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a, Schematic illustration of TT pair dynamics. More than half of the TT pairs form quintet states initially, thus control of SQ coupling can substantially impact triplet fission and fusion efficiencies. b, Energy levels and spin states of nine eigenstates of TT pairs as a function of the distance between two triplet excitons. As shown in the inset, in a separated TT pair, the eigenstates are mixed SQ or triplet–quintet states governed by the zero-field splitting defined by the parameters D and E (ref. 50). The intertriplet exchange interaction, J, dominates as the triplets get closer, eventually yielding pure-spin eigenstates. c, A resonant magnetic field can drive an oscillation between the singlet and quintet states. For SQ splitting on the order of D, the spin transition time is ∼ℏπ/D. d,e, Spatial description of triplet fusion in molecular solids (d) and MOFs (e). Moderate applied magnetic fields μB ≈ D reshuffle the singlet, triplet and quintet character of TT pairs in the shaded magnetic field effect (MFE) regions, generating both the conventional Merrifield MFE and potentially SQ resonances. TT states with weaker exchange splitting J ≈ D may be precursors to exciton formation in MOFs. In a typical densely packed molecular solid, however, triplets migrate rapidly relative to ℏπ/D and fusion typically occurs from a triplet pair with exchange splitting J ≫ D. This means that, in molecular solids, SQ resonances may only be observed under very high applied magnetic fields μΒB ≫ D and low temperature, if at all.

Image courtesy of the researchers.

Dincă and Van Voorhis Collaboration Published in Nature Materials

by Danielle Doughty | Department of Chemistry |
Categories: Faculty, Research

The paper, Exchange controlled triplet fusion in metal–organic frameworks, was published online on October 6, 2022.

A paper authored by Dong-Gwang Ha, Ruomeng Wan, Changhae Andrew Kim, Ting-An Lin, Luming Yang, Troy Van Voorhis, Marc A. Baldo, and Mircea Dincă was published in Nature Materials on October 6, 2022.

Exchange controlled triplet fusion in metal–organic frameworks
Dong-Gwang Ha, Ruomeng Wan, Changhae Andrew Kim, Ting-An Lin, Luming Yang, Troy Van Voorhis, Marc A. Baldo, and Mircea Dincă

Nature Materials, 6 October, 2022
DOI: https://doi.org/10.1038/s41557-018-0159-8

Abstract: Triplet-fusion-based photon upconversion holds promise for a wide range of applications, from photovoltaics to bioimaging. The efficiency of triplet fusion, however, is fundamentally limited in conventional molecular and polymeric systems by its spin dependence. Here, we show that the inherent tailorability of metal–organic frameworks (MOFs), combined with their highly porous but ordered structure, minimizes intertriplet exchange coupling and engineers effective spin mixing between singlet and quintet triplet–triplet pair states. We demonstrate singlet–quintet coupling in a pyrene-based MOF, NU-1000. An anomalous magnetic field effect is observed from NU-1000 corresponding to an induced resonance between singlet and quintet states that yields an increased fusion rate at room temperature under a relatively low applied magnetic field of 0.14 T. Our results suggest that MOFs offer particular promise for engineering the spin dynamics of multiexcitonic processes and improving their upconversion performance.

Read the Full Text at Nature Materials.

The Dincă Lab is focused on addressing research challenges related to the storage and consumption of energy and global environmental concerns.

The Van Voorhis group is developing new methods – primarily based on density functional theory (DFT) – that provide an accurate description of excited electron motion in molecular systems.