Scheme 1.

Chemical structure of the two fluorophores C1 and C2. The double Knoevenagel condensation between 4-nitrophenylacetic acid or 2-(4-nitrophenyl)acetonitrile respectively and the alkoxy-terephthalaldehyde [22,25] was a convenient symmetric pattern to synthesize the 4,4’-dinitrostilbene skeleton. The identification and purity were assessed by mass spectrometry and ¹H-NMR and by comparing melting points with the literature data [22]. The optical properties in solution were analyzed in three solvents with different polarity: dioxane < acetone<DMF (see Table 1).