  The efficiency of an organic light-emitting diode (OLED) is fundamentally
governed by the spin of recombining electron-hole pairs (singlet and triplet
excitons), since triplets cannot usually emit light. The singlet-triplet energy
gap, a key factor for efficient utilization of triplets, is normally positive.
Here we show that in a family of materials with amide donor and carbene
acceptor moieties linked by a metal, this energy gap for singlet and triplet
excitons with charge-transfer character can be tuned from positive to negative
values via the rotation of donor and acceptor about the metal-amide bond. When
the gap is close to zero, facile intersystem crossing is possible, enabling
efficient emission from singlet excitons. We demonstrate solution-processed
LEDs with exceptionally high quantum efficiencies (near-100% internal and >27%
external quantum efficiencies), and current and power efficiencies (87 cd/A and
75 lm/W) comparable to, or exceeding, those of state-of-the-art
vacuum-processed OLEDs and quantum dot LEDs.
