Self-Assembly and Properties of Nonmetalated Tetraphenyl-Porphyrin on Metal Substrates

Abstract

IntroductionThe self-assembly of porphyrins on well-defined surfaces is attracting considerable interest because it promises to create surface patterns with nanometer dimension that exhibit specific electronic, sensoric, optic, or catalytic functionality 1-3 or even interesting magnetic properties. 4,5 The ability of porphyrin to show self-organization and to accommodate metal atoms in their macrocycle is exploited, for instance, to form metal−organic frameworks or adsorbed layers for catalysis. [6][7][8][9] The selfassembly is mainly driven by noncovalent metal−organic coordination interactions, which is well-known and important in solution-based 3D supramolecular chemistry. [10][11][12][13][14][15] Porphyrin molecules have been adsorbed onto surfaces to form supramolecular networks from solution, [16][17][18][19] electrochemically 20,21 or by thermal evaporation under vacuum conditions. [22][23][24][25][26][27][28] While there is a rich literature on the electronic structure of these adsorbates, the surface adlayer structures have also been characterized with scanning force microscopy, scanning tunneling microscopy, or X-ray absorption near-edge structure analysis. 29 The rationale of such experiments on 2D structures has been to study the long-range interactions that determine the self-assembly processes. It has been demonstrated that the bottom-up fabrication of highly organized porphyrin layers, as well as of porphyrin-based multicomponent molecular entities, depends on the interplay of molecule−molecule and substrate−molecule interactions. Molecule−substrate interactions will set limits to the mobility of the adsorbed molecules and may alter the electronic structure of the absorbed molecules, or the electronic states at the surfaces may become locally perturbed by the adsorbate. 60 A consequence is that the established concepts of solution-based coordination chemistry cannot be applied without appropriate modification. The substrate thus becomes an additional parameter to control the adsorption energy of the molecules and, hence, their diffusivity at surfaces. An intriguing demonstration of this effect is the self-assembly of porphyrins, which are decoupled from their metal substrate by insulating NaCl layers of varying thickness. 23 The interaction was shown to be dependent on the NaCl layers, and the thicker the NaCl the weaker the interaction and the more delayed the onset of network formation. The occupation of the center ring of the porphyrin may affect the molecular adsorption at surfaces. As an example, free-base or Cu-incorporated porphyrin molecules show different arrangements along step edges on Cu(100) surfaces. While 2H-TBPP bridges over the step edges, Cu-TBPP sits on either side of step edges. 27 In contrast, no difference in the network architecture was found for differently metalated TPP on Ag(111). 57 Such a subtle dependence of adsorption site on metal incorporation, if fully understood, may become useful to control the self-assembly or the properties of the molecules on surfaces.The goal...

Self-assembly of strongly dipolar molecules on metal surfaces

“…For instance, tetraphenylporphyrin molecules, 2H-TPP, were observed to experience attractive intermolecular forces on Ag(111), but repulsive interactions on Cu(111) due to the charge pillow effect. 49 This is consistent with the formation of ZI nano gratings on Cu(111) in this study.…”

“…As can be seen, the energies per molecule vary from −0.1 to −4 meV. These calculated energies need to be compared to typical energies of H-bonds (170-650 meV), 36 van der Waals bonds (<50 meV), 37 CH-π bonds (60-100 meV), 38 π-π bonds (100 meV), 38 and metal-ligand bonds (500-2000 meV). 38 For an additional comparison, the difference in bonding site energies for adsorbed ZI on the Au(111) surface was reported to be 40 meV, 19 which by itself was declared inconsequentially small.…”

“…These calculated energies need to be compared to typical energies of H-bonds (170-650 meV), 36 van der Waals bonds (<50 meV), 37 CH-π bonds (60-100 meV), 38 π-π bonds (100 meV), 38 and metal-ligand bonds (500-2000 meV). 38 For an additional comparison, the difference in bonding site energies for adsorbed ZI on the Au(111) surface was reported to be 40 meV, 19 which by itself was declared inconsequentially small. A similar assessment of the role of dipolar interactions has been made by Talapin et al, which considered inorganic nanoparticles with an extremely large dipole moment of 100 D (which is an order of magnitude larger than any organic molecule) and a radius of 5.8 nm.…”

Studying the dynamic behaviour of porphyrins as prototype functional molecules by scanning tunnelling microscopy close to room temperature

“…107 The very different behaviour found here for 2HTTBPP on Cu(111) (y = 5 AE 51 and f = 35 AE 51) is a strong indication of considerable stabilizing interactions. For the concave conformation, this interpretation is in line with a strong attractive interaction between the iminic nitrogen atoms and the Cu substrate, by which the porphyrin macrocycle is literally ''pulled'' towards the surface; 34,35,53,55,79 the reason for the convex conformation, however, is not readily accessible. Here lateral interactions also must play a role and it will be later discussed that this conformation is entropically stabilized.…”

Charge-induced distortion and stabilization of surface transfer doped porphyrin films

“…Self-assembled monolayers of metallated porphyrin and other porphyrin derivatives on top of metal substrates have been thoroughly examined in previous studies. [28][29][30] Rojas et al 29 examined a range of metallated and non-metallated porphyrin derivatives on top of metal substrates and found that a subtle balance between molecule-molecule and molecule-substrate interactions governs the occurrence of self-assembly. Whereas most porphyrin molecules selfassemble into closely packed monolayers, H 2 TPP adsorbates are arranged randomly on top of Cu(111) indicating strong molecule-substrate interaction in terms of charge transfer.…”