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

TL;DR

This study demonstrates a method to selectively address and distinguish coupled spin pairs in organic semiconductors using combined spin and optical techniques, revealing site-specific exchange interactions and luminescence properties.

Contribution

It introduces a novel approach for site-selective measurement of spin pairs in organic materials, applicable across various synthetic and biological systems.

Findings

Identified three distinct triplet-pair sites with varying exchange couplings

Demonstrated magnetic field tuning of spin states and luminescence

Revealed exchange-specific luminescence features in a single material

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 (S=0) and dark triplet, quintet (S=1,2) 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.