Stimuli-responsive side chains for new function from conjugated materials

• The expansion of side chains of conjugated materials to impart function is described.
• o-Nitrobenzyl linkers on conjugated backbones yield insolubility upon UV photolysis.
• Varied inductive effects of non-conjugated side chains can perturb HOMO–LUMO gaps.
• Aromatic side chain/main chain interactions can twist phenylene-ethynylenes.
• Piezochromism and mechanofluorochromism occur in a twisted phenylene-ethynylene.

This article describes several projects in our laboratory relating to stimuli-responsive conjugated materials using functional side chain moieties. Although side chain engineering has become increasingly important in controlling the packing of conjugated materials, these groups have typically been reserved for imparting solubility to otherwise insoluble materials. By incorporating solubilizing side chains through photocleavable nitrobenzyl linkers, new conjugated polymers behave as negative photoresists upon exposure to ultraviolet light with minimal photobleaching. Our approach enables photopatterning and solution-based fabrication of multilayer conjugated polymer films without the use of orthogonally soluble materials. Work to understand the electronic effects of the aromatic side chain substituents in these materials led to the subsequent discovery of aromatic interactions between side chains and main chains that control the conformations of solid-state phenylene-ethynylene oligomers, including highly twisted conjugated backbones. Such materials, when appropriately substituted, display reversible piezochromic and mechanofluorochromic behavior.

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