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), −CO 2 Et (5), and −NO 2 (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 timeresolved 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, S n , that rapidly intersystem crosses into a high-lying triplet state, T n . Over the course of 7−22 ps, T n relaxes to the lowest lying triplet state, T 1 , which is metal/ligand-to-ligand charge transfer, 3 Ru(d)/NHC(π) → bpy(π*) in character. These 3 ML-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 CH 3 CN. Both the excited-state lifetime at 77 K and the T n → T 1 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 T n , as only T 1 is observed. In the case of 6, excitation is expected to populate a 1 Ru(d)/NHC(π) → NHC(π*) state localized on the NHC-NO 2 ligand, which decays to a higher energy 3 Ru(d)/NHC(π) → NHC(π*) state followed by internal conversion to the 3 Ru(d)/NHC(π) → bpy(π*) T 1 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 T 1 in the picosecond time range. The relatively slow T n → T 1 internal conversion in these complexes makes the population of the higherenergy 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.

Photosensitive Ru(II) Complexes as Inhibitors of the Major Human Drug Metabolizing Enzyme CYP3A4

“…Complexes 12 and 13 were purchased (Strem Chemicals). Complexes 14 , 15 , 16 , 17 , 18 , 19 , 20 – 22 , 23 and 24 , 25 , and 26 were prepared following literature protocols. For synthesis of 7 – 11 , a solution of [Ru­(tpy)­(L 1 )­Cl]Cl in EtOH was treated with pyridine.…”

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

“…1b ). Although the bonding parameters of these complexes are within the typical bonding parameter ranges, 18 a close scrutiny of acyclic carbene ligands in these complexes reveals that Ru–C(carbene) of 3 is slightly shorter (2.01 Å for 3 vs. 2.03–2.04 Å for 1 and 2 ) with a larger N–C–N angle (116.6° for 3 vs. 111.2–111.5° for 1 and 2 ). Besides, the positions of N-deprotonation in the diaminocarbene ligands are different.…”

Slow 3MLCT Formation Prior to Isomerization in Ruthenium Carbene Sulfoxide Complexes

“…41 Lastly, we see two reversible couples representing bpy 0/− and bpy −/2− at −1.65 V and approximately −1.8 V in accord with other ruthenium polypyridine complexes, 40 and those comprising an NHC ligand. 29 Our analysis of the voltammograms reveals the S → O isomerization rate constants on Ru 3+ and O → S isomerization rate constants on Ru 2+ . The S → O rate constants of isomerization on Ru 3+ are 0.18 ± 0.01 s −1 (RuOTE3), 0.89 ± 0.01 s −1 (RuOTE4), and 0.86 ± 0.01 s −1 (RuOTE9).…”

“…Kender and Turro have also observed slow internal conversion kinetics in their study of related ruthenium polypyridine NHC complexes. 29 They proposed the presence of two distinct triplet excited states, the lower energy of which is assigned to metal/ligand-to-ligand charge transfer (ML-LCT). Indeed, for a range of five complexes with different groups located on the chelating NHC ligand, internal conversion from the higher-lying triplet to the 3 ML-LCT takes place with time constants ranging from 7 to 22 ps, which are similar in magnitude to our τ 2 values reported here.…”