Fine Tuning of Hybridized Local and Charge-Transfer Excited States and Luminescence in Carbazole/Benzothiadiazole Isomeric Emitters by Varying Linking Mode

Abstract

Hybridized local and charge-transfer (HLCT) type emitters can harvest triplet excitons for light emission via high-lying reverse intersystem crossing (hRISC), showing great potential for application in organic light-emitting diodes (OLEDs). Herein, two isomeric emitters NPC3BTD and NPC2BTD were developed by linking two 9-phenyl-9H-carbazole (NPC) donors onto the central benzo[c][1,2,5]thiadiazole (BTD) acceptor. By tuning the linkage from 3-site to 2-site of carbazole ring, the D−A dihedral angle was decreased, then the HOMO delocalization and CT/LE content ratio in HLCT excited states were reduced, thus NPC2BTD revealed fluorescence hypsochromic shift by 30 nm to green region. Simultaneously, the molecular rigidity was increased and non-radiative transition was suppressed, thus the photoluminescence quantum yield was increased to about 90 %. Their HLCT excited state nature was verified by the natural transition orbitals results and detection of two different excited dipole moments in various solvents. It is supposed the hRISC occur from T2 to S1 states. Yellow and green doped OLEDs of NPC3BTD and NPC2BTD exhibited external quantum efficiencies of 5.31 % and 5.79 % with exciton utilization efficiency of 36.12 % and 32.42 %. The superior performance of NPC2BTD over NPC3BTD was attributed to the finely tuned HLCT excited states through structure isomerization.