Synthesis, optical and electrochemical properties of novel copolymers containing oligo(p-phenylene ethynylene) moieties connect at bay position of perylene bisimide

Three conjugated copolymers containing different length of oligo (p-phenylene ethynylene) moieties (OPE) connecting at the bay position of N, N'-dioctylperylene-3, 4:9, 10-tetracarboxylic acid bisimide were designed and synthesized. The three OPE monomers were prepared according to a stepwise synthesis method. This method involves two sets of reaction conditions: desilylation and Pd/Cu-catalyzed coupling. In every two steps, 2, 5- dihexyloxy-4-[(trimethylsilyl)ethynyl]iodobenzene was coupled with the diethynyl compounds which were desilylated from the di(trimethylsilyl)-ethynyl compounds. Through this approach, undesired oligomerization was avoided and OPE monomer, trimer, pentamer were readily prepared in high purity. The bromination of perylene bisanhydride was carried out under reaction condition with a more than two-fold of bromine in 98 % sulfuric acid. After imidization by n-octylamine with acetic acid as catalyst in NMP, the resultant compound was soluble in dichloromethane, which facilitated its purification. The coupling polymerization was performed by a standard Sonagashira reaction between the acetylene-terminated OPE monomers and perylene monomer, leading to the final three alternative copolymers (P1, P2, P3). After polymerization reaction has been finished, all polymers dissolved in chloroform and then reprecipitated in methanol for three times, followed by Soxhlet extraction with acetone for 48 h to remove residual catalyst and monomers. The structures of the three polymers were characterized by IR, Elementary analysis and GPC. P1 has limited solubility in common solvents such as THF, dichloromethane, toluene, etc, due to the high rigidity of the backbone as well as the high content of perylene. But the solubility of P2 and P3 was improved dramatically. The photophysical characteristics of the polymers were investigated by UV-Vis and PL spectra in solution. In UV-Vis, the absorptions of perylene in polymers show a large red-shift (nearly 170 nm) compared to that of the perylene monomer, indicating a strong interaction between electron donor and acceptor on the ground state. Together with a non-interfered conjugated backbone, all polymers absorb from 350 to 900 nm which makes them promising in photovoltaic applications. In PL experiments, when exciting the perylene part, a complete fluorescent quenching was observed in all three polymers, which can be attributed to the electron transfer process between the electron donor and acceptor induced by excitation of the electron acceptor segment to a charge separated state. When exciting the OPE unit in each polymer, only the emission from the OPE chromorphore was observed in all three polymers. Cyclic voltammetry (CV) was used to assess the electrochemical behavior of the polymers. All three polymers show a reversible reduction wave, an irreversible wave in the more negative field and an irreversible oxidation wave. The first reversible reduction wave comes from the n-doping of perylene unit in polymers. From P1 to P3, the onset potential of first reduction wave decreases, which is in accordance with the p-doping degree increasing in each polymer. At the oxidation side, the onset potential of oxidation goes to the opposite way that is increasing from P1 to P3. By changing the amount of electron donating or accepting content in the polymers, the electronic properties of the corresponding polymers can be manipulated, which can serve as a guide for new material design in the kind and facilitating the photovoltaic device fabrication. It is hoped that these new polymers would be a promising candidate used for polymeric solar cells.