Hund’s multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration¹. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, as has been consistent with numerous experimental observations over almost a century. Here we report a fluorescent molecule that disobeys Hund’s rule and has a negative singlet–triplet energy gap of −11 ± 2 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes (OLEDs) using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting and lasers.

Finessing Ferroelectric Liquid Crystals: For a material to show a ferroelectric response, it needs to have segments that can be polarized, with a net polarization that remains when the applied field is removed. However, the fluidity that allows liquid crystal molecules to easily move under an applied force also makes it hard to create a ferroelectric response. Miyajima et al. (p. 209 ) show that a set of columnar liquid crystal molecules, with polar cyano groups tethered to amide-capped nonpolar chains, can assemble into an umbrella-shaped core–shell architecture, in which hydrogen bonding among the amides keeps the cyano groups confined. With only subtle variations in the tether chemistry, the assemblies can be tuned from having a para-electric to a ferroelectric response, which requires only a small coercive field.