Synthesis and Acidichromic Properties of Copolymers of Quinoxaline and Dioctylflourene

2, 5-Dibromo-o-pheneylenedimanine was obtained by the reduction of 4, 7-dibromo-2, 1, 3-benzothiadiazole, and four monoquinoxaline type monomers (M1, M2, M3, M4) were prepared by the reaction of 2, 5-dibromo-o-pheneylenedimanine with 5, 6-dione-1, 10-phenanthroline, phenanthrenequinone, acenaphthenequinone and benzyl, respectively. Using palladium diacetate (Pd(CH₃COO)₂) and ligand tricyclohexylphosphine as catalyst, four donor-acceptor-donor (D-A-D) type polymers (P1, P2, P3, P4) were prepared by Suzuki coupling alternative copolymerization of 2, 7-bis(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-9, 9-dioctylfluorene (M5) with four monoquinoxaline type monomers, respectively. All monomers and polymers were characterized by FT-IR and ¹H-NMR spectra. The optical properties of polymers solution in CHCl₃, at neutral state and when different concentrations of trifluoracetic acid (TFA) was added to the solution, were investigated by UV-Vis and fluorescence spectrometer. The UV-Vis spectra results showed that four D-A-D type polymers had good sensitivity to low concentration of TFA, wherein polymer P1 was sensitive to 2.17×10^-7 mol/L of TFA and polymers P2-P4 were sensitive to 4.32×10^-3 mol/L of TFA. Polymer P1 had better sensitivity to low concentration of TFA than P2-P4, due to the existence of two pyridine rings in quinoxaline unit of polymer P1. The colors of polymers CHCl₃ solution changed significantly after acidified by TFA, a red shift of absorption bands (UV-Vis spectra of polymers P1-P3) followed, and new wide absorption bands on the UV-Vis spectra of P2-P4 appeared. The fluorescence spectra of P1-P4 solution in CHCl₃ were remarkably sensitive to low concentration of TFA, wherein P1 solution (11.0 μg/L) was sensitive to 7.0×10^-10 mol/L of TFA and P2-P4 (32.0, 28.0, 10.0 μg/L) solution were sensitive to 1.73×10^-6, 7.5×10^-7 mol/L and 4.0×10^-6 mol/L of TFA, respectively. The fluorescence spectra intensity of all four polymers decreased to very small value when corresponding concentration of TFA was added to the polymer solution.