Unusually Slow Internal Conversion in N-Heterocyclic Carbene/Carbanion Cyclometallated Ru(II) Complexes: A Hammett Relationship.

Abstract

A series of six [Ru(bpy)2(NHC-R)]+ complexes were synthesized and characterized, where bpy = 2,2'-bipyridine and NHC-R is an N-heterocyclic carbene covalently linked to a carbanion with a number of substituents, R = -OMe (1), -Me (2), -H (3), -Cl (4), -CO2Et (5), and -NO2 (6). The effects of these strongly σ-donating NHC-R ligands on the ground-state electronic structure and on the excited-state character and dynamics were probed using electrochemistry, TD-DFT calculations, and steady-state absorption and emission spectroscopies, along with ultrafast transient absorption and time-resolved IR measurements. The excitation of 1-5 with a 400 nm pulse (irf = 85 fs) results in the population of a high energy singlet state, Sn, that rapidly intersystem crosses into a high-lying triplet state, Tn. Over the course of 7-22 ps, Tn relaxes to the lowest lying triplet state, T1, which is metal/ligand-to-ligand charge transfer, 3Ru(d)/NHC(π) → bpy(π*) in character. These 3ML-LCT states decay to regenerate the ground state with lifetimes, τ, that range from <8 to 15 ns at 298 K and from 10 to 23 ns at 77 K in CH3CN. Both the excited-state lifetime at 77 K and the Tn → T1 rate of internal conversion of 1-5 are dependent on the substituent R, and the latter correlates with the Hammett parameter (σ+p) of the NHC-R ligand. Excitation of 1-5 with low energy light, 550-670 nm, does not result in the population of Tn, as only T1 is observed. In the case of 6, excitation is expected to populate a 1Ru(d)/NHC(π) → NHC(π*) state localized on the NHC-NO2 ligand, which decays to a higher energy 3Ru(d)/NHC(π) → NHC(π*) state followed by internal conversion to the 3Ru(d)/NHC(π) → bpy(π*) T1 state with τ = 250 ps; the population of both states is independent of excitation wavelength in 6. This work demonstrates that the introduction of one NHC-R ligand in these complexes permits the population of a higher energy triplet state that decays to T1 in the picosecond time range. The relatively slow Tn → T1 internal conversion in these complexes makes the population of the higher-energy state potentially useful for more efficient charge injection into semiconductors for solar energy conversion or to aid in accessing dissociative metal-centered states for drug delivery. Overall, this work shows the ability to synthetically access valuable excited-state dynamics using the two different Ru-C bonds of the asymmetric NHC-R ligands.