Pan Zhong-Hua, Zhou Jing-Wei, Luo Geng-Geng
College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, P. R. China.
Phys Chem Chem Phys. 2014 Aug 14;16(30):16290-301. doi: 10.1039/c4cp02151g.
In this study we report about two novel azomethine–BODIPY dyads 1 and 2. The two dyads have been, respectively, synthesized by covalent tethering of tautomeric ortho-hydroxy aromatic azomethine moieties including N-salicylideneaniline (SA) and N-naphthlideneaniline (NA) to a BODIPY fluorophore. Both of the two dyads 1 and 2 show enol-imine (OH) structures dominating in the crystalline state. Dyad 1 in the enol state is the most stable form at room temperature in most media, while enol–keto prototropic tautomerism of the NA moiety in solution is preserved in dyad 2, which can be reversibly converted between enol and keto forms in the environment's polarity. Visible illumination of dyad 2 in the enol state excites selectively the BODIPY fragment and then deactivates radiatively by emitting green light in the form of fluorescence, while the emission intensity of 2 in the keto state is quenched on the basis of the proton-coupled photoinduced electron transfer (PCPET) mechanism. This allows large fluorescence modulation between the two states of dyad 2 and generates a novel tautomerisable fluorescent switch. Theoretical calculations including calculated energies, potential energy surfaces (PESs) and intrinsic reaction coordinate (IRC) analysis further support that the single proton transfer reaction from an enol form to a transition state (TS) and from the TS to a keto form for 2 is easier to occur than that for 1, which accounts for the fluorescence quenching of 2 in methanol. The agreement of the experimental results and theoretical calculations clearly suggests that fluorescent and tautomeric components can be paired within the same molecular skeleton and the proton tautomerization of the latter can be designed to regulate the emission of the former. In addition, preliminary experiments revealed that 1 can be potentially used as a simple on/off fluorescent chemosensor which exhibited higher selectivity for Cu(2+) over other common cations.
在本研究中，我们报道了两种新型的甲亚胺 - BODIPY二元化合物1和2。这两种二元化合物分别是通过将互变异构的邻羟基芳香甲亚胺部分（包括N - 水杨醛苯胺（SA）和N - 萘醛苯胺（NA））共价连接到一个BODIPY荧光团上而合成的。二元化合物1和2在晶体状态下均显示烯醇 - 亚胺（OH）结构占主导。在大多数介质中，处于烯醇状态的二元化合物1在室温下是最稳定的形式，而溶液中NA部分的烯醇 - 酮质子转移互变异构在二元化合物2中得以保留，其可以在环境极性中在烯醇和酮形式之间可逆转换。处于烯醇状态的二元化合物2在可见光照射下选择性地激发BODIPY片段，然后通过以荧光形式发射绿光进行辐射失活，而处于酮状态的2的发射强度基于质子耦合光致电子转移（PCPET）机制而猝灭。这使得二元化合物2的两种状态之间能够实现大的荧光调制，并产生一种新型的可互变异构的荧光开关。包括计算能量、势能面（PESs）和内禀反应坐标（IRC）分析在内的理论计算进一步支持，对于2而言，从烯醇形式到过渡态（TS）以及从TS到酮形式的单质子转移反应比1更容易发生，这解释了2在甲醇中的荧光猝灭现象。实验结果与理论计算的一致性清楚地表明，荧光和互变异构组分可以在同一分子骨架内配对，并且可以设计后者的质子互变异构来调节前者的发射。此外，初步实验表明，1有可能用作一种简单的开/关荧光化学传感器，其对Cu(2+)的选择性高于其他常见阳离子。
