Exploring the Photophysics and Photocatalytic Activity of Heteroleptic Rh(III) Transition-Metal Complexes Using High-Throughput Experimentation

High-throughput synthesis and screening (HTSS) methods
were used
to investigate the photophysical properties of 576 heteroleptic Rh(III)
transition-metal complexes through measurement of the UV–visible
absorption spectra, deaerated excited-state lifetime, and phosphorescent
emission spectra. While 4d transition-metal photophysics are often
highly influenced by deleterious metal-centered deactivation channels,
the HTSS of structurally diverse cyclometalating and ancillary ligands
attached to the metal center facilitated the discovery of photoactive
complexes exhibiting long-lived charge-transfer phosphorescence (0.15–0.95
μs) spanning a substantial portion of the visible region (546–620
nm) at room temperature. Further photophysical and electrochemical
investigations were then carried out on select complexes with favorable
photophysics to understand the underlying features controlling these
superior properties. Heteroleptic Ir(III) complexes with identical
ligand morphology were also synthesized to compare these features
to this family of well understood chromophores. A number of these
Rh(III) complexes contained the requisite properties for photocatalytic
activity and were consequently tested as photocatalysts (PCs) in a
water reduction system using a Pd water reduction cocatalyst. Under
certain conditions, the activity of the Rh(III) PC actually surpassed
that of the Ir(III) PC, uncovering the potential of this often-overlooked
class of transition metals as both efficient photoactive chromophores
and PCs.

本研究采用高通量合成与筛选（High-throughput synthesis and screening, HTSS）方法，通过测试紫外-可见吸收光谱、除氧激发态寿命与磷光发射光谱，对576个异配体铑(III)过渡金属配合物的光物理性质展开了系统探究。尽管4d过渡金属的光物理过程通常极易受到有害的金属中心失活通道影响，但通过对配位在金属中心的结构多样化环金属配体与辅助配体开展高通量合成与筛选，本研究成功发现了具备长寿命电荷转移磷光特性的光活性配合物：这类配合物在室温下的磷光寿命覆盖0.15~0.95微秒，发光波段跨越可见光区的546~620纳米范围。随后，研究人员针对一批具备优异光物理性能的目标配合物开展了进一步的光物理与电化学表征，以揭示调控这类优异性能的内在机制。同时，研究人员合成了配体结构一致的异配体铱(III)配合物，将其光物理特性与这类已被充分认知的发色团家族进行对比。这批铑(III)配合物中有多款具备光催化活性所需的核心性能，因此研究人员以钯基水还原助催化剂构建了水还原体系，将其作为光催化剂（photocatalysts, PCs）开展性能测试。在特定实验条件下，该类铑(III)光催化剂的催化活性甚至超越了铱(III)光催化剂，由此揭示了这一长期被忽视的过渡金属类别，在高效光活性发色团与光催化剂两方面的应用潜力。