Thin films of novel chiral π-conjugated oligomers with outstanding chiroptical features

Chirality is gaining large interest in organic optoelectronics:[1] first, it is a valid tool for controlling the supramolecular order of π-conjugated molecules constituting the active layers of devices;[2] secondly, it opens the way to innovative applications, like producing (CP-OLED),[3] or detecting (CP-OFET)[4] circularly polarized (CP) light. A current research goal is to obtain thin films of organic semiconductors with high discrimination of CP-light in absorption (electronic circular dichroism, ECD) or in emission (circularly polarized luminescence, CPL). Chiral supramolecular architectures of π-conjugated systems could be the key to achieve this goal. We synthesized a set of new oligothiophenes functionalized with inexpensive alkyl chiral groups from natural sources, studying their (chiro)optical features in thin films, both in absorption and in emission. Surprisingly, for some of them we found the uncommon property of ECD signal inversion by sample flipping: a chiroptical heads and tails (Figure 1).[5] This is due to an interference effect between linear dichroism and linear birefringence, called LDLB effect, which is theoretically well understood, but to date rarely reported in the literature. We investigated the impact of LDLB depending on the deposition technique (drop casting vs. spin coating) and the chemical structure (i.e. by changing the alkyl chiral chains or the π-conjugated backbone). For some of them we also found promising CPL properties, with maximum glum values up to 2 •10-2 (quite high for purely organic molecules), opening the door to possible applications in CP-OLED devices.