Solid state optical properties of 4-alkoxy-pyridine butadiene derivatives: reversible thermal switching of luminescence

Abstract

The synthesis and optical properties of a series of alkoxyphenyl-pyridyl butadiene derivatives in solution and in the solid state are described. All the derivatives were practically nonfluorescent in solution but showed good fluorescence in the solid-state. The role of molecular packing in controlling the solid-state fluorescence was investigated by studying the X-ray crystal structure of these molecules. One of the derivatives, 4-((1E,3E)-4-(4-butoxyphenyl)buta-1,3-dienyl)pyridine exhibited polymorphism, with the different polymorphs exhibiting visually distinguishable fluorescence. In the natural state it existed as a polymorph exhibiting blue fluorescence, while it's cooled melt emitted yellow light. The difference could be attributed to a transformation in the molecular packing of the material from a herringbone to a brickstone arrangement, resulting in a change from monomer to J-type aggregate fluorescence. The polymorph exhibiting yellow fluorescence was fairly stable (>6 months) but could be converted back to the original form by keeping the film at 110 °C for a short period of time (~8-10 min) before slowly cooling to room temperature. The thermally induced changes in fluorescence behavior were clearly reproducible over several cycles, indicating the utility of this material for thermal imaging applications.