Models of (a) free-base tetraphenylporphyrin (2H-TPP) and (b) copper tetraphenylporphyrin (Cu-TPP) displayed with conformations obtained by geometry optimization of isolated molecules. The angles α imi , α pyrr , and α coord describe the inclination of the respective pyrrole rings out of the macrocycle plane, while α ph refers to the rotation of a phenyl ring around the C–C bond that connects it to the macrocycle. The model in (b) is characterized by α coord = 0 and α ph = 90°.
The N1s XPS spectra of (a) a 2H-TPP multilayer and (b) a 2H-TPP monolayer adsorbed on Cu(111) show two peaks that are assigned to iminic (A, blue) and pyrrolic (B, green) nitrogen species. After annealing the multilayer sample (3-4 layers) to 420 K, the spectrum (c) shows a single main peak (D, red) which is assigned to the equivalent nitrogen atoms of Cu-TPP. Further annealing to 490 K leads to a Cu-TPP monolayer (d).
The ratio of iminic (blue) and pyrrolic (green) N1s XPS peaks of a 2H-TPP monolayer is angular dependent. Whereas in the normal electron emission mode (b) the peak ratio is just 0.4 it increases when the sample is rotated out of this position by Δ = −25°(a), Δ = 10°(c), and Δ = 25°(d).
Comparing N K-edge NEXAFS spectra of (a) a 2H-TPP multilayer and (b) a 2H-TPP monolayer shows the big influence of the surface on the molecule. After annealing both the multilayer (heating to 420 K, c) and the monolayer (heating to 490 K, d) signatures are different from the 2H-TPP spectra and indicate the metalation to Cu-TPP. Values in the insets denote the incident angle θ.
The differences of the C K-edge NEXAFS spectra between (a) a 2H-TPP multi- and (b) a 2H-TPP monolayer as well as between the annealed multi- (c) and monolayer (d) show the influence of the surface on conformation and electronic structure of the molecules. Values in the insets denote the incident angle θ.
Comparison of experimental (top) and calculated (middle, bottom) NEXAFS spectra for the C-edge 2H-TPP multilayer spectrum (a), the N-edge 2H-TPP multilayer spectrum (b), and the N-edge Cu-TPP multilayer spectrum (c). (d) Shows the calculated N-edge TPP spectrum. The middle panel displays the sum of all corresponding single spectra, in the bottom panel this total spectrum is split up into its single components. The insets show the position and number of the atoms used for the calculation of the respective spectra.
Molecular orbitals obtained from transition state calculations; (a, b) 2H-TPP, (c) Cu-TPP, excitation centers are marked by an arrow.
Fits for the experimental multilayer N-edge (a) and C-edge (c) region (for clarity only the 53° curves are shown). The good agreement of the sum (red continuous line) of all single fits (dashed and dotted lines) with the experimental data (symbols) allows to determine the conformation of the molecules. Therefore, the dependence on the incidence angle θ of the normalized intensities (symbols) obtained from the fit is compared with the theoretical curves of a π system on a threefold symmetric surface (black curves, b and d). The curves for different angles α between orbital and surface normal assumed a linear polarization of 0.9. The inset in (b) illustrates the angles involved in the measurement and analysis. The circle symbolizes the conjugated system which is coplanar to the surface for α = 0.
Fits of the experimental N-edge (a) and C-edge (c) region of a 2H-TPP monolayer (for clarity only the 50° curves are shown) lead to the analysis of the normalized intensities (b,d) which indicates a substantial distortion of the macrocycle. The theoretical curves (black curves, b and d) assumed a threefold symmetric surface and a linear polarization of 0.82.
Side and top view model of (a) a free-base porphyrin with a marked saddle-shaped conformation, where the iminic nitrogen atoms are pointing downwards, i.e., with the nitrogen lone pair towards the substrate and the pyrrolic nitrogen atoms are pointing upwards and (b) a slightly saddle-shaped Cu-TPP conformer with a nearly flat macrocycle (for the atoms the same color code as in Fig. 1 is used).
XPS energies of different nitrogen species obtained from multilayer measurements (E exp ) compared with calculated ionization energies (E calc ); nearly constant energy difference between experimental and theoretical results (ΔE calc, exp ) for 2H-TPP (=N-, -NH-) and Cu-TPP (NCu).
Peak assignment for the N1s peaks in the 2H-TPP NEXAFS spectrum, only main transitions below the ionization energies are listed.
Angles derived from NEXAFS measurements of mono- and multilayers of 2H-TPP and annealed 2H-TPP (Cu-TPP) samples. α imi , α pyrr , and α coord describe the inclination of the pyrrole rings out of the macrocycle plane, whereas α ph refers to the rotation of the phenyl rings (see Fig. 1). To interpret the angles in terms of adsorption geometry further information has to be taken into account (see text).
Peak assignment for the N1s peaks in the Cu-TPP NEXAFS spectrum, only main transitions below the ionization energies are listed.
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