The porphyrin triplet state : from porphyrin-2,2':6',2''-terpyridine conjugates to photocatalysis

The doctoral work we present consists of three separate parts, each dealing with a different project, each is linked to the others by a main theme: porphyrins.
Part I is relative to the synthesis of a new porphyrin-terpyridine conjugate and its application in the preparation of homo- and heteroleptic transition metal complexes. Particularly, attention is focused on a heteroleptic ruthenium complex, which was tested as a light harvester in regards to dye-sensitized solar cell applications. Homoleptic complexes were prepared to study their spectroelectrochemical behaviour.
Part II moves the focus onto photocatalysis. A water-soluble free-base porphyrin catalyst has been prepared and employed to sensitize molecular oxygen to its singlet state. Singlet oxygen is active against a wide selection of substrates, among which we decided to work on two: thiols and sulfides. Studies against simple aminoacids made us interested in assessing whether photocatalytic activity would be retained in polypeptides in vitro. We stepped into photodynamic therapy by applying our photocatalyst to E. coli liquid cultures and following the population change over exposure to singlet oxygen. From a pure synthetic point of view, we screened the photocatalyst on aliphatic and aromatic sulfides. We collected examples on commercial substrates, both symmetric and asymmetric, of natural or pharmaceutical origin. The main goals were the assessment of catalyst’s site-specificity, tolerance to coexisting functional groups and action on diverse structures.
Part III arises from a purely academic interest: is it possible to record the emission spectrum of a redox species, electrochemically generated in situ? In fact, spectroelectrochemistry is widely applied to absorption spectroscopy, where detection of the transmitted light greatly increases sensitivity. It is indeed possible to record such a signal, with the main problem being that reduced or oxidized species mainly show quenching of the emission. For a reversible redox couple, reversing the potential results in the recovery of the original fluorescence band. It remains a challenge to find a molecule whose redox behaviour is associated with emission at different wavelengths or variation in the vibrational pattern of the emission band.