Effect of alkyl position of pyrrole on structures and properties of conjugated polysquaraines

Poly(1-octylpyrrolyl)squaraine and poly(3-octylpyrrolyl)squaraine were synthesized from squaric acid with 1-octylpyrrole and 3-octylpyrrole under identical conditions of condensation. Visible absorption spectra, photoluminescence spectra, TGA curves, and IR spectra were recorded to characterize the structures of these polymers as synthesized; the measured optical and thermal properties are consistent with backbone structures. The polysquaraine from 3-octylpyrrole possesses mostly zwitterionic repeating units (>97%), but the polymer from 1-octylpyrrole consists of zwitterionic and covalent repeating units in a ratio 2:1. Observations from the visible absorption and photoluminescence spectra in varied solvents, TGA curves, and thermal IR measurements for these two polymers reconciled to their structures. This result indicates that the position of the alkyl group in the pyrrole derivative affects the conformation in the condensation of poly(pyrrolyl)squaraine. These polysquaraines are stable below temperature 155 °C and their visible absorption and photoluminescence properties in hydrogen-bonding solvents exhibit negative solvatochromism.