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PhD defense: Yugo Tsuji

Co-director: Gilles Clavier (PPSM)
Co-director: Tsuyoshi Kawai (NAIST)

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Title

Molecular Dimerization Strategy for the Control of Singlet–Triplet Energy Gap via Exciton Coupling: From Thermally Activated Delayed Fluorescence to Phosphorescence

Abstract

Organic electroluminescence has been widely applied in electronic displays and lighting devices. Further enhancement of brightness and energy efficiency represents a crucial technological challenge for the realization of a decarbonized society. As one of the promising solutions, thermally activated delayed fluorescence (TADF), an emission phenomenon induced by precise control of the excited-state energy, has attracted considerable attention. In this study, a design concept based on exciton coupling was proposed to effectively reduce the singlet–triplet energy gap (ΔEST), and its validity was experimentally verified.
Chapter 1 outlines TADF materials, organic molecules exhibiting negative ΔEST, and the background of exciton coupling, thereby clarifying the objective and positioning of this research.
In Chapter 2, a twisted donor–acceptor (D–A) molecule consisting of carbazole (Cz) as the donor and 1,8-naphthalimide (NI) as the acceptor was designed, and its dimer was synthesized to evaluate its photophysical properties. Dimerization induced excitonic coupling, leading to a marked reduction in ΔEST compared to the monomer. Moreover, the introduction of axial chirality through dimerization afforded enantiomeric emitters that simultaneously exhibited TADF and circularly polarized luminescence (CPL). Furthermore, circularly polarized phosphorescence was observed at low temperature, exhibiting a markedly enhanced dissymmetry factor compared to that of CPL at room temperature. This enhancement indicates that the phosphorescence originates from electronically forbidden transitions.
Chapter 3 aimed to further reduce the ΔEST of the D–A dimer system by modulating the substituents on the acceptor unit, NI, and verifying their effects. Introduction of an electron-withdrawing pentafluorophenyl group stabilized the lowest unoccupied molecular orbital and enhanced the charge transfer character, leading to a smaller ΔEST. In frozen toluene and polymethyl methacrylate (PMMA) film, the delayed emission (phosphorescence) appeared at a higher energy than the prompt emission (fluorescence), indicating an inversion of emission energies. This unusual behavior is attributed to the restriction of molecular geometric and orientational relaxation by the semi-rigid environment, which suppresses the low-energy relaxation pathways typically accessible in solution.
In Chapter 4, covalently linked a couple of emitting units, which exhibit a small ΔEST due to the multi-resonance effect were designed and synthesized. By using three different linker units, I systematic investigated how exciton coupling affects ΔEST. As a result, stabilization of the triplet excited state caused an increase in ΔEST. Consequently, long-lived room-temperature phosphorescence (RTP) was observed in PMMA films, particularly in a co-facial type dimer. Transient absorption spectroscopy revealed that this persistent RTP originates from a radiative transition of a stabilized triplet state formed via a singlet excimer.
Chapter 5 describes an attempt to rigidify and planarize the twisted D–A molecule based on the Cz–NI framework, as well as the investigation of the dimer synthesis. The photophysical properties of the resulting planar D–A monomer were also characterized. I rationally characterized optical properties of the monomer with enhanced emissivity. Optimization of the synthetic conditions for the dimer is also presented.
Finally, Chapter 6 summarizes the overall conclusions of this doctoral thesis and discusses future perspectives.

PhD supervision

  • Co-director: Dr Gilles Clavier, Research Director, CNRS - ENS Paris-Saclay
  • Co-director: Pr Tsuyoshi Kawai, Professor, Division of Materials Science, Nara Institute of Science and Technology

Members of the jury

  • Gilles Ulrich, Research Director, ICPEES, CNRS, Strasbourg University, Rapporteur

  • Yoichi Kobayashi, Professor, Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, Rapporteur

  • Gwenaël Rapenne, Professor, Division of Materials Science, Graduate School of Science and Technology, Nara Institute of Science and Technology & CEMES, CNRS, Université de Toulouse, Examinateur

  • Grégory Pieters, CEA Research Director, SCBM, CEA Saclay, Examinateur

  • Marine Louis, Associate Professor, Tokyo University of Agriculture and Technology, Invited