The impact of metalation on adsorption geometry, electronic level alignment and UV-stability of organic macrocycles on TiO₂(110)

Abstract

Metal complexes of the tetradentate bipyridine based macrocycle pyrphyrin (Pyr) have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on stoichiometric TiO(110) is investigated in ultrahigh vacuum by means of scanning tunneling microscopy, photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. In a joint experimental and computational effort, the local adsorption geometry at low coverage, the long-range molecular ordering at higher coverage and the electronic structure have been determined for both the bare ligand and the cobalt-metalated Pyr molecule on TiO. The energy level alignment of CoPyr/TiO supports electron injection into TiO upon photoexcitation of the CoPyr complex and thus renders it a potential sensitizer dye. Importantly, Co-incorporation is found to stabilize the Pyr molecule against photo-induced degradation, while the bare ligand is decomposed rapidly under continuous UV-irradiation. This interesting phenomenon is discussed in terms of additional de-excitation channels for electronically highly excited molecular states.

Atomically Resolved Band Bending Effects in a p-n Heterojunction of Cu2O and a Cobalt Macrocycle

“…Subtracting the resulting spectrum from that of the CoPyr covered surface (dotted blue lines in Figure a) yields a pronounced peak at a binding energy of about 2 eV for molecular coverages less than 1 ML. This value is similar to the HOMO binding energy for CoPyr on a rutile TiO 2 (110) surface . The weak feature at 0.65 eV binding energy could be attributed to an interface state arising from the interaction between the Co 3d z 2 orbital with occupied states from the substrate, as discovered for cobalt porphyrin on Ag(111) .…”

“…For coverages higher than 1 monolayer (ML), all of the spectral features shown in the SI change which indicates layer-by-layer rather than island growth . Previous scanning tunneling microscopy studies of CoPyr evaporated on Au(111) and on rutile TiO 2 (110) surfaces reveal a flat adsorption geometry with the molecular plane, shown in the inset of Figure a, oriented parallel to the surface. , …”

“…This value is similar to the HOMO binding energy for CoPyr on a rutile TiO 2 (110) surface. 16 The weak feature at 0.65 eV binding energy could be attributed to an interface state arising from the interaction between the Co 3d z 2 orbital with occupied states from the substrate, as discovered for cobalt porphyrin on Ag(111). 19 We 2e shows the linear dependence of these positions with respect to each other, which is indicative of band bending effects.…”

“…We observe a similar feature for CoPyr adsorbed on Au(111) (SI). DFT calculations predict a HOMO-LUMO gap of 2.35 eV for adsorbed CoPyr on rutile TiO 2 (110); thus we expect the LUMO of CoPyr to be located at around 0.35 eV above the Fermi level. With a band gap value of 2.17 eV for bulk Cu 2 O, this is well below the conduction band minimum (CBM) of the Cu 2 O­(111) substrate.…”

“…18 Previous scanning tunneling microscopy studies of CoPyr evaporated on Au(111) and on rutile TiO 2 (110) surfaces reveal a flat adsorption geometry with the molecular plane, shown in the inset of Figure 1a, oriented parallel to the surface. 14,16 Due to the p-type character of the oxide, the highest occupied molecular orbital (HOMO) is buried within the 16 According to optical absorption measurements on bulk Cu 2 O, the CBM is plotted 2.17 eV above the VBM. 17 manifold of Cu states.…”