Photoinduced electron transfer in model systems of photosynthesis

This Thesis describes Investigations on photoinduced electron transfer (ET) for several compounds, serving as model systems of the natural photosynthesis. In addition, the properties of the systems, e.g. the conformation in solution and the electronic properties of the photoexcited states are treated.

Chapter 2 discusses present theories of photoinduced electron transfer. The following factors appear to effect the electron transfer rate constants:
- donor-acceptor distance
- nature of the linking chain
- conformation of donor and acceptor
- spin dynamics
- change in free energy, AG, of the process
- solvent

Chapter 3 describes optical and magnetic resonance experiments to study the electronic properties, the conformations and the electron transfer rates in the model systems.

In this Thesis three different classes of donor-acceptor compounds, containing porphyrins as light-sensitive compounds are treated in detail.

Chapter 4 describes the properties of sulfonyloxy-linked porphyrinanthraquinone compounds and the porphyrin moiety. The electronic properties of the porphyrin (free base diphenyletioporphyrin) were examined by fluorescence detected magnetic resonance (FDMR). The triplet state properties and the optical absorption spectra of the diphenyletioporphyrin could be rationalized by applying the fourorbital model of Gouterman. In frozen n-octane at 4.2 K the porphyrin In the two sites exhibits different degrees of configuration interaction, indicating that the medium around the molecule exerts a strong effect on the properties of the excited states. The electron transfer from this porphyrin to covalently linked anthraquinone appeared to be very fast for three porphyrin-anthraquinone compounds, in which the anthraquinone-2-sulfonyloxy group is substituted at a phenylgroup of the porphyrin at the ortho-, meta-, or para- postion. Forward ET in these compounds in CH ₂ Cl ₂ occurred in less then 3 picoseconds after photoexcitation. If the two molecules are not closely folded, so that the through-space electronic coupling is small, ET is most likely to occur through the sulfonyloxy linking bridge. The sulfonyloxy bridge can easily accomodate an electron, enhancing the participation of the empty orbitals of the bridge in a superexchange coupling through the bridge. This illustrates the effect of the nature of the bridge on the electron transfer. In one of the porphyrin- anthraquinone compounds a high triplet yield is found, resulting from a singlet-triplet conversion in the charge-separated state. The same process occurs in natural photosynthesis and is then caused by spin-dephasing due to hyperfine interactions and different g-values. From the triplet ESR spectrum the mechanism of the singlet-triplet conversion for the model compound appears to be different and is attributed to spin-orbit coupling within the charge -separated state. In § 4.3 the covalently linked porphyrin-anthraquinone systems were compared with the freely diffusing systems. The electron transfer rate in the freely diffusing system appears to be diffusion-limited.

Chapter 5 focusses on another class of porphyrin complexes in which electron transfer occurs, namely dimers of oppositely charged watersoluble porphyrins. Heterodimers with two different central atoms can easily be formed by combining different positively and negatively charged porphyrins, resulting in a modular system, of which the energetics can easily be varied by varying the central atoms on the two porphyrin constituents of the dimers. In § 5.1 it is shown that the lowest excited triplet state for those dimers that do not exhibit ET, is localized on one of the constituents of the dimer and has a partly charge transfer character. §§ 5.2 and 5.3 report the structure of the dimers in solution as investigated by ¹H NMR. From ring current shift calculations the porphyrin macrocycles turn out to be at a very close distance (3.1 Å) and shifted along the aryl-aryl axis over 4.2 -Å optimizing the interaction between the porphyrin monomers. For some dimers also higher aggregates can be formed with an interchromophore orientation equal to that in the dimers. §§ 5.1 and 5.4 discuss the observed relation between the fluorescence quenching, caused by electron transfer, with the energetics of these complexes. The porphyrins were extended with extra donor and acceptor groups to separate the charges after the initial formation after photoexcitation of the dimer.

These water-soluble porphyrin dimers are interesting for further research for two reasons: (i) They form oriented stacks of alternatingly charged porphyrins and (ii) their behaviour (ET or not) can be controlled by the central atoms in the components.

Chapter 6 discusses the third class of donor-acceptor compounds that were studied in this work. These were flexibly linked donor-tetraphenylporphyrin-acceptor triads and their components. The average ground-state conformations of these compounds could be determined from 1H NMR by ring current shift calculations. The rate constants for forward ET are found to increase with increasing values of the change in free energy AG: the AG value for forward ET for these compounds are shown to be in the Marcus normal region. The nature of the bridge is found to be very important, because it affects the redoxpotentials of the acceptor anthraquinone and contributes to the electronic coupling between donor and acceptor via a superexchange mechanism through the bridge.

In chapter 7 the quenching of the fluorescence of a free base or zinc porphyrin by a paramagnetic copper porphyrin was studied for different dimers. In dimers of oppositely charged porphyrins and in dimers consisting of zinc tetraphenylporphyrin ligated by the pyridyl group of copper pyridyltriphenylporphyrine, electron transfer acts as quenching mechanism of the fluorescence of the diamagnetic porphyrin. In covalently linked free base and copper tetraphenylporphyrin electron transfer is energetically forbidden and energy transfer occurs from the copper porphyrin to the triplet state of the free base porphyrin.