Development and optimization of spectroscopic methodologies for the investigation of biomimetic compounds

Electron Paramagnetic Resonance (EPR) spectroscopy provides a unique perspective on the behavior of transient paramagnetic species that are created upon exposure to light. This technique offers valuable perspectives into short-lived spin systems involved in various fields, such as paramagnetic intermediates in photoactive proteins, donor-acceptor electron transfer models, and materials for organic photovoltaic devices. The work described in this thesis provides significant insights into the properties and spin dynamics of three types of photoexcited transient paramagnetic systems: spin-correlated radical pairs, triplet states, and triplet-doublet spin systems. The research findings detailed in this document contribute to the development and optimization of advanced spectroscopic methodologies for the investigation of biomimetic compounds. The first three chapters introduce the EPR spectroscopy and the theoretical background of photoexcited systems. Afterward, Chapter 4 explores the use of shaped microwave pulses on two model donor-bridge-acceptor triads for controlled selective and non-selective excitation of spin-correlated radical pairs generated by photoinduced electron transfer. These spin systems exhibit distinctive spin polarization and unique behavior in EPR spectroscopy, and the accurate control of spins with microwave pulses is crucial to unravel their role in biological systems, optoelectronic devices, and quantum information science. Narrowband-selective excitation is accomplished with BURP (Band-selective, Uniform Response, Pure phase) pulses, while frequency-swept chirp pulses ensure broadband excitation of both radical pair spins. The application of frequency-swept chirp pulses in EPR experiments enhances modulation depth and enables correlation of the dipolar frequencies with the EPR spectrum, thereby providing further insights into donor-acceptor geometries. Chapters 5 and 6 explore the properties and spin dynamics of a series of photoexcited porphyrins which, just like other photochemically generated systems, often result in spin states with non-Boltzmann spin polarization. The study demonstrates that their triplet states evolve into long-lived net polarization with lifetimes matching their optical emission. This emphasizes the potential of four-fold symmetric porphyrins to generate net polarization not limited by spin-lattice relaxation, thereby enabling extended manipulation and transfer to remote spin systems. The transfer of net polarization from photoexcited porphyrins to remote weakly-coupled radicals is investigated in peptide-based rulers in Chapter 7. The results reveal a long-lived polarized radical whose lifetime correlates with that of the triplet state. The spin polarization transfer is shown to scale with the chromophore-radical distance with a behavior consistent with the weak-coupling regime. The research extends in Chapter 8 to moderately-coupled triplet-doublet pairs, providing insights into their coupling mechanisms and drawing comparisons with structurally similar weakly-coupled spin systems. In Chapter 9, the study expands to investigate two non-porphyrin chromophores each weakly coupled to a stable radical, providing further evidence and confirming that the electron spin polarization transfer occurs regardless of the net polarization source. Chapter 10 is dedicated to the growing application of photoexcited triplet states in EPR pulsed dipolar spectroscopy. The photoexcited triplet state of a boron-dipyrromethene derivative, coupled with a nitroxide paramagnetic center in a rigid peptide model, is investigated as a suitable spin label for application in light-induced pulsed dipolar spectroscopies. The study introduces a novel approach by combining magnetic and optical orientation selection in Laser-Induced Magnetic Dipole spectroscopy to investigate model compounds containing a photo-switchable probe.