Tunable Interface of Ruthenium Porphyrins and Silver

Abstract

The adsorption and monolayer self-assembly of functional metal–organic blocks on solid surfaces are critical for the physicochemical properties of these low-dimensional materials. Although modern microscopy tools are very sensitive to the lateral arrangement of such blocks, they are still unable to offer directly the complete structural analysis especially for nonplanar molecules containing different atoms. Here, we apply a combinatorial approach for the characterization of such interfaces, which enables unexpected insights. An archetypal metalloporphyrin on a catalytically active surface as a function of its molecular coverage and substituent arrangement is characterized by low-energy electron diffraction, scanning probe microscopy, X-ray photoelectron spectroscopy, normal-incidence X-ray standing waves, and density functional theory. We look into Ru tetraphenyl porphyrin (Ru-TPP) on Ag(111), which is also converted into its planarized derivates via surface-assisted cyclodehydrogenation reactions. Depending on the arrangement of the phenyl substituents, the Ru atoms have distinct electronic structures and the porphyrin macrocycles adapt differently to the surface: saddle shape (pristine Ru-TPP) or bowl shape (planarized Ru-TPP derivates). In all cases, the Ru atom resides close to the surface (2.59/2.45 Å), preferably located at hollow sites and in the interface between the plane of the porphyrin macrocycle and the Ag surface. For the more flexible pristine Ru-TPP, we identify an additional self-assembled structure, allowing the molecular density of the self-assembled monolayer to be tuned within ∼20%. This precise analysis is central to harnessing the potential of metalloporphyrin/metal interfaces in functional systems.

Conformational Control of Chemical Reactivity for Surface‐Confined Ru‐Porphyrins

“…Our earlier structural investigation of Ru-TPP and Ru-TPP pl has shown that the adsorption height of the Ru centre differs only by 0.14 . [20] Nevertheless,t he adsorption height of the Ru centre of Ru-TPP increases upon CO ligation at 200 Kb y0 .59 from 2.59 AE 0.05 [20] to 3.18 AE 0.12 ,a s shown by NIXSW data of the Ru(CO)-TPP (Figure 7A, Table 2). [30] Theh igh coherent fraction indicates av ery welldefined adsorption height for the molecules,c onfirming ar ather uniform geometry.…”

“…When deposited on Ag(111) at room temperature in submonolayer coverages,R u-TPP (Figure 2i nset) molecules self-assemble in as quare phase [19] described by the epitaxial matrix 70 48 . [20] Figure 2A shows the assembly on such asurface cooled down to 5K(overview image in Figure S1). Forn egative bias voltages (%À1V), the single molecule appearance of the pristine Ru-TPP (outlined in orange) is characterised by three bright protrusions along the macrocycle and four less bright in the periphery marking the phenyl substituents.T he central bright protrusion corresponds to af illed electronic state of the Ru centre (cf.U PS below), [21] whereas the outer ones can be assigned to the protruding nwpyrroles (a-pyr) of the macrocycle.…”

“…The substituents are faded to highlight the difference in the conformation of the porphyrin macrocycles. [20] Ru, C, N, H, and Ag are shown in raspberry, grey,blue, white, and silver,respectively. TheC Ol igands can be selectively removed by STM tip manipulations at 150 Kasillustrated in the sequence of STM images in Figure 3: At the position marked by the green cross (Figure 3A), the voltage was ramped from 1.28 Vto2.15 Vin ac onstant current mode (50 pA) while monitoring the tip height (Figure 3B).…”

“…To investigate the effect of the macrocyclic conformation on the CO ligation to the Ru centre,wehave investigated the adsorption behaviour of Ru-TPP on Ag(111) after annealing to 620 K. This process causes cyclodehydrogenation reactions between the macrocycle periphery and the phenyl substituents,l eading to af amily of four planarized Ru-TPP derivatives,R u-TPP pl . [19][20] Based on DFT calculations,t he binding energy of the most commonly occurring Ru-TPP pl 3 [19] to Ag(111) is 5.66 eV,1 .26 eV higher than that of pristine Ru-TPP.T he different derivatives can be identified by matching the characteristic outline of the structural formula (Figure 4A)t ot he STM image (Figure 4B,E), whereas nc-AFM imaging can visualise more directly the chemical identity,a s illustrated for one of the more frequently occurring species in Figure 4C.T he resulting porphyrin macrocycle appears to exhibit asubtle bowl shape with pyrrole tilt angles of 68 8 and 88 8 (see nc-AFM and respective simulation in Figure 4C,D and Table 2) and also offers ac oordinatively unsaturated metal centre.W en ote that the surface depicted in Figure 4E has been exposed to the small amounts of CO at 5Kneeded for the tip functionalisation, however no evidence of al ateral adsorbate stabilisation was found on the Ru-TPP pl molecules by STM/nc-AFM. As the rider ligation is associated with the saddle shape deformation, [11] we would not expect this bowl configuration to permit such ligation.…”

“…[29] On theruthenium centre,adecrease of electron density in thed z 2 orbital, as well as an increase in thed zx andd yz orbitals is observed.Itisnotable that this change in theRu3delectron densityi ss imilar between, both,t he Ru-CO, andR u-Ag, whereasf or Ru-TPP pl 3 thec orrespondingD FT calculations find theelectronaccumulationtobeind z 2 ,and depletioninthe d zx ,d yz orbitals. [20] Thed epletion andg aino fe lectrons in orbitals of both Ru andCOshowaback-donationofelectrons from theRucentretothe CO ligand,which in addition to the decoupling cancontributetothe increaseinbinding energy of theR u3 d 5/2 core leveluponCOligation(Figure 6A).…”