Influence of Axial and Peripheral Ligands on the Electronic Structure of Titanium Phthalocyanines

To discover how molecular changes affect the electronic structure of dye molecules for solar cells, we have investigated four titanium phthalocyanines customized by axial and peripheral ligands (monodentate oxo versus bidentate catechol and tert-butyl versus tert-butylphenoxy, respectively). X-ray absorption spectroscopy and photoelectron spectroscopy were combined with density functional theory (DFT) and crystal-field multiplet calculations to characterize the Ti 3d and N 2p valence electrons that form the frontier orbitals. When a monodentate oxo axial ligand was replaced by a bidentate catechol ligand, the multiplet structure of the Ti 2p-to-3d transitions was found to change systematically. The most noticeable change was an additional transition into the low-lying 3d(xy) level, which is attributed to a reduction in local symmetry from 4-fold to 2-fold at the Ti center. An increase of the Ti 2p core level binding energy was observed in the bidentate complex and compared to a calculated core level stabilization. DFT predicts a change of the LUMO from the inner phthalocyanine ring to the Ti d(xy) orbital and a reversal of the high-lying d(x)(-y)(2)(2) and d(z)(2) orbitals. The N Is edge was calculated using time-dependent density functional theory (TDDFT) and compared to experiment.