Peizhu Tian

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.

Covalently linked porphyrin ubiquinones (O) as model compounds for photosythetic reaction center.

Abstract trans- and cis- cyclohexylene linked porphyrin ubiquinones(O) have been synthesized and characterized by NMR and optical spectroscopy. ENDOR spectra of the corresponding porphyrin semiquinone radical anions are discussed.

Continuous-wave electron spin resonance studies of porphyrin and porphyrin-quinone triplet states

CW ESR studies of the photoexcited triplet states of a series of porphyrins and covalently linked porphyrinquinones have been performed in isotropic and anisotropic (liquid-crystalline) frozen solutions. In frozen nematic solutions, the appearance of the spectra is strongly dependent on the orientation of the sample relative to the external magnetic field. This fact allows a differentiation between the z and x, y axes and hence a more accurate determination of the zero-field splitting parameters D and E. Spectrum simulation and fitting aided in the interpretation of the experimental results and the extraction of the relevant parameters. In the case of some porphyrin-quinones, steady-state triplet ESR signals cannot be observed, which is ascribed to singlet-electron transfer enhanced by folding of the quinone acceptor over the porphyrin donor. The doublet signal of porphyrin or chlorin cation radicals, which is also observed, appears in emission under certain conditions, indicative of the radical triplet pair mechanism.

Mimicking primary processes in photosynthesis—covalently linked porphyrin quinones

Abstract Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studies in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Electron transfer (ET) from the porphyrin donor to the quinone acceptor could be observed by time-resolved picosecond fluorescence spectroscopy (singlet ET) and by time-resolved EPR spectroscopy (triplet ET) in isotropic fluid solution and in anisotropic media (liquid crystals and reversed micelles). Steady-state in situ photoexcitation of P-Qs in CTAB cationic reversed micelles yielded the corresponding semiquinone radical anions. In TRITON X-100 reversed micelles both the radical cation of the porphyrin and the radical anion of the semiquinone could be detected, which occured in complete emission. In covalently linked porphyrin flavins ET from the photoexcited porphyrin fragment to the flavin and, in addition, energy transfer from the photoexcited flavin to the porphyrin could be observed.

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.

Photoinduced reactions of covalently linked porphyrin quinones in reversed micelles as studied by EPR spectroscopy

From covalently linked porphyrin quinones, dissolved in reversed micellar solutions, porphyrin radical cations and semiquinone radical anions can be generated photochemically. At pH 7 of the water pool the radical species are rather short-lived and can thus only be observed duringin-situ light irradiation (optical pumping) of the sample. Different polarization patterns are observed in different types of micelles, e.g., CTAB cationic or Triton X-100 neutral reversed micelles. Specifically, in the former absorptive (either Boltzmann relaxed or enhanced absorptive) spectra are obtained, whereas in the latter both species (porphyrin radical cation and semiquinone radical anion) occur in emission. Complete emission is indicative of a strong triplet mechanism.

Photochemistry of a butylene linked porphyrin-quinone donor-acceptor system studied by time-resolved and steady-state EPR spectroscopy

Radicals generated photochemically from a covalently linked porphyrin-quinone donor-acceptor system dissolved in reversed micelles and isotropic solution have been studied by steady-state and time-resolved EPR spectroscopy. In these systems photoinducedintramolecular as well asintermolecular electron transfer processes occur which result in the formation of semiquinone radical anions and porphyrin radical cations. Disproportionation of the semiquinone leads to the formation of porphyrin-hydroquinone (and porphyrin-quinone). The porphyrin-hydroquinone is itself photoactive and reacts through the photoexcited triplet state of the porphyrin. Reduction of the porphyrin to the dihydro from — probablyvia hydrogen abstraction by the photoexcited porphyrin from the hydroquinone — appears to be the dominant reaction. Once formed the dihydrophyrin undergoes further similar photochemistry. Emissively polarized spectra are observed from these systems in steady-state EPR experiments. Timeresolved EPR indicates that this polarization is essentially due to the radical triplet pair mechanism.

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.