Ferroelectricity and polymorphism in organic materials : Infrared and Raman spectroscopy of charge-transfer crystals and organic semiconductors

We have studied two classes of organic materials: charge-transfer mixed-stack organic crystals and organic semiconductors. Several charge-transfer crystals exibit the so-called neutral-ionic phase transition. We have investigated the temperature-induced phase transition of dimethyltetrathiafulvalene-chloranil by means of vibrational, infrared and Raman, spectroscopy. For another charge-transfer crystal, tetrathiafulvalene-chloranil, that shows ferroelectric properties at low temperature, we have studied the effect of an external electric field. In the field of organic semiconductors we have taken into account the problem of polymorphism. By means of Raman spectroscopy in the lattice phonons region, we have showed the possibility of discriminating the two different polymorphs of quaterthiophene. For the other examined crystal, rubrene, we have not experimental evidence of the existence of different polymorphs.