Electronic structures and optical characteristics of fluorescent pyrazinoquinoxaline assemblies and Au interfaces
AUGUST 20, 2021
Soyeong Kwon, Dong Yeun Jeong, Weon-Sik Chae, Kyungju Noh, P. Devi, Luciano Colazzo, Youngmin You, Taeyoung Choi & Dong-Wook Kim
Scientific Reports 11, 16978 (2021)
The electronic structures and optical properties of supramolecule/Au interfaces were investigated using scanning probe and photoluminescence microscopy techniques. Au is one of the most popular electrode materials for both transport measurements and device fabrication. A pyrazinoquinoxaline derivatibe (DY1) was employed because of its excellent thermal, photochemical, and electrochemical stabilities. DY1 is based on a planar pyrazinoquinoxaline core that exerts a strong intermolecular adhesive force due to π–π interactions. These intermolecular interactions would be beneficial for the formation of supramolecular self-assemblies of DY1. It was also anticipated that the highly symmetrical molecular structure would facilitate the formation of two-dimensional reticular assemblies of DY1 onto crystalline substrates. The propensity for intermolecular packing and lateral layering would provide a valuable opportunity to investigate molecular excitons at DY1-based solid-state heterojunctions. This work can provide us with important clues to answer the following questions: (1) is there any structural modification in the molecular self-assemblies of DY1 by interactions with Au, thereby leading to variations in the molecular energy levels of DY1? (2) Does charge transfer occur at the DY1/Au interface? (3) Do we need to consider chemical reactions and resulting defect formation at the DY1/Au interface?
Understanding the excitonic processes at the interfaces of fluorescent π-conjugated molecules and metal electrodes is important for both fundamental studies and emerging applications. Adsorption configurations of molecules on metal surfaces significantly affect the physical characteristics of junctions as well as molecules. Here, the electronic structures and optical properties of molecular assemblies/Au interfaces were investigated using scanning probe and photoluminescence microscopy techniques. Scanning tunneling microscopy images and tunneling conductance spectra suggested that the self-assembled molecules were physisorbed on the Au surface. Visible-range photoluminescence studies showed that Au thin films modified the emission spectra and reduced the lifetime of excitons. Surface potential maps, obtained by Kelvin probe force microscopy, could visualize electron transfer from the molecules to Au under illumination, which could explain the decreased lifetime of excitons at the molecule/Au interface.