What Can Be Learned from Magnetic Field Effects on Singlet Fission: Role of Exchange Interaction in Excited Triplet Pairs

Abstract

Magnetic field effects (MFEs) on singlet fission were studied by observing fluorescence from organic crystal of 1,6-diphenyl-1,3,5-hexatriene under magnetic fields of up to 5 T. We found anomalous MFE dips at magnetic fields higher than 2 T, in addition to the known MFEs which saturated around 1 T. The observed results were analyzed by using the stochastic Liouville equation (SLE) in which a distance-dependent exchange interaction (J) in triplet pair, hopping of triplet, and geminate fusion in contacted triplet pair were incorporated. The SLE analysis revealed that the observed dips were caused by a MFE due to the level crossing mechanism and strongly suggested that the contacted triplet pair has a large J, which has been ignored in the previous model of MFEs on the singlet fission. Present results lead to the conclusion that the initial dissociation of the singlet exciton to the contacted triplet pair does not show the MFE and the triplet pair at a separated distance produced by hopping of the triplet plays an important role on the generation of the MFE on the singlet fission.

Site-selective measurement of coupled spin pairs in an organic semiconductor

“…Despite their key role in light-emitters and harvesters, triplet pairs have only recently been discovered to form exchange-coupled states [13][14][15][16][17]. We start by outlining how such states can be selectively addressed to provide a site-specific measurement of their exchange interactions and associated optical spectra.…”

“…At these field positions, bright singlet pair states become hybridized with a dark triplet or quintet pair-state, manifesting as a resonant reduction in the relevant PL spectral window (Fig. 1C ) [16][17][18].…”

“…At these field positions, bright singlet pair states become hybridized with a dark triplet or quintet pair-state, manifesting as a resonant reduction in the relevant PL spectral window (Fig. 1C ) [16][17][18].Crucially, the crossings directly address pairs with a specific exchange coupling.For an exchange interaction JS 1 · S 2 , where S 1,2 are the spin operators for the two triplets, the resonances occur at |J| (singlet-triplet crossing), and 3|J|/2, 3|J| (singlet-quintet crossings), giving a direct measurement of the exchange. (Here we take J > 0 -see Supplementary Information.)…”

Structure and dynamics of triplet-exciton pairs generated from singlet fission studied via magnetic field effects

“…In a TP, spin states and spin dynamics are influenced by magnetic fields [1][2][3][4]37,38 . In the case of a correlated TP in which there was a distinct exchange interaction, we recently reported that singlet fission from an organic crystal of 1,6-diphenyl-1,3,5-hexatriene (DPH) showed clear magnetic field effects (MFEs) at magnetic field strengths of 0-5 T 38 .…”

“…In the case of a correlated TP in which there was a distinct exchange interaction, we recently reported that singlet fission from an organic crystal of 1,6-diphenyl-1,3,5-hexatriene (DPH) showed clear magnetic field effects (MFEs) at magnetic field strengths of 0-5 T 38 . These MFEs on the fluorescence of DPH could be explained by the level crossing of correlated TP.…”