Naoya Aizawa1,2, Akinobu Matsumoto1, Takuma Yasuda3,4. 1. INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. 2. PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. 3. INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. yasuda@ifrc.kyushu-u.ac.jp. 4. Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
Abstract
In any complex molecular system, electronic excited states with different spin multiplicities can be described via a simple statistical thermodynamic formalism if the states are in thermal equilibrium. However, this ideal situation has hitherto been infeasible for efficient fluorescent organic molecules. Here, we report a highly emissive metal-free purely organic fluorophore that enables thermal equilibration between singlet and triplet excited states. The key to this unconventional excitonic behavior is the exceptionally fast spin-flipping reverse intersystem crossing from the triplet to singlet excited states with a rate constant exceeding 108 per second, which is considerably higher than that of radiative decay (fluorescence) from the singlet excited state. The present fluorophoric system exhibits an emission lifetime as short as 750 nanoseconds and, therefore, allows organic light-emitting diodes to demonstrate external electroluminescence quantum efficiency exceeding 20% even at a practical high luminance of more than 10,000 candelas per square meter.
In any complex molecular system, electronic excited states with different spin multiplicities can be described via a simple statistical thermodynamic formalism if the states are in thermal equilibrium. However, this ideal situation has hitherto been infeasible for efficient fluorescent organic molecules. Here, we report a highly emissive metal-free purely organic fluorophore that enables thermal equilibration between singlet and triplet excited states. The key to this unconventional excitonic behavior is the exceptionally fast spin-flipping reverse intersystem crossing from the triplet to singlet excited states with a rate constant exceeding 108 per second, which is considerably higher than that of radiative decay (fluorescence) from the singlet excited state. The present fluorophoric system exhibits an emission lifetime as short as 750 nanoseconds and, therefore, allows organic light-emitting diodes to demonstrate external electroluminescence quantum efficiency exceeding 20% even at a practical high luminance of more than 10,000 candelas per square meter.