Boron- and Oxygen-Doped π-Extended Helical Nanographene with Circularly Polarised Thermally Activated Delayed Fluorescence

Chiral nanographenes exhibiting circularly polarized luminescence

Chiral nanographenes constitute an unconventional material class that deviates from planar graphene cutouts. They have gained considerable attention for their ability to exhibit circularly polarized luminescence (CPL), which offers new opportunities in chiral optoelectronics. Their unique π-conjugated architectures, coupled with the ability to introduce chirality at the molecular level, have made them powerful contenders in developing next-generation optoelectronic devices. This review thoroughly explores recent advances in the synthesis, structural design, and CPL performance of chiral nanographenes. We delve into diverse strategies for inducing chirality, including covalent functionalization, helically twisted frameworks, and heteroatom doping, each of which unlocks distinct CPL behaviors. In addition, we discuss the mechanistic principles governing CPL and future directions in chiral nanographenes to achieve high dissymmetry factors (glum) and tunable emission properties. We also discuss the key challenges in this evolving field, including designing robust chiral frameworks, optimizing CPL efficiency, and scaling up real-world applications. Through this review, we aim to shed light on recent developments in the bottom-up synthesis of structurally precise chiral nanographenes and evaluate their impact on the growing domain of circularly polarized luminescent materials.

Boraporphyrins: Unlocking Global Aromaticity in Organoboron Macrocycles

We report the synthesis of 5-bora-21,22-dioxaporphyrin 5 and its main-group and d-block metal complexes 6 and 7, respectively. These macrocyclic boranes constitute the first examples of neutral porphyrins with boron in a meso-position that exhibit global aromaticity. This is evidenced by spectroscopic and structural features as well as calculated nucleus independent chemical shifts (NICS) and anisotropic induced current densities (ACID). The boraporphyrins absorb light strongly in the red spectral region (Q bands) and show enhanced fluorescence with higher quantum yields (5: 29%) compared to conventional porphyrins. DFT calculations reveal that the incorporation of the borane moiety has a distinct impact on the frontier orbital energies, thus leading to altered electrochemical and optical properties.

B,N-Embedded Helical Nanographenes Showing an Ion-Triggered Chiroptical Switching Function

Intramolecular oxidative aromatic coupling of 3,6-bis(m-terphenyl-2'-yl)carbazole provided a bis(m-terphenyl)-fused carbazole, while that of 3,6-bis(m-terphenyl-2'-yl)-1,8-diphenylcarbazole afforded a bis(quaterphenyl)-fused carbazole. Borylation of the latter furnished a B,N-embedded helical nanographene binding a fluoride anion via a structural change from the three-coordinate boron to the four-coordinate boron. The anionic charge derived from the fluoride anion is stabilized over the expanded π-framework, which leads to the high binding constant (Ka) of 1×105 M-1. The four-coordinate boron species was converted back to the parent three-coordinate boron species with Ag+, and the chiroptical switch between the three-coordinate boron and four-coordinate boron species has been achieved via the ion recognition with the change in the color and glum values.

Regioisomeric π-Extended Nanographene with Long-Lived Phosphorescence Afterglow

The cutouts of graphene sheets, particularly those with a nonplanar topology, present vast opportunities for advancement. Even a slight deviation from the planar structure can lead to intriguing (chiro)optical features for helically twisted nanographenes. In this context, we introduce two regioisomeric π-extended nanographenes that exhibit distinct excited-state characteristics. The helicene structure and the photophysical features can be easily tuned by changing the connecting position of the nanographene to the carbazole core (2,7- and 3,6-). Single-crystal X-ray diffraction analysis confirmed the formation of nanographenes with bent and helical conformations. Both derivatives exhibited thermally activated delayed fluorescence at room temperature and phosphorescence at low temperatures. Notably, the nanographene with the bent structure displayed an impressive red afterglow lasting over 30 seconds, in contrast to the very weak afterglow observed in the helical structure. DFT calculations revealed the existence of an isoenergetic higher triplet state (T8) and comparatively weak spin-orbit coupling (T1-S0), thereby enabling the bent nanographene to exhibit a long-lived component and strong afterglow. Our findings highlight the significance of regioisomeric nanographenes with exceptional optical properties and offer a deeper understanding of the structure-property relationship in nonplanar nanographenes.

Ultra-Narrowband Circularly Polarized Luminescence from Multiple 1,4-Azaborine-Embedded Helical Nanographenes

In this manuscript we present a strategy to achieve ultranarrowband circularly polarized luminescence (CPL) from multiple 1,4-azaborine-embedded helical nanographenes. The impact of number and position of boron and nitrogen atoms in the rigid core of the molecule on optical properties─including absorption and emission maxima, photoluminescence quantum yield, Stokes shift, excited singlet-triplet energy gap and full width at half-maximum (fwhm) for CPL and fluorescence─was investigated. The molecules reported here exhibits ultranarrowband fluorescence (fwhm 16-17.5 nm in toluene) and CPL (fwhm 18-19 nm in toluene). To the best of our knowledge, this is among the narrowest CPL for any organic molecule reported to date. Quantum chemical calculations, including computed CPL spectra involving vibronic contributions, provide valuable insights for future molecular design aimed at achieving narrowband CPL.

Tailoring helical ends of π-extended [6]heterohelicenes to control optical, and electrochemical features

The inherent helical chirality and improved π-stacking capabilities endow helicenes with fascinating photophysical characteristics when decorated with lateral π-extensions. Here, we report the synthesis and physicochemical characterization of expanded hetero[6]helicenes fused with thiadiazole and selenadiazole rings at the helical ends. Comparing these heterohelicenes revealed the impact of the heteroatom-embedded aromatic rings on the excited state and redox features. A small structural variation of the terminal rings from thiadiazole to selenadiazole caused a striking change in the heterohelical nanographenes.

Helically chiral multiresonant thermally activated delayed fluorescent emitters and their use in hyperfluorescent organic light-emitting diodes

Chiral multiresonant thermally activated delayed fluorescence (MR-TADF) materials show great potential as emitters in circularly polarized (CP) organic light-emitting diodes (CP-OLEDs) owing to their bright and narrowband CP emission. Here, two new chiral MR-TADF emitters tBuPh-BN and DPA-tBuPh-BN possessing intrinsically helical chirality have been synthesized and studied. The large steric interactions between the tert-butylphenyl groups not only induce the helical chirality but also provide a notable configurational stability to the enantiomers. Racemic mixtures of tBuPh-BN and DPA-tBuPh-BN show narrowband emission at 490 and 477 nm with full-width at half maximum (FWHM) of 25 and 28 nm and photoluminescence quantum yields, Φ PL, of 85 and 54% in toluene. The separated enantiomers of tBuPh-BN and DPA-tBuPh-BN show symmetric circularly polarized luminescence (CPL) with respective dissymmetry factors |g PL| values of 1.5 × 10-3 and 0.9 × 10-3. The hyperfluorescence organic light-emitting diodes (HF-OLEDs) with tBuPh-BN and DPA-tBuPh-BN acting as terminal emitters and 2,3,4,5,6-penta-(9H-carbazol-9-yl)benzonitrile (5CzBN) as their assistant dopant exhibited, respectively, maximum external quantum efficiencies (EQEmax) of 20.9 and 15.9% at 492 and 480 nm with FWHM of 34 and 38 nm. This work demonstrates a strategy for developing intrinsically helically chiral MR-TADF emitters possessing significant configurational stability, which can be used in HF-OLEDs.

Tetraborated Intrinsically Axial Chiral Multi-resonance Thermally Activated Delayed Fluorescence Materials

Chiral multi-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials hold promise for circularly polarized organic light-emitting diodes (CP-OLEDs) and 3D displays. Herein, we present two pairs of tetraborated intrinsically axial CP-MR-TADF materials, R/S-BDBF-BOH and R/S-BDBT-BOH, with conjugation-extended bidibenzo[b,d]furan and bidibenzo[b,d]thiophene as chiral sources, which effectively participate in the distribution of the frontier molecular orbitals. Due to the heavy-atom effect, sulfur atoms are introduced to accelerate the reverse intersystem crossing process and increase the efficiency of molecules. R/S-BDBF-BOH and R/S-BDBT-BOH manifest ultra-pure blue emission with a maximum at 458/459 nm with a full width at half maximum of 27 nm, photoluminescence quantum yields of 90 %/91 %, and dissymmetry factors (|gPL|) of 6.8×10-4/8.5×10-4, respectively. Correspondingly, the CP-OLEDs exhibit good performances with an external quantum efficiency of 30.1 % and |gEL| factors of 1.2×10-3.

Binaphthalene-Assisted Axial Chirality in Porphyrins: Toward Solid-State Circularly Polarized Luminescence from Self-Assembled Nanostructures

Circularly polarized luminescence (CPL) is emerging as an effective tool to study the excited-state optical activity in molecules and their self-assembled nanostructures. Chiral porphyrins are a class of optically active molecules wherein the ground-state chirality has been extensively studied in recent times using circular dichroism (CD) spectroscopy. However, obtaining CPL from porphyrin nanostructures, which would have vast implications in biological applications, has remained an uphill task. In this work, we design and synthesize a pair of chiral porphyrin enantiomers functionalized by axially chiral binaphthalene units at the four meso-positions. The molecule undergoes self-assembly following an isodesmic polymerization model, leading to the formation of a spherical nanostructure possessing opposite chirality. Favorable thermodynamic parameters achieved through the controlled experimental conditions helped drive the self-assembly in the forward direction. The limitations imposed by a large nonradiative decay constant arising due to the aggregation-induced quenching could be overcome by fabricating self-standing polymeric films of the nanostructures. The films exhibited relatively high radiative decay and, more interestingly, good CPL activity with clear mirror image spectra for the nanostructures with opposite chirality. The work on CPL-active solid-state materials opens avenue for the design and synthesis of a variety of porphyrin-based chromophoric systems and their nanoaggregates that can find potential application in the field of chiral biosensing and bioimaging, security tags, and display devices.

Solvent-Free Organic Liquids: An Efficient Fluid Matrix for Unexplored Functional Hybrid Materials

ConspectusThe invention of solvent-free organic liquids (SOLs) was serendipitous. However, the curiosity-driven research in the later stage delivered new soft materials with exciting optical, and optoelectronic properties along with appealing physical characteristics suitable for the futuristic applications. A slight change in the molecular design resulted in a drastic change in the physical state of molecules demonstrating monomer-like features in the bulk. The basic idea of core isolation has been successful in delivering new SOLs with attractive functional properties. The unique fluid matrix associated with SOLs offers a tremendous opportunity for making hybrid materials by simple mixing. The chance to study the fundamentally important electron transfer, energy transfer, charge transfer interactions, triplet-state emissions, and even detailed NMR experiments in the solvent-free neat state is the major attraction of SOLs. Usually, solvents and their polarity control such molecular properties, and in the case of SOLs, it avoids the use of solvents to study such fundamentally important properties. Besides, SOLs protect the triplet emitters and excited state processes involving triplet states from quenchers and make the analysis possible under ambient conditions.Our effort in this direction was focused on tuning the ground and excited state properties by transforming conventional organic molecules to SOLs and further value addition by preparing the hybrid SOLs. We developed a series of hybrid SOLs, exploring room-temperature phosphorescence, thermally activated delayed fluorescence, charge or energy transfer between donor and acceptor SOLs, selective explosive sensing, etc. A slight variation in the chemical structure or optoelectronic properties of the individual components imparted exciting optical features for the hybrid SOLs. It includes nonemissive charge transfer, tunable emission exciplex, room temperature phosphorescence, and thermally activated delayed fluorescence SOLs. The liquid matrix of donor SOLs accommodated varying amounts of acceptor SOLs to tune the ground and excited state features. In all examples of donor-acceptor-based hybrid SOLs, even a low amount of acceptor, such as a donor-acceptor ratio of 1000:1, can cause pronounced optical properties. Hence, the evaluation of the optical properties of SOLs, especially, in the absence of solvents is so special that it avoids the interference of solvent molecules. Still, the major drawback of SOLs remains unsolved until we report polymerizable SOLs. Although a large variety of SOLs have been reported in the literature, the long-lasting problem of surface stickiness of SOLs was resolved by polymerizable SOLs. It enabled the development of flexible, foldable, and stretchable large-area luminescent films suitable for lighting and display devices. In this Account, we summarize our work on SOLs, hybrid SOLs, polymerizable SOLs, and the application of SOLs in selective sensing of explosives. Finally, an outlook on the feasibility of luminescent polymerizable SOLs in futuristic applications is provided.