4 research outputs found
Effect of Perylene Photosensitizer Attachment to [Pd(triphosphine)L]<sup>2+</sup> on CO<sub>2</sub> Electrocatalysis
Two new covalently
linked chromophore–CO<sub>2</sub> reduction catalyst systems
were prepared using a perylene chromophore and a bisÂ[(dicyclohexylphosphino)Âethyl]ÂphenylphosphinopalladiumÂ(II)
catalyst. The primary goal of this study is to probe the influence
of photosensitizer attachment on the electrocatalytic performance.
The position either para or meta to the phosphorus on the phenyl group
of the palladium complex was linked via a 2,5-xylyl group to the 3
position of perylene. The electrocatalytic CO<sub>2</sub> reduction
activity of the palladium complex is maintained in the meta-linked
system, but is lost in the para-linked system, possibly because of
unfavorable interactions of the perylene chromophore with the glassy
carbon electrode used. Following selective photoexcitation of the
perylene, an enhanced perylene excited-state decay rate was observed
in the palladium complexes compared to perylene attached to the free
ligands. This decrease is accompanied by formation of the perylene
cation radical, showing that electron transfer from perylene to the
palladium catalyst occurs. Electron transfer and charge recombination
were both found to be faster in the para-linked system than in the
meta-linked one, which is attributed to stronger electronic coupling
in the former. These results illustrate the need to carefully tune
the electronic coupling between a photosensitizer chromophore and
the catalyst to promote photodriven electron transfer yet inhibit
adverse electronic effects of the chromophore on electrocatalysis
Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers
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<sub>CR</sub> = 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
Cleavage of DNA by Proton-Coupled Electron Transfer to a Photoexcited, Hydrated Ru(II) 1,10-Phenanthroline-5,6-dione Complex
Visible light irradiation of a rutheniumÂ(II) quinone-containing
complex, [(phen)<sub>2</sub>RuÂ(phendione)]<sup>2+</sup> (<b>1</b><sup><b>2+</b></sup>), where phendione = 1,10-phenanthroline-5,6-dione,
leads to DNA cleavage in an oxygen independent manner. A combination
of NMR analyses, transient absorption spectroscopy, and fluorescence
measurements in water and acetonitrile reveal that complex <b>1</b><sup><b>2+</b></sup> must be hydrated at the quinone functionality,
giving [(phen)<sub>2</sub>RuÂ(phenH<sub>2</sub>O)]<sup>2+</sup> (<b>1H</b><sub><b>2</b></sub><b>O</b><sup><b>2+</b></sup>, where phenH<sub>2</sub>O = 1,10-phenanthroline-6-one-5-diol),
in order to access a long-lived <sup>3</sup>MLCT<sub>hydrate</sub> state (τ ∼ 360 ns in H<sub>2</sub>O) which is responsible
for DNA cleavage. In effect, hydration at one of the carbonyl functions
effectively eliminates the low-energy <sup>3</sup>MLCT<sub>SQ</sub> state (Ru<sup>III</sup> phen-semiquinone radical anion) as the predominant
nonradiative decay pathway. This <sup>3</sup>MLCT<sub>SQ</sub> state
is very short-lived (<1 ns) as expected from the energy gap law
for nonradiative decay, and too short-lived
to be the photoactive species. The resulting excited state in <b>1H</b><sub><b>2</b></sub><b>O</b><sup><b>2+</b></sup>* has photophysical properties similar to the <sup>3</sup>MLCT
state in [RuÂ(phen)<sub>3</sub>]<sup>2+</sup>* with the added functionality
of basic sites at the ligand periphery. Whereas [RuÂ(phen)<sub>3</sub>]<sup>2+</sup>* does not show direct DNA cleavage, the deprotonated
form of <b>1H</b><sub><b>2</b></sub><b>O</b><sup><b>2+</b></sup>* does via a proton-coupled electron transfer
(PCET) mechanism where the peripheral basic oxygen sites act as the
proton acceptor. Analysis of the small molecule byproducts of DNA
scission supports the conclusion that cleavage occurs via H-atom abstraction
from the sugar moieties, consistent with a PCET mechanism. Complex <b>1</b><sup><b>2+</b></sup> is a rare example of a ruthenium
complex which ‘turns on’ both reactivity and luminescence
upon switching to a hydrated state
Improving the Efficiency of Mustard Gas Simulant Detoxification by Tuning the Singlet Oxygen Quantum Yield in Metal–Organic Frameworks and Their Corresponding Thin Films
The
photocatalytically driven partial oxidation of a mustard gas simulant,
2-chloroethyl ethyl sulfide (CEES), was studied using the perylene-based
metal–organic framework (MOF) UMCM-313 and compared to the
activities of the Zr-based MOFs: PCN-222/MOF-545 and NU-1000.
The rates of CEES oxidation positively correlated with the singlet
oxygen quantum yield of the MOF linkers, porphyrin (PCN-222/MOF-545)
< pyrene (NU-1000) < perylene (UMCM-313). Subsequently, thin
films of UMCM-313 and NU-1000 were solvothermally grown on a conductive
glass substrate to minimize catalyst loading and prevent light scattering
by suspended MOF particles. Using a conductive glass support, the
initial turnover frequencies of the MOFs in the photocatalytic reaction
improved by 10-fold