In search of efficient solubilizing groups for liquid and luminescent oligo(phenylene-thiophene) chromophores

Abstract

In this work, the synthesis of oligomers having a rigid conjugated 4,4′-bis(2-thienyl)biphenyl fragment end-capped with various types of solubilizing groups (SGs), such as either alkyl or alkylsilyl or alkyl-oligodimethylsiloxane, has been reported. The comprehensive study of their thermal and optical properties as well as rheology in comparison to model highly crystalline oligomers with simple either hexyl or trimethylsilyl SGs allowed us to elucidate structure–property correlations and find the most powerful type of SG in terms of liquefaction for them. It was revealed that oligomers with long and branched alkyl SGs still retain high crystallinity, whereas oligomers with alkyl-oligodimethylsiloxane SGs combine very low glass transition temperatures (up to −111 °C) with a liquid-crystalline behaviour. The alkylsilyl SGs were found to be the most efficient, since the oligomers end-capped with trihexyl- and tri(2-butyloctyl)silyl SGs are liquid and have low values of both the glass transition temperature (up to −60 °C) and viscosity (up to 1.94 Pa s). All the oligomers prepared have similar optical absorption/luminescence spectra and high values of photoluminescence quantum yield in solution (90–95%) without a significant impact of the SG type. In the neat films, the type of SG has a huge impact on the shape and maxima of the absorption and luminescence spectra as well as the photoluminescence efficiency. Among this series of molecules, oligomers with alkylsilyl SGs demonstrate the highest values of photoluminescence quantum yield in the neat form (24–61%) and close to the solution optical characteristics, which indicates their strong capability to suppress aggregation of molecules in the bulk. Thus, for the first time liquid luminescent thiophene/phenylene co-oligomers were reported and the solubilizing capabilities of some of the most promising types of SG were comprehensively investigated and compared to each other. The results obtained can be used as a guideline for the design of functional materials based on conjugated oligomers with a tunable and controllable phase behaviour, solubility and optical properties in the neat state.