Giant Chiroptical Properties in Thin Films of a Chiral Indaceno[1,2-b:5,6-b′]Dithiophene-Based Organic Dye

Abstract

We report a new chiral indaceno[1,2-b:5,6-b′]dithiophene bis-thiophenylpropynone (IDT-TPO) dye exhibiting very noticeable chiroptical properties in thin films. While freshly prepared samples are ECD-silent, aging triggers a giant dichroic response, with ellipticity values reaching approximately 18,000 mdeg, among the highest reported to date for thin films of chiral organic dyes as neat materials. This amplification is accompanied by an unusual flattening of the main UV-Vis absorption band, and the combination of these effects produces very large g_abs values (exceeding 0.1). A thorough investigation, combining microscopy and spatially resolved chiroptical techniques with time-dependent density functional theory (TD-DFT) calculations, reveals that the intense chiroptical activity arises primarily from the consistent formation of right-handed three-dimensional (3D) helical architectures. Notably, the degree of circular polarization exhibits a non-monotonic dependence on film thickness: the maximum ECD intensity was found for a thinner film, rather than for the thickest one. This behavior is particularly promising for optoelectronic applications requiring efficient chiroptical responses in relatively thin active layers.