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, 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, 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 T_n → 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 T_n, 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 T_n → 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.

Photoisomerization and thermal isomerization of ruthenium aqua complexes with chloro-substituted asymmetric bidentate ligands

A series of ruthenium complexes with chloro-substituted bidentate ligands, proximal-[Ru(tpy)(Cl-pyqu)L] n+ [n = 1 for L = Cl, and n = 2 for L = OH2, tpy = 2,2';6',2''-terpyridine, pyqu = 2-(2'-pyridyl)quinoline] were synthesized and their reversible photoisomerizations and thermal isomerizations were investigated experimentally. The crystal structures of the complexes indicated that introduction of a chloro substituent at the 4- or 4'-position of the pyqu ligand did not change the structure around the metal center from that of the non-substituted complex, proximal-[Ru(tpy)(pyqu)L] n+. In contrast, the 6'-substituted complexes had sterically hindered environments around the metal center. The ruthenium aqua complexes showed reversible photoisomerization between the proximal and distal isomers. The quantum yield for photoisomerization of the 6'-substituted ruthenium aqua complex was almost twice as large as those of the other derivatives. This is explained by weakening of the ligand field on the ruthenium center by introduction of a chloro substituent at the 6'-position. Thermal back isomerization from the distal isomer to the proximal one was observed for the 6'-substituted complex, but such reactions were not observed for the other derivatives. The steric hindrance in the 6'-substituted aqua complex enhanced both thermal isomerization and photoisomerization.

Generating Photonastic Work from Irradiated Dyes in Electrospun Nanofibrous Polymer Mats

For solar-driven macroscopic motions, we assert that there is a local heating that facilitates large-scale deformations in anisotropic morphologic materials caused by thermal gradients. This report specifically identifies the fate of heat generation in photonastic materials and demonstrates how heat can perform work following excitation of a nonisomerizing dye. Utilizing the electrospinning technique, we have created a series of anisotropic nanofibrous polymer mats that comprise nonisomerizing dyes. Polymers are chosen because of their relative glass transition temperatures, elastic moduli, and melting temperatures. Light irradiation of these polymer mats with an excitation wavelength matching the absorption characteristics of the dye leads to macroscopic deformation of the mat. Analysis of still images extracted from digital videos provides plots of angular displacement vs power. The data were analyzed in terms of a photothermal model. Analyses of scanning electron microscopy micrographs for all samples are consistent to local melting in low T_g polymers and softening in high T_g polymers. Dynamic mechanical analysis allowed for quantification of the modulus change under a given light fluence. We employ these data to calculate a energy conversion efficiency. These efficiencies for the polymer mats are compared to other nonmuscular systems, including a few natural, biological samples.