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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-((1 E,3 E)-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.

A detailed study on the photophysical properties of a series of alkoxy substituted diphenylbutadienes in solution and in the solid state providing a molecular level understanding of the factors controlling their solid-state luminescence behavior is reported. Our studies provide clear evidence for exciton splitting in the solid state resulting in red-shifted emission for this class of materials. The role of the number of alkoxy substituents and the alkyl chain length in controlling the nature of the molecular packing and consequently their fluorescence properties has been elucidated. Whereas in the di- and tri-alkoxy substituted derivatives, the solid-state fluorescence was independent of the length of the alkyl chains, in the monoalkoxy substituted derivatives, increasing the length of the alkyl chain resulted in a visual change in fluorescence from green to blue. On the basis of the analysis of the molecular packing in the single crystals, this difference could be attributed to fluorescence arising from aggregates with an edge-to-face alignment in the molecules possessing short alkyl chains (methyl and butyl) to monomer fluorescence in the long alkyl chain containing derivatives.