Pressure induced structural change in PbPc studied by infrared and UV–visible spectroscopy and theoretical calculation

Abstract Lead phthalocyanine (PbPc) has a non-planar ‘shuttle-cock’ structure with a C 4 v molecular symmetry and forms a one-dimensional column in the crystal. We measured infrared and UV–visible spectra for the PbPc crystal under high hydrostatic pressure by using a diamond anvil cell. The IR spectrum of PbPc shows three strong peaks in the 1000–1200 cm −1 region. With increasing pressure, the intensity ratio of the middle peak to the other two peaks increased. This result suggests a structural transformation of the PbPc molecule from the shuttle-cock structure toward the planar structure with increasing pressure. In the UV–visible spectra, two remarkable changes were observed under high pressure: the peak intensity of the band at 2.7 eV was decreased, and the band at 1.5 eV was shifted to lower energy and broadened. The former feature suggests that the highest occupied molecular orbital (HOMO) band is not filled perfectly in the solid-state of PbPc under ambient pressure, and that the filling of the HOMO band occurs with increasing pressure. The change on the low energy band at 1.5 eV due to increasing pressure can be attributed to an increase in the intermolecular interaction.