Hot luminescence from single-molecule chromophores electrically and mechanically self-decoupled by tripodal scaffolds.

Control over the electrical contact to an individual molecule is one of the biggest challenges in molecular optoelectronics. The mounting of individual chromophores on extended tripodal scaffolds enables both efficient electrical and mechanical decoupling of individual chromophores from metallic leads. Core-substituted naphthalene diimides fixed perpendicular to a gold substrate by a covalently attached extended tripod display high stability with well-defined and efficient electroluminescence down to the single-molecule level. The molecularly controlled spatial arrangement balances the electric conduction for electroluminescence and the insulation to avoid non-radiative carrier recombination, enabling the spectrally and spatially resolved electroluminescence of individual self-decoupled chromophores in a scanning tunneling microscope. Hot luminescence bands are even visible in single self-decoupled chromophores, documenting the mechanical decoupling between the vibrons of the chromophore and the substrate.

Activating Electroluminescence of Charged Naphthalene Diimide Complexes Directly Adsorbed on a Metal Substrate.

Electroluminescence from single molecules adsorbed on a conducting surface imposes conflicting demands for the molecule-electrode coupling. To conduct electrons, the molecular orbitals need to be hybridized with the electrodes. To emit light, they need to be decoupled from the electrodes to prevent fluorescence quenching. Here, we show that fully quenched 2,6-core-substituted naphthalene diimide derivative in a self-assembled monolayer directly deposited on a Au(111) surface can be activated with the tip of a scanning tunneling microscope to decouple the relevant frontier orbitals from the metallic substrate. In this way, individual molecules can be driven from a strongly hybridized state with quenched luminescence to a light-emitting state. The emission performance compares in terms of quantum efficiency, stability, and reproducibility to that of single molecules deposited on thin insulating layers. Quantum chemical calculations suggest that the emitted light originates from the singly charged cationic pair of the molecules.

Synthesis and Surface Behaviour of NDI Chromophores Mounted on a Tripodal Scaffold: Towards Self-Decoupled Chromophores for Single-Molecule Electroluminescence.

This paper reports the efficient synthesis, absorption and emission spectra, and the electrochemical properties of a series of 2,6-disubstituted naphthalene-1,4,5,8-tetracarboxdiimide (NDI) tripodal molecules with thioacetate anchors for their surface investigations. Our studies showed that, in particular, the pyrrolidinyl group with its strong electron-donating properties enhanced the fluorescence of such core-substituted NDI chromophores and caused a significant bathochromic shift in the absorption spectrum with a correspondingly narrowed bandgap of 1.94 eV. Cyclic voltammetry showed the redox properties of NDIs to be influenced by core substituents. The strong electron-donating character of pyrrolidine substituents results in rather high HOMO and LUMO levels of -5.31 and -3.37 eV when compared with the parental unsubstituted NDI. UHV-STM measurements of a sub-monolayer of the rigid tripodal NDI chromophores spray deposited on Au(111) show that these molecules mainly tend to adsorb flat in a pairwise fashion on the surface and form unordered films. However, the STML experiments also revealed a few molecular clusters, which might consist of upright oriented molecules protruding from the molecular island and show electroluminescence photon spectra with high electroluminescence yields of up to 6×10-3 . These results demonstrate the promising potential of the NDI tripodal chromophores for the fabrication of molecular devices profiting from optical features of the molecular layer.