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

Abstract

From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet ([Formula: see text]) and dark triplet quintet ([Formula: see text]) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3-5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems.

Quintet-triplet mixing determines the fate of the multiexciton state produced by singlet fission in a terrylenediimide dimer at room temperature

“…Magnetic-Field Effects on the Triplet Yield. Previous studies on SF chromophores have observed magnetic-field effects (MFEs) on both fluorescence (20)(21)(22)(24)(25)(26)(27) and device performance (28). The shape of the MFE curve depends on the level-crossing mechanism of the (T 1 T 1 ) pairs and thus provides valuable information on how to tune the mixing of the various (T 1 T 1 ) states.…”

Understanding the Bound Triplet-Pair State in Singlet Fission

“…66 Another emerging technique for probing the role of m (T 1 T 1 ) in SF is time-resolved electron spin resonance (tr-ESR). 40,67,68 This technique has recently been used to directly interrogate m (T 1 T 1 ) in films of TIPS-tetracene or solutions of bipentacene. 13,14 In this experiment, an optical pulse populates excited states that evolve in time, and transiently populated paramagnetic species are probed by a continuous wave microwave source.…”

Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield

“…We consider that the distinct spectral feature at 720 nm could be due to a specific geometric orientation of a pair of TTBP molecules. It has been shown in TIPS-tetracene that emission from triplet pairs with strong exchange coupling is red-shifted relative to that of weak coupling 45 , so this feature could be attributed to very strongly coupled pairs. This would likely originate in pairs of molecules that are slip-stacked face-to-face at only 0.35 nm, consistent with the crystal structures reported by Müller et al 35 , notably closer than any pair in the TIPS-tetracene crystal structure 45 .…”