Conjugated polymers based on poly(fluorenylene ethynylene)s: Syntheses and sensing performance for nitroaromatics

• Four polymers based on poly(fluorenylene ethynylene)s (PFEs) have been synthesized.
• These polymers show high fluorescence quenching efficiency to DNT.
• Introducing thiophenylenes to PFEs improves the fluorescence quenching of polymer.
• Diphenyl groups on PFEs enhance the fluorescence quenching of film in DNT vapor.
• Polymer containing both thiophenylenes and diphenyls show the strongest quenching.

Four conjugated polymers (P1, P2, P3 and P4) based on poly(fluorenylene ethynylene)s (PFEs) have been synthesized by introducing 9,9-diphenylfluorene units and/or thiophene units into the backbone of PFEs. These exotic units remarkably change the behaviors of polymer, such as the solubility, the peak wavelengths of the optical spectra, and the spectral red-shift of solid polymers, etc. As sensing materials for detecting nitroaromatics, all the four polymers show observable fluorescence quenching when DNT exists. For example, in DNT/THF solution, the quenching efficiencies (ηEP) of P2 and P4 are 95% and 90% at quenching equilibrium respectively, which are much higher than those of P1 (77%) and P3 (72%). The high ηEP of P2 and P4 may be ascribed to the electron-rich character of thiophenylene units which can facilitate the electron-transfer between polymer and DNT. However, in another case of polymer films in DNT vapor, P3 shows higher quenching efficiency even than P2 at quenching equilibrium, which may be attributed to the fact that the steric hinderance of 9,9-diphenylfluorene units in P3 can improve the permeability of the film sensor. P4, which contains both thiophenylene and 9,9-diphenylfluorene units, shows the highest quenching efficiency of 91% in DNT vapor at quenching equilibrium.