Confining donor conformation distributions for efficient thermally activated delayed fluorescence with fast spin-flipping

Abstract

Fast spin-flipping is the key to exploit the triplet excitons in thermally activated delayed fluorescence (TADF) based organic light-emitting diodes (OLEDs) toward high efficiency, low efficiency roll-off and long operating lifetime. In common donor-acceptor type TADF molecules, the distribution of dihedral angles in the film state would have significant influence on the photo-physical properties, which are usually neglected by researches. Herein, we found that the excited state lifetimes of TADF emitters are subjected to conformation distributions in the host-guest system. Acridine-type flexible donors have a broad conformation distribution or dual conformations, which could bring about the long-tail conformer with large singlet-triplet energy gap, resulting in long excited state lifetime. Utilization of rigid donors with steric hindrance can restrict the conformation distributions in the film to achieve degenerate singlet and triplet states, which is beneficial to efficient reverse intersystem crossing (RISC). Based on this principle, three prototype TADF emitters with confined conformation distributions were developed, achieving high RISC rate constants greater than 106 s− 1, which enable highly efficient solution-processed OLEDs with significantly suppressed efficiency roll-off.

Rigid and planar π-conjugated molecules leading to long-lived intramolecular charge-transfer states giving thermally activated delayed fluorescence

“…This dispersion arises from dihedral angle inhomogeneity in a twisted D-A molecule, giving a broad-range of excited state lifetimes. 49,50 In DCM, the instantaneous 560 nm band was observed to decay very rapidly within 1 ps, with the formation of an initial ICT ESA at 730 nm, that rapidly relaxes to 820 nm within ca. 1.7 ps (Figure 3 and Supplementary Fig.…”