Effect of pressure on structure and fluorescence of phthalocyanine

Abstract

Phthalocyanine (Pc) is a kind of important photoelectric material, but a lot of questions remain to be clarified, like the relationship between the structure of Pc and its photoelectric property. High pressure is a powerful tool to study the structure transformation. High pressure study on piezochromic materials, which shows color change under high pressure due to structure changing, serves as an effectively way of studying the relationship between materials’ structure and photoelectric property. In this work Raman spectrum is employed to study the phase change of α phase metal-free Pc (α-H₂Pc) under high pressure, meanwhile the effect of pressure on fluorescence (FL) is also studied to show how the Pc’s structure affects the photoelectric property. The diamond anvil cell is employed to achieve the high pressure condition, by using NaCl as a pressure transmitting medium. And Raman and FL measurements are performed by using a LabRam HR Evolution spectrometer equipped with a 473 nm laser. The Raman spectra of α-H₂Pc show to slightly change during compression to 12.0 GPa. The main Raman peaks remain at highest pressure, including the Raman peak from macrocyclic of Pc molecules, which shows the stability of Pc molecules. Note that an enhancement of Raman peak at 623 cm^–1 can be found with the pressure increasing, which appears only in the Raman spectrum of χ phase metal-free Pc (χ-H₂Pc), showing that α-H₂Pc is converted into χ-H₂Pc under pressure. The curve for Raman frequency as a function of pressure shows that no obvious evidence related to bonding or structure transition can be observed, which means that α-H₂Pc is transformed into χ-H₂Pc gradually. For FL spectrum, only the FL of excimer can be found in α-H₂Pc at atmosphere pressure. When the solid α-H₂Pc is compressed, the FL intensity is found to decrease as pressure increases, and it is quenched at 3.0 GPa. The FL of Pc molecule, which is not found in α-H₂Pc at ambient pressure, appears at 0.7 GPa. As the pressure increases, the FL intensity ratio between Pc molecule and excimer is enhanced. Considering the pressure induced phase transition from α-H₂Pc to χ-H₂Pc gradually, the change in FL spectrum should be due to the structure transformation. It is proved that the degree of overlapping between Pc molecules in α-H₂Pc is larger than that in χ-H₂Pc. We think, the degree of overlapping decreases under high pressure, which hinders the formation of excimer. It makes the excimer emission decrease and the FL of Pc molecules appear under high pressure. Our work can explain the relationship between Pc crystal structure and its fluorescence, reveals the kinetic behavior of macromolecules similar to Pc system under high pressure, and provides a new possibility of designing the photoelectric materials with excellent performances.