Mechanism of ligand photodissociation in photoactivable [Ru(bpy)2L2]2+ complexes:: A density functional theory study

A series of four photodissociable Ru polypyridyl complexes of general formula [Ru(bPY)(2)L-2](2+), where bpy = 2,2'-bipyridine and,L = 4-aminopyridine (1), pyridine (2), butylamine (3), and gamma-aminobutyric acid (4), was studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). DFT calculations (B3LYP/LanL2DZ) were able to predict and elucidate singlet and triplet excited-state properties of 1-4 and describe the photodissociation mechanism of one monodentate ligand. All derivatives display a Ru -> bpy metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum and a corresponding emitting triplet (MLCT)-M-3 state (Ru -> bpy). 1-4 have three singlet metal-centered (MC) states 0.4 eV above the major (MLCT)-M-1 states. The energy gap between the MC states and lower-energy MLCT states is significantly diminished by intersystem crossing and consequent triplet formation. Relaxed potential energy surface scans along the Ru-L stretching coordinate were performed on singlet and triplet excited states for all derivatives employing DFT and TDDFT. Excited-state evolution along the reaction coordinate allowed identification and characterization of the triplet state responsible for the photodissociation process in 1-4; moreover, calculation showed that no singlet state is able to cause dissociation of monodentate ligands. Two antibonding MC orbitals contribute to the (MC)-M-3 state responsible for the release of one of the two monodentate ligands in each complex. Comparison of theoretical triplet excited-state energy diagrams from TDDFT and unrestricted Kohn-Sham data reveals the experimental photodissociation yields as well as other structural and spectroscopic features.