Fluolab【Angew.Chem.】调控金属间距0.2Å：精准控制双核铑配合物的近红外二区磷光发射
文章标题： Controlling Near-Infrared Fluorescence-to-Phosphorescence Ratios and Triplet Lifetimes in Rhodium(I) Dimers via Primary and Secondary Coordination Sphere Effects
通讯作者： Vanitha R. Naina, Oliver S. Wenger
文章概要
引言
在现代光物理学研究中，多核金属配合物因其独特的金属-金属相互作用而备受关注。相比于单核体系，多核配合物能够通过金属间的协同效应实现多电子转移和长波长发光。尽管铂(II)二聚体在这一领域已积累了大量研究，但关于双核铑(I)配合物的研究相对匮乏。特别是如何通过合成手段精准调控这类体系的系际穿越过程以及三重态激发态寿命，仍然是化学家面临的巨大挑战。本研究旨在通过设计新型的芳基异腈配体，探索配体环境对双核铑中心光物理性质的影响，力求为近红外二区发光材料的设计提供新的理论指导。
Molecular structures of (a) the homoleptic [Hom-Rh2]tBu and (b) the heteroleptic [Het-Rh2]tBu complexes investigated in this work. Molecular structures of (c) homoleptic [Hom-Rh2]Mes and (d) heteroleptic [Het-Rh2]tBu complexes in the solid state, along with RhI···RhI distances. The [Hom-Rh2]tBu complex could not be crystallized. We used the x-ray crystal structure of [Hom-Rh2]Mes as a proxy to estimate the RhI···RhI distance in [Hom-Rh2]tBu (see text for further information). Counter-anions, hydrogen atoms, and non-coordinating solvent molecules are omitted for clarity. Structural parameters are given in the Supporting Information.
主要实验及结论
研究团队首先合成并对比了均配型与异配型两种双核铑配合物。实验发现，由四个相同芳基异腈配体构成的均配配合物表现出显著的近红外二区（NIR-II）磷光发射，峰值位于900 nm附近，且磷光量子产率高达8.0%。相比之下，引入二膦配体的异配配合物则呈现出以近红外一区荧光为主、近红外二区磷光较弱的双重发射特征。通过超快瞬态吸收光谱分析，研究人员观察到两种配合物在系际穿越路径上存在显著差异。
(a) UV-Vis absorption spectrum of [Hom-Rh2]tBu. Inset: A picture of an NMR tube containing a solution of [Hom-Rh2]tBu. (b) UV-Vis absorption spectrum of [Het-Rh2]tBu. Inset: A picture of an NMR tube containing a solution of [Het-Rh2]tBu. (c) Steady-state photoluminescence spectrum of [Hom-Rh2]tBu showing phosphorescence from the T1 excited state. Inset: Phosphorescence decay of [Hom-Rh2]tBu after pulsed excitation at _λ_exc = 625 nm and detected at _λ_obs = 790 nm. (d) Steady-state photoluminescence spectrum of [Het-Rh2]tBu showing predominant fluorescence from the S1 excited-state, along with some weaker phosphorescence from the T1 state. Inset: Fluorescence decay (_λ_exc = 635 nm) of [Het-Rh2]tBu detected at _λ_obs = 725 nm obtained from time-correlated single-photon counting (TCSPC) measurements. The decay of the T1 excited-state is analyzed below in the nanosecond-transient absorption studies. All measurements were performed in deoxygenated CH2Cl2 at room temperature.
(a) Nanosecond transient UV–Vis absorption spectra at different delay times for [Hom-Rh2]tBu (laser pump pulses at 600 nm, 8 mJ/pulse, OD600 nm = 0.2, and integration time of 200 ns). Inset: decay of the excited-state absorption (ESA) at 750 nm. (b) Transient UV–Vis absorption spectra at different delay times for [Het-Rh2]tBu (laser pump pulses at 625 nm, 8 mJ/pulse, OD625 nm = 0.2, and integration time of 200 ns). Inset: decay of the ESA at 450 nm. All measurements were performed in deoxygenated CH2Cl2 at room temperature.
进一步的研究聚焦于异配体系的外围配体修饰。通过在芳基异腈的邻位或间位引入不同空间位阻的取代基，研究团队实现了对铑-铑金属间距在超过0.2 Å范围内的精准调节。研究结果表明，金属间距的缩短显著增强了双核中心的结构刚性，有效抑制了非辐射跃迁路径。这种结构刚化不仅延长了三重态激发态的寿命，还显著提升了磷光与荧光的比例。特别是当金属距离缩短至约3.00 Å时，配合物表现出更长的磷光寿命和更高的发射效率，有力地证明了二级配位球效应在调控激发态动力学中的重要作用。
Femtosecond transient absorption spectra at different delay times for [Hom-Rh2]tBu after excitation at 620 nm (OD620 nm = 0.2); (a) at shorter delay times; (b) at longer delay times. Additional spectra at extended delay times (up to 890 ps) are given in the Supporting Information. Arrows indicate changes in the spectral features over time. (c) Species-associated spectra (SAS1, SAS2, and SAS3) of the global fit of the ultrafast transient absorption spectroscopy data of [Hom-Rh2]tBu. (d) Schematic energy diagram of excited-state relaxation pathways for [Hom-Rh2]tBu obtained from the transient absorption studies. Measurements were performed in aerated CH2Cl2 at room temperature.
Molecular structures of a series of heteroleptic RhI complexes (with structural modification in the ligand periphery) in the solid state, along with their Rh···Rh distances. Counter-anions, hydrogen atoms and non-coordinating solvent molecules are omitted for clarity. The phenyl groups attached to the phosphine moiety of [Het-Rh2]tBu2 are colored white for visual clarity. Structural parameters are given in the Supporting Information.
Steady-state photophysical characterization of a series of heteroleptic RhI dimers ([Het-Rh2]tBu2, [Het-Rh2]tBu, [Het-Rh2]Me). (a) UV-Vis absorption spectra of the heteroleptic RhI dimers. Inset: Pictures of NMR tubes containing a solution of [Het-Rh2]tBu2 and [Het-Rh2]tBu. (b) Photoluminescence spectra of the heteroleptic RhI dimers, scaled relative to one another according to their respective _ϕ_fl. Inset: Plot depicting the sum of _ϕ_fl and _ϕ_ph of heteroleptic complexes and their ratios. All measurements were performed in deoxygenated CH2Cl2 at room temperature.
总结及展望
本研究通过精巧的分子设计，成功揭示了芳基异腈配体在增强双核铑配合物磷光性能方面的独特性。研究明确了通过调控配位几何结构（如从重叠构型转变为部分交错构型）以及缩短金属间距，可以显著提升近红外发光性能。这一发现不仅为开发新型生物成像和光疗所需的近红外二区探针开辟了新路径，也为光催化领域中调控单重态与三重态的反应活性提供了重要参考。未来，这种基于结构刚性调控光物理性质的策略有望被推广至更具成本优势的第一列过渡金属体系中。