Abstract - GISR 2016

GISR 2016 Padova
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Micro-Raman investigation of the lattice phonon modes of Indigoids:
a route to the characterization of bulk and thin film phases.
Tommaso Salzillo, Alice Fiocco, Arianna Rivalta, Elisabetta Venuti, Raffaele G. Della Valle,
Aldo Brillante
Dipartimento di Chimica Industriale “Toso Montanari” and INSTM-UdR di Bologna, Viale Risorgimento 4, 40136 Bologna (Italy).
[email protected]
In the search of materials for applications in organic electronics, there has been in the past years a growing interest
in small molecule semiconductors of natural origin, to be used in eco-friendly devices where all components are
designed from biocompatible and biodegradable compounds [1]. Ideal candidates must have the same
characteristics found in the best performing organic semiconductors, i.e. aromatic or heteroaromatic conjugated
cores, good chemical stability, crystal packing optimizing π-π stacking and charge transport properties. Good
solubilities are sought to impart solution processability.
Natural pigments such as indigoids, napthoquinones and alizarines have been found very promising, with indigoids,
among these, showing the best charge transport properties and a balanced ambipolar behavior [2]. Indigoids are
probably the most ancient of the natural dyes found in human history and owe many of their interesting properties
to the capability of forming both intra- and inter-molecular strong hydrogen bonds, which are responsible for the
aggregation of the dyes in solution and guide their crystal structures.
Films of indigo [1-4] have been grown by vapor deposition techniques on various substrates and, characterized by
grazing incidence X-ray scattering, reported to be partly crystalline, with a strong dependence of crystallinity on the
nature and conditions of the substrates [4,2]. In fact, a surface-induced crystal structure appears to grow on
aliphatic hydrocarbon dielectrics [2], whereas on silica it is not possible to identify which one of the two known bulkphase polymorphs is present [4].
Using micro-Raman techniques to investigate crystal polymorphism in the bulk and thin film phase is an efficient
method, capable of monitoring physical modifications in domains at the micrometer scale [5]. Lattice phonon
measurements, in particular, are an invaluable tool, as they probe intermolecular interactions and depend
dramatically on the molecular packing.
Indigo, and indigoids in general, with polymorphs characterized by slight changes in packing and the occurrence of
surface-induced structures, appear to be interesting and challenging systems for the Raman investigation here
presented.
Figure 1 Lattice phonon Raman spectra and crystal packing of Indigo (left) and Thioindigo (right).
References
[1] Głowacki, E. D. G. Voss, G Sariciftci, N.S., Adv. Mater. 25, 6783 (2013).
[2] Anokhin, D.V. Leshanskaya, L.I. Piryazev, A.A. Susarova, D.K. Dremova, N.N. Shcheglov, E.V Ivanov,D.A. Razumova, V.
F. Troshin, P. A., Chem. Commun. 50, 763 (2014).
[3] Głowacki, E. D. Leonat, L. N. Voss, G. Bodea, M.A. Bozkurt, Z. Irimia-Vladu, M. Bauer, S. Sariciftci, N. S., AIP Advances 1,
42132 (2011).
[4] Scherwitzl, B Resel, R. Winkler, A., J. Chem. Phys. 140, 184705 (2014).
[5] Brillante A., Bilotti, I. Della Valle, R.G. Venuti, E. Girlando, A., CrystEngComm, 10, 937 (2008).