Engineering inter-triplet exchange coupling allows spin mixing between singlet and quintet manifolds in triplet–triplet pair states in metal–organic frameworks, demonstrating increased room-temperature triplet-fusion rates under relatively small applied magnetic fields

Behrends, Jan

Panjwani, Naitik A.

English

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Document type: PreprintTitle:    Framing fusion and fission   Author(s):   Naitik A. Panjwani & Jan BehrendsCitation:           "Naitik A. Panjwani & Jan Behrends, 2Nature Materials volume 21, pages1221–1222 (2022) ; https://doi.org/10.1038/s41563-022-01387-y" Archiviert unter http://dx.doi.org/10.17169/refubium-38225Terms of Use:     Copyright applies. A non-exclusive, non-transferable and limited right to use is granted. This document is intended solely for personal, non-commercial use. Framing Fusion and Fission 1  2 Naitik A. Panjwani and Jan Behrends 3 Berlin Joint EPR Lab, Fachbereich Physik, Freie Universität Berlin, Berlin, Germany 4 Engineering inter-triplet exchange coupling allows spin mixing between singlet and 5 quintet manifolds in triplet-triplet pair states in metal organic frameworks, demonstrating 6 increased room-temperature triplet-fusion rates under relatively small applied magnetic 7 fields 8  9 Absorption of light in organic molecules can result in the formation of strongly coupled 10 electron-hole pairs referred to as excitons. They play a key role in all types of optoelectronic 11 devices that either produce light (such as light-emitting diodes) or absorb light (such as solar 12 cells or photodetectors). Excitons can have singlet (spin 0) or triplet (spin 1) character, and 13 the probability for their radiative decay strongly depends on the spin state. In most organic 14 molecules with a singlet ground state the decay of triplet excitons is spin-forbidden, 15 resulting in long lifetimes of triplet excitons, which are therefore also called ‘dark states’. 16 Harvesting these non-emissive triplet states by converting them, through reverse 17 intersystem crossing, to emissive singlet excitons, for which radiative decay is spin-allowed, 18 was the key achievement that led to the success of organic light-emitting diodes.1 19 A particularly promising strategy for utilising dark triplet states for optoelectronic 20 applications is based on the conversion of a pair of triplet excitons into one singlet exciton 21 via triplet fusion as well as the reverse process, singlet fission. As the energy of the singlet 22 exciton is higher than the energy of the individual triplet states, triplet fusion is referred to 23 as upconversion. The probability for the upconversion process strongly depends on the 24 electronic coupling between both triplet states. In molecular solids, both triplet-triplet (TT) 25 pair constituents can be very close, resulting in strong inter-triplet exchange coupling. The 26 strongly-coupled TT pair can be in a singlet (spin 0), triplet (spin 1) or quintet (spin 2) state. 27 Quintet TT states have been shown to be relevant for the efficiency of triplet fusion as well 28 as singlet fission. 29 In cases where the two constituent triplets that form the TT pair are in close proximity such 30 that orbital overlap and therefore inter-triplet exchange coupling is large, the quintet TT 31 state is energetically not accessible, and efficient conversion into the emissive singlet state 32 is not possible. However, increasing the probability for the upconversion process is highly 33 desirable for a number of applications. This requires singlet-quintet (SQ) spin mixing, which 34 is most efficient when the energies of the states involved in this process are equal (close to 35 avoided level crossings). This situation can be achieved by applying a magnetic field which 36 affects the energies of the states involved in the conversion process. The field-induced 37 energy changes have to be on the order of the exchange coupling between the TT pair 38 constituents, which usually requires strong magnetic fields (several T), far beyond what 39 would be realised in practical applications.2 40 Ha and colleagues3 came up with a clever strategy to increase the SQ mixing at moderate 41 magnetic fields. They achieved this by engineering the inter-triplet exchange coupling by 42 placing the exciton-accommodating molecules far away from each other in a well-defined 43 distance using metal-organic frameworks (MOFs). The spatial separation results in weak 44 exchange coupling and concomitantly effective mixing between singlet and quintet TT pair 45 states at moderate magnetic field well below 1 T. Further, the fixed arrangement in the 46 MOF prevents the triplet excitons from separating through diffusion, thereby increasing the 47 TT pair lifetime and thus the time available for spin mixing. 48 The weak inter-triplet exchange coupling is quantified in magnetic field effect (MFE) 49 experiments, in which the light emitted by the sample through radiative decay of singlet 50 excitons is measured as a function of the magnetic field strength. The anomalous MFE 51 curves can, in combination with simulations, be considered as smoking-gun evidence for the 52 existence of weak electronic coupling between the triplet states. 53 The distance between neighbouring triplet excitons can also be adjusted in dimers formed 54 by two covalently linked molecules. This has previously been used in molecules undergoing 55 singlet fission.4 However, the possibility to engineer the distance between the 56 chromophores, and thus the exchange-coupling strength, is limited by the flexibility of the 57 dimers, in particular for large distances. In contrast, the rather rigid and tunable structure of 58 MOFs holds particular promise for applications which require precise control of electronic 59 couplings. Furthermore, MOFs reduce molecular vibrations which are known to modulate 60 the exchange interaction during SQ mixing, which is more difficult to realise in dimer 61 structures. 62 The approach by Ha and colleagues provides a very interesting and exciting pathway for 63 future research. The reported upconversion efficiency of <1% in this proof-of-principle study 64 is nevertheless relatively low and not yet sufficient for real-world optoelectronic 65 applications. However, given the fact that MOFs are highly designable in structure and 66 functionality,5 it seems likely that the SQ conversion efficiency can be improved 67 substantially in the near future. The authors point out some of the key parameters that 68 could lead to improved upconversion efficiencies, such as the fusion rate, exciton hopping 69 rate and decoherence time. This could also involve engineering the relative orientation of 70 constituent molecules which are involved in fusion, since the inter-triplet coupling and 71 hopping rates are orientation dependent. 72 Control over SQ mixing can help harvesting low-energy photons in solar cells via triplet 73 fusion6 and high-energy photons via singlet fission.7 It can also improve the performance of 74 organic light-emitting diodes.8 Furthermore, it is conceivable that MOFs with optimised 75 triplet-fusion capabilities may be used as drug-delivery systems for the transport of 76 therapeutic agents that can be activated by upconverted skin-penetrating near-infrared 77 light.9  78  79  1 H. Uoyama, K. Goushi, K. Shizu, H. Nomura, and C. Adachi, Nature 492 (2012), p. 234. 2 S. L. Bayliss, et al., P. Natl. Acad. Sci. USA 115 (2018), p. 5077. 3 D.-G. Ha et al., Nature Materials (2022), this issue. 4 M. J. Y. Tayebjee, S. N. Sanders, E. Kumarasamy, L. M. Campos, M. Y. Sfeir, and D. R. McCamey, Nature Phys. 13 (2017), p. 182. 5 S. Horike and S. Kitagawa, Nature Mater. 21 (2022), p. 983. 6 T. F. Schulze and T. W. Schmidt, Energy & Environmental Science 8 (2015), p. 103. 7 A. Rao and R. H. Friend, Nature Reviews Materials 2 (2017), p. 17063. 8 D. Y. Kondakov, Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences 373 (2015), p. 20140321. 9 L. Huang, E. Kakadiaris, T. Vaneckova, K. Huang, M. Vaculovicova, and G. Han, Biomaterials 201 (2019), p. 77. Figure 1: Schematic structure of MOF NU-1000 as well as photo- and spin- physics involved in triplet fusion. 	The left schematic depicts the MOF NU-1000 structure. Orange circles represent oxo-Zr6 SBU (MOF node), green circles PtOEP (sensitizer molecules located in the pores of the MOF), black ellipses H4TBAPy (MOF linker). Excitation of PtOEP (green curly arrow) results in, after intersystem crossing, a triplet state. Dexter triplet energy transfer results in the triplet energy migrating from the PtOEP to a nearby H4TBAPy molecule (red ellipse). Subsequently triplet exciton hopping occurs until two triplet on nearby H4TBAPy molecule meet, allowing for triplet fusion (blue ellipses). The singlet-quintet spin mixing occurring in NU-1000 under low magnetic fields due to weak inter-triplet coupling is shown on the right. This leads to increased room-temperature triplet fusion and light emission. 

Framing fusion and fission

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Framing fusion and fission

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