1. Development of Various Mechanoresponsive Luminescent Materials
External stimuli-responsive luminescent materials would be nice candidates of sensors, memories, security inks, etc. Our group is currently developing variety types of stimuli-responsive luminescent materials. In particular, we mainly focus on “Mechanoresponsive Luminescence”.
For example, Movie 1 shows that a crystalline compound exhibits an emission color change from green to orange upon grinding on the glass substrate under excitation light. Movie 2 displays a rubbery polymer film changing the emission intensity upon stretching by fingers. These phenomena are typical mechanoresponsive luminescence.
We currently concentrate on preparing “supramolecular mechanophores” that function at a single molecular level based on the concept of “mechanoresponsive luminescent materials based on molecular assemblies (please see below)”. Besides, creating cyclic molecules showing stimuli-responsive luminescence and is also our current targets because Introduction of cyclic structures would be a promising way to obtain mechanoresponsive luminescent materials. Furthermore, researches concerning low-dimensional supramolecular materials are also going on in our group.
2. Fundamental Concept
In general, photoluminescent properties of molecular assembly depend on the assembled structures. In the condensed states, excimer formations or exciton coupling between the luminescent moieties and conformational change of luminescent groups affect on the photoluminescent properties of the molecular materials as a whole. If the molecular assembled structures are different, the photoluminescent properties are also different from each other even though the materials consist of the same molecules. So, if external stimuli such as temperature change, mechanical stimuli, electric or magnetic fields induce change in molecular assembled structures, luminescent properties of whole materials also change.
3. History of our group’s research
Since a 1,3,6,8-tetraphenylpyrene derivative was found to show mechanochromic luminescence, we have prepared various mechanoresponsive luminescent materials from organic crystals, liquid crystals, micelles, and supramolecular mechanophores.