Ultrafast spectroscopy of liquid crystal conjugated polymer composite structures

Organic materials are nowadays more and more exploited in scientific research thanks to their wide flexibility and tunable properties. On one side, conjugated polymers are organic semiconductors that have gained wide popularity in development of new technology due to their promising properties. On the other hand, liquid crystals are materials that possess physical properties that are intermediate between conventional isotropic fluids and anisotropic solids, with various interesting characteristics depending on temperature and electric field applied to them. The main idea of this work is to try to take advantage of both the materials in order to realize a device in which the photophysical properties of the polymer could be changed or activated by the transition of the liquid crystal state. As a specific case, we selected to study a blend of F8BT as polymer and 5CB as liquid crystal. As a preliminary step in view of applications, in this work we have studied the physical behaviour of this blend and mainly the ultrafast dynamics of the photophysical processes which occur after an optical excitation. Due to the fact that the dynamics in organic materials typically take place on the subpicosecond timescale, the most powerful tool for studying them is pump-probe spectroscopy. We thus decided to put our blend in a cell made on purpose in order to be able to apply on it an electric field, which purpose is to align the liquid crystals. We then performed measurements on the blend with and without the electric field applied: the most important result observed is a change in the stimulated emission behaviour of the polymer when a field of suitable intensity is applied. Some hypothesis have been put forward to explain this effect.