accurately controls the spatial distance, but also increases the lifetime of the excited
Ru state. In the following year, the photophysical properties of the assembled R77
were studied [166]. Comparing with the non-substituted [Ru(tpy) 2 ]
2+ , it is found that
the lifetime of the excited states of R77 or [Ru(tpy) 2 ]
2+ containing phenylacetylene
tether substituted by different alkyl groups is greatly enhanced by at least one order
of magnitude. The driving force required for the transfer of electrons from the
excited ruthenium to the dirron portion can be calculated by using the energy of
the excited state and electrochemical data, which is uphill by 0.59 eV. The reason is
that the protonation of N atom on ADT bridge (ADT ¼ azadithiolate
(SÀCH2ÀNRÀCH2ÀS)) will make the reduction potential of the assembly R77
shift more positively, so that the electron transfer is more thermodynamically
advantageous. The disadvantage of this series of research is that the exact single
crystal structure information of R77 has not been obtained, although R77 has been
indirectly demonstrated by other spectroscopic methods such as NMR, IR and
MS. More regrettably, there is a lack of in-depth study on the catalytic properties
of the assembly.
Cobalt complexes are a kind of cheap and efficient molecular catalysts with low
overpotential, which can effectively catalyze proton reduction under light-driven
conditions [28, 46, 47, 167]. In 2010, Tiede and coworkers [168] hoped to explore
the relationship between structure of different supramolecular R78–79 (Fig. 27)and
photo-induced electron transfer in catalytic proton reduction. The assemblies are
composed of different Ru-based photosensitizers and cobaloxime catalyst. In this
study, the differences between assemblies and fragments were compared by
solution-phase X-ray scattering and ultrafast transient optical spectroscopy analyses;
further, the self-assembly process of assemblies was analyzed. Notably, the photosensitizer with push–pull effect in the assembly makes the excited state Ru center
quenched rapidly by the Co-based part after illumination, thus greatly increasing the
chances of solar energy converting into hydrogen energy.
3 Molecular Catalysts with Pincer Ligand for OER
3.1 Noble Metals-Based Water Oxidation Catalysts
Hundreds of molecular ruthenium-based water oxidation catalysts [40, 61] had
emerged since 1982 when Meyer group reported the first water oxidation catalyst
containing binuclear ruthenium named “blue dimer” [169–171]. Under the condition
of pH ¼ 7, the thermodynamic driving potential for water oxidation is as high as
1.23 V (vs NHE). At such a high potential, many ligands containing P and S are
difficult to avoid being oxidized, so most of the catalysts for water oxidation use
N-based ligands such as polypyridine. As a result, pincer-type tpy and its derivatives
ligands have been widely developed and applied to water oxidation catalysts.
In 2000, Tanaka and coworkers reported the first Ru-based water oxidation
catalyst O1 [172] containing tpy units. When the catalyst was anchored on the
The Application of Pincer Ligand in Catalytic Water Splitting
413
Ru state. In the following year, the photophysical properties of the assembled R77
were studied [166]. Comparing with the non-substituted [Ru(tpy) 2 ]
2+ , it is found that
the lifetime of the excited states of R77 or [Ru(tpy) 2 ]
2+ containing phenylacetylene
tether substituted by different alkyl groups is greatly enhanced by at least one order
of magnitude. The driving force required for the transfer of electrons from the
excited ruthenium to the dirron portion can be calculated by using the energy of
the excited state and electrochemical data, which is uphill by 0.59 eV. The reason is
that the protonation of N atom on ADT bridge (ADT ¼ azadithiolate
(SÀCH2ÀNRÀCH2ÀS)) will make the reduction potential of the assembly R77
shift more positively, so that the electron transfer is more thermodynamically
advantageous. The disadvantage of this series of research is that the exact single
crystal structure information of R77 has not been obtained, although R77 has been
indirectly demonstrated by other spectroscopic methods such as NMR, IR and
MS. More regrettably, there is a lack of in-depth study on the catalytic properties
of the assembly.
Cobalt complexes are a kind of cheap and efficient molecular catalysts with low
overpotential, which can effectively catalyze proton reduction under light-driven
conditions [28, 46, 47, 167]. In 2010, Tiede and coworkers [168] hoped to explore
the relationship between structure of different supramolecular R78–79 (Fig. 27)and
photo-induced electron transfer in catalytic proton reduction. The assemblies are
composed of different Ru-based photosensitizers and cobaloxime catalyst. In this
study, the differences between assemblies and fragments were compared by
solution-phase X-ray scattering and ultrafast transient optical spectroscopy analyses;
further, the self-assembly process of assemblies was analyzed. Notably, the photosensitizer with push–pull effect in the assembly makes the excited state Ru center
quenched rapidly by the Co-based part after illumination, thus greatly increasing the
chances of solar energy converting into hydrogen energy.
3 Molecular Catalysts with Pincer Ligand for OER
3.1 Noble Metals-Based Water Oxidation Catalysts
Hundreds of molecular ruthenium-based water oxidation catalysts [40, 61] had
emerged since 1982 when Meyer group reported the first water oxidation catalyst
containing binuclear ruthenium named “blue dimer” [169–171]. Under the condition
of pH ¼ 7, the thermodynamic driving potential for water oxidation is as high as
1.23 V (vs NHE). At such a high potential, many ligands containing P and S are
difficult to avoid being oxidized, so most of the catalysts for water oxidation use
N-based ligands such as polypyridine. As a result, pincer-type tpy and its derivatives
ligands have been widely developed and applied to water oxidation catalysts.
In 2000, Tanaka and coworkers reported the first Ru-based water oxidation
catalyst O1 [172] containing tpy units. When the catalyst was anchored on the
The Application of Pincer Ligand in Catalytic Water Splitting
413