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.