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