Jörg von Gersdorff

Photo‐induced electron transfer in covalently linked donor–acceptor assemblies in liquid crystals. Time‐resolved electron paramagnetic resonance

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time‐domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210<T<320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130<T<160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet‐ and probably singlet‐initiated ET routes can concurrently be detected, and the free...

Time-resolved EPR studies of photoinduced electron transfer reactions in photosynthetic model porphyrin quinone triads

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or...

NOVEL BIOMIMETIC MODELS FOR PHOTOSYNTHESIS : PORPHYRINS COVALENTLY LINKED TO REDOX-ACTIVE CROWN ETHER QUINONES

Abstract The synthesis of biomimetic photosynthetic model compounds, composed of 5,10,15-triphenylporphyrins covalently linked to redox active crown ether quinones via different bridges, is described. Using the zinc complex with the flexible butylene spacer in non polar solvents backfolding of the crown ether quinone over the porphyrin plane is observed by 1H NMR chemical shift studies. This conformational effect is supported by molecular modelling calculations. The backfolding is induced by complexation of the acceptor with the central atom since it is not observed in the metal free-base porphyrin derivative. Due to steric constraints the zinc porphyrin cyclohexylene crown ether quinones lack this backfolding. Cyclic voltammetry measurements show a significant shift of the reduction potential of the quinone crown ether moiety in the presence of alkali metal cations.

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.

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.

Transient optical absorption and time-resolved resonance Raman experiments on covalently linked porphyrin–quinone systems

Light-induced triplet electron transfer (ET) and subsequent triplet radical-pair (RP) recombination in two covalently linked porphyrin–quinone systems in highly viscous ethanol has been investigated by both transient optical absorption and time-resolved resonance Raman spectroscopy with a time resolution of 10 ns. The temperature dependence of the rates is measured between 155 and 200 K and compared with predictions of solvent-controlled adiabatic electron-transfer theory. It is shown that the triplet ET in the normal region (exergonicity ΔG0 λs) depends on the dynamics of the exchange interaction J, on the triplet–singlet mixing of the radical pair states and on the singlet recombination rate. An intermolecular ET process leading to a disproportionation reaction of the quinone moieties is also observed.

Photoinduced electron transfer in Langmuir-Blodgett films between the donor [5,10,15,20-tetra(pentyl-oxy-biphenyl-oxy-m-phenyl)-porphyrinato]zinc(II) and quinone acceptors, either separated or linked by a cyclohexylene-bridge

Abstract Complexes of zinc porphyrins with mesogenic substituents and quinones, as well as the corresponding mixtures of unlinked porphyrins and quinones, have been investigated in Langmuir–Blodgett (LB) mono- and multilayer films by means of steady-state as well as time-resolved fluorescence spectroscopy. The films were deposited onto quartz substrates. They consist of one or more layers of cadmium behenate (cadmium salt of docosanoic acid) containing either the donor (a zinc porphyrin), the acceptor (a quinone), or both. In the latter case, a covalently linked zinc porphyrin–quinone and mixtures of a zinc porphyrin and a quinone have been investigated. Steady-state S 2 -fluorescence of zinc porphyrins was utilized as a reference to determine the rates of photoinduced electron transfer (PET), in order to characterize the intramolecular and intermolecular PET. This approach was found to be particularly useful for monolayer films. For PET of covalently linked zinc porphyrin–quinone compounds in monofilms, the same PET rates as in ethanol were observed (5.3×10 10 s −1 ). In the case of unlinked zinc-porphyrin/quinone mixtures, the PET rates depend on the concentration of quinones in the acceptor layers. Furthermore, the competition of energy transfer (between zinc porphyrins) and PET (between zinc porphyrins and quinones) in the cadmium behenate matrix is discussed.