Chromism and molecular weight of polyaniline derivatives

The striking solvatochromic shift observed in polyaniline derivatives correlates with two solvent polarity scales, donor number (DN) and hydrogen bond acceptor scale (Taft's β-scale). The large shift is caused by conformational changes in solution that result in dramatic differences in relative molecular weight values obtained by gel permeation chromatography (MW_GPC) in different solvents. For example, for poly-o-toluidine base in the emeraldine oxidation state, a low transition energy solvent like NMP (excitonic transition, λ_max 608 nm) yields a very high molecular weight value (M_w  15,000 g mol⁻¹) whereas a high transition energy solvent like CHCl₃ (excitonic transition, λ_max 570 nm) yields an absurdly low value (M_w  650 g mol⁻¹). An absolute molecular weight value, M_w  4700 g mol⁻¹, was obtained for the first time using laser light scattering (MW_LS) suggesting that CHCl₃ promotes a highly coiled chain conformation whereas NMP promotes a more expanded, rod-like conformation. A similar trend is observed for poly-o-toluidine base in the fully oxidized pernigraniline oxidation state (Pierels transition). This suggests that even though the Pierels and excitonic transitions have different molecular origins their conformation driven solvatochromic shifts trend in a similar fashion which is different from thermochromic trends reported in previous studies.