Seth A. Sharber
Mechanofluorochromic (MC) materials, which show switchable fluorescence with mechanical force that is often reversible with heat or solvent vapor, have seen rapid development in recent years and would benefit from the expansion of rational design principles based on non-covalent control strategies to engineer molecular packing and properties on-demand in order to develop functional materials. Our lab has previously studied three-ring phenylene ethynylene oligomers (PEs) bearing perfluorinated benzyl ester pendants on the central terephthalate, a form of side-chain engineering in which the well-studied perfluoroarene-arene (ArF-ArH) supramolecular synthon intramolecularly induces a twist into the conjugated backbone, which significantly affects solid-state optical properties by interrupting conjugation and preventing chromophore aggregation. Many of our PEs show reversible MC behavior, which can be tuned according to two lesser utilized parameters: the length of alkyl chains on terminal substituents, and the regiochemistry of fluorination in the side chains. In a series of N,N-dialkylaniline substituted PEs with thermally reversible MC, we may tune the temperature required to recover the original fluorescence of films after mechanical grinding according to the length of the terminal alkyl chains. These PEs retain their MC properties to various extents when doped into polymethacrylate hosts, and efforts are underway to construct MC polymers incorporating this PE design. In a separate series of compounds that have different fluorination patterns in tetrafluorinated side chains (the ArF ring), four sets each of three regioisomers show regular trends in their solid state optical properties according to the fluorination pattern, as well as the ability to alternate between bathochromic and hypsochromic shifts in the MC transition. Crystallographic analysis in both studies offers significant mechanistic insight into their MC responses and elucidates the utility of side-chain engineering in these stimuli-responsive oligomers that can inform the design of MC polymeric materials.