Yusuke Kuramochi

Energy Transfer Followed by Electron Transfer in a Porphyrin Macrocycle and Central Acceptor Ligand: A Model for a Photosynthetic Composite of the Light‐Harvesting Complex and Reaction Center

A system that models a photosynthetic composite of the light-harvesting complex and reaction center is reported in which light energy collected by cyclic antenna porphyrins is transferred to a central energy-acceptor porphyrin, followed by photoinduced electron transfer to a fullerene positioned above the ring plane. Pyridyl tripodal ligands appended with bis(phenylethynyl)porphyrinatozinc(II) (ZnP-Tripod) and additional fulleropyrrolidine moieties (C(60)-ZnP-Tripod) were synthesized as the reaction center units. The tripodal ligand was strongly accommodated by the light-harvesting porphyrin macrocycle N-(1-Zn)(3) (1-Zn = trisporphyrinatozinc(II)) by using three-point coordination of pyridyl to uncoordinated porphyrinatozinc sites to afford a stable 1:1 composite. The binding constants for ZnP-Tripod and C(60)-ZnP-Tripod in benzonitrile were estimated from steady-state fluorescence titrations to be 1.4x10(7) and 1.6x10(7) M(-1), respectively. The steady-state fluorescence titration, fluorescence lifetime, and transient absorption studies revealed that in both composites the excitation energy collected by the nine porphyrins of N-(1-Zn)(3) was efficiently transferred to a ZnP moiety by means of a through-space mechanism with a quantum yield of approximately 90%. Furthermore, in the composite with C(60)-ZnP-Tripod, the converged energy at the ZnP moiety induced electron transfer to the C(60) moiety, which afforded the stable charge-separated state (Phi(CS)>90%).

Light-Harvesting Supramolecular Porphyrin Macrocycle Accommodating a Fullerene–Tripodal Ligand

Trisporphyrinatozinc(II) (1-Zn) with imidazolyl groups at both ends of the porphyrin self-assembles exclusively into a light-harvesting cyclic trimer (N-(1-Zn)(3)) through complementary coordination of imidazolyl to zinc(II). Because only the two terminal porphyrins in 1-Zn are employed in ring formation, macrocycle N-(1-Zn)(3) leaves three uncoordinated porphyrinatozinc(II) groups as a scaffold that can accommodate ligands into the central pore. A pyridyl tripodal ligand with an appended fullerene connected through an amide linkage (C(60)-Tripod) was synthesized by coupling tripodal ligand 3 with pyrrolidine-modified fullerene, and this ligand was incorporated into N-(1-Zn)(3). The binding constant for C(60)-Tripod in benzonitrile reached the order of 10(8) M(-1). This value is ten times larger than those of pyridyl tetrapodal ligand 2 and tripodal ligand 3. This behavior suggests that the fullerene moiety contributes to enhance the binding of C(60)-Tripod in N-(1-Zn)(3). The fluorescence of N-(1-Zn)(3) was almost completely quenched (approximately 97 %) by complexation with C(60)-Tripod, without any indication of the formation of charge-separated species or a triplet excited state of either porphyrin or fullerene in the transient absorption spectra. These observations are explained by the idea that the fullerene moiety of C(60)-Tripod is in direct contact with the porphyrin planes of N-(1-Zn)(3) through fullerene-porphyrin pi-pi interactions. Thus, C(60)-Tripod is accommodated in N-(1-Zn)(3) with a pi-pi interaction and two pyridyl coordinations. The cooperative interaction achieves a sufficiently high affinity for quantitative and specific introduction of one equivalent of tripodal guest into the antenna ring, even under dilute conditions ( approximately 10(-7) M) in polar solvents such as benzonitrile. Additionally, complete fluorescence quenching of N-(1-Zn)(3) when accommodating C(60)-Tripod demonstrates that all of the excitation energy collected by the nine porphyrins migrates rapidly over the macrocycle and then converges efficiently on the fullerene moiety by electron transfer.

Energy Transfer among Light-Harvesting Macrorings Incorporated into a Bilayer Membrane

Amphiphilic bis(imidazolyl)tris(porphyrinatozinc) complexes having omega-carboxyalkyl meso substituents were prepared. The barrel-type porphyrin macroring organized through their complementary coordination was incorporated into a liposomal bilayer membrane with orientation normal to the surface. Under the conditions concentrated in the membrane, introduction of a fullerotripyridyl ligand into the cavity of the macroring quenched fluorescence not only from the host ring itself but also from other surrounding macrorings. Energy transfer among antenna macrorings in the membrane mimics the assembly of a bacterial light-harvesting antenna system.

Supramolecular Organization of Light-Harvesting Porphyrin Macrorings

Porphyrin-based supramolecular nanostructures have been produced by the self-assembly of porphyrin macrorings with three benzoic acid groups (Acid-R) on each side of the rings through cooperative carboxyl-carboxyl hydrogen bonds. Structures of the organized Acid-R were analyzed by AFM, and two clear distribution peaks were observed at 3 and 27 nm in the height-distribution histogram. From the overall assessment, the higher objects are considered to be one-dimensional structures standing vertically on the mica substrate. The height corresponds to an 11-mer of a unit Acid-R. Light-harvesting functions were examined by using fluorescence titration, whereby an energy-acceptor molecule (Tripod 2) was employed that strongly interacted with Acid-R units (association constant: 2.0×10(8)  M(-1) ), specifically from the inner pore. The titration results showed that the apparent stoichiometry [Tripod 2]/[Acid-R] was <0.5, and that the value was concentration dependent. Titration results reasonably account for the scheme in which Tripod 2 only interacts with each terminal in the organized Acid-R. The number of organization was fitted to a 10-mer of Acid-R in a 6.8×10(-7)  M solution, and was consistent with that estimated from the AFM results. In the composites of organized Acid-R/Tripod 2, a singlet excitation energy transfer occurred among the Acid-R units, and to Tripod 2. The energy-transfer rate constants were estimated by using the decamer model, which employed kinetic parameters obtained from steady-state and time-resolved fluorescence experiments.

Fullerene- and pyromellitdiimide-appended tripodal ligands embedded in light-harvesting porphyrin macrorings.

Three new tripyridyl tripodal ligands appended with either fullerene or pyromellitdiimide moieties, named C(60)-s-Tripod, C(60)-l-Tripod, and PI-Tripod, were synthesized and introduced into a porphyrin macroring N-(1-Zn)(3) (where 1-Zn = trisporphyrinatozinc(II)). From UV-vis absorption and fluorescence titration data, the binding constants of C(60)-s-Tripod, C(60)-l-Tripod, and PI-Tripod with N-(1-Zn)(3) in benzonitrile were estimated to be 3 × 10(8), 1 × 10(7), and 2 × 10(7) M(-1), respectively. These large binding constants denote multiple interactions of the ligands to N-(1-Zn)(3). The binding constants of the longer ligand (C(60)-l-Tripod) and the pyromellitdiimide ligand (PI-Tripod) are almost the same as those without the fullerene or pyromellitdiimide groups, indicating that they interact via three pyridyl groups to the porphyrinatozinc(II) coordination. In contrast, the larger binding constants and the almost complete fluorescence quenching in the case of the shorter ligand (C(60)-s-Tripod) indicate that the interaction with N-(1-Zn)(3) is via two pyridyl groups to the porphyrinatozinc(II) coordination and a π-π interaction of the fullerene to the porphyrin(s). The fluorescence of N-(1-Zn)(3) was quenched by up to 80% by the interaction of C(60)-l-Tripod. The nanosecond transient absorption spectra showed only the excited triplet peak of the fullerene on selective excitation of the macrocyclic porphyrins, indicating that energy transfer from the excited N-(1-Zn)(3) group to the fullerenyl moiety occurs in the C(60)-l-Tripod/N-(1-Zn)(3) composite. In the case of PI-Tripod, the fluorescence of N-(1-Zn)(3) was quenched by 45%. It seems that the fluorescence quenching probably originates from electron transfer from the excited N-(1-Zn)(3) group to the pyromellitdiimide moiety.