Porphyrins photo-electron transfer from

While molecular assembly has proven to be effective for a photoelectric conversion system, coordination reactions are possibly a simple approach for connecting such functional molecules, as presented in the previous section. We applied the stepwise coordination method to prepare a photoelectric conversion system. Since the molecular wire exhibits redox conduction through the wire,11,13 efficient photo-electron transport through the redox sites in the wire is also expected. In this section, we demonstrate the fabrication of a photoelectric conversion system using ITO electrodes modified with M(tpy)2 (M = Co, Fe, Zn) complex wires with a terminal porphyrin moiety as a photosensitizer. The behavior of photo-electron transfer from porphyrin to ITO through the molecular wire was investigated by changing the metal element in the M(tpy)2 moieties.14... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

Selective excitation of the Zn porphyrin unit of [2]-rotaxane 111 at 575 nm resulted in photo-induced electron transfer from the singlet excited state ZnP to the ground state AuP+. Two rates of electron transfer could be extracted from the fast kinetic studies, (83 ps)-1 and (770 ps)-1, with 35% and 65% contributions respectively.75 These rates probably correspond to different conformations of the [2]-rotaxane. In any case the electron transfer rates are much slower than those measured for through bond processes in rotaxanes 102 and 107. Related [2]-rotaxanes with Au porphyrin incorporated into, rather than appended to, the macrocyclic component have been synthesized and studied.2411 Similar conclusions could be drawn from the photophysical studies.76... 

The model of electron transfer in gas-phase metal-molecule reactions can be extended to more complex systems such as the collisions of metastable rare gas atoms with molecules to produce negative molecular ions [306], In surface chemistry the harpoon model describes the forces between the reagents after the electron transfer has been applied to reactions of molecules with metal surfaces [120]. Another domain, involving the reaction of metal ions with complex systems could be interpreted in the framework of electron transfers in the porphyrin site of the heme within hemoglobin, addition of oxygen to the Fe " " results in an electron transfer from the metal to the oxygen. The dynamics of this attachement and of the photo-induced detachment could be viewed in that perspective. 

Figure 41 (a) The arrangement of the central cofactors of the Reaction Centre of Rh. viridis, according to an X-ray crystal structure analysis. SP, the special pair, is the primary electron donor in its excited state, BCh is an accessory bacterio-chlorophyll, and BPh, a bacterio-pheophtyin, is the primary electron acceptor [142-144], Photo-induced electron transfer from SP to BPh takes place in 3 ps [145-147], (b) A synthetic model of the natural system (128). [139, 140, 148] SP and BPh are mimicked respectively by a zinc(II) and a gold(III) porphyrin bridged by a dpp spacer. 

Another example of self-assembly of porphyrin-containing polymer was illustrated by Li et al.73 Polyacetylene functionalized with fullerene and zinc porphyrin pendant groups were synthesized by polymerizing the corresponding fullerene/porphyrin substituted alkyne monomers with rhodium(I) norbomadiene catalyst (Scheme 5.5).74 Polymers with different ratio of C60 and porphyrin were synthesized. The polymers showed photocurrent response when the thin films were irradiated with white light, which was due to the electron transfer from the photo-excited porphyrin to the C60 units. In addition, the copolymers aggregated into ellipse-shaped nanorod structures with a diameter of approximately 100 nm and a length of... 

However, the luminescence measurements show quenching of fluorescence in the trimer, which is attributed to a photo-induced electron transfer from the axial ruthenium(II) porphyrin to the excited state of the basal tin porphyrin. Not only ruthenium(II) porphyrins, but also rhodium(III) porphyrins can easily be incorporated into the arrays with the same strategy [33]. Again, the isonicotinic acid is first reacted with the bis-hydroxy tin porphyrin to give the bis-isonicotinic acid complex. Addition of two equivalents of rhodium(III) porphyrin readily yields the trimeric array of the composition Rh-Sn-Rh. The X-ray structure of this complex, which is shown in Fig. 34c, shows that the ligands on the tin center (carboxylates) are in an off-direction which is close to orthogonal to the porphyrin plane, and the three porphyrins adopt a near coplanar arrangement. The tin porphyrin is tilted by about 8.6° with respect to the rhodium(III) porphyrins. 

A variety of other porphyrin-quinone-based multicomponent systems bearing four or more donor and acceptor moieties have been reported. For example, P-P-P-Q tetrad 53 and related compounds have been reported by Sessler and coworkers [64, 65, 73, 220, 232-236]. Fluorescence and time-resolved absorption experiments with 53 were interpreted in terms of rapid ( 10-ps) singlet-singlet energy transfer between the porphyrin units in the linear array and extremely fast (<350-fs) photo-induced electron transfer to the quinone from the proximal porphyrin excited singlet state to give a charge-separated species. 

FIG. 21 Complex IMPS spectra obtained for the photo-oxidation of DFcET by ZnTPPC4- at the water-DCE interface (a). The opposite potential dependencies of the phenomenological ET rate constant and the porphyrin coverage (b) are responsible for the maximum on the flux of electron injection obtained from IMPS responses for DFcET and Fc (c). The potential dependence of the back electron-transfer rate constant is also shown in (d). (From Ref. 83. Reproduced by permission of The Royal Society of Chemistry.)... 

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