Insight into molecular chemical structure

Octaethylporphyrins (OEPs) are a representative class of the porphyrin molecules with peripheral ethyl substituents. Upon anchoring to a solid substrate, molecules adapt their structural conformation to the local environment, deforming the macrocycle and/or reorienting the substituents. Thereby, the peripheral ethyl groups of the adsorbed OEP molecule can point away from and towards to the substrate, allowing the metal ions in M-OEP to be closer to or farther from the metal substrate, respectively. Together with the surface-driven conformational changes, the porphyrin molecules may undergo on-surface temperature-induced chemical changes of their molecular structure. A surface-assisted ring-closure reaction in the case of metal-OEP involves the peripheral ethyl groups and results in the transformation of OEP in tetrabenzoporphyrin (TBP), which occurs via the dehydrogenation of two neighbouring ethyl groups followed by the formation of six-membered carbon rings. This on-surface reaction can deeply alter the intriguing properties of the porphyrin array, therefore, a proper technique is requested to probe the reaction pathway and final chemical products.

We show that the photoemission tomography (PT) approach based on imaging of molecular orbitals in the reciprocal space can be efficiently used to study of surface-induced intramolecular modifications, i.e. ring-closure reaction, allowing identification the reaction products.

In Fig. 1 we present the angle-integrated valence band spectra of pristine and annealed CoOEP/Ag(100) interfaces. While the valence band spectrum of the bare Ag(100) substrate has a rather featureless plateau associated with the sp-bands, three prominent features can be recognized for the organic/metal interfaces, both before and after annealing. In particular, the feature peaked at higher binding energy (BE=2.15 eV) is present for both pristine and annealed samples, while the two low energy peaks undergo only minor changes in the energy position. While those energy shifts cannot reveal the nature of the chemical modifications, clear differences are observed in the measured momentum maps. To identify the origin of the features observed in the valence spectra and test the possible CoOEPCoTBP on-surface reaction, the experimental maps have been compared to the square modulus of the Fourier transform of the real space molecular orbitals, calculated for gas-phase CoOEP and CoTBP molecules. Indeed, it has been shown for various organic/metal interfaces that there exists a one-to-one correspondence between the momentum distribution of the photocurrent and the molecular orbitals in the reciprocal space. In that way, the features observed for the CoOEP/Ag(100) system can be unequivocally assigned to the molecular states simulated for the gas-phase CoOEP, where the states centered at BE 2.15 eV, 1.75 eV and 0.65 eV are due to the HOMO-1, HOMO and the degenerate LUMO/LUMO+1, respectively. No match for the annealed interface can be found between the measured maps and the CoOEP calculated patterns, while the momentum maps simulated for CoTBP match very well the three measured patterns at 2.15 eV, 1.60 eV and 0.70 eV for the annealed interface, which according to the present calculation can be assigned to the HOMO-1, HOMO and LUMO/LUMO+1 molecular states of CoTBP, respectively. The excellent agreement between the measured and simulated maps using the PT approach is a direct proof that the annealing at 600K on Ag(100) induces the chemical modification of CoOEP and leads to the formation of the tetrabenzoporphyrin array. For both systems, the HOMO-LUMO gaps are significantly lowered after on surface deposition, from 3.0 eV to 1.1 eV (CoOEP) and from 2.5 eV to 0.9 eV (CoTBP), suggesting a strong molecule-surface interaction at both interfaces. Furthermore, the presence of the LUMOs for both the organic systems confirms the charge transfer effect for both metalorganic/Ag(100) systems.

In summary, we show that PT is an efficient tool to probe on-surface reactions in self-assembled molecular arrays. This has been proved on the temperature-induced CoOEP®CoTBP transformation on Ag(100), which goes through the dehydrogenation of two neighbouring ethyl groups in CoOEP followed by the formation of six-membered carbon ring, and the method can be extended to the study of different π-conjugated molecules on various conductive substrates. While PT can be an appropriate method to study on surface complete dehydrogenation reactions in a molecular array, the STM is still a more suitable approach to follow the degree of dehydrogenation at lower temperatures where different molecular intermediates products are still present.

Original Publication:

I. Cojocariu, F. Feyersinger, P. Puschnig, L. Schio, L. Floreano, V. Feyer and C. M. Schneider, "Insight into intramolecular chemical structure modifications by on-surface reaction using photoemission tomography", Chem. Commun. 57 (2021) 3050, https://doi.org/10.1039/D1CC00311A