Formation of Nitrogen-Doped Positively Curved Molecules by π-Extension

Two nitrogen-doped positively curved aromatic molecules bearing doubly fused pentagonal rings were synthesized and characterized. Crystallographic analysis confirms the formation of a bowl-shaped structure, which is induced by the fusion of adjacent pentagons to the rigid aromatic planes. Both compounds demonstrate good photoluminescence. These electron-rich bowl-shaped molecules can associate with C60 to form complexes in 2:1 ratio in toluene with different association constants depending on the molecular dimension of the hosts.

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

Facile Functionalization of Ambipolar, Nitrogen-Doped PAHs toward Highly Efficient TADF OLED Emitters

Despite promising optoelectronic features of N-doped polycyclic aromatic hydrocarbons (PAHs), their use as functional materials remains underdeveloped due to their limited post-functionalization. Facing this challenge, a novel design of N-doped PAHs with D-A-D electronic structure for thermally activated delayed fluorescence (TADF) emitters was performed. Implementing a set of auxiliary donors at the meta position of the protruding phenyl ring of quinoxaline triggers an increase in the charge-transfer property simultaneously decreasing the delayed fluorescence lifetime. This, in turn, contributes to a narrow (0.04-0.28 eV) singlet-triplet exchange energy split (ΔEST) and promotes a reverse intersystem crossing transition that is pivotal for an efficient TADF process. Boosting the electron-donating ability of our N-PAH scaffold leads to excellent photoluminescence quantum yield that was found in a solid-state matrix up to 96% (for phenoxazine-substituted derivatives, under air) with yellow or orange-red emission, depending on the specific compound. Organic light-emitting diodes (OLEDs) utilizing six, (D-A)-D, N-PAH emitters demonstrate a significant throughput with a maximum external quantum efficiency of 21.9% which is accompanied by remarkable luminance values which were found for all investigated devices in the range of 20,000-30,100 cd/m2 which is the highest reported to date for N-doped PAHs investigated in the OLED domain.

Sulfone-Embedded Heterocyclic Narrowband Emitters with Strengthened Molecular Rigidity and Suppressed High-Frequency Vibronic Coupling

Sulfone-embedded heterocyclics are of great interest in organic light-emitting diodes (OLEDs), however, exploring highly efficient narrowband emitters based on sulfone-embedded heterocyclics remains challenging. Herein, five emitters with different sulfur valence state and molecular rigidity, namely tP, tCPD, 2tCPD, tPD and tPT, are thoroughly analysed. With restricted twisting of flexible peripheral phenyl by strengthening molecular rigidity, molecular emission spectra can be enormously narrowed. Further, introducing the sulfone group with bending vibration in low-frequency region that suppresses high-frequency vibration, sharp narrow full-widths at half-maximum of 28 and 25 nm are achieved for 2tCPD and tPD, respectively. Maximum external quantum efficiencies of 22.0 % and 27.1 % are successfully realized for 2tCPD- and tPD-based OLED devices. These results offer a novel design strategy for constructing narrowband emitters by introducing sulfone group into a rigid molecular framework.

π-Extended Pyrrole-Fused Heteropine: Synthesis, Properties, and Application in Organic Field-Effect Transistors

Sulfur-embedded polycyclic aromatic compounds have been used as building blocks for numerous organic semiconductors over the past few decades. While the success is based on thiophene-containing compounds, aromatic compounds that contain thiepine, a sulfur-containing seven-membered-ring arene, has been less well investigated. Here we report the synthesis and properties of π-extended pyrrole-fused heteropine compounds such as thiepine and oxepine. A π-extended pyrrole-fused thiepine exhibited a "pitched π-stacking" structure in the crystal, and exhibited a high charge carrier mobility of up to 1.0 cm²  V^-1  s^-1 in single-crystal field-effect transistors.

Nitrogen-doped polycyclic aromatic hydrocarbons by a one-pot Suzuki coupling/intramolecular SNAr reaction

We report a new method for the synthesis of nitrogen-doped (N-doped) polycyclic aromatic hydrocarbons (PAHs) by a Suzuki coupling/intramolecular S_NAr cascade reaction. A one- or two-fold [3 + 3] naphtho-annulation of halogenated aniline was conducted under Suzuki-Miyaura cross-coupling conditions to yield a series of fully fused N-doped PAHs. In contrast to reported methods to synthesize pyridinic or pyrrolic nitrogen-doped PAHs, our method enables preparation of PAHs doped with graphitic nitrogen, for which few reports are known in the literature. The crystal structure as well as absorption, fluorescence and electrochemical properties of these N-doped PAHs were investigated, which demonstrated the capability of N-doping to adjust optical and electronic properties and alter the LUMO energy level.

Phenanthrene-Extended Phenazine Dication: An Electrochromic Conformational Switch Presenting Dual Reactivity

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.

Modular Nitrogen-Doped Concave Polycyclic Aromatic Hydrocarbons for High-Performance Organic Light-Emitting Diodes with Tunable Emission Mechanisms

Although bowl‐shaped N‐pyrrolic polycyclic aromatic hydrocarbons (PAHs) can achieve excellent electron‐donating ability, their application for optoelectronics is hampered by typically low photoluminescence quantum yields (PLQYs). To address this issue, we report the synthesis and characterization of a series of curved and fully conjugated nitrogen‐doped PAHs. Through structural modifications to the electron‐accepting moiety, we are able to switch the mechanism of luminescence between thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP), and to tune the overall PLQY in the range from 9 % to 86 %. As a proof of concept, we constructed solid‐state organic light‐emitting diode (OLED) devices, which has not been explored to date in the context of concave N‐doped systems being TADF/RTP emitters. The best‐performing dye, possessing a peripheral trifluoromethyl group adjacent to the phenazine acceptor, exhibits yellow to orange emission with a maximum external quantum efficiency (EQE) of 12 %, which is the highest EQE in a curved D‐A embedded N‐PAH to date.

Fully conjugated azacorannulene dimer as large diaza[80]fullerene fragment

A fully conjugated azacorannulene dimer with a large π-surface (76π system) was successfully synthesized from a fully conjugated bifunctional polycyclic aromatic azomethine ylide. This molecule represents an example of diaza[80]fullerene (C78N2) fragment molecule bearing two internal nitrogen atoms. X-ray crystallography analysis shows its boat-shaped structure with two terminal azacorannulenes bent in the same direction. The molecular shape leads to unique selective association with a dumbbell-shaped C60 dimer (C120) over C60 through shape recognition. Owing to its large π-surface and a narrow HOMO–LUMO gap, the azacorannulene dimer exhibits red fluorescence with a quantum yield of up to 31%. The utilization of the fully conjugated bifunctional azomethine ylide is a powerful method for the bottom-up synthesis of large multiazafullerene fragments, providing a step towards the selective total synthesis of multiazafullerenes.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

S,C,C- and O,C,C-Bridged Triarylamines and Their Persistent Radical Cations

This work reports the synthesis, crystal structures, and electronic properties of structurally constrained S,C,C- and O,C,C-bridged triarylamine derivatives and their persistent radical cations. O,C,C-Bridged triphenylamines and a dinaphthylphenylamine were obtained through a straightforward synthetic protocol. Similar to a previously reported S,C,C-bridged triphenylamine, the O,C,C-bridged triarylamines were easily oxidized to afford the corresponding radical cations, which were obtained as hexachloroantimonate salts. X-ray crystallographic analyses showed almost planar structures for these O,C,C-bridged triarylamine radical cations, which represent new members of the family of planar triarylamine radical cations without substituents on the aryl rings. Detailed investigations of the electronic properties of the S,C,C- and O,C,C-bridged triarylamine radical cations demonstrated that the spin and positive charge are sufficiently delocalized over the planar triarylamine scaffolds. The results provide the following insights into the effects of the bridging unit (sulfur vs oxygen) and the dibenzo-annulation on the spin delocalization in the bridged triarylamine radical cations: (1) An effective decrease of the spin density on the nitrogen atom is observed for the sulfur bridge relative to the oxygen bridge; and (2) a moderate decrease of the spin density on the oxygen atom rather than the nitrogen atom is induced by the dibenzo-annulation.