Simple peripheral modification for color tuning of thermally activated delayed fluorescence emitters in OLEDs

Through simple peripheral tert-butyl or methoxy substitution at 3,6-position of carbazole to increase electron donating property, emission colors of donor-acceptor (D-A) type thermally activated delayed fluorescence (TADF) emitters are rationally tuned. Due to enhanced intramolecular D-A interactions, the intramolecular charge transfer (ICT) transition absorption is bathochromically shifted from 451 to 474 and 490 nm in DCM solution and 458 to 476 and 497 nm in film for 4CzCNPy, 4tCzCNPy and 4oCzCNPy, respectively. And the singlet, triplet as well as HOMO energy levels were also gradually down-shifted from bare 4CzCNPy to 4tCzCNPy and 4oCzCNPy. 4CzCNPy shows green emission in DCM, while 4tCzCNPy and 4oCzCNPy demonstrates yellow and deep red, with peak values from 536 to 561 and 634 nm, respectively. TADF OLEDs based on yellow emitter of 4tCzCNPy exhibit a maximum external quantum efficiency (EQE) of 22.2%, while maximum EQE of 17% and 2.9% is achieved for the green 4CzCNPy and orange-red 4oCzCNPy.