What do OLEDs have in common with migratory birds?
Prof. John Lupton, Universität Regensburg
OLEDs are everywhere, but if it were for their inventors – or rather the patent attorneys involved – this would not be so. The original OLED patent explicitly excluded radiative decay from triplet excitations – phosphorescence – and only covered fluorescence from singlets. Three out of four electron-hole recombination events in an OLED end up in triplets, so losing this energy rather limits the technological appeal. The solution to this limitation lies in spin mixing to coax luminescence from otherwise dark states. The simplest way to do this is by spin-orbit coupling through the heavy-atom effect – a seemingly trivial trick which has turned into a billion dollar business. Given sufficiently long spin-coherence times, however, spin precession in tiny magnetic fields may suffice to mix singlets and triplets – an effect which is revealed by resorting to more modest spin-orbit coupling arising in non-bonding orbitals of aromatic heterocycles.
OLEDs operate by spin-dependent recombination of electrically injected charges of opposite sign, the solid-state equivalent to any magnetic-field-dependent reaction: spin chemistry. Spin-dependent recombination may also occur from carrier pairs formed by optical excitation, for example in retinal pigment-protein complexes of certain avian species. While compelling evidence for this radical-pair mechanism exists from behavioural studies, birds rarely avail themselves to probing singlet-triplet interconversion due to magnetic fields directly, let alone to measurements of radical spin coherence.
OLEDs show sensitivities to magnetic fields on scales of one hundredth of the earth’s field, and even exhibit signatures of magnetic resonance in oscillating fields – on energy scales less than a millionth of kT. Coherent spin precession is probed directly, in the time domain, by magnetic resonance to disentangle hyperfine and spin-orbit coupling effects from dipolar and exchange interactions in the radical pair – all of which are manifested in device current or light emission. Current-detected NMR even offers direct isotopic fingerprints of the emitter material. Since the spin pairs of an OLED behave like near-perfect two-level systems, quantum-optical features such as Dicke-type superradiance emerge in the operation of a simple device, at room temperature.