Department of Chemistry, University of California, Riverside, Riverside, California 92521, United States.
Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States.
J Am Chem Soc. 2020 Oct 14;142(41):17581-17588. doi: 10.1021/jacs.0c07727. Epub 2020 Oct 2.
Efficient energy transfer is particularly important for multiexcitonic processes like singlet fission and photon upconversion. Observation of the transition from short-range tunneling to long-range hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This is firmly supported by steady-state photon upconversion measurements, a direct proxy for the efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon upconversion quantum efficiencies increase again to 0.468% and 0.413%, 1.5-1.6 fold higher than that with 3 phenylene units (using the convention where the maximum upconversion quantum efficiency is 100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first observation of the tunneling-to-hopping transition for long-range triplet energy transfer between nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should further be explored in the context of artificial photosynthesis.
能量转移效率对于多激子过程（如单重态裂变和光子上转换）至关重要。本文报道了在 CdSe 纳米晶到蒽的三重态激子转移过程中观察到从短程隧穿到长程跳跃的转变。这一转变得到了稳态光子上转换测量的有力支持，该测量是三重态能量转移（TET）效率的直接代理，以及瞬态吸收测量的支持。当苯并桥最初插入 CdSe 纳米晶给体和蒽受体之间时，TET 的速率呈指数下降，与光子上转换量子效率从 11.6%降至 4.51%再降至 0.284%一致，这与隧穿机制相符。然而，当刚性桥的长度增加到 4 和 5 个苯并环单元时，光子上转换量子效率又增加到 0.468%和 0.413%，比 3 个苯并环单元（采用最大上转换量子效率为 100%的惯例）高 1.5-1.6 倍。这表明激子隧穿到跳跃的转变，导致在传统的 1nm Dexter 距离之外，TET 相对高效且与距离无关。瞬态吸收光谱用于确认 CdSe 向传输器的三重态能量转移，并确认桥三重态态作为跳跃机制的中间态。这是在纳米晶光吸收体和分子受体之间长程三重态能量转移从隧穿到跳跃的转变的首次观察，表明这些混合材料应在人工光合作用的背景下进一步探索。
