Real-space investigation of energy transfer in heterogeneous molecular dimers

Abstract

Given its central role in photosynthesis and artificial energy-harvesting devices, energy transfer has been widely studied using optical spectroscopy to monitor excitation dynamics and probe the molecular-level control of energy transfer between coupled molecules. However, the spatial resolution of conventional optical spectroscopy is limited to a few hundred nanometres and thus cannot reveal the nanoscale spatial features associated with such processes. In contrast, scanning tunnelling luminescence spectroscopy has revealed the energy dynamics associated with phenomena ranging from single-molecule electroluminescence, absorption of localized plasmons and quantum interference effects to energy delocalization and intervalley electron scattering with submolecular spatial resolution in real space. Here we apply this technique to individual molecular dimers that comprise a magnesium phthalocyanine and a free-base phthalocyanine (MgPc and HPc) and find that locally exciting MgPc with the tunnelling current of the scanning tunnelling microscope generates a luminescence signal from a nearby HPc molecule as a result of resonance energy transfer from the former to the latter. A reciprocating resonance energy transfer is observed when exciting the second singlet state (S) of HPc, which results in energy transfer to the first singlet state (S) of MgPc and final funnelling to the S state of HPc. We also show that tautomerization of HPc changes the energy transfer characteristics within the dimer system, which essentially makes HPc a single-molecule energy transfer valve device that manifests itself by blinking resonance energy transfer behaviour.

Single-Molecule Investigation of Energy Dynamics in a Coupled Plasmon-Exciton System

“…We note that the I/I0 spectrum reproduces the following features seen in the typical absorption spectrum of H2Pc 21,23,29,30 : the signal at 1.81 eV (Qx) is stronger than that at 1.92 eV (Qy), and the energy separation of 110 meV between the two dips agrees with the reported value (106-126 meV which varies depending on the environment surrounding H2Pc) 24 measured at low temperature with the matrix isolation method.…”

“…More importantly, the developed technique in combination with STL has established the foundation of an absorption/emission spectroscopy with the unprecedented sensitivity and spatial resolution, which will certainly expand our perception into fundamental energetic processes taking place at the nanometre scale. Indeed, we have recently applied it to individual molecular dimers consisting of H2Pc and MgPc, and successfully revealed the energy transfer dynamics in the coupled molecular system 30 .…”

Atomically precise graphene nanoribbon heterojunctions from a single molecular precursor

“…11,12 While the behavior of heterojunctions in macroscopic semiconductor crystals has been extensively studied, 13 heterojunction behavior in molecular nanostructures is not as well understood. [14][15][16][17] Bottom-up GNR heterojunctions have previously been fabricated by an approach that combines two different molecular precursor types, thus yielding either impurity-modulated 18 or widthmodulated 19 GNR heterojunctions. This fabrication method, however, depends on the random process of precursor self-assembly and does not allow heterojunction fabrication or modification after a GNR has been synthesized.…”

Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation

“…In many fundamental research fields, notably in surface science [1], they serve as model templates and atomically controlled spacers, for the manipulation and study of nanoparticles [2][3][4], molecules [5][6][7][8][9][10], or single atoms [11]. Most of the time, chemical stability of insulating thin films upon charged particle irradiation is required, e.g., for characterization using electron spectroscopy and microscopy.…”