Controlling the Self‐Metalation Rate of Tetraphenylporphyrins on Cu(111) via Cyano Functionalization

Abstract

The reaction rate of the self-metalation of free-base tetraphenylporphyrins (TPPs) on Cu(111) increases with the number of cyano groups (n=0, 1, 2, 4) attached at the para positions of the phenyl rings. The findings are based on isothermal scanning tunneling microscopy (STM) measurements. At room temperature, all investigated free-base TPP derivatives adsorb as individual molecules and are aligned with respect to densely packed Cu substrate rows. Annealing at 400 K leads to the formation of linear dimers and/or multimers via CN-Cu-CN bonds, accompanied by self-metalation of the free-base porphyrins following a first-order rate equation. When comparing the non-cyano-functionalized and the tetracyano-functionalized molecules, we find a decrease of the reaction rate by a factor of more than 20, corresponding to an increase of the activation energy from 1.48 to 1.59 eV. Density functional theory (DFT) calculations give insights into the influence of the peripheral electron-withdrawing cyano groups and explain the experimentally observed effects.

Formation of Highly Ordered Molecular Porous 2D Networks from Cyano‐Functionalized Porphyrins on Cu(111)

“…The molecules are aligned along one of the high‐symmetry crystallographic directions of the substrate, which is due to a strong attractive site‐specific interaction between the iminic nitrogens of the macrocycle and the Cu atoms of the Cu(111) surface . In a previous study of 2H‐ cis DCNPP on Cu(111) at much lower coverages, but otherwise similar conditions, the formation of dimers connected through native Cu adatoms was reported, along with self‐metalation at 400 K . At the high coverage studied here, we do observe some dimers, but neither extended 1D chains nor 2D porous networks are evident.…”

“…The saddle‐shape conformation is common for TPPs and slightly modified TPPs on a variety of surfaces. In contrast, metal‐free 2H‐TPP and the corresponding cyano‐functionalized species 2H‐(5‐mono‐( p ‐cyanophenyl))‐(10,15,20‐triphenyl)‐porphyrin (2H‐MCNPP), 2H‐ cis DCNPP, and 2H‐TCNPP exhibit a very peculiar “inverted” structure on Cu(111), which is characterized by direct N−Cu bonds and two nearly upright pyrrole groups …”

“…Notably, 2H‐TCNPP on Cu(111) did not show the transition to the porous structure upon annealing at elevated temperatures, which was observed here for 2H‐ cis DCNPP. This behavior is attributed to the significantly larger activation barrier for metalation for 2H‐TCNPP …”

“…Notably, cyano groups cannot only contribute in hydrogen bonding and dipolar coupling, but also can coordinate to different metals . Furthermore, it was shown that the electron withdrawing effect of the peripheral cyano groups significantly influences the self‐metalation of corresponding metal‐free porphyrin derivatives on Cu(111) …”

Bifunctional Behavior of a Porphyrin in Hydrogen-Bonded Donor–Acceptor Molecular Chains on a Gold Surface

“…In addition to pure steric arguments, a different charge distribution between acceptor and donor TAPPs could also explain their different behavior with respect to adatoms trapping. In a recent paper, Lepper et al 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 other ( Figure S11). However, we cannot confirm this picture, since, as previously mentioned, DFT simulations performed using the larger unit cell resulted in no chain formation.…”