Rotaxane photoinduced electron transfer

In making rotaxanes usable as parts of molecular devices and with the purpose of studying long range election transfer processes within large molecular systems of well controlled geometries, the introduction of photoactive and electroactive compounds has been a valuable development. Photoinduced electron transfer between porphyrin species has a particular relevance to the primary events occurring in bacterial photosynthetic reaction center complexes, and so is a well studied phenomenon. 

The rates of photoinduced electron transfer between the zinc porphyrin and the gold(m) porphyrin are 1.7 ps for the Cu(i)-complexed [2]-rotaxane, and 36 ps for... 

Kinetic studies have demonstrated that photo-induced electron transfer between the zinc and the gold porphyrin occurs at a rate of (1.7 ps)-1 in Cu(I)-complexed [2]-rotaxane 102, which is much higher than in the case of the free rotaxane 107 (36 ps)-1.73 The higher photoinduced electron transfer rate in the Cu(I) complex 102 than in the demetallated system 107 was explained also in terms of a superexchange mechanism. 

Porphyrinic Rotaxanes Control of the Photoinduced Electron Transfer Rate between PZn and PAu by the Assembler Metal... 

For the copper-free [2]rotaxane the rate of photoinduced electron transfer from the Zn porphyrin excited state to the Au porphyrin is 273 x 10 s (as calculated from Ref. [76c]), which compares well with A i4 = 178 x 10 s. ... 

As in the past two years, fullerenes continue to be a source of considerable photochemical interest and photoinduced electron transfer is a central theme of numerous studies. The field of molecular-scale electronic devices continues to promote interest in the photophysical properties of novel molecular architectures and such aspects of photoactive rotaxanes and catenates have been reviewed (Benniston and Chambron et ai), while Harriman and Ziessel have outlined the design principles associated with the construction of photo-activated molecular wires. 

Figure 15. Photoinduced electron transfer processes that take place in [2]rotaxane 17 + [78, 79].

For space reasons, in this chapter we will only review recent advances in the field of molecular-level machines operating by means of photoinduced electron-transfer processes. Since such machines are based on pseudorotax-anes, rotaxanes, and catenanes, we will first recall some important features of these kinds of supramolecular systems. 

For the sake of space, in this chapter we will only discuss examples of molecular-level machines based on photoinduced electron transfer processes. An extensive review on artificial molecular machines [3c] and more detailed discussions on electron-transfer processes involving pseudorotaxanes [23a], and rotaxanes and catenanes [23b] are reported elsewhere. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Sandanayaka, A. S. D., Sasabe, H., Takata, T. and Ito, O. Photoinduced electron transfer processes of fuUerene rotaxanes containing various electron-donors. J. Photochem. Photobiol. C Photochem. Rev. 11, 73-92, 2010. 

Sasabe, H. et al. Axle charge effects on photoinduced electron transfer processes in rotaxanes containing porphyrin and [60]fullerene. Phys. Chem. Chem. Phys. 11, 10908-10915, 2009. 

Sandanayaka, A. S. D. et al. Photoinduced electron-transfer processes between [C60]fullerene and tri-phenylamine moieties tethered by rotaxane structures. Through-space electron transfer via excited triplet states of [60]fullerene. J. Phys. Chem. A 108, 5145-5155, 2004. 

Rajkumar, G. A. et al. Prolongation of the lifetime of the charge-separated state at low temperatures in a photoinduced electron-transfer system of [60]fullerene and ferrocene moieties tethered by rotaxane structures. J. Phys. Chem. B 110, 6516-6525, 2006. 

Figure 8 Donor-acceplor mulliporphyrin conjugalcs. The donor porphyrin (empty diamond) and the acceptor porphyrin (hatched diamond) are mechanically linked in cither catcnanc (a) or rotaxane (b) structure. Tlie arrows show the direction of photoinduced electron transfer.

Photoinduced Intramolecular Electron Transfer Within Porphyrinic Rotaxanes... 

Figure 16. Photoinduced energy and electron transfer processes occurring in [2]rotaxane 18 + [81], which behaves as a triad for photoinduced charge separation according to the scheme illustrated in Figure 14a. For more details, see text.

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