Porphyrines, intramolecular transfers

As shown by Nango et al. [243] for the porphyrin-quinone complex as an example, even when an electron travels via intramolecular transfer only a part of its way across the membrane, the observed overall rate of transmembrane electron transport can be notably increased as compared to transport via diffusion only. 

The reorientation of partially deuterated rotors, as discussed in Section IV.A.5, is an example of proton displacements between positions that are distinguishable. Even when these positions are equivalent in the isolated molecule, the interaction with an environment of lower symmetry usually makes the positions inequivalent so that the PES becomes asymmetric. This situation applies also for proton translations in intra- and intermolecular PT processes, examples of which are discussed below (Sections IV.B.l and IV.B.2). Prototype examples for intramolecular transfers are the tautomer-ization of free base porphyrines and phthalocyanines and the PT in malonaldehyde, tropolone, 9-hydroxyphenalone, and so on. The tautomer-ization of carboxylic dimers is the best studied example of intermolecular transfer in a (near) symmetric PES. 

There are numerous reports of the existence of electrocatalysis via an attached redox center on a SAM (see Sect. 4.3), but few reports in which the rates of electron transfer between the electrode and the attached redox molecule and between the attached redox molecule and the solution redox molecule are measured. It would be interesting to study the electron-transfer kinetics in SAMs with multiple redox molecules linked along a single tether (such as porphyrins [157] or metal-terpyridine complexes) [122]. From such a system, one could derive the rate of electron transfer between two redox molecules connected by a molecular bridge and check the considerable data available on intramolecular transfer obtained by other methods [254]. For a variety of applications, measurements of the rates of electron transfer between an electrode and metal nanoparticles tethered to the SAM are also of interest [255]. 

Tliere are several reasons for this great interest in the tautomerism of porphyrins (which could justify its own review) (1) their biological significance, (2) their applications in material science ( hole burning is related to their tautomerism), (3) the simplicity of the system (annular tautomerism involving intramolecular proton transfer both in solution and in the solid state), and (4) the possibility of elucidating the kinetic processes in great detail. 

As described before, the rr-electrons of porphyrin are delocalized over the molecule and the energy levels of the HOMO and the LUMO are high and low, respectively. The resultant narrow intramolecular HOMO-LUMO gap causes absorption of the entire region of visible light. Usually, porphyrins are red to purple and phthalocyanines are blue to green. Furthermore, the long lifetime of their excited states is appHcable to the construction of photo-induced electron and/or energy transfer systems. 

Other supramolecular structures such as dendrimers have also been synthesized with zinc-containing porphyrins. Sixteen free base and sixteen zinc porphyrin units were added at the fifth generation of dendritic poly(L-lysine) and intramolecular fluorescence energy transfer was observed.823 Assembly of supramolecular arrays in the solid state has been achieved with the incorporation of an amide group for hydrogen bonding. Zinc meso-tetra(4-amidophenyl)porphyrin... 

T. J. Kemp, University of Warwick Noting the very low quantum yield for intramolecular electron transfer in low temperatures displayed by your porphyrin-quinone model compound, would it not be possible to shock-freeze a solution undergoing irradiation at a higher temperature (and giving a workable concentration of paramagnetic species) in order to determine a low-temperature spectrum with the particular aim of observing a possible Am = 2 transition ... 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach," the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

Ge(TPP)R2, Ge(TPP)(Fc)Ph, and Ge(TPP)Fc2) the spectrum after 2 /us was consistent with a triplet excited state, although this decayed much faster for the fer-rocenyl complexes. Addition of ferrocene to Ge(TPP)R2 also quenches triplet lifetimes. A similar situation was observed for the indium complexes In(Por)R, and the triplet-state quenching was attributed to an energy transfer process from the excited-state triplet to ferrocene. In the case of the germanium porphyrins, the longer-lived triplet state in Ge(TPP)R2 is responsible for the Ge—C bond homolysis, and both inter- and intramolecular quenching by ferrocene is observed. 

The heterometallic system [(bpy)2Ru(234)] exhibits several intramolecular energy-transfer processes (i) ultrafast singlet-to-singlet transfer, (ii) fast triplet-to-singlet transfer and (iii) singlet-to-triplet transfer. Excitation into the Ru(bpy)3 " " domain is followed by rapid energy transfer to the triplet state of the Zn(porphyrin) fragment. There is no evidence for intramolecular electron transfer between the Ru(bpy)3 and Zn(porphyrin) units. 

Bcemoto J, Takimiya K, Aso Y et al (2002) Porphyrin—oligothiophene—fullerene triads as an efficient intramolecular electron-transfer system. Org Lett 4 309-311... 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

This hydroxylation-induced intramolecular migration, known as the NIH shift, was explained by the involvement of arene oxides formed by the attack of electrophilic oxoiron(V) porphyrin on the aromatic ring.753 Intermediate 98 was also suggested to be formed in hydroxylation by the Fenton and related reagents in aprotic media after initial oxidation with an oxoiron(V) complex followed by electron transfer.744 754... 

Figure 4.12 Examples of bichromophoric molecules used for the study of intramolecular electron transfer, (a) Dimethoxynaphthalene electron donors dicyanoethylene electron acceptor with rigid spacers (b) porphyrin donor and quinone acceptors separated by flexible spacers...

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