Restricted Aggregate Formation on Tetraphenylethene-Substituted Polythiophenes

Abstract

Two new polythiophenes functionalized with different degrees of tetraphenylethene, TPE, side groups (with TPE moieties at each thiophene unit, homoPT, and one in which only each second thiophene unit carries a TPE side chain, coPT) were successfully synthesized and characterized together with a poly(3-hexylthiophene) with similar average molecular weight and a tetraphenylethene-thiophene model compound. The study aimed to understand the role of the sterically bulky TPE pendant groups on the inhibition of intra- and/or interchain packing (aggregation) of these conjugated polymers. For the tetraphenylethenethiophene model, compound aggregation induced emission (AIE) is active for water fractions ≥ 90%. An opposite behavior is found for the polythiophenes (homoPT and coPT) where aggregation caused quenching (ACQ) was found to occur both in solution and in the solid state. For the polythiophene with the higher degree of TPE labeling, homoPT, the amount of ACQ is significantly decreased. This is attributed to the high TPE substitution density in the polymer which promotes a more twisted conformation of the polymer backbone as the emissive chromophore. For the TPE-polythiophenes, only the energy migration mechanism was found to be active since the bulky TPE substituents restrict the polymer’s intramolecular motions. The experimental observations of a small shift in the emission spectra of homoPT and significant hindering of ACQ on going from solution to the solid state explain the absence of intermolecular interactions together with the restriction of intramolecular rotations.