Highly efficient luminescence from space-confined charge-transfer emitters

Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m^-2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m^-2. As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties.