Structural, Spectroscopic, and Computational Characterization of the Concomitant Polymorphs of the Natural Semiconductor Indigo

Indigo [2,2′-bis­(2,3-dihydro-3-oxoindolyliden)], a commonly used natural dye, has been shown to exhibit a highly promising semiconducting behavior, allowing for the realization of ambipolar devices. Nevertheless, up to date, it is still unclear which crystal structure is present in the thin films,...

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Published in:Journal of physical chemistry. C 2018-08, Vol.122 (32), p.18422-18431
Main Authors: Salzillo, T, d’Agostino, S, Rivalta, A, Giunchi, A, Brillante, A, Della Valle, R. G, Bedoya-Martínez, N, Zojer, E, Grepioni, F, Venuti, E
Format: Article
Language:English
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Summary:Indigo [2,2′-bis­(2,3-dihydro-3-oxoindolyliden)], a commonly used natural dye, has been shown to exhibit a highly promising semiconducting behavior, allowing for the realization of ambipolar devices. Nevertheless, up to date, it is still unclear which crystal structure is present in the thin films, a piece of information relevant for device applications. In this work, we address this issue by an in-depth characterization of the polymorphs of Indigo in the bulk and in drop-cast films. To do this, X-ray diffraction (XRD) and micro-Raman spectroscopy have been employed jointly, with the support of state-of-the-art density functional theory calculations in the solid state. Structural and spectroscopic characterizations have established that the two known A and B polymorphs grow as concomitant in the bulk under most of the experimental conditions adopted in this work. In the drop-cast films, XRD cannot unambiguously identify the structure, but Raman spectroscopy is effective in establishing that only the B form is present. The calculations augment the experiments, providing valuable insights into the relative thermodynamic stability of the two forms as a function of temperature. They also allow for a more comprehensive characterization of the Raman modes.
ISSN:1932-7447
1932-7455
DOI:10.1021/acs.jpcc.8b03635