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Elucidating the Nature of the Excited State of a Heteroleptic Copper
Photosensitizer Using Time-Resolved X-ray Absorption Spectroscopy
Dooshaye Moonshiram*[a], Pablo Garrido-Barros[a,b], Carolina Gimbert-Suriñach[a], Antonio Picón[c,d],
Cunming Liu[e], Xiaoyi Zhang[e], Michael Karnahl*[f], and Antoni Llobet*[a,g].
Abstract: We report the light-induced electronic and geometric
changes taking place within a heteroleptic Cu(I) photosensitizer,
namely [(xant)Cu(Me2phenPh2)]PF6 (xant = xantphos, Me2phenPh2 =
bathocuproine), by time-resolved X-ray absorption spectroscopy in
the ps-µs time-regime. Time-resolved X-ray absorption near edge
structure (XANES) and extended X-ray absorption fine structure
(EXAFS) analysis enabled the elucidation of the electronic and
structural configuration of the copper centre in the excited state as
well as its decay dynamics in different solvent conditions with and
without triethylamine acting as a sacrificial electron donor. A threefold decrease in the decay lifetime of the excited state is observed in
the presence of triethylamine showing the feasibility of the reductive
quenching pathway in the latter case. A prominent pre-edge feature
is observed in the XANES spectrum of the excited state upon metal
to charge ligand transfer transition indicating an increased
hybridization of the 3d states with the ligand p orbitals in the
tetrahedron around the Cu centre. EXAFS and Density Functional
Theory (DFT) illustrate the local geometrical configuration of the
excited state showing a significant shortening of the Cu-N and an
elongation of the Cu-P bonds together with a decrease in the torsional
angle between the Xantphos and bathocruproine ligand. This study
provides mechanistic time-resolved understanding for the
development of improved heteroleptic Cu(I) photosensitizers, which
can be used for the light-driven production of hydrogen from water.

Introduction
The production of solar fuels by using the energy of the sun
is one of the most urgent scientific challenges in view of the rapid
depletion of fossil fuels, the resulting environmental pollution and
climate change.[1] Although today’s state of the art technology has
made considerable progress in producing electricity using
sunlight and wind, these intermittent sources will only find limited
applications without efficient energy storage.[1c, 2] One capable
way to store solar energy is to convert it into chemical energy
through fuel forming reactions inspired by natural photosynthesis,
such as the light induced splitting of water into hydrogen and
oxygen (2 H2O + 4hʋ = O2 + 2 H2).[3] Indeed, the prospect of using
molecular hydrogen has motivated the development and

application of light harvesting units for solar H2 generation in both
metal-complex chromophores and molecular photocatalysts.[4]

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While most of the molecular photosensitizers are based on
noble-metals, such as Ru(II), Ir(III), Pt(II), Au(III) and Re(I), some
work on cheaper and more abundant 3d transition metals like
Cu(I), Cr(III) and Zn(II) porphyrin complexes have also been
reported.[4a, 4b, 5] In this respect, novel heteroleptic Cu(I)
photosensitizers of the general type [(P^P)Cu(N^N)]+ composed
of a diimine N^N and a bulky diphosphine P^P ligand have
attracted particular interest as they allow for considerable
photocatalytic activities in conjunction with an Fe-based water
reduction catalyst.[6]
The design of these heteroleptic Cu(I) complexes is an
ongoing area of research that has experienced a significant
development during the last years with respect to their well-known
homoleptic counterparts of the type [Cu(N^N)2]+ (Scheme 1A and
1B).[5a, 5d, 6a, 7] The properties of the homoleptic bis-diimine
photosensitizers are already extensively studied since decades[8]
due to their tuneable photophysical and electrochemical
properties. These photosensitizers are found to undergo a
structural reorganization upon photoexcitation from a pseudo
tetrahedral geometry to a flattened state (flattening distortion)[9]
enabling the coordination of a solvent molecule to the Cu centre.
As a result, a low-energy excited state is formed, known as an
“exciplex”, which exhibits a fast emission decay to the ground
state with shortened excited state lifetimes in the order of ps-ns.[5a,
9a, 9b]
. While significant advances have been made in the finetuning of the diimine ligands[8a, 8c, 8g, 10] and the improvement of the
resulting bis-diimine complexes these homoleptic Cu(I)
compounds have only found restricted use in the photocatalytic
reduction of protons from water.[7c, 11] In contrast, a large number
of the heteroleptic ones exhibit increased excited state lifetimes,
which even exceed some of the commonly applied Ru(II)
photosensitizers.[5a, 6b, 7a, 12] In addition, the properties of these
heteroleptic complexes can be tuned in a broader range due to
the presence of two distinct ligands compared with their

[a]

[b]

[c]

[d]

[e]

[f]

[g]

Dr. D. Moonshiram*, Mr. P. Garrido-Barros, Dr. C. Gimbert
Suriñach, Prof. Dr. A. Llobet*
Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països
Catalans 16, 43007 Tarragona, Spain
E-mails: dmoonshi@gmail.com, allobet@iciq.cat
Mr. P. Garrido-Barros
Departament de Química Física i Inorganica, Universitat Rovira i
Virgili, Campus Sescelades, C/Marcellí Domingo, s/n, 43007,
Tarragona, Spain
Dr. A. Picón
Grupo de Investigacion en Aplicaciones del Laser y Fotonica,
Departamento de Fisica, Aplicada, Universidad de Salamanca,
37008, Salamanca, Spain
Dr. A. Picón
ICFO, Institut de Ciences Fotoniques, The Barcelona Institute of
Science and Technology, 08860, Castelldefells, Barcelonia, Spain
Dr. C.Liu, Dr. X.Zhang
X-ray Science Division, Argonne National Laboratory, 9700 S. Cass
Avenue, Lemont, IL, 60439, U.S.A
Prof. Dr. M. Karnahl*
University of Stuttgart, Institute of Organic Chemistry,
Pfaffenwaldring 55, 70569, Stuttgart, Germany
Email: michael.karnahl@uni-stuttgart.de
Prof. Dr. A. Llobet*
Departament de Química, Universitat Autonoma de Barcelona,
08193 Cerdanyola del Valles, Barcelona, Spain

Supporting information for this article is given via a link at the end of the
document. This includes pre-edge XAS calculations, EXAFS simulations,
EXAFS analysis, and DFT optimized coordinates.

homoleptic counterparts. For instance, it was found that both the
photocatalytic activity and emission lifetime strongly improved
upon addition of bulkier substituents at the 2,9-position of the
phenanthroline ligand (Scheme 1).[6b, 7a, 12a, 13] Replacement of the
two methyl groups by sec-butyl considerably increased the
turnover number of hydrogen production from 862 to 1330 as well
as the emission lifetime of the triplet excited state from 6.4 µs to
54.1 µs (in degassed THF solution).[6b, 12a]
However, in spite of emerging design principles and active
synthetic efforts in developing advanced heteroleptic copperbased photosensitizers, there is an urgent need to correlate their
performance and stability to the underlying geometric structures
for rational developments in the future. Although the
photophysical and structural properties of these heteroleptic
copper photosensitizers have been studied by different
techniques such as optical transient absorption[6c, 7c], UV-vis[6b, 6e,
7b, 7d, 7f, 7h-j, 7l, 14]
, resonance Raman[6e], electrochemistry[6e, 7e, 7h, 7j,
7l]
, single crystal X-ray diffraction[7f, 7i, 7l, 14-15], mass
spectrometry[15], NMR[6f, 7f, 7g, 13, 15] and EPR spectroscopy[6f], the
geometrical reorganization of the triplet excited state has not yet
been experimentally demonstrated. Elucidation of the nature of
the excited state through direct experimental proof is however
crucial. Moreover, although disproportionation of heteroleptic
[Cu(N^N)(P^P)]+ complexes to form homoleptic species in
solution has been proposed[6e, 6f, 7g], structural analysis of the
excited state which could lead to the degradation pathway of this
class of Cu(I) photosensitizers has not yet been performed.
In this regard, synchrotron-based X-ray absorption
spectroscopy (XAS) has proven to be a powerful tool for tracking
electronic and geometrical changes within molecules.[16] XAS has
further been extended to the ultrafast time domain for timeresolved laser (pump), X-ray (probe) experiments through the
usage of lasers with high power and high repetition rate. In this
technique, a short laser pulse excites molecules within a liquid
and the ~100 ps X-ray pulses capture the transient states of the
molecule as it evolves.[17] Nowadays, these high-repetition-rate
laser systems can also produce short fs-ps pulses, which when
combined with the high-repetition rate of the X-rays allows full
utilization of the X-ray flux.[17a]. This capability has been
implemented at various beamlines around the world such as the
SLS[18] (Switzerland), BESSY[19] (Germany), Petra III[20]
(Germany), Elettra[21] (Italy), ALS[22] (U.S.A), APS[17b, 23] (U.S.A)
and the Photon factory (Japan)[24], enabling the use of timeresolved hard X-ray absorption spectroscopic (tr-XAS) tools with
ps-µs temporal resolution in both the soft and hard X-ray range.
Tr-XAS has been used to capture snapshots of transient states in
many photocatalytically relevant complexes, and has
considerably enhanced the fundamental understanding of
photochemistry, solar energy conversion, interfacial electron
transfer processes and biological enzymatic systems[22b, 22c, 24-25]
The present study elucidates the light-induced electronic
and structural dynamics leading to the formation of the excited
state (as presented in Scheme 1C) derived from the heteroleptic
copper photosensitizer [(xant)Cu(Me2phenPh2)]PF6 (with xant =

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xantphos and Me2phenPh2 = bathocuproine). XANES, EXAFS
and time-correlated single photon counting experiments in
combination with DFT calculations are used to reveal critical
electronic and structural information about the Cu-MLCT state
and its dynamics in the presence and absence of an electron
donor. To the best of our knowledge, time-resolved X-ray
absorption spectroscopy studies of heteroleptic copper
complexes of the type [(P^P)Cu(N^N)]+ have not been reported
prior to this work. Therefore, the findings of this study constitute
an important step towards elucidating the structural configuration
of the excited state with the aim of improving the design of
heteroleptic copper photosensitizers resulting in higher chemical
stabilities and photocatalytic activities.

Results and Discussion
Time-resolved XANES: copper oxidation and metal-toligand charge transfer transitions. Formation of the excited
state of [(xant)Cu(Me2phenPh2)]PF6 after illumination was probed
in the ps-µs time range by time-resolved laser/X-ray pump/probe
spectroscopy of the copper complex in a THF/H2O solvent mixture
(volume ratio of 9:1) as well as in a mixture of THF/TEA/H2O
(volume ratio of 4:3:1). The latter mixture including triethylamine
as electron donor was chosen as it had previously been proven
to give the best catalytic results in combination with [Fe3(CO)12]
as water reduction catalyst.[6b, 6d, 12a] The Cu(I) complex in both
solution mixtures was optically pumped at 400 nm with a 10 kHz
repetition rate laser and probed with X-ray pulses at varying time
delays ranging from 100 ps to 30 µs.

Figure 1. Experimental transient X-ray absorption spectrum of the heteroleptic
Cu(I) photosensitizer before laser excitation (black) and measured at an
averaged time delay of 100 ps-306 ns after laser excitation (blue). The
reconstructed spectrum of the excited state was calculated considering an
excitation percentage of 17.5 % (red, see main text). Inset. Top. Calculated
Time-dependent Density Functional Theory (TD-DFT) pre-edge simulations for
the ground and excited state from optimized calculations (Table 1). Bottom.
Zoom of the experimental pre-edge features

Scheme 1 A. General structure of common homoleptic Cu(I) bis-diimine
complexes B. Heteroleptic copper photosensitizer investigated in this study,
[(xant)Cu(Me2phenPh2)]PF6 (with xant = xantphos, Me2phenPh2 =
bathocuproine) C. Proposed reaction mechanism for the photocatalytic
production
of
hydrogen
applying
the
copper
photosensitizer
[(xant)Cu(Me2phenPh2)]PF6 D. Simplified molecular orbital scheme of Cu(I)
photosensitizers upon light excitation adapted from [26].

The XANES spectra were collected before (laser off) and
after (laser on) laser excitation. The laser-on XAS spectrum for
an averaged time delay of 100 ps-306 ns between the laser
pump excitation and the X-ray probing is shown in Figure 1.
Upon light excitation, metal-to-ligand-charge-transfer (MLCT)
causes an electronic transition from the 3d orbital of Cu(I) to
the π*-orbital of the phenanthroline ligand followed by
intersystem crossing to a triplet excited state (Scheme 1
C,D).[6c, 9a, 9b] The MLCT transition causes a formal oxidation of
the copper centre from Cu(I) to Cu(II) and a reduction of the
diimine ligand forming an intermediate [(P^P)CuII(N^N-)]PF6
species (see Scheme 1 C,D). The formation of the excited
state was illustrated by the copper XANES spectra, which are
generally characterized by two peaks along the rising
maximum edge, namely the 1s  4p “main” peak along with a

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secondary peak due to a 1s 4p transition with shakedown
contributions.[27] These shakedown effects involve ligand to
metal charge transfer states during the relaxation of the Cu
local electrons in the excited state.[27] The comparison of the
laser on, and the laser off XANES spectra shows a clear
positive energy shift of around 0.53 eV at around half height
and 0.5 normalized absorption, reflecting the higher ionization
energy required for ejecting a core 1s electron from a more
positively charged ion. The excited state fraction of
[(P^P)CuII(N^N-)]+ was determined by comparing its XANES
spectrum to those of previously reported [Cu(dmp)2]+
complexes[8e, 9a, 28] (where dmp = 2,9-dimethyl1,10,phenanthroline) and typical reference Cu(I) and Cu(II)
complexes with similar coordination environment. A relative
chemical shift in energy of around 3 eV is typically obtained
between those Cu(I) and Cu(II) reference complexes [29] such
that a proportion of the excited state of around 17.5 % was
estimated in the laser on spectrum of [(P^P)CuII(N^N-)]+, and
further used to plot the actual or reconstructed spectrum of the
excited state (Figure 1, red line). The reconstructed excited
state exhibits a pre-edge transition at 8979.0 eV and the 1s
(4p+shakedown) rising-edge transition at 8985.7 eV which are
in good agreement with previously reported four-coordinate
Cu(II) ions[29-30], thus confirming the correct proportion of
excited state fraction used in the estimation of the laser on
spectrum.
Interestingly, the XANES spectrum not only shows a
clear change in the formal oxidation state of the copper centre
but also striking changes in the electronic configuration and
local site geometry as demonstrated by the prominent preedge feature (Figure 1 inset). Presence of pre-edge features
correspond to 1s to 3 d quadrupole transitions which are
formally dipole forbidden but gain intensity due to 4 p mixing
into the Cu 3d-shell.[31] The tetrahedral Cu(I) ground state has
a d10 orbital configuration with filled eg and t2g levels. Upon light
excitation, metal to ligand charge transfer triggers an electron
to be promoted to the low lying π*-orbital of the phenanthroline
ligand, thus causing the occupation of the Cu d level to change
as illustrated in Scheme 1D. The d level atomic
rearrangements in the excited state are additionally
accompanied with structure relaxation, and increased
hybridization of the valence 3 d states with N/P ligand p orbitals
explaining the presence of the prominent pre-edge feature at
8979 eV.[31c, 31d] Pre-edge calculations from DFT optimized
coordinates were carried out for both the Cu(I) ground and
excited state and reproduced the experimental data well
(Figure 1 inset, Figure S1). Since the pre-edge peak intensity
of the Cu(I) excited state’s is too large to arise only from electric
quadrupole transitions as illustrated in Figure S1, this feature
gains further intensity through 4p 3d mixing, and the
substantially more intense dipole allowed 1s 4 p character
contribution to this transition (Figure 1, S1). The pre-edge
calculation shown here further ascertains that geometry
optimization and ab-initio XAS simulations used for the Cu
photosensitizer can be reliably used for analysis of the Cu(I)
excited state.

Time-resolved EXAFS and DFT calculations:
Structural configuration around the copper centre. As
previously noted, unlike in homoleptic Cu(I) complexes, the
bulky diphosphine ligand of the heteroleptic copper
photosensitizer inhibits flattening distortion of the 3d9
configuration of the excited state. Thus, due to steric
hindrance, the formally Cu(II) MLCT state maintains its
geometrical configuration instead of the usual flattened
geometry observed for analogous homoleptic complexes. The
copper centre is consequently less susceptible to nucleophilic
attack by solvent molecules and exciplex formation, which
have been known to lead to shorter excited state lifetimes in
their homoleptic counterparts.[5a, 8e, 9a, 28b]

Figure 2 A. Fourier transforms of k2-weighted Cu EXAFS of the laser off
(black) and reconstructed excited state, considering a 17.5 % excited state
fraction (red). Inset: Back Fourier transforms, experimental results (solid
lines), and fitting (dotted marked lines) k2 χ(k) Fourier transforms were
calculated for k values of 2- 10.2 Å-1. C. Calculated structures of the Cu(I)
complex in the ground and the excited state. Hydrogen atoms are omitted
for clarity.

The EXAFS spectra of the heteroleptic Cu(I)
photosensitizer and its derived excited state are shown in
Figure 2 A. Analysis of the Cu(I) complex reveals a prominent
peak in the first coordination sphere corresponding to summed
contribution of the Cu-N and Cu-P bond distances. On the
other hand, two prominent peaks are observed for the excited
state derivative corresponding to the distinct contribution of the
shortened Cu-N and elongated Cu-P bonds. EXAFS fits for the
first coordination sphere and the entire spectrum are shown in
tables 1,S1. Analysis of the first peak in the excited state
clearly resolves the Cu-N distances at 2.09 Å. Furthermore, the
inclusion of two Cu-P distances at 2.27 Å improves the quality
of the fit as shown by the decreased R-factors and reduced chisquared values (Table 1, S1, fit 2). Similarly, fitting of the

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excited state illustrates an improvement of the fit quality after
addition of two shortened Cu-N distances at 1.91 Å and
elongated Cu-P distances at 2.36 Å (Table 1, S1, fit 6). The
fitted Cu-N and Cu-P bond distances are in agreement with the
reported single crystal X-ray diffraction (XRD) structure[32], and
relative trends between the relaxed structures from Density
Functional Theory (DFT) geometry optimization (Table 1, S2).
Table 1. Comparison of structural parameters from EXAFS
Species,
Fit No.
in Table
S1

Cu(I)
Ground
state
(Fit 3)

Excited
State
(Fit 6)

EXAFS
Shell:
N x distance
in Å/
Error bars
(100th Å)
Cu-N, 2: 2.04(1)
Cu-P,2 : 2.27(2)
Cu-C, 4: 2.93(3)
Cu-C, 8: 3.14(3)

Cu-N, 2: 1.91(2)
Cu-P, 2: 2.36(2)
Cu-C, 4: 3.05(3)
Cu-C, 8: 3.14(3)

XRD (Å)[32] [a]

Cu-N1: 2.06
Cu-N2: 2.10
Cu-P1: 2.24
Cu-P2: 2.30
N-Cu-N: 80.6˚
P-Cu-P: 119.9˚
Torsional
Angle = 85.0˚

DFT
Optimized
Coordinates
(Å) (Table
S2,SI)

Cu-N 1: 2.10
Cu-N 2: 2.12
Cu-P1: 2.23
Cu-P2: 2.29
N-Cu-N: 79.9˚
P-Cu-P: 122.3˚
Torsional
angle = 84.6 ˚
Cu-N 1: 1.99
Cu-N 2: 2.01
Cu-P1: 2.38
Cu-P2: 2.38
N-Cu-N: 83.4˚
N-Cu-P: 102.0˚
Torsional
angle = 71.6˚

dissociations in the heteroleptic Cu(I) photosensitizer and its
degradation pathway.
Excited state lifetime. The excited state is formed in less
than ~100 ps, which corresponds to the pulse duration of the Xrays in the tr-XAS experiment. The decay of the excited state’s
transient signal was monitored at varying time points between 100
ps to around 5.4 µs. Features in the tr-XAS spectra obtained by
subtracting the laser-on and laser-off (dark) spectra provide
further information about the excited state. Figures 3 A and B
show the tr-XAS spectra at varying time delays of 100 ps, 612 ns
and 1.683 µs between the laser and X-ray pulses. Two prominent
features are obtained in the transient signal at 8983.2 and 8988.9
eV related to the changes in the 1s  (4p + shakedown) transition
along with the 1s  4p main transition. These transitions point
towards the ground state bleaching of the Cu(I) photosensitizer.
Moreover, two broad peaks at 8999.1 eV and 9016.6 eV relate to
the formation of the formally oxidized Cu(II) species. These
energy transitions show that the K-edge of the Cu centre shifts to
higher energy, indicating the oxidation of Cu(I) and confirming the
electron transfer from the 3d orbital of Cu(I) to the π*-orbital of the
bathocuproine ligand. Lastly a pre-edge peak is observed at 8979
eV related to the increased hybridization of the core electrons into
the valence 3d states hybridized with N/P ligand orbitals as
elaborated above.

[a] XRD data of [(xant)Cu(Me2phenPh2)]PF6 was taken from reference
[32]
for comparison. CCDC1561239 contains the crystallographic data
of this complex, which is provided free of charge by the Cambridge
Crystallographic Data Centre.

DFT calculations of the excited state further show a
pronounced distorted geometry around the copper centre
compared to the ground state, which is in agreement with the
EXAFS results discussed above. For instance, the N-Cu-N bite
angle increases from 79.9˚ to 83.4˚, whereas the P-Cu-P angle
decreases considerably by 20.3˚ from 122.3˚ to 102.0˚. The
high degree of distortion is also evident from the angle between
the two planes which are formed by the xantphos and the
bathocuproine ligand that decreases from 84.6˚ to 71.6˚ as
depicted in Figure 2B. This decrease of the so-called ligand
plane intersection angle is in agreement with previous
findings.[6c, 32] Furthermore, the stepwise decomposition of the
original heteroleptic photosensitizer, and the formation of its
homoleptic [Cu(N^N)2]+ counterpart in solution has been
revealed through by a range of spectroscopic techniques such
as UV-Vvis spectroscopy[6e, 7k], hydrogen evolution studies
under various solvent conditions[7k], electrochemistry[6f, 7h],
resonance rRaman spectroscopy[6e], Electron Paramagnetic
Resonance[6f], and Nuclear Magnetic Resonance[[6f, 7g13]
studies. While the XANES and EXAFS analysis of the
decomposition product is outside the scope of this manuscript,
the elongation of the Cu-P bond distances and P-Cu-P bite
angle could provide a first hint towards the ligands’

Figure 3. Experimental difference spectrum corresponding to the excited state
due to MLCT at a series of time delays between laser and X-ray pulses for a
mixture consisting of A. 1 mM Cu(I) photosensitizer in a solvent mixture of
THF/H2O with a volume ratio of 9:1 . B. 1 mM Cu(I) photosensitizer in a solvent
mixture of THF/TEA/H2O with a volume ratio of 4:3:1.

The kinetics and decay of the excited state in the absence
and presence of triethylamine (TEA), acting as electron donor,
was additionally monitored. As illustrated in Figure 4 A, the photon
energy was fixed to around 9000 eV corresponding to the
formation of the excited state and the time-delays between the
laser and X-ray pulses varied. The formation of the Cu(II) MLCT

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state appears promptly in less than the ~100 ps pulse duration of
the X-rays and decays within 1095 + 87 ns in a solution mixture
of THF/H2O (Figure 3A, 4A). However, in the presence of TEA,
the excited state decays much faster by a factor of around 3,
within 375 ±+ 129 ns, indicating the quicker generation of the Cu(I)
reduced state, [(P^P)CuI(N^N)] (Scheme 1, Figures 3B,4A). The
shorter decay time of the excited state in the presence of TEA
confirms that reductive quenching is fast and may have a
significant contribution in photochemical hydrogen evolution
reactions.[6b] The emission decays of the excited state in the
absence and presence of the sacrificial reductant were also
studied by time-correlated single photon counting (TCSPC)
measurements (Figure 4 B). Decay times of 1170 + 5 ns (without
TEA) and 382 + 1 ns (with TEA) were obtained, which are in
agreement with previous observations and the proposed reaction
mechanism.[6b]

Summary and Conclusions
In summary, we trapped and elucidated the structure of the
excited state of one of the most efficient heteroleptic diiminediphosphine Cu(I) complexes [(xant)Cu(Me2phenPh2)]+ used for
the photocatalytic production of hydrogen by time-resolved X-ray
absorption spectroscopy (tr-XAS). XANES in combination with
EXAFS analysis revealed the local electronic and structural
configuration of the copper centre in the excited state. Importantly,
the excited state reveals a Cu-P bond length elongation of 0.13 Å
and a Cu-N bond length reduction of 0.09 Å. This change is in
agreement with the DFT calculations, which show a much more
pronounced distorted tetrahedral geometry in the excited state
and decreased P-Cu-P bite angle. For instance, the angle
between the two planes formed by the xantphos and
bathocuproine ligand in the ground state is around 84.6˚, whereas
in the excited state this angle distorts only to around 71.6˚ as
compared to around 59˚ in homoleptic photosensitizers of the type
[CuI(dmp)2]+ (with dmp = dimethylphenanthroline).[28b] This is in
agreement with the longer lifetime of the excited state in this class
of heteroleptic Cu(I) photosensitizers due to their lower propensity
to exciplex formation. However, the reduced stability of the
heteroleptic diimine-diphosphine Cu(I) photosensitizers with
regard to the homoleptic bis-diimine counterparts could be due to
the elongation of the Cu-P bond in the excited state that leads
towards a stepwise decomposition during photocatalysis.[6c, 6e, 6f,
7g]

The decay kinetics of the excited state was also studied with
tr-XAS and TCSPC techniques giving lifetimes of the order of
microseconds. This is in contrast to the broad range of analogous
homoleptic bis-diimine Cu(I) complexes, that have vastly been
studied through both ultrafast optical and tr-XAS spectroscopy[8e,
8g, 9a, 28]
revealing emission lifetimes only in the order of
nanoseconds.

Figure 4 A. Pump-probe delay scan recorded at peak energy of the excited
state’s transient signal, corresponding to the excited state’s formation in the
absence (black) and in the presence (red) of the sacrificial electron donor
triethylamine. B. Emission decay of the excited state (monitored at 586 nm) of
0.1 mM Cu(I) photosensitizer in a solvent mixture of THF/H2O with a volume
ratio of 9:1 (black) and a solvent mixture of THF/TEA/H2O with a volume ratio of
4:3:1 (red).

Importantly, this study outlines the necessity to further
enhance the stability of heteroleptic Cu(I) photosensitizers by the
right choice of the diphosphine ligand. Furthermore, the
application of bulkier and more rigid ligands supports to avoid
undesired flattening distortion of the excited state as well as
exciplex quenching. This work also provides future promise of
elucidating key processes in the order of femtosecond-sub
picoseconds times scales such as metal-to-ligand-chargetransfer and intersystem crossing. Such processes which occur
before the formation of the excited state can, for instance, be
studied at free electron laser facilities where femtosecond time
resolution has become possible. These studies can identify
critical factors influencing the structural reorganization of the
copper centre in the excited state and better control the energetics
of the MLCT charge transfer and dynamics of the intersystem
crossing rates upon light excitation.

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Experimental Section
Sample Preparation. The heteroleptic copper photosensitizer
[(xant)Cu(Me2phenPh2)]PF6 was synthesized as previously described and
matched all reported characterization.[6b, 6d] Ultrapure water (type 1, with a
resistivity of 18.2 MΩ.cm at 25°C, total organic carbon content of 4 μg/L)
sourced from a Q-POD unit of Milli-Q integral water purification system
(Millipore) was used. The respective solvent mixtures were prepared
applying anhydrous > 99 % pure tetrahydrofuran (Catalog No. 401757,
Sigma Aldrich) and > 99 % pure triethylamine (Catalog No. T0886, Sigma
Aldrich).
Time-resolved XAS measurements. Time-resolved X-ray absorption
spectra were collected at 11 ID-D[17b] beamline at the Advanced Photon
Source using undulator radiation at electron energy 9.0 keV. The
experiments were carried out using the 24 bunch timing mode of APS (in
top up mode with a constant 102 mA ring current) which consists of a train
of X-rays separated by 153 ns. This mode easily allows for gateable
detectors that selects X-ray pulses.
The sample was pumped at 400 nm wavelength using a regenerative
amplified laser with 10 kHz repetition rate 5ps-FWHM pulse length and
laser power of 397 mW. The sample was circulated through a stainless
steel nozzle into a free-flowing 550 µm cylindrical jet inside an airtight
aluminium chamber, and continuously degassed with nitrogen. The X-ray
and laser beam was spatially overlapped with an X-ray spot size of 100
µm(V) x 450 µm(H) and laser spot size of 170 µm(V) x 550 µm. With a
liquid flow speed of 3 m/s, the pumped laser volume was calculated to
move out of the FWHM region in around 30 µs. This temporal range
ensured that the excited state volume was probed more at the centre and
less at the edges where the excitation fraction would be less, due to
movement of the sample. Beamline 11 ID-D has an automated data
digitization system which allows for all X-ray pulses after laser excitation
to be collected. Such a system, together with the larger X-ray beam spot
size, was very useful for our experiments, as multiple X-ray pulses after
laser excitation were averaged to monitor the dynamics for the formation
and decay of the copper excited state species in the ns-µs time regime. In
addition, by averaging multiple pulses, we obtained a better resolution of
the main features in the pre-edge and edge regions of the transient signals.
The delay between the laser and X-ray pulses was adjusted by a
programmable delay line (PDL-100A-20NS, Colby Instruments) and the Xray fluorescence signals were collected with two APDs positioned at 90˚
on both sides of the liquid jet. Moreover, a combination of Z-1 filters and
soller slits with conical geometry were used to reduce the background from
elastically scattered X-rays. A Cu metal foil was placed between two
ionization chambers downstream to the X-ray beam, and its transmission
recorded with each scan for energy calibration.
Time-correlated single photon counting measurements. Timecorrelated single photon counting experiments (TCSPC) measurements
were carried out on an Edinburgh Instruments LifeSpec-II spectrometer,
equipped with a PMT detector and a double subtractive monochromator.
The Cu(I) L complex was pumped with 405 nm excitation with a 20 KHz
picosecond pulsed diode laser source and its emission probed at 586 nm.
The samples were measured in a 1 cm cuvette sealed with a septum cap
and thoroughly degassed with N2 gas.
EXAFS data analysis. Athena software [33]was used for data processing.
The energy scale for each scan was normalized using copper metal
standard. Data in energy space were pre-edge corrected, normalized, and
background corrected. The processed data were next converted to the
photoelectron wave vector (k) space and weighted by k2. The electron

wave number is defined as, k = [2m(E-E0)/ħ2]1/2 where E0 is the energy
origin or the threshold energy. K-space data were truncated near the zero
crossings (k = 2-10.2 Å-1) in Cu EXAFS before Fourier transformation. The
k-space data were transferred into the Artemis Software for curve fitting.
In order to fit the data, the Fourier peaks were isolated separately, grouped
together, or the entire (unfiltered) spectrum was used. The individual
Fourier peaks were isolated by applying a Hanning window to the first and
last 15% of the chosen range, leaving the middle 70% untouched. Curve
fitting was performed using ab initio-calculated phases and amplitudes
from the FEFF8[34] program from the University of Washington Ab initiocalculated phases and amplitudes were used in the EXAFS equation [35]

 ( k )  S 02 
j

N
kR

2 R
j
2
j

f eff j (  , k , R j ) e

 2

2
j

k

2

e

j

 j (k )

sin( 2 kR j   ij ( k )) (1)

where Nj is the number of atoms in the jth shell; Rj the mean distance
between the absorbing atom and the atoms in the jth shell; feffj (,k, Rj ) is
the ab initio amplitude function for shell j, and the Debye-Waller term
2 2 k 2
e j accounts for damping due to static and thermal disorder in
absorber-backscatterer distances. The mean free path term reflects losses
due to inelastic scattering, where λj(k), is the electron mean free path. The
oscillations in the EXAFS spectrum are reflected in the sinusoidal term ,
sin (2kRj +ɸij(k)) where ɸij(k) is the ab initio phase function for shell j. This
sinusoidal term shows the direct relation between the frequency of the
EXAFS oscillations in k-space and the absorber-back scatterer distance.
S02 is an amplitude reduction factor. The EXAFS equation (Eq. 1) was
used to fit the experimental Fourier isolated data (q-space) as well as
unfiltered data (k-space) and Fourier transformed data (R-space) using N,
S02, E0, R, and 2 as variable parameters. N refers to the number of
coordination atoms surrounding Cu for each shell. The quality of fit was
evaluated by R-factor and the reduced Chi2 value. The deviation in E0
ought to be less than or equal to 10 eV. R-factor less than 2% denotes that
the fit is good enough[35] whereas R-factor between 2 and 5% denotes that
the fit is correct within a consistently broad model. The reduced Chi2 value
is used to compare fits as more absorber-backscatter shells are included
to fit the data. A smaller reduced Chi2 value implies a better fit.
DFT Calculations. DFT optimization calculations were performed using
the ORCA program package developed by Neese and co-workers [36] The
DFT optimization reported in Table 1 was carried out using ORCA ,
B3LYP[37] as functional, the Zeroth Order Regular Approximation (ZORA)
as previously reported[38] with def2-TZVP triple-zeta basis sets[39], and the
atom-pairwise Grimme dispersion correction with the Becke-Johnson
damping scheme[40] (D3BJ). Time-dependent DFT (TD-DFT) calculations
for the XAS spectrum of the Cu pre-edge were performed using the same
functional, basis sets and dispersion correction effects with dense
integration grids, and the fully decontracted def2-TZVP/J[39] auxiliary basis
set. The pre-edge absorption spectra from the TD-DFT calculations were
shifted in energy by -7.5 eV relative to the experimental data and a
broadening of 2.0 eV was applied to all calculated spectra. The donor
orbitals for XAS calculations were chosen as Cu 1s and all virtual orbitals
were selected as acceptor orbitals. Up to 150 roots were calculated. The
calculated pre-edge spectrum contains contributions from electric
quadrupole, electric dipole and magnetic dipole transitions. Further DFT
optimizations with lesser precision as XRD reported coordinates were
carried out using the BP86[41] and BLYP[42] functional, and are summarized
in Table S2 and SI. Importantly, all optimizations trends between the
ground and the excited state agree well with experimental data as
illustrated in Table 1 and Table S2. A consistent decrease in the Cu-N and
elongation in the Cu-P distances is observed upon formation of the Cu(I)*
excited state. Furthermore, an increase in the N-Cu-N bite angle and
considerable decrease in the P-Cu-P angle is obtained in all reported
calculations (Table 1, S2). The degree of distortion between the ground
and excited states is additionally evident from the decrease in the torsional
angle between the xantphos and the bathocuproine ligand.

FULL PAPER
Acknowledgements
MINECO, FEDER and FCI are gratefully acknowledged for
financial support (CTQ2016-80058-R, CTQ2015-73028-EXP,
SEV
2013-0319,
ENE2016-82025-REDT
(FOTOFUEL),
CTQ2016-81923-REDC (INTECAT). The authors thank the
COST Action CM1202. P. G.-B. acknowledges “La Caixa”
foundation for a PhD grant. A.P. acknowledges funding from the
European Union's Horizon 2020 research and innovation program
under the Marie Sklodowska-Curie Grant Agreement No. 702565.
This research used resources of beamline 11 ID-D of the
Advanced Photon Source (APS), a U.S. Department of Energy
(DOE) Office of Science User Facility operated for the DOE Office
of Science by Argonne National Laboratory under Contract No.
DE-AC02- 06CH11357. The laser system at beamline 11-ID-D of
APS was funded through New Facility and Mid-scale
Instrumentation grants (PI: Lin X. Chen). X.Z. and C.L.
acknowledge funding from DOE, Office of Science, Basic Energy
Sciences, Chemical Sciences, Geosciences, and Biosciences
Division under contract no. DE-AC02-06CH11357.
Keywords: Artificial Photosynthesis • Electron Transfer • Copper
heteroleptic photosensitizers • Time-resolved X-ray absorption
spectroscopy. Photocatalysis
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FULL PAPER

D. Moonshiram*, P. Garrido-Barros, C.
Gimbert-Suriñach, A. Picón, C. Liu, X.
Zhang, M. Karnahl*, and A. Llobet*.
Page No. – Page No.

Light-induced electronic and geometric changes within of a heteroleptic Cu(I)
photosensitizer are reported within the pico-microsecond time regime through timeresolved X-ray absorption spectroscopy.

Elucidating the Nature of the Excited
State of a Heteroleptic Copper
Photosensitizer Using Time-Resolved
X-ray Absorption Spectroscopy