Molecular design of DBA-type five-membered heterocyclic rings to achieve 200% exciton utilization for electroluminescence.

Achieving high exciton utilization is a long-cherished goal in the development of organic light-emitting diode materials. Herein, a three-step mechanism is proposed to achieve 200% exciton utilization: (i) hot triplet exciton (T 2 ) conversion to singlet S 1 ; (ii) singlet fission from S 1 into two T 1 ; (iii) and then a Dexter energy transfer to phosphors. The requirement is that S 1 should lie slightly lower than or close to T 2 and twice as high as T 1 in energy. For this, a scenario is put forward to design a series of donor-bridge-acceptor (DBA) type molecules with 2 E (T 1 ) ≤ E (S 1 ) < E (T 2 ), in which the Baird-type aromatic pyrazoline ring is used as a bridge owing to its stabilized T 1 (1.30-1.74 eV) and different kinds of donors and acceptors are linked to the bridge for regulating S 1 (2.35-3.87 eV) and T 2 (2.44-3.96 eV). The ultrafast spectroscopy and sensitization measurements for one compound (TPA-DBPrz) fully confirm the theoretical predictions.