Isomer-Dependent Franck–Condon Blockade in Weakly Coupled Bipyridine Molecular Junctions

Franck–Condon blockade is an attractive functionality of molecular junctions, but its tunability is still a challenge that has not been fully addressed. We show here from first-principles calculations that the electron–vibration coupling strength of a weekly coupled bipyridine molecular junction can be largely tuned from weak to strong coupling regime through isomerization. Electron transport properties of four bipyridine isomers, namely 2,6′-bipyridine, 2,4′-bipyridine, 2,2′-bipyridine, and 4,4′-bipyridine, have been exclusively examined. A very strong Franck–Condon blockade is found to be associated with 2,2′-bipyridine and 4,4′-bipyridine molecules and should be observable experimentally. A gate-controlled conductance switch is proposed for a molecular junction with a 4,4′-bipyridine molecule. Our calculations have clearly demonstrated that bipyridine isomers are excellent candidates for the experimental study of vibration-mediated transport properties in a single molecule

Theoretical Modeling of Plasmon-Enhanced Raman Images of a Single Molecule with Subnanometer Resolution

Under local plasmonic excitation, Raman images of single molecules can now surprisingly reach subnanometer resolution. However, its physical origin has not been fully understood. Here we report a quantum-mechanical description of the interaction between a molecule and a highly confined plasmonic field. We show that when the spatial distribution of the plasmonic field is comparable to the size of the molecule, the optical transition matrix of the molecule becomes dependent on the position and distribution of the plasmonic field, resulting in a spatially resolved high-resolution Raman image of the molecule. The resonant Raman image reflects the electronic transition density of the molecule. In combination with first-principles calculations, the simulated Raman image of a porphyrin derivative adsorbed on a silver surface nicely reproduces its experimental counterpart. The present theory provides the basic framework for describing linear and nonlinear responses of molecules under highly confined plasmonic fields

Fe L‑Edge X‑ray Absorption Spectra of Fe(II) Polypyridyl Spin Crossover Complexes from Time-Dependent Density Functional Theory

L-edge near-edge X-ray fine structure spectroscopy (NEXAFS) has become a powerful tool to study the electronic structure and dynamics of metallo-organic and biological compounds in solution. Here, we present a series of density functional theory calculations of Fe L-edge NEXAFS for spin crossover (SCO) complexes within the time-dependent framework. Several key factors that control the L-edge excitations have been carefully examined using an Fe­(II) polypyridyl complex [Fe­(tren­(py)₃)]²⁺ (where tren­(py)₃ = tris­(2-pyridylmethyliminoethyl)­amine) as a model system. It is found that the electronic spectra of the low-spin (LS, singlet), intermediate-spin (IS, triplet), and high-spin (HS, quintet) states have distinct profiles. The relative energy positions, but not the spectral profiles, of different spin states are sensitive to the choice of the functionals. The inclusion of the vibronic coupling leads to almost no visible change in the resulting NEXAFS spectra because it is governed only by low-frequency modes of less than 500 cm^–1. With the help of the molecular dynamics sampling in acetonitrile at 300 K, our calculations reveal that the thermal motion can lead to a noticeable broadening of the spectra. The main peak position is strongly associated with the length of the Fe–N bond