Electron Storage Capability and Singlet Oxygen Productivity of a RuII Photosensitizer Containing a Fused Naphthaloylenebenzene Moiety at the 1,10-Phenanthroline Ligand

Article abstract of DOI:10.1002/chem.202001564

As a novel rylene type dye a diimine ligand with a fully rigid and extended π-system in its backbone was prepared by directly fusing a 1,10-phenanthroline building block with 1,8-naphthalimide. The corresponding heteroleptic ruthenium photosensitizer bearing one biipo and two tbbpy ligands was synthesized and extensively analyzed by a combination of NMR, single crystal X-ray diffraction, steady-state absorption and emission, time-resolved spectroscopy and different electrochemical measurements supported by time-dependent density functional theory calculations. The cyclic and differential pulse voltammograms revealed, that the naphthaloylenebenzene moiety enables an additional second reduction of the ligand. Moreover, this ligand possesses a very broad absorption in the visible region. In the Ru^II complex this causes an overlap of ligand-centered and metal-to-ligand charge transfer transitions. The emission of the complex is clearly redshifted compared to the ligand emission with very long-lived excited states lifetimes of 1.7 and 24.7 μs in oxygen-free acetonitrile solution. This behavior is accompanied by a surprisingly high oxygen sensitivity. Finally, this photosensitizer was successfully applied for the effective evolution of singlet oxygen challenging some of the common Ru^II prototype complexes.

Full text of DOI:10.1002/chem.202001564

Accepted Article
Accepted Article
Title: Electron storage capability and singlet oxygen productivity of a
Ru(II) photosensitizer containing a fused naphthaloylenebenzene
moiety at the 1,10-phenanthroline ligand
Authors: Yingya Yang, Jannik Brückmann, Wolfgang Frey, Sven Rau,
Michael Karnahl, and Stefanie Tschierlei
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001564
Link to VoR: https://doi.org/10.1002/chem.202001564
01/2020

10.1002/chem.202001564
Chemistry - A European Journal
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Electron storage capability and singlet oxygen productivity of a
Ru(II) photosensitizer containing a fused naphthaloylenebenzene
moiety at the 1,10-phenanthroline ligand
Yingya Yang,a,‡ Jannik Brückmann,a,‡ Wolfgang Frey,b Sven Rau,a Michael Karnahl,*,b and Stefanie
Tschierlei*,a
Abstract: As a novel rylene type dye a diimine ligand with a fully rigid
and extended π-system in its backbone was prepared by directly
fusing a 1,10-phenanthroline building block with 1,8-naphthalimide.
The corresponding heteroleptic ruthenium photosensitizer bearing
one biipo and two tbbpy ligands was synthesized and extensively
analyzed by a combination of NMR, single crystal X-ray diffraction,
steady-state absorption and emission, time-resolved spectroscopy
and different electrochemical measurements supported by time-
dependent density functional theory calculations. The cyclic and
differential pulse voltammograms revealed, that the naphthaloylene-
benzene moiety enables an additional second reduction of the ligand.
Moreover, this ligand possesses a very broad absorption in the visible
region. In the Ru(II) complex this causes an overlap of ligand-centered
and metal-to-ligand charge transfer transitions. The emission of the
complex is clearly redshifted compared to the ligand emission with
very long-lived excited states lifetimes of 1.7 and 24.7 µs in oxygen-
free acetonitrile solution. This behavior is accompanied by a
surprisingly high oxygen sensitivity. Finally, this photosensitizer was
successfully applied for the effective evolution of singlet oxygen
challenging some of the common Ru(II) prototype complexes.
reduction of protons to hydrogen) photosensitizers are desired
that are able to store multiple charges.[17-22] In this context
especially Ru(II) polypyridine complexes containing an additional
electron storage moiety at the diimine ligand have been
extensively studied.[17,19,20,23,24] For instance, different types and
number of methyl viologen, anthraquinone, naphthalimide or
pyrene derivatives had been introduced as an additional
substituent to 2,2’-bipyridine (bpy) or 1,10-phenanthroline
(phen).[23-27,28]
The integration of donor-acceptor units like naphthalene
dicarboxylic acid monoimide (NMI) or other rylene derivatives
enabled easy synthetic access to photosensitizers with high
thermal and (photo)chemical stabilities and to adapt the
photophysical as well as electrochemical properties of the
chromophores in a wide range. In addition, rylene dyes often
provide high fluorescence quantum yields, large extinction
coefficients over a broad spectral range and the option to store
multiple electrons.[29-35] The usage of related dyes like
naphthalene tetracarboxylic acid bisbenzimidazole (perinone)
allows for low energy gaps and light absorption over a wide
range.[36-38] However, due to their low solubility, these ligand
systems received only limited attention.[36-38]
In contrast to the extension of the -system of the diimine ligand
by different substituents, the main aim of this study was the direct
integration of a naphthaloylenebenzene unit in the backbone of
the 1,10-phenanthroline ligand (Figure 1) to obtain a fully
conjugated and rigid ligand. It is expected that this modification
not only affects the redox behavior, but also the charge transfer
Introduction
The capture, storage and usage of solar light for light-driven
reactions in organic synthesis[1-4] and for the production of solar
fuels[5-8] or singlet oxygen (1O2) is of strongly increasing
importance.[9-12] As 1O2 is one of the most relevant reactive
oxygen species (ROS) for photodynamic therapy (PDT), there is
processes
and
efficiency.
To
this
end,
16H-
benzo[4',5']isoquinolino[2',1':1,2]imidazo[4,5-f]-[1,10]-
a
great need for effective singlet oxygen generating
phenanthrolin-16-one (biipo) and its heteroleptic Ru(II) complex
[(tbbpy)2Ru(biipo)]2+ (Rubiipo, with tbbpy = 4,4’-tert.-butyl-2,2’-
bipyridine) were prepared and analyzed (including single crystal
X-ray diffraction (XRD)) for the first time (Figure 1). Cyclic and
differential pulse voltammetry were used to evaluate the effect of
chromophores.[13-16] This requires the design of efficient and
stable photosensitizers with suitable redox potentials and excited
state lifetimes. Moreover, for some applications (e.g. the
the internal electron storage on the redox chemistry.
A
[a]
[b]
Y. Yang, J. Brückmann, Prof. Dr. S. Rau, Dr. S. Tschierlei
Ulm University, Institute of Inorganic Chemistry I
Albert-Einstein-Allee 11, 89081 Ulm, Germany.
E-mail: stefanie.tschierlei@uni-ulm.de.
combination of steady-state and time-resolved emission
spectroscopy along with transient absorption spectroscopy and
(time-dependent) density functional theory (TD-)DFT calculations
were then used to explore the photophysical properties and
charge-transfer processes. Moreover, Rubiipo was successfully
used for the evolution of 1O2 competing some of the parent
Dr. W. Frey, Dr. M. Karnahl
University of Stuttgart, Institute of Organic Chemistry
Pfaffenwaldring 55, 70569 Stuttgart, Germany.
E-mail: michael.karnahl@oc.uni-stuttgart.de
standard
complexes
like
[Ru(bpy)3]2+
(Rubpy)
or
[(tbbpy)2Ru(phen)]2+ (Ruphen).
‡ Both authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document. The supporting information (SI) includes experimental and
synthetic information, crystallographic data, NMR, MS and UV/vis spectra,
cyclic voltammograms as well as results of the (TD-)DFT calculations.
This article is protected by copyright. All rights reserved.

10.1002/chem.202001564
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SI). Rubiipo and biipo both exhibit a characteristic carbonyl
stretch vibration at around 1700 cm-1 in their IR spectra (Figures
S3.15 and S3.16) originating from the biipo ligand framework.
The high-resolution mass spectra of both the ligand and the
complex match very well with theoretical data and simulated
1
isotopic patterns (see Figures S3.9 and S3.10). In the H NMR
spectra the chemical shifts of all aromatic protons appear as
expected.[41,42] The proton signals of the two bipyridines, the
phenanthroline and the naphthaloylene-benzene moiety suggest
an asymmetric octahedral Ru(II) complex. Moreover, the aliphatic
protons of the tertiary butyl (tBu) groups show a small splitting (i.e.
four separated singlet signals with 9 protons each), which is
caused by the asymmetric structure of the biipo ligand. Caused
by this asymmetry the four pyridines and tBu groups of the two
tbbpy ligands are not equivalent within the Rubiipo complex and
possess a different chemical environment. Further, 1H NMR
spectra with different concentrations of Rubiipo (in the range from
1.28 mM to 32.1 mM, Figure S3.6) exhibit considerable shifts of
the proton signals of up to 0.5 ppm. According to literature
protocols, a dimerization constant of 27 M-1 was determined for
Rubiipo (see SI).[43-46] This clearly suggests intermolecular
interactions between different Rubiipo species in solution, which
are caused by the quite large and planar biipo ligand. This
observation is in line with other Ru(II) complexes bearing
polycyclic diimine ligands such as dipyridophenazine,
tetrapyridophenazine (dimerization constant = 289 ± 17 M-1),
di(pyridin2-yl)tetraazaphenazine (83 ± 17 M-1) or tetraazatetra-
pyridopentacene, which reveal even stronger interactions
Figure 1. Schematic representation of a Ru(II) photosensitizer containing an
additional electron storage (ES) moiety (left) and the structure of Rubiipo
investigated in this study (right). The biipo ligand is depicted in black.
Results and Discussion
Synthesis and structural characterization
The synthesis of the biipo ligand was performed via a four-step
procedure (Scheme S2.1) starting from 1,10-phenanthroline. In
the final step 5,6-diamino-1,10-phenanthroline and a slight
excess of 1,8-naphthalic anhydride were condensed to yield biipo
as a yellow solid in 94 % (Figure 2). Subsequently, due to its low
solubility biipo was coordinated to the [(tbbpy)2RuCl2] precursor
using hot ethylene glycol as solvent. This procedure is also
applied for similar poorly soluble diimine ligands and is known in
literature.[39,40] Rubiipo was then purified via silica column
chromatography (for further details see SI) and counter ion
exchange to obtain the complex in a total yield of 71 %.
[45,46,26,27,29]
between the extended π-systems of these ligands.
This might be important, because in case of high intermolecular
association behavior self-quenching of excited states can get
more favorable.[29,47-49]
Figure 2. Preparation of the biipo ligand and its corresponding Ru(II) complex
Rubiipo. Conditions: i) acetic acid, Ar, 140°C, 24 h and ii) [(tbbpy)2RuCl2],
ethylene glycol, 120°C, 20 h.
Figure 3. Solid-state structure (ORTEP representation) of Rubiipo with thermal
-
ellipsoids at a probability level of 50%. Hydrogen atoms, PF6 counter anions
and solvent molecules are omitted for clarity.
Rubiipo was fully characterized by 1H and 13C nuclear magnetic
resonance (NMR) spectroscopy, high-resolution mass
spectrometry (HRMS, electron spray ionization), IR spectroscopy,
elemental analysis and single crystal XRD (see all spectra in the
Single crystals of Rubiipo suitable for X-ray diffraction analysis
were obtained by slow evaporation from
a
saturated
acetonitrile/water mixture. Rubiipo crystallizes in the monoclinic
space group C 2/c. Due to the asymmetric structure of biipo, two
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enantiomers of Rubiipo are possible causing a geometrical
disorder (see Figure S3.11) of the respective solid-state structure.
Please note, that Rubiipo does not have different diastereomers,
but exists as a mixture of enantiomers (Figure S3.13).[50] As
clearly evident from the X-ray structural data, there are the
expected enantiomers of the corresponding regioisomers of
Rubiipo (Figure S3.13). In addition, the biipo ligand facilitates a
certain degree of π-π interactions between the individual metal
complex cations (Figure S3.12) similar to other extended rigid
polyaromatic diimine ligands.[47,51] The obtained unit cell contains
calculations of the reduced ligand, showing that the spin density
is located at the backbone of the biipo ligand (Figure S8.2).
However, the irreversible reduction process at -2.21 V of Rubiipo
seems to be related to the imidazole unit as known from the
literature.[41]
-
8 Ru complexes along with 16 PF6 anions and 30 acetonitrile
molecules. This resulted in a challenging crystal structure, where
the second hexafluorophosphate anion is overlaid by several
acetonitrile molecules. Interestingly, this obstacle and the need
for several restraints was found in multiple measurements, but
prevents a detailed discussion of exact bond lengths and angles.
Nevertheless, with a final R-value of 6.4% the solid-state structure
is still well suited to show the general properties, e.g. the
conventional distorted octahedral type coordination environment
around the ruthenium center[52] and the planar structure of biipo.
Figure 4. Cyclic voltammograms (top) and differential pulse voltammograms
(DPV, bottom) of biipo (black, < 1 mM due to low solubility in acetonitrile) and
Rubiipo (orange, 1 mM) in acetonitrile solution referenced vs. the
ferrocene/ferricenium (Fc/Fc+) couple. The current values of the DPV are
normalized. Conditions: scan rate of 100 mVs-1, [Bu4N][PF6] (0.1 M) as
supporting electrolyte.
Electrochemical Properties
The redox properties of Rubiipo and biipo were studied by cyclic
voltammetry and differential pulse voltammetry (Figure 4, Table
1) in acetonitrile solution containing 0.1 M [Bu4N][PF6]. The
reversible RuII/RuIII oxidation couple is located at 0.80 V vs.
Fc/Fc+, which is in accordance to related Ru(II) complexes, e.g.
Ruphen (0.78 V) or Rubpy (0.89 V).[53,54]
The reversible reduction waves at -1.44, -1.86, -2.04 and -2.31 V
are characteristic for [(bpy)2Ru(L)]2+ type complexes, where L
represents a phen derivative with an extended π-system.[41,47]
These potentials can be assigned to the stepwise reduction of (i)
the naphthaloylene-benzene moiety of biipo, (ii)/(iii) the two tbbpy
ligands and (iv) the phen part of the biipo ligand,
respectively.[55,56] The first reduction event of Rubiipo (-1.44 V) is
150 mV shifted to more positive potentials compared to biipo
(-1.59 V). This reduction is reversible for both compounds as also
evidenced by scan-rate dependent CV measurements (Figure
S5.2). This process can be attributed to the naphthaloylene-
benzene moiety of biipo, which is supported by the DFT
Table 1. Summary of the photophysical and electrochemical properties of biipo, Rubiipo and of selected reference compounds in acetonitrile solution at room
temperature. The electrochemical data were obtained from deaerated acetonitrile solutions at room temperature and are referenced vs. the ferrocene/ferricenium
(Fc/Fc+) couple.
Compound
λꢀꢁꢂ,ꢃꢀꢄ
[nm]
(ε [103 M-1 cm-1])
λꢅꢃ
[nm]
inert
φꢅꢃ
inert
φꢅꢃ
O2
τem
τem
[ns]
O2
Eꢆ/ꢇꢈꢄ
[V]
Eꢆ/ꢇꢉꢅꢊ
[V]
[ns]
inert
biipo
409 (6.2)
540
365
623
602
0.258
< 0.01
0.022
0.03
0.428
< 10 [b]
< 10 [b]
-
-1.59, -2.29[c]
phen[a]
Rubiipo
Ruphen[a]
265 (32.0)
411 (26.3), 460 (24.7)
426 (16), 455 (19)
0.006
-
1668, 24717
272
222
-
+ 0.80
+ 0.79
-1.44, -1.86 -2.04, -2.21[c], -2.31
-1.79, -1.99, -2.26
[a] taken from reference 57; [b] the lifetime was below the detection limit; [c] irreversible redox event.
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Absorption and Emission Properties
in this region of the absorption spectrum. Nevertheless, above
420 nm the MLCT transitions in Rubiipo are not overlaid by the
LC transitions, which is commonly observed for related systems
containing diimine ligands with an extended π-system.[41,49,58-60]
There, the MLCT transitions are often superimposed by the LC
transitions.[41,49]
The biipo ligand absorbs in acetonitrile in a broad visible range
up to 500 nm (Figure 5). This is a significant redshift compared to
the phen ligand, which possesses an absorption maximum at
around 365 nm.[57] The energetically lowest lying band could be
assigned by TD-DFT calculations to a ligand centred (LC) -*
transition from the phenanthroline/imidazole part to the
naphthaloylenebenzene moiety of biipo (480nm, transition 1,
Figure 5 and SI). At higher energies (i.e. the band at 350 nm) the
The emission properties of biipo and Rubiipo differ significantly
from each other. The ligand emits at about 540 nm and is almost
unaffected by the presence of oxygen in the acetonitrile solution
(Figure 5 bottom). In strong contrast, Rubiipo is almost non-
emissive under aerated conditions, but exhibits a clear emission
at 623 nm in the absence of oxygen with a quantum yield of about
2.2% (Table 1). This quantum yield is in the same order of
magnitude as some related Ru(II) complexes,[52] but lower than a
heteroleptic Ru(II) complex with an azabenz-annulated
perylene bisimide (11%).[28] Remarkably, the emission intensity
is drastically reduced already by small traces of oxygen. Thus,
Rubiipo is highly sensitive vs. oxygen and a good oxygen sensor
situation
is
different
and
a
transition
from
the
naphthaloylenebenzene part to the phenanthroline unit occurs.
These results indicate that although in the biipo ligand the
naphthaloylenebenzene moiety is directly fused to the
phenanthroline part, this ligand is still split into two separated
halves concerning its orbitals in the ground and excited states.
This results in a very broad absorption of the pure ligand.
]
due to the strong quenching of emission.[61
In accordance to the steady state emission measurements the
emission lifetimes of Rubiipo were also determined in acetonitrile
(Figure 6). Under aerated conditions a lifetime of 222 ns was
obtained (Figure S6.1), which is comparable to the related
complex [(bpy)2Ru(PNI-phen)]2+ (with PNI = 4-piperidinyl-1,8-
naphthalimide), where the π extension is not directly fused to the
]
phenanthroline part.[25 Under inert conditions the emissive
excited state lives remarkably longer (Figure 6). Analysis of the
kinetics revealed a biexponential decay with two time constants
of τ1 = 1.7 µs and τ2 = 24.7 µs, which belong to LC or MLCT
transitions. Thus, Rubiipo emits about 100 times longer when
changing from ambient to oxygen free conditions. This is again
comparable to [(bpy)2Ru(PNI-phen)]2+ with a lifetime of 27.6 µs
under inert conditions. Nevertheless, to the best of our knowledge
this is a large increase in excited state lifetime for such a Ru(II)
complex containing a diimine ligand with a fully conjugated and
extended π-system. Interestingly, the second time constant is
highly oxygen sensitive. As soon as Rubiipo gets in contact with
O2 τ2 considerably decreases to only sub microseconds. This
again underlines the strong oxygen sensitivity of Rubiipo.
Figure 5. Spectra of biipo (black) and Rubiipo (orange). Top: UV/vis
absorption spectrum in acetonitrile. Middle: Calculated absorption spectrum of
Rubiipo resulting from the convolution (std. deviation 40) of the transitions
above 290 nm (vertical bars) with a Gaussian function. The transition energies
are calculated at full TD-DFT level of theory (B3LYP-D3(BJ)/def2-TZVP.
Bottom: Emission spectra in acetonitrile under oxygen free conditions.
In comparison to Ruphen,[57] having an absorption maximum at
444 nm in the metal-to-ligand charge transfer (MLCT) transition,
Rubiipo possesses a broader MLCT band with two maxima
located at 411 and 460 nm, respectively. This latter prominent
feature can be explained by MLCT transitions from the ruthenium
centre to the two tbbpy ligands and to the phen part of biipo
(Table S9.2) Additionally, the MLCT transition with the lowest
energy ends also directly at the naphthaloylenebenzene moiety
(c.f. inset of Figure 6, SI) as supported by TDDFT calculations.
However, in the calculations the LC -* transitions of Rubiipo
are redshifted from 320 nm to 370 nm compared to the transitions
of the uncoordinated biipo ligand. Contrary, the -* transition to
the naphthaloylenebenzene moiety is clearly blueshifted from
480 nm (biipo) to 417 nm (Rubiipo, transition 12, Table S9.2).
For these reasons, the absorption maximum at around 411 nm is
very intense causing the high attenuation coefficient of Rubiipo
Figure 6. Emission lifetime of Rubiipo in acetonitrile under ambient (green) and
inert conditions (orange). The respective fits and the residuals are depicted in
the SI. The inset (differential density plot of Rubiipo) shows one of the occurring
MLCT transitions from the Ru d orbital to the π* orbital located at the
naphthaloylenebenzene moiety.
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Transient Absorption Spectroscopy
solution of ABDA (9,10-anthracenediyl-bis(methylene)dimalonic
acid) in PBS (phosphate buffered solution) was added to distinct
quantities (c = 1 µM) of Rubiipo, Rubpy and Ruphen (ABDA:Ru
= 50:1), respectively. ABDA is a commonly applied singlet oxygen
detection agent, due to its very selective reaction with this reactive
oxygen species (ROS). 1O2 and ABDA build up an endoperoxide
(Figure S4.5), which loses the characteristic anthracene
absorption bands at around 350 to 410 nm.[64-66] Upon irradiation
with blue light (LED, 470 nm, 50 mWcm-2) under constant cooling
Rubiipo (Figure 8 top) shows very fast depletion of the ABDA
absorbance at around 360, 380 and 400 nm. In order to evaluate
the singlet oxygen productivity, the decrease of this absorption
band was plotted against time until complete consumption of
ABDA (Figure 8 bottom). The observed decay was not linearly,
but seemed to be an exponential decrease, which is much faster
compared to Ruphen (Figure 8 bottom). Whereas also Rubpy
To verify the obtained excited state lifetimes by a different method
time-resolved transient absorption spectroscopy was performed.
Under inert conditions and excited at 355 nm, a very broad excited
state absorption from 480 to 700 nm was obtained for Rubiipo in
acetonitrile solution (Figure 7). This is a typical observation, if
ligand-centred transitions are detected by transient absorption
spectroscopy.[60,62] Due to this feature the expected ground state
bleach was only extended up to 440 nm and not up to 500 nm, as
estimated from the steady state absorption spectra (Figure 5 top).
The kinetics (Figure 7) were analysed globally over the whole
range from 370 to 700 nm by a biexponentially fit function. The
received lifetimes τ1 = 2.5 and τ2 = 24.9 µs are comparable to the
ones obtained for the emission lifetime under oxygen-free
conditions. It should be noted, that especially the longer lifetime
is very oxygen sensitive. As soon as there are traces of oxygen
present, this time constant decreases significantly. Thus, it is very
important to work under fully inert conditions for the determination
of the excited state lifetimes of Rubiipo. Under aerobe conditions
the monoexponential fit of the data results in one time constant,
i.e. τ = 261 ns, which is also comparable to the obtained emission
lifetimes in the presence of oxygen.
1
evolves O2 much slower compared to Rubiipo (Figures S4.6-
S4.10). Nevertheless, a better energy transfer rate from Rubiipo
to oxygen can neither be assessed nor excluded based on the
current data and further investigations concerning this issue are
needed.
Figure 7. Transient absorption spectra (top) from 500 (red) to 40000 ns
(yellow)and respective kinetics (bottom) of Rubiipo in acetonitrile excited at 355
nm under inert conditions. Kinetics at 390 (red), 480 (green), 500 (blue) and 650
nm (light blue), the black line belongs to the fits with two time constants of 2.5
and 24.9 µs.
Figure 8. Top: Singlet oxygen evolution experiment using Rubiipo (1 eq.) and
ABDA (50 eq.) in PBS after irradiation with blue light (50 mWcm-2, 470 nm) in
the range of ABDA absorbance (se also Figure S4.5.). Bottom: Comparison of
the relative ABDA absorbance as function of time during irradiation of Rubiipo
(squares), Rubpy (circles) and Ruphen (triangles) with blue light (50 mWcm-2,
470 nm). It becomes clear, that Rubiipo degrades ABDA the fastest. See for
the specific ABDA test in the SI the section 4.
Oxygen sensitivity and productivity
Motivated by the strong oxygen sensitivity Rubiipo was tested for
1
its ability to produce singlet oxygen upon light irradiation. O2 is
the leading actor in Type II pathway of Photodynamic Therapy
(PDT) actions. An efficient production of reactive singlet oxygen
under oxygen containing conditions is still a crucial point for this
type of application.[5,63] To verify the 1O2 production by Rubiipo, a
Conclusions
In summary, starting from 1,10-phenanthroline a new type of a
polycyclic, fully conjugated and rigid diimine ligand with a
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naphthaloylenebenzene moiety in the back was prepared for the
first time. Based on the special ligand structure, a Ru(II)
photosensitizer (Rubiipo), also verified by single crystal X-ray
diffraction analysis, with some unique properties was formed. For
instance, Rubiipo shows a reversible redox chemistry (i.e. four
reductions and one oxidation) as well as a strong and broad
absorption in the visible with two maxima at 411 and 460 nm,
respectively. More importantly, Rubiipo possesses a clear
emission at 623 nm with very long-lived excited states of 1.7 and
24.7 µs in acetonitrile in the absence of oxygen. Interestingly, the
emission intensity and lifetimes are drastically decreased (by a
factor of >100) by O2, which indicates a high oxygen sensitivity of
this complex. In the future, temperature-dependent emission,
emission lifetime and spectroelectrochemical measurements are
planned, to clarify the nature of the excited states further. In
addition, a detailed investigation of the oxygen sensing ability of
Rubiipo will be assessed by Stern-Volmer quenching
experiments.
Keywords: ruthenium photosensitizer • extended π-system •
electron storage • excited state properties • singlet oxygen
evolution
[1]
M. H. Shaw, J. Twilton, D. W. C. MacMillan J. Org. Chem. 2016, 81 (16),
6898–6926.
[2]
[3]
S. Malzkuhn, O. S. Wenger Coord. Chem. Rev. 2018, 35952–56.
Q. M. Kainz, C. D. Matier, A. Bartoszewicz, S. L. Zultanski, J. C. Peters,
G. C. Fu Science 2016, 351 (6274), 681.
[4]
J. Twilton, C. C. Le, P. Zhang, M. H. Shaw, R. W. Evans, D. W. C.
MacMillan Nat. Rev. Chem. 2017, 1 (7), 0052.
[5]
[6]
R. Eisenberg Science 2009, 324 (5923), 44.
P. D. Frischmann, K. Mahata, F. Würthner Chem. Soc. Rev. 2013, 42 (4),
1847–1870.
[7]
[8]
[9]
S. Berardi, S. Drouet, L. Francàs, C. Gimbert-Suriñach, M. Guttentag, C.
Richmond, T. Stoll, A. Llobet Chem. Soc. Rev. 2014, 43 (22), 7501–7519.
Y.-J. Yuan, Z.-T. Yu, D.-Q. Chen, Z.-G. Zou Chem. Soc. Rev. 2017, 46
(3), 603–631.
T. J. Dougherty, C. J. Gomer, B. W. Henderson, G. Jori, D. Kessel, M.
Korbelik, J. Moan, Q. Peng J. Natl. Cancer Inst. 1998, 90 (12), 889–905.
[10] P. R. Ogilby Chem. Soc. Rev. 2010, 39 (8), 3181–3209.
[11] D. Van Straten, V. Mashayekhi, S. H. De Bruijn, S. Oliveira, J. D.
Robinson Cancers 2017, 9 (2),
Most importantly, beside potential applications as an oxygen
sensor Rubiipo is able to produce singlet oxygen under visible
light irradiation with very fast kinetics. Therefore, the study of
further applications of the Rubiipo photosensitizer as well as of
other transition metal complexes using the new biipo ligand are
currently ongoing.
[12] S. Monro, K. L. Colón, H. Yin, J. Roque, P. Konda, S. Gujar, R. P.
Thummel, L. Lilge, C. G. Cameron, S. A. McFarland Chem. Rev. 2019,
119 (2), 797–828.
[13] M. C. DeRosa, R. J. Crutchley Coord. Chem. Rev. 2002, 233-234351–
371.
[14] C. Mari, V. Pierroz, S. Ferrari, G. Gasser Chem. Sci. 2015, 6 (5), 2660–
2686.
[15] G. Shi, S. Monro, R. Hennigar, J. Colpitts, J. Fong, K. Kasimova, H. Yin,
R. DeCoste, C. Spencer, L. Chamberlain, A. Mandel, L. Lilge, S. A.
McFarland Coord. Chem. Rev. 2015, 282-283, 127-138.
[16] F. Heinemann, J. Karges, G. Gasser Acc. Chem. Res. 2017, 50 (11),
2727–2736.
Experimental Section
CCDC 1993565 (Rubiipo) contains the supplementary
crystallographic data for this paper. This data is provided free of
charge by The Cambridge Crystallographic Data Centre.
[17] R. Konduri, N. R. de Tacconi, K. Rajeshwar, F. M. MacDonnell J. Am.
Chem. Soc. 2004, 126 (37), 11621–11629.
[18] S. Singh, N. R. de Tacconi, N. R. G. Diaz, R. O. Lezna, J. Muñoz Zuñiga,
K. Abayan, F. M. MacDonnell Inorg. Chem. 2011, 50 (19), 9318–9328.
[19] S. Karlsson, J. Boixel, Y. Pellegrin, E. Blart, H.-C. Becker, F. Odobel, L.
Hammarström Faraday Discuss. 2012, 155 (0), 233–252.
[20] J. M. Aslan, D. J. Boston, F. M. MacDonnell Chem. Eur. J. 2015, 21 (48),
17314–17323.
For further details see the Supporting information: Experimental
and synthetic details, further crystallographic data, NMR, MS, IR,
UV/vis, and emission spectra, photostability measurements,
singlet oxygen evolution measurements, cyclic voltammograms,
emission lifetime analysis, transient absorption spectra, TD-DFT
calculation data.
[21] L. Hammarström Acc. Chem. Res. 2015, 48 (3), 840–850.
[22] Y. Zhang, P. Traber, L. Zedler, S. Kupfer, S. Gräfe, M. Schulz, W. Frey,
M. Karnahl, B. Dietzek Phys. Chem. Chem. Phys. 2018, 20 (38), 24843–
24857.
[23] E. H. Yonemoto, R. L. Riley, Y. I. Kim, S. J. Atherton, R. H. Schmehl, T.
E. Mallouk J. Am. Chem. Soc. 1992, 114 (21), 8081–8087.
[24] A. Lavie-Cambot, C. Lincheneau, M. Cantuel, Y. Leydet, N. D.
McClenaghan Chem. Soc. Rev. 2010, 39 (2), 506–515.
[25] D. S. Tyson, C. R. Luman, X. Zhou, F. N. Castellano Inorg. Chem. 2001,
40 (16), 4063–4071.
Acknowledgements
Y.Y., M.K. and S.T. grateful acknowledge the German Science
Foundation (DFG, Priority Program SPP 2102 “Light-controlled
reactivity of metal complexes” (KA 4671/2-1 and TS 330/4-1) and
the DFG project TS 330/3-1) for funding. S.R. is thankful for
financial support from the DFG within the collaborative research
center TRR 234 Catalight. J.B. gratefully acknowledges the
Fonds der Chemischen Industrie (FCI) for a Kekulé-Stipendium.
The authors acknowledge support by the state of Baden-
Württemberg through bwHPC and by the DFG (INST 40/467-1
FUGG). The authors also thank Bernd Maier, Martin Rentschler
and Julian Bösking for their synthetic work and Dr. Dieter Sorsche
for providing X-ray data.
[26] D. E. Polyansky, E. O. Danilov, F. N. Castellano Inorg. Chem. 2006, 45
(6), 2370–2372.
[27] J. Hankache, O. S. Wenger Phys. Chem. Chem. Phys. 2012, 14 (8),
2685–2692.
[28] M. Schulze, A. Steffen, F. Würthner Angew. Chem. Int. Ed. 2015, 54,
1570-1573.
[29] H. Langhals Heterocycles 1995, 40 (1), 477–500.
[30] A. Hermann, K. Müllen Chem. Lett. 2006, 35 (9), 978-985.
[31] T. Weil, T. Vosch, J. Hofkens, K. Peneva, K. Müllen Angew. Chem. Int.
Ed. 2010, 49 (48), 9068-9093.
This article is protected by copyright. All rights reserved.

10.1002/chem.202001564
Chemistry - A European Journal
FULL PAPER
[32] M. Pan, X.-M. Lin, G.-B. Li, C.-Y. Su Coord. Chem. Rev. 2011, 255 (15),
1921–1936.
[51] M. Karnahl, C. Kuhnt, F. W. Heinemann, M. Schmitt, S. Rau, J. Popp, B.
Dietzek Chem. Phys. 2012, 393, 65-73.
[33] F. N. Castellano Dalton Trans. 2012, 41 (28), 8493–8501.
[34] S.-L. Suraru, F. Würthner Angew. Chem. Int. Ed. 2014, 53 (29), 7428–
7448.
[52] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von
Zelewsky Coord. Chem. Rev. 1988, 8485–277.
[53] S. Rau, R. Fischer, M. Jäger, B. Schäfer, S. Meyer, G. Kreisel, H. Görls,
M. Rudolf, W. Henry, J. Vos Eur. J. Inorg. Chem. 2004, 2001–2003.
[54] L. A. Büldt, A. Prescimone, M. Neuburger, O. S. Wenger Eur. J. Inorg.
Chem. 2015, 4666–4677.
[35] F. Würthner, C. R. Saha-Möller, B. Fimmel, S. Ogi, P. Leowanawat, D.
Schmidt Chem. Rev. 2016, 116 (2), 962-1052.
[36] H. Langhals, H. Jaschke Chemistry - A European Journal 2006, 12 (10),
2815–2824.
[55] P. Wintergerst, A. K. Mengele, D. Nauroozi, S. Tschierlei, S. Rau Eur. J.
Inorg. Chem. 2019, 1988–1992.
[37] Z. Yuan, Y. Xiao, Z. Li, X. Qian Org. Lett. 2009, 11 (13), 2808–2811.
[38] M. Mamada, C. Pérez-Bolívar, D. Kumaki, N. A. Esipenko, S. Tokito, P.
Anzenbacher Jr. Chem. Eur. J. 2014, 20 (37), 11835–11846.
[39] M. Karnahl, S. Tschierlei, C. Kuhnt, B. Dietzek, M. Schmitt, J. Popp, M.
Schwalbe, S. Krieck, H. Görls, F. W. Heinemann, S. Rau Dalton Trans.
2010, 39 (9), 2359–2370.
[56] S.-H. Fan, A.-G. Zhang, C.-C. Ju, L.-H. Gao, K.-Z. Wang Inorg. Chem.
2010, 49 (8), 3752–3763.
[57] M. Karnahl, S. Krieck, H. Görls, S. Tschierlei, M. Schmitt, J. Popp, D.
Chartrand, G. S. Hanan, R. Groarke, J. G. Vos, S. Rau Eur. J. Inorg.
Chem. 2009, 4962–4971.
[40] H. Yin, M. Stephenson, J. Gibson, E. Sampson, G. Shi, T. Sainuddin, S.
Monro, S. A. McFarland Inorg. Chem. 2014, 53 (9), 4548–4559.
[41] D. Isakov, R. Giereth, D. Nauroozi, S. Tschierlei, S. Rau Inorg. Chem.
2019, 58 (19), 12646–12653.
[58] R. Giereth, I. Reim, W. Frey, H. Junge, S. Tschierlei, M. Karnahl
Sustainable Energy Fuels 2019, 3, 692-700.
[59] Y Sun, L. E. Joyce, N. M. Dickson, C. Turro Chem. Commun. 2010, 46,
2426-2428.
[42] M. Skaisgirski, X. Guo, O. S. Wenger Inorg. Chem. 2017, 56 (5), 2432–
2439.
[60] J. Schindler, Y. Zhang, P. Traber, J.-F. Lefebvre, S. Kupfer, M.
Demeunynck, S. Gräfe, M. Chavarot-Kerlidou, B. Dietzek J. Phys. Chem.
C 2018, 122 (1), 83-95.
[43] V. Steullet, D. W. Dixon J. Chem. Soc., Perkin Trans. 2 1999 (7), 1547–
1558.
[61] J. N. Demas, B. A. DeGraff, P. B. Coleman Anal. Chem. 1999, 71 (23),
793A–800A.
[44] de Tacconi, R. Norma, R. Chitakunye, F. M. MacDonnell, R. O. Lezna J.
Phys. Chem. A 2008, 112 (3), 497–507.
[62] D. S. Tyson, K. B. Henbest, J. Bialecki, F. N. Castellano J. Phys. Chem.
A 2001, 105 (35), 8154-8161.
[45] M. G. Pfeffer, C. Pehlken, R. Staehle, D. Sorsche, C. Streb, S. Rau
Dalton Trans. 2014, 43 (35), 13307–13315.
[63] K. Plaetzer, B. Krammer, J. Berlanda, F. Berr, T. Kiesslich Lasers Med.
Sci. 2009, 24 (2), 259–268.
[46] F. Huber, D. Nauroozi, A. K. Mengele, S. Rau Eur. J. Inorg. Chem. 2017,
4020-4027.
[64] T. Wang, N. Zabarska, Y. Wu, M. Lamla, S. Fischer, K. Monczak, D. Y.
W. Ng, S. Rau, T. Weil Chem. Commun. 2015, 51 (63), 12552–12555.
[65] O. Planas, N. Macia, M. Agut, S. Nonell, B. Heyne J. Am. Chem. Soc.
2016, 138 (8), 2762–2768.
[47] J. Bolger, A. Gourdon, E. Ishow, J.-P. Launay Inorg. Chem. 1996, 35 (10),
2937–2944.
[48] S. D. Bergman, M. Kol Inorg. Chem. 2005, 44 (6), 1647–1654.
[49] L. Troian-Gautier, C. Moucheron Molecules (Basel, Switzerland) 2014,
19 (24759069), 5028–5087.
[66] S. Chakrabortty, B. K. Agrawalla, A. Stumper, N. M. Vegi, S. Fischer, C.
Reichardt, M. Kögler, B. Dietzek, M. Feuring-Buske, C. Buske, S. Rau,
T. Weil J. Am. Chem. Soc. 2017, 139 (6), 2512–2519.
[50] L. Gong, M. Wenzel, E. Meggers Acc. Chem. Res. 2013, 46 (11), 2635–
2644.
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Entry for the Table of Contents
The constant evolution of phenanthroline leads to a new type of a
polycyclic, fully conjugated and rigid diimine ligand with
a
naphthaloylenebenzene moiety in the back. The so-called biipo ligand
enabled a Ru(II) photosensitizer, which has an excited state lifetime of
up to 24.7 µs and is active for the light-driven singlet oxygen production.
This article is protected by copyright. All rights reserved.