Role of Nanocavity Plasmons in Tunneling Electron Induced Light Emission on and near a Molecule

By using a microscopic quantum model, we study theoretically different roles of nanocavity plasmons in scanning tunneling microscope (STM) induced light emission upon selective initial excitation of molecules or plasmons. The time evolution and spectroscopic properties of the emission from the coupled plasmon-molecule system in each case are studied using time-dependent quantum mater equations. When the STM tip is placed on the molecule to ensure direct carrier injection induced molecular excitation, the major role of the plasmons is to enhance the molecular emission via increasing its radiative decay rate, resulting in sharp molecule-specific emission peaks. On the other hand, when the STM tip is located in close proximity to the edge of the molecule but without direct carrier injection into the molecule, the role of the plasmon-molecule coupling is to cause destructive interferences between the two quantum objects, leading to the occurrence of Fano dips around the energy of the molecular exciton in the plasmonic emission spectra.     

Tunneling Electron Induced Fluorescence from Single Porphyrin Molecules Decoupled by Striped-Phase Octanethiol Self-assembled Monolayer

We investigate tunneling electron induced luminescence from isolated single porphyrin molecules that are decoupled by striped-phase self-assembled monolayer of octanethiol from the underneath Au(111) substrate. Intrinsic single-molecule electroluminescence has been realized by such decoupling at both bias polarities. The photon emission intensity acquired from the molecular lobe is found stronger than that from the molecular center. These re-sults provide useful information on the understanding of electroluminescent behavior and mechanism in molecular tunnel junctions.     

Influence of Atomistic Protrusion on the Substrate on Molecular Luminescence in Tunnel Junctions

Scanning tunneling microscope (STM) induced luminescence can be used to study various optoelectronic phenomena of single molecules and to understand the fundamental photophysical mechanisms involved. To clearly observe the molecule-specific luminescence, it is important to improve the quantum efficiency of molecules in the metallic nanocavity. In this work, we investigate theoretically the influence of an atomic-scale protrusion on the substrate on the emission properties of a point dipole oriented parallel to the substrate in a silver plasmonic nanocavity by electromagnetic simulations. We find that an atomic-scale protrusion on the substrate can strongly enhance the quantum efficiency of a horizontal dipole emitter, similar to the situation with a protrusion at the tip apex. We also consider a double-protrusion junction geometry in which there is an atomic-scale protrusion on both the tip and the substrate, and find that this geometry does provide significantly enhanced emission compared with the protrusion-free situation, but does not appear to improve the quantum efficiency compared to the mono-protrusion situation either at the tip apex or on the substrate. These results are believed to be instructive for future STM induced electroluminescence and photoluminescence studies on single molecules.     

Full Quantum Theory of Molecular Hot-Electroluminescence in Scanning Tunneling Microscope Tunnel Junctions

The pursuit of nanoscale photonics and molecular optoelectronics has stimulated a lot of interests in scanning tunneling microscope (STM) induced molecular emission. In this work, we have introduced a full quantum mechanical approach instead of the previous semiclassical theory to consider the quantized surface plasmon modes in this system. By considering the mutual interactions between a single molecule and the quantized surface plasmon, we have studied the molecular electroluminescence from STM tunnel junctions. Due to the coupling to the surface plasmons, the spontaneous emission rate and the fluorescence intensity of themolecule are both enormously enhanced. In particular, we show that when the radiative decay rate becomes comparable to the vibrational damping rate, hot-electroluminescence can be observed. All these findings are believed to be instructive for further developments of both molecular electronics and photonics.     

Manipulation of Molecular Aggregation States to Realize Polymorphism,AIE, MCL,and TADF in a Single Molecule

Multifunctional emitting materials are scarce and need to be further explored. Now, a newly anthraquinone derivative, 2‐(phenothiazine‐10‐yl)‐anthraquinone (PTZ‐AQ) was designed and synthesized and found to demonstrate polymorphism, multi‐color emission, aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and thermally activated delayed fluorescence (TADF) in its different solid forms. It is shown for the first time that TADF properties of a compound can be systematically tuned via its aggregation state. The optimized PTZ‐AQ crystal shows a small singlet–triplet energy splitting of 0.01 eV and exhibits red TADF with a photoluminescence quantum yield as high as 0.848. This study shows that the unique multiple functions can be integrated into one single compound through controlling the aggregation states, which provides a new strategy for the investigation and application of multifunctional organic materials.     

Selective Expression of Chromophores in a Single Molecule: Soft Organic Crystals Exhibiting Full‐Colour Tunability and Dynamic Triplet‐Exciton Behaviours

Soft luminescent materials are attractive for optoelectronic applications, however, switching dominant chromophores for property enrichment remains a challenge. Herein, we report the first case of a soft organic molecule (DOS) featuring selective expression of chromophores. In response to various external stimuli, different chromophores of DOS can take turns working through conformation changes, exhibiting full‐colour emissions peaking from 469 nm to 583 nm from ten individual single crystals. Dynamic triplet‐exciton behaviours including thermally activated delayed fluorescence (TADF), room‐temperature phosphorescence (RTP), mechanoluminescence (ML), and distinct mechano‐responsive luminescence (MRL) can all be realized. This novel designed DOS molecule provides a multifunctional platform for detection of volatile organic compounds (VOCs), multicolour dynamic displays, sensing, anticounterfeiting, and hopefully many others.     

Polymorph‐Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State

Current research on thermally activated fluorescence (TADF) emitters is mainly based on the molecular levels, while the aggregation states of TADF emitters are to be explored deeply. Now two multifunctional emitters are reported with simultaneous TADF, aggregation induced emission (AIE), and multicolor mechanochromic luminescence (MCL) features. Both emitters also show polymorph‐dependent TADF emission. Crystal structure analysis reveals that the polymorphism is ascribed to the mutable conformations in different aggregation states. This work brings new insight to TADF emitters from a perspective of aggregation states.     

White‐Light Emission from a Single Polymer with Singlet and Triplet Chromophores on the Backbone

Summary: A strategy to generate an efficient white‐light emission has been developed by mixing fluorescence and phosphorescence emission from a single polymer. Fluorene is used as the blue‐emissive component, benzothiadiazole (BT) and the iridium complex [(btp)₂Ir(tmd)] are incorporated into a polyfluorene backbone, respectively, as green‐ and red‐emissive chromophores by Suzuki polycondensation. By changing the contents of BT and [(btp)₂Ir(tmd)] in the polymer, the electroluminescence spectrum from a single polymer can be adjusted to achieve white‐light emission. A white polymeric light‐emitting diode (WPLED) with a structure of ITO/PEDOT:PSS/PVK/PFIrR1G03/CsF/Al shows a maximum external quantum efficiency of 3.7% and the maximum luminous efficiency of 3.9 cd · A⁻¹ at the current density of 1.6 mA · cm⁻² with the CIE coordinates of (0.33, 0.34). The maximum luminance of 4 180 cd · m⁻² is achieved at the current density of 268 mA · cm⁻² with the CIE coordinates of (0.31, 0.32). The white‐light emissions from such polymers are stable in the white‐light region at all applied voltages, and the electroluminescence efficiencies decline slightly with the increasing current density, thus indicating that the approach of incorporating singlet and triplet species into the polymer backbone is promising for WPLEDs.

Structure of PFIrR1G04 and the EL spectra of its devices under various voltages. Device structure: ITO/PEDOT:PSS/PVK/polymer/CsF/Al.