High resolution study of the n = 7–9 p,p’-n-alkylazobenzenes phase transitions by photopyroelectric and adiabatic scanning calorimetries

Photopyroelectric Investigation of the trans–cis Isomerization Effect on the Nematic- Isotropic Phase Transition of a Liquid Crystalline Azobenzene

“…This is related to the broadening of the transition region because of the presence of the cis isomers which act as impurities within a matrix of trans isomers. It is worth noting that the temperature dependence of D obtained for the largest value of the intensity (6 mW/cm 2 ) is practically smooth over the investigated temperature range because of the absence of the formation of the N phase [11]. Also evident is the progressive decrease of the D values in the N phase, indicating a smaller degree of achieved mean molecular alignment.…”

“…7AB normally undergoes phase transitions from the crystalline (K) to the nematic (N) phase and, upon further heating, from the N to the isotropic (I) phase [11]. It has been found that, upon UV irradiation operated with a beam intensity of about 8 mW/cm 2 , the I phase converts directly into the K one [11], being the intermediate ordered phases not able to form because of the presence of a sufficiently large number of cis isomeric molecules. Comparative analysis of the effects obtained at different cis molecule concentrations on the occurrence of the phase sequence in 7AB has however not yet been carried out.…”

“…Accordingly, the I to N phase transition in 7AB samples for varying UV light intensities has been investigated by calorimetric evaluations. In particular, the photopyroelectric (PPE) calorimetric technique [13][14][15][16][17][18][19][20][21][22] has been adopted, which has been shown to be an effective tool for the characterization of phase transitions in PLCs [11,22] since, unlike other calorimetric techniques, it enables the photoisomerization of the molecules to be induced by the UV light even during the calorimetric evaluations. In this respect, it is worth mentioning that a previous Nuclear Magnetic Resonance (NMR) investigation [23], also aimed at characterizing the disorder introduced by the cis isomers in the N phase in 7AB, could be carried out only after the UV light inducing the photoisomerization of the molecules had been switched off.…”