The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1-25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τ = 72-85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore-catalyst systems for solar fuel production that utilize multiple perylene chromophores.
Photodriven charge transfer dynamics are described for an atomic layer deposition-stabilized, organic dye-sensitized photocathode architecture that produces hydrogen.
Improving
stability and slowing charge recombination are some of
the greatest challenges in the development of dye-sensitized photoelectrochemical
cells (DSPECs) for solar fuels production. We have investigated the
effect of encasing dye molecules in varying thicknesses of Al2O3 deposited by atomic layer deposition (ALD) before
catalyst loading on both the stability and the charge transfer dynamics
in organic dye-sensitized TiO2 photoanodes containing iridium-based
molecular water-oxidation catalysts. In the TiO2|dye|Al2O3|catalyst electrodes, a sufficiently thick ALD
layer protects the perylene-3,4-dicarboximide (PMI) chromophores from
degradation over several weeks of exposure to light. The insulating
capacity of the layer allows a higher photocurrent in the presence
of ALD while initial charge injection is slowed by only 1.6 times,
as observed by femtosecond transient absorption spectroscopy. Rapid
picosecond-scale catalyst oxidation is observed in the presence of
a dinuclear catalyst, IrIr, but is slowed to tens of picoseconds for
a mononuclear catalyst, IrSil, that incorporates a long linker. Photoelectrochemical
experiments demonstrate higher photocurrents with IrSil compared to
IrIr, which show that recombination is slower for IrSil, while higher
photocurrents with IrIr upon addition of ALD layers confirm that ALD
successfully slows charge recombination. These findings demonstrate
that, beyond stability improvements, ALD can contribute to tuning
charge transfer dynamics in photoanodes for solar fuels production
and may be particularly useful for slowing charge recombination and
accounting for varying charge transfer rates based on the molecular
structures of incorporated catalysts.
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