Provided are OLEDs and related electronic devices that utilize these OLED devices. The OLED includes an emissive region that includes a sensitizer and an acceptor, where the sensitizer has a lowest excitation energy state ET1, and is capable of harvesting triplet excitons; where the acceptor is capable of receiving energy from the sensitizer and functioning as a fluorescent emitter at room temperature; where the acceptor has a first moiety and a second moiety, the first moiety having a lowest singlet excitation energy state ES1A and a lowest triplet excitation energy state ET1A, and the second moiety having a lowest singlet excitation energy state ES1B and a lowest triplet excitation energy state ET1B; and where ES1A ET1B, and ET1>ES1A.