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Abstract
Despite the proliferation of multiple resonance (MR) emitters with rigid 1,4-borazine-based skeletons, the straightforward and efficient incorporation of nonhexagonal rings, especially for heptagons, to avoid notorious aggregation-induced quenching effect remains elusive. Here, a green-yellow emitter consisting of two azepines was designed and synthesized via a palladium-catalyzed one-pot twofold [5+2]-annulation reaction with high selectivity and efficiency. The tetrabenzene-fused benzo[1,2-b:5,4-b']bis(azepine) (TBBBA) core induced a highly twisted and dynamically helical rim for the novel MR-skeleton, which reduced Π-Π stacking in the solid state. Moreover, the nonalternant topology facilitated the delocalization of frontier molecular orbitals (FMO) within the twisted geometry, thus achieving red-shifted narrow emission. Our work provides a new synthetic strategy towards nonalternant extension of MR-emitters and gives insights into the electronic effects of multiple azepination on FMO distribution.
Highlights
Polycyclic conjugated hydrocarbons (PCHs) are of great interest to the community of chemistry and materials due to their fine-tuned electrochemical and photophysical properties.[1]
The ubiquitous synthetic strategies for de novo construction or π-extension of PCH skeletons are highly important for the development of diversified boron–nitrogen-containing multiple resonance (MR)-emitters with thermally activated delayed fluorescence (TADF).[2]
Π-extension with a nonalternant topology might endorse a twisted geometry with significant delocalization of frontier molecular orbitals (FMO) to achieve red-shifted emission with suppressed intermolecular π–π stacking
Summary
Polycyclic conjugated hydrocarbons (PCHs) are of great interest to the community of chemistry and materials due to their fine-tuned electrochemical and photophysical properties.[1]. The introduction of nonhexagonal rings (pentagons and heptagons) into graphene molecules induced unique geometric deformations and photophysical properties.[3]. Π-extension with a nonalternant topology might endorse a twisted geometry with significant delocalization of FMO to achieve red-shifted emission with suppressed intermolecular π–π stacking. Bin and You developed blue MR-emitters with twisted geometry by borylation of TBA-linked intermediates (Figure 1a).5a The introduction of heptagons enhances spin-orbital coupling to suppress efficiency roll-off. Hatakeyama synthesized a TBA-decorated deep-blue MR-emitter based on υDABNA to achieve fine-tuned energy levels and color output.5b less effective π-conjugation restricted those emitters from long-wavelength gamut. We developed a programmable approach towards nonalternant B,Nembedded helical nanographenes with controllable integration of azepines.5c the extended TBA units resulted in improved π-conjugation and deep-red circularly polarized luminescence was observed, the photoluminescence quantum yield (PLQY) decreased for such large π-system.
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