Models Of The Photosynthetic Reaction Centers Based On Prophyrin And Ruthenium/Rhenium Polypyridyl Complexes

Photosynthesis, a process of converting light energy into chemical energy, involves two major steps, absorption and transportation of light energy of appropriate wavelength by the antenna light harvesting molecules to the reaction center, and photoinduced electron transfer (PET) to generate charge separated entities by using the electronic excitation energy. Mimicking the photosynthetic functions by using synthetic model compounds is important to further our understanding of the process of bioenergetics. Research in this area also holds promise for technological advances in solar energy conversion, and developing optoelectronic wires and switches. In order to mimic the functions of natural photosynthetic reaction center and antenna in photosystems, a series of multicomponent systems based on porphyrin and ruthenium/rhenium polypyridyl compounds are designed and synthesized.In this thesis, we synthesize eight new compounds 5, 11, 12, 16, 5-Zn, 11-Zn, 12-Zn, 16-Zn, which can be absorbed on the surface of nanometer size semiconductors (TiO₂) with four ester groups on the bipyridyl ligands. Their redox potentials, quantum yields, excited-lifetimes and electron transfer are studied by the use of cyclic voltammetry, fluorescence analysis and nanosecond laser flash photolysis.By the studies of nanosecond flash photolysis and estimates of excited state energy level, we deduce that there is a charge-transfer excited state corresponding to the transfer of an electron from the zinc porphyrin to the Ru-based moiety of 5-Zn in acetonitrile when exciting at λ= 532 nm. Such a charge separation state is written as PZn⁺-Ru⁺. For compound 11-Zn, the study of time-resolved kinetic decay shows that ruthenium unit plays a quenching role in zinc porphyrin unit's triplet excited state lifetime, but the effect of rhenium part can be neglected. Other compounds are also investigated in acetonitrile, with their transient absorption spectrums showing characteristics of the triplet excited state of (zinc) porphyrin moiety, but these triplet excited state lifetimes are much lower than that of regular (zinc) porphyrin. This result indicates that these compounds are not quenched directly from triplet excited state of (zinc) porphyrin moiety to the ground state.Primary dynamic studies on the electron injection and backing recombination between these complexes and TiO₂ nanoparticle are carried out by means of transient absorption spectrum, the results indicate long-lived charge separation obtained in these assemblies, and the kinetic decays of these systems exhibit di-exponential behavior. For the 5-Zn-TiO₂, we...