Efficient Narrowband Circularly Polarized Light Emitters Based on 1,4-B,N-embedded Rigid Donor-Acceptor Helicenes.

Narrow-band emission is essential for applicable circularly polarized luminescence (CPL) active materials in ultrahigh-definition CP-OLEDs. One of the most promising classes of CPL active molecules, helicenes, however, typically exhibit broad emission with a large Stokes shift. We present, herein, a design strategy capitalizing on intramolecular donor-acceptor interactions between nitrogen and boron atoms to address this issue. 1,4-B,N-embedded configurationally stable single- and double helicenes were synthesized straightforwardly. Both helicenes show unprecedentedly narrow fluorescence and CPL bands (full width at half maximum between 17-28 nm, 0.07-0.13 eV) along with high fluorescence quantum yields (72-85 %). Quantum chemical calculations revealed that the relative localization of the natural transition orbitals, mainly on the rigid core of the molecule, and small values of root-mean-square displacements between S0 and S1 state geometries, contribute to the narrower emission.

Bis-Olefin Based Crystalline Schlenk Hydrocarbon Diradicals with a Triplet Ground State.

A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m-phenylene bridged electron-rich bis-triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half-field signal. A computational analysis confirms the triplet state to be the ground state. As a proof-of-principle for the modular methodology, the 4,6-dimethyl-m-phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6-dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m-phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m-phenylene-bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m-phenylene, between two radical centers in a crystalline form.

Pyrene-Fused [7]Helicenes Connected Via Hexagonal and Heptagonal Rings: Stereospecific Synthesis and Chiroptical Properties.

In this manuscript, we portrayed a stereospecific synthesis of C2- and C1-symmetric pyrene-fused [7]helicene compounds 1 and 2, respectively. Compounds 1 and 2 were synthesized via a one-pot Suzuki coupling-C-H activation and two-step Suzuki coupling-Scholl reaction, respectively, with complete retention of configuration. The synthesized molecules differ in the fusing mode of [7]helicene units with pyrene via six- and seven-membered rings for 1 and 2, respectively. There was a significant difference in the functional properties and enantiomerization barrier of both compounds because of their distinct molecular symmetry as well as fusing mode to pyrene moiety. The heptagon-containing molecule 2 showed remarkable photophysical and chiroptical properties with commendable configurational stability compared to 1 and pristine [7]helicene as well as its [5]helicene congener.

Nano- and Microspheres Containing Inorganic and Biological Nanoparticles: Self-Assembly and Electron Tomographic Analysis.

Organofullerene amphiphiles show diverse behaviors in water, forming vesicles, micelles, Langmuir-Blodgett films, and anisotropic nanostructures. We found that gradual in situ protonation of an organic solution of (4-heptylphenyl)5C60-K+ by water or buffer generates the corresponding protonated molecule, (4-heptylphenyl)5C60H, which self-assembles to form nano- and microspheres of organofullerene (fullerspheres) with uniform diameters ranging from 30 nm to 2.5 µm that are controlled by the preparation or pH of the buffer. By using an aqueous solution of an organic dye, inorganic nanoparticle, protein, and virus, we encapsulated these entities in the fullersphere. This approach via self-assembly is distinct from other preparations of organic core-shell particles that generally require polymerization for the construction of a robust shell. The sphere is entirely amorphous, thermally stable up to 300 °C under vacuum, and resistant to electron irradiation, and we found the unconventional utility of the sphere for electron tomographic imaging of nanoparticles and biomaterials.

Anionic Boron- and Carbon-Based Hetero-Diradicaloids Spanned by a p-Phenylene Bridge.

Herein we report the synthesis and characterization of anionic boron- and carbon-based Kekulé diradicaloids spanned by a p-phenylene bridge. In contrast to Thiele's hydrocarbon, a closed-shell singlet system, they show an appreciable population of the triplet state at room temperature, as evidenced by both NMR and EPR spectroscopy. Moreover, en route to these anionic boron- and carbon-based hetero-diradicaloids, the formation of an isolable diamino(4-diarylboryl-phenyl)methyl radical was observed.

Carbo[n]helicenes Restricted to Enantiomerize: An Insight into the Design Process of Configurationally Stable Functional Chiral PAHs.

The most important stereodynamic feature of carbo[n]helicenes is the interconversion of their enantiomers. The Gibbs activation energy (ΔG≠ (T)) of this process, which determines the rate of enantiomerization, dictates the configurational stability of [n]helicenes. High values of ΔG≠ (T) are required for applications of functional chiral molecules incorporating [n]helicenes or helicene substructures. This minireview provides an overview of the mechanism, recent developments, and factors affecting the enantiomerization of [n]helicenes, which will accelerate the design process of configurationally stable functional chiral molecules based on helicene substructures. Additionally, this minireview addresses the misconception and irregularities in the recent literature on how the terms "racemization" and "enantiomerization" are used as well as how the activation parameters are calculated for [n]helicenes and related compounds.

Diamidocarbene-Based Thiele and Tschitschibabin Hydrocarbons: Carbonyl Functionalized Kekulé Diradicaloids.

Herein, we report diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, diradicaloids that contain four carbonyl/amido functional groups. The impact of two different π-conjugated spacers, p-phenylene vs p,p'-biphenylene, has been realized. The quantum chemical calculations suggest diamidocarbene (DAC)-based Thiele hydrocarbon (p-phenylene bridged) closed-shell singlet is the ground state, whereas for the diamidocarbene (DAC)-based Tschitschibabin hydrocarbon (p,p'-biphenylene bridged), open-shell singlet is the ground state. The influence of two different π-conjugated spacers also has been reflected in their UV-vis spectra. To gain more information on the diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, we have also carried out cyclic voltammetry investigations along with UV-vis-NIR-spectroelectrochemical studies of their corresponding 2-e oxidized product.

Zinc-[7]helicenocyanine and Its Discrete π-Stacked Homochiral Dimer.

In this communication, we demonstrate a novel approach to prepare a discrete dimer of chiral phthalocyanine (Pc) by exploiting the flexible molecular geometry of helicenes, which enables structural interlocking and strong aggregation tendency of Pcs. Synthesized [7]helicene-Pc hybrid molecular structure, zinc-[7]helicenocyanine (Zn-7HPc), exclusively forms a stable dimeric pair consisting of two homochiral molecules. The dimerization constants were estimated to be as high as 8.96×106  M-1 and 3.42×107  M-1 in THF and DMSO, respectively, indicating remarkable stability of dimer. In addition, Zn-7HPc exhibited chiral self-sorting behavior, which resulted in preferential formation of a homochiral dimer also in the racemic sample. Two phthalocyanine subunits in the dimeric form strongly communicate with each other as revealed by a large comproportionation constant and observation of an IV-CT band for the thermodynamically stable mixed-valence state.

[n]Helicene Diimides (n = 5, 6, and 7): Through-Bond versus Through-Space Conjugation.

The interactions between auxochromic groups in π-conjugated functional molecules dictate their electronic properties. From the standpoint of potential applications, understanding and control of such interactions is a vital requirement for the material design. In this communication, we describe the design, synthesis, and functional properties of a novel class of helically chiral diimide molecules, namely, [n]HDI-OMe (n = 5, 6, and 7), in which two imide units are connected via an [n]helicene skeleton. The experimental results supported by quantum chemical calculations reveal that the helical backbone in these molecules offers not only through-bond but also through-space conjugation between imide groups, which leads to distinct optical and electrochemical properties when compared to the related [n]helicenes and rylene diimides.

Benzo[cd]triangulene: A Spin 1/2 Graphene Fragment.

How does edge modification affect spin distribution in open-shell graphene fragments? We investigated this effect by analyzing spin-delocalization in benzo[cd]-triangulene, a spin 1/2 graphene fragment composed of seven benzenoid rings fused in a hybrid zigzag/armchair fashion. Six rings of this system form the core of Clar's hydrocarbon triangulene, to which an additional ring is annulated in the zigzag region. The singly occupied molecular orbital (SOMO) of this hydrocarbon radical resembles both SOMOs of triangulene, but the spin density is distributed over the core in a nonuniform fashion. The uneven spin distribution is reflected in the reactivity-reaction with oxygen occurs selectively at a position with the highest spin density-and correlates nicely with relative stabilities of the corresponding Clar resonance structures. The spin distribution is different from that of a topologically similar compound composed of the same number of sp2 carbon atoms but featuring six rings only, illustrating the impact of subtle structural changes on spin-density distribution. This compound was characterized by means of UV-vis and electron paramagnetic resonance spectroscopy, cyclic voltammetry, mass spectrometry, and X-ray crystallography. The experimental results are supported by density functional theory calculations.

Stereospecific Synthesis and Photophysical Properties of Propeller-Shaped C90 H48 PAH.

Herein, we have synthesized an enantiomerically pure propeller-shaped PAH, C90 H48 , possessing three [7]helicene and three [5]helicene subunits. This compound can be obtained in gram quantities in a straightforward manner. The photophysical and chiroptical properties were investigated using UV/Vis absorption and emission, optical rotation and circular dichroism spectroscopy, supported by DFT calculations. The nonlinear optical properties were investigated by two-photon absorption measurements using linearly and circularly polarized light. The extremely twisted structure and packing of the homochiral compound were investigated by single-crystal X-ray diffraction analysis.

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch.

We describe the synthesis and properties of 13,14-dimethylcethrene, a prototype of a chiral diradicaloid photochemical switch that can be transformed reversibly via conrotatory electrocyclization to its more stable closed form by light (630 nm) or heat and back to its open form by light (365 nm). This system illustrates how the chemical reactivity of a diradicaloid molecule can be translated into a switching function, which alters substantially all electronic parameters, namely, the HOMO-LUMO and the singlet-triplet (ST) energy gaps, and the degree of helical twist. As a result, distinct changes in the optical and chiroptical properties of this system were observed, which allowed us to monitor the switching process by a variety of spectroscopic techniques, including NMR, UV-vis, and CD. In comparison to the previously reported parent molecule cethrene, this system benefits from two methyl substituents installed in the fjord region, which account for the stability of the closed form against oxidation and racemization. The methyl substituents increase the ST energy gap of 13,14-dimethylcethrene by ∼4 kcal mol-1 in comparison to cethrene. Our DFT calculations reveal that the larger ST gap is a result of electronic and geometric effects of the methyl substituents and show the potential of related systems to act as magnetic switches at room temperature.

Cethrene: The Chameleon of Woodward-Hoffmann Rules.

We demonstrate that the electrocyclic (EC) ring-closure of cethrene in solution proceeds in a conrotatory mode both thermally and photochemically. The facile photochemical EC process promises that cethrene can serve as an efficient chiroptical switch operated solely by light. As for the thermally activated EC reaction, a low reaction barrier and a solvation effect on the EC rate indicate that the C2-symmetric pathway predicted by DFT calculations might not be the correct mechanism. Instead, we argue that the molecular symmetry decreases along the reaction coordinate as a consequence of the low-energy singlet excited state in this diradicaloid molecule, which might lead to a lower activation energy in accord with that determined through kinetic studies. Cethrene, therefore, represents a thought-provoking molecular chameleon of the Woodward-Hoffmann rules that puts our chemical concepts and intuition to test.

π-Extended and Curved Antiaromatic Polycyclic Hydrocarbons.

Synthesis of antiaromatic polycyclic hydrocarbons (PHs) is challenging because the high energy of their highest occupied molecular orbital and low energy of their lowest unoccupied molecular orbital cause them to be reactive and unstable. In this work, two large antiaromatic acene analogues, namely, cyclopenta[pqr]indeno[2,1,7-ijk]tetraphene (CIT, 1a) and cyclopenta[pqr]indeno[7,1,2-cde]picene (CIP, 1b), as well as a curved antiaromatic molecule with 48 π-electrons, dibenzo[a,c]diindeno[7,1,2-fgh:7',1',2'-mno]phenanthro[9,10-k]tetraphene (DPT, 1c), are synthesized on the basis of the corona of indeno[1,2-b]fluorene. These three antiaromatic PHs possess a narrow energy gap down to 1.55 eV and exhibit high kinetic stability under ambient conditions. Moreover, these compounds display reversible electron transfer processes in both the cathodic and anodic regimes. Their cation and anion radicals are characterized by in situ vis-NIR absorption and electron paramagnetic resonance spectroelectrochemistry. The X-ray crystallographic analysis confirms that while CIP and CIT manifest planar structures, DPT shows a curved π-conjugated carbon skeleton. The synthetic strategy starting from ortho-substituted benzene units to construct five-membered rings in this work provides a unique entry to novel pentagon-embedding or curved antiaromatic polycyclic hydrocarbons. In addition, besides the detailed chemical and physical investigations, microscale single-crystal fiber field-effect transistors were also fabricated.

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides.

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through p-ter-phenyl (PPP), ter-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°) in PPP-NN were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for TTT-NN and TPT-NN. The intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated by broken symmetry DFT calculations.

Spin-Delocalization in a Helical Open-Shell Hydrocarbon.

Neutral open-shell molecules, in which spin density is delocalized through a helical conjugated backbone, hold promise as models for investigating phenomena arising from the interplay of magnetism and chirality. Apart from a handful of examples, however, the chemistry of these compounds remains largely unexplored. Here, we examine the prospect of extending spin-delocalization over a helical backbone in a model compound naphtho[3,2,1-no]tetraphene, the first helically chiral open-shell hydrocarbon, in which one benzene ring is fused to [5]helicene, forming a phenalenyl subunit. The unpaired electron in this molecule is delocalized over the entire helical core composed of six rings, albeit in a nonuniform fashion, unlike in phenalenyl. In the case of a monosubstituted derivative, the uneven spin-distribution results in a selective σ-dimer formation in solution, as confirmed by 2D NMR spectroscopy. In contrast, the dimerization process is suppressed entirely when four substituents are installed to sterically hinder all reactive positions. The persistent nature of the tetrasubstituted derivative allowed its characterization by EPR, UV-vis, and CD spectroscopies, validating spin-delocalization through a chiral backbone, in accord with DFT calculations. The nonuniform spin-distribution, which dictates the selectivity of the σ-dimer formation, is rationalized by evaluating the aromaticity of the resonance structures that contribute to spin-delocalization.

Cethrene: A Helically Chiral Biradicaloid Isomer of Heptazethrene.

We report the synthesis and properties of "cethrene", the only helically chiral isomer of heptazethrene with a biradicaloid singlet ground state. Cethrene gives a well-resolved EPR spectrum at room temperature and its structure was confirmed by 2D NMR and absorption spectroscopies. Our experiments and calculations show that the helical twist affects its electronic properties and decreases the singlet-triplet energy gap when compared to that of planar heptazethrene. Cethrene undergoes an intramolecular cyclization within several hours at room temperature.

"Tschitschibabin type biradicals": benzenoid or quinoid?

Our findings provide a better understanding of the discrepancies related to the discussion in the literature on the ground state of Tschitschibabin's hydrocarbon. A series of phenylene bridged bisnitroxide molecules were designed and studied for the comparison. The simple theoretical and experimental methodologies have been developed and utilized to understand the singlet biradicaloids which exist in semi-quinoid form and exhibits characteristics of biradicaloid and quinoid form simultaneously.