In the present study, photophysical properties of [N]phenylenes were studied by means of stationary and time-resolved absorption and fluorescence spectroscopy (in THF at room temperature). For biphenylene (1) and linear (1) and (2a) as "fast IC compounds", with k IC > 10 9 s -1 , and of (3) and (4) as "slow IC compounds", with k IC ≈ 10 7 s -1 , is suggested. This classification cannot simply be related to Hückel's rule-type concepts of aromaticity, because the group of "fast IC compounds" consists of "antiaromatic" (1) and "aromatic" (2a), and the group of "slow IC compounds" consists of "antiaromatic" (3b), (4) and "aromatic" (3a), (3c). The IC in the[N]phenylenes is discussed within the framework of the so-called energy gap law established for non-radiative processes in benzenoid hydrocarbons.2

Tribenzosubporphyrins are boron(III)-chelated triangular bowl-shaped ring-contracted porphyrins that possess a 14π-aromatic circuit. Their flat molecular shapes and discrete molecular orbital diagrams make them ideal for observation by scanning tunneling microscopy (STM). Expanding their applications toward single molecule-based devices requires a fundamental knowledge of single molecular conductance between tribenzosubporphines and the STM metal tip. We utilized a tungsten (W) STM tip to investigate the electronic properties of B-(5-mercaptopentoxy)tribenzosubporphine 1 at the single molecular level. B-(5-mercaptopentoxy)-tribenzosubporphine 1 was anchored to the Au(111) surface via reaction with 1-heptanethiol linkers that were preorganized as a self-assembled monolayer (C7S SAM) on the Au(111) substrate. This arrangement ensured that 1 was electronically decoupled from the metal surface. Differential conductance (dI/dV - V) measurements with the bare W tip exhibited a broad gap region of low conductance and three distinct responses at 2.4,-1.3, and -2.1 V. Bias-voltage-dependent STM imaging of 1 at 65 K displayed a triangle shape at -2.1 < V < -1.3 V and a circle shape at V < -2.1 V, reflecting its HOMO and HOMO-1, respectively. In addition, different conductance behaviors were reproducibly observed, which has been ascribed to the adsorption of a tribenzosubporphine-cation on the W tip. When using a W tip doped with preadsorbed tribenzosubporphine-cation, negative differential resistance (NDR) phenomena were clearly observed in a reproducible manner with a peak-to-valley ratio of 2.6, a value confirmed by spatial mapping conductance measurements. Collectively, the observed NDR phenomena have been attributed to effective molecular resonant tunneling between a neutral tribenzosubporphine anchored to the metal surface and a tribenzosubporphine cation adsorbed on a W tip.

We identified the orientation of individual Lu@C 82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C 82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C 82 molecule, suggesting that thermal rotation of Lu@C 82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn-Sham molecular orbitals of Lu@C 82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C 82 . Spatial mapping of the differential conductance on individual Lu@C 82 molecules revealed that the local conductivity within a molecule became large around the Lu atom at a negative sample bias voltage. From spatial mapping of the differential conductance measurements, we also evaluated the HOMO-LUMO gap of Lu@C 82 to be 0.47 eV. From the results of the spatial mapping of the differential conductance and DFT calculations, the locally high conductivity around the Lu atom was attributed to the HOMO-2 level orbital concentrated on the Lu atom and its six nearest C atoms at 0.055 eV below the HOMO level. We demonstrated changes in the molecular orientation of Lu@C 82 by applying a high electric field (about 1 × 10 7 V/cm) with a large tunneling current (1.5 nA).