In Search of Surface-Induced Crystal Structures: The Case of Tyrian Purple

Recently, structural studies in the thin-film phase of 6,6′-dibromoindigo, the main component of the Tyrian purple dye, revealed the presence of surface-induced structures. This finding is important, as the system is a good candidate for applications in organic electronics, and knowledge of the film structure is needed to explain the physical properties of the conducting layer. In this work, combining the results of low-frequency Raman spectroscopy, X-ray diffraction, and accurate density-functional theory (DFT) calculations of the vibrational properties of the proposed surface-induced structures, we suggest that a newly detected 6,6′-dibromoindigo bulk crystal form is structurally closely related to the surface-induced modifications. Thus, 6,6′-dibromoindigo behavior shows similarities and differences at the same time, with respect to other investigated systems.
We thank CINECA Supercomputing Center for providing computer time through the ISCRA scheme (project C—SCSCDFT). T.S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919). The authors thank Prof. Roland Resel for the fruitful discussions. This work has been funded by the PRIN project “From natural to artificial light-harvesting systems: unveiling fundamental processes toward a bio-inspired materials design” (HARVEST) protocol 201795SBA3.