title: Spectroscopic and computational studies of a Ru(II) terpyridine complex: the importance of weak intermolecular forces to photophysical properties.
abstract: The complex [Ru(tpy)(CO)(2)TFA]+[PF(6)]- (where tpy = 2,2':6',2' '-terpyridine and TFA = CF(3)CO(2)-) (1) has been synthesized and fully characterized spectroscopically. The X-ray structure of the complex has been determined. The photopysical properties of the ruthenium complex and the free ligand tpy have been investigated at room temperature and at 77 K in acetonitrile solution and in the solid state. Their electronic spectra are highly influenced by intermolecular stacking interactions, both in solution and in the solid state. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed to characterize the electronic structure and the excited states of [Ru(tpy)(CO)(2)TFA]+[PF(6)]- and tpy. TDDFT calculations on three different conformations of free ligand have been performed as well. Absorption and emission spectra of tpy have been studied at different temperatures and concentrations in order to have a better understanding of this ruthenium derivative's properties. The absorption spectrum of 1 is characterized by metal-perturbed ligand-centered (LC) bands in the UV region. No metal-to-ligand charge transfer (MLCT) bands are observed in the visible for the complex. Only at high concentrations (10(-4) M) does a very weak band appear at 470 nm. At 77 K and low concentrations, solutions of 1 exhibit a major 3LC emission band centered at 468 nm (21.4 x 10(-3) cm(-1)). When the concentration of the complex is increased, an unstructured narrow emission at 603 nm (16.6 x 10(-3) cm(-1)), with a lifetime of 10 micros, dominates the emission spectrum in glassy acetonitrile. This emission originates from a pi-pi stacked dimeric (or oligomeric) species. TDDFT calculations performed on a tail-to-tail dimer structure, similar to that seen in the solid state, ascribe the transition to a triplet excited state, where intermolecular metal (d) --> ligand (pi*, polypyridine) charge transfer occurs. A good estimate of the transition energy is also obtained (623 nm, 1.94 eV).
authors.affiliation: Dipartimento di Chimica IFM, Università di Torino, Via P. Giuria 7, I-10125 Torino, Italy.
authors.forename: Claudio
authors.initials: C
authors.lastname: Garino
authors.forename: Roberto
authors.initials: R
authors.lastname: Gobetto
authors.forename: Carlo
authors.initials: C
authors.lastname: Nervi
authors.forename: Luca
authors.initials: L
authors.lastname: Salassa
authors.forename: Edward
authors.initials: E
authors.lastname: Rosenberg
authors.forename: J B Alexander
authors.initials: JB
authors.lastname: Ross
authors.forename: Xi
authors.initials: X
authors.lastname: Chu
authors.forename: Kenneth I
authors.initials: KI
authors.lastname: Hardcastle
authors.forename: Cristiana
authors.initials: C
authors.lastname: Sabatini
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D002621:Chemistry / Q000379:methods; D002627:Chemistry, Physical / Q000379:methods*; D003460:Crystallization; D018360:Crystallography, X-Ray / Q000379:methods; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D008024:Ligands; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D008670:Metals / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D010777:Photochemistry / Q000379:methods*; D011725:Pyridines / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry*; D012984:Software; D013053:Spectrophotometry / Q000379:methods*; D013696:Temperature
chemical_list: D000097:Acetonitriles; D008024:Ligands; D008670:Metals; D011725:Pyridines; D012428:Ruthenium; C032159:acetonitrile