RESUMO
We present a Raman spectroscopy study of the vibrational properties of free-base meso-tetra(4-pyridyl) porphyrin polycrystals under various temperature and hydrostatic pressure conditions. The combination of experimental results and Density Functional Theory (DFT) calculations allows us to assign most of the observed Raman bands. The modifications in the Raman spectra when excited with 488 nm and 532 nm laser lights indicate that a resonance effect in the Qy band is taking place. The pressure-dependent results show that the resonance conditions change with increasing pressure, probably due to the shift of the electronic transitions. The temperature-dependent results show that the relative intensities of the Raman modes change at low temperatures, while no frequency shifts are observed. The experimental and theoretical analysis presented here suggest that these molecules are well represented by the C2v point symmetry group.
RESUMO
We report complementary high-pressure Raman and single-crystal X-ray diffraction studies of a perovskite-like dicyanamide framework of [(C3H7)4N][Mn(N(CN)2)3] ([TPrA][Mn(dca)3]). Our studies show that the bulk modulus of the ambient pressure P4[combining macron]21c phase is B0 = 8.1(11) GPa, and the ab-plane compresses by 54.4(15) × 10-3 GPa-1 and the c-axis by 8.0(12) × 10-3 GPa-1, indicating the low stiffness of the framework and its highly anisotropic nature. [TPrA][Mn(dca)3] transforms near 0.4 GPa into the Pbcn phase. The driving forces for this symmetry change are partial ordering of the dicyanamide (dca) linkers, off-center shifts of TPrA+ cations and large changes in the columnar shifts of the MnN6 octahedra within the ab-plane. Upon further increase of pressure, [TPrA][Mn(dca)3] undergoes symmetry-lowering transitions into a monoclinic phase (space group P21/n) near 3 GPa and a triclinic phase near 5 GPa. The observed structural changes are, however, very subtle.
