Molecular distortion and pi-pi interactions in the solid state of titanylphthalocyanine

The influence of intermolecular interactions on the optical absorption has been investigated in single crystals (phases I and II) and evaporated films (amorphous phase, phases I, II and Y) from the standpoint of molecular distortion and interplanar pi-pi interactions. Careful re-examination of the X-ray structure analysis revealed that the TiOPc molecules are considerably distorted in phase II (near-IR active) and there are close atomic contacts along the molecular stack. This molecular distortion (reduction in molecular symmetry: C-4v-->C-1) lifts the doubly degenerate excited state, giving rise to the two absorption bands (660 and 840 nm) whose transition moments are orthogonal. The major part of the optical absorption spectra in phases I and II can be interpreted in terms of the distortion of molecular framework. Furthermore, the molecular distortion is found to be closely linked with interplanar pi-pi interactions, as supported by the temperature dependence of absorption spectra and lattice contraction. The correlation between molecular distortion and pi-pi interactions suggests that the molecule can be distorted in the solid state due to pi-pi interactions along the molecular stack.