Unusually Slow Internal Conversion in N-Heterocyclic Carbene/Carbanion Cyclometallated Ru(II) Complexes: A Hammett Relationship
Unusually Slow Internal Conversion in N-Heterocyclic Carbene/Carbanion Cyclometallated Ru(II) Complexes: A Hammett Relationship
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N-杂环卡宾/碳负离子环金属化 Ru(II) 配合物中异常缓慢的内转化：哈米特关系
DOI：
10.1021/acs.jpca.9b00858
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Turro, Claudia
中科院分区：
文献类型：
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作者：
Kender, William T.;Turro, Claudia
合成并表征了一系列六个[Ru（bpy）2（NHC-R）]+配合物，其中bpy = 2，2 ′-联吡啶，NHC-R是一个与碳负离子共价连接的N-杂环卡宾，R = −OMe（1），−Me（2），−H（3），−Cl（4），− CO2 Et（5），−NO2（6）.利用电化学、TD-DFT计算、稳态吸收光谱和发射光谱、沿着超快瞬态吸收光谱和时间分辨红外光谱研究了这些强σ-供体NHC-R配体对基态电子结构、激发态特征和动力学的影响。用400 nm的脉冲（irf = 85 fs）激发1 - 5导致高能量单重态Sn的布居，Sn迅速地在系统间跨越成高位置的三重态Tn。在7-22 ps的时间内，Tn弛豫到最低三重态T1，这是金属/配体间的电荷转移，其特征为3Ru（d）/NHC（π）→ bpy（π*）。这些3 ML-LCT态在CH 3CN中衰变以再生基态，其寿命τ在298 K时为<8至15 ns，在77 K时为10至23 ns。77 K激发态寿命和1 - 5的内转换速率Tn→ T1都与取代基R有关，后者与NHC-R配体的Hammett参数（σ+p）有关。用550-670 nm的低能光激发1 - 5，不会导致Tn的布居，因为只观察到T1。在6的情况下，预期激发将占据位于NHC-NO2配体上的1 Ru（d）/NHC（π）→ NHC（π*）态，该态衰变为更高能量的3Ru（d）/NHC（π）→ NHC（π*）态，随后内部转换为3Ru（d）/NHC（π）→ bpy（π*）T1态，τ = 250 ps;两个态的布居与6中的激发波长无关。这项工作表明，在这些配合物中引入一个NHC-R配体允许人口的更高能量的三重态，在皮秒的时间范围内衰减到T1。在这些复合物中相对缓慢的Tn→ T1内部转换使得较高能量状态的人口可能用于更有效地将电荷注入半导体用于太阳能转换或帮助进入解离的金属中心状态用于药物递送。总的来说，这项工作显示了使用不对称NHC-R配体的两个不同的Ru-C键来综合访问有价值的激发态动力学的能力。
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 of1–5with 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, Tnrelaxes to the lowest lying triplet state, T1, which is metal/ligand-to-ligand charge transfer,3Ru(d)/NHC(π) → bpy(π*) in character. These3ML-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→ T1rate of internal conversion of1–5are dependent on the substituent R, and the latter correlates with the Hammett parameter (σ+p) of the NHC-R ligand. Excitation of1–5with low energy light, 550–670 nm, does not result in the population of Tn, as only T1is observed. In the case of6, excitation is expected to populate a1Ru(d)/NHC(π) → NHC(π*) state localized on the NHC-NO2ligand, which decays to a higher energy3Ru(d)/NHC(π) → NHC(π*) state followed by internal conversion to the3Ru(d)/NHC(π) → bpy(π*) T1state with τ = 250 ps; the population of both states is independent of excitation wavelength in6. 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 T1in the picosecond time range. The relatively slow Tn→ T1internal 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.