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

Abstract

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.

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

“…Naphthalene diimides are compact, planar, and stable chromophores with electronic and optical properties that can be adjusted by the core substituents 23 , 30 , thus finding promising applications in optoelectronic devices 31 . The intention of the molecular design is to fix the chromophore upright on the substrate, minimizing its interaction with the surface.…”

“…The NDI dyes were identified as the ideal optically active subunit due to their robustness, optical tunability, structural integrity, and synthetic accessibility. In the first series of NDIs mounted on a smaller tripodal platform based on tris( meta -sulfanylphenyl)methane 23 , the physisorption of the chromophore efficiently competed with the intended fixation by the three foot subunits of the tripod, pointing at the subtle balance between the surface interactions involved 23 . As the size of the chromophore in the presently reported second series remains the same, the foot structure was enlarged by three tolane subunits, increasing its lateral dimension and thus also its interaction with the surface.…”

“…For our low-temperature STM studies, the NDI model compounds Tol-Tpd-sNDI -Ac and Tol-Tpd-nNDI -Ac were deposited as dichloromethane solution onto an atomically clean Au(111) surface by an in-house developed spray technique, which was optimized and successfully applied for tripod model compounds which cannot be sublimed, since their degradation temperatures lie below their sublimation temperatures 23 , 24 , 32 , 33 . Subsequent thermal annealing before cooling down to measurement temperatures promotes deprotection of the thiol anchors and the intended upright configuration with the chromophore pointing away from the substrate (details of the sample preparation can be found in Methods).…”

“…The lateral dimension of the platform turns out to be of crucial importance. In a first attempt with a tripodal platform of smaller dimension, the physisorption of the mounted chromophore was favored over the chemisorption of the foot structure, resulting in a flat arrangement of the chromophore on the surface, additionally stabilized by two of the three anchor groups forming covalent bonds 23 , 24 . An effective strategy to increase the interaction of the foot structure with the substrate is to enlarge its size 26 , with the additional advantage that the distance between perpendicularly mounted chromophore is increased, avoiding quenching by inter-chromophore stacking.…”

Light-Driven Charge Transport and Optical Sensing in Molecular Junctions

“…In recent years, luminescence, i.e., photoemission, from organic molecules on metals, has gained considerable interest for its potential applications in the display and lighting industries [97][98][99]. However, when molecules are directly adsorbed on or very close to metal surfaces, the intrinsic molecular fluorescence is often quenched by the strong interaction between the molecules and the substrate [100].…”