Synthesis of azulene fluorophores reveals structure-property relationships.

We report the synthesis, photophysical characterisation, and crystallographic analysis of a series of substituted azulenes. The compounds are analogues of the known fluorophore diethyl 2-amino-6-hydroxyazulene-1,3-dicarboxylate, in which the electron-withdrawing groups at C1 and C3 have been retained, but the electron-donating groups at C2 and C6 have been systematically varied. Correlations were observed between the nature of the C2 and C6 substituents and the fluorescence quantum yields and emission maxima; on this basis trends in the structure-property relationship have been elucidated. A novel azulene exhibiting a five-fold increase in quantum yield over the previously known fluorophore has been identified.