2018 Volume 67 Issue 11 Pages 693-697
Nanographenes have now become one of the most promising candidates for new functional materials because of their high fluorescence properties, electronic properties and opt-electric properties. Therefore, nanographenes have been mostly studied in the field of organic solar cells and organic light emitter diodes. Despite the importance of understanding their electronic character, their exact HOMO-LUMO energy levels were poorly understood. This is because of the difficulty in size and composition selective syntheses of nanographenes. In this study, we synthesized three nanographenes with different sizes of π conjugate systems and molecular distortion by using a bottom-up method. We conducted Photoemission yield spectroscopy in air to measure their HOMO levels directly. By combining those data with the HOMO-LUMO gap energy derived from the UV-Vis absorption spectra, we revealed energy diagrams of nanographenes. We found that the HOMO-LUMO gap energy becomes smaller by extending the π conjugate systems, and the energy levels of HOMO and LUMO become lower by introducing molecular distortion.
