Abstract
Commonly, thermally activated delayed fluorescence (TADF) emitters present a twisted donor–acceptor structure. Here, electronic communication is mediated through-bond via π-conjugation between donor and acceptor groups. A second class of TADF emitters are those where electronic communication between donor and acceptor moieties is mediated through-space. In these through-space charge-transfer (TSCT) architectures, the donor and acceptor groups are disposed in a pseudocofacial orientation and linked via a bridging group that is typically an arene (or heteroarene). In most of these systems, there is no direct evidence that the TSCT is the dominant contributor to the communication between the donor and acceptor. Herein we investigate the interplay between through-bond localized excited (LE) and charge-transfer (CT) states and the TSCT in a rationally designed emitter, TPA-ace-TRZ, and a family of model compounds. From our photophysical studies, TSCT TADF in TPA-ace-TRZ is unambiguously confirmed and supported by theoretical modeling.
Highlights
Thermally activated delayed fluorescence (TADF) emitters present a twisted donor−acceptor structure
The use of metal-free thermally activated delayed fluorescence (TADF) emitters in organic lightemitting diodes (OLEDs) has attracted significant attention within the OLED community following the first reports from Goushi et al.[1] and Uoyama et al.,[2] which demonstrated the potential of E-type delayed emission exciplexes and donor−acceptor emitters to produce highefficiency devices
This is made possible by the small energy gaps (ΔEST), generally less than 100 meV, among the lowest CT singlet, triplet, and local triplet excited states of these materials, enabling efficient upconversion of the electrically generated CT triplet states through a vibrationally coupled spin−orbit coupling mechanism where the local triplet excited state (T1) mediates the otherwise forbidden 3CT triplet to 1CT singlet transition via reverse intersystem crossing (RISC).[4,5]
Summary
While the delayed emission component of TPAace-TRZ is observable only at very long times (tens of milliseconds) and has a very weak contribution ascribed to phosphorescence, TPA-ace-Br makes a strong RTP contribution, confirming that attaching a heavy atom such as Br to the molecule significantly enhances the intersystem crossing (ISC) in this system. To understand why we observe little or no TADF from the TSCT state in TPA-ace-TRZ, even in DCM where the ΔEST becomes rather small, we consider where the lowest-energy triplet state of the molecule resides This is found to be the local triplet state of the ace bridge ( a Br attached to the ace gives a heavy-atom enhancement to the ace phosphorescence), and in TPA-ace-TRZ, the ace unit is orthogonal and electronically decoupled from D and A, especially in the TSCT conformation.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call