Understanding the individual fluorescence response mechanism of covalent organic frameworks (COFs) at a single-crystal level is of great significance for the rational design of COF-based microsensors but unreachable because all previous COF-based sensors are performed with average fluorescence response behavior of various sized polycrystalline COFs. Herein, we design to explore the fluorescence response of a monodisperse single-crystal COF and further reveal the individual heterogeneity of the response mechanism. Three-dimensional single-crystal COF-301 (SCOF-301) with an intramolecular H-bond-induced excited-state intramolecular proton-transfer effect is selected as a proof-of-concept SCOF. With ethanol, benzene, and ammonia as model analytes, three different deformation and competition H-bond site-induced fluorescence response mechanisms related to crystal size are revealed. Small single particles of SCOF-301 (SSCOF-301) exhibit a more flexible structure, leading to the dominant role of deformation in the fluorescence response of small-sized SSCOF-301. The decreasing flexibility of SSCOF-301 with the increase of crystal size results in involvement of competition of the H-bond site to the fluorescence response besides deformation. Further increase of the crystal size makes the large-sized SSCOF-301 difficult to deform; thus, the competition of the H-bond site dominates the fluorescence response. This work provides a deep understanding of the individual fluorescence response mechanism of COFs to guide the design of a functional COF sensor with suitable size and mechanism for different structural analytes.

中文翻译：

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单晶三维共价有机框架的尺寸依赖性变形和竞争氢键位点诱导的个体荧光响应

在单晶水平上了解共价有机骨架（COF）的个体荧光响应机制对于基于COF的微传感器的合理设计具有重要意义，但由于之前所有基于COF的传感器都是以各种不同的平均荧光响应行为来执行的，因此这是无法实现的。尺寸的多晶COF。在此，我们设计探索单分散单晶COF的荧光响应，并进一步揭示响应机制的个体异质性。选择具有分子内氢键诱导的激发态分子内质子转移效应的三维单晶COF-301（SCOF-301）作为概念验证SCOF。以乙醇、苯和氨作为模型分析物，揭示了与晶体尺寸相关的三种不同的变形和竞争氢键位点诱导的荧光响应机制。 SCOF-301 (SSCOF-301) 的小单颗粒表现出更灵活的结构，导致变形在小尺寸 SSCOF-301 的荧光响应中起主导作用。随着晶体尺寸的增加，SSCOF-301 的柔韧性降低，除了变形之外，还导致氢键位点与荧光响应的竞争。晶体尺寸的进一步增大，使得大尺寸的SSCOF-301不易变形；因此，氢键位点的竞争主导了荧光响应。这项工作提供了对COF个体荧光响应机制的深入理解，以指导针对不同结构分析物设计具有合适尺寸和机制的功能性COF传感器。