Bukhanko V. Two-photon induced photorelease of nitric oxide from Ruthenium-nitrosyl complexes containing ligands with various push-pull capabilities

The work “Two-photon induced photorelease of nitric oxide from ruthenium-nitrosyl complexes containing ligands with various push-pull capabilities” was devoted to the revealing of main factors that determine the efficiency of ruthenium-nitrosyl complexes in two-photon absorption (TPA) and to the improvement of TPA cross-section through modification of the ligands nature and their structure. Starting from complexes previously obtained by the research group of Prof. I. Malfant we have modified their structure in several directions. It was shown that substitution of monodentate ligands by 2,2'-bipyridine in Ruthenium complexes with nitrosyl-ligand leads to a slight increase of their TPA cross-section, however this modification decreases the quantum yield of the nitric oxide photorelease. Synthetic part of the work was mainly aimed at the modification of the ligand 4'-(9H-fluoren-2-yl)-2,2':6',2''-terpyridine. The elongation of conjugation path could improve the polarizability of the ligand and the “push-pull” character of the complex. This subsequently results in an increased efficiency of the compounds in TPA that was confirmed in the current work. Insertion of double bond between two parts of the ligand leads to the enhancement of the TPA cross-section by 20%, whereas the triple bond appears better linker leading to 36% improved efficiency of complexes in TPA. Another parameter of compounds, sensitive to the structural modifications, is the difference in dipole moments between the ground and excited states of the molecules. It deserves particular attention whereas it enters the expression of the TPA efficiency as the criterion of “push-pull” character of the molecule. With the purpose of increasing this difference we put as a goal the synthesis of the complexes with ligands bearing donating dimethylamino-groups. Having optimized the methodology of ligands syntheses, we, however, found out that usual way of complex synthesis in this case has not resulted in the desired complex obtention. During the second step of the ruthenium-nitrosyl compound synthesis an incorporated dimethylamino group undergoes the oxidation process with its subsequent demethylation. However, having obtained a series of easier accessible complexes with groups displaying different donating abilities, we were able to discover some tendency. These complexes contain phenyl rings, carrying functional groups starting from accepting nitro- to donating metoxy- and dimethylamino-groups. It was shown that while accepting or neutral groups at didn’t show any significant influence on the TPA cross-sections of complexes, donating moieties increased it with the highest value of 154 GM observed for complex with 4'-(4-methoxyphenyl)-2,2':6',2''-terpyridine ligand. Thus the observation has confirmed the initial hypothesis that increasing of the difference in dipole moments between the ground and excited state would lead to an enhanced efficiency of complexes in TPA. In the current work theoretical calculations on the basis of density functional theory (DFT) are widely involved. Apart from studying the ability of complexes to release NO under irradiation a theoretical investigation of their nonlinear optical properties was made. In order to improve the conjugation and the length of the π-conjugation system we have assessed the effect of the fluorenyl-fragment substitution by 22 different heterocyclic analogues having similar tricyclic skeleton and containing donating Nitrogen atom in the system. It was shown, that in spite of the fact that such modification doesn’t lead to a desired improvement, such compounds could be indeed interesting as nonlinear optics materials. The structure of the photoproduct and Ruthenium oxidation states are discussed in details. In contrast to photoproducts previously observed, complexes having five pyridine rings in the coordination sphere of metal form Ru(II) products during the process of NO-photorelease, whereas the formation of NO-radical is confirmed by EPR-spectroscopy.