Surface-assisted Dehydrogenative Homocoupling of Porphine Molecules

Abstract

The templated synthesis of porphyrin dimers, oligomers, and tapes has recently attracted considerable interest. Here, we introduce a clean, temperature-induced covalent dehydrogenative coupling mechanism between unsubstituted free-base porphine units yielding dimers, trimers, and larger oligomers directly on a Ag(111) support under ultrahigh-vacuum conditions. Our multitechnique approach, including scanning tunneling microscopy, near-edge X-ray absorption fine structure and photoelectron spectroscopy complemented by theoretical modeling, allows a comprehensive characterization of the resulting nanostructures and sheds light on the coupling mechanism. We identify distinct coupling motifs and report a decrease of the electronic gap and a modification of the frontier orbitals directly associated with the formation of triply fused dimeric species. This new on-surface homocoupling protocol yields covalent porphyrin nanostructures addressable with submolecular resolution and provides prospective model systems towards the exploration of extended oligomers with tailored chemical and physical properties.

Iron phthalocyanine on Cu(111): Coverage-dependent assembly and symmetry breaking, temperature-induced homocoupling, and modification of the adsorbate-surface interaction by annealing

“…In these structures, the FePc molecules come significantly closer to each other than what the van der Waals radii of the atoms would suggest is possible. This indicates dehydrogenation and polymerization of the molecules in a homocoupling reaction 18 and, indeed, thermally induced dehydrogenation and polymerization of phthalocyanine molecules have been reported previously for the Ag(111) surface. 16 We note that any appreciable dissociation beyond dehydrogenation is not discernible from the STM images (cf.…”

“…• C. 16,18 Today, similar schemes are also known from a large variety of other compounds and are being used for on-surface covalent synthesis (see, e.g., Ref. 19).…”

Surface chemistry of porphyrins and phthalocyanines

“…ZnPc on Cu(1 0 0) was shown to form oligomers with 901 or 1801 angles during annealing at 500 K [243]. C-H bond dissociation and subsequent covalent linkage can also occur directly at the porphyrin core, as was demonstrated for porphin on Ag(1 1 1) [435]. After annealing of a 2HP multilayer to 573 K, the remaining submonolayer contained covalently linked dimers, trimers, and larger oligomers along with residual 2HP monomers.…”

“…For coverages exceeding 1 ML, second-layer islands with a rhombic unit cell have been observed [41]. The reactive intermolecular coupling observed upon annealing to 573 K [435] will be discussed in Section 4.5. 1.…”

Dehydrogenative Homocoupling of Terminal Alkenes on Copper Surfaces: A Route to Dienes

“…[1][2][3] Generally,t he reactants of these reactions are hydrocarbons functionalized with C À Ho rC À Xg roups (X stands for halogen), and two reactants couple by cleaving either the CÀHo rC À Xb onds and forming an ew carbon-carbon bond with the help of carefully selected catalysts.R ecently,o n-surface chemistry has attracted attention because various well-known chemical reactions have been successfully achieved on surfaces, [4][5][6][7][8][9][10][11][12][13] and more interestingly,some reactions that are hard to mediate by conventional chemistry have also been achieved on surfaces. [14][15][16][17][18][19][20] Specifically,d ehydrogenative coupling of hydrocarbons like alkynes (sp 1 ), [9,21,22] alkanes (sp 3 ), [17,18] and arenes (sp 2 ), [19,20,23] have very recently been introduced onto surfaces with the interest of fabricating novel carbon nanostructures/ nanomaterials or exploring new reaction pathways.T oo ur knowledge,h owever,o n-surface coupling of alkenes (sp 2 ), another important hydrocarbon, has not been reported so far. It is therefore of utmost interest to complement the database to extend our fundamental understanding of the on-surface chemistry of hydrocarbons.…”