Spectroscopic and Photophysical Properties of the Trioxatriangulenium Carbocation and its Interactions with Supramolecular Systems

Trioxatriangulenium (TOTA⁺, 4,8,12-trioxa-4,8,12,12c-tetrahydrodibenzo[cd,mn]-pyrenylium) is a closed shell carbocation which is stable in its crystalline form and in polar solvents at ambient temperatures. Raman and infrared (IR) spectra of TOTA⁺ are reported. In a theoretical effort, ab initio and density function theory (DFT) methods are used to determine the vibrational spectra and the optimised geometry of TOTA⁺. The theoretical results are compared with the experimentally obtained vibrational spectra and with the previously reported X-ray crystal structure. It is concluded that DFT calculations based on the nonlocal exchange-correlation functionals (UB-LYP) yield the most accurate description of the vibrational spectra and the geometry of TOTA⁺. As a result of the calculations it appears that the positive charge in the TOTA⁺ carbocation is substantially delocalised. The physical and chemical properties of the excited singlet and triplet states of TOTA⁺ are investigated. A strong fluorescence is observed at 520 nm, which is readily quenched by 10 aromatics. The quenching rate increases with decreasing oxidation potential of the aromatic molecules signifying a charge transfer mechanism, which falls within the normal region of the Marcus theory. The Stern-Volmer plots derived from steady state quenching measurements all curve upwards indicating partial static quenching due to complex formation between the ground state of TOTA⁺ and the electron donor. The charge transfer complexes are also observed directly by means of electron absorption spectroscopy. The TOTA⁺ triplet – triplet absorption and phosphorescence spectra are reported. The interaction between TOTA⁺ and the DNA double helix is investigated. The mode of binding is determined as intercalation, i.e. the discus shaped carbocation is wedged between the base pairs. Various techniques are employed in order to determine the binding mode such as viscosity, melting point and circular dichroism experiments. According to the viscosity measurements extraordinary small lengthening of the DNA fragments (1.3 Å) is observed when intercalation takes place. TOTA⁺ is a photonuclease, the scission taking place at a GG step in the DNA string. DNO dimers substituted with pyrene as photophysical probes are synthesised. The excimer formation and ground state associations are investigated by means of fluorescence and absorption spectroscopic techniques. A significant difference is observed of the photophysical parameters between the different derivatives, which reflects the structural properties of the DNOs.