Mechanism of Ligand Photodissociation in Photoactivable [Ru(bpy) 2 L 2 ] 2+ Complexes: A Density Functional Theory Study Luca Salassa, Claudio Garino, Giovanni Salassa, Roberto Gobetto,* and Carlo Nervi Dipartimento di Chimica I.F.M., UniVersita ` di Torino, Via P. Giuria 7, I-10125 Torino, Italy Received April 9, 2008; E-mail: roberto.gobetto@unito.it Abstract: 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 γ-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 f bpy metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum and a corresponding emitting triplet 3 MLCT state (Ru f bpy). 1-4 have three singlet metal- centered (MC) states 0.4 eV above the major 1 MLCT 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 3 MC state respon- sible 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. Introduction Photodissociation of ligands from metal complexes has been extensively studied in a variety of derivatives for synthetic reasons 1,2 since the 1980s. 3,4 Photoreactions are characterized by loss of a monodentate ligand and coordination of a solvent molecule. In nonpolar solvents, the solvent molecule is replaced by coordination of counterions, added ions, or residual water. Photodissociation of a second monodentate ligand is generally more difficult. Recently, new interest has grown around transition metal complexes that are able to release one of the coordinated ligands when excited with a specific wavelength of light. 5–9 Such attention is motivated by two different but equally appealing applications: first, the light activation of a specific and desired interaction between the metal complex and its biological target, 10–12 and second, the controlled delivery of small, biologically active, organic or inorganic molecules. 6–9,13 In principle, phototriggering allows spatial control of activated species, which may reduce the negative side effects of metals in certain tissues. 14 Tissue damage may be further reduced by using visible light excitation instead of UV light. 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