Electronic circular dichroism imaging (CDi) maps local aggregation modes in thin films of chiral oligothiophenes

We highlight the power of electronic circular dichroism imaging (CDi), a technique developed at Diamond Light Source B23 beamline for synchrotron radiation circular dichroism (SRCD) to detect local domains endowed with a chiral supramolecular order in thin films of chiral oligomers and polymers. The highly brilliant and collimated beam light of B23 enabled CDi at a spatial resolution of 0.1 mm, unattainable with benchtop electronic circular dichroism (ECD) spectropolarimeters. CDi is bridging the gap between standard ECD spectroscopy and conventional microscopy. Presently, we apply CDi to reveal the local polymorphism of chiral oligothiophene-based molecules. By extensive use of a post-acquisition data-analysis tool called similarity factor, we quantified the polymorphs revealing a manifold of aggregation pathways whose relative weight was a function of the sample preparation protocol. This work uncovers the parameters that need to be optimised in order to obtain reproducible and controllable supramolecular structures in thin films, which is of paramount importance for materials with optoelectronic properties. It is noteworthy that this analysis is based on CDi measurements at high spatial resolutions of thin films of organic chiral semiconductors at the ultimate stage of preparation as the active layer of an optoelectronic device.