
first
time
Linda Zedler, Alexander Klaus
[
c
]
[ ]
Mengele, Karl Michael Ziems, b Ying
Zhang,[ Maria Wächtler,
spectroelectrochemistry using UV-vis,
resonance Raman and transient
absorption spectroscopy has been
employed to investigate the reactive
intermediates of the photocatalytic
a, ]
[a, ]
b
b
Stefanie
[b]
[d]
Gräfe, Torbjörn Pascher, Sven
[ ]
[ ]
Rau,* c Stephan Kupfer* b and
Benjamin Dietzek*
cycle
(tbbpy)
of
the
photocatalyst
[
2
Ru(tpphz)Rh(Cp*)Cl]Cl(PF )
6 2
Page No. – Page No.
in detail. Photo-induced electronic
transitions limiting the catalytic
efficiency have been identified for
guiding structural improvements.
Title
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
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Title
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