Synthesis and Characterization of a Covalent Porphyrin-Cobalt Diimine-Dioxime Dyad for Photoelectrochemical H2 Evolution

Article abstract of DOI:10.1002/ejic.202001111

The utilization of solar energy via photoelectrochemical cells (PEC) towards hydrogen (H₂) production is a promising approach in the field of artificial photosynthesis. In this work, the synthesis and complete characterization of the first covalently linked porphyrin-cobalt diimine-dioxime dyad (ZnP-Co) is reported. The synthetic procedure that was followed is based on simple and high yielding reactions, without using any noble metal. Photophysical investigation of the dyad revealed sufficient electronic communication between the sensitizer and the catalyst in the excited state. Finally, NiO films were sensitized with ZnP-Co and the H₂-evolving photoelectrochemical activity of the resulting photocathode was assessed. The modest performances could be rationalized thanks to photolysis and post-operando characterizations of the system, thus providing some guidelines toward the design of more efficient porphyrin-based assemblies.

Full text of DOI:10.1002/ejic.202001111

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doi.org/10.1002/ejic.202001111
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Synthesis and Characterization of a Covalent Porphyrin-
Cobalt Diimine-Dioxime Dyad for Photoelectrochemical H2
Evolution
Asterios Charisiadis,[a] Emmanouil Giannoudis,[b] Zoi Pournara,[a] Aimilia Kosma,[a]
Vasilis Nikolaou,[a] Georgios Charalambidis,*[a] Vincent Artero,[b]
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Murielle Chavarot-Kerlidou,*[b] and Athanassios G. Coutsolelos*[a]
The utilization of solar energy via photoelectrochemical cells
(PEC) towards hydrogen (H2) production is promising
sufficient electronic communication between the sensitizer and
the catalyst in the excited state. Finally, NiO films were
sensitized with ZnP-Co and the H2-evolving photoelectrochem-
ical activity of the resulting photocathode was assessed. The
modest performances could be rationalized thanks to photolysis
and post-operando characterizations of the system, thus
providing some guidelines toward the design of more efficient
porphyrin-based assemblies.
a
approach in the field of artificial photosynthesis. In this work,
the synthesis and complete characterization of the first
covalently linked porphyrin-cobalt diimine-dioxime dyad (ZnP-
Co) is reported. The synthetic procedure that was followed is
based on simple and high yielding reactions, without using any
noble metal. Photophysical investigation of the dyad revealed
Introduction
The first essential component in a dye-sensitized photo-
cathode is the photosensitizer, which should be cost-effective,
photo-stable and absorb irradiation to relatively wide part of
the sunlight spectrum, possessing mobility and high carrier
lifetime as well. Porphyrin derivatives are ideal candidates for
that purpose since apart from being analogues to the natural
occurring chlorophyll compounds, they also possess all the
above features.[4] Additionally, porphyrins can be easily structur-
ally modified through straightforward synthetic procedures,[5]
rendering them attractive components for the preparation of
covalently linked photosensitizer-catalysts dyads[6] and for the
construction of dye-sensitized photocathodes for H2
production.[7]
Recent literature reports have described the first functional
H2-evolving photocathodes based on a noble metal free
covalent dye-catalyst assembly.[8] Herein, we report the syn-
thesis and full characterization of the first example of a covalent
porphyrin dye-catalyst assembly based on the same cobalt
diimine-dioxime catalyst (ZnP-Co) and its integration in an H2-
evolving photocathode (Figure 1).
Based on our previous work, the photosensitizer and
catalyst moieties were connected via a Cu-catalyzed click
reaction.[8a] This reaction offers facile experimental conditions
and in addition, the triazole bridge is reported to provide
enhanced electronic communication in porphyrin-based molec-
ular assemblies.[10] For the preparation of the ZnP-Co dyad, ZnP
porphyrin (Figure 2) was used as the main building block. This
derivative was also used as reference compound and is
functionalized on one side with an alkyne group for the
coupling reaction with the catalyst and on the other side with a
carboxylic acid group for the anchoring of the dyad onto the
NiO surface. We also prepared the methyl-ester analogues of
the porphyrin precursor and of the final dyad (Figure 2), in
Inspired by photosynthesis, many researchers have developed
artificial photosynthetic systems achieving efficient light-har-
vesting and solar energy conversion into an electrochemical
potential.[1] One step further, the construction of dye-sensitized
photoelectrochemical cells (DSPECs) has recently emerged as a
highly attractive approach to convert and store solar energy
into solar fuels.[2] Water splitting tandem DSPECs are in
particular the subject of intense research efforts to produce
hydrogen from sunlight and water. Their main limitation lies
however in the low efficiency of the photocathodes reported so
far, compared to the photoanodes counterpart. Thus, the
design of novel H2-evolving photocathode architectures with
high catalytic performance is crucial and a large number of
research groups has been motivated and focused their studies
towards this target.[2c,d,3]
[a] Dr. A. Charisiadis, Z. Pournara, A. Kosma, Dr. V. Nikolaou,
Dr. G. Charalambidis, Prof. Dr. A. G. Coutsolelos
Department of Chemistry,
University of Crete,
Laboratory of Bioinorganic Chemistry,
Voutes Campus, Heraklion
70013, Crete, Greece
E-mail: gcharal@uoc.gr
acoutsol@uoc.gr
www.chemistry.uoc.gr/hycore
www.chemistry.uoc.gr/coutsolelos
[b] Dr. E. Giannoudis, Dr. V. Artero, Dr. M. Chavarot-Kerlidou
Univ. Grenoble Alpes, CNRS, CEA,
IRIG, Laboratoire de Chimie et Biologie des Métaux,
38000 Grenoble, France
E-mail: murielle.chavarot-kerlidou@cea.fr
www.solhycat.com
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001111
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of COOMe-ZnP and then the methylester group was hydrolyzed
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under basic conditions to provide compound ZnP. In the
following step, ZnP was coupled with the copper diimine-
dioxime complex 5[12] via copper(I)-catalyzed azide-alkyne cyclo-
addition (CuAAC) according to our previously reported
protocole.[8a,12] Then, metal exchange was performed through
reaction with an excess of CoCl2 in an acetone/THF (1:1)
mixture under continuous air bubbling to obtain a stable Co(III)
complex. This intermediate was purified through silica-gel flash
chromatography and subsequently precipitated in NaBr-satu-
rated aqueous solution to yield the desired dyad (ZnP-Co). The
procedures that were performed for the preparation of the
methyl-ester dyad (COOMe-ZnP-Co) are depicted in Scheme S1.
A click reaction was also carried out in order to connect
COOMe-ZnP with complex 5, followed by a metal exchange
reaction. The purity of all intermediates as well as both final
dyads (ZnP-Co and COOMe-ZnP-Co) was confirmed by MALDI-
TOF mass spectrometry and NMR spectroscopy (1H and 13C)
(Figures S1-S21). Both dyads (ZnP-Co and COOMe-ZnP-Co)
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Figure 1. Energy diagram at pH 5.5 and working principle of the NiO
photocathode based on ZnP-Co. Potentials were converted from Fc to NHE
considering E (Fc+/0)= +0.53 V vs NHE. The NiO valence band edge
°
potential at this pH value was estimated from the 0.37 V vs NHE value
determined at pH 7.[9]
1
order to achieve enhanced solubility in organic solvents, namely
THF, thus enabling their complete characterization in solution.
presented in their H NMR spectra the characteristic signal for
the proton of the oxime bridge in the pseudo-macrocyclic
cobalt diimine-dioxime catalyst at 20.07 and 19.28 ppm,
respectively; the peak of the newly formed triazole ring is
observed at 8.34 and 9.28 ppm, respectively. Moreover, the
peak corresponding to the carboxylic acid anchoring group was
observed at 13.37 ppm for ZnP-Co.
The photophysical characterization of the final dyad as well
as the reference porphyrin was performed in their ester
analogues via absorption and emission spectroscopies. The
absorption spectra of COOMe-ZnP and COOMe-ZnP-Co in THF
solutions are presented in Figure S22. The reference derivative
Results and Discussion
The synthetic route that was followed for the preparation of
ZnP-Co is outlined in Scheme 1 and consisting of six steps.
In the first step, the acid-catalyzed condensation between
dipyrromethane 3[11] and the aldehydes 1 and 2 took place
yielding the free-base porphyrin derivative 4. Consequently, the
macrocycle was metallated with zinc resulting in the formation
Figure 2. Structures of the two reference compounds (COOMe-ZnP and ZnP) along with the two dyads (COOMe-ZnP-Co and ZnP-Co) reported herein.
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Scheme 1. Synthetic scheme for the ZnP-Co dyad. Conditions: i) CHCl3, BF3OEt2, 24 h RT, DDQ, 20 h RT; ii) Zn(CH3COO)2 ·2H2O, MeOH, CH2Cl2; iii) KOH, THF/
MeOH/H2O; iv) CuSO4 5H2O, Sodium ascorbate, CH2Cl2/H2O/MeOH, RT, 24 h; v) CoCl2 6H2O, THF/Acetone, RT, air bubbling, overnight; vi) KBr, THF/Acetone, RT,
5d.
.
.
illustrates typical zinc porphyrin-based features, i.e. one Soret
(425 nm) and two Q bands (557 and 598 nm). These character-
istics are also observed in the dyad accompanied by some weak
contributions at lower wavelengths, which could be attributed
to the cobalt diimine-dioxime catalyst.[8a] Since no significant
shift was observed for any of the absorption peaks, but only a
slightly decreased absorption coefficient for the Soret band in
the dyad, we can safely assume that the two constituent
entities interact weakly in the ground state of the dyad.
In an effort to gain some insight into the photoinduced
processes that can take place in our dyad, fluorescence experi-
ments were conducted and the emission spectra of COOMe-
ZnP and COOMe-ZnP-Co in THF were compared (Figure 3).
Both the reference compound (COOMe-ZnP) and the dyad
(COOMe-ZnP-Co) exhibited characteristic emission peaks for a
zinc porphyrin, located at around 608 and 656 nm. In the case
of the dyad, the intensity of both bands is partly quenched
Figure 3. Emission spectra of isoabsorbing solutions of compound COOMe-
ZnP (black) and COOMe-ZnP-Co (red) in THF upon excitation at 555 nm.
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the irreversible reductive wave (CoII/I) is assigned to the
Table 1. Summary of the electrochemical redox data of COOMe-ZnP, Co-
N3 and COOMe-ZnP-Co in THF. All potentials are reported vs. Fc/Fc+ which
was used as the internal standard.
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formation of the triazole ring in the dyad.[13]
The remaining reductive wave along with both oxidation
peaks were assigned to the porphyrin core by comparing the
voltammograms of our dyad and the reference dye (Figure 4,
top). Both oxidation peaks are slightly cathodically shifted in
the dyad compared to the reference dye (approximately 0.1 V),
due to the different peripheral substituents of the porphyrin
ring, namely triazole ring and ethynyl group, respectively.
Based on the electrochemical data, we estimated a driving
À
II/I
III/II
+
+
2+
Name
E(ZnP/ZnP ) [V] ECo
ECo
[V]
E(ZnP/ZnP
[V]
E(ZnP
[V]
)
/ZnP
)
[V]
COOMe-ZnP
Co-N3
COOMe-ZnP-Co À 1.90
À 1.96
0.44
0.82
0.70
À 1.15 À 0.66
À 1.00 À 0.63 0.34
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°
upon selective excitation of the porphyrin unit at 555 nm. Two
processes might account for this observation: either a thermo-
dynamically allowed electron transfer from the porphyrin
excited state to the Co(III) center (ΔG° =À 1.13 eV, see below),
or an energy transfer from the first singlet excited state of
porphyrin towards the catalyst unit.
The electrochemical properties of COOMe-ZnP-Co were
determined by cyclic voltammetry (Figure 4) in freshly distilled
and deoxygenated tetrahydrofuran with tetrabutylammonium
hexafluorophosphate (TBAPF6) as supporting electrolyte. All the
electrochemical data are summarized in Table 1.
Three reduction processes and two oxidative waves can be
observed in the case of the dyad (Figure 4, bottom). The first
two reductive processes at À 0.63 V vs Fc and À 1.00 V vs Fc
were attributed to the CoIII/II and CoII/I couples of the catalytic
moiety, respectively, by comparison with the cyclic voltammo-
gram of the Co-N3 precursor (Figure 4, middle) and with the
previously reported dye-catalyst assembly, which displayed
similar processes at À 0.55 and À 1.02 V vs Fc.[8a] In Co-N3, the
second process is less reversible and slightly shifted to a more
cathodic potential, a behaviour we attribute to some conforma-
tional constraints created by the azido substituent. As it has
already been reported, this shift of approximately 100 mV of
force ΔG of À 1.13 eV for the photoinduced electron transfer
from ZnP* to the Co(III) center, according to the simplified
Rehm–Weller equation:
DG ¼ Eox ðZnP=ZnPþÞ-E00 ðZnP Þ-EredðCoIII=II
Þ
*
E00 stands for the energy of the singlet excited state of the
ZnP estimated from the intercept of the absorption and
emission spectra as 2.1 eV.
ZnP-Co was then anchored onto homemade F108-tem-
plated NiO films.
A
dyad loading of 6.4�0.9 nmolcmÀ 2
(Table S1) was determined by desorption of the dyad from
freshly-sensitized films in a THF solution of 1 M phenylphos-
phonic acid.[14] Linear sweep voltammograms were recorded in
a 2-(N-morpholino)ethanesulfonic acid (MES) buffer electrolyte
at pH 5.5[8a] under dark, chopped-light and continuous light
irradiation conditions (Figure 5), using the ZnP-Co-sensitized
photocathode as working electrode in a three-electrode setup.
The experimental conditions that were followed herein are
based on our previous light-driven H2 evolution studies with
covalent dye-catalyst assemblies.[8a] Cathodic photocurrents are
clearly generated under visible light irradiation (Figure 5) from
+0.94 V vs RHE, reaching up to 11 μA.cmÀ 2 (dark current
subtracted). To assess whether these photocurrents are corre-
Figure 4. Cyclic voltammograms of COOMe-ZnP (top) compared with those of Co-N3 (middle) and of the dye precursor COOMe-ZnP-Co (bottom).
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Figure 6. UV-vis absorption spectra of desorption solutions (1 M phenyl-
phosphonic acid in THF) of a fresh ZnP-Co-sensitized NiO film (black line)
and the same sample (second half of the same sensitized film; red line) after
a two-hours chronoamperometric measurement under continuous light
irradiation at an applied potential of À 0.4 V vs Ag/AgCl in MES buffer pH 5.5.
Figure 5. Linear sweep voltammograms of ZnP-Co–sensitized NiO electro-
des, recorded in a pH 5.5 MES 0.1 M/NaCl 0.1 M buffer under dark conditions
(black line), continuous visible light irradiation (red line) or chopped light
irradiation (blue line).
lated with photoelectrocatalytic hydrogen production, two
hours chronoamperometric measurements coupled to hydro-
gen detection were carried out (Figure S23) at À 0.4 V vs Ag/
AgCl for direct comparison with our previously-reported photo-
cathodes based on the same cobalt diimine dioxime catalyst.[8]
At the end of the experiments, the amount of hydrogen
produced was measured in the headspace and in the electrolyte
solution,[8b] reaching 8�1% Faradaic Efficiency (F.E.) and 7�
1 nmolcmÀ 2. Similarly low F.E. values, ranging from 6 to 13%,
were also obtained for the previously-reported dye-sensitized
H2-evolving photocathodes based on the same cobalt diimine-
dioxime catalyst;[8,15] the reason behind this is still not clearly
understood although it might be related to the reduction of
some O2 trapped within the mesoporous NiO structure or to
some degradation pathways. By comparison, control experi-
ments performed with ZnP-sensitized NiO films under identical
conditions produced only 3 nmolcmÀ 2 (Table S1).
Post-operando characterization of the dyad was performed
after desorption from the film in a 1 M phenylphosphonic acid
solution in THF. The UV-vis spectrum of the solution displays an
overall lower intensity of the porphyrin characteristic signals by
comparison with the spectrum recorded after desorption of the
freshly-sensitized film (Figure 6); this indicates that some
desorption of ZnP-Co from the NiO surface (estimated to
�40%) occurred during the course of the two-hours photo-
electrochemical experiment. In addition, a clear increase in the
number of Q bands is observed. This can be attributed either to
the demetallation of the porphyrin core,[16] or to the formation
of chlorin species due to the reduction/hydrogenation of the
porphyrin ring.[17] As demetallation of porphyrins is reported to
take place under acidic conditions,[16] the photoelectrocatalytic
activity of the films was also assessed at pH 7. Although the
ZnP-Co-sensitized film displays improved stability at this less
acidic pH (desorption�25%, see Figure S24), the activity was
not enhanced, and the same amount of hydrogen was
produced (Table S1).
The overall performances of this novel dyad are slightly
lower than the ones previously reported for a NiO-based
photocathode based on the same cobalt diimine-dioxime
complex covalently coupled with a push-pull organic dye.[8a] In
order to ascertain whether electrons are transferred to the
catalytic center within the dyad, a UV-vis-monitored photolysis
experiment was performed in a THF solution of COOMe-ZnP-
Co, using triethanolamine (TEOA) as sacrificial electron donor
(Figure S25). Unfortunately, in all conducted experiments we
could neither observe the accumulation of the Co(II) state nor
the one of the Co(I) state of the catalyst, which is in sharp
contrast with our previous dyads.,[8a][18] The newly-formed peak
at approximately 620 nm is ascribed to the reduced form of the
porphyrin macrocycle, generated upon reductive quenching of
the excited state of the latter by TEOA.[19] Accumulation of the
reduced porphyrin during the course of the photolysis experi-
ment thus clearly indicates that electron transfer to the catalyst
is not efficient although this process is thermodynamically
favored. Compared to our dye-catalyst assemblies based on a
push-pull organic dye, a lack of directionality for the electron
transfer from the porphyrin core to the catalytic center can be
invoked to support these observations. Indeed, the electron-
withdrawing ability of the carboxylate anchoring group most
probably stabilizes the electron on the opposite side of the
cobalt complex, preventing the formation of the catalytically
active species and accounting for the low photoelectrocatalytic
activity of the ZnP-Co sensitized photocathode.
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butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) was
selected as matrix.
Conclusion
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A new noble metal-free dyad was synthesized by employing a
click chemistry reaction in order to covalently connect a zinc-
porphyrin sensitizer with a H2-evolving cobalt diimine dioxime
catalyst. We have also prepared the methyl-ester analogue of
the dyad to determine the electrochemical features of our novel
system. In the cyclic voltammogram of COOMe-ZnP-Co we
observed five redox potentials, which were assigned to either
the chromophore or catalytic unit by comparison with the
corresponding voltammograms of both precursors (COOMe-
ZnP and Co-N3, respectively). Additionally, we performed
photolysis experiments in the COOMe-ZnP-Co dyad in order to
further evaluate the electronic communication between the
zinc porphyrin and the catalyst. However, we did not observe
the formation of the desired Co(I) species, hence suggesting
that an electron shift from the porphyrin to the cobalt diimine-
dioxime moiety does not take place easily. Finally, dye-
sensitized photocathodes were constructed incorporating both
the ZnP-Co dyad and the ZnP dye and their photoelectrochem-
ical activity was assessed under pertinent conditions.
This is the first attempt to prepare a noble metal free
covalently connected porphyrin-cobalt dyad targeting its
utilization in PEC for H2 production. All the results in this study
highlight the importance of developing novel photosensitizer-
catalyst entities for photo-electrocatalytic reduction reactions of
interest in the context of a solar-driven chemistry. Several
modifications should be explored in future research works
examining crucial factors that impact the catalytic efficiency as
well as the stability of the dyad onto the photocathode.
Namely, the insertion of a non-conjugated methylene spacer
between the anchoring group and the porphyrin ring will
decrease the electronic coupling with NiO, and in turn slow
down unwanted recombination processes.[20] In addition, in-
troducing electron donating moieties on the periphery of the
porphyrin macrocycle could possibly assist the charge transfer
process from the sensitizer to the catalyst. Finally, a thorough
investigation regarding different anchoring groups of the
photosensitizer, such as phosphonic acid, hydroxamic acid or
alkoxysilane, is required to increase the stability of the dyad
onto the photocathode and enhance the catalytic activity.
Alternatively, atomic layer deposition (ALD) of a thin layer of
alumina would prevent the dyad release in solution. Optimizing
all the above mentioned parameters will allow to develop
better performing photocathodes for H2 evolution based on
noble metal free porphyrin light harvesting units.
Photophysical Measurements. Absorption spectra were obtained
using a Shimadzu UV-1700 spectrophotometer and steady-state
emission spectra were recorded on a JASCO FP-6500 fluorescence
spectrophotometer.
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Electrochemistry. Cyclic voltammetry experiments were carried out
at room temperature using an AutoLab PGSTAT20 potentiostat. All
measurements were carried out in freshly distilled and deoxygen-
ated THF in the presence of 0.1 M of tetrabutylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte, at a
scan rate of 100 mVsÀ 1. A three-electrode cell setup was used with
a glassy carbon working electrode, an Ag/AgCl reference electrode,
and a platinum wire as a counter electrode. In all measurements
the potential of the reference electrode was calibrated vs the
electrolyte ferrocenium-ferrocene (Fc+/0) couple, by adding in the
supporting electrolyte of each experiment, ferrocene as an internal
reference.
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Photoelectrochemical measurements. For a typical photoelectro-
catalytic experiment, irradiation was carried out with a 300 W
ozone-free Xe lamp operated at 280 W equipped with a water-filled
filter for elimination of IR irradiation (Spectra-Physics 6123NS) and a
UV cut-off filter (Spectra-Physics 59472, λ>400 nm). The power
density was calibrated to 65 mWcmÀ 2 (�1 sun) using a 60% optical
density filter using a Newport PM1918-R power-meter. The experi-
ments were performed in a three-electrode cell, using the NiO-
sensitized film as working electrode (3.3 to 3.3 cm2, accurately
measured for each experiment), Ag/AgCl as the reference electrode
and a Pt wire as the auxiliary electrode. The supporting electrolyte
was 0.1 M 2-(N-morpholino)ethanesulfonic acid (MES) / 0.1 M NaCl
buffer at pH 5.5 or Britton-Robinson buffer at pH 7. The volume of
supporting electrolyte (around 5 mL) and the headspace volume
(around 2 mL) were accurately measured for each single experi-
ment. The different compartments of the cell and the electrolyte
were degassed for 30 minutes before the PEC experiments. The
amounts of evolved hydrogen were determined after two hours of
chronoamperometry run by sampling 50 μL of the headspace in
Perkin Elmer Clarus 580 gas chromatograph equipped with a
molecular sieve 5 Å column (30 m–0.53 mm) and by micro-clark
electrode in the electrolyte solution.[8b]
Linear sweep voltammograms (LSV). Prior to the long-term
chronoamperometric experiments, linear sweep voltammograms
were recorded for each sensitized NiO film under dark conditions
(lamp off), then continuous irradiation (lamp on), then chopped
light irradiation (lamp on-off), screening from+0.5 V to – 0.5 V vs
Ag/AgCl with a scan rate of 10 mVsÀ 1
.
Film sensitization. F108-templated NiO films were prepared
according to our previously reported procedure.[8a] Sensitization
was achieved by soaking them for 24 h in 0.1 mM ZnP-Co or ZnP
THF solutions. Finally, the films were soaked in pure THF to remove
physiosorbed compounds from the surface and dried under air
flow.
Determination of the surface concentration in ZnP-Co (or ZnP).
Every film was cut in half after sensitization. One half was used to
determine the surface concentration and the other half was used
for the assessment of the PEC activity. The freshly grafted half of
the film was dipped for 4 hours in 5 mL of 1 M phenylphosphonic
acid THF solution in order to desorb the dyad from the surface,
according to a previously reported procedure.[14] The UV-Visible
absorption spectrum of the desorption solution was then recorded
to determine the amount of grafted complex from the molar
extinction coefficient at the Soret band. Knowing the surface
concentrations in ZnP-Co allowed us to calculate turn-over
Experimental Section
Materials and techniques. Reagents and solvents were purchased
as reagent grade from usual commercial sources and were used
without further purification, unless otherwise stated. NMR spectra
were recorded on a Bruker AVANCE III-500 MHz spectrometer using
solutions in deuterated solvents and the solvent peak was chosen
as the internal standard. A Bruker UltrafleXtreme matrix assisted
laser desorption ionization time-of-flight (MALDI-TOF) spectrometer
was used to record the mass spectra and trans-2-[3-(4-tert-
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numbers (TON) based on the amount of catalyst present at the
filtered and washed with water. The solid was purified by flash
chromatography on silica gel using as eluent a mixture CH3CN/
aqueous KNO3 solution at 10% of the saturating concentration. The
pure product was obtained after evaporation of CH3CN in the
combined fractions and precipitation by addition of a KPF6-
saturated aqueous solution. The precipitate was filtered, washed
with water and dried under vacuum to give dyad 6 as red-brown
brick solid (65 mg, 74% yield), which was directly used for the next
step.
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surface of the film. A similar desorption procedure was applied to
the other half of the film after the PEC measurements for post-
operando analysis.
Synthesis of 4. In a two-neck round bottom flask, dipyrromethane 3
(500 mg, 1.9 mmol), 4-ethynylbenzaldehyde 1 (124 mg, 0.95 mmol)
and methyl-4-formylbenzale (2) (156 mg, 0.95 mmol) were dis-
solved in 250 mL of CHCl3. The reaction mixture was subsequently
bubbled under N2 for 15 min followed by addition of BF3OEt2
(75 μL) and stirring for 24 h. Then, 300 mg of DDQ were added and
the mixture was additionally left stirring for 20 h. The resulting
product was filtered through a silica pad and afterwards purified
via column chromatography (silica gel, Hexane/CH2Cl2 in 7:3 ratio)
Synthesis of ZnP-Co. Dyad 6 (65 mg, 0.05 mmol) and CoCl2 ·6H2O
(132 mg, 0.55 mmol) were solubilized in an acetone/THF (1:1)
mixture (30 mL) and stirred overnight under air bubbling. After the
reaction reached completion, the solution was evaporated under
vacuum and the solid was purified by flash chromatography on
silica gel using as eluent a mixture of CH3CN and aqueous KNO3
solution at 10% of the saturating concentration, then a solution of
acetone saturated in NaI to completely desorb the product from
the silica gel. The desired fractions were combined, and the organic
solvents were evaporated under vacuum, yielding a precipitate that
was filtered and washed with water. The solid was re-solubilized in
an acetone/THF (1:1) mixture. To this solution a NaBr-saturated
aqueous solution (a third of the final volume) is subsequently
added. The mixture was stirred at RT for five days to allow the
organic solvents to slowly evaporate, giving a precipitate. This
precipitate was filtered, washed with water and dried under
vacuum to obtain the expected ZnP-Co dyad as a purple solid
(18 mg, 33% yield). 1H NMR (500 MHz, DMF-d7): δ=20.07 (s, 1H),
13.37 (sb, 1H), 8.94 (m, 2H), 8.84 (d, J=4.5 Hz, 2H), 8.69 (m, 4H),
8.47 (d, J=7.5 Hz, 2H), 8.39 (d, J=7.7 Hz, 2H), 8.34 (sb, 1H), 8.16 (d,
J=8.0 Hz, 2H), 8.02 (m, 2H), 7.38 (s, 4H), 6.14 (m, 1H), 4.61 (db, J=
14.3 Hz, 2H), 4.48 (m, 2H), 2.63 (s, 6H), 2.49 (s, 6H), 2.28 (s, 6H), 1.86
(m, 12H) ppm. 13C NMR (125 MHz, DMF-d7): δ=174.2, 168.0, 156.4,
150.2, 149.9, 149.7, 149.6, 148.1, 139.8, 139.0, 137.6, 134.9, 134.3,
132.5, 132.0, 130.5, 130.2, 127.9, 126.1, 125.5, 123.7, 118.7, 58.9,
53.5, 21.3, 20.9, 17.2, 12.6 ppm. MS (MALDI-TOF): calculated for
C64H56Br2CoN11O4Zn [M]+ 1323.1507; found 1323.1522.
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providing 150 mg of porphyrin 4 (yield=20%). H NMR (500 MHz,
CDCl3): δ=8.79 (d, J=4.7 Hz, 2H), 8.75 (d, J=4.7 Hz, 2H), 8.72 (d,
J=4.7 Hz, 4H), 8.43 (d, J=8.4 Hz, 2H), 8.32 (d, J=8.4 Hz, 2H), 8.20
(d, J=8.3 Hz, 2H), 7.89 (d, J=8.3 Hz, 2H), 7.29 (s, 4H), 4.11 (s, 3H),
3.32 (s, 1H), 2.63 (s, 6H), 1.85 (s, 12H), À 2.64 (s, 2H) ppm. 13C NMR
(75 MHz, CDCl3): δ=167.5, 147.0, 142.7, 140.5, 139.5, 138.9, 138.3,
138.2, 138.0, 134.7, 134.5, 130.7, 129.3, 128.6, 128.0, 127.9, 121.7,
118.8, 118.7, 118.3, 118.2, 83.8, 78.4, 52.6, 21.8, 21.6 ppm. MS
(MALDI-TOF): calculated for C54H44N4O2 [M]+ 780.3464; found
780.3452.
Synthesis of COOMe-ZnP. To a 40 mL CH2Cl2 solution of 4 (78 mg,
.
0.10 mmol), a MeOH (6 mL) solution containing Zn(CH3COO)2 2H2O
(215 mg, 1 mmol) was added and the reaction mixture was stirred
at room temperature (RT) overnight. The solvents were evaporated
under reduced pressure and the resulting residue was purified by
column chromatography (silica gel, using Hexane/CH2Cl2 as eluent
in a 7:3 ratio). The formed metallated porphyrin derivative COOMe-
ZnP was obtained in quantitative yield, namely 98% (80 mg). 1H
NMR (500 MHz, CDCl3): δ=8.87 (d, J=4.6 Hz, 2H), 8.83 (d, J=4.6 Hz,
2H), 8.80 (m, 4H), 8.40 (d, J=8.3 Hz, 2H), 8.32 (d, J=8.3 Hz, 2H), 8.21
(d, J=8.2 Hz, 2H), 7.88 (d, J=8.2 Hz, 2H), 7.29 (s, 4H), 4.08 (s, 3H),
3.31 (s, 1H), 2.64 (s, 6H), 1.84 (s, 12H) ppm. 13C NMR (75 MHz, CDCl3):
δ=167.6, 150.2, 150.1, 149.8, 149.6, 147.9, 143.6, 139.3, 139.0, 138.0,
137.7, 134.6, 134.5, 132.3, 132.1, 131.8, 131.6, 131.2, 131.1, 130.5,
129.9, 129.3, 127.8, 121.4, 119.7, 119.5, 119.2, 119.0, 83.9, 78.3, 52.5,
21.8, 21.6 ppm. MS (MALDI-TOF): calculated for C54H43N4O2Zn [M+
H]+ 843.2677; found 843.2689.
Acknowledgements
Synthesis of ZnP. To a THF (17 mL) and MeOH (8 mL) mixed
solution of COOMe-ZnP (65 mg, 0.08 mmol), an aqueous solution
(9 mL) of KOH (430 mg, 7.70 mmol) was added. The reaction
mixture was stirred at room temperature for two days and then the
solvents were evaporated under vacuum and distilled H2O (10 mL)
was added to the resulting residue. The mixture was acidified using
HCl(aq) 1 M resulting in the precipitation of derivative as a purple
solid which after being filtered, washed with distilled water and
This research was funded by the General Secretariat for Research
and Technology (GSRT) and Hellenic Foundation for Research and
Innovation (HFRI; project code: 508). This research has also been
co-financed by the European Union and Greek national funds
through the Operational Program Competitiveness, Entrepreneur-
ship, and Innovation, under the call RESEARCHÀ CREATEÀ
INNOVATE (project code: T1EDK-01504). In addition, this research
has been co-financed by the European Union and Greek national
funds through the Regional Operational Program “Crete 2014–
2020,” project code OPS:5029187. Moreover, the European
Commission’s Seventh Framework Program (FP7/2007-2013) under
grant agreement no. 229927 (FP7-REGPOT-2008-1, Project BIO-
SOLENUTI), the Special Research Account of the University of Crete
and the French National Research Agency in the framework of the
“Investissements d’avenir” program (ANR-15-IDEX-02, Labex AR-
CANE and CBH-EURGS, ANR-17-EURE-0003) are gratefully acknowl-
edged for the financial support of this research.
1
dried, was collected (62 mg, 97%). H NMR (500 MHz, CDCl3): δ=
8.80 (m, 8H), 8.51 (db, J=5.9 Hz, 2H), 8.38 (d, J=7.4 Hz, 2H), 8.21 (d,
J=7.5 Hz, 2H), 7.89 (d, J=6.9 Hz, 2H), 7.29 (s, 4H), 3.31 (s, 1H), 2.64
(s, 6H), 1.84 (s, 12H) ppm. 13C NMR (125 MHz, CDCl3): δ=171.2,
150.3, 150.2, 149.9, 149.6, 148.8, 143.3, 139.3, 139.0, 137.7, 134.7,
134.5, 132.4, 132.1, 131.3, 130.5, 130.3, 128.5, 127.9, 121.4, 119.8,
119.7, 118.9, 83.9, 78.3, 52.5, 21.8, 21.6 ppm. MS (MALDI-TOF):
calculated for C53H40N4O2Zn [M]+ 828.2443; found 828.2451.
Synthesis of 6: ZnP (62 mg, 0.07 mmol) and copper diamine-
dioxime complex 5 (26 mg, 0.07 mmol) were solubilized in CH2Cl2
.
(5 mL) under Ar. A solution of CuSO4 5H2O (12 mg, 0.05 mmol) and
sodium ascorbate (28 mg, 0.14 mmol) in Ar-purged water (2 mL)
was added. Then, Ar-degassed CH3OH (5 mL) was added to the
biphasic mixture until a single phase was obtained. The reaction
mixture was stirred at RT under Ar. After 24 h the solution was
concentrated under vacuum yielding a precipitate, which was
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Conflict of Interest
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The authors declare no conflict of interest.
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Keywords: Click chemistry
Photocatalysis · Porphyrinoids · Sensitizers
·
Hydrogen evolution
·
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Manuscript received: December 9, 2020
Revised manuscript received: January 15, 2021
Accepted manuscript online: January 24, 2021
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© 2021 Wiley-VCH GmbH