Effect of the electronic structure on the robustness of ruthenium(II) bis-phenanthroline compounds for photodissociation of co-ligand: Synthesis, structural characterization, and density functional theory study

Abstract

Distorted octahedral ruthenium(II) bis-phenanthroline compounds of the type cis-[Ru(phen) sub(2)(L) sub(2)](PF sub(6)) sub(2) (L = isoquinoline 1; phthalazine 2) were synthesized and their photochemistry investigated. The photodissociation of the monodentate N-heterocyclic co-ligands upon blue light irradiation (Lambda sub(irr) = 470 nm) occurs much more readily in 1 than in 2 with the first ligand dissociating faster than the second ligand. The density functional theory (DFT) calculations were performed to investigate geometries of the sup(3) MLCT, sup(3) TS, dissociative sup(3) MC states and the triplet potential energy surfaces. The DFT and time-dependent DFT (TD-DFT) calculations reveal smaller sup(3) MLCT- sup(3) MC energy gap for 1 (0.025 eV) than in 2 (0.090 eV) suggesting the faster dissociation of the first co-ligand in both 1 and 2. On the other hand, the sup(3) MLCT- sup(3) MC energy gap is moderately larger for the monochloro-substituted photoproducts 1a (0.389 eV) and 2a (0.396 eV), leading to conclusion that the second co-ligand dissociation is slower in both 1a and 2a. This is also in agreements with the observed values of quantum yields. Molecular orbital analysis along the triplet potential energy surface scan for Ru-N(co-ligand) bond stretch suggest that the lower energy singly occupied molecular orbital (SOMO sub(1)) of sup(3) MLCT, comprised of a d pi orbital of Ru with little contribution of pi orbital of the co-ligand, does not change much when Ru-N bond stretches to reach sup(3) MC. The higher-energy SOMO sub(2) of sup(3) MLCT state is mostly phenanthroline based pi* orbital. Upon stretching Ru-N distance, pi* orbital of phenanthroline in the sup(3) MLCT state and d sigma* orbital of the dissociative sup(3) MC state are seen to be mixing in an antibonding fashion especially after transition state (sup(3) TS). This mixing of d sigma* orbital with the pi* orbital is one of the important reasons favoring the release of N-heterocyclic co-ligand to give photodissociation products.