"On-off" switch between red thermally activated delayed fluorescence and conventional fluorescence by isomeric regulation.

[Display omitted] • Luminous mechanism switch between TADF and CF was realized by isomer engineering. • Different exciton dynamic processes depend on the interplay of 1CT, 3LE and 3CT and molecular geometry rigidity. • Red TADF OLEDs with external quantum efficiency >20% was achieved. Marching toward highly efficient long-wavelength organic light-emitting diodes (OLEDs) is paramount but formidable challenge, and rational manipulation of molecular configuration and excited state dynamic processes are of great importance to harvest dark triplet exciton and suppress non-radiative transition for excellent light emission properties. Herein, we introduced a novel electron acceptor 11H-indeno [1,2-b] quinoxalin-11-one (IQ) as a building-block to construct two bright red light-emitting regioisomers of IQ-oTPA and IQ-pTPA, which exhibit entirely different exciton dynamic processes of thermally activated delayed fluorescence (TADF) and conventional fluorescence (CF) respectively. According to theoretical calculation and photophysical characterization, the energy gap, energy level alignment and spin–orbit coupling (SOC) effects of charge-transfer singlet state (1CT), local excited triplet state (3LE) and charge-transfer triplet state (3CT) together with the molecular geometry rigidity play a critical role in the triplet exciton up-conversion processes. Maximum external quantum efficiencies of 20.6% and 3.5% with emission peaks of 604 and 642 nm were achieved for the proof-of-concept electroluminescent devices based on IQ-oTPA and IQ-pTPA respectively, which are among the state-of-the-art performance for red-emission TADF and CF OLEDs and provide new insights for the molecular design tactic of high-performance red OLEDs. [ABSTRACT FROM AUTHOR]