In‐Situ Growth of Gadolinium Phthalocyaninato Sandwich Complexes on the Ag(111) Surface

Abstract

We report a low-temperature scanning tunneling microscopy investigation of the in-situ growth of gadolinium phthalocyaninato complexes by combined deposition of free-base phthalocyanines and gadolinium atoms on a smooth Ag(111) substrate. A careful control of the stoichiometry allows the expression of a multilevel structurecomposed of irregularly distributed Gd x-1 (Pc) x complexes, x = 2-5, thus paving new avenues for surfaceconfined columnar growth.Tetrapyrrole compounds such as porphyrins and phthalocyanines are functional pigments that have attracted much interest in the last decades due to their importance in biology and technology. [1] Specifically, phthalocyanines (Pcs) are attractive synthetic molecules for nanotechnology [2][3][4][5] presenting an appealing variety of functional properties in pigments, organic solar cells and optoelectronics. [6] In the last decade, scanning tunneling microscopy (STM) as well as scanning tunneling spectroscopy (STS) techniques have facilitated the study and characterization with atomic-scale precision of the assembly and electronic properties of such compounds. [7][8][9][10][11][12][13][14][15] Contemporarily, the successful synthesis of phthalocyanine sandwich-type double and triple-decker lanthanide complexes reveals distinctive features that cannot be achieved for their non-sandwich analogues, i. e., single-molecule magnetism and axial molecular rotation. On one hand, rare-earth metal atoms promote octa-coordinated sandwich-type complexes [16][17][18][19][20][21] that have been recently under the spotlight due to the large magnetic moment and anisotropy [22,23] of the lanthanide elements, which results in the expression of single molecular magnetism, [24][25][26][27][28][29] thus with great potential for organic fieldeffect transistors, [30] molecular magnets, [26] chemical sensors and liquid crystals, [31] and single-molecule qubits. Steered by these functionalities, surface science studies have addressed the sublimation, self-assembly, electronic and magnetic properties of such compounds when deposited on surfaces. [17,22,23,[32][33][34][35][36][37][38][39] On the other hand, surface-confined multi-decker species have revealed their capability to be used as axial molecular rotors, [40][41][42][43] while simultaneously introducing columnar growth on surfaces, though limited to triple-deckers. [18] In this work, we demonstrate the formation of homoleptic gadolinium phthalocyaninato complexes, up to stacked penta (phthalocyaninato) species, on a smooth Ag(111) substrate under ultra-high-vacuum (UHV) conditions. Figure 1a shows a large-scale high-resolution STM image after deposition of 0.02 ML of Gd atoms onto a precursor multilayer of phthalocyanine species and subsequent annealing to 550 K on the Ag(111) substrate. Herein, three molecular layers of different apparent heights are recognized. A detailed analysis of the first-layer reveals four-lobed species attributed to 2HÀ Pc molecular entities with a two-fold symmetry, D 1 axis deviating by 3°from the close packed ...

On-Surface Synthesis of Nonmetal Porphyrins

“…Porphyrin molecules play an important role in biological systems and are used in diverse technological and medical applications. − The characteristic porphyrin macrocycle can host most elements of the periodic table and can be further functionalized by peripheral substituents. − In a nano- and surface science scenario relying on an ultrahigh vacuum (UHV) environment, porphyrin complexes are extensively studied as building blocks for self-assembled molecular films, , two-dimensional metal–organic coordination networks, , and covalent architectures. − In this context, the on-surface synthesis of porphyrin complexes, and specifically the metalation of free-base (2H) species by intrinsic surface adatoms or deposited atoms, was shown to provide elegant routes to metalloporphyrin architectures with distinct chemical, physical, and structural properties. − In addition to the magnetic and electronic structure of the central atom, its size plays a relevant role in the properties of the resulting complex. While 3d metal ions are generally positioned in or near the macrocycle plane, larger elements such as lanthanides are displaced from the molecular center (e.g., allowing for the in situ formation of double- and multidecker assemblies , ). Importantly, the interaction with the supporting surface can affect the adsorbed porphyrins’ properties, such as the molecular conformation ,,− and the oxidation state of the central atom, which is often reduced by charge transfer from the metallic substrate. − …”

“…15−19 In addition to the magnetic and electronic structure of the central atom, its size plays a relevant role in the properties of the resulting complex. While 3d metal ions are generally positioned in or near the macrocycle plane, 7 larger elements such as lanthanides are displaced from the molecular center 20 (e.g., allowing for the in situ formation of double-and multidecker assemblies 21,22 ). Importantly, the interaction with the supporting surface can affect the adsorbed porphyrins' properties, such as the molecular conformation 7,8,23−26 and the oxidation state of the central atom, which is often reduced by charge transfer from the metallic substrate.…”

Actinide Coordination Chemistry on Surfaces: Synthesis, Manipulation, and Properties of Thorium Bis(porphyrinato) Complexes

“…Here, we introduce the in situ synthesis of Th(TPP) 2 (see Figure 1a−c) in a solvent-free UHV environment on two distinctly different substrates, namely, Ag(111) and h-BN/ Cu(111). Our work thus expands on-surface metalation protocols, which are well-established for 3d metals, 61,62 alkali metals, 63 nonmetals, 64,65 and lanthanides, [58][59][60]66 to the actinide series. Along with extending tetrapyrrole surface science, 67,68 this study provides a rare example of a successful reaction on h-BN.…”

“…50 As a downside, the intact deposition of such complexes by sublimation in a UHV environment poses severe challenges, with thermally induced molecular fragmentation or modifications often interfering with clean, homogeneous adsorbate structures, 46,52,57 even though molecular functionalization can favor successful deposition. 49 In situ synthesis of lanthanide tetrapyrrole double-deckers and multideckers was resorted to as a remedy and alternative, e.g., yielding Ce(TPP) 2 , Ce 2 (TPP) 3 , 58 Gd x−1 Pc x (x = 1−5), 59 TbPc 2 , and Tb(tbu-Pc) 2 60 complexes directly on noble metal supports. Here, we introduce the in situ synthesis of Th(TPP) 2 (see Figure 1a−c) in a solvent-free UHV environment on two distinctly different substrates, namely, Ag(111) and h-BN/ Cu(111).…”

Layer-by-Layer Epitaxy of Porphyrin−Ligand Fe(II)-Fe(III) Nanoarchitectures for Advanced Metal–Organic Framework Growth

“…Directed self-assembly of molecular architectures in three dimensions (3D) is a fundamental process of complex matter. − Its control is crucial for the fabrication of hybrid organic–inorganic molecular heterostructures, which are promising candidates for modular, integrated electronic nanodevices. − While self-assembly in two dimensions (2D) is well-established, research to achieve epitaxial deposition of truly precise supramolecular arrays stacking into the third dimension, is still in its infancy. Popular strategies for 3D-directed self-assembly at well-defined interfaces include templated growth − and solution layer-by-layer (LbL) growth. − The general LbL approach for tailored reticular architectures is highly appealing due to the possibility of incorporating different molecular species at each stratum, thereby approaching biological complexity. However, precision 3D growth of heterostructures is largely limited by the understanding of complexation kinetics and diffusion rates of the individual molecular species, and by the ample self-assembly parameter space, which cannot necessarily guarantee stoichiometrically layered materials.…”