Theoretical study of new nitrosyl ruthenium complexes : mechanisms of photoisomerization and photorelease of NO

Over the last few decades, metal-nitrosyl complexes have gained an ever-growing interest among the pharmaceutical, chemical and material-science communities. This interest arises from their unique physicochemical properties, namely their response to light perturbation. Upon light irradiation, these compounds are able to release the nitric oxide radical, a signaling molecule in the vascular and other important physiological systems. It comes as no surprise that molecules with such properties have drawn the attention of the medical community for its potential use in photodynamic therapy treatment of several diseases such as cancer. This liability of nitric oxide can also be controlled with purely chemical redox reactions, with no electromagnetic perturbations. Reduction of the metal-nitrosyl moiety may trigger the cleavage of NO. Indeed, molecules that show charge transfer bands from a ligand to the metal-nitrosyl moiety in their UV-Vis absorption spectra afford photorelease quantum yields orders of magnitude larger than those who do not. This charge transfer may be considered as a M-NO reduction. Another important property shown by these metal-nitrosyl complexes is their extraordinary photochromic response to electromagnetic irradiation. In solid crystals, the changing color is due to a rearrangement of the NO ligand, going back and forth from the nitrosyl (N-bound) to the isonitrosyl (O-bound) forms. With the appropriate wavelength, the direction of the photoinduced linkage isomerization (forward and backwards) can be controlled. This feature is very appealing for the design of new high-capacity optical storage devices. One of the main goals of this PhD is to unravel the photochemical mechanisms behind both the photoisomerization and the photorelease phenomena of ruthenium-nitrosyl complexes. In order to shed some light into these processes, a full characterization of the electronic structures and potential energy surfaces of the ground and lowest excited states is required. Density Functional Theory calculations have proven to be suitable for the rationalization of the full photoinduced linkage isomerization mechanism of the trans-[RuCl(NO)(py)4]2+ molecule, a complex that yields one of the highest photoconversion rates (ca. 100%) observed among this family of complexes. The full characterization of the singlet ground state and of the lowest triplet excited state, as well as the identification of multiple crossings, allowed the establishment of the sequential two-photon absorption mechanism, involving a sideways-bonded metastable state. This predicted mechanistic picture has been confirmed by very recent experimental data. The proposed mechanism of the backwards reaction triggered by infrared or red irradiation is also consistent with the experimental data. A similar computational approach has been followed in the study of the nitrosyl photorelease. In solution, the trans-[RuCl(NO)(py)4]2+ complex also shows photodissociation of NO, but in this case with low quantum yields. Assuming that photoisomerization and photorelease are competitive processes, both mechanisms have been studied for an ensemble of complexes in an attempt to rationalize which are the molecular features that would favor one mechanism over the other. Analyzing the results, it becomes clear that both processes are triggered by the same irradiation wavelengths, and that photoisomerization is a required process in the sequential photorelease of NO in ruthenium-nitrosyl complexes. Introducing appropriate ligands in the complex can enhance the photoinduced charge transfer to the Ru-NO, which favors the photorelease of NO in solution.