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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