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

Raman spectroscopy of organic molecular materials in the low-wavenumber region gives access to lattice vibrational modes and to the wealth of information on solid state properties that these can provide. In the field of organic electronics a useful application concerns the discrimination of the crystalline forms i. e. polymorphism of the semiconductor. The capability of characterizing and identifying the polymorphs of a compound is in fact the prerequisite for an exhaustive study of the charge transport characteristics which arise from the relationship between molecular, electronic, and crystal structures. Thus, the need is felt of a non-invasive, non-destructive tool such as Raman, which probes the crystal phase by detecting the lattice modes which are sensitive even to subtle variations of the packing. Here we review the contribution of the technique to the study of organic pigments displaying promising semiconducting properties and characterized by polymorphism both in their bulk and thin film phases.