Speakers

2017    Team Leader, Emergent Supramolecular Materials Research Team, Supramolecular Chemistry Division, The Center for Emergent Matter Science (CEMS), RIKEN, Japan

2010    Associate Professor, Graduate School of Organic Materials Science, Research Center for Organic Electronics, Yamagata University, Japan

2006    Assistant Professor, Department of Organic Device Engineering, Yamagata University, Japan

2004    Research Fellow, Department of Chemistry, Oxford University, UK

2002    Assistant Professor, Department of Applied Chemistry, Waseda University, Japan

2002    Ph.D., Department of Applied Chemistry, Graduate School of Science and Engineering, Waseda University, Tokyo, Japan

Excited States Control of Organic Semiconductors

Abstract

Control of excited states of organic semiconductors has been fundamentally important in their light-related applications such as LEDs, solar cells, sensors, photocatalysts, etc. When neutral and closed-shell organic molecules are optically or electrically excited, spin multiplicity of the excited state will be either singlet or triplet. In general, without any heavy atoms, the singlet excited state is bright and the triplet excited state is dark because of a spin selection rule. Therefore, reverse intersystem crossing from triplet excited states to singlet excited states makes light emission efficient. Energetically low and long-lived triplet excited states are involved in unique photophysical characteristics: singlet exciton fission, triplet–triplet annihilation, delayed fluorescence, etc. We designed and synthesized the organic semiconductor molecules showing unique triplet excited states–related phenomena [1–4].

Reference

1. Singlet fission of non-polycyclic aromatic molecules in OPVs (Kawata, Pu et al., Adv. Mater. 2016, 28, 1585). Most singlet fission molecules are based on tetracene or pentacene structure. We synthesized non-acene type singlet fission molecules and investigated their OPV properties.
2. Absence of delayed fluorescence and triplet-triplet annihilation in OLEDs with spatially orthogonal bianthracenes (Pu, Satake et al., J. Mater. Chem. C 2019, 7, 2541).  The anthracene molecules that we synthesized do not show the delayed fluorescence derived from triplet-triplet annihilation in OLEDs, suggesting that they do not electrically produce triplet excitons.
3. Kinetic prediction of reverse intersystem crossing in organic donor–acceptor molecules (Aizawa, Pu et al., Nat. Commun. 2020, 11, 3909). We can precisely predict reverse intersystem crossing rate of TADF materials by quantum chemical calculations with structure optimzation along crossing seam of S₁ and T₂ geometry.
4. Delayed fluorescence from inverted singlet and triplet excited states (Aizawa, Pu et al. 2022, in press). Energetically lower S₁ state than T₁ state is experimentally demonstrated for heptazine type molecules.