Department of Chemistry , Fu-Jen Catholic University , New Taipei City 24205 , Taiwan , ROC.
Department of Medicinal and Applied Chemistry , Kaohsiung Medical University , Kaohsiung 80708 , Taiwan , ROC.
J Phys Chem A. 2019 Nov 7;123(44):9431-9449. doi: 10.1021/acs.jpca.9b05695. Epub 2019 Oct 29.
The 77 K emission spectra of cyclometalated ruthenium(II)-2,2'-bipyridine (CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy complexes with am(m)ine or diimmine ancillary ligands, and density functional theory (DFT) modeling confirms that the lowest energy triplet metal to ligand charge transfer (MLCT) excited states of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic configurations. However, the phosphorescence decay efficiencies of CM-Ru-bpy excited states are about twice those of the conventional Ru-bpy analogues. In contrast to the similar MLCT excited state electronic configurations of the two classes of complexes, the CM-Ru-bpy chromophores have much broader visible region MLCT absorptions resulting from several overlapping transitions, even at 87 K. The emitting excited-state emission efficiencies depend on spin-orbit coupling (SOC) mediated intensity stealing from singlet excited states, and this work explores the relationship between the phosphorescence efficiency and visible region absorption spectra of Ru-bpy MLCT excited states in the weak SOC limit. The intrinsic MLCT emission efficiency, ι, depends on mixing with singlet excited states whose Ru-dπ-orbital angular momenta differ from that of the emitting state. DFT modeling of the MLCT excited-state electronic configurations that contribute significantly to the lowest energy absorption bands have Ru-dπ orbitals that differ from those of their emitting MLCT excited states. This leads to a very close relationship between ι and the lowest energy MLCT band absorptivities in Ru-bpy chromophores. Thus, the larger number of MLCT transitions that contribute to the lowest energy absorption bands accounts for the enhanced phosphorescence efficiency of Ru-bpy complexes with cyclometalated ancillary ligands.
金属钌(II)-2,2'-联吡啶（CM-Ru-bpy）发色团在 77 K 时的发射光谱与具有胺（m）或二亚胺辅助配体的相关 Ru-bpy 配合物非常相似，密度泛函理论（DFT）建模证实 CM-Ru-bpy 和相关 Ru-bpy 配合物的最低能量三重态金属到配体电荷转移（MLCT）激发态具有非常相似的电子构型。然而，CM-Ru-bpy 激发态的磷光衰减效率大约是传统 Ru-bpy 类似物的两倍。与这两类配合物的相似 MLCT 激发态电子构型相反，CM-Ru-bpy 发色团具有更宽的可见区域 MLCT 吸收，这是由于几个重叠的跃迁，即使在 87 K 时也是如此。发射激发态的发射效率取决于从单重激发态的自旋轨道耦合（SOC）介导的强度窃取，本工作在弱 SOC 极限下探索了 Ru-bpy MLCT 激发态的磷光效率与可见区域吸收光谱之间的关系。固有 MLCT 发射效率 ι 取决于与单重激发态的混合，其 Ru-dπ-轨道角动量与发射态的不同。对显著贡献于最低能量吸收带的 MLCT 激发态的电子构型的 DFT 建模表明，Ru-dπ 轨道与它们的发射 MLCT 激发态的轨道不同。这导致 ι 与 Ru-bpy 发色团中最低能量 MLCT 带吸收率之间存在非常密切的关系。因此，贡献于最低能量吸收带的 MLCT 跃迁的数量越多，就会导致具有金属环化辅助配体的 Ru-bpy 配合物的磷光效率增强。
