Thiruvancheril G. Gopakumar

Spin-Crossover Complex on Au(111): Structural and Electronic Differences Between Mono- and Multilayers

Submono-, mono- and multilayers of the Fe(II) spin-crossover (SCO) complex [Fe(bpz)2 (phen)] (bpz=dihydrobis(pyrazolyl)borate, phen=1,10-phenanthroline) have beenprepared by vacuum deposition on Au(111) substrates and investigated with near edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). As evidenced by NEXAFS, molecules of the second layer exhibit a thermal spin crossover transition, although with a more gradual characteristics than in the bulk. For mono- and submonolayers of [Fe(bpz)2 (phen)] deposited on Au(111) substrates at room temperature both NEXAFS and STM indicate a dissociation of [Fe(bpz)2 (phen)] on Au(111) into four-coordinate complexes, [Fe(bpz)2 ], and phen molecules. Keeping the gold substrate at elevated temperatures ordered monolayers of intact molecules of [Fe(bpz)2 (phen)] are formed which can be spin-switched by electron-induced excited spin-state trapping (ELIESST).

Transfer of Cl Ligands between Adsorbed Iron Tetraphenylporphyrin Molecules

Iron tetraphenylporphyrin chloride (FeTPPCl) adsorbed on a Au(111) substrate is investigated using low-temperature scanning tunneling microscopy. Cl is controllably transferred between the Fe center of a selected molecule and the tip of the microscope without disrupting the surrounding molecular pattern. Cl abstraction from FeTPPCl is triggered by removing an electron from the highest occupied molecular orbital. Density functional calculations suggest that the reaction involves a change in the oxidation state of the Fe ion.

Surface control of alkyl chain conformations and 2D chiral amplification.

Trioctyl-functionalized triazatriangulenium (trioctyl-TATA) deposited on Au(111) and Ag(111) surfaces by electrospray ionization was investigated using low-temperature scanning tunneling microscopy. The molecule surprisingly adsorbs with gauche rather than anti conformations of the octyl groups. We observed chiral amplification in the islands. Only one of the eight possible configurations of the octyl groups was found in homochiral hexagonal networks. Quantum-chemical calculations confirmed and explained the preference for the gauche conformations of adsorbed trioctyl-TATA.

Scanning Tunnelling Microscopy on Ultrathin Organic Layers of Phthalocyanine and Naphthalocyanines on Highly Oriented Pyrolytic Graphite (0001)

The adsorption geometry of the monolayers of planar palladium phthalocyanine (PdPc), naphthalocyanine (Nc), and non-planar tin–naphthalocyanine (SnNc) on the basal plane (0001) of graphite have been studied. Monolayers were prepared using organic molecular beam epitaxy (OMBE) under ultrahigh vacuum (UHV) conditions at room temperature. Planar PdPc and Nc molecules form large areas of ordered monolayers with their molecular plane parallel to the substrate. PdPc forms quadratic and Nc forms distorted quadratic superstructures, however the superstructure is non-commensurate with the substrate lattice. Non-planar SnNc shows a commensurism with graphite and forms a hexagonal superstructure, which indicates a stronger molecule–substrate interaction compared to the planar derivatives revealing the influence of geometry on the adlayer structure.

Effect of chain length on the tunneling conductance of gold quantum dots at room temperature

Understanding the electronic structure of nanometer-sized metal particles which bridge the gap between the molecules and bulk materials is important due to the fabrication of many nanodevices, like single-electron transistors and molecular switches. Using a simple core-shell model, here, we investigate the variation of electrostatic charging properties of metallic nanoclusters with the chain length of the passivating molecule and cluster core size. The estimation of capacitance (Cc) and charging energy (Ec) as 2πe0erRc/(1+2r/Rc) and e2(1+2r/Rc)/4πe0erRc, respectively, as a function of the core size as well as the intercluster spacing, reveals that longer chains (8 to 10 methylene units) limit the electron transport beyond usefulness.

Switching of an Azobenzene-Tripod Molecule on Ag(111)

The trans-cis isomerization makes azobenzene (AB) a robust molecular switch. Once adsorbed to a metal, however, the switching is inefficient or absent due to rapid excited-state quenching or loss of the trans-cis bistability. We find that tris-[4-(phenylazo)-phenyl]-amine is a rather efficient switch on Ag(111). Using scanning tunneling and atomic force microscopy at submolecular resolution along with density functional theory calculations, we show that the switching process is no trans-cis isomerization but rather a reorientation of the N-N bond of an AB unit. It proceeds through a twisting motion of the azo-bridge that leads to a lateral shift of a phenyl ring. Thus, the role of the Ag substrate is ambivalent. While it suppresses the original bistability of the azobenzene units, it creates a new one by inducing a barrier for the rotation of the N-N bond.

Broken Symmetry of an Adsorbed Molecular Switch Determined by Scanning Tunneling Spectroscopy

An asymmetric turn: Scanning tunneling spectroscopy has been used to analyze the structure of tris[4-(phenylazo)phenyl)]amine on a Au(111) surface. A degenerate marker state serves as a sensitive probe for the structure of the adsorbed molecules.

DFT study of vibronic properties of d8 (Ni-, Pd-, and Pt-) phthalocyanines.

By means of density functional theory, we have studied the electronic structure and vibronic properties of single neutral NiPc, PdPc, and PtPc molecules and their singly and doubly ionized cations and anions. In particular, the vibronic couplings and reorganization energies of all systems are compared. Partitioning of the reorganization energy, corresponding to the photoelectron spectra of the first and second ionizations of studied molecules, into normal mode contributions shows that the major contributions are due to several vibrational modes with a(1g) symmetry and energies lower than 1600 cm(-1). The results reveal that the reorganization energy due to the singly positive ionization in the studied molecules is up to about one order of magnitude less than other reorganization energies. This makes these metal phthalocyanines, from the perspective of intramolecular reorganization energies, attractive as electron donor for intramolecular electron transfer in electron acceptor-donor systems.

Surface cis Effect: Influence of an Axial Ligand on Molecular Self-Assembly.

Adding ligands to molecules can have drastic and unforeseen consequences in the final products of a reaction. Recently a surface trans effect due to the weakening of a molecule-surface bond was reported. Here, we show a surface cis effect where an axial ligand at adsorbed transition-metal complexes enables lateral bonding among the molecules. In the absence of this ligand, the intermolecular interaction is repulsive and supramolecular patterns are not observed. Fe-tetramethyl-tetraazaannulene on Au(111) was investigated using low-temperature scanning tunneling microscopy and spectroscopy along with density functional theory calculations. At low coverages, the molecules remain isolated. Exposure to CO leads to axial CO bonding and induces reordering into extended clusters of chiral molecular trimers. The changed self-assembly pattern is due to a CO-induced modification of the molecular structure and the corresponding charge transfer between the molecule and the substrate, which in turn changes the lateral intermolecular forces.

Coordination-Controlled One-Dimensional Molecular Chains in Hexapodal Adenine–Silver Ultrathin Films

Growth of a silver coordination polymer of a C3-symmetric hexaadenine ligand is studied on highly oriented pyrolytic graphite (HOPG), using high-resolution atomic force microscopy (AFM). This unusual ligand offers 6-fold multidentate coordination sites, and consequently, a multidimensional growth of coordination polymer is expected. Notably, each discrete hexapodal unit is bridged by two silver ions along one of the crystallographic directions, resulting in high interaction energy along this direction. When the polymer was deposited on an HOPG surface from a dilute solution, we observed abundant one-dimensional (1D) coordination polymer chains, with a minimum width of approximately 4.5 nm. The single-crystal structure using X-ray analysis is compared with the surface patterns to reconcile and understand the structure of the 1D polymer on an HOPG surface. The energy levels of Ag-L1 within the proposed model were calculated, on the basis of the X-ray crystal structure, and compared to the ligand states to gain information about the electronic structure of ligand upon Ag coordination. On the basis of the wave functions of a few molecular orbitals (MOs) near the Fermi energy, it is surmised that unfilled MOs may play a crucial role in the transport properties of the Ag-L1 adlayer.