Spectroscopic and computational studies of a Ru(II) terpyridine complex: The importance of weak intermodular forces to photophysical properties

The complex [Ru(tpy)(CO)₂TFA]⁺[PF₆] ^- (where tpy = 2,2′:6′,2″-terpyridine and TFA = CF₃CO₂^-) (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 intermoleeular 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)₂TFA] ⁺[PF₆]^- 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 × 10^-3 cm^-1). When the concentration of the complex is increased, an unstructured narrow emission at 603 nm (16.6 × 10^-3 cm ^-1), with a lifetime of 10 μs, dominates the emission spectrum in glassy acetonitrile. This emission originates from a π-π 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 intermoleeular metal (d) → ligand (π*, polypyridine) charge transfer occurs. A good estimate of the transition energy is also obtained (623 nm, 1.94 eV).