Advances in nitrogen-containing helicenes: synthesis, chiroptical properties, and optoelectronic applications

Introduction

Helicenes, a class of non-planar polycyclic aromatic hydrocarbons characterized by ortho-fused aromatic rings forming a helical framework, have attracted significant attention due to their inherent chirality, unique optoelectronic properties, and wide-ranging applications in asymmetric catalysis [1,2], molecular recognition [3], and organic electronics [4,5]. In recent years, the incorporation of heteroatoms – particularly nitrogen – into the helicene backbone, giving rise to so-called "azahelicenes", has emerged as a powerful strategy to modulate electronic structures, enhance solubility, and expand functional diversity [6]. Substituting carbon atoms with electron-deficient nitrogen atoms introduces new opportunities to fine-tune redox potentials, charge-transport behavior, and intermolecular interactions [7]. These modifications have proven especially valuable in applications such as organic light-emitting diodes (OLEDs) [8], circularly polarized luminescence (CPL) [9], and chiral photocatalysis [10]. In the past decade, heteroatom-containing helicenes have attracted increasing attention due to their tunable optoelectronic properties and potential applications in chiral optoelectronics. Several comprehensive reviews have examined specific classes of these molecules. Crassous and co-workers provided a detailed overview of heterohelicenes up to 2019, focusing on their structural diversity and functional applications [11]. Nowak-Król and colleagues reviewed boron-doped helicenes, emphasizing their roles in chiral materials design [12], while Maeda and Ema explored the circularly polarized luminescence (CPL) properties of azahelicenes [13]. However, despite these valuable contributions, a dedicated and up-to-date overview of nitrogen-doped helicenes – particularly those incorporating additional heteroatoms within the helical π-conjugated framework – remains lacking.

This review addresses this gap by systematically summarizing recent advances (from the past five years) in the synthesis, structural modification, and chiroptical properties of nitrogen-doped helicenes. Particular attention is given to multi-heteroatom systems co-doped with elements such as boron, oxygen, sulfur, and selenium, highlighting their influence on CPL performance and structure–property relationships. We classify the nitrogen-doped helicenes into only N-containing helicenes, B,N-containing helicenes, and X,N-containing helicenes (X = O, S or Se). In each section, structurally similar compounds are categorized into groups to facilitate comparison. Then, the others are discussed in chronological order based on their reported publication dates, with attribution to the respective research groups. Notably, helicenes bearing nitrogen atoms located outside the conjugated system are excluded from this discussion to maintain a consistent focus on electronically integrated heteroatom-doped architectures.

Review

N-Containing helicenes

Among nitrogen-containing helicenes, HBC-fused azahelicenes represent a particularly significant subclass due to their extended π-conjugation and potential for enhanced chiroptical properties. Over the past few years, multiple research groups have investigated their synthesis, structural characteristics, and optoelectronic behavior. Notably, in 2021, Jux and co-workers reported a series of superhelicenes that combine helical and planar π-systems. However, the structural characterization of compound 1 (Table 1) was impeded by its inherent instability, limiting further analysis [14]. In 2024, Liu’s group developed a series of nonalternant nanographenes 2a–c featuring a nitrogen-embedded cyclopenta[ef]heptalene core [15]. These compounds exhibit λ_abs at 363, 452, and 580 nm, and PLQYs of 0.05, 0.33, and 0.32, respectively. While compounds 2a and 2b display broad emission near 505 nm, 2c shows dual-emission peaks at 588 and 634 nm with an ultranarrow FWHM of 22 nm. Notably, 2b and 2c demonstrate strong chiroptical activity with |g_abs| values of 6.7 × 10⁻³ and 1.0 × 10⁻², |g_lum| of 2.4 × 10⁻³ and 7.0 × 10⁻³, and B_CPL values of 9.1 and 95.2 M⁻¹ cm⁻¹, respectively. Shortly thereafter, Gong’s group further expanded the π-system by constructing a tris-hexabenzo[7]helicene 3 with a carbazole core, which emits at 595/628 nm (PLQY = 0.40), displays |g_abs| = 2.98 × 10⁻³, and achieves a B_CPL of 32.5 M⁻¹ cm⁻¹ [16]. In 2025, Babu’s group synthesized two regioisomeric π-extended azahelicenes, 4a and 4b, which differ in the position of attachment to the carbazole core [17]. Compared to 4a, compound 4b exhibits bathochromic shifts of 12 nm in absorption and 45 nm in emission, as well as a higher Φ_F (0.75 vs 0.68). Both isomers display TADF at room temperature and phosphorescence at 77 K. Notably, 4a demonstrates a long-lived red afterglow persisting for up to 30 seconds. In contrast, 4b exhibits superior chiroptical properties, with |g_abs| and |g_lum| values of 3.91 × 10⁻³ and 1.12 × 10⁻³, respectively, and an impressive B_CPL of 45.77 M⁻¹ cm⁻¹ (Table 1).

In 2021, several research groups reported structurally diverse heterohelicene systems exhibiting distinctive chiroptical and photophysical properties, highlighting the expanding potential of these molecules in chiral optoelectronics. Yorimitsu’s group developed a series of dihetero[8]helicenes through a systematic asymmetric synthesis. Among these, diaza[8]helicene 5 exhibited pronounced chiroptical activity, with absorption and emission maxima (λ_abs = 399 nm, λ_em = 405 nm), a fluorescence quantum yield (Φ_F) of 0.13, and high dissymmetry factors (|g_abs| = 1.9 × 10⁻², |g_abs| = 9.5 × 10⁻³ at 403 nm) [18] (Table 2). Miura and co-workers employed Pd(II)/Ag(I)-catalyzed cyclizations to construct azahelicenes, with compound 6 exhibiting enhanced chiroptical performance and protonation-induced CPL amplification [19]. Meanwhile, Audisio’s team developed heterohelicenes via regioselective [3 + 2]-cycloadditions, with compound 7 displaying pH-responsive CPL sign inversion (|g_lum| = +1.1 × 10⁻³ at 430 nm, −1.2 × 10⁻³ at 585 nm) attributed to reversible intramolecular charge transfer [20]. In parallel, several groups explored the functional versatility of heterohelicenes in device-oriented and sensing applications. Crassous’s group synthesized bipyridine-embedded helicenes via the Mallory reaction, enabling coordination with Ru(II) to form NIR-emissive complexes that exhibit redox-responsive chiroptical switching, notably with complex 8 showing reversible electronic circular dichroism (ECD) upon oxidation [21]. Liao and co-workers introduced a narrowband CP-TADF emitter 9, characterized by a narrow emission bandwidth (FWHM = 36 nm), |g_lum| = 1.1 × 10⁻³, |g_EL| = 1.5 × 10⁻³, and an external quantum efficiency (EQE) of 0.14 – demonstrating promise for CPL-OLED applications [22]. Wanichacheva’s team reported urazole-functionalized aza[5]helicene 10, exhibiting selective Fe(III) sensing, marked solvatochromism, and a large Stokes shift (85 nm) with emission at 530 nm in DMSO [23] (Table 2). Collectively, these studies underscore the structural versatility and functional tunability of heterohelicenes, establishing them as robust platforms for advanced chiral optoelectronic materials. Their diverse response to external stimuli, modular synthetic accessibility, and strong CPL performance render them ideal candidates for applications in molecular sensing, stimuli-responsive switches, and next-generation CPL-active devices.

In 2021, Ema’s group reported the synthesis of carbazole-based azahelicenes 11a–e via intramolecular Scholl reactions [24] (Table 3). All compounds exhibited strong absorption in the UV–vis region (250–450 nm) and fluorescence emission between 400–550 nm. Among these, compound 11c, a saddle-shaped dibenzodiaza[8]circulene, was particularly noteworthy as the first example of its kind synthesized in solution and structurally confirmed via single-crystal X-ray diffraction. It demonstrated the highest CPL performance among the series, with a |g_lum| value of 3.5 × 10⁻³ and a photoluminescence quantum yield (PLQY) of 0.31, indicating its potential as a chiral emissive material. Building upon this foundation, the same group in 2024 developed a series of structurally refined aza[7]helicenes (compounds 12a and 12b) under modified Scholl reaction conditions [25]. These products were obtained as optically active diastereomers, which were successfully separated using silica gel chromatography. Additionally, two cyclic dimers, designated as compounds 12c and 12d, were isolated, exhibiting strong absorption bands at 493 and 474 nm, high PLQYs of 0.61 and 0.54, and notable CPL activity (|g_lum| = 0.74 × 10⁻³ and 1.3 × 10⁻³, respectively), with corresponding brightness values (B_CPL) reaching 19 and 31 M⁻¹ cm⁻¹ (Table 3). Importantly, both dimers displayed selective fluoride ion recognition through hydrogen bonding, with (M,M)-12c exhibiting a high binding constant (K_a = 2 × 10⁵ M⁻¹). The resulting [12c·F⁻] and [12d·F⁻] complexes exhibited red-shifted circular dichroism (CD), fluorescence, and CPL spectra, underscoring the capability of helicene-based frameworks for anion-responsive chiroptical modulation. These findings highlight how precise structural design and supramolecular engineering can facilitate the development of high-performance, stimuli-responsive chiral luminophores.

In 2022, Zhang and co-workers reported a nitrogen-embedded quintuple [7]helicene 13, constructed by hybridizing helicene and azacorannulene π-systems [26] (Table 4). Compound 13 exhibited distinct absorption bands at 408, 611, and 715 nm, with strong near-infrared (NIR) fluorescence centered at 770 nm and a PLQY value of 0.28. Upon coordination with tris(4-bromophenyl)aminium hexachloroantimonate (BAHA), a new absorption band emerged around 900 nm, extending to 1300 nm, indicative of charge-transfer processes. The enantiomers of 13 displayed mirror-image CD signals and showed excellent dispersibility in polar solvents, highlighting their potential for NIR bio-imaging applications. In parallel, Církva’s group synthesized a series of aza[n]helicenes 14a–d via photocyclodehydrochlorination [27]. These compounds exhibited dual fluorescence bands, with emission red-shifting progressively with increasing helical length. Protonation further induced red-shifted emission, with compound 14d-H⁺ emitting at 542 nm. However, PLQYs decreased significantly from 0.078 to 0.006 with longer helicenes. The CD spectra of 14c and 14d were found to resemble their carbohelicene analogues, underscoring the structural fidelity and chiroptical retention upon nitrogen incorporation. Qian’s group developed a series of azahelicenes 15a–d through Bischler–Napieralski cyclization [28]. Notably, compound 15b displayed a high interconversion barrier of 36.0 kcal mol⁻¹, enabling enantiomeric resolution. All compounds exhibited visible-range fluorescence (400–500 nm) and structured UV–vis absorption spectra. Importantly, 15b showed acid/base-switchable UV and CD spectra, suggesting potential for use in responsive optoelectronic systems. Hu’s group reported an X-shaped double [7]helicene 16 functionalized with four triazole units, which demonstrated absorption at 368 and 516 nm, strong emission at 553 nm, a high PLQY of 0.96, |g_abs| of 1.1 × 10⁻², |g_lum| of 9.1 × 10⁻⁴, and B_CPL of 30.1 M⁻¹ cm⁻¹ – surpassing the performance of its all-carbon and thiadiazole counterparts [29]. In a related study, Hu’s team synthesized double aza[5]helicenes 17a and 17b, among which compound 17b exhibited red-shifted emission (538–632 nm in CHCl₃) and the largest Stokes shift (192 nm), attributed to extended conjugation and sulfur incorporation [30] (Table 4). These findings collectively underscore how structural modulation and heteroatom doping can tailor the optical, chiroptical, and stimuli-responsive behavior of azahelicenes, providing strategic design avenues for next-generation chiral optoelectronic materials.

In 2023, Langer’s group synthesized a series of double aza[4,6]helicenes 18a–l featuring diverse peripheral substituents through a one-pot, multistep synthetic protocol [31] (Table 5). Selected compounds such as 18b, 18c, 18d and 18l exhibit similar λ_abs around 410 nm and emit fluorescence centered near 530 nm, demonstrating consistent optical profiles despite structural variation. In a parallel effort, Yang’s group developed an efficient, enantioselective synthetic approach toward azahelicenes via a chiral phosphoric acid-catalyzed multicomponent Povarov reaction or oxidative aromatization [32]. Among the synthesized compounds, compound 19 displayed dual absorption bands at 260 and 325 nm and emission peaks at 420 and 440 nm, which red-shifted to approximately 500 nm upon trifluoroacetic acid treatment. Both the neutral and protonated forms of 19 exhibited mirror-image CD and CPL spectra, with high |g_lum| values of 1.4 × 10⁻³ and 1.3 × 10⁻³, respectively, underscoring their potential for responsive chiral optoelectronic applications. Concurrently, Liu [33] and Ishigaki’s [34] groups independently reported a class of highly twisted nitrogen-doped heptalene derivatives (e.g., compound 20a), which exhibit consistent absorption at 315 nm and blue fluorescence centered near 450 nm, regardless of the substituents. These compounds display redox and electronic behaviors reminiscent of nitrogen-doped azulenes, featuring strong absorption dissymmetry factors (|g_abs|) at 345 nm – 1.2 × 10⁻² for compound 20a, 1.0 × 10⁻² for 20d, and 1.3 × 10⁻² for 20e (Table 5). Notably, the radical cation form of compound 20e (20e^•+) exhibits pronounced CD signals extending into the near-infrared region, suggesting potential for redox-responsive chiral photonic systems.

In 2023, Chen’s group reported three nitrogen–nitrogen (NN)-embedded azahelicenes 21a–c, among which compound 21c, a structurally defined antiaromatic double aza[7]helicene – exhibited distinctive long-wavelength optical and chiroptical properties [35] (Table 6). In the solid state, 21c emitted in the far-red region at 641 nm (Φ_F = 0.10) and demonstrated CPL with |g_lum| = 2.04 × 10⁻⁴. In solution, 21c showed a strong absorption band at 560 nm and a high Ф_F value of 0.86 at 583 nm, yielding a B_CPL value of 13.2 M⁻¹ cm⁻¹. Notably, compound 21c undergoes reversible redox interconversion to its radical cation 21c^•+ and dicationic 21c²⁺ states via chemical oxidation, enabling controllable switching between antiaromatic and aromatic configurations. These results provide a compelling strategy for engineering redox-switchable chiral luminophores. In 2024, the same research group expanded on this redox-responsive platform by constructing a polycationic open-shell cyclophane 22, comprising carbazole-embedded aza[7]helicene subunits [36]. Compound 22 displays intense fluorescence (Φ_F = 0.99), exceptionally high B_CPL as 100.2 M⁻¹ cm⁻¹, and marked chiroptical activity (|g_abs| = 2.50 × 10⁻³ at 435 nm; |g_lum| = 5.00 × 10⁻³ at 460 nm) (Table 6). Upon mild oxidation, neutral 22 undergoes stepwise conversion into highly charged, multispin open-shell species 22^2+2• and 22^4+4•, preserving strong chiroptical signals. This study presents a novel approach to constructing stable, redox-switchable chiral luminophores based on extended azahelicene architectures, offering broad potential for molecular electronics and spintronic devices.

In 2024, Qiu’s group synthesized π-extended diaza[7]helicenes 23a–f incorporating dual heptagonal rings [37]. Compound 23a exhibits dynamic chirality, aggregation-induced emission (AIE), and intense CPL (|g_lum| = 1.7 × 10⁻²), whereas compound 23f, with lateral π-extension, shows enhanced thermal stability and green emission at 517 nm (Table 7). Kuehne and co-workers reported two radical aza[7]helicenes, 24a and 24b, exhibiting distinct photophysical behaviors [38]. Compound 24b features a higher PLQY (0.43), while 24a demonstrates doublet-state CPL (|g_lum| = 5.0 × 10⁻⁴), highlighting the potential of helicene radicals for spintronic applications. Meng’s group synthesized carbonyl-nitrogen embedded hetero[7]helicenes 25a and 25b bearing axial chirality [39]. Compound 25a displays excellent optical characteristics with Φ_F = 0.57, |g_abs| = 1.7 × 10⁻², |g_lum| = 1.4 × 10⁻³, and a B_CPL of 8.94 M⁻¹ cm⁻¹. Then, Chen’s group contributed triple aza[6]helicenes 26a and 26b with |g_lum| values of approximately 3.0 × 10⁻³, offering new architectures for CPL-active helicenes [40]. Singh’s group developed fluorophore-conjugated aza[7]helicenes 27a–d, with 27b demonstrating pronounced intramolecular charge transfer (ICT), a high Φ_F of 0.71 and an extended fluorescence lifetime (τ) of 15.5 ns [41]. Wu’s group synthesized a family of expanded azahelicenes 28a–e, where increasing helical length leads to red-shifted emission, prolonged lifetime, and attenuated PLQY [42]. Nonetheless, these compounds exhibit outstanding chiroptical performance, with |g_abs|_max reaching 4.8 × 10⁻², |g_lum|_max = 2.1 × 10⁻², and B_CPL values up to 76 M⁻¹ cm⁻¹. Collectively, these investigations underscore the efficacy of heteroatom doping, extended π-conjugation, and radical design in advancing azahelicene-based systems. These approaches significantly enhance optical and chiroptical performance, paving the way for high-efficiency chiral optoelectronic and quantum materials.

In 2024, Kivala’s group selectively synthesized highly distorted [6]helicenes 29a and 29b incorporating azocine units via a regioselective Beckmann rearrangement from oxime precursor 29c [43] (Table 8). For comparative evaluation, the corresponding lactams 29d and 29e and amines 29f and 29g were also obtained. Compounds 29a and 29b exhibit λ_abs centered at 513 nm, while the amines 29f and 29g display high Ф_F values of 0.48 and 0.56, respectively. Notably, azocine derivative 29b exhibits the highest CPL activity among the series, with a |g_lum| value of 1.6 × 10⁻³. In addition, both 29a and 29b demonstrate redox activity, undergoing reversible formation of radical anions, dianions, and radical cations. The radical cation 29b^•+, in particular, exhibits a broad near-infrared (NIR) absorption band extending to 3000 nm, highlighting its potential for NIR optoelectronic applications. Building on this work, in 2025 the same group reported the synthesis of a stable N-heterotriangulene dimer (compound 30) bridged by a rigid π-conjugated [5]helicene [44]. This chiral dimer undergoes reversible stepwise oxidation to 30^•+ and 30²⁺, accompanied by pronounced NIR Cotton effects extending up to 2000 nm. These results provide critical insights into the rational design of redox-switchable, NIR-active chiral molecular systems, underscoring their promise in advanced optoelectronic and spintronic technologies.

In 2024, Tanaka’s group synthesized and characterized a series of length-variable aza[n]helicenes 31a–f via a one-pot intramolecular cyclodehydrogenation [45] (Table 9). Notably, compounds 31e and 31f represent the first examples of triple-layered heterohelicenes with fully conjugated frameworks. All members of the series demonstrate high solubility, attributed to intermolecular hydrogen bonding with solvent molecules. With increasing helical length, both the λ_abs and λ_em exhibit progressive bathochromic shifts, while the Ф_F values systematically decline, without clear saturation within the investigated range. Chiroptical measurements of the N-butylated aza[n]helicenes 31g–j reveal |g_abs| and |g_lum| values on the order of 10⁻³. These findings address long-standing challenges in the synthesis and stabilization of extended heterohelicenes, paving the way for the development of structurally persistent, π-extended chiral materials. In a parallel effort, Tanaka’s group synthesized benzannulated double aza[9]helicene 32a and its alkylated derivatives 32b and 32c via a one-pot oxidative fusion strategy [46]. Compared to the parent compound 32a (Φ_F = 0.07), compounds 32b and 32c exhibit significantly enhanced Φ_F (0.35), red-shifted absorption bands, and |g_abs| values of 2.4 × 10⁻³ and 2.3 × 10⁻³ at 345 nm, respectively. Their corresponding B_CPL values reach 16.0 and 19.2 M⁻¹ cm⁻¹. Furthermore, terminus-functionalized aza[9]helicenes 33a, 33b, and 33c were prepared to investigate interlayer interactions [47]. Among them, the pyrene-decorated compound 33c displays red-shifted emission and prolonged fluorescence lifetimes as solvent polarity increases, indicating enhanced excited-state stabilization. Collectively, these studies offer valuable strategies for stabilizing long π-extended helicenes and finely tuning their chiroptical and emissive properties, thereby advancing their application in multifunctional chiral photonic and sensing platforms.

In 2025, Gryko’s group synthesized a series of heterohelicenes 34a–c, featuring a 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) core [48] (Table 10). The compounds exhibit similar absorption and emission profiles. However, compound 34c stands out due to its pronounced solvatofluorochromism (λ_em = 546 nm, Φ_F = 0.42 in DMSO). Among the series, compound 34b exhibits the highest |g_lum| of 7.22 × 10⁻³, while compound 34c shows the greatest B_CPL as 29.3 M⁻¹ cm⁻¹. These studies underscore the importance of regioisomerism and molecular core design in optimizing the chiroptical and emissive properties of heteroatom-rich nanographenes, advancing their potential in next-generation optoelectronic and chiral photonic devices.

B,N-containing helicenes

Enhancing charge transfer between electron-donating and electron-accepting units, as well as extending π-conjugated frameworks, are widely employed strategies for achieving longer-wavelength emission in optoelectronic materials. Inspired by the electronic configuration of borazine, boron has emerged as a valuable electron-accepting counterpart to electron-donating nitrogen in conjugated systems, enabling the design of donor–acceptor helicenes with tunable photophysical properties.

In 2020, Ema and co-workers developed a series of chiral carbazole-based BODIPY analogues 35a–f, derived from helical carbazole-based BF₂ dyes [49] (Table 11). These analogues exhibit red-shifted emission and enhanced CPL compared to their carbazole-based helicene precursors. At λ_abs (≈500 nm), the compounds display |g_abs| values ranging from 1.1 × 10⁻³ to 3.1 × 10⁻³, Ф_F values of 20–36%, and |g_lum| values between 7.0 × 10⁻⁴ and 1.9 × 10⁻³. In a subsequent study, Ema’s group reported an N-containing hetero[7]helicene 36a containing a boron–nitrogen coordination site [50]. Its chiroptical properties could be modulated through the addition of tetrabutylammonium (TBA) salts, which transformed the boron center from a trigonal planar to a tetrahedral geometry, thereby enhancing the |g_lum| from 4.7 × 10⁻⁴ to 1.5 × 10⁻³ (OAc⁻, 36c) and 1.7 × 10⁻³ (F⁻/OH⁻, 36b/36d). Treatment with Ag⁺ ions reversed this coordination, restoring the neutral trigonal boron center and its initial optical characteristics. These findings underscore the potential of boron–nitrogen-embedded helicene frameworks as tunable chiral luminophores with reversible CPL modulation, offering promising strategies for the development of advanced molecular optoelectronic devices.

In 2021, Hatakeyama and co-workers developed an expanded B,N-containing heterohelicene 37 via a one-step synthesis employing excess BBr₃ at 180 °C in an autoclave, achieving a 44% yield [51] (Table 12). In a 1 wt % PMMA-dispersed film, compound 37 exhibited ultra-narrowband emission (FWHM = 16 nm) at 484 nm with an 80% PLQY. OLEDs based on 37 demonstrated excellent external quantum efficiency, current efficiency, and power efficiency. Duan and co-workers reported B,N-containing double hetero[7]helicenes 38a,b, which exhibited deep-red fluorescence emission at 662 and 692 nm, respectively, with narrow emission bandwidths (full width at half maximum, FWHM = 38 nm) and exceptional PLQYs of 100% [52]. Remarkably, they achieved maximum EQEs of 28.1% and 27.6%, representing the highest reported values for thermally activated delayed fluorescence (TADF) emitters operating above 650 nm. Shortly thereafter, Wang’s group reported a related series of B,N-containing compounds 38a–c, which displayed pronounced chiroptical activity in the visible region [53]. These compounds displayed the highest |g_abs| values recorded for helicenes to date – 0.033, 0.031, and 0.026 at 502, 518, and 526 nm, respectively. They also showed near-unity Ф_F values of 100%, 99%, and 90%, with corresponding λ_em at 660, 684, and 696 nm, and |g_lum| values of 2 × 10⁻³. The calculated B_CPL reached 28.5, 37.1, and 40.0 M⁻¹ cm⁻¹, positioning these helicenes among the most efficient red CPL emitters reported to date (Table 12).

However, such long-wavelength emission poses challenges for achieving optimal color purity in OLED devices. To overcome this limitation, Duan’s group subsequently introduced a covalent B–N bond into the helicene framework in 2023, affording compound 39 [54]. This material emits at 617 nm with a FWHM of 38 nm and maintains a near-unity PLQY. Circularly polarized OLEDs (CP-OLEDs) based on 39 exhibit outstanding device performance, achieving a |g_EL| of 1.91 × 10⁻³, a record-high EQE exceeding 36%, and operational stability with an LT₉₅ of approximately 400 h at 10,000 cd m⁻². These findings underscore the efficacy of B–N covalent integration in helicene-based frameworks for realizing high-efficiency, spectrally optimized, and robust red CP-OLED emitters.

In 2022, Yang and co-workers reported a W-shaped double hetero[5]helicene 40, incorporating boron, nitrogen, and sulfur atoms within its framework [55] (Table 13). Compound 40 exhibits exceptional photophysical and electroluminescent performance, including a PLQY value of 100% and a |g_lum| value of 2.1 × 10⁻³. Circularly polarized organic light-emitting diodes (CP-OLEDs) based on 40 demonstrated a |g_EL| of 2.2 × 10⁻³, a narrow emission bandwidth (FWHM = 49 nm), and a maximum external quantum efficiency (EQE) of 31.5%, placing it among the highest-performing multiple-resonance-induced thermally activated delayed fluorescence (MR-TADF) emitters to date. In 2023, the same group introduced the first deep-blue chiral MR-TADF emitters based on heterohelicene scaffolds 41a–c [56]. These compounds exhibited sharp emissions at 440–444 nm in solution and 445–449 nm in doped films, with emission bandwidths as narrow as 23 nm and PLQYs reaching up to 95%. Notably, racemic 41b and 41c displayed excellent chiroptical properties, with |g_lum| values ranging from 1.4 to 1.5 × 10⁻³ and B_CPL values exceeding 22 M⁻¹ cm⁻¹. Compound 41c, in particular, achieved a |g_EL| of 2.6 × 10⁻³ and a maximum luminance exceeding 10,000 cd m⁻². These findings underscore the significant potential of heteroatom-integrated helicene systems as high-efficiency, CPL-active MR-TADF materials for next-generation OLED technologies, particularly in the development of deep-blue emissive devices with high color purity and device efficiency.

In 2022, Marder and co-workers introduced various boryl substituents at both termini of a series of nitrogen-doped [5]helicenes, yielding helicenoids 42a–h [57] (Table 14). The Bpin-substituted derivatives 42a–e exhibited broad emission across the 400–800 nm range, whereas their analogues 42f and 42g showed negligible emission, indicating a strong dependence of photophysical behavior on boryl-substituent identity. Compared to their parent azahelicenes, these compounds displayed significantly larger Stokes shifts, highlighting the pronounced electronic effects of boryl incorporation. Notably, when a CF₃ group was introduced as a substituent on the azahelicene core, the resulting boryl-functionalized compound 42c exhibited an emission maximum at 563 nm in CH₂Cl₂, with a quantum yield of 15%, representing the highest emission efficiency observed among the boron-containing quasi-circulenes.

In 2022, Lu and co-workers developed a series of helical aza-BODIPY analogues 43a–h, featuring a distinctive B–O–B bridge installed within each molecule [58] (Table 15). These compounds display broad chiroptical responses extending from the ultraviolet to the entire visible spectrum – an uncommon characteristic among helicene-type systems. Among them, the phenyl-substituted aza[7]helicene 43f exhibits pronounced chiroptical activity, with |g_abs| and |g_lum| values reaching 3.04 × 10⁻³ and 1.30 × 10⁻³, respectively, and a high B_CPL of 11.5 M⁻¹ cm⁻¹ in the near-infrared region. In contrast, the corresponding aza[5]helicene analogue shows negligible chiral response, with |g_abs| and |g_lum| values in the 10⁻⁵ range. To further enhance chiroptical performance, Lu’s group introduced edge-positioned methyl and ethyl substituents into the helical core, affording 44a and 44b [59]. Compared with 43c, they are with significantly improved |g_abs| values of 1.51 × 10⁻³ and 1.69 × 10⁻³, respectively. This study underscores the critical importance of molecular design in modulating chiroptical properties and provides valuable insights into the development of helicene-based BODIPY systems for near-infrared CPL applications. In 2024, Shimizu’s group reported azabora[6]helicenes 45a and 45b [60]. However, their enantiomers could not be isolated due to low racemization barriers. The F- and Ph-coordinated derivatives displayed moderate PLQYs in solution (0.26 and 0.18, respectively), which dropped markedly in the solid state (0.02 and 0.04) owing to aggregation-caused quenching (ACQ).

In 2023, Yang and co-workers reported a pair of (NBN)₂-containing double and quadruple helicenes 46a–d [61] (Table 16). The neutral compounds exhibited high PLQYs of 99% and 65% in solution, and 90% and 55% in PMMA-doped films, respectively, with exceptionally narrow full-width (FWHM values as 24 nm and 22 nm). Stepwise titration experiments with fluoride ions induced a change in the coordination number of the boron centers from three to four, forming corresponding anionic species. This coordination triggered red-shifted absorption and CPL responses while maintaining excellent PLQYs – 99% and 90% in solution, and 80% and 77% in PMMA-doped films, respectively.

In 2024, Wang’s group developed a B,N-embedded hetero[8]helicene 47, exhibiting narrow green emission at 531 nm (FWHM = 36 nm), a high PLQY of 93%, and outstanding CP-OLED performance (EQE = 32.0%; |g_EL| = 7.74 × 10⁻⁴) [62] (Table 17). Bin’s group introduced orthogonal spiro-structures into hetero[6]helicenes 48a–c, achieving near-unity PLQYs in solution (up to 99%) and OLED external quantum efficiencies (EQEs) exceeding 31% [63]. Chen’s group reported 49, a B,N-containing hetero[9]helicene that emits at 578 nm with a PLQY of 98% and showing excellent chiroptical properties (|g_lum| = 5.8 × 10⁻³; B_CPL = 220.75 M⁻¹ cm⁻¹) [64]. OLEDs incorporating compound 49 demonstrated an EQE of 35.5% and |g_EL| = 6.2 × 10⁻³. Zhang’s group synthesized 50a–f, with and without installed heptagons [65]. The heptagon-containing derivatives showed red-shifted emission, broader FWHM, lower PLQYs, and diminished B_CPL values, indicating a trade-off between extended conjugation and emissive efficiency. Yin’s group introduced 1,4-BN motifs into compounds 51a and 51b, which emitted blue-green light at 474 and 465 nm, respectively, and exhibited moderate CPL activity (|g_lum| ≈ 5 × 10⁻⁴) [66] . OLEDs based on compound 51a emitted at 502 nm and achieved an EQE of 3.18%. Liu’s group positioned B and N atoms on the inner rim of 52a and 52b [67]. While 52b exhibited remarkably high |g_abs| and |g_lum| values (6.1 × 10⁻² and 2.4 × 10⁻², respectively), its PLQY was relatively low (24%). Further molecular optimization led to the development of compounds 53a–c, which demonstrated ultra-narrow emission bands (FWHM = 16–34 nm), high PLQYs (67–82%), and exceptional CPL brightness (B_CPLs of 583, 374, and 349 M⁻¹ cm⁻¹, respectively), with compound 53a setting a new record for BN-containing helicene CPL brightness [68]. These collective findings underscore the critical role of rational BN doping, π-conjugation engineering, and structural rigidity in precisely tuning the photophysical and chiroptical properties of helicene-based materials, thereby advancing the design of next-generation CPL-active optoelectronic systems with superior performance metrics.

However, these findings also suggest that boron may not always be the optimal choice for enhancing charge-transfer properties. The delocalization of electrons between the vacant p-orbital of boron and the electron-rich π-conjugated systems can diminish both the electron-accepting capability of boron and the electron-donating efficiency of the conjugated framework. Additionally, the inherently low electronegativity of boron further limits its effectiveness as an electron acceptor, thereby restricting the achievable red-shift in emission. To overcome these limitations, alternative electron-withdrawing atoms and functional groups have been introduced into nitrogen-doped helicene frameworks to improve their photophysical performance and extend emission into the longer wavelength region.

X,N-containing helicenes (X = O, S or Se)

Imide functional groups are well recognized for their strong electron-accepting character, making them valuable moieties in the design of optoelectronic materials. When incorporated into π-conjugated frameworks, imide groups can significantly modulate electronic structures and enhance properties such as fluorescence efficiency, charge transport, and chiroptical responses. In this section, we begin by summarizing representative imide-functionalized helicenes, highlighting their structural features and photophysical performances. In 2020, Ravat’s group introduced a novel class of helically chiral diimide molecules 54a–c, which integrate the favorable characteristics of arylene diimides within the chiral architecture of [n]helicenes [69]. These compounds exhibit varying PLQYs of 0.22, 0.02, and 0.12 for 54a, 54b, and 54c, respectively, and notably retain fluorescence in the solid state. The |g_abs| in the visible region increase systematically with helical length, reaching values as high as ≈10⁻² for compounds 54b and 54c – among the highest reported to date – highlighting their strong potential in chiral optoelectronic applications (Table 18). In 2023, the same group reported a stable push–pull [7]helicene diimide (compound 55) that exhibited notable chiroptical performance, with |g_abs| and |g_lum| values of 1.12 × 10⁻² and 5.0 × 10⁻³, respectively, in toluene [70]. Furthermore, compound 55 demonstrated solvent-dependent fluorescence and CPL behavior across the visible spectrum, with both emission intensity and chiroptical properties varying in response to solvent polarity. Concurrently, Würthner’s group developed two naphthalimide-annulated [n]helicenes, compounds 56a and 56b (n = 5, 6), via a concise two-step synthetic route that afforded excellent yields and notable photophysical properties [71]. Both helicenes display high Φ_F as 73% for 56a and 69% for 56b. Notably, compound 56b exhibits markedly enhanced |g_abs| and |g_lum| values of 2.1 × 10⁻³ and 2.3 × 10⁻³, approximately 4.5-fold greater than that of compound 56a. Its red CPL emission at 615 nm and high B_CPL of 66.5 M⁻¹ cm⁻¹ underscore its potential for advanced chiral photonic applications.

Heteroatom engineering in double helicenes has emerged as a powerful strategy for tuning chiroptical properties and excited-state dynamics. In 2021, Sakamaki’s group synthesized a novel double N,O-hetero[5]helicene (compound 57b) by coupling two 12H-benzo[b]phenoxazine (BPO) units and systematically compared it to its N,N-analogue (compound 57a) derived from 13H-dibenzo[b,i]phenoxazine (DBPO) scaffolds [72] (Table 19). Compound 57b was obtained in significantly higher yield and, like compound 57a, exhibited electron-rich character and compact molecular packing, both favorable for p-type transistor performance. Importantly, both helicenes displayed strong CPL in CH₂Cl₂, with |g_lum| values exceeding 10⁻². Intriguingly, the CPL signals of the two compounds exhibited opposite signs, underscoring the sensitivity of chiral excited-state properties to heteroatom substitution within the helicene framework. Extending this design principle, the group reported a double N,S-hetero[5]helicene 58 constructed from two benzo[b]phenothiazine units in 2023 [73]. Compared to the N,O-analogue 57b, this new compound showed more intense phosphorescence and an extended emission lifetime in dilute solution. Notably, it demonstrated room-temperature dual-emission CPL originating from both prompt fluorescence and long-lived phosphorescence, a rare feature in helicene systems. In a subsequent study, the same group reported a bis(N,Se)-hetero[4]helicene 59b and systematically compared its structural and dynamic properties with those of its sulfur analogue 59a [74]. Despite their close structural resemblance, the longer C–Se bond in 59b led to a markedly higher racemization barrier (145.7 vs 112.8 kJ/mol), thereby illustrating how subtle atomic substitutions can significantly influence the conformational stability of helical molecules (Table 19). These studies illustrate how precise heteroatom modulation enables fine control over CPL directionality and emission lifetimes, offering promising avenues for the development of multifunctional chiral optoelectronic materials – particularly those capable of simultaneous fluorescence and phosphorescence-based CPL.

Recently, thiadiazole-fused helicenes have gradually come into our view. In 2023, Hirose’s group synthesized a series of tetraazadithia[n]helicenes – 60a, 60b, and 60c – featuring 2,1,3-thiadiazole termini [75] (Table 20). Among them, compound 60c exhibited pronounced CPL activity in toluene (|g_lum| = 0.04, Φ_F = 3%), demonstrating the efficacy of terminal heterocycle incorporation for boosting chiroptical performance. In 2024, Babu and co-workers developed two π-extended hetero[6]helicenes – 61a and 61b – incorporating thiadiazole and selenadiazole moieties, respectively [76]. Substitution of sulfur with selenium enhanced intermolecular interactions and led to a notable reduction in the optical bandgap, highlighting the effectiveness of heteroatom modulation in tuning the electronic and photophysical properties of chiral nanographenes. These studies exemplify how strategic structural and electronic design – through π-extension, end-group heteroatom engineering, and atom-specific substitutions – enables precise tuning of chiroptical and photophysical properties in helicene-based materials, advancing their applicability in next-generation optoelectronic devices.

In 2020, Pittelkow’s group developed a unique synthetic strategy that converts a non-planar hetero[7]helicene into a planar hetero[8]circulene featuring an antiaromatic cyclooctatetraene (COT) core (62a–f) [77] (Table 21). Through controlled oxidation of the thiophene units to sulfones, they achieved a systematic red-shift in both absorption and emission spectra. Remarkably, the emission of these derivatives spans nearly the entire visible spectrum. These studies provide innovative molecular design strategies for constructing helically twisted or planarized chiral π-conjugated systems with tunable optical properties, thereby paving the way for the development of multifunctional materials in advanced photonic and electronic technologies.

In 2021, Viglianisi’s group synthesized a series of thia-bridged triarylamine[4]helicene-functionalized polynorbornenes 63a–c via ring-opening metathesis polymerization (ROMP), introducing helicene chirality into polymer backbones with tunable electrochromic behavior [78]. These polymers exhibit reversible pH-responsive color changes. For instance, 63a transitions from pale yellow to deep blue in the solid state upon exposure to TFA, while 63b and 63c in CH₂Cl₂ exhibit new absorption bands at 570 and 575 nm, respectively – reversibly decolorized upon triethylamine treatment (Table 22). This work demonstrates the potential of helicene-containing polymers as stimuli-responsive chiral electrochromic materials. In the same year, You’s group developed a transition-metal-catalyzed C–H/C–H-type regioselective C3-arylation of benzothiophenes using molecular oxygen as the oxidant [79]. This strategy afforded the TADF-active compound 64a, which exhibits efficient blue emission and excellent OLED performance with a maximum EQE of 25.4%. This example highlights the utility of helicene-related heteroaromatic frameworks in the design of high-efficiency emissive materials. Also in 2021, Ema’s group reported a concise Scholl-type cyclodehydrogenation strategy for synthesizing azahelicenes and diaza[8]circulenes 65a–d [24] (Table 22). These molecules exhibited distinct Cotton effects and CPL, with |g_lum| reaching up to 1.6 × 10⁻³. This approach offers a generalizable route to structurally diverse chiral polycyclic aromatic hydrocarbons (PAHs) with strong chiroptical responses. Concurrently, Tanaka’s group achieved the enantioselective synthesis of aza[6]- and aza[7]helicene-like molecules via Rh(I)/chiral bisphosphine-catalyzed [2 + 2 + 2] cycloaddition [80]. The resulting S-shaped double aza[6]helicene-like compound 66 displayed high enantiomeric excess (up to 89% ee), pronounced chiroptical activity (|g_abs| = 0.0054–0.0056), and substantial Φ_F of 0.21–0.32 under both neutral and acidic conditions. This work exemplifies the power of transition-metal catalysis for constructing enantioenriched helicenes with tunable photophysical properties. These contributions from 2021 underscore the synthetic versatility and functional diversity of helicene-based systems, spanning electrochromism, thermally activated delayed fluorescence, and circularly polarized luminescence. Such structural innovations provide valuable frameworks for the development of next-generation chiral optoelectronic materials.

In 2022, Furuta’s group developed a one-pot synthetic protocol to access (NH)-phenanthridinone derivatives and chiral amide-functionalized [7]helicene-like molecules 67a,b from biaryl dicarboxylic acids, employing a Curtius rearrangement followed by basic hydrolysis [81] (Table 23). Notably, when chalcogen-containing substrates were used, the process afforded phosphorus ester derivatives of aza[5]helicenes. The chiral nature of the products was confirmed by optical rotation and CD measurements. In parallel, Soni’s group established an efficient three-step synthesis of coumarin-containing hetero[5]- and [6]helicene-like structures 68a–g in high yields [82]. These compounds display diverse photophysical behaviors: compound 68d emits yellow fluorescence in both solution and solid state, exhibiting solvatofluorochromism due to a twisted intramolecular charge transfer (TICT) mechanism, while compound 68e emits blue light (Φ_F = 0.37) and demonstrates pronounced AIE in the solid state. Concurrently, Jiang’s group reported 69b, the first hetero[4]helicene-type molecule exhibiting both CPL and TADF [83]. This compound displays a high Φ_F of 0.51 and a |g_lum| of 1.2 × 10⁻³. OLED devices fabricated using 69b emit sky-blue light with a peak EQE of 10.6% and |g_EL| values up to 1.6 × 10⁻³. Collectively, these studies demonstrate the versatility of helicene-inspired architectures for constructing multifunctional chiral optoelectronic materials, highlighting their growing relevance in next-generation circularly polarized OLED technologies.

Takizawa and co-workers have pioneered electrochemical strategies for synthesizing structurally diverse hetero[7]helicenes with tunable chiroptical properties and excellent configurational stability. In 2022, they introduced two electrochemical routes to construct aza-oxa-dehydro[7]helicenes, yielding helicenes with high racemization barriers and notable chiral stability [84]. The quasicirculenes 70a and 70b demonstrated strong blue CPL activity, with |g_lum| values of 2.5 × 10⁻³ at 433 nm and 2.4 × 10⁻³ at 418 nm, respectively (Table 24). Building on this, the team achieved the enantioselective synthesis of heterodehydrospiroenes on a gram scale using chiral vanadium(V) complexes – marking a significant advancement in asymmetric electrochemical catalysis. In a complementary study that same year, they reported a two-step electrochemical synthesis of a double aza-oxa[7]helicene via oxidative coupling followed by dehydrative cyclization [85]. The resulting meso-isomer (P,M)-71 emerged as the major product, exhibiting dual emission bands at 415 and 440 nm and solvent-independent absorption at 407 nm. Expanding the structural diversity, the group developed a two-pot synthesis of unsymmetrical hetero[7]helicenes 72a–g in 2023 [86], employing p-benzoquinone and N-aryl-2-naphthylamines through acid-promoted cyclization followed by electrochemical domino reactions. This method produced six compounds with yields ranging from 33–45%, all featuring extended π-conjugation and distinct photophysical characteristics. Furthermore, they established a mild electrochemical protocol for synthesizing oxaza[7]helicenes incorporating pyrrole and furan units [87]. This method afforded products in 50–86% yield with Faradaic efficiencies up to 77%. Among them, derivative 73 exhibited CPL activity (|g_lum| = 3.0 × 10⁻⁴), showcasing the ability to modulate chiroptical responses via heteroatom integration. These studies underscore the versatility of electrochemical synthesis in enabling precise structural modulation of heterohelicenes, facilitating access to high-performance chiral optoelectronic materials.

In 2023, Zhang’s group introduced a new class of helically chiral double hetero[4]helicenes 74a and 74b exhibiting CP-TADF, constructed on a distinct donor–acceptor core architecture [88] (Table 25). These compounds demonstrate excellent configurational stability and robust CPL signals both in solution and in solid-state films, with a |g_lum| of 3.1 × 10⁻³. Corresponding CP-OLEDs based on compound 74a achieved outstanding device performance, reaching a maximum EQE of 20.03% and a |g_EL| of 2.9 × 10⁻³ – underscoring their considerable potential for advanced chiral optoelectronic applications. Building upon this framework, in 2024, the same group developed a novel cove-region bridging strategy to construct double hetero[4]helicenes with enhanced structural rigidity and persistent chirality [89]. By selectively modifying the bay regions of the SPZ (spiro[fluorene-9,9'-xanthene]) scaffold, they successfully converted initially non-emissive helicenes into efficient TADF luminophores with tunable emission wavelengths ranging from sky-blue to deep red. Particularly, the enantiomeric forms of the 75b derivatives emerged as rare examples of red-emissive CPL materials. This innovative design approach offers a versatile and modular platform for engineering chiral multi-helicene systems with customizable optoelectronic properties, paving the way for their deployment in next-generation CPL-active materials and high-performance CP-OLED devices.

In 2024, Jančařík and co-workers introduced an intramolecular radical cyclization strategy to synthesize highly luminescent tetraceno[6]helicenone and its aza analogue 76 [90] (Table 26). The incorporation of a carbonyl group into the helicene backbone substantially enhanced fluorescence quantum yields and red-shifted the emission into the visible region. The aza analogue demonstrated promising performance in OLEDs, confirming its potential for optoelectronic applications. Concurrently, Shirinian’s group synthesized a series of nitrogen-functionalized quinoline (NFQ)-based aza-oxa[5]helicenes 77a–f exhibiting excellent UV stability and solvent-dependent fluorescence [91]. Protonation significantly enhanced their emission intensity, and the presence of nitrogen facilitated further structural derivatization. In the same year, Alcarazo’s group reported an enantioselective gold-catalyzed synthesis of compound 78, achieving a high enantiomeric excess [92]. They further investigated various post-synthetic modification strategies, demonstrating their potential for application in chiral photonic materials. Collectively, these advances underscore the power of structural tailoring, heteroatom incorporation, and enantioselective strategies in finely tuning the photophysical and chiroptical properties of helicenes, providing a versatile foundation for the development of high-performance chiral optoelectronic materials.

Conclusion

Nitrogen-doped helicenes and their heteroatom co-doped analogues constitute a rapidly advancing class of chiral π-conjugated materials, distinguished by exceptional structural tunability, photophysical diversity, and chiroptical functionality. The integration of nitrogen – and its synergistic pairing with heteroatoms such as boron, oxygen, sulfur, and selenium – has significantly expanded the molecular design space, enabling precise control over redox behavior, emission wavelength, CPL, and responsiveness to thermal or redox stimuli. These heteroatom modifications have led to remarkable breakthroughs, including near-unity PLQYs, ultranarrow emission bands, |g_lum| values exceeding 10⁻³, and unprecedented B_CPL, particularly in the visible to near-infrared (NIR) spectral regions.

Recent advances in synthetic methodology – including electrochemical, Scholl-type, and enantioselective catalytic strategies – have further enabled access to structurally complex helicene topologies with enhanced configurational stability and integrated multifunctionality. These developments have facilitated a growing range of applications in CP-OLEDs, molecular sensing, chiral switches, and photonic devices. Moving forward, key challenges remain, such as mitigating spectral broadening in red/NIR emission, enhancing the chemical and photostability of electron-deficient helicenes, and developing sustainable, scalable synthetic approaches. The integration of computational design with multifunctional molecular engineering is expected to accelerate the deployment of helicene-based materials in next-generation technologies spanning chiral optoelectronics, bioimaging, spintronics, and quantum information science.