Photopyroelectric calorimetry: An advanced tool for the investigation of optically induced phase transitions

In this work, we discuss the unique capability of photopyroelectric (PPE) calorimetry to investigate optically induced phase transitions in phototropic liquid crystals. To this end, we present experimental results obtained for two mixtures consisting of 4-octyl-4-cyanobiphenyl (8CB) liquid-crystal hosting methyl 8-(49-pentylbiphenyl-4-yl)-2-phenyl-2-(4-fluorophenyl)-2H-naphtho[1,2-b]pyran-5-carboxylate and p,p′-diheptylazobenzene, respectively, being such molecules characterized by opposite UV-induced photoisomerization behavior. We demonstrate that the combined analysis of the investigated thermal parameters, supported by the simultaneous optical imaging of the sample texture, provides valuable insights into the underlying photoinduced processes. In particular, the temperature dependence of the specific heat allows us to quantify the light-induced shifts of the phase transition temperatures and to assess the influence played by the different isomeric states of the photoactive molecule on the critical behavior of the 8CB phase transitions. On the other hand, thermal conductivity measurements enable the investigation of optically driven changes in the average molecular orientation and, in addition, the determination of characteristic timescales associated with these reorientation processes. These results highlight the effectiveness of PPE calorimetry as a powerful tool for probing both the thermodynamic and kinetic aspects of optically stimulated phase transitions in liquid crystals.