Triplet excited states (triplets) serve as key intermediates in critical technologies and processes ranging from organic synthesis to biomedicine to molecular electronics. Production of triplets of π-conjugated organic molecules without heavy atoms remains challenging. Spin-orbit, charge-transfer intersystem crossing (SOCT-ISC) directly converts singlet charge-separated states to triplets in an electron donor-acceptor (D-A) pair. Here, using a series of orthogonal D-A type boron dipyrromethene (BODIPY) derivatives as a model system, we show that the formation of triplets is largely controlled by the spin-allowed transitions rather than by SOCT-ISC. Yet, the SOCT-ISC process can still proceed much faster than ordinary ISC between (π, π*) states because the spin-orbit coupling of SOCT-ISC is 2 orders of magnitude stronger. We further show that such a process can produce triplets in a non-triplet-forming molecule, perylene. Our findings reveal a clear physical basis for this spin-forbidden process and provide guidelines for future molecular designs exploiting the process.

三重态激发态（三重态）是从有机合成到生物医学再到分子电子学等关键技术和过程的关键中间体。没有重原子的三重π共轭有机分子的生产仍然具有挑战性。自旋轨道，电荷转移系统间交叉（SOCT-ISC）将单重态电荷分离态直接转换为电子供体-受体（DA）对中的三重态。在这里，使用一系列正交的DA型硼二吡咯亚甲基（BODIPY）衍生物作为模型系统，我们表明三联体的形成很大程度上受自旋允许的跃迁控制，而不是受SOCT-ISC的控制。然而，在（π，π*）状态之间，SOCT-ISC过程仍比普通ISC进行得快得多，这是因为SOCT-ISC的自旋轨道耦合强了2个数量级。我们进一步表明，这样的过程可以在非三重峰形成分子per中产生三重峰。我们的发现揭示了禁止自旋过程的明确物理基础，并为利用该过程的未来分子设计提供了指导。