On-surface synthesis of phthalocyanines with extended π-electron systems

Abstract

Expanded phthalocyanines are a promising class of materials for optoelectronic applications, owing to their unique properties and versatile metal coordination reactivity. The expansion of their π-electron systems and resulting red-shifted absorption are of particular interest for achieving broader applications. Here, we report the on-surface synthesis of metallo-phthalocyanines with extended electron systems and an open-chain polycyanine from ortho-dicarbonitrile precursors on Ag(111) and Au(111), studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The larger 6,7-di(2-naphthyl)-2,3-naphthalenedicarbonitrile (NND) undergoes spontaneous cyclotetramerization on the Ag(111) surface forming the corresponding silver naphthalocyanines (Ag-NPc), contrasting previous reports where a partially aliphatic ortho-dicarbonitrile precursor formed polycyanine chains. In contrast, monolayers of the smaller 6,7-diphenyl-2,3-naphthalenedicarbonitrile (PND) form the corresponding naphthalocyanine only in the presence of co-adsorbed iron atoms (Fe-NPc). In the absence of iron, PND multilayers form polycyanine chains and Ag-NPc. NND and PND further differ in their reactivity due to the supramolecular behavior of their products. While the larger Ag-NPc aggregates to non-covalent one-dimensional ribbons, the smaller Fe-NPc forms an extended non-covalent two-dimensional network. Our study demonstrates the versatility of on-surface dinitrile tetramerization for the synthesis of π-extended cyclic phthalocyanines and their open-chain polycyanine counterparts.

Fig. 1. Chemical structures of ortho-dinitrile precursors for naphthalocyanine synthesis.

a The 5,5,8,8-tetramethyl-5,6,7,8-tetrahydroanthracene-2,3-carbodinitile (ADN) precursor used previously. b Four on-surface conformers of the 6,7-di(2-naphthyl)-2,3-naphthalenedicarbonitrile (NND) precursor. c 6,7-Diphenyl-2,3-naphthalenedicarbonitrile (PND) precursor utilized for the synthesis of Fe-NPcs, the polycyanine chain, and Ag-NPcs.

Fig. 2. Formation of Ag naphthalocyanine from the 6,7-di(2-naphthyl)-2,3-naphthalenedicarbonnitrile (NND) precursor on Ag(111).

a Large-scale image of a NND sub-monolayer (0.40 ML) on Ag(111) at 300 K. b, c Close-up image of the coordination of four precursor molecules to tetramer structures without (b) and with (c) overlaid molecular structures. d Reaction scheme of the formation of Ag-NPc upon annealing to 500 K. e, f Images of the ribbon-like arrangement of the Ag-NPc product after annealing to 500 K (300 K, 0.76 ML; 500 K, 0.40 ML). g Enlarged and rotated cutout of (f) with overlaid models, illustrating the connection of single Ag-NPc units to a ribbon via attractive intermolecular π-stacking. Highlighted in magenta is a single miss-oriented naphthyl-group, which prevents the continuation of the ribbon. Scale bars: (a, e) 10 nm; (b, c) 1 nm; (f) 4 nm; (g) 2 nm. Tunneling parameters: (a) U = −2.67 V, I = −0.10 nA; (b, c) U = −2.93 V, I = −0.14 nA; (e) U = −2.75 V, I = −0.10 nA; (f, g) U = −2.75 V, I = −0.09 nA.

Fig. 3. Formation of Fe naphthalocyanine from the 6,7-diphenyl-2,3-naphthalenedicarbonitrile (PND) precursor and co-adsorbed Fe on Ag(111).

a Large-scale image of a PND sub-monolayer (0.35 ML) on Ag(111) at 300 K. b Close-up image with overlaid molecular model, showing the formation of supramolecular hexamers. c Reaction scheme for the formation of Fe-NPc on Ag(111) upon annealing to 500 K (300 K, 0.52 ML; 500 K, 0.33 ML) in presence of co-adsorbed Fe. d Island formation of regular Fe-NPc molecules after annealing to 500 K. Irregular shaped structures lead to unordered fractions around the islands. e Close-up image of an island as in (d) with an overlaid model, which shows the staggered structure of the Fe-NPcs, stabilized by attractive π-π-interactions. Scale bars: (a, d) 10 nm; (b, e) 1 nm. Tunneling parameters: (a) U = −1.68 V, I = −0.11 nA; (b) U = −2.22 V, I = −0.36 nA; (d) U = −2.93 V, I = −0.08 nA; (e) U = −2.59 V, I = −0.14 nA.

Fig. 4. Formation of polycyanine chains in multilayers of the PND precursor on Ag(111).

a Reaction scheme for the formation of polycyanine chains and Ag-NPc cyclic covalent tetramers in PND multilayers at 450 K. b Large-scale STM image taken after annealing of a PND multilayer (~8 × 10¹⁴ molecules/cm²) to 450 K on Ag(111). c STM image showing that further annealing of the sample in (b) to 470 K leads to desorption of residual monomers and the coexistence of chains and cyclic tetramers. d Close-up STM image of the polycyanine chains in (c). e Zoom-in STM image as indicated in (d) with overlaid chemical model. f Single chain perpendicular to a step edge before and parallel to the edge after (g) repeated lateral tip manipulations along the directions of the arrows in (h–j). k Different ensemble of chains. l Close-up STM image of the chain in (k) before manipulation. m Chain after tip manipulation showing a dent and additional bright protrusions attributed to upstanding chain segment. n Close-up STM image of the tetramer island in (c), showing cross-shaped cyclic covalent tetramers (NPcs) with and without bright central protrusions. o Large-scale STM image of the sample upon annealing to 550 K. Scale bars: (b, f–j, l, m) 5 nm; (c, k) 10 nm; (d) 3 nm; (e, n) 1 nm (o) 20 nm. Tunneling parameters: (b) U = 1.20 V, I = 0.11 nA; (c) U = −1.28 V, I = −0.26 nA; (d, e) U = −1.40 V, I = −0.24 nA; (f–i) U = −2.59 V, I = −0.19 nA; (j) U = −3.02 V, I = −0.21 nA; (k) U = −2.15 V, I = −0.20 nA; (l) U = −2.15 V, I = −0.18 nA; (m) U = −2.15 V, I = −0.15 nA; (n) U = −0.20 V, I = −0.28 nA; (o) U = −3.31 V, I = −0.13 nA.

Fig. 5. XPS spectra of the NND and PND precursors and the corresponding naphthalocyanine products on Ag(111).

N 1 s and C 1s XPS spectra of both precursors (a, e, c, g) and their corresponding M-NPcs (b, f, d, h) on Ag(111). All spectra were taken at 300 K. The N 1 s data show the conversion of the cyano groups to the phthalocyanines by a shift towards lower E_B and peak broadening. The C 1 s data (e, g) were fitted by taking the stochiometric factors of the three different main species (nitrile, tilted and non-special carbon) in account. The products (f, h) show a decreased intensity of the shoulder arising from the contribution of the nitrile groups (blue).

Fig. 6. Formation of Fe naphthalocyanine from the PND precursor and co-adsorbed Fe on Au(111).

a Proposed reaction scheme for PND with Fe on Au(111). b STM image of PND on Au(111) at 300 K (0.61 ML). Two different domains can be distinguished in the adsorbate structure. Upper left: Structure similar to NND on Ag(111) (cf. Fig. S8 for further details); lower right: mixed phase of tetramers and hexamers. c Close-up STM image of the mixed phase from (b) with overlaid model, which shows the coordination of four PND molecules to non-covalent tetramers. d STM image after annealing to 500 K showing the formation of islands with straight edges consisting of regular Fe-NPc molecules (coverages: 300 K, 0.67 ML; 500 K, 0.34 ML). e Close-up STM image of an island as in (d) with an overlaid model, showing the regular pattern of the Fe-NPcs, stabilized by attractive π-π-interactions. f Larger-scale STM image of the sample after annealing to 630 K (0.26 ML). g STM image of a fully planarized Fe-NPc molecule next to non-regular structures. Scale bars: (b, d, f) 20 nm; (c, e, g) 1.5 nm. Tunneling parameters: (b) U = −1.00 V, I = −0.21 nA; (c) U = 2.02 V, I = 0.09 nA; (d) U = −1.85 V, I = −0.13 nA; (e) U = 1.44 V, I = 0.12 nA; (f) U = −2.67 V, I = −0.17 nA; (g) U = −2.84 V, I = −0.13 nA.