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

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.

Probing Interligand Electron Transfer in the 1MLCT S1 Excited State of trans-Mo2L2L'2 Compounds: A Comparative Study of Auxiliary Ligands and Solvents.

The interligand charge dynamics of the lowest singlet metal-to-ligand charge-transfer states (1MLCT S1 states) of a series of quadruply bonded trans-Mo2(NN)2(O2C-X)2 paddlewheel compounds are investigated, where NN is a π-accepting phenylpropiolamidinate ligand and O2C-X (X = Me, tBu, TiPB, or CF3) is an auxiliary carboxylate ligand. The compounds show strong light absorption in the visible region due to MLCT transitions from the Mo2 center to the NN ligands. The transferred electron density was followed by femtosecond time-resolved infrared (fs-TRIR) spectroscopy with vibrational reporters such as the ethynyl groups on the NN ligands. The observed fs-TRIR spectra show that these compounds have asymmetric 1MLCT S1 excited states where the transferred electron mainly resides on a single NN ligand. The presence of interligand electron transfer (ILET) is suggested to explain the shape of the ν(C≡C) bands and the influence of auxiliary ligands and solvents on the interligand electronic coupling. The ILET in the 1MLCT S1 state is shown to be sensitive to the functional groups on the auxiliary ligands while being less responsive to changes in solvents.

Electronic and spectroscopic properties of avobenzone derivatives attached to ditungsten quadruple bonds.

From the reactions between W2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of acid, 4-formylbenzoic acid, HBzald, 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo, or 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, three new compounds W2(T(i)PB)2(Bzald)2, I, W2(T(i)PB)2(Avo)2, II, and W2(T(i)PB)2(AvoBF2)2, III, have been prepared. As solid compounds I and II are blue while compound III is green. Characterization of these compounds has been carried out by means of (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond transient absorption and time-resolved infrared spectroscopies. Compounds I and II have strong metal to ligand charge transfer, MLCT, transitions in the visible region of their spectra while compound III exhibits MLCT absorption in the near-infrared (λmax = 1017 nm). All three have S1 states that have corresponding lifetimes of ∼3-30 ps and are (1)MLCT in character. The triplet states are (3)MLCT with lifetimes in the range 3-10 ns. Density functional theory and time-dependent density functional theory were employed to perform electronic structure calculations in order to aid in the interpretation of these data. The spectroscopic properties of I and II are similar while the planarity of the ligand in III greatly lowers the energy of the MLCT state. The W2 unit enables direct observation of intersystem crossing from the (1)MLCT state to (3)MLCT state via the use of ultrafast spectroscopy.

Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo2 Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation.

From the reactions between Mo2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, the compounds Mo2(T(i)PB)2(Bzald)2, I; Mo2(T(i)PB)2(Avo)2, II; and Mo2(T(i)PB)2(AvoBF2)2, III, have been isolated. Compounds I and II are red, and compound III is blue. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S1 states having lifetimes ∼5-15 ps. The triplet states are Mo2δδ* with lifetimes in the microseconds. The spectroscopic properties of I and II are similar, whereas the planarity of the ligand in III greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo2 unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety.