Molecular Packing and Solid-State Fluorescence of Alkoxy-Cyano Substituted Diphenylbutadienes:  Structure of the Luminescent Aggregates

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.