Mechanofluorochromic Material toward a Recoverable Microscale Force Sensor

Abstract Among the multiple classes of mechanofluorochromic (MFC) compounds reported in recent years, simple donor–acceptor molecules involving oligooxyethylene N‐substituted diphenylamine connected to a dicyanovinyl group by a thienyl spacer constitute a specific class of mechano‐stimulable smart materials with a unique combination of properties including turn‐on mechanofluorochromism with red‐light photoluminescence emission, turn‐off second harmonic generation, and self‐recovery in ambient conditions. While the dynamics of the MFC processes of thin films of these materials have already been investigated, the correlation of the behavior observed at the macroscale with elemental processes taking place at the micro‐ and nanoscopic scales remains largely unexplored. In this work, atomic force microscopy (AFM) coupled with wide‐field and confocal optical microscopies is used to investigate the turn‐on/recovery MFC behavior of microcrystals and nanostructures. It is shown that the red light photoluminescence of microcrystals and nano‐objects can be turned‐on by contact AFM and that the intensity of light emission is linearly correlated to the applied nominal force. Recovery is associated with a decrease in emission intensity, which can be reactivated again by subsequent contact scans. These results thus pave the way toward the development of micro‐/nanoscale recoverable force‐sensing technologies based on this particular class of smart molecular materials.