NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

Non-Alternant Nanographenes Bearing N-Doped Non-Hexagonal Pairs: Synthesis, Structural Analysis and Photophysical Properties

Introduction of non-hexagons and/or heteroatoms allows for finely tuning the physicochemical properties of nanographenes. Heteroatoms doping have dominated the modulation of nanographenes with tunable band gap, rich electrochemical activities and so on. The pair of non-hexagons, for instance, pentagon-heptagon pairs, have furnished nanographenes with aromatic and/or antiaromatic characteristics, open-shell properties and so on. In order to meet the growing demand for versatile nanographenes in materials science, research on novel nanographenes with heteroatom doped non-hexagonal pairs has been aroused in recent years. In this review, we focus on nanographenes with nitrogen-doped non-hexagonal paris including the synthesis, structure analysis, photophysical properties, and potential applications in organic devices.

Perylene with Split-Azulene Embedding

Splitting the five and seven-membered rings of azulene and embedding them separately into a conjugated backbone provides azulene-like polycyclic aromatic hydrocarbons (PAHs), which are of great interest in quantum and material chemistry. However, the synthetic accessibility poses a significant challenge. In this study, we present the synthesis of a novel azulene-like PAH, Pery-57, which can be viewed as the integration of a perylene framework into the split azulene. The compact structure of Pery-57 displays several intriguing characteristics, including NIR II absorption at 1200 nm, a substantial dipole moment of 3.5 D, and head-to-tail alternating columnar packing. Furthermore, Pery-57 exhibits remarkable redox properties. The cationic radical Pery-57⋅+ readily captures a hydrogen atom. Variable-temperature NMR (VT NMR ) and variable-temperature EPR (VT-EPR) studies reveal that the dianion Pery-572- possesses an open-shell singlet ground state and demonstrates significant global anti-aromaticity. The dication Pery-572+ is also predicted to exhibit diradical character. Despite bearing three bulky substituents, Pery-57 displays p-type transport characteristics with a mobility of 0.03 cm2 V-1 s-1, attributed to its unique azulene-like structure. Overall, this work directs interest in azulene-like PAHs, a unique member of nonalternant PAHs showcasing exceptional properties and applications.

A nine-ring fused terrylene diimide exhibits switching between red TADF and near-IR room temperature phosphorescence

Herein, we report the first example of a terrylene diimide derivative that switches emission between thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) in the red region. By design, the molecule TDI-cDBT boasts a symmetrical, consecutively fused nine-ring motif with a kite-like structure. The rigid core formed by the annulated dibenzothiophene moiety favoured efficient intersystem crossing and yielded a narrow-band emission with a full-width half maxima (FWHM) of 0.09 eV, along with high colour purity. A small ΔE S1-T1 of 0.04 eV facilitated thermally activated delayed fluorescence, enhancing the quantum yield to 88% in the red region. Additionally, it also prefers a direct triplet emission from the aggregated state. The room temperature phosphorescence observed from the aggregates has a longer emission lifetime of 1.8 ms, which is further prolonged to 8 ms at 77 K in the NIR region. Thus, the current strategy is successful in not only reducing ΔE S1-T1 to favour TADF but also serves as a novel platform that can switch emission from TADF to RTP depending upon the concentration.

Axially Chiral Nonbenzenoid Nanographene with Second Harmonic Generation Property

Chiral nanographenes (NGs) have garnered significant interest as optoelectronic materials in recent years. While helically chiral NGs have been extensively studied, axially chiral NGs have only witnessed limited examples, with no prior reports of axially chiral nonbenzenoid NGs. Herein we report an axially chiral nonbenzenoid nanographene featuring six pentagons and four heptagons. This compound, denoted as 2, was efficiently synthesized via an efficient Pd-catalyzed aryl silane homocoupling reaction. The presence of two bulky 3,5-di-tert-butylphenyl groups around the axis connecting the two nonbenzenoid PAH (AHR) segments endows 2 with atropisomeric chirality and high racemization energy barrier, effectively preventing racemization of both R- and S-enantiomers at room temperature. Optically pure R-2 and S-2 were obtained by chiral HPLC separation, and they exhibit circular dichroism (CD) activity at wavelengths up to 660 nm, one of the longest wavelengths with CD responses reported for the chiral NGs. Interestingly, racemic 2 forms a homoconfiguration π-dimer in the crystal lattice, belonging to the I222 chiral space group. Consequently, this unique structure renders crystals of 2 with a second harmonic generation (SHG) response, distinguishing it from all the reported axially chiral benzenoid NGs. Moreover, R-2 and S-2 also exhibit SHG-CD properties.

Cycl[2,2,4]azine-embedded non-alternant nanographenes containing fused antiaromatic azepine ring

The development of non-alternant nanographenes has attracted considerable attention due to their unique photophysical properties. Herein, we reported a novel aza-doped, non-alternant nanographene (NG) 1 by embedding the cycl[2,2,4]azine unit into the benzenoid NG framework. Single-crystal X-ray diffractometry suggests saddle or twisted nonplanar geometry of the entire backbone of 1 and coplanar conformation of the cycl[2,2,4]azine unit. DFT calculation together with solid structure indicates that NG 1 possesses significant local antiaromaticity in the azepine ring. By oxidative process or trifluoroacetic acid treatment, this nanographene can transform into a mono-radical cation, which was confirmed by UV/Vis absorption, 1H NMR, and electron paramagnetic resonance (EPR) spectroscopy. The antiaromaticity/aromaticity switching of the azepine ring on 1˙+ from 1 enables the high stability of this radical cation, which remained intact for over 1 day. Due to the electron-donating nature of the nitrogen and the unique electronic structure, NG 1 exhibits strong electron-donating properties, as proved by the intermolecular charge transfer towards C60 with a high association constant. Furthermore, selective modification of NG 1 was accomplished by Vilsmeier reaction, and the derivatives 7 and 8 with substituted benzophenone were obtained. The photophysical and electronic properties can be tuned by the introduction of different electronic groups in benzophenone.

π-Extended benzo[1,2:4,5]di[7]annulene bis(dicarboximide)s - a new class of non-alternant polycyclic aromatic dicarboximides

Aromatic dicarboximides are a class of molecules represented by the well-known rylene bis(dicarboximide)s, in particular perylene or naphthalene bis(dicarboximide)s, which show pronounced optoelectronic properties and are applied as color pigments, fluorescent dyes and organic semiconductors. Herein we extend the family of aromatic bis(dicarboximide)s and report the synthesis of the first series of non-alternant aromatic dicarboximides by twofold Pd-catalyzed [5 + 2] annulation. Characterization by UV/vis spectroscopy and cyclic voltammetry (CV) measurements give insight into the optoelectronic characteristics of the hitherto unexplored substance class of heptagon-containing imides. Theoretical studies by nucleus independent chemical shift (NICS) XY-scans and anisotropy of the induced current density (ACID) plots demonstrate the influence of both the non-alternant carbon framework and the imide moieties on aromaticity of the synthesized bisimides.

Diazulenorubicene as a Non-benzenoid Isomer of peri-Tetracene with Two Sets of 5/7/5 Membered Rings Showing Good Semiconducting Properties

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received a lot of attention because of their unique optical, electronic, and magnetic properties, but their synthesis remains challenging. Herein, we report a non-benzenoid isomer of peri-tetracene, diazulenorubicene (DAR), with two sets of 5/7/5 membered rings synthesized by a (3+2) annulation reaction. Compared with the precursor containing only 5/7 membered rings, the newly formed five membered rings switch the aromaticity of the original heptagon/pentagon from antiaromatic/aromatic to non-aromatic/antiaromatic respectively, modify the intermolecular packing modes, and lower the LUMO levels. Notably, compound 2 b (DAR-TMS) shows p-type semiconducting properties with a hole mobility up to 1.27 cm2  V-1  s-1 . Moreover, further extension to larger non-benzenoid PAHs with 19 rings was achieved through on-surface chemistry from the DAR derivative with one alkynyl group.

Merging of Azulene and Perylene Diimide for Optical pH Sensors

Polycyclic aromatic hydrocarbons (PAHs) have emerged as promising materials for organic electronics, including organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Particularly, non-hexagonal ring-fused PAHs are highly desirable due to their unique optoelectronic properties. Herein, a new redox-active azulene-perylene diimide triad 1 and its ring-fused counterpart, diazulenocoronene diimide 2, were synthesized and fully characterized by a combination of NMR, cyclic voltammetry, and UV-visible absorption spectroscopy. Direct comparison of their electronic properties leads us to the conclusion that a significant change in the localization of HOMO and LUMO occurs upon the fusion of azulene and perylene diimide in 2, leading to the lack of intramolecular charge-transfer character for transitions in the visible spectral region. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to gain further insight into various electronic transitions. Moreover, we found that the adaptive response to acids and bases manifests itself in a reversible two-color change that can be attributed to changes in the chemical structures. Our findings pave the way for manipulating the relative HOMO and LUMO energy levels of organic chromophores by fusing non-alternant azulenes to an otherwise flat PAH, which could possibly lead to applications in organic electronics and optical sensors.

Two-Step Synthesis of B2 N2 -Doped Polycyclic Aromatic Hydrocarbon Containing Pentagonal and Heptagonal Rings with Long-Lived Delayed Fluorescence

Pentagon-heptagon embedded polycyclic aromatic hydrocarbons (PAHs) have aroused increasing attention in recent years due to their unique physicochemical properties. Here, for the first time, this report demonstrates a facile method for the synthesis of a novel B2 N2 -doped PAH (BN-2) containing two pairs of pentagonal and heptagonal rings in only two steps. In the solid state of BN-2, two different conformations, including saddle-shaped and up-down geometries, are observed. Through a combined spectroscopic and calculation study, the excited-state dynamics of BN-2 is well-investigated in this current work. The resultant pentagon-heptagon embedded B2 N2 -doped BN-2 displays both prompt fluorescence and long-lived delayed fluorescence components at room temperature, with the triplet excited-state lifetime in the microsecond time region (τ = 19 µs). The triplet-triplet annihilation is assigned as the mechanism for the observed long-lived delayed fluorescence. Computational analyses attributed this observation to the small energy separation between the singlet and triplet excited states, facilitating the intersystem crossing (ISC) process which is further validated by the ultrafast spectroscopic measurements.

Constructing 1-Ethoxyphenanthro[9,10-e]acephenanthrylene for the Synthesis of a Polyaromatic Hydrocarbon Containing a Formal Azulene Unit

peri-Acenoacenes are attractive synthetic targets, but their non-benzenoid isomeric counterparts were unnoticed. 1-Ethoxyphenanthro[9,10-e]acephenanthrylene 8 was synthesized and converted to azulene-embedded 9, which is a tribenzo-fused non-alternant isomeric motif of peri-anthracenoanthracene. Aromaticity and single-crystal analyses suggested a formal azulene core for 9, which showed a smaller highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap with a charge-transfer absorption band and brighter fluorescence than 8 (quantum yield (Φ): 9 = 41.8%, 8 = 8.9%). The reduction potentials of 8 and 9 were nearly identical, and the observations were further supported by density functional theory (DFT) calculations.

Construction of a 2,2'-biazulene framework via Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans

Dibenzofurans featuring a 2,2'-biazulene framework were prepared in good yields by Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans in 100% H₃PO₄. NMR, UV-Vis, and fluorescence spectroscopies were used to investigate the structural and optical properties of the products prepared. Remarkably, the annulated products exhibited fluorescence, with the longest wavelength of azulene derivatives reported to date, which extended into the near-infrared region under acidic conditions.

Photostable NIR-II Pigments from Extended Rylenecarboximides

A series of near-infrared (NIR) organic absorbers, named FNs and FPs, have been obtained with absorption maxima from 870 nm to 1100 nm and thus falling into the attractive second near-infrared region (NIR-II). The synthesis of their extended aromatic cores utilized an initial aryl-amination between 4-aminonaphthalene-1,8-dicarboximide (NMI-NH2 ) or 9-aminoperylene-3,4-dicarboximide (PMI-NH2 ) with chloro-substituted 9,10-anthraquinones followed by a novel base-induced cyclodehydrogenation. A NIR-II pigment, compound FPP, was obtained through de-alkylation of a soluble precursor. The synthesis of this photostable pigment is high-yielding and avoids column chromatographic purification which is important for many applications.

Tridecacyclene Tetraimide: An Easily Reduced Cyclooctatetraene Derivative

Tridecacyclene tetraimide, TCTI, an electron-deficient non-benzenoid nanocarbon with a C56 N4 polycyclic framework was obtained in a concise synthesis. TCTI has a non-planar structure and forms π-stacked dimers in the solid state. In solution, it undergoes eight single-electron reductions, yielding a range of negatively charged states up to an octaanion. Except for the latter species, which has a remarkably large electronic gap, the anions feature extended near-infrared absorptions, with a particularly strong band at 1692 nm observed for the dianion. A computational analysis of the TCTI anions shows that their stability originates from the combined effects of electron-deficient imide groups and the local aromaticity of reduced acenaphthylene units. The properties of TCTI make it potentially useful in electrochromic and charge storage applications.

Naphthalimide-Fused Dipyrrins: Tunable Halochromic Switches and Photothermal NIR-II Dyes

A family of tunable halochromic switches is developed using a naphthalimide-fused dipyrrin as the core π-conjugated motif. Electronic properties of these dipyrrins are tuned by substitution of their alpha and meso positions with aryl groups of variable donor-acceptor strength. The first protonation results in a conformational change that enhances electronic coupling between the dipyrrin chromophore and the meso substituent, leading to halochromic effects that occasionally exceed 200 nm and switch the absorption between the near-infrared (NIR)-I and NIR-II ranges. A NIR-II photothermal effect, switchable by acid-base chemistry is demonstrated for selected dipyrrins. Further protonation is possible for derivatives bearing additional amino groups, leading to up to four halochromic switching step. The most electron-rich dipyrrins are also susceptible to chemical oxidation, yielding NIR-absorbing radical cations and closed-shell dications.

2,6-Azulene-based Homopolymers: Design, Synthesis, and Application in Proton Exchange Membrane Fuel Cells

Azulene-based homopolymers are of great interest from the point view of chemistry and material science. Herein, by means of Friedel-Crafts acylation to introduce solubilizing chains on the 1-position of azulene, we designed and synthesized two examples of 2,6-azulene-based homopolymers RP(Az-AC16) and P(Az-AC16). The arrangement of 2,6-azulene units is irregular for RP(Az-AC16), while P(Az-AC16) has head-to-head/tail-to-tail arranged 2,6-azulene units. Proton-responsive studies demonstrate that RP(Az-AC16) and P(Az-AC16) show reversible proton responsiveness in both solution and thin film. To utilize the dynamically reversible proton-responsive property of these polymers in thin films, RP(Az-AC16) and P(Az-AC16) were incorporated into a Nafion matrix as proton exchange membranes, wherein the Nafion/P(Az-AC16) composite membrane exhibits significant increases in proton conductivity relative to the Nafion membrane at different temperatures of each relative humidity (RH), which further results in a 64% improvement in hydrogen fuel cell output power under 30% RH at 80 °C. Our studies have realized the first solution synthesis of 2,6-azulene-based homopolymers and the first application of azulene-based π-systems in hydrogen fuel cells.

Bis-periazulene (Cyclohepta[def]fluorene) as a Nonalternant Isomer of Pyrene: Synthesis and Characterization of Its Triaryl Derivatives

Bis-periazulene (cyclohepta[def]fluorene), which is an unknown pyrene isomer, was synthesized as kinetically protected forms. Its triaryl derivatives 1c-e exhibited the superimposed electronic structures of peripheral, polarized, and open-shell π-conjugated systems. In contrast to previous theoretical predictions, bis-periazulene derivatives were in the singlet ground state. Changing an aryl group controlled the energy gap between the lowest singlet-triplet states.

Perylene Five-membered Ring Diimide for Organic Semiconductors and π-Expanded Conjugated Molecules

A perylene five-membered ring diimide PDI39 was developed as a new electron-deficient building block for n-type semiconductors. The π-expanded conjugated molecules entailing azulenes were synthesized from PDI39. These conjuagted molecules...

Controlled Asymmetric Charge Distribution of Active Centers in Conjugated Polymers for Oxygen Reduction

Active center reconstruction is essential for high performance oxygen reduction reaction (ORR) electrocatalysts. Usually, the ORR activity stems from the electronic environment of active sites by charge redistribution. We introduce an asymmetry strategy to adjust the charge distribution of active centers by designing conjugated polymer (CP) catalysts with different degrees of asymmetry. We synthesized asymmetric backbone CP (asy-PB) by modifying B←N coordination bonds and asymmetric sidechain CP (asy-PB-A) with different alkyl chain lengths. Both CPs with backbone and sidechain asymmetry exhibit superior ORR performance to their symmetric counterparts (sy-P and sy-PB). The asy-PB with greater asymmetry shows higher catalytic activity than asy-PB-A with relatively smaller asymmetry. DFT calculations reveal that the increased dipole moment and non-uniform charge distribution caused by asymmetric structure endows the center carbon atom of bipyridine with efficient catalytic activity.

Copper-Catalyzed 3-Positional Amination of 2-Azulenols with O-Benzoylhydroxylamines

A copper-catalyzed ortho-selective amination of 2-azulenols with O-benzoylhydroxylamines (RR'N-OBz) to synthesize ortho-aminoazulenols is reported. A wide range of functional groups on amines are compatible, furnishing the corresponding amino-azulene derivatives in moderate to good yields. The further synthetic elaboration using 3-amino-2-azulenols as starting materials is demonstrated.

Revisiting Acepleiadylene: Two-Step Synthesis and π-Extension toward Nonbenzenoid Nanographene

Acepleiadylene (APD), a nonbenzenoid nonalternant isomer of pyrene, exhibits different electronic properties from pyrene, but has been rarely studied since its first synthesis in 1956, probably due to the difficulties in synthesis and further derivatization. In this work, we revisited this long-known compound and developed a new two-step synthetic route to efficiently access APD on the gram scale. Theoretical and experimental characterizations elucidated the unique properties of APD as compared with its benzenoid isomer pyrene, particularly revealing its dipolar structure with a narrow optical gap. The functionalization of APD was demonstrated for the first time, providing doubly brominated APD as a key precursor for further π-extension. As a proof of concept, a π-extended APD and a cyclotrimer nanographene (C₄₈H₂₄) were constructed, opening up new avenues to nonbenzenoid nanographenes with low HOMO-LUMO gaps.

C4-aldehyde of guaiazulene: synthesis and derivatisation

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

Defective Nanographenes Containing Seven-Five-Seven (7-5-7)-Membered Rings

Defects have been observed in graphene and are expected to play a key role in its optical, electronic, and magnetic properties. However, because most of the studies focused on the structural characterization, the implications of topological defects on the physicochemical properties of graphene remain poorly understood. Here, we demonstrate a bottom-up synthesis of three novel nanographenes (1-3) with well-defined defects in which seven-five-seven (7-5-7)-membered rings were introduced to their sp² carbon frameworks. From the X-ray crystallographic analysis, compound 1 adopts a nearly planar structure. Compound 2, with an additional five-membered ring compared to 1, possesses a slightly saddle-shaped geometry. Compound 3, which can be regarded as the "head-to-head" fusion of 1 with two bonds, features two saddles connected together. The resultant defective nanographenes 1-3 were well-investigated by UV-vis absorption, cyclic voltammetry, and time-resolved absorption spectra and further corroborated by density functional theory (DFT) calculations. Detailed experimental and theoretical investigations elucidate that these three nanographenes 1-3 exhibit an anti-aromatic character in their ground states and display a high stability under ambient conditions, which contrast with the reported unstable biradicaloid nanographenes that contain heptagons. Our work reported herein offers insights into the understanding of structure-related properties and enables the control of the electronic structures of expanded nanographenes with atomically precise defects.

Conformation and Aromaticity Switching in a Curved Non-Alternant sp2 Carbon Scaffold

A curved sp2 carbon scaffold containing fused pentagon and heptagon units (1) was synthesized by Pd-catalyzed [5+2] annulation from a 3,9-diboraperylene precursor and shows two reversible oxidation processes at low redox potential, accompanied by a butterfly-like motion. Stepwise oxidation produced radical cation 1.+ and dication 12+ . In the crystal structure, 1 exhibits a chiral cisoid conformation and partial π-overlap between the enantiomers. For the radical cation 1.+ , a less curved cisoid conformation is observed with a π-dimer-type arrangement. 12+ adopts a more planar structure with transoid conformation and slip-stacked π-overlap with closest neighbors. We also observed an intermolecular mixed-valence complex of 1⋅(1.+ )3 that has a huge trigonal unit cell [(1)72 (SbF6 )54 ⋅(hexane)101 ] and hexagonal columnar stacks. In addition to the conformational change, the aromaticity of 1 changes from localized to delocalized, as demonstrated by AICD and NICS(1)zz calculations.

Porphyrin-Ryleneimide Hybrids: Tuning of Visible and Near-Infrared Absorption by Chromophore Desymmetrization

Unsymmetrically fused porphyrins containing one or two naphthalimide subunits were prepared in modular syntheses relying on electron-rich and electron-poor pyrrole building blocks. These new chromophores show progressive changes in their electron-deficient character, while retaining comparably small optical and electrochemical band gaps. The intrinsic curvature and extended optical absorption of these systems make them of interest as mono- and difunctional components of multichromophoric assemblies.

NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

The first planar π-extended azulene that retains aromaticity of odd-membered rings was synthesized by [3+3] peri-annulation of two naphthalene imides at both long-edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π-extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD-DFT, as well as nucleus-independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X-ray crystallography confirmed the planarity of the reported π-scaffolds and aromaticity of their azulene moiety.