External Heavy-Atom Effect via Orbital Interactions Revealed by Single-Crystal X‑ray Diffraction

Enhanced spin–orbit coupling through external heavy-atom effect (EHE) has been routinely used to induce room-temperature phosphorescence (RTP) for purely organic molecular materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions between the external heavy atom and the luminophore, is of essential importance in molecular design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy atoms serving to realize the EHE-related RTP. In this report, we conduct an investigation on how heavy-atom perturbers and aromatic luminophores interact on the basis of data obtained from crystallography. We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides, where the luminescent molecules are considered as “base” or “acid” relative to the heavy-atom perturbers, respectively. We propose that electron donation from a π molecular orbital (MO) of the carbazole to the σ* MO of the C–X bond (π/σ*) and n electron donation to a π* MO of the quinolinium moiety (n/π*) are responsible for the EHE (RTP) in the solid state, respectively.借助外重原子效应（external heavy-atom effect, EHE）实现的增强自旋轨道耦合，是纯有机分子材料诱导室温磷光（room-temperature phosphorescence, RTP）的常规策略。因此，阐明外重原子效应的本质——即外重原子与发光体之间的特异性轨道相互作用——对于分子设计具有核心意义。在有机体系中，卤素（如氯Cl、溴Br、碘I）是实现外重原子效应相关室温磷光最常用的重原子种类。本研究基于晶体学获取的实验数据，探究了重原子扰动剂与芳香族发光体之间的相互作用机制。我们合成了两类分子体系：N-卤代烷基取代咔唑与卤化喹啉鎓，其中发光分子分别被视为相对于重原子扰动剂的"碱"与"酸"。我们提出，咔唑的π分子轨道（molecular orbital, MO）向C-X键的σ*分子轨道的电子转移（π/σ*），以及n电子向喹啉鎓片段π*分子轨道的电子转移（n/π*），分别是固态下外重原子效应（室温磷光）的核心成因。