The central role of the metal ion for photoactivity: Zn-vs.Ni-Mabiq

Article abstract of DOI:10.1039/d0sc06096h

Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and efficient earth-abundant photoredox catalysts remai

Full text of DOI:10.1039/d0sc06096h

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The central role of the metal ion for photoactivity:
Zn– vs. Ni–Mabiq†
Cite this: DOI: 10.1039/d0sc06096h
b
c
Raphael Lauenstein,a Sophie L. Mader,
Henrieta Derondeau,
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Oaikhena Z. Esezobor,a Matthias Block,c Armin J. Romer,b Christian Jandl,a
¨
c
a
Ville R. I. Kaila, ‡b Jurgen Hauer,a Erling Thyrhaug
*
Eberhard Riedle,
¨
a
*
and Corinna R. Hess
Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as
powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and
efficient earth-abundant photoredox catalysts remains a challenge. We here present the photochemical
and photophysical properties of a Ni–Mabiq catalyst ([NiII(Mabiq)]OTf (1); Mabiq ¼ 2-4:6-8-bis(3,3,4,4-
tetramethyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6)—and of
a
Zn-containing analogue ([ZnII(Mabiq)OTf] (2))—using steady state and time resolved optical
spectroscopy, time-dependent density functional theory (TDDFT) calculations, and reactivity studies. The
Ni and Zn complexes exhibit similar absorption spectra, but markedly different photochemical properties.
These differences arise because the excited states of 2 are ligand-localized, whereas metal-centered
states account for the photoactivity of 1. The distinct properties of the Ni and Zn complexes are manifest
in their behavior in the photo-driven aza-Henry reaction and oxidative coupling of methoxybenzylamine.
Received 5th November 2020
Accepted 21st April 2021
DOI: 10.1039/d0sc06096h
rsc.li/chemical-science
complexes, remain the ‘workhorses’ of photoredox catal-
ysis.1,17–19 However, the scarcity and cost of these elements
Introduction
signicantly limits their applications. Alternative catalysts
based on low-cost, earth-abundant elements are clearly needed.
While chemical behavior is generally similar within a given
group of elements, the simple approach of replacing 2nd and
3rd row transition metal complexes with earth-abundant 1st
row analogues is not straightforward. A major complicating
factor is the oen dramatically different photophysics of the
lighter elements. A striking example is provided by the RuII
polypyridyl complexes, which tend to form metal-to-ligand
charge transfer (MLCT) states with lifetimes approaching
microseconds.17 In contrast, ferrous analogues are notoriously
short lived, with relaxation to high-spin metal-centered (MC)
states oen proceeding on the sub-picosecond timescale.20
Nevertheless, there have been several exciting developments
with respect to earth-abundant photoredox catalysts, including
rst-row transition metal complexes based on Zr, and Cr
through Zn.21–24 Moreover, sophisticated efforts are being made
to design and tailor ligand scaffolds and impose desirable
electronic structure features on the coordinated metal. Varia-
tion of the transition metal clearly can also alter the photo-
physical and chemical properties among complexes based on
a given ligand system. However, systematic investigations with
a broad range of M–ligand complexes in this context are limited.
Our own efforts in this regard center on the development of
photosensitizers based on the macrocyclic biquinazoline
ligand, Mabiq. We recently demonstrated that both
The breadth of light-driven chemical transformations has
rapidly expanded over the last decade, due to a surge of research
in photoredox catalysis.1–8 Photoredox catalysts are used not
only for solar fuel applications,9–13 but also as components of
complex catalytic processes that couple photosensitizers with
secondary organic, transition metal or Lewis acid catalysts.14–16
Consequently, contemporary developments in light mediated
catalysis have enabled unique reactivities beyond what organ-
ometallic catalysts alone can achieve. Photocatalysts thus have
emerged as important tools for sustainable chemistry and
renewable energy needs.
An ensuing challenge within the eld is to develop an array
of photosensitizers for these varied catalytic processes. The
conventional noble metal complexes, Ru and Ir polypyridyl
aDepartment of Chemistry and Catalysis Research Center, Technical University of
Munich, Lichtenbergstr. 4, 85747 Garching, Germany. E-mail: corinna.hess@ch.tum.
de; erling.thyrhaug@tum.de
bDepartment of Chemistry, Technical University of Munich, Lichtenbergstr. 4, 85747
Garching, Germany
cLehrstuhl fur BioMolekulare Optik, Fakultat fur Physik, Ludwig-Maximilians-
¨
¨
¨
¨
¨
Universitat Munchen, Oettingenstr. 67, 80538 Munich, Germany
† Electronic supplementary information (ESI) available: Experimental procedures,
spectroscopic and TDDFT data, crystallographic tables and CIF le. CCDC
1949717. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d0sc06096h
‡ Present address: Department of Biochemistry and Biophysics, Stockholm
University, Stockholm, Sweden.
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[CuII(Mabiq)OTf] and [NiII(Mabiq)]OTf (1, Fig. 1) can be pho- reactivity of 1 and 2—alongside select supporting studies of
toreduced in the presence of a sacricial donor to the corre- the Fe– and Co–Mabiq complexes—provide insight into the
sponding neutral [MII(Mabiqc)]0—formally ‘MI’§—forms.25,26 unique characteristics and potential applications of the
Likewise, we have now veried the ability to photoreduce the metal–Mabiq (M–Mabiq) system.
Fe–, Co–, and Zn–Mabiq complexes (vide infra). Furthermore, 1
acts as an effective photoredox catalyst for the cyclization of
Results and discussion
a bromoalkyl-substituted indole.25 To our knowledge, no other
ligand system can support similar photoredox behavior with
such a wide range of metal ions, but the excited state mecha-
nisms involved in these processes remain elusive.
Compound 2 was generated according to an established
procedure for MII–Mabiq synthesis,27 and its composition was
1
veried by mass spectrometry, H NMR spectroscopy and CHN
In this work, we have focused our attention on the
detailed photochemical and photophysical studies of the Ni
complex 1, a conrmed photocatalyst. We aimed to obtain
a deeper understanding of the root of its photoactivity by
probing the excited state structure and dynamics of this
compound, using spectroscopic and computational
methods. As a benchmark, we compare the results with the
zinc–Mabiq analogue ([ZnII(Mabiq)OTf], 2, Fig. 1). In
contrast to the Ni compound, the fully occupied d-orbitals of
the redox-inert Zn ion are not expected to contribute signif-
icantly to the spectroscopic and redox properties of the
complex. Thus, a comparative study of these complexes is
expected to yield valuable information about how the overall
properties are altered by varied metal–ligand interactions.
We present results for both complexes using a combination
of steady state and transient optical spectroscopy, time-
dependent density functional theory (TDDFT) calculations,
and reactivity studies. In addition, we examined the
comparative activities of the two compounds in select pho-
tocatalytic reactions. The comparative properties and
analysis (see ESI and Fig. S1† for details).
In the solid state, complex 2 adopts a square pyramidal
geometry (Fig. 1), with the triate ligand in the apical position
˚
(Zn–O ¼ 2.0661(15) A). The ligand is signicantly less buckled
than the uncomplexed HMabiq,28 as also observed in the
structure of 1.25 The 19F NMR spectra (Fig. S2 and S3†) of 2 show
a single resonance in both DCM-d2 (ꢀ79 ppm) and MeCN-d3
(ꢀ78 ppm), similar to the spectrum of 1 (ꢀ79 ppm, MeCN-d3;
Fig. S4†). Thus, the triate molecule of 2 may dissociate in
1
solution. The H NMR of the complex in MeCN-d3 with added
Et3N (500 equiv., Fig. S6†) shows two species, which indicates
that the amine also may coordinate at high concentrations.
Optical properties: steady-state spectra
The absorption spectra of 1 and 2 (Fig. 2, top) are overall similar
in structure. Both feature a moderately intense progression of
bands in the blue-green spectral region (lmax ¼ 489 nm/2.53 eV
and 457 nm/2.71 eV, respectively), followed by more intense
transitions in the near-UV. These characteristic absorption
features also dominate the visible spectra of CoIII– and CuII–
Mabiq complexes,26,29 and are recognizable in the spectra of
Fig. 2 Top: Absorption spectra (1 : 1, THF : MeCN) of 1 (red) and 2
(blue). The emission spectrum of 2 (lmax ¼ 505 nm) is shown by the
Fig. 1 (Top) Chemical structure of 1 and 2. (Bottom) Molecular dotted trace. Bottom: Absorption spectra of the photoreduced forms
structure of the newly synthesized 2 (50% ellipsoids; solvent molecules of 1 (1red; red) and of 2 (2red; blue); 1 : 1, THF : MeCN (see Fig. S13 and
and hydrogen atoms omitted for clarity).
S16† for photoreduction data).
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other Mn–Mabiq compounds.27 The similarity among a fairly
large range of coordination compounds suggests that these
features predominantly involve ligand-localized (singlet) states.
In support of this, we note that for both 1 and 2 (Fig. S8 and S9†)
the absorption spectra are essentially solvent-insensitive,
implying that at least the initial excitation has negligible
charge-transfer (CT) character. Despite the similarities among
the absorption spectra, the photo-chemical and -physical
behavior of the complexes differ markedly and are strongly
dependent on the metal center, as illustrated by our studies
with 1 and 2 (vide infra).
Appreciable luminescence is only observed for 2, which has
an emission quantum yield approaching 0.7 in DCM. The
Stokes shi is modest and the emission spectrum (Fig. 2, top,
dotted line) exhibits mirror-image symmetry with the absorp-
tion. The emission wavelength is close in energy to the very
weak emission observed for the [CuII(Mabiq)OTf] (513 nm),26 as
well as of the uncoordinated Mabiq ligand (501 nm; Fig. S7b†),
as a further indication of the ligand-centered character of the
transitions. In stark contrast to this dye-like optical behaviour,
the Ni-containing 1—as well as the other open d-shell Mn–
Mabiq compounds that we have examined (vide infra)—is
effectively non-emissive (quantum yield < 10ꢀ4). This might be
expected, as the open-shell conguration of 1 allows for relax-
ation pathways involving optically dark MC and MLCT/LMCT
states.
Scheme 1 The series of M–Mabiq complexes (M ¼ Fe, Co, Ni, Cu, Zn)
can be photoreduced upon irradiation at l ¼ 455 nm, in the presence
of a sacrificial donor.
S15 and S16†). The product spectrum is identical to that of the
chemically generated [ZnII(Mabiqc)] (2red),27 conrming that 2
undergoes one-electron reduction during the photochemical
reaction. The photoreduction to 2red is both fast, reaching
completion within seconds (at [2] ¼ 77 mM, Pref ¼ 248 mW,
[Et3N] ¼ 38.5 mM, MeCN : THF ¼ 1 : 1; Fig. S15†), and efficient
(quantum yield of 0.79 as determined at Pref ¼ 1.4 mW, [Et3N] ¼
345 mM; Fig. S16 and S17†). The yield is three orders of
magnitude larger than the photoreduction yield of 1 (see ref. 25
and Fig. S13 and S14;† F ¼ 2.9 ꢁ 10ꢀ4 at [1] ¼ 67 mM, Pref ¼ 245
mW, [Et3N] ¼ 33.5 mM). However, the photogenerated 2red
decays shortly aer its formation, as seen by the disappearance
of its 550–800 nm features, and the concomitant emergence of
new bands below 500 nm. These latter bands also subsequently
decrease in intensity. We note that the ability to chemically
synthesize and isolate [ZnII(Mabiqc)]27 indicates that the
reduced form of the Zn complex is not inherently
unstable, but that the decomposition is light-driven under the
photoreduction conditions. The photo-decomposition of 2red is
unique in the Mn–Mabiq series, as the corresponding Ni and Cu
complexes are stable indenitely under analogous condi-
tions.25,26 Similarly, the photoreduction of the two open-shell
FeII– and CoII–Mabiq complexes, [FeII(Mabiq)(MeCN)2]PF6 and
[CoII(Mabiq)(THF)]PF6, also leads to photostable reduced
products (Fig. S18 and S21†) with quantum yields comparable
to that of 1 (Fe: 1.8 ꢁ 10ꢀ4; Co: 3.6 ꢁ 10ꢀ4). The ability of Et3N to
coordinate to 2, as evidenced by 1H NMR, may contribute to the
rapid photoreduction of this particular complex in the series, as
O2 sensitization
Structurally related Zn pyrrin- and pyrrole-based complexes act
as singlet oxygen photosensitizers, which has prompted interest
in such compounds for photodynamic therapy.30–32 To probe the
potential formation of triplet states in our system, we
examined singlet oxygen generation by
1 and 2, using
dihydroxynapthalene (DHN) as a 1O2 scavenger (Fig. S28 and
S30†). Based on the absorption of the oxidized product, juglone,
a reaction yield of 10% was determined using 2 as the photosen-
sitizer. In contrast to 2, 1 does not act as a 1O2 photosensitizer: no
changes were observed in the absorption spectrum upon irradia-
tion of an aerated 1/DHN solution (Fig. S30†). This could suggest
that intersystem crossing (ISC) is not the primary luminescence
quenching route for 1. However, other factors should be taken into
consideration: the triplet excited state may be too short-lived to
efficiently participate in diffusion-controlled reactions with O2; the
3NiII* energy may be insufficient for the sensitization; or triplet–
triplet annihilation may be sterically hindered. Our work (vide
infra), as well as recent work on other NiII photocatalysts,33 implies
that this lack of sensitization is predominantly due to the low
energy of the 3NiII* state.
well as the subsequent instability of 2red
.
In order to assess potential specic interactions
between the reducing agent and the M–Mabiq compounds, we
additionally examined the photoreduction of 1 by two
other sacricial donors: dimethylaniline (DMA) and
N-(2,4,6-trimesitylbenzyl)dicyclohexylamine (N(CH2Mes)Cy2).
Structurally, the donors differ signicantly from Et3N. The
nitrogen atom of N(CH2Mes)Cy2 is hindered by the bulky
cyclohexyl and mesityl groups, which might (partially) inhibit
collisional quenching and ground-state pre-association. The
aniline moiety of DMA also is a poor ligand relative to trie-
thylamine. However, these molecules have comparable
oxidation potentials to Et3N (Eox (vs. SCE, MeCN): DMA ¼ ca.
0.7 V;34 N(CH2Mes)Cy2 ¼ 0.78 V (ref. 25)). Therefore, the
Photoreduction
Compound 2 can be photoreduced in the presence of Et3N as
a sacricial donor, in accord with the photochemical behaviour
of 1 and the Cu–Mabiq complexes (Scheme 1).25,26 Upon irra-
diation of a solution of 2 (l ¼ 455 nm), a series of new bands
appears in the red spectral region, coincident with a decrease in
the higher energy absorption bands of [Zn(Mabiq)OTf] (Fig. 2,
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amines should readily reduce NiII* if photoreduction occurs
in an outer-sphere, diffusion-controlled manner.
Time-evolution of transient spectra. The time-resolved uo-
rescence of 2 shows that the initially populated state—a ligand-
centered singlet—decays mono-exponentially with a 1.1 ns
lifetime (Fig. S27†). While uorescence provides only limited
information about the excited-state behaviour, it suggests that
at least the initial photophysics is relatively simple.
The uorescence properties can be compared with observa-
tions from TA. Immediately on excitation, both 1 and 2 feature
stimulated emission (SE) to the red of the excitation wave-
length—i.e., the longest wavelength absorption band—and
The NMR spectrum of the Ni complex was unaltered upon
addition of >100 equiv. (>0.5 M) of any of the three reductants
to solutions of 1 (see ESI and Fig. S5a and b†). Thus, as ex-
pected for the square planar d8 complex we nd no evidence of
ground-state coordination or non-covalent interactions of the
reductants to 1. Nevertheless, we observe substantial differ-
ences in the photoreduction efficiency. The photoreduction of
1 by N(CH2Mes)Cy2 proceeds with a yield that is nearly iden-
tical to that using triethylamine (F ¼ 2.1 ꢁ 10ꢀ4 at [1] ¼ 67 mM,
Pref ¼ 245 mW, [(N(CH2Mes)Cy2)] ¼ 33.5 mM; Fig. S23†).
However, only low yields of 1red were obtained using DMA,
even with 10-fold higher concentrations of the reductant (F ¼ 5
ꢁ 10ꢀ5 at [1] ¼ 51 mM, Pref ¼ 245 mW, [DMA] ¼ 255 mM;
Fig. S24†). The oxidized forms of all three donor molecules
examined here should have limited chemical stability; a factor
which is critical to efficient photoreduction.34 Nonetheless,
factors beyond the redox potential clearly have an impact on
the photoreduction.
a
broad featureless excited-state absorption (ESA) band
extending into the near-IR. These early time spectra can be
1
assigned to ligand-localized singlet states, referred to as NiII*
1
and ZnII* respectively from here on (singlet and triplet nota-
tions refer to the overall spin state of the complex).
We can follow the dynamics of these initial singlet excited
states by the transient kinetics around the excitation wave-
length and in the long-wavelength region (magenta and brown
vertical cuts in Fig. 3a and d, respectively).
The initial TA kinetics of 2 semi-quantitatively matches the
time constant of the uorescence decay, as the SE decays with
a time-constant close to the 1 ns length of the optical delay
stage. Beyond this loss of stimulated emission, only
subtle changes occur in the spectra during this time period, as
illustrated by both vertical (Fig. 3e) and horizontal cuts (Fig. 3f)
through the TA data. These changes can be attributed to
solvation dynamics and small-scale geometric relaxation.
The corresponding kinetic traces of 1 (Fig. 3b), on the other
hand, exhibit much faster and more complicated behaviour,
with a concomitant decay of the long-wavelength ESA and SE on
a timescale of only ꢂ1 ps. This rapid decay of the SE signal of 1
explains the negligible luminescence: the population transfers
out of the initial, optically bright, 1NiII* state with a rate
constant far larger than the radiative relaxation rate.
Excited-state relaxation dynamics
The steady state absorption/emission spectroscopy, oxygen
sensitization, and photoreduction experiments described above
imply that, although the initial excitation of 1 and 2 is to states
of similar character, the subsequent relaxation dynamics are
substantially different. To elucidate these relaxation processes,
time-resolved optical spectroscopies are the method of choice.
Using time-resolved uorescence spectroscopy, only the
excited state dynamics of 2 can be addressed conveniently
(Fig. S27†). As 1 is essentially non-emissive, its relaxation
dynamics must be followed by ultrafast transient absorption
(TA) spectroscopy.35,36 To investigate both 1 and 2 on common
footing, and to monitor dynamics beyond the relaxation of the
Mabiq-localized excited singlet state, we recorded their fs/ps TA
spectra in the 1 : 1 MeCN : THF solvent system relevant to the
photocatalytic experiments (Fig. 3; noteworthy delay times and
spectral positions are indicated by the horizontal and vertical
lines in (a) and (d)). Spectra for both complexes during the
subsequent ns/ms time range are shown in Fig. S35 and S38.† All
decay constants determined from a comprehensive analysis,
along with our assignments, are summarized in Table 1. While
the statistical errors (see Table S11†) in the multi-exponential t
model are small, multi-component models are delicate. We
therefore supply this uncertainty estimate with an estimate of
the upper bound of possible parameter ranges from the peak-
widths of a maximum entropy analysis (vide infra). This anal-
ysis yields ranges of approximately 50% of the mean value.
Notably, even using this highly conservative upper bound the
time constants remain well separated and we can safely
consider the reported time-constants as meaningful.
1
Aer the decay of the initial ZnII* excited state, the relaxa-
tion of 2 continues at longer timescales (see Fig. S38 and S39†).
We assign these slower dynamics, as described in Table 1, to
processes within a ligand-centered 3ZnII state. This triplet state
decays on timescales of 580 ns and 1 ms, followed by formation
of lower-amplitude signals that decay on timescales up to 140
ms. A possible explanation of the latter processes is ligand
dissociation and nally non-geminate re-association; unlike 1,
the Zn complex can coordinate a h ligand in the axial
position.
In contrast to 2, 1 displays substantial spectral dynamics
over much of the sub-ns range—also aer the initial ꢂ1 ps
decay phase. The complexity of the spectral time-evolution
clearly demonstrates that the 1NiII* state relaxes via multiple
distinct intermediates before the ground state is recovered. This
complexity contrasts with the essentially two-state relaxation
dynamics observed for bipyridyl (bpy) based NiII
photocatalysts.33
A number of signicant differences in the spectral evolution
of the two complexes are readily apparent—in particular, the
dynamics of 1 are faster and more complicated on the fs/ps time
scale. We outline these differences in detail in the following
sections.
To shed light on these dynamics, we show the transient
spectra of 1 at selected delay times in Fig. 3c. As the spectra
demonstrate signicant time-dependent changes in the excited
states, we have subtracted the ground state bleach contribution
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Fig. 3 (a) and (d) Transient absorption data of 1 and 2 in degassed 1 : 1 MeCN : THF after excitation at 460 nm (70 fs pulse) and 493 nm (49 fs
pulse), respectively. Red-hued colors denote increased absorption due to, e.g., excited absorption or product absorption, blue-hued colors
denote decreased absorption due to, e.g., ground state bleach or stimulated emission. (b) and (e) Kinetic traces extracted along the areas marked
by the vertical lines. (c) and (f) Transient spectra extracted at selected probe-delays according to the horizontal lines. Note that in (c), the ground
state bleach was subtracted for clarity (see text for details), while the cuts in (f) were unaltered. The ESA features marked by the vertical dashed
lines in (c) are discussed in detail in the text.
Table 1 Summary of the dynamics observed for 1 and 2 in the tran- excited organic molecules—with a largely featureless photo-
sient spectroscopya
induced absorption extending into the near-infrared (NIR).
Over the next ten ps, well-dened spectral structures emerge.
Two features of particular interest are highlighted with vertical
Assignment
1 (NiII)
0.37 ps
1.1 ps
2.5 ps
14 ps
dashed lines in Fig. 3c: First, a relatively narrow ESA feature
appears at approximately 465 nm concomitant with the decay of
Solvation dynamics
1NiII* / MC/CT
1
the ligand-localized NiII* state. This is followed by the emer-
MC/CT / MC
gence of a feature at 490 nm. Accordingly, kinetic traces from
the main ESA bands at 487 and 364 nm, which capture both of
these contributions (Fig. 3b, violet and green traces), are rela-
tively complex; an indication of the formation and decay of
several distinct spectral species. Such dynamics—fast decay of
SE and broad ESA features, followed by well-dened ESA bands
slightly red-shied relative to the absorption spectrum—are
oen seen in association with energy-transfer from optically
bright ligand-localized states to dark metal-centered states in
open d-shell transition metal complexes (exemplied, e.g., by
the photoinduced dynamics of nickel porphyrin).39,40 The time-
evolution of the spectra here shows, however, that this transi-
tion from ligand-localized to metal-centered states involves at
least one short-lived intermediate state.
Global kinetic analysis. A more complete picture of the
spectral evolution of 1, and thus the overall relaxation
dynamics, requires global analysis of the whole dataset. In order
to ensure maximal reliability of the extracted model, we per-
formed two independent analyses: a model independent
maximum entropy analysis (Fig. S34†),41,42 as well as singular-
value decomposition followed by a global t to a sum-of-
exponentials model.43 We nd good agreement of timescales
from both approaches, implying good consistency of the
extracted physical model. In agreement with the complex
Spectral narrowing, e.g., vibrational relaxation
Combined MC / GS and MC / MC/CT
MC/CT / ground state (GS)
81 ps
772 ps
2 (ZnII)
2 ps
62 ps
1.1 nsb
580 ns
1 ms
Solvation dynamics
Geometric relaxation
3
SE loss/1ZnII* / ZnII
Relaxation within 3ZnII manifold
Decay of ligand-centered 3ZnII state
Ligand dissociation (tentative)
Ligand re-association (tentative)
5 ms
140 ms
a
Fit statistics, selected traces and residuals can be found in the ESI, Fig.
S66, S67 and Table S11. b Time constant determined from uorescence
decay. The same constant as determined from the fs–ps TA
measurements is 770 ps with a large error, since this is close to the
full scanning range. For the ns–ms experiment a value of 1.5 ns is
obtained, shorter than the 2.7 ns excitation pulse.
using a published weighting procedure37,38 to clearly illustrate
these changes. As such, these spectra directly show the time-
evolution of the excited-state absorption spectra, distorted
only at early times by the stimulated emission contribution.
The excited-state spectrum we observe at the earliest times
(Fig. 3c, red trace, 0.2 ps) is, as noted above, typical for locally
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dynamics suggested by the single-wavelength kinetic traces, we the average solvation times of the two pure solvents.44 The 1.1 ps
nd that six components are necessary to describe the observed spectra on the other hand prominently contain the decay of the
spectra within an exponential decay model.
SE contribution as well as the appearance of a characteristic ESA
The wavelength-dependence of this multi-component t is feature close to the excitation wavelength. Thus, these spectral
best represented by the spectra associated with the decay dynamics reveal the transfer of the initially excited ligand-
1
components. The two most common representations of these localized NiII* state to states that are optically dark from the
“global” spectra are the Decay Associated Spectra (DAS) and the ground state.
Evolutionary Associated difference Spectra (EAS), where the
This energy-transfer step is accompanied by the (quasi-)
former is simply a plot of the amplitude of each decay compo- ground state recovery of the Mabiq ligand: i.e., aer this initial
nent as a function of wavelength, and the model can be inter- relaxation phase, the excited population resides almost
preted as representing the parallel decay of six independent completely on the metal center, while the ligand is in a ground
species. In the EAS model, we rather assume a strictly sequential state distorted by the excited metal center. Consequently, we
organization of compartments—or “states” (not necessarily can assign the spectra of the intermediate time components (14
electronic eigenstates of the system)—and the time-constants and 81 ps lifetimes. Fig. 4, central panels) to transitions of
refer to the (unidirectional) population transfer between ligand-localized p–p* character. The 14 ps component is
these. Hence, the EAS can be considered the spectra of these primarily associated with spectral narrowing—as demonstrated
individual “states”. In Fig. 4 we plot the DAS and EAS corre- by the second-derivative prole of the DAS and slightly broad-
sponding to the decay-component of 1. The corresponding ened EAS spectral prole—which suggests vibrational cooling45
global component spectra for 2 can be found in Fig. S41.† The or heat dissipation.46
effective picture emerging from the analysis of these global
spectra is summarized in Table 1 and depicted in Fig. 6.
Interestingly, the fast appearance and subsequent decay of
a 465 nm feature highlighted with a vertical dashed line in
At early times, the dynamics are dominated by components Fig. 3c is clearly mirrored in the global analysis: immediately
with characteristic times of 370 fs and 1.1 ps (Fig. 4, top). The aer the 1NiII* decay, the transient spectrum in Fig. 3 takes on
faster component can be assigned to solvation, as it occurs with a shape similar to—although strongly distorted—the 81 ps EAS.
The global analysis captures this behaviour as a corresponding
EAS with 1.1 ps growth and 2.5 ps decay (Fig. 4, bottom panel).
The presence of a short-lived intermediate in the ligand / MC
transfer demonstrated by this component is by itself inter-
esting; in addition, we nd that a state with an essentially
identical spectrum re-emerges at long times as the MC states
decay. This state subsequently decays on a much longer time-
scale, with a time constant of approximately 770 ps.
While short-lived intermediates as observed here
can be assigned to, e.g., vibrational cooling, we stress that the
re-emergence of long-lived states with essentially identical
spectral characteristics—as shown by the strong resemblance
between the 2.5 ps and 770 ps EAS—makes such an assignment
here highly implausible.
Earlier studies on macrocyclic nickel systems instead point
to contributions from charge-transfer (CT) states in the elec-
tronic structure—both indirectly through the relaxation rate
dependency on solvent polarity,39 and through spectroscopic
signatures in ultrafast XANES experiments.47 While a complete
one-electron ligand-to-metal charge transfer in 1 is unlikely, as
one would then expect to observe the Mabiq cation spectrum,
we nd admixture of states with CT character to the MC states to
be plausible in the relatively polar solvent environment in these
experiments. This contrasts with photocatalytically active NiII–
bpy complexes, where the long-lived lowest energy states appear
to be of purely d–d character.33,48
Fig. 4 Decay associated- (DAS, left column) and evolutionary asso-
Comparing the proles of the DAS spectra, we nd that both
ciated- (EAS, right column) decay spectra extracted from a global fit of
the relaxation dynamics of 1. DAS amplitudes are shown on absolute the 80 ps component assigned to population of MC states and
scale, while the EAS are normalized to their maximum for easier
comparison of spectral shapes. The component spectra are organized
to highlight similarities in EAS component spectra. The time-constants
are component lifetimes extracted from the relaxation dynamics of 1.
the 772 ps component clearly contain the GSB signature in the
400–460 nm range. In other words, the ground state of the
complex recovers bi-exponentially, implying a branching in the
relaxation pathway: only a fraction of the initial excited state
population reaches the long-lived states with admixture of CT
SE contributions to the spectra are found only around the 460 nm
excitation wavelength at the earliest two times.
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states, while the remainder presumably relaxes directly from the transition metal–Mabiq complexes.27–29 The calculated excita-
fully metal-centered states. tion energies of 1 and 2 are of similar magnitude (Fig. 5a; 1:
We can compare the observed excited-state dynamics with 2.85 eV S0 / S4; 2: 2.77 eV S0 / S1), and the density differences
those inferred from the photoreduction of 1. The dependence of associated with these transitions (Fig. 5b; corresponding
the reaction quantum yield on TEA concentration was deter- natural transition orbitals (NTO) in Fig. S48†) demonstrate
mined in our previous study.25 Based on analysis of the results ligand-localized motion of charge. Similar vertical excitation
using a standard diffusion model49 and a diffusion-limited energies are also obtained at the second-order correlated
quenching rate typical of molecules comparable to our system (ADC(2)/TZVP) level (Table S4†).
(ꢂ1010 Mꢀ1 sꢀ1),50 the lifetime of the reactive excited state is
The DFT results also suggest an explanation for the experi-
predicted to be between 580 and 900 ps. This is in excellent mentally observed differences in the relaxation dynamics of the
agreement with the measured lifetime of 770 ps, which indi- two complexes. The spin densities of the 3ZnII triplet excited
cates that the outer-sphere electron transfer (ET) process with states (Fig. 5c) show that the excitation remains on the ligand,
TEA can occur from this mixed MC/CT state.
regardless of spin state, upon excited-state relaxation of 2. The
minimal contribution of MC states to the electronic structure
and dynamics again suggests that 2 can be considered as an
organic chromophore (i.e., Mabiq) conned to a limited
conformational range by the coordinating Zn2+ ion. The ꢂ1 ns
intersystem crossing to the Mabiq triplet state is reasonable for
an organic chromophore with an associated metal.
Electronic structure
To correlate the spectroscopic signals with the electronic states
involved in the optical response of the complexes, we employed
electronic structure calculations at the linear response time-
dependent density functional theory (DFT) level. The TDDFT-
derived energy level diagrams for 1 and 2 are shown in Fig. 5a
(see also Fig. S42 and S43†). To highlight the electronic struc-
ture of the states—i.e., the parts of the complexes involved in
the photo-excitation and relaxation—we computed the electron
density differences between the ground and excited states (the
density differences; Fig. 5b, S44–S47†), the spin density distri-
butions of the lowest energy triplet states (Fig. 5c), and per-
formed a Mulliken population analysis (Table S5†).
Our interpretation that the absorption consists of p–p*
singlet transitions between Mabiq localized states is supported
by the TDDFT calculations. This is also consistent with the
observation that the optically bright transitions for 1 and 2 are
of similar character in both complexes, as well as in other
The structure and relaxation effects of 1 contrast with this
1
simple behaviour. The NiII*–3NiII* energy gap is substantially
larger than the corresponding singlet-triplet energy difference
of 2. While the electronic structure of the 1ZnII* and 3ZnII states
are both predominantly ligand-localized, the corresponding
3
1NiII* and NiII states are clearly different (Fig. 5b and c), with
a
3NiII state that is essentially metal-centered. Thus, the
computational data suggest that intersystem crossing in 1
involves energy transfer from the Mabiq ligand to the metal
center, in agreement with our experimental observations. A
large number of additional states are located between the
ground and the optically bright 1NiII* state, many of which are
also metal-centered, of both singlet and triplet character. The
Fig. 5 (a) Vertical excitation energies (VEEs) derived at the TDDFT/B3LYP level for 1 and 2 at the optimized singlet (S0) and lowest energy triplet
(T1) geometries. Optically allowed states are shown with solid lines, optically dark states are denoted by dashed lines; MC states are shown in red,
and ligand-centered states in black. Note that the lowest energy optically allowed state in 1 is the ligand-centered S4 state, whereas in 2 the
lowest optically active singlet is the ligand-centered S1 state. (b) Total electron density difference upon photo-excitation into the lowest energy
optically bright state for 1 (S4, top) and 2 (S1, bottom), with positive and negative differences between the excited and ground states marked in
blue and red, respectively. The charge migration takes place almost exclusively on the ligand for both species. (c) Spin density (a–b) for the lowest
energy triplet state of 1 (top) and 2 (bottom), with positive and negative differences between alpha and beta spin densities marked in red and blue,
respectively. Note that the excess spin is almost entirely metal localized for 1, while it is distributed on the ligand for 2.
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Table 2 Photoredox-catalysed cyclization of an N-(u-bromoalkyl)-
substituted indole (top), aza-Henry reaction (middle) and oxidative
coupling of 4-methoxybenzylamine (bottom)
large number of states facilitates the observed rapid ligand-to-
metal energy transfer. A similar electronic picture is also ob-
tained at the TDDFT/CAM-B3LYP level (Fig. S43†), suggesting
that the results are robust and do not arise from, e.g., DFT
charge transfer problems. The calculations also suggest that
spin–orbit coupling (SOC) effects are small for the Zn–Mabiq
complex, but could contribute into transition between singlet
and triplet states for Ni–Mabiq (Fig. S49†).
The implicit assumption that intersystem crossing is an
efficient quenching channel for the 1NiII* state is well-founded,
given the typical behaviour of open-shell transition metals.21
The optical spectra themselves only reveal that the nal state in
the relaxation cascade is (i) optically dark from the ground state
(at least in the UV/Vis spectral region), and (ii) has a prominent
ESA band slightly redshied from, but close in energy to, the
Entry
Catalyst
% yielda
1
2
1
2
86 ꢃ 5 (13 h)
35 (13 h)
1
(1NiII / NiII*) ground state absorption band. In this regard,
the calculations support our expectation of relaxation via metal
centered states. All MC transitions with energies in the range of
the TA measurements, either from the singlet ground state or
from the triplet state, are dark; any optically active transitions
primarily involve the Mabiq ligand.
With the assignment of triplet character to the low-lying
energy levels of 1 and 2, we can reconcile and at least semi-
quantitatively explain the photoreduction quantum yields and
the time constants from the TA measurements. The quantum
yield, QY, of the photoinduced reduction of the M–Mabiq
catalysts by Et3N is given by:
Entry
Catalyst
1
1/without irradiation
None
2
% yieldb (time)
1
2
3
4
82 ꢃ 7 (5 h)
—
14 ꢃ 6 (5 h)
36 ꢃ 13 (30 min)
kred
QY ¼
ktot
Entry
Catalyst
% yieldc (time)
1
2
3
4
1
27 ꢃ 0 (17 h)
— (12 h)
— (12 h)
73 ꢃ 12 (30 min)
with the effective quenching rate, kred, and the total relaxation
rate, ktot, as determined from the TA measurements. With a QY
of 2.9 ꢁ 10ꢀ4 for Ni–Mabiq and ktot ¼ 1/772 ps, we obtain
a surprisingly long quenching time of t ¼ 1/kred ¼ 2.7 ms. The
low QY is therefore the consequence of a low electron transfer
rate and the rapid decay of the excited 1. Using the same value
of kred as a rst guess for 2, we predict a total decay time of 10 ms,
which is well within the range of the slow time constants ob-
tained from the TA data for the Zn complex. In summary, the
reduction is rather slow, and the QY is limited by the lifetime of
the triplet states.
1/without irradiation
None
2
a
For details see ref. 25 and 50. b Yields are mean ꢃ standard deviation
of measurements performed at least in duplicate. Reactions were run on
a 0.05 mmol scale. Product yields were determined by 1H NMR using
c
mesitylene as an internal standard. For the oxidative coupling of
benzylamine, ESI-MS was further used to conrm the presence of the
desired product ([M + H]+ m/z ¼ 256.09).
obtained and degradation of the metal complex occurred, as
evidenced by bleaching of the solution throughout the reac-
tion.51 Conversely, while the photoreduction quantum yield is
relatively low, 1 is an effective photoredox catalyst, as it appears
resistant to degradation and remains active throughout the
extended reaction period.25 In related preliminary work, we
observed that the FeII–, CoII– and CuII–Mabiq complexes all
exhibit varying reaction yields and selectivities,51 further
demonstrating that the choice of metal center clearly inuences
the activity.
We next examined the photocatalytic ability of 1 and 2 in two
reactions that do not rely on initial reduction of the catalyst by
a sacricial donor: the oxidative C–H functionalization of N-
phenyl-tetrahydroisoquinoline (THIQ), and the oxidative
coupling of 4-methoxybenzylamine (Table 2; see ESI† for
details).
Catalysis
Given the distinctive photophysics, differing behaviour with
respect to photoreduction, and activity toward O2, we examined
the comparative ability of 1 and 2 to act as a photoredox catalyst
in a series of photo-driven reactions. The rst of these is the
previously reported cyclization of an N-(u-bromoalkyl)-
substituted indole (dimethyl 2-(3-(1H-indol-1-yl)propyl)-2-
bromomalonate) by 1 (Table 2, top scheme; mechanism in
Scheme S1†), which gave the product in high yield.25 In this
case, irradiation of the reaction mixture facilitates reduction of
1 by Et3N, and the resultant 1red triggers the ring closing in
a diffusive manner. A catalytic cycle for the formation of the
cyclized indole should also be feasible using 2, based on its
redox properties alone. However, low product yields were
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The THIQ functionalization represents a photoredox cata-
To supplement the basic reactivity studies, we examined the
lysed aza-Henry reaction (Table 2, middle scheme; mechanism ability of 4-methoxybenzylamine to photoreduce the Mabiq
in Scheme S2†).52 In the initial step, reductive quenching of the compounds—the rst step in the photoredox pathway. Both 1
excited state of the catalyst by the isoquinoline leads to and 2 can be photoreduced using 4-methoxybenzylamine under
formation of a cationic amine radical. As discussed above, this inert atmosphere, indicating that an electron transfer pathway
step is expected to proceed on the low ms timescale. The solvent is feasible (Fig. S62 and S63†). However, aer the initial
nitromethane (CH3NO2) can serve to regenerate the catalyst, formation of some 2red by the amine, the spectrum of the Zn-
and facilitates iminium ion formation. The resultant reduced complex is further altered. Evaluation of the photoreduction
oxidant and amine cation radical will likely remain in close data shows that the reduction of NiII* by methoxybenzylamine
proximity due to the lack of time for diffusion; alternatively the is photo-reversible, with comparable quantum yield values for
reduced oxidant readily combines with CH3NO2 via a hopping the forward and back electron transfer processes (ca. 1.7 ꢁ
process. Either way, catalyst reoxidation and iminium ion 10ꢀ5; Fig. S62†). The low yield is consistent with the low
formation via a quasi-geminate step should be fast and coupling product yields obtained with 1, even aer 17 h. From
efficient.
the illumination conditions of 4-methoxybenzylamine with 1 we
The nal product of the aza-Henry reaction is generated can estimate a lower limit of the overall reaction quantum yield
upon addition of the nitromethane-derived CH2NO2 to the of 4.3 ꢁ 10ꢀ4. In comparison, for the quenching QY—taking
iminium. Thus, nitromethane also acts as the co-substrate. The into account the concentration of methoxybenzylamine under
aza-Henry reaction exploits the direct reaction of MII* with the the reaction conditions (25 mM), and the linear dependence of
substrate, unlike the previously examined indole cyclization, the process on the quencher concentration25—we obtain a value
which reected the interaction of a substrate with the reduced of 8.5 ꢁ 10ꢀ4. Within the uncertainty of this estimate the two
‘MI’–Mabiq forms. The reaction also allowed us to probe the yields are equal, showing that indeed all steps aer the rst ET
ability to regenerate the reduced catalyst forms in the presence occur with near unity.
of an oxidant, which would allow application of the Mabiq
complexes in a broader range of chemistry.
The quenching of the photo-excited 2 is associated with QYs
of 0.12 to the reduced form, and 0.20 to an unidentied side
The product yields obtained in the photoredox catalysed aza- product, both of which rapidly convert further to some nal
Henry reaction by 1 and 2, using nitromethane as the solvent, species with an optical spectrum that is distinct from that of 2
were 82% (1, aer 5 h) and 36% (2, aer 30 min). With the or 2red (Fig. S63†). Clearly any accumulation of 2red also leads to
illumination power and total time, we can estimate a lower limit degradation of the original form of the Zn catalyst. A slight shi
1
of the overall reaction quantum yield of 1.5 ꢁ 10ꢀ3. The yields in both the H NMR resonances and absorption spectrum of 2
obtained for the Ni-complex are comparable to those reported in the presence of the substrate is observed (Fig. S64 and S65†),
using [Ru(bpy)2]2+ and [Ir(ppy)2(dtbbpy)]+.52 Thus, 1 shows which suggests that the ability of the amine to coordinate to the
promise as an effective catalyst for such C–H functionalization Zn may contribute to the activity and degradation of 2. However,
reactions. The lower yields of 2 in the reaction are again due to overall, the high reduction yields together with the ability of O2
degradation of the Zn-complex. A signicant decrease in the to reoxidize any accumulated 2red enables signicant product
absorption bands of 2 was already observed within the rst 30 formation before the catalyst is fully consumed.
minutes reaction time (Fig. S55†). This contrasts with the
While the above reactions are quite distinct, the photoredox
behaviour of 1, which remained intact throughout the reaction, catalytic activity of the M–Mabiq complexes in all of the reac-
as evidenced by both absorption and NMR spectroscopies tions hinges on the ability of the excited states to engage in
(Fig. S56 and S57†). We note that irradiation of a solution of 1 single electron transfer with a sacricial donor or the substrate.
and the isoquinoline in the absence of a suitable oxidant also Our model for the excited state relaxation pathways for 1 and 2,
leads to rapid decomposition of the Ni-complex (Fig. S58 and based on spectroscopic characteristics and the calculated elec-
S59†), which we speculate is due to attack of 1red by the iso- tronic structure, is summarized in Fig. 6. ET involving the
quinoline radical.
singlet states is precluded by the short lifetimes—the close
The nal reaction that was examined, the oxidative coupling vicinity of the resulting ion pair also would likely lead to a rapid
of 4-methoxybenzylamine (Table 2, bottom scheme), depends back-ET and therefore be unproductive. Single electron transfer
on the participation of O2 in the catalytic cycle (Scheme S3†). between the amine substrates and M–Mabiq will occur via
Both energy transfer and photoredox mechanisms have been the triplet states, where back ET is consequently blocked by
proposed for this reaction—in the latter case, O2 would serve as the Pauli principle.54 In the Zn-containing 2, all relevant
an oxidant and to generate an imine intermediate.53 The Ni– excited states, including the triplet states, are ligand-localized.
Mabiq complex was able to catalyse the coupling reaction, albeit As a result, photoreduction results in formation of what is
with only 27% product yield even aer 17 h. No signicant essentially a Mabiq radical anion coordinated by a ZnII;
amount of side product was detected. Nevertheless, 1 remained decomposition proceeds as is typical for organic radical ions.
stable under these conditions (Fig. S60†). Using 2 as the pho- The longer excited state lifetime of 2 should favor catalytic
tocatalyst,
a larger amount of N-(4-methoxybenzyl)-1-(4- activity, but the instability of 2 hinders its use as a photoredox
methoxyphenyl)methanimine was obtained (73% yield, 30 catalyst. Oxidants, as used in the light-driven aza-Henry and
min). However, complete degradation of the Zn compound was oxidative coupling reactions, regenerate 2 to some degree.
again observed aer 30 min (Fig. S61†).
However, even in the presence of excess oxidant the degradation
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photophysical properties have been studied in great depth.17
Excitation of [Ru(bpy)3]2+ in the visible region leads to pop-
ulation of long-lived 3MLCT excited states, which subsequently
engage in single electron transfer. The sub-ps excited state
3
lifetimes of the MLCT states of [Fe(bpy)3]2+ on the other hand
can be attributed to relaxation to MC states, which in contrast to
those of the Ru complex lie below the MLCT states.55 This
altered ordering reects the general problem of rst row tran-
sition metal complexes, where low lying d–d states lead to short
excited state lifetimes and limit the photoredox capacity of the
compounds. A notable exception is the behavior of the other
class of NiII photoredox catalysts (tBubpy)NiII(Ar)(X); X ¼ halide
3
or ORꢀ, also based on the bpy ligand.33,48,56 The low-lying d–
d states are not only long lived, but allow this complex to
undergo photoinduced Ni–aryl bond homolysis.
The Mabiq ligand likewise contains
a bipyridine-like
moiety—as well as a second redox-active functionality in the
diketiminate. The presence of multiple redox-active units in the
macrocyclic M–Mabiq systems results in photophysics that are
more complex than the bipyridyl systems, with ligand-centered,
CT and MC character all involved (Fig. 6). The initially accessed
ligand-centered p–p* state of 2 can also engage in single elec-
tron transfer, whereas the CT and MC states become more
relevant for 1. Our ndings suggest that these latter states lie
closer in energy than what is observed in bpy-based
compounds. Interactions between CT, MC, and p–p* states
account for the complex photodynamics of 1, and further
underscores the synergy between the redox-active ligand and
the metal centers. The nature of the bound metal center may
alter the relative energies of these CT and MC states. For the Fe,
Co and Cu complexes, which possess additional axial ligands,
the d–d states might also enable M–L bond homolysis. Overall,
the unique features of the Mabiq ligand allow for rich and
varied photochemistry.
Fig. 6 Effective relaxation model of the compounds 1 (a) and 2 (b) as
extracted from the time-resolved absorption data. Components
assigned to solvation and vibrational cooling omitted for clarity. Metal-
centered states are depicted in red; ligand-centered states are indi-
cated in black.
of 2red appears to outcompete its reoxidation. In contrast, the
initial ligand-centered excited state of 1 relaxes rapidly towards
states of metal-centered and partial CT character. The involve-
ment of metal-centered states in 1red appears to make this
complex more robust, further highlighting the synergy between
the metal and redox-active ligand.
Our preliminary studies with alternate electron donors and
select substrates demonstrate, however, that the nature of the
donor is crucial. Electron transfer between 1 and either DMA or
4-methoxybenzylamine was sluggish; in the latter case this
signicantly limited the catalysis. Moreover, the isoquinoline
radical can destroy the complex if subsequent transformations
of the intermediate are not fast enough (Fig. S58 and S59†).
More detailed studies will be required to determine the specic
donor/M–Mabiq interactions and their importance in the
mechanism of excited state electron transfer. Finally, the
generally greater stability of 1red allows this species to be more
effectively regenerated by oxidants, allowing 1 to engage in
various stages of a photoredox cycle.
Conclusions
The nature of the coordinated metal ion clearly has a signicant
inuence on the photophysical properties and reactivity of the
Mabiq complexes, as exemplied by the Ni and Zn complexes 1
and 2. Our ndings unravel contributions of the various ligand-
centered, charge transfer and MC states, and provide initial
insight into how the interactions between the metal and redox-
active ligands inuence activity and stability of photoredox
catalysts. With the exception of the Zn complex, the chemical
stability of the M–Mabiq series is a notable feature of this class
of photocatalysts. Light-driven access to the reduced, stable
forms of MII–Mabiq complexes containing Fe / Cu may also
permit more diverse organic transformations.
In our system, the Mabiq system scaffold provides a highly
unique degree of freedom in a second metal binding site at the
complex periphery. Incorporation of a second metal ion may
enable alterations to the electronic structure of the core metal
complex, while circumventing the need to modify the ligand
3
The ZnII* state of the Zn complex may be more suited for
select energy transfer reactions, as for singlet O2 sensitization.
With respect to singlet-oxygen sensitization, the (1NiII–3NiII)
energy gap is much smaller than the (3O2–1O2) energy gap, and
consequentially the process is energetically disfavored. Thus, 1
may be less effective for energy transfer processes.
Overview of photodynamics
It is worth comparing the photophysics of our Mabiq complexes backbone. As such, the second coordination site may have an
to that of M-bpy based compounds. The bpy ligand is essential inuence on stability and electron transfer efficiencies. The
to the functionality of [Ru(bpy)3]2+, whose underlying photochemical properties of a series of bimetallic Mabiq
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compounds can be systematically studied. The second coordi-
nation site also opens up entirely new possibilities for bifunc-
tional photoredox catalysis. Overall, we believe the properties of
the Mabiq system will allow these complexes to become valu-
able additions to the growing repertoire of earth-abundant
photoredox catalysts.
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Author contributions
RL carried out synthesis and characterization of compounds,
and performed photoreduction experiments. RL & HD per-
formed data analysis for photoreduction experiments. OZE
performed photocatalytic addition- and coupling-reactions. RL,
HD, and MB performed the time-resolved optical experiments.
SLM and VRIK designed and performed the DFT calculations.
RL, SLM, HD and OZE made comparable and substantial
contributions to experiments and analysis. AJR carried out
additional DFT calculations. CJ was responsible for X-ray crys-
tallography. ER, ET, JH, and CRH planned, designed, and
analyzed the experiments. CRH and ET conceived the project
and wrote the paper with contributions from all authors.
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
We are grateful to Prof. Jonathan Finley and Lukas Hanschke
for the use of instrumentation, and assistance with sample
measurements for uorescence lifetime data. RL thanks the
TUM International Graduate School of Science and Engineering
(IGSSE) for nancial support. We are grateful for nancial
support from the DFG (Deutsche Forschungsgemeinscha,
German Research Foundation) through the excellence cluster e-
conversion, under Germany's Excellence Strategy – EXC 2089/1 –
390776260, as well as funding from DFG grant 426785626.
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Notes and references
§ In our previous studies, we have shown that all of the previously isolated, formal
MI–Mabiq complexes are best described by [MII(Mabiqc)], containing the reduced
Mabiq radical, rather than a monovalent metal center.
¨
28 E. Muller, G. Bernardinelli and A. von Zelewsky, Inorg.
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