Characterization of porphyrin-based biomimetic systems for artificial photosynthesis using Electron Paramagnetic Resonance (EPR) spectroscopy coupled with optical techniques

This work focuses on advanced Electron Paramagnetic Resonance (EPR) techniques for the investigation of the photoexcited triplet state of porphyrins. EPR spectroscopy is a powerful tool that enables to study the photophysics, with a special emphasis on this state, otherwise not easily accessible via optical spectroscopies. In the present work, porphyrins with different chemical structures have been considered and investigated by means of advanced EPR techniques. The photophysical properties of 5,10,15,20-tetrakis(4-sulphonatophenyl) porphyrin (TPPS) have been investigated by means of coupled optical and magnetic spectroscopy. TPPS is an interesting biomimetic model since it can form J-aggregates in acidic environment, mimicking pigment aggregates in natural light harvesting systems. TPPS has been used as a model system for the study of the phototautomerism process. This is an important process for the free-base porphin and its derivatives, since it has a great impact on the molecular and electronic structure. In conjunction with EPR, optical spectroscopies have been employed to gain insight into the photophysics of the process. In particular, fluorescence anisotropy has been employed to study phototautomerism in the photoexcited singlet state, while Time-Resolved EPR spectroscopy coupled with magnetophotoselection has allowed to investigate this process in the triplet state. Magnetophotoselection allows to determine the orientation of the transition dipole moment in the zero-field splitting frame by using linearly polarized light oriented with a parallel or a perpendicular configuration with respect to the external magnetic field. Both fluorescence and EPR experiments have been performed in different conditions: protic and deuterated environment and adding potassium bromide in order to favour intersystem crossing via external heavy atom effect. A detailed investigation has been performed on a cofacial μ-oxo-bridged porphyrin heterodimer, consisting of an aluminium-porphyrin and a phosphorus-based porphyrin, along with its monomers. This heterodimer is the synthetic analogue of the “special pair” chlorophyll dimer found in Photosystem II. Along with the heterodimer system, a fluorinated porphyrin and its non-fluorinated counterpart have been studied, as fluorination provides enhanced electron acceptor properties, which are important for the optimization of the electron transfer process in biomimetic compounds for photosynthesis. The electronic structure of such porphyrins has been studied using pulsed hyperfine spectroscopy, in particular Electron Nuclear Double Resonance (ENDOR). The ENDOR technique allows the measurement of the hyperfine couplings, which are local probes of the electron spin density. Triplet state ENDOR spectroscopy has been performed at Q band at Universität des Saarlandes in the group of Prof. C.W.M. Kay, in the frame of the NExuS programme. Subsequently, at the University of Padua porphyrin radical cations have been produced and investigated by ENDOR spectroscopy at Q-band to have a better understanding of the charge transfer process in the heterodimer. Lastly, a porphyrin-based dye system for dye sensitized solar cells has been examined. It is made up of a 5,10,15,20-tetraphenyl porphyrin moiety covalently bound to a peptide. The peptide acts as a linker for the binding of Titania nanoparticles and also as a mediator for long-range electron transfer. The photoinduced electron transfer process has been investigated using CW-EPR spectroscopy under light excitation.