The first rational synthesis of a BN-doped coronene derivative in which the central benzene ring has been replaced by a borazine core is described. This includes six C-C ring-closure steps that, through intramolecular Friedel-Crafts-type reactions, allow the stepwise planarization of the hexaarylborazine precursor. UV/Vis absorption, emission, and electrochemical investigations show that the introduction of the central BN core induces a dramatic widening of the HOMO-LUMO gap and an enhancement of the blue-shifted emissive properties with respect to its all-carbon congener.
White‐light‐emitting electrochemical cells (WLECs) still represent a significant milestone, since only a few examples with moderate performances have been reported. Particularly, multiemissive white emitters are highly desired, as a paradigm to circumvent phase separation and voltage‐dependent emission color issues that are encountered following host:guest and multilayered approaches. Herein, the origin of the exclusive white ternary electroluminescent behavior of BN‐doped nanographenes with a B3N3 doping pattern (hexa‐perihexabenzoborazinocoronene) is rationalized, leading to one of the most efficient (≈3 cd A−1) and stable‐over‐days single‐component and single‐layered WLECs. To date, BN‐doped nanographenes have featured blue thermally activated delayed fluorescence (TADF). This doping pattern provides, however, white electroluminescence spanning the whole visible range (x/y CIE coordinates of 0.29–31/0.31–38 and average color rendering index (CRI) of 87) through a ternary emission involving fluorescence and thermally activated dual phosphorescence. This temperature‐dependent multiemissive mechanism is operative for both photo‐ and electroluminescence processes and holds over the device lifespan, regardless of the device architecture, active layer composition, and operating conditions. As such, this work represents a new stepping‐stone toward designing a new family of multiemissive white emitters based on BN‐doped nanographenes that realizes one of the best‐performing single‐component white‐emitting devices compared to the prior‐art.
Self-heating in light-emitting electrochemical cells (LECs) has been long overlooked, while it has a significant impact on i) device chromaticity by changing the electroluminescent band shape, ii) device efficiency due to thermal quenching and exciton dissociation reducing the external quantum efficiency (EQE), and iii) device stability due to thermal quenching of excitons and formation of doped species, phase separation, and collapse of the intrinsic emitting zone. Herein, we reveal, for the first time, a direct relationship between self-heating and the early changes of the device chromaticity as well as the magnitude of the error comparing theoretical/experimental EQEs -i.e., overestimation error of ca. 35 % at usual pixel working temperatures of around 50 °C. This has been realized in LECs using a benchmark nanographene i.e., a substituted hexa-peri-hexabenzocoronene -as an emerging class of emitters with outstanding device performance compared to the prior-art of small molecule LECs -e.g., luminances of 345 cd/m 2 and EQEs of 0.35%. As such, this work is a fundamental contribution highlighting how self-heating is a critical limitation towards the optimization and wide use of LECs.
The first rational synthesis of a BN-doped coronene derivative in which the central benzene ring has been replaced by a borazine core is described. This includes six CÀC ringclosure steps that, through intramolecular Friedel-Crafts-type reactions, allow the stepwise planarization of the hexaarylborazine precursor. UV/Vis absorption, emission, and electrochemical investigations show that the introduction of the central BN core induces a dramatic widening of the HOMO-LUMO gap and an enhancement of the blue-shifted emissive properties with respect to its all-carbon congener. Figure 1. HBC and its borazino-doped analogue HBBNC. Rue de Bruxelles 61, Namur 5000 (Belgium) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.doi.org/10.1002/anie.201700907. Scheme 1. Retrosynthetic strategies toward HBBNC.
The synthesis and
isolation of one of the few examples of a π-extended
diamagnetic phenazine dication have been achieved by oxidizing a phenanthrene-based
dihydrophenazine precursor. The resulting dication was isolated and
fully characterized, highlighting an aromatic distorted structure,
generated by the conformational change upon the oxidation of the dihydrophenazine
precursor, which is also correlated with a marked electrochromic change
in the UV–vis spectrum. The aromaticity of the dication has
also been investigated theoretically, proving that the species is
aromatic based on all major criteria (structural, magnetic, and energetic).
Moreover, the material presents an intriguing dual reactivity, resulting
in ring contraction to a π-extended triarylimidazolinium and
reduction to the dihydrophenazine precursor, depending on the nature
of the nucleophile involved. This result helps shed light on the yet
largely unexplored reactivity and properties of extended dicationic
polycyclic aromatic hydrocarbons (PAHs). In particular, the fact that
the molecule can undergo a reversible change in conformation upon
oxidation and reduction opens potential applications for this class
of derivatives as molecular switches and actuators.
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