Probing the electronic structure of titanium phthalocyanines via XAS, XES, RIXS and UV-visible spectroscopy Janine Grattage, Tsu-Chien Weng, Marcin Sikora, Kristina Kvashnina and Pieter Glatzel ID26, ESRF. 6 Rue Jules Horowitz, BP 220, 38043 Grenoble, France
ESRF
The aim of this research is to understand the electronic structure in titanium (and other metal) complexes, particularly phthalocyanines, using K-edge XANES, nonresonant and resonant XES spectroscopy1, and UV-visible spectroscopy. We discuss the relative advantages and disadvantages of these different techniques, and aim to correlate optical-range peaks in both UV-visible and RIXS spectra. Ti phthalocyanines (TiPc)
2-D RIXS maps
TiPcCl2
TiPcO
PMCP-TiCl3
Non-resonant XANES and valence to core XES spectra K-edge XANES records information about the crystal field splitting and local coordination in the pre-edge region. A intense single pre-edge peak in TiPcO is due to Ti 5-coordination and p-d mixing. Valence to core XES probes the ligand effect on the metal and is an elementspecific equivalent to VB-PES. Ligand peaks can be assigned as shown. FEFF calculations are also included to show calculated peak contributions. N,O 2p O2s N2s
Energy Transfer Ω−ω (eV)
Energy Transfer Ω−ω (eV)
Region:
Energy Transfer Ω−ω (eV)
TiPcO is a common choice for IR sensitised devices, and is used in molecular solar cell development2. Understanding the electronic structure of TiPcO and related compounds facilitates directed engineering of future devices. Powder TiPcO, and the related TiPcCl2 were studied with PMCP-TiCl3 included as a model Ti-Cl system.
Optical peak Elastic peak
RIXS maps were measured using the Kβ2,5 emission line over the Ti K edge. RIXS is element specific, unlike UV-visible, so we can attribute transitions directly to the metal atom being probed. Also, some forbidden optical transitions (d-d excitations) can be accessed in RIXS3 as there are different selection rules for the two processes. Note the optical peak at 2.7eV for PMCP-TiCl3 and the relative intensity increase in region 2 of the elastic peak for the Pc molecules.
Cl3p 1.6eV
Cl3p
Ti K-edge XANES
UV-visible spectra Unlike RIXS, UV-visible spectra are not influenced by momentum transfer, and have much higher energy resolution, but a shorter energy range. Here cobalt Pc data is included as a interesting case of UV-visible vs RIXS. We want to compare and identify features in UVvisible spectra with those found in RIXS. In the Ti samples there are no obvious features present in both spectra types, perhaps due to molecular distortion caused by axial groups (O, Cl) affecting the possible transitions.
Non-resonant Kβ valence to core XES
Total energy diagram for valence band RIXS The energy diagram below shows an optical transition accessed through a RIXS process (black) and UV-visible spectroscopy (red). Possible molecular orbitals (MOs) accessed via RIXS are shown calculated using DFT. The electrons in RIXS are excited into the pd type MOs. The pre-edge is found to be mostly Ti p-like from DFT calculations. There is some Ti d-like contributions also, due to the low symmetry of the molecules. Selection rules determine which MOs can be accessed. After excitation, an electron relaxes from another pd MO to fill the 1s core hole resulting overall in an optical range transition, detected in RIXS and UV-visible spectra.
However, CoPc (planar Pc) data shows a weak optical peak in RIXS at 2.4eV (marked *) coinciding with the CoPc UV-visible peak which is not present for free base Pc. Resonant XES spectra from 2-D RIXS maps, with UV-visible data
Conclusions •UV-visible and RIXS spectroscopy are complementary probes of the electronic structure of organnometallic molecules. •The challenge is to interpret these measurements to determine the electronic structure of these interesting materials.
References: 1. P. Glatzel and U. Bergmann, “High resolution 1s core hole spectroscopy in 3d transition metal complexes–electronic and structural information”, Coord Chem Rev, 249(1-2), 2005, pp65 2. E. Palomares et al, “State selective electron injection in non-aggregated titanium phthalocyanine sensitised nanocrystalline TiO2 films”, Chem Comm, 18, 2004, pp2112 3. C. N. Kodituwakku et al, “Resonant inelastic X-ray scattering studies of copper phthalocyanine”, Physical Review B, 77(12), 2008, art. 125205
