Conjugated polymers (CP) are promising materials for modern organic electronics, combining structural and electronic adaptability to deliver efficient and low-cost devices. The molecular-scale characterisation of CPs is, however, still unsatisfactory, as analytical techniques that can provide high-resolution information on polymers are scarce. We report an innovative approach to study CPs, <i>i.e.</i> the combination of ultrahigh vacuum (UHV) electrospray deposition (ESD) and low-temperature scanning tunnelling microscopy (LT-STM). This method allows for intact deposition of thermally fragile macromolecules and their characterisation at the ultimate spatial resolution of UHV-STM, revealing their self-assembly, length distribution, sequence and exact chemical structure.<br/>Following the recent employment of CPs in OECTs, the typically used alkyl side-chains, that provide stability to the polymers through inter-strand interdigitation, have been substituted with polyethylene glycol (PEG) side-chains for optimal ion transport in solution. Despite the competitive performance of PEG-functionalised materials, the effect of PEG side-chains on the microstructure has not yet been examined. However, a precise understanding of the inter- and intra-molecular interactions is fundamental, as structural modifications of CP assembly impact order properties and charge-transport mechanisms. Here, we present the results of a comparative study on the widely employed poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT) and two PEG derivatives. The polymers were independently synthetised, deposited in UHV by ESD and studied by STM. Our findings allow to derive parallels and differences between alkyl and PEG side-chains, leading to the formulation of a new hypothesis for PEG side-chain interactions in polymeric films. New insight into the relationship between side-chains and polymer microstructure is also attained, allowing for predictions on optimised chemical structures.

Effect of side-chains on the assembly of conjugated polymers

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.