Effect of Perylene Photosensitizer Attachment to [Pd(triphosphine)L]²⁺ on CO₂ Electrocatalysis

Two new covalently linked chromophore–CO₂ reduction catalyst systems were prepared using a perylene chromophore and a bis­[(dicyclohexylphosphino)­ethyl]­phenylphosphinopalladium­(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 CO₂ 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_CR = 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 ruthenium­(II) quinone-containing complex, [(phen)₂Ru­(phendione)]²⁺ (<b>1</b>^<b>2+</b>), 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>^<b>2+</b> must be hydrated at the quinone functionality, giving [(phen)₂Ru­(phenH₂O)]²⁺ (<b>1H</b>_<b>2</b><b>O</b>^<b>2+</b>, where phenH₂O = 1,10-phenanthroline-6-one-5-diol), in order to access a long-lived ³MLCT_hydrate state (τ ∼ 360 ns in H₂O) which is responsible for DNA cleavage. In effect, hydration at one of the carbonyl functions effectively eliminates the low-energy ³MLCT_SQ state (Ru^III phen-semiquinone radical anion) as the predominant nonradiative decay pathway. This ³MLCT_SQ 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>_<b>2</b><b>O</b>^<b>2+</b>* has photophysical properties similar to the ³MLCT state in [Ru­(phen)₃]²⁺* with the added functionality of basic sites at the ligand periphery. Whereas [Ru­(phen)₃]²⁺* does not show direct DNA cleavage, the deprotonated form of <b>1H</b>_<b>2</b><b>O</b>^<b>2+</b>* 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>^<b>2+</b> 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