Investigating the effects of planarized versus twisted donors on a multiple resonance thermally activated delayed fluorescence core

Abstract

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are known for their short-range charge transfer excited states and rigid, planar π-frameworks. In contrast, donor–acceptor TADF materials are typically dominated by long-range charge transfer excited states and twisted π-frameworks. An emerging design strategy that seeks to combine these two approaches can be achieved by appending a donor moiety to an MR-TADF core. This strategy has been shown to enhance desirable photophysical properties, such as increasing the rate of reverse intersystem crossing, due to spin-vibronic coupling between excited states of differing orbital symmetry. Here, two donor-appended materials based on the BNCz MR-TADF core are investigated using the donors HMAT (hexamethylazatriangulene) and ACR (9,9-dimethyl-9,10-dihydroacridine). Though structurally similar, the HMAT donor is a fused π-skeleton that provides a higher degree of planarity. In contrast, the ACR donor remains unfused and adopts a twisted structure. This difference leads to altered solution-state photophysical properties which are further investigated using a Lippert–Mataga analysis and theoretical calculations. Solid-state measurements of both emitters, HMAT-BNCz and ACR-BNCz, are then performed to elucidate the impact of donor rigidity on their TADF properties.