Caratterizzazione di stati fotoeccitati di tripletto di cromofori organici interagenti mediante spettroscopia EPR risolta nel tempo

The aim of the thesis has been the characterization of the photoexcited paramagnetic states of two classes of organic molecules by means of time-resolved EPR spectroscopy (TR-EPR) coupled to photoexcitation with pulsed laser. More specifically, the investigation involved a derivative of helicene and four derivatives of p-azaquinodimethane (pAQM). TR-EPR experiments allow to characterize transient paramagnetic species both in terms of the relevant magnetic interactions responsible for the position and intensity of EPR transitions and the non-equilibrium electron spin polarization (ESP) of these species by simulation of the EPR spectrum. ESP derives from the selective population of the spin sublevels due to the presence of symmetry constraints or conservation of physical quantities in the mechanism of formation of these paramagnetic states and can be described by a non-Boltzmann distribution. ESP can be exploited in the magnetophotoselection experiment, in which TR-EPR is performed with photoexcitation with linearly polarized light for in order to derive the orientation of the optical transition dipole moment (TDM) in the triplet frame. This technique is based on exploitation of the dependence of the shape of the EPR spectrum, which depends on the ESP, upon the relative orientation between the external magnetic field and the electric field of the radiation. In the first part of the project, this approach has been used to investigate the excitonic interaction in a chiral porphyrin dimer characterized by an helicene motif as a linker. This molecule belongs to a family of compounds (helicene-porphyrin conjugates) that display enhanced circular dichroism in the visible region thanks to the synergic effect of the excitonic coupling between the porphyrins and the chiral environment due to the bridge and have been recently investigated due to their intriguing spin-filtering properties. Optimization of the chiroptical properties in terms of molecular geometry and functionalization of the porphyrins-helicenes derivatives requires an accurate knowledge of the orientation of the main excitonic TDMs, which has been achieved by spectral simulation and is in satisfactory agreement with computational results. Lastly, TR-EPR experiments with circularly polarized light have also been performed with the aim to explore whether the circular dichroism displayed by these molecules was associated with an effect on the ESP of their triplet states. ESP is also strongly dependent upon the mechanism of formation of the triplet state, and simulation of the TR-EPR spectrum allows to deduce this mechanism unambiguously. In the second part of the project, this property has been exploited to prove that the triplet state of the pAQM derivatives, which differed in their peripherical electron donating groups, is populated via singlet fission, in order to find an alternative to acene derivatives and carotenoids as chromophores associated with efficient triplet formation through singlet fission. In this photophysical process, an excited singlet state is converted in two localized triplet states via an intermediate with singlet character and is observed when a strong interaction between chromophores can occur such as in crystals, aggregates or films. This process provides an important pathway for high yield triplet formation (up to 200%) to be exploited for efficient charge separation in photovoltaic applications or for photosensitizing singlet oxygen and radical oxygen species in photodynamic therapy. The investigation on the pAQM derivatives has confirmed that for all compounds the singlet fission process is active in films, since for three of the four pAQM derivatives a quintet state has been detected, while for one molecule uncorrelated triplets deriving from the quintet state after separation of the interacting triplets have been observed.