High-spin states play a key role in chemical reactions found in nature. In artificial molecular systems, singlet fission produces a correlated triplet-pair state, a spin-bearing excited state that can be harnessed for more efficient solar-energy conversion and photocatalysis. In particular, triplet-pair states with overall quintet character (total spin $S=2$) have been discovered, but many of the fundamental properties of these biexciton states remain unexplored. The net spin of these pair states makes spin-sensitive probes attractive for their characterization. Combined with their surprisingly long spin coherence (of order microseconds), this opens up techniques relying on coherent spin control. Here we apply coherent manipulation of triplet-pair states to (i) isolate their spectral signatures from coexisting free triplets and (ii) selectively couple quintet and triplet states to specific nuclear spins. Using this approach, we separate quintet and triplet transitions and extract the relaxation dynamics and hyperfine couplings of the fission-borne spin states. Our results highlight the distinct properties of correlated and free triplet excitons and demonstrate optically induced nuclear spin polarization by singlet fission.

中文翻译：

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单峰裂变材料中相干控制的五重态多激子态的波函数和动力学

高自旋态在自然界发现的化学反应中起着关键作用。在人工分子系统中，单线态裂变会产生相关的三线态对态，即自旋轴承激发态，可以利用该态进行更有效的太阳能转换和光催化。特别是具有整体五重态特征的三重态对（总自旋$\mathrm{小号}=2$）已经被发现，但是这些双激态的许多基本性质仍未得到探索。这些对态的净自旋使自旋敏感探针的特征吸引人。结合其惊人的长自旋相干性（数量级微秒），这开启了依靠相干自旋控制的技术。在这里，我们将三重态对状态的相干操纵应用于（i）从共存的游离三重态中分离它们的光谱特征，以及（ii）将五重态和三重态选择性地耦合到特定的核自旋。使用这种方法，我们分离了五重态和三重态转变，并提取了裂变传播自旋态的弛豫动力学和超精细耦合。