Anions play a variety of roles in biology, bioinorganic chemistry, organic chemistry and mineralogy. As such, the directed coordination of anions in molecular complexation and the development of synthetic receptor molecules represent a burgeoning field for supramolecular chemistry in solution phase recognition and solid-state crystal engineering.
The desire to develop solution phase anion sensors utilizing the optoelectronic properties of tetraphenylethylene (TPE), in particular its capacity to participate in aggregation-induced emission (AIE) enhancement has resulted in the synthesis of two new classes of TPE-based anion sensors. The first class features a series of ureas off of a tetrasubstituted TPE framework capable of detecting a range of anions with its sensitivity toward these anions directly influenced by the basicity of the anion. The second class of TPE-based sensors displays a unique sensitivity toward the presence of pyrophosphate anion and is the first known example of a neutral TPE-based pyrophosphate sensor that does not require a zinc cofactor as a requisite of detection.
Our interest in utilizing anions in the self-assembly of organic materials resulted in the synthesis of twelve N-halophenyl pyridinium salts that are capable of assembling the solid state as a direct consequence of a combination of hydrogen- and halogen-bonding intermolecular forces and their interaction with a halide counterion. In four examples, the abundance of iodine halogen-bond donors relative to counterion acceptors resulted in the formation of extended halogen-bonded networks in the crystal structures that proved to be sufficiently strong to avoid disruption by an abundance of competing hydrogen bond donors.
Five additional examples of TPE-based halopyridinium salts were analyzed via crystallographic methods to examine what role the counterion will play, if any, in the directed assembly of these materials with the aid of potentially strong halogen bond donors. Three examples feature the presence of a traditionally coordinating anion and two examples study the types of assembly motifs that may be encountered when the anion is noncoordinating. Conclusions from this study led to attempts to synthesize TPE-based pyridinium- and imidazolium salts capable of participating in anion recognition through solution phase halogen bonding. Though the results did not imply success, the potential to continue to refine our synthetic methodology and analytical techniques is being explored.