Cyclophane‐Based Fluorescence Tuning Induced by Hydrostatic Pressure Changes
Cyclophane‐Based Fluorescence Tuning Induced by Hydrostatic Pressure Changes
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DOI：
10.1002/cptc.201800163
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发表时间：
2018-11
影响因子：
4
通讯作者：
Yoshimitsu Sagara;N. Tamaoki;G. Fukuhara
中科院分区：
文献类型：
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作者：
Yoshimitsu Sagara;N. Tamaoki;G. Fukuhara
与高压下线性发色团单元的氯仿溶液通常繁琐的荧光增强形成鲜明对比，荧光环芳烷溶液在高压下表现出剧烈的荧光猝灭，其中压力诱导的分子内π堆叠构象的增加很可能是造成这种现象的原因。机械刺激可以改变溶液和/或固态有机材料的光致发光特性。机械响应有机化合物是活细胞施加力指示器、光学数据存储和机械生物成像探针的有前途的候选者。在作为机械力之一的静水压力下对各种荧光团的均匀溶液进行简单和基础的研究一直是长期的科学课题。一般来说，有机溶剂中荧光团的光致发光强度由于所用溶剂粘度的增加而在高压下增强，有时遵循福斯特-霍夫曼方程。因此，这些事实表明压力诱导的荧光调谐、溶液淬灭显然是一项艰巨的任务。另一方面，机械刺激直接应用于固态发光化合物近十年来引起了广泛关注。研磨、破碎和拉伸变形显着影响分子组装材料的光物理性质。在迄今为止研究的机械致色发光材料的几种方法中，源自特定结构特征的环状、树枝状和机械联锁化合物在凝聚态下表现出机械响应发光。然而，这些结构有趣的分子的静水压力诱导的荧光行为尚未得到研究。特别是，通过柔性连接体带有两个芳香烃的荧光环芳基是令人着迷的目标，因为环状结构中两个芳香族部分之间的分子内相互作用可能仅通过改变静水压力来调节。因此，在本研究中，我们重点关注不对称光致发光环芳烷 1（图 1），它在静水压力下表现出对比鲜明的荧光猝灭行为。与非环芳烷 2 获得的结果进行比较，我们可以评估环芳烷结构对单个分子水平压力下荧光响应的影响。定量研究施加的机械研磨力与结晶或液晶 (LC) 状态下分子排列变化之间的相关性仍然具有挑战性，因为结晶颗粒或 LC 域的尺寸和分子组装体不同。最终，这项对静水压力下分散的“孤立”分子材料的研究可能会为复杂分子组装状态下的机械响应发光特性提供深入的见解。环芳1具有1,6-双(苯基乙炔基)芘和萘基团，两个芳香烃通过六乙二醇连接基桥联。柔性连接体可以使环芳烷能够在溶液中形成各种构象，通过改变静水压力来调节光致发光性能。事实上，在凝聚态下，化合物 1 形成过冷向列液晶相，根据热处理程序，该相在室温下显示出绿色光致发光和蓝色发射晶态。绿色发射归因于准分子的形成，另一方面，发光团不再形成晶态的准分子，导致蓝色发射。此外，芘芬1在固态下表现出机械致色发光。线性参比化合物 2 与环芳烷 1 具有相同的发光体单元。 图 1. 不对称环芳烷 1 和非环状参比化合物 2 的化学结构。 化合物 1 和 2 在环境条件下在氯仿 (c = 1.0 × 10 M) 中的吸收和荧光光谱如图 2 所示。两种氯仿溶液均显示出与 1,6 双(苯乙炔基)芘相对应的 350 和 1,6-二(苯乙炔基)芘的吸收带。 450 nm（图 2a）。 [a] Y. Sagara 博士、N. Tamaoki 电子科学研究所教授、北海道大学 N20, W10, Kita-Ku, Sapporo, Hokkaido 001-0020（日本）电子邮件：sagara@es.hokudai.ac.jp [b] Y. Sagara 博士、G. Fukuhara 博士 JST-PRESTO Honcho 4-1-8、 Kawaguchi, Saitama 332-0012 (Japan) [c] Dr. G. Fukuhara Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-Ku, Tokyo 152-8551 (Japan) 电子邮件：gaku@chem.titech.ac.jp 本文作者的支持信息和 ORCID 识别号可在 XXX 下找到
In keen contrast to a usual, tedious fluorescence augment of a chloroform solution of a linear chromophore unit under high pressure, a fluorescent cyclophane solution shows a drastic fluorescence quenching under elevated pressure for which a pressure-induced increment of an intramolecular π-stacked conformation is highly likely to be responsible. Mechanical stimuli can vary photoluminescence properties of organic materials in solution and/or solid states. Mechanoresponsive organic compounds are promising candidates of indicator of applied force, optical data storage and mechano-bioimaging probes for living cells. Simple and basic studies on homogeneous solutions of various fluorophores under hydrostatic pressure as one of mechanical forces have been long-term scientific subjects. In general, photoluminescence intensities of fluorophores in organic solvents augment under elevated pressure due to increasing viscosity of solvents used, and sometimes follow the Förster– Hoffmann equation. Therefore, these facts indicate that pressure–induced fluorescence–tuning, quenching of solutions is an apparently difficult task. On the other hand, direct applications of mechanical stimuli to luminescent compounds as solid states have attracted much attention in a decade. Grinding, crushing and tensile deformation significantly affect the photophysical properties of molecular assembled materials. Of several approaches to mechanochromic luminescent materials hitherto examined, cyclic, dendritic, and mechanically-interlocked compounds, originated from particular structural features, show mechanoresponsive luminescence in the condensed states. Nevertheless, hydrostatic pressure-induced fluorescence behavior of these structurally interesting molecules has not been examined yet. In particular, fluorescent cyclophanes bearing two aromatic hydrocarbons through a flexible linker are fascinating targets since it is likely that an intramolecular interaction between two aromatic moieties in the cyclic structure may be tuned just by changing the hydrostatic pressure. In the present study, we thus focus on an asymmetric photoluminescent cyclophane 1 (Figure 1) that shows a contrasting fluorescence quenching behavior under hydrostatic pressure. Comparison with the results obtained from noncyclophane 2 allows us to assess the effects of the cyclophane structure on the fluorescence responses under pressure at individual molecule level. It is still challenging to quantitatively investigate the correlation between applied mechanical grinding forces and molecular arrangement changes in crystalline or liquid-crystalline (LC) states, since crystalline particles or LC domains are divergent size and molecular assemblies. Eventually this study on the dispersed, "isolated" molecular materials under hydrostatic pressure may provide deep insights into the mechanoresponsive luminescence properties in complicate molecular assembled states. The cyclophane 1 has 1,6-bis(phenylethynyl)pyrene and naphthalene groups, and the two aromatic hydrocarbons are bridged through hexaethylene glycol linkers. The flexible linkers can provide the cyclophane with ability to form various conformations in solution, modulating photoluminescence properties by varying hydrostatic pressure. Indeed, in the condensed states, compound 1 forms a supercooled nematic LC phase that shows a green photoluminescence and blueemissive crystalline state at room temperature depending on a thermal treatment procedure. The green emission is ascribed to an excimer formation, on the other hand, the luminophores no longer form the excimers in the crystalline states, leading to the blue emission. Furthermore, the pyrenophane 1 shows mechanochromic luminescence in the solid state. The linear reference compound 2 has the same luminophore unit as cyclophane 1. Figure 1. Chemical structures of asymmetric cyclophane 1 and the acyclic reference compound 2. Absorption and fluorescence spectra of compounds 1 and 2 in chloroform (c = 1.0 × 10 M) under ambient condition are shown in Figure 2. Both chloroform solutions display absorption bands corresponding to 1,6bis(phenylethynyl)pyrene between 350 and 450 nm (Figure 2a). [a] Dr. Y. Sagara, Prof. N. Tamaoki Research Institute for Electronic Science, Hokkaido University N20, W10, Kita-Ku, Sapporo, Hokkaido 001-0020 (Japan) E-mail: sagara@es.hokudai.ac.jp [b] Dr. Y. Sagara, Dr. G. Fukuhara JST-PRESTO Honcho 4-1-8, Kawaguchi, Saitama 332-0012 (Japan) [c] Dr. G. Fukuhara Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-Ku, Tokyo 152-8551 (Japan) E-mail: gaku@chem.titech.ac.jp Supporting information and the ORCID identification numbers for the authors of this article can be found under XXX