Molecular aggregation of poly[(1,3-phenyleneethynylene)-alt-oligo(2,5-dialkoxy-1,4-phenyleneethynyl ene)]: Effects of solvent, temperature, and polymer conformation

摘要

By altering solventonsolvent ratio and temperature, a comprehensive study is carried out to understand the molecular aggregation of poly[(1,3-phenyleneethynylene)-alt-oligo(2,5-dialkoxy1,4-phenyleneethynyle ne)]s 1 and 2 in solution, which possess different lengths of linear phenyleneethynylene (PE) fragments. Although both polymers exhibit random-coil conformation in THF solution, PPE 2 of longer rigid-rod fragment exhibits a higher tendency to form aggregate than 1. In a good solvent, optical spectra of PPEs reveal a gradual spectral shift in responding to the temperature change, which is attributed to molecular conformational change (planarity of chromophore). With addition of methanol to polymer solutions in THF, UV-vis absorption spectroscopy detects the aggregation band from 2 but not from 1. The combination of solventonsolvent and low temperature provides improved sensitivity to probe molecular aggregation. Although chromophore planarity is anticipated to play an important role, current results favor the pi-pi interaction in the aggregate formation of 1 and 2. [References: 17]