jp8b03635_si_004.avi (426.11 kB)
Structural, Spectroscopic, and Computational Characterization of the Concomitant Polymorphs of the Natural Semiconductor Indigo
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posted on 2018-07-11, 00:00 authored by T. Salzillo, S. d’Agostino, A. Rivalta, A. Giunchi, A. Brillante, R. G. Della Valle, N. Bedoya-Martínez, E. Zojer, F. Grepioni, E. VenutiIndigo
[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.