Valence band RIXS spectroscopy of phthalocyanine complexes - probing excitations in the optical range Janine
Grattage1,
Weng1,
Sikora1,
Kvashnina1,
Tsu-Chien Marcin Kristina Schulte2, Andrei Khlobystov2 and Pieter Glatzel1 1 - ID26, ESRF. 6 Rue Jules Horowitz, BP 220, 38043 Grenoble, France
ESRF
Karina
2 - MAX-lab, Lund Universitet , Ole Römersväg 1, 223 63 Lund, Sweden 3 - School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
New and powerful spectroscopy techniques are currently being developed to probe the electronic structure of a variety of samples. Beamline ID26 at the ESRF can record resonant inelastic X-ray scattering (RIXS) spectra at the metal K-edge over the Kbeta satellite emission lines to selectively access electronic transitions in the valence band region. In this study we used valence band RIXS (VB RIXS) to study transitions in Ti and Co phthalocyanine systems.
Why study phthalocyanines ? Phthalocyanines (Pcs) are a fascinating class of molecules which can be used in a vast array of applications including organic electronics, pigments, and cancer treatments [1,2]. They consist of an organic macrocycle with a metal atom in the centre. The metal atom, and possible side or axial groups can be easily changed to tune the molecular properties. Thus understanding the electronic structure of these molecules is vital to fully exploit their properties. Titanium phthalocyanines
CoPc and CoPc@CNT
The Ti molecules studied (TiPcO, TiPcCl2 and PMCP-TiCl3) are of interest in photovoltaic and organic electronics applications. The geometries and axial ligands of the two Pc compounds permit different electronic transitions to take place. PMCP-TiCl3 was measured as a comparison to TiPcCl2
CoPc was measured, both as a powder and as an encapsulated species inside carbon nanotubes (CNT) [3,4]. CoPc alone shows an optical range excitation in the valence band RIXS plane which is suppressed upon encapsulation. The origins of these optical peaks is under discussion and we are comparing VB RIXS with UV-visible measurements.
VB RIXS of molecules
TiPcCl2
TiPcO
PMCP-TiCl3
In VB RIXS of 3d atoms in molecules, a 1s electron is excited into the pd type MOs. An electron from another pd type MO relaxes, giving an overall energy transfer (ET) to the system which is recorded in the VB RIXS plane.
VB RIXS for Ti complexes
d-d peak
Extracting RXES scans Resonant XES line spectra can be extracted from the RIXS planes to show distinct electronic transitions (shown by arrows and below in a total energy diagram (Pcs only).
The pre-edge for TiPcCl2 and PMCP-TiCl3 is split, where there is only one pre-edge peak in TiPcO. Only PMCPTiCl3 shows an optical range peak at 2.7eV ET.
CoPc is a planar molecule and upon encapsulation in CNT it stacks in a similar way to the bulk beta-phase structure [3]. The TEM image here shows a filled CNT.
Optical excitations and suppression due to CNT The valence to core RIXS plane of CoPc (right) shows a charge transfer peak at 10eV ET and a dd excitation at 2.4eV ET, at 7715.5eV incident energy. RXES line scans were recorded for a CoPc/CNT mixture and for CoPc@CNT at 7715.5eV. The 2.4eV peak is suppressed in the CNT samples (below) possibly due to CoPcCNT bonding interactions. d-d peak
The Pc molecules also show an elastic peak enhancement at 4970eV. The two Pcs have peaks/features at 4970eV with similar profiles.
The CoPc VB RIXS plane shows the weak optical range feature at 2.4eV ET at the same incident energy as the edge peak at 10eV ET. MO4pd MO5pd MO6pd
MO3pd
MO MO4pd 5pd MO6pd
UV-visible comparison The optical RIXS peak at 2.4eV coincides with an additional peak in the UV-visible which is not present for the CNT samples or free base Pc. This peak in the UV-visible may therefore originate from the Co atom.
Conclusions •VB RIXS and UV-visible spectroscopy are complementary probes of the valence electronic structure of organnometallic molecules. •Electronic transitions can be extracted from VB RIXS of Ti Pcs and PMCP-TiCl3 to indicate the valence electronic structure. •CoPc measured using VB RIXS and UV-visible shows suppression of a d-d (optical) excitation upon CNT encapsulation. References: [1] C. G. Claessens, U. Hahn and T. Torres, Chem. Rec. 8, 75–97, (2008). [2] Y. Shirota, J. Mat. Chem 10, 1-25, (2000) [3] K. Schulte et al, Adv. Mater 19, 3312-3316 (2007)
[4] A. N. Khlobystov, D. A. Britz and G. A. D. Briggs, Acc. Chem. Res. 38, 910-909 (2005) [5] C. N. Kodituwakku et al, Phys. Rev. B, 77(12), 2008, art. 125205
