Outstanding chiroptical features in thin films of new chiral π-conjugated oligomers

Abstract

Chirality in organic optoelectronics is a topic which is rapidly gaining interest:1 in fact, it is a means to drive and control the supramolecular order of π-conjugated molecules constituting the active layers of the devices,2 and secondly it opens the way to highly specialized applications, such as producing (CP-OLED),3 or detecting (CP-OFET)4 circularly polarized (CP) light or specifically responding to enantiomers in analytical sensors.5 A current research goal is to obtain thin films of organic semiconductors displaying high discrimination of CP-light in absorption (electronic circular dichroism, ECD) or in emission (circularly polarized luminescence, CPL). Chiral supramolecular architectures of π-conjugated oligomers could be the key to achieve this goal. Starting from this consideration, we synthesized a set of new oligothiophenes functionalized with inexpensive alkyl chiral groups from natural sources (Figure 1a), studying their (chiro)optical features in thin films. Surprisingly, for some of them we found the uncommon property of ECD signal inversion by sample flipping (Figure 1b)6. This is due to the interference between linear dichroism and linear birefringence, called LDLB effect, which is theoretically well understood, but to date only rarely reported in the literature. In particular, we investigated the impact of LDLB in thin films depending on the deposition technique (drop casting vs. spin coating) and the chemical structure (by changing the alkyl chiral chains or the π-conjugated backbone)