Effects of molecular weight on the optical and electrochemical properties of EDOT-based π-conjugated polymers

Absolute molecular weight values obtained by gel permeation chromatography with multi angle light scattering of PEDOTF8 were approximately 65% of the relative molecular weight values obtained by gel permeation chromatography using polystyrene standards. Both light absorption and molecular weight measurements showed the effective conjugation lengths (absolute molecular weight <2600, relative molecular weight <4000, number of EDOT-F8 units: n < ca. 5 unit). The low molecular weight polymers showed higher energy absorption and fluorescence bands. Molecular weight also affected the electrochemical process of the polymer films. The high molecular weight PEDOTF8 (number average molecular weight > 70000) showed a higher redox stability than the low molecular weight one. The two polymers with number average molecular weights of 70200 and 40000 retained 65% and 25% of the charge storage capacity after 100 electrochemical scans, respectively. Square-wave potential step absorptiometry measurements of the polymers with number average molecular weights of 70200 and 40000 showed that the optical contrasts remain stable after ca. 55 and three cycles, respectively. The high molecular weight polymer has a high electrochemical stability and would be a good material for optoelectronic devices.