Intramolecular Hydrogen Bonding in Thermally Activated Delayed Fluorescence Emitters: Is There Evidence Beyond Reasonable Doubt?

Abstract

Intramolecular hydrogen bonding between donor and acceptor segments in thermally activated delayed fluorescence (TADF) materials is now frequently employed to—purportedly—rigidify the structure and improve the emission performance of these materials. However, direct evidence for these intramolecular interactions is often lacking or ambiguous, leading to assertions that are largely speculative. Here we investigate a series of TADF-active materials incorporating pyridine, which bestows the potential ability to form intramolecular H-bonding interactions. Despite possible indications of H-bonding from an X-ray analysis, an array of other experimental investigations proved largely inconclusive. Instead, after examining computational potential energy surfaces of the donor–acceptor torsion angle we conclude that the pyridine group primarily alleviates steric congestion in our case, rather than enabling an H-bond interaction as elsewhere assumed. We suggest that many previously reported “H-bonding” TADF materials featuring similar chemical motifs may instead operate similarly and that investigation of potential energy surfaces should become a key feature of future studies.

Uncovering the Mechanism of Thermally Activated Delayed Fluorescence in Coplanar Emitters Using Potential Energy Surface Analysis

“…This is confirmed by single-crystal X-ray diffraction and variable-temperature NMR studies (details in ESI). A similar conclusion was drawn recently about a family of nonplanar TADF emitters, where intramolecular H-bonding had previously been assumed to impact the geometry, but it was determined that the ground state geometry was simply sterically preferred . Our results suggest that dihedral rotation between the donor and acceptor moieties is not restricted by intramolecular hydrogen bonding, such that H-bonding is not a likely source of molecular rigidification in these coplanar systems.…”

Role of the Intramolecular‐Locking Strategy in the Construction of Organic Thermally Activated Delayed Fluorescence Emitters with Rotation‐Restricted Acceptors

“…Most rigid TADF acceptors have been constructed using covalent-bond-type "intramolecular locks" to restrict the rotation of the single bonds. However, TADF systems with multiple covalent-bond-type "intramolecular locks" per molecular unit have been relatively less investigated and would appear [182] Reproduced with permission. [182] Copyright 2022, American Chemical Society.…”

“…More representative TADF emitters with potential intramolecular CH•••N interactions can be accessed in the cited paper. [182] Adv. Optical Mater.…”

“…Recently, the research of Bryce et al has put intramolecular hydrogen bonding in TADF emitters into question. [182] They suggested that the state that improved quantum yields, narrowed the emission spectra, or enhanced the SOC was the D-A molecular planarization or rigidification of the molecular structure that was induced by N•••H or similar intramolecular interactions in the heteroatom-substituted material can only ever be inferred rather than conclusively demonstrated. To investigate intramolecular H-bonding in TADF materials, they synthesized a set of model D-A compounds 1-3 featuring pyridine and pyrimidine groups, which allowed the possibility to form intramolecular H-bonding interactions (Figure 21).…”

Azaborine as a Versatile Weak Donor for Deep Blue Thermally Activated Delayed Fluorescence

“…22 In contrast, the presence of the larger 6-member central ring of DMAC results in near perpendicular conformations that are less sensitive to steric environment 14,23 although it is simultaneously a much stronger electron-donor. 24 It would therefore be appealing to design weak donors akin to Cz, but that share the useful steric properties associated with DMAC. 9 Previously we have reported a CF3-substituted DMAC derivative that is a weaker electron-donor due to the presence of the inductively electron-withdrawing trifluoromethyl groups, and which yielded a blue-shifted emission compared to the reference DMAC-based material; however, this emitter was electrically unstable in the OLED.…”