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

Highlights: • Photopyroelectric and adiabatic calorimetry enable to study p,p'-n-alkylazobenzenes. • The UV induced trans-cis isomerization influences the nAB phase transitions. • Upon cooling, 7AB phase transition sequence is: isotropic-nematic-crystal 2-crystal 1. • Trans-cis isomerization in 7AB and 8AB makes the intermediate mesophases disappear. A study of phase transitions of photochromic p,p'-n-alkylazobenzenes compounds was carried out by means of adiabatic scanning calorimetry (ASC) and photopyroelectric calorimetry (PPE) techniques, yielding accurately the temperature dependence of the enthalpy and of the specific heat capacity across the observed phase transitions, and the corresponding enthalpy of transition values. In addition, polarization microscopy imaging of the sample texture was performed together with the PPE calorimetric evaluations to establish the nature of the involved phases. By performing PPE calorimetric evaluations during UV irradiation of the samples, the influence of the photoinduced trans to cis isomerization of the nAB molecules on the occurrence of the phase transitions was assessed.