Twisting and piezochromism of phenylene-ethynylenes with aromatic interactions between side chains and main chains

This paper describes a series of three-ring phenylene-ethynylenes (PEs) in which specific, non-covalent arene-arene interactions control conformation in the solid-state. As determined by single crystal X-ray structures, edge-face interactions between benzyl ester side chains and conjugated main chains are observed. In contrast, perfluorobenzyl ester side chains interact cofacially with main chains, resulting in similar to 60 degrees torsional angles between neighboring aryl rings in crystalline PEs. Absorbance and fluorescence spectra of films of these compounds reflect these conformational effects, with the spectra of perfluorobenzyl-substituted compounds shifting hypsochromically from solution-to solid-state. In a demonstration of how balancing non-covalent interactions can open the way to new responsive materials, a main chain twisted derivative with octyloxy substituents displayed significant piezochromic behavior.