Abstract
Aggregation of cationic isothiouronium polythiophenes with alkoxy-spacers of different lengths at the 3-position of the thiophene ring was studied in solvents of different polarities. Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aqueous solutions was studied by electron paramagnetic resonance spectroscopy, using paramagnetic probes of different polarities. The two polymers displayed similar features in respect to conformation, effect of cosolvents on aggregation, unstructured absorption–fluorescence spectra, Stokes shifts when aggregated, solvatochromic effect, and self-quenching concentration. However, these polymers also showed different specific interactions with water, Stokes shifts in water, effect of the solvent on the extent of dominant state of the S1 level, and also different inner cavities and hydrophobic–hydrophilic surface area in aqueous solution aggregates. Water maximized the difference between the polymers concerning the effect of specific increases in concentration, whereas the presence of 1,4-dioxane generated almost identical effects on both polymers.
Highlights
Molecular aggregation of π-conjugated molecules is relevant because the functional properties and electronic interactions of these systems are modulated by varying factors such as temperature, solvent polarity, and concentration.[1]
Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aqueous solutions was studied by electron paramagnetic resonance spectroscopy, using paramagnetic probes of different polarities
As a brief resume of results, on one hand, regardless of the spacer length, the two polymers, PT1 and PT2, display several similarities, some of which have been widely associated to charged polythiophenes in solution in the literature
Summary
Molecular aggregation of π-conjugated molecules is relevant because the functional properties and electronic interactions of these systems are modulated by varying factors such as temperature, solvent polarity, and concentration.[1] Conjugated polyelectrolytes (CPEs) possess an electron delocalizing πconjugated backbone, which confers these materials semiconducting, chromophoric, and fluorophoric properties, whereas functionalization with hydrophilic excess charge confers these molecules solubility in polar solvents (e.g., water) These materials possess the interaction driving forces of conjugated polymers (π−π stacking and hydrophobic interactions) and those of polyelectrolytes, such as coordination through electrostatic forces[2] and hydrogen bonding (H-bonding),[3] with surrounding water molecules, allowing their aqueous solubilization.[4] Recently, transmission electron microscopy studies demonstrated that the aggregation in solution of an ethylene oxide polythiophene is directed by the π−π interactions of backbones and by the H-bonding capacity of the side chains.[5]. Films present a relatively limited number of physical parameters to tune.[6]
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