Anion-Responsive Molecules That Exhibit Switching of Two-Photon Optical Properties

Two-photon-excited fluorescent probes are important for two-photon microscopy for biomedical studies. In contrast to the many examples of probes for cationic species, such as metal ions, there have been fewer reports on the control of two-photon optical properties by anions because in such systems it is difficult to control the associated π-electronic states. This Minireview summarizes anion-responsive molecules that exhibit changes in two-photon optical properties and describes their molecular design and anion-response mechanisms, which are driven by changes in covalent bonds and noncovalent interactions. Results from a recent study of two-photon systems, where geometries and optical properties are modulated by anion binding, are also discussed.

Pyrrole-based anion-responsive π-electronic molecules as fluorescence sensors responsive to multiple stimuli

Introduction of electron-donating N,N-dimethylaminophenyl groups in dipyrrolyldiketone BF2 complexes as anion-responsive π-electronic molecules resulted in fascinating fluorescence properties. The fluorescence properties, which depended on the degree of photo-induced electron transfer, could be controlled by solvent polarity, anion binding and protonation.

Arylethynyl Groups That Modulate Anion-Binding and Assembling Modes of Rod- and Fan-Shaped π-Electronic Systems

The introduction of arylethynyl moieties at the pyrrole α- and β-positions of dipyrrolyldiketone BF₂ complexes as anion-responsive π-electronic molecules was investigated. The arylethynyl-substituted derivatives formed a variety of anion complexes with planar [1+1]- and interlocked [2+1]-type structures in solution and in the solid state. The derivatives with long alkyl chains in the introduced arylethynyl groups also formed mesophases in the form of ion pairs of the anion complexes and a counter cation. The geometries of the constituent anion complexes affected the packing modes of the dimension-controlled assemblies.

Computational simulation of anion binding association mechanisms contributing toward rotation of pyrrole rings in dipyrrolyldiketone BF2 complexes

Dipyrrolyldiketone BF₂ complexes have a characteristic association mechanism with anions; an anion is tightly captured by the NH bonding of the two rotated pyrrole rings and the CH bonding of the backbone.

Multiply aryl-substituted dipyrrolyldiketone boron complexes exhibiting anion-responsive emissive properties

Multiply aryl-substituted pyrrole-based anion-responsive π-electronic molecules were synthesized. The effects of the introduced aryl moieties on solution-state emissive behaviours were examined, depending on the solvent polarity and anion binding.