Photon Factory Activity Report 2018 #36 (2019)

BL-27A/2018G073

Electronic structure of axial ligand chain in chloroaluminum phthalocyanine
Hiromi IKEURA-SEKIGUCHI 1,* and Tetsuhiro SEKIGUCHI 2
1
Research Institute for Measurement and Analytical Instrumentation (RIMA),
National Metrology Institute of Japan (NMIJ),
National Institute of Advanced Industrial Science and Technology (AIST),
1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan
2
Japan Atomic Energy Agency (JAEA), Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
1 Introduction
Chloroaluminum phthalocyanine (Cl-AlPc) is a metal
Pc complex with an axial ligand Cl normal to the Pc ringplane. Cl-AlPc can form various structural configurations,such as a cofacial stack (head-to-tail) [1] and interleaved
slipped stacks (head-to-head or tail-to-tail) [2]. Two major
types of intermolecular interactions in Cl-AlPc are πconjugated ring-ring interactions and metal-axial ligand
dipole interactions based on the crystal structure [2].
Therefore, charge transfer can occur either via π–π
stacking or through oligomer-like (–Al–Cl–Al–) metal
chain sites. In order to understand electron transferprocesses, site-selective X-ray absorption spectroscopy(XAS) spectra were measured using the normal (NA) andthe spectator (SA) KL2,3L2,3 Auger electron yields (AEY) of
Cl as a function of photon energy near the Cl K-edge toseparate contributions from the delocalized ground statelike unoccupied partial density of state (DOS) and the
localized core-excitonic DOS.

area ratio of 1a–1b was approximately 1:1. This ratio is
expected to result from a competition between the corehole lifetime and the lifetime of an electron bound to aparticular atom in the conduction band. Such an ultrashort
lifetime of an initially bound electron at the σ*(Cl–Al)
state indicates that the Cl axial ligand participates informing the conduction band of polycrystalline Cl-AlPc.
These results also suggest that one-dimensional electrontransfer or delocalization along the periodic Cl–Al chain isprobable.

2 Experiment
Experiments were performed at beamline BL-27A withan InSb(111) double-crystal monochromator with anenergy resolution of ca. 0.9 eV around Cl K-edge. Auger
spectra were measured using a hemispherical analyzer(CLASS-100; Vacuum Science Workshop), with a passenergy of 44 eV. The X-ray energy was calibrated byreference to the lowest sharp peak of NaClO3 (2830.5 eV).
3 Results and Discussion
Figure 1 shows Cl K-edge XAS recorded in AEY modes.
Peak 1 and peak 2 could be assigned to the Cl 1s → σ*(Cl–Al) transition consisting of the Cl 3pz molecular orbital
and the Cl 1s → Cl 3px,y/Al 3p transition, respectively.
Peak 3 could also be assigned to the Cl 1s → Cl 4p coreto-Rydberg transition. Peak 1 is split into two peaks
labelled 1a and 1b. Peak 1b is clearly resolved by the NAAEY below the continuum edge jump, while peak 1a is
resolved by the SA-AEY. The origin of both peaks isconsidered to be the same transition state of σ*(Cl–Al),
whereas the exciton peak 1a is pulled down below theconduction band by the coulomb interaction between thecore hole and the excited electron. Such doublet peaks
indicate that these are the result of both resonant (peak 1ain SA-AEY) and DOS-like (peak 1b in NA-AEY)
contributions based on previous observations of XAS
features in the band insulator [3]. In Cl-AlPc, the coreexciton binding energy was measured to be ∼0.9 eV, which
depends on the degree of core-hole screening. The peakFig. 1: Cl K-edge XAS of polycrystalline Cl-AlPc
measured in AEY modes. Integrated intensities of SA (redclosed diamond) and NA (blue closed square) components
are shown with results of curve fitting (red and blue dottedlines).
References[1] H. Yanagi, et. al., J. Phys. Chem. 96, 1366 (1992).
[2] K. J. Wynne, Inorg. Chem. 23, 4658 (1984).
[3] W. Drube, R. Treusch, and G. Materlik, Phys. Rev. Lett.
74, 42 (1995).
* ikeura-sekiguchi@aist.go.jp