Synthesis of Low Band Gap Zwitterionic and Planar Conjugated Pyrrole-DerivedPolymers. Reversible Optical Absorbances From the UV to the Near-IR

摘要

Described is the synthesis of a zwitterionic pyrrole-derived polymer. The monomer(1) is formally an azomethine ylide and it was synthesized in 3 steps from pyrrole. 1 was polymerized using copper-bronze in DME. The polymer had a strong optical absorbance at 512 nm in THF and an inter-unit ionic interaction is proposed to explain this large red-shifted band. If aqueous NaOH is added to the THF solution, or a Lewis basic solvent used instead of THF/NaOH, then a tremendous bathochromic shift occurs to 881 or 901 nm, respectively. The large shifts are attributed to a dramatic structural change in the polymer induced by hydroxide or Lewis base attack on the iminium unit. This causes a cascade of electron shifts to form a fully conjugated polymer that is fixed into a planar conformation. The planarization process is reversible so that addition of acid or shifting to a weaker Lewis basic solvent causes the polymer to recover its zwitterionic form and its original optical absorption characteristics. Moreover, upon mild reduction of the zwitterionic polymer with Pd/C and H2 (1 atm), some of the extended conjugation is lost but a polymer forms that has an enormous optical absorption maximum range from the near-UV to the near-IR (Lambda sub max = 886 nm) depending on the solvent or hydroxide concentration. Further, a sample of the reduced polymer can be dispersed in plasticized poly(vinyl chloride). The flexible polymer composite reversibly changed from dark blue-brown in aqueous hydroxide to bright yellow-orange in aqueous HCl.