Soft x-ray spectroscopy study of the element and orbital contributions to the electronic structure of copper hexadecafluoro-phthalocyanine

The electronic structure of copper hexadecafluorophthalocyanine $\left({\text{F}}_{16}\text{CuPc}\right)$ has been measured using soft x-ray emission spectroscopy and x-ray absorption spectroscopy at the C, N, and $\text{F}K$ edges. Our element- and orbital-specific measurements confirm that substitution of hydrogen for fluorine leads to profound changes in the electronic environment of the carbon atoms in contrast to that of the nitrogen atoms. These findings are supported by simulated x-ray absorption and emission spectra of ${\text{F}}_{16}\text{CuPc}$ calculated by density-functional theory methods. The experimental results are directly compared with those of CuPc. Additional information regarding specific carbon sites is obtained from the evolution of the $\text{C}K$-edge resonant x-ray emission spectra of ${\text{F}}_{16}\text{CuPc}$.

Figure 1

Schematic representation of the (a) ${\text{F}}_{16}\text{CuPc}$ and (b) CuPc molecules. The four distinct carbon environments are labeled ${\text{C}}_1{\text{-C}}_4$ for ${\text{F}}_{16}\text{CuPc}$. The distinct N and F sites are also labeled.

Figure 2

(Color online) The $\text{C}1s$ XPS spectrum of ${\text{F}}_{16}\text{CuPc}$ (filled circles). The Voigt function fit and its components are also displayed, identifying the three carbon environments associated with the C-N $\left({\text{C}}_1\right)$, C-C $\left({\text{C}}_2\right)$, and C-F (${\text{C}}_3$, ${\text{C}}_4$), bonds and their respective $\pi \text{−}{\pi }^{\ast }$ shake-up satellites. For reference, the $\text{C}1s$ XPS spectrum of CuPc is shown above.

Figure 3

(Color online) The (a) C and (b) $\text{N}K$-edge x-ray absorption spectra of ${\text{F}}_{16}\text{CuPc}$ (red lines). Also displayed underneath each spectrum is the CuPc XAS for the C and $\text{N}K$ edges (blue lines).

Figure 4

(Color online) (a) Angular variation in the $\text{C}K$-edge XAS of F16CuPc. (b) The plotted variation of the experimental ${\pi }^{\ast }:{\sigma }^{\ast }$ peak intensity (black square) against the calculated variation for a range of planar tilt angles.

Figure 5

(Color online) The experimental and simulated x-ray absorption spectra for the (a) $\text{C}K$ edge, (b) $\text{N}K$ edge, and (c) $\text{F}K$ edge of ${\text{F}}_{16}\text{CuPc}$. The ${\pi }^{\ast }$ (red) and ${\sigma }^{\ast }$ (green) contributions to the total (black) simulated spectra are shown for each $K$ edge. The resonant excitation energies used for the $\text{C}K$ edge RXES are also shown for the $\text{C}K$-edge as triangles marked (a)–(e).

Figure 6

(Color online) The (a) N and (b) $\text{F}K$-edge x-ray emission spectra of ${\text{F}}_{16}\text{CuPc}$. The corresponding StoBe calculated emission spectra (with site-specific contributions) are displayed beneath each experimental spectrum.

Figure 7

(Color online) (a) $\text{C}K$-edge XES from a stationary ${\text{F}}_{16}\text{CuPc}$ film (black dots) and a translated ${\text{F}}_{16}\text{CuPc}$ film (red dots). Also displayed is the StoBe calculated nonresonant emission spectrum (red line). (b) $\text{C}K$-edge XES from a stationary CuPc film (black dots) and a translated CuPc film (blue dots), with a calculated nonresonant emission spectrum (blue line). A 3 point-binomial smoothing has been used (thin lines) for each experimental spectrum to guide the eye.

Figure 8

(Color online) The experimental $\text{C}K$-edge RXES spectra ${\text{F}}_{16}\text{CuPc}$ (a)–(e) as a function of excitation energy (filled circles). The elastic peak lies at the incident energy used in each case and for (b) $h\nu =285.7\text{eV}$ the intensity of the elastic peak has been truncated. Also shown underneath the experimental spectra are the StoBe simulated emission spectra corresponding to contributions from, in ascending order, solely those transitions arising from $a_{1\text{g}}$ symmetry MOs on the link ${\text{C}}_2$ site (C-C bond) in face-on geometry; then the spectrum arising solely from $b_{1\text{g}}$ symmetry MOs on the ${\text{C}}_2$ sit in face-on geometry; then the total spectrum arising from the ${\text{C}}_2$ carbon site (blue line) shown for both face-on emission geometry (thick line) and edge-on emission geometry (thin line); then the sum of the spectra arising from ${\text{C}}_1$ (C-N) and ${\text{C}}_2$ (C-C) link and pyrrole) carbon sites (red line) again in both face-on and edge-on geometries; and finally the sum of all (sum) carbon sites (black line) in both face-on and edge-on geometries.