Crystallization of Tyrian purple (6,6′-dibromoindigo) thin films: The impact of substrate surface modifications

Abstract

The pigment 6,6'-dibromoindigo (Tyrian purple) shows strong intermolecular hydrogen bonds and the film formation is, therefore, expected to be influenced by the polar character of the substrate surface. Thin films of Tyrian purple were prepared by physical vapor deposition on a variety of substrates with different surface energies: from highly polar silicon dioxide surfaces to hydrophobic polymer surfaces. The crystallographic properties were investigated by X-ray diffraction techniques such as X-ray reflectivity and grazing incidence X-ray diffraction. In all cases, crystallites with "standing" molecules relative to the substrate surface were observed independently of the substrate surface energy. In the case of polymer surfaces, additional crystallites are formed containing "lying" molecules with their aromatic planes parallel to the substrate surface. Small differences in the crystallographic lattice constants were observed as a function of substrate surface energy, the corresponding small changes in the molecular packing are explained by a variation of the hydrogen bond geometries. This work reveals that despite the limited influence of the surface energy on the molecular orientation, the crystalline packing of Tyrian purple within thin films is altered and slightly different structures form.

Bulk and Surface‐Mediated Polymorphs of Bio‐Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation

“…The main component of ancient pigment Tyrian purple is the indigoid of natural origin 6,6-dibromoindigo (Scheme 1), which has in the past years garnered a renewed interest for its ambipolar semiconducting properties in thin-film OFETs. [23,25] The Raman investigation was stimulated by the report of the existence for this system of genuine surface induced or thin film phases, all very similar to each other, identified on several substrates, [87,88] and very different from the single packing configuration so far reported in the literature. In this respect, Tyrian purple constitutes an interesting case among indigoids, because, unlike the previously discussed indigo and thioindigo, the effect of the surface appears to give rise to structures that otherwise would not exist.…”

“…Here less than perfect matches should not surprise because even in the presence of structural similarities the spectral features can differ greatly (e. g. the indigo polymorphs). Hence, the relevant piece of information conveyed by the spectroscopic data is that the origin of the phases detected in thin films as reported in refs [87,88] can be tracked down to bulk Form II, to which they appear to be closely structurally related.…”

“…Tyrian Form I has a monoclinic P2 1 /c structure with two molecules per unit cell (Figure 7a). [79,87,88] The thin-film cell obtained by combining grazing incidence X-ray diffraction data of films deposited on Si/SiO 2 and molecular dynamics simulations for crystal structure prediction [87,88] is instead triclinic P � 1 with one molecule per cell on an inversion centre. In both cases intermolecular hydrogen bonds and π À π stacking interactions are the driving forces for the molecular packing, but only Form I displays a strong similarity with indigo polymorphs.…”

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…Post-deposition thermal treatment leads to a reorganization into layered ultrathin crystalline nanostructures, exhibiting structural and photophysical properties similar to those of microscopic crystals obtained by solvent-induced precipitation. The commonality of the influence of surface energies and polarities of the substrate surfaces on the organization of organic molecules has also been reported [228].…”

Solution of an elusive pigment crystal structure from a thin film: a combined X-ray diffraction and computational study

“… 5 The second peak gives a d -spacing similar to the intermolecular distance of π–π interactions, suggesting that molecules lying parallel to the substrate surface are also present. The presence of molecules with two distinct orientations in the same film is not uncommon 38 – 41 and the broad, weak nature of the diffraction peaks observed for films prepared at 200 K shows that the film is not well ordered. 42 The fact that films produced at 200 K are mostly disordered can potentially explain, and is consistent with, the observed poor device performance of EPI transistors prepared at substrate temperatures of 200 K, 5 as the degree of molecular ordering is strongly correlated with charge transport mobility in thin films.…”