Copper Photosensitizers Containing P^N Ligands and Their Influence on Photoactivity and Stability

Article abstract of DOI:10.1002/chem.201703672

Driven by the intention to improve classic heteroleptic copper photosensitizers two novel Cu(I) complexes applying a hetero-bidentate P^N ligand were prepared. A combined photophysical, electrochemical, and theoretical study gives insights into structure–activity relationships and revealed an increased absorptivity. Both complexes were tested for the light-driven production of H₂.

Full text of DOI:10.1002/chem.201703672

A Journal of
Accepted Article
Title: Copper photosensitizers containing P^N ligands and their
influence on photoactivity and stability
Authors: Robin Giereth, Wolfgang Frey, Henrik Junge, Stefanie
Tschierlei, and Michael Karnahl
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Copper photosensitizers containing P^N ligands and their
influence on photoactivity and stability
Robin Giereth,[a] Wolfgang Frey,[a] Henrik Junge,[b] Stefanie Tschierlei,*[c] and Michael Karnahl*[a]
to the S1 state, where a flattening distortion of the Cu(I) center
Abstract: Driven by the intention to improve classic heteroleptic
copper photosensitizers two novel Cu(I) complexes applying a hetero-
bidentate P^N ligand were prepared. A combined photophysical,
electrochemical and theoretical study gives insights into structure-
activity-relationships and revealed an increased absorptivity. Both
complexes were tested for the light-driven production of H2.
occurs followed by the population of a triplet excited state.[21,22]
This excited state exhibits a long lifetime primarily due to the
presence of the bulky diphosphine ligand.[8,21] However, these
diphosphines bind less strongly to the copper center, and hence,
the formation of homoleptic bis-diimine complexes is
thermodynamically favored.[20,23,24,25] It was shown that the extent
of dissociation mainly depends on the size and kind of the
phosphine ligands as well as on the solvent.[20,23,26] This dynamic
ligand exchange represents one of the major drawbacks of
current heteroleptic [(P^P)Cu(N^N)]+ photosensitizers. Therefore,
it is of significant interest to enhance the complex stability and
absorptivity. This might be realised by exchanging the
The increasing use of sunlight represents a promising option
which might contribute to solve the current energy challenge.[1-3]
Therefore, photocatalysis and the storage of solar energy in the
form of chemical energy carriers are becoming more and more
important.[1-3] This also requires the design of efficient,
inexpensive and sustainable photosensitizers for light
harvesting.[4,5] In addition, a suitable photosensitizer should
possess a strong absorption, long-lived excited states, a
reversible electrochemical behavior and a high stability which all
need to be adjusted with the respect to their costs and activity.[6]
In this context, heteroleptic Cu(I) complexes of the general type
[(P^P)Cu(N^N)]+, where N^N indicates a diimine and P^P a
diphosphine ligand, are of particular interest.[7-11] Starting with the
pioneering work by McMillin et al. a rising number of (heteroleptic)
copper photosensitizers (CuPS) were developed and successfully
applied in intermolecular systems for the photocatalytic
production of hydrogen from water.[7-10,12-18]
As a common feature of such heteroleptic Cu(I) complexes it has
been recognized, that bulky and rigid diphosphine ligands with
wide bite-angles are beneficial in order to obtain photophysical
properties like long-lived triplet excited states and high quantum
yields.[7,8,19,20] DFT and spectroscopic studies revealed that the
photophysical activity solely originates from the diimine ligand, as
the electron density in the S1 state is located on the diimine moiety
compared to S0.[11,21] This often results in smaller extinction
coefficients compared to classic homoleptic bis-diimine
complexes [Cu(N^N)2]+.[10,14,15] In contrast, the diphosphine ligand
predominantly ensures a maximum site occupancy and shields
the system from exciplex quenching after light-induced excitation
diphosphine with
a
heterobidentate P^N ligand[27,28] while
maintaining favourable steric (e.g. rigidity) and electronic (e.g.
nitrogen and phosphorus donor/acceptor ability) features
important for photocatalytic applications.
Consequently, the synthesis of two novel Cu(I) complexes 1 and
2 (Figure 1) was successfully performed. The solid state
structures, photophysical and electrochemical properties as well
as the photocatalytic activities are presented and compared to the
standard complex [(xant)Cu(bcp)]+ 3 (with xant = xantphos and
bcp = bathocuproine). The experimental results are accompanied
by DFT/TD-DFT (density functional theory/time-dependent DFT)
calculations. Within this framework the electronic ground-state
(S0) and excited state structures (S1, T1), the electronic densities
as well as associated properties were determined.
Results and Discussions
The already known phosphinooxazoline (phox) ligand was
chosen as P^N ligand,[27] which potentially fulfils the requirements
for a suitable photosensitizer. Subsequently, the respective
homo- and heteroleptic Cu(I) complexes 1 and 2 (Figure 1) were
prepared following a modified synthesis procedure (see SI)
starting from [Cu(MeCN)4]PF6 (MeCN
=
acetonitrile) as
precursor.[8,11] The homoleptic complex 1 was obtained in a one-
step reaction of the copper precursor with two equivalents of the
phox ligand in dichloromethane solution under inert conditions.
Full conversion of the P^N ligand was already accomplished after
a few minutes under reflux. This gives a first indication of an
enhanced binding of the P^N ligand compared to the previously
applied diphosphines, because full formation of the
[(P^P)Cu(MeCN)2]+ intermediate typically requires several
hours.[8,11] Then, the heteroleptic complex 2 was prepared by a
one-pot two-step synthesis where the phox ligand is added first,
followed by addition of the diimine ligand. However, in contrast to
the synthesis of [(P^P)Cu(N^N)]+ complexes, the risk of formation
of the homoleptic by-product [Cu(N^N)2]+ is substantially
minimized and almost independent from the reaction time of the
first step. The pure complexes were obtained as yellow to orange
crystalline solids in excellent to good yields of 87 % (1) and 54 %
(2), respectively.
[a]
R. Giereth, Dr. W. Frey, Dr. M. Karnahl
Institute of Organic Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart, Germany
E-mail: michael.karnahl@oc.uni-stuttgart.de
Homepage: www.karnahl-group.de
[b]
[c]
Dr. H. Junge
Leibniz-Institute for Catalysis at the University of Rostock (LIKAT)
Albert-Einstein-Str. 29a, 18059 Rostock, Germany.
Dr. S. Tschierlei
Institute of Inorganic Chemistry I, Ulm University
Albert-Einstein-Allee 11, 89081 Ulm, Germany.
E-mail: stefanie.tschierlei@uni-ulm.de
Supporting information for this article is given via a link at the end of
the document. These includes experimental and synthetic details,
crystallographic data, MS and NMR spectra as well as detailed results
of the (TD-)DFT calculations.
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Table 1. Selected crystallographic bond lengths (pm) and angles (°) of the
complexes 1-3. For atom labeling compare with Figure 2, further values are
given in the SI. The respective CCDC reference numbers are 1561237 (1),
1561238 (2) and 1561239 (3).
1
2
3
Cu-N1
209.4(2) Cu-N1
226.40(8) Cu-N2
207.0(2) Cu-N3
227.26(7) Cu-P1
86.38(6) N1-Cu-N2
86.98(7) N3-Cu-P1
120.65(3) N2-Cu-P1
207.5(4)
Cu-N1
Cu-N2
Cu-P1
206.5(3)
Cu-P1
205.0(4)
207.5(4)
209.9(3)
Cu-N2
230.48(10)
224.25(10)
80.41(10)
119.68(4)
128.84(8)
Cu-P2
220.44(15) Cu-P2
Figure 1. General structure of the homo- and heteroleptic Cu(I) complexes 1
and 2 applied in this study.
N1-Cu-P1
N2-Cu-P2
P1-Cu-P2
80.38(17) N1-Cu-N2
88.97(12) P1-Cu-P2
134.42(14) N2-Cu-P1
plane angle 71.1 [a]
plane angle 88.3 [a]
plane angle 85.0 [a]
[a] calculated angle between the two ligand planes
The cyclic voltammogram of 2 in acetonitrile exhibits a reversible
reduction wave at -2.02 V vs. Fc/Fc+ and two quasireversible
oxidation processes (Figure 3). These findings are in accordance
with related Cu(I) complexes.[8,11,25] The reduction potential of 1
(-2.27 V) is 250 mV more anodic than that of 2 (-2.02 V), whereas
the oxidation of both complexes occurs around 0.38 - 0.56 V. The
additional redox event at -0.92 V appears only after the previous
reduction of 1 (Figure SI19) and can be therefore assigned to a
newly formed side-product. Comparison of 2 with 3 reveals that
the reduction potentials of these two heteroleptic complexes are
almost the same (Table 2). This is due to the fact that the
reversible one-electron reduction is assigned to the reduction of
the diimine ligand,[8,25] which is identical in 2 and 3.
Furthermore, the small other redox-wave at approx. -2.1 V of 2,
can be attributed to the progressive build-up of the homoleptic
species 1 upon redox cycling.[24,25] However, from previous
experiments with related heteroleptic [(P^P)Cu(N^N)]+ complexes
it is known, that the equilibrium (Keq) between hetero- and
homoleptic Cu(I) complexes is comparably low in acetonitrile.[20]
Hence, no or only little formation of the homoleptic [Cu(P^N)2]+
and [Cu(N^N)2]+ species is expected as long as the complexes
are not electro- or photochemically oxidized.[20,24,25]
Figure 2. Solid state structures (ORTEP representation) of complex 1 (left) and
2 (right) with thermal ellipsoids at a probability level of 50 %. Hydrogen atoms,
counter anions and solvent molecules are omitted for clarity.
Single crystals suitable for X-ray crystallography were received
either by slow diffusion of n-pentane in
a
saturated
dichloromethane solution (for 1) or in ethanol (for 2). In both
complexes the ligand environment displays a distorted tetrahedral
geometry around the copper center (Figure 2). The angle between
the two ligand planes, which are spanned through the chelating
hetero atoms and the copper center, differs from 90° (Table 1 and
Figure SI10). However, with 88.3° the deviation of the heteroleptic
complex 2 is smaller than in 3 with 85.0°. This could be caused
by the smaller size and less steric demand of the phox ligand in
contrast to xantphos. Another reason for distortion is given by the
quite small bite angle of phox in 2 (N3-Cu-P1: 88.97(12)°)
compared to that of xant in 3 (P1-Cu-P2: 119.48(4)°), which
strongly deviates from the ideal tetrahedral angle of 109.5° (Table
1 and SI2). The smaller binding angle is also reflected by a shorter
Cu-P distance in 2 (e.g. Cu-P1: 220.44(15) pm) in comparison to
3 (e.g. Cu-P2: 230.48(10) pm). Furthermore, the coordinated
phox ligand shows a strong twist in relation to the respective N-
Cu-P plane (see visualization of DFT calculation in Figures SI4
and SI5). As a result, in the homoleptic complex 1 an interaction
between the two phox ligands takes place, resulting in a
considerably distorted geometry with an angle between the
planes of 71.1°. The extent of distortion is also supported by the
DFT calculation yielding in 76.1° (Table SI3). Further structural
analysis of 1 and 2 also reveals a slight electronic effect. In 1,
where the π-acceptor bcp is missing, the Cu-P bond lengths (Cu-
P1: 226.40(8) and Cu-P2: 227.26(7) pm) are elongated compared
to 2 (Cu-P1: 220.44(15) pm), which is again confirmed by DFT
calculations where the Cu-P bonds are increased from 224.1 pm
in 2 to 230.0 pm in 1 (for details see SI).
Figure 3. Cyclic voltammograms of 1 (black) and 2 (grey) in acetonitrile solution
referenced vs. the ferrocene/ferrocenium (Fc/Fc+) couple. Conditions: scan rate
of 100 mVs-1, 0.1 M Bu4NPF6 as supporting electrolyte. The arrow indicates the
scan direction.
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A comparison of the absorption spectra (Figure 4) of 1 and 2 in
acetonitrile solution reveals clear differences between both
complexes. The spectrum of 2 possesses bands at 280 and 450
nm, which are assigned to ligand-centered and metal-to-ligand
charge transfer (MLCT) transitions, respectively.[10,14,21] Instead,
in 1 the ligand centered π-π* transitions are blue-shifted and the
MLCT transitions occur only as broad shoulder of the first-
mentioned. Further, the molar extinction coefficient of complex 1
is significantly lower than of 2 (Table 2) as also approved by TD-
DFT calculations with lower oscillator strengths (cf. Figure SI11
and further transition energies in Table SI4). This highlights the
importance of the diimine ligand on the absorptivity of the resulting
CuPS. Intrinsic basis bonding analysis (IBBA) of the TD-DFT
calculation shows, that the HOMO of 2, which has the highest
orbital contributions in the lowest-lying excitations, is
predominantly located on the copper center (54 %) as well as on
the phosphorous (15 %) and the nitrogen (7 %) atom of the phox
ligand (see Table SI6). A closer inspection on the orbital
contributions of the acceptor orbitals of 2 revealed that the LUMO
and LUMO+1 are mainly located on the bcp scaffold (Table SI5
and Figure SI15). Furthermore, the respective transitions to the
LUMO possess the largest oscillator strength in the visible range
as also obtained for related CuPS containing a bcp and xant
ligand or derivatives thereof.[21] However, there are also small
contributions originating from the phox ligand which are located at
419 and 441 nm (compare Table SI5) as obtained by TD-DFT
calculation. This additional participation of the P^N ligand in the
absorption of visible light increases the molar extinction
coefficient of 2 by about 40 % compared to 3. However, the
contribution of the phox ligand to the light absorption process is
significantly lower than of the bcp ligand. Hence, the absorption
of 1 is considerably decreased in relation to that of 2. In order to
examine the effects of the P^N ligand on the ability of these novel
Cu(I) complexes to function as photosensitizers they were tested
in the light-driven hydrogen production.
Figure 4. UV/vis absorption spectrum of 1 (black solid) and 2 (grey dashed) in
acetonitrile. The enlargement represents the range of the metal-to-ligand
charge transfer transitions.
The photocatalytic reduction of protons to hydrogen was chosen
to study the influence of the P^N ligand on the photocatalytic
activity. Therefore, a fully noble-metal-free system composed of
[Fe3(CO)12] as WRC, triethylamine (TEA) as SR and the Cu(I)
complexes was used.[7,8,11] Previously developed standard
conditions, i.e. a solvent mixture of THF/TEA/H2O in the ratio
4:3:1, were applied (for further details see Table 3 and the
SI).[7,8,11] Both complexes 1 and 2 show a low, but fairly constant
production of H2 within 24 h (Figure 5). The development of the
respective turnover numbers over time is presented in the
supporting information (see Figure SI22). The catalytic behavior
might be explained by the reduced ability of 1 and 2 to undergo
electron transfer from the SR compared to 3 (see excited-state
redox potentials below and Figure SI18). Another reason are the
adverse effects of the smaller steric demand and low rigidity of
the phox ligand on the excited state properties (see Figures SI16
and SI17). The resulting sensitivity to fast exciplex-quenching
also reduces the turnover numbers compared to 3. However, the
additional contribution of the phox ligand to MLCT transitions as
well as the increased stability of 1 and 2 are of high importance in
order to develop a sustainable catalytic system in the future. A
further elucidation of this issue is currently ongoing in our
laboratories.
Table 2. Summary of the photophysical and electrochemical properties of the
complexes 1-2 and of the reference 3 in acetonitrile solution at room temperature.
complex

abs [nm]

abs, max [M-1 cm-1]
Eox [V] [a]
Ered [V] [a]
1
2
400 sh
408
1.450 [c]
7.300
+0.56 [b]
-2.27
-2.02
+0.38 [b]
+0.52 [b]
3 [d]
389
5.700
+0.93
-2.05
[a] in acetonitrile vs. Fc/Fc+, [b] quasireversible oxidation, [c]  at 400 nm,
[d] taken from ref. [8].
Figure 5. Hydrogen evolution curves for the photocatalytic reduction of protons
using the copper photosensitizers (ca. 3.5 µmol) 1 (black solid) and 2 (grey
dashed) with [Fe3(CO)12] (ca. 5.0 µmol) as water reduction catalyst (for
experimental details see SI).
In addition to the investigation of the different MLCT transitions
and of the involved ligands therein, the analysis of the extent of
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[b] V24h corrected by a blank volume of 2.5 mL (entry 1) or 2.7 mL (entries 2 and
3) [c] TONH,Cu = n(H) / n(CuPS) with Vm,H2,25°C = 24.48 mL/mmol., [d] taken from
ref. [8] for comparison.
distortion of the ligands through photoexcitation is of high interest.
Calculation of the singlet and triplet excited states at full TD-DFT
level of theory reveals a significant change of the dihedral angle
between the planes,[29,30] e.g. of the homoleptic CuPS
[Cu(phen)2]2+ and [Cu(dmp)2]2+ (dmp = 2,9-Me2phen) from 90°
Conclusions
(S0) to 41.7° (T1) and 67.4° (T1), respectively.[31] As
a
consequence of this large distortion the triplet excited states can
be efficiently quenched by solvent molecules.[10,14] A reduced
distortion and a shielding through the diphosphine ligand was
First insights in the effect of exchanging a classic diphosphine
P^P by a heterobidentate P^N ligand on the photophysical and
catalytic properties of the resulting Cu(I) complexes are presented.
The employed phox ligand leads to an increase in absorptivity and
a bathochromic shift of the main MLCT absorption compared to
the heteroleptic reference complex [(bcp)Cu(xant)]+. With regard
to the photocatalytic hydrogen production an increased stability
could be observed, enabling a catalytic activity of more than 24 h.
However, further P^N ligands with more steric demand and an
increased rigidity have to be tested, as this renders the major
drawback of the current system.
found
for
[(DPEphos)Cu(dmp)]+
(DPEphos
=
bis[2-
(diphenylphosphino) phenyl]ether) with a change from 82.5° (S0)
to 69.0° (S1) and 70.2° (T1),[32] resulting in a considerably
evolution of H2.[7,8,11,18] In contrast, the angles between the planes
of 1 and 2 in the excited states were obtained by TDDFT
calculation to be 60.6° (59.6° in T1) and 50.9° (55.4° in T1),
respectively (see Table SI7 and Figure SI17). These smaller
values indicate, that the phox ligand lacks the high steric demand
of the DPEphos or xant ligand.[21] Thus, the copper center is more
prone to nucleophilic attack, and hence, fast exciplex quenching.
This might explain the absence of emission at room temperature
in solution and the relatively weak evolution of hydrogen.
Acknowledgements
Another possible reason for the reduced photocatalytic activity is
the lability of the oxazoline Caryl-Coxazoline and phosphine Caryl-PPPh2
joints. In addition to the steric constraints due to the small bite
angle, the six-membered phox-Cu chelate ring gets twisted in the
S1 state in both, complex 1 and 2 (see Figs. SI16 and SI17).
Especially in 1, the strong geometrical changes lead to a more
distinct definition of a trans-position. Hence, the Cu-N bond
opposite to the more powerful acceptor atom gets weakened
which leads to a difference in Cu-N bond lengths of 20.6 pm (Cu-
N1: 195.9 pm, Cu-N2: 216.5 pm). In complex 2 this effect is less
pronounced and the deviations between the Cu-N1 and Cu-N2
distances is only about 4.8 pm.
The authors thank A. Kammer (LIKAT Rostock) for performing the
catalytic experiments. M.K. is thankful to the University of
Stuttgart and the Fonds der Chemischen Industrie (FCI) for
financial support. S.T. grateful acknowledges the German
Science Foundation (DFG, TS 330/3-1) and the FCI for funding.
The authors acknowledge support by the states of Mecklenburg-
Western Pomerania and Baden-Württemberg through bwHPC
and by the DFG (INST 40/467-1 FUGG).
Keywords: copper complexes • heterobidentate P^N ligands •
photocatalysis • hydrogen • photophysics • DFT calculations
In combination with the catalytic experiments the ability of the new
CuPS to undergo electron transfer with the water reduction
catalyst (WRC) and the sacrificial reductant (SR) is estimated by
the calculation of the excited state redox potentials.[33] For 1 these
can be determined to +0.15 V and -1.86 V (Table SI9), which only
renders the reduction of the WRC [HFe3(CO)11]- (-1.64 V vs.
Fc/Fc+) possible, whereas the electron transfer from the SR
triethylamine (+0.46 V vs. Fc/Fc+) is not likely.[8] This also holds
true for complex 2, where the excited state redox potentials are
predicted as +0.20 V and -1.84 V vs. Fc/Fc+. Therefore, both 1
and 2 appear to be potent photoreductants, where an electron can
be transferred from the CuPS to the WRC after light excitation.
Unfortunately, 1 and 2 are not able to act as photooxidant, which
is in contrast to complex 3.
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[a] Conditions: CuPS (ca. 3.5 µmol), [Fe3(CO)12] (ca. 5.0 µmol), THF/TEA/H2O
(4:3:1, 10 mL), 25°C, Xe light irradiation (output 1.5 W) without light filter, 24 h
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10.1002/chem.201703672
Chemistry - A European Journal
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The more the better: Conventional
diphosphine P^P ligands were
Robin Giereth, Wolfgang Frey, Henrik
Junge, Stefanie Tschierlei,* and Michael
Karnahl*
replaced by a heterobidentate P^N
ligand in order to enhance stability
and absorptivity of the resulting
copper photosensitizers. A combined
photophysical, electrochemical and
theoretical study gives insights into
structure-activity-relationships and
explains the photocatalytic activity.
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Copper photosensitizers containing
P^N ligands and their influence on
photoactivity and stability
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