Emissive Materials: OLEDs: rotation propels crossing

If you are reading this article on an electronic device, the chances are that you are benefitting from organic light-emitting diode (OLED) technology. Emissive materials in OLEDs undergo excitation from their singlet electronic ground state (S₀) to either their first excited triplet (T₁) or singlet (S₁) states. Although the T₁ state is dark, S₁ relaxes to the ground state (S₁ → S₀) through fluorescence, a rapid spin-allowed process that is desirable for electronics applications. A problem with present emitters in OLEDs is that three-quarters of their excitation events result in promotion to the T₁ state, from which intersystem crossing (ISC; here, T₁ → S₁) is unlikely. Yet, this transition, and the fluorescence that follows, is enabled by conformational dynamics in carbene amido complexes of copper and gold as described in Science by a team led by Dan Credgington, Manfred Bochmann and Mikko Linnolahti.

The fabrication of low-latency, durable devices requires an OLED material to make quick use of its T₁ excited state, which might otherwise undergo deleterious bimolecular reactions, particularly when in densely packed arrays and when it is high in energy (as would be required if blue light is desired). Technological applications exploit the T₁ state through phosphorescence (T₁ → S₀), a process facilitated by spin–orbit coupling in complexes of heavy metals, such as Ir, Pt or Os. Another way of harnessing the T₁ state is to effect ISC by thermal upconversion, allowing the resulting S₁ state to fluoresce. These two approaches are not fast (emission times are >1 μs), and are not amenable to emitting blue light, motivating Credgington and co-workers to search for new materials and mechanisms to convert triplets to singlet excited states. On excitation, the linear complexes ([M{(amino)(alkyl)carbene}NR₂] (M = Cu^I, Au^I)) studied undergo charge transfer from the amido (carbazolido, for example) to the carbene ligand and relax through photoluminescence. Certain Cu^I and Au^I complexes had a striking glow, initially assigned to phosphorescent or thermal emission, in line with the common wisdom described above. “We now think something far more interesting is going on,” explains Credgington.

Credit: ©Edgar Schilter

Unlocking blue could have a huge impact on the display and lighting industries, finally making large-area, efficient OLED light sources a reality

How can emission from a T₁ state occur in under 1 μs? One possibility is that the S₁ and T₁ states are sufficiently close in energy such that ISC is facile, as was revealed for the Cu^I and Au^I complexes using time-resolved spectroscopy. Monitoring the dynamics on timescales from femtoseconds to milliseconds, Credgington’s team noted that the carbene and amido ligands — the ‘blades’ of the linear ‘propeller’ complex — rotate such that the dihedral angles between their planes vary from close to 0° in the virtually planar S₀ and T₁ states, to almost 90° in the rotated S₁ state. When the ligands are co-planar, the T₁ state is lower in energy than the productive S₁ state, an order that is reversed when the ligand planes are perpendicular. In between lies a geometry — readily accessible owing to free rotation — at which the S₁ and T₁ states are degenerate and ISC is rapid. A single delayed-emission channel is responsible for the glow; one cannot distinguish whether excitation first affords the S₁ state or the T₁ state that must rotate to cross to S₁. However, regardless of the initial spin of the complex on excitation, it contributes to luminescence such that efficiency can approach 100%.

The thermal stability and photostability of the complexes are boosted by the weakness of their intermolecular interactions, which also enables solution processing for the fabrication of emissive films sandwiched between donor and acceptor layers — motifs that are useful in plastic electronics. The complexes emit green light, and the team aims to extend the design to emit near-IR (for medical diagnostics) as well as blue light (for RGB displays and white lighting). “Unlocking blue could have a huge impact on the display and lighting industries, finally making large-area, efficient OLED light sources a reality,” posits Credgington.