Hisahiro Sasabe

Photoinduced Electron Transfer Processes in Rotaxanes Containing [60]Fullerene and Ferrocene: Effect of Axle Charge on Light-Induced Molecular Motion

Two rotaxanes containing [60]fullerene (C60) as pendants on a crown-ether necklace, to which ferrocene (Fc) as axle stoppers were added, have been synthesized. One rotaxane has an ammonium cation in the centre of the axle (C60;Fc)Rot+ and the other has a neutral axle (C60;Fc)Rot. Optimized structures, calculated using a molecular orbital method, suggest that in the ground states (C60;Fc)Rot+ has a shorter distance between C60 and Fc than that of (C60;Fc)Rot. In both rotaxanes, efficient intra-rotaxane photoinduced electron-transfer processes have been observed by the selective excitation of C60 which acts as a photosensitized electron acceptor. The rates and efficiencies of the charge-separation and charge-recombination processes were evaluated by time-resolved fluorescence and transient absorption measurements with changing solvent polarity. From the different kinetic parameters between (C60;Fc)Rot+ and (C60;Fc)Rot, the light-induced molecular motions of these rotaxanes in the excited states and charge-separated states were separately revealed.

Axle charge effects on photoinduced electron transfer processes in rotaxanes containing porphyrin and [60]fullerene

Two rotaxanes containing zinc porphyrin (ZnP) with crown ether and [60]fullerene (C(60)) with cationic and neutral axles are synthesized. Optimized structures calculated by molecular orbital methods indicate that the rotaxane with an ammonium cation in the center of the axle has a shorter distance between the C(60) and ZnP moieties than that of the rotaxane with a neutral axle because of acylation of the ammonium cation, which draws away the C(60) and ZnP moieties by releasing the interaction with the crown ether. The charge-transfer interaction is revealed by absorption spectra for the rotaxane with a short distance, but not for the rotaxane with a long distance, which strongly affects the rates and efficiencies of photoinduced electron-transfer and energy-transfer processes via the excited singlet states of the ZnP and C(60) moieties and their triplet states, as revealed by the time-resolved fluorescence and absorption measurements. The rate of the charge recombination of the radical ion pair of the rotaxane with a neutral axle is slower than that of the rotaxane with a cationic axle, due to the loose structure of the former rotaxane, which results from the long distance between the C(60) anion radical and the ZnP cation radical.

Photoinduced electron transfer processes in three component rotaxanes with porphyrins, [60]fullerene and triphenylamine

Electron transfer processes occurring through-space and through-bond were studied for novel mechanically linked triad [2]rotaxanes, which contain a porphyrin (MP) unit as a pendant and [60]fullerene (C60) and triphenylamine (TPA) moieties as stopper groups at the axle ends (abbreviated as (MP;C60-TPA)rot+ with MP = H2P or ZnP. The photophysical properties were investigated by means of time-resolved fluorescence and transient absorption measurements with changing solvent polarity. The charge separation took place mainly via(1MP*;C60-TPA)rot+ and (MP;1C60*-TPA)rot+ in polar solvents. Within the charge-separated states of triads (MP;C60-TPA)rot+, hole-shift and/or back electron transfer took place competitively. The lifetime of the charge separation state of (ZnP;C60-TPA)rot+ was similar to that of dyad (C60-TPA)rot+ (140 ns in dimethylformamide), whereas that of (H2P;C60-TPA)rot+ was as long as 230 ns, suggesting two final charge separation states such as (MP;C60•--TPA•+)rot+ and (MP•+;C60•--TPA)rot+ depending on the kind of porphyrin.

Diastereotopic relationship between planar and central chiralities in the formation of Ru(η3-allyl)(CO)(PPh3)(L–L′) complexes

Abstract Hydride ruthenium complexes, RuHCl(CO)(PPh3)2(L–L′) 3 (L–L′=bidentate ligand having nitrogen and oxygen) react with allenes to give Ru(η3-allyl)(CO)(PPh3)(L–L′) complexes 5 in good yields via hydrometalation reaction. The complexes 5 have planar chirality at the η3-allyl ligand and central chirality at the Ru metal, and consist of one pair of enantiomers. Ligand substitution reaction of Ru(η3-allyl)Cl(CO)(PPh3)2 complexes 6 with bidentate ligands (L–L′) also afford the complexes 5 which have the same stereochemistry as those formed by the hydrometalation reaction. The planar chirality is controlled by the central chirality at the Ru metal in both the formations of the complexes 5. The structure of 5a (L–L′=N–N bidentate ligand) was determined by the X-ray crystal structure analysis.

Preparation of Ru(η3-2-alkenylallyl)(CO)Cl(PPh3)2 complexes via carbometallation of allenes with alkenyl ruthenium complexes

Abstract Alkenyl ruthenium complex, Ru(CHCHR)(Cl)(CO)(PPh3)2 1, reacted with allenes 2 to give η3-allyl ruthenium complexes, Ru(η3-2-alkenylallyl)(Cl)(CO)(PPh3)2 3, in good yields. The reaction depends on the structure of the alkenyl group. When R was phenyl or methoxycarbonyl group, the carbometallated complex 3 was yielded as a sole product. However, when R was butyl or trimethylsilyl group, besides the carbometallation product as main product, was obtained a small amount of 2-unsubstituted η3-allyl ruthenium complex which was formed via β-elimination of the alkenyl complex followed by the reaction with allene. Structure of 3 was determined by the X-ray crystal structure analysis.