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, 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.