Abstract
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 ) 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.
- Received 13 January 2020
- Revised 16 April 2020
- Accepted 30 April 2020
DOI:https://doi.org/10.1103/PhysRevX.10.021070
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
In molecular materials, absorption of light generates excitons—bound pairs of electrons and their positively charged holes. Excitons are important in a range of processes, from solar energy harvesting to catalysis. Until recently, molecular excitons had been observed to have no spin (a singlet state) or a spin of 1 (a triplet state). However, the interaction of two triplet excitons can lead to a quintet state, with a spin of 2. This multiexciton state acts as an intermediary in the conversion between light and free excitons, with potential applications in optoelectronic or spintronic devices. It is therefore important to understand the fundamental properties of this state. However, probing quintet states can be challenging since their spectroscopic signatures often overlap with other species, and they are optically dark. Here, we use the difference in spin between quintet and triplet states to coherently separate these states and probe their properties.
Using pulsed electron spin resonance, we separately isolate photoexcited quintet and triplet states coexisting within the same organic semiconductor film. We use this coherent separation to measure the spin dynamics of these states, highlighting the differences between coupled and uncoupled excitons. We then selectively couple quintet and triplet states to specific nuclear spins on the molecules on which they reside, probing their spatial wave functions and polarizing the nuclei.
These results demonstrate how molecular spin coherence can be used to probe multiexciton intermediates and present new opportunities for organic spintronics with photoexcited quintet states.