Five different colours solid-state fluorescence of azastilbenes: a new push–pull π-conjugated system

Five push–pull azastilbene derivatives [R = −OH (1), −OCH3(2), −OCH2CH3(3), −N(CH3)2 (4) and −N(CH2CH3)2(5)] have been synthesized and successfully characterized by 1H NMR, ultraviolet–visible (UV–Vis) and Fourier-transform infrared (FT-IR) spectroscopy. Compounds 1–5 possess solid-state fluorescence in five different colours ranging from blue to orange (λex at 400 nm) in which blue (λem at 474 nm) for 1, bright-blue (λem at 469 nm) for 2, green (λem at 537 nm) for 3, yellow (λem at 568 nm) for 4 and orange (λem at 540, 630 nm) for 5. Compounds 4 and 5 exhibited strong fluorescence with quantum yield values 0.61 and 0.84, respectively, higher than those of 1–3. Moreover, liquid-state fluorescence performed in non-polar solvent in order to investigate the effect of dipolar intermolecular interactions on their fluorescence properties, the results revealed that the emission spectra of 1 −5 in liquid state are similar to those of solid state, indicating that no significant dipolar interactions were observed. Therefore, the substituent with electron-donating group as N- or O-donor and electron-withdrawing group as –SO3H, on the azastilbene skeleton play a major role on their solid-state fluorescence.