Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels

Abstract

An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a(1u) and b(1g) symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a(1u) orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b(1g) orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.

Subspace Representations in Ab Initio Methods for Strongly Correlated Systems

“…55. Turning first to the valence band edge, the UPS spectra confirm that single-molecule CuPc possesses a doublyoccupied HOMO of pyrrole-delocalised a 1u character, while a further weak ionisation peak at 800 meV above that is consistent with the magnetisation-carrying copper 3d x 2 −y 2 -based b 1g orbital seen, albeit substantially closer to the valence band edge for moderate U values, in .…”

“…Due to the differing localisation regions of the carbon 1s and singly-occupied b 1g orbital, as noted in Ref. 55, such excitations are neglected and so we cannot compare our prediction that the latter orbital lies somewhat below the e g level for all except the "dual" representation at high U values. We agree on the proximity of these latter levels with previous monomer calculations using the PBE functional 53,54 .…”

“…The accepted understanding 51,54,55 of the spectroscopic nature of the gap in the copper phthalocyanine monomer is that the HOMO level is dominated by a doubly-occupied a 1u orbital which consists of a superposition of Carbon p z orbitals delocalized on the pyrrole rings of both monomer units, while the spectroscopically correct LUMO level is also a delocalized doublydegenerate orbital, of e g symmetry composed of a superposition of π orbitals on pairs of macrocycle Carbon atoms. We may expect some minor differences in the spectroscopic properties in the dimer system with respect to the monomer, due to σ-bonding between moieties, but for the main features to be preserved.…”

Copper-phthalocyanine based metal–organic interfaces: The effect of fluorination, the substrate, and its symmetry

“…1. 26,27 Inspection of the spatial distribution of occupied and unoccupied molecular orbitals of F 16 CuPc and CuPc as obtained from DFT calculations reveals a remarkable resemblance, with little influence of the fluorine atoms (Fig. 1).…”

Modeling the interactions of phthalocyanines in water: From the Cu(II)-tetrasulphonate to the metal-free phthalocyanine

“…The CuPc complex is a neutral open-shell system, whose most stable electronic state is a doublet, the radical character mainly associated to the Cu 3d x 2 −y 2 orbital, which overlaps the Cu-N bond regions in the molecular plane, with small contributions from the in-plane 2p orbitals of the ligand atoms. 31 The model complex, Cu(NH 3 ) 2 (NH 2 ) 2 , is also a neutral open-shell system, where the most stable electronic state is also a doublet. As in the case of the CuPc complex, the highest spin density is associated to the Cu 3d x 2 −y 2 orbital with small contributions from the in-plane N 2p orbitals of the amino-ligands.…”