Aizawa Naoya, Matsumoto Akinobu, Yasuda Takuma
INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
Sci Adv. 2021 Feb 12;7(7). doi: 10.1126/sciadv.abe5769. Print 2021 Feb.
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 10 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.
在任何复杂的分子系统中，如果处于热平衡状态，具有不同自旋多重性的电子激发态可以通过简单的统计热力学形式来描述。然而，对于高效荧光有机分子而言，这种理想情况迄今为止是不可行的。在此，我们报道了一种高发射性的无金属纯有机荧光团，它能够实现单重态和三重态激发态之间的热平衡。这种非常规激子行为的关键在于从三重态到单重态激发态的异常快速的自旋翻转反向系间窜越，其速率常数超过每秒10次，这大大高于单重态激发态的辐射衰变（荧光）速率。目前的荧光体系表现出短至750纳秒的发射寿命，因此，即使在超过每平方米10000坎德拉的实际高亮度下，也能使有机发光二极管展现出超过20%的外部电致发光量子效率。
