Ring-in-Ring(s) Complexes Exhibiting Tunable Multicolor Photoluminescence

One ring threaded by two other rings to form a non-intertwined ternary ring-in-rings motif is a challenging task in noncovalent synthesis. Constructing multicolor photoluminescence systems with tunable properties is also a fundamental research goal, which can lead to applications in multidimensional biological imaging, visual displays, and encryption materials. Herein, we describe the design and synthesis of binary and ternary ring-in-ring(s) complexes, based on an extended tetracationic cyclophane and cucurbit[8]uril. The formation of these complexes is accompanied by tunable multicolor fluorescence outputs. On mixing equimolar amounts of the cyclophane and cucurbit[8]uril, a 1:1 ring-in-ring complex is formed as a result of hydrophobic interactions associated with a favorable change in entropy. With the addition of another equivalent of cucurbit[8]uril, a 1:2 ring-in-rings complex is formed, facilitated by additional ion–dipole interactions involving the pyridinium units in the cyclophane and the carbonyl groups in cucurbit[8]uril. Because of the narrowing in the energy gaps of the cyclophane within the rigid hydrophobic cavities of cucurbit[8]urils, the binary and ternary ring-in-ring(s) complexes emit green and bright yellow fluorescence, respectively. A series of color-tunable emissions, such as sky blue, cyan, green, and yellow with increased fluorescence lifetimes, can be achieved by simply adding cucurbit[8]uril to an aqueous solution of the cyclophane. Notably, the smaller cyclobis(paraquat-p-phenylene), which contains the same p-xylylene linkers as the extended tetracationic cyclophane, does not form ring-in-ring(s) complexes with cucurbit[8]uril. The encapsulation of this extended tetracationic cyclophane by both one and two cucurbit[8]urils provides an incentive to design and synthesize more advanced supramolecular systems, as well as opening up a feasible approach toward achieving tunable multicolor photoluminescence with single chromophores.

构建由两个环穿入单环以形成非互穿三元套环基序的结构，是非共价合成（noncovalent synthesis）领域极具挑战性的研究课题。构建具备可调谐特性的多色光致发光（multicolor photoluminescence）系统同样是一项基础研究目标，该类系统可应用于多维生物成像、可视化显示及加密材料等领域。在此，我们报道了基于扩展四阳离子环蕃（extended tetracationic cyclophane）与葫芦[8]脲（cucurbit[8]uril）的二元及三元套环复合物的设计与合成。此类复合物的形成伴随可调谐多色荧光发射信号。当将等摩尔量的环蕃与葫芦[8]脲混合时，依托伴随有利熵变的疏水相互作用（hydrophobic interactions），可形成1:1型套环复合物。当再加入一当量葫芦[8]脲后，借助环蕃上吡啶鎓单元（pyridinium units）与葫芦[8]脲羰基之间的额外离子-偶极相互作用（ion–dipole interactions），可生成1:2型套环复合物。由于环蕃在葫芦[8]脲刚性疏水空腔内的能隙（energy gaps）收窄，二元与三元套环复合物分别发射绿色与亮黄色荧光。仅需向环蕃水溶液中添加葫芦[8]脲，即可获得一系列荧光寿命递增的可调谐发射色，涵盖天蓝色、青色、绿色与黄色。值得注意的是，与该扩展四阳离子环蕃拥有相同对亚二甲苯基连接臂的更小尺寸的环双(对百草枯-对亚苯基)（cyclobis(paraquat-p-phenylene)），无法与葫芦[8]脲形成套环复合物。通过一个与两个葫芦[8]脲分别包合该扩展四阳离子环蕃，不仅为设计与合成更先进的超分子系统（supramolecular systems）提供了契机，同时也为实现单发色团（single chromophores）可调谐多色光致发光开辟了可行路径。