Photoluminescent organic materials responsive to macroscopic mechanical stimuli

Abstract

Organic solid materials that exhibit tunable photoluminescence in the solid state under macroscopic mechanical stimuli have potential applications in sensors and imaging devices. So far, no rational designs have been proposed that impart these mechano-responsive luminescent properties to π-conjugated compounds. We have demonstrated an effective strategy for mechanoresponsive luminescent materials by imparting amphiphilic and dipolar characteristics to a luminescent π-conjugated molecule. The oligo(p-phenylenevinylene) luminophore with a didodecylamino group at one end and a tri(ethylene glycol) ester group at the other end self-assembled into segregated solid structures by separately aggregating its hydrophobic and hydrophilic moieties. The segregated structures force the dipolar π-systems to align in the same direction, thereby generating a conflict between the side-chain aggregation and dipolar stabilization. As a result, these metastable solid structures can be transformed through mechanical stimulation to a more stable structure, from a π−π stacked aggregate to a liquid crystal and further to a crystalline phase with variable luminescence. We also showed the control of the liquid crystalline-crystalline phase transition by the complexation with lithium salt, and the application of such a property to an inkjet-imaging device with a concept of “molecular decoder”.