Dominique Niethammer

Electron paramagnetic resonance and ENDOR study of semiquinones in reversed micelles

A series of substituted p-benzosemiquinone radical anions have been investigated in reversed micelles by EPR and ENDOR spectroscopies. Compared with EPR studies in homogeneous aqueous phases or in alcoholic solutions, small but significant changes in g values and hyperfine splittings were observed. Particularly interesting is the observation of asymmetric line-broadening effects in reversed-micellar solutions. Computer simulations of the EPR spectra demonstrate that different molecular positions make distinctly different contributions to this effect, and the ENDOR spectra exhibit a selective broadening of some of the signal pairs. It is concluded that the semiquinone anion radicals are immobilized at the surfactant–water interface. Models for the preferred location and anisotropic motion are discussed. The effect of bulky alkyl groups such as t-butyl on the spin density distribution in semiquinones in polar protic solvents is rationalized in terms of a hydrophobic effect.

Mimicking primary processes in photosynthesis photochemistry of covalently linked porphyrin quinones studied by EPR spectroscopy

Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studied in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Time-resolved and steady-state photoexcitation experiments showed that intra- and intermolecular electron transfer (ET) processes occur in these systems, both in isotropic and reversed micellar solution. Analysis of the experimental data showed the occurrence of photochemical redox processes which result in the formation of hydroquinoid and chlorin type derivatives. Strong polarisation effects were observed for the doublet species - generated by intermolecular ET - under steady-state illumination. It is demonstrated that the polarisation pattern can be explained by the encounter of a triplet and a doublet species allowing the radical triplet pair mechanism to occur. Using a porphyrin linked to a redox active crown ether quinone, complexation of sodium cations in the crown gave rise to well resolved sodium ENDOR lines with an unusual high spin density at the alkali metal ion. Moreover, interesting photochemically induced conformational changes, such as folding and unfolding of donor and acceptor with respect to one another in the case of a flexible butylene bridge, could be detected.

ENDOR studies of alkyl substituted p-Benzosemiquinones in reversed micelles and in 2-propanol

Various substituted p-benzosemiquinone radical anions, inter alia ubisemiquinone and derivatives, have been investigated in 2-propanol and in reversed micelles by EPR and ENDOR spectroscopy. Unsymmetrical semiquinones, with respect to the oxygen atoms, experience remarkable hyperfine shifts depending on the medium. This effect even allows differentiation between stereoisomers. Immobilization of the semiquinone molecules at the water-surfactant interface in reversed micelles gives rise to pronounced asymmetric linewidth effects. In the case of 2-cyclohexyl-3-methyl-1,4-benzosemiquinones, mixtures of two species (conformers) have been observed.

EPR and ENDOR of radicals of chlorin‐ and bacteriochlorin‐quinone model compounds for electron transfer in photosynthesis

A series of covalently linked (bacterio)chlorin‐quinones were synthesized as model compounds for light reactions in photosynthesis. By replacing the more commonly used porphyrin by chlorins, a closer match to the photosynthetically active chlorophylls is achieved. Eight compounds are presented, in which the quinone is linked directly or via a cyclohexylene spacer to the meso position 5 or 10 of the chlorin. The electronic structure of donor and acceptor portions were investigated by EPR and electron–nuclear double resonance (ENDOR) of the cation radicals of bacteriochlorin and chlorin and of the cation and anion radicals of the (bacterio)chlorin‐quinones. Semi‐empirical molecular orbital calculations (methods: AM1 or PM3 on the neutral compounds, RHF‐INDO/SP on the radicals) were performed for the assignment of hyperfine coupling constants, to determine the conformations of the model compounds and to estimate electron transfer properties. The directly linked quinone has a local effect on the electronic structure and conformation of the chlorin. The magnitude of this effect depends on the linking position. Linking the quinone to the cyclohexylene spacer has no measurable effect on the electronic structure of the chlorin. The electronic structure suggests that ratios of charge separation to recombination rates could differ by factors of 20 for different linking positions for the acceptor due to the electronic matrix element. Copyright

Porphyrins linked to high acceptor strength cyano quinones as models for the photosynthetic reaction center

Abstract The synthesis of biomimetic photosynthetic model compounds, composed of 10, 15, 20-tritolylporphyrins linked to different quinones by cyclohexylene bridges, is described. Cyclic voltammetry measurements show a significant change of the reduction potential of the quinone moiety through introduction of cyano groups as electron withdrawing substituents. EPR and ENDOR spectra of the semiquinone anion radical derivatives reveal a considerable spin density redistribution within the semiquinone moiety caused by the substituents. Lamp irradiation in situ of the porphyrin hydroquinones, dissolved in reversed Triton X-100 micelles, through the resonator slits of the EPR spectrometer does not only yield hyperfine resolved absorptive, but also emissive EPR spectra. This is indicative of strong electron spin polarization effects. From the polarization pattern it can be deduced that the radical/triplet pair mechanism between two photoactive species, located in one micelle, gives rise to this effect.

Isomeric porphyrin phenanthrenequinones: synthesis, NMR spectroscopy, electrochemical properties, and in situ EPR/ENDOR studies of the o-semiquinone anion radicals

Porphyrin quinones are attractive model compounds for mimicking natural electron transfer processes. While the overwhelming majority of studies have been performed with porphyrin p-quinones the isomeric porphyrin o-quinones have been mostly neglected. Using phenanthrene-9,10-quinones as the acceptor component we have prepared several porphyrin o-quinones (1, 2, 5, 6) and show that a facile and simple variation of ΔGET can be achieved by using the in situ formed semiquinones for metal chelatization. Additionally, detailed NMR studies show that for asymmetrically substituted porphyrins a complete assignment of the 1H- and 13C-chemical shifts is possible. Complete NMR assignments were necessary for an unambiguous structure determination.