Researching On Synthesis And Properties Of All-Conjugated Polyelectrolytes

All-conjugated polyelectrolytes' potential application in the field of fluorescence sensing and optoelectronic devices has developed very quickly in recent years, because of their unique photoelectric activity and excellent solubility in polar solvents. Molecular design and synthesis of conjugated polyelectrolytes with novel functional chains, researching on the relationship between the structure of aggregations and photo-physical properties of polymers in different solution environment, have significant impacts on the design of the fluorescent materials and the development of organic photovoltaic devices.with two different monomers 2,5-dibromo-3-(6-bromohexyloxymethyl)thiophene and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di-n-octylfluorene, neutral alternating copolymer poly[(9,9-di-n-octyl-2,7-fluorene)-2,5-(3-(6-bromohexyloxymethyl) thiophene)](PFTBr) was synthesized by Suzuki-type cross-coupling. Then, the side chain of thiophene monomers of copolymer PFTBr was quaternized, getting the target product conjugated polyelectrolyte poly[(9,9-di-n-octyl-2,7-fluorene)-2,5-(3-(6-trimethylammonium hexyloxymethyl)thiophene)] (PFTNBr)?1H NMIL 13C NMIR?XPS?and EA all demonstrated the right structures of copolymers, and PFTNBr was 96.7% quaternized. GPC result shows the average molecular weight (38000) and narrow PDI (1.2) of copolymers. TGA results reveal that the thermal decomposition temperatures of the two copolymers are higher than 200?.To investigate the relationship between the structure of aggregations and photo-physical properties of conjugated polyelectrolyte PFTNBr in different solution environment, and the electrochemical characteristics of copolymers, we primarily combined the methods of UV absorption spectra (UV-vis), photoluminescence spectra (PL), steady and transient fluorescence spectra, transmission electron microscopy (TEM) and cyclic voltammetry.Firstly, we explored the photo-physical properties of conjugated polyelectrolyte PFTNBr in single solvents and mixed solvents. PFTNBr in methanol solution exhibits a kind of extended state; while PFTNBr in water, chloroform and tetrahydrofuran solution, aggregation is easy to occur. In mixed solvents of methanol/water and methanol/tetrahydrofuran, with increasing volume fraction of water or tetrahydrofuran in the solvent, the absorption band is blue-shifted and UV-vis intensity is decreased; while the emission band is red-shifted and PL intensity is decreased. TEM result demonstrates that uniform spherical aggregations are formed in water.Secondly, we investigated the photo-physical properties of conjugated polyelectrolyte PFTNBr in solutions with different ions.In concentrated alkaline solution, UV-vis intensity and PL intensity are decreased. While in acidic solution, UV-vis intensity and PL intensity are increased, but in a certain range of the hydrogen ion concentration, PL intensity is reinforced and then weakened.In methanol solution with metal cation, aggregations are formed and UV-vis intensity is decreased. However, when Fe3+is added to solution, with increasing addition of Fe3+, UV-vis intensity is increased significantly, while PL intensity is reinforced, then weakened, and finally quenched.With increasing concentration of potassium ferrocyanide in methanol solution, UV-vis intensity is decreased and PL intensity is reinforced, then weakened, but no obvious fluorescence quenching has occurred.Finally, the electrochemical characteristics of copolymer PFTBr and PFTNBr were analyzed by the cyclic voltammetry, and the ionization potential(HOMO) and electron affinity(LUMO) were calculated. The results show that compared to copolymer PFTBr, the energy levels of conjugated polyelectrolyte PFTNBr decline, which is consistent with the results of our simulation and measured results. Also the HOMO levels of copolymers are lower than that of P3HT, which means that both PFTBr and PFTNBr can be good cathode interfacial layers in photovoltaic devices.