Metal-to-Ligand Charge-Transfer Emissions of Ruthenium(II) Pentaammine Complexes with Monodentate Aromatic Acceptor Ligands and Distortion Patterns of their Lowest Energy Triplet Excited States

Abstract

This is the first report of the 77 K triplet metal-to-ligand charge-transfer ((3)MLCT) emission spectra of pentaammine-MDA-ruthenium(II) ([Ru(NH3)5(MDA)](2+)) complexes, where MDA is a monodentate aromatic ligand. The emission spectra of these complexes and of the related trans-[Ru(NH3)4(MDA) (MDA')](2+) complexes are closely related, and their emission intensities are very weak. Density functional theory (DFT) calculations indicate that the energies of the lowest (3)MLCT excited states of Ru-MDA complexes are either similar to or lower than those of the lowest energy metal-centered excited states ((3)MC(X(Y))), that the barrier to internal conversion at 77 K is large compared to kBT, and that the (3)MC(X(Y)) excited states are weakly bound. The [Ru(NH3)5py](2+) complex is an exception to the general pattern: emission has been observed for the [Ru(ND3)5(d5-py)](2+) complex, but its lifetime is apparently very short. DFT modeling indicates that the excited state distortions of the different (3)MC excited states are very large and are in both Ru-ligand bonds along a single Cartesian axis for each different (3)MC excited state, nominally resulting in (3)MC(X(Y)), (3)MC((X)Y), and (3)MC(Z) lowest energy metal-centered states. The (3)MC(X(Y)) and (3)MC((X)Y) states appear to be the pseudo-Jahn-Teller distorted components of a (3)MC((XY)) state. The (3)MC(X(Y)) states are distorted up to 0.5 Å in each H3N-Ru-NH3 bond along a single Cartesian axis in the pentaammine and trans-tetraammine complexes, whereas the (3)MC(Z) states are found to be dissociative. DFT modeling of the (3)MLCT excited state of [Ru(NH3)5(py)](2+) indicates that the Ru center has a spin density of 1.24 at the (3)MLCT energy minimum and that the (3)MLCT → (3)MC(Z) crossing is smooth with a very small barrier (<0.5 kcal/mol) along the D3N-Ru-py distortion coordinate, implying strong (3)MLCT/(3)MC excited state configurational mixing. Furthermore, the DFT modeling indicates that the long-lived intermediate observed in earlier flash photolysis studies of [Ru(NH3)5py](2+) is a Ru(II)-(η(2)(C═C)-py) species.

Assigning Optical Absorption Transitions with Light-Induced Crystal Structures: Case Study of a Single-Crystal Nanooptomechanical Transducer

“…Indeed, the geometrically opposing Ru–N1 and Ru–N3 bonds display a pseudo-Jahn–Teller effect throughout their light-exposure time profile (Figure (top left)). Previous work on the closely related [Ru­(NH 3 ) 5 (pyridine)] 2+ ion reported a similar pseudo-Jahn–Teller feature in the equatorial plane of its Ru–N ammine bonds; this was attributed to a metal-centered (MC) charge transfer, and a substantial level of configurational mixing between MLCT and MC transitions was noted in this ion. Such mixing explained its lack of emissive characteristics.…”

“…It is therefore tempting to suggest that the newly resolved optical absorption peak is likely to be dominated by Ru–N ammine MC charge transfer. However, it should be borne in mind that the Ru–N pyridyl MLCT transition exhibits an intense band in the closely related [Ru­(NH 3 ) 5 (pyridine)] 2+ ion, while the intensities of its MC transitions depend largely on the extent of MLCT and MC configurational mixing …”

From curiosity to applications. A personal perspective on inorganic photochemistry

“…The longer values of τ were seen for those complexes for which the LEES are MLCT in character. Recently, Endicott and coworkers 9 detected very weak emissions ( Φ r ∼ 5 × 10 –5 ) from several of these complexes in 77 K organic glasses. Under these conditions, lifetimes were found to be on the 0.1–3 μs timescale, so the weak emissions were attributed to very low values of k r rather than to short lifetimes under these conditions.…”

“…In support of this view, a recent TDDFT calculation on the metal centered 3 MC* state of Ru(NH 3 ) 5 (py) 2+ confirms that this state is nearly dissociative in character. 9 …”

Synthesis, spectroscopic characterization, thermal behaviour, in vitro antimicrobial and anticancer activities of novel ruthenium tricarbonyl complexes containing monodentate V‐shaped Schiff bases

“…The UV–visible spectra of the ligands showed two bands in the regions 276–300 and 307–332 nm which were assigned to π–π* and n–π* transitions (Table ), respectively. Moreover, all the complexes showed a lowest energy absorption band, which may be ascribed to metal‐to‐ligand charge transfer …”