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

Direct Observation of Ultrafast Excimer Formation in Covalent Perylenediimide Dimers Using Near-Infrared Transient Absorption Spectroscopy

Energy transfer in perylene-3,4:9,10-bis­(dicarboximide) (PDI) aggregates is often limited by formation of a low-energy excimer state. Formation dynamics of excimer states are often characterized by line shape changes and peak shift dynamics in femtosecond visible transient absorption spectra. Femtosecond near-infrared transient absorption experiments reveal a unique low-energy transition that can be used to identify and characterize this state without overlapping excited singlet-state absorption. Three covalently bound PDI dimers with differing PDI–PDI distances were studied to probe the influence of interchromophore electronic coupling on the PDI excimer transient spectra and dynamics

Photoinitiated Electron Transfer in Zinc Porphyrin–Perylenediimide Cruciforms and Their Self-Assembled Oligomers

Two X-shaped, cruciform electron donor₂–acceptor–acceptor′₂ (D₂-A-A′₂) molecules, <b>1</b> and <b>2</b>, in which D = zinc 5-phenyl-10,15,20-tripentylporphyrin (ZnTPnP) or zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), respectively, A = pyromellitimide (PI), and A′ = perylene-3,4:9,10-bis­(dicarboximide) (PDI), were prepared to study self-assembly motifs that promote photoinitiated charge separation followed by electron and hole transport through π-stacked donors and acceptors. PDI secondary electron acceptors were chosen because of their propensity to form self-ordered, π-stacked assemblies in solution, while the ZnTPnP and ZnTPP donors were selected to test the effect of peripheral substituent steric interactions on the π-stacking characteristics of the cruciforms. Small- and wide-angle X-ray scattering measurements in toluene solution reveal that <b>1</b> assembles into a π-stacked structure having an average of 5 ± 1 molecules, when [<b>1</b>] ≅ 10^–5 M, while <b>2</b> remains monomeric. Photoexcitation of the π-stacked structure of <b>1</b> results in formation of ZnTPnP^•+-PI-PDI^•– in τ_CS1 = 0.3 ps, which is nearly 100-fold faster than the formation of ZnTPnP^•+-PI^•– in a model system lacking the PDI acceptor. The data are consistent with a self-assembled structure for <b>1</b> in which the majority of the intermolecular interactions have the ZnTPnP donor of one monomer cofacially π-stacked with the PDI acceptor of a neighboring monomer in a crisscrossed fashion. In contrast, <b>2</b> remains monomeric in toluene, so that photoexcitation of ZnTPP results in the charge separation reaction sequence: ^1*ZnTPP-PI-PDI → ZnTPP^•+-PI^•–-PDI → ZnTPP^•+-PI-PDI^•–, where τ_CS1 = 33 ps and τ_CS2 = 239 ps. The perpendicular orientation of ZnTPnP and ZnTPP relative to PDI in <b>1</b> and <b>2</b> is designed to decrease the porphyrin–PDI distance without greatly decreasing the overall number of bonds linking them. This serves to decrease the Coulomb energy penalty required to produce D^•+-PI-PDI^•– relative to the corresponding linear D-PI-PDI array, while retaining the weak electronic coupling necessary to achieve long-lived charge separation, as evidenced by τ_CR = 24 ns for ZnTPP^•+-PI-PDI^•–

Singlet Exciton Fission in Thin Films of <i>tert</i>-Butyl-Substituted Terrylenes

Two terrylene chromophores, 2,5,10,13-tetra­(<i>tert</i>-butyl)­terrylene (<b>1</b>) and 2,5-di­(<i>tert</i>-butyl)­terrylene (<b>2</b>), were synthesized and studied to determine their singlet exciton fission (SF) efficiencies. Compound <b>1</b> crystallizes in one-dimensional stacks, whereas <b>2</b> packs in a slip-stacked, herringbone pattern of dimers motif. Strongly quenched fluorescence and rapid singlet exciton decay dynamics are observed in vapor-deposited thin films of <b>1</b> and <b>2</b>. Phosphorescence measurements on thin films of <b>1</b> and <b>2</b> show that SF is only 70 meV endoergic for these chromophores. Femtosecond transient absorption experiments using low laser fluences on these films reveal rapid triplet exciton formation for both <b>1</b> (τ = 120 ± 10 ps) and <b>2</b> (τ = 320 ± 20 ps) that depends strongly on film crystallinity. The transient absorption data are consistent with formation of an excimer state prior to SF. Triplet exciton yield measurements indicate nearly quantitative SF in thin films of both chromophores in highly crystalline solvent-vapor-annealed films: 170 ± 20% for <b>1</b> and 200 ± 30% for <b>2</b>. These results show that significantly different crystal morphologies of the same chromophore can both result in high-efficiency SF provided that the energetics are favorable

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of <i>N</i>,<i>N</i>-bis­(<i>n</i>-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis­(dicarboximide), <b>1</b>, obtained by X-ray diffraction reveals that <b>1</b> has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of <b>1</b> results in a 140 ± 20% yield of triplet excitons (^3*<b>1</b>) in τ_SF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of <i>N</i>,<i>N</i>-bis­(<i>n</i>-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis­(dicarboximide), <b>1</b>, obtained by X-ray diffraction reveals that <b>1</b> has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of <b>1</b> results in a 140 ± 20% yield of triplet excitons (^3*<b>1</b>) in τ_SF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis