Porphyrin quinone

Chapter 13 is devoted to the PLC of namral pigments, which encompass fla-vonoids, anthocyanins, carotenoids, chlorophylls and chlorophyll derivatives, porphyrins, quinones, and betalains. Chromatography of pigments is especially difficult because many are photo- and air-sensitive and can degrade rapidly unless precautions are taken. 

Our current work is concentrated on the series of porphyrin-quinone molecules (V). These are very similar to I in that amide linkages have replaced the ester linkages. We chose to study these molecules not only because they provided a definite change... 

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 ... 

Scherer, P. L. J. and Fischer, S. F. Theoretical analysis of the photoinduced electron transfer in porphyrin-quinone cyclophanes, Chem.Phys.Lett., 190(1992), 574-580... 

M. Antolovich, P. J. Keyte, A M. Oliver, M. N. Paddon-Row, J. Kroon, J. W. Verhoeven, S. A Jonker, J. M. War-man, Modelling Long-Range Photosynthetic Electron Transfer in Rigidly Bridged Porphyrin-Quinone Systems ,/. Phys. Chem. 1991, 95,1933-1941. 

Distance-Dependent Rates of Photoinduced Charge Separation and Dark Charge Recombination in Fixed-Distance Porphyrin-Quinone Molecules... 

We recently prepared a series of restricted distance porphyrin-quinone donor-acceptor molecules designed to study the dependence of the rate of electron transfer proceeding from the lowest excited singlet state of the porphyrin on the free energy of reaction.(3-4) We now report measurement... 

Investigation of photoinduced intramolecular ET processes in such systems has in general shown that, compared to analogous porphyrin/quinone systems, i. charge separation occurs with higher efficiency and ii. charge-separated states have longer lifetimes (Scheme 9.4). Such a behavior is the result of the combination of two... 

Porphyrin quinones serve as biomimetic models for fundamental studies on photo-induced ET. Most studies have been performed with the well-known porphyrin p-quinones and the isomeric porphyrin o-quinones have been mostly neglected due to their higher chemical reactivity but in fact they should be better electron acceptors. Speck et al. synthesized several porphyrin o-quinones and have shown that a facile and simple variation of AGEt can be achieved by using in situ formed semiquinones for metal chelation230. 

Processes of charge separation in porphyrin-quinone compounds with flexible bonding... 

Kong and Loach [30, 31] were the first to synthesize porphyrin-quinone compounds of the P-L-Q type. The structure of the compounds they obtained (n — 2, 3) is shown in Fig. 8(a). A strong quenching of the fluorescence of the... 

Charge separation processes in porphyrin-quinone compounds with several flexible bridges... 

The high conformational mobility of porphyrin-quinone compounds with flexible bonding makes it difficult to elucidate in sufficient detail the mechanism of electron transfer between porphyrin and quinone fragments. Far greater possibilities for determining the role of mutual orientation of P and Q and the distance between them are offered by P-Q compounds in which the P and Q fragments are linked by several bridges. A P-Q compound of this... 

Another example of compounds with the fixed mutual location of porphyrin and quinone are the porphyrin-quinone compounds with a rigid bridge. Charge photoseparation in P-L-Q molecules in which L is the trip-ticene bridge, P is tetraphenylporphin, TPP, or its zinc complex, and Q is benzoquinone, BQ, naphthoquinone, NQ, or anthraquinone, AQ, has been studied [55]. The distance between the centres of P and Q fragments in these... 

The charge photoseparation in porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge, ensuring a distance between the centres of P and Q of about 16 A, has been studied [57]. The rate constant of intramolecular electron transfer from P to Q was found to depend on the dielectric properties of the medium and reached 3.3 x 107s 1 for a solution of P-L-Q in propionitrile. 

Fig. 10. Structure of porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge.

Fig. 13. The kinetics [67] of the dark intramolecular electron transfer in a porphyrin-quinone compound (P Lr Q -> P-L-Q ) at T - 77 K. The structure of the compound is given in the upper part of the figure.

Modelling of the intermediate step of the charge separation process during photosynthesis with porphyrin-quinone systems... 

The results of ref. 67 show that it is indeed possible to use solutions of porphyrin-quinone compounds in electron donor solvents for modelling the stage of electron transfer from pheophytin to quinone during photosynthesis (cf. Chap. 8, Sect. 1.2). Further research on these relatively simple model systems may provide still deeper insight into the mechanisms of this stage of photosynthesis. 

A number of covalently bound porphyrin-quinone systems have recently been synthesized as models for carefully spaced donor-acceptor systems. These are designed to test the possibility of controlling electron transfer rates by spatial separation of donor-acceptor pairs. Among these exceedingly clever studies (30) are molecules designed to separate the donor and acceptor by rigid insulating molecules, for example, 2 (31),... 

Numerous systems of this type, such as the carotenoid-porphyrin-quinone triad 94 [8.62a] (for PeT in a pentad see [8.62b]), have been extensively studied in many laboratories from the photochemical point of view and as models of natural photosynthetic centres [8.62-8.69, A. 10, A.20], especially in order to achieve very fast charge separation [8.69] and slow recombination, for instance in multiporphyrin... 

Electron Tunneling in Bridge Porphyrin-Quinone Compounds... 

Processes of Charge Separation in Porphyrin-Quinone Compounds with Flexible Bonding... 

Charge Separation Processes in Porphyrin-Quinone Compounds with Several Flexible Bridges... 

Despite the presence of four bridging groups the quinone is not held rigidly above the plane of the porphyrin. Two channels of electron transfer from the singlet excited state of the porphyrin were found for this compound [145]. They were ascribed to slowly equilibrating introverted and extroverted conformers in which the estimated interplanar porphyrin.quinone separation is respectively, 6.5 and 8.5 A. The faster of the two rate constants is independent of the temperature over the range 80-300 K. 

---