Oxa[7]superhelicene: A π-Extended Helical Chromophore Based on Hexa-peri -hexabenzocoronenes

Heavy-atom-free π-twisted photosensitizers for fluorescence bioimaging and photodynamic therapy

As the field of preclinical research on photosensitizers (PSs) for anticancer photodynamic therapy (PDT) continues to expand, a focused effort is underway to develop agents with innovative molecular structures that offer enhanced targeting, selectivity, activation, and imaging capabilities. In this context, we introduce two new heavy-atom-free PSs, DBXI and DBAI, characterized by a twisted π-conjugation framework. This innovative approach enhances the spin-orbit coupling (SOC) between the singlet excited state (S1) and the triplet state (T1), resulting in improved and efficient intersystem crossing (ISC). Both PSs are highly effective in producing reactive oxygen species (ROS), including singlet oxygen and/or superoxide species. Additionally, they also demonstrate remarkably strong fluorescence emission. Indeed, in addition to providing exceptional photocytotoxicity, this emissive feature, generally lacking in other reported structures, allows for the precise monitoring of the PSs' distribution within specific cellular organelles even at nanomolar concentrations. These findings underscore the dual functionality of these PSs, serving as both fluorescent imaging probes and light-activated therapeutic agents, emphasizing their potential as versatile and multifunctional tools in the field of PDT.

Cyclic Azahelicene Dimers Showing Bright Circularly Polarized Luminescence and Selective Fluoride Recognition

Although helicenes are promising molecules, the synthetic difficulty and tediousness have often been problems, and only small amounts of optically pure helicenes have been obtained by using chiral HPLC in most cases. Herein, aza[7]helicenes or closed-aza[7]helicenes with (1R)-menthyl substituents were selectively synthesized via the intramolecular Scholl reaction, and the diastereomeric pairs were separated by silica gel column chromatography. The optically pure helicenes were further transformed into the corresponding cyclic dimers, and the chiroptical properties were investigated. The rigid π-frameworks of the dimers led to the high molar extinction coefficients and fluorescence quantum yields, while the twisted helicene moieties induced clear Cotton effects and CPL in the visible region, and the high CPL brightness (BCPL) was achieved. Furthermore, the cyclic dimers were found to have the macrocyclic cavity with the two NH groups suitable for the selective binding of a fluoride anion, which induced significantly redshifted fluorescence and CPL in the red region.

Nonalternant Nanographenes Containing N-Centered Cyclopenta[ef]heptalene and Aza[7]Helicene Units

Introducing helical subunits into negatively curved π-systems has a significant effect on both the molecular geometry and photophysical properties; however, the synthesis of these helical π-systems embedded with nonbenzenoid subunits remains challenging due to the high strain deriving from both the curvature and helix. Here, we report a family of nonalternant nanographenes containing a nitrogen (N)-doped cyclopenta[ef]heptalene unit. Among them, CPH-2 and CPH-3 can be viewed as hybrids of benzoannulated cyclopenta[ef]heptalene and aza[7]helicene. The crystal structures revealed a saddle geometry for CPH-1, a saddle-helix hybrid for CPH-2, and a twist-helix hybrid for CPH-3. Experimental measurements and theoretical calculations indicate that the saddle moieties in CPHs undergo flexible conformational changes at room temperature, while the aza[7]helicene subunit exhibits a dramatically increased racemization energy barrier (78.2 kcal mol-1 for CPH-2, 143.2 kcal mol-1 for CPH-3). The combination of the nitrogen lone electron pairs of the N-doped cyclopenta[ef]heptalene unit with the twisted helix fragments results in rich photophysics with distinctive fluorescence and phosphorescence in CPH-1 and CPH-2 and the similar energy fluorescence and phosphorescence in CPH-3. Both enantiopure CPH-2 and CPH-3 display distinct circular dichroism (CD) signals in the UV-vis range. Notably, compared to the reported fully π-extended helical nanographenes, CPH-3 exhibits excellent chiroptical properties with a |gabs| value of 1.0 × 10-2 and a |glum| value of 7.0 × 10-3; these values are among the highest for helical nanographenes.

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.

Cationized orthogonal triad as a photosensitizer with enhanced synergistic antimicrobial activity

Single-molecule-based synergistic phototherapy holds great potential for antimicrobial treatment. Herein, we report an orthogonal molecular cationization strategy to improve the reactive oxygen species (ROS) and hyperthermia generation of heptamethine cyanine (Cy7) for photodynamic and photothermal treatments of bacterial infections. Cationic pyridine (Py) is introduced at the meso‑position of the asymmetric Cy7 with intramolecular charge transfer (ICT) to construct an atypical electron-transfer triad, which reduces ΔES1-S0, circumvents rapid charge recombination, and simultaneously enhances intersystem crossing (ISC) based on spin-orbit charge-transfer ISC (SOCT-ISC) mechanism. This unique molecular construction produces anti-Stokes luminescence (ASL) because the rotatable CN bond enriched in high vibrational-rotational energy levels improves hot-band absorption (HBA) efficiency. The obtained triad exhibits higher singlet oxygen quantum yield and photothermal conversion efficiency compared to indocyanine green (ICG) under irradiation above 800 nm. Cationization with Py enables the triad to target bacteria via intense electrostatic attractions, as well as biocidal property against a broad spectrum of bacteria in the dark. Moreover, the triad under irradiation can enhance biofilm eradication performance in vitro and statistically improve healing efficacy of MRSA-infected wound in mice. Thus, this work provides a simple but effective strategy to design small-molecule photosensitizers for synergistic phototherapy of bacterial infections. STATEMENT OF SIGNIFICANCE: We developed an orthogonal molecular cationization strategy to enhance the reactive oxygen species and thermal effects of heptamethine cyanine (Cy7) for photodynamic and photothermal treatments of bacterial infections. Specifically, cationic pyridine (Py) was introduced at the meso‑position of the asymmetric Cy7 to construct an atypical electron-transfer triad, which reduced ΔES1-S0, circumvented rapid charge recombination, and simultaneously enhanced intersystem crossing (ISC). This triad, with a rotatable CN bond, produced anti-Stokes luminescence due to hot-band absorption. The triad enhanced antimicrobial performance and statistically improved the healing efficacy of MRSA-infected wounds in mice. This site-specific cationization strategy may provide insights into the design of small molecule-based photosensitizers for synergistic phototherapy of bacterial infections.

Construction of hexabenzocoronene-based chiral nanographenes

The past decade witnessed remarkable success in synthetic molecular nanographenes. Encouraged by the widespread application of chiral nanomaterials, the design, and construction of chiral nanographenes is a hot topic recently. As a classic nanographene unit, hexa-peri-hexabenzocoronene generally serves as the building block for nanographene synthesis. This review summarizes the representative examples of hexa-peri-hexabenzocoronene-based chiral nanographenes.

Superhelicenes in the gas phase: experimental and computational evidence of stable radical cation dimers

Electrospray-ionization mass spectrometry (ESI-MS) readily produces stable radical cation π-dimers of the superhelicenes. Energy-resolved collision experiments reveal the dissociation of the dicationic dimer into two singly charged superhelicenes. DFT calculations indicate that open-shell dications composed of two radical cations are thermochemically more attractive than the closed-shell dimer formed by a doubly charged and a neutral superhelicene.

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S₀-S₁ transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (B_CPL) up to 66.5 M^-1 cm^-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (g_abs) and luminescence dissymmetry (g_lum) factor.

Edge-Decorated Polycyclic Aromatic Hydrocarbons by an Oxidative Coupling Approach

Oxidative phenol coupling reduces reliance on halo/metalated substrates used in conventional redox neutral couplings. A new strategy for constructing polycyclic aromatic hydrocarbons (PAHs) that incorporates oxidative phenol coupling is outlined in a three-stage approach: oxidative fragment coupling, linking of the two resultant units, and oxidative cyclization. The protocol allows rapid assembly of both planar and helical systems with a high degree of edge functionalization. The incorporation of 12 alkoxy groups on systems with 12 rings gave rise to lower optical gaps compared to systems with a lesser degree of edge functionalization.

Azepine-Embedded Seco-Hexabenzocoronene-Based Helix Nanographenes: Access to Modification of the Core by N-H Functionalization

Contorted polycyclic aromatic hydrocarbons (PAHs) or nanographenes (NGs) have received increasing attention and are mostly prepared by "bottom-up" strategies. Apparently, systematically tuning the properties of NGs for application is important but challenging. Here, a new type of helix, azepine-embedded NGs, were designed and synthesized by the introduction of NH into the hexa-peri-hexabenzocoronene (HBC) core. We demonstrate that this nitrogen-doped NG can be functionalized via N-H derivatization. Through modifications to the NH site with a chiral auxiliary reagent, optical resolution of the chiral NG was achieved. Meanwhile, it was found that by introducing various aryl groups with electron-donating or electron-withdrawing substituents, the emission intensity and the fluorescence mechanism can be modulated. Compared to the original NH-containing NG, the modified derivative exhibited improved fluorescence efficiency and tunable emission wavelength. A functionalized structure of benzoic acid with considerably improved fluorescence efficiency, hydrophilicity, and membrane permeability to stain the live cells was proved.

Helical Trilayer Nanographenes with Tunable Interlayer Overlaps

The synthesis of well-defined nanocarbon multilayers, beyond the bilayer structure, is still a challenging goal. Herein, two trilayer nanographenes were synthesized by covalently linking nanographene layers through helicene bridges. The structural characterization of the trilayer nanographenes revealed a compact trilayer-stacked architecture. The introduction of a furan ring into the helicene linker modulates the interlayer overlap and π-conjugation of the trilayer nanographenes, enabling the tuning of the interlayer coupling, as demonstrated by optical, electrochemical, and theoretical analyses. Both synthesized trilayer nanographenes are rigid chiral nanocarbons and show a chirality transfer from the helicene moiety to the stacked nanographene layers. These helical trilayer nanographenes reported here represent the covalently linked multilayer nanographenes rather than bilayer ones, showing the tunable multilayer stacking structure.

Helical Synthetic Nanographenes with Atomic Precision

ConspectusUnderstanding and harnessing the properties of nanoscale molecular entities are considered as new frontiers in basic chemistry. In this regard, synthetic nanographene with atomic precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of carbon, flat, curved, ribbon-type, or cone-shaped nanographenes have been prepared in highly controllable and elegant manner, allowing one to explore fascinating molecular architectures with intriguing optical, electrochemical, and magnetic characteristics. This stands in stark contrast to other carbon-rich nanomaterials, such as graphite oxides or carbon quantum dots, which preclude thorough investigations because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern aromatic chemistry and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine.Nonetheless, in many cases, synthesis and characterization of nanographene compounds are highly demanding. Low solubility, high molecular strain, undesired selectivity, as well as incomplete or excessive C-C bond formation are common impediments, that require formidable efforts to control the molecular geometry, to modulate the edge structure, to achieve accurate doping, or to push the upper size boundary. These endeavors are indispensable for establishing structure-property relationships, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications.In this Account, we summarize our contributions to this field by presenting a series of helical synthetic nanographenes, such as hexapole [7]helicene (H7H), nitrogen-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of carbon quantum dots, albeit has precise atomic structure. It provides a unique platform to study aromatic chemistry and chirality at the nanoscale. We discuss synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidation, which are expected to be valuable for making synthetic nanographenes in general. In addition, we illustrate their exciting electrochemical and photophysical performance, which include, but are not limited to, reversible multielectron redox chemistry, record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their three-dimensional conjugated architectures, highlighting the critical roles of π-electron delocalization, heteroatom-doping, substitution, and molecular symmetry in determining nanographenes' properties and functions. Lastly, we put forward our understanding on the challenges and opportunities that lies ahead and hope this Account will inspire ever more ambitious achievements from this attractive area of research.

Synthesis of Highly Luminescent Chiral Nanographene

Chiral nanographenes with both high fluorescence quantum yields (Φ_F ) and large dissymmetry factors (g_lum ) are essential to the development of circularly polarized luminescence (CPL) materials. However, most studies have been focused on the improvement of g_lum , whereas how to design highly emissive chiral nanographenes is still unclear. In this work, we propose a new design strategy to achieve chiral nanographenes with high Φ_F by helical π-extension of strongly luminescent chromophores while maintaining the frontier molecular orbital (FMO) distribution pattern. Chiral nanographene with perylene as the core and two dibenzo[6]helicene fragments as the wings has been synthesized, which exhibits a record high Φ_F of 93 % among the reported chiral nanographenes and excellent CPL brightness (B_CPL ) of 32 M^-1  cm^-1 .

Synthesis and Properties of Achiral and Chiral Dipyrenoheteroles and Related Compounds

Achiral and chiral isomers of dipyrenoheteroles were synthesized via alkyne benzannulation. The electronic properties of these compounds were examined using cyclic voltammetry and differential pulse voltammetry. The enantiomers of the chiral isomers were separated, and their optical properties were examined in circular dichroism and circularly polarized luminescence studies. The chiral isomers exhibited a large bathochromic shift, relative to the achiral isomer, in both absorbance and fluorescence, resulting from decreased symmetry, rather than a change in the size of the backbone.

Supramolecular Assembly of Edge Functionalized Top-Down Chiral Graphene Quantum Dots

The construction of supramolecular assemblies of heterogeneous materials at the nanoscale is an open challenge in science. Herein, new chiral graphene quantum dots (GQDs) prepared by amidation reaction introducing chiral amide groups and pyrene moieties into the periphery of GQDs are described. The analytical and spectroscopic data show an efficient chemical functionalization and the morphological study of the supramolecular ensembles using SEM and AFM microscopies reveals the presence of highly ordered fibers of several micrometers length. Fluorescence studies, using emission spectroscopy and confocal microscopy, reveal that the fibers stem from the π-π stacking of both pyrenes and GQDs, together with the hydrogen bonding interactions of the amide groups. Circular dichroism analysis supports the chiral nature of the supramolecular aggregates.

Highly contorted superhelicene hits near-infrared circularly polarized luminescence

Herein we describe a novel superhelicene structure consisting of three hexa-peri-hexabenzocoronene (HBC) units arranged in a helical geometry and creating two carbo[5]helicenes and a carbo[7]helicene. The central HBC bears a tropone moiety, which induces a saddle-helix hybrid geometry into the 3D structure of the prepared nanographene. The introduction of multiple helicenes and the position of the tropone unit trigger near-infrared circularly polarized luminescence (NIR-CPL, up to 850 nm, |g lum| = 3.0 × 10-3) combined with good photoluminescence quantum yields (ϕ F = 0.43) and upconverted emission based on two-photon absorption (TPA). Compared to previously reported superhelicenes of similar size, higher quantum yields, CPL brightness, and red-shifted absorption and emission spectra are achieved. Besides, chiroptical properties of enantiopure thin films were evaluated. These findings place this novel superhelicene as the first NIR-CPL superhelicene ever reported and make it a promising candidate for use as a chiral luminescent material in optoelectronic devices.

The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics

The past decade has witnessed remarkable success in the synthesis of curved polycyclic aromatics through Scholl reactions which enable oxidative aryl-aryl coupling even in company with the introduction of significant steric strain. These curved polycyclic aromatics are not only unique objects of structural organic chemistry in relation to the nature of aromaticity but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topology. Moreover, they have received considerable attention in the fields of supramolecular chemistry and organic functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aromatics, the outcome of a newly designed substrate in the Scholl reaction still cannot be predicted in a generic and precise manner largely due to limited understanding on the reaction mechanism and possible rearrangement processes. This review provides an overview of Scholl reactions with a focus on their applications in synthesis of curved polycyclic aromatics with interesting structures and properties and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aromatics.

Nanographenes and Graphene Nanoribbons as Multitalents of Present and Future Materials Science

As cut-outs from a graphene sheet, nanographenes (NGs) and graphene nanoribbons (GNRs) are ideal cases with which to connect the world of molecules with that of bulk carbon materials. While various top-down approaches have been developed to produce such nanostructures in high yields, in the present perspective, precision structural control is emphasized for the length, width, and edge structures of NGs and GNRs achieved by modern solution and on-surface syntheses. Their structural possibilities have been further extended from "flatland" to the three-dimensional world, where chirality and handedness are the jewels in the crown. In addition to properties exhibited at the molecular level, self-assembly and thin-film structures cannot be neglected, which emphasizes the importance of processing techniques. With the rich toolkit of chemistry in hand, NGs and GNRs can be endowed with versatile properties and functions ranging from stimulated emission to spintronics and from bioimaging to energy storage, thus demonstrating their multitalents in present and future materials science.

Synthesis of a magnetic π-extended carbon nanosolenoid with Riemann surfaces

Riemann surfaces are deformed versions of the complex plane in mathematics. Locally they look like patches of the complex plane, but globally, the topology may deviate from a plane. Nanostructured graphitic carbon materials resembling a Riemann surface with helicoid topology are predicted to have interesting electronic and photonic properties. However, fabrication of such processable and large π-extended nanographene systems has remained a major challenge. Here, we report a bottom-up synthesis of a metal-free carbon nanosolenoid (CNS) material with a low optical bandgap of 1.97 eV. The synthesis procedure is rapid and possible on the gram scale. The helical molecular structure of CNS can be observed by direct low-dose high-resolution imaging, using integrated differential phase contrast scanning transmission electron microscopy. Magnetic susceptibility measurements show paramagnetism with a high spin density for CNS. Such a π-conjugated CNS allows for the detailed study of its physical properties and may form the base of the development of electronic and spintronic devices containing CNS species.

Fabrication of large π-conjugated carbon nanosolenoid materials with helicoid topology remains a challenge. Here the authors demonstrate synthesis of a metal-free π-extended carbon nanosolenoid material with a helical structure, exhibiting unique photophysical and magnetic properties.

Hexabenzocoronene functionalized with antiaromatic S- and Se-core-modified porphyrins (isophlorins): comparison with the dyad with regular porphyrin

The important and perspective molecular building blocks composed of hexaphenylbenzenes (HPBs) or their oxidized derivatives, hexa-peri-hexabenzocoronenes (HBCs), and metalloporphyrins have recently received significant attention of the researchers. In this study, motivated by recent findings, we have addressed the modifications of structures and properties of HBC-porphyrin compounds by using instead of aromatic porphyrins antiaromatic 20π isophlorin derivatives of thiophene or selenophene. We have reported the first comparative computational investigation of the following systems: (i) HBC with one non-metallated aromatic porphyrin, P(N4H2), unit, HBC-P(N4H2), (ii) HBC with one S-core-modified antiaromatic porphyrin (S-isophlorin), PS4, unit, HBC-PS4, and (iii) HBC with one Se-core-modified antiaromatic porphyrin (Se-isophlorin), PSe4, unit, HBC-PSe4. The study has been done employing the B3LYP/6-31G* approach (in the gas phase and in the implicit solvents, benzene and dichloromethane), and comparison with the B3LYP/6-31G** and B3LYP/6-311G* approaches was performed, where relevant. The effects of the core-modified antiaromatic isophlorins on the structures, electronic, and other properties, potentially including reactivity, of the whole building block HBC-isophlorin have been shown to be quite pronounced and to be noticeably stronger than the effects of the original aromatic non-metallated porphyrin. Thus, we have demonstrated theoretically that the complete porphyrin core-modification with other elements, this time with S and Se leading to the formation of the antiaromatic isophlorins, should be considered as a promising way for modifying and tuning structures, electronic properties and reactivity of the hexabenzocoronene-porphyrin(s) building blocks.

In Situ Formation of Polycyclic Aromatic Hydrocarbons as an Artificial Hybrid Layer for Lithium Metal Anodes

Nonuniform Li deposition causes dendrites and low Coulombic efficiency (CE), seriously hindering the practical applications of Li metal. Herein, we developed an artificial solid-state interphase (SEI) with planar polycyclic aromatic hydrocarbons (PAHs) on the surface of Li metal anodes by a facile in situ formation technology. The resultant dihydroxyviolanthron (DHV) layers serve as the protective layer to stabilize the SEI. In addition, the oxygen-containing functional groups in the soft and conformal SEI film can regulate the diffusion and transport of Li ions to homogenize the deposition of Li metal. The artificial SEI significantly improves the CEs and shows superior cyclability of over 1000 h at 4 mAh cm^-2. The LiFePO₄/Li cell (2.8 mAh cm^-2) enables a long cyclability for 300 cycles and high CEs of 99.8%. This work offers a new strategy to inhibit Li dendrite growth and enlightens the design on stable SEI for metal anodes.

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).

Diels-Alder Cycloaddition with CO, CO2, SO2, or N2 Extrusion: A Powerful Tool for Material Chemistry

Phenyl, naphthyl, polyarylphenyl, coronene, and other aromatic and polyaromatic moieties primarily influence the final materials' properties. One of the synthetic tools used to implement (hetero)aromatic moieties into final structures is Diels-Alder cycloaddition (DAC), typically combined with Scholl dehydrocondensation. Substituted 2-pyranones, 1,1-dioxothiophenes, and, especially, 1,3-cyclopentadienones are valuable substrates for [4 + 2] cycloaddition, leading to multisubstituted derivatives of benzene, naphthalene, and other aromatics. Cycloadditions of dienes can be carried out with extrusion of carbon dioxide, carbon oxide, or sulphur dioxide. When pyranones, dioxothiophenes, or cyclopentadienones and DA cycloaddition are aided with acetylenes including masked ones, conjugated or isolated diynes, or polyynes and arynes, aromatic systems are obtained. This review covers the development and the current state of knowledge regarding thermal DA cycloaddition of dienes mentioned above and dienophiles leading to (hetero)aromatics via CO, CO2, or SO2 extrusion. Particular attention was paid to the role that introduced aromatic moieties play in designing molecular structures with expected properties. Undoubtedly, the DAC variants described in this review, combined with other modern synthetic tools, constitute a convenient and efficient way of obtaining functionalized nanomaterials, continually showing the potential to impact materials sciences and new technologies in the nearest future.

Infinitene: A Helically Twisted Figure-Eight [12]Circulene Topoisomer

New forms of molecular nanocarbon particularly looped polyarenes adopting various topologies contribute to the fundamental science and practical applications. Here we report the synthesis of an infinity-shaped polyarene, infinitene (1) (cyclo[c.c.c.c.c.c.e.e.e.e.e.e]dodecakisbenzene), comprising consecutively fused 12-benzene rings forming an enclosed loop with a strain energy of 60.2 kcal·mol^-1. Infinitene (1) represents a topoisomer of still-hypothetical [12]circulene, and its scaffold can be formally visualized as the outcome of the "stitching" of two homochiral [6]helicene subunits by both their ends. The synthetic strategy encompasses transformation of a rationally designed dithiacyclophane to cyclophadiene through the Stevens rearrangement and pyrolysis of the corresponding S,S'-bis(oxide) followed by the photocyclization. The structure of 1 is a unique hybrid of helicene and circulene with a molecular formula of C₄₈H₂₄, which can be regarded as an isomer for kekulene, [6,6]carbon nanobelt ([6,6]CNB), and [12]cyclacene. Infinitene (1) is a bench-stable yellow solid with green fluorescence and soluble to common organic solvents. Its figure-eight molecular structure was unambiguously confirmed by X-ray crystallography. The scaffold of 1 is significantly compressed as manifested by a remarkably shortened distance (3.152-3.192 Å) between the centroids of two π-π stacked central benzene rings and the closest C···C distance of 2.920 Å. Fundamental photophysical properties of 1 were thoroughly elucidated by UV-vis absorption and fluorescence spectroscopic studies and density functional theory calculations. Its configurational stability enabled separation of the corresponding enantiomers (P,P) and (M,M) by a chiral HPLC. Circular dichroism (CD) and circularly polarized luminescence (CPL) measurements revealed that 1 has moderate |g_CD| and |g_CPL| values.

Chiral Distorted Hexa-peri-hexabenzocoronenes Bearing a Nonagon-Embedded Carbohelicene

A new family of chiral saddle-helix hybrid nanographenes is reported. The first hexa-peri-hexabenzocoronene (HBC) analogues bearing a nine-membered carbocycle are presented. Furthermore, for the first time, π-extended carbo[n]helicenes containing a nine-membered ring as part of the helical moiety have been synthesized. The combination of a [5]helicene moiety and a nonagon ring in a single chiral motif induces a tremendous distortion from planarity into the nanographenic structures compared to other saddle-helix hybrids such as heptagon- and octagon-containing π-extended carbo[5]helicenes. In fact, the interplanar angle of the two terminal rings reaches the largest angle (134.8°) of a carbohelicene reported to date, thus being by far the most twisted helicene yet prepared. Photophysical properties evaluation showed improved absorption dissymmetry factors (|gabs |=4.2×10-3 ) in the new family of nonagon-containing π-extended carbo[5]helicenes.

Synthesis of thiophene-fused porphyrin dimers as effective π-extended helical chromophores

We synthesized thiophene-fused porphyrin dimers as effective π-extended helical chromophores. The porphyrin dimers exhibit a red-shifted absorption with the edge extending up to 1100 nm, implying strong electronic communication over the two porphyrin moieties through the thiophene-fused structure. Importantly, their racemic inversion barriers can be modulated by the central metal ions.

A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems

We designed a straightforward synthetic route towards a full-fledged family of π-extended helicenes: superhelicenes. They have two hexa-peri-hexabenzocoronenes (HBCs) in common that are connected via a central five-membered ring. By means of structurally altering this 5-membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In-depth physico-chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded-off by crystal structure analyses. Mixed stacks of M- and P-isomers led to twisted molecular wires. Using such stacks, charge-carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.

Synthesis of [2.2]Paracyclophane/9-Alkylfluorene Hybrids and the Discovery of a Solvent-assisted Rearrangement

Work on the synthesis of [2.2]paracyclophane/9-alkylfluorene hybrids led to the discovery of the rearrangement of cyclopentadienone 7 to cyclophane 6. A DFT computational study revealed that this formal 1,3-alkyl shift occurs in two steps, but requires the participation of a solvent molecule (ethanol). The rearrangement could be avoided by generating 7 under mild conditions and using benzynes as dienophiles to afford the targeted cyclophanes 14 and 16, the latter of which exhibits dual fluorescence emission.

Regioselectivity in the Scholl Reaction: Mono and Double [7]Helicenes

We employed the density functionaly theory (DFT)-predicted regioselectivity of the intramolecular Scholl reaction in phenanthrene and dibenzo[g,p]chrysene frameworks to obtain π-extended mono and double [7]helicenes, respectively. The formation of these helical structures occurs despite the buildup of a large strain energy up to 30 kcal/mol compared with their most stable isomers. The twisted and strained structures were characterized and analyzed by experimental (NMR, UV-vis, emission, electrochemistry, and single-crystal X-ray diffraction) techniques and were further supported by DFT calculations.

Helically and Linearly Fused Rylenediimide-Hexabenzocoronenes

Perylene- as well as naphthalenediimides were fused to hexabenzocoronenes (HBCs) at their imide position to realize highly π-extended donor-acceptor (D-A)-hybrids. Successful isomer separation in the first step was decisive to guarantee a straightforward synthetic sequence. Hexaphenylbenzenes as precursors were accessed via Diels-Alder reactions and reacted in a Scholl oxidation to yield the respective HBC derivatives. The fully conjugated benzimidazole linker, which separates the electron donating HBC from the electron accepting rylenediimide, enabled the formation of either a linear or a helical configuration. Largely different chemical, physical, and optoelectrical characteristics were noted for the two configurations. What stood out was their aggregation and their excited state deactivation depending on the solvent polarity. Results from global analysis of the femtosecond transient absorption data corroborated the formation of a charge-transfer (CT) state that is stabilized in the helically fused configuration relative to the linear analogue. However, a comparison with spectroelectrochemical experiments failed to disclose evidence for a charge-separated (CS) state.

Double π-Extended Undecabenzo[7]helicene

This work reports the first double π-extended undecabenzo[7]helicene 1, which is a large chiral nanographene, composed of 65 fused rings and 186 conjugated carbon atoms. The molecular identity of 1 has been confirmed by single crystal X-ray diffraction. A wine coloured solution of 1 in dichloromethane absorbs light from ultraviolet to the near infrared, featuring an extremely large molar absorption coefficient of 844 000 M^-1  cm^-1 at 573 nm. Optically pure 1 shows a record high electronic circular dichroism intensity in the visible spectral range (|Δϵ|=1375 M^-1  cm^-1 at 430 nm) known for any discrete polycyclic aromatic hydrocarbon. These unusual photophysical properties of 1 contrast sharply with those of a mono-undecabenzo[7]helicene derivative 2.

Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes

π-Extended helicenes constitute an important class of polycyclic aromatic hydrocarbons with intrinsic chirality. Herein, we report the syntheses of π-extended [7]helicene 4 and π-extended [9]helicene 6 through regioselective cyclodehydrogenation in high yields, where a “prefusion” strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure analysis. Compared to the parent pristine [7]helicene and [9]helicene, these novel π-extended helicenes display significantly improved photophysical properties, with a quantum yield of 0.41 for 6. After optical resolution by chiral high-performance liquid chromatography, the chiroptical properties of enantiomers 4-P/M and 6-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approximately 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M^–1 cm^–1 as one of the highest among carbohelicenes.

Electrochemical Synthesis, Deposition, and Doping of Polycyclic Aromatic Hydrocarbon Films

Polycyclic aromatic hydrocarbons (PAHs) are employed as organic semiconductors because their delocalized π-electron systems and strong intermolecular interactions endow them with an exceptional charge-transport ability. However, the deposition of PAHs from solution onto high-quality thin films is often difficult. Here, we report a one-step electrochemical method to synthesize and deposit unsubstituted PAHs, starting from twisted oligophenyl precursors. The cyclodehydrogenated products were analyzed by matrix-assisted laser-desorption time-of-flight mass spectrometry as well as Fourier transform infrared and Raman spectroscopy. With this electrosynthesis and deposition, the PAHs stack into compact and ordered supramolecular structures along the π-π direction to form thin films with controllable thicknesses and doping levels. The direct fabrication of PAH films opens new pathways toward PAH-based optoelectronic devices.

Octagon-Embedded Carbohelicene as a Chiral Motif for Circularly Polarized Luminescence Emission of Saddle-Helix Nanographenes

We report a new family of hexa-peri-hexabenzocoronene (HBC)-based helical nanographenes incorporating π-extended carbo[5]helicenes bearing an octagonal carbocycle. This family represents a new kind of highly distorted saddle-helix hybrid nanographenes. For the first time, the eight-membered ring becomes a constituent of both a carbo[5]helicene and a HBC and thus, the negative curvature is responsible for twisting both units. This novel chiral motif, namely, oct-[5]helicene results in the largest torsion angle recorded so far for a carbo[5]helicene (θ=79.5°), as it has been suggested by DFT-calculations and confirmed by X-ray crystallography. Consequently, the barriers of isomerization become exceptionally high for a [5]helicene unsubstituted in the fjord region since neither racemization nor decomposition were observed at 200 °C for 1 or 3 during 5 h. Therefore, racemic resolutions allowed subsequent chiroptical studies showing the ECD and CPL responses of this novel family of chiral nanographenes.

π-Extended Diaza[7]helicenes by Hybridization of Naphthalene Diimides and Hexa-peri-hexabenzocoronenes

The synthesis of an unprecedented, π-extended hexabenzocorene (HBC)-based diaza[7]helicene is presented. The target compound was synthesized by an ortho-fusion of two naphthalene diimide (NDI) units to a HBC-skeleton. A combination of Diels-Alder and Scholl-type oxidation reactions involving a symmetric di-NDI-tolane precursor were crucial for the very selective formation of the helical superstructure via a hexaphenyl-benzene (HPB) derivative. The formation of the diaza[7]helicene moiety in the final Scholl oxidation is favoured, affording the symmetric π-extended helicene as the major product as a pair of enantiomers. The separation of the enantiomers was successfully accomplished by HPLC involving a chiral stationary phase. The absolute configuration of the enantiomers was assigned by comparison of circular dichroism spectra with quantum mechanical calculations.

Synthesis and crystal packing of perylene-derivatives with extreme sterically demanding pentaphenylbenzene bay-substituents

An unprecedented library of single and double bay-substituted perylene-pentaphenylbenzenes (PPBs) is presented. An extreme core-distortion as well as successful elimination of aggregation interactions of perylenes was confirmed by X-ray analysis. The isomerically pure perylene-PPB hybrids show remarkable differences in their photophysical properties with respect to their regiochemistry as well as different peri-functionalization.

On the photophysics of nanographenes - investigation of functionalized hexa-peri-hexabenzocoronenes as model systems

In this study, we report on hexa-peri-hexabenzocoronenes (HBCs) as representative models for nanographenes. To this end, we synthesized a family of functionalized HBCs and investigated the impact of the substituents on the π-extended systems of the HBCs. DFT and TD-DFT calculations suggested a charge transfer character, which intensified as the electron density withdrawing effects of the substituents (-M-effect) increased. Unambiguous corroboration of the charge transfer character in the case of NO2-substituents was realized via steady-state absorption and fluorescence experiments, which focused on the dependencies on the solvent polarity and temperature featuring. Going beyond HBCs with NO2-substituents, time-correlated single photon counting, and femtosecond and nanosecond transient absorption spectroscopy unveiled long-lived singlet and triplet excited states. As a complement, we performed electrochemical and spectroelectrochemical measurements. These measurements were carried out to shed light onto the nature of the functionalized HBCs as electron acceptors and/or donors, on the one hand, and their corresponding spectroscopic signatures, on the other hand. All of the aforementioned information enabled intermolecular charge separation assays with, for example, suitable electron acceptors by steady-state and time-resolved spectroscopy.

Heavy-Atom-Free Photosensitizers: From Molecular Design to Applications in the Photodynamic Therapy of Cancer

Photodynamic therapy (PDT) is a clinically approved therapeutic modality that has shown great potential for the treatment of cancers owing to its excellent spatiotemporal selectivity and inherently noninvasive nature. However, PDT has not reached its full potential, partly due to the lack of ideal photosensitizers. A common molecular design strategy for effective photosensitizers is to incorporate heavy atoms into photosensitizer structures, causing concerns about elevated dark toxicity, short triplet-state lifetimes, poor photostability, and the potentially high cost of heavy metals. To address these drawbacks, a significant advance has been devoted to developing advanced smart photosensitizers without the use of heavy atoms to better fit the clinical requirements of PDT. Over the past few years, heavy-atom-free nonporphyrinoid photosensitizers have emerged as an innovative alternative class of PSs due to their superior photophysical and photochemical properties and lower expense. Heavy-atom-free nonporphyrinoid photosensitizers have been widely explored for PDT purposes and have shown great potential for clinical oncologic applications. Although many review articles about heavy-atom-free photosensitizers based on porphyrinoid structure have been published, no specific review articles have yet focused on the heavy-atom-free nonporphyrinoid photosensitizers.In this account, the specific concept related to heavy-atom-free photosensitizers and the advantageous properties of heavy-atom-free photosensitizers for cancer theranostics will be briefly introduced. In addition, recent progress in the development of heavy-atom-free photosensitizers, ranging from molecular design approaches to recent innovative types of heavy-atom-free nonporphyrinoid photosensitizers, emphasizing our own research, will be presented. The main molecular design approaches to efficient heavy-atom-free PSs can be divided into six groups: (1) the approach based on traditional tetrapyrrole structures, (2) spin-orbit charge-transfer intersystem crossing (SOCT-ISC), (3) reducing the singlet-triplet energy gap (ΔE_ST), (4) the thionation of carbonyl groups of conventional fluorophores, (5) twisted π-conjugation system-induced intersystem crossing, and (6) radical-enhanced intersystem crossing. The innovative types of heavy-atom-free nonporphyrinoid photosensitizers and their applications in cancer diagnostics and therapeutics will be discussed in detail in the third section. Finally, the challenges that need to be addressed to develop optimal heavy-atom-free photosensitizers for oncologic photodynamic therapy and a perspective in this research field will be provided. We believe that this review will provide general guidance for the future design of innovative photosensitizers and spur preclinical and clinical studies for PDT-mediated cancer treatments.

500-Fold Amplification of Small Molecule Circularly Polarised Luminescence through Circularly Polarised FRET

Strongly dissymmetric circularly polarised (CP) luminescence from small organic molecules could transform a range of technologies, such as display devices. However, highly dissymmetric emission is usually not possible with small organic molecules, which typically give dissymmetric factors of photoluminescence (gPL ) less than 10-2 . Here we describe an almost 103 -fold chiroptical amplification of a π-extended superhelicene when embedded in an achiral conjugated polymer matrix. This combination increases the |gPL | of the superhelicene from approximately 3×10-4 in solution to 0.15 in a blend film in the solid-state. We propose that the amplification arises not simply through a chiral environment effect, but instead due to electrodynamic coupling between the electric and magnetic transition dipoles of the polymer donor and superhelicene acceptor, and subsequent CP Förster resonance energy transfer. We show that this amplification effect holds across several achiral polymer hosts and thus represents a simple and versatile approach to enhance the g-factors of small organic molecules.

Synthesis of diverse heterocyclic frameworks using cyclopentadienones via the Diels–Alder strategy

In this review article, we briefly summarize the versatility of Diels–Alder reactions of cyclopentadienones and concise routes to diverse hetero-atom bearing PAHs using cyclones as building blocks.