Phase behaviour of the discotic mesogen 5,10,15,20-tetrakis (4-n-dodecylphenyl)porphyrin under pressure

The phase behaviour of the discotic mesogen 5,10,15,20-tetrakis(4-n-dodecylphenyl)porphyrin (C12TPP) was investigated under hydrostatic pressures up to 300MPa by high pressure DTA and wide angle X-ray diffraction methods. The typical enantiotropic phase transitions of C12TPP, low- to high-temperature crystal (Cr-2-Cr-1), Cr-1-discotic lamellar phase (D-L), and D-L-isotropic liquid (I) are observed at pressures up to 10MPa. Application of hydrostatic pressure to the sample generates a pressure-induced crystal polymorph (Cr-3) between the Cr-2 and Cr-1 phases, and the phase transitions Cr-2-Cr-3-Cr-1-D-L-I occur reversibly in the pressure region between 10 and 180MPa. On heating at higher pressures above 180MPa, the fourth crystal polymorph (Cr-4) is formed between the Cr-2 and Cr-3 phases at lower temperatures, and at the same time the fifth crystal polymorph (Cr-5) appears abruptly between the Cr-1 and D-L phases at high temperatures. The Cr-2-Cr-4-Cr-3-Cr-1-(Cr-5)-D-L-I transition processes were observed at 180 similar to 200 MPa. Further increasing the pressure above 270 MPa induces entirely different thermal behaviour: only two peaks for the pressure-induced transition between the sixth and fifth polymorphs (Cr-6-Cr-5) and the Cr-5-I transitions are detected at low and high temperatures on heating, while both the DTA and WAXD experiments on cooling show the formation of the D-L phase as a monotropic phase between the I and Cr-5 phases, indicating the I --> D-L --> Cr-5 --> Cr-6 process. The thermal behaviour was ambiguous and complex in the pressure region between 200 and 260MPa because the peaks for the intermediate crystal transitions were too small to detect with confidence. The two different sequences of the Cr-2-Cr-4-Cr-3-Cr-1-D-L-I and Cr-6-Cr-5-(D-L)-I processes seems to occur competitively. The T vs. P phase diagram of a sample cooled at 300MPa was studied to determine the triple point of the D-L phase and to investigate the phase stability of the pressure-induced crystal polymorphs. The Cr-6-Cr-5-I transition process was observed on heating at 200 and 300MPa, while the Cr-6-Cr-5-D-L-I process was detected at lower pressures below 100MPa. Since the Cr-5-D-L transition temperature changes linearly with a slope dT/dP congruent to 40 degrees C/100MPa, while the D-L-I transition temperature changes slightly (dT/dP congruent to 5.5 degrees C/100MPa), the D-L phase forms a triangle in the T vs. P diagram. The triple point of the D-L phase was found to be 240.8MPa and 168.8 degrees C. The Cr-6 polymorph reorganized to the stable Cr-2 form under atmospheric pressure on annealing at room temperature overnight.