摘要/Abstract

不含芳(杂)环、单键-重键交替单元等大共轭结构的纯有机发光材料引起了人们的广泛关注, 但其结构与发光性质间的关系仍需进一步明确. 本文研究了氧杂酸酐与氧杂酰亚胺两种非芳香性发光化合物. 通过在不同溶剂中培养单晶, 得到了具有不同发光颜色及荧光-磷光双发射的同质多晶体, 发光效率最高达17.0%. 这种高效发射可归因于含π电子与n电子基团的簇聚产生的空间共轭及构象刚硬化. 尽管这些同质多晶体结构差异微小, 但少量溶剂分子的存在导致簇聚体微环境改变, 调节了单线态与三线态发射比例, 从而产生了不同颜色的发光. 这些结果有助于人们进一步了解非典型发光化合物构象与发光间的关系, 为其机理研究与分子设计提供了新的启示.
关键词: 非芳香性发光化合物, 同质多晶, 室温磷光, 簇聚诱导发光, 空间共轭
Recently, nonconventional luminophores have received increasing attention, owing to their fundamental importance, advantages in outstanding biocompatibility, easy preparation, environmental friendliness and potential applications in sensing, imaging, encryption, etc. In order to provide more information about relationship among molecular conformation, molecular packing and emission, and moreover, to guide the design of nonconventional luminophores, molecules with definite structures and explicit molecular packing are highly desired. In this contribution, we report two nonconventional luminophores, namely F-MA and F-MI, consisting of carbonyls (C=O), electron-rich heteroatoms (O/N), and unsaturated C=C subgroups. They are nonluminous in dilute solutions while being emissive in concentrated ones. Furthermore, through crystallization in different solvents, polymorphs of both compounds with various emission colors along with distinct room temperature phosphorescence (RTP) are successfully obtained. Under 312 nm UV irradiation, three polymorphs of F-MA emit bluish-violet, blue and white lights, accompanying photoluminescence (PL) and RTP quantum efficiencies ( Φ _c/ Φ _p) of 10.6%/1.8%, 9.4%/2.1% and 2.9%/1.7%, respectively. To acquire more efficient emission, hydrogen bonds are introduced via amidation of F-MA, leading to the target compound F-MI with strikingly improved PL performance. Notably, F-MI is also polymorphic, whose Φ _c and Φ _p are of up to 17.0% and 4.8%, respectively. Meanwhile, the RTP lifetimes of F-MI polymorphs are significantly prolonged by 10- to 56-fold, as compared with their corresponding F-MA counterparts. The above PL properties can well be rationalized by the clustering-triggered emission (CTE) mechanism, namely through-space electronic delocalization of π and n electrons among different molecules in concentrated solutions or crystals alongside with sufficiently rigidified conformations is accountable for the emission, which is also verified by single crystal analysis and theoretical calculation. Besides, the noticeable RTP emission should be ascribed to the presence of C=O and heteroatoms and the clustering of such subgroups, which are ready to enhance spin-orbit coupling (SOC) and subsequent intersystem crossing transitions with effective through-space conjugation. Surprisingly, subtle changes caused by trace solvents in molecular conformations and packing modes significantly impact on intra/intermolecular interactions, which alter the relative intensity of singlet (fluorescence) and triplet (RTP) emissions, thus resulting in polymorphism-dependent emission colors. For these unorthodox luminescent molecules, their PL properties of polymorphs will deepen the understanding of the relationship between subtle structure variation and emission, thus enlightening further luminescent mechanism understanding and future rational design of novel nonconventional luminophores.
Key words: nonaromatic luminophores, polymorphism, room temperature phosphorescence, clustering-triggered emission, through-space conjugation


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